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 Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni) and SrTi0.3Fe0.7O3 (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 SrOx 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.