Metal–Organic Frameworks (MOFs) and MOF-Derived Materials ... · 30 Electrochemical Energy...

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Electrochemical Energy Reviews (2019) 2:29–104 https://doi.org/10.1007/s41918-018-0024-x

REVIEW ARTICLE

Metal–Organic Frameworks (MOFs) and MOF‑Derived Materials for Energy Storage and Conversion

Xu Zhang1 · An Chen1 · Ming Zhong1 · Zihe Zhang1 · Xin Zhang1 · Zhen Zhou1 · Xian‑He Bu1

Received: 10 August 2018 / Revised: 20 September 2018 / Accepted: 19 October 2018 / Published online: 17 November 2018 © Shanghai University and Periodicals Agency of Shanghai University 2018

AbstractAs modern society develops, the need for clean energy becomes increasingly important on a global scale. Because of this, the exploration of novel materials for energy storage and utilization is urgently needed to achieve low-carbon economy and sustainable development. Among these novel materials, metal–organic frameworks (MOFs), a class of porous materials, have gained increasing attention for utilization in energy storage and conversion systems because of ultra-high surface areas, controllable structures, large pore volumes and tunable porosities. In addition to pristine MOFs, MOF derivatives such as porous carbons and nanostructured metal oxides can also exhibit promising performances in energy storage and conversion applications. In this review, the latest progress and breakthrough in the application of MOF and MOF-derived materials for energy storage and conversion devices are summarized, including Li-based batteries (Li-ion, Li–S and Li–O2 batteries), Na-ion batteries, supercapacitors, solar cells and fuel cells.

Keywords MOFs · Batteries · Supercapacitors · Fuel cells · Solar cells

PACS 82.45.FK Electrodes · 88.40.H- Solar cells · 88.80.F- Energy storage technologies · 88.80.ff Batteries · 88.80.fh Supercapacitors

1 Introduction

Globally, we are facing multiple challenges arising from the depletion of fossil fuels and the emission of large amounts of CO2. As a result, the shift to low-carbon economy is becom-ing increasingly important. Based on this, the development and utilization of clean energy sources such as wind and solar energy has attracted great attention from researchers all over the world. However, although these energy sources are prom-ising, one of the most challenging issues faced by researchers is the intermittent nature of wind and solar power. To deal

with this issue, researchers have developed electrochemical and photoelectric energy storage and conversion systems, such as Li-ion batteries (LIBs), supercapacitors (SCs), fuel cells and solar cells, to store and utilize these intermittent clean energy resources. Despite the promising performance of these storage methods, the service life, rate performance, energy density, as well as safety of these batteries and SCs need to be further improved and costs need to be reduced. Because of this, the exploration of novel materials for energy storage and conversion systems to provide better performances and lower costs is a crucial area of study in materials science.

Of the various materials studied thus far, porous materi-als have been successfully applied to energy storage and conversion systems and shown promise because these mate-rials can provide large surface areas for chemical reactions and interfacial transport pathways for shortened diffusion paths [1]. Among the various classes of porous materials, metal–organic frameworks (MOFs), first defined by Yaghi et al. [2] in 1995, have gained wide attention over the past two decades for their promising properties in which in the synthesis of MOFs, the combination of a wide variety of organic ligands and metal ions or clusters is desirable and

* Zhen Zhou [email protected]

* Xian-He Bu [email protected]

1 School of Materials Science and Engineering, National Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300350, China

http://orcid.org/0000-0003-3232-9903

http://crossmark.crossref.org/dialog/?doi=10.1007/s41918-018-0024-x&domain=pdf

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have led to the discovery of over 20,000 MOFs, with more being discovered [3, 4]. Because of the nature of these com-binations, it is possible to obtain a large variety of control-lable structures with ultra-high surface areas [5], large pore volumes and tunable porosities [3] that can be applied in many fields related to porous materials, including traditional areas such as gas separation and storage [6–9], magnetism [10–13], catalysis [14–19], sensors [20, 21], proton conduc-tion [22, 23] and drug delivery [24, 25].

Transition metal ions or clusters can also serve as redox-active sites for electrochemical reactions. Although most pristine MOFs are insulators or semiconductors, the infiltra-tion of pores with redox-active conjugated guest molecules can be used to tune electrical conductivity [26], allowing pristine MOFs to be applied in energy storage and conver-sion systems such as fuel cells, batteries, supercapacitors and solar cells. Furthermore, because of the unique struc-ture of MOFs which possess ordered micro-/mesopores and abundant organic linkers, MOFs are also one of the most promising candidates to be used as templates or precursors to derive porous carbon with various morphologies through proper treatments [27]. In addition, MOFs contain abundant metal ions or clusters, allowing MOF alternatives to be capa-ble of obtaining metal/metal oxide materials with large sur-face areas and porous structures under suitable calcination conditions. These MOF-derived materials can subsequently inherit the advantages of MOFs, including inherent mor-phology, high surface area and tunable porosity, which are suitable for electrochemical and photoelectric applications. Finally, MOFs can be synthesized from relatively cheap pre-cursors through inexpensive and high-yield methods [28], further reducing costs in future large-scale applications.

Although several early reviews have summarized the application of MOFs in the field of energy storage and conversion, including fuel cells, LIBs and supercapacitors [29–42]; in recent years, investigations have increased at an exponential rate, with many important breakthroughs being reported. However, these breakthroughs have not been com-prehensively summarized, and therefore, a review into the latest progresses and breakthroughs concerning the appli-cation of pristine MOFs and derived materials in energy storage and conversion systems is urgently needed. Because of this, this review will summarize the latest reports on elec-trochemical and photoelectric applications of MOF-based materials to promote further developments in this field.

2 Applications of MOF‑Based Materials in Li‑Based Batteries

For decades, there have been many investigations into Li-based batteries including LIBs, Li-S batteries and Li-air bat-teries. In particular, LIBs are commercially successful energy

storage devices that have been widely applied in portable electronics. However, current commercial LIBs suffer from high costs, which restrict large-scale applications. This is further compounded by the development of electric vehicles (EVs), which increases the demand for energy density, power density and safety in Li-based batteries. Because of this, the development of promising, high-performance materials for Li-based batteries is an important goal in materials science.

2.1 MOFs for Li‑Ion Batteries

The cathode, the anode and the electrolyte are main com-ponents of LIBs [43, 44], and in anodes, Li is oxidized to Li+ and moves to the cathode through the electrolyte, with graphite and lithium cobalt oxide being the most commonly used materials for anodes and cathodes in commercial LIBs, respectively. However, the low theoretical capacities of these materials (372 mAh g−1 for graphite and 148 mAh g−1 for LiCoO2) are insufficient to meet the demands of future energy storage systems. Alternatively, MOFs are generated through organic ligands and metal ions or clusters with abundant pores and cavities and can provide Li+ storage sites and miti-gate volume changes during charge and discharge processes. In addition, metal ions or clusters can also serve as redox-active sites during the electrochemical process. Because of these desirable properties, MOFs are outstanding candidates for batteries. The various results obtained from the direct use of pristine MOFs as electrode materials are listed in Table 1.

2.1.1 Pristine MOFs for Li‑Ion Batteries

Cathodes Although there have been several attempts to apply pristine MOFs to LIBs [45, 46], lithium insertion usually leads to the irreversible decomposition of host materials (VSB-1 or MOF-177) and the formation of Ni or Zn-based nanocompos-ite matrixes containing Li2O, which causes large irreversible capacity loss. Despite these drawbacks however, these attempts have shed light on the application of pristine porous MOFs to LIBs, and researchers have reported that an effective method to overcome these poorly reversible reactions is to use earlier 3d transition metals (such as V, Cr and Fe) with lower occupations of 3d-electron orbitals, causing higher metal–O bond stability. In addition, the stability of mixed-valence states can also bring about long-range electron delocalization and is conducive to the storage and release of electrons. Based on this, Férey et al. [47] successfully applied [FeIII(OH)0.8F0.2(O2CC6H4CO2)] (MIL-53 (Fe)); a pristine MOF, as an intercalation cathode for LIBs and reported better reversibility and stability. In this study, in situ X-ray absorption fine structure (XAFS) analysis was conducted and indicated that the process of lithium extrac-tion was reversible at x = 0.5 (Li0.5MIL-53 (Fe)), demonstrating the redox-active flexibility of this MOF [48]. Furthermore, subsequent density functional theory (DFT) computations

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were conducted and disclosed the detailed mechanism of Li insertion in MIL-53 (Fe), in which four different sites were proposed for Li insertion [49–51]. These computational results also revealed that excessive Li insertion can lead to the com-plete loss of cohesive interaction between organic and inor-ganic networks, causing irreversible decomposition, which can explain the irreversible behavior of the resulting LIBs. Another serious issue of pristine MOFs is the poor electronic conduc-tivity, which can affect high rate performances. To improve this, de Combarieu et al. [52] introduced an electroactive guest molecule; 1,4-benzoquinone to MIL-53 (Fe), to increase π–π interactions. Despite promising initial performances however, the extra capacity associated with this guest molecule rapidly degrades during cycling because of the progressive exchange between quinone and dimethyl carbonate (DMC) molecules in the electrolyte. In another study, Shin et al. [53] applied another Fe-based MOF, MIL-101 (Fe), as the cathode material in LIBs and through the use of ex situ and in-operando X-ray absorption spectroscopy (XAS), reported that the relaxation of Fe2+ to Fe3+ can result in the irreversible accumulation of Li+, leading to capacity degradation. Recently, Yamada et al. [54] also investigated the application of MIL-101 (Fe) as the cathode material for LIBs and reported reversible charge and discharge cycles with a capacity of 0.62 Li/Fe after 100 cycles. In addition to Fe-based MOFs, Kaveevivitchai et al. [55] employed a V-based MOF, VIV(O)(BDC) (BDC = 1,4-benzen-edicarboxylate)), and MIL-47 (V), an iso-structure of MIL-53 (Fe), as the cathodes for LIBs and reported that because of the large open channels and higher V–O bond stability, more Li+ can be reversibly inserted (0.7 Li per V).

Aside from metal ions or clusters as redox-active sites, organic ligands can also be used. For example, Zhang et al. [56] used a Cu-based MOF, Cu(2,7-AQDC) (AQDC = anth-raquinone dicarboxylate), as the cathode for LIBs, which was the first microporous MOF material with independent

redox-active sites on both the metal and ligands. Here, the researchers reported that within a voltage range of 1.7–4.0 V, an initial specific capacity of 147 mAh g−1 can be obtained, which rapidly decreases to ~ 105 mAh g−1 after 50 cycles. The researchers in this study attributed this capacity loss to the insufficient extraction of Li+ during the charge process as well as the large charge transfer resistance which further hindered the movement of Li+.

In the MOFs discussed above, the general mechanism of Li insertion involves the reduction in active sites and the insertion of Li ions into the void space of the MOFs. However, because of the high resistivity of MOFs, MOFs can only be partially reduced in the discharge step and Li+ are insufficiently extracted in the charge process, causing capacity losses and low gravimetric capacities. To address this, Zhang et al. [57] prepared a flexible redox-active MOF, Mn7(2,7-AQDC)6(2,6-AQDC)(DMA)6 (DMA = N,N-dimethylacetamide), as a cathode for LIBs and reported a new mechanism during the charge–discharge process which the researchers referred to as “bipolar charging.” In this mechanism, Mn(II) cations are oxidized to Mn(III) with the intercalation of PF6

− ions from the electrolyte during the charge process. During the discharge process, PF6

− ions are released back into the electrolyte, followed by the insertion of Li ions and the reduction of organic ligands (Fig. 1). The researchers in this study also reported that both anions and cations can contribute to capacity in this “bipolar charging” mechanism, and that it is beneficial to reduce the ionic traffic of the cathode with the same theoretical capacity.

MOFs containing lithium ions have also been investigated and recently, Schmidt et al. [58] synthesized lithium iron methylenediphosphonate (Li1.4Fe6.8[CH2(PO3)2]3[CH2(PO3)(PO3H)]·4H2O) as the cathode for LIBs and reported that a capacity of 128 mAh g−1 can be preserved after 200 cycles

Table 1 Rate (mA g−1), initial discharge capacity (DC, mAh g−1), charge capacity (CC, mAh g−1) cycle number (CN) and capacity after cycles (AC, mAh g−1) of pristine MOFs for LIBs

MOF Rate DC/CC CN AC Reference

Cathode MIL-101 (Fe) 0.2 C 60/– 100 72 [54] Cu(2,7-AQDC) – 147/~ 35 50 105 [56] Mn7(2,7-AQDC)6(2,6-AQDC)(DMA)6 – 205/~ 60 50 ~ 190 [57] Lithium iron methylenediphosphonate 20 97/~ 210 200 128 [58]

Anode MOF-177 50 400/105 2 105 [46] Mn-BTC MOF 100 1717/694 100 576 [60] Zn(IM)1.5(abIM)0.5 100 –/– 200 190 [61] CaC8H4O4 14 340/140 100 118 [62] SrC8H4O4 13.7 –/– 50 91 [63] BaC8H4O4 13.7 –/– 50 131 [63] Hetero-triple-walled MOF 50 1108/345 70 300 [64] NNU-11 50 1322/810 200 750 [65]


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as a result of the little change in lattice parameters during Li+ insertion.

Anode MOFs have also been investigated as anodes for LIBs. For example, Ogihara et al. [59] proposed an interca-lated metal–organic framework (iMOF) (2,6-naphthalene dicarboxylate dilithium) for Li storage and reported based on crystal structure analysis during Li intercalation that lay-ered frameworks can be maintained and volume changes are remarkably small. As a result, the iMOF anode exam-ined in this study exhibited favorable cycling stabilities with 96% capacity retention after 100 cycles. Furthermore, a 4-V cell fabricated by using this iMOF as the anode and LiNi0.5Mn1.5O4 spinel as the cathode reportedly exhibited a high specific energy of 300 Wh kg−1 and a high specific power density of 5 kW kg−1. Mn-based MOFs have also been suc-cessfully applied as anodes. For example, Maiti et al. [60] syn-thesized a Mn-based MOF (Mn-1,3,5-benzenetricarboxylate (Mn-BTC MOF)) and applied it as the anode for LIBs. Here, the researchers suggested that the COO− groups can play an important role in Li insertion (proposed sites of Li insertion are shown in Fig. 2a) and that the conjugated carboxylate-containing aromatic cores with strong π–π interactions can stabilize the MOF structure, resulting in this MOF-based anode providing a high specific capacity of 694 mAh g−1 at 0.1 A g−1 with ~ 83% retention after 100 cycles. Furthermore, the grafting of carbon nanotubes (CNTs) into Mn-BTC MOF networks is an effective method to increase electrical con-ductivity and improve electrochemical performance. Based on this, Lin et al. [61] introduced a mixture of imidazole (IM) and 2-aminobenzimidazole (abIM), and synthesized a bi-functionalized MOF (BMOF, Zn(IM)1.5(abIM)0.5) that possessed hydrophobic and polar functionalities with remark-able thermal and chemical stabilities. In this study, the sub-sequent DFT computations and XRD patterns revealed that Li ions can be stored in the functionalized pores and that the framework remained intact during the charge and discharge process. Host–guest interactions between Li ions and amine

groups/N can also greatly affect Li storage; therefore, increas-ing active N-rich functional groups and surface area per pore volume can further enhance Li storage capacity. As an exam-ple, Wang et al. [62, 63] employed different types of alkaline earth metal-based MOFs (MC8H4O4 (MTPA, M = Ca, Sr, Ba)) as the anode of LIBs and reported that the electrostatic interaction between the metal cations and terephthalate anions mainly affected the crystallography and molecular structure, in which MOFs with smaller cationic radii exhibited stronger ionic bonding, lower discharge potentials and higher discharge capacities with less capacity degradation.

Using the mixed-molecular building block (MBB) strat-egy, Tian et al. [64] constructed a hetero-triple-walled MOF

Fig. 1 The mechanism and conceptual scheme of “bipolar charging” chemistry. Reprinted with permission from Ref. [57]

Fig. 2 a Probable de-lithiation sites for coordination with Li in the organic moiety of the Mn-BTC MOF; b cycling performance of the Mn-BTC MOF. Reprinted with permission from Ref. [60]


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with high chemical stability and reported that due to its stability, this MOF can avoid capacity decay and improve cyclability if used as the anode of LIBs. In addition, theses researchers reported that due to the multiple ligands within the confined pores, this hetero-triple-walled MOF can also provide more redox-active sites, resulting in good Li storage performances, and a high capacity of ~ 300 mAh g−1 after 70 cycles at 50 mA g−1 with a Coulombic efficiency of > 95% after 15 cycles. Based on these results, the preparation of multi-walled MOFs as electrodes is a promising method to enhance electrochemical performance, and especially cyclic performance. In another example, Huang et al. [65] recently synthesized a polyoxometalate-based MOF (NNU-11) and reported that because of the multi-electron redox and func-tionalization of the MOF, NNU-11 can achieve a capacity of 750 mAh g−1 at 50 mA g−1 after 200 cycles.

Protection In addition to the direct application of pristine MOFs as electrode materials, Han et al. [66] suggested that MOFs can also be used as a type of protection “net” or a “cushion” for silicon anodes. This is because although Si is one of the most promising anode materials for LIBs due to its ultra-high theoretical capacity, large volume changes during the lithiation/de-lithiation process result in poor cyclability [67]. Therefore, if MOF films can be casted onto the surface of Si (Fig. 3a), the resulting electrode can exhibit improved cycling performances, in which this novel MOF sandwich coating (MOF-SC) can serve as a protective cushion for the large volume change in Si anodes during the lithiation/de-lithiation process, reducing the direct exposure of Si to electrolytes and help form stable solid electrolyte interphase (SEI) films. Based on this, several representative MOFs such as zeolitic imidazolate framework-8 (ZIF-8), ZIF-67, MOF-5, HKUST-1, MIL-53 and NH2-MIL-53 (Fig. 3b) have been investigated and the results suggest that flexible MOFs such as MIL-53 and NH2-MIL-53, or MOFs with small apertures such as ZIF-8 and ZIF-67 can provide better improvements in cycling stability as compared with MOFs with open chan-nels such as MOF-5 and HKUST-1 (Fig. 3c). Moreover, the results in this study also suggest that large pore volumes and ultra-high surface areas can also help in holding more elec-trolytes, promoting the diffusion of Li+. To further improve the Li+/electronic conductivity of the interface between Si and ZIF-8, Yu et al. [68] introduced a TiN/Ti buffer (Fig. 3d) and reported enhancements in the capacity of ZIF-8/TiN/Ti/Si nanorods. Here, DFT computations were conducted and revealed that the diffusion barrier of Li+ in the channels of ZIF-8 (~ 0.29 eV) was much lower than that in Si (~ 0.60 eV).

MOFs can be also applied to modify Li-rich layered oxide cathodes to alleviate oxygen evolution and improve the cycle stability and initial Coulombic efficiency of Li-rich layered oxides. This is because although Li-rich layered oxides have attracted substantial attention as LIB cathodes due to large capacities, oxygen loss causes a series of problems

such as large initial irreversible loss, structure transforma-tion and electrolyte oxidation or decomposition. In one example, Qiao et al. [69] modified a Li-rich layered oxide (Li(Li0.17Ni0.20Co0.05Mn0.58)O2) using a Mn-based MOF and reported that the MOF-modified sample exhibited better performances including a larger discharge capacity, higher initial Coulombic efficiency and better thermal stability than the sample without the MOF modification. The researchers in this study suggested that these improvements result from the good oxygen storage capability of the MOF layer stabi-lizing the Li-rich layered oxide.

The direct utilization of Li metal as an anode is a promis-ing option for next-generation LIBs due to leading theoreti-cal capacities of 3860 mAh g−1 or 2061 mAh cm−3 [70]. However, dendrite growth and low Coulombic efficiency limit practical application. To address these issues, Liu et al. [71] reported the use of MOF materials to improve the cyclability of Li metal anodes. In their study, the researchers coated a NH2-MIL-125 (Ti) MOF onto a commercial poly-propylene separator membrane (Celgard 3501) and reported that the obtained composite separator enabled dendrite-free and dense Li deposition as well as long-term reversible Li plating/stripping, contributing to higher Li+ transference numbers and uniform Li nucleation and providing a new approach of using MOF materials for Li anode protection.

Electrolytes Aside from electrodes, the electrolyte is also an important component for improving energy/power den-sity, cycle life, costs and safety of batteries [72]. Because traditional liquid electrolytes suffer from potential safety issues due to volatility and flammability, the design and synthesis of novel solid electrolytes is important. How-ever, insufficient ionic conductivity restricts the applica-tion of solid electrolytes, and therefore, the exploration of possible MOFs as solid electrolytes has gained increasing attention in recent years. For example, Wiers et al. [73] applied Mg2(dobdc) (dobdc = 1,4-dioxido-2,5-benzenedi-carboxylate) as a solid lithium electrolyte and reported that the uptake of LiOiPr in Mg2(dobdc) followed by soaking in a 1 M solution of LiBF4 in 1:1 EC/DEC (EC = ethylene carbonate, DEC = diethyl carbonate) can result in a solid electrolyte of Mg2(dobdc)·0.35LiOiPr·0.25 LiBF4·EC·DEC that can provide a conductivity of 3.1 × 10−4 S cm−1. In a subsequent study, these researchers also proposed a new solid electrolyte, tert-butoxide (LiOtBu) grafted UiO-66 (Zr6O4(OH)4(bdc)6), for Li-based batteries [74]. In another example, Liu et al. investigated the effects of nanosized MOF-5 [75] and MIL-53 (Al) [76] as fillers in poly(ethylene oxide) (PEO) and lithium bistrifluoromethane sulfonylimide (LiTFSI) based electrolytes, which were tested in LiFePO4/electrolytes/Li cells. Subsequently, Stephan et al. applied three types of MOFs (Al-BTC MOF [77], Mg-BTC MOF [78] and Cu-BDC MOF [79]) as fillers in PEO and LiTFSI matrixes. Overall, the results in these studies suggest that


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the incorporation of MOFs in PEO and LiTFSI matrixes is useful in the enhancement of electrolyte ionic conductivity and thermal stability. In particular, in the case of MIL-53 (Al), the researchers reported that the Lewis acidic sur-face of MIL-53 (Al) can interact with TFSI to promote the dissolution of LiTFSI and increase the lithium ionic con-ductivity. Moreover, these researchers also suggested that MOF particles can act as cross-linking centers for PEO and increase the thermal stability and mechanical strength of the electrolyte due to a more robust network. In addition, Wang et al. [80] recently reported a MOF-based solid-like electrolyte (Li-IL@MOF SLE) that can be used in recharge-able Li/LiFePO4 solid-state battery systems for the first time. In this study, the researchers attributed the high ionic con-ductivity (3.0 × 10−4 S cm−1 at normal temperatures) and excellent compatibility and stability of the Li-IL@MOF SLE to the unique nano-wetted interfacial mechanism. In addition, the researchers here also reported that this notable

performance can be retained over a wide temperature range (−20–150 °C), which they attributed to the rapid 3D-con-nected Li+ transport that is facilitated by the high ionic conductivity and nano-wetted interface of the Li-IL@MOF SLE.

Modified traditional liquid electrolytes can also protect Li anodes to improve battery performances. For example, Bai et al. [81] successfully prepared a novel MOF-modified electrolyte that possessed considerable angstrom-level pores to achieve homogeneous Li electro-deposition in pristine liquid electrolytes, efficiently inhibiting Li dendrite growth. In this study, the tested Li||Li/Li||Cu symmetric cell as well as the Li-Li4Ti5O12 battery using this type of MOF-modified electrolyte demonstrated remarkable long-term cycling sta-bility at high current densities (~ 7 mA cm−2). Furthermore, inspired by the ionic channels of biological systems, Shen et al. [72] investigated a new class of solid-state electrolytes with biomimetic ionic channels in which they used a MOF

Fig. 3 a Representative procedure of the coating method, the struc-ture of the sandwich electrode and the possible protection mecha-nism; b crystal structures of HKUST-1, MOF-5, ZIF-8, ZIF-67, MIL-53 and NH2-MIL-53; c capacity of the 6th cycle and capac-

ity retention after 30 cycles; d schematic capacity of Si, TiN/Ti/Si, ZIF-8/Si and ZIF-8/TiN/Ti/Si NRs as anodes for LIBs. Reprinted with permission from Refs. [66, 68]


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(HKUST-1) as a scaffold for the novel electrolytes to create ionic channels and reported that these new electrolytes pos-sessed similar structures to glutamate-like ionic channels, allowing for low energy barrier (< 0.21 eV) Li+ transport.

2.1.2 MOF‑Derived Materials for Li‑Ion Batteries

MOFs are made up of tunable metal ions and organic link-ers and have controllable structures and porosity, as well as large pore volumes and high surface areas. Because of these properties, MOFs are one of the most promising can-didates for templates or precursors to prepare different types of derived materials such as carbon and metal oxides for electrochemical applications. Of the preparation methods for these derived materials, calcination is the most effec-tive. Moreover, heteroatoms such as N, S and P atoms gen-erally exist in organic linkers and can be used to prepare heteroatom-doped carbon materials, in which heteroatom doping can be effective in enhancing electronic properties and electrochemical reactivities and can provide active sites for Li storage. Overall, by combining conductive materials such as graphene or CNTs with heteroatom doping, elec-tronic conductivities can be increased and the use of in situ composite methods can further reduce agglomeration and collapse during the calcination process or cycling. In addi-tion, the regulation of morphologies after calcination is also important to achieve high capacity and stability [82] in which calcined products can inherit nanoscale cavities and open channels from MOF precursors, which is conducive to the enhancement of electrochemical performances. This is because increased specific surface areas can provide larger electrode/electrolyte interfaces which promote mass and electron transfer and hollow [83] or porous structures can relax volume changes during cycling which results in good cycling performances.

MOF-derived carbon Carbon-based materials are widely used as commercial anodes for LIBs because of their excel-lent cycling stability and electrical conductivity. In addition, porous carbon materials are among the most promising can-didates for anode materials because their porous nature and high surface areas can provide more active sites.

MOF-derived porous carbon materials were first reported by Liu et al. [27], who synthesized these materials by car-bonizing polyfurfuryl alcohol (PFA)/MOF-5 composites. Since then, MOFs have been extensively investigated as templates for preparing carbonaceous materials. For exam-ple, Zheng et al. [84] fabricated a N-doped graphene par-ticle analogue with a high nitrogen content of 17.72 wt% through the pyrolysis of ZIF-8 polyhedrons for lithium stor-age and reported that the resulting high nitrogen content carbon anode exhibited a high capacity of 2132 mAh g−1 after 50 cycles at 100 mA g−1 and 785 mAh g−1 after 1000 cycles at 5 A g−1 (Fig. 4a). Remarkably, most reported

N-doped carbon materials for LIBs possess a nitrogen con-tent of only ~ 10 wt% because higher nitrogen content in two-dimensional (2D) honeycomb lattices can result in structural instabilities. However in this study, the researchers attributed the resulting high performance to the graphene-analogous particles with high nitrogen content in the hexagonal lattice and edges. Moreover, DFT computations in this study dem-onstrated that the edge doping N atoms can provide extra Li storage capacities (Fig. 4b).

In another study, Xie et al. [85] prepared a sandwich-like graphene-based N-doped carbonaceous material as the anode for LIBs (Fig. 4c) through the pyrolysis of ZIF-8 in situ grown on graphene oxide (GO) and reported that the resulting porous N-doped carbon (PNC)/Gr material pos-sessed an ultra-high nitrogen content of 21.5 wt%, which is significantly higher than those of most reported N-doped carbon materials. The researchers also reported that this resulting anode exhibited high capacities with an outstand-ing rate capacity and cycling performance, and attributed these improved performances to the sandwich-like struc-ture and appropriate nitrogen content of the material. In a further study, Song et al. [86] also used ZIF-8 to prepare a porous cage-like nano-Si@C composite through the pyroly-sis of nano-Si@ZIF-8 and subsequent washing with HCl. Here, the researchers suggested that the resulting porous cage-like structure can relax the volume change of Si and provide more sites for Li+ insertion, resulting in high spe-cific capacities and good cycling performances. Zuo et al. [87] in their study prepared a pure three-dimensional (3D) porous carbon with a high surface area of 1880 m2 g−1 by calcining MOF-5 and reported that their 3D porous struc-ture with its high surface area can provide more sites for Li+ insertion/extraction and more channels for charge and ion transport. As a result, the material exhibited a high revers-ible capacity of 1015 mAh g−1 after 100 cycles and a good rate capability.

MOF-derived metal oxides Metal oxides have generated increasing interests as promising electrode materials for LIBs because they are inexpensive, eco-friendly and possess high theoretical capacities (approximately three times that of graphite). However, poor electrical conductivity, large vol-ume expansion and aggregation during the charge and dis-charge process result in rapid capacity decay and poor rate performances, limiting further application. Therefore, the rational design and controllable preparation of metal oxide materials with desirable morphology, size, porosity, surface area and composition are significant challenges hindering application in highly reversible and high-rate lithium storage systems. The application of MOF-derived metal oxides in LIBs is summarized in Table 2.

Co3O4 is considered to be a promising anode candidate for LIBs due to its high theoretical capacity and facile prepa-ration [88], and ZIF-67 is a typical MOF that can produce


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cobalt oxides with different morphologies under specific annealing treatments. Based on this, Wu et al. [89] prepared a ZIF-67 template possessing a rhombic dodecahedral morphology and subsequently prepared a highly symmet-ric porous Co3O4 hollow dodecahedron through a two-step thermal annealing process. In another study, Yu et al. [90] prepared nanostructured ZIF-67 with different morpholo-gies (bead-on-string morphology, core–shell structure, nano-flakes and nanotubes) by controlling experimental condi-tions including temperature, reaction time and concentration of solutions. After an annealing treatment, the researchers reported that the ZIF-67 nanotube array (ZIF-NTA) con-verted to a mesoporous Co3O4 nanotube array (Co3O4-NTA) and exhibited a high specific capacity, good rate capabil-ity and cyclic stability, which the researchers attributed to the special structure of the ZIF-67-derived Co3O4-NTA. In another study, Zhao et al. [91] used Ti nanowire arrays as the substrate to solve the issues of discontinuous growth and poor adherence of metal–organic framework films on

substrates using an electrochemically assistant method. Furthermore, Qu et al. [92] were able to obtain graphene-supported ultra-fine sized Co3O4 nanocrystallites (Fig. 5a) through the self-assembly growth of ZIF-67 dodecahedron on GO nanosheets and subsequent pyrolysis, which the researchers reported were capable of producing a high rate capacity (877 mAh g−1 at 5 A g−1) and long-term cycling stability (714 mAh g−1 after 200 cycles). Shao et al. [93] in their study fabricated a series of MOF (ZIF-67)-derived Co3O4 with different morphologies and graphene foam (GF) and reported that among these composites, the leaf-Co3O4/GF provided a high capacity of 986 mAh g−1 after 250 cycles at a current density of 100 mA g−1.

