Elucidation of the mechanism of dioxygen reduction on metal-free carbon electrodes

On a glassy carbon (GC) surface covered with a covalently bonded methylphen yl monolayer, O-2 reduction to superoxide was observed and shown to be chem ically reversible above pH 10. The voltammetry is completely explained by e lectron tunneling through the organic monolayer, then degradation of O-2(.- ) in aqueous solution by known homogeneous mechanisms. As the pn is decreas ed below about 10, O-2(.-) in solution decays to O-2 and H2O2 by routes pre viously deduced from pulse radiolysis experiments. In contrast, a GC surfac e cleaned with isopropanol and activated carbon is very active toward adsor ption, and a two electron reduction to peroxide is observed. An adsorbed in termediate is proposed to be a surface hydroperoxide analogous to stable or ganic peroxides of the general formula ROOH. On clean, unmodified GC, the p H dependence of the O-2 reduction mechanism is consistent with control of t he reduction process by adsorbed O-2(.-) or (O2H)-O-.. In the absence of ad sorption sites on the carbon surface, degradation of electrogenerated super oxide occurs entirely in solution. (C) 2000 The Electrochemical Society. S0 013-4651(99)12-021-4. All rights reserved.