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Unique Hierarchical Mo2C/C Nanosheet Hybrids as Active Electrocatalyst for Hydrogen Evolution Reaction Can Wu, and Jinghong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13822 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Unique

Hierarchical

Mo2C/C

Nanosheet

Hybrids

as

Active

Electrocatalyst for Hydrogen Evolution Reaction

Can Wu, Jinghong Li* Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, China *Corresponding author: [email protected]

ABSTRACT: A facile and effective pyrolysis stratagem is proposed for the synthesis of hierarchical Mo2C/C nanosheet hybrids with sodium chloride (NaCl) crystals as a template. Large numbers of well-dispersed sheet-like Mo2C nanoparticles with the thickness of about 20 nm were anchored on the surface of carbon nanosheets. Benefitting from the ideal synergistic catalytic effect between the highly active sheet-like Mo2C nanoparticles and the conductive graphitic carbon, and strong charge transfer ability, the unique hierarchical sheet-like structure of Mo2C/C hybrids demonstrated excellent hydrogen evolution reaction (HER) performance in both alkaline and acid medias with small overpotential (125 mV for 10 mA cm-2 in 1 M KOH and 180 mV for 10 mA cm-2 in 0.5 M H2SO4) and remarkable stability, which is comparable to most reported non-noble metal HER electrocatalysts. With the simplicity and low-cost of the synthetic approach, the strategy presented here can be extendable to the preparation of other transition metal-based/carbon nanosheet hybrids for versatile applications. 1 / 36

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KEYWORDS: Molybdenum carbide, carbon nanosheet, hierarchical structure, electro-catalysis, hydrogen evolution reaction

1. Introduction As the global energy crisis and environmental pollution are worsening, clean and sustainable energy sources and carriers have attracted considerable attention. Hydrogen (H2) is a clean and renewable energy source that is regarded as a promising energy carrier to replace fossil fuels.1,2 Electrocatalytic water splitting is considered as a sustainable and promising way for large-scale hydrogen production.3 In order to efficiently drive the electrocatalytic water splitting reaction, active and stable hydrogen evolution reaction (HER) electrocatalysts are essential. Precious Pt-based materials are considered as the most efficient hydrogen evolution reaction (HER) electrocatalysts, which is active at a nearly zero overpotential.4 However, the scarcity and high cost of Pt-based electrocatalysts seriously hinder its widespread applications. Therefore, designing and fabricating cost effective and earth-abundant HER electrocatalysts with high-performance is highly imperative. Up to now, many earth-abundant transition metal-based electrocatalysts such as sulfides,5-7 nitrides,8-10 phosphides,11-14 carbides15-17 and other materials18-20 have been studied as high-performance HER electrocatalysts in acid media. However, few HER electrocatalysts display high catalytic activity both in acid and alkaline medias, especially in alkaline electrolytes. So far, molybdenum compounds, such as 2 / 36


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molybdenum disulfide,21 molybdenum boride,22 molybdenum carbide23 and so on have exhibited remarkable HER catalytic properties in alkaline solution. Among these molybdenum-based HER electrocatalysts, molybdenum carbide, as one of the most representative carbides, has attracted great interests for its low cost, chemical stability, good catalytic activity and similar d-band electronic density-of-state to that of noble metal Pt.24 Since the early 1980s, molybdenum carbide has been extensively studied as desulfurization, hydrogenation, and the water gas shift reaction catalysts.25 Subsequently,

commercially

available

molybdenum

carbide

microparticles

(com-Mo2C) were first realized to be highly active for HER in 2012 by the Hu group.26 Since then, extensive effort has been devoted to improving the HER catalytic activity by optimizing the structures of molybdenum carbide at nanoscale, for instance, molybdenum carbide nanotubes,27 nanowires,28 and nanoparticles29,30 have been designed as HER catalysts. What’s more, various carbon-supported molybdenum carbide nanomaterials have also been reported as highly efficient HER catalysts for the enhanced electric conductivity and decreased charge-transfer resistance.31-33 However, excess growth and coarsening often occur during the high-temperature annealing process, which will lead to the serious decrease of exposed active sites and poor catalytic activity.34 Though much effort has been made, the controllable synthesis of well-defined structure of molybdenum carbide nanomaterials with abundant active sites and high conductivity is still highly challenging. In this study, a facile approach is proposed for fabricating a unique hierarchical Mo2C/C nanosheet hybrid employing water-soluble NaCl cube crystals-template 3 / 36



