In Situ Characterization of Strontium Titanium Ferrite Perovskites for Application as Electrodes of Solid Oxide Cells

This study investigates the redox behavior, the surface composition, and the electrochemical performance of the Ni-doped Sr_0.95(Ti_0.3Fe_0.63Ni_0.07)-O₃ (STF-Ni) and SrTi_0.3Fe_0.7O₃ (STF) perovskites under conditions relevant to solid oxide cell applications. The effect of Ni doping on exsolution and on its reversibility is examined with synergistic characterization techniques. In situ XRD experiments (in 5% H₂, up to 750 °C) reveal that FeNi and FeNi₃ coexist in alloyed Ni-Fe nanoparticles and that the exsolved Ni is only partially reincorporated in the lattice of STF-Ni on reoxidation in air. In contrast, the reduction of STF leads to the segregation of nonalloyed metallic Fe particles. In situ near-ambient pressure XPS experiments (20 mbar, 550 °C) show that Sr segregates on both perovskites as SrO _x during reduction in pure H₂, and that the exsolution of Ni and Fe enhances the segregation. Subsequent exposure to CO₂ causes the formation of SrCO₃ and compositional changes of the STF-Ni nanoparticles, which become richer in Ni due to the back-diffusion of Fe in the lattice. When applied as fuel electrodes of electrolyte-supported solid oxide cells, STF-Ni and STF exhibit distinct behaviors in H₂ electro-oxidation and CO₂ electrolysis. STF-Ni shows better performance than STF with 3% humidified H₂ supply (450 vs 350 mW/cm² at 0.5 V), while both electrodes achieve similar current density (-450 mA/cm² at 1.4 V) in reversible CO₂ electrolysis with a 50/50 CO/CO₂ mixture. These performance differences primarily arise from the interaction with CO₂, which causes SrCO₃ formation, electrode passivation, and compositional modifications of the nanoparticles.