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Self-Supported Biocarbon Fiber Electrode Decorated with Molybdenum Carbide Nanoparticles for Highly Active Hydrogen Evolution Reaction Jian Xiao, Yan Zhang, Zheye Zhang, Qi-Ying Lv, Feng Jing, Kai Chi, and Shuai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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ACS Applied Materials & Interfaces

Self-Supported Biocarbon Fiber Electrode Decorated with Molybdenum Carbide Nanoparticles for Highly Active Hydrogen Evolution Reaction

Jian Xiao†, #, Yan Zhang†, #, Zheye Zhang†, Qiying Lv†, Feng Jing†, Kai Chi†, Shuai Wang†, §, *

†

State Key Laboratory of Digital Manufacturing Equipment and Technology. Key laboratory of

Material Chemistry for Energy Conversion and Storage, Ministry of Education. School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China §

Flexible Electronics Research Center (FERC), School of Mechanical Science and Engineering,

Huazhong University of Science and Technology, Wuhan 430074, P. R. China

KEYWORDS:

electrocatalysis,

self-supported

electrode,

hydrogen

evolution

reaction,

molybdenum carbide, biocarbon fiber

ABSTRACT To devise and facilely synthesize an efficient noble metal-free electrocatalyst for accelerating the sluggish kinetics in the hydrogen evolution reaction (HER) are still a big challenge for electrolytic water splitting. Herein, we present a simple one-step approach for constructing self-supported biocarbon fiber cloth decorated with molybdenum carbide nanoparticles

ACS Paragon Plus Environment


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(BCF/Mo2C) electrodes by a direct annealing treatment of the Mo oxyanions loaded cotton T-shirt. The Mo2C nanoparticles not only serve as the catalytic active sites toward the HER but also enhance the hydrophilicity and conductivity of resultant electrodes. As an integrated three-dimensional HER cathode catalyst, the BCF/Mo2C exhibits outstanding electrocatalytic performance with extremely low overpotentials of 88 mV and 115 mV to drive a current density of 20 mA cm-2 in alkaline and acidic media, respectively. In addition, it can continuously work for 50 h with little decrease in the cathodic current density in both alkaline and acidic solutions. Even better, self-supported tungsten carbide- and vanadium carbide-based electrodes also can be prepared by similar synthesis process. This work will illuminate an entirely new avenue to prepare various self-supported three-dimensional electrodes made of transition metal carbides for various applications.


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INTRODUCTION As a hopeful renewable chemical fuel, hydrogen (H2) is a perfect energy carrier to replace the conventional fossil fuel in the future.1 Electrocatalytic water splitting is regarded as a practical technology to yield highly pure H2 by using electrical power produced from wind, solar energy or other fitful renewable energy resources.2, 3 Platinum (Pt)-based compounds are the most active and stable catalysts toward the hydrogen evolution reaction (HER),4, 5 but the high cost and scarcity restrict their widespread application. Thus, it is still an urgent task to develop a high-activity noble metal-free catalyst toward the HER. In the past decades, many works have been done to prepare cost-efficient non-noble metal based catalysts, such as metal oxides,6, 7 phosphides,8, 9 sulfides,10, 11 selenides,12,

13

carbides,14,

15

nitrides,15,

16

and heteroatom-doped nanocarbons17, among which

transition metal carbides (TMCs), especially molybdenum carbide-based materials, have been considered as a class of plentiful excellent catalysts toward the HER. Molybdenum carbide has long been expected to be high-activity noble metal-free catalyst toward the HER due to its Pt-like d-orbital electronic structure, high electrical conductivity and good corrosion resistance.18 In 2012, Hu group first reported commercial Mo2C microparticles show good electrocatalytic performance for the HER in both of acidic and alkaline media.19 Since then, much effort has been made to improve their catalytic activity. Lou et al. reported a metal-organic framework assisted method to synthesize porous MoC octahedral nanoparticles, which have a good electrocatalytic activity for HER.20 Molybdenum carbide with other nanostructural forms, such as porous nanowire,21 nanorod,22 hierarchical nanotube constructed from porous nanosheets,23 and nanoparticle24, were also designed to achieve excellent HER performance by maximizing the number of their catalytic active sites. Another effective method to



