Controllable Edge Exposure of MoS2 for Efficient Hydrogen Evolution

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Controllable edge exposure of MoS2 for efficient hydrogen evolution with high current density Zexia Zhang, Yuanxi Wang, Xiangxing Leng, Vincent H. Crespi, Feiyu Kang, and Ruitao Lv ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00010 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Energy Materials

Controllable edge exposure of MoS2 for efficient hydrogen evolution with high current density Zexia Zhang,1,3,5# Yuanxi Wang,2# Xiangxing Leng,4 Vincent H. Crespi,2,6* Feiyu Kang1,3,4∗ and Ruitao Lv1,3* 1

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 2 2-Dimensional Crystal Consortium, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA 3 Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 4 Graduate School at Shenzhen, Tsinghua University, Shenzhen, Guangdong Province, 518055, China 5 School of Physics and Electronic Engineering, Xinjiang Normal University, Urumqi, Xinjiang Province, 830046, China 6 Department of Physics, Department of Materials Science and Engineering, Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA ABSTRACT: MoS2-based electrocatalysts are promising cost-effective replacements for Pt-based catalysts for hydrogen evolution by water splitting, yet achieving high current density at low over-potential remains a challenge. Herein, a binder-free electrode of MoS2/CNF (carbon nanofiber) is prepared by electrospinning and subsequent thermal treatment. The growth of MoS2 nanoplates contained within or protruding out from the CNF can be controlled by adding urea or ammonium bicarbonate to the electrospinning precursors, due to the crosslinking effects of urea and the increased porosity caused by pyrolysis of ammonium bicarbonate allowing growth through pores in the CNF. By virtue of the abundant exposed edges in this microstructure and strong bonding between the catalyst and the conductive carbon network, the composite material exhibits ultra-high electrocatalytic hydrogen evolution activity in acidic solutions, with current densities of 500 and 1000 mA/cm2 at overpotentials of 380 and 450 mV, respectively, exceeding the performance of many reported MoS2-based catalysts and even commercial Pt/C catalysts. Thus MoS2/CNF membranes show potential as efficient and flexible binder-free electrodes for electrocatalytic hydrogen production.

KEYWORDS: MoS2, hydrogen evolution reaction, electrocatalyst, carbon nanofiber, controllable synthesis.

INTRODUCTION

structures include three-dimensional mesoporous double-gyroid MoS 2 thin films15 and MoS2 nanosheets made defective by various methods including oxygen doping,16 excess thiourea,17 ball milling18 or solvothermal treatment.19 However, the semiconducting nature of MoS 2 impairs charge transport, especially between basal planes, and thereby limits its performance as a HER catalysis. Thus improvements in electrical conductivity are important to the development of MoS2-based catalysts. As a conducting framework that can support catalytic MoS2 nanoparticles, carbon materials are ideal choices due to their excellent conductivity, structural diversity, environmental stability, and earth abundance. To date, HER catalysts that combine MoS 2 with different carbon materials include: a composite catalyst of MoS2 nanoparticles with reduced graphene oxide that shows excellent electrical conductivity and improved catalytic activity,20 and also MoS2 nanoparticles on mesoporous graphene foams,21 carbon nanofibers,22,23 carbon cloth,24 graphene film,25 carbon fiber paper,26 single-walled carbon nanotube

Motivated by the high cost of Pt-group catalysts, transitionmetal dichalcogenides (TMDs) based earth-abundant hydrogen evolution reaction (HER) electrocatalysts have been attracting great attention.1-3 TMD-based electrocatalytic materials were designed into hierarchical or polycomponent nanostructures in order to expose higher curvature surface and more active sites for enhanced electrocatalytic performance.4-10 Among them, molybdenum sulfides (MoS2) are proposed to be the most promising HER catalyst following the theoretical prediction of high efficiency as a hydrogen evolution reaction (HER) catalyst.11 Diverse MoS2-based nanostructures have been synthesized and investigated for their HER catalytic activity.1, 12 Engineering the microstructure to contain abundant MoS2 edges and defective regions improves catalytic efficiency,13 likely because edges and defect sites are more active than the basal planes.14 Such

