12576 | Chem. Commun., 2017, 53, 12576--12579 This journal is©The Royal Society of Chemistry 2017

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53, 12576

Highly efficient hydrogen evolutionelectrocatalysts based on coupled molybdenumphosphide and reduced graphene oxide derivedfrom MOFs†

Ji-Sen Li, ab Jing-Quan Sha, b Bing Dub and Bo Tang *a

A coupled hybrid of molybdenum phosphide (MoP) and reduced

graphene oxide has been prepared for the first time utilizing

Mo-MOFs as precursors through a facile method. The nanocompo-

site exhibits superior electrocatalytic performance towards the HER,

and is one of the best high-performance MoP-based electrocatalysts

under acidic conditions reported so far.

Hydrogen, as the cleanest and a renewable form of energy, isthe most ideal alternative to fossil fuels due to zero CO2

emissions and high gravimetric energy density.1–4 In terms ofsustainable hydrogen generation, electrochemical water-splittinghas attracted major attention worldwide owing to its environ-mentally friendly features.5–10 Hitherto, Pt has been consideredas the state-of-the-art electrocatalyst for the hydrogen evolutionreaction (HER) because of its superior electrocatalytic performance.Unfortunately, its large-scale application has been substantiallyhindered owing to high cost and scarcity.11–17 Therefore, the searchfor inexpensive and Earth-abundant HER catalysts to replace Ptis of great urgency but still remains a considerable challenge.

Molybdenum phosphide (MoP), as a highly efficient electro-catalyst, has recently received great interest because of its Pt-likeelectronic structure.18–21 Nonetheless, during preparation and long-term storage, MoP usually suffers from degradation of catalyticperformance due to aggregation and surface oxidation.22,23 Thus, toenhance the catalytic activity and stability of MoP, conductivematerials such as graphene,22–25 porous carbon26–31 and so on,32

have been introduced into MoP-based catalysts for manycatalytic reactions.

In recent years, metal–organic frameworks (MOFs), as novelporous materials, have become a hot topic because of theirstructural diversity.33,34 In particular, when utilizing MOFs asprecursors/templates, MOF-derived nanomaterials have beenapplied extensively to clean energy storage and conversionsystems.35–45 Despite great progress, to the best of our knowl-edge, until now the rational design and fabrication of MoP-based electrocatalysts using Mo-based MOFs (Mo-MOFs) asprecursors has not been reported. Consequently, we anticipatethat MoP nanoparticles (NPs) can be obtained through facilecarbonization of Mo-based MOFs followed by phosphidation.Meanwhile, the organic ligands in the MOFs could offer carbonand heteroatom sources for the formation of heteroatom-dopedcarbon layers, which may effectively inhibit agglomeration andsurface oxidation of MoP NPs, further boosting their catalyticbehavior and durability.

In this work, we first prepare a coupled hybrid composedof an N,P co-doped carbon/reduced graphene oxide-wrappedMoP (denoted as MoP@NPC/rGO) hybrid by directly mixingMo-MOFs with graphene oxide (GO) followed by carbonizationand phosphidation processes, respectively. Interestingly, dueto the synergetic contributions of MoP, NPC, and rGO, theMoP@NPC/rGO nanocomposite demonstrates excellent HERactivity, which is even comparable to that of the best MoP-based electrocatalysts reported to date.

The synthetic process of the MoP@NPC/rGO nanocompositeand the corresponding characterization are indicated in Scheme 1and Fig. S1 (ESI†), respectively. Fig. 1a and b show transmissionelectron microscopy (TEM) images of MoP@NPC/rGO, clearlyrevealing that the rod-like product consists of small NPs andthe surface is coated with a thin layer of wrinkled rGO. The high-resolution TEM (HRTEM) image of MoP@NPC/rGO is presentedin the inset of Fig. 1a, in which clear lattice fringes with ad spacing of 0.32 nm are detected, corresponding to the (001)plane of the MoP NPs. Noticeably, it was found that the MoP NPsare surrounded by carbon layers, which could effectively protectthe MoP NPs from agglomeration or dissolution.39 In addition,a great deal of pores (yellow arrow) are obviously observed in

a College of Chemistry, Chemical Engineering and Materials Science, Collaborative

Innovation Center of Functionalized Probes for Chemical Imaging in Universities

of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of

Education, Shandong Provincial Key Laboratory of Clean Production of Fine

Chemicals, Shandong Normal University, Shandong 250014, China.

