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

TL;DR

This paper studies how adding nickel to a perovskite material affects its performance as an electrode in solid oxide cells, focusing on structural and electrochemical changes under different gas conditions.

Contribution

The study reveals how Ni doping influences exsolution, surface composition, and electrochemical performance in perovskite electrodes under operational conditions.

Findings

Ni doping leads to partial reincorporation of exsolved Ni during reoxidation, unlike non-doped STF.

Exsolution of Ni and Fe enhances Sr segregation and alters nanoparticle composition upon CO2 exposure.

STF-Ni outperforms STF in H2 electro-oxidation but both show similar performance in CO2 electrolysis.

Abstract

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 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.

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