Nano Energy 26 (2016) 603–609

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Nano Energy

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journal homepage: www.elsevier.com/locate/nanoen

Vanadium carbide nanoparticles encapsulated in graphitic carbonnetwork nanosheets: A high-efficiency electrocatalyst for hydrogenevolution reaction

Xiang Peng a,1, Liangsheng Hu b,1, Lei Wang c, Xuming Zhang a,d, Jijiang Fu d, Kaifu Huo c,n,Lawrence Yoon Suk Lee b, Kwok-Yin Wong b,n, Paul K. Chu a,n

a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, Chinab Department of Applied Biology and Chemical Technology and the State Key Laboratory of Chirosciences, The Hong Kong Polytechnic University, Hung Hom,Kowloon, Hong Kong, Chinac Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, Chinad The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China

a r t i c l e i n f o

Article history:Received 5 May 2016Received in revised form6 June 2016Accepted 11 June 2016Available online 14 June 2016

Keywords:Vanadium carbideHydrogen evolution reactionElectrocatalysisHierarchical nanosheetsMagnesium thermic reaction

x.doi.org/10.1016/j.nanoen.2016.06.02055/& 2016 Elsevier Ltd. All rights reserved.

esponding authors.ail addresses: kfhuo@hust.edu.cn (K. Huo),in.wong@polyu.edu.hk (K.-Y. Wong), paul.chu@ese authors contributed equally to this work

a b s t r a c t

A hierarchical nanosheet structure comprising isolated vanadium carbide nanoparticles encapsulated ina highly conductive mesoporous graphitic carbon network (VC-NS) is synthesized by a hydrothermalreaction and subsequent low-temperature magnesium thermic reaction. It has a large specific surfacearea and boasts highly efficient HER (hydrogen evolution reaction) activity such as very small over-potential, fast proton discharge kinetics, and excellent durability. The small Tafel slope of 56 mV dec�1

with a low overpotential of only 98 mV at 10 mA cm�2 is quite close to that of the commercial 20% Pt/Ccatalyst. The excellent durability is indicated by the overpotential shift of only 10 mV after 10000 cyclicvoltammetric cycles at a current density of 80 mA cm�2. The high-performance precious-metal-freeelectrocatalyst is promising in HER and related energy generation applications.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

The hydrogen evolution reaction (HER) produces clean andrenewable hydrogen as an alternative to fossil fuel and an efficientelectrocatalyst is the key to accomplishing high electricity-to-hy-drogen conversion efficiency [1–3]. At present, precious metalsdeliver the most efficient electrocatalytic activity in HER, espe-cially platinum (Pt) which boasts a small overpotential and longlife time. However, the high cost and small abundance of Pt havehampered more widespread application to HER and hence, pre-cious-metal-free alternatives such as transition metal carbideshave aroused interest since they often share similar electronic andcatalytic properties with the Pt-group metals [4,5].

Transition metal (groups IVB–VIB) carbides, particularly tung-sten and molybdenum carbides, have been extensively used inHER [5–7]. For instance, Ham et al. have reported that tungstencarbide microspheres exhibit an onset potential of 0.05 V versusthe reversible hydrogen electrode (RHE) with a Tafel slope of

cityu.edu.hk (P.K. Chu)..

118 mV dec�1 [8]. Leonard et al. have demonstrated that β-Mo2Chas high electrocatalytic activity and stability in HER in an acidicsolution showing a Tafel slope of 120 mV dec�1 [9]. With regard toreduction of the HER overpotential, there are very few reports onusing group VB metal carbides, especially vanadium carbide (VC),which have higher natural abundance and cheaper than preciousmetals [3,10]. Recently, theoretical density-functional calculationpredicts that hybridization between V atoms and p-orbitals of Catoms would yield metallic properties such as low electrical re-sistivity (comparable to Mo2C) and similar hydrogen binding en-ergy as Mo2C and WC [11–15]. More importantly, the smallerdensity of VC compared to Mo2C and WC offers a larger exposedsurface area for the same mass loading and particle size, thusmaking VC a suitable candidate as an electrocatalyst in HER.

Unfortunately, precious-metal-free electrocatalysts are prone tocorrosion attack in acidic media during HER resulting in lowdurability [16–18]. Recently, construction or encapsulation ofelectrocatalysts in carbon nanotubes (CNTs)[17–19], graphene[20,21], and other carbon-based materials [22–26] has been de-monstrated to mitigate corrosion in acidic media thus improvingthe long-term activity and electrocatalytic activity as a result of theimproved electrical contact between the active electrocatalystwith the conductive carbon substrate. However, it is not easy to

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Scheme 1. Schematic illustration of the synthesis of hierarchical VC-NS and VC-P for enhanced hydrogen evolution reaction activity.

