IMPEDANCE OF ANODICALLY PASSIVATED FE80B20 OVER POTENTIALS FROM PASSIVE STATE TO OXYGEN EVOLUTION

The electrochemical impedance of anodically grown passive films on gla ssy metal Fe80B20 in berate buffered solutions, was measured in the ra nge between +0.50 V vs. see, the potential of surface film formation, and + 1.10 V, which is deep in the range of the oxygen evolution react ion (OER). By modelling, numerical fitting, and validation procedure, equivalent circuits in particular ranges of potentials were establishe d. The values of elements of these were determined and their potential dependence analysed. Results of both the experimental and modelling p rocedures show that: (i) at passive potentials, between +0.50 and +0.7 0 V, the electrode impedance exhibits pure capacitative behaviour due to the non-separable film, and Helmholtz double-layer capacitance, wit h a low ion transfer anodic current density, i(a); (ii) at E greater t han or equal to +0.75 V, the pure capacitance model is complemented by surface (adsorbed) state impedance; (iii) at potentials between +0.95 and + 1.05 V the impedance data and a steep increase in i(a) point to wards two-step kinetics of the OER, in which surface (adsorbed) states mediate electron transfer. Comparison of impedance data for Fe80B20 w ith those obtained for Fe indicates a lower rate constant for the form ation of surface (adsorbed) states, a different control step and a low er electrocatalytic efficiency for the OER on Fe80B20.