Transition-metal oxides with the perovskite structure are promising catalysts to promote the kinetics of the oxygen evolution reaction (OER). To improve the activity and stability of these catalysts, a deeper understanding about the active site, the underlying reaction mechanism, and possible side reactions is necessary. We chose smooth epitaxial (100)-oriented La 0.6 Sr 0.4 MnO 3 (LSMO) films grown on Nb:SrTiO 3 (STNO) as a model electrode to investigate OER activity and stability using the rotating ring−disk electrode (RRDE) method. Careful electrochemical characterization of various films in the thickness range between 10 and 200 nm yields an OER activity of the epitaxial LSMO surface of 100 μA/cm 2 ox at 1.65 V vs RHE, which is among the highest reported for LSMO and close to (110)-oriented IrO 2 . Detailed post-mortem analysis using XPS, XRD, and AFM revealed the high structural and morphological stability of LSMO after OER. The observed correlation between activity and Mn vacancies on the surface suggested Mn as the active site for the OER in (100)-oriented LSMO, in contrast to similar perovskite manganites, such as Pr 1−x Ca x MnO 3 . The observed Tafel slope of about 60 mV/dec matches the theoretical prediction for a chemical ratelimiting step that follows an electrochemical pre-equilibrium, probably O−O bond formation. Our study established LSMO as an atomically flat oxide with high intrinsic activity and high stability.

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.

Studying catalysts in situ is of high interest for understanding their surface structure and electronic states in operation. Herein, we present a study of epitaxial manganite perovskite thin films (Pr 1−x Ca x MnO 3 ) active for the oxygen evolution reaction (OER) from electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES) at the Mn L-and O K-edges, as well as X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states have been performed in ultra-high vacuum and in water vapor under positive applied bias at room temperature. It is shown that under the oxidizing conditions of the OER a reduced Mn 2+ species is generated at the catalyst surface. The Mn valence shift is accompanied by the formation of surface oxygen vacancies. Annealing of the catalysts in O 2 atmosphere at 120 °C restores the virgin surfaces.

Environmental Transmission Electron Microscopy (ETEM) studies offer a great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants but also pose big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr 0.64 Ca 0.36 MnO 3 (PCMO) thin film electro-catalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modelling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, sample-conductivity and grounding, and gas properties. We present evidence that two observed electro-chemical reactions are driven by the beam induced electrostatic potential of the order of a volt. The first reaction is an anodic electrochemical oxidation reaction of oxygen depleted amorphous PCMO which results in a recrystallization of the perovskite structure. The second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO 2-x at catalytically active surfaces. Recently published in-situ XANES observation of subsurface oxygen vacancy formation during oxygen evolution at a positive potential [ 32 ] is confirmed in this work. The quantification of beam induced potentials is an important step for future controlled electro-chemistry experiments in an ETEM. 1. Introduction: Electro-catalysts are of high importance in speeding up electro-chemical reactions via the control of reaction steps and activation barriers [ 1 ]. Typically, electro-catalysts are used in the form of electrodes that enable the transfer of electrical charges into absorbed reactants. An ideal catalyst is not involved in the catalysed electro-chemical reaction. However, high catalytic performance typically requires materials with a sufficiently flexible atomic and electronic surface structure in order to affect the reaction processes [ 2 , 3 ]. In their active state, catalysts often undergo significant changes in surface and defect structure. A recent example even shows that a water splitting electro-catalyst can be formed during its activity [ 4 ]. Whether the catalyst forms desired highly active states, undergoes undesired changes which reduce its activity or even shows irreversible corrosion can sensibly depend on the specific ambient conditions. In-situ atomic scale studies of electro-catalysts in working conditions can contribute substantially to provide insights into the underlying mechanism [ 5 , 6 ].

The electronic structure of Pr1−xCaxMnO3 has been investigated using a combination of firstprinciples calculations, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions, x = 0, 1/2, 1, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study has been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental spectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the coexistence of magnetic orders respectively to the occupation of local orbitals. A consistent treatment of the Coulomb interaction indicate a partial cancellation of Coulomb parameters and support the dominance of the electron-phonon coupling.