X. Peng et al. / Nano Energy 26 (2016) 603–609604

encapsulate electrocatalysts in CNTs and multi-step procedures arenormally required.

Herein, hierarchical nanosheets (NS) composed of VC nano-particles encapsulated in highly conductive mesoporous graphiticcarbon network (VC-NS) are synthesized by a hydrothermal re-action followed by the low-temperature and environmentallyfriendly magnesium thermic reduction (MTR) using V2O5 na-nosheets as both the template and precursor, as schematically il-lustrated in Scheme 1. Low-temperature MTR is frequently utilizedto synthesize carbides (especially SiC) without involving hightemperature and costly carbonaceous gases [27,28]. Moreover, inMTR, carbides converted from the corresponding oxides retain thestructural regularity [29] and V2O5 nanosheets have been preparedby the traditional hydrothermal reaction [30]. During the MTRprocess, Mg is incorporated into V2O5 which is reduced to highlyactive metallic vanadium while CO2, the thermic decomposedproduct of NaHCO3, is converted to carbon. Carbon is deposited onthe surface of the nanosheets forming a conductive carbon net-work on the template of V2O5 nanosheets and then VC is formedby the reaction between highly active metallic vanadium andcarbon. The detailed formation mechanism of the hierarchical VC-NS is discussed in the supporting information (Figs. S1, S2). Thetwo-dimensional (2D) hierarchical nanosheets composed of na-noparticles encapsulated in the conductive network with a largespecific surface area provide a large number of exposed activesites, fast electron transport paths, and good electrical contactwith the active sites to facilitate HER. As a result, the hierarchicalVC-NS electrocatalyst delivers outstanding HER performance in-cluding very small overpotential, fast proton discharge kinetics,stability, as well as durability, suggesting promising application toHER and related energy generation schemes.

2. Experimental section

2.1. Samples preparation

The V2O5 nanosheets were synthesized by the procedures de-scribed in our previous paper [30]. In brief, 2 mmol of NH4VO3powders (Sigma-Aldrich) were mixed with 39 mL of deionizedwater (DW) under stirring and 1 mL of concentrated HCl (Sigma-Aldrich, Z37%) was added to the mixture dropwise and stirred for30 min before transferring to 50 mL Teflon-line autoclave. Thereaction proceeded at 200 °C for 1 h and the V2O5 gel was freeze-

dried for more than 48 h. The VC nanosheets were synthesized bylow-temperature MTR of the as-prepared V2O5 �nH2O nanosheets(V2O5-NS). 182 mg of the as-prepared V2O5-NS, 336 mg of sodiumbicarbonate (Sigma Aldrich), and 300 mg of magnesium powders(Sigma Aldrich) were mixed and sealed inside a stainless steelsealed can which was heated at 700 °C for 5 h and then cooled toroom temperature (RT) in a tube furnace filled with Ar. Thereafter,the product was washed with 3 M HCl to remove the by-product(MgO), rinsed with DW and ethanol several times, and vacuumdried at 80 °C overnight. The final product was designated as VC-NS. For comparison, NH4VO3 powders were calcined in air directlyto obtain V2O5 particles (V2O5-P) and then the same MTR protocolwas adopted except replacing the V2O5-NS with V2O5-P powdersas the precursor to prepare VC particles (VC-P).

2.2. Materials characterization

The samples were characterized by scanning electron micro-scopy (SEM, JEOL SM-31010), X-ray diffraction (XRD, Bruker AXSD2 Phaser) with the Cu Kα line, transmission electron microscopy(TEM, JEOL JEM-2100F), X-ray photoelectron spectroscopy (XPS,ESCALB MK-II, VG Instruments, UK) using monochromatic Mg Kαradiation, and Raman scattering (HR RamLab with the 514.5 nmArþ laser as the excitation source). The specific surface area andpore size distribution were determined by N2 adsorption anddesorption using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods on the Micromeritics ASAP 2020 at77 K.

