How Nickel Oxidizes in Alkaline Media: The Initial Stages of Nickel (Hydr)oxide Formation

Oxygenated species formed by the anodic oxidation of nickel play a central role in diverse electrochemical technologies from passivation to electrocatalysis. Four mechanisms of hydroxide formation have been proposed in the literature: direct OH adsorption, place-exchange, dissolution-precipitation, and nucleation-and-growth, but the dominant pathway remains unresolved. Here, we probe the initial stages of Ni oxidation using three complementary electrochemical experiments, supported by DFT calculations and ToF-SIMS surface characterization. We track Ni pseudo-capacitance as a function of scan rate to resolve processes across different time scales. Next, we examine voltammograms recorded with varying anodic vertex potentials, showing that the cathodic charge increases with the extent of prior oxidation. Finally, potential holds of different durations reveal the time-dependent redistribution of cathodic features, supporting the presence of multiple oxidized surface species.