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Sulfur Decorated Molybdenum Carbide Catalysts for Enhanced Hydrogen Evolution Chaoyun Tang, Wei Wang, Aokui Sun, Chengkang Qi, Dezun Zhang, Zhuangzhi Wu, and Dezhi Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01803 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015
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Sulfur Decorated Molybdenum Carbide Catalysts for Enhanced Hydrogen Evolution Chaoyun Tanga, b, Wei Wangc, Aokui Sun a, Chengkang Qia, Dezun Zhang a, Zhuangzhi Wua, b*, Dezhi Wang a, b* a
School of Materials Science and Engineering, Central South University, Changsha 410083,
China b
Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering,
Central South University, Changsha 410083, China c
School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China
ABSTRACT: A highly active and stable electrocatalyst (MoSx@Mo2C) for hydrogen evolution is developed via the sulfur decoration of molybdenum carbide for the first time. Although the decoration of sulfur reduced the electrochemically active surface area and slightly enlarged the impedance resistance of Mo2C substrates, the turnover frequency (TOF) was remarkably enhanced, resulting in the great improvement of the final hydrogen evolution reaction (HER) activity. More importantly, there is a synergistic effect between MoSx and Mo2C, making the MoSx@Mo2C catalyst exhibit an excellent activity with a small Tafel slope of 44 mV dec-1, which is among the best records for the Mo2C-based catalysts.
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KEYWORDS: Molybdenum carbide, Molybdenum disulfide, Sulfuration, Synergistic effect, Hydrogen evolution reaction. ■ INTRODUCTION
The increasing energy demand caused by rapid development along with the environmental crisis has prompted the exploitation of hydrogen.1 Sustained hydrogen production from water splitting is one of the most attractive methods.2, 3 Platinum-based catalysts have been considered to be the most effective hydrogen evolution reaction (HER) electrocatalysts, but the high cost and lack of abundance limited their scalable industrial applications.4-6 As a result, more alternatives have been widely explored, including transition metal carbides, sulfides, nitrides, phosphides, and so on. Among these, Molybdenum carbide (Mo2C) has also been proposed as a highly effective electrocatalyst for HER under both acidic and basic conditions.7 And great efforts have been made over the component and structure design of Mo2Cbased catalysts to further enhance the final HER activity.8-11 Cui et al synthesized Mo2C nanoparticles decorated graphitic carbon sheets via a biopolymer-derived solid-state reaction, exhibiting a high HER activity with a Tafel slope of 62.6 mV dec-1.8 Liao et al developed a nanoporous Mo2C nanowire with a large surface area of 63.9 m2 g-1, resulting in a Tafel slope of 53 mV dec-1.9 Zhang et al have successfully synthesized ultrathin MoS2 nanosheets on Mo2Cembedded and N-doped carbon nanotubes, showing a Tafel slope of 69 mV dec-1.10 Ge et al even compared the HER activity between Mo2C nanosheets and nanowires, and found that the latter showed better activity with smaller Tafel slope of 55.8 mV dec-1.11 Despite of great efforts, more work is still strongly required to further optimize the Mo2C-based catalysts, especially over the functional design of components and structures.12
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As typical HER electrocatalysts, molybdenum sulfides have been widely reported due to the excellent HER activity, including crystalline MoS2 and amorphous MoSx.13-17 However, molybdenum sulfides show poor electric conductivity between stacking layers, greatly limiting the charge transport and hence efficiency of electrocatalysis.18 Therefore, highly conductive supports with a large effective surface area are technically desirable to improve the electrocatalytic activity of molybdenum sulfides.19-21 Considering the optimization of conductivity and active sites simultaneously, in this work, we propose a functional design: sulfur decorated molybdenum carbide (MoSx@Mo2C) nanostructures (Scheme 1), which mitigate the deficiencies of separated components, exhibiting superior HER activity with a small Tafel slope of 44 mV dec-1 and a large current density of 178 mA cm-2 obtained at η=-400 mV. Different from the previous designs of MoS2/MoO3 and MoSx/Ni, in which the substrates show no obvious activity toward HER, the current MoSx@Mo2C adopts a highly conductive and active catalyst of Mo2C as the support, providing excellent charge transfer ability and more extra active sites.22, 23 Meanwhile, the supported MoSx exposes more efficient active sites due to the large surface area of the Mo2C substrate (120 m2 g1
), 24 resulting in the enhancement of HER functions.
Scheme1. Formation process and structure mode of MoSx@Mo2C.