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strategy. It should be noted that NaCl cube crystals template owns the advantages of low cost, environmental friendliness and ease of fabrication and removal. The novel preparation process mainly involves carburizing ammonium molybdate and citric acid coated NaCl cube crystals under an Ar gas atmosphere and the removal of NaCl templates. It is worth noting that the formed two-dimensional carbon nanosheets with a thickness of about 25 nm are decorated by numerous sheet-like Mo2C nanoparticles. The obtained sheet-like Mo2C nanoparticles are expected to expose more active sites than other nanostructures while the carbon nanosheets carrier can maintain superior conductivity. Additionally, the robust hierarchical structure also owns high specific surface area. Benefiting from the above-mentioned merits, the as-prepared hierarchical sheets structures exhibit abundant exposed active sites and fast electron transport pathways, which are in favor of electrocatalytic activity. As a result, when used as a novel HER catalyst in 1 M KOH electrolyte, the synthesized Mo2C/C nanosheet hybrids exhibit superior catalytic activity, with onset overpotential of 60 mV, Tafel slope of 72 mV/dec, and 125 mV overpotential at current density of 10 mA cm-2, which is comparable to the benchmark Pt/C catalyst. Moreover, it also shows high activity and durability in 0.5 M H2SO4 media with overpotentials of 180 mV to achieve 10 mA cm-2.

2. Experimental Section 2.1 Chemicals and materials Sodium chloride (NaCl), citric acid (C6H8O7), ammonium molybdate 4 / 36


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tetrahydrate ((NH4)6Mo7O24·4H2O), sulfuric acid (H2SO4, 98%), potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Pt/C (20 wt% Pt on Vulcan XC-72R) and Nafion (5 wt%) were obtained from Sigma-Aldrich. All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. 2.2 Materials synthesis Synthesis of NaCl templates: First, NaCl powders were completely dissolved into deionized water to form a saturated solution, and then abundant ethanol was injected under stirring, milky suspension was observed for the significantly decreased solubility. Finally, the obtained white NaCl crystals were collected, dried and followed by fine grind for further use. Synthesis of Mo2C/C nanosheet hybrids: Typically, 50 mg ammonium molybdate tetrahydrate and 50 mg citric acid were dissolved in 1 mL double-distilled water, then 2 g of NaCl templates were added into the above solution, and the mixture was treated with ultrasonication for about 10 min. Then, the obtained mixture was completely dried in a 80 oC oven overnight, and the resultant solid mixture was grinded into very fine powders. After pyrolysis at 750 oC for 2 h with a ramp rate of 5 oC min-1 in Ar atmosphere, the obtained product was repeatedly washed with distilled water to remove the NaCl templates under sonication and centrifugalization. Finally, the black powders were harvested and dried at 80 oC, and the Mo2C/C nanosheet hybrids were readily obtained. For the synthesis of Mo2C/C nanosheet electrocatalyst, the mass ratio of ammonium molybdate tetrahydrate (50 mg) and citric acid (50 mg) was 5 / 36



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defined as 2:2, thus the electrocatalyst was denoted as Mo2C/C(2:2). Similarly, 25 mg, 75 mg and 100 mg ammonium molybdate tetrahydrate were also used with 1:2, 3:2, 4:2 initial mass ratios of ammonium molybdate tetrahydrate and citric acid to prepare the samples by a similar procedure, and the obtained samples were denoted as Mo2C/C(1:2), Mo2C/C(3:2) and Mo2C/C(4:2), respectively. Synthesis of pure C nanosheet: Pure carbon nanosheet matrix was prepared without the addition of ammonium molybdate tetrahydrate by a similar procedure. Synthesis of molybdenum carbide-C composite: Molybdenum carbide-C composite was prepared without the addition of NaCl templates with the similar procedure. 2.3 Materials Characterization Powder X-ray diffraction (XRD) patterns were collected out on a Bruker D8-Advance diffractometer with Cu Kα radiation (λ = 1.5418 Å). A field-emission scanning electron microscopy (SEM, Hitachi SU8010) with an accelerating voltage of 10 kV was used to characterize the morphology. Transmission electron microscopy (TEM) and corresponding energy-dispersive X-ray (EDX) elemental mapping images were performed using a JEM 2010 transmission electron microscope at an accelerating voltage of 200 kV. Raman spectrum was carried out on a micro-Raman spectrometer (Reinshaw Raman Scope RM3000) under ambient conditions with excitation by an argon ion laser (532 nm). X-ray photoelectron spectroscopy (XPS) was conducted on a PHI Quantera scanning X-ray microprobe spectrometer with Al Kα (λ = 1486.7 eV). The Brunauer-Emmett-Teller (BET) measurement was carried 6 / 36