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film,27 and graphene-carbon nanotube composites.28 Synergistic effects between the carbon-based supports and co-catalysts remarkably boost the performance of semiconducting MoS2. In particular, CNF have some unique advantages, including percolating networks, free-standing structures, and easy production scale-up. However, many reported MoS 2-CNF composites can not be used as free-standing electrodes for HER catalysis, since they need to be loaded onto glassy carbon electrodes with binders such as Nafion solution.20, 22, 23, 27, 28 Although small Tafel slopes has been achieved for MoS 2 nanoplates with lateral sizes smaller than 10 nm embedded in CNF, the current density and thus overall HER activity is still limited by an inadequate number of active sites. 22, 23 For the quantitative comparison of electrocatalytic activity, the overpotential at a current density of 10 mA/cm2 (expected for a solar water-splitting efficiency of 12.3%)29 is usually used to evaluate performance. Commercial hydrogen production expects high current densities across a range of overpotentials30 (like oxygen evolution reaction catalysts31, 32) to rapidly generate large amounts of gas with limited energy consumption. However, even for 20 wt% Pt/C, it is difficult to obtain such high current densities.33, 34 At overpotentials higher than 0.1 V, the current density of a Pt/C electrode fluctuated and its increase became slowing down.33 In addition, it is vital for electrodes to reach high current density while withstanding intensive gas evolution reaction for high-performance HER catalysts (e.g., the commercial requirement for oxygen evolution reaction catalysts is to exhibit current density of more than 500 mA/cm2 at overpotentials lower than 0.3V).35 Metallic 1T phase of MoS2 could reach 200 mA/cm2 at −0.4 V, with a current density increase rate faster than that of Pt/C.34 Nanostructured MoS2 thin films on Ti foil also showed a superior current increase rate of 56 mA/cm2 per 100 mV from −0.2 V to −0.5 V, which yields a higher efficiency than Pt/C near −0.5 V.33 However, semiconducting MoS2-based binder-free HER catalysts that could stably bear high current densities remain unknown. Here we report the controlled growth of free-standing MoS 2/CNF membranes for electrocatalytic hydrogen production. The edges of MoS2 nanoplates are embedded in or exposed outside of the CNFs depending on the choice of precursors; the MoS 2 growth mechanism of the latter is modeled using Kinetic Monte Carlo. Both types of composites demonstrate excellent catalytic efficiencies at high current density, especially the edge-embedded ones, which yield remarkably high current densities of 155, 313 and 675 mA/cm2 at overpotentials of 300, 350 and 400 mV. More importantly, ultra-high current densities of 500 and 1000 mA/cm2 could be achieved at overpotentials of 380 and 450 mV, respectively. Kinetics analysis indicates that the reaction rate should be determined by a Volmer–Heyrovsky mechanism and mass diffusion.

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“HH” stand for “Polyvinyl alcohol” and “Hexaammonium Heptamolybdate tetrahydrate”, respectively. Urea or ammonium bicarbonate (NH4HCO3) were added to the electro-spinning precursors (Figure 1b), and the corresponding samples are called PHH-U and PHH-AB, respectively. Electrochemical measurements were conducted in 0.5 M H2SO4 using the samples as binder-free working electrodes, a graphite rod as counter electrode, and a saturated calomel reference electrode (see experiment details in Supplementary Information). The scanning electron microscopy (SEM) images in Figure S3 show that all three types of samples can form interconnected fiber networks. In the case of PHH, sparse nanoplates grow externally on fibers with nonuniform spatial distribution and sizes, including large MoS2 sheets (＞1 µm) with curled edges. Figure 1b and Figure S3c-d show that the CNFs of PHH-U are smooth, with MoS2 nanoplates encapsulated inside. In contrast, the CNFs of PHH-AB are decorated with smaller and more uniformly dispersed MoS2 nanoplates that stand vertically on the fiber surface. The edges of these MoS 2 nanoplates are sharper than those of PHH, suggesting higher crystallinity of MoS2. We propose that the gas released during the pyrolysis of NH4HCO3 (NH4HCO3 → NH3 + CO2 + H2O) gives rise to a large number of pores on the fibers, out of which the nanoplates could grow, thus yielding the uniform dispersion of nanoplates seen for PHH-AB. The detailed growth mechanism will be discussed in the following sections.

Figure 1. (a) Schematic illustration of MoS2/CNF synthesis and its hydrogen evolution reaction (HER) catalysis. (b) Controlled growth of MoS2 nanoplates in the CNFs by adding urea or NH4HCO 3 in the precursors. PHH, PHH-U and PHH-AB denote MoS2/CNFs samples synthesized with different precursors, with P=Polyvinyl alcohol and HH=Hexaammonium Heptamolybdate tetrahydrate. By adding urea (PHH-U) or ammonium bicarbonate (PHH-AB), structures with MoS2 nanoplates embedded inside (the top right SEM image) or exposed outside (the bottom right SEM image) of CNFs are obtained.

RESULTS AND DISCUSSION The two-step process of Figure 1a was used to synthesize freestanding MoS2/CNF membranes. Hexaammonium heptamolybdate tetrahydrate (HAHM) was used as the MoS 2 precursor and was mixed in polyvinyl alcohol (PVA) aqueous solution to prepare nanofibers by electrospinning. Subsequent sulfurization and carbonization were performed in Ar atmosphere (Figure S1a and b) and free-standing membranes were collected (Figure S1c). As-prepared samples are denoted as PHH where “P” and

Transmission electron microscopy (TEM) of PHH-U and PHHAB in Figure 2 show morphological details of MoS 2 embedded in or exposed outside of CNFs. For PHH-U, the fiber includes interleaving oriented layered stripes (Figure 2a), and HRTEM imaging indicates the layered stripes are lateral edges embedded in the fibers (Figure 2b). The lattice fringe interplanar distance (0.610 nm) is consistent with the interlayer spacing of hexago-

