Pr$_{1-x}$Ca$_x$MnO$_x$ for Catalytic Water Splitting - Optical Properties and In Situ ETEM Investigations

Abstract

In this thesis, Ca-doped PrMnO$_3$ (PCMO) is evaluated as a catalyst for (photo)electrochemical water oxidation. The investigations are focused on elementary steps of the overall process. First, the optical absorption in PCMO is studied as a function of temperature and doping level in order to understand the impact of correlation effects on the optical properties. The results show that the formation of small polarons in PCMO as a result of strong correlation interactions provokes a broad absorption maximum in the near-infrared to visible energy range. This absorption maximum is discussed in the framework of photon-assisted polaron hopping and excitation between Jahn-Teller split states. The doping dependence of the optical spectra indicates the relevance of Mn 3d-O 2p hybrid states for the electronic structure near the Fermi energy and that the relative O 2p contribution varies with the Ca-doping level. Second, the active state of PCMO in contact with water or water vapor is studied for different doping levels using cyclovoltammetry as well as in situ environmental transmission electron microscopy (ETEM). The results of both methods reveal that the catalytic water oxidation according to $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4 \text{H}^+$ competes with a corrosion process in terms of a Pr/Ca dissolution and amorphization of the PCMO electrode. The highest catalytic activity, as well as chemical stability, has been found for intermediate doping levels. In the framework of the in situ ETEM experiments, it is demonstrated that the addition of monosilane to a water containing gas electrolyte allows detection of an electron beam driven water oxidation at active PCMO surfaces via the secondary reaction $\text{SiH}_4+2\text{O}_2\rightarrow\text{SiO}_2+2\text{H}_2\text{O}$. Electron energy loss spectroscopy before and after the reaction in the gas electrolyte shows that the active state of PCMO involves the generation of oxygen vacancies as well as their annihilation via intercalation of the oxygen evolved by catalytic water oxidation. By means of electron holography, electrical measurement, and theoretical considerations based on the secondary electron emission, the role of the electron beam as the driving force for catalytic water oxidation in the ETEM is identified as a positive electron beam induced electric potential.