0D/2D heterojunctions of molybdenum carbide-tungsten carbide quantum dots/N-doped graphene nanosheets as superior and durable electrocatalysts for hydrogen evolution reaction

Lili Huo a, Baocang Liu ab, Zhiqing Gao a and Jun Zhang *ab
aCollege of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P. R. China. E-mail: cejzhang@imu.edu.cn
bInner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot 010021, P. R. China

Received 2nd April 2017 , Accepted 13th June 2017

First published on 14th June 2017


It is of great importance to exploit and design efficient and low-cost alternatives to platinum-based electrocatalysts for the hydrogen evolution reaction (HER). In this work, we report novel well-defined 0D/2D heterojunctions of uniform molybdenum carbide-tungsten carbide quantum dots ((Mo2C)x–(WC)1−x–QDs, ∼3–5 nm)/N-doped graphene (NG) nanosheets with a two-dimensional layered structure obtained via a nanocasting method using KIT-6/graphene (G) as a template. By controlling the molar ratio of the Mo and W precursors, (Mo2C)x–(WC)1−x–QDs (0 < x < 1)/NG nanohybrids with different Mo/W molar ratios can be obtained, which exhibit superior activity in the HER to individual Mo2C/NG and WC/NG nanohybrids. The superior activity in the HER may be attributed to redistribution of the valence electrons of Mo and W elements, nitrogen-coordinating sites, and highly dispersed Mo2C–WC nanocrystals, as well as strong coupling between Mo2C–WC nanocrystals and NG. Excitingly, the optimal electrocatalyst, namely, (Mo2C)0.34–(WC)0.32/NG, exhibited low overpotentials (100 and 93 mV) to achieve a cathodic current density of 10 mA cm−2, small Tafel slopes (53 and 53 mV dec−1), and high exchange current densities (0.419 and 0.804 mA cm−2) in acidic and alkaline media, respectively. More importantly, it also displayed excellent long-term durability for 25 h of stable catalytic current at different pH values. This work is expected to provide a feasible route for the fabrication of 0D/2D earth-abundant nanocomposites for the HER.


Introduction

Hydrogen, as a clean and sustainable energy sources, is perceived to be a promising alternative to non-renewable fossil fuels on account of its lack of carbon emissions and high gravimetric energy density.1,2 Electrochemical water splitting is an environmentally friendly and efficient strategy for the scalable production of hydrogen, whereas highly active and durable electrocatalysts are urgently desired to improve its energy conversion efficiency.3–5 Hitherto, Pt-based catalysts have generally been regarded as the best electrocatalysts for the HER, but their scarcity and high cost have impeded their commercialization.6–8 Therefore, the search for and discovery of earth-abundant and inexpensive non-precious metal HER catalysts with significant activity and durability as potential substitutes for Pt group catalysts are in great demand.9–11

In recent decades, considerable promising efforts have been concentrated on the development of non-precious metal electrocatalysts, which mainly contain earth-abundant 3d transition metals12–15 and metal-free carbon nitride-based alternatives.16,17 Among these catalysts, transition metal carbides have long been expected to be potential substitutes for noble metal electrocatalysts because their electronic and catalytic properties are similar to those of Pt group metals.18,19 As two of the most efficient catalysts for the HER, molybdenum carbide and tungsten carbide have been extensively investigated. So far, many strategies have been employed to improve the activity of carbides in the HER, including forming nanostructures to decrease their grain size,20 creating abundant porosity to improve their mass transfer efficiency,21 and interacting with nanocarbons (e.g., graphene and carbon nanotubes) to increase their electrical conductivity.22 Very recently, bimetallic compounds such as NiMoNx/C,15 Mo–W–P hybrid nanosheets,23 Ni–Mo–S,24 and Co–Mo–N25 have been found to exhibit superior activities and stabilities as candidates for the HER in comparison with individual monometallic compounds owing to the synergistic effect of the two metals. Nevertheless, dual transition metal carbides have rarely been reported until now.35 It is well known that the preparation of carbides usually makes use of the high-temperature pyrolysis of precursors, and the aggregation and coarsening of grains is inevitable, which may lead to the loss of active sites. Moreover, by the introduction of mesopores into catalysts to increase their surface area their catalytic performance can be improved.18 Unfortunately, until now, the controllable preparation of nanostructured metal carbides with small crystal sizes and large surface areas has still remained a great challenge.