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nal MoS2 (002) planes (0.615 nm).36 The surface of PHH-U fibers also contains polycrystalline MoS2 layers (Figure S4). For PHH-AB, edge-exposed hexagonal nanoplates are clearly seen in Figure 2c; these consist of approximately ten S-Mo-S layers. The interplanar distance of 0.620 nm is slightly larger than that of PHH-U, which presumably results from the different degrees of constraint imposed by the carbon matrix to the embedded or protruding plates. The spots in selected area electron diffraction (SAED) patterns (inset in the Figure 2d) indicate the single crystalline nature of the exposed nanoplates.37-39 For PHH, irregular MoS2 sheets that consist of overlapping layers are hosted in or exposed outside of the fibers (Figure S5). SAED patterns verified the polycrystallinity of these sheets. Elemental mapping by energy-dispersive X-ray spectroscopy (EDS) demonstrates the homogeneity of Mo and S across entire nanoplates (Figure 2e-g), with a S/Mo atomic ratio of nearly two: slightly larger than two for PHH-U and slightly smaller than two for PHH and PHH-AB (Figure S6). The excess may be attributed to more abundant S-rich edges of the MoS 2 flakes in PHH-U. Different from agglomerated structures of bulk MoS 2 (Figure S7a and c), highly dispersed MoS2 nanoplates were obtained by compositing with CNFs.

MoS 2/CNF composites were further characterized by Raman spectra (Figure S8b). Peaks at 380 cm–1 and 405 cm–1 can be assigned in-plane E12g and out-of-plane A1g phonons,40, 41 where the higher intensity of the A 1g mode than E12g mode suggests a diversity of basal plane orientations.42 The 25 cm–1 difference in peak position reflects multilayer MoS2.40 The peak broadening in PHH-AB may be due to impurities or defects produced during the growth of these larger single-crystal plates. X-ray photoelectron spectroscopy (Figure S8c-e) confirmed the presence of Mo4+ and S2–.43 The slight offset towards lower binding energy for Mo 3d and S 2P in PHH-AB may be due to the dislocation or defect derived from the unordered crystal surfaces.44 The most interesting results derive from the ability to control the composite structure of MoS 2/CNF, i.e. the locations of MoS2 nanoplates inside or outside the fibers. In sol-gel chemical processes, PVA is used as fuels for pyrolysis. 45 Cross-linking chains form between PVA and urea because the hydroxyl (-OH) of PVA and the amide (-NH2) of urea can form hydrogen bonds, as shown in Figure S9. The cross-linking chains may divide the gel into dense micro-regions46 where MoS2 precursors can be trapped and well dispersed, forming ultrafine and highly dispersed particles; the enclosed borders of the fibers prevent the formation of pores. Thus the growth of MoS 2 plates in PHH-U was restricted inside of the fibers and can not extend across fiber walls as in the case of PHH-AB. This is further supported by a Brunauer-Emmett-Teller specific surface area (SSA) analysis (Figure S10), showing that the SSA of PHH-U (161 m2/g) is smaller than that of PHH-AB (225 m2/g). Although vertically standing MoS 2 flakes have been reported previously, they were either the result of neighboring islands colliding (thereby deflecting the growth fronts of each out-ofplane),36 the result of an epitaxy between the substrate and the edge plane (in contrast to the basal plane),47 or unspecified.49, 50 It appears unlikely that either mechanism applies for growth of MoS 2 fins on CNFs (in the case of PHH-AB) due to the disorder of the CNF surface and the lack of epitaxy between CNF and MoS 2. We therefore propose an alternative mechanism of vertical growth that exploits the porosity of the growth substrate: 2D growth of MoS2 can take place not only on the exterior surface, but may also nucleate in the interior and thereafter project outward through the pores of the CNF host, forming vertical finlike structures (Figure 3a). We simulate such a growth scenario using a 2D kinetic Monte Carlo growth model. Due to the complexity of synthesis conditions, the growth model does not aim to be material-specific (for example, the final fin structures could be due to growth of a MoS2 nanoplate or the growth of MoO3-x nanoplates which are subsequently sulfurized); instead this minimal model is defined by coordination-dependent energies and a deposition rate (see SI for details). The simulation space consisting of a 2D triangular lattice that is partitioned into two regions representing the interior (lower) and the exterior (upper) of the CNF, as shown in Figure 3d. Since each lattice point corresponds to an MoS 2 unit cell, the sublattice asymmetry of MoS 2 does not appear in the model, leaving only one type of energetically favorable edge termination (i.e. final morphology according to Wulff construction will be hexagons instead of sharp or truncated triangles). Growth is forbidden at the white rectangular regions and only allowed through a hole at the center connecting the two partitions, representing a pore in a CNF shell. Growth is initiated with a nucleation seed in the

Figure 2. Transmission electron microscopy (TEM) characterization of the edge-embedded and edge-exposed MoS 2/CNF samples. (a) An individual MoS2/CNF fiber with embedded MoS 2 edges. (b) Enlarged TEM image of MoS2 nanoplates embedded in the fiber. (c) MoS2 nanoplates incorporated vertically in the CNF with exposed edges. (d) Enlarged TEM image of MoS2 nanoplate edges in (c). Inset showing the corresponding selected area electron diffraction pattern. (e-g) Elemental mappings of the MoS2 nanoplates shown in (c). Oxygen is almost invisible in the scanned area.

