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 c arried out in order to investigate the nature and the mechanism of the oxygen reduction process at a gold electrode in 62/38 mol% and 42.7/5 7.3 mol% Li2CO3 + K2CO3. An original aspect of these experiments is th at 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 t he electrode to molecular oxygen. Subsequently, the oxygen reacted wit h carbonate ions in the diffusion layer forming in-situ one or more re duced oxygen species. The effect of the positive limit potential, pree lectrolysis time at this potential, scan rate, acidity of the melt, te mperature, gas composition and peroxide and superoxide additions, in t he form of Li2O2 and KO2, on the reduction process was thoroughly anal yzed. The gas dependency of the reduction wave was also investigated i n conditions where the start potential was the open-circuit potential. Under acidic conditions superoxide species, with a law stability in t he bulk of the melt, is probably reduced according to a net three elec tron 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 peroxycarbona te mechanism is not likely in this melt. The agreement between the res ults in both laboratories, ENSCP and TU Delft, although not complete, was such that similar conclusions could be obtained from them. (C) 199 7 Elsevier Science S.A.