Numerical and experimental studies on bubble behaviour and electrochemical reaction effects in an alkaline water electrolyser

Alkaline water electrolysis is a promising technology for sustainable hydrogen generation. In this work, both Computational Fluid Dynamics (CFD) and experimental approaches were applied to provide a better understanding of the two-phase flow behaviour within an electrolyser cell, and thus, a better design was proposed to reduce the mass transfer limitation often reported in literature. A hybrid Eulerian-Eulerian-VOF numerical model was previously applied to investigate the two-phase flow and bubble behaviour, coupling two frameworks in a separate computational domain, which may lead to model instability. In Chapter 3, a singleframework Eulerian-Eulerian-Multi-Fluid-VOF model was adopted instead, offering a insightful understanding of the internal behaviour during electrochemical reactions, thus providing a predictive tool for performance improvement. The kinetic parameters found within the Butler-Volmer equation are crucial in determining electrochemical reaction rates. The kinetic data previously reported for CFD modelling work showed inconsistencies; thus, the selection of correct values is critical to ensure model reliability. Chapter 4 focused on the experimental extraction of kinetic data based on an actual system, providing reliable predictions of polarisation curve behaviour, rather than relying on previous literature data, which was the common practice in past simulation work. Adopting the commonly reported literature data for predicting the polarisation curve in my current CFD work resulted in mean average relative errors up to 11%, while incorporating tailored experimental data reduced the errors to less than 3%. The CFD numerical model was then used for parametric analysis. This study was essential to identify the optimum electrolyte concentration and inlet flow rate to enhance mass transport, thereby improving the overall efficiency and operational stability of the electrolyser. At 30 wt% and 0.5 m/s, the electrolyser cell exhibits the most optimal performance, characterised by a more uniform electrolyte distribution, resulting in improved electrolyser performance. Finally, new flow channel designs that addressed the mass transport limitations identified in the modelling work were proposed, and their performance was evaluated experimentally. The proposed circular-edged design improved electrolyser performance at higher flow rates by enhancing bubble removal rate, with 18% improvement over the parallel design at the same operating voltage.