Ultra fine molybdenum carbide nanoparticles supported on nitrogen doped carbon nanosheets for hydrogen evolution reaction

Kedong Xia,Junpo Guo,Cuijuan Xuan,Ting Huang,Zhiping Deng,Lingxuan Chen,Deli Wang*

Key laboratory of Material Chemistry for Energy Conversion and Storage(Huazhong University of Science and Technology),Ministry of Education,Hubei Key Laboratory of Material Chemistry and Service Failure,School of Chemistry and Chemical Engineering,Huazhong University of Science and Technology,Wuhan 430074,China

Keywords:Molybdenum carbides Ultra fine nanoparticles Nitrogen doping Carbon naonosheets Hydrogen evolution reaction

ABSTRACT Molybdenum carbides(Mox C)/nitrogen doped carbon nanosheets(NCS)composites are synthesized via simple mixing melamine and ammonia molybdate,followed by a high-temperature treatment.Metal carbide nanoparticles with ultra-small size(13nm)are uniformly supported on nitrogen doped carbon nanosheets.The hydrogen evolution reaction(HER)is investigated in both 0.5mol/LH2SO4 and 1 mol/L KOH media.Mo2C/NCS-10(melamine/ammonia molybdate weight ratio of 10:1)exhibits excellent performance with a low overpotential of130 m V in 0.5 mol/LH2SO4 solution and 108 m V in 1 mol/LKOH solution at the current density of 10[46_TD DIFF]m A/cm 2.The better electrocatalytic activity could be ascribed to N-doped carbon nanosheets,small particle size,mesoporous structure,and large specific surface area,which could provide the large electrochemical active surface area and facilitate mass transport.

Hydrogen as an environmental-friendly renew able energy source,has become one of the promising alternatives for fossil fuels due to its high gravimetric energy density[1–3].Electrochemical w ater splitting is one of most attractive approach for hydrogen production.Although Pt-group noble metals exhibit the predominant HERperformance with small overpotential and large current density,the high cost and limited reservation seriously hinder their ranges of applications[4].Tremendous efforts have been made to develop high-ef fi ciency and noble-metal free catalysts,and many 3d-transition metal based materials(such as nickel[5–7],iron[8–10],cobalt[11–13],molybdenum[14–26])are w idely investigated.However,the HER performance on most of these materials is disappointing in comparison to Pt-group electrocatalysts.

Molybdenum carbides(MoC or Mo2C)have attracted great interest since the fi rst report by Huet al.[27]ow ing to their low cost,unique d-band electronic structure,chemical stability and similar catalytic properties to Pt-group metals[28,29].However,poor conductivity and readily fall off MoxC nanoparticles from supports is still a challenge to maintain their activity and stability during cycles.Thus,considerable efforts have been made to improve the electrochemical properties of MoxC.Decreasing particle size to nanoscale or ultrasmall size and construction of different nanostructure(e.g.,nanow ires,nanoparticles)are effecive methods to improve HERactivity for the enhanced exposure of active sites to electrolyte[17,30,31].Creating porous structure in MoxCis favorable for increase the specific surface areas of carbides[32–34].A porous Mo2C nanow ires were developed by Liaoet al.showed better HERactivity and stability in acidic electrolyte[35].To improve the electronic conductivity of MoxC,by coupling it with carbon materials(e.g.,graphene and carbon nanotubes)is another promising route[21,36,37].Chenet al.reported that nanotubesupported Mo2C catalysts exhibit an overpotential of 63m V at 1 m A/cm2of current density[17].Besides,chemical doping of carbon materials by heteroatom(e.g.,N,S and P)can not only provide large numbers of active sites,but also improve the electronic conductivity and wettability of material surface[19,38].

As compared with bulk carbon materials,synthesis of carbon nanosheet is an effective method to increase the specific surface area and relieve the aggregation of carbides during the carburization process,making more active sites exposed and utilized.Herein,a simple method is developed to synthesize the tw odimensional MoxC/N-doped carbon nanosheet hybrids(Mo2C/NCS and MoC/NCS)for the HER.The different structured MoxC/NCS composites were prepared by tuning the proportion of melamine and ammonium molybdate precursors.The HER performance of these composites was conducted both in 0.5mol/L H2SO4and 1 mol/L KOH media.The relationship between structure and electrochemical properties was also discussed.

