Toshiki Kahara

Performance and life of 10-kW molten-carbonate fuel cell stack using Li/K and Li/Na carbonates as the electrolyte

NiO cathode dissolution is a serious problem with molten carbonate fuel cells (MCFC). The target life-time of such cells is 40,000 h, but shorting by NiO cathode dissolution markedly decreases cell performance. NiO cathode dissolution depends on the composition of the molten carbonate electrolyte. The electrolyte generally comprises a mixture of lithium carbonate and potassium carbonate. Since the solubility of NiO in a mixture of lithium carbonate and sodium carbonate is lower than in lithium and potassium carbonate, it is expected that shorting by NiO cathode dissolution will take longer in a mixture of lithium and sodium. Therefore, a mixture of lithium carbonate and sodium carbonate is a strong candidate electrolyte. A unique 10-kW class stack, which uses a mixture of lithium and potassium carbonate and mixture of lithium and sodium carbonate as the electrolyte, has been developed and tested. The basic performance and life time of both electrolyte cells of the stack are reported. In particular, the change in cathode polarization caused by NiO cathode dissolution is evaluated quantitatively.

NiO Cathode Dissolution and Ni Precipitation in Li/Na Molten Carbonate Fuel Cells: Distribution of Ni Particles in the Matrix

Due to the dissolution of the lithiated nickel oxide cathode, the life expectancy of a molten carbonate fuel cell is reduced. The use of a Li/Na carbonate electrolyte is expected to lead to a higher voltage and a longer life expectancy due to its higher ionic conductivity and its lower nickel oxide cathode solubility. Using the Li/Na electrolyte, single cells have been tested to evaluate their performance and their life expectancy. Empirical equations for these cells have been presented to determine the temperature, the CO 2 partial pressure in the cathode gas, and the matrix thickness. The results prove that the life expectancy of LilNa cells is reduced by nickel short-circuiting in comparison to Li/K cells, for which the life expectancy is many times longer. The dependence of the nickel-containing particle distribution in the matrix on the temperature has been evaluated using an image processing method. At 973 K, most of the particle distribution moves toward the anode more rapidly than at 873 K, because the rate of particle growth is lower at the higher temperature, and the particles move toward the anode due to the convection of the molten carbonate in the matrix. The initiation time for nickel short-circuiting was derived from the results of this study to explain the relationship between the shorting conductance and the volume of nickel-containing materials in the matrix porosity. Moreover, the results show that the predominant element contributing to short-circuiting is the nickel oxide, and not the metal.