Nanoconfined materialen en strategieën voor ionische koppeling: richting elektrolysers in de dampfase

Electrochemical water splitting is a key technology for sustainable hydrogen production. However, adapting conventional liquid-phase electrolyzer designs to vapor-fed configurations present substantial challenges. Most state-of-the-art low-temperature vapor-phase water electrolyzers suffer from suboptimal electrode designs, ambiguous data acquisition protocols, and, at times, inappropriate cell assemblies. These issues often constrain performance, limiting current densities to below 50 mA/cm² (geomteric). This thesis addresses these limitations by developing and evaluating novel nanostructured electrode architectures and ionic coupling strategies specifically tailored for vapor-fed water electrolysis. The research begins with the fabrication of high surface area, micrometer thin platinum nanomesh electrodes, consisting of billions of interconnected 50 nm nanowires. These nanomesh structures enable confined in-situ electrolyte generation from deionized (DI) water, allowing electrolysis without the use of conventional ionomers or added electrolytes. Integrated into a zero-gap proton exchange membrane (PEM) electrolyzer, these electrodes achieve current densities of approximately 1 A/cm² using only DI water. Furthermore, the bare Pt nanomesh, without any additional water absorbing layers, reaches current densities of 140 mA/cm², outperforming many current vapor-fed electrolyzer systems. To further introduce ionic coupling, we demonstrate an electrochemical method to conformally coat nanomesh scaffolds with mesoporous silica thin films using electrochemically assisted self-assembly. These coatings improve water retention and establish continuous ionic pathways through adsorbed water layers. Using interdigitated electrode arrays (IDEAs), the ionic conductivity of these thin films is evaluated to be around 0.1 and 0.2 mS/cm at high relative humidity. Notably, conductivity is tunable via surface functionalization, offering insight into ion transport mediated by adsorbed water layers in the absence of bulk liquid electrolyte. In our journey towards enabling vapor-fed electrolyzers, we systematically eliminated the electrolyte supply to the electrolyzer. This thesis demonstrates catholyte-free electrolysis using an anion exchange membrane (AEM). By leveraging in-operando K⁺ ion crossover, an alkaline environment is established at the cathode, enabling sustained electrolysis in the absence of a catholyte. The thesis also critically reevaluates the terminology and assumptions surrounding 'vapor-phase electrolysis.' It distinguishes truly vapor-fed systems from those unintentionally dependent on residual liquid water or pre-soaked components. A more rigorous experimental framework is proposed, along with the concept of 'pocket-mediated electrolysis', highlighting the crucial role of confined water pockets in sustaining electrolysis. In conclusion, this thesis provides a comprehensive strategy for overcoming longstanding limitations in vapor-phase water electrolysis and provides insights to further the field. It combines innovative electrode design, ionic coupling strategies, and electrolyzer-level insights to pave the way for efficient vapor-fed electrolyzers. Moreover, the principles and architectures developed here lay a strong foundation for future applications in gas-phase CO₂ and N₂ electroreduction.