Mixed conductivity, thermochemical expansion and electrochemical activity of Fe-substituted (La,Sr)(Cr,Mg)O3−δ for solid oxide fuel cell anodes

Abstract

The effect of iron substitution in perovskite-type (La0.9Sr0.1)0.98Cr0.9−xFexMg0.1O3−δ (x = 0–0.3) is evaluated with emphasis on the properties relevant for solid oxide fuel cell anode application including the phase stability, oxygen nonstoichiometry, mixed ionic–electronic transport, thermochemical expansion and electrochemical activity. Thermogravimetric analysis, Mössbauer spectroscopy and electrical measurements in combination with X-ray diffraction confirm the stability of perovskite phase for x = 0.3 down to p(O2) as low as 10−19 atm at 1223 K. Mössbauer spectroscopy results indicate also that iron cations substitute in 3+ oxidation state in both oxidized and reduced material. The total conductivity is predominantly p-type electronic, with negligible contribution of ionic transport under oxidizing conditions. Substitution with iron decreases electronic transport, but also leads to higher oxygen deficiency and ionic conductivity under reducing conditions. The oxygen nonstoichiometry variations, determined by coulometric titration, and defect chemistry of (La0.9Sr0.1)0.98Cr0.6Fe0.3Mg0.1O3−δ can be described by non-ideal solution model and site-exclusion effects. The materials exhibit moderate thermal expansion coefficients (10.1–11.5) × 10−6 K−1 in air, nearly independent of iron content and p(O2), and favorably small chemical expansion on reduction. Porous (La0.9Sr0.1)0.98Cr0.6Fe0.3Mg0.1O3−δ anodes applied onto LaGaO3-based solid electrolyte with thin Ce0.8Gd0.2O2−δ interlayers show a better electrochemical performance compared to (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ under identical conditions.