Mediation at High Potentials for the Reduction of Oxygen to Water by Cobalt Porphyrin-Quinone Systems in Porous Aerogel Carbon Electrodes

Reduced p-, m-, and o-benzoquinones: hydroquinone (HQ), resorcinol (Res), and catechol (Cat), undergo irreversible monolayer adsorption in aerogel carbon (AEC) electrodes with rates of 1.7 x 10(-4), 7.1 x 10(-5), and 1.4 x 10(-4) s(-1) for HQ, Res, and Cat, respectively. The adsorbed species showed electrochemical quasi-reversible behavior in 1 M H(2)SO(4) with half-wave potentials (E(1/2)) of +0.45, +0.31, and +0.58 V vs Ag/AgCl/KCl (saturated) for AEC/HQ, AEC/Res, and AEC/Cat, respectively. Upon adsorption of Co (III) tetra(p-sulfonatophenyl)porphyrin in these electrodes, a single reduction wave was observed and its E(1/2)(similar to+ 0.45 V) was independent of the nature of the adsorbed reduced quinone. This was attributed to a metalloporphyrin/quinone complex, which formed and stabilized at the electrode surface. This species, after being reduced, reacted with oxygen with a rate of 1.8 x 10(5) M(-1) s(-1). Mediation of oxygen reduction by these systems occurred at a relatively high potential (similar to+ 0.5 V) almost completely via a four-electron-transfer process. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3247582] All rights reserved.