Tungsten-Assisted Phase Tuning of Molybdenum Carbide for Efficient

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Tungsten-Assisted Phase Tuning of Molybdenum Carbide for Efficient Electrocatalytic Hydrogen Evolution Kai Zhang, Gong Zhang, Jiuhui Qu, and Huijuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14733 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Tungsten-Assisted Phase Tuning of Molybdenum Carbide for Efficient Electrocatalytic Hydrogen Evolution Kai Zhang,a,d Gong Zhang,b Jiuhui Qu,b,c Huijuan Liu*a,b a

State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. b

c

School of Environment, Tsinghua University, Beijing 100084, China Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. d

University of Chinese Academy of Sciences, Beijing 100049, China.

KEYWORDS: nonprecious metal electrocatalysts, molybdenum carbide, phase tuning, electronic structure, hydrogen evolution reaction (HER)

ACS Paragon Plus Environment 1

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ABSTRACT: Phase tuning during crystal phase transformation is an important but challenging step toward the development of effective hydrogen evolution reaction (HER) catalysts. Herein, we report on a feasible approach to achieve effective polycrystalline molybdenum carbides during transformation from α-phase to β-phase through the regulation of additive (tungsten) amount. Benefiting from the optimal Mo-C bond lengths and abundant active sites, molybdenum carbide prepared with a moderate addition of tungsten enhanced electrocatalytic activity and stability compared with pure α-phase and β-phase Mo2C in the HER, and only required an overpotential of 148 and 93 mV to drive 20 mA cm-2 of current density in 0.5 M H2SO4 and 1.0 M KOH, respectively.

1. INTRODUCTION Hydrogen produced by water splitting is considered a promising clean energy for replacing traditional fossil fuels and ameliorating environmental contamination.1-4 Although platinum (Pt) or Pt-based materials are the most active electrocatalysts for hydrogen evolution reaction (HER), their scarcity and high cost substantially hinders their global application.5,

6

Exploration of

effective, abundant, and inexpensive HER electrocatalysts remains an urgent priority. Due to their unique d-band electronic structures, considerable effort has been made to develop molybdenum-based electrocatalysts (e.g., Mo2C, MoS2, and MoP) that exhibit comparable catalytic properties to Pt group metals.7-12 Molybdenum carbides (Mo2C) with superior catalytic performance have been investigated in light of their intrinsic low cost and high abundance.13, 14 To improve HER activity, researchers have devoted substantial attention to different structures, such as nanoparticles, nanowires, nanosheets, nanorods, and nanotubes.15-22 In addition, through tuning surface properties, such as the electronic band structure and synergistic coupling effects,


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researchers have examined the integration of Mo2C with other elements or the introduction of doping elements.23-27 However, in view of the heterogeneous carburization process required, it is difficult to achieve well-defined Mo2C nanostructures using a universal synthesis process, thereby restricting their practical application. Furthermore, doping or electronic structure regulation processes can result in structural damage to the catalysts. Alternative methods related to phase regulation have been attempted recently for the development of highly effective catalysts. Four crystal structures of molybdenum carbide have been produced by different fabrication approaches, including α-MoC1-x, η-MoC, γ-MoC, and βMo2C, which show increasing catalytic HER activity in the order of α-MoC1-x < η-MoC < γ-MoC < β-Mo2C.13,

28

Notably, large differences in HER activity are observed among these

molybdenum carbide phases, with β-phase molybdenum carbide (β-Mo2C) considered to be a highly active HER catalyst.29, 30 It is suggested that polycrystalline molybdenum carbide might possess unexpected HER activity. Crystal-phase control of α-MoC1-x and β-Mo2C has also been explored by precisely adjusting the type of hydrocarbon used as the reducing and carburizing agent as well as the carburizing temperature.31 Nevertheless, no relevant studies have focused on the phase transformation process of molybdenum carbide. In addition, very few studies have reported on the HER activities of polycrystalline molybdenum carbides. We successfully developed a method to transform from α-phase to β-phase molybdenum carbide by tuning the crystal phase using an additive-assisted approach in which tungsten content was gradually increased. Due to differences in the carbonizing temperature used for synthesizing different phases of molybdenum carbide, the α-phase to β-phase transition was achieved by regulating the tungsten content to initiate temperature changes for the formation of Mo-C bonds. In comparison to pure α-phase and β-phase Mo2C, the polycrystalline molybdenum carbide