In our previous research, we developed a cost-efficient and environmentally benign method for synthesizing mesoporous Mo2C nanocrystals hybridized with graphene sheets with a two-dimensional layered structure, which possessed abundant mesopores, a large surface area and ultrasmall Mo2C nanoparticles owing to the confinement effect of the KIT-6/G template.26 However, m-Mo2C/G electrocatalysts still have a high overpotential, which might stem from the negative hydrogen binding energy image file: c7ta02864d-t1.tif of Mo2C, which gives rise to restricted desorption of Hads.11,27 In addition, the activity of Mo2C or WC mainly relies on the establishment of a “Pt-like” d-band electronic density of states by the intercalation of C into the lattice of Mo or W, and the electron density around the active sites in Mo or W will be reduced with an increase in the C content because of the transfer of electrons from Mo or W to C.28,29 For the above reasons, the incorporation of a second transition metal into Mo2C, such as W, may cause the redistribution of valence electrons, increase the hydrogen binding energy image file: c7ta02864d-t2.tif, and subsequently achieve a synergistic enhancement in activity in the HER.11,23

Here, we report highly active 0D/2D heterojunctions of ternary uniform (Mo2C)x–(WC)1−x QDs decorated on NG for the HER and obtained by an identical strategy. (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts with different Mo/W molar ratios were denoted as (Mo2C)0.41–(WC)0.18–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs/NG and (Mo2C)0.24–(WC)0.52–QDs/NG as calculated by ICP measurements (Table S1), respectively. In comparison with our previously reported m-Mo2C/G electrocatalysts, the as-prepared (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts possess similar two-dimensional layered structural features with abundant mesopores, a large surface area, and ultrasmall nanocrystals, whereas their performance in the HER is remarkably improved. The enhanced activity in the HER may be ascribed to the redistribution of valence electrons and an increase in conductivity on the introduction of highly conductive tungsten carbide. In addition, the abundant N dopants from the precursors could not only lead to a downshift in the valence bands of active carbon atoms in graphene with the accompanying formation of structural defects but also function as an electron acceptor to assist the C atoms adjacent to molybdenum–tungsten carbide,30–33 which is immensely conducive to enhancements in activity in the HER. The optimal (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst exhibited low overpotentials of 100 mVacid/93 mValkaline and 128 mVacid/123 mValkaline to achieve cathodic current densities of 10 mA cm−2 and 20 mA cm−2 in acidic and alkaline media, respectively, and excellent long-term durability for 25 h of stable catalytic current.

Results and discussion

The phase compositions of (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts with different Mo/W molar ratios were analyzed by X-ray diffraction (XRD). As seen in Fig. 1a, the diffraction peaks at 34.36°, 37.97°, 39.39°, 52.11°, 61.52°, and 69.60° correspond well to the (100), (002), (101), (102), (110) and (103) planes of hexagonal Mo2C (JCPDS no. 00-035-0787), whereas the peaks at 31.5°, 35.6°, 48.2°, 64.0°, 73.1° and 75.4° can be indexed to the (001), (100), (101), (110), (111), and (200) planes of hexagonal WC (JCPDS no. 00-073-0471). It is remarkable that hexagonal Mo2C is present in all the (Mo2C)x–(WC)1−x–QDs/NG composites, whereas hexagonal WC is only present in (Mo2C)0.24–(WC)0.52–QDs/NG. This phenomenon may be caused by the large differences in the lattice constants, which are 2.906 × 2.906 × 2.8369 Å and 3.012 × 3.012 × 4.735 Å for hexagonal WC and Mo2C, respectively. Therefore, the coexistence of hexagonal WC and hexagonal Mo2C or the formation of their hexagonal solid solution is difficult owing to the large lattice matching energy. Thus, only when the content of WC is higher than that of Mo2C can phase separation occur, and diffraction peaks attributed to hexagonal WC can be observed. Furthermore, cubic WC (marked by triangles) gradually appears with an increase in the Mo content and the diffraction peak located at 42.9° shifts to 41.2°, which indicates that Mo atoms are possibly dissolved in the WC lattice and may form an MoWC solid solution owing to the subtle difference between MoC and WC (4.273 vs. 4.235 Å).34,35
image file: c7ta02864d-f1.tif
Fig. 1 (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption/desorption isotherms, and (d) pore size distribution of Mo2C–QDs/NG, (Mo2C)0.41–(WC)0.18–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs/NG, (Mo2C)0.24–(WC)0.52–QDs/NG, and WC–QDs/NG electrocatalysts.

