Surface electrochemical transformations on spinel iron oxide electrodes in aqueous solutions

The effect of potential and electrolyte composition on the surface stoichiometry in magnetite oxide electrodes has been investigated by a variety of electrochemical techniques: open-circuit potential, chronoamperometry, surface redox titration by galvano-static pulses, electrochemical impedance spectroscopy and electroreflectance. These studies revealed a continuous change in surface iron composition with potential that results in a low-frequency, potential-dependent pseudo-capacitance. The capacitance–potential curves for different electrolyte pH were analysed in terms of: (a) the iron spinel system Fe3–δO4(0 ⩽δ⩽ 0.33) and (b) a modified Fe3O4 electrode with octahedral surface iron complexes, to accommodate the change of surface stoichiometry. The second model is consistent with the experimental data and the resulting dependence of FeII surface sites on potential and pH predicts the kinetics of O2 electroreduction on Fe3O4. Similar electrochemical behaviour of oxidized magnetite electrodes and passive iron in alkaline solutions has been found, which parallels the analogous kinetics observed for oxygen electroreduction on these oxides.