Amorphous-Ni(OH)<sub>2</sub> on a Vertically Grown Lamellar Ag-Modified MoS<sub><i>x</i></sub> Thin-Film Electrode with Surface Defects for Hydrogen Production in Alkaline Solutions
Hairus Abdullah, Noto Susanto Gultom, Chiao-Chen Hsu, Hardy Shuwanto, Dong–Hau Kuo
Abstract
A low-temperature radio-frequency-sputtered Ag-modified MoSx lamellar thin-film electrode with a home-made target was deposited on a carbon-cloth substrate and electrochemically modified with amorphous Ni(OH)2. All the as-prepared thin-films’ properties were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and electrochemical measurements. Electrochemical measurements of a-Ni(OH)2/Ag-modified MoSx lamellar thin films exhibited promising hydrogen evolution reaction properties in alkaline solutions. The linear sweep voltammetry and electrochemical impedance spectroscopy measurements indicated a relatively low Tafel slope of 95.3 mV/dec and a low electron-transfer resistivity of 12.26 Ω, respectively. The observed overpotentials were 0.12 and 0.26 V at 10 and 100 mA, respectively. Furthermore, cyclic voltammetry measurements revealed that the thin-film electrode had the highest double-layer capacity (Cdl) of 11.3 mF/cm2, which also implied a highly active electrochemical surface area. Finally, the stability of a-Ni(OH)2/Ag-modified MoSx thin films was examined with a constant current density of 10 mA/cm2 for 20 h. It was found that the non-stoichiometric MoSx played an essential role in the water-activation process due to the presence of sulfur-vacancy sites (VS2+). During the electrochemical process, VS2+ acts as an active site to pin water and weaken the OH–H bonding, in which the water-originated OH is further bonded with periodically terminated surface defect of Ni(OH)x on the electrode. The defected Ni(OH)x on the catalyst surface helps the dissociation of OH bonding in water adsorbed on Ag-MoSx to generate H* intermediates for further Heyrovsky steps. The low-cost thin-film electrode with a relatively low overpotential is promising for industrial applications based on the experimental data.