Oxygen electrodes in fused salts: Investigation and mechanistic remarks on the system (Ni)O 2, H 2O/OH − in the (Na, K)NO 3 eutectic melt

Abstract

A potentiometric investigation on the system (Ni)O 2, H 2O/OH − was carried out within the temperature range 513⩽ T⩽636 K in the (Na, K)NO 3 equimolar mixture containing OH − ions in the concentration range 5×10 −6<[OH −]<10 −1 m and flushed with a mixture of O 2 and H 2O at variable partial pressures. The system has been found to behave reversibly in all hydroxide concentration and temperature intervals studied with respect to all the species involved in the over-all electrode reaction ½ O 2+H 2O+2 e −=2OH − so that the following nernstian relationship could be written E= E O 2 ,H 2O/OH −+ RT/Fln{[O 2] 1/4[H 2O] 1/2/[OH −]} This potentiometric behaviour was tentatively interpreted on the basis of mechanistic models involving, in some steps, solid nickel oxides formed on the electrode surface by contact with the melt. The actual formation and existence of these compounds on the electrode surface under the given experimental conditions was proved by a proper XPS investigation.