Proton Exchange Membranes with Reduced Methanol Transport Funded by Defense Advanced Research Projects Agency (DARPA) Principal Investigator: James E. McGrath

ABSTRACT:The utilization of sulfonated polymers in applications such as ion exchange resins, desalinization, and proton exchange membranes has been known for some time. Inevitably, the polymers have been made by a post-reaction with typically SO3 or related compounds or complexes. The post-reactions can, in many cases, be reasonably well controlled. However, in a number of situations it is known that the post reaction could produce materials with substituted or less stable activated sites. We have recently successfully synthesized wholly aromatic poly(arylene ether sulfone)s and phosphine oxides by direct polymerization of sulfonated monomers, which have the ion conductors on the more stable deactivated sites (Structure 1). Techniques have been developed for the efficient synthesis of the commercially available (50 MM lbs/year) monomer, as well as the polymerization to high molecular weight utilizing variations of well-established step or polycondensation procedures. The resulting materials are soluble in aprotic, dipolar solvents, such as NMP or DMAC, and tough transparent films can be generated, which show (as expected) an increasing hydrophilic character as the concentration of sulfonic acid groups are increased. The stability of the aromatic sulfonic acids has been investigated by combinations of high temperature NMR, non-aqueous potentiometric titrations, and thermal analysis. The sodium salt forms are very stable and are equivalent or better than the behavior observed for the unsulfonated materials. The free acid form that would be of greatest interest in proton exchange membranes for fuel cells is significantly more stable than expected from model small molecule experiments. Thus, exposing Structure 1 to temperatures as high as 220°C does not alter the molecular structure, as proven by quantitative characterization of the sulfonic acid structures, ionic equivalent measurements, and simple intrinsic viscosity values. It is argued that the deactivated rings will be better defined locations for the ionic conductor, since they are placed on the monomer prior to polymerization, and more stable to desulfonation since the anticipated intermediate carbocation required for desulfonation is difficult to stabilize on such a sulfone deactivated ring. Related materials incorporating phenyl phosphine oxide (Structure 2) have also been prepared and characterized and they show the additional interesting feature of extensive hydrogen bonding between the P=O and the pendant sulfonic acid, as indicated. This also allows for efficient dispersion of inorganic acid additives, such as phosphotungstic acid, which may permit the utilization of these ion conductor proton exchange membranes at temperatures well above 100°C, where the efficiency of hydrogen or direct methanol fuel cells is higher. Characterization and preliminary evaluations of these materials will be reported. The influence of morphology, conductivity and mechanical strength retention at elevated temperatures (e.g., 120°C) under hydrated conditions is particularly interesting (Figure 1). The 40-50% copolymers show a good combination of conductivity and retention of mechanical strength, even at 120°C. Complete phase inversion is observed for the 60% copolymers.

 

Structure 1.

Figure 1.

For more information on this project
contact Dr. McGrath or Laurie Good