Correlation of Time-Dependent Oxygen Surface Exchange Kinetics with Surface Chemistry of La0.6Sr0.4Co0.2Fe0.8O3-delta Catalysts

Abstract

The Sr segregation at the surface of a perovskite La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) oxygen electrode is detrimental to the electrochemical performance and durability of energy conversion devices such as solid oxide fuel cells. However, a quantitative correlation of degradation of the oxygen surface exchange kinetics with Sr precipitation formation at the LSCF surface is not clearly understood yet. Herein, the correlation of the time-dependent degradation mechanisms of the LSCF catalysts with respect to Sr segregation phenomenon at the surface were investigated at 800 degrees C for a prolonged annealing time (similar to 800 h) by combining in situ electrochemical measurements, and ex situ chemical and structural analyses at the multiscale. The in situ monitored surface exchange coefficient (k(chem)) was found to drastically drop by similar to 86% over the 800 h, and it was accompanied by the formation of Sr-containing secondary phases on the bulk LSCF surface, as expected. However, the estimated coverage of Sr segregation on the LSCF surface was only similar to 15%, even after 800 h of aging time, showing significant deviation from the k(chem) degradation rate (similar to 86%). The surface chemistry evolution at the clean surface area, which is believed to be electrochemically active, was further analyzed on the nanoscale. The quantified results showed that the Sr elemental fraction of the A-site at the outermost surface of the LSCF samples was becoming deficient from similar to 4.0 at 0 h to similar to 0.27 at 800 h annealing. Interestingly, the time-dependent behavioral tendencies between k(chem) degradation and surface Sr fractional changes were highly analogous. Thus, our results suggest that this Sr deficiency at the clean surface region more dominantly impacts the degradation process rather than an electrochemical activity passivation by the SrOx precipitates, which has been shown to be a major degradation mechanism of LSCF performance.