Fig. 1b shows the Raman spectra of (Mo2C)x–(WC)1−x–QDs/NG nanocomposites. It can be observed that the D band (1350 cm−1) and G band (1590 cm−1) assigned to disordered graphitic carbon and a graphitic carbon layer appear for all samples. The higher G/D ratio indicates that more sp2-hybridized carbon atoms are present in all the synthesized nanocomposites, which may be beneficial for accelerating the charge transfer rate.36 In addition, the characteristic peaks of Mo2C gradually became weaker until they vanished on an increase in the amount of W,36 accompanied by the appearance of a stretching mode attributed to WC,37 which is approximately consistent with the results of XRD. N2 sorption/desorption curves reveal that all the (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts possess obvious mesoporous features, as illustrated by typical type IV isotherms (Fig. 1c).38 Moreover, an average pore diameter centered at around 3.5 nm can be determined from the BJH curves (Fig. 1d). It is worth mentioning that (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts with different Mo/W molar ratios exhibit similar large surface areas and pore volumes (Table S2).

The morphology and microstructure of (Mo2C)x–(WC)1−x–QDs/NG were further characterized by transmission electron microscopy (TEM). As shown in Fig. 2a–c, S2a–c and S3a–e, monodisperse (Mo2C)x–(WC)1−x nanoparticles (black dots) with a size of ∼3–5 nm and abundant mesopores (white dots) with a size of ∼3.5 nm are uniformly dispersed on graphene sheets, which implies a uniform distribution of the active components, which may facilitate the accessibility of the active sites to the electrolyte. Notably, the highly dispersed nanoparticles and abundant mesopores are derived from the successful synthesis of the KIT-6/G template with a highly ordered mesoporous structure and mesoporous channels with a size of ∼5–6 nm parallel to or vertically spread over the graphene layers, which is demonstrated in Fig. S1. HRTEM images of (Mo2C)0.41–(WC)0.18–QDs/NG and (Mo2C)0.34–(WC)0.32–QDs/NG (Fig. S2c and 2c) reveal crystal lattice fringes with approximate interplanar spacings of 0.259, 0.229, 0.283, and 0.251 nm, which are attributed to the (100) and (101) planes of Mo2C and the (001) and (100) planes of WC, respectively. This result suggests the formation of (Mo2C)x–(WC)1−x–QDs/NG. In addition, the corresponding fast Fourier transform (FFT) images (Fig. S2d and 2d) show obvious diffraction points assigned to the (100) planes of hexagonal Mo2C and orthorhombic WC, which further indicates the formation of (Mo2C)x–(WC)1−x–QDs/NG. Nevertheless, an HRTEM image (Fig. S3c) and an FFT image (Fig. S2f) of (Mo2C)0.24–(WC)0.52–QDs/NG show not only the crystal faces of Mo2C but also the (100) crystal planes associated with the lattice fringe spacing of 2.53 Å for hexagonal WC, which is consistent with the XRD results. Fig. 2e and f, S2e and f show STEM images and the corresponding EDX elemental mapping images of Mo, W, C, and N in the (Mo2C)x–(WC)1−x–QDs/NG nanocomposites, which confirm that Mo, W, C, N and O elements are uniformly distributed throughout the entire sample. The actual contents of Mo, W, and N were determined by EDS and are shown in Table 1. The doped N atoms could enable their adjacent carbon atoms to play dual roles as both electron acceptors and electron donors, which can not only strengthen the synergy between (Mo2C)x–(WC)1−x and the N dopants but also make these carbon atoms become catalytically active sites.23,33 Thus, the (Mo2C)x–(WC)1−x–QDs/NG nanocomposites exhibit enhanced activity in the HER. The presence of oxygen species should be due to the superficial oxidation of (Mo2C)x–(WC)1−x in air.


image file: c7ta02864d-f2.tif
Fig. 2 (a and b) TEM images, (c) HRTEM image, (d) fast Fourier transform (FFT) image, (e and f) STEM images and (g–k) elemental mapping images of the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst.
Table 1 Comparison of the catalytic performance of different catalysts in the HER. η: overpotential required to achieve the stated current density, b: the Tafel slope, and J0: the exchange current density
Electrocatalyst Mo (wt%) W (wt%) N (wt%) Electrolyte η 10 (mV) η 20 (mV) b (mV dec−1) J 0 (mA cm−2)
(Mo2C)0.41–(WC)0.18–QDs/NG 20.02 8.58 1.78 0.5 M H2SO4 100 134 54 0.278
1.0 M KOH 116 146 58 0.610
(Mo2C)0.34–(WC)0.32–QDs/NG 12.06 10.67 1.67 0.5 M H2SO4 100 128 53 0.419
1.0 M KOH 93 123 53 0.804
(Mo2C)0.24–(WC)0.52–QDs/NG 7.82 14.38 1.23 0.5 M H2SO4 119 153 58 0.304
1.0 M KOH 133 163 59 0.349
Mo2C–QDs/NG 37.75 2.13 0.5 M H2SO4 146 181 60 0.050
1.0 M KOH 134 175 59 0.242
WC–QDs/NG 26.68 0.88 0.5 M H2SO4 175 204 62 0.047
1.0 M KOH 160 199 60 0.178