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nanoparticles (0.5 W-MoxC, including α-Mo2C and η-MoC) with optimal tungsten addition showed superior HER performance, with an overpotential of only 148 and 93 mV required to drive 20 mA cm-2 of current density in 0.5 M H2SO4 and 1.0 M KOH, respectively. The high performance of the 0.5 W-MoxC for HER was attributed to the optimal Mo-C bond length, large surface area, and abundant active sites. 2. EXPERIMENTAL SECTION 2.1 Materials. Sodium tungstate dihydrate (Na2WO4·2H2O), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), potassium dichromate (K2Cr2O7), glucose, and platinum on graphitized carbon (20% Pt/C) were purchased from Sinopharm Chemical Reagents Beijing Co., Ltd. Ultrapure water (18.2 MΩ·cm) was used as the solvent. 2.2 Synthesis of materials. We dissolved 30 mmol glucose and 1.5 mmol (NH4)6Mo7O24·4H2O in 40 ml of ultrapure water. After that, n mmol (n=0, 0.5, 1.0, 2.5, and 4) Na2WO4·2H2O was added to form homogeneous solutions (Table S1). The mixed solutions were transported into a 50ml Teflon-lined stainless autoclave, followed by hydrothermal reaction at 200°C for 10h. The mixture precursors were then collected from the autoclave and dried at 60°C. Subsequently, the precursors were calcined at 850°C for 2.5 h with a heating rate of 5°C min-1 in an argon (Ar) atmosphere. A successive H2 flow at 150s.c.c.m was provided for an additional 30 min at 850°C, with the final products cooled to room temperature naturally. The obtained products were denoted as n W-MoxC (n=0.5, 1.0, 2.5, and 4.0, Table S1), where n is the addition amount (mmol) of tungsten (Na2WO4·2H2O). For comparison, we substituted chromium for tungsten (Table S2). The synthetic method of Cr-MoxC remained the same as that of W-MoxC with K2Cr2O7 as the second metal source.


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2.3 Characterization. Powder X-ray diffraction (XRD) with Cu Kα irradiation operating at 40 kV and 40 mA with a fixed slit was used to characterize crystal structure. Morphology was determined by field emission scanning electron microscopy (FE-SEM, SU8010, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, 2100F, Hitachi, Japan) coupled with an energy dispersive X-ray spectroscope (EDS). Nitrogen sorption isotherms and pore size analysis were performed using an ASAP 2020 instrument at liquid nitrogen temperature (77K). X-ray photoelectron spectroscopy (XPS) was performed in a PHI5000 Versa Probe system, using C1s (284.8 eV) as a reference. The evolved gases during HER were quantified by a gas chromatograph (Agilent 6890, USA) equipped with a thermal conductivity detector. The gas chromatograph was connected to an airtight H-cell with two tubes as the Ar carrier gas inlet and outlet. The W and Mo contents of W-MoxC nanoparticles were determined by inductively coupled plasma (ICP), and C content was determined by CHN elemental analysis using a Vario EL Elementar analyzer. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were measured on a TGA/DSC 1/1600 thermogravimetric analyzer in an Ar flow. Raman spectra were obtained using a confocal Raman microscope at 532 nm. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker ER 300E spectrometer at room temperature. The Mo K-edge X-ray absorption fine structure (XAFS) spectroscopy was carried out at the BL 14W1 beam line at the Shanghai Synchrotron Radiation Facility (SSRF), China. 2.4 Electrochemical measurements. The catalytic activity of the prepared samples was evaluated in a three-electrode setup via a computer controlled CHI electrochemical workstation. A glass carbon electrode (GCE, 3 mm in diameter) was used as the working electrode, Ag/AgCl/3 M KCl was used as the reference electrode, and a graphite rod was used as the auxiliary electrode. For the preparation of the catalyst suspension, 5 mg of catalyst was dispersed