E-mail: tangb@sdnu.edu.cnb Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of

Chemistry and Chemical Engineering, Jining University, Qufu, 273155, Shandong,

P. R. China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cc06660k

Received 25th August 2017,Accepted 27th October 2017

DOI: 10.1039/c7cc06660k

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the MoP@NPC/rGO nanocomposite, which is beneficial fordecreasing the diffusion resistance and improving the HERactivity.12,28 The scanning TEM (STEM) and correspondingenergy dispersive X-ray spectroscopy (EDX) elemental mappingshown in Fig. 1c suggest the presence of C, N, Mo, and P. Inparticular, Mo and P elements are uniformly distributed withinthe MoP@NPC/rGO composite.

In contrast experiments, MoO2@NC/rGO and MoP@NPCwere also prepared under identical conditions without phos-phidation and GO, respectively, and characterized by TEM andHRTEM in detail (Fig. S2, ESI†).

Fig. 2a shows the powder X-ray diffraction (PXRD) pattern ofMoP@NPC/rGO, where seven distinct peaks at around 27.91,32.01, 43.11, 57.11, 64.91, 67.61, and 74.11 are indexed to the(001), (100), (101), (110), (111), (102), and (201) planes of MoP

(JCPDS, No. 65-6487), respectively. The weak peak at 22.51 couldbe ascribed to carbon. The PXRD profile of MoP@NPC isdepicted in Fig. S3a (ESI†), which is similar to that ofMoP@NPC/rGO. For MoO2@NC/rGO, all visible peaks belongto MoO2 (JCPDS, No. 76-1807). As observed from Fig. S4 (ESI†),the pore size distribution obtained from the density functionaltheory (DFT) method is mainly centered in the range from 5 to35 nm, characteristic of the hierarchical mesoporous structure.These results are in good agreement with the data obtainedfrom the TEM images. The Raman spectra of MoO2@NC/rGOand MoP@NPC/rGO demonstrate two characteristic peaks at1348 and 1590 cm�1 (Fig. 2b), corresponding to D and G bands,respectively. Compared to that of MoO2@NC/rGO, the ID/IG

value of MoP@NPC/rGO is higher, suggesting that more defectswere introduced into the MoP@NPC/rGO nanocomposite afterphosphidation.23,45 Inductively coupled plasma-atomic emissionspectrometry (ICP) analysis for the as-synthesized MoP@NPC/rGO reveals that the atomic ratio of Mo and P is about 1.29 : 1.X-ray photoelectron spectroscopy (XPS) analysis was appliedto probe the valence state and elemental composition ofMoP@NPC/rGO. From Fig. S5a (ESI†), the XPS survey spectrumclearly shows that the MoP@NPC/rGO catalyst is composed ofC, N, O, P, and Mo elements. The high-resolution C 1s spectrumindicates three peaks of C–C/CQC (284.6 eV), C–P (285.1 eV),and C–N (286.2 eV) (Fig. 2c).27,45 The XPS N 1s spectrum can befitted into the three separated peaks at 398.1, 399.6, and401.8 eV, accompanied by pyridinic-N, pyrrolic-N, and graphitic-N,respectively (Fig. 2d).30,42 As can be seen from Fig. 2e, the two peaksat 129.5 and 130.4 eV in the P 2p spectrum are indicative of thebinding energies of P 2p1/2 and P 2p2/3, respectively, along with the

Scheme 1 Preparation process of the MoP@NPC/rGO nanocomposite.

Fig. 1 (a and b) TEM and (c) STEM images and the corresponding EDXelemental mapping of C, N, Mo, and P of MoP@NPC/rGO. Inset in (a): theHRTEM image of MoP@NPC/rGO. The yellow arrows show the existenceof pores in MoP@NPC/rGO.

