Highly Dispersed Mo2C Nanoparticles Embedded in Ordered

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Highly-Dispersed Mo2C Nanoparticles Embedded in Ordered Mesoporous Carbon for Efficient Hydrogen Evolution Jianying Wang, Weiwen Wang, Lvlv Ji, Steffen Czioska, Lixia Guo, and Zuofeng Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00191 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Energy Materials

Highly-Dispersed Mo2C Nanoparticles Embedded in Ordered Mesoporous Carbon for Efficient Hydrogen Evolution ⊥

⊥

Jianying Wang,†, Weiwen Wang,‡, Lvlv Ji,† Steffen Czioska,† Lixia Guo† and Zuofeng Chen†,* †

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China. ‡ Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry Institute, Fudan University, Shanghai 200433, China Supporting Information ABSTRACT: The development of non-noble metal-based electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. As a promising HER catalyst candidate, the preparation of molybdenum carbide requires high temperature for carbothermal reduction, which often causes nanoparticles sintering, leading to low exposed active sites. In this work, highly dispersed Beta(β)-Mo2C nanoparticles of approximately 5 nm embedded in ordered mesoporous carbon (Mo2C@OMC) have been synergistically synthesized. During the synthesis process, the resol precursor for OMC template could serve as carbon source for the formation of Mo2C and mitigate the sintering of Mo2C nanoparticles. The resultant well-defined Mo2C possesses high exposed active sites of approximately 26.5% and exhibits an excellent performance for the HER in both acidic and alkaline solutions. The synthetic procedure developed in this study may be extended to fabricate other metal carbide@OMC nanocomposites for the HER and other electrocatalytic applications. KEYWORDS: molybdenum carbide, electrocatalysis, hydrogen evolution reaction, ordered mesoporous carbon, pyrolysis

Introduction Hydrogen produced from water splitting has been intensely investigated as one of the promising alternatives to conventional fossil fuels.1-3 The precious Pt-based materials have been considered as the most active electrocatalysts for the hydrogen evolution reaction (HER), but the high cost and scarcity limit their scale-up application.4-6 To obtain more economical and efficient catalysts for practical applications, intensive efforts have been devoted to design and fabricate inexpensive and earthabundant HER electrocatalysts with comparable activity and stability.4-9 Molybdenum carbide, an important member of transitionmetal carbides, has attracted extensive research attention as an efficient HER electrocatalyst due to its d-band electronic structure which is similar to that of Pt-group metals.10,11 Many effective strategies have been developed to fabricate MoxC-based materials with high catalytic activity and excellent stability for the HER.4,12,13 To improve the catalytic performance, the materials are required to expose more catalytically active sites which may be realized through nanostructuring and surface engineering of the materials. However, the high temperature (> 700 °C) during preparation of MoxC by carbothermal reduction often induces severely nanoparticles sintering, leading to a low exposing surface area and subsequently low density of catalytic active sites.14 In order to mitigate this drawback, MoxC nanoparticles were usually dispersed by the assistance of carbonbased supports such as carbon nanotubes, graphene nanosheets, and polymers.12,15-21 Despite this, the controllable synthesis of well-defined MoxC nanoparticles with small crystalline size and uniform distribution still remains a great challenge. In recent years, the ordered mesoporous carbon (OMC) materials have triggered increasing interest in applications such as energy storage, separation, and catalyst supports, due to their uniform mesoporous structures, high surface areas, large pore volumes, and inert surfaces.22-24 The earlier studies have shown

