We describe the voltammetric behavior of an anion-exchange membrane, hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI). The anion-exchange properties of HMT-PMBI chemically modified electrodes were investigated using K4Fe(CN)6 and K2IrCl6 as redox probes. The permselectivity properties of HMT-PMBI chemically modified electrodes were ascertained using tris(2-2’)bipyridyl-ruthenium(II) chloride Ru(bpy)32+. Cyclic voltammetry and chronoamperometry were utilized to extract parameters such as the concentration of the redox mediators inside the films and the apparent diffusion coefficients. We found the concentration of K4Fe(CN)6 and K2IrCl6 redox species within HMT-PMBI-coated films to be on the order of 0.04–0.1 mol·dm−3, and values of Dapp ca. 10−10–10−9 cm2·s−1. To evaluate the possibility of using such a polymer coating in electroanalysis, HMT-PMBI-modified electrodes were utilized for the voltammetric detection of uric acid in artificial urine, Surine® and ascorbic acid in Vitamin C samples. The results showed that HMT-PMBI-coated electrodes can detect uric acid in Surine® with a limit of detection (LoD) of 7.7 µM, sensitivity of 0.14 µA·µM−1·cm−2, and linear range between 5 μM and 200 μM, whereas for Vitamin C tablets, the LoD is 41.4 µM, the sensitivity is 0.08 µA·µM−1·cm−2, and the linear range is between 25 μM and 450 μM.

An understanding of water permeation through solid polymer electrolyte (SPE) membranes is crucial to offset the unbalanced water activity within SPE fuel cells. We examine water permeation through an emerging class of anion exchange membranes, hexamethyl-p-terphenyl poly (dimethylbenzimidazolium) (HMT-PMBI), and compare it against series of membrane thickness for a commercial anion exchange membrane (AEM), Fumapem® FAA-3, and a series of proton exchange membranes, Nafion®. The HMT-PMBI membrane is found to possess higher water permeabilities than Fumapem® FAA-3 and comparable permeability than Nafion (H+).

We have studied the morphology of a novel series of benzimidazole-based ionenes, methylated poly(hexamethyl-p-terphenylbenzimidazolium) (HMT-PMBI), in halide form. Materials with anion-exchange capacities ranging from 0 to 2.5 mequiv/g were studied. X-ray scattering reveals three length scales in the materials: ion-polymer spacing (4 Å), polymer-polymer interchain spacing (6 Å), and an intrachain repeat distance (20 Å). No long-range structure is apparent above the monomer length, which is rare in ion-conducting polymer membranes. In preliminary molecular dynamics simulations, water molecules were observed forming chains between ions, even at a modest level of hydration, providing an interpenetrating network where conductivity can occur.

This study describes the use of a benzimidazolium-based anion exchange membrane for creating bipolar membranes and the assessment of their suitability for solar-driven water splitting. Bipolar membranes were prepared by laminating anion exchange membrane with Nafion NR-211 membrane without modification of the interface. Under acidic and basic conditions, proton and hydroxide ion conductivities of 103 and 102 mS cm-1 were obtained for Nafion and benzimidazolium-based membranes, respectively. The fabricated bipolar membranes have an average thickness of 90 μm and show high transmittance, up to 75% of the visible light. The findings suggest that the two membranes create a sharp hydrophilic interface with a space charge region of only a few nanometers, thereby generating a large electric field at the interface that enhances water dissociation.

Hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) is a hydroxide-resistant, high-performing anion-conducting polymer. Such materials are very interesting, as they may allow the use of hydrogen fuel cells without platinum-group catalysts [1]. We have investigated the structure of this material using small-angle x-ray scattering (SAXS,) small-angle neutron scattering (SANS,) and computational methods. Our SAXS results revealed three distinct length scales, which we assigned to ion-polymer, interchain, and monomer length scales. A molecular dynamics (MD) simulation reproduced these features of the scattering curve and corroborated our interpretations of them. No structure at larger length scales was observed; the lack of large watery regions, common in high-performance ion-exchange membranes, may contribute to the material’s remarkable mechanical stability. [1] A.G. Wright and S. Holdcroft, ACS Macro Lett. 3, 444 (2014). Figure 1

Steric hindrance is employed as a design strategy of polybenzimidazoles for ion-solvating polymer electrolyte membrane alkaline water electrolysis.

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.

The application of highly charged anion exchange membranes is often limited by strong water sorption leading to excessive swelling and eventual dissolution, especially at elevated temperatures. The cross-linking of polymers has been shown to be an excellent mitigation strategy but this often restricts membrane and ionomer processing methods. Here, we explore the reaction, stability, and utility of a cross-linking agent for the recently discovered class of cationic polymers: methylated, C2-protected poly(benzimidazolium)s. In situ reaction and formation of p-xylyl cross-linking groups is found to provide novel, highly functionalized, hydroxide-stable membranes and films with enhanced anion conductivities and superior mechanical properties compared to un-cross-linked polymers. The versatility of this strategy will be important in the design of polymers for the preparation of stable, hydroxide-conducting films, membranes, and ionomers.

