Mo2C

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Letter

In-situ Engineering of Double Phase Interface in Mo/Mo2C Heteronanosheets for Boosted Hydrogen Evolution Reaction Jie Xiong, Jing Li, Jiawei Shi, Xinlei Zhang, Nian-Tzu Suen, Zhao Liu, Yunjie Huang, Guoxiao Xu, Weiwei Cai, Xinrong Lei, Ligang Feng, Zehui Yang, Liang Huang, and Hansong Cheng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01180 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Energy Letters

In-Situ Engineering of Double Phase Interface in Mo/Mo2C Heteronanosheets for Boosted Hydrogen Evolution Reaction Jie Xiong,†,# Jing Li,† Jiawei Shi,† Xinlei Zhang,† Nian-Tzu Suen,‡ Zhao Liu,† Yunjie Huang,† Guoxiao Xu,† Weiwei Cai,*,† Xinrong Lei,† Ligang Feng,*,‡ Zehui Yang,*,† Liang Huang*,∥ and Hansong Cheng† †

Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University

of Geoscinces (Wuhan), 388 Lumo Road, Wuhan, 430074 (P.R. China). ‡

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002

(P.R. China). ∥

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and

Technology, Wuhan, 430081(P.R. China). #

College of Chemistry, Chemical Engineering and Material Science, Zaozhuang University,

Zaozhuang, 277160 (P.R. China).

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ABSTRACT: Two-dimensional Mo/Mo2C heteronanosheets (Mo/Mo2C-HNS) were successfully prepared via a NaCl template-assisted synthesis route followed by a controllable simultaneously reduction and carbonization of MoO3 nanosheets for efficient hydrogen evolution reaction (HER) catalysis under both acid and alkaline conditions. The Mo specie in the atomic-thin Mo/Mo2C-HNS not only guarantees the rapid transport of electrons but also optimizes the electronic configuration of β-Mo2C. Besides, the abundant Mo/β-Mo2C heterointerfaces in nanodimension afford large numbers of additionally heterogeneous catalytic sites. HER electrocatalytic performance with overpotential of merely 89 mV to drive a current density of 10 mA/cm2 in 0.5 M H2SO4 is therefore achieved. Strikingly, stable chronoamperometric electrolysis for 20 h and also an impressive cycling stability with negligible overpotential decay over 4000 sweeps demonstrate its prominent durability in an acidic environment. These findings highlight the promising potential of Mo/Mo2C-HNS catalyst as an efficient and stable noble metal-free electrocatalyst towards HER.


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Graphical Table of Contents


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Hydrogen is a clean and efficient energy carrier, which has the merits of the highest massspecific energy density and zero emissions of greenhouse gases and environmental contaminants, and has been recognized as a potential alternative to traditional fossil fuel. 1-5 Meanwhile, it’s readily available mostly from steam reforming or direct thermal pyrolysis of hydrocarbons, partial oxidation of heavy oil, coal gasification and electrolysis of water. Hydrogen generation from splitting the abundant and renewable water is a safe, sustainable and environmentally friendly strategy among all the hydrogen production processes. 3, 6-9 Pt based catalysts were proved to be the most effective electrocatalyst to boost hydrogen evolution reaction (HER), cathodic reaction in the water electrochemical splitting devices, especially under acidic condition due to the proper adsorption energy of H atom on Pt. 10-11 Unfortunately, high cost of Pt hinders the large scale application of HER related technologies including the promising water electrolysis and solar-driven water splitting 12-15. Therefore, cost-effective transition metal (TM) carbides, 1, 4, 6, 9, 15-23 nitrides, 3, 8, 24-29 sulfides, 5, 14, 30-37 phosphates 7, 12, 38-45 and borides 46-49 with various nano-structures were widely designed for efficient HER catalysis in demand. By considering the varied electronic state of molybdenum and the tunable phases, 50-51 molybdenum carbides, represented by Mo2C, have attracted special attention towards HER. Enormous efforts were therefore made on the Mo2C catalyst to enrich the highly accessible reactive sites via nano morphology design 6, 15 or / and to overcome the poor electronic conductive performance issue of Mo2C via composition structure design 6, 9, 16. Lou and colleagues 6 proposed a “metal-organic frameworks (MOFs)-assisted strategy” to synthesize molybdenum carbide nano-octahedrons from polyoxometalate-embedded Cu-MOFs through the confined carburization into MOFs matrix. The derived mesoporous MoxC nano-octahedrons composed of ultrafine nanocrystallites within in situ-incorporated carbon matrix exhibited