X-ray diffraction (Figure S8a) of PHH, PHH-U and PHH-AB samples reveal the hexagonal 2H-MoS2 phase (JCPDS No. 371492), consistent with the pattern of bulk MoS 2 (Figure S7b). The sharp (002) peaks imply well-stacked layers of MoS 2. The

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Figure 3. Kinetic Monte Carlo and density functional theory (DFT) simulations of MoS2 nanoplate growth. (a) MoS2 nanoplates embedded partly in CNF with exposed edges. Schematic of a hexagonal MoS 2 nanoplate (b) in a constrained growth scenario, crossing a pore on the fiber wall and (c) in an unconstrained growth scenario, growing in the interior of a fiber. (d) Kinetic Monte Carlo growth model of MoS2 nanoplate growing past a CNF wall (white rectangular regions) through a pore. (e-f) Charge transfer from a graphene sheet to the edges of a MoS2 ribbon.

interior region and proceeds under equilibrium at a constant influx of depositing particles descending from the upper edge of the box into the simulation cell (see SI for details). The growth front of the interior nanoplate that reaches the CNF shell stops (Figure 3b), but is able to extend through the pore and serve as a new nucleation site for an exterior nanoplate (Figure 3c), which subsequently develops into a shape with sharp edges consistent with the Wulff construction. For the same total mass of material, the resulting structure features more exposed MoS 2 edges compared with the growth result without the CNF constraint, which is beneficial for HER. In addition to more exposed edges, charge transfer between MoS 2 flakes and the CNF host may also increase HER efficiency. Although there is no charge transfer between pristine graphene (representing the CNF here) and MoS2 (since the charge neutrality point for graphene resides in the bandgap of MoS 2), the metallic exposed edges in finite-sized MoS2 nanoplates can transfer charge to or from a graphitic environment. Density functional theory calculations of a MoS2 ribbon with two exposed edges (see side view in Figure 3f) located next to a periodic graphene sheet reveals a charge transfer from the graphene sheet to the MoS2 edges of roughly 0.2 electrons/Å along the ribbon direction (x, into the screen) (Figure 3e). This charge transfer may change the interaction of H atoms or H3O+ ions with MoS 2 edge sites and thus affect the HER rate, especially for the PHH-U case, where all MoS 2 edges are in contact with graphitic surfaces. The electrocatalytic HER performance of these free-standing MoS 2/CNF samples was evaluated using linear sweep voltmmetry (LSV) in a typical three-electrode system. For reference, 10 wt% Pt/C was also measured (Figure S11); it showed a near-zero over-potential and a Tafel slope of 28 mV/decade. Figure 4a shows the polarization curves of PHH, PHH-U and

PHH-AB with corresponding onset potentials of −155, −120 and −150 mV. These values are much smaller than the bulk MoS 2 value of ~−400 mV (Figure S7d) and CNF of ~-500 mV (Figure S12). The Tafel slopes of PHH, PHH-U and PHH-AB are 131, 69 and 86 mV/decade, as shown in Figure 4b. These results indicate a significant improvement in catalytic activity induced by anchoring MoS2 nanoplates to the CNF conductive framework. More importantly, after −200 mV, the activity of PHH-U and PHH-AB delivers a remarkable current density of up to ~1000 mA/cm2 at overpotentials less than 500 mV. Specifically, PHH-U yields current densities of 155, 313 and 675 mA/cm2 at overpotentials of 300, 350 and 400 mV; the corresponding current values for PHH-AB are 56, 142 and 372 mA/cm2. The stability of PHH-U and PHH-AB was assessed by performing over 1000 cycles and then comparing final and initial LSV curves (Figure 4c). Although the current densities beyond −350 mV show ~20% reduction (for example, for PHHU, the initial current density at -0.4 V is 666.5 mA/cm2 and the final data is 545.1 mA/cm2, which induce a decay of 18.2%) because of rapid bubble emission, those lower than −350 mV have negligible loss. Figure S13 demonstrates that the HER performance remained almost unchanged for 10 h. The XRD, SEM and TEM characterizations of these samples were collected after 1000 electrochemical cycles (Figures S14-16). No obvious changes were detected, which further evidenced their good stability. Comparing our materials to various other MoS2-based catalysts (Figure 4d), PHH-U and PHH-AB show prominent advantages at an overpotential of 400 mV. For overpotentials lower than 300 mV, the activity of PHH-U is equal to or greater than that of a similar MoS 2/CNF catalyst,22 whereas that of PHH-AB shows no advantage. As a free-standing electrode, the cathodic current of PHH-U at −300 mV is comparable to those reported

