The significance of electrochemical impedance spectra recorded during active oxygen evolution for oxide covered Ni, Co and Fe electrodes in alkaline solution

Abstract

Although the mechanism for the oxygen evolution reaction (OER) is likely to be similar for oxidised electrodes of the adjacent transition metals Ni, Co and Fe, the electrochemical impedance spectra, recorded for each of these anodes during oxygen evolution in 1.0 M NaOH, are significantly different in appearance. This impedance data is analysed using an approach similar to that of Cahan and Chen [B.D. Cahan, C.T. Chen, J. Electrochem. Soc. 129 (1982) 700?705] in an earlier important article on this topic. A controversial proposal by these workers that OER ?Tafel slopes? (measured by either ac or dc techniques) for many electrode systems are not in fact characteristic of the kinetics of the reaction, but instead of the potential dependent conductivity of the oxide, is dismissed on the basis of our data. In the case of Co however, it is found that the kinetics of the OER are masked by a dominant low frequency pseudo-capacitance, arising most likely from a non-homogeneous surface charge distribution associated with the oxidation of Co(III)?Co(IV) oxy-hydroxide species. A consistent interpretation of the data is provided, which envisages that Fe exhibits the most general impedance response with the spectra for the other anodes taking the form of limiting cases. Consideration is also given to the most appropriate choice of equivalent circuit model (with a firm physical basis) to aid our understanding of the complex interface at which oxygen evolution proceeds.