The synthesis process of MoxC/NCSis schematically depicted in Fig.1.Melamine is fi rst transformed into yellow graphitic carbon nitride(g-C3N4)at 550[49_TD DIFF]?C,which could be veri fied by the X-ray diffraction(XRD)pattern described in Fig.2a.Tw o obvious diffraction peaks at 13.7?and 27.4?are detected which are consistent with previously reported g-C3N4[39].However,the mixed melamine/ammonium molybdate precursors were transformed into g-C3N4supported MoOxspecies(MoOx/g-C3N4)at 550[49_TD DIFF]?C.This MoOx/g-C3N4intermediate phase is converted into black pow der with increasing the heat treated temperature,indicating the formation of carbon and/or MoxC at 800[50_TD DIFF]?C.XRD patterns of the resulting products are show n in Fig.2b.The different cystal structure of MoxC-based materials is obtained by adjusting the mass ratio of melamine to ammonium molybdate to be 5:1,10:1,15:1 and 20:1,respectively.For the mass ratio of melamine to ammonia molybdate is 5:1 and 10:1,diffraction peaks located at 34.4?,37.8?,39.4?,52.1?,61.5?,69.6?,72.4?,and 74.6?is detected,which is indexed to the(100),(002),(101),(102),(110),(103),(200)and(112)lattice planes of Mo2C(JCPDSNo.01-071-242).However,w hen the mass ratio is increased to 15:1 and 20:1,XRD show s the gradually conversion of Mo2Cto MoC,which is due to the suf fi cient carbon source.The diffraction peaksat 36.4?,42.3?,61.4?and 73.5?are corresponding to the characteristic(111),(200),(220)and(311)lattice planes of MoC(JCPDSNo.01-089-2868).By calculating from Scherrer equation,the average particle size of Mo2Cand MoCin these composites is calculated to be 6.3,2.5,2.1 and 1.6 nm,respectively,which is estimated from the highest intensity diffraction peaks of the prepared four catalysts(Table S1 in Supporting information),indicating the fact that massive melamine is favorable to reduce the particle size of MoxC.

Fig.1.Schematic illustration of the synthetic procedure for Mox C/NCScomposites.

Fig.2.(a)XRD patterns of g-C3N4 and MoOx/g-C3N4.(b)XRD patterns,(c)XPSC 1s and(d)Mo 3d spectra of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 catalysts.

X-ray photoelectron spectroscopy(XPS)was performed to analysis the electronic structures of the catalysts.The full scan XPS spectra suggested that the Mo,O,C and N as main elements are presented on the surface of catalysts(Fig.S1 in Supporting information).The relative atomic content is given in Table S2 in Supporting information.The Mo,N and O content decreased with increasing the ratio of melamine and ammonium molybdate.However,the C content increased obviously with increasing of melamine content,and thus resulting in the improvement of electronic conductivity and the conversion of MoxC.The existence of N element implies the doping of C with N atom,which is favorable for improving the interaction of active sites with H+[21].The fi tting C 1s spectra are show n in Fig.2c.The peaks located at 284.6[51_TD DIFF]eV,285.1 eV,286.3 eV and 288.8 eV are attributed to C??C bond,C?N bond,C??O bond and O?C??O bond[40],respectively.The existence of C?N bonding indicates that the N atoms is incorporated into carbon substrate,which plays a vital role for HER.In addition,the C-Mo bonding is identi fied in fi tting C1s spectra of obtained catalysts which demonstrates the existence of MoxC.The fi tting Mo 3d spectra is given in Fig.2d,and there are four valence states(+2,+3,+4 and+6)of Mo on the surface.The oxidation states of Mo6+are likely attributed to surface oxidation(MoO3)on the surfacesof catalystsduring the measurement[41],and the MoO3is thought to be inactive for HER.Meanwhile,the peaksof Mo2+,Mo3+and Mo4+are recorded at 228.0–229.0 eV,231.0–232.0 eV and 233.0–235.0 eV,which can be attributed to Mo2C,Mo N and Mo C[42,43],respectively.The XPSanalysis suggeststhat melamine canact as both carbon source and nitrogen source to obtain N doped carbon and MoxC.Thus,the obtained catalysts are denoted as Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20,respectively.Additionally,Mo2+and Mo3+species are considered to be active sites for electrocatalytic HER,and the Mo3+/Mo2+ratio on the MoxCsurface could provide useful information to acquire the nature of the active-sites[43].The relative amount of Mo2+and Mo3+species is estimated by the fi tted peak area.As depicted in Table S3 in Supporting information,the Mo3+/Mo2+ratios in Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15,and MoC/NCS-20 are calculated to be 0.28,0.54,0.91 and 1.55,respectively,which suggests the fact that Mo2+isdominant in Mo2C/NCS-5 and Mo2C/NCS-10 while Mo3+is prevailing in MoC/NCS-15 and MoC/NCS-20.Thus,the variation of the Mo3+/Mo2+ratio w ill result in different HER activity,which is attributed to the different electron densities around Mo3+and Mo2+.