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Figure 4. (a) Linear scan voltammetry (LSV) polarization curves for MoS2/CNFs synthesized by PHH, PHH-U and PHH-AB precursors tested in 0.5 M H2SO4. (b) Tafel plots of the corresponding samples in (a). (c) Polarization curves of the MoS2/CNFs synthesized by PHHU and PHH-AB in the initial scan and after 1000 cycles. (d) Comparison of current density at different voltage with previously reported HER catalytic materials, such as oxygen-incorporated MoS217, 1T-MoS234, MoS2/C 51, MoS2/Ti foil33, MoS2/CNF22, MoS 2/CC 24, MoS2/SWNT27. Here, CNF, CC and SWNT denote carbon nanofibers, carbon cloth and single-walled carbon nanotubes, respectively.

for a Ti foil33 and pyrolytic sponge51 support (MoS2/Ti foil and MoS2/C in Figure 4d). Beyond 350 mV our samples show a great advantage over the MoS 2/Ti foil33 and 1T-MoS 2 (powder).34 These results indicate MoS2/CNF decorated with abundant edge sites is a highly efficient HER electrocatalyst in acidic media. Their high current density at low applied potentials could open applications for these non-noble catalysts in industrial water electrolysis, which requires current densities above 500 mA/cm2 at overpotentials lower than 300 mV.35 Finally, we discuss possible mechanisms for this high activity at moderate Tafel slopes. For HER in acidic electrolyte, the possible steps are:52

discharge and electrochemical desorption. The exchange current density j0, an important kinetic parameter for evaluating electrochemical reaction rate, can be obtained by an extrapolation of Tafel plot (Figure S8f). PHH-U and PHH-AB yield j0 of 0.068 and 0.023 mA/cm2, which are competitive with known catalysts. 16, 53 Electrochemical impedance spectra (EIS) were measured to further investigate reaction kinetics (Figure S11). Equivalent circuit models fit from Nyquist plots consisted of a solution resistance (Rs), charge transfer resistance (Rct), constant phase element (CPE) and Warburg impedance (W). The resulting values of these parameters are listed in Table S1, in which Rp is the sum of R ct and W. Different samples are distinguished mainly on the basis of W, which suggests that these electrochemical reactions are controlled by mass diffusion. According to a semiinfinite diffusion model,54 the slope of the linear part of the EIS spectra is inversely proportional to the diffusion rate, i.e. the steeper the slope, the lower the diffusion rate. The different slopes (Figure S18) are in accord with the diffusion impedance listed in Table S1. The results show that the superior activity of PHH-U and PHH-AB also benefits from a lower diffusion resistance (Table S1). More importantly, the very low Rct of PHHU and PHH-AB (0.11 and 0.04 Ω, respectively) reflects a very fast faradaic reaction process that we attribute to the stacking MoS 2 nanoplates perpendicular to the conductive CNF support, which allows electron transport to occur predominately within a single MoS2 layer (the resistivity through the S-Mo-S basal

–

(1) Volmer reaction: H3O+ (aq) + e + * → H* + H2O (l) –

(2) Heyrovsky reaction: H* + H3O+ (aq) + e → H2 (g) + * (3) Tafel reaction: H* + H* → H2 (g) + 2* Here * labels an empty active site and H* denotes a hydrogen atom occupying an active site. H3O+ denotes an aqueous hydronium ion. The Tafel slope is determined by the rate-limiting step of HER: when the Volmer, Heyrovsky or Tafel reaction is the rate-limiting step, Tafel slopes of ~120, ~40 and ~30 mV/decade respectively should be observed at low overpotential. The Tafel slope of PHH is 131 mV/decade, hence its HER rate is determined by the Volmer reaction. The Tafel slopes of PHHU and PHH-AB are 69 and 86 mV/decade respectively, characteristic of a Volmer–Heyrovsky hybrid reaction that combines

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Z161100004916099) and the Tsinghua University Initiative Scientific Research Program (No. 20151080367). Z.Z. acknowledges the financial support from the National Natural Science Foundation of China (No. 11364043). Y. W. and V. H. C. acknowledge support from the National Science Foundation Materials Innovation Platform under DMR-1539916.

planes has been measured to be 2200 times larger than that parallel to the planes) and conductive edges.12 The lower Tafel slope of PHH-AB (69 mV/decade) compared to PHH-U (86 mV/decade) may stem from the greater number of active MoS 2 edges associated with a larger number of exposed nanoplates, which is evidenced by the results that PHH-AB possesses higher electrochemical surface area (ECSA) than PHH-U (Figure S19). Nevertheless, owing to the higher exchange current density and lower diffusion resistance, a lower onset potential and considerably larger current density are obtained for PHHU, likely due to a shorter charge transfer distance for embedded edges in PHH-U fibers in contrast to longer distance for the MoS2 surfaces of exposed plates on PHH-AB fibers. In short, the abundant dispersed edges (producing a large number of active sites) and excellent electrical coupling with conductive support (supplying rapid charge transfer pathways) synergistically improve the catalytic active of MoS2/CNF for HER applications.