2.3. Electrochemical measurements

The commercial Pt/C electrocatalyst with 20 wt% Pt in graphi-tized carbon was purchased from Sigma-Aldrich. The electro-chemical measurements were conducted based on a three-elec-trode system on the CHI 6144D electrochemical workstation(Shanghai CH Instrument Company, China) using a Pt foil(1 cm�1 cm) as the counter electrode, a saturated calomel elec-trode (SCE) as the reference electrode, and the electrocatalystmodified glassy carbon electrode (GCE) as the working electrode.The SCE was calibrated referenced to the RHE. It was present in thehydrogen-saturated electrolyte with a Pt wire as the workingelectrode as shown in Fig. S3. Here, all the potentials were con-verted to RHE by adding a value of 0.23 V. To prepare the workingelectrode, 24 mg of the as-prepared electrocatalyst were dispersed

X. Peng et al. / Nano Energy 26 (2016) 603–609 605

in 6 mL of DW by probe sonication (SCIENTZ-IID, Ningbo ScientzBiotechnology Co., Ltd., China). 5 μL of the catalyst dispersion wasloaded onto the GCE with a diameter of 3 mm. The catalyst loadingdensity was �0.28 mg cm�2. After drying in air, 5 μL of the 5%Nafion were coated on the GCE to protect the catalyst fromdropping off during the electrochemical measurements. Linearsweep voltammetry (LSV) was performed at a scanning rate of5 mV s�1 in N2-saturated 0.5 M H2SO4 and electrochemical im-pedance spectroscopy (EIS) was carried out in the frequency rangebetween 100 kHz and 0.1 Hz with an AC perturbation of 5 mV. Toevaluate the stability of the catalyst, cyclic voltammetry (CV) wasperformed at a scanning rate of 200 mV s�1 between 0.05 and�0.6 V vs. RHE. All the potentials and voltages in polarizationcurves were iR corrected (Fig. S4) using Rs obtained in EIS.

3. Results and discussion

The morphologies of the samples examined by SEM are de-picted in Figs. 1 and S5. The product of air-calcined NH4VO3 hasthe particulate form (Fig. S5a, b), designated as V2O5-P, and afterMTR (Fig. S5c, d), the particle size becomes smaller (designated asVC-P). Compared to the morphology of V2O5-P, the hydrothermalproduct designated as V2O5-NS consists of large-area nanosheetswith a smooth surface (Fig. 1a, b). After MTR (Fig. 1c, d), the na-nosheet structure is preserved and the surface becomes hier-archical and thicker consisting of nanosheets with inter-connectednanoparticles (designated as VC-NS). The results indicate that thehierarchical nanosheet structure can be prepared by the magne-sium thermic reaction on the precursor template.

The XRD patterns in Fig. 2a reveal the crystal structure andphases of the samples. The diffraction peaks from the hydro-thermal product can be assigned to the (001), (003), and (006)lattice planes of V2O5 �nH2O [30,31]. After MTR, the diffraction

Fig. 1. SEM images: (a) and (b) V

peaks at 37.4°, 43.4°, 63.1°, 75.7°, and 79.7° can be assigned to the(111), (200), (220), (311), and (222) lattice planes of the cubic VCcrystal (JCPDS card No. 73-0476), respectively, indicating that V2O5has been converted to VC. For comparison, the XRD patterns ofV2O5-P and corresponding VC-P are also displayed in Fig. 2a. Allthe diffraction peaks of V2O5-P, the product of air-calcinedNH4VO3, belong to the orthorhombic phase V2O5 (JCPDS card No.41-1426). After MTR, V2O5-P is also converted to VC, indicatingthat V2O5 can be converted to VC by low-temperature MTR. Aweak peak at 26° (2θ) is observed from both VC-NS and VC-P dueto the carbon species providing evidence that the final productscontain carbon. XPS is performed to determine the composition ofthe electrocatalyst as shown in Fig. S6. Fig. S6a reveals that thecomposite is composed of C, V, and O. The high-resolution V 2pspectrum in Fig. S6b shows two strong peaks at 513.3 and 521.0 eVstemming from VC [32–34]. The V-O species can be attributed toslight surface oxidation of carbide since no oxide is detected byXRD [35,36]. The high-resolution C 1s spectrum (Fig. S6c) showstwo strong peaks corresponding to the carbon species and VC. TheRaman spectra exhibit the typical D and G bands (Fig. S1c) ofcarbon, suggesting that the final product is a composite of carbonand VC. To determine the distribution of carbon and VC in thecomposite, TEM is performed. The TEM images (Fig. 2b and c)indicate that a large number of nanoparticles with diameter of 10–30 nm are encapsulated in the carbon network forming a hier-archical nanosheet structure. The high-resolution TEM image inFig. 2d shows lattice fringes with spacings of 0.24 nm and 0.34 nmthat are associated with the (111) planes of VC and graphitic car-bon, respectively. The graphitic carbon serves as a highly con-ductive support for the VC nanoparticles forming hierarchicalnanosheets with a large exposed specific surface area, paths forfast electron transport, and improved electrical contact with theactive sites.

