Facile synthesis of hollow carbon microspheres embedded with molybdenum carbide nanoparticles as an efficient electrocatalyst for hydrogen generation

Abstract

In an attempt to exploit efficient and stable non-precious-metal electrocatalysts for hydrogen production from water electrolysis in both acid and basic solution, hollow carbon microspheres embedded with molybdenum carbide nanoparticles are prepared via ultrasonic spray pyrolysis. The as-synthesized catalyst exhibits superior activity in hydrogen evolution reaction (HER) with a small overpotential of 203 mV in acidic solution and 346 mV in basic solution to reach a current density of 20 mA cm⁻². The enhanced electrochemical activity should be ascribed to the effects of the anchored structure. The catalyst can work stably in both acidic and basic solution with 100% faradaic efficiency. These excellent properties make the catalyst a promising electrocatalyst in the HER.

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Introduction

Developing renewable clean energy sources is of critical importance as the energy crisis and environmental pollution become severe. Molecular hydrogen generated from water electrolysis could play a major role as an energy carrier. The efficiency of the process is dictated by the overpotential that has to be applied to overcome the energy barriers, and inexpensive electrocatalysts with high efficiency for the hydrogen evolution reaction (HER) are required. Though noble-metal catalysts are the state-of-the-art catalysts, they are expensive and in low abundance. The development of efficient and low-cost non-precious-metal electrocatalysts is an important and urgent precondition for practical utilization. In the past decade, transition metal chalcogenides,^1–3 nitrides,^4,5 phosphides^6–12 and carbides^13–15 have been reported to be promising electrocatalysts towards the HER. In particular, transition metal carbides (e.g., molybdenum carbide) display remarkable catalytic activities, ascribed to their unique electronic and catalytic properties similar to noble metals (by inducing carbon into the metal lattice), and have attracted considerable attention as alternative catalysts.^16–18

Enormous research efforts on the incorporation of electrocatalysts with highly conductive materials (e.g., carbon materials) have been devoted to improve the electrocatalytic HER activity. Several kinds of ideal carbon materials with high electrical conductivity and large surface area have been reported, such as graphene or doped graphene with heteroatoms (e.g., N, B, P),^1,14 carbon nanotubes,¹³ carbon nanospheres⁶ and Vulcan XC-72R.¹⁸ The approaches to synthesize the hybrids of electrocatalysts anchored to carbon supports include in situ carburization of transition metal salts supported on pre-synthesized carbon materials,^18–21 the thermolysis of metal–organic frameworks (MOFs) or other preformed precursors,²² and so on. Hence, the one-pot synthesis of carbon-loaded electrocatalysts has been emerged and attracted increasing attention, markedly reducing the time and cost of synthesis. The successful examples include Fe₂P/N-doped graphene,⁹ Fe–N-doped carbon nanofibers,²³ Mo₂C/graphene porous foam,²⁴ Mo₂C/Mo₂N composite,²⁵ etc.

Herein, we describe a one-step protocol for the synthesis of hollow carbon microspheres embedded with molybdenum carbide nanoparticles (Mo₂C/HCMs). The resulting material acts as a highly active non-precious HER electrocatalyst, exhibiting a low η₂₀ (overpotential required to reach a current density of 20 mA cm⁻²) of 203 mV in acidic solution, and 346 mV in basic solution. The Mo₂C/HCMs afford stable water electrolysis over 24 h in both acidic and basic solution. The Tafel slope is 83.9 mV dec⁻¹ in acidic solution and 143.4 mV dec⁻¹ in basic solution, implying that the HER process might proceed along Volmer–Heyrovsky mechanism in acidic solution and Volmer reaction in basic solution. The faradaic efficiency of the Mo₂C/HCMs is 100%. The performance of the Mo₂C/HCMs compares favorably to those of other carbon-based Mo₂C nanostructures reported.

