Molecular Copper and Ruthenium Water Oxidation Electrocatalysts for Light-Driven Water Splitting

Abstract: In recent years, there has been a growing awareness of the need to transition to renewable energy to mitigate the effects of climate change and reduce greenhouse gas emissions. Governments, businesses, and individuals are investing in renewable energy infrastructure and technologies to create a more sustainable energy future. This thesis explores transition metal-based molecular complexes for water oxidation catalysis and their integration into water-splitting devices. Water oxidation (WO) is a key reaction in all processes intended to produce chemical fuels from water. When water splitting is driven by solar light, it is also referred to as artificial photosynthesis.The work focuses on a molecular copper catalyst with a tetra-amidate macrocyclic ligand (TAML), called [CuMac]2−, which can be isolated as a (Me4N)2[CuMac] salt. Derivatives of the organic ligand scaffold bearing either two ester or two carboxylic acid functionalities were synthesized to enable heterogenization of [CuMac]2−. The compounds were evaluated for their oxygen evolution reaction (OER) activity at different operational pH. However, introducing the carboxylic moieties appeared to be kinetically detrimental to the OER activity compared to that of the parent [CuMac]2−. A study on identifying possible short-lived intermediates in the catalytic cycle of [CuMac]2−is presented. UV-visible transient absorption spectroscopy was used to deduce the electron transfer mechanism and kinetics of catalyst oxidation in an attempt to understand the catalytic cycle's early steps. Using [Ru(bpy)3]Cl2 and persulfate as the photosensitizer and the oxidative quencher, respectively, flash-photolysis was employed to gather evidence of the one-electron oxidized catalyst and to follow intermediates that could previously not be resolved temporally by electrochemical means.Printed electrodes (PEs) were fabricated using graphite as the conductive material and a polymer binder P(MMA-s-HEMA). The PEs were then covalently functionalized with [CuMac]2−using an in-situ electroreduction of the corresponding bis(diazonium) compound (N2)2[CuMac]. The resulting heterogenized catalyst's activity was evaluated by cyclic voltammetry to find that it was impeded compared to that of [CuMac]2− in homogeneous solution phase.Finally, a ruthenium-based coordination oligomer water oxidation catalyst was integrated into a flow-electrolyzer powered by a perovskite/perovskite tandem solar cell to drive water splitting by light. An operational voltage of 1.4 V was achieved, and a solar-to-hydrogen efficiency (STH) of 12.5 %. However, the stability of the system remains to be improved, which could be achieved by changing the membrane separating the two electrolyte compartments. Such adjustments can fine-tune the local pH surrounding the catalyst, which is an essential parameter for catalyst activity and integrity.