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 Sr0.93Ti0.3Fe0.63Ni0.07O3−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.