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considerable electrocatalytic activity for HER with low overpotentials of 142 and 151 mV to yield a current density of 10 mA/cm2 under acidic and basic conditions, respectively, due to the altered electron transfer property and the exposure of more active sites stemming from the small particle size. Apart from developing various novel nanostructured molybdenum carbide, heteroatom doping, for instance phosphorus 1, cobalt 51 and nickel 18, 20, 52, is another effective strategy for promoting HER kinetics, which could modify the d-orbitals features as well as adjust the Fermi level, and in turn favors a more thermo-neutral hydrogen-binding energy (△GH*) to achieve a desirable electrochemical activity, as the binding strength between MoxC and adsorbed hydrogen atoms (Hads) are usually too strong that makes the desorption of Hads difficult 10-11, 51-54. Moreover, heteroatom dopants themselves and their neighbor atoms can also provide extra active sites. 16, 18, 53, 55-57 Here in this letter, large-size atomic thin heteronanosheets (HNS) of Mo/Mo2C with abundant heterointerfaces in nanoscale were synthesized via a salt-templated process for high efficient HER catalysis (Figure 1a). An ultralow η10 value (defined as the HER overpotential at a cathodic current density of 10 mA/cm2) of 89 mV was obtained, which is among the lowest reported ones of MoxC based catalysts. 6, 11, 17, 51-52, 54, 58-61 As illustrated in Figure 1a, the MoO3 nanoflakes grown on the NaCl crystals were synchronously reduced to Mo and carbonized to Mo2C to get Mo/Mo2C-HNS on a large scale. The metal Mo and Mo2C was therefore interconnected in nanoscale to generate abundant Mo/Mo2C heterointerfaces. The Mo/Mo2C heterointerface provided an opportunity to assemble the double-phase interface (DPI) comprised of catalytic sites (exposed β-Mo2C defects) and electron transfer channel (metallic Mo). Electrochemical reaction kinetics can be therefore boosted due to the significantly increased effective active sites compared with the β-Mo2C nanosheets. The subsequent theoretical simulation revealed that


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Mo/β-Mo2C heterointerfaces can not only increase the effective active sites but also alter the intrinsic binding energy between H* (* denoted as adsorbate) and catalysts, and the △GH* for Mo/β-Mo2C heterointerfaces is much suitable for HER comparing to those of sole Mo and βMo2C.The specially constructed DPI can therefore lead to a remarkable HER performance under wide pH range. The alkaline HER activity on the Mo/Mo2C-HNS-750 catalyst can rival to the commercial 20% Pt/C catalyst at high overpotential. Great HER durability resulted from the large size of the HNS was also confirmed by the 20 h long-term operation.

Figure 1. (a) Schematic illustration of the synthesis progress of Mo/Mo2C-HNS catalysts; (b) The XRD patterns of the Mo/Mo2C-HNS catalysts; (c) TEM image and (d) HR-TEM image of molybdenum and carbon of a single sheet Mo/Mo2C-HNS-750 catalyst. Mo/Mo2C-HNS catalysts with different carbonization degrees were prepared by tuning the annealing temperature (650-800 oC) of MoO3 nanoflakes precursor in CH4/H2 atmosphere. According to the X-ray diffraction (XRD) patterns of the as-obtained Mo/Mo2C-HNS catalysts


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(Figure 1b), the MoO3 nanosheets coated on NaCl surface were successfully converted to Mo/Mo2C-HNS at the annealing temperature range of 650-750 oC with the detected main character peaks belonging to not only β-Mo2C but also metallic Mo. The incomplete carbonization of precursors below 750 oC possibly resulted from the sluggish carburetion kinetics induced by the slow pyrolysis process of methane, leading to a satisfactorily partial carbonization accompanied by a synchronous reduction of MoO3 precursor to form Mo/Mo2CHNS in the presence of CH4/H2 mixture. In addition, strength of XRD peaks assigned to Mo2C was enhanced with the increased calcination temperature, suggesting the improved carbonization degree and resultantly promoted contents of β-Mo2C ingredient in the hybrid catalysts. While for Mo/Mo2C-HNS-800, the distinct peaks indexed as metallic Mo at 40.5°, 58.6° and 73.7° were no longer observed, whose well-crystallized diffraction peaks could be only ascribed to the hexagonal β-Mo2C phase (JCPDS No. 35-0787), indicating the accomplishment of full carbonization of MoO3 at 800 oC. A Rietveld refinement 11, 55 was further carried out to estimate the proportion of metallic Mo and β-Mo2C in the Mo/Mo2C-HNS catalysts. The atomic ratio of Mo to β-Mo2C was confirmed to be 89:11, 72:28 and 61:39, respectively, for Mo/Mo2C-HNS650, Mo/Mo2C-HNS-700 and Mo/Mo2C-HNS-750 as expected. Scanning electron microscopy (SEM) images on the Mo/Mo2C-HNS, represented by Mo/Mo2C-HNS-750 and Mo/Mo2C-HNS-800 (Figure S1a,b) reveal the successful retention of the 2D nanosheet morphology of the MoO3 in the Mo/Mo2C-HNS catalysts. Size of the Mo/Mo2C-HNS is large to 1-2 µm, which is similar to that of the MoO3 templates.62 The wrinkled structure with folded edge and a high transparency of the transmission electron microscopy (TEM) images observed individual heteronanosheet of the Mo/Mo2C-HNS-750 and Mo/Mo2C-HNS-800 as depicted in Figure 1c and Figure S1c, respectively, illustrate the ultrathin


