Electrospinning PFSA + PVDF Nanofibers for Fuel Cell Membrane Fabrication

In 2008, Pintauro and co-workers [1] showed how nanofiber electrospinning can be used to fabricate composite fuel cell membranes.  The method of membrane fabrication and the membrane morphology were alternatives to traditional membranes based on polymer blends and copolymers.  These early membranes were made by electrospinning an ionomer fiber mat followed by the impregnation of an uncharged, reinforcing polymer into the inter-fiber voids. A series of post-electrospinning processing steps were required to convert a porous mat into a dense and defect-free fuel cell membrane, namely (a) mat compression, (b) fiber welding, (c) ionomer annealing, and (d) inert polymer impregnation. The resultant nanofiber composite membrane morphology decoupled the proton conduction function of the ionomeric nanofibers from the mechanical support and swelling control functions of the uncharged matrix polymer. Dual-fiber electrospinning was introduced in 2011 by Ballengee and Pintauro [2] as a means to eliminate a separate interfiber void-filling impregnation step during nanofiber composite membrane fabrication. Here, the ionomer and uncharged polymers were simultaneously electrospun as separate fibers that co-deposited as a well-mixed mat on the rotating drum collector surface. Subsequent processing via hotpressing and either annealing or solvent vapor exposure, induced the flow of one of the polymer components into the interfiber void space while retaining the nanofiber morphology of the second polymer.  Two membrane morphologies were produced from a single dual fiber mat:  (i) an interconnected network of ionomer nanofibers embedded in an uncharged polymer reinforcing polymer matrix and (b) a network of uncharged reinforcing polymer nanofibers surrounded by an ionomer matrix.  In the dual fiber membrane work, the ionomer was Nafion^Ò perfluorosulfonic acid polymer and the uncharged polymer was polyphenylsulfone.

Recently our group has begun fabricating dual-fiber Nafion-PVDF and Aquivion^Ò-PVDF composite proton conducting membranes (Aquivion is a short side chain 825 EW PFSA polymer from Solvay Solexis). These membranes were designed for use in regenerative hydrogen-bromide and conventional hydrogen/air fuel cells. Two general membrane structures were prepared: (1) membranes with “melted” PVDF fibers that fill the voids between PFSA fibers, and (2) membranes with “melted” PFSA which fills the voids between PVDF nanofibers. These two membrane types were obtained from the same as-spun two-component mats using the procedures described in the preceding paragraph. The PFSA content of membranes was typically in the range 30-60% PVDF for morphology #1 and 20-50% for morphology #2.

This presentation will provide additional details regarding the fabrication and properties of PFSA/PVDF fuel cell membranes.  Specific topics to be covered in the talk are:

1. New ways of electrospinning Nafion and Aquivion  nanofibers
2. Nanofiber composite membranes where an interconnecting network of PFSA nanofibers is embedded in a PVDF matrix.
3. Nanofiber composite membranes where PVDF nanofibers are embedded in a PFSA matrix
4. PFSA/PVDF blended membranes with no nanofiber morphology.

The various membranes in items 2-4 will be contrasted with one another and with commercial Nafion 212 films in terms of the following fuel cell properties: proton conductivity, gravimetric and areal water swelling, wet/dry mechanical properties, and gas permeability.  Physical property differences will be linked with changes in membrane morphology.

References

[1]  J. Choi, K.M. Lee, R. Wycisk, P.N. Pintauro and P.T. Mather, Macromolecules, 41, 4569, 2008.

[2]  J.B. Ballengee and P.N. Pintauro, Macromolecules, 44, 7307, 2011.