Mathematical Modeling of the Ni(OH)₂/NiOOH Reaction on a Nickel Rotating Disk Electrode: A Foundation for Studying Organic Electrooxidation

Source abstract

Abstract Nickel is a low-cost, widely available metal well suited to large-scale applications such as wastewater treatment, sensors, and electrosynthesis, where the electrochemically active species is a thin Ni(OH) 2 /NiOOH surface layer. Existing models are largely battery-oriented, limiting their transferability across these applications. Here, the Ni(OH) 2 /NiOOH reaction was modeled on a nickel rotating disk electrode (RDE) to simulate cyclic voltammetry, combining species conservation equations, Butler–Volmer kinetics, and a Nernst expression with composition-dependent activity coefficients, with kinetic and thermodynamic parameters fitted to experimental data. Hysteresis between oxidation and reduction was captured by assigning distinct equilibrium-potential parameters to the forward and reverse scans. A kinetics-limited Surface Model was compared to a proton-diffusion-limited Layer Model using a block permutation test, which found no statistically significant difference in fit quality; the Surface Model was therefore preferred for its simpler structure and better-constrained parameters. The dominant factor in reproducing experimental voltammograms was the description of the activity coefficients in the equilibrium-potential expression; among several candidates, a Wohl expansion using four proton–proton self-interaction coefficients was selected. Model predictions were extended across 0.1–1.0 M NaOH by introducing an empirical correction that made the maximum concentration of electroactive material depend on surface OH⁻ concentration. Simulated and experimental voltammograms were both insensitive to rotation speed, indicating that OH⁻ transport does not limit the reaction at 100 mV s − 1 . Because the RDE geometry defines mass transport unambiguously, the model provides a foundation, extensible to organic substrates, for mathematical modeling of organic electrooxidation on nickel electrodes.