In Situ Electrochemical Electron Microscopy Study of Oxygen Evolution Activity of Doped Manganite Perovskites

Fundamental studies of catalysts based on manganese oxide compounds are of high interest since they offer the opportunity to study the role of variable valence state in the active state during O2 evolution from H2O. This paper presents a study of doping dependent O2 evolution electrocatalysis of Pr-doped CaMnO3 via in situ environmental transmission electron microscopy (ETEM) combined with ex situ cyclic voltammetry studies. ETEM studies of heterogeneous catalysis are a challenge, since the reactions in the H2O vapor phase cannot directly be observed. It is shown that the oxidation of silane by free oxygen to solid SiO2-x can be used to monitor catalytic oxygen evolution. Electron energy loss spectroscopy (EELS) as well as the in situ X-ray absorption study of near edge structures (XANES) in H2O vapor reveals that the Mn valence is decreased in the active state. Careful TEM analysis of samples measured by ex situ cyclic voltammetry and an in situ bias-controlled ETEM study allows us to distinguish between self-formation during oxygen evolution and corrosion at the Pr1-xCaxMnO3-H2O interface. Including density functional theory (DFT) calculations, trends in O2 evolution activity and defect chemistry in the active state can be correclated to doping induced changes of the electronic band structure in A-site doped manganites.