Electrochemical partial reduction of Ni(OH)2 to Ni(OH)2/Ni via coupled oxidation of an interfacing NiAl intermetallic compound for robust hydrogen evolution

Abstract

Ni-based porous electrocatalysts have been widely used in the hydrogen evolution reaction (HER) in alkaline water electrolysis, and the catalysts are produced by selective leaching of Al from Ni-Al alloys. It is well known that chemical leaching of Ni-Al intermetallic compound (IMC) generates a high surface area in Ni(OH)2. However, the Ni(OH)2 produced by leaching the Ni-Al intermetallic compound retards the hydrogen evolution reaction, which is attributed to its weak hydrogen adsorption energy. In this study, we controlled the chemical state of Ni using plasma vapor deposition (PVD) followed by heat treatment, selective Al leaching, and electrochemical reduction. X-ray diffraction (XRD), scanning microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) were used to confirm the phase evolution of the electrocatalysts during fabrication. We reveal that the heat-treated Ni-Al alloy with a thick Ni2Al3 surface layer underwent selective Al leaching and produced biphasic interfaces comprising Ni(OH)2 and NiAl IMCs at the edges of the grains in the outermost surface layer. Coupled oxidation of the interfacing NiAl IMCs facilitated the partial reduction of Ni(OH)2 to Ni(OH)2/Ni in the grains during electrochemical reduction, as confirmed by X-ray photoelectron spectroscopy (XPS). An electrocatalyst containing partially reduced Ni(OH)2/Ni exhibited an overpotential of 54 mV at 10 mA/cm2 in a half-cell measurement, and a cell voltage of 1.675 V at 0.4 A/cm2 for single-cell operation. A combined experimental and heoretical study (density functional theory calculations) revealed that the superior HER activity was attributed to the presence of partially reduced metallic Ni with various defects and residual Al, which facilitated water adsorption, dissociation, and finally hydrogen evolution.