Challenges in determining the electrochemically active surface area of Ni-oxides in the oxygen evolution reaction

Abstract

Thin films of Ni-oxy-hydroxides of different composition and thickness were electrochemically grown on Ni foil electrode: hydrated α-phase, anhydrous β-phase, and two mixed α- and β-phase electrodeposited by potential cycling. The characterization of bare Ni and the deposited films by cycling voltammetry (CV) and electrochemical impedance spectroscopy (EIS) provided data of the initial electrochemically active surface area (ECSA), an amount of Ni-active oxide species, double-layer capacitance, ionic adsorption capacitance and the adsorption pseudocapacitance in the oxygen evolution reaction (OER) potential range. Ni-oxy-hydroxide films were deposited with loading up to 18 monolayers or 25 nmol cm−2. The specific pseudocapacitance was calculated from a linear correlation of the capacitance and the film thickness. Steady-state polarization curves of OER were presented as specific activity obtained by normalization of the currents by ECSA derived from the maximum adsorption pseudocapacitance. Also, polarization curves were given as turnover frequency calculated from the amount of Ni-active oxide species derived from CV. Both method of evaluation of intrinsic activity of Ni-oxy-hydroxide films towards OER gave consistent results and can be used to compare the activity of similar Ni-oxide catalysts.