Phase and Microstructure Stability of Electrolyte Matrix Materials for Molten Carbonate Fuel Cells

The molten-carbonate fuel cell (MCFC) is being developed as a next-generation power system in the near future. The MCFC has many advantages, including very high thermal/electrochemical conversion efficiency, unnecessary of novel-metal electrode, diversity of fuel, and pollution-free operation. The electrolyte matrix of the MCFC is used for electronic insulation and ionic (CO₃²⁻) communication between the electrodes. In a porous plate manufactured from LiAlO₂ powder, the electrolyte matrix is comprised of the molten carbonate, which is solid at room temperature and paste-like at the operation temperature of 650 ^oC. The phase and microstructure stabilities of LiAlO₂ for improving the matrix reliability of molten carbonate fuel cells are investigated. The phase transformation and particle growth of LiAlO₂ are examined via immersion tests of pure α-LiAlO₂ matrix, Al-reinforced α-LiAlO₂ matrix, Al-reinforced γ-LiAlO₂ matrix, aqueous γ-LiAlO₂ matrix, and α-/β-LiAlO₂ mixture powder in molten carbonate at 650 ^oC in air atmosphere. In case of γ-LiAlO₂ and α-/β-LiAlO₂ mixture, the growth of particles takes place continuously from an early stage of heat-treatment after 10000 h. Also, crystalline phase transformation of these powders and matrices occur γ-LiAlO₂ and β-LiAlO₂ to α-LiAlO₂ and γ-LiAlO₂, respectively. By contrast, the size of these particles and the crystalline phase of α-LiAlO₂ did not change during immersion test. These results show that α-LiAlO₂ is more stable than β-/γ-LiAlO₂ in molten carbonate.