Enhancing Hydrogen Evolution Reaction by Controlling Chemical States Through Electrochemical Reduction of Raney Nickel in Alkaline Water Electrolysis

Porous electrocatalysts based on nickel (Ni) have found extensive application in alkaline water electrolysis for the hydrogen evolution reaction (HER). The selective leaching of aluminum (Al) from Ni-Al alloys is widely employed as a prominent approach to increase the surface area of porous Ni during fabrication, resulting in high surface area in Ni(OH)₂. However, the Ni(OH)₂ hampers the HER due to its weak hydrogen adsorption energy. To enhance the hydrogen adsorption energy, it is essential to control the chemical state of the Ni by incorporating metallic Ni, which exhibits strong hydrogen adsorption characteristics. In this study, we controlled the chemical states of Ni through plasma vapor deposition (PVD), heat treatment, chemical leaching, and electrochemical reduction. The phase evolution of the electrocatalysts during fabrication was confirmed using a range of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). We revealed that the heat-treated Ni-Al alloy, possessing a thick Ni₂Al₃ surface layer, underwent selective Al leaching, resulting in the formation of biphasic interfaces comprising Ni(OH)₂ at the grain and NiAl IMCs near the grain boundary of the outermost surface layer. X-ray photoelectron spectroscopy (XPS) analysis confirmed that the coupled oxidation of the NiAl IMCs facilitated the partial reduction of Ni(OH)₂ to Ni(OH)₂/Ni at the grains during electrochemical reduction. The electrocatalyst containing partially reduced Ni(OH)₂/Ni exhibited an overpotential of 54 mV at 10 mA/cm², and in single-cell operation, it demonstrated a cell voltage of 1.675 V at 0.4 A/cm² at 50 °C.