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Molybdenum Diselenide Nanolayers Prepared on Carbon Black as An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction Ning Xue, and Peng Diao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09590 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017
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Molybdenum Diselenide Nanolayers Prepared on Carbon Black as An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction Ning Xue a, Peng Diaoa,* a
School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China
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ABSTRACT As a green and renewable energy source, hydrogen will play a more and more important role in lessening our reliance on fossil fuels. Electrochemical and photoelectrochemical reduction of water are two critical strategies for sustainable hydrogen production. Exploring low-cost, efficient and stable catalysts to replace the expensive noble metal-based catalysts for hydrogen evolution reaction (HER) is practically significant in large-scale hydrogen production. In this work, we report the facile synthesis of a novel MoSe2/carbon black (MoSe2/CB) composite with few-layered MoSe2 grown on commercially available carbon black. The obtained MoSe2/CB composite displays a high catalytic activity toward HER in acidic media. The uppermost activity is obtained on the MoSe2/CB composite with a CB percentage of 50% (denoted as MoSe2/CB-50%). The MoSe2/CB-50% catalyst exhibits a current density of 31.5 mA·cm-2 at -0.225 V vs. RHE and a small Tafel slope of 62 mV·dec-1, which is superior to most MoSe2-based catalysts reported previously. Moreover, the MoSe2/CB-50% catalyst also shows an excellent stability for HER during a long-term reaction. In view of the facile preparation, the high activity, good stability and the low cost of CB, our MoSe2/CB catalyst enables a more prospective application for industrial and renewable hydrogen production compared to other MoSe2-based catalysts.
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INTRODUCTION Ever-increasing demand of fossil fuels and the resulting environmental issues call for the innovation in energy conversion field. Hydrogen, a clean energy carrier, has been considered as a promising candidate to replace fossil fuels in energy conversion systems in the future due to its highest energy density (143 kJ·g-1)1 and zero pollutant emission upon combustion. Electrochemical as well as photoelectrochemical reduction of water are two prospective methods to produce hydrogen.2-4 Electrocatalysts play an important role in improving the efficiency of hydrogen evolution reaction (HER) in these two strategies, since the overpotential for HER is rather high on most metal and other conductive materials. Currently, the state-of-the-art electrocatalysts for HER are noble metals and their alloys, such as Pt,3, 5 Pd,6 Ag/Pt,7 Au/Pd,8 and etc. However, the rarity and high cost of these noble metal-based catalysts severely limit their large-scale application in hydrogen production.9-11 Hence, development of efficient and inexpensive noble-metal-free electrocatalysts based on earth-abundant materials is of great practical significance for HER. Recently, layered transition metal dichalcogenides, such as MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2, have aroused great interests as a promising alternative to noble metal-based catalysts because of their high activity toward HER.12-20 Among these materials, molybdenum disulfide (MoS2), which is composed of covalently bonded Mo and S atoms in three parallel planes,21 has been widely studied as an effective HER electrocatalyst.18-19, 22-32 Terminating disulfide and sulfide (S22- and S2-) groups at the edges of MoS2 layers have been proved to be the active sites for HER in MoS2.18, 32 As an analogue to MoS2, MoSe2 has a similar three atom layers which are held together by van der Waals interactions: a Mo plane sandwiched between two selenium planes. Recently, Tsai et al. demonstrated that both the Mo-edges and Se-edges in MoSe2 layers are active for electrochemical HER.33 Density functional theory calculation also revealed that the Gibbs free energy for H2 adsorption on MoSe2 edges is low (-0.14 eV). Thus, a high coverage of hydrogen adsorption (75%) was occurred for MoSe2, which is beneficial for a high HER activity.13 Moreover, according to the partial metallic nature of Se, MoSe2 has a relatively high intrinsic 3
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electrical conductivity.34 This is also advantageous for MoSe2 to show high electrocatalytic activity toward HER. As a result, it is meaningful to prepare MoSe2-based electrocatalysts to realize excellent catalytic activity for HER. Until now, several works have been carried out to prepare MoSe2 related catalysts for HER. Cui et al. reported a series of studies about forming vertically aligned MoSe2 nanosheets with many exposed edges on the surface of Si nanowires or Si wafers to achieve high activity.35-36 Wang and coworkers prepared MoSe2 on reduced graphene oxide (rGO)/polyimide composite by electrochemical deposition. The small size of MoSe2 nanoparticles and high conductivity of rGO/polyimide substrate resulted in the high catalytic performance of the composite.37 Tang et al. synthesized MoSe2/graphene hybrid via a hydrothermal reaction followed by a low temperature annealing. The resulting MoSe2/graphene hybrid, which were composed of MoSe2 nanosheets grown on graphene, displayed high catalytic performance for HER.13 All these works demonstrated that, to obtain excellent catalytic activity, it is rational to design MoSe2-based catalyst with both the maximum exposed edges and the improved electrical conductivity. Growing layered MoSe2 on highly conductive substrate such as carbon-based materials provides a convenient strategy to achieve this object. So far, several carbon-based materials, such as carbon nanotubes,38-39 carbon fibers,40 and graphene,13, 27, 37, 41-43 have been employed as supports to load layered MoSe2 catalyst because of their high conductivity and excellent chemical stability. However, the mostly investigated carbon materials are usually expensive carbon supports such as graphene, rGO and carbon nanotubes. Therefore, it is challenging but worthwhile to develop efficient MoSe2-based HER catalysts that are composed of layered MoSe2 and the low-cost and commercially available carbon materials. Carbon black (CB) is a commonly used carbon material that is composed of nanoscaled paracrystalline carbon spheres. CB usually has chemisorbed oxygen complexes (i.e., carboxylic, carbonyl, and phenolic groups) on its surface to varying degrees depending on the conditions of manufacture and treatments. Therefore, it is easy to modify the surface of CB with other materials. This property together with its high conductivity and low cost make CB a good support for electrocatalysts. For example, CB has been widely 4