Fig.3.TEM and HRTEM imagesof Mox C/NCScatalysts.(a,b)Mo2C/NCS-5,(c,d)Mo2C/NCS-10,(e,f)MoC/NCS-15 and(g,h)MoC/NCS-20.The inset in(a)show s a typical SAED pattern of Mo2C/NCS-5.The inset in(d),(f)and(h)are corresponding to the particle size distribution of Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20,respectively.

TEM measurements were performed to observe the microstructure of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20,asshow n in Fig.3.The particle size of Mo2C/NCS-5 islarger and accumulated obviously,which is due to the reaction of melamine derived carbon with Mo Ox,and only few amount of carbon remains.The selected area electron diffraction(SAED)pattern inserted in Fig.3a illustrates the presence of Mo2C nanoparticles.The bright diffraction rings indicate the nanostructure is polycrystalline.It is observed from high resolution transmission electron microscopy(HRTEM)image of an individual particle that the inter-planar spacing between adjacent fringes for Mo2C/NCS-5 are measured to be 0.26[54_TD DIFF]nm and 0.24 nm(Fig.3b),which are corresponding to the(100)and(101)planes of Mo2C,and the result is consistent the SAED pattern.The exposed(101)lattice plane of Mo2C is coordinatively unsaturated and have negative H2adsorption energies[44],which plays a signi fi cant role for the HERperformance.Ultra-thin carbon sheets supported MoxC particles are oberved clearly with increasing the weight ratio of melamine to ammonium molybdate up to 10:1,15:1 and 20:1.TEM images of Mo2C/NCS-10,MoC/NCS-15,and MoC/NCS-20 show the uniform distribution of MoxC nanoparticles on the carbon nanosheets,as given inFigs.3c–h.The average particle size is estimated to be 2.7 nm,2.0 nm and 1.4 nm for Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20,respectively.The decrease of particle size may attribute to the more decentralization of amorphous molybdenum precursor with increasing the amount of melamine,which is also consistent with the result calculated from Scherrer equation in XRD patterns.Furthermore,Fig.3h show s the inter-planar spacing of MoC/NCS-20 to be 0.25 nm,which is corresponding to(111)plane of MoC phase[16].The lattice fringe measured in HRTEM image is in consistence with the theoretical inter-planar spacing calculated from the XRD patterns(Fig.2b),and which also con fi rmed the phase transformation from Mo2C to MoC as well.The unique nitrogen doped carbon nanosheets and tiny MoxC nanoparticles could provide more active sites and higher electrochemically active surface area for HER.

Brunauer-Emmett-Teller(BET)was applied to analyze the specific surface area of MoxC/NCScomposites.Fig.S2(Supporting information)show s that the specific aurface area is determined to be 53.4[57_TD DIFF]m2/g,79.2 m2/g,91.7 m2/g and 110.8 m2/g for Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20,respectively.The increase of specific surface area could provide more reaction interfaces for the HER,and higher carbon content facilitates the electron transport to active sites[21].The similar pore-size distribution is obtained for MoxC/NCSsamples.Micropores peaks at about 1.5[58_TD DIFF]nm and 1.7 nm,and the main mesoporespeak at 3.2[59_TD DIFF]nm is detected.The mesoporous structure enable the fast diffusion of electrolyte and promote mass transfer during the HER process.

