Isotope and surface preparation effects on alkaline dioxygen reduction at carbon electrodes

The reduction of dioxygen in base was examined on several carbon electrode surfaces, particularly polished and modified glassy carbon (GC). Electrochemical pretreatment, fracturing, and vacuum heat treatment shifted the reduction peak positive, while adsorption of several covalent and physisorbed organic compounds shifted it negative. A reverse wave for O⁻₂ oxidation was observed in tetraethylammonium hydroxide electrolyte, and on GC surfaces preadsorbed with Co(II) phthalocyanine. An isotope effect was observed when H₂O + KOH and D₂O + KOD electrolytes were compared, with the largest effect observed on surfaces exhibiting the most positive reduction peak potential. The results indicate involvement of proton transfer in the rate limiting step of reduction, and a strong dependence of the electron transfer rate on the carbon surface condition. The results support a mechanism involving adsorption of O⁻₂ and associated enhancement of proton transfer from water to O⁻₂. Activation of the dioxygen reduction by surface pretreatment is attributed to increasing the concentration of adsorbed O⁻₂.