that, by embedding metal particles in the carbon walls of ordered mesoporous carbons, thermally stable and catalytically active catalysts could be obtained.18,25 However, the synergistical synthesis of metal carbides of small crystalline size supported on ordered mesoporous carbons and their use for the efficient HER catalysis have rarely been explored so far. Herein, we design and prepare a Mo2C@OMC electrocatalyst via an evaporation induced self-assembly (EISA) procedure followed by carbothermal reduction, as shown in Scheme 1. The synthesis involves introduction of Mo salts into the reaction mixture of OMC precursors containing a resol and a surfactant. After being subjected to pyrolysis at high temperature, the resol polymer was converted into the carbon framework by decomposition of the surfactant F127 micelles to form the mesoporous structure. At the same time, the Mo ions were carbothermally in situ reduced to form Mo2C nanoparticles which were embedded in the carbon walls. Thanks to the confinement effect of ordered mesoporous carbons, the Mo2C nanoparticles are endowed with sufficient stability at high temperature without sintering and the nanoparticle size is around 5 nm. Moreover, by virtue of the unique structural advantages, the Mo2C@OMC nanocomposites have high exposed active sites of approximately 26.5% with a low atomic percentage of Mo ~2.6 at.%, and exhibits an excellent performance for the HER in acidic and alkaline solutions.

Experimental Chemicals. Phenol, formalin (37 wt% formaldehyde), triblock copolymers Pluronic F127 (Mw = 12600, EO106-PO70-EO106), phosphomolybdic acid (PMo12), HCl, H2SO4, KOH, NaOH and commercial Mo2C were all purchased from Sigma-Aldrich Co. LLC. All electrolyte solutions were prepared with deionized water (18 MΩ·cm) unless stated otherwise.

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Scheme 1. Schematic illustration of the synthesis procedure of Mo2C@OMC nanocomposites.

Synthesis of Mo2C@OMC nanocomposites. The precursors of Mo2C@OMC were synthesized by the typical solvent evaporation induced self-assembly (EISA) method with copolymers F127 as a template in an ethanol solution. In a typical procedure, 1.0 g of F127 and different amounts of PMo12 - 0.05 g, 0.1 g, 0.25 g and 0.5 g - were dissolved in 20.0 g of ethanol. Then 5.0 g of 20 wt% resol precursors in an ethanol solution was added. Soluble resol precursors were pre-prepared from phenol and formaldehyde in a base-catalyzed process, according to a previously reported procedure.10 After stirring for 10 min, a homogeneous solution was obtained. The solution was then poured into dishes to evaporate ethanol at room temperature for 5 - 8 h, followed by heating in an oven at 100 oC for 24 h. The as-prepared yellowish films were scraped from the dishes and crushed into powders. Mo2C@OMC nanocomposites were prepared by pyrolyzing the powders under nitrogen atmosphere at 800 oC for 2 h with a ramping rate 1 oC /min. Characterization. Scanning electron microscope (SEM) images were obtained at Hitachi S-4800 (Hitachi, Japan). Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM) images, selected area electron diffraction (SAED) pattern, energy dispersive X-ray spectroscopy analysis (EDS) data and EDS elemental mapping images were obtained using Tecnai G2 F20 S-Twin. Powder Xray diffraction (XRD) was measured by Bruker D8 Focus via ceramic monochromatized Cu Kα radiation of 1.54178 Å, operating at 40 kV and 40 mA. The scanning rate was 6° per min in 2θ and the scanning range was from 20 - 80°. Raman spectra were acquired using a confocal microscope laser Raman spectrometer (Rainshaw invia). The excitation source was a helium-neon laser with a wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) for elemental analysis was conducted on a Kratos Axis Ultra DLD X-ray Photoelectron Spectrometer using 60 W monochromated Al Kα radiation as the X-ray source for excitation. The carbon 1s peak (284.6 eV) was used for internal calibration. The peak resolution and fitting were processed by XPS Peak 41 software. Surface area measurements were performed on an ASAP 2020 Brunauer-Emmett-Teller (BET) analyzer. Thermogravimetry (TG) measurement of the samples was carried out on a thermal analyzer (TGA Q500), and the samples were heated at a rate of 10 °C/min in an air flow. Electrochemical Studies. All electrochemical measurements were performed on a CHI 660E electrochemical workstation

