Phase stabilities in molten Li/K carbonate of efficient matrix materials for molten carbonate fuel cells: Thermodynamic calculations and experimental investigations

In this study, we investigated the thermodynamics and experimental performance of Al, Zr, and Ce species under anode and cathode gas conditions in Li/K carbonate at 650 °C. Among the Al, Zr, and Ce species investigated, we found that lithium aluminate (LiAlO₂), lithium zirconate (Li ₂ZrO₃), and cerium/ceria oxide (CeO₂) were the most stable materials. Experimentally, we performed immersion tests in molten (Li_0.62/K_0.38)₂CO₃ at 650 °C to evaluate the phase and microstructure stabilities of these materials. The γ-LiAlO₂ phase transformation, determined using X-ray diffractometry, was dependent on the immersion time. We performed similar measurements for α-LiAlO₂, Li₂ZrO₃, and CeO₂ materials in molten Li/K carbonate at 650 °C. From immersion tests, the presence of the α-LiAlO₂ phase revealed that phase transformation of γ-LiAlO₂ occurs in Li/K carbonate melts under cathode gas atmospheres; in contrast, no phase transformation was evident after immersion of the pure α-LiAlO₂ phase in molten carbonate for 5,000 h. Furthermore, we found that Li₂ZrO₃ and CeO ₂ were stable phases after immersion in molten carbonate at 650 °C, under both anode and cathode gas atmospheres, for more than 5,000 h.