Fig.4.(a)HERpolarization curves of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 in 0.5 mol/LH2SO4 solution.(b)Tafel plots of the corresponding catalysts.(c)Capacitive current at 0.3[38_TD DIFF]V as a function of scan rate for Mox C/NCS.(d)HERpolarization curves of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 before and after 3000 potential cycles in 0.5 mol/LH2SO4 solution.(e)HERpolarization curves of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 in 1 mol/LKOH solution.(f)Tafel plots of the corresponding catalysts.(g)HER polarization curves of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 before and after 3000 potential cycles in 1 mol/LKOH solution.(h)[39_TD DIFF]Current density versus time curves(at a constant potential of 130 m V and 108 m V,respectively)on Mo2C/NCS-10 in 0.5 mol/LH2SO4 and 1[40_TD DIFF]mol/LKOH solution.

The electrocatalytic performance of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 catalysts for HER was investigated in 0.5 mol/LH2SO4solution.As show n from the polarization curves in Fig.4a,Pt/Cexhibits perfect catalytic performance with nearly zero onset overpotential,and Mo2C/NCS-10 exhibits a higher HERactivity as compared with the other three molybdenum carbides catalysts.The overpotential obtained at the current density of 10 m A/cm2is an important parameter for comparison catalysts in solar hydrogen production[36].The corresponding overpotentials at a current density of 10[46_TD DIFF]m A/cm2for Mo2C/NCS-5,Mo2C/NCS-10,Mo C/NCS-15 and Mo C/NCS-20 are 155[60_TD DIFF]m V,130 m V,172 m V and 197 m V,respectively,indicating a faster hydrogen evolution rate on Mo2C/NCS-10 electrode,although it is higher than that of Pt/C(46[61_TD DIFF]m V).Such prominent HER activity of Mo2C/NCS-10 in acid media is better in the list among the current noble metal-free HERcatalysts(Table S4 in Supporting information).The Tafel plot was gained by plotting the logarithm of the kinetic current density obtained from the corresponding HERpolarization curves(Fig.4b).The Tafel slope of Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20 is calculated to be 80[62_TD DIFF]m V/dec,71 m V/dec,103 m V/dec and 118 m V/dec respectively,indicating a Volmer-Heyrovsky mechanism for HER,and the HER rate is determined by the electrochemical desorption of H and H+from the catalyst surface to form hydrogen[16].In addition,the lowest value of Tafel slope for Mo2C/NCS-10 means the excellent HER catalytic activity with increasing potential as compared with other MoxC based catalysts.

The nitrogen doped carbon nanosheets and fine MoxC nanoparticles could provide more active sites for HER.Besides,the enhanced HERperformance of Mo2C/NCS-10 could be attributed to the large electrochemically active surface area(ECSA),which can be derived by calculating the double-layer capacitances(Cdl).The double-layer capacitances were obtained using cyclic voltammograms between 0.2[63_TD DIFF]V and 0.4 V at scan rates ranging from 20m V/s to 180[64_TD DIFF]m V/s(Fig.S3 in Supporting information).The measured Cdlis 30.4,32.1,29.2 and 22.4[65_TD DIFF]m F/cm2for Mo2C/NCS-5,Mo2C/NCS-10,MoC/NCS-15 and MoC/NCS-20,respectively(Fig.4c).It can be seen that the Cdlvalue of Mo2C/NCS-10 is 1.06,1.10 and 1.44 timeshigher than that of Mo2C/NCS-5,MoC/NCS-15 and MoC/NCS-20,indicating a relative more quantitive of active sites.The result show s that the larger the specific surface area,the smaller the electrochemical active surface area,which is due to the different N content.Although the specific surface area increased,the N/C ratio decreased with the increase of melamine content(Table S2).The Ncontent w ould affect the double-layer capacitances.Ahigher Ncontent could increase the electrochemical active area and active sites.Therefore,N-doped carbon nanosheets,ultra fine Mox C nanocrystallites,mesoporous structure and large electrochemical active surface area work together for the better electrocatalytic HERperformance.By comparing the electrochemical property and physical characterization,it is found that Mo2C/NCS catalysts possess superior HERactivity to Mo C/NCScomposites.The stability of MoxC/NCScatalysts were measured by potential cycling for 3000 cycles at a sweep rate of 100[66_TD DIFF]m V/s in 0.5 mol/LH2SO4solution,as illustrated in Fig.4d.There is negligible current decay for all asprepared materials,suggesting the excellent electrochemical stability of these catalysts in acid solution.