Aside from graphene, multi-walled carbon nanotubes (MWCNTs) have also been used to improve conductivity and mechanical/chemical stability. For example, Huang et al. [94] fabricated hierarchical porous MWCNT/Co3O4 nanocomposites through the thermal treatment of MWCNTs inserted ZIF-67 in air (Fig. 5b) and extended this method to

Fig. 4 a Cycling performance of the N-doped graphene parti-cle analogue at 100 mA g−1 and 5 A g−1; b top and side views of a simplified model with a large hole inside the 2D graphene and 18 Li atoms stored in the space; c the synthesis route for the sandwich-like PNC/Gr. Reprinted with permission from Refs. [84, 85]


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Table 2 MOF-derived metal oxides for Li-ion batteries

MOFs Rate (mA g−1)

DC/CC (mAh g-1)

CN AC (mAh g-1)

Reference

Cobalt oxides ZIF-67 Co3O4 dodecahedra 100 1371/921 100 780 [89] ZIF-67 Co3O4/graphene 5000 1865/988 200 714 [92] ZIF-67 MWCNTs/Co3O4 100 1171/812 100 813 [94] ZIF-67 (Zn, Co) MWCNTs/ZnCo2O4 100 ~1100/~ 700 100 755 [94] ZIF-67 CoO nanoparticle cookies 100 2687/2271 200 1383 [95] Co-NTCDA Co3O4 hexagonal rings 100 1324/~ 1050 30 1370 [97] Co(mIM)2 Co3O4 nanocages 500 975/~ 800 100 810 [98] MOF-71 Ball-in-dodecahedra 200 1286/879 60 913 [99] [Co3(HCOO)6]·DMF Co3O4 parallelepipeds 100 1680/1080 50 1100 [100] Co–Co PBA Co3O4 microframes 5000 –/– 200 1296 [101] [Co3L2(TPT)2·xG]n Co3O4 hollow tetrahedra 50 1370/975 60 1196 [102] ZIF-67 Co3O4 film 20,000 –/– 2000 – [91] ZIF-67 Leaf-Co3O4/GF 100 979/~ 530 250 986 [93] Co-MOFs (NUM-6) Co/Co3O4@N–C-700 100 1535/830 100 903 [170]

Fe2O3

MIL-88-Fe Spindle-like α-Fe2O3 200 1372/940 50 911 [104] MIL-53, MIL-88B α-Fe2O3 nano-spindles 100 1487/1024 40 ~900 [105] Fe-based ZIFs Fe2O3@N-C 100 1696/1368 50 1573 [106] PB Fe2O3 nanocubes 200 1294/– 50 800 [107] PDA-coated PB HPHNF nanocages 200 944/– 200 878 [108]

Mn2O3

Mn-LCP Porous Mn2O3 1000 1158/852 250 705 [110] Mn-based MOFs Mini-hollow Mn2O3 1000 –/– 1200 819 [111]

TiO2

MIL-125 (Ti) Porous TiO2 168 ~300/~ 170 500 166 [116] MIL-125 (Ti) Porous TiO2-C 5000 190/– 10,000 140 [117] MIL-125 (Ti) Mesoporous anatase 1680 126/123 1100 127 [118] MIL-125 (Ti) HPT 840 –/– 200 ~140 [119]

CuO MOF-199 CuO nanoparticles 100 1208/538 40 ~480 [121] MOF-199 CuO hollow octahedra 100 1208/483 100 470 [122] MOF-199 CuO@C 500 1140/598 300 512 [123] MOF-199 CuO/Cu2O 100 727/513 250 740 [124]

SnO2

Sn-based MOFs SnO2 nanoparticles 400 1600/832 100 541 [125] HKUST-1 SnO2@C 100 2134/1208 200 880 [126]

ZnO ZIF-8 ZnO@ZnO QD/C NRAs 500 785/722 100 699 [128] ZIF-8 Amorphous ZnO/carbon 50 ~1300/~ 800 50 750 [129] Zn–MS–Zn ZnO/C microboxes 100 1289/528 100 716 [131] Zn–Ni MOFs ZnO/NiO microspheres 100 1222/769 200 1008 [171] ZIF-67 V2O5@carbon 735 –/– 800 98 [133]

Bimetallic materials Zn–Co–ZIFs Porous ZnxCo3-xO4 100 1272/969 50 990 [134] Zn–Fe–ZIFs ZnFe2O4/C@NCNTs 100 2192/1121 100 844 [140] Co/Ni-MOF-74 Ni0.3Co2.7O4 2000 –/– 500 812 [141] Cu–Co–ZIFs CuCo2O4 100 977/793 50 740 [142]

Co-V-MOFs Co3V2O8 1000 1536/934 700 501 [145]


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fabricate MWCNT/ZnCo2O4 nanocomposites using MWC-NTs/ZIF-67 (Zn, Co) as the precursor. Here, the research-ers reported that the MWCNT/Co3O4 composite exhibited a high reversible capacity of 813 mAh g−1 at 100 mA g−1 after 100 cycles, whereas the MWCNT/ZnCo2O4 exhibited a capacity of 755 mAh g−1 at 100 mA g−1 after 100 cycles. The excellent electrochemical performances observed in this study can be attributed to the hierarchical porous struc-ture as well as the synergistic interaction between MCo2O4 (M = Zn, Co) and MWCNTs. Furthermore, Wang et al. [95] reported that a porous nanomaterial with excellent capacity and cycling stability consisting of CoO nanoparticles and a N-doped carbon matrix can be prepared (Fig. 5c) if ZIF-67 nanocrystals were annealed under a flow of Ar and air (50 mL min−1 including 48.5 mL Ar and 1.5 mL air). Here, the researchers attributed the superior performances to the unique structure and composition of the nanomaterial.

Other types of Co-based MOFs have also been used as precursors to obtain Co3O4 with different morphologies. For example, Su et al. [97] fabricated Co3O4 with different morphologies including hexagonal nanorings, nanoplates and nanoparticles by controlling the spatial hindrance of organic linkers and the release rate of Co2+ during the hydrolysis process of Co-based MOFs. In this study, Co-NTCDA (NTCDA = 1,4,5,8-naphthalenetetracarboxylic dianhydride) and Co-PTCDA (PTCDA = perylene-3,4,9,10-tetracarboxylic dianhydride), both of which are types of Co-based MOFs, were prepared and treated with organic amine solution and subsequently hydrolyzed to form the cor-responding hydroxides. These hydroxides were subsequently calcinated to obtain the final Co3O4, in which the Co3O4 hexagonal nanoring displayed a high reversible capacity of 1370 mAh g−1 after 30 cycles and good stability due to its

special structure which can facilitate the transfer process of Li+. Recently, Feng et al. [96] in their study used a liquid-phase deposition method to grow Co-based MOFs onto 3D nickel foam (3DNF) and annealed the resulting mate-rial to prepare Co3O4 nanosheet coated 3DNF (Fig. 5d) and reported that this as-prepared Co3O4/3DNF hybrid can be used as an anode without the need of any additional con-ductive materials or binders. In addition, the researchers in this study reported that the resulting 3DNF can increase the electrical conductivity and decrease the inner resistance of LIBs, resulting in the hybrid material exhibiting outstanding electrochemical performances, and especially an excellent cyclic stability and high rate capability. Co3O4 with differ-ent morphologies can also be obtained through the use of differing MOFs as precursors, such as nanocage structures through the annealing of Co(mIM)2 (mIM = 2-methylimi-dazole) [98], ball-in-dodecahedra through the pyrolysis of Co(bdc)(DMF) (MOF-71, DMF = N,N-dimethylformamide) (Fig. 6a) [99], porous hollow parallelepipeds through the calcination of Co-based MOF ([Co3(HCOO)6]·DMF) tem-plates (Fig. 6b) [100], microframes through the use of Co–Co Prussian blue analogue (PBA) microcubes (Fig. 6c) [101] and porous hollow tetrahedra through the use of [Co3L2(TPT)2·xG]n (TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine, G = guest molecular) self-sacrificing templates (Fig. 6d) [102, 103], and these structures all share similar characteristics. The first similarity is that they all possess large surface areas that can provide more active sites for Li+ insertion/extraction and sufficient contact between elec-trodes and electrolytes, facilitating the transport of Li+. The second similarity is that these materials all possess void spaces that can effectively alleviate large volume expansions. The last similarity is that these materials all possess robust

Table 2 (continued) MOFs Rate (mA g−1)

DC/CC (mAh g-1)

CN AC (mAh g-1)

Reference

Co–Ni-BTC MOFs CuO@NiO 100 1218/856 200 1061 [146] Zn-Co-MOFs ZnO/ZnCo2O4 2000 ~1800/~ 1100 250 1016 [147] Zn-Co-MOFs ZnO/ZnCo2O4/C 500 1278/974 250 669 [148] Zn-Co-MOFs ZnCo2O4-ZnO-C 100 930/667 150 1184 [149] Zn-based MOFs ZnO/Ni3ZnC0.7/C 500 1743/1015 750 1002 [150] Zn3[Co(CN)6]2 ZnO/Co3O4 100 2049/1164 100 957 [151] FeIII-modified MOF-5 ZnO/ZnFe2O4/C 500 1385/1047 100 1390 [152] ZIF-67 NiCo2O4/NiO HD 200 1622/1030 100 1497 [153] PBA Fe2O3@NiCo2O4 100 1311/902 100 1080 [154] α-MnO2/ZIF-8 MnO@ZnMn2O4/N–C 1000 –/– 200 595 [155] MIL-88 NiFe2O4/Fe2O3 100 1400/989 100 937 [156] MIL-88 NiFe2O4@TiO2 100 1550/933 100 1034 [157] Ni/Mn-1,3,5-BTC

BMOFsNiMn2O4 126 1738/1049 200 621 [172]


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structures that can provide high durability, resulting in good cycling stabilities.

Iron oxides are also promising anode materials for LIBs because of their much lower costs than cobalt oxides, which is important for commercial applications. As an example, Xu et al. [104] reported a two-step calcination method to prepare spindle-like mesoporous α-Fe2O3 through the use of MIL-88-Fe as a template (Fig. 7a). In this study, MIL-88-Fe was first heated at 500 °C and calcinated at 380 °C in air. For comparison, a one-step method in which MIL-88-Fe was directly heated at 380 °C in air was also conducted to prepare a reference α-Fe2O3. As a result, the researchers reported

that the as-prepared α-Fe2O3 using the two-step calcination method possessed a higher Brunauer–Emmett–Teller (BET) surface area of 75 m2 g−1 and demonstrated better electro-chemical performances (Fig. 7b) in which the capacity of the spindle-like mesoporous α-Fe2O3 retained 911 mAh g−1 after 50 cycles at 200 mA g−1 and even achieved 424 mAh g−1 at 10 A g−1. In another example, Banerjee et al. [105] prepared high electrochemically performing α-Fe2O3 nano-spindles using a one-step pyrolysis of Fe-based MOFs at 550 °C (Fig. 7c,d) and reported that the resulting α-Fe2O3 nano-spindles exhibited a reversible capacity of 1024 mAh g−1 at 100 mA g−1 with a capacity retention of over 90% after 40

Fig. 5 Schematic illustration of the preparation of a graphene/Co3O4, b MWCNTs/Co3O4, c CoO nanoparticle cookies, and d Co3O4/3DNF composites. Reprinted with permission from refs. [92, 94–96]


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cycles, indicating that calcination conditions play an impor-tant role in determining the electrochemical performance of obtained materials.

Fe2O3/carbon composites can also be obtained through the annealing of Fe-based MOFs. For example, Zheng et al. [106] prepared ultra-small Fe2O3 nanoparticles uni-formly embedded in N-doped hollow carbon sphere shells (Fe2O3@N–C) through a one-step pyrolysis of Fe-based ZIF at 620 °C in air. Here, the researchers reported that the unique structure of the resulting material effectively avoided

the aggregation of Fe2O3 particles during the lithiation/de-lithiation process, resulting in reduced volume changes and rapid electrochemical kinetics, thus exhibiting a high capacity of 1573 mAh g−1 after 50 cycles at 100 mA g−1 and 1142 mAh g−1 after 100 cycles at 1 A g−1. Prussian blue (PB) can also be applied as the template or precursor to obtain iron oxides. For example, Zhang et al. [107] pre-pared porous Fe2O3 nanocubes through the heat treatment of PB nanocubes in air and reported high lithium storage capacities and excellent cycling stabilities due to the porous

Fig. 6 Schematic illustration of the formation of Co3O4 with dif-ferent morphologies. Reprinted with permission from refs. [99], [100], [101], and [102]


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structure. Moreover, annealing MOFs under the protection of inert gases such as N2 or Ar can be used to prepare metal oxide/carbon composites. As an example, Wang et al. [108] prepared hollow N-doped Fe3O4/carbon (HPHNF) nanoc-ages with a hierarchical porous structure by calcining poly-dopamine (PDA)-coated PB at 550 °C in N2 atmosphere and reported that the HPHNF nanocages displayed good electrochemical performances because of the N-doped car-bon framework and the porous structure. In another study, Li et al. [109] prepared carbon-decorated Fe3O4 (C–Fe3O4) with micro-cuboid-like morphologies through the annealing of Fe-based MOFs at 400 °C under flowing N2 and reported that the carbon network facilitated the formation of stable SEI films as well as improved conductivities. Overall, the results in these studies demonstrate that the introduction of proper carbonaceous materials can serve to enhance electro-chemical performances.

Mn2O3 is another promising metal oxide anode for LIBs because it can provide higher energy densities as compared with other transition metal oxides due to the low potential of manganese [110]. In one example, Bai et al. [110] pre-pared porous Mn2O3 through the calcination of Mn-MOF ([Mn(Br4-bdc)(4,4′-bpy)(H2O)2]n (Mn-LCP, Br4-bdc = tetra-bromoterephthalate and 4,4′-bipyridine)) at 600 °C in air and reported that the as-prepared Mn2O3 provided excellent electrochemical performances due to its hierarchical porous morphology, in which the reversible capacity remained at 705 mAh g−1 after 250 cycles at 1 A g−1 and even at 4 A g−1, a specific capacity of 450 mAh g−1 can be obtained. Recently, Cao et al. [111] prepared a polyhedron Mn2O3 with a small interior cavity (the mini-hollow polyhedron Mn2O3) by heating Mn-MOFs at 750 °C in air and reported that the mechanism of lithium storage in the mini-hollow polyhedron Mn2O3 can be presented as follows:

Fig. 7 a SEM image of MIL-88-Fe and illustration of the fabrication of spindle-like porous α-Fe2O3; b rate performance of spindle-like α-Fe2O3 and bulk α-Fe2O3 (1 C = 1 A g−1); c SEM image of pyro-

lyzed α-Fe2O3; d rate performance of the Li/α-Fe2O3 nano-spindles half-cell. Reprinted with permission from refs. [104] and [105]


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For comparison purposes, the electrochemical performance of a bulk polyhedron Mn2O3 electrode was also investigated in this study and compared with the mini-hollow Mn2O3 electrode, the bulk polyhedron Mn2O3 electrode exhibited sluggish kinetics and worse electrochemical performances. Here, the researchers suggested that the cavity space of the mini-hollow Mn2O3 can mitigate volume expansions and that during the first cycle, the mini-hollow feature is beneficial for the reconstructed hierarchical nanostructure which can remain stable for long cycles (Fig. 8a). As a result, the mini-hollow Mn2O3 electrode achieved higher specific capacities, longer cycle life spans and better rate capacities (Fig. 8b). Similarly, Mn2O3 octahedra with a hierarchical porous struc-ture, obtained through the heat treatment of Mn-MOFs (Mn-MIL-100), can also display high capacities, cycling stabilities and remarkable rate performances [112]. In addition, Mn-BTC MOFs have also been used as templates or precursors to prepare manganese oxides in which Mn2O3 porous nanobars can be prepared by calcining Mn-BTC MOFs in air [113] and porous MnO@carbon nanorods can be obtained in N2 atmos-phere [114]. Here, the carbon matrix of the MnO@carbon was found to be able to enhance conductivity and release volume change, resulting in good electrochemical performance.

Titanates have also gained great attention in recent years as potential anodes for LIBs due to their excellent safety and rate capability. Among various titanates, Li4Ti5O12 spinel is already being used as a commercial anode. Furthermore, because TiO2 possesses a higher theoretical specific capac-ity than Li4Ti5O12 [115], TiO2 is a particularly attractive anode material for LIBs and MIL-125 (Ti) is a promising template or precursor to prepare TiO2-based materials. For example, Wang et al. [116] were the first to prepare a mod-erately porous TiO2 through the calcination of MIL-125 (Ti) in air (Fig. 9a) in which TiO2 particles can be gener-ated without long-range atomic migration in MIL-125 (Ti), allowing TiO2 to easily inherit the porous structure of MIL-125 (Ti). In another study, Wang et al. [117] conducted a two-step heat treatment of MIL-125 (Ti) involving a pre-heating step at 300 °C followed by calcination at high tem-peratures to produce porous TiO2–C composites (Fig. 9b) that produced a high reversible capacity of ~ 400 mAh g−1 at 100 mA g−1 and a capacity of 140 mAh g−1 at 5 A g−1 even after 10,000 cycles. In a further study, Xiu et al. [118] pre-pared mesoporous anatase TiO2 (MAT) (Fig. 9c) by directly

(1)First discharge: 3Mn2O3 + 2Li+ + 2e− → 2Mn3O4 + Li2O

(2)Mn3O4 + 8Li+ + 8e− → 3Mn + 4Li2O

(3)Afterward: 3Mn + 4Li2O ↔ M3O4 + 8Li+ + 8e−

(4)Mn3O4 + (3x − 4) Li2O ↔ 3MnO

x+ (6x − 8) Li+ + (6x − 8) e−

calcining MIL-125 (Ti) in air and reported that the MAT electrode exhibited good electrochemical performances due to the mesoporous characteristics. In a subsequent study, the same group prepared hierarchical meso-/macroporous anatase TiO2 (HPT) (Fig. 9d) through the hydrolysis of MIL-125 (Ti) followed by calcination in air [119] and reported superior rate capabilities as compared with their previous sample obtained through the direct calcination of MIL-125 (Ti). Here, the researchers attributed this improvement in performance to the hierarchical porous nano-architecture which shortens electron and Li+ transport distances and increases electrode and electrolyte contact areas.

CuO is another promising metal oxide anode for LIBs because of its abundant reserves, environmental friendli-ness and a high theory capacity of 674 mAh g−1 [120] and Banerjee et al. [121] were the first to apply CuO nanostruc-tures derived from MOFs as anodes for LIBs. In this study, pure-phase CuO nanoparticles were prepared by pyrolyzing a Cu-based MOF (MOF-199) at 550 °C in air, resulting in a reversible capacity of ~ 538 mAh g−1 at 100 mA g−1 and a ~ 90% initial reversible capacity retention after 40 cycles, demonstrating the good electrochemical performance of MOF-derived CuO. Subsequently, Wu et al. [122] obtained CuO hollow octahedra through a two-step thermal anneal-ing of MOF-199 in which MOF-199 was heated at 300 °C in nitrogen gas and subsequently in air and reported that the as-prepared CuO octahedra exhibited excellent anodic per-formance in LIBs, demonstrating that by controlling calcina-tion conditions, derived metal oxide materials can acquire various morphologies and exhibit different electrochemical performances. To buffer volume change and prevent CuO aggregation during the charge and discharge process so as to promote electron transport and Li+ diffusion, Chen et al. [123] prepared CuO@mesoporous carbon multi-yolk-shell octahedra using a two-step annealing treatment of MOF-199/polyvinylpyrrolidone (PVP), in which MOF-199/polyvi-nylpyrrolidone (PVP) was first heated at 700 °C in nitrogen gas and subsequently annealed at 350 °C in air, leading to the resulting CuO@C multi-yolk-shell octahedra display-ing outstanding cycling stabilities and a high capacity of 512 mAh g−1 at 500 mA g−1 after 300 cycles. In another study, Hu et al. [124] conducted the direct thermal decom-position of MOF-199/polyvinylpyrrolidone (PVP) at 350 °C in air to prepare CuO/Cu2O hollow polyhedra with porous shells and reported not only a fascinating architecture, but also synergistic effects between the different components, contributing to excellent electrochemical performances in which a reversible storage capacity of 740 mAh g−1 at 100 mA g−1 after 250 cycles was obtained.

SnO2 is also considered to be a promising anode candi-date for LIBs due to its ultra-high theoretical capacity (as large as 1494 mAh g−1). Sun et al. [125] were the first to prepared SnO2 nanoparticles by heating Sn-based MOFs and


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reported that the as-prepared nano-SnO2 achieved a high capacity of ~ 541.8 mAh g−1 at 400 mA g−1 after 100 cycles due to the nanostructure of SnO2 which can shorten the elec-tron transport and Li+ diffusion distances and increase the electrode and electrolyte contact area.

Aside from using Sn-based MOFs as precursors to obtain SnO2, utilizing the adsorption properties of MOFs to prepare metal oxide materials was also proposed by Wang et al. [126] and in their study, HKUST-1, a type of Cu-based MOFs, was first prepared, to which Sn was introduced by means of adsorption followed by calcination at 600 °C in air to prepare Sn–Cu@C. This Sn–Cu@C composite was subsequently washed with nitric acid to obtain the final SnO2@C (Fig. 10). Here, the high conductivity of carbon was effectively com-bined with the high capacity of SnO2 nanoparticles, contrib-uting to good electrochemical performances, leading to the resulting nanocomposite being capable of achieving a high reversible specific capacity of 880 mAh g−1 after 200 cycles at 100 mA g−1 and an excellent rate capability (Fig. 10c). Furthermore, this facile method of using the adsorption

properties of MOFs can also be extended to prepare other nanomaterials through the rational combination of injected species and recipient MOFs. Similarly, Li et al. [127] used the adsorption properties of MOFs to prepare NiP2@C nano-particles through the calcination of Ni-MOF-74 with adsorp-tive red phosphorus at 600 °C under Ar atmosphere and reported that the in situ introduced porous carbon around the NiP2 nanoparticles can greatly improve electronic conductiv-ity and buffer volume expansion during the de-intercalation of Li+, leading to a high capacity of 359 mAh g−1 after 700 cycles at 1 A g−1 for the NiP2@C anode.

Many other metal oxides have also been prepared through MOFs as precursors. For example, Zhang et al. [128] devel-oped a method to grow ZnO@ZnO quantum dot (QD)/C core–shell nanorod arrays (NRAs) on flexible carbon cloth in which ZnO NRAs were grown on carbon cloth and used as sacrificial templates to controllably prepare ZIF-8 nanorods (NRs). In a further step, ZnO@ZnO QD/C NRAs were obtained after annealing in N2 gas (Fig. 11a). In this study, the researchers reported that the flexible carbon cloth

Fig. 8 a Schematic illustration of the structural evolution of mini-hollow, bulk and large-hollow polyhedron Mn2O3 elec-trodes with cycling; b cycling performance of the mini-hollow polyhedron Mn2O3 electrode at different current densities. Reprinted with permission from Ref. [111]


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can act as a current collector and improve electrical conduc-tivity and that the special morphology of the resulting mate-rial facilitates the fast transport of electrons. In addition, the researchers also reported that the outer carbon layer can suppress the volume expansion of ZnO, and that overall, the as-prepared material exhibited remarkable rate capabili-ties and cycling performances. In another study, Han et al. [129] prepared amorphous ZnO decorated porous carbon materials by pyrolyzing chitosan coated ZIF-8 nanocrystals and reported that the composite, possessing a hierarchal pore structure, exhibited a high capacity of 750 mAh g−1 at 50 mA g−1 with a capacity retention of 739 mAh g−1 after 50 cycles. Furthermore, Jung et al. [130] prepared ZnO/C

microboxes by annealing Zn-based MOFs (Zn–MS–Zn) in N2 atmosphere which exhibited a high specific capacity of 716 mAh g−1 at 100 mA g−1 after 100 cycles, excellent cyclability and high rate capability which Shen et al. [131] attributed to carbon which can buffer volume variations and prevent ZnO particle aggregation. Zhang et al. [132] also prepared NiO through the heating treatment of Ni-based MOFs and reported good electrochemical performances due to well-defined morphologies and structures, and recently, Zhang et al. [133] from another group prepared a V2O5@carbon composite by absorbing vanadium precursors into ZIF-67 and annealing the resulting composite in air. Here, the researchers reported that due to the synergistic effects

Fig. 9 Schematic illustration of the preparation of TiO2 with different morphologies. Reprinted with permission from refs. [116–119]


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of V2O5 and carbon, the resulting V2O5@carbon compos-ite exhibited good electrochemical performances with a

capacity of 130 mAh g−1 at 735 mA g−1 and 75.7% reten-tion after 800 cycles.

Fig. 10 a Schematic illustra-tion of the preparation of SnO2@C composites from MOFs; b cycling performance of SnO2@C at 100 mA g−1; c rate performance of SnO2@C. Reprinted with permission from Ref. [126]


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The adjustable metal ions of MOFs allow for the prepa-ration of bimetallic oxides, in which the complex chemi-cal compositions of the bimetallic oxides can lead to high electrochemical activities and the synergetic effects of bimetals can contribute to high specific capacities [135]. In particular, researchers have reported that Zn-based and Co-based bimetallic oxides have exhibited enhanced elec-trochemical performances as anodes for LIBs [135–138]. In addition, Zn–Co bimetallic oxides can be prepared through MOFs. For example, Wu et al. [134] fabricated highly sym-metric porous spinel ZnxCo3 − xO4 hollow polyhedra using a two-step annealing treatment of Zn–Co–ZIFs in which Zn–Co–ZIFs are heated at 400 °C under a N2 gas flow and subsequently in air (Fig. 11b), and reported that as-prepared ZnxCo3 − xO4 material exhibited a reversible capacity as high as 990 mAh g−1 at 100 mA g−1 after 50 cycles and dis-played excellent cycling stability and rate capability which the researchers attributed to the unique structure. This is because the ultra-fine building subunits of a few nanometers in size can facilitate the transport of Li+ and electrons and the hollow polyhedra with the porous shells and void spaces can endure volume changes. In addition, the high symmetry of the structure can provide more freedom to produce atomic

steps than asymmetric structures, facilitating the reaction of Li and ZnxCo3 − xO4, and improving electrochemical perfor-mances [139]. In another example, Wu et al. [140] prepared ZnFe2O4/carbon@N-doped carbon nanotube (NCNT) com-posites by calcining Zn–Fe–ZIF in N2 and obtained a high specific capacity and remarkable rate performances.

As for Co-based bimetallic oxides, they are also prom-ising anodes due to large theoretical capacities, and based on this, Li et al. [141] prepared mesoporous NixCo3 − xO4 (mainly NiCo2O4 and Ni0.3Co2.7O4) nanorods composed of interconnected nanoparticles with uniform distribution of the two metal species through the annealing of Co/Ni-MOF-74 at 450 °C in air (Fig. 12a). For comparison, the researchers also prepared mesoporous Co3O4 nanorods using the same procedure through the pyrolysis of Co-MOF-74. Here, the resulting electrochemical measurements indicated that the Co–Ni bimetallic nanorods performed better than the Co3O4 nanorods (Fig. 12b), which the researchers attributed to the synergistic effects between Ni and Co, and the increased electrical conductivity in which the two metal oxides assisted each other at different voltages and collectively improved electrical conductivity. In addition, this study also reported that the Ni0.3Co2.7O4 can retain reversible capacities

Fig. 11 Schematic illustration of the preparation of ZnO@ZnO QD/C NRAs on carbon cloth (a) and bimetallic ZIFs, and their conversion to spinel ZnxCo3-xO4 hollow polyhedral (b). Reprinted with permission from refs. [128] and [134]


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of 812 and 656 mAh g−1 after 500 cycles at 2 A g−1 and 5 A g−1, respectively (Fig. 12b), demonstrating not only the beneficial synergistic effects of the two species in the bimetallic oxide, but also that the ratio of the two species can influence electrochemical performances. Furthermore, other Co-based bimetallic oxides such as CuCo2O4 [142] and FeCo2O4 [143, 144] have also be prepared by using MOFs as templates or precursors with promising results.

Metal vanadates are widely investigated as cathode mate-rials for LIBs, and recently, Co3V2O8 with a sponge mor-phology has been prepared through the heat treatment of Co–V-based MOFs [145], in which the researchers reported that the as-prepared Co3V2O8 produced a specific capacity of 501 mAh g−1 at 1 A g−1 after 700 cycles and attributed this performance to the unique morphology arising from the

ordered array of MOF networks acting as the intermediate to prepare Co3V2O8.

MOFs can also be utilized as templates or precursors to prepare bi-component active metal oxides due to their capability to accommodate multiple metal atoms. These resulting materials can subsequently allow volume changes to occur in a stepwise manner and provide richer redox chemical kinetics and better electronic conductivity than corresponding single-metal materials. For example, Guo et al. [146] prepared CuO@NiO microspheres possessing a three-layer ball-in-ball hollow morphology by annealing Co–Ni–BTC MOFs in air (Fig. 13) and compared it with a NiO yolk–shell composite. Here, the researchers reported that the CuO@NiO anode exhibited a larger reversible capacity of 1061 mAh g−1 at 100 mA g−1, and attributed this to the synergistic effects of the dual-metal oxides and the

Fig. 12 a Schematic illustration of the preparation of a bimetal-organic framework and its derived mesoporous NixCo3-xO4 nanorods for lithium storage; b cycling performance and high rate cycling performance of NixCo3-xO4. Reprinted with permission from Ref. [141]


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multiple-layer yolk–shell nanostructure, in which Li+ can be inserted step-by-step as induced by the matching CuO@NiO composition from shell to core, and Li+ and electrons can be transported and volume changes can be accommodated as facilitated by the multiple-layer yolk–shell architecture.

