Behaviour of surface intermediate states in anodic O2 evolution electrocatalysis at Co3O4 on Ni and Ti substrates

Abstract Determination of the behaviour of the adsorbed species that are the kinetically‐involved intermediates in multi‐step Faradaic reactions of the electrocatalytic type has remained an undeveloped area of electrode kinetics, yet is important in the evaluation of electrocatalysis and reaction mechanisms of electrode processes as a function of current‐density ( i ) and electrode potential ( V ). — From values of the complementary derivatives d V /d log i , A V ( t )/d log t and d V ( t )/d log —d V ( t )/d t , obtained from potential‐relaxation measurements, the electrochemical behaviour and change of steady‐state coverage with potential of the intermediate states in the OER at Co 3 O 4 is derived. The behaviour observed is related to changes of oxidation state of the bulk oxide and its surface region. — The observed potential‐relaxation and a.c. impedance behaviour is different at Co 3 O 4 on Ni and Ti substrates. It is clearly shown that this effect arises from the additional component of impedance at the interface which is introduced by the presence of a thin underlying oxide film of TiO 2 on the Ti, on which the Co 3 O 4 film is formed. These observations have practical interest regarding preparation of composite electrocatalytic surface films on Ti substrates. Over most of the potential range studied, however, the mechanism and electrode kinetic characteristics of the OER are the same for the two metal substrates. — The O 2 evolution kinetics and associated potential‐relaxation behaviour on open‐circuit, following current interruption, are determined by two pseudo‐capacitances: one arising from the potential‐dependent state of oxidation of metal ions in the oxide film and a second, coupled contribution from the potential‐dependence of coverage by OH/O intermediates on the oxide film.