Mechanical characterization of anion exchange membranes under controlled environmental conditions

While proton exchange membrane (PEM) fuel cells have been the focus of development in the past, anion exchange membranes (AEM) have the potential to dramatically lower the cost of fuel cells by utilizing non-noble catalysts and a variety of fuel sources. Although chemical degradation typically dominates membrane failure pathways in a fuel cell, mechanical breakdown due to humidity cycling is a common occurrence. This thesis aims to understand the mechanical properties of anion exchange membranes under fuel cell operating conditions. A humidity delivery system was developed for the TA Instruments ARES-G2 rheometer to allow for testing at a range of temperatures (30-100°C) and relative humidity conditions (0-95% RH). A modified Sentmanat Extensional Rheometer (SER) was used to perform tensile-like testing using less than 5% of material needed for a traditional tensile tester. These tools established metrics for a robust membrane through mechanical characterization across temperatures and humidities.;A pentablock AEM with a balance of stiff and elastic blocks was shown to have adequate conductivity (up to 60 mS/cm at 90°C and 95%RH), low water uptake (<25%), and good mechanical integrity under dry and hydrated conditions, showing potential for being durable under hydration and mechanical stresses.;Complementing the destructive tensile testing, a "water stress" test was explored to measure the tension and durability under hygral cycles. Membranes with a low (<5 MPa) and near constant water stress absorb and desorb water reversibly. The materials that performed poorly in the water stress tests also had elongation <50% under dry conditions and swelled with water. Membranes performing well in the water stress test also had an elongation to break 10 times that of its in-plane water swelling (in liquid water). Ion exchange membranes need to be able to mechanically stretch in the elastic region well above the in-plane swelling with water to withstand hygral stresses in an electrochemical device. By identifying a relationship between the mechanical and hygral stretching to predict durability in a working device, this thesis advanced the understanding of mechanical performance under relevant temperature and humidity conditions, which is critical to the development of durable anion exchange membranes.