Electroreduction of oxygen in aprotic media

Abstract

A detailed investigation on the electrochemical reduction of oxygen was carried out using cyclic voltammetry (CV) or voltammetry at a rotating disc (graphite) electrode in three different dipolar aprotic solvents, namely, N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO) and acetonitrile (AN) containing 0.1 M tetrabutylammonium perchlorate (n-Bu4NClO4) as the supporting electrolyte with glassy carbon (gc), graphite and platinum as the working electrodes. A diffusion-limited, reversible one-electron peak (or wave) corresponding to the oxygen-superoxide ion (O2O−·2) couple is seen in each medium, with the cathodic peak potential being a function of the medium and the electrode material. The reduction of oxygen carried out at 50 °C in DMF + 0.1 M n-Bu4NClO4 revealed a higher solubility and/or diffusion coefficient D of oxygen but the reversibility on the time scale of CV measurements was maintained. Further reduction of O−·2 to the peroxide (O2−2) is observed as a highly irreversible peak at a potential which is more than 1 V more negative. Controlled potential electrolysis (CPE) of dissolved oxygen gave stable pale yellow coloured solutions of O−·2 with λmax at 253 nm (ϵ = 1483 M−1cm−1) and OO stretching frequency at 1140 cm−1. The colour disappeared immediately on the addition of n-butyl bromide. The formal potentials of the redox couple O2O−·2 referred to that of ferrocene/ferrocenium ion varies from −1.16 V (DMSO) over −1.22 V (DMF) to −1.25 V (AN). This sequence is expected owing to decreasing dielectric constants and the decreased dipolar solvation energies of O−·2 in the respective solvents. The heterogeneous charge-transfer rate constants ks determined from CV data show a decreasing trend in the rates on going from DMSO over DMF to AN and are low at a Pt electrode.