Experimental

Synthesis of Mo₂C/HCMs

Typically, the precursor solution was prepared by dissolving glucose (C₆H₁₂O₆·H₂O, 5.945 g), sodium chloride (NaCl, 7.0128 g) and hexaammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O, 3.3368 g) in deionized water. The solution was atomized with an ultrasonic nebulizer and the resulting mist was carried into a preheated (800 °C) quartz tube reactor in an insulated tube furnace with inert stream (2 L min⁻¹, 99.99% N₂). The product was collected by deionized water bubblers. Then the product was isolated by centrifugation and dried under vacuum at 60 °C overnight. Finally, the product was annealed at 850 °C for 2 h in 5% H₂/95% N₂ atmosphere.

Characterization

Scanning electron microscopy (SEM, S-4800, Hitachi) and transmission electron microscopy (TEM, 300 kV, Tecnai G2 F30 S-TWIN, FEI) were used in the morphology characterization of the resultant sample. The X-ray energy dispersive spectroscopy (EDS) spectra were recorded using an EDXA Instruments' TIA system equipped on the TEM. The composition and microstructure of the as-prepared sample were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation. λ = 1.54178 Å), Raman spectra (ThermoFisher, USA, λ = 532 nm), X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, ThermoFisher) and thermal gravimetric analysis (TGA, STA449C, NETZSCH, Germany, heating rate 10 °C min⁻¹, O₂ atmosphere).

Electrochemical measurement

All electrochemical measurements were performed using an electrochemistry workstation (CHI 614D Instruments) and a standard three-electrode system. A graphite rod, a mercury/mercurous sulfate electrode (MSE) or mercury/mercury oxide electrode (MOE), and a glass carbon electrode (GCE) were used as the counter, reference electrode, and working electrode, respectively. A porous glass frit was used to separate the working electrode and the counter electrode. The H₂-purged 0.5 M H₂SO₄ or 1 M KOH aqueous solution was used as electrolyte. The reversible hydrogen evolution potential (RHE) was determined to be −0.699 V versus MSE for the 0.5 M H₂SO₄ and −0.83 V versus MOE for the 1 M KOH. All of the potentials reported in our manuscript were referenced to a RHE by adding a value of 0.699 V for the 0.5 M H₂SO₄ and 0.83 V for the 1 M KOH. The scan rate is 5 mV s⁻¹ for linear sweep voltammetry (LSV) and 50 mV s⁻¹ for cyclic voltammograms (CV). Electrochemical impedance spectroscopy (EIS) measurements were carried out at −200 mV vs. RHE in 0.5 M H₂SO₄ in the frequency range of 10⁻² to 10⁶ Hz. To evaluate the electrochemical surface area (ECSA), CV scans were obtained from 0.1 to 0.2 V vs. RHE at pH 7 with sweep rates of 60, 80, 100, 120, 140, 160 and 180 mV s⁻¹.

Results and discussion

The structural details of Mo₂C/HCMs were revealed by XRD experiment (Fig. 1a) and the peaks are consistent with the reference XRD patterns of hexagonal Mo₂C (JCPDS no. 35-787, a = 3.0124 Å, c = 4.7352 Å). The Scherrer equation was used to estimate the average grain size of Mo₂C nanoparticles, being (39.9 ± 0.6) nm.²⁶ The presence of the carbon was confirmed by the Raman spectrum (Fig. 1b), in which the peaks corresponding to D band (1345 cm⁻¹) and G band (1589 cm⁻¹) are well distinguished. The intensity ratio between the D band and the G band (I_D/I_G) is 1.000, which is analogous to that of reduced graphene oxide (1.024)²⁷ and that of graphite structure thermolysis from glucose (1.010).²⁴ A TGA experiment (Fig. S1, ESI†) suggests that the mass ratio of Mo₂C nanoparticles in the Mo₂C/HCMs is ca. 56.1%.

Fig. 1 (a) XRD pattern and (b) Raman spectrum of the Mo₂C/HCMs. (c) Mo 3d window and (d) C 1s window of the XPS spectrum of Mo₂C/HCMs.

