Kinetics of Dioxygen Reduction on Gold and Glassy Carbon Electrodes in Neutral Media

The electrochemical reduction of dioxygen (O2) has been studied on bulk gold (Au) and glassy carbon (GC) electrodes in aqueous neutral solution close to blood ionic composition. The mechanism was found to involve two successive bielectronic steps with hydrogen peroxide (H2O2) as the reaction intermediate whatever the electrode material used. On Au, O2 and H2O2 were reduced at close potentials. The determination of the kinetic parameters of O2 electroreduction was thus achieved after removing the cathodic current corresponding to H2O2 reduction. Cyclic voltammograms exhibited one cathodic peak whose both current density (jp) and potential (Ep) evolution as a function of potential scan rate (r) was in accordance with Randles-Sevcik and Nicholson-Shain equations, respectively. Rotating disk electrode (RDE) voltammetry was also performed and the data were analyzed using the Koutecky-Levich relationship. The effective number of electrons (n) was found to be roughly independent of the potential and close to n = 2 when removing H2O2 reduction current whereas it gradually increased up to n = 4 while considering the total current. The Tafel slopes allowed the cathodic transfer coefficients (ꞵn) to be calculated in several neutral aqueous electrolytes. Values varied from 0.25 to 0.49 and were systematically higher on Au than on GC electrode. Similar results were obtained with Tafel slopes deduced from Butler-Volmer exploitation of the current-potential curves.