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in 1 ml of Nafion solution containing 750 µl of ultrapure water, 200 µl of isopropanol, and 50 µl of 5% Nafion solution, followed by ultrasonication for 30 min. Then, 5 µl of the dispersion was pipetted onto the polished GCE to form a catalyst film with a loading of 0.35 mg cm-2 after drying overnight at ambient temperature. Linear sweep voltammetry (LSV) was recorded at a scan rate of 2 mV s-1 in 0.5 M H2SO4 solution or 1.0 M KOH solution to obtain polarization curves. The polarization curves were corrected with iR-compensation regarding ohmic resistance of the solution. Long-term stability of the catalyst was tested using chronopotentiometry and chronoamperometry

measurements.

Electrochemical

impedance

spectroscopic

(EIS)

measurements were performed at η = 100 mV in the frequency range of 0.1 Hz to 100 kHz. The electrical double-layer capacitance was determined from the CV curves measured in a potential range without redox processes according to the equation: Cdl = Ic / ν, where Cdl, Ic, and ν are the double-layer capacitance (mF cm-2) of the electroactive materials, charging current (mA cm-2), and scan rate (mV s-1), respectively. 3. RESULTS AND DISCUSSION In the absence of tungsten, the orthorhombic crystal structure of α-Mo2C was well-indexed to the reference XRD pattern (JCPDS No. 31-0871), whereas hexagonal phase β-Mo2C (JCPDS No. 35-0787) was achieved with suitable tungsten addition (Figure 1a).17, 25 As the W/Mo molar ratio increased to 0.048, peaks at 36.9° and 42.8° (labeled by blue dots) appeared, and were assigned to η-MoC (JCPDS No. 89-4305).13 Simultaneously, the evolution of narrow peaks was due to the transformation of α-Mo2C to β-Mo2C. After that, the peaks at 36.9° and 42.8° gradually vanished as the addition of tungsten further increased, with the XRD pattern finally well-matched to pure β-phase Mo2C (Figure 1a). Thus, significant α-Mo2C → β-Mo2C phase transition was clearly induced by the progressive increase of tungsten. Furthermore, a similar transformation pathway


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was observed when the W source was substituted by a Cr source, suggesting a feasible congenerassisted phase transition mechanism (Figure S1). To gain insight into the underlying mechanism for the role of tungsten in molybdenum carbide crystal growth, the formation pathways of the five carbide electrocatalysts were explored using a TG-DSC analyzer. As shown in Figure 1b, the DSC profiles of the five carbide electrocatalysts demonstrated small exothermic reactions over the ranges 720–750, 735–778, 737–817, 758–857, and 762–861°C for α-Mo2C, 0.5 W-MoxC, 1.0 W-MoxC, 2.5 W-MoxC, and 4.0 W-MoxC (βMo2C), respectively, which was likely due to the reduction and formation of Mo-C bonds.32 To preliminarily exclude the interference of W-C bonds during polyposis, Raman spectra for graphitic carbon were observed to maintain the same intensities as the pristine one (Figure S2). We also found that W-C bonds could not be formed under 850°C polyposis conditions, further confirming that the exothermic reactions resulted from the formation of Mo-C bonds (Figure S3). Therefore, higher temperatures were necessary to form the Mo-C bonds with increasing tungsten content. As expected, the temperature to synthesize α-Mo2C was relatively low, whereas that to synthesize β-Mo2C was high.13, 31 Thus, the temperature required for Mo-C bond formation was a critical factor affecting the phase transformation of molybdenum carbide, with the α-Mo2C → βMo2C phase transition in the current study realized by increasing the amount of tungsten to elevate the temperature required for Mo-C bond formation. This was further confirmed by thermogravimetric analysis (TGA, Figure S4). As shown in Figure S5, similar trends of TGA, DSC, and the first derivative weight profiles in the Cr-MoxC samples were found, further verifying that α-Mo2C → β-Mo2C phase transition occurred with the formation of Mo-C bonds due to temperature variations following the addition of congeners (e.g., tungsten or chromium).



