Nonlinear Impedance Analysis of La[subscript 0.4]Sr[subscript 0.6]Co[subscript 0.2]Fe[subscript0.8]O[subscript3-[subscript[delta]] Thin film Oxygen Electrodes

Abstract

Linear and nonlinear electrochemical impedance spectroscopy (EIS, NLEIS) were used to study 20 nm thin film La[subscript 0.6]Sr[subscript 0.4]Co[subscript 0.2]Fe[subscript 0.8]O[subscript 3][subscript -][delta] (LSCF-6428) electrodes at 600°C in oxygen environments. LSCF films were epitaxially deposited on single crystal yttria-stabilized zirconia (YSZ) with a 5 nm gadolinium-doped ceria (GDC) protective interlayer. Impedance measurements reveal an oxygen storage capacity similar to independent thermogravimetry measurements on semi-porous pellets. However, the impedance data fail to obey a homogeneous semiconductor point-defect model. Two consistent scenarios were considered: a homogeneous film with non-ideal thermodynamics (constrained by thermogravimetry measurements), or an inhomogeneous film (constrained by a semiconductor point-defect model with a Sr maldistribution). The latter interpretation suggests that gradients in Sr composition would have to extend beyond the space-charge region of the gas-electrode interface. While there is growing evidence supporting an equilibrium Sr segregation at the LSCF surface monolayer, a long-range, non-equilibrium Sr stratification caused by electrode processing conditions offers a possible explanation for the large volume of highly reducible LSCF. Additionally, all thin films exhibited fluctuations in both linear and nonlinear impedance over the hundred-hour measurement period. This behavior is inconsistent with changes solely in the surface rate coefficient and possibly caused by variations in the surface thermodynamics over exposure time.