Perfluorosulphonic acid (PFSA) ionomer membranes are commonly used as the electrolyte in proton exchange membrane fuel cells (PEMFC) and have time, temperature and hydration dependent mechanical properties. The operating conditions for these membranes include temperatures ranging from subzero to +85oC, hydration from dry vapor to liquid water, and varying load rates in the fuel cell. The mechanical properties of the membranes are highly sensitive to these conditions and the membranes experience significant cyclic swelling/shrinkage during operation in a fuel cell, which may lead to high tensile stress. These high tensile stresses obtained during cyclic loading can be detrimental to the durability of the fuel cells, and are suspected to cause crack initiation and/or pinholes formation in the membrane. Therefore, it is critical to characterize the mechanical response of the membrane material and understand its behavior when subjected to hygrothermal cyclic loading as seen in a typical fuel cell operation.

In this dissertation work, mechanical properties of two PFSA membrane materials are characterized:​ one is a commercially available PFSA membrane and the other is an experimental e-PTFE reinforced PFSA membrane. The stress response of each material, and also a composite membrane created using both materials is investigated using a representative fuel cell finite element model.

For each membrane material, a viscoelastic-plastic model is developed to characterize the constitutive response. These models are based on uniaxial tensile and stress-relaxation tests conducted in our lab at University of Delaware. For the PFSA membrane, an existing two-layer model is extended to incorporate the effect of rate varying instantaneous modulus. For the experimental e-PTFE reinforced PFSA membrane, the time-dependent response is characterized by developing a three-layer model. This model takes into account the anisotropic mechanical behavior of the reinforced membrane material.

The swelling strains (and their rates) in the PFSA membranes are a function of the water sorption behavior of the membranes. The amount of water uptake by the membrane and the associated rate influence the stress levels during fuel cell operation. In order to characterize the sorption behavior of the PFSA membranes, a humidity-dependent net sorption coefficient is determined using a reverse analysis based on Fickian diffusion. The results from simulations conducted using slower sorption coefficient suggest that residual tensile stresses in the membrane are considerably reduced.

Using the determined mechanical and the sorption properties for the two PFSA membranes, the mechanical response of a membrane constrained in a fuel cell is investigated. A representative volume element of a single fuel cell unit assembly is modeled using finite element analysis. A standardized relative humidity (RH) protocol developed for testing the mechanical durability, and its variations are used as loading condition for the model. The results for the PFSA membrane suggest that large residual in-plane stresses develop in the membrane after dehydration. Slower feed rates and slower sorption rates are found to be critical in reducing stress levels. The mechanical behavior of a composite membrane based on the unreinforced and the e-PTFE reinforced PFSA membrane with various configurations is investigated. This analysis shows that it may be possible to optimize a composite membrane with respect to the layered configuration (e.g., thickness and location of the reinforced vs. the non-reinforced part) to enhance the lifetime of the fuel cell membrane.