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

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-pterphenylene 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.A lkaline anion exchange membrane (AAEM) fuel cells have received considerable interest as a high efficiency, low emission, and low cost energy converter. 1 Their use of nonprecious metal catalysts provides a potential advantage over incumbent proton exchange membrane fuel cells. 2 AAEMs may also find use in water electrolyzers for energy storage. 3 Typical cationic functional groups for AAEMs include ammonium, 4 sulfonium, 5 phosphonium, 6 pyridinium, 7 imidazolium, 8 benzimidazolium, 9 and metal cations, such as ruthenium. 10 However, these generally degrade when exposed to solutions of high pH and at elevated temperature. 8,11−13 Recently, an exceptionally hydroxide-stable polymer was discovered, mes-PDMBI-OH − (Figure 1), 14 which showed no observable degradation in 6 M KOH at rt or in 2 M KOH at 60°C. Hydroxide stability is conferred by introducing steric crowding around the C2-position of the benzimidazolium units. Mes-PDMBI-OH − , however, is water-soluble, requiring it to be blended to form water-insoluble membranes. Blending reduces the ion-exchange capacity (IEC) from 4.5 mequiv g −1 to 1−2 mequiv g −1 , which limits the conductivity. Moreover, blends require the use of a high-boiling solvent, DMSO, for casting, which greatly limits processability, especially catalyst layer fabrication.In this paper, we report novel ionenes, classically defined as polymers which contain ionic amines in the backbone, 15 that are stable in hydroxide form. These ionenes are soluble in alcohol/water solvents but insoluble in water, making them candidates for ionomers in catalyst layers for fuel cells and electrolyzers. Such selective solubility of hydroxide-conducting polymers has previously proven elusive. 16 Of the few ammonium-based ionomers developed, most are soluble in solvents such as NMP and DMSO, requiring higher temperatures for spray-coating and are more likely to contaminate the catalyst layer. 17−19 A recent paper by Li et al. described an ammonium-based poly(2,6-dimethyl-phenylene oxide) with solubility in methanol and ethanol solvents; but these appear to degrade to 90% of their original conductivity after 60 ...

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 KOH aq at 100 °C (t 1/2 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.

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.