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feature of the Mo/Mo2C-HNS catalysts. Atomically thin nanoflake was further corroborated by the atomic force microscopy (AFM, Figure S2) analysis with a thickness of 1.229 nm. For Mo/Mo2C-HNS-750, two sets of distinct lattice spacing d=1.57 Å and d=2.37 Å which correspond to the (200) of Mo and (002) of β-Mo2C grains, respectively, could be verified by high-resolution TEM imaging presented in Figure 1d, agreeing well with the results detected from XRD patterns. Most importantly, a clear dividing boundary, i.e. the Mo/Mo2C heterointerface was found, providing convincing tangible evidence for the symbiotic relationship of the two different phases. The fast Fourier transfer (FFT) generated diffraction patterns (Figure S3a-c) with the square-shaped and six-fold symmetric profiles further confirmed the existence of cubic metallic Mo and hexagonal β-Mo2C in the Mo/Mo2C-HNS-750, whereas the Mo/Mo2CHNS-800 (Figure S3d,e) exhibits sole hexagonal β-Mo2C phase. The corresponding energy dispersive spectrum (EDS) analysis (Figure 2a) on the selected region of the TEM image reveal the even distribution of the elements of both Mo and C in the composite, and the homogeneous absence of C in the mapping compared to that of Mo element, where the blanks are filled with metallic Mo ingredient correspondingly, suggesting Mo and βMo2C components are uniformly and inextricably interwoven with each other on nanoscale in the heteronanosheet. Such a unique 2D planar configuration with equably inlaid constituents of Mo and β-Mo2C throughout the nanoflake would create plenty of double-phase interfaces (DPI) composed of reactive catalytic sites (exposed β-Mo2C edges) and charge transfer channel (metallic Mo), which is believed to be greatly advantageous for the electrochemical hydrogen evolution. The valence state and chemical composition information could be further provided by XPS spectra in Figure 2b. The deconvolution of Mo 3d core level signals for Mo/Mo2C-HNS reveals


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that there are four states (0, +2, +4 and +6) for exterior molybdenum element, among which the doublets of Mo0 originated from metallic Mo, Mo2+ derived from β-Mo2C, whereas Mo4+ and Mo6+ arose from the inactive MoO2 and MoO3 phases, since nanostructured molybdenum carbides are inevitably contaminated with molybdenum oxides upon exposure to air. 15-16, 63 Notably, all peaks for oxidation state molybdenum are visibly negative-shifted in Mo/Mo2CHNS catalysts acquired below 750 oC as compared with that of Mo/Mo2C-HNS-800, i.e. pure βMo2C, in which the more Mo amounts derived from the lower carbonization temperatures result in the lower binding energy, implying the enriched electrons around β-Mo2C. According to the previous study and calculated free-energy diagram of the HER at the equilibrium potential 1, 11, 51, the metallic Mo with enriched electrons could function as an electron donor, whereas the βMo2C serves as an electron acceptor, which would remarkably increase the electronic density around Fermi level in β-Mo2C, resulting in the weakened Mo-H bond strength and shifts to lower binding energy for oxidation state molybdenum, and thus brings a more thermo-neutral hydrogen adsorption free energy (△GH*) and enhances adsorbed hydrogen atom (Hads) desorption. Such a favorable modulation on the intrinsic binding energy between H* (* denoted as adsorbate) and catalysts is further evidenced by a theoretical simulation based on density functional theory (Figure S4). The calculation indicates that the free energy of H* for Mo/β-Mo2C heterointerfaces is nearly zero, up to -0.04 eV, which is much suitable for HER as compared to those of individual Mo and β-Mo2C with more zero-alienated △GH* of -0.39 and -0.19 eV, respectively. As a consequence, the more positive-shifted binding energy of Mo0 against increased carbonization degree are caused by the more electron migration from metallic Mo to adjacent molybdenum ions owing to the increased β-Mo2C contents, which leads to its own partial oxidation of metallic Mo species. The ligand effect induced by the presence of metallic Mo in