In another study, Xu et al. [147] prepared hierarchical porous ZnO/ZnCo2O4 (ZZCO) nanosheets through the one-step thermal annealing of Zn–Co–MOFs in air and suggested that besides the porous hierarchical close-packed nanosheet structure, the synergistic effects between ZnO and ZnCo2O4 can contribute to outstanding electrochemical performances and that the Zn and Co nanoparticles formed in the cathodic process can act as a mutual buffer matrix that can alleviate structural strain during the charge and discharge process. As a result, the as-prepared ZZCO nanosheets exhibited a high specific capacity of 1016 mAh g−1 at 2 A g−1 after 250 cycles. Furthermore, Ge et al. [148] heated Zn–Co-MOFs using a two-step calcination method in which Zn–Co-MOFs were first heated in N2 and subsequently in air to prepare ZnO/ZnCo2O4/C with a porous core/shell structure and reported that the resulting material maintained a specific capacity of 669 mAh g−1 after 250 cycles at 0.5 A g−1. In another study, Li et al. [149] prepared a reduced graphene oxide (RGO)/ZnCo2O4–ZnO–C/Ni sandwich-structured material by growing a Zn–Co–MOF onto Ni foam and wrapping GO onto the MOF followed by annealing in N2. Here, the researchers reported that the RGO and Ni sub-strate can effectively provide two highways for charge trans-fer and that the carbon layers and open pores can buffer volume change and the pores can guarantee large contact areas with the electrolyte. In addition, the RGO used in this study can help to fix the polyhedra onto the Ni foam, allowing the sandwich-structured anode to exhibit good cycling stabilities, rate capabilities and high specific capaci-ties. In another example, Zhao et al. [150] prepared ZnO/Ni3ZnC0.7/C spheres by annealing yolk–shell Zn–MOF/Ni in an Ar flow and reported that besides the unique yolk–shell structure which can improve electrochemical performances,

synergistic effects were also present and provided apparent performance improvements. In addition, the metallic Ni in the resulting Ni3ZnC0.7 can serve as a catalyst to decom-pose Li2O products and promote the reversible reaction of ZnO + Li ↔ Zn + Li2O. As a result of these properties, a high reversible capacity of 1002 mAh g−1 after 750 cycles at 500 mA g−1 was retained. Other hybrid metal oxides con-taining ZnO, such as ZnO/Co3O4 [151] and ZnO/ZnFe2O4/C [152], have also been prepared by using MOFs as templates or precursors and have reportedly exhibited excellent cycling performances and high capacities, and in these metal oxides, the improved performances can also be attributed to the syn-ergistic effects of the two individual metal oxides and the special morphology arising from the MOFs.

Using ZIF-67 as a precursor and a self-sacrificing tem-plate, Sun et al. [153] fabricated porous NiCo2O4/NiO hol-low dodecahedra (HD) (Fig. 14a) and tested the material as a LIB anode. Here, the researchers reported that NiCo2O4/NiO HD exhibited high capacities, cycling stabilities and outstanding rate performances (Fig. 14d, e), indicating that NiCo2O4/NiO HD can provide higher electrochemi-cal activities and better conductivity as compared with single-metal materials. In addition, NiCo2O4 can also be hybridized with other metal oxides. For example, Huang et al. [154] used a method to prepare Fe2O3@NiCo2O4 porous nanocages through the pyrolysis of core–shell PBA (Ni3[Co(CN)6]2@Co3[Fe(CN)6]2) and reported that the resulting Fe2O3@NiCo2O4 nanocages retained a high capacity of 1079.6 mAh g−1 at 100 mA g−1 after 100 cycles. Recently, Zhong et al. [155] prepared yolk-shell MnO@ZnMn2O4/N–C nanorods using a one-step carbonization of α-MnO2/ZIF-8 and suggested that due to the special struc-ture of the yolk-shell nanorods as well as the effects of the N–C bond and the nanoscale size, the resulting yolk–shell nanorods produced excellent electrochemical performances. Many other hybrid metal oxides such as NiFe2O4/Fe2O3 [156], NiFe2O4@TiO2 [157] and Cr2O3@TiO2 [158] have

Fig. 13 Schematic illustra-tion of the preparation of NiO and CuO@NiO microspheres. Reprinted with permission from Ref. [146]


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also been prepared from well-designed MOFs, and all have been reported to display good electrochemical performances.

In addition to carbon and metal oxide-based materi-als, other materials can also be prepared from MOFs. For example, Xiu et al. [159] prepared a porous TiN/N-doped carbon composite electrode through the heat treatment of a mixture of MIL-125 (Ti) and melamine in Ar atmosphere, and in comparison with TiO2/C composites which were also prepared through the directly annealing of MIL-125 (Ti) in Ar, the TiN composited with N-doping carbon reportedly exhibited higher electronic conductivities, and therefore superior rate capabilities. In another example, Tan et al. [160] prepared N-doped graphene/Fe-Fe3C nanocompos-ites through the pyrolysis of a mixture of MIL-100 (Fe) and dicyandiamide (DCDA) and reported that Fe–Fe3C can not

only improve electronic conductivity but can also facilitate Li+ de-intercalation and promote the reversible formation/decomposition of SEI film, exhibiting a high reversible capacity and good cycle stability. Recently, metal sulfides, especially cobalt sulfides, have also been prepared from MOFs as anodes for LIBs. For example, Wang et al. [161] prepared a C/CoS2 composite by carbonizing ZIF-67 to form a C/Co composite and subsequently heating a mixture of the C/Co composite with sulfur. Here, the researchers reported that the resulting CoS2 nanoparticles possessed bet-ter adaptability to strain effects and that the carbon matrix possessed small cages which can buffer volume expansion during Li+ de-intercalation, thus avoiding pulverization. In addition, the researchers also reported that the N-rich hier-archical porous carbon can provide electrolyte pathways to

Fig. 14 a Schematic illustra-tion of the preparation of porous NiCo2O4/NiO hollow dodecahedra (HD); b–c low and high magnified field-emission scanning electron microscopy (FESEM) images of ZIF-67 (b) and NiCo2O4/NiO HD particles (c); d cycling performance of NiCo2O4/NiO HD and Co3O4 HD at 0.2 A g−1; e rate capability of NiCo2O4/NiO HD. Reprinted with permission from ref [153]


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the inside CoS2 particles and adsorb polysulfides. Because of these properties, the resulting composite exhibited a high capacity of 560 mAh g−1 after 50 cycles at 100 mA g−1, and even 410 mAh g−1 at 2500 mA g−1. Similarly, hollow Co9S8 nanoparticles embedded in graphitic carbon nanoc-ages [162] and coated/sandwiched RGO/CoSx composites [163] can also be prepared by using ZIF-67 as the precursor and display advantages to improve electrochemical perfor-mance. These advantages include not only the unique porous structures, but also the synergetic effects between the carbo-naceous materials and the sulfides, all of which contribute to higher capacities, outstanding rate capabilities and stable cycling performances. Furthermore, composites contain-ing other metal sulfides such as β-NiS nanoparticles or ZnS nanoparticles embedded in porous carbon matrixes have also been prepared from Ni-based (MOF-74) [164] or Zn-based (MOF-5) [165] MOFs, respectively.

Aside from metal sulfides, Hu et al. [166] also reported that ZIF-67 can be used to prepare metal selenides through the heat treatment of a mixture of ZIF-67 nanocubes and Se powder at 350 °C in N2 followed by further annealing at 600 °C in N2 (Fig. 15a). Here, the special inner hollow structure of the as-prepared CoSe@carbon nanoboxes can buffer stress caused by CoSe-involved conversion reactions

during cycling and the outer carbon layer can protect CoSe nanoparticles to prevent pulverization and shedding, leading to high initial Coulombic efficiencies. Moreover, the overall porous carbon matrix in this material can provide pathways for charge transport, facilitating effective lithium insertion/extraction. As a result, CoSe@carbon reportedly exhibited excellent lithium storage performances in terms of capac-ity, cyclability, initial Coulombic efficiency and rate perfor-mance (Fig. 15b).

MOF-derived materials can be used to protect electrodes as well. For example, Han et al. [167] proposed a method to assemble ZIF-8 onto the surface of nano-Si (Si@ZIF-8) in which the as-prepared Si@ZIF-8 was pyrolyzed in N2 atmos-phere (Si@ZIF-8-700N). Here, the researchers reported that the Zn(II) and N-doped carbon derived from ZIF-8 was uniformly distributed on the surface of Si particles and facilitated the lithiation and de-lithiation process and pre-vented the volume expansion of Si, allowing for remarkable improvements in the performance of Si anodes. The protec-tion capabilities of MOF-derived composites can also be extended to cathodes. For example, Han et al. [168] prepared LiCoO2 coated with MOF-derived metal oxide compos-ites by annealing LiCoO2@UiO-66 and LiCoO2@MIL-53 and reported that in comparison with LiCoO2@ZrO2 and

Fig. 15 a Schematic illustra-tion of the formation of CoSe@carbon nanoboxes; b cycling performance at 0.2, 0.5 and 1.0 A g−1 and corresponding Coulombic efficiency at 0.2 A g−1. Reprinted with permission from Ref. [166]


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LiCoO2@Al2O3 which were prepared by treating commer-cial LiCoO2 with ZrO2 and Al2O3, the LiCoO2 coated with MOF-derived material displayed better electrochemical per-formances, demonstrating that the structural degradation of LiCoO2 can be inhibited by MOF-derived coating layers. Moreover, the researchers in this study tested corresponding batteries at 55 °C under 155 mA g−1 and reported that the LiCoO2@UiO-66 was capable of retaining a high capacity of 147 mAh g−1 after 100 cycles, corresponding to a capacity retention of 84.8%. Furthermore, this group recently fabri-cated a well-dispersed amorphous alumina protection layer for LiNi0.6Co0.2Mn0.2O2 (NCM-622) by annealing an Al-based MOF (NH2-MIL-53@NCM-622) [169] and reported that the MOF-derived alumina (MDA)-coated NCM-622 exhibited greatly enhanced electrochemical performances in which 214.6, 196.5 and 168.5 mAh g−1 at 28, 140 and 700 mA g−1 in the voltage range of 3–4.5 V can be obtained and a 92.7% capacity can be retained after 100 cycles at 140 mA g−1. Here, the researchers attributed these perfor-mances to the fact that the well-dispersed amorphous MDA coating can inhibit side reactions between NCM-622 and the electrolyte without blocking Li+ pathways.

2.2 MOFs for Li–S Batteries

Lithium–sulfur (Li–S) batteries can potentially surpass the lithium storage limitations of traditional LIBs [173] in which theoretically, remarkably high capacities (1672 mAh g−1) and energy densities (2600 Wh kg−1) can be obtained if sulfur can be completely reduced based on the formula of: S + 2Li++2e− → Li2S [174]. In addition, sulfur is one of the most abundant elements on the Earth [175] and is therefore extremely inexpensive, and more importantly is environmen-tally benign. Because of these properties, S is regarded as one of the most promising cathode materials for next-generation Li-based batteries. However, several issues must be addressed to ensure good cycle performances and safety. One issue is that both sulfur and the final discharge product of Li2S are electrical and ionic insulators, meaning that thick sulfur films are unfavorable for electrochemical reactions. For this reason, host materials with high surface areas are important because they can effectively minimize the sulfur thickness between the electrolyte and the host to facilitate the transport of electrons and Li+ [176]. Another issue is that the inter-mediate species generated during the conversion reactions of sulfur cathodes are polysulfides (Li2Sx) [177], which are soluble in most common organic electrolytes (particularly at 3 ≤ x ≤ 6 ) [178], meaning that they can transfer to the Li metal anode (shuttle effect) and react to form insoluble and insulating Li2S, resulting in low Coulombic efficiencies and poor recyclability [179]. A further issue is that solubility can lead to the redistribution of sulfur on host materials, causing the uneven distribution of sulfur. Finally, Li–S batteries can

experience volume expansions as large as 80% of the active materials due to the lower density of Li2S (1.66 g cm−3 for Li2S and 2.03 g cm−3 for sulfur). Therefore, because of these issues, the design of effective host materials that can store and trap polysulfide intermediates provides large conductive surface areas for electrochemical reactions and buffer volume expansions is of great importance to improve the cycle per-formance and safety of Li–S batteries.

2.2.1 Pristine MOFs for Li–S Batteries

MOFs, possessing ultra-high surface areas, controllable structures, large pore volumes and tunable porosities are promising platforms if rationally designed for the storage and immobilization of sulfur and dissolved polysulfides. Typically, these designs involve the mixing of pristine MOFs and sulfur, followed by heat treatment, in which sulfur can be injected into the pores of MOFs. Here, MOFs with large pore volumes and proper windows are vital for the encapsu-lation of sulfur and polysulfide intermediates.

In one example, Demir-Cakan et al. [180] proposed the use of MIL-100 (Cr) with a mesoporous structure and a high BET surface area of 1485 m2 g−1 as a confining matrix for sulfur impregnation. Here, based on the melt diffusion con-cept, sulfur was injected into the pores of MIL-100 (Cr) in which the large pore volume (~ 1 cm3 g−1) and small win-dows (5–8.6 Å apertures) guaranteed that MIL-100 (Cr) can be used for the encapsulation of sulfur and polysulfide inter-mediates. Moreover, the polar sections of the inorganic moi-eties in MIL-100 (Cr) can provide further interactions with highly polar polysulfide species. Because of these properties, the MIL-100 (Cr)/S@155 composite as an electrode material reportedly exhibited significant improvements in capacity retention. However, the sulfur content in this composite was relatively low (∼ 48 wt%), making the effective evaluation of MOF functions difficult. In another example, Zheng et al. [181] reported a novel highly porous Ni-based MOF (Ni-MOF: Ni6(BTB)4(BP)3 (BTB = benzene-1,3,5-tribenzoate and BP = 4,4′-bipyridyl)) that possessed a higher BET sur-face area of 5243 m2 g−1 for sulfur impregnation (Fig. 16a). Here, the researchers used the melt diffusion strategy to prepare a Ni–MOF/S composite with ~ 60 wt% sulfur and reported that this composite possessed micropores with a pore size of 1.38 nm which can effectively confine soluble polysulfides during long-term cycling and mesopores with a pore size of ~ 2.8 nm which can further buffer the leak-age of dissolved polysulfides from the MOF. In addition, the researchers in this study also suggested that the strong interaction between Lewis acidic Ni(II) and polysulfides can further inhibit the movement of soluble polysulfides out of the pores. For comparison, these researchers prepared a Co-MOF (Co6(BTB)4(BP)3)/S composite and evaluated both as the cathodes for Li–S batteries and found that although


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Co-MOFs/S exhibited inferior cycling stabilities (Fig. 16c, d), electronic conductivity was better, which was revealed to be due to the weaker coordination between Co(II) and polysulfides according to first-principles computations. Alternatively, the Ni-MOF/S composite demonstrated excellent cycling performances with a high capacity reten-tion of 89% after 100 cycles at 168 mAh g−1 (Fig. 16b). In a further study, Jiang et al. [182] reported a new type of MOF composite (polypyrrole-MOF (ppy-MOF)) which can properly confine sulfur. In this study, the researchers focused on the different pore geometries of MOFs and inves-tigated the importance of MOF structure at high rates in Li–S batteries in which three MOFs (MIL-53, MIL-101 and PCN-224) with distinct structures were synthesized. Here, the BET surface area of the three samples was 1370, 3250 and 2660 m2 g−1, respectively, and among them, ppy-S-in-PCN-224, which possessed cross-linked pores and tun-nels, produced the best performance with high capacities of 670 and 440 mAh g−1 at 10 C after 200 and 1000 cycles, respectively. The researchers attributed such excellent per-formances to the short ion diffusion pathways and the large pore apertures (19.3 Å) of the ppy-S-in-PCN-224. This is because more ions can pass through pores in a certain time period if electrode materials possess shorter ion diffusion pathways and larger pore apertures, thus proving that ppy-S-in-PCN-224 possesses better ion transfer efficiencies.

ZIF-8 is another MOF that has demonstrated promise as a sulfur host material. For example, Wang et al. [183] synthesized S@ZIF-8 as the cathode for Li–S batteries and reported that because of the confining effects of the cage-like pores of ZIF-8, polysulfide dissolution can be alleviated, allowing the as-prepared S@ZIF-8 composite to exhibit a better cycle performance of ~ 510 mAh g−1 after 100 cycles at 168 mA g−1. To investigate the influence of MOF structure and size on Li–S battery performance, Zhou et al. [184] prepared four MOFs, including ZIF-8 with cage-type pores and small apertures, breathing network MIL-53 with one-dimensional (1D) channels, NH2-MIL-53 with amine functionality and HKUST-1 with unsaturated metal sites (Fig. 17a) as well as ZIF-8-M and ZIF-8-L with an average size of 1 μm and 3 μm, respectively, and compared their performance with nanosized counterparts with an average size of 150 nm. Here, the S/MOF composites were pre-pared by using a melt diffusion process, and electrochemi-cal measurements demonstrated that the ZIF-8 with a small window size of 3.4 Å (Fig. 17c) and a uniform particle size of 100–200 nm exhibited the best performance in Li–S bat-teries (Fig. 17), suggesting that the small particle size of MOFs and the small apertures with functionalities in the open frameworks that have affinity with polysulfides can contribute to high capacities and cycling stabilities. A more detailed investigation into the effects of host MOF particle size on Li–S battery performance was conducted by Zhou

Fig. 16 a Crystal structure of Ni-MOFs; b cycling per-formance of the Ni-MOF/S composite at 168, 336 and 840 mA g−1 with a voltage range of 1.5–3.0 V. The inset is a schematic diagram illustrat-ing the interaction between polysulfides and the paddle-wheel unit in the Ni-MOF. The gray, red, blue, yellow, pink and green spheres represent C, O, N, S, Li and Ni atoms, respectively; c charge/dis-charge profiles and d cycling performances of Ni-MOF/S and Co-MOF/S composites at 336 mA g−1 with a voltage range of 1.5–3.0 V. Reprinted with permission from Ref. [181]


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et al. [185] using five sets of ZIF-8 possessing different particle sizes (from < 20 nm to > 1 μm) for sulfur storage, suggesting that decreasing ZIF-8 particle size can increase sulfur utilization (< 20 nm: > 950 mAh g−1 at 840 mA g−1). However, the results in this study also revealed that moderate particle sizes (~ 200 nm) led to the best cycling stability with a capacity retention of 75% after 250 cycles at 840 mA g−1. Therefore, finding the “golden size” is essential for enhanc-ing sulfur utilization and cycling stability. Yue et al. [186] also prepared ZIF-8 nanocrystals dispersed homogeneously within multi-walled carbon nanotubes (MWNTs) for sulfur storage and reported that MWNTs can also provide high sur-face areas and microporous channels, and therefore increase conductivity.

MIL-100 (Cr) is another promising sulfur host candidate due to high surface areas (~ 5000 m2 g−1) and pore volumes (> 1.6 cm3 g−1) [187], benefiting the high dispersion of sul-fur and polysulfides into pores with interaction. For exam-ple, Bao et al. [188] prepared MIL-101(Cr)@RGO compos-ites for sulfur storage by depositing graphene sheets onto MIL-100 (Cr) surfaces in which MIL-101(Cr)@RGO was mixed with CS2-containing sulfur followed by evaporation of CS2 to prepare MIL-101(Cr)@RGO/S. Similarly, Zhou et al. [187] prepared MIL-101 (Cr)/S composites using the melt diffusion method and subsequently prepared graphene-wrapped MIL-101 (Cr)/S composites using a liquid process, taking advantage of strong electrostatic attraction. Here,

both studies reported that MIL-101(Cr) with its micropo-rous windows and large pore volumes can effectively con-fine polysulfides. Moreover, the graphene coating in these composites can effectively improve conductivity and build physical barriers to restrain polysulfide diffusion and dis-solution, allowing for enhanced cycling stabilities.

To investigate the effects of Lewis acid sites (LASs) on Li–S battery performance, Wang et al. [189] synthesized mixed-metal–organic frameworks (MMOFs, MOF-525 (M), M = 2H+, Fe3+-Cl, Cu2+) as sulfur hosts for Li–S batteries in which MOF-525 (M) is composed of Zr6(OH)4O4 clusters linked by [5,10,15,20-Tetrakis(4-carboxyphenyl)porphy-rin]M (MTCCP) (Fig. 18a). Because Zr(IV) centers cannot provide accessible LAS, ligands without central metal sites [MOF-525 (2H)], with Fe3+-Cl [MOF-525 (FeCl)] and with Cu2+ [MOF-525 (Cu)] can provide zero, one and two LASs for sulfur binding, respectively. Furthermore, the researchers in this study synthesized S@MOF-525 (M) using the melt diffusion method (Fig. 18a) and subsequent electrochemical tests revealed that S@MOF-525 (Cu) can exhibit superior electrochemical performances with a reversible capacity of 704 mAh g−1 at 840 mA g−1 between 1.5 and 3 V after 200 cycles (Fig. 18b). Here, a combination of advantages includ-ing a highly porous structure and two LASs enhanced the ability of MOF-525 (Cu) to take up more sulfur therefore demonstrating superior performances. HKUST-1 is another Cu-containing MOF that was prepared for sulfur storage

Fig. 17 a Schematic of selected MOFs; b rate capacities of S/MOFs; c long-term cyclability of S/MOFs at 840 mA g−1, and schematic of the largest apertures of the four MOFs; d performances of S/ZIF-8 with different ZIF-8 particle sizes. Reprinted with permission from Ref. [184]


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(Fig. 18c) [190] in which the suitably sized pores and cages and the Cu2+ sites in HKUST-1 can effectively trap sulfur and inhibit polysulfide dissolution into the electrolyte.

Aside from sulfur matrixes, MOFs have recently been used as separators in Li–S batteries to mitigate shuttling issues by Bai et al. [191] (Fig. 19a). In this study, HKUST-1 was selected to fabricate the MOF@GO separator because of its highly ordered micropores with a size window of ~ 9 Å, which is significantly smaller than the diameter of lithium polysulfides (The fabrication process is shown in Fig. 19b.) Because of this property, the resulting separator was reported to be suitable for blocking polysulfides and selectively sieving Li ions, in which under electrochemical conditions, structural stability and reliability was observed. Here, the researchers attributed these enhanced perfor-mances to size effects derived from the homogeneous porous framework of the separator, leading to remarkable stability

over long-term cycling with low capacity-fading rates of ~ 0.019% per cycle after 1500 cycles.

2.2.2 MOF‑Derived Materials for Li–S Batteries

MOF-derived carbon Besides the direct use of pristine MOFs for sulfur storage, MOF-derived carbons, with large surface areas and high electrical conductivity, are also suit-able to host sulfur, in which metal atoms with low boiling points such as Zn can evaporate during calcination and increase the porosity and surface area of resulting carbon material. Therefore, MOFs containing these types of metal atoms can be used to prepare porous carbon after calcination.

In one study, Xi et al. [192] selected four Zn-based MOFs, including ZIF-8, room temperature synthesized MOF-5 (RT-MOF-5), solvothermal synthesized MOF-5 (solo-MOF-5) and [Zn3(fumarate)3(DMF)2] (ZnFumarate)

Fig. 18 a Schematic illustration of S@MOF-525 (M) synthe-sized using the melt diffusion method at 155 °C in Ar atmos-phere; b cycle performance and rate capacity of S@MOF-525 (2H), S@MOF-525 (FeCl), and S@MOF-525 (Cu); c schematic illustration of the preparation of HKUST-1 ⊃ S. Reprinted with permission from Ref. [189] and [190]


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to prepare MOF-derived carbon based on the fact that the evaporation of Zn atoms can prepare porous carbon. Here, the results revealed that MOF-derived carbon with higher mesopore (2–50 nm) volumes exhibited higher initial dis-charge capacities, whereas MOF-derived carbon with higher micropore (< 2 nm) volumes displayed better cycling stabili-ties. Zn-based MOFs, especially MOF-5 and ZIF-8, are also commonly used as self-sacrificial templates or precursors to prepare carbon materials for Li–S batteries, and based on this, Xu et al. [193] prepared hierarchically porous car-bon nanoplates (HPCNs) with a high BET surface area of 1645 m2 g−1 for sulfur storage through a one-step pyrolysis of MOF-5 at 900 °C under N2. Here, the researchers reported that the porous structure of the HPCN effectively suppressed the diffusion of polysulfides and buffered volume expansion during the charge and discharge process, allowing the as-prepared HPCN with its unique porous structure and high surface area to exhibit good performances in Li–S batteries. In further studies, MWCNTs and GO have also been com-posited with MOF-5-derived carbons because of their large surface areas and good electronic conductivities [194, 195].

In another study, Li et al. [196] prepared nitrogen-doped carbon (NDC) spheres with abundant mesopores (~ 22 nm) and micropores (0.5 nm) by heating ZIF-8 at 1000 °C under Ar followed by immersion in HF solution to remove remaining metal components. Here, the researchers reported that small S2-4 molecules can be impregnated into the

micropores, avoiding the loss of active sulfur. In addition, N doping can not only improve interfacial charge transfer kinetics but can also reinforce interactions with sulfur spe-cies. Recently, Li et al. [197] fabricated a porous carbon with a desired porous structure tuned by the in situ treatment of ammonia. Here, the researchers reported that the in situ ammonia treatment led to a hierarchically mesoporous struc-ture that provided a capacity that was over twice that of pristine ZIF-8-derived carbon materials. To further improve conductivity and accommodate volume change, Chen et al. [198] hybridized graphene sheets with ZIF-8-derived car-bon using an ultrasonic method to prepare a hierarchical sandwich-type graphene sheet-sulfur/ZIF-8-derived carbon composite and reported that the synergistic effects of the microporous carbon and the highly conductive graphene sheets contributed to good cycling stabilities (561 mAh g−1 after 120 cycles at 168 mA g−1).

MOFs containing other metals have also been used to pre-pare carbon materials for sulfur storage [173]. For example, Li et al. [199] synthesized RGO-wrapped ZIF-67-derived cobalt-doped porous carbon polyhedra for use as sulfur immobilizers (RGO/C–Co–S) using a carbonization process (Fig. 20a) and reported that the porous carbon with its large surface area can physically adsorb sulfur and polysulfides, whereas the chemical interaction between Co and sulfur species can further alleviate polysulfide dissolution and the RGO nanosheets can build physical barriers to inhibit

Fig. 19 Schematic illustration of a a MOF@GO separator in Li–S batteries and b the fabrica-tion process to prepare the MOF@GO separator. Reprinted with permission from Ref. [191]


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polysulfide diffusion, further suppressing the shuttle effect. In addition, the researchers also reported that the carbon matrix and the Co particles can provide high conductivities, ensuring fast charge transfer. As a result, the RGO/C–Co–S electrode displayed an excellent capacity of 949 mAh g−1 after 300 cycles at 0.3 A g−1 and good rate capabilities with capacities of 772, 704, 606 mAh g−1 at 0.5, 1 and 2 A g−1, respectively. Furthermore, by using ZIF-67 as precursors, Li et al. [200] prepared Co and N-doped graphitic carbon (Co–N–GC) as sulfur immobilizers (Fig. 20b) and com-pared their performance with the performance of S@Co-GC and S@N-GC. Here, the researchers reported inferior rate capacities and lower Coulombic efficiencies for S@Co-GC and S@N-GC, indicating that Co is beneficial for increasing capacities and N doping can improve cycling performances by facilitating the oxidation of Li2S6 → Li2S8 → S8, improv-ing the utilization of sulfur (Fig. 20b). As a result, the syn-ergetic effects of Co and N reportedly led to a high capacity of 625 mAh g−1 after 500 cycles at 1680 mA g−1 and an outstanding high rate response. Other MOF-derived carbons, such as Fe-based MOFs (Basolite F300) [201] and Al-based metal–organic gel (MOG) [202], have also been employed for sulfur storage. Recently, Chen et al. [173] utilized ZIF-8 and ZIF-67 to prepare a sulfur–nitrogen-doped porous car-bon/graphene (S-NPC/G) cathode which produced an excel-lent specific capacity of 1372 mAh g−1 with good stability (over 300 cycles). Here, the commendable properties of the cathode were thought to be the result of the special structure of the MOF-derived nitrogen-doped porous carbon which is finely dispersed, allowing the material to both physically confine and chemically adsorb polysulfides.

In the same group as sulfur, selenium has also attracted considerable attention as a cathode in Li–Se batteries. Sele-nium possesses a similar de-lithiation mechanism to sulfur: Se + 2Li++2e− ↔ Li2Se, with a corresponding gravimetric capacity of 675 mAh g−1. In addition, due to a higher den-sity than S, Se can provide a comparably high volumetric capacity density of 3253 mAh cm−3 (S is 3467 mAh cm−3) [203, 204]. More importantly, Se possesses a much higher conductivity of 1 × 10−3 S m−1 than S (5 × 10−28 S m−1) and thus potentially higher active material utilizations and better rate performances. In one example, Lai et al. [205] prepared a Se@mesoporous carbon matrix (Se@meso-C) for use as the cathode of Li–Se batteries using the one-step pyrolysis of MOF-5 at 900 °C in N2 followed by heat treatment of the mixture with Se. Here, the researchers reported that not only can the resulting mesoporous carbon matrix improve conductivity and provide transmission channels for Li+, but also confine the diffusion of Se and polyselenides, suppress-ing the shuttle effect. As a result, the Se@meso-C composite cathode exhibited a much better cycling performance and Coulombic efficiency than a pristine Se cathode in which a capacity of 306.9 mAh g−1 after 100 cycles at 337.5 mA g−1

can be maintained. Using a similar method, Li et al. [206] prepared a N-doped carbon sponge (NCS)/Se composite by carbonizing Al-based MOFs under Ar and NH3 flow fol-lowed by a selenylation step and reported that the N doping can further facilitate electron transfer, improving electro-chemical performances, especially at high rates. As a result, the reported NCS/Se composite exhibited a high capacity of 443.2 mAh g−1 after 200 cycles at 337.5 mA g−1 with a Coulombic efficiency up to 99.9%, and retained a capac-ity of 286.6 mAh g−1 after 60 cycles even at 3375 mA g−1. Furthermore, He et al. [207] used MOF-derived cobalt and nitrogen-doped porous carbon (Co–N–C) polyhedra as SeS2 immobilizers and reported that due to the substantial amount of micropores and the uniform distribution of Co nanoparti-cles, the Co–N–C/SeS2 cathode possessed more active sites and efficiently adsorbed polysulfides and polyselenides. Here, the high SeS2 loading (66.5 wt% SeS2) Co–N–C/SeS2 composite produced a reversible capacity of 1165.1 mAh g−1 and maintained over 84.1% of the initial capacity with a high coulombic efficiency of ~ 100% after 200 cycles.

