Identification and electrochemical characterization of in situ produced and added reduced oxygen species in molten Li2CO3+K2CO3

In two different laboratories, electrochemical experiments have been carried out in order to investigate the nature and the mechanism of the oxygen reduction process at a gold electrode in 62/38 mol% and 42.7/57.3 mol% Li2CO3 + K2CO3. An original aspect of these experiments is that cyclic voltammetric measurements were performed in which potentials higher than the positive stability limit of the molten carbonate were applied. It has been shown that the carbonate ions were oxidized at the electrode to molecular oxygen. Subsequently, the oxygen reacted with carbonate ions in the diffusion layer forming in-situ one or more reduced oxygen species. The effect of the positive limit potential, preelectrolysis time at this potential, scan rate, acidity of the melt, temperature, gas composition and peroxide and superoxide additions, in the form of Li2O2 and KO2, on the reduction process was thoroughly analyzed. The gas dependency of the reduction wave was also investigated in conditions where the start potential was the open-circuit potential. Under acidic conditions superoxide species, with a law stability in the bulk of the melt, is probably reduced according to a net three electron reversible process. Under basic conditions peroxide species, well stabilized in the melt, undergoes a two-electron reversible reduction. It has also been deduced from experimental data that a peroxycarbonate mechanism is not likely in this melt. The agreement between the results in both laboratories, ENSCP and TU Delft, although not complete, was such that similar conclusions could be obtained from them. (C) 1997 Elsevier Science S.A.