The Oxygen Evolution Reaction at Hydrous Iron Oxide Films in Base: Kinetics and Mechanism

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials, respectively, with the former reducing to 40 mV dec(-1) for very thick films. A reaction order of ca. 1.0 is observed irrespective of overpotential and film thickness. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution.