Operando Imaging Techniques for Probing Local Electrochemical Response in Solids

Abstract

Global energy supplies are transitioning to renewable sources, such as solar and wind; however, widespread adoption is contingent upon a new generation of energy conversion and storage devices, such as fuel cells and batteries. Kinetic and transport properties at the micro- and nanoscale govern these devices' performance; therefore, directly measuring them is crucial for continued development. Conventional characterization techniques–like impedance spectroscopy–must infer the microscopic behavior from an aggregate response–an average of rates over the entire sample geometry –and may be distorted by local inhomogeneities. Others have developed chemically sensitive in situ or operando techniques targeting these length scales, but these methods focus on steady-state or stepwise responses that simultaneously probe a broad range of timescales. In this work, we introduce frequency-resolved X-ray absorption spectroscopy (FR-XAS) to probe electrode response both spatially and temporally during a global impedance perturbation. As a model system, we studied a thin film solid oxide fuel cell cathode material (La1-xSrxCoO3-δ) with a patterned insulating layer to restrict contact between the electrolyte and electrode. With FR-XAS, we captured 1-dimensional images of oxygen vacancy profiles in the patterned film, representing the first direct measurements of concentration distributions associated with a Gerischer or Warburg impedance. We successfully extracted vacancy diffusion and surface exchange rate parameters using a 1-dimensional model, independent of the impedance response, which deviated from the expected Gerischer shape. Unexpected contributions from the insulating layer explain most of the discrepancy, though the exact response mechanism is an open question. Finally, we use strain-based atomic force microscopy methods to probe the space charge region (SCR) at grain boundaries in polycrystalline acceptor-doped ceria. We report images of ceria under DC polarization using scanning thermo-ionic microscopy–a recently developed technique that probes ionic concentration via thermal stress. The results show promising signs of manipulating the SPR; however, inconsistent responses across some grain boundaries and large topographic changes prevent unambiguous conclusions.