Mo₂C/HCMs were analysed by XPS to probe the electronic structures of the catalysts. Four doublets can be deconvoluted from the Mo 3d window (Fig. 1c). The peak at 228.7 eV was attributable to Mo²⁺, stemming from Mo₂C. In parallel, the peaks at 229.4, 232.4 and 233.0 eV were attributable to Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺, which can be associated with a thin layer of molybdenum oxide developed on the surface of the Mo₂C nanoparticles due to air exposure.^24,28,29 Three peaks are suggested by peak fitting from the C 1s window (Fig. 1d), which are assigned to C–Mo (284.2 eV), C–C/C=C (284.8 eV), and C=O (285.9 eV), respectively.^24,28

The overall morphology of sample Mo₂C/HCMs is revealed in Fig. S2a (ESI†). It is shown that the sample is composed of a large quantity of microspheres, average diameter being 1.08 ± 0.03 μm (Fig. S2b, ESI†). A low-magnification SEM image (Fig. 2a) shows that the microspheres exhibit a hollow structure with obvious porous on their surface (as indicated by arrows). A SEM image with larger magnification (Fig. 2b) shows that the surface of the microspheres is crumpled and rough. From the low-magnification TEM image (Fig. 2c), some Mo₂C nanoparticle aggregates with size about 100 nm (as indicated by rectangle) are found embedded in the shell of the carbon hollow sphere. To further investigate the distribution of the Mo₂C in the carbon spheres in the Mo₂C/HCMs, scanning TEM (STEM) and EDS elemental-mapping experiments were performed. The typical dark-field STEM image (Fig. 2d) demonstrates that the Mo₂C nanoparticles (the brighter component in the image) were embedded in the carbon hollow microspheres, which is further confirmed by the corresponding EDS mapping images for C (Fig. 2e) and Mo (Fig. 2f).

Fig. 2 (a) Low- and (b) high-magnification SEM images of the Mo₂C/HCMs. (c) Low-magnification TEM image of the Mo₂C/HCMs. (d) Dark-field STEM image of the Mo₂C/HCMs and corresponding elemental mapping images of (e) C and (f) Mo.

The electrochemical polarization experiments were performed to evaluate the electrocatalytic activity of the Mo₂C/HCMs composite toward the HER in 0.5 M H₂SO₄. For comparison, a bare GCE, commercial Mo₂C (200 nm diameter from Yao Tian), commercial Pt/C (20 wt% Pt on carbon black from Johnson Matthey) and the physical mixture of the commercial Mo₂C and Vulcan XC-72R (denoted as Mo₂C/XC-72R) were also assessed. The weight content of Mo₂C nanoparticles in Mo₂C/XC-72R is 56.1%, analogous to that of Mo₂C in Mo₂C/HCMs. The corresponding polarization curves are shown in Fig. 3a. The Mo₂C/HCMs loaded on GCE exhibit a remarkable catalytic activity toward HER, with a large current density when the potential is more negative than −0.15 V vs. RHE, while the bare GCE shows poor catalytic activity with almost no reduction current. Moreover, the current density of the Mo₂C/XC-72R is smaller than that of the Mo₂C/HCMs. It is apparent that the formation of hollow carbon spheres could enhance the electrochemical activity of the Mo₂C/HCMs. The Mo₂C/HCMs afford a current density of 10 and 20 mA cm⁻² at overpotentials of 179 and 203 mV, respectively. The Tafel slope of the Mo₂C/HCMs is 83.9 mV dec⁻¹ in 0.5 M H₂SO₄ (Fig. 3b), suggesting that the HER process in the Mo₂C/HCMs proceeds along a Volmer–Heyrovsky mechanism. Meanwhile, the Mo₂C/HCMs shows a lower Tafel slope (83.9 mV dec⁻¹) than Mo₂C (140.9 mV dec⁻¹) and Mo₂C/XC-72R (89.5 mV dec⁻¹), indicating a fast increase of the hydrogen generation rate with the applied overpotential.

