La_0.9Sr_0.1Ga_0.8Mg_0.2O_{3 - δ}-La_0.6Sr_0.4Co_0.2Fe_0.8O_{3 - θ} composite cathodes for intermediate-temperature solid oxide fuel cells

Solid oxide fuel cells with thin La_0.9Sr_0.1Ga_0.8Mg_0.1O_{3 - δ} (LSGM) electrolytes have the potential to provide high power densities down to at least 600 °C. The present study was on a candidate cathode material for such SOFCs: composites containing La_0.6Sr_0.4Co_0.2Fe_0.8O_{3 - θ} (LSCF) and LSGM. Symmetrical cathodes with LSCF contents of 30-70 wt.% were fired at temperatures from 1000 to 1300 °C on both sides of bulk LSGM electrolytes. No secondary phases were detected by X-ray diffraction for the firing temperatures tested. Cathode polarization resistance R_p, measured using impedance spectroscopy, varied little with firing temperature despite substantial changes in cathode microstructure as observed by scanning electron microscopy. R_p varied only slightly with LSCF content, reaching a minimum of ∼ 0.18 Ohm.cm² at 650 °C for 40-60 wt.% LSCF fired at 1100 °C. The R_p value was stable over several hundred hours at a current density of 0.5 A/cm². The temperature dependence of R_p yielded an activation energy of ≈ 1.7 eV for all compositions. Measurements versus oxygen partial pressure P_O yielded R_p ∝ P_O2^0.20. SOFCs with LSCF-LSGM cathodes and thin LSGM electrolytes yielded a maximum power density of 0.57 W/cm² at 650 °C.