Benjamin Britton

Hexamethyl-p-terphenyl poly(benzimidazolium): a universal hydroxide-conducting polymer for energy conversion devices

A hydroxide-conducting polymer, HMT-PMBI, which is prepared by methylation of poly[2,2′-(2,2′′,4,4′′,6,6′′-hexamethyl-p-terphenyl-3,3′′-diyl)-5,5′-bibenzimidazole] (HMT-PBI), is utilized as both the polymer electrolyte membrane and ionomer in an alkaline anion-exchange membrane fuel cell and alkaline polymer electrolyzer. A fuel cell operating between 60 and 90 °C and subjected to operational shutdown, restarts, and CO2-containing air demonstrates remarkable in situ stability for >4 days, over which its performance improved. An HMT-PMBI-based fuel cell was operated at current densities >1000 mA cm−2 and power densities of 370 mW cm−2 at 60 °C. When similarly operated in a water electrolyzer with circulating 1 M KOH electrolyte at 60 °C, its performance was unchanged after 8 days of operation. Methodology for up-scaled synthesis of HMT-PMBI is presented, wherein >½ kg is synthesized in six steps with a yield of 42%. Each step is optimized to achieve high batch-to-batch reproducibility. Water uptake, dimensional swelling, and ionic conductivity of HMT-PMBI membranes exchanged with various anions are reported. In the fully-hydrated chloride form, HMT-PMBI membranes are mechanically strong, and possess a tensile strength and Youngs modulus of 33 MPa and 225 MPa, respectively, which are significantly higher than Nafion 212, for example. The hydroxide anion form shows remarkable ex situ chemical and mechanical stability and is seemingly unchanged after a 7 days exposure to 1 M NaOH at 80 °C or 6 M NaOH at 25 °C. Only 6% chemical degradation is observed when exposed to 2 M NaOH at 80 °C for 7 days. The ease of synthesis, synthetic reproducibility, scale-up, and exceptional in situ and ex situ properties of HMT-PMBI renders this a potential benchmark polymer for energy conversion devices requiring an anion-exchange material.

Structurally-Defined, Sulfo-Phenylated, Oligophenylenes and Polyphenylenes

We report the synthesis and molecular characterization of structurally defined, sulfo-phenylated, oligo- and polyphenylenes that incorporate a novel tetra-sulfonic acid bistetracyclone monomer. The utility of this monomer in the [4 + 2] Diels-Alder cycloaddition to produce well-defined, sulfonated oligophenylenes and pre-functionalized polyphenylene homopolymers is demonstrated. Characterization of the oligophenylenes indicates formation of the meta-meta and para-para adducts in a ∼ 1:1 ratio. These functionalized monomers and their subsequent coupling provide a route to prepare novel, sterically encumbered, sulfonated polyphenylenes possessing unprecedented structural control.

Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes

Two classes of novel sulfonated phenylated polyphenylene ionomers are investigated as polyaromatic-based proton exchange membranes. Both types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures. They exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity, and are markedly resilient to free radical attack. Fuel cells constructed with membrane-electrode assemblies containing each ionomer membrane yield high in situ proton conductivity and peak power densities that are greater than obtained using Nafion reference membranes. In situ chemical stability accelerated stress tests reveal that this class of the polyaromatic membranes allow significantly lower gas crossover and lower rates of degradation than Nafion benchmark systems. These results point to a promising future for molecularly designed sulfonated phenylated polyphenylenes as proton-conducting media in electrochemical technologies.

Sulfo-phenylated terphenylene copolymer membranes and ionomers.

The copolymerization of a prefunctionalized, tetrasulfonated oligophenylene monomer was investigated. The corresponding physical and electrochemical properties of the polymers were tuned by varying the ratio of hydrophobic to hydrophilic units within the polymers. Membranes prepared from these polymers possessed ion exchange capacities ranging from 1.86 to 3.50 meq g-1 and exhibited proton conductivities of up to 338 mS cm-1 (80 °C, 95 % relative humidity). Small-angle X-ray scattering and small-angle neutron scattering were used to elucidate the effect of the monomer ratios on the polymer morphology. The utility of these materials as low gas crossover, highly conductive membranes was demonstrated in fuel cell devices. Gas crossover currents through the membranes of as low as 4 % (0.16±0.03 mA cm-2 ) for a perfluorosulfonic acid reference membrane were demonstrated. As ionomers in the catalyst layer, the copolymers yielded highly active porous electrodes and overcame kinetic losses typically observed for hydrocarbon-based catalyst layers. Fully hydrocarbon, nonfluorous, solid polymer electrolyte fuel cells are demonstrated with peak power densities of 770 mW cm-2 with oxygen and 456 mW cm-2 with air.