Preparation and Study of Sulfonated Co-Polynaphthoyleneimide Proton-Exchange Membrane for a H2/Air Fuel Cell

The sulfonated polynaphthoyleneimide polymer (co-PNIS70/30) was prepared by copolymerization of 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid (ODAS) and 4,4'-methylenebisanthranilic acid (MDAC) with ODAS/MDAC molar ratio 0.7/0.3. High molecular weight co-PNIS_70/30 polymers were synthesized either in phenol or in DMSO by catalytic polyheterocyclization in the presence of benzoic acid and triethylamine. The titration reveals the ion-exchange capacity of the polymer equal to 2.13 meq/g. The membrane films were prepared by casting polymer solution. Conductivities of the polymer films were determined using both in- and through-plane geometries and reached ~96 and ~60 mS/cm, respectively. The anisotropy of the conductivity is ascribed to high hydration of the surface layer compared to the bulk. SFG NMR diffusometry shows that, in the temperature range from 213 to 353 K, the ¹H self-diffusion coefficient of the co-PNIS_70/30 membrane is about one third of the diffusion coefficient of Nafion^® at the same humidity. However, temperature dependences of proton conductivities of Nafion^® and of co-PNIS_70/30 membranes are nearly identical. Membrane-electrode assemblies (MEAs) based on co-PNIS_70/30 were fabricated by different procedures. The optimal MEAs with co-PNIS_70/30 membranes are characterized by maximum output power of ~370 mW/cm² at 80 °C. It allows considering sulfonated co-PNIS_70/30 polynaphthoyleneimides membrane attractive for practical applications.