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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com1605336 (1 of 30)

Asymmetric Supercapacitor Electrodes and Devices

Nitin Choudhary, Chao Li, Julian Moore, Narasimha Nagaiah, Lei Zhai, Yeonwoong Jung,* and Jayan Thomas*

DOI: 10.1002/adma.201605336

1. Introduction

Due to the rapid increase in the global energy consumption of fossil fuels and the world’s population explosion, there has been an urgent demand for the development of alternative energy sources that are clean, low cost, compact, and efficient. The intermittent nature of conventional energy sources such as wind, solar, hydroelectric, and biomass prevents them from being selected for the provider of sustainable energy technolo-gies in any near future. Certainly, sustainable electric-power sources such as batteries and supercapacitors are expected to continuously remain as the dominant energy models for world-wide energy usage, as they have been so for several decades.[1–4]

The world is recently witnessing an explosive development of novel electronic and optoelectronic devices that demand more-reliable power sources that combine higher energy density and longer-term durability. Supercapacitors have become one of the most promising energy-storage systems, as they pre-sent multifold advantages of high power density, fast charging–discharging, and long cyclic stability. However, the intrinsically low energy density inherent to traditional supercapacitors severely limits their widespread applications, triggering researchers to explore new types of supercapacitors with improved performance. Asymmetric supercapacitors (ASCs) assembled using two dissimilar electrode materials offer a distinct advantage of wide operational voltage window, and thereby significantly enhance the energy density. Recent progress made in the field of ASCs is critically reviewed, with the main focus on an extensive survey of the materials developed for ASC electrodes, as well as covering the progress made in the fabrication of ASC devices over the last few decades. Current challenges and a future outlook of the field of ASCs are also discussed.

Dr. N. Choudhary, Dr. C. Li, J. Moore, Prof. L. Zhai, Prof. Y. Jung, Prof. J. ThomasNanoScience Technology CenterUniversity of Central FloridaOrlando, FL 32826, USAE-mail: [email protected]; [email protected]. N. NagaiahCenter for Advanced Turbines and Energy Research (CATER)Mechanical and Aerospace Engineering University of Central FloridaOrlando, FL 32826, USAProf. L. Zhai, Prof. Y. Jung, Prof. J. ThomasDepartment of Materials Science and EngineeringUniversity of Central FloridaOrlando, FL 32826, USA

Prof. L. ZhaiDepartment of ChemistryUniversity of Central FloridaOrlando, FL 32826, USAProf. Y. JungDepartment of Electrical and Computer EngineeringUniversity of Central FloridaOrlando, FL 32826, USAProf. J. ThomasCREOLCollege of Optics and PhotonicsUniversity of Central FloridaOrlando, FL 32826, USA

It has also been estimated that the world-wide net electric-power generation will nearly double over the next 25 years, i.e., from 21.6 trillion kilowatt-hours (kW h) in 2012 to 25.8 trillion kW h in 2020 and to 36.5 trillion kW h in 2040.[5] In spite of a plethora of research carried out in the field of batteries (lithium-ion batteries, in particular), spanning from small gadgets to heavy electric vehicles, we are on the verge of a power revolution with renewed interest in supercapacitors.[6–9] Unlike bat-teries, supercapacitors not only charge faster but are more reliable owing to their indefinite lifespan, nonvulnerability to temperature change, and nontoxicity in nature.[10–13]

A supercapacitor generally consists of two electrodes (anode and cathode) separated by an electrolyte (aqueous or organic) and a separator that permits the transfer of ions while keeping the elec-

trodes electrically insulated from each other. They are further classified into electric double-layer capacitors (EDLCs) and pseudocapacitors, depending on the type of charge-storage mechanism. In EDLCs, the charges are stored due to the sur-face adsorption of the ions from the electrolyte as a result of the electrostatic attraction, thus forming two charged layers (double layer). Electrode materials play a significant role in providing high perfromance for supercapacitors. For EDLC electrodes, the main requirements are their high surface area, fast charge/discharge rates, and high conductivity. In most cases, EDLCs are constructed using carbon-based electrode materials (porous carbon, carbon nanotubes (CNTs), and graphene) as they repre-sent high surface area, nontoxicity, controllable porosity, good

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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (2 of 30) 1605336

electronic conductivity, and availability in various forms like tubes, powders, composites, sheets, and aerogels.[14] Unlike EDLCs, pseudocapacitors store charges through fast and revers-ible oxidation/reduction (Faradiac) reactions occuring at the electrode/electrolyte interfaces, as well as in the bulk near the surface of the electrode. Pseudocapacitors show higher capaci-tance as compared to EDLC-type devices due to the additional charges transferred within the defined potential, but they typi-cally have inferior cycle life due to active material degradation as a result of Faradaic reactions. Metal oxides and conductive polymers are widely used as pseudocapacitive electrode mate-rials due to their fast reversible redox reactions, cost-effective-ness, easy processability, and relatively longer cyclic stabillity.[15] Despite the advantages of high power density and much higher cycle life in EDLCs and pseudocapacitors, their intrinsically low energy density (amount of energy stored per unit weight) have held them back from outperforming batteries for widespread commercial applications.[16–18] Currently, supercapacitors are mainly used in heavy machinery and other types of equipment that require high power. As a signature of its reliability, NASA uses supercapacitor packets in the drill that astronauts use for their space walk to perform repair work on the International Space Station.[19] In additon, Airbus 380 uses supercapacitors on its emergency doors as a testimony to their reliability.[12] How-ever, supercapacitors are used only as backup power sources to aid batteries in electric cars, since their energy density is not as high as batteries.[15,20] For example, Toyota uses ultracapacitors to add about 480 horsepower to their hybrid race car, TS040.[21] Maxwell technologies, a well-known supercapacitor manufac-turer partially rely on supercapacitors for the operation of their hybrid buses with stop–start engines.[22] Besides electric vehi-cles, supercapacitors are in demand for remotely located wind turbines that need bursts of power to adjust the turbine blades during wind changes.[23] Batteries fail to serve these needs due to unwanted chemical reactions and physical changes to the active chemicals used during high-power requirements. Fur-thermore, the advent of next-generation portable, flexible, and wearable electronics and optoelectronics devices requires min-iaturized energy-storage systems with unique advantages of flexibility and light weight.[9,24–27] Hence, there is a huge surge for further improving the energy density of supercapacitor tech-nologies (which is currently 5–35% of the Li-ion batteries) to meet industrial demands.

Consider a win–win situation where the fundamental prin-ciples behind batteries and supercapacitors work together to reach the common goal of higher energy density and power density. One such design is “asymmetric supercapacitors (ASCs)”, which, unlike traditional supercapacitors, consist of two dissimilar electrodes, i.e., a battery-type Faradaic electrode (cathode) as an energy source and a capacitor-type electrode (anode) as a power source. Typically, in a symmetric superca-pacitor, the working voltage is limited to less than 1.0 V because of the thermodynamic breakdown potential of water molecules when aqueous electrolytes are used. Nevertheless, the working voltage can be improved beyond 2.5 V by using organic electro-lytes. However, these organic electrolytes are sometimes toxic and not enviornmentally benign for certain applications. There-fore, a feasible approach to achieve higher working voltage for aqueous electrolytes is to use two different electrode materials

for the anode and the cathode. The reason for the higher energy density (E) achieved in ASCs is due to the higher operating voltage (V). The energy density (E)[28–32] is given by:

12

2=E CV (1)

Nevertheless, the specific capacitance (C) of a superccapac-itor can be enhanced by optimizing the intrinsic properties, like porosity, electrical conductivity, and chemical stability of the electrode materials, as well as rationally engineering them into low-dimensional nanostructures (quantum dots, nano-onions, nanorods, sheets, foams, etc.) and making novel electrode designs like composites, core/shells, and heterostructures.[33–37] Figure 1a illustrates the principle of ASCs, where two dissimilar materials are assembled together as anode and cathode. The Ragone plot given in Figure 1b compares the energy and power densities of various energy-storage devices. It is apparent that ASCs deliver significantly higher power density as comapred to batteries, fuel cells, and symmetric supercapacitors. In addi-tion, the energy density of asymmetric devices comparable to LIBs suggests their widespread use for next-generation elec-tronics and energy-storage devices. Figure 1c shows a year-wise

Yeonwoong Jung is an assistant professor at NanoScience Technology Center (NSTC), Materials Science and Engineering, and Electrical and Computer Engineering at the University of Central Florida (UCF). He received his Ph.D. in mate-rials science and engineering from the University of Pennsylvania, Philadelphia,

USA. He joined UCF in 2015 after completing his postdoc-toral training at Yale University. His research group at UCF currently focuses on developing 2D layered materials and their hybrid systems for electronics, energy, and environ-mental applications.

Jayan Thomas is an associate professor at NanoScience Technology Center (NSTC), College of Optics and Photonics (CREOL) and College of Engineering and Computer Science at the University of Central Florida (UCF). After receiving his Ph.D. from Cochin University of Science and Technology in India, he joined the College

of Optical Sciences, University of Arizona in 2001 as a research faculty. He moved to UCF in 2011 and is currently working on the development of energy-storage devices, wearables, solar cells, and photorefractive polymers.

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publication list demonstrating the increasing trend of studying ASCs. Some of the application areas that demand the increase in the use of ASCs include flexible electronics, e-textiles, trans-portation, etc., and are shown in Figure 1d.

Here, we discuss the recent developments in the field of ASCs with the main focus on the synthesis, properties, and per-formances of the state-of-the-art materials for anode and cath-odes. Carbon-based materials such as activated carbon (AC), porous carbon, carbon nanotubes (CNTs), graphene, and gra-phene oxide (GO) are generally employed for negative (capaci-tive) electrodes owing to their high surface area and electro-static charge-storage mechanisms at electrode/electrolyte inter-faces. Being pseudocapacitive in nature, some metal oxides and nitrides have also been utilized as anode materials. The positive electrode counterparts are generally pseudocapacitive in nature, which include metal oxides and carbonaceous mate-rials. In addition to the material-property considerations, the design aspects of electrode structures are extensively reviewed, covering tailored nanostructures such as nanorods, nanowires, nanospheres, nanosheets, and nanoribbons. Recently discov-ered novel two-dimensional (2D) materials such as transition-metal dichalcogenides (TMDs) and transition-metal carbides

(MXenes) are are also introduced. Finally, the fabrication of next-generation ASC electrodes and devices are extensively dis-cussed in the context of emerging technologies such as flexible and wearable technologies.

Electrochemical symmetric supercapacitors made using carbon electrodes exhibit low energy density in aqueous electro-lytes as a result of the small operating-voltage window. There-fore, all the commercially available supercapacitors use organic electrolytes, attaining a cell voltage of 2.5–2.85 V, with their energy density reaching as high as 5–10 W h kg−1. However, ASCs using aqueous electrolytes can reach a cell voltage of more than 2.0 V[39] and are likely to replace symmetric supercapaci-tors employing organic electrolytes. In addition, large operation voltages with extremely high energy densities can be achieved in ASCs by using non-aqueous or ionic-liquid electrolytes owing to their higher dissociation voltage. However, studies on ASCs in organic electrolytres are still sparse. Khomenko et al.[40] studied the perfromance of a hybrid ASC made with all-carbon-based hybrid electrodes i.e., graphite and AC for the negative and posi-tive electrodes, respectively, in two organic electrolytes: 1 mol L−1 tetraethylammonium tetrafluoroborate (Et4NBF4) in acetoni-trile (AN) and 1 mol L−1 lithium hexafluorophosphate (LiPF6)

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Figure 1. a) Schematic showing typical construction of an asymmetric supercapacitor. b) Ragone plot comparing the energy and power densities of various state-of-art supercapacitors and batteries with ASCs. c) Increasing trend in the number of publications on ASCs during the last decade (2006–2015). d) Some possible application areas of ASC devices. c) Searched by SciFinder Scholar: https://scifinder.cas.org and American Chemical Society database: https://www.acs.org/content/acs/en.html), September 20th, 2016. d) Reproduced with permission.[38] Copyright 2015, Wiley-VCH.

https://scifinder.cas.org

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in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). It was found that the hybrid ASC could be operated at a maximum voltage of 4.5 V and exhibited a very high energy den-sity of 103.8 W h kg−1. Considering the fact that high pseudoca-pacitance in metal oxides might result in significant increase in the energy density, Wang et al.[41] reported a non-aqueous activated-mesocarbon microbead (AMCMB)//MnO2 nanowire-sphere (negative//positive electrodes) hybrid supercapacitor using 1 m Et4NBF4 in AN as an electrolyte. This hybrid super-capacitor operates over a wide voltage range of 0–3 V and dis-plays a high specific capacitance of 228 F g−1 and a high specific energy of 128 W h kg−1. The high ionic resistance of organic electrolytes is one of the potential problems while assembling a supercapacitor device, as it tends to increase the equivalent series resistance (ESR) and thereby lowers the capacitance. In this regard, Foo et al.[42] demonstrated an ASC assembled using free-standing V2O5–rGO as the anode and free-standing rGO as the cathode with LiClO4 as the organic electrolyte. The flex-ible ASC device, without using separate current collectors and binders, facilitated a large ionic conduction and a wide poten-tial window of (1.5–4.0 V) with a high areal capacitance of 511.7 mF cm−2. Recently, Wang et al.[43] reported a high energy density of 58.2 W h kg−1, superior cyclic stability (88% retention after 10 000 cylces) in organic ASCs with CNT-paper-supported PPy (CNT@PPy) and poly(1,5-diaminoanthraquinone) (CNT@PDAA) as the cathode and anode, respectively. Besides organic electrolytes, ionic-liquid electrolytes have been used for higher voltage operation of supercapacitors (≥3.0 V).[44,45] Ionic elec-trolytes are nonflammable and nontoxic in nature, as well as highly desirable for their high working-temperature range.