Fig. 20 a Schematic illustration of the preparation of RGO/C–Co–S nanohybrid polyhedra; and b the formation of Co–N–GC and its interaction with polysulfides during the charge/discharge process in a Li–S battery. Reprinted with permission from Refs. [199, 200]


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2.3 MOFs for Li–O2 Batteries

Li–O2 batteries are also considered to be a promising alterna-tive to traditional Li–ion batteries for powering EVs due to their ultra-high theoretical specific energy in which 11,400 Wh kg−1 and 6080 Wh L−1 can be achieved in the fully charged state of Li–O2 batteries corresponding to the elec-trochemical reaction of 2Li + O2 ↔ Li2O2 [208] based on the assumption that metallic lithium is used as the anode and oxy-gen comes from the air instead of being storage in the battery. Furthermore, carbonaceous materials are commonly applied as the air cathode in Li–O2 batteries due to their excellent electrical conductivity and high porosity which are favora-ble for the transport of gas and electrons and the immersion of electrolytes. Despite these advantages however, reaction kinetics (oxygen evolution reaction (OER) and oxygen reduc-tion reaction (ORR)) of Li–O2 batteries are sluggish [209].

2.3.1 Pristine MOFs for Li–O2 Batteries

MOFs, possessing tunable porosities, open channels and large surface areas, can exhibit better oxygen selectivity, facilitating the transport of electrons and oxygen, and pro-moting the formation of thin film Li2O2. In addition, MOFs with open metal sites can enhance the interaction between MOFs with small molecules and ions, and catalyze corre-sponding reactions.

Wu et al. [210] were the first group to apply pristine MOFs as air cathodes for Li–O2 batteries in which they selected three MOFs, including MOF-5, HKUST-1 and M-MOF-74 (M = Mn, Mg, Co). Here, the researchers obtained the air cathodes by mixing 40 wt% MOFs with 40 wt% super P as the conductive agent and 20 wt% poly-vinylidene fluoride (PVDF) as the binder and found that MOFs play an important role in enhancing capacity as compared with batteries using super P alone. In particular, these researchers found that M-MOF-74 possessed a high density of open metal sites available on the inner surface and uniform 11.0-Å-diameter 1D channels which were large enough to permit the entry of O2 molecules and can act as reservoirs of O2. In addition, this study reported that among the three MOFs, Mn-MOF-74 exhibited the highest capacity of 9420 mAh g−1 under 1 atm of oxygen, demonstrating the possible catalytic activity of Mn as compared with Mg and Co containing MOFs. Furthermore, Hu et al. [211] demon-strated Ni-based MOFs as cathodes for Li–O2 batteries in which the open catalytic sites and high specific surface area of the Ni-based MOF with a 3D micro-nano structure facili-tated effective contact between the electrolyte and the cata-lytic sites and facilitated mass transfer, allowing a Ni-MOF-based Li–O2 battery to exhibit a high capacity of 9000 mAh g−1, a high round-trip efficiency of 80% and a good cycling stability of 170 cycles without obvious voltage drops.

However, considering the practical application of Li-air batteries in ambient environments, atmospheric moisture can lead to serious oxidation of Li anodes and fast deg-radation of batteries, along with the formation of Li2CO3 caused by CO2. To prevent this, Cao et al. [212] prepared a mixed-matrix membrane (MMM) by incorporating polydo-pamine-coated MOF CAU-1-NH2 into a polymethylmeth-acrylate (PMMA) matrix and reported that the abundant –OH groups of the polydopamine and the –NH2 groups on the CAU-1-NH2 framework can prevent CO2 molecules and that the intrinsic hydrophobic behavior of the PMMA can prevent the ingress of H2O. As a result, the as-prepared MMM allowed Li-air batteries to operate well under ambi-ent atmospheres with a high humidity of ~ 30% by repelling CO2 and H2O in air.

2.3.2 MOF‑Derived Materials for Li–O2 Batteries

MOF-derived materials can be applied to Li–O2 batteries as well. Because N doping in carbon materials can effec-tively improve ORR performance, N-containing MOFs are excellent templates to prepare catalysts for Li–O2 batteries, in which active sites are uniformly dispersed after calcina-tion. In addition, high surface areas and porous structures inherited from MOF precursors can further enhance Li–O2 battery performances.

In one example, Li et al. [213] prepared N-doped gra-phene/graphene tube-rich N–Fe–MOF catalysts by heat treating a mixture of dicyandiamide, iron acetate and cage-containing MOFs. Here, the cage-containing MOFs are needed for the formation of Fe3C species, which further catalyzes growth of N-doped graphene/graphene tube com-posites. As a result, the N–Fe–MOF catalyst exhibited a high discharge voltage plateau of 2.8 V, yielding a high energy density for the battery, in which a high initial discharge capacity of 5300 mAh g−1 at 50 mA g−1 and good cycling stability up to 16 cycles without significant capacity loss at 400 mA g−1 was achieved. Furthermore, Chen et al. [214] used MIL-100(Fe) as a precursor and template to prepare hierarchical mesoporous γ-Fe2O3/carbon nanocomposites with a high BET surface area of 194.3 m2 g−1 and reported that the high electrocatalytic activity of γ-Fe2O3 for OER and ORR and the hierarchical mesoporous nanostructure which can provide more active sites and faster electron trans-port can collectively contribute to excellent electrochemical performances with a discharge capacity of up to ~ 5970 mAh g−1 at 0.1 mA cm−2, and a low overpotential and good cycle performance with over 30 cycles under a specific capacity limit of 600 mAh g−1.

However, most catalysts with high ORR/OER activ-ity can also accelerate side reactions, such as electrolyte degradation. To resolve this, Yin et al. [215] prepared ZnO/ZnFe2O4/C (ZZFC) nanocages using Fe(III)-MOF-5


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nanocages as a template and reported that the ZZFC nanoc-age with its lower ORR/OER catalytic activity did not cata-lyze the degradation of organic electrolytes during operation. In addition, the researchers reported that the large surface area, hierarchical porosity and uniformly dispersed active sites of the resulting nanocages benefited mass and electron transport, allowing the ZZFC cathode to deliver a primary discharge/charge capacity exceeding 11,000 mAh g−1 at 300 mA g−1 and battery operations at 5000 mAh g−1 for 15 cycles in a “deep discharge + fixed capacity” mode. In another example, Zhang et al. [216] prepared spinel-type porous cobalt–manganese oxide (Co–Mn–O) nanocubes by annealing nanocube-like MOF precursors and reported that the resulting Co–Mn–O nanocube electrode exhibited good ORR and OER catalytic activities with reduced overpoten-tials (200 mV) and excellent cycle stability up to 100 cycles with a limited capacity of 500 mAh g−1.

Furthermore, Tang et al. [217] prepared a cage-type highly graphitic porous carbon–Co3O4 (GPC–Co3O4) using a two-step annealing treatment in which ZIF-8@ZIF-67 was first carbonized at 900 °C in N2 and subsequently at 400 °C in air and reported that because of its high electronic conductivity, the GPC–Co3O4 composite can be employed as air cathodes without conductive agents. Here, the high conductivity of the carbon framework and the uniformly embedded catalytic sites of the Co3O4 contributed to sat-isfactory performances, including a low charge overpo-tential of ~ 0.58 V and a long cycle life of 50 cycles at a limited specific capacity of 500 mAh g−1. Other cathode catalysts such as Cr2O3@octahedral porous carbon derived from MIL-101 (Cr) [218] and Fe/Fe3C decorated 3D porous N-doped graphene derived from MIL-100 (Fe) [219] have also been investigated as Li–O2 battery cathodes. Recently, Zhong et al. [220] prepared a N-doped porous carbon on GO by pyrolyzing ZIF-8 and GO and reported that the result-ing material exhibited high capacity, good Coulombic effi-ciency and satisfactory cyclic stability over 125 cycles at 200 mA g−1, which they attributed to the proper pore struc-ture, high electrical conductivity and abundant active sites as a result of N doping.

3 Applications of MOF‑Based Materials in Na–Ion Batteries

Currently, to effectively store and utilize renewable resources such as solar and wind power, and accommodate peak loads, large-scale energy storage systems for power grids are neces-sary. In order to meet the different requirements and func-tions these grids, electrochemical energy storage systems are considered viable solutions possessing adequate charac-teristics [221]. However, although LIBs can potentially be applied in these large-scale electrochemical energy storage

systems, the feasibility of lithium needs to be considered [222]. This is because lithium is a relatively scarce resource with an abundance of only 20 ppm in the Earth’s crust [223], which restricts large-scale application [224]. As a result, SIBs have been widely investigated as promising alternatives to LIBs due to the natural abundance of sodium resources (23,600 ppm in the Earth’s crust) and the standard electrode potential [223]. However, because the size of Na+ is much larger than that of Li+, host frameworks with larger intersti-tial spacing are required. Based on this, MOFs with highly open structures are promising candidates for SIBs.

3.1 Pristine MOFs for Na–Ion Batteries

Linear (C≡N)− molecules in Prussian blue (PB) and its analogues (PBAs) consist of different transition metal ions (M, Fe, Mn, Ni, Cu, Co and Zn) and can provide a long M(II)–N≡C–Fe(III) bond length (Fig. 21) to allow Na+ to insert reversibly into empty large ion sites. There-fore, PB and its analogues are widely investigated as SIB cathodes. For example, Lu et al. [225] synthesized a series of KMFe(CN)6 (M = Fe, Mn, Ni, Cu, Co and Zn) as SIB cathodes and reported that these cathodes presented a flat reversible discharge capacity of over 70 mAh g−1, and that in particular, KFe2(CN)6 exhibited a reversible capacity of nearly 100 mAh g−1 with no fading after 30 cycles, further indicating that SIBs with Prussian blue framework cath-odes are feasible. Furthermore, Lee et al. [226] investigated Na2Zn3[Fe(CN)6]2·xH2O (NZH), a modified PBA, as a SIB cathode material and reported that by simply replacing Fe3+ in PB with Zn2+, the as-prepared NZH was endowed with larger ionic channels and open sites which are favorable for repeated Na+ diffusion. Moreover, the robust nature of the resulting crystal structure ensured cycle stability. As a result, the NZH displayed good electrochemical activities with a reversible capacity of 56.4 mAh g−1 and 85.2% retention of initial capacity after 50 cycles at 10 mA g−1 in the redox potential range of 2.0–4.0 V versus Na/Na+.

Previous reports have indicated that KMnIIFeIII(CN)6 can produce reversible plateaus at 3.82 and 3.56 V [225] and Wang et al. [228] reported that a 3.4-V cathode (vs. Na/Na+) can be obtained from the removal of Na+ in PBAs. In this study, Wang et al. compared rhombohedral Na1.72MnFe(CN)6 (NMHFC-1) with cubic Na1.40MnFe(CN)6 (NMHFC-2) and found that Na+ concentrations can signifi-cantly affect the composition and structure of the product, resulting in different electrochemical performances in which at higher concentrations of Na+, Na+ displacement along the [111] direction can lead to the transition from cubic to rhom-bohedral symmetry, which is reversible during cycling. As a result, this rhombohedral NMHFC-1 exhibited a reversible capacity of 134 mAh g−1 with 120 mAh g−1 retained after 30 cycles at 6 mA g−1.


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Vacancies and water content in PB crystals can also have significant influences on the electrochemical perfor-mance of PB cathodes for SIBs. This is because vacancies can decrease the electronic conductivity of PB and cause framework collapse and lattice disorder during cycling. In addition, water molecules in interstitial sites can inhibit the insertion and transport of Na+ and water decomposi-tion can reduce Coulombic efficiency, resulting in poor cycling performances [227]. To address these issues, You et al. [227] prepared Na0.61Fe[Fe(CN)6]0.94 (HQ-NaFe), a high-quality PB nanocrystal, with a small number of vacan-cies and a low water content by using Na4Fe(CN)6 as the single iron-source precursor and reported that in the redox mechanism (Fig. 21b), a two-electron redox reaction occurs: cubic FeIIIFeIII(CN)6 ↔ cubic NaFeIIIFeII(CN)6 ↔ rhombo-hedral Na2FeIIFeII(CN)6. Here, the perfect structure of the HQ-NaFe with high stability was found to be capable of accommodating the mechanical strain and volume change experienced during Na+ de-intercalation, thus favoring the redox reaction. As a result, the HQ-NaFe reportedly exhib-ited a high capacity of 170 mAh g−1 without any apparent capacity loss after 150 cycles at 25 mA g−1 and a high Cou-lombic efficiency of ~ 100%, demonstrating that HQ-NaFe is a promising cathode for long lifespan SIBs. Porous struc-tures can also greatly affect electrochemical performances. Based on this, Yue et al. [229] synthesized a mesoporous PBA (KNiFe(CN)6 (NiHCF)) using a template-free method and compared this with a series of hierarchical porous PBAs that were prepared by controlling the reaction time. Here,

the results showed that rate performances improved with increasing pore diameters, which the researchers attributed to the fact that large pores are favorable to the diffusion of Na+ into particle bulk through filled electrolytes, therefore improving rate performances. As a result, the as-prepared NiHCF analogue in this study exhibited a reversible capac-ity of ~ 65 mAh g−1 at 10 mA g−1 and displayed good cycling stability. In another study, Nie et al. [230] prepared PBA FeIIIFeIII(CN)6·xH2O nanoparticles on carbon fiber paper as a SIB cathode without any binders or conductive additives and reported that the intimate contact between the FeFe(CN)6 nanoparticles and the carbon cloth ensured fast electron transport between the electroactive materials and the carbon current collectors and that the open spaces and loose textures between the carbon fibers not only facili-tated contact between the FeFe(CN)6 nanoparticles and the electrolyte but also buffered volume change during cycling. In addition, the researchers also reported that the nanosize of the FeFe(CN)6 further shortens the diffusion lengths of Na+ and electrons, and that the absence of binders and con-ductive additives reduced internal resistances and improved charge transfer kinetics. As a result, the FeFe(CN)6/carbon hybrid exhibited outstanding electrochemical performances including a capacity of 82 mAh g−1 at 24 mA g−1 with 81.2% capacity retention after 1000 cycles and a good rate capability.

Recently, Meng et al. [231] synthesized PB mesocrystals using a hydrothermal reaction of a Na4[Fe(CN)6] solution which exhibited different electrochemical performances

Fig. 21 a Crystal structures of PBAs; b schematic illustration of the redox mechanism of high-quality Na0.61Fe[Fe(CN)6]0.94 nanocrystals. Reprinted with permission from Refs. [225, 227]


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as compared with PB single crystals in PB/Na half cells. Here, in comparison with PB single crystals, the researchers reported that the diffusion of Na+ into the PB mesocrystals can temporally expand the framework and cause local disor-dering. In addition, the PB mesocrystals reportedly shorten Na+ diffusion pathways as compared with PB single crystals, causing smaller sections of local disordering in the mes-ocrystals. As a result of all of this, the initial capacity of the PB mesocrystals (115 mAh g−1) was higher than that of the single crystals (95 mAh g−1) at 25 mA g−1. However, the cycling and rate performances of the PB mesocrystals were worse, which may result from rich defects such as grain boundaries. Despite this, the results in this study suggest that mesostructuring can be an efficient method to regulate electrochemical performances. Other PBAs have also been investigated as SIB cathodes. For example, by replacing the transition metal with Ti, Xie et al. [232] prepared sodium titanium hexacyanoferrate (NTH) and reported a capacity of 92.3 mAh g−1 with 65.0 mAh g−1 retention after 50 cycles at 50 mA g−1.

In addition to PB and PBAs, Aubrey et al. [233] also proposed a dual-ion cathode for SIBs through the oxida-tive insertion of anions into MOFs. Here, the redox-active MOF (Fe2(dobpde)(dobpde4− = 4,4-dioxidobiphenyl-3,3′-dicarboxylate)) was prepared and half-cell performances with a 0.6 M NaPF6 electrolyte in 3:7 EC/DMC were inves-tigated. As shown in Fig. 22a, anions (PF6−) are inserted into MOFs with electrochemical oxidation, resulting in a capacity plateauing at ~90 mAh g−1 after 10 cycles. The intercalating cycling performance at 140 mA g−1 (Fig. 22b) also demonstrates its long-term reversibility of PF6−.

3.2 MOF‑Derived Materials for Na–Ion Batteries

MOF-derived carbon MOF-derived carbon materials have shown promise in Na–ion batteries. For example, Qu et al. [234] prepared a microporous carbon (ZIF-C) with a homo-geneous pore size of 0.5 nm by directly heating ZIF-8 at 930 °C in N2 followed by removing residual inorganic com-ponents with a HCl solution. In comparison with a prepared SBA-15-templated mesoporous carbon, the small pore size of the ZIF-C greatly weakened the reductive decomposition of electrolytes, thus reducing the irreversible capacity of the first cycle and enhancing the reversibility of Na+ storage. Shi et al. [235] also used ZIF-8 as a precursor to synthesize N-doped porous carbon as the anode for SIBs and reported good electrochemical performances. Recently, Li et al. [236] reported a strategy to improve the Na storage performance of red P through the confinement of nanosized amorphous red P into a ZIF-8 derived N-doped microporous carbon matrix (P@N-MPC). Here, the researchers reported that the MPC not only improve conductivity but also relieve strain from volume variations. In addition, the highly porous structure also ensured efficient access of electrolytes to red P nanopar-ticles, allowing the P@N-MPC to display a high capacity of ~ 600 mAh g−1 at 0.15 A g−1 with a good rate capacity. Aside from ZIF-8, Zou et al. [237] prepared a cube-shaped porous carbon (CPC) by pyrolyzing MOF-5 at 1000 °C under Ar and reported that due to a high specific area (2316 m2 g−1, the BET method), good conductivity and abundant micro/mesopores, the resulting CPC exhibited a high capacity of ~ 100 mAh g−1 at 3.2 A g−1 after 5000 cycles. Kong et al. [238] also synthesized nitrogen-doped wrinkled carbon foil (NC-1) by pyrolyzing 2D Mn-based MOFs and reported that due to the hierarchical pores and channels, their resulting NC-1 anode showed efficient capability and high stability

Fig. 22 a Schematic illustration of Fe2(dobpdc) operation in an electrochemical cell in which A− can denote tetrafluorobo-rate, hexafluorophosphate, bis(trifluoromethylsulfonyl)imide, or

tetrakis(perfluorophenyl)borate and M+ can denote lithium, sodium or potassium; b cycling performances of Fe2(dobpdc) at 140 mA g−1. Reprinted with permission from Ref. [233]


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in sodium-ion capacitors, achieving a high capability of 150 mAh g−1 at a current density of 10 A g−1 and retaining 72.8% after 1000 cycles at 1.0 A g−1.

Compared with Li+, Na+ is too large to be easily inserted; therefore, carbon nanofibers (CNFs) and CNTs have always provided poor performance as anodes of SIBs due to the narrow spacings. To resolve this, Chen et al. [239] prepared N-doped carbon hollow tubules (CHTs) with a larger inter-layer spacing required for Na+ intercalation through a com-bination of the self-etching and graphitization of Zn and Co bimetallic ZIF-based nanocomposites. Here, the researchers reported that the metal salt in the ZIF composite can etch carbon cores into a hollow tubular structure. Benefiting from these combined improvements, including a high degree of graphitization, a multi-level hierarchical porous structure, large enough interlayer spacing and N doping, the resulting CHT demonstrated excellent electrochemical performances as an anode for SIBs with a high capacity of 346 mAh g−1, good capability and a long cycle life of 10,000 cycles with-out capacity fading.

MOF-derived metal oxides Composite materials contain-ing metal oxides derived from MOFs have also been inves-tigated for SIBs. For example, Zou et al. [240] were the first to report MOF-derived transition metal oxides as anodes for SIBs. In their study, Ni-based MOFs with a unique hierar-chical hollow ball-in-ball nanostructure were first prepared through solvothermal reactions. Following this, a two-step annealing process was carried out in which the Ni-based MOFs were first heated at 450 °C in Ar and subsequently at 200 °C in air to obtain the resulting hierarchical NiO/Ni nanocrystals covered by graphene shells (NiO/Ni/Gra-phene) with a hollow ball-in-ball nanostructure (Fig. 23a). Here, the researchers reported that the unique structure effectively buffered volume expansions and facilitated elec-trolyte penetration. In addition, the graphene coating not only improved conductivity but also provided a buffer layer to further mitigate volume expansion, promoting the for-mation of stable SEI films. Furthermore, the ultra-fine size of the NiO/Ni improved conversion reaction kinetics and reduced mechanical strain caused by volume expansions. As a result of these improvements, the NiO/Ni/graphene anode exhibited excellent storage capacities for both Li and Na, in which if employed in SIBs, exhibited good cycling stabili-ties with 0.2% fading per cycle and high rate capacities, with 207 mAh g−1 being obtained even at 2 A g−1 (Fig. 23b). In another example, Wang et al. [241] prepared N-doped carbon-coated Co3O4 nanoparticles (Co3O4@NC) through the two-step annealing of ZIF-67 in which ZIF-67 was first heated at 550 °C in Ar, followed by heating at 150 °C in air (Fig. 23c). In comparison with pure Co3O4 which was prepared by calcining a Co3O4@NC intermediate at 350 °C in air, the Co3O4 encapsulated in the NC shell reportedly avoided aggregation and pulverization, with the NC shells

being able to accommodate volume change during cycling, decreasing charge transfer resistances. Moreover, the researchers reported that the NC also contributed to a large capacity and led to high rate capabilities and outstanding cycling stabilities, resulting in the as-prepared Co3O4@NC achieving a high capacity of 175 mAh g−1 after 1100 cycles at 1 A g−1 as the anode of a SIB (Fig. 23d).

Cu-based oxides such as CuO and Cu2O have also attracted much attention as the anode material for SIBs. For example, Zhang et al. [242] prepared porous CuO/Cu2O composite hollow octahedra by annealing Cu-based MOF templates and reported that the resulting stable hollow porous structure can improve electrochemical performances in which the synergistic effects between the CuO and Cu2O can contribute to high capacities. In this study, the CuO/Cu2O composites reportedly displayed a good sodium stor-age capacity of 415 mAh g−1 after 50 cycles at 50 mA g−1 and maintained a capacity of ~ 165 mAh g−1 after 1000 cycles at a higher current density of 2 A g−1. In another study, Ramaraju et al. [243] prepared CuO/Cu2O hollow pol-yhedra on RGO (Cuox–RGO) by sintering a Cu-based MOF embedded with exfoliated graphene oxide. Here, the RGO not only stabilized the structure and buffered volume expan-sion, but also improved conductivity. If used as a SIB anode, the obtained Cuox–RGO composite reportedly delivered long-term cycling stabilities without obvious capacity fading after 3400 cycles at 500 mA g−1 and good rate capabilities. As for other metal oxides, Qi et al. [244] recently reported an in situ quantization process to obtain Fe3O4 quantum dots uniformly embedded into a micro-carbon coating (Fe3O4 QD@C-GN) by using a Fe-based MOF (MIL-88-Fe-NH2) as the template. Here, the researchers reported that the small size of the Fe3O4 shortened transport pathways for Na+ and electrons and that the integrated hierarchical conductive network of Fe3O4 QD@C-GN can further facilitate the fast transport of Na+ and electrons. Because of these proper-ties, the Fe3O4 QD@C-GN anode demonstrated reversible capacities of 343, 234 and 149 mAh g−1 after 1000 cycles at 2, 5, and 10 A g−1, respectively, combining supercapacitor-like rate performances with battery-level capacities.

Furthermore, tiny ilmenite FeTiO3 nanoparticle-embedded carbon nanotubes (FTO/CNTs) have recently been proposed as anodes for SIBs by Yu et al. [245] in which good cycling sta-bilities (358 mAh g−1 after 200 cycles at 100 mA g−1), excel-lent rate capabilities (210 mAh g−1 at 5000 mA g−1) and high Coulombic efficiencies (~ 99%) were observed. Other materi-als, including phosphides [246] and sulfides [247], have also been prepared through the heat treatment of MOFs as anode materials for SIBs, and all these obtained anode materials reportedly inherited the structure of the precursors through the phosphidation/sulfidation process. By benefiting from these unique structures, the obtained materials all exhibited good electrochemical performances as well.


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4 Applications of MOF‑Based Materials to Supercapacitors

Supercapacitors (SCs), also known as electrochemical capacitors (ECs) and ultracapacitors, are widely investi-gated due to their high power densities, long cycle life spans and rapid charging/discharging rates [248, 249]. Because of the high power outputs, SCs can be used in hybrid electric vehicles and fuel cell vehicles as the aux-iliary start, power compensation, start–stop or temporary storage systems during braking. Therefore, SCs can poten-tially play an equally important role as batteries in future energy storage systems [250]. However, the optimiza-tion of energy densities without the sacrifice of valuable

characteristics such as high power densities and long cycle life spans is still challenging.

Based on the energy storage mechanism, there are two main types of SCs. One is the electrochemical double-layer capacitor (EDLC), in which capacitance comes from the pure electrostatic charge stored at the interface of the electrode and electrolyte. In this type of SCs, the surface area of the electrode material, which can be acces-sible to electrolyte ions, is crucial for the electrochemical performance of EDLCs. Because of this, porous carbo-naceous materials, such as activated carbon, templated carbon, carbide-derived carbon, carbon nanotubes and graphene, are commonly used as the electrode material due to their large specific surface area, good chemical and

Fig. 23 a Schematic illustration of the synthesis of NiO/Ni/gra-phene; b cycling performance of NiO/Ni/graphene at 1 A g−1 (200 mA g−1 was used for acti-vation in the first 5 cycles) and the specific capacities at current densities ranging from 200 to 2000 mA g−1; c illustration of the formation of Co3O4@NC; d cycling stability of the Co3O4 electrode at 1 A g−1 Reprinted with permission from Refs. [240, 241]


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thermal properties and electrical conductivity [251]. The other type of SCs is the pseudocapacitor which is based on fast and reversible Faradic processes. For pseudoca-pacitors, the most commonly used electroactive materi-als are various noble or cheap transition metal oxides or hydroxides, conducting polymers and carbon materials with oxygen- or nitrogen-containing functional groups on the surface [251], in which the size of the redox-active species, the porosity and the specific surface area of the electrode material mainly determine the characteristics of the pseudocapacitor.

4.1 Pristine MOFs for Supercapacitors

Pristine MOFs with large surface areas, adjustable pore sizes and abundant redox-active species demonstrate great potential as electrode materials for SCs. This is because high surface areas can ensure large interfaces between electrodes and electrolytes, resulting in good electrochemical perfor-mances as electrodes for EDLCs. In addition, metal ions in MOFs can endow pseudocapacitance activity properties. Table 3 provides a summary of the performance of pristine MOFs in SCs.

In one example, Liao et al. [252] prepared Ni-based MOFs as electrode materials for SCs and reported a spe-cific capacitance of 634 F g−1 with a 16% decrease after 2000 cycles. Diaz et al. [253] subsequently prepared

Co8-MOF-5 through the partial substitution of Zn by Co in the MOF-5 structure and reported better electrical con-ductivities. In this study, the electrodes were prepared by mixing Co8-MOF-5 with carbon black and polytetrafluoro-ethylene (PTFE) at a ratio of 75:15:10 wt%. In acetonitrile, the as-prepared electrode reportedly behaved as an EDLC that was mainly dominated by the influence of carbon black additives but also limited by the unsuitable conductivity and stability of Co8-MOF-5. In a later study, Lee et al. [254] synthesized Co-based MOFs as promising materi-als for SCs and reported that their doctor-bladed Co-based MOF film exhibited good pseudocapacitor behavior with an energy density of 7.18 Wh kg−1 and a specific capacitance of 206.76 F g−1 at 0.6 A g−1 in 1 M LiOH electrolyte. In addition, the electrochemical redox switching was reversible with a loss of only 1.5% in capacitance after 1000 cycles. The researchers also used other electrolytes such as LiCl, KCl and KOH, and reported worse performances for the MOF, suggesting that the presence of CoO is the cause of the poor performance in LiCl, KCl and KOH electrolytes. In another study, the same group [255] used organic link-ers with different molecular lengths to manipulate the pore size and surface area of MOFs to investigate the relationship between supercapacitance behaviors and MOF pore sizes or surface areas. Here, the results revealed that MOF films with larger pores, larger surface areas and continuously inter-connected leaflet-like microstructures with less structural

Table 3 Electrolyte, rate, initial capacitance (IC, F g−1), cycle number (CN) and capacitance after cycles (AC, F g−1 or the percentage of initial capacity) of pristine MOFs for supercapacitors

MOFs Electrolyte Rate IC CN AC Refs

Ni-based MOFs 6 M KOH 50 mV s−1 ~ 125 2000 ~ 100 [252]Co-based 1 M LiOH 0.6 A g−1 206 1000 ~ 203 [254]437-MOFs 6 M KOH 4 A g−1 64 6000 ~ 64 [257]Ni-MOF-24 6 M KOH 10 A g−1 668 – – [258]Zn-doped Ni-MOFs 6 M KOH 2 A g−1 ~1250 3000 ~1200 [259]nMOF-867 1 M (C2H5)4NBF4 12.7 mA cm−3 726 10,000 ~ 700 [260]UiO-66 6 M KOH 10 mV s−1 ~ 900 2000 654 [261]UiO-66 Polymer-gel 80 mV s−1 – 1000 ~89% [262]Ni-based MOFs 6 M KOH 0.5 A g−1 1765 – – [264]Ni3(HITP)2 1 M TEABF4/ACN 2 A g−1 – 10,000 ~ 90% [265]Cu-CAT NWAs 3 M KCl 800 mV s−1 – 5000 80% [267]Co-LMOFs 1 M KOH 2 A g−1 1978 2000 94.3% [268]Layered Co-MOFs 5 M KOH 2 A g−1 – 3000 95.8% [269]DABCO-MOFs 2 M KOH 10 A g−1 – 16,000 98% [270]PANI-ZIF-67-CC 3 M KCl 0.05 mA cm−2 35 mF cm−2 2000 80% [272]ZIF-67 1 M KOH 2 A g−1 188 3000 105% [273]MnOx-MHCF 1 M Na2SO4 5 mA cm−2 127 10,000 94.5% [277]Accordion-like Ni-MOFs 3 M KOH 1.4 A g−1 – 5000 96.5% [278]ZIF-LDH/GO 6 M KOH 5 A g−1 – 5000 90.7% [279]VIV(O)(bdc) 1 M Na2SO4 1 A g−1 521 10,000 92.8% [280]CoNi-MOFs 1 M KOH – – 5000 94% [281]CoMn-MOFs 2 M KOH 100 mV s−1 – 1500 96% [282]


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interfaces have better supercapacitive performances. Based on redox-active L (N,N′-bis-(4-pyridyl)phthalamide ligand), Gong et al. [256] synthesized two porous MOFs; Zn6(BPC)(L)3·9DMF (H2BPC = 4,4′-iphenyldicarboxylic acid) and Cd2(TDC)2(L)2·4H2O (H2TDC = 2,5-thiophenedicarboxylic acid), as electrode materials for supercapacitors and reported that both MOFs can retard the electrolysis of water, pro-viding large voltage windows as high as 2.6 V in aqueous electrolytes.

