(423b) Proton Exchange Membranes for Low Relative Humidity Fuel Cell Operation Based on Poly(Phenylene Sulfonic Acid)

Novel, composite, fuel cell membranes have been fabricated from a non-fluorinated polymer using a pore filling technique. The membranes consist of a mechanically stabilizing skeleton from an electrospun poly(phenylene sulfone) (PPSU) fiber mat and a thermally crosslinkable poly(phenylene sulfonic acid) (cPPSA) proton conducting ionomer that fills the interfiber voids. The work builds on two prior studies carried out in the labs of Prof. M. Litt and Prof. P.N. Pintauro demonstrating: (1) Exceptionally high proton conductivity, even at very low RH (down to 20%RH), of cPPSA solution cast films, and (2) Excellent durability of electrospun perfluorosulfonic acid (PFSA)/PPSU composite membranes, exceeding that of the pristine PFSA. Unfortunately, the high proton conductivity of cPPSA films, especially at low relative humidity, was offset by their significant brittleness, which made handling of those materials, including fabrication of membrane-electrode-assemblies, extremely difficult. The proposed pore-filling strategy should alleviate the brittleness problem leading to a robust, low RH-conductive PEM that is a true competitor to PFSA membranes and suitable for commercial applications.

Experiments focused on optimizing the cPPSA/PPSU ratio to achieve the desired balance of tensile strength and proton conductivity, and on minimizing membrane thickness to obtain area specific resistance (ASR) lower than 0.02 S/cm in the 40-90% RH range at 80^oC. cPPSA copolymer was synthesized using Ullmann coupling copolymerization of 4,4′-dibromobiphenyl 3,3′-disulfonic acid with 1,4-dibromobenzene-2,5-disulfonic, followed by grafting certain fraction of backbone sulfonic acid groups with biphenyl linker [1]. The PPSU fiber mat was electrospun from NMP/acetone solution. Pore-filling was carried out by pouring a solution of cPPSA in methanol over the framed mat, followed by heating at 70^oC to evaporate solvent. The cPPSA was crosslinked by an additional heating step, in a vacuum oven at 210^oC for 1-2 hours. The resultant membranes had excellent proton conductivity, 5-10 times greater than that of Nafion^Ò 211, in the 40-90% RH range at 80^oC. Details of the membrane fabrication and characterization work will be presented in this talk.

[1] M. Litt and R. Wycisk, Poly(Arylenesulfonic Acids) with Frozen-In Free Volume as Hydrogen Fuel Cell Membrane Materials, Polymer Reviews, 55, 307 (2015).