Solubility of Nickel Ferrite in Li-K Binary Molten Carbonate

Abstract

NiO has been utilized as the cathode of molten carbonate fuel cells (MCFC′s). But NiO is reported to be unstable in long-time operation, since it dissolves in the carbonate melt. In order to improve the stability of a cathode of MCFC, a new cathode material of low solubility should be developed. In this study, the solubilities of nickel ferrites (Ni_δFe_3-δO₄)of several compositions (=0.33, O.53, O.75, 1.00) have been measured in a Li-K binary eutectic carbonate melt (Li₂CO₃: K₂CO₃=62: 38 mol%) under a carbon dioxide pressure ranging from O.1 to 1 atm in a temperature range from 873 to 1023 K, in order to estimate their stability when used as a cathode of MCFC.
The solubilities of nickel and iron in th e ferrites increased as the carbon dioxide pressure increased. This might be explained by the acid fluxing mechanism. The dependence of the solubilities of nickel ferrites on the pressure was smaller than that of nickel oxide. The solubilities of nickel decreased with a raise in temperature in any composition. In the ferrites of lower nickel content the solubility of iron decreased at higher temperatures.
The solubilities of iron in the higher nickel ferrites increased at higher temperatures. The solubilities of nickel ferrites were much smaller than that of nickel oxide by a factor of 3-18 under any conditions of this work. Considering the solubilities, the nickel ferrite might be a more stable cathode material of MCFC than nickel oxide. However, the ferrites reacted with the carbonate melt and the reaction products had a low electrical conductivity, although the nickel ferrites themselves have a good electrical conductivity, especially for the low nickel ferrites. These points should be improved in utilizing these materials as the cathode of MCFC.