The specific surface area and pore size distribution of the

2O5-NS; (c) and (d) VC-NS.

Fig. 2. (a) XRD patterns of V2O5-NS (1), V2O5-P (2), VC-NS (3) and VC-P (4); (b, c) TEM and (d) high-resolution TEM images of VC-NS.

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samples are determined by the BET and BJH methods, as shown inFig. S7. VC-NS has a large specific surface area of 90.5 m2 g�1

compared to the 78.6 m2 g�1 of VC-P. Both VC-NS and VC-P have asimilar pore size distribution of �2.5 nm, possibly resulting fromthe removal of MgO during HCl rinsing. The large specific surfacearea of the hierarchical VC-NS electrocatalyst stems from the largeamount of exposed active sites.

The HER activity is evaluated using a three-electrode cell in N2-saturated H2SO4 (0.5 M). Before the assessment, the SCE is cali-brated referenced to the RHE as shown in Fig. S3 and all the po-tentials and voltages in the polarization curves are iR corrected(Fig. S4). The polarization curves of VC-NS and VC-P in comparisonwith the bare GCE and commercial Pt/C electrocatalyst are dis-played in Fig. 3a. VC-NS shows a small overpotential η1 (defined asthe overpotential at a current density of 1 mA cm�2 as shown inTable S1) of 40 mV (vs. RHE, all the potentials are referenced toRHE in this work) which is less than those of Mo2C/CNT (64 mV)[37], mechanically activated MoS2 (103 mV) [38], NiMoNx/C(�160 mV) [39], and pure Mo2C (340 mV)[37] as listed in Table S2.Even at a current density of 10 mA cm�2, VC-NS shows an over-potential of only 98 mV which is much lower than that of commonHER electrocatalysts such as WSe2 nanoflakes on carbon nanofi-bers (158 mV) [40], Co2P (134 mV) [41], CoS2 nanowires (145 mV)[42], Ni2P particles (121 mV) [43], porous MoCx (142 mV) [44], γ-Mo2N (381 mV) [45], MoB (�220 mV) [6], and c-MoC (293 mV)[45] as shown in Table S2. To investigate the effects of the carbonnetwork on the HER activity of VC-NS, the polarization curve ofpure carbon prepared by the same method as VC-NS but usingonly Mg and NaHCO3 as precursors is presented in Fig. S8, whichshows that the electrocatalytic activity of pure carbon is worsethan that of VC-NS.

The HER kinetics of the electrocatalysts is investigated based onthe Tafel slopes determined from the Tafel plots in the linearportions that can be fitted with the Tafel equation (η¼b� log jþa,where j is the current density, b is the Tafel slope), as shown inFig. 3b. The Tafel slope of commercial Pt/C is 34 mV dec�1, whichis very close to those reported previously [46,47]. HER on the Ptsurface is considered to proceed by the Volmer–Tafel reactionmechanism. The proton discharge reaction is fast and H2 is evolvedin a rate-determining combination reaction or Tafel step [48,49].With regard to VC-NS, the Tafel slope is 56 mV dec�1, which iscomparable to those of MoP, MoCx and CoP/Ti [44,46,50] butsmaller than those of WS2, MoS2, Mo2N, MoSe2, and WSe2-C[40,45,51,52], listed in Table S2, indicating that HER occurs via aVolmer-Heyrovsky HER mechanism and electrochemical deso-rption is the rate-limiting step [48,49], The exchange currentdensities j0 (defined as the current density at zero overpotential)of the electrocatalysts determined from the Tafel plots by extra-polation are listed in Table S1. VC-NS shows a j0 value of0.193 mA cm�2 which is larger than that of VC-P of0.092 mA cm�2. In general, a larger exchange current density j0and smaller Tafel slope imply superior HER activity.

