Abstract
Linear and nonlinear electrochemical impedance spectroscopy (EIS, NLEIS) were used to study 20 nm thin film La0.6Sr0.4Co0.2Fe0.8O3-δ (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.
Highlights
The MIT Faculty has made this article openly available
In order to address some of these challenges, this paper focuses on the application of nonlinear electrochemical impedance spectroscopy (NLEIS) to study O2 reduction/oxidation on LSCF thin films
Thermodynamic behavior.—The magnitude of the V SC, as well as close agreement between the measured thin film capacitance and that predicted based on the measured oxygen nonstoichiometry in bulk LSCF suggests that the majority of the measured V SC arises from the oxygen storage capacity of LSCF
Summary
The MIT Faculty has made this article openly available. Please share how this access benefits you. The thermodynamic understanding of La1-xSrxCo1-yFeyO3-δ derives largely from measurements of equilibrium oxygen nonstoichiometry (δ0) upon exposure to different oxygen environments and temperatures.[4,9,10,11] In the limit of high B-site iron content (y > 0.6), workers have interpreted this δ0 − pO2 − T relationship using a ptype point-defect model originally developed for LSF (LSCF, with y = 1),[12] where electrons are localized on iron centers of discrete charge and defects are considered dilute.[4,9] For Co-rich compositions (y < 0.4), a metallic model used to describe LSC (LSCF, with y = 0) yields better agreement.[4] While La0.6Sr0.4Co0.2Fe0.8O3-δ exhibits semiconducting behavior over a wide range of pO2 and temperature, the observed δ0 − pO2 relationship deviates substantially from both p-type and metallic models under oxidizing conditions ( pO2 > 10−4, lower temperature) The cause for this departure from a p-type model is unclear, though some workers have sought to hybridize p-type and metallic models to describe LSCF.[11] Others speculate that LSCF’s surface is more reducible than its bulk, based on first-principles calculations[13] and an experimentally observed increase in oxygen nonstoichiometry with pellet surface area under oxidizing conditions.[9]. Low energy ion scattering (LEIS) revealed a nearly Sr and O-saturated surface monolayer for polished pellets and films alike when annealed above 600◦C.22,23
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