Electrocatalytic Oxygen Evolution on Electrochemically Deposited Cobalt Oxide Films: Comparison with Thermally Deposited Films and Effect of Thermal Treatment

Abstract

Electrocatalytic cobalt oxide layers have been prepared on nickel substrates using thermal decomposition and electrochemical deposition methods. Importantly, it was confirmed that the electrochemical deposition method could be applied to nickel foam substrates for use in zero-gap alkaline water electrolysis cells. The oxide layers produced were then investigated for their activity towards the oxygen evolution reaction in 30 wt % KOH solution and found to be superior compared with the uncoated nickel substrate. Layers produced by both methods had similar electrochemical behaviour, provided that the layers were annealed at temperatures ≥350 ∘C. This thermal treatment was required to mechanically stabilise the electrochemically deposited cobalt oxide layer. Due to this finding, the effect of annealing temperature was investigated for the electrochemically deposited layer, and it was found that the overpotential for oxygen evolution increased with increasing annealing temperature. Using cyclic voltammetry and impedance spectroscopy, it is concluded that the decrease in performance with increasing annealing temperature is largely caused by the corresponding decrease in active surface area. However, for annealing temperatures ≥400 ∘C, additional resistances are introduced that cause lower performance. The impedance data suggest that these additional resistances are caused by either a decrease in the conductivity of the cobalt oxide layer itself, or the formation of a passivating-like nickel oxide layer between the active cobalt oxide and the nickel substrate, or both. The resistances’ dependence on potential suggests that they originate from a semi-conducting material and these additional resistances ultimately give rise to non-linear Tafel behaviour.