VC-P shows larger overpotentials at all current densities andTafel slope compared to VC-NS because of the relatively smallerspecific surface area and less exposed active sites on VC-P. Thelarger contact resistance between the particles in VC-P also givesrise to the inferior performance. Since a large number of VC na-noparticles are encapsulated in the highly conductive graphiticcarbon network forming a 2D hierarchical nanosheet structure,electrons can be transported through the large-area nanosheetsdirectly and the contact resistance decreases sharply. In contrast,VC-P is composed of small particles and electrons must be

Fig. 3. (a) Polarization curves of VC-NS, VC-P, and commercial Pt/C compared to bare GCE; (b) Corresponding Tafel plots of VC-NS, VC-P, and commercial Pt/C; (c) Nyquistplots of VC-NS, VC-P, and commercial Pt/C obtained at the overpotential η¼100 mV with inset showing the magnified plots at high frequencies; (d) Stability evaluation of VC-NS showing the polarization curves before and after CV scanning.

X. Peng et al. / Nano Energy 26 (2016) 603–609 607

transported from one particle to another to overcome the largecontact resistance between particles, as schematically illustratedin Scheme 1. The charge transfer resistance of the electrocatalystsis determined by EIS. The Nyquist plots in Fig. 3c are fitted withthe equivalent circuit in Fig. S9 (fitting parameters shown in TableS3). VC-NS shows a small charge transfer resistance (Rct) of�19.0Ω cm2 when an overpotential of 100 mV is applied. It issmaller than those of VC-P (�98.3Ω cm2) and other electro-catalysts reported previously including three-dimensional orderedmacro-/mesoporous Ni (33Ω cm2) [53], indicating highly con-ductive and fast proton discharge kinetics of the as-preparedhierarchical VC-NS electrocatalyst.

The durability of the hierarchical VC-NS electrocatalyst is eval-uated by CV at a scanning rate of 200 mV s�1. The polarization curvesof the initial cycle and after certain CV cycles are displayed in Fig. 3dwhich shows a mere 5 mV shift in the overpotential for a currentdensity of 80 mA cm�2 in the initial 5000 CV cycles. Even after 10000CV cycles, the overpotential shifts by only 10 mV for 80 mA cm�2

indicating excellent stability in long-term application, which maypartially be due to protection rendered by the carbon network pre-cluding the electrocatalyst from corrosion in acidic media.

4. Conclusion

Hierarchical VC-NS composed of VC nanoparticles encapsulatedin a graphitic carbon network is prepared by a simple hydro-thermal reaction in combination with the low-temperaturemagnesium thermic reaction. Since the hierarchical VC-NS elec-trocatalyst has a large specific surface area and small pore size

distribution, there are more exposed active sites to boost the HERactivity in acidic media. The hierarchical VC-NS electrocatalystshows an overpotential of only 98 mV at a current density of10 mA cm�2 with a small Tafel slope of 56 mV dec�1. On accountof the excellent durability, the overpotential shifts by only 10 mVafter 10000 CV cycles at a current density of 80 mA cm�2. Theprecious-metal-free electrocatalyst, which can be mass producedeconomically with less environmental impact than common pre-cious-metal based electrocatalysts, delivers outstanding HER per-formance and has large potential in HER and related energy gen-eration schemes.

Acknowledgements

This work was financially supported by City University of HongKong Applied Research Grant (ARG) Nos. 9667104 and 9667122,the Innovation and Technology Commission of Hong Kong, as wellas National Natural Science Foundation of China (NSFC Nos.51572100, 51504171 and 31500783). The TEM experiments wereconducted at the Hong Kong Polytechnic University ResearchFacility in Materials Characterization and Device Fabrication(UMF)-centre for Electron Microscopy. We acknowledge the use ofthe facilities and engineering support by Dr. Wei Lu.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.nanoen.2016.06.020.

http://doi:10.1016/j.nanoen.2016.06.020

X. Peng et al. / Nano Energy 26 (2016) 603–609608

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(2011) 7296–7299.[50] X.B. Chen, D.Z. Wang, Z.P. Wang, P. Zhou, Z.Z. Wu, F. Jiang, Chem. Commun. 50

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J. Mater. Chem. A 3 (2015) 11367–11375.

Xiang Peng presently is a PhD candidate under thesupervision of Prof. Paul K. Chu in Department of Phy-sics and Materials Science, City University of HongKong. His research interests focus on synthesis offunctional nanomaterials and fabrication of electro-chemical energy storage devices for supercapacitor, Li-ion battery and electrocatalysis applications.