2. Distinction between Terminologies Used: Pseudocapacitors, Asymmetric Supercapacitors and Hybrid Supercapacitors

Recently, Brousse et al.[46] reported a clear distinction between pseudocapacitors and hybrid supercapacitors. In a three-elec-trode geometry, pseudocapcitance is provided by electrode materials that can show activated-carbon-like capacitance. Even though the capacitance is due to Faradaic reactions, within a given voltage window, these materials show a linear depend-ence of charges stored with changing potential. Unlike acti-vated carbon in EDLCs, the charge-storage mechanism in pseudocapacitors involves the exchange of electrons (Faradaic reaction). Materials like RuO2 and MnO2, which clearly show activated-carbon-like (EDLC-like) curves in cyclic voltammetry (CV) and galvanic charge–discharge (GCD) measurements can be easily categorized as pseudocapacitive materials. It has been proposed that battery-type electrodes like LiMn2O4, PbO2, etc. should not be considered as pseudocapacitive electrode mate-rials. In addition, the terms “ASCs” and “hybrid supercapaci-tors” are used without a clear distinction in many reports. In several instances, hybrid supercapacitors are also referred to as ASCs, as they use two separate electrodes, even though the CV curves show battery-type behavior. Brousse et al.[46] suggested that the term “asymmetric supercapacitor” should be used only for those devices where a pseudocapacitive electrode is used

and that the term “hybrid supercapacitor” should be used for those devices with a battery electrode. We completely agree with these suggestions; however, to avoid confusion for the reader, we follow the terminology used in the respective cited paper. The reader should bear in mind that many reports interchange-ably use these two terms.

3. Selection of Electrode Materials

As discussed above and according to Equation (1), the working voltage of an ASC should be maximized to achieve a high energy density. The selection of the electrode materials plays a critical role in determining the voltage window of the device. A fundamental understanding of the different ways to widen the voltage window in supercapacitors is not completely known. However, the work function of the metal oxides used as elec-trode materials is directly related to the oxidation–reduction reaction.[47] The electrode materials with the largest difference in work function can provide the highest voltage window in asymmetric supercapacitors. An estimate of the working poten-tial can be obtained from Equation (2):[48,49]

ω ω( )= + ∆ + ∆ = − + ∆ + ∆β α1/0 1 2 A 1 2E E E E F N E E (2)

where ΔE1 and ΔE2 represent the positive and negative sur-face electrode potentials repectively; ωα and ωβ are their respective work functions, and NA is the Avogadro’s number. Chang et al.[50] showed that ASC electrodes (anode and cathode) with the largest difference in work function provide the highest working voltage. For example, among various metal oxides, they selected MnO2 and CoO3 as positive and negative electrodes having work functions of −4.4 eV and −6.2 eV (both from vacuum level), respectively. The assem-bled ASCs exhibited a large operation voltage window of 2 V. A slight increase in the operating voltage from the work-function difference is due to the adsorption of ions on the surface of the electrode, which modifies the work-function values of the electrodes.

However, from the materials’s perspective, the factors that decide the appropriateness of an electrode materials are: i) sur-face area: since most of the charge via EDLC or pseudocapaci-tance is stored at or near the surface of the electrode material, nanoarchitecturing can highly enhance the surface area of the electrode materials. ii) Electronic/ionic conductivity: limited conduction of electrons/ions through the electrode materials leads to low rate capability and higher electrochemical series resistance (ESR), thereby resulting in low specific capacitance. Binder-free integration of electrodes, open pore structures, and composites with highly conductive materials are critical approaches to enhance the electronic conductivity. iii) Mechan-ical/chemical stability: the cyclic stability of a supercapacitor depends on the mechanical/chemical robustness of the elec-trode materials. Direct fabrication of electrode materials on current collectors, surface passivation, and composites with chemically/mechanically more stable electroactive materials are viable approaches for better cycle life. In addition, depending on the application, the toxicity and cost effectivenss of the active

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materials used in an electrode design should also be considered during materials selection.

4. Negative Electrode Materials (Anode)

4.1. Carbon-Based Materials

The 21st century has predominantly become the carbon age, as carbon is the building block of almost all new energy devices, including Li-ion batteries, supercapacitors, ultracapacitors, and hydrogen-storage devices. Carbon-based materials are the most prospective candidates for anode materials in ASCs owing to their low cost, abundance, nontoxicity, and environmentally friendly nature, as well as high electronic conductivity and outstanding mechanical stability. These materials generally work as elec-trochemical double-layer capacitors (EDLCs); a tunable porous structure and carbon surface chemistry, as well as large surface area and high electrical conductivity, are desired attributes in tai-loring electrode properties to achieve optimum performance.

The general electrode reaction at the carbon negative elec-trode in an ASC can be represented as:

+ + →← + −Carbon K Carbon/Kcharging

discharginge (3)

where, K+ is a cation.

4.1.1. Activated Carbon

Among the various carbonaceous materials, activated carbons (ACs) have been the first choice from research and commer-cial perspectives over the last 40 years due to their merits of low cost, large theoretical surface area (≈3000 m2 g−1) and a broad pore-size tunability, ranging from macropores (>50 nm) to nanopores (<2 nm).[14,51–54] Since the capacitive mechanism for ACs is mainly physical adsorption/desorption, a large por-tion of the micropores remains inaccessible to the electrolyte because of the incompatible pore size with electrolyte ions, which significantly drops the usable surface area and the spe-cific capacitance (Csp).[57] The capacitance of ACs with different pore sizes greatly depends on the type of the electrolyte used, and it has been suggested that ACs with pore sizes in the range 0.4–0.7 nm are readily available for electroadsorption with aqueous electrolytes, whereas a pore size of about 0.8 nm is best suited for most organic electrolytes.[51,52,55] Largeot et al.[55] sug-gested that a maximum EDLC could be observed in ACs with pores of size similar to that of electrolyte ions (Figure 2a,b), while pores whose size is largely deviated from ion size can result in significant loss of the capacitance. Hence, it is impera-tive to control the pore-size distribution in ACs to maximize the energy/power density. Templates made of metal–organic frame-works (MOFs) such as silica, zeolite, and MgO, have been suc-cessfully employed to tailor the pore size in ACs.[58,59] Besides

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Figure 2. a) AC capacitance as a function of pore size. The images on the right show the size of the electrolyte ions that are within the range of the AC pore size. The maximum capacitance was obtained when the pore size was similar to the size of the ions used. b) C–V curve showing typical EDLC behavior in AC. c) SEM image showing the 3D hierarchical porous carbon electrode structure. d) Csp as a function of current density ranging from 0.5 to 50 A g−1. a,b) Reproduced with permission.[55] Copyright 2008, ACS Publishing. c,d) Reproduced with permission.[56] Copyright 2013, RSC Publishing.

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the large surface area and controlled porosity, the surface func-tionalization of ACs plays an important role in improving their capacitive performance. Surface-functionalization methods introduce heteroatoms (oxygen, nitrogen, boron, and sulfur) into the carbon scaffold and facilitate better adsorption of ions by making them hydrophilic in nature.[60–62] It is expected that the presence of functional groups on ACs endows Faradaic charge-transfer reactions, resulting in ≈5–10% increase in Csp.[63] For example, Kim et al.[64] observed that a mild thermal oxidation of ACs enhances the Csp to ≈37%. Unlike oxygen-functionalized ACs, nitrogen-doped ACs are more stable, and a significant pseudocapacitance reaction with both aqueous and organic electrolytes can be realized.[62,65] Huang et al.[66] intro-duced sulfur-modified ACs for supercapacitors and verified almost 40% increment in the Csp.

ACs form efficient negative electrode materials in ASCs when combined with metal oxides as positive electrode mate-rials. Some of the AC-based ASCs designs developed so far include AC//Ni(OH)2,

[67] AC//MnO2,[68] AC//LiMn2O4,

[69] and AC//V2O5.

[70] Among the various metal oxides, MnO2 is con-sidered the most promising positive-electrode material due to its high abundance, nontoxic nature, and high theoretical Csp (1370 F g−1).[71,72] Kim et al.[64] fabricated an ASC with function-alized ACs as the negative electrode and MnO2/SiC nanoneedle composites as the positive electrode. The optimized ASC could be operated in a voltage window of 0–1.9 V and exhibited a Csp of 59.9 F g−1 with excellent maximum energy and power den-sity of 30.06 W h kg−1 and 113.92 W kg−1, respectively. Three-dimensional (3D) porous ACs are desired for better perfor-mances as they not only facilitate continuous electron pathways for good electrical contacts, but also provide shorter diffusion paths for electrolyte ions.[73] Quie et al.[56] presented function-alized 3D hierarchical porous carbon structures (Figure 2c) with large surface area (2870 m2 g−1) and excellent electrical conductivity (5.6 S cm−1). The 3D hybrid electrodes showed a high capacitance (≈318 F g−1) (Figure 2d), excellent rate perfor-mance, and capacitance retention above 90% in both aqueous and organic electrolytes. Recently, carbon cloth (CC) is drawing significant attention as a promising negative electrode material because of its high conductivity, low-cost, and high mechanical flexibility, suitable for flexible solid-state supercapacitors.[74,75] Wang et al.[76] demonstrated a high areal capacitance of 756 mF cm−2 in functionalized CC electrodes. The CC//TiN@MnO2 ASC delivered a remarkable cyclic stability without any capacitance decay after 70 000 cycles.

4.1.2. Carbon Nanotubes

Despite the great use of AC materials as ASC electrodes, the inaccessibility of the electrolyte ions into their micropores and/or interior atoms at higher scan rates still remains the central issue that limits their effective capacitances.[77] In addition, their poor electrical conductivity leads to a higher internal resistance that prevents them from being used in high-power-density supercapacitors.[78] Alternatively, carbon nanotubes (CNTs) have been suggested as a supercapacitor electrode material, initially proposed by Niu et al.[79] back in

the 1990s. In spite of their moderately small theoretical sur-face areas (≈50–1315 m2 g−1), they exhibited higher capaci-tances over other ACs. This is attributed to their unique tubular structures and the high density of mesopores, which allow fast charge transport and large accessibility of electrolyte ions.[80,81] The initial work on supercapacitors based on CNTs were mainly carried out with randomly oriented and entangled multiwalled CNTs (MWCNTs), exhibiting a Csp in the range of 102–135 F g−1.[82,83] Compared to MWCNTs, single-walled nanotubes (SWNTs) show better electrochemical performances due to their large specific surface area (≈1 600 m2 g−1), high aspect ratio, and better accessibility to the electrolyte ions.[84–86] An et al.[87] found a maximum specific capacitance of 180 F g−1 in arc-discharge-produced SWNT electrodes. Aligned CNTs are highly preferred over entangled CNTs as they present unbun-dled structures, providing more mesopores and accessible sur-faces. Moreover, they provide lower contact resistance during the course of the charging–discharging process, leading to large energy and power densities.[88,89] Chen et al.[90] reported a high capacitance of 365 F g−1 in highly ordered MWNT-array electrodes fabricated using the anodic aluminum oxide (AAO) template. Later, Lu et al.[91] reported a template-free chemical vapor deposition (CVD) growth of aligned MWCNTs elec-trodes, exhibiting a higher capacitance of 440 F g−1. Kim and co-workers directly grew vertically aligned CNTs on conductive carbon papers using water-assisted CVD,[92] which exhibited a Csp of about 200 F g−1 at 20 A g−1.

In addition to the inherent EDLC mechanism offered by CNTs, their capacitive performances have been further enhanced by combining them with other materials such as metal oxides, ACs, and CPs.[93–95] These additives introduce the advantages of high specific capacitance (pseudocapacitance), excellent conductivity, and/or large surface areas to the CNT matrices. Negative ASC electrodes based on nitrogen-doped AC-coated MWCNT composites show a high specific capaci-tance of 311.7 F g−1.[96] The improved electrical conductivity in asymmetric devices resulted in a high operational voltage window of 1.9 V and an energy density of 26.4 mW h g−1. Zhang et al.[97] investigated synergetic effects of Co–Al lay-ered double hydroxide (LDH) mixed with MWCNTs, achieving a specific capacitance of 342 F g−1. Binders have been used to incorporate CNTs with other pseudocapacitive materials, which inevitably increases the dead volume in the composite electrodes.[98] Zhang et al.[99,100] grew a 3D CNT-array network directly on a tantalum (Ta) foil current collector, followed by the deposition of PANI and MnO2 nanoflowers to construct hybrid composites (Figure 3a). The transmission electron microscopy (TEM) image in Figure 3b shows that MnO2 nanoflowers tend to nucleate at the junctions of CNTs rather than at their curved surfaces. These binder-free composites presented good syner-gistic effects; a very high Csp of 1030 F g−1 was achieved for PANI–CNTs, and a long cycle life with only 3% capacity loss after 20 000 cycles for a MnO2–CNTs electrode (Figure 3c).

