Oxygen reduction with hydroxy-1,4-naphthoquinones immobilized at carbon electrodes

Abstract

Oxygen reduction with naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) has been studied by cyclic voltammetry and by flow-through electrolysis. The mechanisms of oxygen reduction were studied thermodynamically. At pH 7 the semiquinone anion is the most likely candidate for oxygen reduction among the reduced species of naphthazarin, including quinols, and the superoxide is mainly produced by this species. By flow-through electrolysis e.s.r. the semiquinone has been detected under anaerobic conditions. Its concentration depends on the potential because of disproportionation. Distribution of the semiquinone has been calculated against the potential, and the results are compared with the potential dependence of the e.s.r. intensity. The redox activity of other quinones has also been studied. Naphthazarin is the most active in oxygen reduction.