REFERENCES (1) Yan, Y.; Xia, B.; Xu, Z.; Wang, X. Recent Development of Molybdenum Sulfides as Advanced Electrocatalysts for Hydrogen Evolution Reaction. ACS Catal. 2014, 4, 1693-1705. (2) Merki, D.; Hu, X. Recent Developments of Molybdenum and Tungsten Sulfides as Hydrogen Evolution Catalysts. Energy Environ. Sci. 2011, 4, 3878-3888. (3) Wang, F.; Shifa, T. A.; Zhan, X.; Huang, Y.; Liu, K.; Cheng, Z.; Jiang, C.; He, J. Recent Advances in Transition-Metal Dichalcogenide Based Nanomaterials for Water Splitting. Nanoscale 2015, 7, 19764-19788. (4) Yu, L.; Xia, B. Y.; Wang, X.; Lou, X. W. General Formation of M-MoS 3 (M = Co, Ni) Hollow Structures with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28, 92-97. (5) Zhang, L.; Wu, H. B.; Yan, Y.; Wang, X.; Lou, X. W. Hierarchical MoS 2 microboxes Constructed by Nanosheets with Enhanced Electrochemical Properties for Lithium Storage and Water Splitting. Energy Environ. Sci. 2014, 7, 3302-3306. (6) Park, S.-K.; Chung, D. Y.; Ko, D.; Sung, Y.-E.; Piao, Y. Three-Dimensional Carbon Foam/N-Doped Graphene@ MoS2 Hybrid Nanostructures as Effective Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2016, 4, 12720-12725. (7) Miao, J.; Xiao, F. X.; Yang, H. B.; Khoo, S. Y.; Chen, J.; Fan, Z.; Hsu, Y. Y.; Chen, H. M.; Zhang, H.; Liu, B. Hierarchical NiMo-S Nanosheets on Carbon Fiber Cloth: A Flexible Electrode for Efficient Hydrogen Generation in Neutral Electrolyte. Sci. Adv. 2015, 1, e1500259. (8) Xu, K.; Wang, F. M.; Wang, Z. X.; Zhan, X. Y.; Wang, Q. S.; Cheng, Z. Z.; Safdar, M.; He, J. Component-Controllable WS2(1X)Se2x Nanotubes for Efficient Hydrogen Evolution Reaction. ACS Nano 2014, 8, 8468-8476. (9) Shifa, T. A.; Wang, F.; Cheng, Z.; Zhan, X.; Wang, Z.; Liu, K.; Safdar, M.; Sun, L.; He, J. A Vertical-Oriented WS 2 Nanosheet Sensitized by Graphene: An Advanced Electrocatalyst for Hydrogen Evolution Reaction. Nanoscale 2015, 7, 14760-14765. (10) Shifa, T. A.; Wang, F.; Liu, K.; Xu, K.; Wang, Z.; Zhan, X.; Jiang, C.; He, J. Engineering the Electronic Structure of 2D WS2 Nanosheets Using Co Incorporation as CoxW(1- X )S 2 for Conspicuously Enhanced Hydrogen Generation. Small 2016, 12, 3802-3809. (11) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308. (12) Laursen, A. B.; Kegnæs, S.; Dahl, S.; Chorkendorff, I. Molybdenum Sulfides: Efficient and Viable Materials for Electro- and Photoelectrocatalytic Hydrogen Evolution. Energy Environ. Sci. 2012, 5, 5577. (13) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355. (14) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102. (15) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis. Nat. Mater. 2012, 11, 963-969. (16) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering

CONCLUSION In summary, the edge geometries and electrical connectivity of MoS2/CNF composites are controlled by a strategy of electronspinning and subsequent thermal treatment. For HER catalysts, MoS2/CNF exhibits excellent performance as compared to free MoS2 particles and 10 wt% Pt/C, especially at high current densities. The most optimized sample could deliver 500 and 1000 mA/cm2 at overpotentials of 380 and 450 mV. The porous or enclosed CNF are likely key factors to selectively grow MoS 2 nanoplates anchored to or embedded in the CNF, which provides a fibrous support hosting small and highly dispersed MoS 2 nanoplates in an interconnected conductive network whose abundant active edges and excellent electrical conductivity lead to outstanding HER catalytic activity. This work may open up new avenues to engineer the composite structures of non-noble metal catalysts (MoS 2, MoSe2, WS 2, etc.) and conductive carbon-based supports for the rational design of binder-free advanced catalysts for different electrochemical reactions, such as oxygen evolution or oxygen reduction.

ASSOCIATED CONTENT Supporting Information. Experimental and simulation details, Figures S1-S12, Table S1, and references.

AUTHOR INFORMATION Corresponding Author * [email protected] (R. Lv) * [email protected] (V. Crespi) * [email protected] (F. Kang) Author Contributions # These authors contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors are grateful to the financial supports from the National Natural Science Foundation of China (No. 51722207) , 973 program of China (No. 2015CB932500), Beijing Nova Program (No.

