Solid polymer electrolyte electrochemical energy conversion devices that operate under highly alkaline conditions afford faster reaction kinetics and the deployment of inexpensive electrocatalysts compared with their acidic counterparts. The hydroxide anion exchange polymer is a key component of any solid polymer electrolyte device that operates under alkaline conditions. However, durable hydroxide-conducting polymer electrolytes in highly caustic media have proved elusive, because polymers bearing cations are inherently unstable under highly caustic conditions. Here we report a systematic investigation of novel arylimidazolium and bis-arylimidazolium compounds that lead to the rationale design of robust, sterically protected poly(arylimidazolium) hydroxide anion exchange polymers that possess a combination of high ion-exchange capacity and exceptional stability.

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.

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The structure–property relationship of sulfonated phenylated poly­(phenylene)­s possessing either angled or linear backbone moieties was investigated. Polymers were synthesized using either bent (ortho or meta) or linear (para) biphenyl linkages and evaluated for differences in physical and electrochemical properties. Model compounds, structurally analogous to the polymers, were prepared and characterized using spectroscopic and computational methods to elucidate structural differences and potential impacts on the properties of the respective polymers. A highly angled ortho biphenyl linkage resulted in a sterically hindered, rotationally restricted molecule. When incorporated into a polymer, the angled ortho biphenyl moiety was found to prevent membrane formation. The angled meta biphenyl-containing polymer, while forming a membrane, exhibited a 74% increase in volumetric expansion, 31% reduction in tensile strength, and 72% reduction in elongation at break when compared to the linear para biphenyl-containing analogue. The differences observed are attributed to a rotationally restricted backbone in the angled biphenyl systems. Collectively, this study suggests that incorporating angled biphenyl linkages into sulfonated phenylated poly­(phenylene)­s leads to highly rigid, inflexible backbones that prevent chain entanglement and the formation of free-standing membranes.

We systematically investigated the effect of incorporating a sterically hindered pyridyl group into a sulfophenylated polyphenylene to control the polymer's physicochemical properties through acid−base interactions. Homopolymers with similar molecular weights and comparable structures that vary by only one atom (N− vs C−) per repeat unit along the polymer chain were prepared. Compared to a non-pyridyl reference membrane, incorporation of a pyridyl group improves the oxidative stability against free radicals, increases the elongation at break to 55% (from 37%), and enhances the thermal stability to 326 °C (from 246 °C). In an accelerated fuel cell degradation test, polymeric membranes containing the sterically encumbered pyridyl unit exhibited exceptional stability (0.16 mV h −1 degradation rate over 1000 h) and retained ∼80% of their peak power density over this time.