Wartner, Garlef: Interfacial Photoelectron- and Absorption Spectroscopy Studies of Earth Abundant Catalyst Materials for the Oxygen Evolution- and Reduction Reaction. , Humboldt Universität zu Berlin, 2022
Abstract:
The future use of renewable energy sources is accompanied by the problem of spatially and temporally limited energy supply. To adjust supply- and demand fluctuations, especially on the seasonal scale, requires efficient storage solutions. Electrolysis-derived fuels, such as hydrogen, are suitable for this purpose, but the efficiency of electrolysis applications has so far been limited by the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The development of suitable catalysts is focused on transition metal-based compounds due to their widespread availability and low financial costs. In order to further optimize these materials and design more efficient catalysts, deeper insights into the catalytic processes on the atomic scale are required. In this work, advanced spectroscopic methods are used to characterize the structure of catalyst-electrolyte interfaces in different states. For this, I used photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS) as suitable methods to study the coordination environment and electronic structure. I have exploited the short mean free path of detected electrons in PES to achieve a high sensitivity to interfacial processes. For this purpose I follow different trategies: The condensation of a liquid layer at near-ambient pressure (NAP), the combination of ante- and post-mortem measurements, and operando PES based on a graphene-covered ionomer membrane-electrode setup tailored for NAP operation. Along with direct PES, I avail resonant Auger-decay channels to gain insights into the occupied as well as unoccupied density of states simultaneously. Meso- and nanostructuring of the catalysts facilitates photon-detection based XAS also to be interface-sensitive. Using spectroelectrochemical cells, I performed operando XAS measurements of transition metal K- and L-edges. The studies presented in this work focus on the Ni-Fe (oxy)hydroxides material system, which represents one of the most active transition metal-based catalysts for OER in the alkaline regime. In addition, iron-nitrogen-carbon (Fe-N-C) compounds are investigated as promising materials for ORR. As a first step, I investigated the interaction between a condensed water-film and electro-deposited Ni0.75Fe0.25(OH)2 using near-ambient pressure PES. My results do not indicate dissociative adsorption of water molecules under these conditions. This suggests, that water dissociation can only be observed with an oxidizing electrode potential applied to the interface. As a step closer to operating conditions, I studied the impact of defect structure on the electrocatalytic properties of reactively sputtered Ni0.75Fe0.25Oy by coupling electrochemical measurements to thorough ante and post mortem investigations based on X-ray spectroscopy and complementary methods. I determined changes of the defect structure within a series of samples prepared under systematically varied deposition conditions, which exhibit a constant Fe/Ni-ratio. Ante- and post mortem PE-spectra reveal the formation of a hydroxide phase during continuous activation triggered by cyclic voltammetry (CV). Moreover, I observed a distinct activity trend with changed deposition conditions, which can be correlated to the observed differences in defect chemistry. My results show, that the defect structure clearly influences the intrinsic activity and redox-kinetics, as the amount of active hydroxide phase formed on the surface during potential cycling was similar for different samples and can therefore not explain the trend in activity. Based on the graphene-capped ionomer approach, I have measured photoelectron spectra from the Fe-Ni (oxy)hydroxide-electrolyte interface, while well defined electrode potentials were applied. Potentialdependent measurements reveal a reversible oxidation of the Ni species, which is accompanied by the formation of metal-oxygen hybridized holes.