Investigation of the oxygen electrode reaction in basic molten carbonates using electrochemical impedance spectroscopy

The oxygen electrode reaction at a gold electrode in a molten 53 mol% Li2CO3 + 47 mol% Na2CO3 eutectic has been studied at 923 and 1000 K by electrochemical impedance spectroscopy. The studies were generally performed in a basic melt, i.e. at the level of the CO2 partial pressure corresponding to the thermal decomposition pressure of the carbonate ion. II was found that a semi-infinite diffusion controlled the rate of the electrode process and a modified Randles-Ershler model described the data obtained well. The dependence of the Warburg coefficient at the rest potential on the partial pressure of oxygen over the melt was analysed to verify an involvement of peroxide/oxide ions in the reduction/oxidation process at both the temperatures. Further analysis of the data allowed us to determine gamma = D(O-2(2-))C-1/2*(O-2(2-))/D(O2-)C-1/2*(O2-), where C*(O-2(2-)) and C*(O2-) are the bulk concentrations of O-2(2-) and O2- ions and D(O-2(2-)) and D(O2-) are their respective diffusion coefficients. The C*(O-2(2-)) and C*(O2-) were also estimated. The conclusion about the reaction mechanism was corroborated by the effect of electrode potential on the Warburg coefficient. (C) 2001 Elsevier Science Ltd. All rights reserved.