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out a Quantachrome NOVA 1000 system at liquid-N2 temperature. 2.4 Electrochemical measurements The electrochemical experiments were conducted using a three-electrode system on a CHI 660B electrochemical workstation (Chenhua Instrument, Shanghai, China) at ambient temperature. KCl saturated calomel electrode (SCE) and carbon rod were used as the reference electrode and the counter electrode, respectively. For the working electrode, 2 mg of Mo2C/C nanosheet was dispersed in the mixture of 30 μL Nafion (5 wt%) and 500 μL distilled water followed by ultrasonication for 1 h. Then 5 μL catalyst suspensions were coated on the clean surface of glassy carbon electrode (GCE) with the diameter of 3 mm. Then the electrode was dried under an infrared lamp in air before measurement, the loading amount of Mo2C/C was calculated to be 0.28 mg cm-2. The Linear sweep voltammetry was recorded in 1 M KOH and 0.5 M H2SO4 with saturated N2 at scan rate of 10 mV s-1. All potentials were referenced to a reversible hydrogen electrode (RHE) according to E (RHE) = E (SCE) + 0.241 V+ 0.059 pH. The long-term stability tests were conducted by continuous cycle voltammetry sweep from -0.2 to 0.1 V (versus RHE, in 1 M KOH) and -0.25 to 0 V (versus RHE, in 0.5 M H2SO4) at a scan rate of 100 mVs-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out on Biologiclab SP-150 (Biologic Science Instruments, France) from 100 kHz to 0.1 Hz with amplitude of 10 mV. All data are presented without iR compensation. To estimate the electrochemically active surface area (ECSA), the electrocatalysts were cycled in the potential region from 0.15 to 0.25 V vs RHE (the non-Faradaic region) under various 7 / 36



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scan rates from 10 to 300 mV s-1. According to the linear relationship between the capacitive currents and the scan rate, the slope k of the fitting line can be obtained. The value of capacitance of the double layer (Cdl) was equal to k/2, which was linearly proportional to the electrochemically active surface area of the electrode. Further, the specific capacitance C can be calculated by C = Cdl/m, where m is the areal loading amount of the catalyst. In addition, the ECSA can be calculated by assuming a standard value of 60 μF/cm2: ECSA = C/60 μF/cm2.25

3. Results and Discussion The typical preparation process of the Mo2C/C nanosheet is described in Scheme 1. Ammonium molybdate-citric acid coated NaCl composite was firstly formed by the evaporation of the water. It should be noted that the amount of ammonium molybdate and citric acid was far less than the added NaCl templates, which can effectively avoid the agglomeration of the precursors on the surface of NaCl templates during the process of recrystallization. Moreover, the smooth surface of NaCl can direct the growth of sheet-like structure. Then, the ammonium molybdate-citric acid coated NaCl powders were directly annealed at 750 oC under Ar atmosphere, leading to the formation of Mo2C/C hybrids coated NaCl composite. During the annealing process, citric acid serves as the carbon source of the carbon matrix and Mo2C. The in situ carburization reaction took place between ammonium molybdate and citric acid on the surface of NaCl templates, which resulted in the formation of uniform Mo2C nanocrystallites embedded in a carbon matrix. Finally, the NaCl templates were 8 / 36


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removed by washing with water, and Mo2C/C nanosheet hybrids were obtained. For convenience, the Mo2C/C electrocatalysts derived from different initial mass ratios of ammonium molybdate tetrahydrate and citric acid at 1:2, 2:2, 3:2, and 4:2 are nominated as Mo2C/C(1:2), Mo2C/C(2:2), Mo2C/C(3:2), Mo2C/C(4:2), respectively, in the following discussion. Uniform NaCl crystal templates with a cube-like structure were prepared via a facile organic solvent-induced recrystallization method.35,36 As revealed by powder X-ray diffraction (XRD) pattern shown in Figure S1a, the as-prepared NaCl templates owned very high crystallinity. Moreover, typical field-emission scanning electron microscopy (SEM) further confirmed the prepared NaCl templates had a cubic morphology with a size of ~10 μm and very smooth surface (Figure S1b and c). By contrast, the morphology of the pristine NaCl powder was rather irregular and disordered (Figure S1d), which is believed to be unconducive for the formation of well-defined two-dimensional sheet-like structure.36 In order to confirm ammonium molybdate-citric acid complex was coated on the surface of NaCl template after the evaporation of water, SEM characterization was performed. As shown in Figure S2, it is obvious that the smooth surface of NaCl became rather rough, and the formed shell-like structure was located on the outer surface, indicating the formation of ammonium molybdate-citric acid-coated NaCl composite. Figure 1a displays the crystal structure of the as-synthesized Mo2C/C(2:2) hybrids, the XRD peaks at 34.5°, 38.1°, 39.5°, 52.3°, 61.8°, 69.8°, 74.9° and 75.8° were attributed to the diffractions of (021), (200), (121), (221), (040), (321), (240) and (142) planes of orthorhombic Mo2C 9 / 36



