Effect of molten carbonate composition on oxygen reduction under pressurized conditions: cyclic voltammetry and equivalent circuit analysis

The oxygen reduction mechanism was investigated in carbonate melts of varia ble composition ranging from the eutectics Li-K, Li-Na to ternary Li-Na-K m elts. The effect of pressure on electrode kinetics was also examined. Cycli c voltammetry, convolution voltammetry and electrochemical impedance spectr oscopy have shown that oxygen reduction is mass-transfer limited in all mel t compositions investigated. It was also found that pressurizing increased the concentration of oxygen electroactive species. To gain information abou t the reaction mechanism, the impedance spectra have been fitted to four po ssible equivalents circuits: (i) Randles circuit, (ii) Randles circuit exte nded by a parallel RC circuit due to film formation, (iii) Gerisher (homoge neous reaction) impedance, and (iv) Modified Randles circuit. The discussio n of the results is based on careful analysis of fit quality and meaningful ness of fit parameters.