Anion exchange membranes (AEM) 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. Development of thin polymer films is critical to reduce membrane resistance, but maintaining membrane integrity becomes increasingly difficult. Additionally, humidity changes during fuel cell operation causes water content changes in the polymer. Repeated swelling and contraction puts significant stresses on the membrane. Water taken up by the polymers at high humidity acts as a plasticizer increasing elasticity of the films. However, previously tested AEMs displayed low elasticity and elongation under dry conditions, which could lead to pinholes and cracks during humidity cycles. A new polyethylene-b-poly(vinylbenzyl trimethylammonium) diblock AEM has been synthesized with improved elongation and elasticity. This polymer can be solution processed into thin (10 micron) films. Extensional tests of the polyethylene diblock showed that at dry conditions the polyethylene diblock has a good Young’s modulus (210 ± 30 MPa) and elongation to break (150 ± 20 %), and reasonable strength (28 ± 4 MPa). When hydrated, water in the membrane acts as a platicizer reducing the modulus by 95% and increasing elongation by 47%, but only reducing strength by 29%. While the changes in strength and elongation are acceptable, the dramatic reduction in modulus from dry to hydrated states requires improvement and warranted further investigation. Dynamic mechanical analysis was utilized to investigate moduli changes while ramping humidity. The moduli go through a dramatic reduction when humidity is ramped and shows signs of hysteresis during dehumidification. Continued study will further investigate the effect of hydration on the mechanical performance of the polyethylene diblock and relate this effect to ionic transport through the membrane. Advanced knowledge of AEM mechanical properties will aid in design of robust membranes to improve fuel cell lifetime.

Effect of hydration on mechanical properties of anion exchange membranes

249th National Meeting (2015)

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