KINETIC-STUDY OF OXYGEN REDUCTION IN MOLTEN LI2CO3-K2CO3 UNDER PRESSURIZED CONDITIONS

The electrode kinetics and mechanism of oxygen reduction at gold electrodes in (62 + 38)mol% (Li + K)CO3 melt at 923 K under pressurized conditions have been studied to elucidate the effect of pressure on molten carbonate fuel cell (MCFC) performance. The partial pressure of oxygen, P(O2), was changed from 0.1 to 3.6atm with a constant P(CO2) fixed at 0.9 atm, and vice versa. Cyclic voltammetric measurements showed that the reaction was highly reversible, and the current density increased with increasing P(O2) indicating that the concentration of the reactive species for O2 reduction increased. To clarify the oxygen reduction path, reaction order analysis of Warburg coefficients (sigma), which were obtained by ac impedance measurements, has been carried out. It was found that the reaction process is dominated by the mixed diffusion of superoxide ions, O2- and CO2. Therefore, sigma(O2) and sigma(CO2) were separated from the apparent sigma value, sigma(app), using the pressure dependence of O2- and CO2 concentrations in the melt. The a 1/sigma(O2-) values, which were proportional to the CD1/2 value of O2- were found to be smaller than 1/sigma(CO2) values at 3.6 greater-than-or-equal-to P(O2) greater-than-or-equal-to 0.1 with a constant P(CO2) of 0.9 atm and at 3.6 greater-than-or-equal-to P(CO2) greater-than-or-equal-to 0.9 with a constant P(O2) of 0.9 atm. It was concluded that the diffusion of O2- is the controlling factor in the overall mass transfer process. Based on a relationship between sigma(app), and partial pressures of the oxidant gas, the optimum cathode gas composition of air and CO2 mixtures was estimated as a function of total gas pressure, to minimize the diffusion impedance.