Quantifying Dynamic Changes of Oxygen Nonstoichiometry and Formation of Surface Phases of SrCoO_<i>x</i> Electrocatalysts by <i>Operando</i> Characterizations

Perovskite electrocatalysts like strontium cobaltite (SrCoO_<i>x</i>, denoted as SCO) experience dynamic changes in both surface and bulk during the oxygen evolution reaction (OER), rather than remaining static. This dynamic, electrochemically driven evolution in composition, structure, and ionic defects (<i>e.g.,</i> oxygen vacancies) can strongly impact the OER activity and stability. Yet, the current lack of quantitative information on these processes impedes a precise and predictive evaluation of the individual and combined effect of both bulk and surface transformations. Here, using epitaxial SCO thin films as a model system, we demonstrate that SCO is a bulk and surface redox-active OER electrocatalyst that undergoes a bulk phase transition via electrochemically induced oxygen intercalation, as well as a surface phase transition toward Co (oxy-)hydroxide. Specifically, applying a suite of <i>operando</i> and <i>ex situ</i> characterization we established a reliable relationship between oxygen nonstoichiometry, optical density, and conductivity as a function of applied potentials. We further accurately quantify the evolution of oxygen stoichiometry in the SCO bulk and the thickness of the formed surface secondary phase. Our work provides a reliable and generalizable workflow and <i>operando</i> characterization toolbox for quantitative assessment of surface and bulk transformations in oxygen-deficient perovskite electrocatalysts.