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 La(2)NiO(4+delta) (n = 1) phase and the higher-order Ruddlesden-Popper phases, La(n+1)Ni(n)O(3n+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 14/mmm in the hyperstoichiometric LaNiO(4.15) (n = 1) to orthorhombic Fmmm in the oxygen-deficient phases, La(3)Ni(2)O(6.95) (n=2) and La(4)Ni(3)O(9.78) (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 La(4)Ni(3)O(9.78). Impedance spectroscopy measurements on symmetrical cells with La(0.9)Sr(0.10)Ga(0.80)Mg(0.20)O(3-delta) (LSGM-9182) as the electrolyte show a systematic improvement in the electrode performance from La(2)NiO(4.15) to La(4)Ni(3)O(9.78) with similar to 1 Omega cm(2) observed at 1073 K for the latter. Long-term thermal stability tests show no impurity formation when La(3)Ni(2)O(6.95) and La(4)Ni(3)O(9.78) are heated at 1123 K for 2 weeks in air, in contrast to previously reported data for La(2)NiO(4.15). The relative thermal expansion coefficients of La(3)Ni(2)O(6.95) and La(4)Ni(3)O(9.78) were found to be similar at similar to 13.2 x 10(-6) K(-1) from 348 K to 1173 K in air compared to 13.8 x 10(-6) K(-1) for La(2)NiO(4.15). 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 La(2)NiO(4+delta) (n = 1) phase. (c) 2006 Elsevier B.V. All rights reserved.