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(JCPDS card No. 72-1683). According to the XRD pattern analysis, it can be observed that the relative intensity ratio of (021) and (040) lattice plane to the main diffraction peak (121) of Mo2C/C(2:2) is remarkably stronger than that of standard map, therefore, it can be roughly speculated that (021) and (040) lattice plane may be preferably exposed in the anisotropic Mo2C nanoparticles. In addition, the peak at around 26° could be attributed to the (002) plane of carbon matrix in the Mo2C/C(2:2) hybrids (Figure S3a). In order to further monitor the graphitic structure in the hybrids, Raman spectroscopy test was conducted (Figure 1b). As expected, two obvious peaks at 1350 and 1590 cm-1 were observed, which could be attributed to the D and G band of graphitic carbon, respectively. The intensity ratio of D band to G band (ID/IG) was about 1.5, suggesting the carbon component in the product was rather disordered with many defects. Therefore, the formation of Mo2C/C(2:2) hybrids was confirmed by XRD and Raman spectra. Figure 1c presents the low-magnification SEM image of the resulting Mo2C/C(2:2) hybrids, lots of well-dispersed nanosheet-like structure were observed. High-magnified SEM images (Figure 1d) reveal that numerous small nanosheet-like nanoparticles were uniformly anchored on the surface of ~25 nm thick nanosheet matrices. The morphology and structure of the hierarchical Mo2C/C(2:2) hybrids were further investigated by TEM. As depicted in Figure 1e and f, uniformly shaped nanosheet-like nanoparticles were embedded in lamellar carbon matrices with thickness of about 20 nm. TEM image illustrated in Figure S3b further clearly reveals that the sheet-like nanoparticles were vertically or horizontally anchored on the sheet-like matrices, and the radial size of the sheet-like nanoparticle was about 100 10 / 36


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nm with thickness of about 20 nm. In addition, high-resolution TEM image (Figure 1g) taken on the surface of single sheet-like Mo2C nanoparticle reveals obvious lattice fringes with d-spacings of 0.23 nm, which can be indexed to the (121) crystal plane of Mo2C phase. The diffraction rings were well consistent with the (021), (200), (121), (221) and (040) planes of the orthorhombic Mo2C crystal phase, which was in agreement with the XRD pattern (Inset in Figure 1g). Moreover, the polycrystalline nature of the sheet-like Mo2C nanoparticle was also confirmed by the obvious grain boundaries (Figure S4). Besides, elemental mapping of Mo2C/C(2:2) hybrids revealed that the Mo and C atoms were homogeneously distributed over the entire structure (Figure 1h), and the embedded sheet-like nanoparticles were composed of Mo2C nanocrystallites, suggesting that the sheet-like Mo2C nanocrystals were uniformly distributed within the carbon matrices. In addition, the original Mo2C content in the Mo2C/C(2:2) hybrids was measured to be 77.5% according to the ICP-AES analysis, implying high amount of loading of Mo2C, which may suggest high HER activity. For comparison, molybdenum carbide-C composite was also prepared in the absence of NaCl templates, however, only micrometer sized bulk products were obtained, as shown in Figure S5a. The diffraction peaks of the bulk molybdenum carbide-C hybrid indicated the formation of molybdenum carbide phase (Mo3C2) and graphitic phase (Figure S5b). The presence of graphitic carbon in the bulk products was further evidenced by Raman spectrum in Figure S5c. Furthermore, molybdenum carbide is well known as active electrocatalysts for HER, the introduction of graphitic carbon is able to greatly increase the electron transfer during the HER process, thus promoting 11 / 36



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the HER performance. In the synthesis process of Mo2C/C(2:2) hybrids, citric acid serves as the carbon source of the graphitic carbon and molybdenum carbide, so the relative ratio of active molybdenum carbide and conductive graphitic carbon can be readily tuned by fixing the amount of citric acid while adjusting the mass of ammonium

molybdate

tetrahydrate.