Du et al. [257] designed an In-based MOF ((In(BTTB)2/3(OH))(NMF)5(H2O)4)n (437-MOF) (NMF = N-methylformamide, H3BTTB = 4,4′,4′′-[ben-zene-1,3,5-triyl-tris(oxy)]tribenzoic acid) with a mesoporous structure and perfect 1-D hexagonal channels of ~ 3 nm pore size and reported that due to the large surface area, high stability and good corrosion resistance, the resulting MOF can potentially be used as an electrode for SCs. Here, the researchers reported specific capacitances of 150.2, 92, 81, 72, 64 and 56 F g−1 at 0.2, 0.5, 1, 2, 4, and 8 A g−1, respectively, for the 437-MOF electrode, which they attrib-uted to the large surface area, hierarchical pore structure and pseudocapacitance provided by the redox reaction of In(III). Furthermore, Yang et al. [258] synthesized a layered Ni-based MOF which delivered capacitances of 1127 and 668 F g−1 at rates of 0.5 and 10 A g−1, respectively. To fur-ther improve capacitance, this same group [259] also synthe-sized a layered structural Zn-doped Ni-based MOF in which the doped Zn served as a pillar to enlarge interlayer distances and prevent collapse, providing enough space for the diffu-sion of electrolytes and facilitating the de-intercalation of OH− (Fig. 24a). Because of this, the Zn-doped MOF mate-rial displayed smaller charge transfer resistances, allowing for faster electron transfer. In addition, the researchers also reported that the interconnected open pores were beneficial for the diffusion of electrolytes and buffered volume vari-ations during charge–discharge cycles. As a result of these properties, the Zn-doped Ni-based MOF material exhibited better electrochemical performances as the electrode for SCs, achieving high capacitances of 1620 and 860 F g−1 at 0.25 and 10 A g−1, respectively, with an excellent cycling stability of only 8% capacitance loss after 3000 cycles.

Recently, Choi et al. [260] prepared a series of 23 nanocrystal MOFs (nMOFs) with various organic link-ers and metal ions and composited them with graphene to incorporate into devices and function as symmetric SCs (Fig. 24b). Of these MOFs, a zirconium-based MOF, MOF-867, achieved high gravimetric, stack and areal capacitances of 726 F g−1, 0.64 F cm−3 and 5.09 mF cm−2, respectively, which was higher than that of benchmarked commercially available activated carbon and graphene (Fig. 24c). In addi-tion, this performance was maintained for at least 10,000 charge/discharge cycles. Here, the researchers attributed the performance of this supercapacitor to the crystallinity and

porosity of the nMOFs, and especially the sp2 nitrogen atoms in the nMOF-867, which can increase the interaction with ions and enhance device performances. By changing reac-tion temperatures, Tan et al. [261] also fabricated a series of Zr-based MOFs (UiO-66) with different particle sizes and BET specific surface areas and reported that the MOF sam-ple prepared at a reaction temperature of 50 °C possessing a minimum particle size of ~ 100 nm and the highest BET specific surface area of 1047 m2 g−1 exhibited a maximum specific capacitance as high as 1144 F g−1 at 5 mV s−1 and a capacitance of 654 F g−1 at 10 mV s−1 after 2000 cycles. Recently, Fu et al. [262] synthesized and coated UiO-66 onto carbon nanotube films (CNTFs) (U-C) and electrochemi-cally co-deposited poly(3,4-ethylenedioxythiophene)-gra-phene oxide (PEDOT-GO) onto the obtained U-C to pre-pare a flexible porous electrode (PEDOT-GO/U-C). Here, the researchers reported that the porosity of the resulting UiO-66 is favorable for electrolyte access and that PEDOT can improve conductivity. Moreover, the large surface area of GO can facilitate the deposition of PEDOT in thin lay-ers. As a result of the synergistic effects among UiO-66, PEDOT and GO, an exceptionally high areal capacitance of 102 mF cm−2 at 10 mV s−1 was obtained.

To improve conductivity, carbonaceous materials can generally be hybridized with MOFs. For example, Srimuk et al. [263] prepared a composite of RGO-HKUST-1 through the ultrasonic mixing of RGO and HKUST-1 and reported that the as-prepared composite possessed a high BET surface area of 1241 m2 g−1 and a specific pore volume of 0.78 cm3 g−1 with an average pore diameter of 8.2 nm, which can be beneficial for the uptake and release of elec-trolytes. To further examine practical uses, the researchers in this study coated RGO-HKUST-1 onto flexible carbon fiber paper (CFP) to assemble a symmetric solid-type SC and reported that the as-fabricated supercapacitor demon-strated a potential window of ~ 1.6 V and a high capacity of ~ 193 F g−1 over a consecutive testing period of 60,000 s with a capacity retention of 98% after 2000 cycles. Fur-thermore, Wen et al. [264] grew a Ni-based MOF onto a carbon nanotube surface (Ni-MOFs/CNTs) using a solvo-thermal method and reported that the surface of the CNTs can be easily modified with the carboxyl groups, promot-ing uniform growth of Ni-based MOFs on the surface. As a result, the Ni-MOF/CNT composite achieved a high specific capacitance of 1765 F g−1 at 0.5 A g−1. To further test the performance of this composite, an asymmetric supercapaci-tor device was fabricated by using the Ni-MOFs/CNTs as a cathode and RGO/C3N4 as the anode and evaluated at an operating voltage of ~ 1.6 V in 6 M KOH aqueous elec-trolyte. Here, the resulting supercapacitor exhibited a high energy density of 36.6 Wh kg−1 at 480 W kg−1 with only 5% initial specific capacitance loss after 5000 cycles.


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The addition of carbonaceous materials can reduce the surface area of composites. However, the use of MOFs as the sole electrode material can maximize surface areas, result-ing in extremely high capacitances for EDLCs. Recently, Sheberla et al. [265] made a significant breakthrough in which they applied Ni3(HITP)2 (HITP = 2,3,6,7,10-hexaimi-nortriphenylene), a MOF with high electrical conductivity [266], as the sole electrode material in an EDLC without any conductive additives or binders. During testing in a two-electrode symmetric supercapacitor, the MOF-based device produced a high area capacitance of ~ 18 μF cm−2, exceeding those of most carbon-based materials. After 10,000 cycles, a capacitance retention > 90% was achieved, confirming the chemical stability of the material as well. This study provides a new direction for the application of conductive MOFs as sole active materials without conductive additives or binders for EDLCs. In another example, Li et al. [267] recently prepared conductive MOF nanowire arrays (NWAs) and applied them as a conductive additive and binder free electrode for solid-state supercapacitors. Here, the as-fab-ricated solid-state supercapacitor delivered a capacitance of ~ 22 μF cm−2, revealing that the electrochemical perfor-mance of MOFs can be improved through the engineering of morphologies.

Co-based MOFs are also viable electrodes for SCs. For example, Liu et al. [268] synthesized a 2D layered Co-based MOF (Co-LMOF) and evaluated it as an electrode

for SCs. Here, the researchers reported that the 2D layered structure of the MOF can increase the contact between the active materials and the electrolyte, ensuring efficient Fara-daic reactions and therefore, resulting in high capacitances. In addition, the nanoscale size of the Co-LMOF particles can further increase the contact area with the electrolyte and shorten the diffusion distance of electrolytes, provid-ing more active sites for electrochemical reactions. Fur-thermore, the electric double-layer capacitance originating from the charge separation on the surface of the electrode and the electrolyte can also contribute to high specific capacitances. As a result, the tested Co-LMOF electrode exhibited a high specific capacitance of 2474 F g−1 at 1 A g−1 in 1 M KOH, and a specific capacitance as high as 1978 F g−1 with 94.3% initial capacitance retention after 2000 cycles even at 2 A g−1. Subsequently, Yang et al. [269] prepared another layered structural Co-MOF with conduc-tive network frames as the electrode for supercapacitors and achieved high capacitances of 2564 and 1164 F g−1 at 1 and 20 A g−1, respectively. In another study, Qu et al. [270] synthesized Ni-based pillared DABCO-MOFs (DMOFs) (DABCO = 1,4-diazabicyclo[2.2.2]-octane) with different kinetic water stabilities and applied them as SC electrodes. Here, the stabilities of DMOF-ADC ([Ni(9,10-anthracenedi-carboxylic acid)(DABCO)0.5]) and DMOF-TM ([Ni(2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid) (DABCO)0.5]) in humid environments were examined and the results revealed

Fig. 24 a Possible model of the structure of the Ni-based MOF before and after Zn doping; b schematic illustration of the construc-tion of nMOF supercapacitors; c the highest nMOF supercapacitor

(nMOF-867) and a comparison with activated carbon and graphene. Reprinted with permission from Refs. [259, 260]


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that DMOF-ADC exhibited the highest stability, whereas DMOF-TM was relatively less stable. During testing as the electrode for SCs in a KOH aqueous solution, the DMOF-ADC converted into highly functionalized nickel hydrox-ide in the presence of alkali, inheriting high stabilities and exhibiting an excellent cycling performance with only 2% capacitance loss after 16,000 cycles. These cycling results were superior to that of DMOF-TM and other Ni-based elec-trodes. In addition, the DMOF-ADC electrode also exhibited high specific capacitances of 552 and 438 F g−1 at 1 and 20 A g−1, respectively. Worrall et al. [271] also obtained Zn and Co-based MOFs (ZIF-4, ZIF-7, ZIF-8, ZIF-14 and ZIF-67) as electrode coatings through the anodic growth method and reported that among these, the additive-free ZIF-67-coated Co electrode displayed the best areal capacitance.

ZIF-67 has been widely investigated as the electrode for SCs. However, it possesses disadvantages such as poor con-ductivity and bulk electrical resistance. To overcome these issues, Wang et al. [272] synthesized ZIF-67 onto carbon cloth (CC) and further electrically deposited polyaniline (PANI) (PANI-ZIF-67-CC), interweaving ZIF-67 crystals with PANI chains (Fig. 25a, b). Here, the resulting PANI-ZIF-67-CC provided an extraordinary areal capacitance of 2146 mF cm−2 at 10 mV s−1 in a three-electrode system, and furthermore, a flexible solid-state supercapacitor (SSC) was assembled and tested based on two symmetric freestand-ing PANI-ZIF-67-CC electrodes (Fig. 25c), achieving a high areal capacitance of 35 mF cm−2 and a power den-sity of 0.83 W cm−3 at 0.05 mA cm−2 with more than 80% retention of the initial capacitance after 2000 cycles. The researchers in this study also tested the performance of this device in various practical conditions and even after bend-ing and twisting, electrical performances did not deteriorate, in which a red light-emitting diode (LED) was lit by the SC (Fig. 25c), indicating the potential of MOF-based SSCs for flexible and wearable electronics. Zhang et al. [273] also synthesized ZIF-67 with a micro-flower structure and applied it as the electrode for supercapacitors. Here, the hier-archical micro-flower structure possessed high chemical and thermal stability, a large BET surface area of 412.87 m2 g−1 and small ion-transport resistances, contributing to a high specific capacitance of 188.7 F g−1 at 1 A g−1. In addition, other ZIF-67 composites such as Ni2CO3(OH)2@ZIF-67 [274] and ZIF-67/GO [275] have also been prepared for pseudocapacitor applications.

Metal oxides as pseudocapacitive materials have also been introduced into MOF systems to improve capacitance. For example, Wang et al. [276] performed electro-deposition to cover MnO2 onto a Ni-based MOF (NiHCF) to prepare a NiHCF@MnO2 dual-layer structure. Here, the resulting composite electrode exhibited an enhanced capacitance of 224 F g−1 at 50 mV s−1 that was larger than the sum of each part. The researchers in this study attributed this enhanced

capacitance to the synergistic effects of the dual-layer struc-ture as well as the complementarity of the reticular battery-type MOF structure and the high power pseudocapacitive MnO2. However, because the electro-deposition method is instrument-dependent, the full potential of the syner-gistic effects of the dual-layer structure fabricated by this approach could not be reached. To resolve this, Zhang et al. [277] introduced MnOx into MOFs using a one-step chemi-cally induced self-transformation process. Here, instead of direct addition, the pseudocapacitive materials come from the in situ growth of MnOx from Mn-containing MOFs in response to the addition of NH4F. By adding NH4F, the Mn in the MOFs can grow into MnOx with a nanoflower mor-phology, decorating the surfaces of manganese hexacyano-ferrate hydrate (MHCF) cubes. As a result of the more inti-mate connection between the pseudocapacitive material and the MOF, the obtained MnOx-MHCF composite electrode in this study exhibited a significantly increased capacitance of ~ 1200 F g−1 at 10 A g−1 as compared with that of the pristine MOF (~ 300 F g−1), demonstrating the viability of this method.

4.2 MOF‑Derived Materials for Supercapacitors

MOF-derived carbon As mentioned above, porous carbon materials are commonly used as electrode materials for SCs and especially for EDLCs. Therefore, MOFs are widely used to prepare carbonaceous materials with high surface areas. In addition, the porosity and structure of derived carbonaceous materials can be adjusted by controlling the structure of MOFs and calcination conditions. Therefore, the exploration of suitable MOF precursors and calcination conditions is significant for the preparation of outstanding carbonaceous electrode materials for SCs. Various experi-mental data for supercapacitors with MOF-derived carbon are listed in Table 4.

Liu et al. [27] were the first to prepare nanoporous carbon with a high BET surface area of 2872 m2 g−1 and a large pore volume of 2.06 cm3 g−1 by employing MOF-5 as the template. In this study, degassed MOF-5 was heated under an atmosphere of furfuryl alcohol (FA) in which FA was polymerized into the pores of MOF-5. Following this, the PFA/MOF-5 composite was carbonized at 1000 °C under Ar flow to prepare the desired nanoporous carbon (NPC). If used as an electrode material for EDLCs, the as-prepared NPC achieved specific capacitances of 204–159 F g−1 with increasing sweep rates from 5 to 50 mV s−1 and at 250 mA g−1, even a capacitance of 258 F g−1 can be reached. To further investigate the influence of carbonization tem-peratures, this group [283] prepared five NPCs by polymer-izing and carbonizing the precursor of FA accommodated in MOF-5 at temperatures from 530 to 1000 °C. Here, the results indicate that the dependence of BET surface area on


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carbonizing temperature presents in a “V” shape in which at higher temperatures (such as 1000 °C), obtained NPCs possess high mesoporosity and good electrical conductivity, allowing for ideal capacitor behaviors and almost constant specific capacitances. However, the results in this study also revealed that although NPCs obtained at carbonization tem-peratures of 530 °C possessed the highest BET surface area, it also demonstrated poor electrochemical performances which may be attributed to the poor conductivity due to low carbonization temperatures.

Recently, Wen et al. [284] fabricated an RGO/carbonized MOF-5 (RGO/CMOF-5) hybrid possessing an RGO inner layer and a CMOF-5 outer cover by annealing GO/MOF-5 at 1000 °C under Ar and reported that the RGO/CMOF-5 exhibited a high BET specific surface area and mesoporous features. In addition, the intercalated graphene layer could also reportedly serve as mini-collectors to shorten electron

transportation lengths and improve conductivity. Further-more, the RGO improved the electrochemical performance of the RGO/CMOF-5 hybrid, resulting in a high specific capacitance of 312 F g−1 with a retention of 89% after 5000 cycles. To further explore the RGO/CMOF-5 compos-ite in practical applications, the researchers in this study assembled a symmetric supercapacitor based on the RGO/CMOF-5 as the cathode and anode and reported that the as-fabricated supercapacitor displayed an energy density of 17.2 Wh kg−1 at a power density of 250 W kg−1 with an excellent cycling stability of 81% capacitance retention after 5000 cycles at 2 A g−1. In another study, Yu et al. [285] prepared 3D interconnected porous carbons (IPCs) from MOF-5 using microwave-assisted KOH activation and reported that the resulting IPC exhibited a high capacitance of 212 F g−1 in 6 M KOH at 0.05 A g−1. Furthermore, Yan et al. [286] prepared three types of porous carbon through

Fig. 25 a Schematic illustration of the migration of electrons and electrolytes before and after interweaving with PANI; b schematic illustration of the fab-rication process of PANI-ZIF-67-CC and corresponding SEM images; c schematic illustration of the PANI-ZIF-67-CC flexible SSC, and testing in practical conditions. Reprinted with permission from Ref. [272]


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the carbonization of HKUST-1, MOF-5, and Al-based MOFs (Al-PCP) without additional carbon precursors at 800 °C. Here, the carbon material derived from Al-PCP reportedly possessed the largest BET surface area and exhibited the highest capacitance of 232.8 F g−1 at 100 mA g−1, whereas the HKUST-1-derived carbon displayed the smallest BET surface area and demonstrated poor capacitive behaviors.

Aside from MOF-5, ZIF-8 has also been widely used as precursors or templates to prepare carbon materials. For example, Jiang et al. [287] prepared porous carbon with a high BET surface area of 3405 m2 g−1 and a large pore vol-ume of 2.58 cm3 g−1 by using ZIF-8 as the precursor and template and FA as the other carbon precursor (Fig. 26a). Here, the porous carbon material achieved specific capaci-tances of ~ 200 F g−1 at 250 mA g−1 as the electrode material for EDLCs. In addition, the tested SC in this study demon-strated a slight sweep-rate dependence in which at higher sweep rates, meso-/macropores contribute to the specific capacitance and at lower sweep rates, contributions are made by micropores. In a further study, Chaikittisilp et al. [288] prepared nanoporous carbon through the direct carboniza-tion of commercially available ZIF-8 without any additional carbon sources and reported that the resultant nanoporous carbon possessed lower BET surface areas ranging from 520 to 1110 m2 g−1 as compared with carbon materials obtained from ZIF-8/FA. However, the obtained nanoporous carbon in this study exhibited a capacitance of 130 F g−1 at 50 mV s−1 with no loss of capacitance after 250 cycles, and moreover, at lower scan rates, the capacitance sharply increases, indi-cating the diffusion limitation inside the micropores. Amali

et al. [289] in their study introduced meso-/macropores as additional second-order structures through the assembly of microporous ZIF-8 particles prepared by ultrasonica-tion. The researchers subsequently carbonized the resultant ZIF-8 followed by KOH activation to obtain 3D hierarchi-cally porous carbon with micro-, meso- and macropores (AS-ZC-800) (Fig. 26b) and reported that the AS-ZC-800 possessed a high BET surface area of 2972 m2 g−1 and a large pore volume of 2.56 cm3 g−1. Because the integrated micro-, meso- and macropores can contribute to the improve-ment of electrochemical, and especially kinetic performance, the AS-ZC-800 exhibited specific capacitances of 251 and 204 F g−1 at 0.25 and 50 A g−1, respectively, and demon-strated high cycling stabilities at 5 A g−1. Furthermore, Yu et al. [290] proposed a method to generate bimodal poros-ity in porous carbon by using ZIF-8 as sacrificial precur-sors and silica colloids as extra porogenes through further self-assembly (Fig. 26c). This as-synthesized hierarchical porous carbon possessed micropores ~ 1.0 nm in size which were inherited from the intrinsic cavities of ZIF-8 crystals and mesopores 3–20 nm in size which were produced with the assistance of self-extra porogenes. As a result, a high capacitance of 181 F g−1 with good capacitance retention was obtained which the researchers attributed to the large surface area, high microporosity and large portion of mesoporosity of the resulting material. In another study, Gao et al. [291] obtained carbon materials by calcining ZIF-8 at 450 °C in N2 and 300 °C in air and reported that calcined ZIF-8 under N2 atmosphere at 450 °C can exhibit better electrochemical performances as electrodes for SCs.

Table 4 MOF-derived carbon for supercapacitors

MOFs Derivatives Electrolyte Rate IC (F g−1) CN AC Reference

MOF-5 NPC 1 M H2SO4 5 mV s−1 204 – – [27]MOF-5 RGO/CMOF-5 6 M KOH 5 A g−1 ~ 220 5000 89% [284]MOF-5 IPC 6 M KOH 50 mA g−1 212 – – [285]Al-PCP NPC 30 wt % KOH 1 A g−1 173.6 – – [286]ZIF-8 NPC 1 M H2SO4 0.25 A g−1 ~200 – – [287]ZIF-8 Z-900 0.5 M H2SO4 50 mV s−1 130 250 130 F g−1 [288]ZIF-8 AS-ZC-800 1 M H2SO4 5 A g−1 ~ 200 2000 ~ 200 F g−1 [289]ZIF-8 NPC 6 M KOH 100 mV s−1 – 2000 97% [290]ZIF-8 NPC 1 M H2SO4 7.5 A g−1 – 2000 92% [292]ZIF-8 NPC 6 M KOH – – 5000 98% [293]ZIF-8 N-doped carbon 6 M KOH 10 A g−1 ~180 1000 98% [295]ZIF-67 NPC 6 M KOH 200 mV s−1 163 – – [296]ZIF-7 NPC 6 M KOH 5 A g−1 – 5000 94% [298]IRMOF-3 N-doped carbon 1 M H2SO4 50 mV s−1 ~ 190 10,000 ~ 190 F g−1 [299]Zn-MOFs ZMP-0.04 1 M H2SO4 1 A g−1 477 100 ~ 90% [301]MOF-74 GNR 1.0 M H2SO4 10 mV s−1 193 500 ~ 190 F g−1 [302]HKUST-1 HPCFs 6 M KOH 10 A g−1 – 10,000 95% [303]K-MOFs Nanoporous

carbon sheets1 M H2SO4 – – – – [304]


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Salunkhe et al. [292] designed symmetric supercapaci-tors based on NPC materials as electrodes through the direct carbonization of ZIF-8 at 800 °C under N2 gas followed by washing with HF aqueous solution to remove Zn nano-particles. Here, the as-fabricated supercapacitor exhibited a specific energy of 10.86 Wh kg−1 at a specific power of 225 W kg−1 and good cycling stability with 92% capaci-tance retention after 2000 cycle at 7.5 A g−1. In a subsequent study, these researchers [293] fabricated an asymmetric supercapacitor in which nanoporous NiO (NPN) derived from cyano-bridged coordination polymers were used as the cathode and NPC derived from ZIF-8 as the anode, and reported that the synergistic cooperation between the pseu-docapacitive NPN nanoflakes and the porous NPC particles enhanced the performance of the asymmetric supercapaci-tor. Recently, this group [294] also fabricated an all-carbon supercapacitor by using ZIF-8 derived nanoporous carbon as the electrodes and 2 M NEt4BF4/PC as the organic elec-trolyte and reported that the high surface area and micro- and mesoporous structure of the MOF-derived nanoporous

carbon contributed to good performances in which volumet-ric and gravimetric capacitances of the nanoporous carbon-based supercapacitor reached 9.24 F cm−3 and 21.0 F g−1, respectively, at 10 mV s−1. Furthermore, Zhong et al. [295] prepared a N-doped porous carbon through the co-carbon-ization of ZIF-8 with additional carbon sources including melamine, urea, xylitol and sucrose at 950 °C, and the results indicated that the macromolecular carbon source, sucrose, can effectively prevent nitrogen loss from the back-bone of ZIF-8 because the pre-melting and polymerization of sucrose adsorbed on the ZIF surface can form a protective layer around ZIF, therefore avoiding nitrogen loss during the carbonization process. As a result, the carbon derived from the ZIF-8/sucrose demonstrated the highest capacitance of 285 F g−1 at 0.1 A g−1 and good cycling stability with only 2.2% capacitance lost after 1000 cycles at 10 A g−1, which can be attributed to the graphene-like structure, hierarchical pore structure and high nitrogen content.

Salunkhe et al. [296] also used ZIF-67 as the single pre-cursor to prepare nanoporous carbon and nanoporous Co3O4

Fig. 26 Schematic illustration of a the preparation of nanoporous car-bon from ZIF-8/FA; b hierarchically porous carbon from the assem-bly of ZIF-8 particles; and c porous carbon with bimodal porosity

through self and assisted assembly approaches. Reprinted with per-mission from Refs. [287, 289, 290]


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by optimizing annealing conditions (Fig. 27a). Here, the obtained nanoporous carbon possessed highly graphitic walls and a BET surface area of 350 m2 g−1, whereas the derived Co3O4 possessed a lower BET surface area of 148 m2 g−1 with only 1.7 at.% carbon. In application as the electrode for SCs, the nanoporous carbon and the Co3O4 exhibited capacitances of 272 and 504 F g−1, respectively, at 5 mV s−1. In further testing, the researchers in this study also fabri-cated symmetric (i.e., carbon//carbon and Co3O4//Co3O4) and asymmetric (Co3O4//carbon) supercapacitors using the nanoporous carbon and Co3O4 as electrodes (Fig. 27b) and reported that the presence of nanoporous pseudocapacitive Co3O4 assisted in attaining higher current sweeps and nano-porous graphitic carbon provided a stable and wide potential window, therefore allowing the asymmetric supercapacitor (Co3O4//carbon) to achieve both a high specific energy of 36 Wh kg−1 and a high specific power of 1600 W kg−1 at 2 A g−1. In addition, this asymmetric supercapacitor also exhibited excellent rate capabilities and long-term stabilities up to 2000 cycles at 5 A g−1. Furthermore, researchers [297] have also reported that metallic cobalt/carbon composites can be achieved using ZIF-67 as the precursor with promis-ing results.

ZIF-7 can also be used as precursors to prepare carbon materials. For example, Zhang et al. [298] prepared porous carbon materials through the co-carbonization of ZIF-7 with additional carbon sources including glucose, ethylene glycol, glycerol and furfuryl alcohol. Here, the use of glucose as the additional carbon source was found to be beneficial for the formation of graphene-like structures due to the pre-melting and polymerization of glucose before the complete carboni-zation of ZIF-7. As a result, the ZIF-7/glucose-derived car-bon exhibited large capacitances of 228 F g−1 and 178 F g−1 at 0.1 and 10 A g−1, respectively, and good cycling stabilities with 94% capacitance retention after 5000 cycles at 5 A g−1, which researchers attributed to the high BET surface area, inherent micropores, proper mesoporosity and graphene-like structure of the ZIF-7/glucose-derived carbon.

In terms of N doping, Jeon et al. [299] used nitrogen-containing IRMOF-3 as a self-sacrificial precursor to pre-pare N-doped porous carbon and revealed that the nitrogen content and surface area can be controlled by tuning car-bonization temperatures. Here, by increasing the carboni-zation temperature from 600 to 950 °C, nitrogen content decreases from 7 to 3.3 at%, whereas BET surface areas increase from 391 to 553 m2 g−1. Furthermore, because of

Fig. 27 Schematic illustration of a the formation of nanopo-rous carbon and Co3O4 from the ZIF-67 polyhedron as the single precursor and b the asymmetric supercapacitor with Co3O4 as the cathode and carbon as the anode. Reprinted with permis-sion from Ref. [296]


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the higher percentage of graphitic carbon as well as the high surface area and pore volume of the N-doped porous car-bon, electrochemical measurements confirmed that porous carbon obtained under 950 °C achieved the highest specific capacitance of 213 F g−1 at 0.5 A g−1. For comparison, the researchers also prepared an analogous nitrogen-free car-bon from MOF-5 which displayed a lower capacitance of 24 F g−1 at 0.5 A g−1, demonstrating the significant role of N doping in the charge storage process. Furthermore, Jayara-mulu et al. [300] used IRMOF-3 as a template and sucrose as an external carbon precursor to obtain graphitic nanoporous carbon and also reported excellent reversible charge storage.

Guo et al. [301] in their study prepared a carbonized Zn-based MOF/PANI composite using in situ chemical oxidative polymerization, and their results revealed that the synergistic effects between carbonized MOFs and PANI can contribute to good electrochemical perfor-mances, in which the carbonized MOF demonstrated high electrical conductivities and maintained the structure of PANI during the charge–discharge process. In addition, PANI can provide high capacitances and a sandwich-like structure that can reduce electron transport distances and electrolyte diffusion resistances, resulting in the composite exhibiting a high capacitance of 477 F g−1 at 1 A g−1.

A recent breakthrough made by Pachfule et al. [302] in which the researchers used MOFs as self-templates to prepare carbonaceous materials as the electrode for SCs. In this study, carbon nanorods were obtained through the thermal transformation of MOF-74-rods at 1000 °C in Ar flow. These carbon nanorods were subsequently subjected to a sonochemical treatment followed by chemical activa-tion to obtain graphene nanoribbons (Fig. 28a). Here, the researchers reported that their self-templated and catalyst-free strategy to prepare carbon nanorods and graphene nanoribbons can overcome the disadvantages of traditional methods such as complex procedures and large energy consumption. Furthermore, to examine the feasibility of the as-prepared carbon nanorods and graphene nanorib-bons, a two-electrode symmetric supercapacitor with 1.0 M H2SO4 as the electrolyte was constructed in this

study and tested (Fig. 28b), revealing that the graphene nanoribbons can exhibit high specific capacitances of 193 and 123 F g−1 at 10 and 400 mV s−1, respectively, and was higher than that of the carbon nanorods. Moreover, the graphene nanoribbons displayed high cycling stabilities at 10 mV s−1. In this study, the excellent supercapacitor performance of the graphene nanoribbons was attributed to the ease of access for ions between the layers of the gra-phene nanoribbons, the high electrical conductivity which enabled the fast transport of electrons and the presence of a 2D lamellar structure which enriched connectivity. In addition, the curvy nature of the graphene nanoribbons prevented restacking, guaranteeing high capacitances for a large number of cycles. Overall, this study provided a facile method to effectively fabricate 1D or 2D carbon materials with promising applications in electrochemical devices.

MOF-derived metal oxides As discussed above, various noble or cheap transition metal oxides, such as RuO2, NiO, Co3O4, Fe3O4 and MnO2, are widely used as electroactive materials for pseudocapacitors [251]. This is because the fast and reversible redox reactions of pseudocapacitors can provide much higher specific capacitances compared with EDLCs. In addition, MOF-derived metal oxides possess large surface areas and controlled porosities, which can be advantageous to the infiltration of electrolytes and the fast diffusion of electrons and ions. In Table 5, representative data of MOF-derived metal oxides applied in supercapaci-tors are listed.