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Mo/Mo2C-HNS hybrids is expected to be significantly beneficial to the HER kinetics. In detail, the specific binding energies of various molybdenum species in the four samples are compared in Table S1.

Figure 2. (a) EDS elemental mappings of molybdenum and carbon of a single sheet Mo/Mo2CHNS-750 catalyst; (b) High-resolution XPS spectra of Mo 3d of the Mo/Mo2C-HNS catalysts; (c) LSV curves and (d) corresponding Tafel curves of HER on the Mo/Mo2C-HNS catalysts and on the commercial Pt/C catalyst in 0.5 M H2SO4;(e) Electrochemical double-layer capacitances of HER on the Mo/Mo2C-HNS catalysts in 0.5 M H2SO4.


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To probe the electrocatalytic activities of as-synthesized materials, the HER performances on Mo/Mo2C-HNS were subsequently investigated. As illustrated in Figure S5, the N-doped C (NC) and bare Mo/Mo2C-HNS-750 yielded a current density of 10 mA/cm2 at an overpotential (η10) of 305 and 127 mV, respectively, while their composite gave the same current density at a much smaller η10, reflecting the desirable synergy between the molybdenum-based catalysts and NC as elucidated in our previous study 29 as well as similar reports from elsewhere 3, 9, 17, 20. For this reason, all through this text Mo/Mo2C-HNS catalysts are incorporated with NC unless otherwise mentioned. The polarization curves and Tafel plots of the Mo/Mo2C-HNS electrocatalysts in 0.5 M H2SO4 are shown in Figure 2c and d, respectively, along with the performance of the commercially available benchmark Pt/C catalyst with the same mass loading of 0.285 mg/cm2 as reference. As expected, the Pt/C catalyst displays brilliant catalytic activity towards HER in acidic solution with an overpotential at 1 mA/cm2 (η1) of ca. 1 mV. The electrocatalytic performance of Mo/Mo2C-HNS enhances with increased carburization temperature initially resulting from the increasing percentage of catalytic active β-Mo2C phase and the accordingly extra exposed DPI, which peaks at Mo/Mo2C-HNS-750 and then descends owing to the absence of metallic Mo and corresponding heterointerfaces. The Mo/Mo2C-HNS-750 electrocatalyst exhibits an ultra-low η1 of ca. 16 mV, beyond which the cathodic current density rises sharply. To drive a current density of 10 mA/cm2, the Mo/Mo2C-HNS-750 just requires an overpotential of 89 mV. Furthermore, the smallest Tafel slope of 70.72 mV/dec was also acquired for Mo/Mo2C-HNS-750, indicating that the hydrogen evolution reaction on it proceeds through the Volmer-Heyrovsky mechanism in which the electrochemical desorption is the rate-determining step. Encouragingly, these performances are among the best ever reported ones for molybdenum carbide based acidic HER electrocatalysts when graphite rods were used as the counter


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electrodes instead of Pt wire or foil during electrochemical tests (Table S2). To better understand the origin of such unprecedented catalytic activities, the electrochemical double-layer capacitances (Cdl, Figure 2e) are determined by the CV method (Figure S6), which are widely applied to estimate the electrochemical active surface area (ECSA). Generally, the ECSA has strong correlationship with the HER activity. The calculated Cdl values are ranked as follows, that is, Mo/Mo2C-HNS-750 (21.66 mF/cm2) > Mo/Mo2C-HNS-700 (19.58 mF/cm2) > Mo/Mo2C-HNS-800 (17.33 mF/cm2) > Mo/Mo2C-HNS-650 (9.86 mF/cm2), in excellent agreement with their LSV results. The highest Cdl value for Mo/Mo2C-HNS-750, reflects the most enriched active sites on carbide surface for hydrogen evolution. Meanwhile, their electrochemical impedance spectroscopy (EIS) measurements are performed at given potentials of η10 in 0.5 M H2SO4, and Nyquist plots (Figure S7) show the charge transfer resistance (Rct) follows a reverse pattern as compared to that of Cdl, in which the smallest Rct as low as 38.3 Ω delivered by Mo/Mo2C-HNS-750 implies the rapid electron diffusion for HER. Moreover, the Faradaic efficiency is of crucial importance to assess the performance of the electrocatalysts. Figure S8 compares the amount of experimentally yielded H2 from the HER over Mo/Mo2CHNS-750 versus that of the theoretically evolved H2 from electrochemical HER, from which it can be seen that they are well matched, and the Faradaic efficiency of the Mo/Mo2C-HNS-750 sample is close to 100%. Since alkaline media is also widely used for industrial application to electrolysis of water, detailed analysis was therefore further conducted to assess the HER performance of Mo/Mo2CHNS catalysts in 1 M KOH. As demonstrated in Figure 3, alkaline HER activity ranking of the Mo/Mo2C-HNS catalysts is identical to their order in acid solution. The best performed Mo/Mo2C-HNS-750 exhibits an ultra-low η1of ca. 10 mV, a low η10 of 79 mV and a Tafel slope