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Notwithstanding the elusive mechanism therein, the choice of congeners as the additive is important for tuning HER electrocatalysts. We used SEM and TEM to analyze the nanostructure of different W-MoxC samples. The SEM images primarily indicated that the materials had porous surfaces loosely covered by graphitic carbon (Figure S6). Based on ICP and EDS analyses, a high W/Mo molar ratio in the MoxC was observed with increasing additive (W) (Figure S7 and Table S1). The selected area electron diffraction (SAED) patterns of α-Mo2C, 0.5 W-MoxC, 1.0 W-MoxC, and 4.0 W-MoxC (β-Mo2C) corresponded to α-Mo2C, α-Mo2C and η-MoC, β-Mo2C and η-MoC, and β-Mo2C phase compositions, respectively (Figure S8). Accordingly, the HR-TEM images revealed an orthorhombic phase structure of α-Mo2C with lattice spacing values of 0.260, 0.229, and 0.175 nm assigned to the (002), (121), and (221) planes, respectively (Figure 1c). The (021), (002), and (121) lattice fringes of 0.5 W-MoxC were identified by HR-TEM (Figure 1d). Figure 1e shows that the 4.0 W-MoxC sample (β-Mo2C) exhibited hexagonal nanoparticles under HR-TEM, with lattice fringes of 0.26, 0.24, and 0.23 nm corresponding to the (100), (002), and (101) planes, respectively. The absence of tungsten nanostructure in TEM analysis was consistent with the XRD results. To distinguish the distribution of Mo and W elements in the 0.5 W-MoxC and 4.0 W-MoxC (β-Mo2C) samples, line-scan EDS profiles were determined, indicating that molybdenum was located in the nanoparticle core and tungsten primarily resided at the edges (Figure 1f, 1g). Moreover, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed the even distribution of the W element on the outer shell of the 4.0 W-MoxC (β-Mo2C) particles, coinciding with the EDS results (Figure S9). To further understand the role of tungsten, N2 desorption isotherms were subsequently measured for the W-MoxC samples. In the presence of W, the Brunauer-Emmett-Teller (BET) surface area


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in 0.5 W-MoxC (345.7 m2 g-1) was four times higher than that for α-Mo2C (86.2 m2 g-1) (Figure S10, Table S3). However, further increases in tungsten content decreased the surface area, with values of 196.1, 256.3, and 261.0 m2 g-1 for 1.0 W-MoxC, 2.5 W-MoxC, and 4.0 W-MoxC (βMo2C), respectively. In agreement with the Barrett-Joyner-Halenda (BJH) results, the decreased surface area was mainly induced by excessive W aggregates in the pores. The amount of carbon in W-MoxC was quantitatively determined by CHN elemental analysis, which exhibited a positive correlation to surface area (Table S3). The role of the carbon precursor was also investigated, indicating that the variable C/Mo ratio was independent of the formation of MoxC (Figure S11). The HER performance of the as-prepared α-Mo2C and W-MoxC electrocatalysts was investigated on glassy carbon (GC) electrodes with a mass loading of 0.35 mg cm-2 in acidic aqueous solution. The polarization curve for 0.5 M H2SO4 is shown in Figure 2a, along with that of a benchmark 20% Pt/C catalyst for reference. As expected, the commercial 20% Pt/C exhibited the highest HER performance in acidic media, with onset overpotential of nearly zero and a low overpotential of 51 mV to achieve a current density of 20 mA cm-2. For the W-MoxC series, however, 0.5 W-MoxC exhibited the highest HER performance. To drive a current density of 20 mA cm-2 for HER, the overpotential value increased from 148 mV for 0.5 W-MoxC to 160 mV for α-Mo2C, 177 mV for 1.0 W-MoxC, 189 mV for 2.5 W-MoxC, and 216 mV for 4.0 W-MoxC (β-Mo2C). Linear Tafel plots are often employed to evaluate HER kinetics and elucidate the HER mechanism by fitting the data to the equation (η=b log (j) +a, where j is the current density and b is the Tafel slope). Generally, two mechanisms are involved in the HER process in acidic media.33, 34 First, a hydrated proton combines with an electron to form adsorbed hydrogen at the Tafel slope of 118 mV dec-1 (Volmer-reaction, H3O+ + e- → Hads + H2O). The adsorbed H then