The intrinsic mechanical flexibility of CNTs makes them suit-able for flexible energy-storage systems toward flexible superca-pacitors.[101,102] In an early report, Chen et al.[103] demonstrated a facile method to grow 3D MWNT flexible, lightweight electrodes directly on various substrates (carbon, metal foils, etc.), which

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essentially shaped the idea of integrating CNTs for flexible supercapacitors. Recently, Qiu et al.[104] fabricated a 3D electrode structure with vertically aligned CNTs directly grown on carbon nanofibers (VACNTs/CNFs). The combination of high charge-storage capacity in the branched VACNTs with the highly con-ducting CNFs enabled a high specific energy of 70.7 W h kg−1 and excellent cycle performance to be delivered, with 97.0% retention even after 20 000 charging/discharging cycles. In recent years, the applications of CNT fibers to supercapacitors have evolved toward the development of highly flexible woven or knitted fabrics/textiles with excellent wearability.[105] ASCs made with CNT yarn as the negative electrode and CNT@MnO2 composite yarn as the positive electrode delivered about five-times-higher energy density (42.0 W h kg−1) and power density (483.7 W kg−1) as compared to a symmetric supercapacitor made of CNT//CNT.[106] Despite enormous research efforts on CNTs, the bottleneck of transferring this technology to the marketplace is the lack of efficient and scalable manufacturing methods. Entanglement in CNTs is a major problem that results in poor efficiency as it limits ionic transport. Furthermore, purification of CNTs and its associated high cost are another limiting factor.

4.1.3. Graphene

Graphene, an atomically thick 2D carbon layer, is an emerging negative-electrode material owing to its exceptionally large theoretical surface area (2630 m2 g−1), excellent EDLC mecha-nism, ballistic electrical and thermal conductivities, and great mechanical strength.[107–110] The high intrinsic double-layer capacitance (≈21 µF cm−2) in graphene can provide a large spe-

cific capacitance of up to 550 F g−1, outperforming almost every EDLC electrode material, including ACs, CNTs, mesoporous carbon, and xerogels.[31,111,112] The schematic of Figure 4 illus-trates the merits and demerits of ACs, CNTs, and graphene for EDLC supercapacitors.[113] The nonuniform distribution of pore size in AC limits the access of electrolyte ions: a large fraction of the ACs remains unused, resulting in small capaci-tance (Figure 4a). Although CNTs present higher electrical conductivity than ACs and graphene, they tend to easily stack into bundles, and only the outer portion of the stack is available for ion adsorption (Figure 4b). This in turn leads to poor elec-trochemical performance. Graphene and its derivatives have shown a great combination of desired properties (conductivity, surface area, mechanical strength), which leads to better elec-trochemical performance as compared to ACs and CNTs.[111,114] However, the tendency of graphene sheets to form irreversible agglomerates of graphite via van der Waals interactions leads to significant surface area loss (Figure 4c), which, in turn, limits ion diffusion and Csp.[34,115,116] Several methods, including gas–solid reduction processes, treatment with weak reducing agents, and adding a spacer between graphene nanosheets, have been addressed to counter the graphene restacking problem.[45,117,118]

Liu et al.[119] proposed a unique curved morphology that prevents the re-stacking of graphene nanosheets, as well as enables the formation of mesopores accessible to ionic elec-trolytes, pushing the energy-density level to 90 W h kg−1. Qin et al.[113] proposed the idea of using CNT spacers in between graphene sheets to reduce their agglomeration and internal resistances, as well as improving the accessibility for electro-lyte ions (Figure 4d). In addition, CNTs function as a binder for graphene, rendering mechanical stability and longer cycle

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Figure 3. a) Schematic showing the procedure for the preparation of a MnO2 nanoflowers/CNTA composite electrode. b) TEM image showing MnO2 nanoflowers grown at the CNT junctions. c) Capacitance retention curves up to 20 000 cycles. A–c) Reproduced with permission.[100] Copyright 2008, ACS Publishing.

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life. SWNT–graphene composite electrodes prepared by this method reached a Csp of 290 F g−1 in aqueous electrolyte.[113]

Metal oxides and/or CPs have been widely used to make composites with graphene for high-performance supercapaci-tors.[120–123] MnO2 has been incorporated into graphene, as compared to other metal oxides, due to its environmentally friendly nature and ability to work as spacer sheets.[123–125] Yu et al.[126] developed graphene/MnO2 nanocomposites wrapped with CNTs and demonstrated improved electrochemical per-formances by ≈45%, offering a specific capacitance value as high as 380 F g−1. Wang et al.[127] synthesized a hybrid elec-trode material by loading Ni(OH)2 pseudocapacitive nanoplates on graphene sheets. This electrode exhibits a very high Csp of ≈1335 F g−1 at the charge and discharge current density of 2.8 A g−1. Usually, chemically reduced or functionalized gra-phene suffers from poor electrical conductivity (100–200 S m−1) and thereby lower electrochemical performances have been observed in several instances.[128] Composites of graphene and conducting polymers (CPs) have been employed to mitigate this problem, as CPs offer low cost, high conductivity, and high theoretical capacitance.[129,130] Zhang et al.[131] prepared gra-phene/PANI nanofiber composites by in situ polymerization of aniline with graphene oxide. These composites showed a high Csp of 480 F g−1 at a current density of 0.1 A g−1.

Recently, Wang et al.[133] achieved a large Csp of 356 F g−1, even at a high current desnity of 20.0 Ag−1 after 1000 cycles in PANI/graphene oxide composites. Even though graphene-based composite electrodes using metal oxides and/or CPs have shown great improvements in the Csp, these materials usually suffer from inferior power performance and cyclicity due to their pseudocapacitive nature.[120,134] Recently, 3D graphene-based frameworks (3DGFs) such as sponges, foams, hydro-gels, and aerogels have been emerging as potential materials

for ultralightweight electrode materials, owing to their large surface area, intercon-nected micro-/macroporous structures, excellent electrical conductivity, and sta-bility.[135–138] Their unique 3D architecture can effectively prevent the agglomeration of graphene nanosheets, as well as facilitate fast ionic transportation. In addition, their self-supporting nature circumvents the use of electrochemically inactive binders.[139–141] Gao et al.[34] prepared graphene hydrogels via the reduction of well-dispersed graphite oxide solution, achieving Csp of ≈150 F g−1 at 20 mV s−1. Tang et al.[142] fabricated a novel ASC based on a highly porous graphene foam (GF) negative electrode prepared by mild reduction. The fabricated ASC device was found to work in a voltage window of 1.6 V with an excellent energy density of 34.5 W h kg−1 at the power density of 547 W kg−1. Recently, our research group developed a functionalized graphene aerogel (GA) by conformally doping palladium (Pd) nanoparticles on graphene nanosheets (P-GA) followed by a lyophilization and two-step reduction method, as shown in

Figure 5a.[132] Figure 5b–d show the scanning electron micro-scopy (SEM) images of GA and P-GA samples. It is evident from the figures that Pd nanoparticles were embedded into the GA with a uniform dispersion. The introduction of Pd nanopar-ticles enabled a significant reduction of the electrical resistivity by 50 times (i.e., from 950 to 16 Ω cm). Additionally, a negative electrode based on P-GA showed a Csp of 175.8 F g−1 at a scan rate of 5 mV s−1, which is more than 3 times enhancement as compared to that without Pd doping (51.9 F g−1). The MnO2//P-GA ASC delivered an average energy density of ≈13.9 W h kg−1 at a power density of ≈13.3 kW kg−1 (Figure 5e,f).

4.2. Metal Oxides

Although carbon-based materials hold great potential as nega-tive electrodes, their intrinsically low Csp and low energy den-sity are major drawbacks. Hence, it is imperative to explore new materials that simultaneously exhibit high capacitance, as well as high conductivity. In this regard, metal oxides are promising, as they rely on a pseudocapacitive charge-storage mechanism, in which fast reversible redox reactions occur near the elec-trode surfaces, offering high capacitance and energy densities. Among various metal oxides, ferric oxide (Fe2O3) presents a high theoretical Csp, an ideal voltage window, cost-effectiveness, abundance, and environmental friendliness.[143–146] However, its conductivity (≈10−14 S cm−1) severely limits its resultant capacitance and power capability. Constructing Fe2O3 in nano-structures, such as nanorods/nanotubes,[147] quantum dots,[30] nanosheets,[144,148] and nanoparticles[101,149,150] has been pursued to alleviate this issue, as it can provide short diffusion paths to electrons, thereby improving the electronic conductivity. Yang et al.[151] designed a flexible ASC based on Fe2O3 nanotubes

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Figure 4. Schematic illustration of the intrinsic limitations of different carbon materials. a) AC: having large surface area, but many pores are inaccessible by electrolyte ions. b) CNTs: form bundles and only the outer surface is available for electrolyte–CNT interaction. c) Graphene sheets: tend to agglomerate due to van der Waal interactions and thereby electrolyte ions refrain from reaching small pores. d) Graphene/CNT composite: CNTs act as spacer and binder from graphene sheets, as well as provide large conduction pathways to electrons and ions. A–d) Reproduced with permission.[113] Copyright 2011, RSC Publishing.

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grown on a flexible carbon cloth as an the anode, and MnO2 nanorods as the cathode. The flexible device worked in a large voltage window of 1.6 V and exhibited excellent energy density of 0.55 mW h cm−3. Another strategy employed to circumvent the electrical-conductivity issue is to make Fe2O3 nanocompos-ites with conductive agents like carbonaceous materials and conducting polymers. Xia et al.[30] fabricated large-scale Fe2O3 quantum dots (QDs) (≈2 nm) decorated with functionalized graphene sheets (FGS). The well-dispersed Fe2O3 QD elec-trodes without graphene agglomeration exhibited a very high Csp of 347 F g−1 in liquid electrolytes. Recently, Liu et al.[152] developed Fe2O3 nanodot negative electrodes supported on N-doped graphene sheets that resulted in a long cyclic stability of about 75.3% capacitance retention after 100 000 cycles. The bottleneck of using carbon-based materials for Fe2O3 compos-ites is their affinity in forming organic compounds that are not environmentally friendly. CP coatings have been alternatively pursued to enhance the Csp and the stability of Fe2O3. Li and co-workers[147] fabricated a novel 3D α-Fe2O3@PANI core–shell

nanowire array structure in which the α-Fe2O3 nanowire array was electrodeposited on carbon fiber followed by thin-film dep-osition of PANI polymer. A significant enhancement in the Csp from 33.93 mF cm−2 to 103 mF cm−2 in the α-Fe2O3@PANI electrode was noticed, as well as a cyclic stability of 95.77% after 10 000 cycles. Recently, Zeng et al.[153] reported a flexible Ti-doped Fe2O3/PEDOT core–shell nanoarray on carbon cloth, exhibiting a significantly high areal capacitance of 1.15 F cm−2 with 96% capacitance retention after 30 000 cycles.

Molybdenum trioxide (MoO3) and tungsten trioxide (WO3) are also promising candidates for anode materials in ASCs due to their high theoretical specific capacity and thin sheet-like structure that facilitate the fast insertion/removal of even smaller electrolyte ions.[154,155] Similar to Fe2O3, they also suffer from poor electrical conductivity, and, thereby, loading addi-tional active materials with high surface area and high conduc-tivity is required to overcome this problem. Sun et al.[156] assem-bled an ASC device based on free-standing CNT–tungsten oxide (WO3) hybrid films prepared by vacuum filtration and physical

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Figure 5. a) Schematic illustration showing the fabrication process of P-GA. b,c) SEM images of the GA (b) and P-GA (c). d) HRTEM images of P-GA. e) Schematic showing ASC device construction using MnO2 and P-GA electrodes. f) Ragone plot reflecting the superiority of P-GA nanostructures over other electrode materials. The inset shows a red LED powered by using two devices connected in series. a–f) Reproduced with permission.[132] Copyright 2015, Elsevier Publishing.

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vapor deposition (PVD) methods (Figure 6a). The resulting device showed a high capacitance (2.6 F cm−3), an extended operating voltage (1.4 V), and high power/energy densities. In addition, the ASC exhibits 75.8% capacitance retention after 50 000 cycles. (Figure 6b). These devices were able to charge a mobile phone, as shown in Figure 6c. Despite the enhanced capacitive performance, the poor corrosion resistance of carbon materials in the electrochemical window of some metal oxides is a potential drawback. Therefore, several research groups have developed carbon MoO3 or WO3 anodes for high performance ASCs. Li et al.[157] demonstrated a ZnO@MoO3 core–shell nanocable supercapacitor electrode where the ZnO nanorod core provided a high electrical conductivity whereas the thin MoO3 layer shell enabled a fast, reversible Faradaic reaction and provided a short ion diffusion path, leading to a Csp of 236 F g−1, which is much higher than that of pristine MoO3 nanoparticles. Recently, Wang et al.[158] reported WO3@Ppy core@shell nanowire arrays for novel negative electrodes in ASCs. The core–shell nanowire electrode exhibited a high areal capacitance of 253 mF cm−2 and the ASC device showed a high volumetric capacitance up to 2.865 F cm−3 and good sta-bility. Besides these, other metal oxides, such as manganese oxides (e.g., MnO2),[159] indium oxides (e.g., In2O3),[160] bis-muth oxides (e.g., Bi2O3),[162] etc. have been recently explored as emerging negative-electrode materials for advanced ASC applications.

