Effect of Ba and Ca additions on the oxygen solubility properties of a (70/30) mol% Li2CO3/Na2CO3 carbonate melt

The oxygen solubility properties of a lithium-rich Li-Na carbonate melt of 70-30 mol% composition was investigated at 923 K under different O2-CO2 atmosphere mixtures. A small addition of equimolar amounts of alkaline-earth elements like Ba and Ca to the alkali carbonate melt was found to deeply raise the oxygen solubility properties, whereas additions larger than 6 mol fraction considerably lowered the ability of the carbonate melt to dissolve the oxygen gas. The mechanism of oxygen dissolution was studied at the maximized conditions for the oxygen solubility, which were obtained for equimolar 1.5 Ba + 1.5 Ca additions (on a molar fraction scale). By using a curve-fitting method oxygen gas was predicted to prevalently react with the carbonate ion to form peroxymonocarbonate ion co-existing in the melt with a small but appreciable quantity of unreacted physically dissolved molecular oxygen. The important role of molecular oxygen in depolarizing the oxygen electrochemical reduction is emphasized to account for the reported high performances of molten carbonate fuel cells using this Li-rich electrolyte composition.