Among the various metal oxides, Co3O4 is considered to be a promising electrode material due to its lower costs than noble metal oxides (RuO2), higher redox activities and environmental benignity. In one example, Meng et al. [305] prepared porous Co3O4 with a BET surface area of 47.12 m2 g−1 and a pore size of 10.8 nm through a two-step calcination of Co-based MOFs involving heating in Ar flow followed by heating in air. Here, the large surface area of the porous Co3O4 can increase the contact area between the electrode and the electrolyte, and the hierarchical porous channels can further ensure efficient contact between the electroactive

Fig. 28 Schematic illustration of a the formation process of MOF-74-Rod, carbon nanorods and graphene nanoribbons and b the supercapacitor cell. Reprinted with permission from Ref. [302]


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particles and the electrolyte, improving the surface adsorp-tion–desorption process of alkali cations. In addition, the amorphous porous structure of Co3O4 can provide more surface electroactive sites for redox pseudocapacitance and facilitate the transport of ions and electrons, leading to fast kinetics. As a result, the as-prepared porous Co3O4 electrode achieved a maximum specific capacitance of 150 F g−1 at 1 A g−1 with excellent cycling stabilities over 3400 cycles in 2 M KOH electrolyte. Following this study, Zhang et al. [306] prepared a porous hollow Co3O4 with a rhombic dodecahedral structure through the calcination of ZIF-67 rhombic dodecahedral microcrystals at 450 °C in air and reported that the obtained Co3O4 exhibited a high BET sur-face of 128 m2 g−1 and possessed abundant mesopores that guaranteed good electrolyte access. As a result, the Co3O4 electrode provided high specific capacitances of 1100 F g−1 at 1.25 A g−1 and 437 F g−1 even at 12.5 A g−1. Even after 6000 cycles at 6.25 A g−1, the Co3O4 electrode retained more than 95.1% of the specific capacitance. Recently, Wang et al. [307] prepared mesoporous Co3O4@carbon compos-ites with an octahedral shape inherited from the precursors through the carbonization of ZSA-1 and reported that the obtained Co3O4 displayed a high specific capacitance of 205.4 F g−1 at 0.2 A g−1 and good cycle performances which the researchers attributed to the good electronic conductivity of the porous carbon. Furthermore, Hu et al. [308] prepared ZIF-67/Ni–Co layered double hydroxides (LDHs) with a

yolk-shell structure using an etching and deposition process with an ethanol solution of Ni(NO3)2. This yolk-shell struc-ture was subsequently annealed in air to form the desired Co3O4/NiCo2O4 double-shell nanocages (DSNCs). Here, the researchers reported that the obtained Co3O4/NiCo2O4 DSNCs achieved a specific surface area of 46 m2 g−1 with a suitable pore size distribution that increased pseudoca-pacitive performance, in which the Co3O4/NiCo2O4 DSNC exhibited high capacitances of 972 F g−1 at 5 A g−1 and 615 F g−1 even at 50 A g−1 in 2.0 M KOH aqueous electro-lyte. In addition, this Co3O4/NiCo2O4 DSNC also demon-strated excellent electrochemical stability with a capacitance retention of 92.5% after 12,000 cycles at 10 A g−1 which might be related to its excellent structural stability.

NiO-based materials have been exploited as alternative candidate materials for SCs. For example, Xia et al. [309] prepared N-doped mesoporous interlinked carbon/NiO nanosheets consisting of monodisperse NiO homogene-ously embedded in carbon nanosheets by annealing Ni-based ZIF-8 at 350 °C in Ar. Here, the doped nitrogen atoms can offer active sites and improve electrochemical performance. Moreover, the combination of the pseudocapacitive behav-ior of NiO and the EDLC behavior of the nitrogen-doped mesoporous carbon can contribute to providing a capaci-tance of 414 F g−1 after 3000 cycles at 5 A g−1 and high rate performances with 390 F g−1 at 25 A g−1.

Table 5 MOF-derived metal oxides for supercapacitors

MOFs Derivatives Electrolyte Rate IC (F g−1) CN AC Reference

Co3O4

Co-MOFs Porous Co3O4 2 M KOH 1 A g−1 ~ 140 3400 ~ 160 F g−1 [305] ZIF-67 Porous Co3O4 3 M KOH 6.25 A g−1 ~ 700 6000 95% [306] ZIF-67/LDH Co3O4/NiCo2O4 2 M KOH 10 A g−1 870 12,000 805 F g−1 [308]

NiO ZIF-8 Carbon/NiO 1 M LiPF6 5 A g−1 ~ 450 3000 414 F g−1 [309]

Mn2O3

Mn-BTC 3DGN/Mn2O3 0.5 M Na2SO4 1 A g−1 374 1800 ~ 100% [310]Fe3O4

Fe-MIL-88B-NH2 Fe3O4/carbon 1 M KOH 2 A g−1 95 4000 83.3% [311]Fe2O3

GO/Fe-MOFs RGO/Fe2O3 6 M KOH 20 A g−1 – 5000 ~ 100% [313]Molybdenum oxides GO/Mo-MOFs RGO/MoO3 1 M KOH 6 A g−1 – 6000 87.5% [314]

Others MOF-199 Cu 6 M KOH 2 mA cm−2 ~ 700 3000 94.5% [315] PPF-3 MOFs CoSNC 2 M KOH 12 A g−1 – 2000 ~ 90% [316] Ni-BTC NixPyOz 2 M KOH 6 A g−1 – 10,000 53.65% [319] MOF-74 Hybrid metal oxides

(NiO/NiCo2O4(1:1))6 M KOH 10 A g−1 – 3000 92.5% [320]

ZIF-67 Co3O4/ZnCo2O4/CuO 2 M KOH 10 A g−1 – 1000 89.7% [321]


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Mn2O3 is also an attractive electrode material for superca-pacitors because of its low cost, abundance and environmen-tally friendly nature. In one example, Ji et al. [310] in situ grew Mn-BTC MOFs onto a 3D graphene network (3DGN) through a solution immersion method and calcinated the resulting 3DGN/Mn-BTC MOF to form a 3DGN/Mn2O3 composite with a nanowire stacking flower-like morphology. Here, the researchers reported that the synergistic effects of the large surface area and high electrical conductivity of the 3DGN as well as the superior pseudocapacitive activity of the Mn2O3 contributed to a high specific capacitance of 471.1 F g−1 at 0.2 A g−1 and good cycle stability without capacitance decay after 1800 cycles.

Fe3O4 also possesses many advantages as electrode mate-rials including extremely low costs, high natural abundance, high electrical conductivity (~ 2 × 104 S m−1) and environ-mental benignity. However, large volume expansions during cycling can cause poor kinetics and cycling stability, limit-ing application in supercapacitors. To address this issue, the design and preparation of nano-scaled structures and the addition of conductive materials are effective. For example, Meng et al. [311] prepared a porous Fe3O4/carbon composite through the calcination of Fe-MIL-88B-NH2 at 500 °C under N2 gas flow. Here, the carbon in the composite increased the conductivity of Fe3O4 and improving the electrochemical performance of the composite, in which the composite dem-onstrated a specific capacitance of 139 F g−1 at 0.5 A g−1 and an excellent cycling stability with 83.3% capacitance reten-tion after 4000 cycles at 2 A g−1 under varied temperatures. Moreover, the specific capacitance improved with increasing working temperatures from 0 to 60 °C. Recently, Mahmood et al. [312] also prepared nanoporous carbon with and without Fe3O4/Fe nanoparticles by optimizing calcination tempera-tures as the electrode material for asymmetric SCs (Fig. 29a). Here, the obtained Fe3O4/Fe/C composites exhibited a high specific capacitance of 600 F g−1 at 1 A g−1 and a capacitance of 500 F g−1 at 8 A g−1. The researchers in this study attrib-uted these enhanced performances to the short electrolyte dif-fusion pathways, the high mechanical stability and the car-bon coating protection that can reduce stress during cycling. As for the nanoporous carbon, its high BET surface area of 1757 m2 g−1 and well-interconnected pores also contributed to capacitances of 272 and 258 F g−1 at 2 and 10 mV s−1, respec-tively. To further evaluate viability in practical applications, the researchers in this study also assembled an asymmetric supercapacitor based on Fe3O4/Fe/C as the cathode, nano-porous carbon as the anode and 6 M KOH as the electrolyte (Fig. 29a) and reported a high energy density of 17.5 Wh kg−1 at 388.8 W kg−1. Recently, Fe2O3 derived from MOFs has also been applied as the electrode for supercapacitors and has displayed capacitances of 869.2 and 289.6 F g−1 at 1 and 20 A g−1, respectively, with a long cycle life without obvious capacitance decrease after 5000 cycles [313].

Other metal oxides such as molybdenum oxides have also proven to be promising candidates for energy stor-age applications. However, like Fe3O4, MoO3 suffers from fast degradation and poor rate performances. Therefore, combining MoO3 with carbon materials is commonly used to solve these issues. For example, Cao et al. [314] pre-pared an RGO-wrapped MoO3 composite (RGO/MoO3) by annealing GO-wrapped Mo-based MOFs under Ar and air, respectively, (Fig. 29b) and reported that the RGO not only increased conductivity and charge transfer but also provided additional EDLC. As a result, the RGO/MoO3 electrode exhibited a high specific capacitance of 617 F g−1 at 1 A g−1 and retained a specific capacitance of 374 F g−1 at 10 A g−1. In addition, this composite also displayed a high capacitance retention of ~ 87.5% after 6000 cycles at 6 A g−1. To further evaluate the practical application of the RGO/MoO3 electrode, the researchers in this study fab-ricated an all-solid-state flexible supercapacitor (Fig. 29c) which exhibited a high specific capacitance of 404 F g−1 at 0.5 A g−1 and a good cycling stability with ~ 80% capaci-tance retention after 5000 cycles at 2 A g−1 along with a high energy density of 14 Wh kg−1 at 500 W kg−1, demon-strating the great potential of RGO/MoO3 composites for practical applications. Copper oxides derived from MOFs have also been investigated as electrode materials for SCs [315], and the application of MoO3@CuO and MoO2@Cu@C derived from polyoxometalate@MOF as electrodes for all-solid-state supercapacitors has also been investi-gated by Zhang et al. [316, 317] with promising results.

Aside from MOF-derived carbon and metal oxides, other types of MOF derivatives can also be applied as electrodes for supercapacitors. For example, Cao et al. [318] prepared a 2D nanocomposite of CoS1.097 nanoparticles (NPs) and N-doped carbon (CoSNC) through the simultaneous sulfida-tion and carbonization of paddle-wheel framework-3 (PPF-3) MOF nanosheets and reported that the obtained 2D CoSNC produced a capacitance of 360 F g−1 at 1.5 A g−1 with good rate capabilities. Recently, phosphates [319] have also been prepared from MOFs for supercapacitors in which phosphate ions were found to be capable of effectively enhancing con-ductivities, therefore improving electrochemical performances.

5 Applications of MOF‑Based Materials to Fuel Cells

Fuel cells are electrochemical devices that can directly con-vert chemical energy to electrical energy. Of the various fuel cells, hydrogen fuel cells are considered to be one of the best alternatives to fossil fuels because water is the only product, ensuring a clean and carbon free process. Moreover, the efficiency of hydrogen fuel cells (~ 60%) is almost twice that of fossil fuels (34%) [322].


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5.1 Pristine MOFs as Oxygen Reduction Reaction Catalysts

In hydrogen fuel cells, electrons offered by the hydrogen oxidation reaction at the anode travel to the cathode through an external circuit and participate in the oxygen reduction reaction (ORR) [323]. Compared with the hydrogen oxida-tion reaction, ORR is the rate-determining step in hydrogen fuel cells [322] and therefore plays an important role in performance. Although Pt and Pt alloys are considered to be the best catalysts for ORR [324, 325], high costs and scarcity of these precious-metal catalysts hinder large-scale application [326]. As a result, there is an urgent need to find efficient and low-cost catalysts for fuel cells especially as ORR catalysts. MOFs possess ultra-high surface areas that can ensure the access of O2 species. In addition, MOFs possess metal centers that can act as active sites for cata-lytic reactions. Because of these properties, MOF-based materials have attracted great interest as promising candi-dates for ORR electrocatalysts [83]. For comparison, the

performances of various pristine MOF ORR catalysts are listed in Table 6.

Pristine MOFs exhibit poor electrical conductivity, how-ever, limiting catalytic performances. To resolve this, Jahan et al. [327] reacted pyridine-functionalized graphene with Fe-porphyrin to prepare a graphene metalloporphyrin MOF and reported that the resulting composite exhibited good sta-bilities, high selectivity and 4-electron transfer mechanism for ORR in alkaline solutions. In a further study, Jahan et al. [328] coated a paddle-wheel Cu-centered MOF onto GO as a tri-functional catalyst for ORR, OER and hydrogen evolution reaction (HER) and reported that compared with pure MOF or GO alone, the composite demonstrated higher catalytic activities for ORR, OER and HER, which the researchers attributed to the synergistic effects of the framework poros-ity, the larger bond polarity of the oxygen ligands in the GO and the catalytically active copper in the MOF. As a result of this high ORR activity, the electrocatalytic performance of the GO/Cu-based MOF composite was evaluated in a sin-gle polymer electrolyte membrane fuel cell (PEMFC) and

Fig. 29 Schematic illustration for a MOF xerogel (MOX) derived Fe3O4/Fe/C as cathodes and nanoporous carbon as anodes for config-uration in an aqueous asymmetric SC; b the preparation of an RGO/

MoO3 composite and c the configuration of an all-solid-state flexible supercapacitor device. Reprinted with permission from Refs. [312, 314]


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produced a power density that was 76% of a commercial Pt/C catalyst.

Aside from compositing with carbonaceous materials, metal oxides can also be embedded into MOF matrixes to improve ORR activity. For example, Yin et al. [329] used a hydrothermal method to embed α-MnO2 nanopar-ticles into a MIL-101 (Cr) matrix to prepare an α-MnO2/MIL-101 (Cr) composite and reported that the resulting abundant micropores and high surface area improved the accessibility of the reactant to the catalytically active sites, enhancing ORR and OER activity in alkaline electrolytes. Moreover, the strong interaction between the α-MnO2 and the MOF ensured structural stability, resulting in excel-lent stability. In another study, Wang et al. [330] synthe-sized an ε-MnO2/Fe-based MOF composite by integrating ε-MnO2 with a Fe-based MOF support. Here, the resulting composite showed enhanced ORR activities and superior stabilities compared with ε-MnO2 in alkaline electrolytes and was comparable to that of a commercial Pt/C elec-trode. Conducting polymers (CPs) can also be composited with MOFs. For example, Rafti et al. [331] grew a ZIF-8 microporous film over a CP substrate (poly(3-aminoben-zylamine-co-aniline), PAPA) to prepare a composite electrode in which ZIF-8 acted as a gas reservoir, causing O2 to be available for ORR through the preconcentration effect at the electrode surface. As a result of the synergistic effects between the electrocatalytic properties of PAPA and the oxygen storage capabilities of ZIF-8, the obtained ORR activities were better as compared with bare CP.

Recently, Miner et al. [332] used an intrinsically con-ductive 2D layered MOF (Ni3(HITP)2) as a tunable oxygen reduction electrocatalyst in alkaline solution (Fig. 30a) by directly growing the Ni3(HITP)2 film onto a glassy carbon electrode without the use of binders and conductive addi-tives. Here, the inherently high surface area and porosity of Ni3(HITP)2 can facilitate easy access to catalytic active sites, resulting in notable ORR activities. In addition, under an O2 atmosphere, the electrode reportedly reduced oxygen at an onset potential of 0.82 V versus RHE. (Fig-ure 30b) in 0.10 M KOH aqueous solution, which is 0.18 V lower than that of Pt (1.00 V). Other MOFs, such as bime-tallic Fe/Co-based MOFs [333] and Cu-BTC mesoMOFs

[334], have also exhibited good ORR activities. For exam-ple, Du et al. [335] synthesized PB crystalline nanograins mosaiced within an amorphous membrane (PB CNG-M-AM) to enhance the electrocatalysis of Pt for ORR through electrostatic self-assembly. Here, the researchers reported that the PB CNG-M-AM can reduce the formation of H2O2 and prevent Pt NPs from migration, dissolution and detachment during ORR. Furthermore, the synergistic effects between Pt NPs and PB CNG-M-AM can alter ORR routes, lowering activation energies and enhancing ORR kinetics. As a result, the obtained catalyst displayed excel-lent ORR activities and stability. In addition to these above solutions, other methods to improve ORR activities have also been reported in the literature, such as the fabrication of novel MOFs with distinctive frameworks [336].

5.2 MOF‑Derived Materials for Oxygen Reduction Reaction Catalysts

Metal/nitrogen/carbon catalysts (M/N/C) derived from the pyrolysis of transition metal macrocycles or mixtures of C, N and M precursors are one of the most promising non-platinum-group-metal (non-PGM) catalysts for ORR. In par-ticular, Fe/N/C and Co/N/C have been widely investigated due to excellent performances and high stabilities. Because MOFs possess ultra-high surface areas, tunable porosities and controllable structures that inherently possess metal, carbon and nitrogen atoms, they are promising precursors to prepare M/N/C-based catalysts. Notably, the synthetic methods of MOF-derived carbon catalysts always influ-ence catalytic performances [323] in which materials pos-sess higher degrees of graphitization at high temperatures, and gain better conductivities. However, high temperatures also come with disadvantages, such as reduced active sites. Therefore, suitable temperatures and MOF precursors need to be both considered to design ideal materials. The perfor-mances of various MOF-derived materials as ORR catalysts are listed in Table 7.

In one example of MOF-derived materials being used as ORR catalysts, Proietti et al. [337] prepared ZIF-8, which possesses a high nitrogen content and a high microporous surface area, as a host for Fe and N precursors. In this

Table 6 The ORR onset potential (OP), electron transfer number (ETN) and limiting current density (LCD, mA cm−2, the rotation rate is listed in the brackets in rpm) for pristine MOF ORR catalysts

MOFs Electrolyte OP ETN LCD Reference

(Fe–P)n MOFs 0.1 M KOH ~ − 0.05 V versus Ag/AgCl ~4 ~ − 3 (250) [327]Cu-centered MOFs 0.5 M H2SO4 0.29 V versus RHE ~ 4 – [328]MIL-101 (Cr) 0.1 M KOH − 0.07 V versus Ag/AgCl 3.75 ~ − 1 (100) [329]Fe-based MOFs 0.1 M KOH 0.84 V versus RHE Near 4 − 5.56 (1600) [330]Ni3(HITP)2 0.1 M KOH 0.82 V versus RHE 2.25 – [332]Fe-Co-MOFs 0.1 M KOH − 0.13 V versus Ag/AgCl 2–4 – [333]Cu-BTC 0.1 M KOH 0.82 V versus RHE – – [334]


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study, the researchers prepare a Fe/N/C catalyst by mix-ing iron(II) acetate, 1,10-phenanthroline (Phen) and ZIF-8 through ball milling, followed by pyrolysis in Ar and in ammonia. Here, the resulting material was used as a cath-ode catalyst for a H2–O2 PEMFC and provided a power density of 0.75 W cm−2 at 0.6 V, which is comparable to that of Pt-based cathodes with a loading of 0.3 mg cm−2. The researchers in this study attributed this high catalytic activity mainly to the enhanced mass transport arising from the interconnected alveolar carbon nanostructure containing numerous pores that can facilitate the circulation of oxy-gen and water. The pyrolysis of FA-impregnated ZIF-8 can also produce carbons that possess higher microporosity than ZIF-8 pyrolyzed alone. Based on this and to investigate the effects of FA on MOF-derived Fe/N/C catalysts, Morozan et al. [338] used two different methods to prepare precursors and reported that the catalyst obtained from the pyrolysis of ZIF-8, iron(II) acetate and Phen (NC_Ar + NH3) exhibited high activities for ORR. Furthermore, Zhang et al. [339]

investigated whether iron in the catalyst was the origin of the first rapid decay through a Fenton reaction with H2O2 which was generated through the incomplete reduction of O2 in the fuel cells. The results indicate that the iron in Fe/N/C catalysts is not the origin of the first rapid decay of these catalysts in fuel cells and H2O2 is also not. However, the slow electro-oxidation of the carbonaceous support of the catalyst occurring in about 15 h at 0.6 V in H2/O2 fuel cells transforms the initial hydrophobic catalyst layers into hydro-philic. This phenomenon can cause micropore flooding and induce mass transport problems, which is the origin of the first fast decay. The electro-oxidation of the carbon surface can also lead to a decrease in mass activity. The catalytic sites electro-oxidized from the surface of the catalyst and electrically disconnected from initially conducting carbon support might cause the decrease of the kinetics of electron transfer.

Yang et al. [340] also reported that chloroiron-tetrameth-oxyporphyrin (ClFeTMPP) can be applied as an iron and nitrogen precursor to prepare Fe/N/C catalysts for ORR if ZIF-8 is used as the main carbon precursor, in which the choice of the Fe precursor ball milled with ZIF-8 is reported to be crucial to the improvement of catalyst performance. More recently, Wang et al. [341] obtained a Fe/N/C catalyst through the direct heat treatment of Fe-doped ZIF-8 under Ar without the need for any additional post-treatments such as acid leaching or secondary heating, in which the O2-free environment played a key role in stabilizing Fe(II) rather than Fe(III). In addition, the researchers in this study sug-gested that the elimination of zinc species helped to yield a highly porous morphology and that their synthesis strategy effectively prevented the formation of inactive iron aggre-gates. As a result, the obtained Fe/N/C catalyst exhibited a high ORR activity with a half-wave potential of 0.82 V vs. RHE, good cycling stability in acid and low H2O2 yields (< 1%).

Shui et al. [342] prepared an interconnected porous nanonetwork Fe/N/C catalyst by electrospinning a polymer solution containing tris-1,0-phenanthroline iron(II) perchlo-rate (TPI) and ZIF-8 followed by thermal activation and found that the macropores over the continuous nanonetwork can improve mass/charge transport and that the micropo-res of the carbon-based nanofibrous catalyst can host many catalytic sites. In testing of this Fe- and N-containing non-precious-metal catalyst in the membrane electrode for a PEMFC, a high volumetric activity of 3.3 A cm−3 at 0.9 V or 450 A cm−3 extrapolated at 0.8 V can be obtained.

Fur thermore, Zhao et a l . [343] employed [Fe3(imid)6(imidH2)]x (FeIM) as a precursor to prepare a non-PGM electrocatalyst by heat treatment under Ar, fol-lowed by washing in H2SO4 and pyrolysis under NH3. Here, the researchers found that as pyrolysis temperatures increased, N content and specific surface area decreased.

Fig. 30 a Perspective view of the 2D layered structure of Ni3(HITP)2 and b the polarization curves of Ni3(HITP)2 under N2 and O2 atmos-phere as well as the blank glassy carbon electrode under N2 and O2 atmosphere. Reprinted with permission from Ref. [332]


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Table 7 MOF-derived materials for ORR catalysts

MOFs Derivatives Electrolyte OP ETN Reference

Fe/N/C ZIF-8 Fe/N/C 0.1 M HClO4 0.95 V versus RHE 3.98 [341] ZIF-8 Fe/N/C 0.5 M H2SO4 0.93 V versus RHE 3.9 [342] FeIM/ZIF-8 Fe/N/C 0.1 M HClO4 0.92 V versus RHE ~3.93 [343] ZIF-70 Fe/N/C 0.5 M HClO4 0.80 V versus NHE ~3.8 [344] IRMOF-3 Fe/N/C 0.1 M KOH 0.93 V versus RHE ~3.85 [345] Zn(eIm)2 Fe/N/C 0.1 M HClO4 0.914 V versus RHE ~3.8 [346]

PB Fe/Fe3C@C/RGO 0.1 M KOH 1 V versus RHE ~3.52 [349]CPM-99 CPM-99Fe/C 0.1 M KOH 0.95 V versus RHE ~4 [348]CPM-99 CPM-99Fe/C 0.1 M HClO4 0.875 V versus RHE ~4 [348]MIL-88B-NH3 CNPs 0.1 M KOH 1.03 V versus RHE ~4 [350]HKUST-1 Fe2N@NPC 0.1 M KOH − 0.038 V versus Ag/AgCl ~4 [351]Co/N/CZIF-67 ZIF-67-900 0.1 M KOH 0.91 V versus RHE ~4 [352]ZIF-67 Co/N/C PBS ~0.15 V versus SCE ~3.9 [353]ZIF-67 PCP/NRGO 0.1 M KOH − 0.97 V versus RHE ~3.9 [354]ZIF-67 LDH@ZIF-67-800 0.1 M KOH 0.94 V versus RHE ~4 [355]ZIF-67 Co/N/C 0.1 M KOH – 3.99 [356]ZIF-67 Co/N/C 0.1 M KOH – ~4 [357]Zn, Co-ZIF C-95Zn5Co 0.1 M KOH ~ 0.95 V versus RHE ~3.5 [358]Zn, Co-ZIF Co/N/C 0.1 M KOH ~ 0.98 V versus RHE ~3.9 [359]Zn, Co-ZIF P-CNCo-20 0.1 M KOH − 0.04 V versus Ag/AgCl 3.9 [360]Zn, Co-ZIF Co25Zn75-NPC 0.1 M KOH – ~ 4 [361]ZIF@mSiO2 Co, N-CNF 0.1 M KOH − 0.082 V versus Ag/AgCl 3.8 [362]CoP-CMP CoP-CMP800 0.1 M KOH − 0.12 V versus Ag/AgCl ~ 3.8 [363]ZIF-67 MDC 0.1 M HClO4 0.86 V versus RHE 3.7 [364]Co phosphonate-based MOFs Co3(PO4)2C-N/RGOA 1.0 M KOH 0.968 V versus RHE ~ 4 [365]ZIF-8 Co-Zn-ZIF/GO-800 0.1 M KOH 0.96 V versus RHE ~ 4 [366]CPM-24 Co@C 0.1 M KOH ~ 0.85 V versus RHE ~ 4 [367]Bimetallic catalysts PBAs Fe, Co@NGC 0.1 M KOH 0.88 V versus RHE ~ 3.8 [370] PCN-Fe, Co Fe, Co/C 0.1 M KOH 1 V versus RHE ~ 4.2 [371] Ni, Co-MOFs Ni, Co/N/C 0.01 M PBS 0.347 V versus SCE ~ 4 [372]

Spinel-type mixed-valence oxides Co-MOFs Co3O4C-NA 0.1 M KOH – ~ 3.9 [377] ZIF-9 C–Co-oxide 0.1 M KOH – ~ 4 [378] Co-MOFs Co3O4@N–C 0.1 M KOH 0.95 V versus RHE ~ 3.9 [379] ZIF-9 Co@Co3O4@C 0.1 M KOH 0.93 V versus RHE ~ 3.9 [381] MPFs Fe3O4@N/Co–C 0.1 M PBS 0.11 V versus Ag/AgCl ~ 3.6 [383] MIL-101-Fe NC@Fe 0.1 M KOH 0.058 V versus Hg/HgO ~ 3.8 [384]

Metal chalcogenideCu-BTC CuxSy 0.1 M KOH 0.835 V versus RHE 3–4 [386]Cu-BTC CuS@Cu-BTC 0.1 M KOH 0.91 V versus RHE 3.82 [387]ZIF-67 ZIF-TAA-p 0.1 M HClO4 0.90 V versus RHE ~ 3.9 [388]MIL-101-NH2 Co9S8@CNS900 0.1 M KOH − 0.05 V versus Ag/AgCl 3.97 [389]ZIF-67 Co–C@Co9S8 0.1 M KOH 0.96 V versus RHE ~ 3.8 [390]Metal carbon composites Zn–Fe-MOFs Au@Zn-Fe–C 0.1 M KOH 0.94 V versus RHE ~ 3.8 [391] PBAs Co3ZnC/Co@CN 1 M KOH 0.91 V versus RHE 3.8 [392] ZIF-8 FePhen@MOF-ArNH3 0.1 M KOH 1.03 versus RHE 2 [393]


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Following this, the researchers mixed in ZIF-8, which pro-vided both a high N content and a high surface area and found that the shortcomings can be circumvented, in which the high surface area can facilitate mass transfer, improving ORR. In testing as a cathode catalyst in a single PEMFC, an onset potential of 0.977 V and a volumetric current density of 12 A cm−3 at 0.8 V can be obtained. In another study, Palaniselvam et al. [344] prepared Fe/N/C catalysts by embedding [Fe(Phen)3]2+ into ZIF-68, ZIF-69 and ZIF-70, followed by pyrolysis in Ar and washing in 3 M H2SO4 to remove Zn and uncoordinated Fe. Here, the results indicated that the [Fe(Phen)3]2+@ZIF-70 (FeNC-70) derived catalyst can exhibit higher catalytic activities with an onset potential of 0.80 V and a half-wave potential of 0.58 V versus NHE. Furthermore, Wu et al. [345] used FeIII-modified IRMOF-3 in their study as a precursor to prepare a Fe and N co-doped porous carbon hybrid (Fe–Nx/C) catalyst through the ther-mal treatment of FeIII-modified IRMOF-3 at 800 °C in Ar. Here, the resulting hybrid displayed a high onset potential of 0.93 V and a half-wave potential of 0.78 V versus RHE for ORR, which the researchers attributed to the synergistic effects of the high surface-volume ratio, uniform pore size of ~ 3.9 nm, multi-active sites (M–Nx and N-doped C), and highly exposed catalytic active sites. In addition, this cata-lyst also reportedly exhibited superior durability and excel-lent tolerance to methanol. Zhou et al. [346] in their study prepared four different Zn–ZIF-based precursors by mixing ZnO with N-containing ligands and 5 wt% TPI followed by heat treatment at 180 °C (Fig. 31). The obtained products were subsequently pyrolyzed under Ar, washed with acid

and pyrolyzed a second time under NH3 to obtain the final products of Zn(ligand)2TPIP. Here, the results indicated that imidazole ligands clearly influenced the ORR activity of catalysts and that among them, Zn(eIm)2TPIP exhibited the highest volumetric current density of 88.1 A cm−3 at 0.8 V in a PEMFC.

