• Hein, Dennis Matthias: Photoelectron spectroscopy of transition metal oxide–electrolyte interfaces relevant for catalytic water splitting. , Dissertation Humboldt-Universität Berlin, 2021

The increasing use of renewable energies requires long-term storage systems to cope with their non-constant availability. Solar-fuels, in particular hydrogen, are the most promising candidates. Its large-scale production can be covered by catalytic water splitting, in which a voltage is applied between two electrodes in an electrolyte solution, producing hydrogen and oxygen. However, the high losses during the oxygen evolution reaction (OER) at the anode are one bottleneck, calling for the improvement, development, and design of suitable and abundant electrocatalysts. Noble metals like platinum, iridium or palladium offer excellent efficiency and stability, but lacking in cost-effectiveness and abundancy for large -scale applications. Transition metal oxides are a promising material class to replace noble-element-based electrocatalysts. However, a complete understanding of the catalyst–electrolyte interface is still missing, necessary for the development and design of better oxygen reaction catalysts. To gain insight into the catalyst–electrolyte interface I used partial electron yield X-ray absorption spectroscopy (PEY-XAS) and (resonant) photoelectron spectroscopy (PES) because they offer high element-specific sensitivity to the local electronic structure and the chemical bonding. PES is a surface-sensitive method that requires vacuum conditions and tunable soft X-ray photons, asking for a synchrotron light facility and sophisticated experimental designs. I used the following three techniques for my investigations: 1) liquid microjet PES, 2) condensation of a few monolayer thin liquid H2O film on top of a solid sample in a near-ambient pressure water atmosphere (~2 mbar), and 3) the free-standing graphene approach, using a graphene bilayer that is thinner than the electron mean-free path, transparent for X-ray photons, and robust enough to withstand a pressure difference between electrolyte (1 bar) and vacuum (10-5 mbar). With these experimental techniques I focused on studying the catalyst–electrolyte interface of three promising OER catalyst: CoOx nanoparticles, nickel-ferrite (Fe2NiO4) nanoparticles, and mixed iron-nickel oxides. Using near-ambient pressure PES, I identified a reversible phase change in size-selected CoOx nanoparticles on a SiOx/Si(111) substrate. Under high vacuum conditions these NPs consist of an octahedrally coordinated (Oh) Co2+ and a tetrahedrally coordinated (Td) Co2+ phase, with a proposed Td shell. In a water atmosphere, the composition is altered to an Oh Co3+ and an Oh Co2+ phase. Additionally, resonant PES revealed that the cobalt-oxide valence band consist of Oh Co2+ + Td Co2+ contributions at high vacuum and of Oh Co3+ + Oh Co2+ species under near-ambient pressure conditions. A similar near-ambient pressure PES study of nickel-ferrite nanoparticles revealed the water-induced reversible formation of Fe2+ from Fe3+, while Ni2+ ions showed no oxidation or phase change. The associate change in the Fe–O bond in the nickel-ferrite lattice was indicated by oxygen PEY-XAS. This oxidation state change of iron might be caused by the adsorption of hydroxyl groups, due to water dissociation on the surface without any applied voltage. Additionally, I performed operando TFY-XAS measurements at the nickel L3-edge that showed the oxidation of Ni2+ into Ni3+/4+ under OER conditions. In the third part of my thesis, I used the free-standing graphene membrane approach to perform operando PES at iron-nickel oxides, where the quality of the used graphene bilayers is crucial for its stability. I present the successful in-situ electrodeposition of an iron-nickel oxide catalyst film on a graphene bilayer and first results from the catalyst–electrolyte interface with PES. The subsequent operando study under OER conditions indicated the oxidation of nickel under applied voltages. An operando C 1s PES study of post-mortem samples demonstrate the challenges of this method due to the oxidation of graphene.