Decreasing the polarization resistance of (La,Sr)CrO_3-δ solid oxide fuel cell anodes by combined Fe and Ru substitution

The perovskite compounds La_0.33Sr_0.67Cr_1-x-yFe_xRu_yO_3-δ (LSCrFeRu, x = 0.62, 0.57, and 0.47; y = 0.05, 0.14, and 0.2, respectively) were synthesized and assessed as a new type of solid oxide fuel cell (SOFC) anode in composite with Gd_0.1Ce_0.9O_2-β (GDC) in La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-ε/La_0.4Ce_0.6O₂ bilayer electrolyte-supported cells. By comparing anode polarization resistance R_P,A values for the LSCrFeRu compounds to the either exclusively Fe- or Ru-substituted (La,Sr)CrO_3-δ perovskites, the present results demonstrate that the two substituent cations work synergistically to provide further reduction in R_P,A from 0.290 Ω·cm² for La_0.33Sr_0.67Cr_0.33Fe_0.67O_3-δ (LSCrFe) and 0.235 Ω·cm² for La_0.8Sr_0.2Cr_0.8Ru_0.2O_3-δ (LSCrRu) to 0.195 Ω·cm² for LSCrFeRu (all measured in humidified hydrogen at 800 °C). These impedance results also strongly suggest that hydrogen dissociative adsorption was the rate-limiting step in the hydrogen oxidation reaction sequence for LSCrFe anodes at some of the pH₂ and temperatures measured. However, the formation of Ru nanoparticles on LSCrFeRu and LSCrRu surfaces, observed by scanning and transmission electron microscopy, appears to promote hydrogen dissociation. Substituting even small amounts of Ru into (La,Sr)(Cr,Fe)O_3-δ perovskites is thus sufficient to make hydrogen electrochemical oxidation the rate-limiting step, resulting in anodes with significantly reduced R_P,A.