(452a) Nanofiber Composite Membranes for Improved Durability and Power In Hydrogen/Air Fuel Cells

High temperature operation of a hydrogen/air proton-exchange membrane (PEM) fuel cell (i.e. a temperature of 80-120^oC) allows for faster electrode kinetics and minimizes catalyst poisoning by carbon monoxide.  Above 100^oC, gas humidification is difficult, requiring pressurized systems that decrease overall system efficiency.  Thus, high-temperature/low-humidity PEM fuel cell operation is highly desirable. While, DuPont’s Nafion® perfluorosulfonic acid (PFSA) has proven adequate for fuel cell operation up to 80^oC at high humidity, it cannot operate at harsher hot and dry conditions.  Nafion’s proton conductivity dramatically decreases at temperatures > 80^oC and low humidity because there is insufficient water content to deprotonate sulfonic acid sites and an insufficient number of water-filled channels for proton migration under the influence of an electric field. Furthermore, the mechanical durability of Nafion and other PFSA materials remains suspect for long-term fuel cell operation.  PFSAs undergo large dimensional changes (swelling/shrinking) when the membrane is heated/cooled and hydrated/dried. The lack of dimensional stability is a source of mechanical membrane failure in automotive operating environments.

Recently, a new class of nanofiber composite membranes have been introduced to address the current limitations of fuel cell PEMs.  These membranes are fabricated by electrospinning ionomer nanofibers and then impregnating an uncharged/inert polymer into the inter-fiber void space.¹  This talk will present a new strategy for fabricating nanofiber composite membranes which eliminates the need for an inert polymer impregnation step.  PFSA and polyphenylsulfone (PPSU) polymers are simultaneously electrospun from separate needle-spinnerets onto a common collecting surface, intimately mixing the two polymers in the dry-state.  Electrospining of PFSA ionomer requires the addition of a small amount of carrier polymer (≤1 wt% poly(ethylene oxide)).²    Follow-on processing “melts” (i.e. induces flow of) either the PFSA or PPSU to fill the interfiber void space, resulting in a fully dense membrane, where PPSU nanofibers are embedded in a PFSA matrix or PFSA nanofibers are surrounded and stabilized by a matrix of PPSU.  These membranes exhibit high proton-conductivity and low volumetric and in-plane water swelling.  Low in-plane swelling is important for improving a membrane’s mechanical durability in a fuel cell.^{3, 4}  Furthermore, the high proton-conductivity of these membranes (for both high and low relative humidities) makes them ideal candidates for a range of practical fuel cell applications.

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, Journal of the Electrochemical Society, 158, B568 (2011).

3.  A. Kusoglu, A. M. Karlsson, M. H. Santare, S. Cleghorn and W. B. Johnson, Journal of Power Sources, 161, 987 (2006).

4.  A. Kusoglu, A. M. Karlsson, M. H. Santare, S. Cleghorn and W. B. Johnson, Journal of Power Sources, 170, 345 (2007).