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in Oxygen-Incorporated MoS 2 Ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881-17888. (17) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25, 5807-5813. (18) Wu, Z.; Fang, B.; Wang, Z.; Wang, C.; Liu, Z.; Liu, F.; Wang, W.; Alfantazi, A.; Wang, D.; Wilkinson, D. P. MoS2 Nanosheets: A Designed Structure with High Active Site Density for the Hydrogen Evolution Reaction. ACS Catal. 2013, 3, 2101-2107. (19) Yan, Y.; Xia, B.; Ge, X.; Liu, Z.; Wang, J. Y.; Wang, X. Ultrathin MoS2 Nanoplates with Rich Active Sites as Highly Efficient Catalyst for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2013, 5, 12794-12798. (20) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS 2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. (21) Liao, L.; Zhu, J.; Bian, X.; Zhu, L.; Scanlon, M. D.; Girault, H. H.; Liu, B. MoS 2 Formed on Mesoporous Graphene as a Highly Active Catalyst for Hydrogen Evolution. Adv. Funct. Mater. 2013, 23, 5326-5333. (22) Zhu, H.; Du, M.; Zhang, M.; Zou, M.; Yang, T.; Wang, S.; Yao, J.; Guo, B. S-Rich Single-Layered MoS 2 Nanoplates Embedded in N-Doped Carbon Nanofibers: Efficient Co-Electrocatalysts for the Hydrogen Evolution Reaction. Chem. Commum. 2014, 50, 1543515438. (23) Guo, Y.; Zhang, X.; Zhang, X.; You, T. Defect- and S-Rich Ultrathin MoS2 nanosheet Embedded N-Doped Carbon Nanofibers for Efficient Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 1592715934. (24) Yan, Y.; Xia, B.; Li, N.; Xu, Z.; Fisher, A.; Wang, X. Vertically Oriented MoS2 and WS 2 nanosheets Directly Grown on Carbon Cloth as Efficient and Stable 3-Dimensional Hydrogen-Evolving Cathodes. J. Mater. Chem. A 2015, 3, 131-135. (25) Behranginia, A.; Asadi, M.; Liu, C.; Yasaei, P.; Kumar, B.; Phillips, P.; Foroozan, T.; Waranius, J. C.; Kim, K.; Abiade, J.; Klie, R. F.; Curtiss, L. A.; Salehi-Khojin, A. Highly Efficient Hydrogen Evolution Reaction Using Crystalline Layered Three-Dimensional Molybdenum Disulfides Grown on Graphene Film. Chem. Mater. 2016, 28, 549-555. (26) Choi, Y.-H.; Lee, J.; Parija, A.; Cho, J.; Verkhoturov, S. V.; Al-Hashimi, M.; Fang, L.; Banerjee, S. An in Situ Sulfidation Approach for the Integration of MoS2 nanosheets on Carbon Fiber Paper and the Modulation of Its Electrocatalytic Activity by Interfacing With nC 60. ACS Catal. 2016, 6, 6246-6254. (27) Liu, D.; Xu, W.; Liu, Q.; He, Q.; Haleem, Y. A.; Wang, C.; Xiang, T.; Zou, C.; Chu, W.; Zhong, J.; Niu, Z.; Song, L. UnsaturatedSulfur-Rich MoS2 Nanosheets Decorated on Free-Standing SWNT Film: Synthesis, Characterization and Electrocatalytic Application. Nano Res. 2016, 9, 2079-2087. (28) Khan, M.; Yousaf, A. B.; Chen, M.; Wei, C.; Wu, X.; Huang, N.; Qi, Z.; Li, L. Molybdenum Sulfide/Graphene-Carbon Nanotube Nanocomposite Material for Electrocatalytic Applications in Hydrogen Evolution Reactions. Nano Res. 2016, 9, 837-848. (29) Liu, Y.; Yu, G.; Li, G. D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites. Angew. Chem. Int. Ed. 2015, 54, 10752-10757. (30) Carmo, M.; Fritz, D. L.; Merge, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901-4934. (31) Lu, X.; Zhao, C. Electrodeposition of Hierarchically Structured Three-Dimensional Nickel-Iron Electrodes for Efficient Oxygen Evolution at High Current Densities. Nat. Commun. 2015, 6, 6616. (32) Li, X.; Walsh, F. C.; Pletcher, D. Nickel Based Electrocatalysts for Oxygen Evolution in High Current Density, Alkaline Water Electrolysers. Phys. Chem. Chem. Phys. 2011, 13, 1162-1167.

