Revealing timescale-dependent oxygen vacancy distributions in solid oxide fuel cell electrodes using frequency-resolved X-ray absorption (FR-XAS)

Development of materials for electrochemical energy conversion requires a deep understanding of the factors governing chemical and physical rates at submicron length scales. Many workers have sought to develop chemically sensitive in situ or operando imaging techniques targeting these length scales. However, current methods focus on steady-state or stepwise response. To probe electrode processes both spatially and temporally, we have developed a frequency-resolved implementation of X-ray absorption imaging (FR-XAS) that can measure local electrochemical response in operando during a global sinusoidal impedance measurement. Frequency-resolved 1-D images of the oxygen vacancy distribution in a thin film SOFC cathode material ($La_{1-x}Sr_xCoO_{3- \delta}$) reveal, for the first time experimentally, the defect concentrations associated with a Warburg and Gerischer impedance. Analysis of these images allows direct extraction of diffusion and kinetic rate parameters, independent of the global impedance.