Sulfophenylated 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.

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.

Cationic Polyelectrolytes, Stable in 10 M KOHaq at 100 °C.

We report on poly(arylene-imidazoliums), which were synthesized by microwave polycondensation of dialdehyde with bisbenzil and quantitatively functionalized by alkylation. This cationic polyelectrolyte is sterically protected around the C2-position and is stable in 10 M KOHaq at 100 °C (t1/2 of >5000 h). Alkaline stability is rationalized through analyses of model compounds, single crystal X-ray diffraction, and density functional theory. The polyelectrolytes form tough, pliable, transparent, ionically conductive films.

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.

Selective formation of hydrogen and hydroxyl radicals by electron beam irradiation and their reactivity with perfluorosulfonated acid ionomer.

Selective formation and reactivity of hydrogen (H(•)) and hydroxyl (HO(•)) radicals with perfluorinated sulfonated ionomer membrane, Nafion 211, is described. Selective formation of radicals was achieved by electron beam irradiation of aqueous solutions of H2O2 or H2SO4 to form HO(•) and H(•), respectively, and confirmed by ESR spectroscopy using a spin trap. The structure of Nafion 211 after reaction with H(•) or HO(•) was determined using calibrated (19)F magic angle spinning NMR spectroscopy. Soluble residues of degradation were analyzed by liquid and solid-state NMR. NMR and ATR-FTIR spectroscopy, together with determination of ion exchange capacity, water uptake, proton conductivity, and fluoride ion release, strongly indicate that attack by H(•) occurs at the tertiary carbon C-F bond on both the main and side chain; whereas attack by HO(•) occurs solely on the side chain, specifically, the α-O-C bond.

A stable hydroxide-conducting polymer.

A stable hydroxide-conducting membrane based on benzimidazolium hydroxide and its analogous anion-exchange polymer is reported for the first time. The molecular and polymeric analogues possess unprecedented hydroxide stability in neutral and KOH solutions as the soluble benzimidazolium salt, made possible by steric crowding around the benzimidazolium C2 position, which is usually susceptible to nucleophilic attack by OH(-). The polymers were cast and insolubilized for the purpose of forming membranes by blending with a poly(benzimidazole) followed by hydroxide-activated electrostatic interactions. The resulting membranes possess ionic (OH(-)) conductivities of up to 13.2 mS cm(-1) and represent a new class of anion-exchange polymers and membranes.

Structure-morphology-property relationships of non-perfluorinated proton-conducting membranes.

A fundamental understanding of structure-morphology-property relationships of proton exchange membranes (PEMs) is crucial in order to improve the cost, performance, and durability of PEM fuel cells (PEMFCs). In this context, there has been an explosion over the past five years in the volume of research carried out in the area of non-perfluorinated, proton-conducting polymer membranes, with a particular emphasis on exploiting phase behavior associated with block and graft copolymers. This progress report highlights a selection of interesting studies in the area that have appeared since 2005, which illustrate the effects of factors such as acid and water contents and morphology upon proton conduction. It concludes with an outlook on future directions.

Relationships of Acid and water content to proton transport in statistically sulfonated proton exchange membranes: variation of water content via control of relative humidity.

An in-depth analysis for proton exchange membranes to examine the effects of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content was carried out on two main-chain, statistically sulfonated polymers at 25 degrees C. These polymer systems consisted of poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (1) and sulfonated trifluorostyrene (BAM) membrane (2). Nafion (3) was used for comparison. Water content (as represented by Xv, the water volume fraction, where Xv = volume of water in hydrated PEM / volume of hydrated PEM), for each sample was varied by adjusting the relative humidity (RH) of the membrane environment from 50% to 98%. It was found that, at low RH (RH < 70%), the major factor determining proton conductivity is proton mobility. In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 degrees C were calculated and found to be 2.6 +/- 0.2 x 10-3 (average of 1a-c), 1.6 +/- 0.3 x 10-3 (average of 2a-e), and 2.32 +/- 0.01 x 10-3 cm2 s-1 V-1 (3). These were then compared to the theoretical value for the mobility of a free proton at infinite dilution and to previously reported data. Possible differences in tortuosity and the juxtaposition of acid groups are proposed in order to account for the significant deviations of all samples from the theoretical value.