(33) Lu, Z. Y.; Zhu, W.; Yu, X. Y.; Zhang, H. C.; Li, Y. J.; Sun, X. M.; Wang, X. W.; Wang, H.; Wang, J. M.; Luo, J.; Lei, X. D.; Jiang, L. Ultrahigh Hydrogen Evolution Performance of under-Water “Superaerophobic” MoS 2 Nanostructured Electrodes. Adv. Mater. 2014, 26, 2683-2687. (34) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS 2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274-10277. (35) Smith, R. D.; Prevot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K.; Trudel, S.; Berlinguette, C. P. Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis. Science 2013, 340, 60-63. (36) Tributsch, H.; Bennett, J. C. Electrochemistry and Photochemistry of MoS 2 Layer Crystals. J. Electroanal. Chem. Interfac. 1977, 81, 97-111. (37) Lee, Y. H.; Zhang, X. Q.; Zhang, W.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y. C.; Wang, J. T. W.; Chang, C. S.; Li, L. J. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320-2325. (38) Liu, K.-K.; Zhang, W.; Lee, Y.-H.; Lin, Y.-C.; Chang, M.T.; Su, C.-Y.; Chang, C.-S.; Li, H.; Shi, Y.; Zhang, H. Growth of LargeArea and Highly Crystalline MoS2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12, 1538-1544. (39) Wang, X.; Feng, H.; Wu, Y.; Jiao, L. Controlled Synthesis of Highly Crystalline MoS 2 Flakes by Chemical Vapor Deposition. J. Am. Chem. Soc. 2013, 135, 5304-5307. (40) Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single and Few-Layer MoS 2. ACS Nano 2010, 4, 2695-2700. (41) Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D. From Bulk to Monolayer MoS 2: Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22, 1385-1390. (42) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Lett. 2013, 13, 1341-1347. (43) Nielsen, J. H.; Bech, L.; Nielsen, K.; Tison, Y.; Jørgensen, K. P.; Bonde, J. L.; Horch, S.; Jaramillo, T. F.; Chorkendorff, I. Combined Spectroscopy and Microscopy of Supported MoS2 Nanoparticles. Surface Science 2009, 603, 1182-1189. (44) Mahatha, S. K.; Menon, K. S. Inhomogeneous Band Bending on MoS2(0001) Arising from Surface Steps and Dislocations. J. Phys. Condens. Matter 2012, 24, 305502. (45) Lu, C.-H.; Hong, H.-C.; Jagannathan, R. Sol-Gel Synthesis and Photoluminescent Properties of Cerium-Ion Doped Yttrium Aluminium Garnet Powders. J. Mater. Chem. 2002, 12, 2525-2530. (46) Subramania, A.; Angayarkanni, N.; Vasudevan, T. Effect of Pva with Various Combustion Fuels in Sol-Gel Thermolysis Process for the Synthesis of LiMn2O2 Nanoparticles for Li-Ion Batteries. Mater. Chem. Phys. 2007, 102, 19-23. (47) Li, H.; Wu, H.; Yuan, S.; Qian, H. Synthesis and Characterization of Vertically Standing MoS 2 Nanosheets. Sci. Rep. 2016, 6, 21171. (48) Vilá, R. A.; Momeni, K.; Wang, Q.; Bersch, B. M.; Lu, N.; Kim, M. J.; Chen, L. Q.; Robinson, J. A. Bottom-up Synthesis of Vertically Oriented Two-Dimensional Materials. 2D Mater. 2016, 3, 041003. (49) Dumcenco, D.; Ovchinnikov, D.; Lopez Sanchez, O.; Gillet, P.; Alexander, D. T. L.; Lazar, S.; Radenovic, A.; Kis, A. Large-Area MoS2 Grown Using H2S as the Sulphur Source. 2D Mater. 2015, 2, 044005. (50) Zhou, G.; Xu, X.; Yu, J.; Feng, B.; Zhang, Y.; Hu, J.; Zhou, Y. Vertically Aligned MoS2/MoO x heterojunction Nanosheets for Enhanced Visible-Light Photocatalytic Activity and Photostability. CrystEngComm 2014, 16, 9025-9032. (51) Chang, Y. H.; Wu, F. Y.; Chen, T. Y.; Hsu, C. L.; Chen, C. H.; Wiryo, F.; Wei, K. H.; Chiang, C. Y.; Li, L. J. Three-Dimensional

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Molybdenum Sulfide Sponges for Electrocatalytic Water Splitting. Small 2014, 10, 895-900. (52) Conway, B.; Tilak, B. Interfacial Processes Involving Electrocatalytic Evolution and Oxidation of H 2, and the Role of Chemisorbed H. Electrochim. Acta 2002, 47, 3571-3594. (53) Wang, H.; Lu, Z.; Kong, D.; Sun, J.; Hymel, T. M.; Cui, Y. Electrochemical Tuning of MoS 2 Nanoparticles on Three-Dimensional Substrate for Efficient Hydrogen Evolution. ACS Nano 2014, 8, 49404947.

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(54) Ter Heijne, A.; Schaetzle, O.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Strik, D. P. B. T. B.; Barrière, F.; Buisman, C. J. N.; Hamelers, H. V. M. Identifying Charge and Mass Transfer Resistances of an Oxygen Reducing Biocathode. Energy Environ. Sci. 2011, 4, 5035.

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A graphic for the Table of Contents (TOC)

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Controllable Edge Exposure of MoS2 for Efficient Hydrogen Evolution

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