Oxygen Overpotential in Molten Carbonates

Abstract

The overvoltage of the oxygen evolution reaction on Pt electrodes has been measured in a ternary mixture of molten Na~CO~-Li~CO~-K2CO3 with current densities up to 50 ma/cm ~ and temperatures up to 1000 ~ Two well distin-guished regions in the overvoltage-log current density relation are observed. At current densities less than 1 ma/cm ~ the marked dependence of the oxygen overpotential on the composition of the gas flow and on temperature are char-acteristic of a concentration-type overpotential most probably due to the bui ld-up of oxide-ion in the melt. In the range of about 1-50 ma/cm ~, the oxygen overpotential seems activation controlled. From the temperature co-efficient of the overpotential, a value for the energy of activation for oxygen evolution of 1[}.4 • 1.1 kcal mole- ~ is obtained. The role of the plat inum elec-trode as an oxygen ion electrode in the over-all process corresponding to the oxygen evolution reaction: CO~-~--> CO ~ + 0 + 2 e