(Chenhua Corp., Shanghai, China). The three-electrode system consisted of a working electrode, a carbon rod counter electrode, and a saturated calomel reference electrode (SCE, ~0.244 V vs. NHE). Unless stated otherwise, all potentials in cyclic voltammetry were reported vs. RHE with 90% iR compensation and all controlled potential electrolysis were conducted without iR compensation. All experiments were performed at 22 ± 2 °C. Tafel plot. The current-potential data of the electrocatalyst were obtained by linear sweep voltammetry (LSV) at a very slow scan rate of 0.1 mV/s. The Tafel slope was obtained from the LSV plot using a linear fit applied to points in the Tafel region. The solution resistance measured prior to the data collection (using iR test function) was used to correct the Tafel plot for the iR drop. Calculation of ECSA. The electrochemically active surface area (ECSA) of the electrocatalyst is evaluated by measurement of their double layer charging capacitance. Briefly, a potential range where no apparent Faradaic process occurred was determined firstly using cyclic voltammetry (CV). Electrochemical impedance spectroscopy (EIS). The EIS was recorded under a given overpotential over a frequency range from 0.01 Hz to 1 MHz at the amplitude of the sinusoidal voltage of 5 mV. The explicit Nyquist plots were obtained based on the EIS data.

Results and discussion Preparation and characterization of electrocatalysts. An outline of the steps involved in the synthesis of Mo2C@OMC is presented in Scheme 1. In a typical synthesis process, the Mo2C@OMC materials were synthesized from the assembly of the triblock copolymer Pluronic F127, resol, and phosphomolybdic acid, subjected to successive thermal curing and in situ carbothermal reduction.10 In this process, the formation of hydrogen bond between the resol and the surfactant F127 has been well understood, which is key to the self-assembly and the formation of ordered mesoporous structures.18 With the introduction of PMo12O403− to the reaction mixture, the formation of the Mo-containing polymer precursor was evidenced by the membrance color changed from colorless to yellowish and their FT-IR spectra (Figure S1 in the Supporting Information). The content of Mo has a significant effect on the structure and morphology of the resultant mesoporous nanocomposites, as well as their


