Publications

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Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm−2 at 160 °C and 1,740 mW cm−2 at 240 °C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.

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Multiple computational and experimental techniques are used to understand the nanoscale morphology and water/proton transport properties in a series of sulfonated Diels-Alder poly(phenylene) (SDAPP) membranes over a wide range of temperature, hydration, and sulfonation conditions. New synthetic methods allow us to sulfonate the SDAPP membranes to much higher ion exchange capacity levels than has been previously possible. Nanoscale phase separation between the hydrophobic polymer backbone and the hydrophilic water/sulfonic acid groups was observed for all membranes studied. We find good agreement between structure factors calculated from atomistic molecular dynamics (MD) simulations and those measured by X-ray scattering. With increasing hydration, the scattering ionomer peak in SDAPP is found to decrease in intensity. This intensity decrease is shown to be due to a reduction of scattering contrast between the water and polymer and is not indicative of any loss of nanoscale phase separation. Both MD simulations and density functional theory (DFT) calculations show that as hydration levels are increased, the nanostructure morphology in SDAPP evolves from isolated ionic domains to fully percolated water networks containing progressively weaker hydrogen bond strengths. The conductivity of the membranes is measured by electrical impedance spectroscopy and the equivalent proton conductivity calculated from pulsed-field-gradient (PFG) NMR diffusometry measurements of the hydration waters. Comparison of the measured and calculated conductivity reveals that in SDAPP the proton conduction mechanism evolves from being dominated by vehicular transport at low hydration and sulfonation levels to including a significant contribution from the Grötthuss mechanism (also known as structural diffusion) at higher hydration and sulfonation levels. The observed increase in conductivity reflects the impact that changing hydration and sulfonation have on the morphology and hydrogen bond network and ultimately on the membrane performance.

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Post-polymerization reactions of Diels-Alder polyphenylene with ring-substituted benzoyl chloride derivatives using triflic acid as the catalyst, effected selective Friedel-Crafts acylation of the lateral phenyl groups attached to the polyphenylene backbone. Using 4-(trifluoromethyl) benzoyl chloride gave a polymer with increased hydrophobicity. Using 4-fluorobenzoyl chloride afforded lateral 4-(fluorobenzoyl)phenyl substituents, which were further functionalized by nucleophilic aromatic substitution of the reactive fluoro substituent by 4-methoxyphenol.

Cheap and efficient ion conducting separators are needed to improve efficiency and lifetime in fuel cells, batteries, and electrolyzers. Current state-of-the-art polymeric separators are made from Nafion, which is too expensive to be competitive with other technologies. Sandia has developed unique polymer separators that have lower cost and equivalent or superior ion transport compared to Nafion. These membranes consist of sulfonated Diels-Alder poly(phenylene) (SDAPP), a completely hydrocarbon polymer that conducts protons when hydrated. SDAPP membranes are thermally and chemically robust, with conductivities rivaling those of Nafion at high sulfonation levels. However, rational design of new separators requires molecular-level knowledge, currently unknown, of how polymer morphology affects transport. Here we describe the use of multiple computational and experimental techniques to understand the nanoscale morphology and water/proton transport properties in a series of sulfonated SDAPP membranes over a wide range of temperature, hydration, and sulfonation conditions.

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