Equilibrium relationships in the system NiO-CoO-O2

As part of a programme to investigate the possibility of using conducting oxide electrodes in the fused carbonate fuel cell, equilibrium relationships in the system NiO-CoO-O₂ were investigated using a thermobalance. An isobaric ternary diagram of the system was constructed and it is shown that the variation in the temperature of the boundary between the single phase (NiO-CoO solid solution) and the two phase (Co₃O₄ + NiO-CoO solid solution) fields, with cationic composition, is consistent with approximately ideal behaviour. A mechanism based on the established phase relationships is also suggested to explain the observation that loss of oxygen from two-phase mixtures on heating results in the formation of NiO-CoO solid solution in the interior of the Co₃O₄ grains. Results are also presented to show the effects of composition and heat-treatment on the electrical conductivities at high temperatures of selected mixtures.