Atom Surface Dynamics of Manganese Oxide under Oxygen Evolution Reaction-Like Conditions Studied by In Situ Environmental Transmission Electron Microscopy

Hydrogen production by electrochemical water splitting is limited by the sluggish oxygen evolution reaction (OER). In order to improve our understanding of the underlying mechanisms, information about the atomic surface structure of the active state of the electrode is required. Here, we present environmental transmission electron microscopy studies of Ca-birnessite (K0.20Ca0.21MnO2.21·1.4H2O) electrodes under conditions close to those of the OER. Remarkably, in H2O vapor, a highly dynamic state of the surface and subsurface develops with a thickness of the formed dynamic layer of up to 0.6 nm, which is absent in O2 and inert gases. Electron beam-induced effects are carefully studied, showing high stability of the material against radiation damage in high vacuum until a dose rate of 42,000 e–/(Å2 s). In contrast, in H2O, the dynamic surface layer develops and forms a stationary state even at low dose rates, down to 5000 e–/(Å2 s). Electron energy-loss spectroscopy reveals an increase in the Mn oxidation state in H2O and in O2 ambient. Our results are interpreted as the formation of a few-angstrom-thick, dynamic, and hydrated surface layer of birnessite in H2O, with an increased Mn valence state. Such a dynamic surface layer with a flexible Mn coordination and valence state might be optimal for oxygen evolution due to the higher effective interaction volume beyond the surface area and a flexible bond coordination of partially hydrated Mn species.