  Furthering the understanding of the catalytic mechanisms in the oxygen
reduction reaction (ORR) is critical to advancing and enabling fuel cell
technology. In this work, we use multimodal operando synchrotron X-ray
diffraction (XRD) and resonant elastic X-ray scattering (REXS) to investigate
the interplay between the structure and oxidation state of a Co-Mn spinel oxide
electrocatalyst, which has previously shown ORR activity that rivals Pt in
alkaline fuel cells. During cyclic voltammetry, the electrocatalyst exhibited a
reversible and rapid increase in tensile strain at low potentials, suggesting
robust structural reversibility and stability of Co-Mn oxide electrocatalysts
during normal fuel cell operating conditions. At low potential holds, exploring
the limit of structural stability, an irreversible tetragonal-to-cubic phase
transition was observed, which may be correlated to reduction in both Co and Mn
valence states. Meanwhile, joint density-functional theory (JDFT) calculations
provide insight into how reactive adsorbates induce strain in spinel oxide
nanoparticles. Through this work, strain and oxidation state changes that are
possible sources of degradation during the ORR in Co-Mn oxide electrocatalysts
are uncovered, and the unique capabilities of combining structural and chemical
characterization of electrocatalysts in multimodal operando X-ray studies are
demonstrated.