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employed in the preparation of electrocatalysts for oxygen reduction reaction.44-47 Recently, we reported the fabrication of few-layered MoS2 nanosheets on CB and demonstrated that the obtained MoS2/CB composites were highly active toward electrochemical HER.32 The high activity originated from the terminating disulfide and sulfide (S22- and S2-) groups in the composite.18, 32 This work indicated that CB could also be a good support for HER electrocataysts. In this work, we report a facile one-step solvothermal method to prepare the composite of few-layered MoSe2 grown on CB. The obtained MoSe2/CB composite catalyst exhibits a high activity toward HER. By varying the content of CB in the composite, the activity of the catalyst could be tuned, and the catalyst with 50% mass percentage of CB (denoted as MoSe2/CB-50%) showed the highest activity. The improvement of the catalytic activity for the MoSe2/CB composite is ascribed to the synergetic effect between MoSe2 and CB, where CB hinders the aggregation of MoSe2 nanosheets, and maximizes the number of exposed active sites and then promotes the electron transfer efficiency for HER at the electrode/solution interface. This work presents a novel and efficient MoSe2/CB HER catalysts, in which the utilization of low cost CB as the support makes it more competitive to other MoSe2-based electrocatalysts in practical application. EXPERIMENTAL SECTION Materials. Sodium molybdate dihydrate (Na2MoO4·2H2O, 99.99%) was purchased from Alfa Aesar company. Hydrazine hydrate (N2H4·H2O, 80%) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Selenium powder (Se, ≥ 99.99%) was purchased from Dongfang Chemical Co., Ltd. Potassium permanganate (KMnO4, A.R.), sodium nitrate (NaNO3, A.R.), hydrogen peroxide (H2O2, 30%), concentrated sulfuric acid (H2SO4, 98%),
concentrated hydrochloric
acid (HCl,
36~38%),
N,N-dimethylformamide (DMF, 99.5%) and isopropanol (IPA, 99.7%) were all purchased from Beijing Chemical Reagents Co.. All the chemicals were used as obtained without further purification. Carbon black was purchased from Tianjin Jinqiushi Chemical Co., Ltd, and the carbon fiber paper was purchased
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from Shanghai Hesen Electric Co., LTD. All aqueous solutions were prepared with ultra-pure water (18 MΩ·cm). Solvothermal Synthesis of MoSe2/CB Composites. CB was mildly oxidized by KMnO4 before use to introduce oxygen-containing functional groups on its surface through our recently developed method32. The details are described in the Supporting Information. The MoSe2/CB composites were prepared by a facile one-step solvothermal strategy. In detail, 60.5 mg of the oxidized CB was firstly dispersed in 17.5 mL DMF under sonication for 10 min at room temperature. Then 121.0 mg (0.5 mmol) of Na2MoO4·2H2O was added into the suspension and sonicated for 20 min. The mixture of 78.96 mg Se powder and 12.5 mL hydrazine hydrate was added slowly to the 17.5 mL Na2MoO4 and CB suspension. The resulting mixture was then transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed tightly, and heated at 200 oC for 10 h. After reaction, the Teflon-lined stainless steel autoclave was naturally coolled to room temperature. The resulting black precipitates were collected by centrifugation, and successively rinsed with ethanol and ultra-pure water for several times, and then dried in an oven at 70 oC for 24 h. The influence of CB content on the electrocatalytic performance of the MoSe2/CB composites was investigated by adding different weight percentage of CB in the reaction system (5, 10, 20, 30, 40, 50, 60, 70 and 80 wt %). The corresponding MoSe2/CB samples were labeled as MoSe2/CB-x%, where x = 5, 10, 20, 30, 40, 50, 60, 70 and 80 respectively. Pure MoSe2 particles were prepared by the same procedure but without adding CB in the reaction system. Electrochemical Measurements. All the electrochemical measurements were performed at room temperature in a three-electrode cell where a Pt foil and a saturated calomel electrode (SCE) was used as the counter and the reference electrodes, respectively. The catalyst modified glassy carbon (GC) electrode (5mm diameter) was used as the working electrode. The GC electrode was polished to a mirror finish and thoroughly cleaned before use. Details for the preparation of the working electrode were described in the Supporting Information. All electrochemical measurements were performed on the CHI660A workstation (CH Instruments Co.). Linear sweep voltammetry (LSV) was conducted in 0.5 M H2SO4 solution (pH = 0) 6
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with a scan rate of 5 mV·s-1. Electrochemical impedance spectroscopic measurements were performed at -0.45 V vs. SCE with an AC amplitude of 5 mV and a frequency region of 100 kHz to 0.02 Hz. All potentials were reported relative to the reversible hydrogen electrode (RHE) according to the following equation: E (V vs. RHE) = E (V vs. SCE) + 0.244 + 0.0591pH. Characterization. Field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Japan) were employed to characterize the morphology and microstructure of the products. The SEM images were obtained at an acceleration voltage of 10 kV and the TEM and high resolution (HRTEM) images were obtained at an accelerating voltage of 200 kV. The crystal structure of all samples was characterized by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer with filtered Cu Kα (λ = 1.5405 Å) radiation. X-ray photoelectron spectroscopic (XPS) analyses were performed on a PHI Quantera SXM scanning X-ray microprobe (ULVAC-PHI, Japan) using a monochromatic Mg Kα source (1253.6 eV) under a base pressure of