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either associates with a hydrated proton to evolve to H2 at the Tafel slope of 40 mV dec-1 (Heyrovsky-reaction, Hads + H3O+ + e-→ H2 + H2O) or binds with another adsorbed H to give H2 at the Tafel slope of 30 mV dec-1 (Tafel-reaction, Hads + Hads → H2), followed by desorption from the catalyst. The Tafel slope of commercial 20% Pt/C was 26 mV dec-1, indicating a VolmerTafel mechanism (Figure 2a, inset).4, 35 The 0.5 W-MoxC sample exhibited a Tafel slope of 56 mV dec-1, which showed higher HER activity than that of α-Mo2C (59 mV dec-1) and 4.0 WMoxC (β-Mo2C, 95 mV dec-1), thus following the Volmer-Heyrovsky mechanism.4 Under the same conditions, the HER kinetics were inhibited due to the effect of excessive tungsten on the 1.0 W-MoxC (66 mV dec-1) and 2.5 W-MoxC (70 mV dec-1) samples (Figure 2a, inset). We further analyzed electrical double-layer capacitance (EDLC, Cdl) to evaluate the electrochemically effective surface area (ECSA). Derived from cyclic voltammograms from 0.2 to 0.4 V at rates varying from 20 to 200 mV s-1, the Cdl of the 0.5 W-MoxC sample was 50.41 mF cm-2, which was much larger than that of α-Mo2C (37.34 mF cm-2), 1.0 W-MoxC (19.58 mF cm2

), 2.5 W-MoxC (7.87 mF cm-2), and 4.0 W-MoxC (β-Mo2C, 2.54 mF cm-2) (Figure 2c, S12).36, 37

The EIS results for 0.5 W-MoxC showed the smallest semicircle (Rct as low as 87.8 Ω) in the Nyquist plot compared with the other samples, demonstrating lower impedance and superior charge transport kinetics for hydrogen production, as shown in Figure 2e.38, 39 The HER activities of the α-Mo2C and W-MoxC electrocatalysts in acidic media are summarized in Table S4. In addition, the electrocatalytic properties of the W-MoxC series for HER were investigated in 1.0 M KOH. The 0.5 W-MoxC sample exhibited dramatically enhanced HER performance in basic media. As shown in Figure 2b, an overpotential of only 93 mV was required for 0.5 WMoxC to produce a current density of 20 mA cm-2, which markedly outperformed that of commercial 20% Pt/C (η=144 mV). To achieve the same current density, the α-Mo2C, 1.0 W-


