In Situ X-ray Spectroscopy and Environmental TEM Study on Manganite Water Oxidation Catalysts

This thesis presents in situ studies on the active states of manganite water oxidation catalysts in water vapor. The study in chapter 2 probes the surface electronics of the strongly correlated Pr1-xCaxMnO3 (0<x<0.8) by in situ X-ray absorption and photoemission spectroscopy (XANES, XPS). Chapter 3 presents an environmental transmission electron microscopy (ETEM) study comparing the microscopic processes in Pr1-xCaxMnO3 (x = 0.1, 0.3) and the related layered Ruddlesden-Popper system Pr0.5Ca1.5MnO4. A discussion about gas phase electrochemistry and further information on the experimental techniques is provided in Chapter 1

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

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 (Pr1−xCaxMnO3) 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 Mn2+ 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 O2 atmosphere at 120 °C restores the virgin surfaces

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr_0.64Ca_0.36MnO₃ Catalysts for Oxygen Evolution

Environmental transmission electron microscopy (ETEM) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants. It also poses big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr_0.64Ca_0.36MnO₃ (PCMO) thin film electrocatalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modeling 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, the sample’s electric conductivity and grounding, and gas properties. We present evidence that two observed electrochemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic oxidation of oxygen depleted amorphous PCMO which results in recrystallization of the oxide. The second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO_2–<i>y</i> at catalytically active surfaces. The quantification of beam induced potentials is an important step for future controlled electrochemical experiments in an ETEM