Frequency-Resolved Evaluation of Electrode Oxidation State in La0.6Sr0.4CoO3 Thin Films with Operando XAS

Abstract

Commercial adoption and implementation of renewable, but intermittent, energy sources relies on a new generation of high-performance energy conversion and storage devices, such as solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC). Performance of the oxygen electrode in SOFC is a limiting factor; however, improvements are hindered by inadequate understanding of electrochemical reaction and ionic diffusion processes. AC impedance and DC polarization techniques are typically used to infer relationships between driving energy and reaction rate, but uniquely understanding reaction mechanisms without direct chemical state measurements of the electrode is challenging. In recent years, workers have developed in operando X-ray absorption spectroscopy (XAS) to study oxygen chemical potentials in SOFC cathodes under operating temperatures and Po2. Determining the oxygen chemical potential is based on shifts in the X-ray absorption edge of 3d transition metals due to valence changes to compensate for oxygen vacancies. So far, operando XAS has only been measured under DC polarization, and thus lacked the ability to probe processes of different timescales [1]. In this work, we extend operando XAS to AC impedance measurements, allowing direct frequency-resolved measurement of electrode oxidation state. This approach can potentially be applied to nonlinear harmonic responses, and the first step in the spatially-resolved mapping of the local steady-periodic electrode response. A dense film of La0.6Sr0.4CoO3 (LSC) was deposited on a mirror polished Gd-doped ceria pellet by pulsed laser deposition as the working electrode with porous Pt counter and reference electrodes. Absorption spectra of the Co K-­edge were collected from fluorescent X-rays to determine the detection energy most sensitive to changes in Co valence state. Operando XAS measurements were conducted at 973 K in 10% O2 at the predetermined X-ray energy, typically around 7.718 keV, under electrical perturbations large enough to excite moderate nonlinearity. The frequency resolved oxidation state response of the electrode is obtained by converting X-ray absorption data to oxygen chemical potentials via sets of calibration XAS spectra at various effective Po2. As a result, the data can be treated by the same analysis methods developed for the voltage response in AC impedance measurements.