Oxygen electrodes in fused salts Investigation and mechanistic remarks on the system (Ni)CO2, O2/CO2−3 in molten (Na, K)NO3

Abstract

A potentiometric investigation on the system (Ni)CO2, O2/CO32− was carried out at 507–637 K in the (Na, K)NO3 equimolar mixture containing carbonate ions in the range 10−5≤[CO32−]≤10−2 mol kg−1 and under a mixture of O2 and CO2 at variable partial pressure. The potential behaviour of the nickel electrodes was found largely dependent on the working temperature. At the highest tested temperature the system behaves irreversibly under potentiometric conditions and the potential was found to be independent of the oxygen concentration. At lower temperatures a large irreversibility of the system was still found, while a certain dependence (different from the theoretical one) of the potential on both oxygen and carbon dioxide partial pressure was experimentally demonstrated. The entire set of results was interpreted on the basis of the two following mechanistic models involving, in the potential determining step, solid species formed on the electrode surface by contact with the melt. Mechanism 1 (high temperature) Mechanism 2 (low temperature)