The electrocatalytic activity of (Mo2C)x–(WC)1−x–QDs/NG and Pt/C, which was used as a benchmark (loading: 0.28 mg cm−2), in the HER was evaluated in solutions of N2-saturated 0.5 M H2SO4 and 1.0 M KOH using a typical three-electrode system at a scan rate of 5 mV s−1. Fig. 3a and c show their polarization curves on the reversible hydrogen electrode (RHE) scale in acidic and alkaline media, respectively. Obviously, the Pt/C catalyst exhibited the expected high activity with an overpotential of nearly zero, and the bimetallic carbides displayed superior catalytic activity to the monometallic carbides. To achieve current densities of 10 and 20 mA cm−2 in acidic conditions, (Mo2C)0.41–(WC)0.18–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs/NG, and (Mo2C)0.24–(WC)0.52–QDs/NG only required corresponding overpotentials of 100 and 134 mV, 100 and 128 mV, and 119 and 153 mV, respectively, which were less than those for Mo2C–QDs/NG (146 and 181 mV) and WC–QDs/NG (175 and 204 mV). Actually, they were also lower than those reported for transition metal carbide HER catalysts such as nw-W4MoC (η10 mA cm−2 = 135 mV, η20 mA cm−2 = 150 mV),11 Mo2C/CC (η10 mA cm−2 = 140 mV, η20 mA cm−2 = 200 mV),39 Mo2C nanotubes (η10 mA cm−2 = 172 mV, η20 mA cm−2 = 197 mV),40 WC–CNTs (η10 mA cm−2 = 145 mV, η20 mA cm−2 = 175 mV),41 Mo2C@NC (η10 mA cm−2 = 124 mV, η15 mA cm−2 = 140 mV),33 Mo0.06W0.94C/CB (η10 mA cm−2 = 210 mV),34 W2C@GL (η10 mA cm−2 = 135 mV, η15 mA cm−2 = 158 mV),42 MoC–Mo2C heteronanowires (η10 mA cm−2 = 126 mV, η20 mA cm−2 = 140 mV),11 C–CoWC (η10 mA cm−2 = 200 mV),43 and meso-Mo2C/G (η10 mA cm−2 = 135 mV).26 In addition, the overpotentials for the (Mo2C)0.41–(WC)0.18–QDs/NG (116 and 146 mV), (Mo2C)0.34–(WC)0.32–QDs/NG (93 and 123 mV), and (Mo2C)0.24–(WC)0.52–QDs/NG (133 and 163 mV) electrocatalysts were lower than those for the Mo2C–QDs/NG (134 and 175 mV) and WC–QDs/NG (160 and 199 mV) electrocatalysts under alkaline conditions, which also indicates that the addition of the second metal facilitated improvements in activity in the HER. Remarkably, catalysts with different Mo2C or WC contents exhibited different activities, which implies that variations in the catalytically active sites in the various catalysts may lead to differences in catalytic performance. In order to determine their intrinsic activity, the turnover frequency (TOF) for each active site in the (Mo2C)x–(WC)1−x–QDs/NG catalysts was also calculated on the basis of methods reported in the previous literature.44 As seen in Fig. S4, it can be seen clearly that the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst possessed the highest TOF value at the same overpotential among the various (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts. For example, to reach a TOF of 0.8 s−1, the (Mo2C)0.34–(WC)0.32–QDs/NG electrode needed an overpotential of 126 mV, which was less than those for (Mo2C)0.24–(WC)0.52–QDs/NG (137 mV), (Mo2C)0.41–(WC)0.18–QDs/NG (154 mV), Mo2C–QDs/NG (178 mV) and WC–QDs/NG (211 mV). This result indicates that the active sites in the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst possess the highest catalytic activity. Because the Mo content in (Mo2C)0.34–(WC)0.32–QDs/NG is higher than those in (Mo2C)0.24–(WC)0.52–QDs/NG and WC–QDs/NG but lower than those in (Mo2C)0.41–(WC)0.18–QDs/NG and Mo2C–QDs/NG, it can be inferred that the intrinsic catalytic activity of Mo2C is higher than that of WC and can be promoted by incorporating WC into Mo2C to enhance its performance in the HER.


