Thermomechanical, transport and anodic properties of perovskite-type (La0.75Sr0.25)0.95Cr1−xFexO3−δ

Effects of iron doping on the functional properties of (La,Sr)CrO3-based electrode materials have been appraised in a range of conditions relevant for SOFCs and other electrochemical applications. Mössbauer spectroscopy of perovskite-type (La0.75Sr0.25)0.95Cr1−xFexO3−δ (x = 0.3–0.4), combined with thermogravimetry and X-ray diffraction, shows that the prevailing oxidation state of iron cations in both oxidizing and reducing atmospheres remains 3+. The redox behavior and transport properties are, therefore, essentially governed by Cr3+/4+ couple, leading to dominant p-type electronic conduction in the oxygen partial pressure range from 0.5 down to 10−20 atm at 973–1223 K. The total conductivity and Seebeck coefficient variations indicate that the electronic transport decreases with iron additions as the hole concentration and mobility become lower. The partial ionic conductivity estimated from the steady-state oxygen permeability under air/H2–H2O–N2 gradients is 0.05–0.08 S cm−1 at 1223 K and p(O2) = 10−17 atm, and increases with x due to rising oxygen deficiency. A similar tendency was observed for chemical expansion studied by the controlled-atmosphere dilatometry, whereas the average thermal expansion coefficients are almost independent of x and vary from 11.1–11.3 × 10−6 K−1 in air down to 10.3–10.5 × 10−6 K−1 in CO–CO2 at 350–1370 K. The electrochemical activity of porous (La0.75Sr0.25)0.95Cr0.7Fe0.3O3−δ anodes with Ce0.8Gd0.2O2−δ interlayers, applied onto LaGaO3-based solid electrolyte, is higher compared to (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ when no metallic current-collecting layers are used. Increasing iron content lowers the electrode performance in wet H2-containing atmospheres, in correlation with electronic conduction.

► Trivalent state of iron stabilized in LaCrO3 matrix dominates in a wide p(O2) range.
► No reductive decomposition of Fe-substituted lanthanum chromite was detected.
► Moderate Fe doping increases SOFC anode performance compared to Mn-doped analogs.
► Large additions lead to lower electronic transport and higher chemical expansion.
► Total conductivity governed by Cr3+/4+ redox couple affects electrochemical activity.