Different

molybdenum

carbide/carbon

electrocatalysts may display different synergistic catalytic effect for the change of relative ratio of active molybdenum carbide and graphitic carbon. On the other hand, the adjusting of the ratio of metal precursor to carbon precursor may also have an effect on the morphology and physical property of the obtained samples, which in turn affect the HER performance. Therefore, in order to obtain the optimal HER electrocatalyst, pure carbon matrices and samples derived from ammonium molybdate tetrahydrate and citric acid with 1:2, 3:2, 4:2 initial mass ratios were also prepared via the similar synthesis process. Figure S6a displays the XRD pattern of pure carbon matrix, the characteristic diffraction peak at about 26.0° revealed the formation of graphitic carbon with high purity. In addition, samples derived from other initial mass ratios of ammonium molybdate tetrahydrate and citric acid also showed high crystallization of Mo2C (Figure S6b). However, according to the amplifying XRD patterns of different samples, the diffraction peak intensity of graphitic carbon was weaken gradually along with the increasing of initial mass ratios of ammonium molybdate tetrahydrate and citric acid (Figure S6c-f), it is clear that (002) diffraction peak of samples obtained with 3:2 and 4:2 nearly disappeared, implying the content of graphitic carbon was gradually decreased, which is also consistent with the Raman 12 / 36


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characterization (Figure S7). The structure of pure carbon matrix and samples obtained from different initial mass ratios of ammonium molybdate tetrahydrate and citric acid were further investigated by SEM. It was clear that the pure carbon matrix consisted of numerous well-dispersed nanosheets with a thickness of about 30 nm (Figure S8a). The as-synthesized hybrids obtained with 1:2 maintained the sheet-like structure with nano Mo2C nanoparticles anchored on the surface of carbon nanosheets (Figure S8b). Sample obtained with 3:2 was composed of many micrometer-scale sheet-like Mo2C nanoparticles while the micrometer-scale sheet-like Mo2C nanostructure consisted of numerous number of nano Mo2C array (Figure S8c). When further increasing the initial mass ratios of ammonium molybdate tetrahydrate and citric acid to 4:2, the formed Mo2C displayed irregular and disordered morphology (Figure S8d). It is likely that a thermodynamic driving force allows to decrease the system surface energy and make the structure more stable, which leads to the formation of sheet-like Mo2C nanoparticles.37 In addition, it should be noted that only spherical Mo2C nanoparticles were formed at 1:2 initial mass ratios of ammonium molybdate tetrahydrate and citric acid, thus, the growth of sheet-like Mo2C nanoparticles was also related to its intrinsic characteristics in the present experiments. This result indicates that the morphology and structure were seriously affected by the ratio of ammonium molybdate and citric acid, the optimal HER performance of catalyst may be obtained only the Mo2C content was accurately controlled. Moreover, the significant structure difference between Mo2C/C hybrids and molybdenum carbide-C composites confirms the important role of NaCl in directing the formation 13 / 36



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of sheet-like nanostructure. The composition and surface chemical state of the as-synthesized Mo2C/C(2:2) hybrids were further probed by X-ray photoelectron spectroscopy (XPS). From the survey spectrum displayed in Figure 2a, elements of C, O, and Mo can be clearly identified in the hybrid. The high resolution spectra of Mo 3d can be deconvoluted into three doublets (Figure 2b). The peaks centered at binding energy of 228.0 and 231.0 eV were assigned to Mo2+ (3d5/2 and 3d3/2, respectively), which can be attributed to the carbide phase, indicating the presence of Mo2C.38,39 Other two pairs of peaks (Mo 3d5/2/3d3/2) at binding energies of 228.7/231.9 and 232.7/235.2 eV can be ascribed to Mo4+ and Mo6+ in molybdenum oxidized phases, which was probably caused by surface oxidation of Mo2C due to air contact.40 The deconvolution of the C 1s peaks is shown in Figure 2c, whereas peaks at 284.2 and 285.0 eV can be assigned to C-C/C=C and C-O bonds, respectively.41 Additionally, no carbidic peak (approximately 282.7 eV) was observed. This may be due to the signal of the carbidic carbon was covered by the graphitic carbon in a similar binding energy region.42 In addition, the electronic structure of Mo2C can also be explained by hybridization of atomic orbitals, where Mo with the coordination number 3 selects d3 hybridization mode and C with the coordination 6 selects sp3sp hybridization mode. Therefore, Mo-C bonds have strong ionic characteristics. Furthermore, O2s2p orbitals, C2s2p orbitals and Mo4d orbitals are involved in chemical bonding.43 Nitrogen adsorption/desorption measurements were carried out to elucidate the porosity

and

pore

size

distribution

in

the

Mo2C/C(2:2)

hybrids.