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MoxC, 2.5 W-MoxC and 4.0 W-MoxC (β-Mo2C) samples required 130, 144, 167, and 194 mV, respectively. The corresponding Tafel slopes of the five electrocatalysts varied in line with HER activity (Figure 2b, inset). It is thought that HER activity in an acidic environment is superior to that in a basic environment because HER is a multistep process under alkaline conditions involving the dissociation of water molecules and desorption of atomic hydrogen.40, 41 We found that HER activity of the MoxC species in basic media, especially that of 0.5 W-MoxC, was much better than that in acidic electrolytes. The same behavior has also been found with β-Mo2C nanotubes, Mo2C@N-doped carbon, and MoCx nanowires, which is probably attributable to the exposure of more active sites for HER due to the dissolution of surface oxidized species on Mo2C by KOH.17, 22, 42 Accordingly, the electrocatalytic activities of the Cr-MoxC samples were also tested for HER performance in both acidic and alkaline media. The LSV curves and corresponding Tafel slopes are displayed in Figure S13, suggesting that the HER performance of 0.5 Cr-MoxC was best among the chromium-containing MoxC species in 0.5 M H2SO4 and 1.0 M KOH. The order of electrocatalytic performance for the samples in 1.0 M KOH was consistent with their Cdl (Figure 2d, S14) and Rct (Figure 2f, Table S5). The 0.5 W-MoxC sample, with a small addition of tungsten, promoted HER activity compared with α-Mo2C, whereas the performances of 1.0 W-MoxC, 2.5 W-MoxC and 4.0 W-MoxC (β-Mo2C) were suppressed due to excessive tungsten. Remarkably, the 0.5 W-MoxC nanoparticles displayed excellent electrocatalytic activity for HER, which was higher than that of other comparable catalysts reported in the literature, such as molybdenum-based compounds, dichalcogenides and phosphides (Table 1, S8). Finally, generated hydrogen in 1.0 M KOH by a loaded 0.5 W-MoxC electrode was determined using gas chromatography. Comparing the theoretical calculation with experimental



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measurement, the Faradaic efficiency (FE) was determined to be close to 100% for the HER (Figure S15). HER performance is closely related to the nature of active sites. XPS is often performed to investigate the surface properties of electrocatalysts.43 Herein, the elements C, Mo, and W were present in the W-MoxC species (Figure S16). Notably, the addition of tungsten in W-MoxC changed the electronic features of Mo. The high-resolution Mo 3d portion of the XPS spectra displayed three doublets, suggesting three valence states (+2, +4 and +6 ) for Mo on the surface of the catalysts (Figure 3a). The dominant Mo2+ peak was attributed to the Mo-C bonds in the molybdenum carbides.44, 45 Previous studies have also shown that Mo2+ species are the active centers for electrocatalytic HER.17,

23

The surface Mo2+ percentage was confirmed by

quantitative XPS analysis, indicating that the Mo2+ percentage reached a maximum of 46.4% in 0.5 W-MoxC (Figure 3a, Table S6). As mentioned above, the amorphous structure indicated that W-C bonds could not be formed under 850 °C polyposis conditions, excluding the possibility of substitution of Mo by W (Figure S3). Furthermore, lattice distortion was not observed in the HRTEM images of the molybdenum carbides with various amounts of W, confirming the absence of interstitial doping. Therefore, the changed electronic structure of Mo induced by W doping was not likely to be the dominant reason for the increased catalytic property. Furthermore, we confirmed the chemical state of W, with the coexistence of W6+ and W4+ embedded in the carbon matrix (Figure S17).46 The changes in W valence state might result from a different coordination mode of W with O in the polyposis process, and the high Mo2+ ratio might be attributable to tungsten being a suitable scavenger of oxygen. The HER performance of α-Mo2C was markedly higher than that of β-Mo2C, as shown in Figure 2a, b. Although the surface area(SBET) increased from 86.2 m2 g-1 (α-Mo2C) to 256.3 m2 g-1 (4.0 W-MoxC, β-Mo2C) (Table S3), the percentage of


