Hydrous Nickel Oxide: Redox Switching and the Oxygen Evolution Reaction in Aqueous Alkaline Solution

The redox switching reaction and the oxygen evolution reaction at multi-cycled nickel oxy-hydroxide films in aqueous alkaline solution are discussed. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. The dynamics of redox switching has been quantified in terms of a diffusive frequency using the Laviron-Aoki diffusion model. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution with slopes of ca. 60 mVdec -1 and ca. 120 mVdec -1 at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity remains invariant at ca. 1.0 as the potential is increased. These observations are rationalized in terms of a kinetic scheme involving surfaquo groups.