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catalytic performance. The obtained samples are denoted as The surface area and pore features of the Mo2C@OMC Mo2C@OMC-1, -2, -3, and -4, respectively, according to nanocomposites were investigated by N2 sorption isotherms. As the feeding content of phosphomolybdic acid hydrate (0.05 shown in Figure 1D, typical type-IV curves with sharp capilliary g, 0.1 g, 0.25 g and 0.5 g). condensation steps at P/P0 = 0.4 - 0.7 are observed for Mo2C@OMC-1, -2 and -3, which is typical for mesoporous The Mo2C/OMC materials were first investigated by XRD measurements. As shown in Figure 1A, XRD patterns of the structures of these materials.31,32 As Mo2C loading is increased mesoporous nanocomposites after being pyrolyzed at 800 °C from sample-1 to -3, the BET surface area is gradually decreased reveal characteristic diffraction peaks of hexagonal beta(β)from 706 to 473 m2/g and the pore size is slightly decreased from Mo2C located at approximately 34.3°, 37.3°, 39.1°, 52°, 61.5°, 6.4 to 5.5 nm, as shown in Figure S4 and Table S1. However, with continuing increase of the feeding content of PMo12 during 69.5°, 74.4° and 75.4° (PDF #35-0787), which are consistent with those reported in literature26 and the commercial β-Mo2C. EISA process, the N2 sorption isotherms of sample-4 do not XRD patterns of the Mo2C/OMC samples also exhibit two broad show apparent type-IV profile, implying that the pore channels might be destroyed. diffraction peaks centered at approximately 27° and 42°, characteristics of carbon-based materials.27,28 By increasing the B A Mo C@OMC - 1 Mo C@OMC - 1 feeding content of PMo12, both peaks are gradually weakened Mo C@OMC - 2 Mo C@OMC - 2 G Mo C@OMC - 3 Mo C@OMC - 3 D relatively to Mo2C peaks. No diffraction peaks corresponding to Mo C@OMC - 4 Mo C@OMC - 4 Mo metal, MoC or MoOx are detected. Commercial Mo C Commercial Mo C 816 The Mo2C/OMC materials were further characterized by 992 657 Raman spectroscopy. In Figure 1B, all Mo2C/OMC samples exhibit two distinct Raman peaks at around 1342 and 1595 cm−1, attributed to the D-band and G-band of the carbon-based matrix. 500 1000 1500 2000 20 30 40 50 60 70 80 In addition, three Raman peaks appear at around 657, 816 and 2θ (degree) Raman Shift (cm ) 992 cm−1 and the intensities of these peaks are enhanced with D 400 Mo C@OMC - 1 C 100 Mo C@OMC - 2 increasing the Mo content, which is consistent with the 80 300 Mo C@OMC - 3 formation of β-Mo2C.29,30 No peaks corresponding to other Mo C@OMC - 4 60 200 molybdenum compounds are observed. The carbonization 41% Mo C@OMC - 1 40 Mo C@OMC - 2 25% temperature is essential for the formation and embedment of 100 Mo C@OMC - 3 20 13.5% Mo C@OMC - 4 well-dispersed nanoparticles inside the carbon framework. In 7.5% 0 0 controlled experiments, the Mo2C/OMC composites were also 0.0 0.2 0.4 0.6 0.8 1.0 100 200 300 400 500 600 Temperature(°C) P/Po synthesized under varied carbonized temperatures. While the sample prepared at 700 °C exhibits poor crystallinity, the sample Figure 1. (A) XRD patterns, (B) Raman spectra, (C) TGA curves obtained at higher temperature of 900 °C displays XRD pattern and (D) N2 sorption isotherm curves of different samples. TGA and Raman spectroscopy similar to that at 800 °C (Figure S2). In measurements were conducted under air atmosphere. addition, the samples obtained at 800 °C and 900 °C exhibit The morphology and structure of Mo2C@OMC were close catalytic activity toward the HER, as shown in Figure S3. investigated by scanning electron microscopy (SEM) and Therefore, the samples prepared at 800 °C were used throughout transmission electron microscopy (TEM). The SEM and TEM the study. To confirm the content of Mo2C in the nanocomposite, thermogravimetry analysis (TGA) were conducted under air atmosphere. Figure 1C shows TGA profiles of Mo2C@OMC, by which Mo contents can be determined by eliminating carbon through combustion in air. For all samples, the initial weight loss below 150 o C is ascribed to the loss of adsorbed water. The slight weight gain between 150 o C and 300 oC should be caused by the gradual oxidation and transformation of Mo2C to MoO3, which is followed by a weight loss due to the combustion of carbon at higher temperature. The remaining weights of samples after being heated to 600 oC are about 7.5 wt.%, 13.5 wt.%, 25 wt.%, and 41 wt.%, respectively, which are belonged to the as-formed “pure” MoO3. Accordingly, the Mo2C content can be calculated and the corresponding weight percentage of Mo2C at Mo2C@OMC-1 to -4 are 5.3 wt.%, 9.6 wt.%, 17.7 wt.%, and 29 wt.%, and atomic percentage of Mo are 0.7 at.% 1.3 at.%, Figure 2. SEM images (A, C) , TEM images (B, D) and HRTEM images (E, F) of 2.6 at.%, and 4.9 at.%, respectively, Mo2C@OMC-3 prepared at 800 °C under N2 atmosphere, viewed from [110] (A, B, E) and pointing to a relatively low content of Mo [001] (C, D, F) directions. (G-J) TEM-EDS mapping images of Mo2C@OMC-3. Insets: the corresponding FFT diffractograms (B, D), and SEAD pattern (E). in this materials. 2

