Time-resolved absorption spectroscopy and electrochemistry have been applied to the investigation of catalyst materials that facilitate water splitting for the conversion of water and renewable energy into hydrogen and oxygen. The goal of this thesis has been to better understand structure-property relationships in different water splitting catalysts by identifying multi-redox states and establishing their role in the catalytic mechanism and the catalytic activity.
Chapters 1-2 investigate photocatalytic electrodes based on molecular organometallic catalysts for water reduction and carbon dioxide reduction immobilised on mesoporous TiO2. This research seeks to parametrise the interplay between charge accumulation, recombination, and catalytic reaction pathways, and their impact on the catalytic efficiency of photocatalytic systems. Chapter 1 focuses on investigating the charge transfer and accumulation processes from the TiO2 to the catalyst in acetonitrile. Chapter 2 studies the activity of H2-production systems in the presence of water to monitor protonated multi-reduced catalytic intermediates. It has been possible to minimise recombination and to optimise the catalytic production by regulating the applied potential, the excitation light intensity, and the photoelectrode surface coverage.
Chapters 4-5 focus on iridium-based water oxidation electrocatalysts and aim at measuring their activity per iridium atom or state. In Chapter 4, a mathematical method is developed to deconvolve the absorption coefficient, the potential-dependent concentration and the water-oxidation kinetics of different redox states in electrodeposited hydrous iridium oxide IrOx under different conditions. Chapter 5 compares the results of IrOx with different iridium content to those of a molecular dimeric iridium complex immobilised on mesoporous ITO. Active redox states are identified under different applied potentials, and their kinetics are measured. Their mechanism and the role of the coordination sphere and chemical surrounding is discussed. Finally, Chapter 7 explores the photoreducing properties of an iridium-based molecular photocatalyst by measuring the reaction kinetics of the photoexcited reduced state.