Elucidation of the Mechanism of Dioxygen Reduction on Metal-Free Carbon Electrodes

On a glassy carbon (GC) surface covered with a covalently bonded methylphenyl monolayer, reduction to superoxide was observed and shown to be chemically reversible above pH 10. The voltammetry is completely explained by electron tunneling through the organic monolayer, then degradation of in aqueous solution by known homogeneous mechanisms. As the pH is decreased below about 10, in solution decays to and by routes previously deduced from pulse radiolysis experiments. In contrast, a GC surface cleaned with isopropanol and activated carbon is very active toward adsorption, and a two electron reduction to peroxide is observed. An adsorbed intermediate is proposed to be a surface hydroperoxide analogous to stable organic peroxides of the general formula ROOH. On clean, unmodified GC, the pH dependence of the reduction mechanism is consistent with control of the reduction process by adsorbed or . In the absence of adsorption sites on the carbon surface, degradation of electrogenerated superoxide occurs entirely in solution. © 2000 The Electrochemical Society. All rights reserved. ©2001 American Geophysical Union