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images of Mo2C@OMC-3 taken along [110] (Figures 2A and 2B) and [001] (Figures 2C and 2D) zone axes display highly ordered parallel mesoporous channels and the hexagonal arrays of mesoporous in this nanomaterial, indicating high-quality twodimensional hexagonal mesostructure.33 Small nanoparticles are homogeneously distributed within the rigid mesoporous carbon matrix. The additional images in Figures S5 and S6 show that Mo2C@OMC-1 and -2 also possess the ordered mesoporous structure, while the mesoporous structure of Mo2C@OMC-4 is destroyed due to the excess feeding content of PMo12, consistent with the results of N2 sorption isotherms. In Figure 2E, the high-resolution TEM (HR-TEM) image of Mo2C@OMC-3 taken along the [110] zone axe reveals that the nanoparticles have ultrasmall sizes ranging from 3 to 5 nm with lattice fringes having an interplane spacing of 0.23 nm, consistent with (101) crystal planes of Mo2C.34 The selected-area electron diffraction (SAED) pattern in the inset displays diffraction spots, indicating further the single-crystalline nature of the Mo2C nanoparticles. The HRTEM image in Figure 2F taken along the [001] zone axe shows further that the obtained nanocomposites have a hexagonally arranged pore structure with an uniform pore size of 5.5 nm, which is consistent with the pore size calculated by BET. The TEM-energy dispersive X-ray spectroscopy (EDS) elemental mapping images confirm the presence of C, Mo and P elements, which were uniformly distributed in the Mo2C@OMC nanomaterial (Figures 2G-2J). In addition, the elemental analysis in Figure S7 shows that the Mo content is increased with increasing the feeding content of PMo12 during the preparation process. The elemental compositions and oxidation states of Mo2C@OMC-3, as a representative sample, were analyzed by XPS. Figure 3A shows its survey XPS spectrum, which reveals the presence of C, Mo, P and O in this sample. Figure 3B shows the high-resolution C 1s XPS spectrum. The main peak at 284.6 eV implies that graphitic carbon is the major species, which is consistent with the successful conversion of the organic polymer to carbon framework. No carbide peak at the lower binding energy of ~ 282.7 eV is observed. This can be ascribed to the fact that the graphitic carbon has covered the signal of the carbidic carbon in the similar binding energy region.19 Since Mo 3d XPS peaks are located far from C 1s peaks in terms of their binding energy, it is possible to see Mo 3d signal at their binding-energy regions. Figure 3C displays the deconvoluted high-resolution Mo 3d XPS. There is no evidence for the presence of metallic Mo. The peaks located at binding energies of 228.5 and 231.5 eV are assigned to Mo2+ 3d5/2 and 3d3/2, respectively, consistent with the presence of Mo in the carbide phase.27 The other peaks located at 229.3 and 232.6 eV are assigned to Mo4+, and 232.9 and 236 eV to Mo6+. The presence of these high-oxidation-state Mo4+/6+ is presumably due to the superficial oxidation through air contact.35 In Figure 3D, the main peaks of P 2p XPS are located at 133.5 and 134.8 eV, which are due to P-C and P-O bonding states, respectively.21

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Figure 3. (A) Survey XPS spectrum and high-resolution XPS spectra of (B) C 1s, (C) Mo 3d and (D) P 2p of Mo2C@OMC-3.

Electrocatalytic HER performance. The electrocatalytic performance of Mo2C@OMC catalysts for the HER are studied in both acidic and alkaline solutions. The current density was calculated based on the geometric area of the glassy carbon electrode unless stated otherwise. Figure 4A shows the linear sweep voltammetry (LSV) curves of the as-prepared Mo2C@OMC electrocatalysts at a scan rate of 2 mV/s in 0.5 M H2SO4. As a benchmark electrocatalyst, the commercial 20 wt% Pt/C was examined with the same mass loading under the same experimental conditions. As expected, 20 wt% Pt/C exhibits an excellent HER activity with nearly zero onset overpotential. The electrocatalytic activity of the as-prepared sample is gradually enhanced with increasing Mo loading, consistent with the HER catalysis of molybdenum carbide. In this series of samples, the highest HER activity was obtained at Mo2C@OMC-3 with a small onset overpotential of 66 mV (0.2 mA/cm2) and an overpotential of 160 mV to reach a current density of 10 mA/cm2. As shown in Table S2, this performance is comparable or superior to those well-known MoxC-based electrocatalysts. By further increasing the Mo loading, Mo2C@OMC-4 displays an inferior catalytic activity over Mo2C@OMC-3. This is probably caused by the fact that the mesoporous structure has been destroyed with excess Mo loading, leading to low exposing active sites of electrocatalyst. The HER process was further investigated by Tafel measurements. Figure 4B gives the Tafel plots of different electrocatalysts in 0.5 M H2SO4 solution. In accordance with the results of the polarization curves, the Pt/C electrocatalyst displays the smallest Tafel slope of 30 mV/decade. The Tafel slope of Mo2C@OMC-3 is 51 mV/decade, much lower than those of Mo2C@OMC-1, -2 and -4, and close to the theoretical value of 40 mV/decade when the Volmer−Heyrovsky reaction pathway is operative with the electrochemical desorption of hydrogen as the rate-limiting step.5 At a given overpotential, the turnover frequency (TOF) is defined as the number of products generated at one active site per unit time.36,37 The estimated TOF of Mo2C@OMC-3 is about 0.38 s–1 at η = 200 mV (see details in the Supporting Information), implying that the mesoporous nanocomposites can efficiently generate H2 products.6 Electrochemical impedance spectroscopy (EIS) was also carried out in 0.5 M H2SO4 to gain further insight into the catalytic activity of the electrocatalysts for the HER. Figure 4C and the inset display the Nyquist plots of different Mo2C@OMC electrocatalysts. It is found that all the samples exhibit two