4.3. Metal Nitrides

Metal nitrides are emerging anode materials that are supe-rior to metal oxides in terms of electrical conductivity (4000–55 500 S cm−1) and pseudocapacitive behavior, providing high power and energy densities for ASCs.[163,164] Among the various nitrides, titanium nitride (TiN) has been the most widely studied anode due to its high electrical conductivity and mechanical stability. Despite the development of TiN in various morphologies, such as nanotubes,[165] nanosheets,[166] and mesoporous microspheres,[167] the fast irreversible oxidi-zation of these materials in aqueous solution impedes their electrochemical stability.[168] Choi et al.[169] reported that a TiN electrode could retain only 28% of its initial capacitance in alkaline electrolyte solution after just 400 cycles. Lu et al.[170] demonstrated a novel core–shell structure in which TiN nanow-ires were covered with ultrathin amorphous carbon protective layers.[171] The TiN–carbon core–shell electrode demonstrated a remarkable 91.3% retention in aqueous electrolyte after 15 000 cycles without any oxidation. Similarly, TiN was com-bined with more stable materials like graphene,[172,173] TiO2,[174] VN,[175] MnO2,[176] and CPs to form core–shell structures to improve its cycle stability and rate performance. For example, Zhu et al.[177] fabricated an all-nitride-based ASC device using atomic layer deposition (ALD)-grown TiN and Fe2N on verti-cally aligned graphene-nanosheet electrodes. The ASCs based

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Figure 6. a) Schematic diagram showing the preparation of a WO3–CNT electrode. b) Cycle performance and Coulombic efficiency of the ASC device for 50 000 cycles at a current density of 1 mA cm−2. The inset shows the CV curves of the 1st cycle and the 50 000th cycle. c) Three ASC devices connected in series can charge a cell phone. d) SEM images of the as-prepared WON nanowires. e) TEM image showing the rough surface of the WON nanowire. The inset shows its polycrystalline nature. c) Cyclic stability of WON electrode over 100 000 cycles, measured at 100 mV s−1. The insets are CV curves of the WON electrode after 100, 50 000, and 100 000 cycles. A–c) Reproduced with permission.[156] Copyright 2015, RSC Publishing. d–f) Reproduced with permission.[161] Copyright 2015, Wiley-VCH.

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on them yielded a capacitance of 58 F g−1 in the solid elec-trolyte, which was stable up to 20 000 cycles. The substantial improvement was attributed to the ultrahigh surface area of the vertically aligned graphene and the enhanced coverage of the active nitride materials over graphene enabled by the ALD.

Vanadium nitride (VN) is considered another prom-ising electrode material due to its high theoretical Csp (up to 1340 F g−1), reversible and fast redox Faradaic response, and excellent electrical conductivity (≈104 S cm−1).[178,179] The pio-neer work by Choi et al.[180] demonstrated a pseudocapacitive behavior of nanostructured VN electrodes offering a high Csp of ≈400 F g−1 at a scan rate of 50 mV s−1. However, VN electrodes become unstable in aqueous electrolytes due to the irreversible formation of vanadium oxide, which leads to significant perfor-mance decay upon cycling.[178,180,181] Li et al.[178] utilized LiCl/poly(vinyl alcohol)(PVA) polymer electrolyte to stabilize the porous VN nanowires. The ASC device employing stabilized VN nanowires as the anode and vanadium oxide (VOx) NWs as the cathode achieved a high capacitance retention of 95.3% after 10 000 cycles. Furthermore, VN composites with CNTs were developed to enhance the electrical conductivity and rate capabilities.[182] Ghimbeu et al.[183] prepared highly porous VN/CNTs composites by sol–gel and temperature-programmed ammonia-reduction methods, demonstrating high capacitance retention (58%) as compared to pure VN (7%) at a higher cur-rent densiy of 30 A g−1.

Dong et al.[184] synthesized TiN/VN composite electrodes and reported a Csp of 170 F g−1 at a scan rate of 2 mV s−1. The excel-lent performance was attributed to the fast transportation of electrons owing to the structural integrity of the VN interfaced with TiN. Similarly, Zhou et al.[185] reported the fabrication of mesoporous coaxial TiN–VN core–shell structures. These hybrid electrodes exhibited a high Csp of 247.5 F g−1 at 2 mV s−1 with an excellent rate capability owing to their high surface area and electrical conductivity.

Besides TiN and VN, molybdenum- and tungsten-based nitrides have also been pursued to improve electrochemical stabilities.[186,187] Li et al.[188] studied the electrochemical per-formance of molybdenum nitride (MO2N) nanoparticles as ASC electrodes, and achieved a Csp of 172 F g−1 in 1 m H2SO4 electrolyte. Very recently, Yu et al.[161] synthesized tungsten oxynitride (WON) nanowires on carbon cloths by the nitrida-tion of WO3 precursor nanowires (Figure 6d,e). The WON ASC anode showed a high volumetric capacitance of 4.95 F cm−3 at 12.5 mA cm−3. Most importantly, an unprecedented cycling sta-bility with only 7% capacitance loss even after 100 000 cycles was reported, which is amongst the best cycling performance ever reported for metal nitrides or oxynitrides (Figure 6f).

5. Positive Electrode Materials (Cathode)

Positive electrodes for ASCs generally utilize the materials that exhibit a large amount of pseudocapacitance originating from the Faradaic charge transfer via fast and reversible redox reac-tions, electrosorption/desorption, or via intercalation/deinterca-lation of the electrolyte with the electrode. The pseudocapacitive behavior is generally accompanied by a high Csp and relatively high energy density, as compared to the EDLC mechanism.

It is because the bulk of the material is exposed to the redox reactions, unlike EDLC where the adsorption of ions takes place only at the surface layers. In this section, we will discuss important redox-active materials like conductive polymers (e.g., polyaniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy)) and transition-metal oxides/hydroxides (MnO2, RuO2, V2O5, and Ni(OH)2).

The general cathode reaction can be given as:

→← + ++ −MnO /M MnO M2charging

discharging2 e (4)

where M+ is the cation.

5.1. Conductive Polymers

Conducting polymers (CPs) are promising ASC electrode materials because of their high electrical conductivity, large charge/discharge cycles and high pseudocapacitance.[189–191] PANI, PEDOT, polythiophene (PTh), and PPy are the com-monly used CPs electrodes. One main drawback is that CPs expand/shrink during the intercalation/deintercalation process, which leads to the mechanical failure of the electrodes when subjected to longer cycling, resulting in fading of the electro-chemical performance and stability. Sharma et al.[192] observed a 50% decrement in initial capacitance just after 1000 cycles in a PPy-based supercapacitor electrode. Similarly, the loss of Csp of PANI nanorods was ≈29.5% after 1000 cycles.[193] Hence, low cycling stability is the major concern associated with CP-based electrodes.

Proper design of the polymer microstructure and mor-phology has been explored to enhance the electrochemical energy-storage ability and cycling stability of CPs-based elec-trodes. It has been observed that constructing CPs in the forms of nanofibers, nanorods, nanowires, and nanotubes can signifi-cantly overcome their poor cyclic stability by providing short diffusion lengths to maximize the electrolyte exposure. For example, Wang et al.[194] observed 95% capacitance retention after 3000 cycles in ordered nanoscale PANI whiskers in 1 m H2SO4 solution. The vertical nanowire construction of CPs is even more intriguing for energy storage, as compared to dis-ordered networks, because: i) each nanowire contributes to the capacitance, as it is directly connected to conducting substrates; ii) the 1D structure allows fast charge transport with short diffusion paths; and iii) the interface between the nanowires can accommodate large volume expansions without cracking. Huang et al.[195] fabricated PPy nanowire arrays using one-step electropolymerization and compared their performances with disordered nanowires and compact films made of same materials. Figure 7a–c show the SEM images of the different morphologies of the PPy in the forms of film, nanowire net-work, and nanowire arrays. Figure 7d shows that ion transport is much easier in the nanowire arrays as compared to the film or entangled nanowire network, achieving a high capacitance of about 566 F g−1, while the networks and the film showed capac-itances of 414 and 378 F g−1, respectively (Figure 7e). Ordered PANI nanowire arrays were also explored, and Wang et al.[196] reported a dramatically high capacitance of 950 F g−1 in PANI

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nanowire arrays, attributed to their unique structure and mul-tiple oxidation states.

Another strategy explored to enhance the mechanical strength, conductivity, and cyclic stability of CPs is to make composites with carbon-based materials.[197] CNTs are con-sidered as an effective additive because of their exceptional mechanical properties, large surface areas, and superior elec-trical conductivities, all of which can help to mitigate the cycle degradation problems in pristine CPs.[198] Zhang et al.[99] used a hierarchical porous PANI/CNT composite for the elec-trodes, and demonstrated a high Csp (1030 F g−1) and supe-rior rate capability and stability (95% capacity retention after 5000 cycles). Similarly, graphene has also been pursued, owing to its excellent conductivity, large surface area, and superior chemical stability. Wang et al.[194] reported the first graphene/PANI-composite-based supercapacitor electrodes with a capaci-tance of 233 F g−1, which was about two times greater com-pared to pure graphene electrodes. Yan et al.[199] synthesized graphene/PANI composites and demonstrated a very high capacitance of 1046 F g−1 at 1 mV s−1. In addition, the energy density of the graphene/PANI composite reached 39 W h g−1 at a power density of 70 kW kg−1.

CPs have demonstrated a great potential for low-cost, light-weight, and highly flexible supercapacitor electrodes as stand-alone materials or composites. Wei et al.[200] demonstrated a flexible supercapacitor device made of PANI nanowires deposited on a cloth-supported SWCNT flexible substrate. The PANI/SWNT/cloth composite electrode showed a capaci-tance of 410 F g−1, which is much higher than that of SWNT/cloth (60 F g−1) and PANI/cloth (290 F g−1) electrodes. In addi-tion, a high capacitance retention of 90% was observed in the

composite up to 3000 cycles. Very recently, Kurra et al.[201] built a flexible solid-state ASC based on all-nanostructured CPs, i.e., PEDOT was used as the anode and PANI was used as the cathode. The asymmetric device showed a maximum power density of 2.8 kW L−1 at an energy density of 9 W h L−1.

5.2. Metal Oxides

5.2.1. Ruthenium Oxide (RuO2)

RuO2 is a promising electrode material for ASCs due to its high specific capacitance, reversible redox reactions with large poten-tial window, and longer cycle life. Among its two phases, i.e., crystalline phase (rutile RuO2) and amorphous hydrous phase (RuO2·xH2O), the latter is generally more promising owing to its ultrahigh pseudocapacitance and large active reaction sites, as well as the high electron and proton conductivities.[202,203] Especially, nanostructured RuO2 in various morphologies like nanoparticles, nanoneedles, nanorods, and nanofibers have shown remarkable electrochemical performance.[204,205] Hu et al.[206] reported a nanotubular (NT) arrayed porous elec-trode made of RuO2·xH2O. This 3D mesoporous architec-tured material with hydrous nature and metallic conductivity exhibited a Csp of 740 F g−1, which was increased to 1300 F g−1 after annealing in air at 200 °C for 2 h. Despite the exceptional pseudocapacitive properties, RuO2 is not suitable for large-area commercialization due to its low abundance and high cost. As an alternative, RuO2-based composites, with cheaper materials possessing high capacitive properties, were pur-sued, and carbonaceous materials (AC, CNTs, carbon aerosols,

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Figure 7. a–c) SEM images of conducting PPy film (a), PPy nanowire network (b), and PPy nanowire arrays (c). d) Ion-transport pathways through different PPy geometries. e,f) Csp (e) and capacitance retention (f) of PPy with different morphologies. a–f) Reproduced with permission.[195] Copyright 2010, RSC Publishing.

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graphene) have been widely utilized.[207–209] Lee et al.[210] fab-ricated MWNT/RuO2 composite electrodes by impregnating RuO2 into MWNTs in an acidic solution. The capacitance was measured for the films of different RuO2 loading, revealing Csp per mass as high as 628 F g−1. Das et al.[211] showed even more promising results by electrodepositing RuO2 on highly porous SWNT films, achieving a Csp of 1715 F g−1 with regard to the weight of RuO2. Another potential problem with RuO2-based electrodes is the possible cracking of the material due to the strain developed during the charging–discharging cycles, leading to a poor long-term stability.[212,213] To solve this problem, Wang et al.[214] developed a novel core–shell-templated approach to fabricate CNT-supported hollow-struc-tured (hRuO2) nanoparticles. The hRuO2/CNT nanocomposite electrode showed a high specific capacitance of 655.0 F g−1 at a current density of 5 A g−1 without any crack formation. Sev-eral variations of RuO2 nanostructures anchored, wrapped, or encapsulated with graphene have shown good synergy and significant improvement in their electrochemical properties.

RuO2 composites anchored to graphene not only suppress the agglomeration and restacking of the graphene but also increase the available surface area and provide electron-con-ductive networks. Wu et al.[120] showed that hydrous RuO2 nan-oparticles (size ≈5–20 nm) were homogeneously anchored on the graphene sheets and demonstrated a high Csp of 570 F g−1, enhanced rate capability, excellent electrochemical stability (97.9% retention after 1000 cycles), and high energy density (20.1 W h kg−1). In a recent report, Kaner et.al.[215] demon-strated miniaturized and interdigitated supercapacitors made of 3D porous RuO2 nanoparticles anchored to graphene sheets (Figure 8a). The high-resolution TEM (HRTEM) image in Figure 8b reveals that multiple layers of the graphene sheets wrapped around each RuO2 nanoparticle. These electrodes exhibited an ultrahigh Csp of 1139 F g−1 with excellent rate capability (Figure 8c). Furthermore, ASCs employing the gra-phene sheet/RuO2 as the positive electrode and an AC negative electrode demonstrated an extremely high energy density of 55 W h kg−1 at a power density of 12 kW kg−1.

