Timothy J. Peckham

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.

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.

Main-chain, statistically sulfonated proton exchange membranes: the relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity

An in-depth analysis has been developed for proton exchange membranes to examine the effect of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content. The analysis was carried out on a series of main-chain, statistically sulfonated polymers with varying ion-exchange capacities. These polymer systems consisted of: sulfonated poly(ether ether ketone) (1), poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (2), sulfonated polyimide (3) and BAM® membrane (4) with Nafion® (5) as baseline. They represent membranes comprising polyaromatic polymers (1 and 3), one of which is also a rigid-rod polymer (3), vinylic polymers (4) and a vinylic polymer polymerized inside a polymer matrix (2). In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 °C were calculated and found to be 3.2 (±0.4) × 10−3 cm2 s−1 V−1 (1), 2.9 (±0.4) × 10−3 cm2 s−1 V−1 (2), 1.6 (±0.7) × 10−3 cm2 s−1 V−1 (3) and 2.1 (±0.2) × 10−3 cm2 s−1 V−1 (4). These were then compared with the theoretical value for the mobility of a free proton at infinite dilution. Significant deviations from this value were theorized to be due to possible differences in tortuosity and proximity of acid groups.

Structural and Morphological Features of Acid-Bearing Polymers for PEM Fuel Cells

Chemical structure, polymer microstructure, sequence distribution, and morphology of acid-bearing polymers are important factors in the design of polymer electrolyte membranes (PEMs) for fuel cells. The roles of ion aggregation and phase separation in vinylic- and aromatic-based polymers in proton conductivity and water transport are described. The formation, dimensions, and connectivity of ionic pathways are consistently found to play an important role in determining the physicochemical properties of PEMs. For polymers that possess low water content, phase separation and ionic channel formation significantly enhance the transport of water and protons. For membranes that contain a high content of water, phase separation is less influential. Continuity of ionic aggregates is influential on the diffusion of water and electroosmotic drag within a membrane. A balance of these properties must be considered in the design of the next generation of PEMs.

Anion conducting poly(dialkyl benzimidazolium) salts

Derivatization of poly(benzimidazole) (PBI) with methyl groups generates poly(dimethyl benzimidazolium) (PDMI), an anion exchange material. By using a simple ion exchange process, it is possible to produce PDMI salts with a variety of counter-ions. Anionic conductivity (2.7 ± 0.33 to 8.5 ± 0.5 × 10−3 S cm−1) for the PDMI membranes was found to be surprisingly high even though the membranes generally exhibit very low water uptakes. To the best of our knowledge, this represents the first report on the anionic conductivity of poly(dialkyl benzimidazolium) salts, despite the large body of literature on PBI and on molecular imidazolium salts.

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.

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.

Poly(3-hexylthiophene) bearing pendant fullerenes: aggregation vs. self-organization

Novel graft copolymers are reported based on poly(3-hexylthiophene) (P3HT) bearing side chains of poly(styrene-stat-chloromethylstyrene), onto which a fullerene C60 or PCBM is covalently attached. P3HT was brominated at the 4-position to various extents (1–30 mol%), subsequently Suzuki-coupled with the boronic ester of 1-(4′-bromophenyl)-1-(2″,2″,6″,6″-tetramethyl-1-piperidinyloxy)ethyl (tempo) to form a nitroxide-functionalized P3HT macroinitiator, which was then used to initiate the nitroxide-mediated radical polymerization (NMRP) of chloromethylstyrene (CMS)-stat-styrene (ST) side chains. CMS units were functionalized with azide units and used to attach fullerene. The polymers contained a relatively high mass content of fullerene (20–41 wt%). Photoluminescence of P3HT is strongly quenched by the fullerene. The absorption of P3HT maximum shifts toward shorter wavelengths with increasing graft density. Films of PCBM/C60 graft copolymers form a bicontinuous morphology with feature sizes <5 nm. Grafting fullerene-bearing side chains directly to P3HT is found to reduce the semi-crystallinity of the P3HT domains, reduce the hole charge mobility, and significantly reduce their photovoltaic activity. This is believed to be due to the poorer solubility of the fullerene units relative to the polymer chains which aggregate during film casting and restricts self-organization of the conjugated polymer.

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.