image file: c7ta02864d-f3.tif
Fig. 3 (a and c) Polarization curves and (b and d) Tafel plots of Mo2C–QDs/NG, (Mo2C)0.41–(WC)0.18–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs/NG, (Mo2C)0.24–(WC)0.52–QDs/NG, WC–QDs/NG, and Pt/C electrocatalysts with a scan rate of 5 mV s−1 under a rotating speed of 1600 rpm in 0.5 M H2SO4 and 1.0 M KOH solution.

Tafel plots were obtained by fitting the linear portions of the polarization curves to explain the reaction mechanism of the HER in acidic and alkaline (Fig. 3b and d) conditions. The Tafel slopes for (Mo2C)0.41–(WC)0.18–QDs/NG (54 and 58 mV per decade), (Mo2C)0.34–(WC)0.32–QDs/NG (53 and 53 mV per decade), and (Mo2C)0.24–(WC)0.52–QDs/NG (58 and 59 mV per decade) were smaller than those for Mo2C–QDs/NG and WC–QDs/NG in acidic and alkaline media, respectively (Table 1), which indicates that the HER that occurred on (Mo2C)x–(WC)1−x–QDs/NG may obey the Volmer–Heyrovsky mechanism with the Heyrovsky step as the rate-determining step. Moreover, the exchange current densities (J0), which represent the intrinsic activities of the electrocatalysts in the HER, are summarized in Table 1. The higher exchange current densities of (Mo2C)x–(WC)1−x–QDs/NG indicate more favourable proton discharge kinetics. Thus, the above results undoubtedly prove the importance of the incorporation of the second metal for enhancing the catalytic activity in the HER. In addition, the bimetallic (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts exhibited much higher values of double-layer capacitance (Cdl) of 39.4 mF cm−2 for (Mo2C)0.41–(WC)0.18–QDs/NG, 42.5 mF cm−2 for (Mo2C)0.34–(WC)0.32–QDs/NG, and 32.6 mF cm−2 for (Mo2C)0.24–(WC)0.52–QDs/NG in comparison with the Mo2C–QDs/NG (30.0 mF cm−2) and WC–QDs/NG (23.0 mF cm−2) electrocatalysts (Fig. S5f) in an acidic medium, which directly reflects the electrochemically active surface area (ECSA) (Fig. S5a–e). The higher values of double-layer capacitance (Cdl) indicate that there are more exposed active sites, which is beneficial for improving the electrocatalytic activity.

To further investigate the catalytic kinetics of the (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts with superior performance in the HER, the electrochemical impedance spectroscopy (EIS) technique was applied from 10[thin space (1/6-em)]000 to 0.1 Hz at overpotential of 150 mV in 0.5 M H2SO4 and 1.0 M KOH solution (Fig. 4). The Nyquist plots for the (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts clearly show the presence of one time constant fitted by the experimental data. The one time constant model is composed of Rs, which comprises the resistance in the wiring (Rwiring) and carbon support (Rcarbon), a resistance due to (Mo2C)x–(WC)1−x–QDs/NG (Rcarbide), and the solution resistance (Rsolution), and one parallel branch related to the charge transfer process (CdRct), which depends strongly on the potentials at lower frequencies. It can be seen from Fig. 4 that among the (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts, the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst exhibited the lowest charge transfer resistance at the catalyst/electrolyte interface, which indicates that it possessed the fastest charge transport kinetics. The lower charge transfer resistance of the (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts may be associated with their unique 0D/2D heterostructure between N-graphene and (Mo2C)x–(WC)1−x–QDs and the synergistic effect of Mo and W atoms in hybrid structures.


image file: c7ta02864d-f4.tif
Fig. 4 Nyquist plots of the various catalysts at overpotentials of 150 mV in solutions of (a) 0.5 M H2SO4 and (b) 1.0 M KOH. (c) The equivalent circuit model for the impedance spectroscopy.

