In electrochemical energy conversion systems such as proton exchange membrane fuel cells, ionomers are essential components because of their role in facilitating proton conduction. The ionic resistance within the membrane electrode assembly largely governed by the properties of the ionomer material, has a pronounced impact on the overall cell performance. Therefore, a comprehensive investigation of ionomer materials is critical for enhancing system efficiency and operational durability. In PEMFC systems, ionomers are employed in the membrane and as a binder material within the catalyst layer. While both applications rely on the ionomer´s ability to conduct protons, each imposes distinct functional requirements. As a membrane, the ionomer must exhibit high proton conductivity, low gas permeability to minimize hydrogen crossover, and electronic insulation. In contrast, when employed as a binder in the catalyst layer, the ionomer facilitates proton transport from the membrane to the catalytic active sites, while simultaneously enabling the diffusion of reactant gases from the gas diffusion layer to the catalyst surface. Thus, the binder ionomer must balance ionic conductivity with optimal dispersion, thin film formation, and strong interfacial adhesion to both catalyst particles and the membrane. Nafion-based materials currently represent the state-of-the-art in proton-conducting ionomers for PEMFC applications. However, they exhibit several limitations, particularly under elevated operating temperatures in the range of 100°C - 180°C. The proton conductivity in a Nafion-based material is based on sulfonic acid groups, which heavily rely on the presence of water to enable effective proton transport. Therefore, at temperatures above 100°C maintaining sufficient hydration becomes challenging, leading to a significant decline in proton conductivity and fuel cell performance. To develop an ionomer suitable for HT-PEMFC operation (100°C - 180°C), the initial approach involved the modification of a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) backbone with aliphatic side chains to improve mechanical stability. Building on this, a two acidic functionalization strategy was employed to introduce two acidic moieties in the aromatic backbone, thereby improving proton conductivity under low-humidity conditions. Blending with polybenzimidazole was employed to prevent water solubility of the polymer containing a high density of ionic groups. The resulting blend membrane demonstrated superior performance compared to Nafion-based membranes under conditions at low relative humidity and operating temperatures exceeding 100°C. Furthermore, an ionomer material tailored for binder application in the catalyst layer was synthesized based on a polymer of intrinsic microporosity (PIM). This polymer features a rigid backbone structure that inhibits dense chain packing, thereby promoting high free volume and facilitating enhanced gas permeability. When employed as binder in the catalyst layer, the PIM-based ionomer demonstrated an elevated oxygen diffusion coefficient, confirmed though ex-situ and in-situ measurements.
Theresa Stigler (Thu,) studied this question.