Fig. 2 (a) PXRD pattern of MoP@NPC/rGO. (b) The Raman spectraof MoO2@NC/rGO and MoP@NPC/rGO. High resolution XPS spectraof (c) C 1s, (d) N 1s, (e) P 2p, and (f) Mo 3d of MoP@NPC/rGO, respectively.

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additional peaks with a high BE of 133.9 and 134.9 eV attributed toP–C and P–O, respectively.21,28 Remarkably, the presence of C–N,C–P, and P–C is indicative of the successful doping of N and P intothe catalyst. Also, the high-resolution XPS spectrum in the Mo 3dregion demonstrates that the two double peaks at 228.3 and231.5 eV are assignable to MoP (Fig. 2f).23,25,30 Nevertheless,due to the surface oxidation, the remaining two doublets at228.69/232.29 eV (MoIV 3d5/2/3d3/2) and 233.02/235.74 eV(MoVI 3d5/2/3d3/2) are attributable to MoO2 and MoO3, respectively,which are unfavorable to the HER activity.18,20 For comparison,MoO2@NC/rGO and MoP@NPC were also characterized by XPSand the corresponding data are analogous to that ofMoP@NPC/rGO (Fig. S5–S7, ESI†).

The electrocatalytic activity of MoP@NPC/rGO for the HERin N2-saturated 0.5 M H2SO4 was investigated using a three-electrode system with a scan rate of 2 mV s�1. MoO2@NC/rGO,MoP@NPC, and commercial Pt/C (20 wt% Pt on carbon black fromJohnson Matthey) were also evaluated under identical conditions asreferences. In this work, all polarization curves without IR com-pensation are shown in Fig. 3a and all potentials are reported vs. areversible hydrogen electrode (RHE). As expected, the commercialPt/C demonstrates the best catalytic performance with an onsetoverpotential of about 0 mV. In contrast to MoO2@NPC andMoP@NPC, the MoP@NPC/rGO catalyst shows a prominent HERactivity with a small onset overpotential of 66 mV. Moreover, it isnoteworthy that the cathodic current density increases sharply withthe increase in the overpotential. Particularly, to derive the currentdensity of 10 mA cm�2, MoP@NPC/rGO only needs an over-potential of 218 mV, much lower than that of MoO2@NPC(590 mV) and MoP@NPC (401 mV), respectively.

To gain further insight into the HER mechanism, the catalytickinetics of the HER was investigated by extracting the slopes fromthe Tafel plots. Fig. 3b displays the corresponding Tafel plots of

different samples in 0.5 M H2SO4, respectively. Impressively, theTafel slope of MoP@NPC/rGO towards the HER is measured to be57.0 mV dec�1, characteristic of faster electron transport in theHER process. That is, the hydrogen evolution on the MoP@NPC/rGO electrode occurred via a typical Volmer–Heyrovsky mecha-nism, where the Volmer step is the rate-limiting step.5,45 In sharpcontrast, the Tafel slopes of MoO2@NC/rGO and MoP@NPC are98.3, and 136.5 mV dec�1, respectively. To our knowledge, theelectrocatalytic activity of MoP@NPC/rGO is superior to that of themost commonly reported MoP-based HER catalysts in acidicsolutions so far (Table S1, ESI†),20,23,25,31 especially for the valuesof the Tafel slope. Accordingly, this result undoubtedly validatesthat the introduction of GO and phosphidation are key to theexcellent electrocatalytic performance for the HER. Besides, theelectrochemically active surface area (EDSA) measured usingcyclic voltammetry (CV) was used to unravel the origin of theHER behavior (Fig. 3c). In comparison with MoO2@NC/rGO(0.96 mF cm�2) and MoP@NPC (0.55 mF cm�2) (Fig. S8, ESI†),the double-layer capacitance (EDLC, Cdl) of 3.13 mF cm�2 forMoP@NPC/rGO is much larger, implying more exposed activesites in the catalyst. Moreover, the turnover frequency (TOF) ofMoP@NPC/rGO is 0.031 s�1 at an overpotential of 150 mV, whichis larger than that of MoP@NPC (0.014 s�1).