It is worth noting that (Mo2C)0.34–(WC)0.32–QDs/NG exhibited superior activity in the HER to (Mo2C)0.41–(WC)0.18–QDs/NG and (Mo2C)0.24–(WC)0.52–QDs/NG, which may be related to its larger ECSA and enhanced conductivity. Furthermore, the surface distribution of electrons and the elemental valence states have a very significant impact on the electrocatalytic activity. Therefore, X-ray photoelectron spectroscopy (XPS) was performed to get an insight into the mechanism of the HER. Fig. 5a and b show the high-resolution XPS spectra in the Mo 3d and W 4f regions, respectively. The three coupled peaks for Mo2C–QDs/NG located at 228.7 and 231.8 eV, 229.3 and 232.7 eV, and 232.4 and 235.6 eV in the Mo 3d profiles correspond to Mo2C, MoO2 and MoO3, respectively.44,45 In addition, the doublet for WC–QDs/NG in the W 4f region at 31.6 and 33.7 eV can be assigned to WC, whereas the peaks at higher binding energies of 35.4 and 37.6 eV are associated with WOx species owing to surface oxidation on exposure to air. Apparently, the bimetallic carbides (Mo2C)x–(WC)1−x–QDs/NG also exhibit the same characteristic peaks in the Mo 3d and W 4f regions, whereas the peaks undergo slight positive shifts, which implies stronger electron donation from Mo and W to C. This may result in enhanced performance in the HER, because the activity of a catalyst in the HER largely depends on the value of image file: c7ta02864d-t3.tif on its surface, and moderate adsorption of H on the catalyst surface is the key to achieving the best performance in the HER.27,46 Considering that the value of image file: c7ta02864d-t4.tif on Mo2C is affected by the electron density around the active sites on Mo, and a high value of image file: c7ta02864d-t5.tif restricts the desorption of Hads,11 the stronger electron donation from Mo and W to C that occurred in the (Mo2C)x–(WC)1−x–QDs/NG composite may therefore reduce the hydrogen binding energy and thereby promote the desorption of Hads in comparison with that on pure Mo2C and WC and consequently facilitate the Heyrovsky and Tafel steps and lead to an enhancement in activity in the HER.47,48 Notably, the binding energies of Mo 3d3/2 and W 4f5/2 in (Mo2C)0.34–(WC)0.32–QDs/NG displayed a slightly larger positive shift than those in (Mo2C)0.41–(WC)0.18–QDs/NG and (Mo2C)0.24–(WC)0.52–QDs/NG, which suggests that more electron transfer occurred in the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst. Thus, the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst exhibited the best catalytic activity among the (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts. The high-resolution N 1s XPS spectra of (Mo2C)x–(WC)1−x–QDs/NG can be fitted to three peaks at 398.1, 399.2, and 401.5 eV, which were assigned to pyridinic N, pyrrolic N, and graphitic N (Fig. S6), respectively, and suggest that N was in fact doped into the graphene sheet.


image file: c7ta02864d-f5.tif
Fig. 5 XPS spectra of (a) Mo 3d and (b) W 4f of Mo2C–QDs/NG, (Mo2C)0.41–(WC)0.18–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs/NG, (Mo2C)0.24–(WC)0.52–QDs/NG, and WC–QDs/NG.

To illustrate their structural advantages, two comparable catalysts comprising bare N-graphene (NG) and N-graphene-free (Mo2C)x–(WC)1−x–QDs ((Mo2C)0.34–(WC)0.32–QDs) nanoparticles with a Mo/W molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 were synthesized. Fig. 6 shows their performance in the HER in acidic and alkaline electrolytes. It can clearly be seen that N-graphene displayed negligible activity in the HER at different pH values. Moreover, the catalytic activity of (Mo2C)0.34–(WC)0.32–QDs/NG was obviously superior to that of N-graphene-free (Mo2C)0.34–(WC)0.32–QDs. For example, the (Mo2C)0.34–(WC)0.32–QDs/NG nanocomposite needed lower overpotentials of only 100 and 93 mV than those for (Mo2C)0.34–(WC)0.32–QDs (188 and 168 mV) to achieve a current density of 10 mA cm−2 in solutions of 0.5 M H2SO4 and 1.0 M KOH, respectively. In addition, the Tafel slopes for (Mo2C)0.34–(WC)0.32–QDs/NG were 53 and 54 mV dec−1, which were also lower than those for (Mo2C)0.34–(WC)0.32–QDs of 96 and 108 mV dec−1 in the corresponding electrolytes. Therefore, the improvement in the activity in the HER of the (Mo2C)x–(WC)1−x–QDs/NG catalysts was mainly based on strong coupling between Mo2C–WC nanocrystals and N-doped graphene rather than individual NG or (Mo2C)x–(WC)1−x–QDs.


image file: c7ta02864d-f6.tif
Fig. 6 (a and c) Polarization curves and (b and d) Tafel plots for (Mo2C)0.34–(WC)0.32–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs, and NG electrocatalysts at a scan rate of 5 mV s−1 and a rotation speed of 1600 rpm in solutions of 0.5 M H2SO4 and 1.0 M KOH.