The 14 / 36


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Brunauer-Emmett-Teller (BET) specific surface area of the as-prepared Mo2C/C(2:2) hybrids was calculated to be 101.71 m2 g-1 (Figure S9a), such high BET surface area can provide more active surface sites, which was believed to be beneficial for their application in electrocatalysis. In addition, the BJH pore size distributions revealed a typical mesoporous character with the pore diameters mainly centered at 3 and 10 nm (Figure S9b), which was favorable for mass transport and adsorption. By contrast, the molybdenum carbide-C hybrids displayed rather low BET specific surface area (Figure S9c), and no mesoporous character was exhibited (Figure S9d). This result further highlighted the important role of NaCl templates for the synthesis of well-defined sheet-like nanostructure with high surface area. The HER activities of different catalysts including pure C nanosheets, Mo2C/C(1:2), Mo2C/C(2:2), Mo2C/C(3:2), Mo2C/C(4:2), molybdenum carbide-C composite, and Pt/C were first evaluated by linear scanning voltammetry (LSV) in N2-saturated 1 M KOH. Figure 3a shows the polarization curves without iR-compensation at a scan rate of 10 mV s-1. As expected, the Pt/C catalyst displayed superior HER activity with a near zero overpotential, whereas C nanosheets exhibited relatively poor HER activity. In addition, the Mo2C/C(2:2) exhibited optimal electrocatalytic HER performance with 125 mV overpotential at a current density of 10 mA cm-2 among these molybdenum carbide-based catalysts, which was possibly ascribed to its optimal synergistic catalytic effect between the sheet-like Mo2C nanoparticles and the conductive C nanosheet substrates. Notably, the overpotential for 10 mA cm-2 current density was comparable to the values of most recently 15 / 36



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reported nonprecious metal catalysts for HER in alkaline electrolyte, including MoCx nano-octahedrons (151 mV),44 MoC-Mo2C heteronanowires (120 mV),45 MoS2 nanosheet arrays (190 mV),46 MoP nanoparticles (130 mV),47 CoP nanowire arrays (209 mV),48 NiP2 nanosheet arrays (102 mV)49 and so on. Moreover, it is worth noting that the HER catalytic performance of molybdenum carbide-C composite was inferior to all the other Mo2C/C hybrids catalysts, indicating the important role of NaCl templates in synthesizing high active molybdenum carbide-based electrocatalysts. The HER performance of various electrocatalysts was also studied by Tafel slope. As shown in Figure 3b, the Tafel slope of Mo2C/C(2:2) was calculated to be 72 mV/dec, which was lower than Mo2C/C(1:2) (135 mV/dec), Mo2C/C(3:2) (119 mV/dec), Mo2C/C(4:2) (94 mV/dec), molybdenum carbide-C (96 mV/dec). The lower Tafel slope value suggests faster reaction dynamic process, Mo2C/C(2:2) showed the smallest Tafel slope value, indicating high HER activity. The Tafel slope value of 72 mV/dec for Mo2C/C(2:2) suggests the HER occurs through a mixed Volmer and Heyrovsky mechanism, which indicates the HER rate is determined by both discharge of H2O molecule and desorption of bonded hydrogen atoms from the catalyst surface.44,50,51 Furthermore, the onset overpotential can be speculated from the low current density region of the Tafel plot, the onset potential of the hierarchical Mo2C/C(2:2) was estimated to be about 60 mV in 1 M KOH solution (Figure S10a). The electrochemical surface area (ECSA) and electrochemical impedance spectroscopy (EIS) were further probed to provide insight into the performance of different electrocatalysts. To assess the effective ECSA, the as-synthesized 16 / 36


