(439h) Effect of Hydration on Mechanical Properties of Anion Exchange Membranes
Anion exchange membranes facilitate ion transport in polymer electrolyte membrane fuel cells. An AEM must have a high ionic conductivity, low gas/fuel crossover, and be chemically and mechanically stable over the lifetime of the fuel cell. While chemical degradation typically dominates membrane failure pathways in a fuel cell, mechanical breakdown due to humidity cycling is a common occurrence. The sorption and desorption of water can cause pin hole cracks, which lead to fuel crossover and stack failure. Development of thin polymer films is critical to reduce membrane resistance. As membrane thickness is decreased, maintaining membrane integrity becomes increasingly difficult. 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 on films 10-100 microns thick. 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). The humidity oven was built to use with all other rheometer accessories, including dynamic mechanical analysis. Dynamic mechanical analysis is used to investigate moduli changes while ramping humidity at constant temperature and while ramping temperature at saturated gas conditions. These tools established metrics for a robust membrane through mechanical characterization across temperatures and humidities. A series of block, random, and crosslinked polymers have been characterized with these tools Complementing 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 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. Identifying a relationship between the mechanical and hygral stretching to predict durability in a working device is critical to the development of durable anion exchange membranes.