Durability is also an important evaluation parameter for a good catalyst. Therefore, the long-term stability of (Mo2C)0.34–(WC)0.32–QDs/NG was investigated in solutions of 0.5 M H2SO4 and 1.0 M KOH. As shown in Fig. 7a and b, a current density of ∼10 mA cm−2 at static overpotentials of −100 and −90 mV could be maintained for at least 25 h from a curve of the time-dependent current density in acidic and alkaline media, respectively.


image file: c7ta02864d-f7.tif
Fig. 7 Time-dependent current density curve for (Mo2C)0.34–(WC)0.32–QDs/NG under static overpotential of 100 and 90 mV in (a) 0.5 M H2SO4 and (b) 1.0 M KOH solutions.

Furthermore, an accelerated degradation test (ADT) (Fig. 8) of the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst demonstrated that only a slight decrease in performance was observed in the HER even after successive CV scanning for 3000 cycles in acidic and alkaline media. These results suggest that an (Mo2C)x–(WC)1−x–QDs/NG hybrid with excellent stability was obtained and consequently has prospects for potential application in the HER.


image file: c7ta02864d-f8.tif
Fig. 8 Stability tests of the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst via a CV scanning for 3000 cycles in (a) 0.5 M H2SO4 and (b) 1.0 M KOH solution.

Conclusions

In summary, 0D/2D heterojunctions of ternary uniform (Mo2C)x–(WC)1−x QDs decorated on NG electrocatalysts with abundant mesopores, large surface areas, and ultrasmall nanocrystals have been successfully synthesized as superior electrocatalysts for the HER in acidic and alkaline media. Unexpectedly, (Mo2C)x–(WC)1−x–QDs/NG displayed excellent activity and stability in the HER, which were superior to those of Mo2C–QDs/NG and WC–QDs/NG nanohybrids. The results of characterization reveal that the enhancement in the activity in the HER of (Mo2C)x–(WC)1−x–QDs/NG may be based on two reasons. On the one hand, the incorporation of molybdenum–tungsten carbide could lead to the redistribution of valence electrons and an increase in conductivity. On the other hand, the N dopants could not only lead to a downshift in the valence bands of active carbon atoms in graphene with the accompanying formation of structural defects but also function as an electron acceptor to assist the C atoms adjacent to molybdenum–tungsten carbide. Its outstanding catalytic performance makes (Mo2C)x–(WC)1−x–QDs/NG an ideal candidate for practical applications in the HER.