Apart from the catalytic performance, durability is alsoan important parameter for the application of HER electro-catalysts. Long-term stability was determined in 0.5 M H2SO4 bycontinuous CV. As depicted in Fig. 3d, the polarization curveshows negligible difference after 1000 cycles. Meanwhile, thestructure and morphology of MoP@NPC/rGO were also investigatedin detail (Fig. S9, ESI†). In parallel, the time-dependent currentdensity curve under a constant overpotential of 218 mV (inset ofFig. 3d) shows that MoP@NPC/rGO could maintain an outstandingelectrocatalytic activity for at least 10 h. This could be associated withthe overcoating of NPC and rGO, which can protect MoP NPs againstcorrosion and oxidation by air.

In addition, the effect of phosphidation temperature onHER behavior was also examined in detail (Fig. S10–S14, ESI†).As a result, 750 1C was selected as the optimal phosphidationtemperature in this work.

Taking into account the above-mentioned facts, the excellentHER performance of the MoP@NPC/rGO catalyst could berationalized as follows: (1) the MoP crystalline phase exhibitsan outstanding electrocatalytic activity towards the HER;18,25,26

(2) rGO could increase the electrical conductivity, and provide alarger active surface area, further boosting the HER activity.24,25

Also, introducing rGO into MoP@NPC/rGO plays a pivotalrole in the formation of an interconnected network structure,beneficial for electron transport and decreasing the internalresisitance.46 (3) The existence of pores is favorable to moreexposed active sites and facilitates sufficient transport of reac-tants and products.32 Just as importantly, the presence of thecarbon layer can not only prevent MoP NPs from agglomeratingand corroding,30 but also promote electron penetration fromMoP to rGO. (4) Last but not least, the co-doping of hetero-atoms (N,P) can provide additional active sites, which furtherimproves the activity for the HER.13,25 Overall, taking advantage

Fig. 3 (a) Polarization curves and (b) the corresponding Tafel plots fordifferent samples in 0.5 M H2SO4. (c) The CVs of MoP@NPC/rGO withdifferent rates from 20 to 200 mV s�1. Inset: The capacitive current at�0.009 V as a function of the scan rate for MoP@NPC/rGO. (d) Polariza-tion curves for MoP@NPC/rGO initially and after 1000 CV cycles. Inset:The time-dependent current density curve for MoP@NPC/rGO under aconstant overpotential of 218 mV for 10 h.

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of the synergetic effects of MoP, NPC, and rGO, the hybrid hasbeen considered as a highly efficient and stable electrocatalysttowards the HER.

In summary, we elaborately fabricate a novel nanocompositeMoP@NPC/rGO using Mo-MOFs as precursors by a facile methodfor the first time. Benefiting from the synergetic contributions ofMoP, NPC, and rGO, the MoP@NPC/rGO catalyst exhibits remark-able HER activity with a lower onset overpotential, a smaller Tafelslope and long-term stability, and is one of the best highlyefficient MoP-based electrocatalysts reported so far. Impressively,this work not only endows us with a highly efficient electrocatalystfor the HER, but paves a new avenue for exploring advanced androbust non-precious metal catalysts based on MOFs for energyconversion systems.

This work was financially supported by the 973 Program (No.2013CB933800), the National Natural Science Foundation ofChina (No. 21535004, and 21390411), the China PostdoctoralScience Foundation (No. 2016M602178), the Natural ScienceFoundation of Shandong Province (No. ZR2014BQ037), theProject of Shandong Province Higher Educational Science andTechnology Program (No. J16LC02), the Foundation of BasicDiscipline Construction and Theoretical Research Center ofJining University (2017JCXK005), and the Talent Team CulturingPlan for Leading Disciplines of University in Shandong Province.

Conflicts of interest

There are no conflicts to declare.

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