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Figure 4. (A) LSV curves at a scan rate of 2 mV/s and (B) Tafel plots of Mo2C@OMC-1, -2, -3, -4 and 20 wt % Pt/C electrocatalysts in 0.5 M H2SO4 solution. (C) Nyquist plots of different samples of Mo2C@OMC-1, -2, -3, and -4 in 0.5 M H2SO4 at η = 150 mV. Inset: a magnified view in the high frequency region. (D) Capacitive current at 0.05 V vs. SCE as a function of scan rate for different samples in 0.5 M H2SO4. (E) CV curve of Mo2C@OMC-3 in the positive potential region showing its irreversible electrochemical oxidation in 0.5 M H2SO4 at a scan rate of 2 mV/s. (F) LSV curves of Mo2C@OMC-3 before and after 1000 CV cycles. Inset shows long-term electrolysis of Mo2C@OMC-3 in 0.5 M H2SO4 under an overpotential of 200 mV for 20 h.

semicircles, which correspond to a two-time-constant behavior of the materials.38 The first semicircle at the high frequency region is related to the porous property of the electrodes, while the second one at the low frequency region represents the charge transfer resistence during electrocatalytic reactions.12 The lowest transfer resistance is obtained at Mo2C@OMC-3, which is consistent with its highest catalytic activity in this series of materials. To gain further insight into the catalytic activity of Mo2C@OMC-3, the EIS measurements were also conducted at various overpotentials in 0.5 M H2SO4, as shown in Figure S8. The charge transfer resistance is decreased as the overepotential is increased. It is worth to note that the Tafel slope can also be obtained by analyzing these EIS data, which can eliminate the effects of some arbitrary factors, such as different choices of overpotential region and different means for iR-compensation. According to the Nyquist plots, a model of two-time-constants was used to calculate the charge transfer resistances (Rct).39 By fitting the extracted data, the plot of η versus logRct−1 shows a Tafel slope of 53 mV dec−1 for the electrode in 0.5 M H2SO4, which is consistent with that obtained based on the polarization curve. To estimate the electrochemically active surface area (ECSA) of Mo2C@OMC electrocatalysts under working conditions, we calculated their double layer capacitance from cyclic voltammetry (CV) curves at different scan rates between 0 and 0.1 V versus SCE (Figure S9). Linear correlations are obtained when the current density at 0.05 V is plotted against the scan rate, as shown in Figure 4D. From the slope, the specific capacitances of ∼176 F/g, 215 F/g, 228 F/g and 145 F/g are derived from the different samples of Mo2C@OMC. If we assume a standard value of 60 µF/cm2,5,40 the ECSAs of various Mo2C@OMC samples are estimated to be ∼293 m2/g, 358 m2/g, 380 m2/g, and 241 m2/g, respectively (see details in the Supporting Information). This result is in good agreement with their performance in electrochemical measurements. The mesoporous structure of the nanomaterials enlarges the specifc surface area