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■ EXPERIMENTAL
1. Preparation of Mo2C: Ammonium heptamolybdate ((NH4)6Mo7O4·4H2O) and glucose with a molar ratio of 1: 20 were dissolved into 50 ml distilled water to form a homogeneous solution. After being stirred for 10 min, the mixture solution was transported into a 100ml Teflon-lined stainless autoclave, heated up to 473 K and maintained for 10 hours to get a mixture precursor. Followed by centrifugation and drying, a black precursor can be collected. Then, the precursor was heated in a tube furnace with a rate of 5 K min-1 from 297 K to 1073 K and kept for 3 hours in the atmosphere of Ar. To remove the carbon residue, a successive H2 flow was also provided for another 20 min. Finally, black Mo2C can be obtained. 2. Preparation of MoSx@Mo2C: 0.2 g as-prepared Mo2C and different amounts of thioacetamide (TAA) associated with different Mo/S molar ratios (1: 2, 1: 8, 1: 16, 1: 24 and 1: 32) were mixed in 50 ml distilled water. After ultrasonication for 30 min, the solution was transported into a 100 ml Teflon-lined stainless autoclave, and maintained at 473 K for 10 hours. The final products were obtained by centrifugation, washing with distilled water and ethanol for several times, and drying at 313 K for 6 hours. 3. Preparation of contrast samples: For comparison, pure MoS2 was also prepared following the same procedure using (NH4)6Mo7O4·4H2O as the raw material with the same amount of Mo molar weight to replace the as-prepared Mo2C. Meanwhile, certain amount of active carbon (Vulcan XC-72) was added in
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the same procedure as substrates of pure MoS2 to prepare the MoS2 supported on active carbon (MoS2@C). Moreover, the as-prepared pure MoS2 and pure Mo2C were milled and physically mixed to form the physical mixture of MoS2 and Mo2C (MoS2+Mo2C-PM) with the same MoS2/Mo2C mole ratio as that in the MoSx@Mo2C-1: 8. 4. Preparation of electrodes: Typically, 3 mg of catalyst and 80 µl of 5 wt% Nafion solution were dispersed in 1 ml of a solution of deionized water and ethanol (4:1 in volume ratio). After stirring by ultrasonication for 30 min, 5 µl catalyst slurry was dropped onto smooth glassy carbon electrodes with a diameter of 3 mm and dried in air. The mass loading was about 0.213 mgcm-2. 5. Characterizations: X-ray diffraction (XRD) patterns were recorded using D/max-2500 system with a Cu Kα irradiation source (λ=0.154 nm). Raman spectroscopy was recorded using the instrument LabRAMHR-800 of French company HORIBA with an excitation wavelength of 633 nm. The morphology and microstructure were observed by the transmission electron microscope (TEM, Tecnai G220). Moreover, X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was also adopted to analyze the surface compositions. The S content of samples was tested using carbon sulfur analyzer (American LECO CS-444). 6. Electrochemical measurements: All the electrochemical measurements were performed on an electrochemical workstation (CHI 660E). For determination of activity toward HER for various catalysts, a three-electrode electrochemical cell (i.e., half-cell) was employed, and linear sweep voltammetry (LSV)
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measurements were conducted from 0.1 V to -0.4 V at room temperature in 0.5 M H2SO4 with a scan rate of 2 mV s-1 using saturated calomel electrode (SCE) as the reference electrode and glassy carbon as the counter electrode. Before the electrochemical measurements, the electrolyte solution was purged with N2 for 1 h to remove the oxygen completely, and stable polarization curves were recorded after 20 cycles. The onset potential was determined as the potential required to reach a specific current density of 1 mA cm-2.The electrochemical impedance spectroscopy analysis (EIS) was carried out from 1 MHz to 1 Hz at -100 mV,-150 mV and -200 mV with an amplitude of 5 mV, respectively. 7. Electrochemically active surface area: To measure the electrochemical capacitance, the potential was swept between 0 to 0.3 V vs RHE three times at each of seven different scan rates (20, 40, 60, 80, 100, 120 and 140mVs-1). We measured the capacitive currents in a potential range where no faradic processes are observed, i.e. at 0.15 mV vs. RHE. The specific capacitance can be converted into an electrochemically active surface area (ECSA) using the specific capacitance value for a flat standard with 1 cm2 of real surface area. The specific capacitance for a flat surface is generally found to be in the range of 20-60 µF cm-2. Because the surface area normalized capacitance associated with double layer charging is expected to be similar (i. e, within an order of magnitude) for many metallic and semiconducting materials in the same aqueous electrolyte, the resulting surface area estimated for our work presented here is accurate with an order of magnitude or better.25 In the following calculations of TOF, we assume 40µFcm-2 as a moderate value.26 Calculated electrochemical active surface area:
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A =
specific capacitance 40 μF cm per cm
8. Turnover frequency calculations: To calculate the per-site turnover frequency (TOF), we used the following formula: #total hydrogen turnovers/cm geometric area TOF = #active sites/cm geometric area The total number of hydrogen turnover was calculated from the current density according to #%&
mA 1 C s 1 mol e 1 mol H = 'j ) * .' )' ) cm 1000 mA 96485.3 C 2 mol e
6.022 × 108 H molecules H /s mA * . = 3.12 × 10-9 per 1 mol H cm cm Since the exact hydrogen binding site is not known, the Mo atoms as the active sites don’t change after sulfuration process. Based on the roughness factor together with the unit cell (volume=37.2 Å3) of the Mo2C crystal structure in the case of Mo2C and MoSx@Mo2C. A similar approach was used to estimate TOF for Mo2C and MoSx@Mo2C. Actually, due to the existence of MoS2 which was reported to have a lower surface site density (1.164× 27 10-9 atoms cm :;?@AB F;== I CDE? ÅI 8G. F;== CDE?
. =1.42× 10-9 atoms cm :;