Liangsheng HU received his BS and MS degrees fromWuhan University of Science and Technology in 2007and 2011 respectively. Currently, he is a PhD candidatein Chemistry of The Hong Kong Polytechnic University.His research interests focus on the synthesis and ap-plication of metal-semiconductor nanostructuredcomposites for (electro)photochemical water-splittingand carbon dioxide reduction.

Lei Wang earned his BS degree in physics from Huaz-hong University of Science and Technology (HUST) in2012. He is currently pursuing his PhD in the WuhanNational Laboratory for Optoelectronics (WNLO) atHuazhong University of Science and Technology(HUST), under the supervision of Prof. Kaifu Huo. Hispresent research interest is the synthesis of synthesisand characterization of nanostructured electrode ar-chitectures and materials for electrochemical energyconversion and storage.

Xuming Zhang received his PhD in materials scienceand engineering from City University of Hong Kong in2015 and worked for one year as a senior research as-sociate in the Plasma Laboratory at City University ofHong Kong under the supervision of Prof. Paul K. Chu.He is now working as professor in Wuhan University ofScience and Technology. His research interests on thesynthesis of nanomaterials for application in bioma-terials, electrochemical sensors, and electrochemicalenergy storage devices.

Jijiang Fu received his BS in Chemistry from NanchangUniversity in 1993 and PhD in Physical Chemistry fromNanjing University (China) in 2005. He is currently aprofessor of materials science at Wuhan University ofScience and Technology. His current research focuseson bioactive nanomaterials and nanostructured elec-trode materials for electrochemical biosensors andenergy storage devices.

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X. Peng et al. / Nano Energy 26 (2016) 603–609 609

Kaifu Huo received his BS in Applied Chemistry fromChina University of Petroleum in 1997 and a PhD inPhysical Chemistry from Nanjing University (China) in2004. He is currently a Professor in the National La-boratory for Optoelectronics at Huazhong University ofScience and Technology. He is an associate editor ofNanoscience and Nanotechnology Letters (NNL). He hasauthored/co-authored more than 100 papers in inter-national refereed journals, which are cited more than2500 times (current H-index¼29). His main researchactivities encompass bioactive nanomaterials and na-nostructured electrode materials for electrochemical

biosensors and energy storage devices.

Lawrence Yoon Suk Lee received his PhD degree fromMcGill University, Canada in 2006. He is now a researchassistant professor at the Hong Kong Polytechnic Uni-versity. His research interests include development ofnovel semiconductor nanomaterials for photocatalyticwater splitting and CO2 reduction, as well as electrodematerials for lithium ion batteries.

Kwok-yin Wong is Patrick S. C. Poon Endowed Pro-fessor in Applied Chemistry, Chair Professor of Chemi-cal Technology and director of the Partner State KeyLaboratory of Chirosciences at The Hong Kong Poly-technic University. He obtained his BSc(Hons) and PhDdegrees in chemistry from The University of Hong Kongin 1981 and 1986 respectively. He was a postdoctoralresearch fellow at California Institute of Technology(USA) from 1986 to 1987. His main research areas in-clude electrochemistry, green chemistry, biosensorsand antibiotics development. He has published over170 SCI journal articles and is the holder of 6 interna-

tional patents.

Paul K. Chu received his PhD in Chemistry from CornellUniversity. He is Chair Professor of Materials En-gineering in the Department of Physics and MaterialsScience at City University of Hong Kong. He is Fellow ofthe American Physical Society (APS), American VacuumSociety (AVS), Institute of Electrical and ElectronicsEngineers (IEEE), Materials Research Society (MRS), andHong Kong Institution of Engineers (HKIE). He is alsoFellow of the Hong Kong Academy of Engineering Sci-ences (HKAES). His research interests are quite diverseencompassing plasma surface engineering, materialsscience and engineering, surface science, and func-

tional materials.

1

Supporting Information

Vanadium Carbide Nanoparticles Encapsulated in Graphitic Carbon Network

Nanosheets：A High-Efficiency Electrocatalyst for Hydrogen Evolution Reaction

Xiang Penga,1, Liangsheng Hub,1, Lei Wangc, Xuming Zhanga, d, Jijiang Fud, Kaifu Huoc,*,

Lawrence Yoon Suk Leeb, Kwok-Yin Wongb,*, Paul K. Chua,*

a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue,

Kowloon, Hong Kong, China.

E-mail: paul.chu@cityu.edu.hk; Fax: +852 34420538; Tel.: +852 34427724

b Department of Applied Biology and Chemical Technology and the State Key Laboratory of

Chirosciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

E-mail: kwok-yin.wong@polyu.edu.hk

c Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan 430074, China.