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surface Mo2+ declined from 38.8 to 16.5% (Table S6), indicating that the surface concentration of Mo2+ plays a key role in improving HER performance. Notably, W-MoxC exhibited higher HER activity compared with that of Cr-MoxC with the same phase compositions. As shown in Figure 3a, S18, the higher percentage of active Mo2+ in W-MoxC might be due to the protective role of tungsten, thus benefitting HER activity. Mo4+ and Mo6+ species were derived from molybdenum oxide on the surface of W-MoxC due to air contact, which were usually thought to be inactive for HER.16, 47 However, molybdenum dioxide (Mo(IV)O2) possesses good electrical conductivity, which is advantageous for electron transport.48,49 The EPR spectra of the W-MoxC species were measured to clarify the oxidation state of molybdenum. The EPR analysis showed a signal at g = 1.933, implying the presence of Mo4+; however, the signal of 0.5 W-MoxC was relatively weaker than that of the other samples, suggesting lower concentration of Mo4+ (Figure 3b).50 In addition, the extended X-ray absorption fine structure (EXAFS) indicated that the Mo-C bond lengths of the W-MoxC species were elongated, increasing from 1.98 to 2.12 Å with the increase in tungsten content (Table S7). Due to its optimal Mo-C bond length (2.06 Å), 0.5 W-MoxC likely facilitated electron transfer from molybdenum to carbon, thus lowering the d-band center of molybdenum, as well as resulting in relatively moderate Mo-H binding strength. This, in turn, should favor the desorption of Hads and promote the transformation of Hads into hydrogen molecules (H2), thus enhancing HER activity. The local atomic arrangement and electronic structure of the W-MoxC series were obtained from the Mo K-edge XAFS spectra, as shown in Figure 3c. The differences among the catalysts were apparent in the Mo K-edge oscillation curves, indicating their different local atomic arrangements, which were further demonstrated by the corresponding R-space curves. The k2weighted Fourier transform showed Mo-C and Mo-Mo peaks in the range of 1 to 6 Å for the W-



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MoxC species (Figure 3c). Moreover, curve-fitting of 0.5 W-MoxC and 4.0 W-MoxC (β-Mo2C) was performed to illustrate fit quality and obtain quantitative structural parameters, which were fitted with models constructed from the molybdenum carbide structure (Figure S19). Long-term durability is also important for a practical catalyst for efficient HER. To investigate the performance stability of the as-prepared 0.5 W-MoxC, a continuous cyclic voltammetry test was carried out in both acidic and alkaline electrolytes. We compared the polarization curves measured before and after 2000 cycles. The final cathodic current exhibited almost no difference from the initial one in both the 0.5 M H2SO4 and 1.0 M KOH solutions (Figure 4a). The stability of the electrocatalysts was also assessed by prolonged electrolysis at a fixed potential. As shown in Figure 4b, the current density of 0.5 W-MoxC held steady for 24 h in both 0.5 M H2SO4 and 1.0 M KOH solutions. Similarly, at constant current, the potential remained stable for 24 h in both acidic and basic media (Figure S20). This clearly indicated that the carbide material had high stability, with tolerance toward both acidic and basic solutions. Earlier work reported that molybdenum carbide exhibits poor stability when exposed to alkaline solutions.15, 22 In our study, however, the excellent stability of the 0.5 W-MoxC nanoparticles probably originated from the additional carbon on the surface of the molybdenum carbide in alkaline media.3 4. CONCLUSIONS In summary, a simple but effective tungsten addition strategy was developed to assist α-Mo2C → β-Mo2C phase transition, resulting in enhanced electrocatalytic activity for molybdenum carbide materials in HER in both acidic and alkaline media. The HER performance of α-Mo2C was better than that of β-Mo2C, but the 0.5 W-MoxC nanoparticles with optimal addition of tungsten exhibited superior activity and favorable stability in both acidic and basic media. Our research


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provides a new strategy for realizing phase transformation of molybdenum carbides, as well as insight into the synergistic effects between tungsten and molybdenum carbides. Most importantly, the correlations uncovered among crystal structures, different Mo-C bond lengths, and enhanced HER properties may provide a theoretical foundation for the design of highly efficient electrocatalysts. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. XRD pattern, Raman analyses, TGA and DSC analysis, SEM morphology, SAED patterns, TEM energy dispersive spectroscopy analysis, Nitrogen sorption analysis, HER polarization curves and CV curves, XPS and EXAFS analyses, Stability analysis, Literature comparison. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was financially supported by the Key Program of the National Natural Science Foundation of China (No. 51708543, 51438011). REFERENCES (1) Turner, J. A. Sustainable Hydrogen Production. Science. 2004, 305, 972-974.



