Anhydrous proton conducting materials for use in high temperature polymer electrolyte membrane fuel cells
Polymers containing tethered benzimidazole revealed that the mitigating factors for proton conduction are segmental mobility and charge carrier density. In general increased mobility will result in increased proton conduction especially at temperatures below 160°C, however, reductions in charge carrier density (benzimidazole content) to achieve increased mobility adversely affect proton conduction above 160°C. Further improvements in proton conduction were achieved by copolymerization of a benzimidazole acrylate with 2-Acrylamido-2-methylpropanesulfonic acid, however, ionic crosslinking between sulfate and benzimidazolium negated any conductivity increases. Additional experiments with benzimidazole were not pursued due to inherently high Tg values of the resulting polymers.
With the understanding that maximization of mobility and charge carrier density are necessary, the nature of the heterocycle was investigated. Polymers containing tethered 1,2,3-triazole were prepared and characterized. While some synthetic difficulties limited the impact, initial results indicated some improvements in proton conductivity in neat materials and large improvements in conductivity could be achieved by doping with up to 100 mol% trifluoroacetic acid (TFA). Additionally, inherently lower Tg materials result when using 1,2,3-triazole in place of benzimidazole. Improvements in monomer synthesis allowed further probing of 1,2,3-triazole as a protonic charge carrier, it was revealed that 1,2,3-triazole given equal charge carrier density and nearly identical Tg values, the use of 1,2,3-triazole as a substitute for benzimidazole results in materials with reduced conductivity. It was found through X-ray crystallographic studies and literature searching that a higher concentration of 1,2,3-triazole is required for productive proton transport due to tautomeric shifts in 1,2,3-triazole.
The effect of charge carrier density and the effect of reduced heterocycle basicity for 1,2,3-triazole was determined by preparing polysiloxanes with tethered 1,2,3-triazole units. The polysiloxane backbone provided inherently low Tg materials to maximize mobility and allowed direct comparison with analogous imidazole containing polysiloxanes reported by Meyer. It was found that increasing weight fraction of triazole results in dramatic conductivity improvements, the pKa of the heterocycle does not play a significant role in proton conduction, and 1.5 to 2 orders of magnitude conductivity improvements are observed when the system is doped with up to 100 mol% TFA. Conductivity values of TFA doped siloxanes containing 1,2,3-triazoles are equal or slightly better than the analogous imidazole materials. The benefits to utilizing 1,2,3-triazoles are the versatility of synthesis using "click" chemistry and the ability to absorb equimolar amounts of an external acid, opening the possibility to further conductivity improvements using alternate low Tg backbones and complex systems utilizing both hydrated and anhydrous proton conduction domains.