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enhance the electrocatalytic performance of molybdenum carbide-based catalysts is to couple them with carbon materials (such as graphene, CNT, and N-doped carbon).25-32 However, the above catalysts were all prepared in a powder form, which always required the time-consuming film coating or casting process with the help of extra conductive additives and polymeric binders to construct a working electrode. Conversely, self-supported electrocatalysts, forming free-standing films or directly growing on conductive substrates (carbon cloth, nickel foam etc.), don’t require any additives and binders. This new class of electrocatalysts always shows large electrochemically surface areas, good electrical conductivity and excellent mechanical stability.33-37 However, there are few molybdenum carbide-based self-supported electrocatalysts to be made for HER.38-40 Herein, we propose a very simple, straightforward, and cost-efficient strategy to prepare self-supported biocarbon fiber (BCF) cloth decorated with molybdenum carbide nanoparticles (BCF/Mo2C) electrodes for HER. The BCF/Mo2C electrodes are synthesized by annealing treatment of (NH4)6Mo7O24•4H2O solution treated cotton textile (T-shirt) under a flowing argon atmosphere. The Mo2C nanoparticles, as active material for HER, not only enhance the wettability but also improve the electrical conductivity of the resultant biocarbon fiber cloth. When directly used as a self-supported three-dimensional electrode toward the HER, the BCF/Mo2C exhibits perfect electrocatalytic performance with a very low overpotential of 115 mV to drive a current density of 20 mA cm-2 in acidic media, and it can work at the overpotential of 150 mV up to 50 h with little decrease in the cathodic current density. The BCF/Mo2C electrode also shows remarkable catalytic activity toward HER in alkaline media. Furthermore, tungsten carbide- and vanadium carbide-based composites can be prepared by using the similar method.


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EXPERIMENTAL SECTIONS Preparation of the Mo2C/BCF electrodes. A piece of cotton textile (8 × 8 cm2) cut from a commercial T-shirt was cleaned by ultra-pure water in an ultrasonic bath, and then dipped into (NH4)6Mo7O24•4H2O solution and kept for 15 min. Afterwards, the sample was dried at 60 oC for 120 min in an electric oven. Finally, the (NH4)6Mo7O24-loaded cotton textile was heated at 900 oC (heating rate is 10 oC min-1) for 5 h into an electric tube furnace under a flowing argon atmosphere (200 sccm). To probe the effect of concentration of (NH4)6Mo7O24•4H2O on catalytic performance, the concentration was varied from 0.2 g mL-1 to 0.8 g mL-1. The corresponding resultant electrodes were referred to as BCF/Mo2C-0.2, BCF/Mo2C-0.4 and BCF/Mo2C-0.8, respectively. The optimal concentration was found to be 0.4 g mL-1, so, unless indicated otherwise, the presented data was obtained from the sample soaked in the (NH4)6Mo7O24•4H2O solution of 0.4 g mL-1. The actual loading of active Mo2C on BCF/Mo2C-0.4 electrode was 10.7 mg cm-2, calculated by thermogravimetric analysis. The biocarbon fiber decorated with vanadium carbide nanoflakes (BCF/VC) and biocarbon fiber decorated with tungsten carbide nanoparticles (BCF/WC-W2C) were synthesized by using (NH4)6(H2WI2O40)•4H2O (0.4 g mL-1) and NH4VO3 (0.4 g mL-1, oxalic acid was added to increase the solubility) in replace of (NH4)6Mo7O24•4H2O in the synthesis process.

