Tracking the nanoparticle exsolution/reoxidation processes of Ni-doped SrTi0.3Fe0.7O3−δ electrodes for intermediate temperature symmetric solid oxide fuel cells

Abstract

The development of redox stable oxide perovskite – based electrodes for cost-effective symmetric solid oxide fuel cells (SOFCs) that can work at intermediate temperatures and compete with state-of-the-art cathodes and anodes is becoming a concrete possibility. The Ni-doped STF perovskite Sr_0.93Ti_0.3Fe_0.63Ni_0.07O_3−δ meets such requirements by exsolving catalytically active Ni–Fe nanoparticles in reducing atmospheres that boost anode performance. This work aims at clarifying whether exsolution is a reversible process, which could extend the lifetime of SOFCs. Element-specific synchrotron – based near-ambient pressure X-ray photoelectron and absorption spectroscopies are key to understanding the exsolution/reoxidation processes of the Ni–Fe nanoparticles during redox cycling in the atmosphere. This study reveals that Ni exsolves easily, dragging along the more stable Fe to form nanoalloyed Ni–Fe even under mild reducing conditions. A significant Sr-surface segregation indicates that the initial Sr-deficiency cannot fully compensate for the B-site cation depletion during exsolution. Switching to an oxidizing atmosphere results in a reoxidation-induced reconstruction of the electrode, in which a Fe- and Sr-rich oxide layer forms on the surface, leaving the Ni segregated from the perovskite. This reoxidized electrode shows a significantly improved cathode response in comparison to the pristine perovskite, indicating changes in the mechanisms that activate the oxygen reduction reaction.