(Invited) Optimizing the Structure and Composition of PFSA/PVDF Fuel Cell Membranes

Abstract

In order to control water uptake and improve the mechanical properties of a highly charged cation or anion exchange membrane, an uncharged polymer is often added to the membrane in one of two ways: (1) the uncharged polymer is added to the membrane casting solution to create a blended polymer film or (2) a composite membrane is made where ionomer is impregnated into a porous uncharged polymer network/support. Improvements in both approaches can be achieved by use of nanofiber electrospinning. Thus, one can electrospin a blended polymer solution of mutually incompatible charged and uncharged polymers, where good shear mixing at the spinneret tip and fast fiber drying minimizes phase separation and where hot-pressing of the resultant fiber mat results in a dense and defect free film. Similarly, dual fiber electrospinning of separate ionomer and uncharged polymer solutions followed by hot pressing/densification circumvents problems associated with impregnating an ionomer solution into a pre-formed reinforcing porous mat. In this talk, three methods of membrane fabrication will be described and contrasted: (1) solution-cast films of a blended polymer solution, (2) membranes from blended electrospun fibers, and (3) membranes from dual fiber electrospun mats. Membranes 20-25 μm in thickness were prepared using a perfluorosulfonic acid (PFSA) ionomer from 3M Co. (660, 725, or 825 EW) and poly(vinylidene fluoride) (PVDF). The effects of membrane composition, i.e., the EW of the PFSA and the PFSA/PVDF weight ratio, and the membrane preparation procedure on proton conductivity, water swelling (gravimetric and in-plane), and membrane mechanical properties will be discussed. In general, it was found that membranes prepared by separately electrospinning PFSA and PVDF exhibited the best combination of high proton conductivity and low in-plane water swelling – two properties relevant to membrane use in a H2/air fuel cell. Acknowledgement: The authors thank the US DOE (EERE Cooperative Agreement No. DE-EE0006362) for funding this work, as well as Dr. Michael Yandrasits at 3M for providing the PFSA ionomers used in this study.