Solar thermal decoupled water electrolysis process III: The anodic electrochemical reaction in a rotating disc electrode cell

The electrochemical oxidation of Co²⁺ is studied at 45 °C using a rotating disc electrode to elucidate the impacts of fluid motion and solid Co³⁺ product formation on the anode reaction rate. The electrolyte is 40% KOH saturated with Co²⁺ and the anode is nickel. Inducing laminar flow with rotation at speeds up to 2500 RPM is shown to increase the current density from <1 mA cm^-2 to 2–5 mA cm^-2 at potentials greater than −0.21 Volts vs. Ag/AgCl. At higher current densities anticipated for commercial application, electrode passivation is a relevant concern. However, bulk electrolysis and cyclic voltammetry—with the latter interpreted using a reaction model to account for the fluid motion—demonstrate that the solid Co³⁺ deposit is not passivating, but electrochemically active. Deposits up to ≈1 mm thick increase the current, with a sixfold increase demonstrated at 2000 RPM.