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In electrochemical cells, the main reactions usually proceed at the surfaces of electrodes and catalysts and their interfaces with the electrolyte. Hence, changes there can have a huge impact on the efficiency of the cell. This thesis concerns elementary processes at such surfaces and interfaces, which affect the electronic band structure and, thus, potentially the reactivity of the surface. Using two-photon photoelectron spectroscopy, I investigate such processes in three model systems for electrode surfaces and electrolyte/electrode interfaces:
ZnO is discussed as anode material in photoelectrochemical water splitting. Its bulk exhibits several coherent phonon modes, but so far no coherent surface phonons are known. In time-resolved photoelectron spectra, I observe oscillations of the (10-10) surface dipole, which are assigned to coherent surface phonons. Their generation relies on the resonant excitation of a defect state from a metastable defect exciton. Furthermore, I develop a method to quantify ultrafast surface dipole changes from the intensity of the secondary electron tail of a photoelectron spectrum.
At the D2O/ZnO(10-10) interface, I examine several processes induced by water adsorption. I demonstrate that some of the coherent surface phonon modes are stable against water adsorption but shift to slightly higher energies. Furthermore, the dissociation of a part of the water molecules within the first monolayer leads to a non-monotonic change in work function upon increasing coverage. Unlike in a previous study, I do not observe surface metallization upon water adsorption. Moreover, there is no clear indication of electron solvation as found at some water/metal interfaces.
At the DMSO/Cu(111) interface, a model system for the electrolyte/cathode interface in metal-air batteries, I determine the elementary steps of superoxide formation. Small polarons are formed in the DMSO adlayer within 200 fs and partly trapped in surface defects. These trapped electrons exhibit lifetimes on the order of several seconds. After O2 is co-adsorbed and diffused into DMSO, these trapped electrons react with O2 to from O2- . Modelling the diffusion process yields estimates for the electrode-reactant distance for electron transfer in DMSO.
The elementary processes examined might appear manifold, in some instances, even unrelated to each other. However, as I conclude, they are connected by underlying physical phenomena.