E-mail: kfhuo@hust.edu.cn

d The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and

Technology, Wuhan 430081, China

1 These authors contributed equally to this work.

mailto:paul.chu@cityu.edu.hkmailto:kwok-yin.wong@polyu.edu.hkmailto:kfhuo@hust.edu.cn

2

MTR mechanism investigation

To investigate the formation mechanism of the hierarchical VC-NS, several experiments were

performed. Firstly, Mg and V2O5·nH2O were mixed and reacted at 700 oC in the stainless steel

sealed cans. The products were MgO and metallic vanadium as shown in Figure S1a, suggesting

that the V2O5·nH2O could be reduceed to metallic vanadium by magnesium. Mg and NaHCO3

were also mixed and reacted at 700 oC in the stainless steel sealed cans. The products were MgO,

Na2CO3, and carbon as shown in Figure S1a (XRD) and Figure S1c (Raman). Generally,

NaHCO3 would be thermally decomposed at 400-500 oC to form CO2, Na2CO3, and H2O and so

the carbon species resulted from reduction of CO2 by Mg at 700 oC. When using Mg, V2O5·nH2O,

and NaHCO3 as the precursors, the final products were MgO, VC, Na2CO3, and carbon as verified

by XRD and Raman scattering as shown in Figures S1a-c. Mg served as the reducing agent during

MTR in which V2O5·nH2O was converted to metallic vanadium and CO2 to carbon species. VC

was formed by the reaction between metallic vanadium and carbon. The possible reactons in MTR

are expressed as follows:

5Mg+V2O5=5MgO+2V (Eq 1)

2NaHCO3=Na2CO3+H2O+CO2 (Eq 2)

2Mg+CO2=2MgO+C (Eq 3)

V+C=VC (Eq 4)

The morphology and composition of the products synthesized for different MTR time were

characterized, as shown in Figure S2. When the MTR time duration was 5 min, the NS structure

became thicker (Figure S2a) compared to the precursor V2O5-NS in Figures 1a and b, which

resulted from the incorporation of Mg into the V2O5 (JCPDS 41-1426, volume 179.6 Å3)

3

nanosheets and formation of MgV2O4 (JCPDS 77-2130, volume 597.0 Å3) as shown by the XRD

pattern in Figure S2d. During formaton of MgV2O4, vanadium in V2O5 was reduced to smaller

valence. By increasing the MTR time, V2O5 would be reduced to metallic vanadium as indicated

by the XRD pattern in Figure S1a. If the MTR time was increased to 30 min, the final product

(Figure S2b) had a nanosheet structure consisting of hollow spheres instead of a smooth surface

observed from the pristine V2O5. The product was a composite of carbon species and VC (Figure

S2d). If the MTR time is further extended to 5 h, the nanosheet structure remained but composed

of nanoparticles (Figure S2c) containing carbon species and VC as shown in Figure S2d.

It can be concluded that Mg is incorporated into V2O5 first to thicken the nanosheets and form

MgV2O4. After more Mg is corporated, V2O5 is reduced gradually and finally, highly acitve

metallic vanadium is produced (Eq 1). At the same time, carbon species are formed (Eqs. 2 and

3) and deposited on the surface of the nanosheets forming a conductive carbon network on the

template of V2O5 nanosheets and then VC is formed by the reaction between metallic vanadium

and carbon (Eq 4). Generally, VC is first formed on the surface of metallic vanadium and then

diffuses into the core. If the MTR time is limited (30 min), VC has a hollow spherical morphology

since the core has not been converted into VC and removed in the HCl rinsing. If the MTR time

is long enough (5 h), all the metallic vanadium is converted into VC and the unreacted C serves as

the backbone of the final nanosheets. It is noted that if the reaction time is short, the newly formed

carbon species cannot react with vanadium in time and the ratio of carbon to VC is larger than that

observed from the product reacted for a longer time (Figure S2d).

4

Figure S1 (a) XRD patterns of the products using different precursors. The assignment of the

diffraction peaks are indicated as follows: △: MgO, ◇: Mg, ▲: Na2CO3, ●: VC, ☆: metallic

vanadium. (b) Detailed analysis of the XRD patterns of the product using Mg, V2O5-NS, and

NaHCO3 as the precursors. (c) Raman scattering spectra of the products obtained from different

precursors. All the products are characterized directly after reacting at 700 oC without rinsing.

