Molten carbonate fuel cell research II. Comparing the solubility and the in-cell mobility of the nickel oxide cathode material in lithium/potassium and lithium/sodium carbonate melts ☆

Abstract The solubility of (lithiated) nickel oxide in Li/K- and Li/Na-carbonate eutectic melts was reinvestigated. NiO solubility depends on oxygen partial pressures or the cathode potential respectively, and is lower by approximately a factor of three at the Ni/NiO equilibrium potential than at the O 2 equilibrium potential at 0.33 bar O 2 . Under operating conditions, cathodically polarized nickel oxide cathodes seem to dissolve mainly as divalent nickel, whereas in standby at O 2 equilibrium potential, more trivalent nickel is dissolved and transferred into the matrix of the molten carbonate fuel cells. At 650°C and 0.65 bar CO 2 in the presence of some tenths of a bar of oxygen, the solubility of nickel oxide is lower by almost a factor of two in Li/Na- than in Li/K-carbonate. The saturation concentration depends on the carbon dioxide pressures according to the power of 0.5 in Li/Na- and 0.7 in Li/K-eutectic melts and, therefore, at 0.13 bar CO 2 , the NiO solubility in both melts is almost the same. Doping the cathode material with different oxidic additives, such as Fe 2 O 3 and MnO 2 , leads, as a transient effect, to a certain retardation of nickel oxide dissolution, but not to a thermodynamically defined decrease in solubility. In isothermal 250 cm 2 cross-flow cells, the distribution of nickel deposition in the LiAlO 2 matrix across the cell surface and in the depth of the matrix was determined. The rate of nickel transport into the matrix is highest at the corner of the fuel and oxidant inlet and lowest at the vicinal corner of the fuel inlet/oxidant outlet with a ratio of more than two. Nickel transfer is shown to be strongly retarded by using LiCoO 2 -coated NiO cathodes. A 140 μm coating diminishes the rate of nickel transfer by a factor of approximately two.