Origin of Enhanced Chemical Capacitance in La_0.8Sr_0.2CoO_3-δThin Film Electrodes

Abstract

Films of La0.8Sr0.2CoO3-δ (LSC-82) with 45 and 90 nm thickness were grown epitaxially on (001) oriented single crystal yttriastabilized zirconia with a thin gadolinium-doped ceria interlayer using pulsed laser deposition, and characterized using X-ray diffraction (XRD), depth-profile secondary mass spectrometry (DP-SIMS), and linear and nonlinear electrochemical impedance spectroscopy (EIS and NLEIS). The films were found to exhibit in-plane tensile strain and normal compressive strain (with overall increased lattice volume) relative to freestanding cubic LSC-82. The films also possess a compositional La/Sr gradient across their thickness, with enhanced Sr2+ composition (within the perovskite lattice) at the gas-exposed surface. The oxygen storage capacity of the films at 450-600°C (as measured using EIS capacitance) is greatly enhanced relative to freestanding cubic LSC-82, consistent with an apparent increase in oxygen-vacancy concentration at the gas-exposed surface of the films (as revealed by NLEIS). These results can be explained by a two-layer model in which a finite thickness of the perovskite lattice near the surface has an enhanced Sr dopant concentration, leading to a much higher concentration of oxygen vacancies than the underlying bulk material of nominal Sr composition. The nonlinear electrochemical response of the film is consistent with a dissociative adsorption rate law, provided the enhanced bulk vacancy concentration near the surface is included in the analysis. © 2013 The Electrochemical Society. All rights reserved.