Fig. 3 (a) Polarization curves and (b) corresponding Tafel plots of Mo₂C/HCMs, Mo₂C/XC-72R, Mo₂C, bare GCE and Pt/C measured in 0.5 M H₂SO₄ (loading amount: 0.285 mg cm⁻²). All potentials were corrected with the iR drop. (c) I–t curve (overpotential: 230 mV vs. RHE), and CV curves of the initial and 4000th scans in CV sweeps. (d) The theoretical and measured volume of hydrogen during potentiostatic electrolysis.

The performances of different Mo₂C nanostructures reported previously are listed in Table S1 (ESI†). It is shown that the catalytic activity of the Mo₂C/HCMs (η₂₀ = 203 mV) is more active than Mo₂C nanoparticles supported on Vulcan carbon black (η₂₀ = 210 mV),¹⁸ commercial Mo₂C particles (η₁₀ = 225 mV),²⁹ Mo₂C nanoparticles supported on XC-72R carbon black (η₈ = 200 mV),²¹ Mo₂C nanowires (η₂₀ = 220 mV) and Mo₂C (η₂₀ = 260 mV) nanosheets synthesized via pyrolysis of their MoO_x/p-phenylenediamine hybrid precursors,¹⁷ Mo₂C nanoparticles synthesized with soybeans as the carbon source (η₁₀ = 177 mV)²⁵ and so on. The Mo₂C/HCMs shows slightly inferior performance than the 3D hierarchical porous Mo₂C framework obtained by template-assisted approach,¹⁹ mesoporous Mo₂C nano-octahedrons synthesized from metal–organic frameworks,²² Mo₂C–WC composite nanowires prepared by a hydrothermal method followed by a carburization reaction.³⁰ The comparison reveals that the Mo₂C/HCMs exhibit comparable HER catalytic activity in comparison with other Mo₂C with a similar morphology.

Potentiostatic electrolysis was used to evaluate the stability of the Mo₂C/HCMs. After potentiostatic electrolysis for 24 h (Fig. 3c), the current density maintained at 85% of the initial value. CV sweeps between −0.25 and 0 V vs. RHE in a 0.5 M H₂SO₄ solution were also performed (inset of Fig. 3c). After 4000 CV sweeps, the increase of η₂₀ is as small as 20 mV.

The faradaic efficiency of the Mo₂C/HCMs during H₂ evolution was evaluated by the comparison of the experimental and the theoretical volumes of the generated hydrogen in a potentiostatic electrolysis measurement (Fig. 3d). After 4000 s, 13.2C of charge passes through the cycle and the theoretical volume of H₂ should be 1643 μL. The experimental volume of H₂ was measured to be 1649 μL, which is very close to the theoretical value, suggesting the 100% faradaic efficiency.

EIS experiments were carried out to get further insight into the HER process of Mo₂C/HCMs. The Nyquist plots of Mo₂C/HCMs and Mo₂C/XC-72R at given overpotentials (200 mV vs. RHE) in 0.5 M H₂SO₄ are compared and fitted to an equivalent electrical circuit with two time constants (Fig. 4a and its inset). The Mo₂C/HCMs has smaller diameter of the semicircle than that of the Mo₂C/XC-72R, in line with the more efficient HER activity of Mo₂C/HCMs than that of the Mo₂C/XC-72R. Furthermore, the electrochemical capacitance was measured by CV scans to evaluate the ECSA of various catalysts (Fig. S3, ESI†).^31,32 Analysis shows that Mo₂C/HCMs exhibit much larger specific capacitance (17.2 mF cm⁻²) than the Mo₂C/XC-72R (1.4 mF cm⁻²), implying more effective active sites can be exposed for the Mo₂C/HCMs, which is responsible for the excellent HER activity (Fig. 4b).

