Effects of carbon electrode surface states for the electro-reduction of (hmc)CoOOH²⁺ which is an intermediate during the catalytic reduction of O₂

The reduction of the cobalt(III) complex with a macrocyclic ligand C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (hmc) dissolved in solution or adsorbed on a graphite electrode in the presence of O₂ showed two cathodic peaks. As discussed in earlier reports, an intermediate (hmc)CoOOH²⁺ produced by the first two-electron reduction of (hmc)Co³⁺ in the presence of O₂ was further reduced to (hmc)Co²⁺ and HOOH. This process appeared as a second cathodic wave and represents a barrier in the overall reduction of O₂. In relation to these studies, it was found that the second cathodic reduction was greatly affected by the surface states of the carbon electrode. The reduction potential of the (hmc)CoOOH²⁺ intermediate when it was adsorbed on a pyrolytic edge plane graphite electrode (EPG) surface was more positive than the corresponding value of the dissolved species measured at a glassy carbon (GC) electrode polished with alumina. It was also shown that the reduction potential shifted in a positive direction when the solution pH was lowered or the surface of the glassy carbon electrode was heavily electro-oxidized. It is proposed that a proton transfer is involved in the electro-reduction of the (hmc)CoOOH²⁺ complex. The EPG surface which has more surface functional groups has a more acidic environment than the GC electrode and the reduction of (hmc)CoOOH²⁺ was facilitated. The oxidation of the glassy carbon electrode gives the same effect.