Materials characterization. The Nova NanoSEM 450 with an accelerating voltage of 10 Kv (SEM) and Tecnai G2-F20 operating at 200 Kv (TEM) were used to examine the product morphologies. The Philips PW-1830 with Cu Kα radiation (λ = 1.5406 Å) was used to obtain the X-ray powder diffraction (XRD) data. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PerkinElmer model PHI 5600 XPS system with an Al Kα X-ray source



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(1486.6 eV). Thermogravimetric analysis was conducted on the Pyris1 TGA (PerkinElmer Instruments) under a flowing air atmosphere (25 sccm) and the rate of heating is 10 oC per minute. Raman spectra were obtained by using the LabRAM HR800 instrument with a Nd:YAG laser source (532 nm).

Electrocatalytic Performance Measurements. The electrochemical measurements were carried out on a CHI 760E electrochemical analyzer (CH Instruments, Inc., Shanghai) at room temperature in a standard three-electrode setup with a graphite rod as counter electrode and a saturated calomel (SCE) as reference electrode in 0.5 M H2SO4 solution (in 1.0 M KOH solution, the Hg/HgO was used as reference electrode). The resultant electrodes were employed as working electrodes. For comparison's sake, we also measured the electrocatalytic performance of commercial Pt/C (20 wt %) deposited on glassy carbon electrode (1.41 mg cm-2). Linear sweep voltammetry (LSV) tests were performed at a scan rate of 5 mV s-1. Unless otherwise specified, all potentials were iR-corrected to 95% with the built-in programme. The electro-active surface areas were measured by a series of cyclic voltammetry measurements performed in 150-250 mV vs. RHE region with various scan rates (2, 4, 6 mV s-1, etc.), and eight continuous sweep cycles were done to ensure consistency. Electrochemical impedance spectroscopy (EIS) analysis was conducted in the frequency range of 10 mHz-100 kHz with a 5 mV ac amplitude at the overpotential of 100 mV in 0.5 M H2SO4. The accelerated degradation test (ADT) was conducted using cyclic voltammetry with a scan rate of 50 mV s-1 between -250 mV and 150 mV (vs. RHE) for 1000 and 5000 continuous cycles. The long-term stability tests were conducted using chronoamperometry measurements at the overpotentials of 100 mV and 150 mV in 1000 mM


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KOH and 500 mM H2SO4 solution, respectively. All potentials measured were calibrated to reversible hydrogen electrode (RHE) according to the following equations: E (RHE) = E (Hg/HgO) + 0.098 + 0.059 pH (in 1.0 M KOH), E (RHE) = E (SCE) + 0.241 + 0.059 pH (in 0.5 M H2SO4).

RESULTS AND DISCUSSION Cotton textile has been considered as an excellent platform for constructing flexible self-supported electrodes for energy related applications,41,

42

Here, cotton T-shirt and

(NH4)6Mo7O24•4H2O are used as raw materials for fabricating BCF/Mo2C electrodes. Scheme 1 shows the schematic illustration of the design and synthesis procedures of BCF/Mo2C. Typically, a piece of cotton textile is first immersed into (NH4)6Mo7O24•4H2O solution of different concentrations and then dried in the oven to obtain Mo oxyanions loaded cotton textile. Finally, the Mo oxyanions loaded cotton textile is heated at 900 oC for 5 h in an electric tube furnace under the argon atmosphere. When heated in the inert atmosphere, Cotton T-shirt, comprised of natural fibers with an average diameter of 8 µm (Figure S1a and inset), is converted into BCF cloth and simultaneously releases many gaseous products, such as H2, CH4, CO, and CO2.42 Although the white cotton T-shirt turns to black and shrinks (Figure S1b) after thermal treatment, its three-dimensional braided structure (Figure S1b, inset) can be maintained. Thanks to the high water absorption of natural fibers, the cotton T-shirt can absorbed an amount of Mo oxyanions when it is soaked in the (NH4)6Mo7O24•4H2O solution. The Mo oxyanions decompose into MoO3 first, then, react with gaseous products to form Mo2C nanoparticles at high temperature. The Mo2C nanoparticles don’t change the three-dimensional structure and flexibility of BCF cloth (Figure 1b and inset) and the BCF/Mo2C can be directly used as an electrode for HER without any



