ELECTROREDUCTION OF OXYGEN IN APROTIC MEDIA

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-dimethylformanide (DMF), dimethylsulphoxide (DMSO) and acetonitrile (AN) containing 0.1 M tetrabutylammonium perchlorate (n-Bu(4)NClO(4)) 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 (O-2/O-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 degrees C in DMF + 0.1 M n-Bu(4)NClO(4) 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 (O-2(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 lambda(max) at 253 nm (epsilon = 1483 M(-1) cm(-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 O-2/O--. 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 k(s) 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.