In Situ, Non-Contact Studies of Oxygen Exchange Kinetics in Thin Film Mixed Conducting Electrodes

Abstract

More rapid and durable surface exchange kinetics at the oxygen electrode are desirable for long-term operation of reversible solid oxide cell (R-SOC) devices. Understanding and improving the oxygen exchange kinetics would benefit from measurement techniques that enable in situ and continuous evaluation of both the defect equilibria and kinetics of native electrodes (without potentially blocking or catalytic metal current collectors). The optical transmission relaxation (OTR) technique is one such approach, which relies on the change in optical absorption signatures of thin film electrode materials upon changing their oxygen contents, typically corresponding to changes in transition metal cation valence states. In this work, the OTR method was applied to thin film electrodes of Sr(Ti,M)O3-x (M=Fe, Co) to evaluate both point defect equilibria and surface exchange kinetics over a range of temperatures and oxygen partial pressures. While the thin film defect equilibria derived this way showed reasonable agreement to established bulk defect chemistry, the surface exchange kinetics were additionally compared to those measured by other methods (curvature relaxation, simultaneous ac-impedance spectroscopy, and simultaneous conductivity relaxation). Again reasonable agreement was found, though the rapid aging behavior of the films' surface exchange rate likely was the cause of some discrepancy. Additionally the surface exchange kinetics measured by ac-impedance spectroscopy were slightly slower than those measured optically, a result that may be attributed to a potentially blocking role of porous current collectors in the former approach. The benefits and limitations of the OTR approach will be discussed, including its particular utility in evaluating the impact of electrode microstructure on exchange kinetics.