The HERactivity of MoxC/NCScatalysts was also conducted in basic media(1[67_TD DIFF]mol/L KOH).The Mo2C/NCS-10 exhibits better electrocatalytic HER activity in 1 mol/L KOH solution,with an smaller overpotential of 108m V at the current density of 10[46_TD DIFF]m A/cm2,compared with Mo2C/NCS-5(125m V),MoC/NCS-15(147 m V)and MoC/NCS-20(194m V),as show n in Fig.4e,demonstrating a faster hydrogen evolution rate on Mo2C/NCS-10 in alkaline.The Tafel slope of Mo2C/NCS-10 in Fig.4f is calculated to be 83m V/dec,which is comparable to Pt/C(60m V/dec),much lower than that of Mo2C/NCS-5(91m V/dec),MoC/NCS-15(107m V/dec)and MoC/NCS-20(133m V/dec),respectively,indicating a Volmer-Heyrovsky mechanism process.Besides,by comparing the HERperformance of recent reported MoxC-based composites in alkaline media,it is also con fi rmed the good HERactivity of the obtained Mo2C/NCS-10(Table S4).The stability of MoxC/NCScatalysts in alkaline medium was evaluated by potential cycling(3000 cycles)at a sweep rate of 100m V/s.Ascan be seen from the polarization curvesin Fig.4g,the Mo2C/NCS-10 catalyst exhibits about 8m V decay after the 3000 potential cyclesin alkaline condition,which issmaller ascompared with Mo2C/NCS-5,Mo C/NCS-15 and Mo C/NCS-20.Furthermore,[39_TD DIFF]Current density versus time curves for Mo2C/NCS-10 in both 0.5 mol/LH2SO4and 1[40_TD DIFF]mol/LKOH solution was conducted at constant potentials of 130 and 108m V for 20 h,respectively.There is a negligible current decay in 0.5[79_TD DIFF]mol/Lh2SO4,while a slight current decrease in 1mol/L KOH during 20 h testing (Fig.4h).The electrochemical stability of Mo2C/NCS-10 catalyst in KOH solution is inferior to that in H2SO4,which is probably attributed to the corrosion of carbon support in KOH solution,and the active sites w ould be detached from the carbon support[45].

In summary,MoxC/NCS catalysts were synthesized through a facile mechanical mixing and subsequent high-temperature treatment strategy.The different structure and grain size of molybdenum carbides are obtained by adjusting the mass ratio of melamine and ammonium molybdate precursors.Mo2C is gradually converted to MoC with increasing the amount of melamine.The electrochemical performance for HER of these MoxC/NCScomposites is investigated in both 0.5mol/LH2SO4and 1 mol/L KOH media.The Mo2C/NCS-10 presents a good HER activity,and exhibits a small overpotential of 130 m V in 0.5[79_TD DIFF]mol/L H2SO4and 108m V in 1 mol/LKOH for delivering current density of 10[46_TD DIFF]m A/cm2,respectively.The superior HERactivity is attributed to the N-doped carbon nanosheets,ultra fine Mo2C nanoparticles,moderate surface area and mesoporous structure.

Acknow ledgm ents

This work was supported by the National Natural Science Foundation(No.21706086),1000 Young Talent(to Deli Wang).The authors thank the Analytical and Testing Center of HUST for allow ing use its facilities.

Appendix A.Supplem entary data

Supplementary materialrelated to thisarticlecan befound,in the online version,at doi:https://doi.org/10.1016/j.cclet.2018.05.009.