Semiquinone intermediates in the two-electron reduction of quinones in aqueous media and their exceptionally high reactivity towards oxygen reduction.

We report the catalytic anthraquinone-mediated reduction of oxygen at a boron-doped diamond electrode. Scheme of squares modelling confirms the existence of and reveals the role of the semiquinone intermediates, which are shown to have an exceptional reactivity towards oxygen (as compared to the di-reduced anthraquinone).