Modeling and analysis of chemical degradation and gas crossover in proton exchange membrane electrolyzers

This thesis investigates the performance and long-term durability of Proton Exchange Membrane (PEM) electrolysers, a key technology for sustainable hydrogen production. PEM electrolysers offer high efficiency, fast dynamic response, and compact system design, making them promising candidates for large-scale integration into future energy systems; their widespread deployment, however, is challenged by high system costs, operational uncertainties, and limited long-term durability, with degradation phenomena playing a central role. The objective of this dissertation is to advance the mechanistic understanding of the chemical, transport, and impurity-related processes that govern PEM electrolyser degradation. To this end, the thesis combines three complementary models with experimental validation: (i) a dynamic model of chemical membrane degradation, (ii) an experimentally validated model for hydrogen and oxygen gas crossover under industrially relevant pressures, and (iii) a multi-species ion exchange model for the removal of dissolved iron impurities. Taken together, the results support a coherent mechanistic interpretation of how oxygen crossover influences hydrogen peroxide formation and radical chemistry, how iron ingress amplifies chemical membrane degradation via Fenton reactions, and how ion exchange mitigates degradation by limiting the availability of catalytic impurities. Chemical membrane degradation is examined using a dynamic lumped model that combines the hydroxyl-radical reaction network, hydrogen-peroxide formation pathways, and a semi-empirical oxygen crossover formulation, reproducing experimentally reported fluoride release rates and characteristic degradation trends such as a maximum at intermediate current densities, while quantifying the influence of operating conditions such as pressure and current density. To address the scarcity of high-pressure experimental data, new measurements of hydrogen and oxygen crossover are presented for a six-cell PEM electrolyser operated at 60\,°C and pressures up to 8.5\,bar, revealing a nonlinear dependence of oxygen crossover on pressure that is not captured by existing approaches and motivating a modified model incorporating current-dependent supersaturation and electro-osmotic water drag. A modified oxygen crossover model incorporating current-dependent supersaturation and electro-osmotic water drag is introduced and shown to provide improved agreement with experimental results. In addition, a spatially resolved multi-species ion exchange model is developed to describe the competitive adsorption of Fe²⁺ and Fe³⁺ ions in electrolyser process water; the model extends multi-component Langmuir theory and captures the essential interaction between ferrous and ferric ions observed in equilibrium and breakthrough experiments, providing insights into exchanger performance and impurity transport dynamics. Overall, the models and experimental results presented in this thesis contribute to a more comprehensive understanding of degradation-relevant processes in PEM electrolysers. The three investigated topics are physically connected by their roles in chemical membrane aging, addressing complementary aspects of the same degradation pathway spanning transport phenomena, chemical reaction mechanisms, and mitigation strategies.