A comparative study of the Ruddlesden-Popper series, Lan+1NinO3n+1 (n=1, 2 and 3), for solid-oxide fuel-cell cathode applications

A comparative investigation of the much-studied La2NiO4+delta (n=1) phase and the higher-order Ruddlesden-Popper phases, Lan+1NinO3n+1 (n=2 and 3), has been undertaken to determine their suitability as cathodes for intermediate-temperature solid-oxide fuel cells. As n is increased, a structural phase transition is observed from tetragonal I4/mmm in the hyperstoichiometric La2NiO4.15 (n=1) to orthorhombic Fmmm in the oxygen-deficient phases, La3Ni2O6.95 (n=2) and La4Ni3O9.78 (n=3). High temperature d.c. electrical conductivity measurements reveal a dramatic increase in overall values from n=1, 2 to 3 with metallic behavior observed for La4Ni3O9.78. Impedance spectroscopy measurements on symmetrical cells with La0.9Sr0.10Ga0.80Mg0.20O3-delta (LSGM-9182) as the electrolyte show a systematic improvement in the electrode performance from La2NiO4.15 to La4Ni3O9.78 with 1Omega cm2 observed at 1073K for the latter. Long-term thermal stability tests show no impurity formation when La3Ni2O6.95 and La4Ni3O9.78 are heated at 1123K for 2weeks in air, in contrast to previously reported data for La2NiO4.15. The relative thermal expansion coefficients of La3Ni2O6.95 and La4Ni3O9.78 were found to be similar at 13.2X10-6K-1 from 348K to 1173K in air compared to 13.8X10-6K-1 for La2NiO4.15. Taken together, these observations suggest favourable use for the n=2 and 3 phases as cathodes in intermediate-temperature solid-oxide fuel cells when compared to the much-studied La2NiO4+delta (n=1) phase.