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Figure 8. a) Microfabrication process of LSG/RuO2 interdigitated micro-supercapacitors via direct laser writing on a DVD disc using a LightScribe DVD burner. b) A high-magnification TEM image illustrating complete wrapping of the RuO2 nanoparticles (NP) by the LSG sheets. c) The gravimetric capacitance retention of LSG and LSG/RuO2 electrodes as a function of the applied current density. d) Facile redox method for controlled nanostruc-tured MnO2 growth on CNT surface. e) TEM images showing MnO2 coverage over the surface of a CNT. f) The cyclic curve of a MnO2–CNT nanowire composite at 2 A g−1. a–c) Reproduced with permission.[215] Copyright 2015, Elsevier Publishing. d–f) Reproduced with permission.[216] Copyright 2015, Elsevier Publishing.

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CPs were incorporated into RuO2,[217–220] which helps in pre-venting the aggregation of hydrous RuO2 by steric and electro-static effects, as well as improving the adhesion of RuO2 to the current collectors. A tubular RuO2/PEDOT composite electrode exhibited a dramatically high Csp of 1217 F g−1, which was attrib-uted to their high surface area, providing short diffusion paths and low ionic resistance for the diffusion of counter-ions.[221] In order to make more-cost-effective composites, RuO2 has been combined with cheap metal oxides such as MnO2, VOx, TiO2, MoO3, and SnO2 to form composite oxide electrodes.[222–225]

5.2.2. Manganese Oxide (MnO2)

MnO2 is a promising electrode material for pseudocapacitor applications owing to its higher electrochemical performance, low cost, and environment benign nature.[226–229] The low sur-face area and poor electronic/ionic conductivity of MnO2 are two major problems that hinder their practical uses. Engineering the morphology of MnO2 into nanostructures is considered to be a viable approach to enhance its electrochemical performance. 1D MnO2 nanostructures generally provide short diffusion paths for ions and electrons, as well as offer large surface areas, resulting in high charge/discharge capacities. Qu et al.[68] tested the per-formance of MnO2 nanorods in different electrolytes (Li2SO4, Na2SO4, and K2SO4) prepared by precipitation reaction methods. The nanorods show the largest capacitance (201 F g−1) in Li2SO4, while an excellent cycling behavior with only 6% loss of initial capacitance after 23 000 cycles. MnO2 nanowires were preferred over nanorods as they provide a larger specific surface area and shorter diffusion paths for protons.[230] Wu et al.[231] reported that MnO2 nanowires (≈8–16 nm in diameter) exhibited a higher capacitance of 350 F g−1 as compared to MnO2 nanorods (≈15–35 nm in diameter) with a capacitance of 243 F g−1. Similarly, several studies showed that single-crystal α-MnO2 nanowires with a pore size between 3 and 30 nm exhibited a Csp value as high as 466 F g−1 along with good cyclic stabili-ties.[232–234] However, the use of extensive processes, expensive reagents, and time-consuming operations are the major con-cerns that restrict them for industrial-scale production.

Incorporating MnO2 with other materials of high surface area and electrical conductivity have been extensively inves-tigated to extend the working voltage and to improve the sta-bility of the resulting electrodes. Most of the reports pertaining to MnO2 composites are with carbonaceous materials such as ACs, CNTs, and graphene.[235–237] Gao et al.[238] fabricated a hybrid supercapacitor based on MnO2/AC composite as the pos-itive electrode and AC as the negative electrode. They showed that the AC not only worked as a conducting agent but also served as an active element in enhancing the capacitance, as well as the power and energy densities. Supercapacitors based on mesoporous-MnO2/AC composites are receiving signifi-cant attention because the Csp and the rate capability of MnO2 greatly depend on the pore size of the active materials.[239,240] CNTs are other promising materials to be incorporated, owing to their outstanding structural and electrical properties, high external surfaces, and good chemical stability. Initial studies on MnO2/CNT nanocomposites[241–243] exhibited a capacitance comparable to AC-based MnO2 composites. Chen et al.[244]

demonstrated the direct growth of MnO2 in a CNT matrix by a facile thermal decomposition of manganese nitrates. The MnO2/CNT composite exhibited a Csp of 568 F g−1 and an excellent cyclic stability of 88% after 2500 charge–discharge cycles. Besides the thermal-decomposition method, other effec-tive methods, such as electro-deposition,[245] the microwave-assisted method,[246] and hydrothermal[247] and redox deposition methods[248] have been extensively employed to attach MnO2 onto the side walls of CNTs. For example, MnOx nanoparticles electro-deposited onto CNT nanosheets resulted in improved performance, exhibiting a high Csp of 1250 F g−1 with a high rate capability.[249] In a recent study, Huang et al.[216] demon-strated a facile redox approach for the controlled growth of MnO2 nanostructures on CNTs, revealing the influence of replacing CNTs on electrochemical properties (Figure 8d,e). The resulting MnO2–CNT electrode exhibited a maximum Csp of 247.9 F g−1, and an extraordinary cycle life, retaining 92.8% retention of initial capacitance after 5000 cycles (Figure 8f). Furthermore, aligned CNTs have been preferred over non-aligned CNTs because of their low contact resistance, large specific surface area, and fast electron-transfer kinetics. Zhang et al.[100] realized well-dispersed MnO2 nanoflower structures on vertically aligned CNTs, achieving excellent rate capability (50.8%), high capacitance, and long cycle life (3% capacitance loss after 20 000 cycles). Amade et al.[250] further optimized the parameters to fabricate dense, long, and vertically aligned CNT composites with a thin layer of MnO2, reporting a significant enhancement in Csp (642 F g−1). MnO2–CNT composites have also been used to make flexible supercapacitor devices.[251] Gu et al.[252] reported an all-solid-state asymmetric stretchable supercapacitor using a wrinkled MnO2/CNT hybrid film as the positive electrode and a wrinkled Fe2O3/CNT composite film as the negative electrode. The ASC devices showed a supreme energy density of 45.8 W h kg−1 along with very high cyclic sta-bility, retaining 98.9% Csp even after 10 000 cycles.

Graphene nanosheets and its derivatives have also been explored to make MnO2 nanocomposite electrodes.[253,254] Wu et al.[31] studied the performance of ASCs using a MnO2 nanowire/graphene composite as the positive electrode and gra-phene as the negative electrode in aqueous Na2SO4 solutions. It was discovered that nanostructured MnO2 prevented the aggre-gation of graphene sheets caused by van der Waals interactions, generating a large electrochemical active surface area and porous structures suitable for energy storage. Moreover, the ASC device based on graphene/MnO2 operated in a high voltage window of 0–2.0 V and exhibited a high energy density of 30.4 W h kg−1. Cheng et al.[255] fabricated binder-free graphene/MnO2 superca-pacitor electrodes in which MnO2 was deposited directly on gra-phene using in situ anodic electrodeposition. The Csp reported in this study was 328 F g−1 and the power density was 25.8 kW kg−1, which is suitable for high-power applications. The advent of 2D planar capacitive materials enables the fabrication of much thinner, compact, and flexible ASC devices,[256,257] while the non-2D structures of MnO2 restrict it from being integrated into planar devices. Peng et al.[258] prepared ultrathin 2D MnO2/gra-phene hybrid planar supercapacitors based on ultrathin δ-MnO2 nanosheets integrated with graphene sheets. This unique design enabled a high Csp of 267 F g−1 at a current density of 0.2 A g−1 with capacitance retention of 92% after 7000 charge/discharge

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cycles. In addition, the planar supercapacitors showed supe-rior flexibility and robust cyclability with a capacitance decay of only 10% after 1000 times of folding/unfolding. Later, Zhang et al.[259] reported highly flexible, all-solid-state ASCs based on MnO2/graphene as the positive electrode and CNT/graphene as the negative electrode. Their ASC devices could be cycled revers-ibly in a high-voltage region of 0–1.8 V with a maximum energy density of 31.8 W h kg−1. Recently, Zhang et al.[260] demonstrated fiber ASCs using hierarchical MnO2/graphene/carbon fiber (CF) as the positive electrode and graphene hydrogel (GH)/copper wire (CW) as the negative electrode. The as-prepared superca-pacitor was highly flexible and could be operated reversibly in a voltage window of 0–1.6 V, delivering a high areal energy density of 18.1 µW h cm−2.

5.2.3. Vanadium Pentoxide (V2O5)

V2O5 is an intercalation compound that has attracted significant attention as a potential candidate for pseudocapacitors due to its high energy density and low cost, as well as variable oxida-tion states (+2 to +5) and ease of fabrication.[261,262] Engineering V2O5 into nanostructures has been pursued to overcome the limitation of poor electrical conductivity (10−2 to 10−3 S cm−1) in V2O5 by improving the electrochemical kinetics, shortening the diffusion length of the electrolyte ions.[263,264] Lee et al.[265] synthesized V2O5 electrodes by quenching V2O5 fine powders at 950 °C into deionized water. They reported a Csp of 346 F g−1 in aqueous KCl solution. V2O5 nanofibers prepared by Srinivasan et al.[266] showed a Csp of 190 F g−1 in aqueous KCl. To further improve the capacitive properties, 3D porous V2O5 nanostruc-tures have been developed by several research groups.[267,268] For example, Zhu et al.[269] reported a freeze-drying process for the large-scale production of 3D V2O5 electrodes that achieved a high Csp of 451 F g−1. Interestingly, the 3D V2O5 renders

more than 90% capacitance retention after 4000 cycles and the energy density was reported upto 107 W h kg−1 at a power den-sity of 9.4 kW kg−1. Very recently, Liang et al.[270] developed a facile approach to fabricate ultrafine porous V2O5 nanowires on a 3D Ni foam current collector. The porous Ni base not only served as a 3D conductive framework for the large loading of active V2O5 materials but also helped V2O5 to expose a greater number of active sites for electrochemical reactions. A high capacitance of 832 F g−1 with high energy density and power densities of 115.7 W h kg−1 and 25 kW kg−1, respectively, was observed in 1 m Na2SO4 solution. Carbonaceous materials such as CNTs, graphene, carbon nanofibers, and CPs have been employed to boost the electrical conductivity, as well as loading amount of active materials.[271–274] Chen et al.[275] developed supercapacitor composites based on V2O5 nanowires interpen-etrating into conductive porous CNT scaffolds via an in situ hydrothermal process. The nanocomposite electrode achieved a capacitance upto 440 F g−1 at a current density of 0.25 A g−1. ASCs based on V2O5/CNT hybrid composites exhibited an excellent energy density of 16 W h kg−1 at a power density of 75 W kg−1. A very high capacitance (1308 F g−1) has been reported for self-standing carbon-nanofiber papers with 3 nm-thick deposited V2O5 layers through cyclic voltammetry meas-urements.[276] Xu et al.[277] utilized a hydrothermal approach to fabricate graphene/V2O5-xerogel nanocomposites. The pres-ence of the graphene improves the specific surface area of the V2O5 xerogels almost 2 times as compared to V2O5-only xero-gels, along with enhanced electrical conductivity, which in turn delivered a good capacitance of 195.4 F g−1 at a current density of 1 A g−1. Guo et al.[278] fabricated a novel electrode structure in which PEDOT was sandwiched between layered MnO2 (LMO) with layered V2O5 (LVO) into an LVO/PEDOT/LMO geometry (Figure 9a). ASCs built from an LVO/PEDOT/LMO cathode and AC anode showed an extraordinary energy density of 39.2 W h kg−1 (based on the active materials) in a

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Figure 9. a) Schematic images showing the fabrication of process of LVO\PEDOT\LMO. b) Energy and power densities of ASC. c) Camera images showing a green LED powered using two LVO\PEDOT\LMO||AC supercapacitors connected in series. d) Schematic representation for the coaxial coating of Ni(OH)2 onto the o-CNT surface. e) SEM images of the o-CNT/Ni(OH)2 composite. f) Comparison of the Csp between pure Ni(OH)2 and 0-CNT/Ni(OH)2 composite. a–c) Reproduced with permission.[278] Copyright 2015, Elsevier Publishing. d–f) Reproduced with permission.[280] Copyright 2015, Elsevier Publishing.

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Na2SO4 aqueous electrolyte (Figure 9b), and good cycle sta-bility (93.5% capacitance retention after 3000 cycles). Figure 9c shows a green LED powered by connecting two ASCs in series. Zhou et al., recently reported an all-solid-state flexible ASC with bamboo-like composites of V2O5/polyindole@activated carbon cloth (ACC) as the positive electrode while rGO@ACC was used as the negative electrode material.[279] The ACC cur-rent collector provided an excellent flexibility and high conduc-tivity, as well as a greater number of open channels for rapid ion transport. Moreover, highly conducting polyindole (PIn) was incorporated, and the V2O5/PIn@ACC electrode exhibited a high Csp of 535.5 F g−1 at 1 A g−1, much higher than that of V2O5@ACC (304.7 F g−1). The assembled ASC device exhibited an excellent cycling performance with a high retention rate of 91.1% after 5000 cycles.