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electrocatalysts were cycled in the potential region from 0.15 to 0.25 V vs RHE at scan rates varying from 10 to 300 mV s-1 (Figure S11). The double-layer capacitance (Cdl) of C nanosheets, Mo2C/C(1:2), Mo2C/C(2:2), Mo2C/C(3:2), Mo2C/C(4:2) and molybdenum carbide-C composite were measured to be 50 mF cm-2, 9.5 mF cm-2, 6.0 mF cm-2, 5.5 mF cm-2, 7.5 mF cm-2 and 3.0 mF cm-2, respectively, according to the linear relationship between the capacitive currents at 0.2 V and the scan rate (Figure S12). Further, the specific capacitance C can be calculated by C = Cdl/m, where m is the areal loading amount of the catalyst, and the ECSA values of C nanosheets, Mo2C/C(1:2), Mo2C/C(2:2), Mo2C/C(3:2), Mo2C/C(4:2) and molybdenum carbide-C composite can be calculated to be about 297.7 m2/g, 56.5 m2/g, 35.7 m2/g, 32.7 m2/g, 44.7 m2/g and 17.8 m2/g by assuming a standard value of 60 μF/cm2.25 It is believed that the high ECSA value generally implies abundant active sites, however, in consideration of the HER performance of different materials displayed in Figure 3a, it can be concluded that Mo2C is highly active for HER and the active specific area is not the determining factor for the excellent HER performance. Therefore, the optimal HER activity of Mo2C/C(2:2) can be ascribed to the optimal synergistic catalytic effect between the active sheet-like Mo2C nanoparticles and the conductive graphitic carbon. In order to further access the intrinsic activity of different catalysts, the electrocatalytic HER currents were normalized by ECSA. As displayed in Figure S13, Mo2C/C(2:2) still exhibited the best HER activity among all the molybdenum carbide-based catalysts, further demonstrating the synergistic catalytic advantage of this hierarchical Mo2C/C nanosheet structure for HER. Electrochemical impedance 17 / 36



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spectroscopy (EIS) analysis was also performed to investigate the transport kinetics of various catalysts (Figure 3c). Inset in Figure 3c is the corresponding equivalent circuit, where R1 is the solution resistance, R2 is the the catalyst layer resistance and R3 in the low frequency region reflects the charge transfer resistance during HER. As shown in Figure 3c, the R3 value of 72 Ω for Mo2C/C(2:2) iwa lower than other molybdenum carbide-based catalysts, indicating faster electron transfer rate and higher activity of Mo2C/C(2:2), which is highly consistent with the LSV curves illustrated in Figure 3a. Besides the catalytic activity, another significant indicator to evaluate the performance of electrocatalyst is durability. The long-term stability of Mo2C/C(2:2) was first evaluated by chronoamperometry, the current density has a minor drop at the beginning, and then almost keeps constant without any obvious debasement even after 20 h at a constant overpotential of 130 mV (Figure 3d), demonstrating good stability of the hierarchical Mo2C/C nanosheet during the HER process. It should be noticed that the minor decrease of the catalytic performance at the initial phase may be related to the part corrosion of Mo2C in alkaline media, which is similar with the previous reports.25 The electrochemical stability of the hierarchical Mo2C/C nanosheet was also verified by polarization curves of the Mo2C/C nanosheet treated before and after 2000 cycles cyclic voltammetry scanning ranging from -0.2 to 0.1 V vs RHE at a scan rate of 100 mV s-1 (Figure S14a). Moreover, the diffraction rings and TEM image in Figure S15a further confirm the morphology and structure of the as-synthesized catalyst can be well maintained after long time durability test. The as-synthesized Mo2C/C hybrids were also investigated as HER 18 / 36


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electrocatalysts in acid media (0.5 M H2SO4). The sample with 2:2 still exhibited the optimal electrocatalytic HER activity among all these molybdenum carbide-based catalyst, which is similar to that in alkaline media. The overpotential required to drive the cathodic current densities of 10 mA cm-2 was 180 mV for Mo2C/C(2:2) (Figure 4a). What is more, the overpotential at 10 mA cm-2 current densities of Mo2C/C nanosheet is also competitive relative to other different previously reported catalysts, such as Mo2C-graphene composites (150 mV),52 Mo2C/CNT (152 mV),53 defect-rich MoS2 ultrathin nanosheet (200 mV),54 MoS2/N-doped carbon nanoboxes (165 mV),55 Co-embedded N-rich CNTs (260 mV),56 ferric phosphide spherical clusters (104 mV)57 and so on. In line with this result in Figure 4a, Pt/C gave the lowest Tafel slope (30 mV/dec), and Mo2C/C(2:2) had a smaller Tafel slope (71 mV/dec) than Mo2C/C(1:2) (133 mV/dec), Mo2C/C(3:2) (83 mV/dec), Mo2C/C(4:2) (79 mV/dec) and bulk molybdenum carbide-C (121 mV/dec), 71 mV/dec of Tafel slope suggests Mo2C/C(2:2) catalyst proceeds via a Volmer-Heyrovsky mechanism (Figure 4b).58 Moreover, the onset overpotential of Mo2C/C nanosheet in 0.5 M H2SO4 was estimated to be about 102 mV (Figure S10b). In addition, the charge-transfer resistance (R3) value of Mo2C/C(2:2) (98 Ω) was also much lower than other molybdenum carbide-based catalysts, implying Mo2C/C(2:2) had good electron transfer ability, which was in agreement with the polarization curve in Figure 4a. In addition, the Mo2C/C(2:2) also demonstrated excellent performance, morphology and structure stability, which can be concluded from the long-time of current-time curve (Figure 4c), LSV curves (Figure S14b) and the structure analysis (Figure S15b). 19 / 36



