Phase Formation during Reduction-Oxidation of Ni-Substituted Sr(Ti,Fe)O3-δ Solid Oxide Fuel Electrodes

Abstract

High performance solid oxide fuel cell anodes have recently been achieved via the exsolution of bimetallic (Ni,Fe) particles from a parent Sr(Ti,Fe,Ni)O3-δ (STFN) matrix. However, a number of unknowns remain regarding the detailed nature of the exsolution process, the role of initial A-site deficiency, and the effect of the change in perovskite stoichiometry during exsolution. In this study, the dynamics of exsolution are explored via thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM), both during exsolution and redox cycling. The diffraction results show that the (Ni,Fe) phase starts Ni-rich during the initial exsolution, gradually shifting to more Fe-rich with time; this result indicates that Ni diffusion towards the surface is occurring faster than for Fe. The TGA results suggest that holding STFN under too-reducing conditions can lead to continued Fe exsolution instead of reaching a stable equilibrium. TEM images display regions where SrO formed after Fe-Ni exsolution, likely as a consequence of increased A-site excess in the perovskite. In-situ re-oxidation and subsequent re-incorporation of exsolved species into the parent oxide have been suggested as a means to “reset” exsolution electrodes after degradation by, e.g., particle coarsening. The present TGA results taken during redox cycling suggest that a shorter oxidation step results in faster exsolution, a hint that, in this case, the exsolved surface species do not have sufficient time to fully re-incorporate into the oxide. The effects of new phase formation, changes in exsolved particle composition, and redox cycling on electrochemical characteristics will also be reported.Figure 1. Ex-situ X-ray diffraction scans from Sr(Ti,Fe,Ni)O3-δ after reduction in 30% H2 – 3% H2O – 67% Ar, showing peaks from the perovskite matrix along with a peak associated with exsolved (Ni,Fe) metal alloy; the latter shifts with increasing reduction time indicating that the alloy becomes increasingly Fe-rich. Figure 1