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of 62.86 mV/dec, even better than those in acid electrolyte, validating that the synthesized Mo/Mo2C-HNS-750 material is also by far one of the most efficient nonprecious metal catalysts for HER in basic media. Noticeably, the commercial Pt/C catalyst possesses overwhelming advantage under acidic condition, while the activity of Mo/Mo2C-HNS is prone to approach that of Pt/C catalyst at high overpotential in alkaline solution. Together with the forementioned characterizations, the excellent HER electrocatalytic activity of Mo/Mo2C-HNS-750 hybrid could be attributed to the following aspects: i) the two dimensional plane structure and presence of metallic Mo render the facilitated charge transfer and mass transport; ii) the excessive electrons in metal Mo would transfer to the orbitals of molybdenum ions in β-Mo2C, leading to the relatively moderate Mo-H for the easier Hads desorption and promoted HER activity; iii) the atomic thin feature of the nanoflakes and nanosized intersections between Mo and β-Mo2C, which allows the full access of the electrolyte and imparts catalyst a great many extra exposed heterogeneous catalytic sites on the surface.

Figure 3. (a) LSV curves and (b) corresponding Tafel curves of HER on the Mo/Mo2C-HNS catalysts in 1 M KOH. Durability is another crucial descriptor for evaluating a HER catalyst. The measurement for long-term cycling stability of Mo/Mo2C-HNS-750 (Figure 4a) in 0.5 M H2SO4 showed slight deterioration of cathodic currents after 4000 cycles and the post-test polarization curve remained


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almost the same to the initial one with an ignorable increment of 3.1 mV and 2.1 mV, respectively on η10 and η50 of HER. Meanwhile, the result of I-t chronoamperometric (CA) response in acid electrolyte for Mo/Mo2C-HNS-750 as displayed in Figure 4b revealed that the catalytic current density maintained at ca. 12.0 mA/cm2 at the overpotential of 100 mV over 20 h, indicating the excellent stability and durability under experimental conditions employed for this HER process. The large lateral dimension of heteronanosheet should account for such splendid stability, which functioned as a mechanically robust scaffold and effectively inhibited the abundant and isolated heterogeneous catalytic sites, i.e. the double-phase heterointerfaces consisting of reactive sites (exposed Mo2C defects) and electronic transport channel (metallic Mo) from aggregation or interface collapse. The prominent stability is also manifested by the indistinguishable phase change (Figure S9a) as well as the remained large size nanosheet (Figure S9b) and well-retained originally micro-continuous morphology (Figure S9c) after 20 h potentiostatic electrolysis. Although the somewhat inferior stability and durability for Mo/Mo2CHNS-750 in 1 M KOH was monitored (Figure 4a), which is attributed to the imperfect corrosion resistivity of molybdenum carbide under alkaline condition as elucidated in previous studies 6, 15, great operation durability was evidenced by the stable cathodic current density around 13.9 mA/cm2 during 20 h on-going electrolysis.