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post-processing steps. In contrast to the smooth surface of pure BCF (Figure 1a), the surface of BCF/Mo2C electrode is rough and many nanoparticles embed into the BCF (Figure 1c). We further use TEM to characterize the structure of the as-prepared BCF/Mo2C electrodes. As shown in Figure 1d, many nanoparticles with a uniform size of about 20 nm decorate the BCF. The observed adjacent plane distance of the nanoparticles is 2.3 Å, which is ascribed to the (101) crystal plane of β-Mo2C phase. The corresponding EDS spectrum indicates that except for a little amount of nitrogen and oxygen elements, the sample only contains molybdenum and carbon elements (Figure 1f). Moreover, the SEM-EDS elemental mapping reveals that the molybdenum and carbon elements are evenly distributed over the BCF surface (Figure 1g-i). The XRD patterns in Figure 2a show nine diffraction peaks for BCF/Mo2C electrode at 2θ = 34.4o, 38.0o, 39.4o, 52.1o, 61.5o, 69.6o, 72.4o, 74.6o and 75.5o, which can be assigned to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of the pure β-Mo2C phase with a hexagonal closed packed structure (PDF#:35-0787). The diffraction peaks observed at 26o, 43o are attributed to the BCF. No other diffraction peaks are determined for impurities and the BCF/Mo2C displays a good crystallinity. To further clarify the chemical composition of the above-mentioned molybdenum-based material, XPS measurement is performed. Owing to the spin-orbital coupling, the high-resolution XPS spectrum of Mo 3d core level is split into 3d5/2 and 3d3/2 peaks (Figure 2b). The peaks of the binding energies at 228.7 and 231.9 eV can be ascribed to the Mo 3d5/2 and Mo 3d3/2 of Mo2+ spectral lines, respectively; these are attributable to the presence of Mo2C and agree well with the measured data reported in the previous works.43, 44 As the result of surface oxidation of Mo2C nanoparticles when exposed to air, the peaks at 232.9 and 236.1 eV are assignable to MoO3 and that at 232.8 and 229.4 eV are attributable to MoO2; both of these species


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are thought to be inactive for HER.19,

27

The content of Mo2C increase with increasing

concentration of the (NH4)6Mo7O24•4H2O solution, and is calculated to be 39%, 51% and 73% for BCF/Mo2C-0.2, BCF/Mo2C-0.4 and BCF/Mo2C-0.8, respectively, by thermogravimetric analysis (TG) (Figure S2). All of these results clearly confirm the successful preparation of BCF/Mo2C electrodes after thermal treatment. The electrocatalytic activity of BCF/Mo2C as a self-supported three-dimensional electrode toward the HER is firstly assessed by linear sweep voltammetry (LSV) measurement conducted in a classic three-electrode system. The analogous tests of the commercial 20 wt % Pt/C and bare BCF are also performed for comparison. Here, a graphite rod rather than a Pt foil/mesh/wire is used as counter electrode to prevent contamination from dissolved Pt.45 Figure 3a shows the iR-corrected LSV curves, recorded in 0.5 M H2SO4 at a scan rate of 5 mV s-1, for different electrodes on the RHE scale. Similar to previous reports, commercial Pt/C presents perfect electrocatalytic performance toward the HER with an onset potential of near zero.4, 5 In contrast, the bare BCF begins to exhibit a small current density at the overpotential of more than 300 mV. However the BCF/Mo2C electrodes are highly active toward the HER. Remarkably, BCF/Mo2C-0.4 electrodes always need the lowest overpotential to gain the same current density and can present 20 mA cm-2 at the overpotential of only 115 mV (η20), which is far less than that needed by bare BCF (η20 = 542 mV). Although the activity is slightly inferior to that of Pt/C, the BCF/Mo2C-0.4 electrode rivals the performance of reported molybdenum carbide-based catalysts, including commercial Mo2C microparticles (210 mV),19 porous molybdenum carbide octahedral nanoparticles (162 mV),20 nanoporous Mo2C nanowires (150 mV),21 Mo2C nanorods (175 mV),22 hierarchical Mo2C nanotubes (197 mV),23 Mo2C nanoparticles (190 mV),24 Mo2C/GCSs (250



