The Oxygen Evolution Reaction on Passive Oxide Covered Transition Metal Electrodes in Alkaline Solution. Part III – Iron.

The kinetics of the oxygen evolution reaction (OER) at passive oxide covered polycrystalline Fe electrodes in aqueous alkaline solution were examined using both dc steady state polarisation and ac impedance techniques. It proved difficult to obtain reproducible polarisation data for bright anodes, and so an electrochemical pre-treatment routine was devised. Upon ageing of a given electrode specimen, and with application of the pre-treatment regime before each experiment, it was possible to determine reproducible values of the Tafel slope and the reaction order with respect to OH- ion activity. The link between OER polarisation behaviour and the changing nature of the passive oxide film, with repeated electrode pre-treatment and experimental utilisation, was characterised by cyclic voltammetry. As was previously found for oxidised Co electrodes, it is necessary to admit a dual energy barrier model in order to rationalise some of the experimental Tafel slope values. By considering the kinetic data presented here, in tandem with complimentary results obtained for extensively aged and also for multicycled iron electrodes, a preferred reaction pathway emerges. The electrode active surface area for the OER has been been estimated using the same current transient decay technique that we have applied to Ni and Co anodes. On this basis, the “real” surface area normalised activity series for the OER amongst the passive oxides of these first row transition metals is outlined.