5.2.4. Nickel Hydroxide (Ni(OH)2)

Nickel hydroxide Ni(OH)2 has been widely used as a posi-tive electrode material in nickel-based batteries but it is also suitable for electrochemical supercapacitors due to its high theoretical Csp (2082 F g−1), cost effectiveness, and avail-ability in various morphologies.[127,281,282] The early studies reported that hydrous Ni(OH)2 makes highly capacitive posi-tive-electrode materials with Csp as high as 1000 F g−1.[283,284] However, similar to other metal oxides, Ni(OH)2 also suffers from poor stability, low conductivity, and poor cycle perfor-mance due to the large volume change during the charging–discharging processes. Heteroatom doping of cobalt (Co) and Zinc (Zn) has been employed to enhance the electrochemical performance of Ni(OH)2.[285,286] Co helps in improving the electrical conductivity, whereas Zn introduces disordering into the Ni(OH)2 lattice, maximizing the active-material uti-lization and ion-conduction paths. For better performances, porous Ni(OH)2 structures are of immense importance as they provide short diffusion paths for ion-diffusion, high active area, and good strain accommodation during the charge–discharge process. Kong et al.[287] reported loosely packed Ni(OH)2-based nanoflakes, which allowed facile elec-trolyte penetration for significant redox reactions, delivering a high Csp of 2055 F g−1 at 0.625 Ag−1. A 3D conducting Ni foam has been widely used to directly grow 3D porous Ni(OH)2 thin films without the use of any binders.[288–290] Yang et al.[291] reported a maximum capacitance of 3108 F g−1 in electrodeposited Ni(OH)2 coatings on Ni foams in porous and 3D nanostructures. However, the large weight of the Ni foam limits the gravimetric capacitances, and lighter conduc-tive backbone elements are required to mitigate this issue. Ruoff et al.[292] reported a binder-free composite of Ni(OH)2 with ultrathin graphite (UGF), in which the free-standing 3D UGF network offers high electrical conductivity and porosity. ASCs employing Ni(OH)2/UGF as the positive electrode and graphite oxide as the negative electrode exhibited a very high power density of 44 kW kg−1, which was much higher than those of many of the commercially available traditional super-capacitors. Su et al.[293] fabricated amorphous Ni(OH)2@3D Ni core–shell electrodes which exhibited a very high Csp of 2848 F g−1 at 1 mV s−1 and an electrochemical stability over

1000 cycles. ASCs based on amorphous Ni(OH)2@3D Ni//AC in aqueous electrolytes could be cycled reversibly in a high voltage window of 0–1.3 V, achieving Csp of 92.8 F g−1 at 1 A g−1. Yang et al.[294] fabricated 3D nanoporous Ni(OH)2 thin films that delivered a high capacitance of ≈1765 F g−1. ASC devices using porous-AC negative electrodes delivered a capacitance of 192 F g−1 with energy and power densities of 68 W h kg−1 and 44 kW kg−1, respectively.

Ni(OH)2 can also form composites with ACs, CNTs, and graphene nanosheets to enhance electrochemical perfor-mance. Tang et al.[295] reported a binder-free, nanocomposite electrode of Ni(OH)2 with CNTs grown on nickel foams. Without CNTs, these nanocomposites showed excellent capac-itive behaviors, but the low Ni(OH)2 loading of the nickel form considerably reduced the areal capacitance down to 1.6 F cm−2. The nanoarchitectured Ni(OH)2/CNT/NF electrode exhibited a high Csp of 3300 F g−1 owing to the high Ni(OH)2 loading of 4.85 mg cm−2 with a substantial improvement in areal capacitance to 16 F cm−2. In a recent study, Salunkhe et al.[280] reported a CNT/Ni(OH)2 composite where Ni(OH)2 is deposited coaxially on oxidized CNTs to achieve 3D structures for an easy access of electrolyte ions (Figure 9d). Figure 9e shows an SEM image of the 3D coaxial composite. A very high capacitance (≈1368 F g−1) was observed in comparison to bare Ni(OH)2, which exhibited a Csp of 265 F g−1 (Figure 9f). ASCs with CNT–Ni(OH)2 and rGO electrodes operated at a high voltage window of 1.8 V, with a high energy density of 35 W h kg−1 at a power density of 1.8 kW kg−1. Graphene and graphene oxide (GO) nanosheets have been employed as excel-lent 2D supports to load Ni(OH)2 for better electrochemical performance. Yan et al.[296] prepared a flowerlike Ni(OH)2 and decorated it with rGO to make positive electrodes for ASCs, and porous graphene as negative electrodes. The Ni(OH)2/graphene hybrid electrode showed a high Csp of 1735 F g−1 and the Ni(OH)2/graphene//porous graphene ASC device retained 94.3% Csp after 3000 cycles. In another effort by Wu et al.[297] a facile electrostatic-induced method was employed to grow highly crystalline and stacked Ni(OH)2 nanosheets on graphene. The as-prepared hybrid Ni(OH)2/graphene com-posite demonstrated a high Csp (1503 F g−1 at 2 mV s−1) and an excellent cycle stability up to 6000 cycles at higher scan rates. The superior performance was attributed to the intimate binding between Ni(OH)2 and graphene nanosheets with good wettability.

Amorphous Ni(OH)2 nanosphere electrodes have been explored for ASCs, which resulted in high capacitance (153 F g−1) and high energy density (35.7 W h kg−1) at a power density of 490 W kg−1.[77] Ni(OH)2 composites with other oxides have also shown intriguing electrochemical properties. Co(OH)2–Ni(OH)2 composites have been fabricated to achieve higher Csp compared to Ni(OH)2 or NiO.[298,299] Zhong et al.[300] showed that Co3O4/Ni(OH)2 composite mesoporous nanosheet networks achieved a high Csp of 1144 F g−1 at 5 mV s−1 and long-term cyclability. Ni(OH)2 composites with CPs were devel-oped to improve electrical conductivity. Yang et al.[301] fabricated NiO/Ni(OH)2 nanoflowers encapsulated in 3D PEDOT on contra wires through a mild electrochemical route. This hybrid electrode delivered a high Csp of 404.1 mF cm−2 (or 80.8 F cm−3) at a current density of 4 mAcm−2.

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6. Emerging 2D Supercapacitor Electrodes

Beyond the paradigm set by the capacitive performance of gra-phene, there has been a surge of interest in other exotic 2D materials for ASCs. In this context, inorganic 2D materials like transition-metal dichalcogenides (TMDs) are gaining sig-nificant interest owing their large surface area, unique crystal structures, and extraordinary electrochemical properties.[302–304] Molybdenum disulfide (MoS2), in particular has become one of the promising materials, owing to its large electrical double-layer capacitance (EDLC) and pseudocapacitance with varying Mo oxidation states (+2 to +6).[305–307] The presence of dif-ferent MoS2 polytypes, i.e., hexagonal 2H (semiconducting), 1T (metallic), and rhombohedral 3R (stable at standard con-ditions) makes it even more intriguing for ASCs. 2H and 3R phases are not preferred due to their limited electrical conduc-tivities, which generally requires electrically conducting addi-tives (CNTs, graphene, etc) or surface functionalizations.[308,309] Chhowalla et al.[310] developed supercapacitor electrodes based on mechanically exfoliated 1T MoS2 nanosheets that were about

107 times more conductive than their semiconducting counter-parts. Figure 10a shows an SEM image of the electrode based on the 1T-MoS2 nanosheets. These binder-free electrodes exhib-ited outstanding electrochemical performances, delivering a high volumetric capacitance of ≈400–700 F cm−3 (Figure 10b,c) with an excellent capacitance retention of 97% over 5000 cycles in a variety of aqueous and organic electrolytes. This superi-ority in performance was attributed to facile ion intercalation into the 1T-MoS2 layers due to their hydrophilicity coupled with their metallic characters. Choudhary et al.[314] reported supercapacitor performances of porous 3R-MoS2 in a 3D structure directly deposited on flexible substrates (i.e., copper, polyimide). These uniquely structured 3D porous materials facilitated a large intercalation of electrolyte ions and exhib-ited a high areal capacitance of 33 mF cm−2 (≈330 F cm−3) in 0.5 m H2SO4 electrolyte and a ≈97% capacitance retention over 5000 cycles. Vertically aligned MoS2 nanosheets are gaining sig-nificant interest as promising supercapacitor electrodes due to their high aspect ratio and extensively exposed sides with 2D edges having high chemical reactivity. Tour et al.[315] fabricated

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Figure 10. a) SEM image of the mechanically stacked 1T-MoS2 nanosheets. b) CV curves of 1T MoS2 electrode in 0.5 M sulphate-based electrolytes at scan rates of 20 mV s−1. c) Volumetric capacitance of 1T MoS2 as a function of scan rate in different electrolyte solutions. d) The structure of the different phases of the MAX family during MXene growth. e) SEM images of Ti3AlC2 before (untreated MAX phase) particle and after (MXene) the HF treatment. f) CV measurements for a 5 mm-thick MXene electrode at different scan rates in 1 M H2SO4. a–c) Reproduced with permission.[310] Copyright 2015, Nature Publishing. d) Reproduced with permission.[311] Copyright 2014, Wiley-VCH. e) Reproduced with permission.[312] Copyright 2012, ACS Publishing. f) Reproduced with permission.[313] Copyright 2014, Nature Publishing.

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edge-oriented/vertically aligned MoS2 nanosheets and utilized them for flexible supercapacitor electrodes, demonstrating a high areal capacitance up to 12.5 mF cm−2.

A new family of 2D materials recently developed are the ter-nary layered carbides and nitrides called MXenes with the gen-eral formula of Mn+1AXn (MAX), where M stands for early tran-sition metal, A is an element from the groups IIIA or IVA (Al, Si, Sn, In), X for carbon or nitrogen, and n = 1, 2, or 3. MXenes are derived by etching away the element A from the MAX group. Figure 10d represents the structural evolution of the different phases of a MAX family during the MXnene growth.[311] Naguib et al.[312] presented the first experimental evidence of MXene 2D layers exfoliated by the immersion of various MAX phases into hydrofluoric acid (HF). Figure 10e shows typical SEM images of a Ti2AlC2 particle before and after successful exfoliation of its MAX phase, demonstrating a nicely stacked layered struc-ture. MXenes show high metallic conductivity and hydrophi-licity analogous to graphene, and have tremendous potential for ASCs. These studies suggest the exploration of ≈60 other mate-rials belonging to the family of Mxenes. Mxenes combine the properties of ceramics and metals and exhibit high hardness and melting points, high stability, and good corrosion resistance at high temperatures. The excellent electrical conductivity and unique layered morphology of Mxenes make them particularly suitable for ASCs. Ghidiu et al.[313] synthesized Ti3C2Tx clays that could be rolled and shaped into the conductive solids of desired forms. The rolled films used as supercapacitor elec-trodes exhibited an extraordinary volumetric capacitance of 900 F cm−3 in 1 m H2SO4 (Figure 10f). Yohan et al.[316] demonstrated the influence of intercalation surface treatments on the capaci-tive performance of Ti3C2. The modified surface chemistry gen-erated oxygen-containing functional groups leading to a high capacitance of 415 F cm−3 at 5 A g−1. Delaminated MXenes have a high aspect ratio and are used as nanofillers in polymer nano-composites for flexible supercapacitors. Ling et al.[317] reported a highly flexible and conductive Ti3C2Tx/PVA-KOH composite electrode that achieved a volumetric capacitance of 530 F cm−3 at 2 mV s−1 with an electrical conductivity of 2.2 × 104 S m−1, a tensile strength of 30 Mpa, and a Young’s modulus of 3 GPa. Despite the interesting properties of MXenes, their easy oxida-tion under anodic potential in aqueous electrolytes is a potential problem that limits their further applications.

7. Devices

Generally, ASCs are made of positive pseudocapacitive elec-trodes and negative EDLC-based electrodes with electrolyte in between. In order to design ASC devices that are lightweight, robust, and reliable with exceptional functional characteristics; the configuration of the two dissimilar electrodes with tailored properties and geometries is of prime importance and must be considered seriously. In the following sections, various device configurations will be discussed.

7.1. Sandwich-Type

Sandwich-type ASCs are the most typical device configura-tion, which consists of two flat electrodes sandwiched together,

separated by liquid/gel electrolytes and an ion-porous separator material. The main benefits of this configuration are the ability to directly deposit, grow, and precisely implement different types of materials and architectures. In this section, three sub-categories of sandwich-type device configurations are described, namely devices built on carbon-cloth materials, metal scaffolds, and other conductive substrates.

7.1.1. Carbon-Cloth-Material Configuration

Being significantly low in cost, highly conductive, and mechanically flexible, carbon cloth has been widely used for ASCs.[151,318–321] Carbon cloth is compatible with various depo-sition techniques, as well as electrode materials. Zhai et al.[322] have recently reported a carbon cloth with incorporated oxygen-deficient V6O13−x nanowires as a novel anode material for ASCs. V3O7 nanowires were hydrothermally grown on carbon-cloth substrates followed by annealing in a nitrogen atmosphere in the presence of a sulfur vapor source to produce sulfur-doped V6O13−x. Likewise, the cathode also employed a carbon-cloth current collector, which served as a scaffold for rGO and electrodeposited MnO2 nanoparticles. ASC devices based on these anode and cathodes delivered an excellent volumetric energy density of 45 W h kg−1 with an average power density of 478.5 W kg−1. Another recent study reports flexible ASCs based on Co9S8 nanorods as the cathode and Co3O4@RuO2 nanosheets as the anode integrated on woven carbon-cloth sub-strates (Figure 11a).[323] KOH solutions and poly(vinyl alcohol) (PVA)/KOH were used as a liquid electrolyte and a solid elec-trolyte, respectively. The resulting ASCs showed an energy den-sity of 1.21 mW h cm−3 and power density of 13.29 W cm−3 in aqueous electrolytes.

