Study on the anodic reaction of Ni in an alkaline solution by transient pH detection based on scanning electrochemical microscopy (SECM)

The anodic reaction of Ni in an alkaline solution was studied by the tip-substrate voltammetry mode of scanning electrochemical microscopy (SECM) and cyclic voltammetry (CV). A platinum microdisc electrode was selected as the tip electrode, which functioned as a pH sensor with transient response capability. The pH value of the solution near the Ni electrode surface varied while the Ni substrate oxidation reaction occurred, and the pH variation could be detected by the tip faradic current. The cyclic voltammogram results showed that two types of hydroxides: i.e. alpha-Ni(OH)(2) and beta-Ni(OH)(2) were formed during Ni oxidation in the lower potential region. In the proceedings of alpha-Ni(OH)(2) -> gamma-NiOOH and beta-Ni(OH)(2) -> beta-NiOOH, the process of OH- concentration decrease in the solution was ahead and behind of electron transfer in the solid phase, respectively. These results indicate that the OH- adsorption process occurs as an elementary step in the former reaction and the H+ diffusion process from the inner to the outer layer of the solid phase occurs as a subsequent step in the latter reaction. The results also revealed that the oxide film on the Ni surface has a two-layer structure. The real potential of the oxygen evolution reaction (OER) on the Ni surface with different cycles is also analyzed in the paper. Copyright (C) 2007 John Wiley & Sons, Ltd.