Microblock Ionomers: A New Concept in High Temperature, Swelling‐Resistant Membranes for PEM Fuel Cells

Abstract

A novel series of polyaromatic ionomers with similar equivalent weights but very different sulphonic acid distributions along the ionomer backbone has been designed and prepared. By synthetically organising the sequence‐distribution so that it consists of fully defined ionic segments (containing singlets, doublets or quadruplets of sulphonic acid groups) alternating strictly with equally well‐defined nonionic spacer segments, a new class of polymers which may be described as microblock ionomers has been developed. These materials exhibit very different properties and morphologies from analogous randomly substituted systems. Progressively extending the nonionic spacer length in the repeat unit (maintaining a constant equivalent weight by increasing the degree of sulphonation of the ionic segment) leads to an increasing degree of nanophase separation between hydrophilic and hydrophobic domains in these materials. Membranes cast from ionomers with the more highly phase‐separated morphologies show significantly higher onset temperatures for uncontrolled swelling in water. This new type of ionomer design has enabled the fabrication of swelling‐resistant hydrocarbon membranes, suitable for fuel cell operation, with very much higher ion exchange capacities (>2 meq g–1) than those previously reported in the literature. When tested in a fuel cell at high temperature (120 °C) and low relative humidity (35% RH), the best microblock membrane matched the performance of Nafion 112. Moreover, comparative low load cycle testing of membrane ‐electrode assemblies suggests that the durability of the new membranes under conditions of high temperature and low relative humidity is superior to that of conventional perfluorinated materials.

Phase separation of supramolecular and dynamic block copolymers

“…Recently, sulfonated polyether ketones have been established as promising fuel cell membrane candidates due to their ability to form phase-separated ionic structure in the bulk state. 33 Polymer-small molecule complexes. In all of the previous examples, polymer-based supramolecular building blocks are employed to construct complex polymer architectures and morphologies.…”

Anion conducting multiblock poly(arylene ether sulfone)s containing hydrophilic segments densely functionalized with quaternary ammonium groups

“…[8][9][10] This has for example been achieved by directing the sulfonic acid groups to side chains [11][12][13][14] and by precisely sequencing the sulfonic acids along the polymer backbone. 15,16 However, the most successful results have been achieved with block copolymers having very high concentrations of sulfonic acid attached to one of the blocks. 17,18 Because of the immiscibility of the ionic and non-ionic blocks, these copolymers characteristically self-assemble to form various well-ordered phase structures on the nanometer level, thus forming a separate and highly sulfonated phase for proton transport.…”

Degradation aspects of water formation and transport in Proton Exchange Membrane Fuel Cell: A review

“…A new class of polymers, known as microblock ionomer, was developed by Zhu et al [300] as a potential swelling-resistant membrane. The new design is based on sequence-distribution of sulphonic acid in ionomer backbone forming ionic and non-ionic spacer segments.…”