Andrew G. Wright

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.

Poly(phenylene) and m-Terphenyl as Powerful Protecting Groups for the Preparation of Stable Organic Hydroxides

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.

Towards a stable ion-solvating polymer electrolyte for advanced alkaline water electrolysis

Advanced alkaline water electrolysis using ion-solvating polymer membranes as electrolytes represents a new direction in the field of electrochemical hydrogen production. Polybenzimidazole membranes equilibrated in aqueous KOH combine the mechanical robustness and gas-tightness of a polymer with the conductive properties of an aqueous alkaline salt solution, and are thus of particular interest in this field of research. This work presents a comprehensive study of ternary alkaline polymer electrolyte systems developed around a polybenzimidazole derivative that is structurally tailored towards improved stability in alkaline environments. The novel electrolytes are extensively characterized with respect to physicochemical and electrochemical properties and the chemical stability is assessed in 0–50 wt% aqueous KOH for more than 6 months at 88 °C. In water electrolysis tests using porous 3-dimensional electrodes completely free from noble metals, they show polarization characteristics comparable to those of commercially available separators and good performance stability over several days.

Transparent Bipolar Membrane for Water Splitting Applications

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.

Morphology of Anion-Conducting Ionenes Investigated by X-ray Scattering and Simulation

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.