Hydrogen and Oxygen Gas Electrode Reactions in Molten Alkali Carbonate

Abstract

Electrode kinetics of hydrogen oxidation and oxygen reduction in molten alkali carbonate have been studied extensively from the viewpoints of i) kinetic techniques, ii) electrode materials and iii) reaction order analyses of gas partial pressures. Comparative studies of kinetic parameters obtained by the AC impedance (AC) method, the coulostatic relaxation (CS) method, the potential step (PS) method and the chronocoulometry of PS transients have been carried out at various electrode materials, and kinetic features of both reactions in Li/K melts at 650°C have been disclosed. In order to reveal the reaction mechanism of hydrogen oxidation, the stoichiometric number (ν) was determined by the chronocoulometry. We obtained ν=2 on Ni, Pt, Ir, Au and Ag electrodes, supporting a CEC mechanism; H₂+2M_??_2MH, 2MH+2CO₃^2-→2M+2CO₂+2OH^-+2e, 2OH^-+CO₂_??_CO₃^2-+H₂O. The Allen-Hicklign plots with nlν=1 resulted in well-linear plots over the whole range of polarization and gave reasonable values of the exchange current density. It was concluded that the metal electrodes examined here have the same rate determining step. As to the oxygen reduction, two processes are known, the superoxide path and the peroxide path, but the actual path in Li/K melts is still an arguing point. In order to resolve this point, we made the reaction order analysis of the Warburg coefficients obtained from AC method, and reached the conclusion that the mass transfer is a mixed diffusion process of superoxide ions and dissolved carbon dioxide.