On the Electrochemical Reduction of β-Ni(OH)2 to Metallic Nickel

Abstract

Nickel (Ni) and its (oxy)hydroxide species are an important class of materials that attract significant attention from the scientific community due to their application in alkaline electrochemical energy technologies. Despite their increasing importance, Ni electrochemistry and electrocatalysis are not very well understood. In this article, we contribute to the understanding of the interfacial electrochemical phenomena occurring at polycrystalline Ni electrodes by conducting the first ever systematic analysis of the electrochemical reduction of β-Ni(OH)2. The procedure for the reduction of surface β-Ni(OH)2 is as follows. First, a cyclic voltammetry (CV) measurement is performed in the potential region of β-Ni(OH)2 formation to grow a reproducible hydroxide layer, followed by a linear sweep voltammetry (LSV) measurement conducted between an upper limit potential of Eu = 0.00 V and a lower limit potential (El) ranging between −0.15 V and −0.60 V. The LSV measurement is carried out using very low potential scan rates (s = 0.10, 0.20, 0.50, and 1.0 mV s−1). The analysis of the electrochemical reducibility of the surface β-Ni(OH)2 layer is accomplished by comparing the CV transient of a freshly polished Ni electrode and the CV transient recorded after the LSV experiment that is designed to reduce the β-Ni(OH)2 layer. The influence of different experimental parameters of the above-described procedure is analyzed independently, namely, (i) the lower potential limit of the LSV measurement, (ii) the potential scan rate of the LSV measurement, (iii) the direction of the LSV measurement (anodic or cathodic), (iv) the thickness of the surface layer of β-Ni(OH)2, and (v) the cathodic polarization of the electrode for different polarization times. Our findings demonstrate that, contrary to an existing belief, the reduction of β-Ni(OH)2 can be achieved electrochemically. Features characteristic of the formation and reduction of α-Ni(OH)2 are observed in the CV transients acquired after employing the procedure for the electrochemical reduction of β-Ni(OH)2, indicative that a metallic Ni surface is regenerated. The analysis of the charge density value of the anodic peak in the CV transient also points to the electrochemical reduction of β-Ni(OH)2 to metallic Ni. These findings lead to an updated version of the Bode diagram and might have significant implications for alkaline electrochemical energy technologies using Ni-based materials.