A Relationship Study of Aromatic Ring Structures, Nanostructure Morphology, and Fuel Cell Performance of Anion Exchange Membranes

The rational design of anion exchange membranes and ionomers is crucial for advancing high-performance, cost-effective anion exchange membrane fuel cells and water electrolyzers. This study systematically investigates the structure–property–performance relationships of a series of polyaromatic anion exchange polymers, including biphenyl (BPN1-100), m- and p-terphenyl (m-TPN1-100, p-TPN1-100), and 9,9-dimethylfluorene (FMN1-100 and FMN2-50) backbones, to elucidate how backbone rigidity and ionic side chain characteristics influence material properties. Small-angle X-ray scattering (SAXS) measurements confirmed that while all polymers form nanophase-separated hydrophilic channels, good correlations with ion exchange capacity (IEC) and backbone flexibility were observed. FMN1-100, which has the most rigid nonrotatable aromatic rings in the backbone, exhibited the most well-defined hydrated nanochannels, while m-TPN1-100, which has the lowest IEC and a folded aromatic backbone, exhibited the least uniform nanochannels. The unique differences in the backbone structure also affected the density and gas permeability of polymer membranes. The structurally similar BPN1-100 and FMN1-100 were evaluated as an ionomer catalyst binder in fuel cell tests. A significantly better performing peak power density was obtained from FMN1-100 due to a combination of enhanced gas permeability and minimal π-electron interaction with electrocatalysts. These findings underscore the importance of purposively designing the polymer backbone to tune nanoscale phase-separated morphology and membrane properties.