Impedance Analysis for Oxygen Reduction in a Lithium Carbonate Melt: Effects of Partial Pressure of Carbon Dioxide and Temperature

Abstract

Effects of partial pressure of carbon dioxide and temperature on oxygen reduction kinetics on a gold electrode in a lithium carbonate melt were examined using electrochemical impedance spectroscopic (EIS) and linear sweep voltammet-ric techniques. The impedance spectra were analyzed by a complex nonlinear least squares method, using the Randles-Ershler equivalent circuit model, to determine the electrode-kinetic and the mass-transfer pa ameters such as the charge-transfer esistance and the Warburg coefficient9 The cyclic voltammetric measurements indicated that the oxygen reduction process in lithium carbonate melt is "reversible " up to 200 mV/s. The product D~o/2Co determined by cyclic voltammetry agreed well with those estimated by the EIS method. The reaction order with respect to carbon dioxide and the activation energy for the exchange current density were determined to be-0.52 and 132 k J/tool, respectively. Also, the reaction order with respect o carbon dioxide and the activation energy for D~o/~Co were calculated to be-0.8 and 185 kJ/mol, respectively. The oxygen reduction reaction in molten alkali carbon-ate electrolytes has been studied for the past quarter cen-tury because of its vital role in the performance of the molten carbonate fuel cell9 Borucka and Sugiyams '2stud-