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mV),29 Mo2C/CNT-GR (147 mV),30 Mo2C@N-CNFs (222 mV),32 Mo2C/CC (193 mV),40 and Co-Mo2C nanowires (160 mV)46. It also compares favorably with other noble metal-free HER elecctrocatalysts in acidic media (Table S1). To figure out different catalytic activities of BCF/Mo2C catalysts, we test their electrical double-layer capacitances (Cdl), which are in proportion to the electro-active surface areas (ECSA), by a simple cyclic voltammetry (CV) method (Figure S3a).28, 47 As shown in Figure S3b, the Cdl of BCF/Mo2C-0.4 (306.8 mF cm-2) is higher than that of BCF/Mo2C-0.8 (183.1 mF cm-2), BCF/Mo2C-0.2 (46.5 mF cm-2), and bare BCF (5.1 mF cm-2). It indicates that increasing Mo2C content leads to an increase in surface roughness, but excessive amount can cause the formation of large Mo2C particles (Figure S4), which reduce the ECSA. The variations of the Cdl correlate well with the electrocatalytic performance of the catalysts, which decreased in the order of BCF/Mo2C-0.4 (η20 = 115 mV)>BCF/Mo2C-0.8 (η20 = 129 mV)>BCF/Mo2C-0.2 (η20 = 180 mV)>BCF (η20 = 542 mV). On the other hand, the EIS tests are conducted to further explain their electrocatalytic performance. The Nyquist plots of BCF/Mo2C electrodes are presented in Figure 3b and fitted with the 2CPE model (Figure S5).48 The series resistance (Rs) decreases with increasing the content of Mo2C in catalysts, indicating that the Mo2C particles can effectively improve the conductivity of BCF. The smallest charge transfer resistance (Rct) of BCF/Mo2C-0.4 electrode demonstrates the fastest charge transfer process and fastest hydrogen evolution rate on the catalyst surface. The hydrophilic surface of BCF/Mo2C-0.4 is verified by the contact angle between water and corresponding electrode (Figure 3c), which is attributable to the rough surface of resultant electrode and the good hydrophilicity of Mo2C. Such rough hydrophilic surface not only exposes more active sites, but also facilitates the release of generated H2 gas and the diffusion


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of electrolyte. The large ESCA, high conductivity and good hydrophilicity jointly promote the supreme electrocatalytic performance of the BCF/Mo2C-0.4 electrode. Tafel slope is another valuable parameter in the evaluation of electrocatalysts for HER. The Tafel plots of resultant electrocatalysts evolved from corresponding LSV curves are presented in Figure 3d. A linear fit using the Tafel equation yields 99.4, 84.8, 154.7, 204.1, and 30.6 mV per decade for the BCF/Mo2C-0.8, BCF/Mo2C-0.4, BCF/Mo2C-0.2, bare BCF, and 20% Pt/C, respectively. It is established that Tafel slope would be 120, 40, or 30 mV per decade when the Volmer, Heyrovsky, or Tafel reaction is the rate-limiting step, respectively.49, 50 Thus, the Tafel slope of 84.8 mV per decade observed from BCF/Mo2C-0.4 electrode suggests that the HER reaction rate is determined by the Volmer reaction. The ADT is executed by CV measurement to evaluate the stability of HER catalysts. The electrodes only show a slight degradation after the fast cycling for one thousand cycles and the overpotentials increase 2.5 and 4 mV to achieve current densities of 20 and 100 mA cm-2, respectively (Figure 3e), and then maintain the same catalytic activity. Furthermore, the BCF/Mo2C-0.4 electrodes can continuous work for 50 h in 0.5 M H2SO4 at a static overpotential of 150 mV, driving a steady current density of about 33 mA cm-2 (Figure 3f). We further investigated the HER performance of the BCF/Mo2C-0.4 electrodes in 1.0 M KOH solution. The electrodes also show excellent electrocatalytic performance, stability and durability under alkaline condition (Figure 4). To achieve the current density of 10 mA cm-2, the BCF/Mo2C-0.4 electrodes only need the overpotential of 71 mV (η10) (Figure 4a). This value is much lower than that required by other reported noble metal-free HER electrocatalysts in alkaline solution.6-8, 11, 19, 20, 23, 51, 52 Further increasing overpotential causes a sharp increase in current