Fig. 4 (a) Nyquist plots of EIS spectra of Mo₂C/HCMs and Mo₂C/XC-72R (overpotential: 200 mV vs. RHE) (inset of (a)). Equivalent circuit used to fit the EIS data. (b) The differences in current density variation (ΔJ = J_a − J_c) at an overpotential of 0.15 V vs. RHE plotted against scan rate.

The influence of the annealing temperature on the size of Mo₂C nanoparticles and the catalytic activity of Mo₂C/HCMs was investigated. The Mo₂C/HCMs obtained at different annealing temperatures are denoted as Mo₂C/HCMs-xxx (xxx is the annealing temperature). The polarization curves of Mo₂C/HCMs-750, Mo₂C/HCMs-850 and Mo₂C/HCMs-950 are shown in Fig. S4a (ESI†), indicating that the catalytic activity of Mo₂C/HCMs correlates heavily with the annealing temperature. In detail, the η₂₀ value decreased from 361 mV for 750 °C to 203 mV for 850 °C. With a further increase of the annealing temperature to 950 °C, the η₂₀ value increased to 299 mV. The XRD patterns of the samples synthesized at the different temperatures are shown in Fig. S4b (ESI†), indicating the existence of carbon and Mo₂C phases in all samples. Meanwhile the content of the Mo₂C phase increases with the annealing temperature increasing from 750 to 950 °C. The average diameter of the Mo₂C particles, which was estimated from XRD patterns, increased with the increasing of the annealing temperature and is 21.1 nm for Mo₂C/HCMs-750, 39.9 nm for Mo₂C/HCMs-850, and 53.9 nm for Mo₂C/HCMs-950. Therefore, the influence of the annealing temperature on the catalytic of the different samples is mainly induced by the weight content of Mo₂C in the composite and the diameter of the active Mo₂C nanoparticles.

It is found that the Mo₂C/HCMs can also work well in basic solution (KOH, 1 M). The η₂₀ value is 346 mV (Fig. 5a), and the Tafel slope is 143.4 mV dec⁻¹ (Fig. 5b). The current density does not decrease after potentiostatic electrolysis for 24 h (Fig. 5c). The η₂₀ value increases from 345 to 364 mV after 4000 CV sweeps (inset of Fig. 5c). In addition, the faradaic yield is also 100% (Fig. 5d). These experiments demonstrate that the Mo₂C/HCMs can work efficiently and stably in HER in basic solution.

Fig. 5 (a) Polarization curves and (b) corresponding Tafel plots of Mo₂C/HCMs, Mo₂C/XC-72R, Mo₂C, bare GCE and Pt/C measured in 1 M KOH (loading amount: 0.285 mg cm⁻²). All potentials were corrected with the iR drop. (c) I–t curve (overpotential: 350 mV vs. RHE), and CV curves of the initial and 4000th scans in CV sweeps. (d) The theoretical and measured volume of hydrogen during potentiostatic electrolysis.

Conclusions

In summary, we introduce a one-step protocol for the fabrication of uniform Mo₂C nanoparticles embedded in the hollow carbon microspheres. The Mo₂C/HCMs exhibit excellent catalytic performance in both acidic and basic solutions for hydrogen generation. The η₂₀ is as small as 203 mV in acidic solution and 346 mV in basic solution. The performance compares favorably to those of other Mo₂C nanostructures reported. The Mo₂C/HCMs can work stably in both acidic and basic solution. The Tafel slope is 83.9 mV dec⁻¹ in acidic solution and 143.4 mV dec⁻¹ in basic solution. The faradaic efficiency of Mo₂C/HCMs is 100%. This work will open up new opportunities to develop high-performance electrocatalysts via facile ultrasonic spray pyrolysis method.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (61006049, 51172100, 51432006, 21505118), the Ministry of Science and Technology of China (2011DFG52970), the Ministry of Education of China (IRT14R23), 111 Project (B13025), Jiangsu Province (2011-XCL-019 and 2013-479), and the Natural Science Foundation of Jiangsu (BK20131252).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16490k

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