  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.
