Mechanical-Enhanced Composite PEM And Its Response To Temperature And Relative Humidity

The durability of polymer electrolyte membranes (PEMs) is one of critical factors to influence the performance of polymer electrolyte membranes fuel cells (PEMFCs) and their life span. The preparation of high tensile strength, low swelling and high proton conductivity is one of the hot-topics in PEMFCs research. The Nafion membranes from DuPont are the the most widely used PEMs for their high chemical stability, however, the mechanical properties (elastic modulus, creep strain, etc.) of Nafion decayed apparently especially at high temperatures and high relative humilities. Thus, the improvement of PEMs with high tensile strength and good mechanical stability is favorable to push forward the development of PEMFCs. In this dissertation, we developed durable perfluorosulfonic polymer electrolyte membranes through alkaline-ion-assisted heat treatment. The composite membranes prepared this way demonstrated excellent performance in mechanical properties and PEMFC out-put performance. Furthermore, we studied the enhancement mechanism, creep-recovery behaviors in different temperature and relative humidity range as well as proton conductivity in the according environments. The conclusions are summarized as follows:(1) The durable coposite membranes were prepared alkline-ionic modified perfluorosulfonic acid (PFSA).Due to the heat treatment at elevated temperature(at Tg of the modified PFSA), the crystallinity and solvent resistance were improved obviously, as well as the water-generated shrinkage stress was lowed. The performance and properties of membrane prepared by Na-PFSA is the best in all the modified PFSA/ePTFE composite membranes.(2) The elastic modulus of the composite membranes decreased with increasing of temperature and relative humidity (RH). For the creep behaviors of the membranes at different temperatures and RH, the ePTFE matrix can improve the creep resistance especially at high temperature dry state and low temperature high RH. This may result from the high elastic modulus of ePTFE fibers and high hydrophobic property.(3) The proton conductivity of PFSA/ePTFE membranes depends highly on the environmental temperature and RH as well as the volume fraction of ePTFE in the composite systerm. The proton conductive percolation value for PFSA/ePTFE membrane were proposed. The percolation value at the humidities of 30%RH、50%RH、70%RH are 70%、62%、47%(volume fraction of ePTFE), respectively. The decreasment of the proton conductive percolation value may come from the swelling effect and the increasement of proton conductivity of PFSA resin under high RH conditions. Take the water volume in the membrane into consideration, the percolation value at the humidities of 100%RH、50%RH、30%RH at temperature of 80℃are 17%、30%、45%(volume fraction of ePTFE), respectively. The proton conductive percolation threshod of hydration level for PFSA/ePTFE membrane isλ=7.