Aliphatic/Aromatic Polyimide Ionomers as a Proton Conductive Membrane for Fuel Cell Applications

TLDR

The study synthesizes sulfonated polyimide ionomers with aliphatic groups to create a proton‑conductive, durable membrane for fuel cells. The ionomers were prepared by polycondensation to yield transparent, ductile, flexible membranes, which were then incorporated into a fuel cell operating at 80 °C and 0.2 A cm⁻² with H₂ and air at 60–90 % RH. The ionomer achieved a maximum conductivity of 0.18 S cm⁻¹ at 140 °C, comparable to Nafion above 100 °C, with lower H₂/O₂ permeability and 5,000 h durability, attributed to well‑dispersed <5 nm hydrophilic domains.

Abstract

To produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide ionomers containing aliphatic groups both in the main and in the side chains have been synthesized. The title polyimide ionomers 1 with the ion exchange capacity of 1.78−2.33 mequiv/g were obtained by a typical polycondensation reaction as transparent, ductile, and flexible membranes. The proton conductivity of 1 was slightly lower than that of the perfluorinated ionomer (Nafion) below 100 °C, but comparable at higher temperature and 100% RH. The highest conductivity of 0.18 S cm-1 was obtained for 1 at 140 °C. Ionomer 1 with high IEC and branched chemical structure exhibited improved proton conducting behavior without sacrificing membrane stability. Microscopic analyses revealed that smaller (<5 nm) and well-dispersed hydrophilic domains contribute to better proton conducting properties. Hydrogen and oxygen permeability of 1 was 1−2 orders of magnitude lower than that of Nafion under both dry and wet conditions. Fuel cell was fabricated with 1 membrane and operated at 80 °C and 0.2 A/cm2 supplying H2 and air both at 60% or 90% RH. Ionomer 1 membrane showed comparable performance to Nafion and was durable for 5000 h without distinct degradation.