Li et al. [347] in their study prepared a N-doped graphene tube (N-GT)-rich Fe/N/C catalyst through the high-tempera-ture treatment of dicyandiamide (DCDA) and iron(II) acetate templated by MIL-100 (Fe) to improve the activity and sta-bility of Pt catalysts in which the ORR active N-GT was used as a matrix to disperse Pt nanoparticles. As a result, the composite of the highly active Fe/N/C catalyst and the Pt nanoparticles demonstrated significantly enhanced ORR activities and H2-air fuel cell performances compared with 20% Pt/C, which the researchers attributed to the intrin-sic activity of N-GT and the possible synergistic effects between non-PGM and Pt nanoparticles. In addition, this hybrid also displayed excellent stabilities due to the unique highly graphitized graphene tube which can provide strong interactions with Pt nanoparticles, preventing agglomera-tion. In a study by Lin et al. [348], zirconium-porphyrin frameworks (ZPFs) were pyrolyzed with CPM-99 (H2, Zn, Co, Fe) to obtain CPM-99X/C (X = H2, Zn, Co, Fe) in which the Zr6-polyoxo-cluster in the CPM-99X can serve as a hard template to impede the agglomeration of the resulting elec-trocatalytic nanofragments. After leaching with dilute HF, the zirconia residue can be removed, which helps to cre-ate nanoscale cavities. Here, the researchers reported that among the different CPM-99X/C, CPM-99Fe/C exhibited

Table 7 (continued)

MOFs Derivatives Electrolyte OP ETN Reference

ZIF-8 FePhen@MOF-ArNH3 0.1 M HClO4 0.93 versus RHE 4 [393] MIL-88B@ZIF-8 Fe3C@NCNTs 0.1 M KOH – ~ 4 [394] MnO2@ZIF-67 MnO@Co–N/C 0.1 M KOH 0.96 V versus RHE ~ 4 [16]

Metal-free catalysts ZIF-8 NC900 0.1 M KOH 0.83 versus RHE 3.3 [397] ZIF-8 P-Z8-Te-1000 0.1 M KOH − 0.07 V versus Ag/AgCl ~4 [398] ZIF-8 NGPC-1000-10 0.1 M KOH − 0.02 V versus Ag/AgCl ~3.85 [399] ZnO@ZIF-8 NCNTs 0.1 M KOH 0.06 V versus Ag/AgCl ~ 3.98 [400] MOF-5 MOFCN900 0.1 M KOH 0.035 V versus Hg/HgO 3.12 [402] MOF-5 NPS-C-MOF-5 0.1 M KOH − 0.006 V versus Ag/AgCl ~ 3.8 [403] MOF-5 NGPC/NCNTs 0.1 M KOH − 0.051 V versus Ag/AgCl ~3.9 [404] ZnFumarate MPC-np 0.1 M KOH ~ − 0.1 versus SCE ~3.9 [405]

NH2-MIL-53 (Al) PC-Al-1000 0.1 M KOH − 0.13 V versus Ag/AgCl ~ 3.8 [406] UiO-66-NH2 N-P-carbon 0.1 M KOH – ~ 3.9 [407] [Zn(bpdc)DMA]·DMF NPC800 0.1 M KOH ~ − 0.14 versus SCE ~ 2.6 [408] MOF-74 N-doped carbon 0.1 M KOH 1.02 V versus RHE ~4 [409] ZIF-67 NCNTFs 0.1 M KOH 0.97 V versus RHE ~4 [410] Pyridyl-ligand based MOF PNPC 0.1 M KOH 0.89 V versus RHE 3.82 [411]


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the best ORR activity with onset potentials of 0.950 and 0.875 V vs. RHE in alkaline and acidic electrolytes, respec-tively, and is comparable to 20% Pt/C catalyst. In this study, this performance was attributed to the unique precursor with its large cavities, as well as the high proportion of the heme-like center and the hard polyoxozirconate cluster template. Moreover, other Fe-based MOFs, such as PB [349], MIL-88B-NH3 [350], and HKUST-1 [351], can also reportedly be applied as precursors to prepare Fe/N/C catalysts.

Aside from Fe/N/C, Co/N/C can also be obtained from MOFs and exhibit good ORR activities in which ZIF-67, a type of Co-based MOF, is commonly applied as precursors to synthesize Co/N/C catalysts. For example, Wang et al. [352] used ZIF-67 as a precursor to prepare noble metal-free catalysts for ORR in which by acid leaching the cal-cined product, excessive Co particles can be removed to expose more CoNx sites and improve surface area. In this study, the resulting catalyst provided a high positive onset potential of 0.91 V, a half-wave potential of 0.85 V versus RHE and a saturation current density of ~ 5 mA cm−2 in alkaline electrolytes. In another example, You et al. [353] heated ZIF-67 in the range of 600 to 1000 °C under N2 gas to prepare Co/N/C catalysts and indicated that the Co/N/C catalyst pyrolyzed at 900 °C exhibited a maximum power density of 1665 mW m−2. Here, the researchers attributed the excellent performance of the catalyst to its large surface area (237.8 m2 g−1), high ratio of micropores and uniform pore size, which ensures the rapid transport of oxygen and protons. In addition, the researchers also suggested that the well-dispersed Co–Nx active sites can improve catalytic

activities for oxygen reduction in neutral electrolytes. Fur-thermore, Hou et al. [354] prepared N/Co-doped porous car-bon polyhedron (PCP)/N-doped RGO (NRGO) by pyrolyz-ing GO/ZIF-67 composites and suggested that the increased defects (such as the CNx structure) in the resulting hybrid originating from the NRGO can serve as additional active sites to contribute to the improvement of catalytic activi-ties, facilitating ORR. As a result of the synergistic effects between the N/Co-doped PCP and NRGO, the resulting cata-lyst in this study demonstrated excellent catalytic activities for ORR, HER, and OER with good stabilities in both acidic and basic media.

In another example, Li et al. [355] used Co, Al LDH nanoplatelets as a scaffold to directly grow ZIF-67 arrays, followed by heat treatment to prepare a carbon-based architecture and reported that the steady and even binding between the ZIF-67 array and the surface of the LDH scaf-fold allowed for the control of the composition and morphol-ogy of the ZIF-derived carbon-based arrays, allowing the obtained carbon-based network to possess effective active sites such as N/C and Co/N/C, as well as a high surface area of 220 m2 g−1 and a hierarchical micro-/mesoporous structure (mainly centered at 1.5–3 nm and 10–20 nm) that can contribute to the fast diffusion of O2 and electrolytes during ORR. As a result, the catalyst exhibited an excellent ORR activity with a high onset potential of 0.94 V versus RHE and a good stability with ~ 99% current retention over 20,000 s, which is superior to commercial Pt/C catalysts. In another study, Guan et al. [356] prepared Co/N-carbon hollow particles (Co/NC) that possessed large single holes

Fig. 31 a The chemical struc-tures of N-containing ligands and iron additives; the crystal structures of b Zn(Im)2; c Zn(mIm)2; d Zn(eIm)2 and e Zn(4abIm)2. Reprinted with permission from Ref. [346]


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through the pyrolysis of yolk–shell polystyrene@ZIF-67 (PS@ZIF-67). Here, due to the unique shell architecture, the obtained single holed Co/NC exhibited excellent elec-trocatalytic performances toward ORR. Furthermore, ZIF-67-coated commercial carbon (VXC72) was also recently prepared in which the pyrolysis of the materials reportedly led to enhanced ORR activities and stabilities [357].

ZIF-67 possesses a Zn-based isomorph referred to as ZIF-8 (Fig. 32a) in which by adjusting the ratio of Co and Zn, a Zn, Co bimetallic ZIF can be prepared. By heating this Zn, Co bimetallic ZIF at 1000 °C in Ar, Wang et al. [358] were able to prepare a Co/N/C catalyst in which they reported that Zn can be eliminated during pyrolysis, yielding a highly porous structure. In addition, the Zn in the MOF precursor can also effectively inhibit the aggregation of Co after pyrolysis, with results revealing an achievable high half-wave potential of 0.90 V versus RHE in 0.1 M KOH solution, which the researchers mainly attributed to the high dispersion of Co active sites. Similarly, You et al. [359] pre-pared a Co/N/C nanopolyhedron electrocatalyst that pos-sessed a high surface area of 484 m2 g−1, well-dispersed Co nanoparticles of ~ 9.5 nm and a high N content of 8.5% through the pyrolysis of the Zn, Co bimetallic ZIF and also reported that the obtained Co/N/C demonstrated excellent ORR activities with a half-wave potential of 0.871 V ver-sus RHE and a kinetic current density of 39.9 mA cm−2 at 0.80 V in 0.1 M KOH. Furthermore, Chen et al. [360] annealed Co/N/C catalysts with triphenylphosphine in N2 atmosphere to introduce P doping, resulting in improved

catalyst ORR activities that surpassed Pt/C catalysts. Over-all, the results in these studies indicate that factors such as CoNx, high surface area, hierarchical pores, high graphitiza-tion and N, P doping are all favorable for improving ORR activities. Additionally, the relationship between MOF pre-cursor calcination temperature and Co/N/C catalytic activity has also been explored [361].

In the synthesis of Zn, Co bimetallic ZIF, tetraethyl orthosilicate can also be injected to obtain Zn, Co-ZIF@mesoporous silica (mSiO2). For example, Shang et al. [362] pyrolyzed Zn, Co-ZIF@mSiO2 and immersed the resulting material in aqueous HF to remove the mSiO2 shell to prepare a hierarchically porous Co, N co-doped carbon nanoframe-work (Co, N-CNF) (Fig. 32b). Here, the mSiO2 protective coating effectively prevented the irreversible fusion and aggregation of Co and N-CNF during the high-temperature pyrolysis process, allowing the obtained Co/N/C catalyst to exhibit a large specific surface area, rich pore structure and high dispersity, all of which contributed to superior electro-catalytic activities as compared with the samples pyrolyzed without mSiO2 protection.

Other MOFs such as cobalt porphyrin-based conjugated mesoporous polymer (CoP-CMP) frameworks can also be applied as precursors to prepare Co/N/C hybrids for ORR. In a study by Wu et al. [363], the researchers obtained Co–N-doped carbons possessing a ribbon-shaped morphol-ogy with high surface areas and a mesoporous structure (4–20 nm) in which cobalt nanoparticles were uniformly embedded into the N-enriched carbon matrix. As a result,

Fig. 32 a Crystal structures of ZIF-67, ZIF-8 and the Zn–Co bimetallic ZIF and the prepara-tion of porous carbon from bimetallic ZIFs; b synthesis procedure of Co,N–CNFs. Reprinted with permission from Refs. [360, 362]


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this catalyst exhibited outstanding catalytic performances for ORR in alkaline media with a half-wave potential of − 0.18 V versus AgCl/Ag, a high and stable limiting cur-rent of ~ 4.62 mA cm−2 and excellent stability with ~ 96.7% retention after 1000 cycles. In addition, the size of MOF precursors can also affect catalytic performances [364] and recently, Zhou et al. [365] synthesized N-coordinated cobalt phosphate supported on RGO sheets (Co3(PO4)2C–N/RGOA) through the pyrolysis of phosphonate-based MOFs and reported that the phosphate groups can stabilize the Co–N center and provide protons to enable proton coupled electron transfer, facilitating the ORR process, and resulting in the Co3(PO4)2C–N/RGOA catalyst exhibiting a high onset potential of 0.968 V versus RHE.

In another example, Wei et al. [366] prepared a ZIF-8/GO/ZIF-8 sandwich-like structure with ultra-small ZIFs (~ 20 nm) fully covering GO using a process involving homogenous nucleation followed by uniform deposition and confined growth. Here, the researchers reported that the uni-form ZIF coating can effectively prevent the agglomeration of GO during the heating process and that after carboniza-tion and acid etching, N-doped carbon/graphene nanosand-wiches with a high surface area of 1170 m2 g−1 as well as a nanoporous structure and good electrical conductivity can be prepared. Because all these features can increase mass trans-port and electron transfer, ORR activities with a high onset potential of 0.92 V versus RHE and a limiting current den-sity of 5.2 mA cm−2 at 0.60 V in 0.1 M KOH was obtained. Furthermore, these researchers used cobalt nitrate and zinc nitrate to introduce Co–Nx species into the carbon mate-rial and further obtained a higher onset potential of 0.96 V, indicating that Co–Nx moieties are more active than sole N-doped carbon for ORR. Other Co-based MOFs [367–369] have also been applied as precursors to prepare Co/N/C cata-lysts and have also achieved high ORR activities.

MOFs can be used to prepare bimetallic catalysts for ORR as well. For example, Xi et al. [370] synthesized Fe, Co@N-doped graphitic carbon (NGC) NCs through the pyrolysis of polydopamine-encapsulated Fe3[Co(CN)6]2 nanocubes (NCs) at 700 °C, and reported that the resulting NGCs at the surface can effectively improve catalytic performance, allowing the catalyst to exhibit outstanding catalytic selec-tivity and superior durability. In another example, Lin et al. [371] prepared a Fe, Co/N/C catalyst through the pyrolysis of porous conjugated metalloporphyrinic networks (PCNs) (Fig. 33) and reported that the resulting large surface area, substantial graphitization, and high density of uniformly dis-tributed heterometallic active moieties of the catalyst con-tributed to high ORR activities and electrochemical stability in both alkaline and acidic electrolytes, and was comparable to benchmark Pt/C. Furthermore, Tang et al. [372] synthe-sized dual-metal and nitrogen co-doped carbon catalysts (M, Co/N/C) in which the researchers first mixed metal chloride

(FeCl2, NiCl2, CuCl2 or ZnCl2) hexane solutions with Co-based MOF powder under vacuum followed by drying at room temperature and subsequently at 150 °C under vacuum to prepare metal-loaded Co-based MOFs. Through the direct carbonization of the obtained M-Co-MOFs (M = Fe, Ni, Cu, Zn) at 800 °C in N2 atmosphere, the final catalysts were obtained in which the researchers reported large surface areas, highly porous structures and uniform compositions of metal and nitrogen well encapsulated within graphite structures, all of which can ensure high-density active sites for ORR and pathways for O2 and electrolytes, providing superior ORR performances in alkaline and neutral media. Among these catalysts, the researchers noted that the Ni, Co/N/C catalyst exhibited the best ORR performance with an onset potential of 0.347 V versus Hg/HgO in a phos-phate-buffered solution (PBS, pH = 7) with good stability. In further testing, the Ni, Co/N/C catalyst was used as the air cathode for a microbial fuel cell (MFCs) and the as-fab-ricated device reportedly exhibited a high power density of 4335.6 mW m−2 and excellent durability.

The integration of multiform MOFs with functional nanomaterials can produce complex MOF hybrids that pos-sess many unexpected properties. Based on this, Guan et al. [373] reported a distinct synthetic method called the “MOF-in-MOF hybrid” confined pyrolysis approach to construct porous Fe–Co alloy/N-doped carbon cages as the catalyst for ORR. In their study, ZIF-8 and FeOOH nanorods were used as co-precursors during the pyrolysis process, offering degradable templates and new metal species to the resulting ZIF-67-based hybrid particles and promoting the formation of interior cavities and Fe-Co alloy nanocrystallites. Here, the researchers reported that among a series of composites, Fe0.3Co0.7/NC possessed the best catalytic activity with the most positive onset potential and better ORR activities than commercial Pt/C electrocatalysts.

Aside from M/N/C catalysts, spinel-type mixed-valence oxides of transition metals (M′M2O4, M′ and M = transition metals), especially Co3O4, also exhibit ORR catalytic activi-ties in alkaline media [374–376]. In addition, carbonaceous materials can improve electrical conductivity and N-doped carbon can also contribute to ORR activities. Therefore, car-bon/metal oxide composites have been widely investigated as electrocatalysts for ORR in which MOFs containing both metal and organic ligands are considered to be promising precursors to prepare carbon/metal oxide catalysts.

For example, by carbonizing Co-based MOF grown on Cu foil, Ma et al. [377] prepared hybrid Co3O4–carbon porous nanowire arrays (Co3O4C–NAs) (Fig. 34a) that pos-sessed a high surface area of 251 m2 g−1 and a carbon con-tent of 52.1 wt% and reported that this obtained catalyst can be directly used as a working electrode without any extra substrates or binders. As a result of the in situ carbon incor-poration and the porous nanowire array configuration, the


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electrode reportedly exhibited strong structural stability and enhanced mass/charge transport, allowing the Co3O4C–NA electrode to not only display superior OER activities and stronger durability, but also effectively catalyze ORR in alkaline solutions. In another example, Chaikittisilp et al. [378] selected ZIF-9 as a precursor to prepare carbon/cobalt oxide catalysts due to the thermal stability and high carbon and nitrogen content of ZIF-9. In this study, the researchers prepared a nanoporous N-doped carbon/cobalt oxide hybrid material through thermal treatment in inert atmosphere followed by heating in air and reported that the resulting material exhibited excellent catalytic activities for both ORR and OER. Furthermore, Zhang et al. [379] selected [Co3(μ2-OH)4(I)2]·2H2O (Co-I-MOF, I = hypoxan-thine) [380] with its chemically robust and thermally stable structure and ordered cavities as a precursor to synthesize Co3O4@N-doped carbon (Co3O4@N–C) nanocomposites and found that the space between the Co3O4 particles and the N-doped carbon shell can act as nanoreactors to enhance catalytic activity. This study also found that –CNx groups can contribute to ORR activities through the structural and electronic modification of carbon. As a result, the as-pre-pared Co3O4@N–C catalyst exhibited a high ORR activity with an onset potential of 0.95 V in alkaline solutions and even displayed better long-term stability and higher metha-nol tolerance than commercial Pt/C catalysts. Xia et al. [381] in their study prepared Co@Co3O4@C core@bishell nanoparticles in situ encapsulated into a highly ordered porous carbon matrix (CM) (Co@Co3O4@C–CM) by pyro-lyzing MOF–CM composites (Fig. 34b) and reported that the organic ligands of the MOF can transform into porous graphitic carbon which can in situ wrap the metal oxide

nanoparticles, causing strong interactions between the metal oxide and the carbon shell linked to the CM. The researchers here reported that this not only improved the electron trans-fer between the nanoparticles and the CM, but also enhanced ORR stability.

Fe3O4-based materials have also been actively pursued as alternative catalysts to Pt and Pt alloys [382]. In one exam-ple, Cao et al. [383] used the “spontaneous bubble-template” method through a one-step carbonization to prepare Co/N dual-doped hierarchically porous carbon/Fe3O4 nanohybrids as ORR catalysts. Here, the Fe3O4 nanoparticles were in situ grown and embedded onto the N/Co self-doped porous car-bon (Fe3O4@N/Co–C) and the obtained Fe3O4@N/Co–C exhibited a 3D interpenetrating morphology with a high electrochemical active area that can provide more active sites for ORR. As a result, the Fe3O4@N/Co–C exhibited remarkable ORR activities in bio-fuel cells with an output voltage of 576 mV and power density of 918 mW m−2, both of which are higher than those of Pt (0.5 mg cm−2) catalysts. Recently, Gao et al. [384] also prepared a N-doped-carbon-coated Fe3O4 (NC@Fe3O4) through the pyrolysis of PANI-coated MIL-101-Fe and reported that PANI can provide a conductive skeleton in the electrocatalyst to allow for higher ORR activity and stability.

Metal chalcogenide catalysts have also attracted great interest as ORR catalysts [385]. This is because many metals can form chalcogenides with S, Se and Te and exhibit good ORR activity. For example, Seredych et al. [386] prepared CuxSy/nanoporous carbon composites through the carboni-zation of a mixture of Cu-BTC MOF/GO composites and poly(4-styrenesulfonic acid-co-maleic acid) sodium salt and found that the small pores with a hydrophobic surface

Fig. 33 Synthetic procedure of a, b TIPP-M and c, d TEPP-M (M = Fe, Co) monomers; e, f porphyrinic conjugated networks; and g carbonization products. TIPP = 5,10,15,20-tetrakis(4-iodophenyl)

porphyrin, TEPP = 5,10,15,20-tetrakis(4-ethynylphenyl)porphyrin. Reprinted with permission from Ref. [371]


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of the carbon was mainly covered by copper sulfides and that sulfur can attract O2 from the electrolyte. Moreover, the researchers also found that the large surface of the cata-lyst can provide high dispersions of active phases, allowing the catalyst to demonstrate good electrocatalytic activities for ORR in alkaline media with a high kinetic current den-sity of 30 mA cm−2 at 0.18 V versus RHE. Recently, Cho et al. [387] fabricated CuS nanoparticles (nano-CuS) with Cu-BTC and discovered that as the amount of nano-CuS increases, the electrical conductivity exponentially increases as well, leading to increases in onset potential, kinetic cur-rent density and electron transfer number, whereas materials with poor porosity led to decreases. As a result, their CuS(28 wt %)@Cu-BTC sample exhibited the best ORR activity with an onset potential of 0.91 V versus RHE.

Furthermore, Zhang et al. [388] synthesized sulfurated catalysts through the pyrolysis of ZIF-67 loaded with sulfur-containing molecules including thioacetamide (TAA), thio-urea (TU), NH4SCN (SCN), thiophene (TP) and sublimed sulfur (S) and reported that the as-prepared catalyst pos-sessed a N/S-co-doped conductive carbon matrix, encap-sulated Co nanoparticles and CoSx/CoNx, all of which can contribute to high ORR activities. As a result, the optimized sulfurated catalyst exhibited excellent electrocatalytic activi-ties for ORR in both acid and alkaline media, in which a high

onset ORR potential of 0.90 V and a half-wave potential of 0.78 V can be achieved in 0.1 M HClO4 and an onset and half-wave potential of 0.98 V and 0.88 V, respectively, can be achieved in 0.1 M KOH.

Recently, Zhu et al. [389] constructed N, S dual-doped honeycomb-like porous carbon immobilizing cobalt sulfide nanoparticles from MOF composites (Fig. 35). In this study, the researchers used a double-phase encapsulation approach to uniformly and quantitatively introduce TU and CoCl2 into MIL-101-NH2 as secondary precursors without aggregation on the external surface of the MOF. Follow-ing this, the encapsulated TU molecules can coordinate to Co(II) ions to form a [Co(TU)4]Cl2 compound. After the pyrolysis of the Co(II)TU@MIL-101-NH2 under Ar and subsequent HF washing, N, S dual-doped honeycomb-like porous carbon with Co9S8 nanoparticles immobilized inside the Co9S8@CNST (T is the pyrolysis temperature) can be prepared. Here, the researchers reported that the resulting Co9S8@CNS900 provided the maximum BET surface area of 1791 m2 g−1 and a high pore volume of 2.217 cm3 g−1, demonstrating that high pyrolysis temperatures can promote the formation of ordered graphitic structures for carbon which can improve electronic conductivities and corrosion resistances in electrocatalysis. Moreover, the researchers suggested that the high content of active species and the

Fig. 34 Synthetic procedure of a Co3O4–carbon porous nanowire arrays and b Co@Co3O4@C–CM. Reprinted with permission from Refs. [377, 381]


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synergistic interactions between the Co9S8 nanoparticles and the N, S doped carbon can also play an important role in the enhancement of ORR activities. Based on these fac-tors, the resulting Co9S8@CNS900 exhibited remarkably comparable ORR catalytic activities, superior long-term stabilities and better methanol tolerance to commercial Pt/C catalysts in alkaline media. In another example, Hu et al. [390] fabricated a ZIF-67@amorphous CoS yolk–shell structure through the reflux reaction between ZIF-67 poly-hedrons and thioacetamide (TAA). Here, the obtained ZIF-67@amorphous CoS can convert to Co–C@Co9S8 double-shell nanocages (Co–C@Co9S8 DSNCs) through thermal annealing under N2 in which the inner Co nanoparticles can act as catalytic centers with extra active sites, whereas the outer permeable Co9S8 can ensure rapid access of reactants to active sites and prevent aggregation and leaching out. In addition, the double-shell Co–C@Co9S8 nanocages can confine reactants in the complex interior with high instan-taneous concentrations, providing a high driving force for ORR. As a result of these structural merits, the Co–C@Co9S8 DSNC composite provided excellent electrocatalytic

activities for ORR with good durability and methanol toler-ance capability.

Metal carbon composites are currently being widely investigated as efficient electrocatalysts in alkaline media for ORR. For example, Lu et al. [391] prepared multi-level core–shell Au nanoparticles@Zn–Fe embedded porous carbon (Au@Zn–Fe–C) through the direct pyrolysis of a Zn–Fe-based MOF coated onto gold nanoparticles under Ar atmosphere, and reported that the encapsulated Au nano-particles can promote electrocatalytic reactions and that the Zn–Fe–C conducting shell can facilitate electron transport and provide active sites. As a result, the obtained catalyst exhibited excellent activities for ORR with a high onset potential at 0.94 V versus RHE, outstanding stabilities and methanol tolerance. In another example, Su et al. [392] pre-pared Co3ZnC/Co nanojunctions encapsulated in N-doped graphene layers (Co3ZnC/Co@CN) through the annealing of PBAs in N2 and reported that due to the unique architec-ture, this catalyst exhibited stable and robust electrocatalytic performances for both ORR and OER.

Iron can act as a catalyst for the graphitization of N-doped carbon during the carbonization process and can be inte-grated into carbon to enhance electrocatalytic properties. Based on this, Strickland et al. [393] prepared a Fe-based non-PGM electrocatalyst without direct metal-nitrogen coordination by using ZIF-8 as a host for Fe-based sites and characterizations revealed that Fe presents as Fe/FeC3 nanoparticles which are the subsurface to N-doped carbon overlayers, imparting synergistic effects on the N-doped carbon, stabilizing peroxide intermediates and enabling the 4e− process of O2 to water, resulting in a high onset potential of 1.03 V versus RHE and a half-wave potential of 0.86 V. Recently, Guan et al. [394] also prepared an iron carbide nanoparticle-embedded N-doped carbon nanotube assem-bly supported by a porous N-doped carbon matrix (Fe3C@NCNT assemblies) through the confined carburization of MIL-88B nanorods embedded in a ZIF-8 polyhedron. In this study, the researchers reported that the introduction of ZIF-8 into MIL-88B ensured the in situ homogeneously con-fined pyrolysis of MIL-88B nanorods and the preparation of Fe3C nanocrystallite embedded NCNTs, in which due to its unique structure, the obtained Fe3C@NCNT assembly demonstrated excellent ORR activities.

In addition to metal-containing materials as catalysts for ORR, metal-free catalysts based on carbon nanomaterials are also promising alternative ORR catalysts that can drastically reduce costs and even improve fuel cell efficiencies [395]. For example, Shui et al. [396] recently reported that carbo-naceous metal-free ORR catalysts can exhibit outstanding operational stabilities and high energy efficiencies in acidic PEMFCs. Because of this, carbonaceous metal-free catalysts can potentially play a critical role in fuel cells and green economy in general. The graphitization of carbon is induced

Fig. 35 Schematic illustration of the formation of the honeycomb-like porous Co9S8@CNST catalyst. Reprinted with permission from Ref. [389]


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by high-temperature treatments; however, the degree of gra-phitization is limited and pore structures and carbon-heter-oatom bonds are destroyed if temperatures are too high. To resolve this, metal atoms in MOFs such as Fe, Ni and Mn can be used as catalysts to promote the formation of high graphitization degrees under relatively low temperatures (< 1000 °C). In addition, high surface areas inherited from MOF precursors can improve the accessibility of molecules and ions to active sites and heteroatom doping can enhance ORR electrocatalytic performances.

ZIF-8 is a commonly used precursor and template to prepare N-decorated nanoporous carbons due to a high N content and robust framework. Based on this, Aijaz et al. [397] synthesized a N-decorated nanoporous carbon with a high surface area by using ZIF-8 as the template and pre-cursor along with FA and NH4OH as the secondary car-bon and nitrogen sources, respectively. After calcining the FA–NH4OH/ZIF-8 composite at 900 °C and 1000 °C and subsequently immersing it in HF acid, the products (NC900 and NC1000) reportedly exhibited the moderate N content of 2.7 and 0.7 wt%, high surface areas of 2724 and 3268 m2 g−1, respectively, and large numbers of mesopores favor-ing the four-electron reduction pathway. In a later study, Zhang et al. [398] proposed an approach to controllably synthesize uniform 1D ZIF-8 nanofibers through the use of ultrathin tellurium nanowires (TeNWs) with excellent dis-persity as a template. Here, the researchers reported that after calcination, ZIF-8 nanofibers can be converted into porous doped carbon nanofibers with a complex network structure, hierarchical pores and a high surface area, lead-ing to enhanced ORR activities. In a further step, by using triphenylphosphine as a phosphorus source, N, P co-doped carbon nanofibers exhibiting excellent electrocatalytic per-formances for ORR that are even better than benchmarked Pt/C catalysts can be obtained.

Zhang et al. [399] also synthesized a N-doped graphitic porous carbon (NGPC) by using nanoscale ZIF-8 as a self-sacrificing template. Here, the as-obtained NGPC main-tained the nanopolyhedral morphology of the parent ZIF-8 and possessed a high surface area, nitrogen content and hier-archical porosity with well-conducting networks. As a result, the optimized NGPC exhibited remarkable ORR activities with an onset potential of −0.02 V and half-wave potential of − 0.20 V versus Ag/AgCl. Recently, Shi et al. [400] used core–shell ZnO@ZIF-8 nanorods as a template and precur-sor to prepare hierarchically porous N-doped carbon nano-tubes (NCNTs) and reported that because of the high degree of pyridinic and graphic N species, along with the nanotube morphology with a hierarchically porous structure and large surface area, the resulting NCNTs displayed excellent ORR performances with high stability.

MOF-5 is another commonly used template or precur-sor to prepare carbon-based metal-free catalysts for ORR

due to its high surface area and carbon content [401]. For example, Pandiaraj et al. [402] prepared a N-doped carbon with a nitrogen content of 7.0 wt% and a high surface area of 765 m2 g−1 from MOF-5 through the in situ g-C3N4 for-mation and reported that the as-prepared metal-free catalyst exhibited high durability, high fuel selectivity and excellent ORR activity comparable to that of commercial Pt catalysts. In another example, Li et al. [403] fabricated N, P and S ternary doped metal-free porous carbon catalysts by using MOF-5 as a template and DCDA, triarylphosphine (TPP) and dimethyl sulfoxide (DMSO) as the N, P and S precur-sors (NPS-C-MOF-5), respectively. Here, the researchers reported that as the heteroatoms are doped into the carbon, the pore structure changes and the percentage of mesopores and active sites increases, leading to different ORR activi-ties, in which the synergistic effects of N, P and S ternary doping contribute to high ORR activities. Furthermore, Zhang et al. [404] used MOF-5 and urea as carbon and nitro-gen precursors, and nickel as a graphitization catalyst to pre-pare N-doped porous carbon and carbon nanotubes (NGPC/NCNTs). Here, the high degree of graphitization increased electron transfer rates and improved corrosion resistances. In addition, the NGPC/NCNT catalyst possessed a unique heterostructure involving interconnected 3D conductive networks of NCNTs bridging dissociative NGPCs. Because of these properties, the obtained metal-free catalyst dem-onstrated superior ORR activities with an onset potential of − 51 mV and a half-wave potential of − 171 mV versus Ag/AgCl, which were slightly more positive than those of Pt/C. More recently, Wang et al. [405] prepared porous car-bons with different pore structures by carbonizing ZnFu-marate and MOF-5 at 1000 °C under Ar. Here, because of the lack of heteroatoms in both precursors and the vaporiza-tion of Zn during the high-temperature carbonization pro-cess, the obtained carbons contained no heteroatom dop-ing or metal atoms, suggesting that the dopant-/metal-free mesoporous carbon derived from ZnFumarate can produce more mesopores than that from MOF-5 and exhibit higher specific surface areas which can provide wider channels for the diffusion of O2 and electrolytes as well as more active sites for ORR. As a result, the ZnFumarate-derived carbon can be used as a highly efficient metal-free electrocatalyst for ORR with an onset potential of ~ − 0.10 V versus SCE.

