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 × 10^-4, 7.1 × 10 ^-5, and 1.4 × 10^-4 s^-1 for HQ, Res, and Cat, respectively. The adsorbed species showed electrochemical quasi-reversible behavior in 1 M H₂SO₄ 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 (∼+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 × 10⁵ M^-1 s^-1. Mediation of oxygen reduction by these systems occurred at a relatively high potential (∼+0.5 V) almost completely via a four-electron-transfer process.