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density, and the BCF/Mo2C-0.4 electrode only needs overpotential of 88 (η20) and 137 mV (η100) to obtain 20 and 100 mA cm-2, respectively. The Tafel slope is calculated to be 52.4 mV per decade (Figure 4b) and comparable to many previously reported catalysts (Figure 4d). The catalyst maintains its excellent catalytic activity except for a little decay at the region of the high current density after continuous cyclic voltammetry (CV) conducted from -250 mV to 150 mV (vs. RHE, without iR-corrected) for the first 1000 cycles in 1.0 M KOH at 50 mV s-1 (Figure 4a). Even better, the BCF/Mo2C-0.4 electrode is able to maintain a steady current density with about 22 mA cm-2 for 50 h at the constant overpotential of 100 mV in 1.0 M KOH (Figure 4c). Remarkably, the three-dimensional structures of the BCF/Mo2C electrodes remain unchanged after long-term stability tests (Figure S6a, inset). The Mo2C nanoparticles don’t fall off the biocarbon fiber and also not dissolve in the electrolyte (Figure S6a). As we known, the chemical environments of the metals in the transition-metal-based materials are influencing their electrocatalytic activities. The high-resolution Mo 3d core-level XPS spectrum of the BCF/Mo2C after long-term stability tests is presented in Figure S6b. It indicates that the chemical environment of molybdenum does not change. In short, the as-prepared BCF/Mo2C electrode exhibits excellent stability, and the high catalytic activity can be perfectly retained in long-term practical applications. Given that the method for preparing BCF/Mo2C catalysts is simple, cost-effective and easy to be extended to other functional materials, we prepared other self-supported transition metal carbide electrodes, such as BCF/VC and BCF/WC-W2C, by using corresponding metal salts in replace of (NH4)6Mo7O24•4H2O in the synthesis process. BCF/VC and BCF/WC-W2C electrodes also have a three-dimensional braided structure. SEM images revealed that lots of


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vertically-aligned nanoflakes are grown on BCF surface for BCF/VC electrode (Figure 5a) and densely-packed nanoparticles are grown on BCF surface for BCF/WC-W2C electrode (Figure 5b). The XRD patterns of as-prepared composite nanomaterials can be completely indexed to the cubic VC phase (PDF#03-065-7885) and the mixture of hexagonal WC (PDF#03-065-4539) and W2C (PDF#03-065-3896), respectively (Figure 5c). The HER performance of BCF/VC and BCF/WC-W2C electrodes are investigated in acidic media. As shown in Figure 5d, the LSV curve recorded with BCF/WC-W2C electrode shows excellent activity and can obtain 10 mA cm-2 at the overpotential of 154 mV toward the HER. Although this potential is higher than that for BCF/Mo2C, it is lower than that obtained from most tungsten carbide-based solid-state catalysts, including tungsten carbide@carbon (303 mV),27 WC@CNS (217 mV),38 porous WC (274 mV),53 W2C@C (435 mV),54 To achieve the current density of 10 mA cm-2, the BCF/VC electrode needs the overpotential of 294 mV, which is in line with that reported in the previous studies.15

CONCLUSIONS In summary, the self-supported BCF decorated with Mo2C nanoparticles is directly prepared by anneal treatment of (NH4)6Mo7O24•4H2O solution treated cotton T-shirt in a flowing argon atmosphere. As a self-supported water splitting cathode, the BCF/Mo2C exhibits high electrocatalytic performance and superior long-term stableness in both acidic and alkaline media. The perfect electrocatalytic performance toward the HER can be ascribed to the large ECSA, high conductivity and good hydrophilicity. In addition, other self-supported carbide electrodes can be prepared by this effective method. Hence, this study will open up an exciting new avenue to design various self-supported three-dimensional electrodes made of TMCs for various applications as electrocatalysts for water splitting, electrode materials for Li-ion battery and catalysts for



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organic reactions.