5

Figure S2 (a: 5 min, b: 30 min, c: 5 h) SEM images and (d) XRD patterns of the products using

Mg, HaHCO3 and V2O5-NS as precursors after reacting for different time durations from 5 min to

5 h in MTR with the insets showing the high-magnification images.

6

Figure S3 CV of SCE calibrated with respect to RHE at a scanning rate of 1 mV s-1. The inset is

the enlarged CV curve. The average of the two potentials at which the current crosses zero is taken

to be the thermodynamic potential of the hydrogen electrode reaction.[1]

7

Figure S4 Polarization curves of the hierarchical VC-NS electrocatalyst in 0.5 M H2SO4 with and

without iR correction.

8

Figure S5 SEM images: (a) and (b) V2O5-P; (c) and (d) VC-P.

9

Figure S6 (a) XPS full spectra of the as-prepared hierarchical VC-NS electrocatalyst; (b and c)

High-resolution XPS of the VC-NS electrocatalyst: (b) V 2p and (c) C 1s.

10

Figure S7 N2 Adsorption-desorption isotherms of VC-NS and VC-P with the inset showing the

corresponding BJH pore size distribution.

11

Table S1 Overpotential at 1 mA cm2 and 10 mA cm2 and calculated exchange current density j0

values of the electrocatalysts.

Electrocatalysts η1 (mV) η10 (mV) j0 (mA cm-2)

VC-NS 40 98 0.193

VC-P 84 167 0.092

Pt-C 9 45 0.544

12

Table S2 Comparison of representative precious-metal-free HER electrocatalysts.

Mass

loading

(mg cm-2)

media η1

[a]

(mV)

η10[b]

(mV)

Tafel slope

(mV dec-1) ref

VC-NS 0.28 0.5 M H2SO4 40 98 56 This

work

VC-P 0.28 0.5 M H2SO4 84 167 81 This

work

MA-MoS2 0.28 0.5 M H2SO4 200 -- 104 [2]

Mo2C/CNT 2 0.1 M HClO4 64 -- 55.2 [3]

Mo2C 2 0.1 M HClO4 340 -- 87.6 [3]

c-Mo2C 0.102 1 M KOH -- 293 58 [4]

γ-Mo2N 0.102 1 M KOH -- 353 108 [4]

Porous Mo2C 0.8 0.5 M H2SO4 87 142 53

[5] 1 M KOH 92 151 59

WSe2-C -- 0.5 M H2SO4 -- 158 98 [6]

NiMoNx/C 0.25 0.1 M HClO4 ~150 -- 35.9 [7]

Co2P 1 0.5 M H2SO4 >70 134 71 [8]

CoS2

nanowire ~1.7 0.5 M H2SO4 -- 145 51.6 [9]

CoS2

microwire ~25 0.5 M H2SO4 -- 158 58 [9]

Ni2P ~1 0.5 M H2SO4 -- ~120 46 [10]

MoB 2.5 1 M H2SO4 -- 220 55 [11]

CoP 2 0.5 M H2SO4 -- 75~ 50 [12]

MoP 0.071 0.5 M H2SO4 >100 246 60 [13]

MoSe2/RGO 0.16 0.5 M H2SO4 >50 ~120 69 [14]

[a] Overpotential at current density of 1 mA cm-2. [b] Overpotential at current density of 10 mA cm-

2.

13

Figure S8 Polarization curves of the hierarchical VC-NS electrocatalyst and pure carbon prepared

by the same MTR procedures but using only Mg and NaHCO3 as the precursors.

14

Figure S9 Electrical equivalent circuit used to simulate the Nyquist plots in Figure 3c, where Rs is

the electrolyte resistance, Rct is the charge-transfer resistance, and Cdl represents the double-layer

capacitance.[15-17]

15

Table S3 Fitted parameters of the electrocatalysts at an overpotential of 100 mV according to the

equivalent circuit in Figure S9.

Electrocatalysts Rs (Ω cm2) Rct (Ω cm

2) Cdl (S sn/cm2)

VC-NS 0.6 19.0 5.628*10-4

VC-P 0.54 98.3 3.45*10-4

Pt-C 0.51 2.6 5.747*10-5

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Vanadium carbide nanoparticles encapsulated in graphitic carbon network nanosheets: A high-efficiency electrocatalyst for...IntroductionExperimental sectionSamples preparationMaterials characterizationElectrochemical measurements

Results and discussionConclusionAcknowledgementsSupporting informationReferences