Other MOFs have also been used as templates or pre-cursors to prepare metal-free electrocatalysts for ORR. For example, Zhao et al. [406] prepared a N-doped micropo-rous carbon material through the direct carbonization of NH2-MIL-53 (Al) followed by HF washing which pos-sessed a high surface area and doped N atoms, contributing to good ORR performances in alkaline medium. In another example, Fu et al. [407] prepared a P–N-rich porous carbon by adding UiO-66-NH2 into an aqueous solution of glypho-sine, followed by carbonization and subsequent HF washing


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and reported that the high electrical conductivity and N, P doping contributed to an ORR activity that was similar to commercial Pt/C catalysts but was more stable and tolerated methanol crossover effects better. Zhu et al. [408] in their study prepared N-doped porous carbon as catalysts for ORR through the direct carbonization of [Zn(bpdc)DMA]·DMF and reported that the obtained N-doped porous carbon exhibited an ORR onset potential of − 0.14 V versus SCE that was similar to that of commercial Pt/C and produced a higher catalytic current than that of Pt/C. In addition, Ye et al. [409] used O-abundant and Zn-containing MOF-74 as a precursor to prepare porous carbon with mesopores and suitable N doping and reported that the N doping and favora-ble pores contributed to excellent ORR activities with an onset potential of 1.02 V vs. RHE.

Recently, Xia et al. [410] synthesized N-doped carbon nanotube frameworks (NCNTFs) through the thermal treat-ment of ZIF-67 particles under Ar/H2 atmosphere, followed by acid treatment with 0.5 M H2SO4 to remove accessible Co nanoparticles and reported that the ZIF-67 particles not only provided a C and N source for growth of CNCTFs catalyzed by in situ formed metallic Co nanoparticles, but also acted as a template for the formation of the hollow framework. As a result, the obtained NCNTF catalyst exhibited higher electrocatalytic activities and stabilities for ORR and OER than commercial Pt/C catalysts which the researchers attrib-uted to the synergistic effects between the chemical com-positions and the robust hollow structure of the crystalline NCNTs. However, further research is required to understand the detailed catalytic mechanisms of active sites in order to precisely control density and distribution.

6 Applications of MOF‑Based Materials in Solar Cells

The harvest of energy directly from sunlight with photovol-taic technology is regarded as one of the most promising routes to resolve the global energy crisis and environment pollution. Among solar cells, dye-sensitized solar cells (DSSCs) have attracted a great deal of interest due to its low cost and high efficiencies [412] and recently, the power con-version efficiency of DSSCs can reach 13% at full sun illu-mination [413]. In DSSCs, dye molecules are anchored onto the surface of a semiconductor such as TiO2, SnO2, ZnO and Nb2O5 to enhance light absorption, and therefore, semicon-ducting materials are important for solar cells. By changing the metal type, controlling the metal cluster size and organic linker, MOF band gaps can be adjusted to endow MOFs with electronic and light absorption properties. Therefore, the exploration of different combinations of metal ions and ligands to tune the electronic and light absorption properties of MOFs is an effective method to improve the performance

of photovoltaic devices. In addition, the combination of 3D perovskite structures and porous MOF coatings, which exhibit important advantages such as large absorption coef-ficients, high stability and carrier mobility, is helpful in the development of high-performance photovoltaic applications.

6.1 Pristine MOFs for Solar Cells

In a pioneering study, Xamena et al. [414] reported the prep-aration of solar cells by depositing a thin layer of MOF-5/DMF paste onto a transparent indium tin oxide (ITO) elec-trode with a layer area and thickness of 1 × 1 cm2 and 50 μm. The performance of this electrode was tested in a solar cell that did not contain either sensitizing dyes or electrolytes, and exhibited an open-circuit voltage (VOC) of 0.33 V, a short-circuit current (JSC) of 0.7 μA and a fill factor (FF) of 44%, proving that MOF-5 can be used as a photoactive mate-rial for photovoltaic cells and that MOF-5 can exhibit activ-ity as an active component in photovoltaic cells. This same group subsequently investigated the intrinsic photo-response of four commercially available MOFs, including Al2(BDC)3, ZIF-8, Fe-BTC and Cu3(BTC)2 [415] and reported that only Al2(BDC)3 exhibited signals derived from the photochemi-cal generation of charge separated states. Furthermore, the researchers in this study reported that if Al2(BDC)3 was used as a semiconductor to build photovoltaic cells, the presence of organic guest 1,4-dimethoxybenzene (DMB) can strongly enhance the efficiency of the device, which the researchers attributed to the high porosity of MOFs and the large vol-ume available for inclusion. In addition, these researchers also stated that film thicknesses can affect performance in which thinner films can provide higher photo-responses. As a result, the DMB@Al2(BDC)3 with the lowest thickness of 2.7 μm exhibited better performances with a current density of 36 μA cm−2, a VOC of 0.36 V and a FF of 40% as com-pared with those without DMB or with higher thicknesses.

In another study, Li et al. [417] for the first time used ZIF-8 with different thicknesses to coat TiO2 to enhance the VOC of DSSCs through immersion in a fresh methanol solution containing 1 mM Zn(NO3)2 and 2 mM 2-methyl imidazole with different reaction times. Here, the researchers reported that the thickness of the ZIF-8 film increased lin-early with growth time and VOC was enhanced linearly with increasing ZIF-8 layer thicknesses in which the value of VOC increased by 55 and 66 mV for dye N719 and D131, respec-tively, which the researchers ascribed to the inhibited charge recombination due to the introduction of the ZIF-8 shell. Although the adsorption of dyes increased due to the over-layer of ZIF-8, the short-circuit current decreased because the overlayer of ZIF-8 inhibited the injection of electrons from the dye into the conduction band edge of TiO2. These researchers subsequently used ZIF-8 in the interfacial modi-fication of DSSCs using a post-treatment strategy to resolve


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the issue of decreasing short-circuit currents [416]. Here, a TiO2 photoanode was first sensitized with dyes, and sub-sequently, the ZIF-8 overlayer was grown to allow for dye sensitization to obtain the TiO2/dye/ZIF-8/dye electrode (Fig. 36). In comparison with the pretreatment method, the dyes in the resulting electrode were tightly adsorbed onto the TiO2, and as a result, short-circuit current reduction is avoided. In addition, the researchers reported that the energy barrier effect of the ZIF-8 can improve VOC and electron life spans, therefore improving overall performances of DSSCs.

In another example, by using a layer-by-layer technique, Lee et al. [418] fabricated a thin layered Cu-based MOF (copper(II) benzene-1,3,5-tricarboxylate) to sensitize doctor-bladed TiO2 nanoparticle films on a fluorine-doped tin oxide (FTO) glass substrate. Here, the researchers reported that with iodine doping, the MOF film can switched from an insulator to an electrical conductor and therefore improve the charge transfer reaction across the TiO2/MOF/electrolyte interface. As a result, the MOF film possessed the highest occupied molecular orbital (HOMO) and the lowest unoc-cupied molecular orbital (LUMO) energy states of ~ − 5.37 and − 3.82 eV (vs. vacuum), respectively, and exhibited a well-matched energy cascade with TiO2. Furthermore, in testing with the iodine-doped MOF, the corresponding solar cell displayed a higher energy conversion efficiency than that of MOFs without iodine doping. Subsequently, this group used the same technique to also synthesize Ru-based MOFs as a sensitizer in TiO2-based solar cells [419] in which they reported that after iodine doping, the energy conversion efficiencies of the Ru-based MOF sensitized TiO2 solar cell improved. This group also prepared cobalt(II)

2,6-naphthalenedicarboxylic acid (Co3(NDC)3DMF4), another MOF, using the layer-by-layer and doctor blade coat-ing technique [420] and reported similarly findings in which iodine doping improved electrical conductivity. In addition, the positions of the HOMO and the LUMO of the doped MOF were found to be suitable for the efficient interfacial photoelectron injection from the MOF to the ITO or TiO2, indicating that iodine-doped Co-based MOFs can be applied to harvest solar radiation. Moreover, other Co-based MOFs, including Co-BDC, Co-NDC [421] and Co-DAPV [422], have also been synthesized as light-harvesting absorber films in fully devised TiO2-based solar cells with promising performances.

Liu et al. [423] also used a liquid-phase epitaxy method to grow surface-grafted MOFs (SURMOFs) in a layer-by-layer fashion onto an appropriately functionalized sub-strate (Fig. 37a) and reported that due to the well-defined anchoring of the MOF on the electrode, the SURMOF was a promising candidate for electrical applications. Here, in a photovoltaic device using the FTO substrate as the bot-tom electrode and the iodine/triiodine electrolyte as the top electrode (Fig. 37b), the addition of Pd into the center of the polycyclic (Fig. 37a) can result in better performances, in which the researchers attributed this enhanced performance to the high photocarrier generation efficiency of the MOF. In a further study, Liu et al. [424] again used the liquid-phase epitaxy method to prepare Zn(II) porphyrin-based MOF thin films with photophysical properties that can be adjusted by the introduction of electron donating diphenylamine (DPA) into the porphyrin skeleton. Here, the introduction of DPA

Fig. 36 Schematic illustration of photoanode statuses after sensitization in pretreatment and in post-treatment approaches. Reprinted with permission from Ref. [416]


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can decrease band gaps, improve solar light absorption and allow for higher photocurrents.

In another study, Du et al. [425] synthesized [In0.5K(3-qlc)Cl1.5(H2O)0.5]2n and [InK(ox)2(H2O)4]n (3-Hqlc = quinoline-3-carboxylic acid; H2ox = oxalic acid), both heterometallic MOFs, and reported that the HOMO–LUMO energy states of the two MOFs are matched for sensitizing TiO2. Because of this, both MOFs were used in combination with N719 in DSSCs, resulting in overall efficiencies of 8.07% and 7.42%, respectively, which are higher than the results obtained from only using N719 (6.61%). Here, the researchers attributed the enhanced photovoltaic performances to the fact that Ti4+ can be coordinated to the MOFs to form Ti–O coordination bonds, therefore facilitating the injection of excited elec-tron into the conduction band of TiO2 and the absorption of blue–violet light that N719 can only absorb marginally. In addition, the MOF unit cells can be assembled together, forming high surface area arrays for photon collection. Moreover, the MOFs can also help to anchor the dyes tightly onto the TiO2 surface to ensure high efficiencies of photo-current output, with additional dyes adsorbed onto the MOFs improving JSC.

Maza et al. [426] in their study prepared a series of Ru(II)L2L′ (L = 2,2′-bipyridyl, L’ = 2,2′-bipyridine-5,5′-dicarboxylic acid) and RuDCBY containing zirconium(IV)-based MOFs as sensitizing materials for TiO2 based solar cells and reported that the MOF sensitized solar cells (MOFSC) exhibited better performances than RuDCBY-TiO2 DSC under the same conditions, indicating that MOF-SCs are promising platforms for photovoltaic applications. More recently, Spoerke et al. [427] proposed that increasing the interfacial contact between TiO2 and MOFs and tailor-ing the crystallography for optimal charge transfer by using a pillared porphyrin framework as a MOF sensitizer can potentially improve the efficiency of charge harvesting and increase the production of photocurrents.

MOFs can also be used to improve solar cell stabilities. Because the diffusion of Cu ions into CdS layers can rapidly

degrade the performance of Cu2 − xS/CdS solar cells, Nevru-zoglu et al. [428] prepared Zr-based MOFs as the copper source to improve the stability of Cu2 − xS/CdS photovoltaic cells. Here, the as-prepared MOF material with its inherently high porosity, chemical stability and density of Lewis basic sites from bipyridine moieties can effectively store Cu(I) ions to compensate for the diffused Cu ions, and therefore, improve the stability of the Cu2 − 0S/CdS photovoltaic cells.

Furthermore, researchers have reported that MOFs can be used as the electrolyte for quasi-solid DSSCs as well. For example, Bella et al. [429] prepared a polymer com-posite containing Mg-based MOFs using a UV-induced free-radical process as an effective candidate to enhance DSSC performance. Here, the researchers reported that the interactions between the terminal carboxyl groups of the Mg-based MOF particles and the surface groups of the TiO2 layer can shield the trap states of the TiO2 electrode, reducing the charge carrier recombination rate. As a result, the presence of Mg-MOF can increase solar energy conversion efficiencies up to 4.8% and provide outstanding long-term durability. In another example, Fan et al. [430] prepared a metal–organic skeleton based gel electrolyte for quasi-solid-state DSSCs through the coordination of Al3+ and 1,3,5-benzenetricarboxylate (H3BTC). Here, the obtained Al-BTC matrix with a multi-scale porous structure accommodated electrolyte ingredients, and therefore preserved the properties of the liquid electrolyte. In addition, the metal–organic gel (MOG) electrolyte was also capable of penetrating into the photoanode film to ensure good interfacial contact. As a result, a high power conversion efficiency of 8.69% was obtained, which is only a little lower than that of a liquid-state cell (9.13%).

Since the first perovskite sensitized solar cells emerged in 2009 [431], hybrid halide perovskite solar cells have attracted great attention due to their relatively low costs and high power conversion efficiencies and in 2012, the power conversion efficiencies of solid-state perovskite

Fig. 37 a Idealized sche-matic representation of the liquid-phase epitaxy deposition process and b the architecture of a porphyrin SURMOF-based photovoltaic device. Reprinted with permission from Ref. [423]


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solar cells exceeded 10% [432]. Recently, a higher effi-ciency of 22.1% was achieved, which even exceeded that of multicrystalline silicon [433]. Here, it was found that the crystallization process of growth of perovskite thin films can strongly influence the efficiency of the hetero-junction thin film perovskite solar cell [434], in which poor morphologies and crystallinities of the perovskite thin film can cause electrical shorting and harm charge transport. Therefore, to resolve this issue, Chang et al. [435] synthesized MOF-525 nanocrystals (~ 140 nm) as additives to incorporate into perovskites to enhance the crystallinity of the obtained perovskite thin film using a one-step deposition. Here, the researchers used a 5 v/v% MOF-525 suspension (20 mg mL−1) in the MOF/perovs-kite precursor solution and the resulting solar cell exhib-ited high performances with a high power conversion of ~ 12%, which was higher than that of the film using the pristine perovskite precursor solution (~ 10%). More recently, MOF-525 was introduced as an electrocatalyst for the counter electrode of DSSCs [436]. Vinogradov et al. [437] applied MOFs to perovskite solar cells in which TiO2-MIL-125 composites were synthesized and applied to produce depleted perovskite/TiO2-MOF het-erojunction solar cells. Here, the researchers reported that if the MOF content with respect to the TiO2 was 3%, the corresponding solar cell exhibited high stabilities over time up to 30 days with a high power conversion of ~ 6.4%.

6.2 MOF‑Derived Materials for Solar Cells

MOF-derived materials such as metal oxides can also be used as semiconductor materials in DSSCs. For example, Kundu et al. [438] obtained rod-shaped, hexagonal column shaped and elliptical aggregations of ZnO morphologies through the thermolysis of Zn-based homochiral MOFs with two types of anions (–Cl and –Br) at 800 °C in air or N2, with the various ZnO films ~ 12 μm in thickness being prepared using the doctor blade method, followed by annealing at 450 °C and impregnation with 0.5 mM N719 dye. Here, the researchers reported that the ZnO prepared in N2 displayed no DSSC activity due to poor conductivi-ties, whereas the ZnO prepared in air exhibited enhanced power conversion efficiencies. Subsequently, Li et al. [439] prepared hierarchical ZnO parallelepipeds with thickness of ~ 300–500 nm through the decomposition of MOF-5 precur-sors and reported that if used as the light scattering layer in DSSCs, remarkable enhancements in cell performance can be achieved.

TiO2 can also be prepared from MOFs for DSSCs. For example, Chi et al. [440] used poly(ethylene glycol) digly-cidyl ether (PEGDGE) as a structure directing agent to prepare a shape- and morphology-controlled MIL-125 (Ti)

and reported that with increasing PEGDGE amounts, the morphology of the MOF changes from 200 nm circular plates to 1 μm bipyramids. The researchers further depos-ited the obtained TiO2 onto a nanocrystalline TiO2 layer as the scattering layer and reported that the DSSC with a quasi-solid-state polymer electrolyte exhibited an improved conversion efficiency of 7.1% as compared with a DSSC using nanocrystalline TiO2 only (4.6%) or with commer-cial scattering TiO2 (5.0%). Furthermore, the researchers also reported that if poly((1-(4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII) was used as the sin-gle component solid electrolyte, a high efficiency of 8.0% can be achieved. Here, the excellent performances can be attributed to the fact that the mesoporous hierarchical TiO2 can reflect light back into the dye as well as the fact that high surface areas can allow high dye loading. As a result, a pure anatase hierarchical TiO2 with a BET surface area of 147 m2 g−1 and a pore size of 10 nm was produced and the corresponding DSSC exhibited an efficiency of 7.2%, which was higher than that of P25 TiO2. Here, the improvements can be ascribed to the stronger dye adsorption, faster elec-tron transport and better charge collection efficiency.

ZIF-67 can also be used to prepare materials for DSSCs. For example, Hsu et al. [441] reported that obtained CoOx derived from ZIF-67 by using sulfide conversion can be converted to CoS nanoparticles which can subsequently be used as the counter electrode for Pt-free DSSCs. Here, the as-synthesized CoS nanoparticles possessed a larger exter-nal surface area and roughness factor than Pt counter elec-trodes, and can enhance interactions with dye molecules. As a result, the CoS-based DSSC achieved a higher open-circuit voltage and greater fill factor, allowing for a high efficiency of 8.1% that is comparable to that of Pt-based DSSCs (~ 8.0%). Cui et al. [442] in their study annealed sulfurized ZIF-67 at 450 °C in N2 to prepare CoS2 embed-ded carbon nanocages and reported that the resulting amor-phous carbon matrix improved conductivities and that the ultra-small particle size facilitated ion diffusion. As a result, a DSSC consisting of the CoS2 embedded carbon nanoc-ages exhibited a high efficiency of 8.2%. Recently, Liu et al. [443] also prepared a ZIF-67-derived carbon framework to support PbS QD sensitized solar cells and found that the carbonizing ZIF-67 significantly improve conductivities and reduce series resistance, therefore enhancing light to electric conversion efficiencies.

MIL-101 is another promising material and Li et al. [444] in their study fabricated highly dispersed and small-sized polyoxometalate nanoparticles loaded onto TiO2 film by using MIL-101 to prevent agglomeration. Here, K6CoW12O40 was first immobilized in MOF-101 to obtain K6CoW12O40@MIL-101, which was composited with TiO2 and subsequently calcinated at 450 °C to prepare the final composite CoW12·Cr2O3@TiO2. The researchers in this


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study suggested that this process ensured the high dispersion of polyoxometalate clusters. In subsequent testing in which the CoW12·Cr2O3@TiO2 composite was introduced into a quantum dot sensitized solar cell (QDSCs), a high power conversion efficiency of 6.0% was achieved.

7 Conclusions and Perspectives

The search for novel materials in the storage and utilization of green energy to achieve low-carbon economy and sustain-able development is a vital mission in materials science, and the emergence of MOFs can push this search to new levels. By adjusting structures through changing metal ions or clus-ters and organic ligands during synthesis processes, MOFs can exhibit versatile properties to satisfy the demand of vari-ous applications. In this review, the application of MOFs in energy storage and conversion systems including Li-based batteries (Li–ion batteries, Li–S batteries and Li–O2 batter-ies), Na–ion batteries, supercapacitors, fuel cells and solar cells are summarized.

In terms of using pristine MOFs in various energy stor-age and conversion systems, the stability of MOFs plays a key role. After decades of research, several chemically stable MOFs have been designed and synthesized which exhibit good cycle life spans and can be directly used as electrode materials. In addition, with the rapid development of MOFs, more chemically and structurally stable MOFs will be designed and synthesized. However, the low elec-tronic conductivity of MOFs due to the existence of organic linkers limits application in batteries. To address this, effec-tive methods such as the infiltration of MOF nanopores with redox-active, conjugated guest molecules such as 7,7,8,8-tet-racyanoquinodimethane (TCNQ) [26] have been developed to improve the electronic conductivity of MOFs. Moreover, synthesizing multivariate MOFs with more than one func-tional group can also improve MOF conductivities [445, 446]. For examples, Ni3(HITP)2, possessing high electrical conductivity [266], can be applied to ORR catalysts [332] and SCs [265]. Based on this, highly conductive MOFs will be designed, synthesized and applied in electrochemical devices in the future.

The abundant pores of pristine MOFs can provide Li+ storage sites and mitigate volume change during the charge and discharge process, whereas the metal ions or clusters can serve as redox-active sites during the electrochemical process. Because of this, MOFs are outstanding candi-dates for Li-ion batteries. The strong interaction between polysulfides and MOFs with high surface areas, suitable pore volumes and small window sizes also make MOFs promising candidates as host materials in S cathodes of Li–S batteries. Here, polar sections of the inorganic moie-ties in MOFs can provide stronger interactions with highly

polar polysulfide species, and MOF Lewis acid sites can accommodate more sulfur, leading to better performances in Li–S batteries. Therefore, the design and synthesis of MOFs with the above features is a potential direction to obtain Li–S battery cathodes with good electrochemical performances. In addition, MOFs with highly ordered pores and proper window sizes can also be used as separa-tors to mitigate the shuttling problem. As for Li–O2 batter-ies, the open metal sites of MOFs can provide more polar-ized surfaces that can enhance the interaction between MOFs and small molecules such as O2, whereas the large channels that permit the entry of O2 molecules can also play a key role in Li–O2 batteries. Therefore, the design and synthesis of novel MOFs with suitable pores and chan-nels through the optimization of metal ions or clusters and organic linkers to select and collect gases such as O2 and CO2 is promising for Li–air batteries. In terms of Na–ion batteries, PB and PBAs are widely investigated pristine MOFs in which long M(II)–N≡C–Fe(III) (M, e.g., Fe, Mn, Ni, Cu, Co, Zn) bond lengths can allow Na+ to reversibly insert into empty large ion sites. With the development of MOFs, more MOFs with large channels for the de-inter-calation of Na+ can be prepared and used in SIBs. As for supercapacitors, MOFs with multivalence metal ions, large pores, high surface areas and continuously interconnected microstructures can exhibit good electrochemical perfor-mances. Although most MOFs exhibit high surface areas, the effective use of this increased surface area is difficult due to the difficulty of electrolyte infiltration into all pores. As a result, adjusting the length of the organic linkers to prepare MOFs with large pores is a feasible method to achieve high capacitances.

Pristine MOFs can also be directly used as catalysts for ORR in fuel cells. However, one major challenge is the low conductivity of pristine MOFs. Currently, an effec-tive approach to address this issue is to composite or grow pristine MOF layers onto highly conductive substrates such as graphene. In addition, embedding metal oxides such as α-MnO2 into MOF matrixes can also improve ORR activity. Furthermore, adjusting metal ions and organic linkers and integrating functional species into frameworks can homog-enize active sites and therefore improve ORR performances. To improve the performance of MOF-based photovoltaic devices, an effective method is to explore different combina-tions of metal ions or clusters and organic ligands to regulate and control electronic and light absorption properties.

Aside from the direct use of pristine MOFs for energy storage and conversion systems, MOF-derived materials such as metal oxides, metal sulfides, carbon or composites, can also be adopted. Due to the organic ligands and metal ions of MOFs, MOFs are a promising choice as precursors and templates to synthesize the above-mentioned materials, in which advantages of MOFs such as high surface areas,


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controllable structures, large pore volumes and tunable porosities can be inherited.

One issue with MOFs is that porous structures can col-lapse after carbonization, leading to decreased surface areas and pore volumes. Here, an effective method to address this issue is to introduce guest molecules into MOFs that can serve as templates to maintain the porous structure. In addi-tion, these guest molecules can be removed using acid treat-ment after calcination. Moreover, using MOF containing metal ions or clusters with low boiling points such as Zn as precursors and templates is another useful approach in which the metals can evaporate during heat treatment, increasing the porosity and surface area of the resulting carbon material.

Heteroatom doping such as N, S and P is another method to improve electrochemical performances. By introducing heteroatoms into organic ligands in the synthesis of MOFs or by using extra nitrogen, sulfur or phosphorus sources during the carbonization process, heteroatom-doped MOF-derived materials can effectively be prepared. Here, heteroatom dop-ing can create more defects, allowing for synergistic effects between the doping atoms and the defects to further improve performances.

High degrees of graphitization can improve electron transfer rates and increase conductivity. However, pore structures and carbon-heteroatom bonds can be destroyed if heating temperatures are too high. Therefore, the intro-duction of catalytic metals such as Fe, Mn and Ni to pro-vide higher degrees of graphitization under relative lower temperatures is an effective approach to prepare MOF-derived carbons with high degrees of graphitization.

In the preparation of MOF-derived metal oxides, con-trolling the size and morphology of metal or metal oxide nanoparticles derived from MOF precursors is significant to achieve advantageous electrochemical performances and currently, the control of calcination temperatures and morphology of MOF precursors is a viable method. How-ever, the exploration of detailed formation mechanisms of the morphology and size is necessary for future improve-ments. Moreover, utilizing the adsorption properties of MOFs, sulfur or phosphorus precursors can be introduced into MOFs during the calcination process to prepare metal sulfides or phosphides that can be used as electrode mate-rials for batteries or catalysts for ORR in fuel cells.

As for metal/carbon-based composite materials, the in situ approach using MOFs as precursors and templates can allow for highly dispersed metal or metal oxides nan-oparticles on carbon with less aggregation as compared with other methods. Because of this, composites derived from MOFs should exhibit excellent performance as elec-trode materials for batteries and catalysts for ORR in fuel cells. However, the precise control of metal or metal oxide particle sizes and the morphology of the compos-ites is a major challenge in which adjusting calcination

temperature might be a possible solution. Despite this, more exploration is required.

A common issue faced by both MOFs and MOF-derived materials for application in batteries is that their high spe-cific surface area can lead to the formation of large SEI layers, causing large irreversible capacity loss during the first cycle. Another issue hindering the application of MOFs in batteries and supercapacitors is the relatively low volumetric energy density and power density due to low mass densities. Because of this, the resolution of these issues is crucial to the development of MOFs for energy storage and conversion systems. Furthermore, the process-ing and formulation of MOF precursors and derived mate-rials need to be improved to further simplify production processes and allow for cost reductions.

In conclusion, MOFs- and MOF-derived materials have demonstrated great potential in the field of energy storage and conversion as a result of their controllable structures with high surface areas, large pore volumes and tunable porosities. The emergence of MOFs and their derivatives present promising opportunities in the design and synthe-sis of novel materials with better electrochemical perfor-mances. However, there is still much to improve in terms of both the synthesis and pyrolysis processes of MOFs. Therefore, further investigations into MOFs are required to develop advanced materials for energy storage and con-version systems.

Acknowledgements This work was financially supported by NSFC (21421001) and Tianjin Municipal Science and Technology Commis-sion (16PTSYJC00010) in China.

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Xu Zhang obtained his B.Sc. in chemistry at Nankai University in 2014. Now he is pursuing his Ph.D. at Nankai University with Professor Zhen Zhou, majoring in materials physics and chemis-try. His research interest mainly focuses on high-throughput screening and computational design of low-dimensional mate-rials for energy storage and conversion.


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An Chen gained her bachelor’s degree in materials chemistry at Northeast Forestry University in 2018. She is studying for her master degree in Prof. Zhen Zhou’s group at Nankai Univer-sity. Her research focuses on computational investigation and design of inorganic energy stor-age materials.

Ming Zhong obtained his M.S. in Prof. Bitao Su’s group at Northwest Normal University in 2014 and Ph.D. in 2018 from Nankai University under the supervision of Prof. Xian-He Bu. He is now a lecturer at Lanzhou University of Technology. His research interest focuses on MOFs and MOFs derived mate-rials for energy storage and conversion.

Zihe Zhang received his bache-lor’s degree in chemistry at Nan-kai University in 2014. He has been studying for his Ph.D. at Nankai University in Prof. Zhen Zhou’s group. His research focuses on computational inves-tigation and design of energy storage materials.

Xin Zhang obtained his B.Sc. from Nankai University in 2014. He is currently pursuing a Ph.D. degree in materials physics and chemistry at Nankai University, under the supervision of Prof. Zhen Zhou. His academic inter-est focuses on Li–air batteries.

Zhen Zhou was born in Shan-dong, China. After he received his B.Sc. (applied chemistry, in 1994) and Ph.D. (inorganic chemistry, in 1999) at Nankai University, China, he joined the faculty at Nankai University as a lecturer in 1999. Two years later, he began to work in Nagoya Uni-versity, Japan, with Professor Masahiko Morinaga (Ex-Presi-dent of the Japan Institute of Metals) as a postdoctoral fellow, under the support of the Japan Society for the Promotion of Sci-ence (JSPS), and then he contin-

ued his research at EcoTopia Institute, Nagoya University for another 2 years. In 2005, he returned to Nankai University as an associate professor of Materials Chemistry. In 2011, he was promoted as a full professor of Materials Science and Engineering at Nankai University. In 2014, he was appointed as Director of Institute of New Energy Mate-rial Chemistry, Nankai University. His main research interest is design, preparation and application of advanced materials for energy storage and conversion.

Xian‑He Bu is a full professor of chemistry (Chang-Jiang Scholar) at Nankai University and serves as Dean of School of Materials Science and Engineering. His research interest includes func-tional coordination chemistry, crystal engineering, MOFs and magnetic materials.

Metal–Organic Frameworks (MOFs) and MOF-Derived Materials ... · 30 Electrochemical Energy...

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