Electrochemical deposition is another highly appreciated approach to fabricate sandwich-type ASCs on carbon mate-rials. A novel one-step electrodeposition of nickel cobalt sulfide (Ni–Co–S) onto carbon cloth exhibited an excellent Csp of 1418 F g−1 at 5 A g−1 and 1285 F g−1 at 100 A g−1 as the cathode material.[324] The schematic in Figure 11b shows this cathode along with a porous graphene film on a carbon cloth as the anode, demonstrating a typical sandwich-type configuration. ASC devices based on them demonstrated outstanding electrochem-ical performance, such as a high energy density of 60 W h kg−1 at a power density of 1.8 kW kg−1 with robust long-term cycling stability up to 50 000 cycles. These deposition techniques have been applied to fabricate more-complex nanostructured elec-trodes, such as one-dimensional core–shell nanowire electrodes. Li et al.[318] reported high-performance and flexible ASC devices by hydrothermally growing hydrogen-treated titanium dioxide (H-TiO2) nanowires as the core (conducting scaffold) on carbon cloths. An electrochemical deposition of MnO2 onto the H-TiO2 nanowires realized positive electrodes in a configuration of core–shell-structured H-TiO2@MnO2 and negative electrodes of H-TiO2@C were fabricated by another hydrothermal method. These core–shell nanowire ASC devices achieved a maximum energy density of 0.30 mW h cm−3. Physical vapor deposition (PVD) presents another route for forming high-surface-area nanostructures. Wang et al.[74] reported a WO3−x nanowire struc-ture grown radially on carbon fabrics by a catalyst-free PVD

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method in an oxygen environment. Furthermore, a pseudoca-pacitive core–shell structure was obtained by electrochemically depositing MoO3−x onto these WO3−x nanowires.

Other forms of carbon, such as carbon papers also serve as potential current collectors in ASC devices.[47,142,325,326] A flex-ible rGO/MnO2 cathode was developed by adding MnO2 pre-cursors into graphene oxide solutions followed by the vacuum filtration of the GO/MnO2 suspension, resulting in rGO/MnO2 in the form of a paper. The anode was prepared by the same method but without including MnO2 into the rGO paper mate-rial. The resulting free-standing ASC devices with large flex-ibility and high conductivity offered a large areal capacitance as high as 897 mF cm−2, which is considerably higher than that of other flexible carbon-based ASCs.[327] Similarly, SWNT membrane-based electrodes were prepared by filtering an SWNT solution through a porous alumina filter.[160] MnO2 and In2O3 nanowires were subsequently grown on the SWNT mem-brane to form MnO2 nanowire/SWNT as the positive electrode and In2O3 nanowire/ SWNT as the negative electrode. These hybrid nanostructured ASC devices exhibited a Csp of 184 F g−1, an energy density of 25.5 W h kg−1 and a power density of 50.3 kW kg−1.

7.1.2. Metal Scaffolds

Metal scaffolds refer to any type of metal foam or foil that is used as a current collector for the active material in an ASC system.[34,282,328,329] Among the many available ones, nickel is

frequently used due to its high conductivity, structural robust-ness, and low cost. A common method of loading active mate-rials onto metal scaffolds is to prepare a slurry of the materials and coat them onto the current collectors.[50] Figure 11c shows a schematic of an ASC device where a graphene/MnO2 nano-sphere cathode and a graphene/MoO3 nanosheet anode were prepared by mixing 80 wt% electroactive material with 15 wt% carbon black and 5 wt% poly(vinylidene fluoride) in the form of a slurry. The slurry of the active material was coated onto a Ni foil substrate and excess slurry was removed by heating at 120 °C for 12 h. Figure 11d,e show the HRTEM images of the active anode and the cathode materials where MnO2 nano-spheres and MoO3 nanosheets were uniformly wrapped with graphene. The ASC device delivered a high energy density of 42.6 W h kg−1 at a power density of 276 W kg−1 and a maximum Csp of 307 F g−1. In many cases, it is necessary to physically press the active materials onto the metal scaffolds to ensure good mechanical contact with the current collectors.[330–332] For example, Wei et al.[333] reported an excellent cycle life with 97.3% capacitance retention after 1000 cycles in ASCs based on graphene/MnO2 and activated carbon nanofibers with firmly pressed slurries of electrode materials. The device yielded a maximum energy density of 51.1 W h kg−1 and power density of 198 kW kg−1 with a high voltage range upto 1.8 V and a Csp of 113.5 F g−1.

Instead of the mechanical pressing, the direct growth of active materials on the metal scaffolds can make full use of their inherently high surface area.[334–341] Liu et al.[342] devel-oped a 3D CoO@polypyrrole nanowire array cathode grown on

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Figure 11. Carbon-cloth configurations. a) Schematic illustration of the as-assembled ASC on carbon cloth. The zoomed-in SEM image is for Co9S8 and Co3O4@RuO2. b) Schematic representation of a Ni–Co–S nanosheet-array cathode grown on carbon cloth sandwiched between a porous gra-phene anode on carbon cloth. c) The concept of metal-scaffold ASCs based on graphene/MnO2 nanosphere composite as the positive electrode and graphene/MoO3 nanosheet as the negative electrode. d,e) HRTEM images of graphene–MnO2 cathode (d) and graphene–MoO3 anode (e) materials. a) Reproduced with permission.[323] Copyright 2013, ACS Publishing. b) Reproduced with permission.[324] Copyright 2014, ACS Publishing. c–e) Repro-duced with permission.[50] Copyright 2013, Wiley-VCH.

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Ni foams by using a hydrothermal synthesis. This asymmetric device with AC as the anode material showed high energy den-sity of 43.5 W h kg−1 and an energy density of 11.8 W h kg−1 at a power density of 5.5 kW kg−1. A more recent device based on Ni(OH)2 directly grown on a Ni foil by using anodic elec-trodeposition and hydrothermal synthesis showed a maximum energy density of 68 W h kg−1 and a maximum power density of 44 kW kg−1, while maintaining 90% of its capacitance over 10 000 cycles.[294] Other chemical methods have also been used to directly grow electrode materials on metal-scaffold current collectors. A thermal CVD to grow CNTs on a Ni foam current collector followed by a chemical bath deposition (CBD) to coat the CNTs with Ni(OH)2 was utilized to fabricate cathode elec-trodes. When paired with an AC on a Ni foam anode, the ASC device achieved a maximum energy density of 50.6 W h kg−1.[295]

7.1.3. Other Substrates

Several other materials have been investigated as substrates to fabricate sandwich-type ASC devices including indium tin oxide (ITO),[343] conducting polymers,[201,344–347] organic materials, etc.[348,349] ITO is a notable current collector due to its high conductivity and availability. Recently, ultrathin films of poly(3,4-ethylenedioxythiophene) (PEDOT) has been prepared by electropolymerization on steel and ITO substrates, achieving a superior Coulomb efficiency of 97% and Csp of 270 F g−1.[350] Moreover, MnO2 (cathode) and polypyrrole (anode) were electrodeposited on rGO-coated ITO substrates to fabricate solid-state ASC devices. The ITO substrates were initially dipped in positively charged 1 wt% poly(ethyleneimine) (PEI) solution for 1 h to promote the adhesion of negatively charged rGO nanosheets. The rGO was then deposited onto the ITO using cyclic voltammetry followed by electrodeposition of MnO2 and polypyrrole. This ASC device based on these electrodes exhibited a power den-sity of 7.4 kW kg−1 and energy density of 16 W h kg−1.[351] Carbon nanofibers derived from bacterial-cellulose (BC) have also been employed as low-cost, abundant, and environ-mentally friendly electrodes for ASCs. Yu et al.[352] reported the annealing of BC pellicles (p-BC) at 1000 °C for 2 h in nitrogen followed by immersion in a reaction solution and to obtain p-BC@MnO2. By replacing the immersion step with a hydrothermal reduction, nitrogen-doped p-BC (p-BC/N) was also obtained. An ASC device based on this material yielded an energy density of 32.91 W h kg−1 and an outstanding power density of 284.64 kW kg−1 maintaining 95.4% of its initial capacitance after 2000 cycles.

7.2. Fiber-Type

With the increasing demand for wearable technologies, highly flexible, bendable, and twistable energy-storage devices have been explored. The planar-type supercapacitors discussed above do not offer sufficient flexibility and/or bendability required to weave them into e-textiles or wearables. In addition, planar devices will occupy more space on the human body and cause unease by blocking air flow. In this context, 1D cylindrically

shaped electrodes held together in different configurations can form a new class of fiber-type supercapacitors, which form suit-able building blocks for wearable energy textiles. These fiber-type supercapacitors include side-by-side (parallel design), twist-type, coaxial-type, and wrap-type configurations. These unique designs make the resulting textiles lightweight, allowing for their easy integration into wearable devices to achieve mul-tifunctionality, including sensing,[353–355] communication,[356,357] and storage.[358] Here, we discuss the recent development of fiber-type ASCs in various configurations.

7.2.1. Side-by-Side (Parallel Configuration)

Side-by-side fiber supercapacitors are fabricated by keeping two fiber-shaped electrodes parallel to each other, separated by a gel/polymer electrolyte and supported by a planar substrate.[359] Yu et al.[360] developed micro-sized cable-type ASCs using a nitrogen-doped rGO/SWCNT fiber as a capacitive-type fiber (anode) and MnO2-deposited rGO/SWCNT fiber as a Faradaic-type fiber (cathode). A schematic of the fabrication process and concept of the parallel configuration is demonstrated in Figure 12a. This device was capable of operating in a voltage window of 1.8 V and showed excellent cycling stability of 87% capacitance retention after 10 000 cycles with the volumetric energy and power density of 5 mW h cm−3 and 929 mW cm−3, respectively. These side-by-side ASC devices can be scaled up by incorporating more fiber electrodes on the planar sub-strate. Zhang et al.[361] reported another fiber-type asymmetric device assembled by two different functionalized carbonaceous fibers: carbon fiber (CF)@RGO@MnO2 and CF@thick rGO (TRGO). The device exhibited excellent stability and flexibility in a 1.6 V high voltage window, yielding a high volume energy density of 1.23 mW h cm−3 along with an areal energy density of 18.5 µW h cm−2.

Jin et al.[363] also developed a fiber-based micro-ASC device using carbon nanoparticle (CNP)-coated-carbon fibers (CFs) and achieved high porosity, flexibility, light weight, and simple processing. MnO2-nanosheet-grown CF@CNPs (MCNP) and functionalized CF@CNPs (FCNP) were employed as posi-tive and negative electrodes. The assembled ASC device could operate at 1.8 V, exhibiting a high volumetric energy density and power density of 2.1 mW h cm−3 and 8 W cm−3, respec-tively. A commercial LED could be powered for 2 min by three ASCs connected in series with 2-second charging.

7.2.2. Twist-Type Configuration

Due to the emerging demand for fabric electronics for var-ious applications, like sensors,[364–367] solar cells,[368–370] and batteries,[371–374] stand-alone fiber supercapacitors without any supports are desirable, as they can be easily woven into the fabric. A twist-type configuration is one such design that is based on two fiber electrodes loaded with different active materials and which are twisted together to construct a fiber-shaped ASC device. Su et al.[106] developed the first twist-type ASC by assembling as-spun CNT yarn as a negative electrode and CNT@MnO2 composite yarn as a positive electrode. This

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asymmetric twist architecture was operable upto 2.0 V and was mechanically stable even after 2000 cycles of folding and unfolding. Moreover, this ASC device resulted in high energy and power density of 42 W h kg−1 and 483.7 W kg−1, respec-tively. Another example of a twisted ASC device was developed by Jin et al.[362] They prepared carbon-fiber-thread@polyani-line as a positive electrode and functionalized carbon-fiber thread as a negative electrode (Figure 12b). An ASC device based on these electrodes achieved a volumetric energy den-sity up to 2 mW h cm−3 with a 1.6 V high voltage window. For the purpose of demonstration for practical applications, this device was woven into a glove and its mechanical flex-ibility was tested (Figure 12c). The capacitance of the stretch-able twisted ASC device remained almost unchanged even at a strain of 100% (Figure 12d). Cheng et al.[375] designed a ternary hybrid positive electrode by growing MnO2 nanosheets onto a PEDOT:Polystyrene Sulfonate (PSS)-coated CNT fiber. The ASC device developed from the twisted MnO2/PEDOT:PSS/CNT electrode with an ordered-microporous-carbon/CNT hybrid negative electrode achieved a high working voltage of 1.8 V along with a high energy density of 11.3 mW h cm−3. The presence of the good adhesive agent (i.e., PEDOT:PSS) in between the MnO2 and CNT renders an excellent cyclic stability of 85% capacitance retention after 10 000 charge–dis-charge cycles and unchanged performance after bending and releasing 5000 times.

7.2.3. Coaxial-Helix Type Configuration

ASCs based on electrodes in coaxial-helix configurations have been recently explored, where a typical wire- or fiber-type elec-trode is wrapped with another electrode resembling a core–shell configuration.[260,376,377] Dong et al.[378] recently developed a coaxial-helix ASC device by a Ni(OH)2-nanowire fiber elec-trode with an ordered-mesoporous-carbon fiber electrode. Due to the high aspect ratio of the Ni(OH)2-nanowire and high cell voltage (1.5 V), this device displayed a high Csp of 6.67 mF cm−1 (35.67 mF cm−2) and a high specific energy density of 0.01 mW h cm−2 (2.16 mW h cm−3). Wang et al.[379] integrated a coaxial-helix ASC device onto a photodetector by employing graphene as both negative electrode and light-sensitive mate-rial. The positive electrode was fabricated by growing Co3O4 nanowires on nickel fibers. More recently, Thomas et al.[380] developed a novel 2-in-1 coil-type ASC electrical cable based on the coaxially assembled electrodes, which was capable of both storing energy and transmitting electricity independently. The cathode was fabricated by growing CuO nanowhiskers (NWs) on the surface of a copper wire via air oxidation followed by MnO2 electrodeposition to form the core–shell structure. A thin layer of a AuPd current collector was sputter-coated on CuO NWs prior to MnO2 deposition. The anode was prepared by hydrothermally growing FeOOH nanorods onto carbon fibers followed by conversion of the FeOOH to Fe2O3. The fabrication

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Figure 12. a) Schematic representation of micro-supercapacitor fabrication in a side-by-side configuration. b) Schematic illustration of twisted CNT@PANI positive electrode and functionalized CNT negative electrode. c) Image of an ASC woven into a glove at 0° and 180° bending angles. d) Capaci-tance ratio of the ASC under different applied strains (0–100%) at a scan rate of 100 mV s−1. The inset shows the CV curves of the FASC measured at different applied tensile strains. a) Reproduced with permission.[360] Copyright 2014, Wiley-VCH. b–d) Reproduced with permission.[362] Copyright 2015, Elsevier Publishing.