Page 16 of 29

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Figure 1. a) XRD patterns and b) DSC thermograms of (I) α-Mo2C, (II) 0.5 W-MoxC, (III) 1.0 W-MoxC, (IV) 2.5 W-MoxC, and (V) 4.0 W-MoxC (β-Mo2C). HR-TEM images of samples c) α-Mo2C, d) 0.5 W-MoxC, and e) 4.0 W-MoxC (β-Mo2C). Line-scan profiles of f) 0.5 W-MoxC and g) 4.0 W-MoxC (β-Mo2C), showing Mo and W elemental distributions.


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Figure 2. Polarization curves of (I) α-Mo2C, (II) 0.5 W-MoxC, (III) 1.0 W-MoxC, (IV) 2.5 W-MoxC, (V) 4.0 W-MoxC (β-Mo2C), and (VI) commercial 20% Pt/C on GC electrodes for HER performance in a) 0.5 M H2SO4 and b) 1 M KOH at a scan rate of 2 mV s-1. Estimation of Cdl in c) 0.5 M H2SO4 and d) 1 M KOH by plotting the current density variation between the anodic and cathodic sweep (∆j) at 300 mV vs. RHE (data obtained from the CV in Figure S12 and S14 in the Supporting Information) against scan rate to fit a linear regression. Nyquist plots of the above carbide electrocatalysts measured at η = 100 mV in e) 0.5 M H2SO4 and f) 1 M KOH. Data obtained by electrochemical impedance spectroscopy (EIS).



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Figure 3. a) High-resolution XPS profiles of Mo 3d in (I) α-Mo2C, (II) 0.5 W-MoxC, (III) 1.0 W-MoxC, (IV) 2.5 W-MoxC, and (V) 4.0 W-MoxC (β-Mo2C), and surface Mo2+ percentages determined by XPS (inset). b) EPR spectra measured on (I) α-Mo2C, (II) 0.5 W-MoxC, (III) 1.0 W-MoxC, and (V) 4.0 W-MoxC (β-Mo2C). c) k2-weighted EXAFS of the Mo K edge in k-space and Fourier transform magnitudes in R space (inset) for (I) α-Mo2C, (II) 0.5 W-MoxC, (III) 1.0 W-MoxC, and (V) 4.0 W-MoxC (β-Mo2C).


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Figure 4. a) Stability test of 0.5 W-MoxC showing initial polarization curve and corresponding curve after 2000 cycles in 1.0 M KOH and 0.5 M H2SO4. b) Time-dependent catalytic current for 0.5 W-MoxC in 1.0 M KOH and 0.5 M H2SO4.



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Table 1. Activities of recently reported Mo-based electrocatalysts for HER in acidic or basic media

Catalyst

0.5W-MoxC

Loading -2

(mg cm )

0.35

Li-MoS2

3.4

MoP

0.86

MoCx nano-octahedrons

MoC-Mo2C heteronanowires

Mo2C nanotubes

0.8

0.14

0.75

Current

Corresponding

density

overpotential -2

(j, mA cm )

(mV dec-1)

148

56

0.5 M H2SO4

93

44

1.0 M KOH

200

200

62

0.5 M H2SO4

30

180

54

0.5 M H2SO4

10

130

48

1.0 M KOH

142

53

0.5 M H2SO4

151

59

1.0 M KOH

126

43

0.5 M H2SO4

120

42

1.0 M KOH

197

62

0.5 M H2SO4

127

55

0.1 M KOH

~225

55-56

1.0 M H2SO4

210-240

54-59

1.0 M KOH

Reference

this work

10

11

10

15

17

10

20

22

20 0.8-2.3

Electrolyte

20

1.4-2.5 Mo2C microparticles

(η, mV)

Tafel slope

30


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Tungsten-Assisted Phase Tuning of Molybdenum Carbide for Efficient

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