and thus provides abundant interspace to expose more active sites for electrocatalysis. In order to probe the fraction of the exposed surface of the electrocatalytically active Mo2C, an anodic potential sweep prior to the oxygen evolution reaction was conducted. As shown in Figure 4E, a series of oxidation waves are evident between 0.28 and 0.75 V due to the stepwise oxidation of surface Mo2C to MoO3 following a 16-electron process.5,41 From the charge integrated over the waves, we estimate that ~ 26.5% of Mo2C is electrochemically accessible (see details in the Supporting Information), which is higher than the value of 12.5% for Mo2C nanoparticles dispersed in hierarchical carbon microflowers.5 The high exposed surface fraction is significant and consistent with the very small size of Mo2C nanoparticles on the mesoporous carbon with a large surface area and an abundant pore distribution. To probe the durability of Mo2C@OMC-3, continuous CV scans were carried out between –0.3 and 0 V vs. RHE at a scan rate of 100 mV/s, and the long-term electrolysis was conducted at an overpotential of 200 mV for 20 h, both measured in 0.5 M H2SO4. Figure 4F shows that the catalytic activity was maintained with a negligible activity degradation after 1000 CVs. In addition, the inset shows that the catalytic current was sustainable with no current loss during electrolysis. These results indicate that Mo2C@OMC-3 is a highly stable HER electrocatalyst in acidic solutions. As shown in Figure S10, the morphology of the electrode material after 20 h electrolysis still maintains the highly ordered mesoporous structure with embedded Mo2C nanoparticles, indicating that this nanocomposite material possesses high structural strength.


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-20

34 mV/dec

-0.6 -0.4 -0.2 E (V vs. RHE)

0.0


-0.2 -30 -0.8

96 mV/dec 85 mV/dec 64 mV/dec

20 wt% Pt/C

0

1 2 3 2 log |j| (mA/cm )

Figure 5. (A) LSV curves at a scan rate of 2 mV/s and (B) Tafel plots of Mo2C@OMC-1, -2, -3, -4 and 20 wt % Pt/C electrocatalysts in 1 M KOH solution.

To couple with water oxidation and realize water splitting, HER electrocatalysts that can operate in alkaline solution are highly desirable. Figure 5A shows the LSV curves of Mo2C/OMC electrocatalysts toward the HER in 1 M KOH solution. As can be seen, the electrocataltic activities increase with increasing the content of Mo initially and the best performance is obtained at Mo2C/OMC-3 in 1 M KOH solution, consistent with the results in 0.5 M H2SO4 solution. For Mo2C/OMC-3, it requires an overpotential of 175 mV to deliver a cathodic current density of 10 mA/cm2, which is generally comparable with the performance in 0.5 M H2SO4. Figure 5B shows that the Tafel slope of Mo2C@OMC-3 obtained from the polarization curve (0.1 mV s−1) is 64 mV dec−1.

Conclusions In summary, we have developed a feasible method to fabricate Mo2C@OMC nanocomposites as an excellent electrocatalyst for electrochemical hydrogen evolution. The nanocomposites were synthesized via an EISA procedure by using polymer precursor as the template of OMC, which in addition could serve as carbon source for the formation of Mo2C and mitigate the sintering of Mo2C nanoparticles during the synergistical preparation. The Mo2C nanoparticles are highly distributed and embedded in the ordered mesoporous carbon matrix, which gives rise to the appealing features, such as small-sizes of approximately 5 nm and large exposed electrochemically active sites of approximately 26.5%. By virtue of the unique structural and morphological advantages, the Mo2C@OMC nanocomposites with a low content of Mo exhibit excellent performance toward the HER in both acidic and alkaline solutions. The synthetic procedure developed in this study may open up new avenues for design of various novel metal carbide cathodes for the HER and other electrocatalytic applications.

ASSOCIATED CONTENT Supporting Information Available As noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION ⊥

These authors contributed equally.

Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21573160, 21405114), the Fundamental Research Funds for the

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Central Universities, and the Science & Technology Commission of Shanghai Municipality (14DZ2261100) for support.

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