Development of heteropoly acid containing polymer electrolytes for use in electrochemical energy conversion devices

Abstract

Polymer electrolyte fuel cells are nearing commercialization due to large strides towards improved performance, durability, and cost targets. Some of the major breakthroughs include the discovery of more active and stable catalysts with reduced platinum loading, as well as the utilization of thin (<15 µm), mechanically supported membranes. While such advances have been impressive, a need still exists for highly conductive and chemically stable polymer electrolytes. As film thicknesses are reduced, more stable materials are needed to retain durability due to increased hydrogen crossover. In this work, new materials based on covalently attached heteropoly acid containing polymers are investigated. The prospect of making a perfluorocyclobutyl containing block polymer was investigated, using quantum chemistry calculations to discover and analyze the cycloaddition reaction mechanism involved in polymerization. The calculated activation energies are in close agreement with experimental data, giving validation to this purposed reaction pathway. A twisting motion occurs in the reaction mechanism and it is proposed that high temperatures or the addition of solvent could both be effective ways to achieve high molecular weight. The experimental portion of this work was hindered due to challenges with monomer purity, thermal stability, and finding a suitable solvent. The next polymer platform that was investigated involved the functionalization of a commercial poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) fluoroelastomer (FC-2178). The synthesis route was based on chemistry that is utilized in crosslinking reactions for producing O-rings and has been used previously to add functional side chains to PVDF-HFP. Although the resulting material was very brittle, it showed promise in proton transport and fuel cell testing. Small angle x-ray scattering (SAXS) data suggests that changing the processing conditions can alter the morphology, but two characteristic features of ca. 6.5 and 1.0 nm always persisted in these materials. Next, the synthesis was altered to allow low temperature side chain attachment, which resulted in much stronger polymer films. These films still gravitated towards the morphology seen in the previous approach. Engineering of thin films resulted in superior performance and durability when compared to the incumbent technology, perfluorinated sulfonic acid polymers (PFSA). While the covalently attached silicotungstic acid (HSiW) is stable in acidic conditions and water at room temperature, some HSiW leaches out in warm water (80 °C). The final set of studies looks into utilizing [γ-SiW10O36]-4 (HSiW10), as the chemical bonds are hypothesized to be stronger than the ones formed through hybridization of the [α-SiW11O39]-4 (HSiW11) moieties used in the previous studies. In the final chapter, the hypotheses are revisited and a new HSiW containing polyimide chemistry is proposed as a promising material for future work.