Electrolysis is a key technology for the production of clean hydrogen that will enable the hydrogen economy and expand the renewable energy landscape. Among the most viable candidates for low-temperature electrolysis is the proton-exchange membrane (PEM) water-splitting electrolyzer (PEMWE), which uses an ion-conductive polymer solid-electrolyte. To be commercially viable, PEMs in these devices must perform over a long operational time in liquid operating environments under compression, which makes stability an important aspect of design considerations. While a hydrated environment is an intrinsic result of the operation and key to the membrane’s conductive function, it undermines PEM stability. Hydration and pressure are used as design parameters for optimizing electrolyzer performance; however, their role in durability is not well established. This creates a gap between the performance and lifetime assessment of electrolyzer membranes. Thus, there is a need for new metrics that could capture ion-exchange membrane durability by accounting for performance operators. This dissertation shows the oft-used tensile testing of membrane samples cannot correspond to compression behavior and identifies compression creep as a potential material stability metric that can account for operation-dependent variables such as pressure and hydration. This work guides a better assessment of life-limiting issues in electrolysis and structurally analogous energy and environmental devices wherein the components are under compressive stress and/or pressurized.

The mechanical stability of PEMs is commonly characterized by tensile testing, and the applicability to electrolyzers wherein the membrane is held under compression is unclear. Chapter 2 introduces the custom-designed compression setup used for all compression tests within this dissertation, including monotonic, cyclic, and creep of dry and hydrated samples while temperature is controlled. Our results show compressive response significantly differs from tensile behavior in both dry and hydrated states.

Chapter 3 aims to improve the understanding of the mechanical interactions between cell components by exploring the monotonic compression response of the various electrolyzer cell components, such as the porous transport layers and gaskets, and compares them with the PEM response under quasistatic conditions at room temperature and 80 $\degree$C. The compressive stress-strain responses of all electrolyzer cell components have a degree of nonlinearity owing to each component's unique morphologies and deformation mechanisms. The acquired mechanical properties are used to analyze the compression of the cell components under applied assembly stress with a simplified 1-D mechanical spring network representation.

Chapter 4 expands on the monotonic compression of the PEM. The chapter presents a comprehensive analysis of the influence of membrane hydration on the mechanical behavior of PFSA membranes under compression and at different temperatures, offering valuable insights for electrochemical devices. The study confirms the crucial role of membrane hydration in mechanical response by quantifying the amount of water in hydrated membranes and examining the relationship between volumetric change and swelling strain. The research highlights the differences in the mechanical responses of dry and wet PFSA membranes, attributing the nonlinear response of the latter to various phenomena, including the plasticizing effect of water, free water squeezing out of the membrane under high compression, and softening of the polymer matrix. In addition, the study delves into the cyclic response of PFSA under both dry and hydrated conditions, revealing significant strain ratcheting and time-dependent residual strain, thus contributing to a deeper understanding of polymer membrane mechanical response and properties. In electrolyzer systems, state-of-the-art industry trends are to increase operational pressure while also reducing membrane thickness to minimize the need for additional pumps to pressurize the generated hydrogen and reduce Ohmic resistance and cost. This poses a challenge as higher operational pressure requires higher sealing pressure of the cell, thereby increasing the internal compressive stress experienced by the active area of the membrane. This higher stress, maintained for long time periods on thin membranes, makes compressive creep and thinning a concern since thinning could impact the interfacial resistance between the membrane and electrodes. Chapter 5 explores the compression creep of membranes under various environmental conditions. Chapter 5 shows that PEMs exhibit creep response under compression with a continuous decrease in thickness over 24 hours, with a dependence on the applied pressure, hydration state, pretreatment or thermal history, among other things.

Chapter 6 investigates the influence of geometry on material behavior under compression using two finite element analysis (FEA) models: a simple uniaxial compression model and a complex model with non-uniform surfaces. The simpler model, incorporating a Mooney-Rivlin hyperelastic model, demonstrated uniform stress and deformation in a frictionless scenario, while friction introduced non-uniform stress distribution. The complex model, which accounted for surface roughness and varying thicknesses, revealed that smaller surface features led to higher average stress but a more uniform distribution, highlighting the critical role of surface quality and membrane thickness in material response. The findings emphasize the importance of considering these factors in engineering design and material selection, particularly for applications requiring compressive strength and durability, such as electrolyzer cells. This research provides valuable insights into the complex interactions between surface features, friction, and material deformation, which are difficult to capture through experimental methods alone.

This dissertation demonstrates the importance of studying the mechanical properties of PEMs under compression, which is more relevant to the membrane's stress states during operation in electrolysis and similar electrochemical technologies.