A benchmark hydroxide-conducting polymer is utilized in alkaline hydrogen fuel cell and water electrolyzer devices at 60 °C for >100 hours.

Four benzimidazolium hydroxide compounds, in which the C2-position is attached to a phenyl group possessing hydrogen, bromine, methyl groups, or phenyl groups at the ortho positions, are prepared and investigated for stability in a quantitative alkaline stability test. The differences between the stability of the various protecting groups in caustic solutions are rationalized on the basis of their crystal structures and DFT calculations. The highest stability was observed for the m-terphenyl-protected benzimidazolium, showing a half-life in 3 M NaOD/CD3OD/D2O at 80 °C of 3240 h. A high-molecular-weight polymer analogue of this model compound is prepared that exhibits excellent mechanical properties, high ionic conductivity and ion-exchange capacity, as well as remarkable hydroxide stability in alkaline solutions: only 5% degradation after 168 h in 2 M KOH at 80 °C. This is the most stable hydroxide-conducting benzimidazolium polymer to date.

Immobilized benzimidazolium cations as functional groups in anion exchange polymers can be used in alkaline anion exchange membrane fuel cells (AAEM-FCs), electrolyzers, or water purification systems, but are prone to hydroxide attack. Steric protection by proximal methyl groups has been shown to drastically increase hydroxide stability (A. Wright, S. Holdcroft ACS Macro Lett. 2014, 3, 444-447.). To further improve stability, model compounds, representing the ion exchange sites of AAEMs, were investigated for their hydroxide stability. By means of density functional theory (DFT), we studied degradation mechanisms, such as de-methylating SN2 reaction of methylated benzimidazolium cations with hydroxide ions and the attack of hydroxide on the C2 position of the benzimidazolium. Some of these results have also been compared to experimental stability tests of model compounds and polymers (A. G. Wright, T. Weissbach, S. Holdcroft Angew. Chem. Int. Ed. 2016, 55, 4818-4821.). The findings of this study enable the design of new materials for AAEM-FCs.

Alkaline anion exchange membrane fuel cells have become a topic of substantial interest in recent years, opening up a new electrochemical environment for hydrogen fuel cells. Facile kinetics for the oxygen reduction reaction open the promise of non-PGM or even non-metal AEMFCs, and radical stability. The most challenging aspect for the field, as defined by the 2016 DOE AMFC III Workshop, is membrane and ionomer stability in alkaline conditions at elevated operating temperatures. Few papers report endurance data, and best practices for in situ fuel cell conditioning and electrochemical characterization are still being developed by the community. Here, we report that HMT-PMBI exhibits membrane and ionomer stability in situ as AEMFCs in relevant conditions for device operation. These fuel cells demonstrate re-equilibration from extensive carbonation and complete re-conditioning in a shut-down / start-up cycle. We further report operation in typically challenging conditions, e.g. increased temperature and reduced humidity. Finally, we report on our attempts to define best-practices for electrochemical characterization.

Vier Benzimidazoliumhydroxide mit einem C2‐Phenylsubstituenten, der an seinen ortho ‐Positionen Wasserstoff‐ oder Bromatome sowie Methyl‐ oder Phenylgruppen trägt, wurden hergestellt, und ihre Hydroxidstabilität wurde quantitativ analysiert. Die Unterschiede der Schutzgruppen hinsichtlich ihrer Stabilität in alkalischen Lösungen wurden mit Kristallstrukturen und Dichtefunktionalrechnungen erklärt. Die höchste Stabilität wurde für m ‐Terphenyl‐geschütztes Benzimidazolium ermittelt, das eine Halbwertszeit von 3240 h in 3 m NaOD/CD 3 OD/D 2 O bei 80 °C aufwies. Ein Polymeranalog dieser Verbindung mit hohem Molekulargewicht wurde hergestellt, das exzellente mechanische Eigenschaften, eine hohe Ionenaustauschkapazität sowie eine bemerkenswerte Hydroxidstabilität in alkalischen Lösungen aufweist: Nach 168 h in 2 m KOH bei 80 °C wurde nur 5 % Zersetzung ermittelt. Dies ist das bis jetzt stabilste hydroxidleitende Benzimidazoliumpolymer.

In the pursuit of stable, hydroxide ion-exchange ionomers and solid polymer electrolytes for fuel cells and electrolyzers, we present a novel, sterically C2-protected poly(benzimidazole) derivative incorporating a hexamethyl-p-terphenylene group. Using a new, scalable, and air-insensitive methylation procedure, N-methylation of the polymer is controlled to yield an unprecedented hydroxide-stable, methanol-soluble, and water-insoluble poly(benzimidazolium) ionene. This original polymer is also soluble in aqueous ethanol, which makes it suitable for use as a processable ionomer for catalyst layers. The water uptake and ionic conductivity is correlated to the degree of methylation. The anionic conductivity reached 9.7 ± 0.6 mS cm-1 for polymers with a 92% degree of methylation. Additionally, the hexamethyl-p-terphenylene unit shows interesting atropisomerism, which may influence their physical properties.