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Figure 4. (a) Long-term cycling stability and (b) Time dependence of HER current density on operation time at the overpotential of 100 mV under both acid and alkaline conditions. In summary, we designed and developed a novel approach to synthesis Mo/Mo2C-HNS catalysts on the basis of a salt-template directed method combined with an in situ synchronously reduction and carburization process. Thanks to the ultrathin character of as-prepared heteronanosheets with maximumly exposed electrochemical accessible surface areas, the sufficient electrical conductivity needed for electrocatalysis benefited from the large size 2D architecture and metallic Mo, the ligand effect of Mo on adjusting the electronic structure and the Fermi level of Mo2C and the additionally exposed active sites originated from the abundant


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Mo/Mo2C heterointerfaces at nanoscale, the Mo/Mo2C-HNS-750 behaved outstanding electrochemical HER performance requiring an overpotential of only 89 mV and 79 mV to produce a current density of 10 mA/cm2 in 0.5 M H2SO4 and 1 M KOH, respectively. Moreover, the Mo/Mo2C-HNS-750 exhibited a superior chronoamperometric (I-t) durability during a 20 h continuous HER operation and also a prominent cycling stability with indistinguishable overpotential loss after 4000-cycle CV examination. These performances are all among the best reported ones, demonstrating prospective application potential for Mo/Mo2C-HNS-750 in wide pH range of water splitting devices operated in both acid and alkaline media. METHODS Materials. All chemical agents were of reagent grade and used as obtained without further purification. Molybdenum metal powder, ethanol, hydrogen peroxide (H2O2, 30 wt%), concentrated nitric acid (HNO3, 68 wt%), concentrated sulfuric acid (H2SO4, 98 wt%), sodium chloride (NaCl) crystal and potassium hydroxide (KOH) powder were offered by Sinopharm Chemical Reagent Co., Ltd., China. The filter membrane (Celgard, USA), Vulcan XC-72R carbon black, commercial 20 wt% Pt/C catalyst, glassy carbon (GC) electrode, graphite rod (99.9995%), Hg/HgO reference electrode and saturated calomel electrode (SCE) were purchased from Alfa Aesar. The 5 wt% Nafion solution was provided by Sigma-Aldrich. All solutions were prepared using deionized water generated from a water purifier (Milli-Q, USA). Synthesis of Mo/Mo2C-HNS catalysts. The Mo/Mo2C heteronanosheets (Mo/Mo2C-HNS) were prepared following a modified salt-templated method. Briefly, 0.40 g of Mo powder was dispersed in 30 mL of anhydrous ethanol under vigorously magnetic stirring for 60 min to form uniform dispersion, followed by a dropwise addition of 1.4 mL of H2O2 into the above


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suspension. After on-going stirring for 48 h, a dark blue peroxomolybdate precursor solution was formed. Subsequently, the obtained Mo precursor dispersion was mixed with 640 g of NaCl crystal powder thoroughly and then dried at 70 oC with continuous stirring. Afterwards, the assynthesized mixture was further annealed at 280 oC for 120 min with a calcinating ramp rate of 2 o

C/min under argon atmosphere to achieve the 2D h-MoO3@NaCl, which was thereafter subject

to a carbonization procedure under CH4/H2 gas mixture at feed gas flow rates of 20 and 80 mL/min, respectively, maintained at 650-800 oC for 2 h at a slow heating ramp of 1 oC/min to gain various degrees of carbonated Mo/Mo2C-HNS@NaCl. Finally, the harvested products were rinsed and filtered through a Celgard membrane by repetitive wash with Milli-Q water to remove NaCl completely and dried at 60 oC for 2 h to obtain 2D Mo/Mo2C-HNS-t (t denotes the calcination temperature of samples) powders. Details on material characterizations, theoretical simulation and electrochemical measurements are provided in Supporting Information. ASSOCIATED CONTENT Supporting Information. Catalyst characterization, theoretical simulation and electrochemical measurement; SEM micrograph, AFM topography and FFT-generated SAED patterns on the catalysts; Calculated free energy diagram for HER on the catalysts; LSV curves of the N-doped C (NC), bare Mo/Mo2C-HNS-750 and their composite; Cyclic voltammograms with different sweeping rates and EIS spectra of the catalysts; Faraday efficient of HER on Mo/Mo2C-HNS-750; XRD spectra, SEM image and TEM image of Mo/Mo2C-HNS-750 after 20 h potentiostatic electrolysis;


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Comparison of XPS peak position of the catalysts; Comparison of HER performance of state-ofthe-art MoxC-based electrocatalysts. AUTHOR INFORMATION Corresponding Author * [email protected] (Weiwei Cai). * [email protected] (Ligang Feng). * [email protected] (Zehui Yang). * [email protected] (Liang Huang). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for financial support from the National Natural Science Foundation of China (Nos. 21703211, 21503197, 21603041 and 21703212) and Fundamental Research Funds for the Central University, China University of Geosciences (Wuhan) (Nos. CUG150615 and CUG150627).


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