Behavior of a macrocyclic cobalt complex adsorbed on an electro-oxidized glassy carbon electrode for the electrocatalytic reduction of O₂

Voltammograms for the catalytic reduction of O₂ at an electrochemically oxidized glassy carbon electrode, on which a cobalt complex with a macrocyclic ligand C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (hmc) was adsorbed, were analyzed. In the presence of O₂ in the solution, the reduction of adsorbed (hmc)Co³⁺ showed two cathodic waves. The catalytic reduction current of O₂ was observed before the second cathodic wave where the (hmc)CoOOH²⁺ intermediate was reduced. The (hmc)Co²⁺ produced from the reduction of the fraction of (hmc)CoOOH²⁺ at the starting potential of the second cathodic wave could react with additional O₂ and the catalytic wave shifted to the positive direction to appear as the first cathodic wave. The reactivities for the present system were compared between the edge plane pyrolytic graphite and the electro-oxidized glassy carbon electrodes. The surfaces of the two electrodes showed almost the same properties as each other for the spontaneous adsorption of the present cobalt complex and for the electro-reduction of O₂ catalyzed by the adsorbed cobalt complex.