ELECTROCATALYSIS OF OXYGEN REDUCTION AT FE3O4 OXIDE ELECTRODES IN ALKALINE-SOLUTIONS

The reduction of O2 at Fe3O4 cathodes in alkaline solutions takes place at electrode potentials where partial reduction of the hydrated Fe(III) surface layer occurs. Simultaneous electroreduction of the surface Fe(III) oxide and dioxygen suggests that octahedrally coordinated surface ferrous ions [Fe(II)OH-], are the active sites for oxygen electrocatalysis as proposed earlier for passive iron. The electroreduction of O2 on Fe3O4 cathodes is a two-electron process: O2+H2O+2e- --> HO2-+HO-Further decomposition of the intermediate peroxide is catalysed by reduced magnetite with a high turnover: 2HO2- --> O2+2HO-As a consequence of oxygen recycling the apparent number n* of electrons exchanged per oxygen molecule is strongly dependent on mass transport conditions. These results are relevant to understanding the mechanisms of reactions involved in the overall complex corrosion processes of ferrous alloys in aqueous aerated environments and in atmospheric corrosion.