Study of Chromite Perovskite Oxides as Solid Oxide Fuel Cell Anodes

Mixed LaCrO3 perovskites, which have the favorable properties of mixed ionic/electronic conductivity and high stability, were investigated as alternative solid oxide fuel cell (SOFC) anodes. While state-of-the-art Ni/Y2O3-stabilized-ZrO2 (Ni-YSZ) anodes currently outperform perovskite oxide anodes, they suffer performance losses from coking, sulfur poisoning, and redox cycling instability. Reasons for perovskites' comparably poor performance were considered with the goal to enhance their performance.;Lanthanum chromite compounds in the series La1-xSrxCr 1-xFexO3-delta (x = 0.2-0.67, LSCrFe) were assessed. While electrochemical impedance spectroscopy results showed that mid to high levels of Fe (x = 0.3-0.67) yielded equally low polarization resistances, diffraction experiments determined that perovskites with x = 0.4 were more stable in the anode fuel environment than perovskites with x = 0.67. This suggests intermediate amounts of Fe result in stable, well performing anodes.;The mixed chromites La0.33Sr0.67Cr1-x-yFexRuyO 3-delta were also examined as novel SOFC anodes alongside Fe- and Ru-free (La,Sr)CrO3 compounds. When combined, the two substituents were found to reduce anode polarization resistances below those attained by exclusively Fe- or Ru-substituted (La,Sr)CrO3-delta perovskites through a combination of ionic conduction enhancement by Fe and increased hydrogen oxidation activity by small amounts of Ru.;The hydrogen pressure dependence of anode resistance and the current-limiting behavior observed in perovskite anode SOFCs were explained by a model where adsorption limited the rate of hydrogen oxidation. The model, which accounted for H2 dissociative adsorption in series with electrochemical hydrogen oxidation, fitted experimental data well for SOFCs with perovskite anodes, and explained performance improvements in oxide anodes like LSCrFeRu as being due to the promotion of hydrogen adsorption by Ru nano-particles.;Chromites with Pd added to promote hydrogen oxidation were studied to assess the anodes' structural behavior. Anodes with more Pd led to SOFCs with lower anode resistance. A substantial amount of the Pd was located in phases secondary to the perovskite while the remainder (∼1 atomic %) was found to be substituted in the perovskite phase.;The present work suggests that SOFCs with optimized perovskite oxide anode compositions have the potential to provide power outputs comparable to SOFCs with Ni-YSZ anodes.