ASSOCIATED CONTENT Supporting Information Additional photograph, SEM, TG, XPS, electrochemical test data and supplementary tables. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Tel: +86 87792464 Author Contribution #

These authors contributed equally to this study.

Note The authors declare no competing financial interest. Acknowledgements The study is supported by some National Natural Science Foundations (Project Nos. 51173055, 51572094, 21401060) and Postdoctoral Science Foundation in China (Project No. 2015M572135). We really appreciate for the help from the Analytical and Testing Center of HUST (Huazhong University of Science and Technology), the Wuhan National Laboratory for Optoelectronics.


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(54) Yan, G.; Wu, C. X.; Tan, H. Q.; Feng, X. J.; Yan, L. K.; Zang, H. Y.; Li, Y. G. N-Carbon Coated P-W2C Composite as Efficient Electrocatalyst for Hydrogen Evolution Reaction at All pH Range. J. Mater. Chem. A 2017, 5, 765-772.


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Figure captions

Scheme 1. Illustration of the synthesis of BCF/Mo2C electrodes.

Figure 1. (a) SEM image of BCF. (b, c) SEM images of BCF/Mo2C, inset: optical photograph of BCF/Mo2C electrode under folded state. (d, e) TEM images of BCF/Mo2C electrode. (f) EDX spectrum and (g-i) elemental mapping of BCF/Mo2C electrode.



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Figure 2. (a) Wide-angle XRD patterns of BCF and BCF/Mo2C electrodes. The standard XRD pattern of Mo2C is also given for reference. (b) The high-resolution Mo 3d core-level XPS spectrum of BCF/Mo2C electrode.


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Figure 3. (a) LSV curves of BCF, BCF/Mo2C, and commercial Pt/C (20%) catalysts measured in 0.5 M H2SO4. (b) Nyquist plots of electrochemical impedance spectroscopy (EIS) for obtained electrodes at the overpotential of 100 mV. (c) Pictures of water droplet on the surface of bare BCF (left) and BCF/Mo2C-0.4 (right). (d) Tafel plots of corresponding electrodes (e) LSV curves of BCF/Mo2C-0.4 electrode before and after cyclic voltammetry for 1000 and 5000 cycles in 0.5 M H2SO4. (f) Chronoamperometric curve of BCF/Mo2C-0.4 electrode measured at the overpotential of 150 mV in 0.5 M H2SO4.



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Figure 4. (a) LSV curves of the BCF/Mo2C-0.4 electrode before and after cyclic voltammetry for 1000 and 5000 cycles in 1.0 M KOH. (b) Tafel plots of the BCF/Mo2C-0.4 electrode measured in 1.0 M KOH. (c) Chronoamperometric curve of the BCF/Mo2C-0.4 electrode measured at the overpotential of 100 mV in 1.0 M KOH. (d) Comparison of the overpotential and Tafel slope between our BCF/Mo2C-0.4 and other reported non-Pt HER catalysts in alkaline media.


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Figure 5. (a, b) SEM images of BCF/VC and BCF/WC-W2C electrodes. (c) Wide-angle XRD patterns of BCF/VC and BCF/WC-W2C electrodes. The standard XRD patterns of VC, WC and W2C are also given for reference. (d) LSV curves of BCF, BCF/VC, BCF/WC-W2C, and commercial Pt/C (20%) catalysts measured in 0.5 M H2SO4.



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Table of Contents Graphic


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Self-Supported Biocarbon-Fiber Electrode ... - ACS Publications

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