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Based on the above results of electrochemical study, it is obvious that the as-synthesized Mo2C/C nanosheet hybrids are highly efficient HER electrocatalyst. The remarkable electrocatalytic HER activity can be attributed to the following factors: (1) numerous number of nano-sized sheet-like Mo2C nanoparticles uniformly anchored on the carbon matrix can effectively expose the active sites; (2) the hierarchical sheet-like structure with porous character and large specific surface area can enlarge the contact of catalyst with the electrolyte, which facilitates the mass transfer; (3) the ideal synergistically effect between highly active of sheet-like Mo2C nanoparticles and the highly conductive carbon nanosheet can facilitate the electron transfer, which provides a ideal conducting network in micrometer scale during the HER process; (4) the unique and robust hierarchical structure enables high stability during long-term operation.

4. Conclusions In summary, we report a facile and effective pyrolysis route to synthesize sheet-like Mo2C nanoparticles-embedded carbon sheets using NaCl cubes as the template. The outstanding electrocatalytic performance for hydrogen production in both alkaline and acidic conditions can be mainly attributed to the optimal synergistic catalytic effect between the highly active sheet-like Mo2C nanoparticles and the conductive graphitic carbon and fast charge transport ability. Such facile approach offers a general methodology for design and preparation of two-dimensional transition metal-based/carbon nanosheet hybrids for versatile applications. 20 / 36


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ASSOCIATED CONTENT Supporting Information Available: The Supporting Information is available free of charge on the http://pubs.acs.org. More results of physical characterization (XRD patterns, SEM images, TEM images, HRTEM image, selected area electron diffraction patterns, Raman spectrum, N2 adsorption-desorption isotherms, pore size distributions), cyclic voltammograms curves, linear sweep voltammetry, Tafel plots and ECSA for different materials. Corresponding Author: [email protected] Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by National Key Research and Development Program of China (No. 2016YFA0203101), National Natural Science Foundation of China (No. 51572139), and the Open Project of National & Local United Engineering Lab for Power Battery, Northeast Normal University (No. 130017501).

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Scheme 1. Illustration of the fabrication of hierarchical Mo2C/C nanosheet hybrids.

Figure 1. (a) XRD pattern and (b) Raman spectra of Mo2C/C(2:2). (c) Low- and (d) high-magnified SEM images of Mo2C/C(2:2). (e) Low- and (f) high magnified TEM images of Mo2C/C(2:2). (g) High-resolution TEM image of Mo2C/C(2:2). Inset is the SAED pattern. (h) EDX element mapping images of Mo2C/C(2:2).

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Figure 2. (a) XPS survey spectrum of Mo2C/C(2:2) hybrids and high-resolution of (b) Mo 3d, (c) C 1s XPS spectra of Mo2C/C(2:2).

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Figure 3. (a) Linear polarization curves of C nanosheets, Mo2C/C(1:2), Mo2C/C(2:2), Mo2C/C(2:3), Mo2C/C(2:4), molybdenum carbide-C and Pt/C electrodes in 1 M KOH with scan rate of 10 mV s-1. (b) Corresponding Tafel plots of the as-prepared materials. (c) Nyquist plots of various molybdenum carbide-based catalysts over the frequency range from 100 kHz to 0.1 Hz at -1.28 V vs SCE in 1 M KOH. Inset is the equivalent circuit. (d) Time dependence of the current density for Mo2C/C(2:2) at a static overpotential of 130 mV for 20 h.

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Figure 4. (a) HER performance of C nanosheets, Mo2C/C(1:2), Mo2C/C(2:2), Mo2C/C(2:3), Mo2C/C(2:4), molybdenum carbide-C and Pt/C electrodes in 0.5 M H2SO4 at 10 mV s-1. (b) Corresponding Tafel plots of the as-prepared materials. (c) Nyquist plots of different molybdenum carbide-based catalysts over the frequency range from 100 kHz to 0.1 Hz at -0.5 V vs SCE in 0.5 M H2SO4. Inset is the equivalent circuit. (d) Time-dependent current density curve of Mo2C/C(2:2) under overpotential of 190 mV in 0.5 M H2SO4.

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Table of Content (TOC) Graphic

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C Nanosheet Hybrids ... - ACS Publications

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