High-temperature transport properties, thermal expansion and cathodic performance of Ni-substituted LaSr2Mn2O7−δ

The substitution of manganese with nickel in LaSr2Mn2O7−δ, where the solubility limit corresponds to approximately 25% Mn sites, enhances the Ruddlesden–Popper phase stability at elevated temperatures and atmospheric oxygen pressure. The total conductivity of LaSr2Mn2−yNiyO7−δ (y=0–0.4) decreases with nickel additions, whilst the average thermal expansion coefficients calculated from dilatometric data in the temperature range 300–1370 K increase from (11.4–13.7)×10−6 K−1 at y=0 up to (12.5–14.4)×10−6 K−1 at y=0.4. The conductivity and Seebeck coefficient of LaSr2Mn1.6Ni0.4O7−δ, analyzed in the oxygen partial pressure range 10−15–0.3 atm at 600–1270 K, display that the electronic transport is n-type and occurs via a small polaron mechanism. Reductive decomposition is observed at the oxygen pressures close to Ni/NiO boundary, namely ∼2.3×10−11 atm at 1223 K. Within the phase stability domain, the electronic transport properties are essentially p(O2)-independent. The steady-state oxygen permeability of dense LaSr2Mn1.6Ni0.4O7−δ membranes is higher than that of (La,Sr)MnO3−δ, but lower if compared to perovskite-like (Sr,Ce)MnO3−δ. Porous LaSr2Mn1.6Ni0.4O7−δ cathodes in contact with apatite-type La10Si5AlO26.5 solid electrolyte exhibit, however, a relatively poor electrochemical performance, partly associated with strong cation interdiffusion between the materials.