(Invited) Polymers for Proton and Anion Exchange Membrane Fuel Cells
Authors: Herring Andrew M. 
Published: February 2020

The polymers used for polymer electrolyte fuel cells have many demands on them. Fundamentally these materials are ion conductors and should have ionic conductivities >100 mS cm-1under the conditions of operation. To date polymer electrolytes still achieve practical ionic conductivities by water mediated ion conduction. So as membranes the material needs to be thin to enable a low area specific resistance (ASR) and to facilitate water diffusion from the anode to the cathode so that external humidification of inlet streams is not necessitated. In addition, the material must be an electrical insulator, be a barrier to fuel and oxidant and be mechanically and chemically stable under the transient operation of a typical fuel cell. For proton exchange membranes (PEMs) the primary chemical degradation mechanism is through attack by oxygenated radicals and so current PEMs are pefluorinated sulfonic acid (PFSA) polymers. For anion exchange membranes (AEMS) the primary chemical degradation mechanism is through attack by the hydroxyl anion, a strong nucleophile, and so these materials tend to be wholly hydrocarbon based with advanced cations designed to resist nucleophilic attack. The water transport in AEM fuel cells becomes more critical as water is both a product and a reactant in these systems. For both situations the polymer should be scalable, processable and manufacturable if membrane at the correct price point for market adoption is to be achieved.
Th polymer electrolyte in the catalyst layer, the ionomer, has subtly different properties while it must still be a good ion conductor and it no longer needs to be mechanically stable it must now not be a barrier to the product and reactants. Through many years of research for PEMS it is often a low equivalent weight PFSA that can be added to the catalyst layer that enhances ion transport and when thin does not inhibit product and reactant transport. For AEM fuel cells the development of ionomers is still at an early stage, and it is not obvious that cathode and anode ionomers will have the same chemistry. In addition, as many AEM polymers are extremely insoluble finding polymers that can be dispersed for application of the catalyst layer is a non-trivial task. Some groups have even reported god performance in fuel cells by the application of a ground powder of the AEM polymer to the fuel cell electrode.
To date a vast number of PEM and AEM polymer chemistries have been proposed. Generally, for PEMs these are PFSAs with bicontinuous phase separated morphologies. For AEMs the variety of morphologies has been quite astonishing. Because of the ease of synthesis of hydrocarbon polymers, and especially block co-polymers, almost all morphologies that can be synthesized have been studied. In this paper we will briefly review the literature and draws some general conclusions. We will then use of own work to illustrate how an experimental polymer is scaled up and processed into device ready films. Using developer feedback, we will the show how modifications to the polymer can tailor the material for specific devices. An interplay between cross-linking, ion exchange capacity, and polymer chemistry and morphology becomes apparent to achieve a successful outcome. Our group makes much use of environmental SAXS and WAXS to study membranes under fuel cell relevant conditions and we have recently begun to expand these studies to environmental GISAXS for catalyst layer relevant work.