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process of the devices is shown in the schematic in Figure 13a. The 2-in-1 ASC device possessed a maximum potential window of 1.8 V and a volumetric Csp of 1.6 F cm−3 at 0.13 A cm−3, and it could be easily bent at various angles (Figure 13b) without sacrificing much of its performance. The SEM and TEM images in Figure 13c and 13d, respectively, reveal that MnO2 was conformally deposited onto each CuO NW (precoated with metal alloy). Being able to combine the functionality of an ASC and a typical electrical cable, the unique design demonstrates a new wave of integrated electrical components.

7.2.4. Wrap-Type Configurations

Wrap-type ASCs are designed by wrapping up two electrodes together in parallel with nonconducting polymer electrolytes.

With this design, a good protection has been provided for ASC devices by avoiding any direct damage to the outer electrode. Selvan et al.[381] recently demonstrated a wire-type ASC utilizing a stainless-steel and a copper wire as current collectors. The cathode materials were prepared by coating the copper wire in β-Ni(OH)2 while the anode (stainless steel) was coated with AC. The as-prepared electrodes were coated with a PVA–KOH gel electrolyte and subsequently inserted into a rubber tube, which creates a cable-shaped ASC as shown in Figure 13e. The electrochemical reliability of the device was tested under bending conditions. Figure 13f shows that the CV curves of the wrap-type ASC remained unchanged irrespective of mechanical bending. This device exhibited a maximum potential window of 1.4 V and a gravi-metric capacitance of 37.5 F g−1 at 2 mA, delivering a max-imum energy density of 9.8 W h kg−1 at a power density of

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Figure 13. a) Reconstructed 3D image of coaxial-helix ASC. b) Camera images showing different bending states. c,d) SEM (c) and TEM (d) images showing conformal growth of MnO2 on CuO NWs. e) Schematic showing the steps involved in making the wrap-type device. f) CV curves of the device under normal and bending states. a–d) Reproduced with permission.[380] Copyright 2015, Wiley-VCH. e,f) Reproduced with permission.[381] Copyright 2014, RSC Publishing.

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154 W kg−1. Liu et al.[382] designed another wrap-type ASC employing MnO2 and V2O5 as two electrode materials. The device was packed with a piece of transparent silicon tube that wrapped the two electrodes together. Recently, Cai et al.[383] demonstrated an ASC device by packing two elec-trodes into poly(tetrafluoroethylene) (PTFE) tubes, where graphene fiber (GF)/NiCo2S4 and GF worked as the positive and negative electrodes, respectively. At a voltage of 1.5 V, a maximum energy density of 12.3 mW h cm−3 and power den-sity of 1.6 W cm−3 were reached. Yu et al.[384] demonstrated an ASC device wrapping two carbon-fiber electrodes with a PVA/LiCl solid electrolyte. They used MnO2 nanosheets arrays on carbon fiber prepared by a hydrothermal method as the positive electrode and graphene/carbon fibers prepared by the dip-coating method as the negative electrode. This ASC device exhibited a maximum operational voltage window of 1.5 V, a high Csp of 87.1 F g−1, and a promising energy density of 27.2 W h kg−1. Table 1 summarizes the various parameters of the ASC devices based on the different configurations of the electrode materials discussed here.

8. Conclusions and Outlook

We have extensively reviewed various materials developed for positive and negative electrodes in ASCs and compared their advantages and disadvantages. These materials include metal oxides, conductive polymers, traditional carbonaceous materials, and recently developed carbon-based nanomate-rials, such as CNT and graphene. Metal oxides such as MnO2, RuO2, Fe3O4, and V2O5 have made significant progress as both the anode and cathode materials with reasonably high specific capacitance, environmental friendliness, and abun-dance. However, their poor electrical conductivity undermines their cyclic stability and power density. Conductive polymers present added advantages of enhanced conductivity, flex-ibility, and ease of processing, while their volume expansion/shrinking during the charge–discharge process limits their cyclic stability. Recently developed carbon-based nanomaterials such as CNT and graphene present unprecedented opportuni-ties owing to their unique structures. CNTs have been widely explored owing to their extremely high electrical conduc-tivity and fast ion-transport properties, but their integration with commercial processing and high cost associated with their purification remain challenging. Graphene-based mate-rials are expected to offer distinct advantages owing to their unique combination of excellent mechanical, electrical, and thermal properties. 2D materials such as TMDs and MXenes have demonstrated remarkable potential exhibiting large-surface areas for EDLC, and the presence of multiple oxida-tion states for pseudocapacitance. The main drawback is their inferior electrical conductivity and their tendency to re-stack via van der Waals forces, which impedes the ion transport. With significant progress in improving the quality of electrode materials, there has been a parallel research interest in devel-oping ASC devices with enhanced properties and novel multi-functionalities, such as lightweight, ultrathin, highly flexible, portable/wearable energy-storage devices. Several approaches to design ASC devices in more-efficient configurations have

been reviewed and novel fiber-type ASC devices are particu-larly highlighted. With the advent of miniaturized/flexible electronics, these devices are anticipated to be very promising as they can be easily knitted into fabrics/textiles, presenting excellent wearability.

In the present age of electric and hybrid vehicles, smart electricity grid systems, and advanced consumer electronics, supercapacitors can play a crucial role in providing the required high energy and power density for these systems. Currently, supercapacitors play a supporting role in many of these applications, basically because of their inferior energy density. Li-ion batteries can provide an energy-storage capacity of more than 200 W h kg−1, whereas that for normal symmetric supercapacitors ranges only 5–10 W h kg−1. However considerable research progress in ASCs has wit-nessed the accomplishment of an energy density of about 40 W h kg−1, making them prospective energy-storage sys-tems next to Li-ion batteries. It is our outlook that the next-generation supercapacitors could be able to provide an energy density close to that of thin-film batteries (about 100 W h kg−1) soon. However, significant efforts are needed to compete or outperform existing or newly developed Li–air batteries, as their research is simultaneously growing. In addition to the large operational voltage window in ASCs, the development of new electrode materials with rational designs/superior architectures is required to achieve highly enhanced surface area, good electronic conductivity, and better ion-transport pathways to further accomplish better energy density. The combination of different capacitive materials in the form of composites should be a viable approach to achieve enhanced specific surface area, high porosity, large potential window, and superior electron/ion conductivity through synergistic effects. However, a deep understanding of these synergisitic effects/material interactions and optimization of parameters like composition of constituent materials and mass loading of active materials is required to ensure real capacitive per-fromances. In this context, composites or hybrid electrode designs made by employing 1D, 2D, and 3D nanostruc-tured materials like nanospheres, nanowires/nanorods, and nanoaerogels would be more intriguing as nanomaterials pos-sess high surface area, provide short diffusion paths for ions/electrons, and offer more electrochemically active sites for large pseudocapacitance. However, integration of electrodes on metal current collectors requires mechanically/chemi-cally stable interfaces (ideally, binder-free), which will ensure long cycle life, as well as high electronic/ionic conductivities. The advancement of novel 2D nanomaterials like TMDs[388] and MXenes is greatly expected to reshape the entire energy research, provided their stability issues and high produc-tion costs are resolved. Another promising direction in ASC research would be their integration with electrical-energy transmission[389] or energy-harvesting devices like solar cells.[390] These self-powered multifunctional systems should find niche applications in wearable technologies. Moreover, the fabrication of high-performance ASCs from laboratory research to industrial-scale fabrication is highly desirable. We sincerely hope that the existing and future endeavors in ASCs can accomplish the ambitious goal of achieving energy den-sity close to thin-film batteries.

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Table 1. Performance of ASCs in various device configurations.

Device Configuration

Category Electrode Operating Voltage [V]

Device performance

Ref.

Anode Cathode Energy density Power density

Sandwitch type Carbon-based VOS-NWs on carbon cloth MnO2@graphene 1.8 0.87 mW h cm−3 9 mW cm−3 [322]

Co3O4@RuO2 nanosheet Co9S8 nanorod

on carbon cloth

1.6 1.44 mW h cm−3 0.89 W cm−3 [323]

Oxygen-deficient Fe2O3 nanorods MnO2 on carbon cloth 1.6 0.41 mW h cm−3 0.1 W cm−3 [145]

Porous graphene on carbon cloth Ni–Co–S on carbon cloth 1.8 60 W h kg−1 1.8 kW kg−1 [324]

H-TiO2@carbon cloth Core–shell structures

H-TiO2@ MnO2

1.8 59 W h kg−1 45 kW kg−1 [318]

rGO paper rGO@ MnO2 1.5 11.5 µW h cm−2 3.8 mW cm−2 [327]

In2O3 NWs@SWNTs MnO2 NWs@SWNTs 2.0 25.5 W h kg−1 50.3 kW kg−1 [160]

Ni(OH)2 on carbon fiber CNT on carbon fiber 1.4 41.1 W h kg−1 1.4 kW kg−1 [321]

Ppy carbon fiber paper rG @cMWCNT 1.6 28.6 W h kg−1 15.1 kW kg−1 [325]

AC nanofibers Graphene/MnO2 Slurry

pressed on nickel foam

1.8 51.1 W h kg−1 198 kW kg−1 [333]

Porous graphene Ni(OH)2/graphene slurry on

nickel foam

1.6 77.8 W h kg−1 174.7 kW kg−1 [296]

Activated polyaniline-derived

carbon

Ni–Co oxide nanosheets

slurry on nickel gauze

1.6 41.6 W h kg−1 16 kW kg−1 [331]

AC Ni@CoO@Ppy on nickel

foam

1.8 11.8 W h kg−1 5.5 kW kg−1 [342]

Porous activated carbon Ni(OH)2 grow on nickel foam 1.6 68 W h kg−1 44 kW kg−1 [294]

AC Ni(OH)2@CNT grow on

nickel foam

1.8 50.6 W h kg−1 95 kW kg−1 [295]

Other substrates Ppy@rGO MnO2@rGO

on PEI/ITO substrate

1.8 16 W h kg−1 7.4 kW kg−1 [351]

PEDOT@Au@PEN PANI@Au@PEN 1.6 9 mW h cm−3 2.8 mW cm−3 [201]

Ni@AC@filter paper Ni@MnO2@filter paper 2.5 0.78 mW h cm−3 2.5 mW cm−3 [385]

Ppy@rGO MnO2@carbon nanofiber on

bacterial-cellulose substrate

2.0 32.91 W h kg−1 284.64 kW kg−1 [352]

Fiber-type Side-by-side N-doped rGO@SWCNT fiber MnO2@rGO@SWCNT fiber 1.8 5 mW h cm−3 929 mW cm−3 [360]

Graphene–CNT hybrid fiber MnO2@core–sheath gra-

phene fiber

1.6 11.9 µW h cm−3 _____ [359]

CF@thick rGO CF@rGO@MnO2 1.6 1.23 mW h cm−3 0.27 W cm−3 [361]

CF@CNPs MnO2@CF@CNPs 1.8 2.1 mW h cm−3 8 W cm−3 [363]

Twist-type CNT yarn MnO2@CNT composite yarn 2.0 42 W h kg−1 483.7 W kg−1 [106]

Carbon-fiber thread (CFT) CFT@polyaniline 1.6 2 mW h cm−3 11 W cm−3 [362]

Porous carbon@CF Copper hexacyanoferrate@CF 2.0 10.6 W h kg−1 50.6 W kg−1 [386]

Microporous carbon@CNT MnO2@PEDOT:PSS@CNT 1.8 11.3 mW h cm−3 0.03 W cm−3 [375]

Co-axial helix type Mesoporous carbon fiber Ni(OH)2 nanowire fiber 1.5 2.16 mW h cm−3 1.6 W cm−3 [378]

Graphene Co3O4 NW on nickel fiber 1.5 0.62 mW h cm−3 1.47 W cm−3 [379]

rGO CNT@Ni(OH)2 1.8 45 W h kg−1 1.8 kW kg−1 [280]

Fe2O3@carbon fiber CuO NW on copper wire 1.8 0.85 mW h cm−3 0.1 W cm−3 [380]

Wrap type Activated carbon@stainless steel β-Ni(OH)2@Cu wire 1.4 9.8 W h kg−1 154 W kg−1 [381]

V2O5&PANI@CF MnO2@CF 2.0 0.34 mW h cm−2 1.5 mW cm−2 [382]

GF-802 Graphene fiber@NiCo2S4 1.5 12.3 mW h cm−3 1.6 mW cm−3 [383]

GH/CW MnO2@graphene/CF 1.6 0.63 mW h cm−3 0.2 W cm−3 [260]

graphene@CF MnO2@CF 1.5 27.2 W h kg−1 979.7 W kg−1 [387]

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AcknowledgementsJ.T. acknowledges the National Science Foundation (CAREER: ECCS-1351757) for partial financial support. Y.J. acknowledges the financial support from a start-up fund from the University of Central Florida.

Received: October 3, 2016Revised: November 27, 2016

Published online: February 28, 2017

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