Hydroxide-Stable Ionenes

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.