A COMPARATIVE-STUDY OF NIO(LI), LIFEO2, AND LICOO2 POROUS CATHODES FOR MOLTEN-CARBONATE FUEL-CELLS

Porous cathodes of NiO(Li), Co-doped LiFeO2, and LiCoO2 for the molten carbonate fuel cell (MCFC) were examined in a comparative study using electrochemical impedance spectroscopy at temperatures of 923, 973, and 1023 K. Using this technique the contributions of charge transfer and diffusion to the impedance could be separated. The impedance results as a function of gas composition were compared with theoretical predictions using the thin-film model leading to the conclusion that the most predominant diffusing species in porous MCFC electrodes are molecular oxygen and carbon dioxide. The reaction mechanism is probably the same for all three cathodes involving either the reduction of peroxycarbonate or the reduction of dissociated oxygen. The remaining difference in gas dependencies can then be explained by assuming a low coverage of oxide ions on LiFeO2 while NiO(Li) and LiCoO2 have intermediate coverage by oxide ions. From the temperature dependence of the impedance an estimate may be given of the activation energies of the polarization processes.