Experimental section

Materials preparation

Synthesis of graphene oxide (GO). Graphene was prepared by a modified Hummers' method. Typically, 1 g graphite powder was slowly added to a mixed solution of concentrated sulfuric acid (130 mL) and phosphoric acid (30 mL) containing 6 g KMnO4 in a 250 mL ground glass flask in an ice bath. Then, the flask was transferred to an oil bath and kept at 50 °C for over 12 h under mechanical stirring. After naturally cooling to ambient temperature, the turbid liquid was slowly poured into a 500 mL beaker containing 15 mL H2O2 and 100 mL deionized water with continual stirring, and the solution in the beaker immediately turned light yellow, which indicated the formation of graphene oxide (GO). After the turbid liquid was completely added, the solution was further stirred for 2 h to ensure the complete oxidation of graphite. Subsequently, the yellow turbid liquid that was obtained was centrifuged at 2000 rpm in a 10 mL tube, and the black particles (unreacted graphite) were discarded. The turbid liquid in the top of the tube was washed with concentrated HCl, deionized water, and ethanol three times, respectively. Then, the products were freeze-dried overnight to obtain GO.
Synthesis of KIT-6/G template. A KIT-6/G nanocomposite was prepared according to a modified method reported previously.49 In brief, 6.0 g Pluronic P123 triblock copolymer, 1 g GO, 6 g n-butanol, and 11.8 g concentrated HCl were added together to a 150 g aqueous solution and continuously stirred at 45 °C for 24 h. Then, 12 g TEOS was added to the above solution under vigorous stirring at a speed of 0.5 mL min−1. Then, the mixed solution was kept under stirring at 45 °C for a further 24 h followed by treatment by refluxing at 80 °C for 6 h. Finally, the dark yellow solid products were collected by filtration and washed with water and ethanol before being calcined at 350 °C for 4 h in air.
Synthesis of (Mo2C)x–(WC)1−x–QDs/NG, (Mo2C)x–(WC)1−x–QDs, and NG catalysts. For the synthesis of (Mo2C)x–(WC)1−x–QDs/NG, 0.15 g glucose and 0.3 g metallic precursors (ammonium molybdate and ammonium metatungstate with different Mo/W molar ratios of 3[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]3) were dissolved in 15 mL distilled water. Then, 0.2 g KIT-6/G template was added to the above solution. The mixture was ultrasonicated until the water was completely removed. The precipitate was calcined at 450 °C for 2 h at a heating ramp rate of 2 °C min−1 and then heated to 950 °C for 5 h at the same temperature ramp rate in a high-purity nitrogen atmosphere. Finally, after cooling to room temperature naturally, the resulting black solid was washed with a 2 M aqueous solution of NaOH at 80 °C to remove the KIT-6 template. (Mo2C)x–(WC)1−x–QDs/NG electrocatalysts with different Mo/W molar ratios of 3[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]3 were denoted as (Mo2C)0.41–(WC)0.18–QDs/NG, (Mo2C)0.34–(WC)0.32–QDs/NG, and (Mo2C)0.24–(WC)0.52–QDs/NG, respectively. The synthesis of (Mo2C)0.34–(WC)0.32–QDs followed a similar process to that of the (Mo2C)0.34–(WC)0.32–QDs/NG electrocatalyst except that the assembled KIT-6/G template was replaced by an individual KIT-6 template during the synthesis process. NG was synthesized by the direct calcination of GO at 950 °C for 2 h under an NH3 atmosphere.

Materials characterization

Electrochemical measurements were carried out on a Zennium electrochemical workstation (Zahner, Germany) assembled with a modulated speed rotator (RRDE-3A) in a standard three-electrode system with a carbon rod as the counter electrode, a catalyst-modified glassy carbon electrode (GCE) with an area of 0.0707 cm2 as the working electrode, and a saturated calomel electrode (SCE, 0.241 V vs. RHE) in acidic media or an Ag/AgCl (3 M KCl) electrode (0.209 V vs. RHE) in alkaline media as the reference electrode. All potentials were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.241 + 0.059 × pH) or (0.209 + 0.059 × pH) V, respectively. The catalyst ink was prepared by ultrasonically mixing 5.0 mg of the as-prepared catalyst with 25 μL Nafion® (5%) solution, 0.25 mL water, and 0.25 mL ethanol for 30 min to form a homogeneous suspension. A total of 2 μL well-dispersed catalyst ink was pipetted and spread onto the surface of a pre-polished rotating disk electrode (RDE) (d = 3 mm) and dried in air for 30 min before measurement, corresponding to a catalyst loading of 269 μg cm−2. Linear sweep voltammetry (LSV) polarization curves in the potential ranges of −0.6 to 0 V (vs. SCE) and −1.5 to −0.8 V (vs. SCE) for the HER were recorded in N2-saturated 0.5 M H2SO4 and 1.0 M KOH electrolytes, respectively, at a sweep rate of 5 mV s−1 at 1600 rpm. Amperometric current density–time (it) curves were recorded for 25 h in N2-saturated solutions of 0.5 M H2SO4 and 1.0 M KOH at controlled potentials. Electrochemical impedance spectroscopy (EIS) measurements were carried out from 10[thin space (1/6-em)]000 to 0.1 Hz in 0.5 M H2SO4 and 1.0 M KOH solution at overpotential of 150 mV.

Acknowledgements

This study was supported by grants from NSFC (21261011, 21373259, 21661023, and 21601096), Program for New Century Excellent Talents in University (NCET-10-0907), Application Program from Inner Mongolia Science and Technology Department approved in 2016, and Program of Higher-level Talents of Inner Mongolia University (21300-5155105).

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Footnote

Electronic supplementary information (ESI) available: Nitrogen sorption analysis data for (Mo2C)x–(WC)1−x–QDs/NG samples; summary of Mo, W, and N concentrations of different samples; TEM images, cyclic voltammograms and XPS spectroscopy. See DOI: 10.1039/c7ta02864d

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