Decoupled electrolysis using silicotungstic acid as a redox mediator for zero-carbon hydrogen production

Water electrolysis for hydrogen production has substantial potential to address the current energy crisis while mitigating environmental pollution. However, achieving truly green hydrogen production requires using materials better suited to intermittent power generation, as existing systems pose serious concerns over gas mixing and cell component degradation. This thesis examines the concept of decoupled water electrolysis, utilising silicotungstic acid (H4SiW12O40) as a redox mediator in a flow-cell system to generate hydrogen. Decoupled electrolysis offers exceptional flexibility by enabling the separate production of hydrogen and oxygen at different times and rates, reducing gas crossover issues to a minimum. Chapter 1 explores the green hydrogen production route in a net-zero world and the various methods employed in green hydrogen production. Furthermore, we introduce the concept of decoupled water electrolysis, an emerging approach for green hydrogen production via electrochemical processes, and discuss how it can potentially address some of the challenges associated with renewable-driven hydrogen production. Chapter 2 provides background information on the theory underlying the experimental techniques used throughout this study. Various electrochemical and analytical methods were employed to monitor changes in potential, current, charge passed, and the electrode surface during the reduction of the redox mediator. Chapter 3 highlights the application of decoupled water electrolysis in a flow system using silicotungstic acid as the redox mediator by assembling two electrochemical cells for hydrogen production (mediator re-oxidation) and oxygen production (mediator reduction) while applying a range of commercially relevant current densities (0.05–1.35 A/cm2 ) to monitor the liquid mediator behaviour as it was circulated continuously between the two cells as hydrogen was produced. In Chapter 4, regenerated cellulose dialysis membranes were employed in the oxygengenerating cell, in order to compare the resulting electrochemical system with the one from Chapter 3 (which used only perfluorinated membranes). A comparative analysis of the membrane (before vs after electrolysis), current density, and the decoupling efficiency (%) obtained in this section was conducted. Chapter 5 contains overall conclusions and discusses future work. It provides a summary of the work and suggestions for future research.