Proton-Conducting Sulfonated and Phosphonated Polymers and Fuel Cell Membranes by Chemical Modification of Polysulfones

Abstract: The proton exchange membrane fuel cell (PEMFC) is currently emerging as an efficient and environmentally friendly power source. The technology is very complex and relies ultimately on materials and components which need further development. One of the major hurdles for advancing the PEMFC technology is currently the demand for new durable low-cost polymeric membranes that will allow fuel cell operation at high temperatures without extensive humidification requirements. Thus, the design and preparation of functional high-performance proton-conducting membranes with the critical set of properties is a major challenge for polymer and materials chemists around the world today. In this context, major efforts are directed towards different durable aromatic main-chain ionomers. In the present thesis project, new fuel cell membrane materials based on polysulfones (PSUs) functionalized with sulfonated or phosphonated moieties via lithiation chemistry have been designed, synthesized and investigated. PSUs are high performance thermoplastics with excellent chemical, mechanical and thermal properties. By isolating the ionic sites on side chains, away from the polymer main chain, the nanophase separation between the hydrophobic and the hydrophilic domains of the hydrated membrane may be manipulated and influenced, which in turn may provide membranes with balanced water sorption characteristics. Membranes with controlled water uptake were obtained by attaching the sulfonic acid unit to stiff aromatic side chains. This was conveniently achieved by reacting lithiated PSU with 2-sulfobenzoic acid cyclic anhydride in a one-pot reaction. In order to increase the length of the aromatic spacers, a new pathway was developed where lithiated PSU was reacted with 4-fluorobenzoyl chloride. This afforded PSUs with pendant fluorobenzoyl side chains in which the fluoride groups are activated for nucleophilic substitution. As a result, a wide range of nucleophiles may be used to further substitute the polymer. In an initial effort, the activated fluoride groups were replaced by sulfophenoxy or sulfonaphthoxy units in a potassium carbonate-mediated nucleophilic substitution reaction. This reaction proceeded under full conversion and the degree of substitution was easily controlled by the degree of lithiation in the first step. Using a similar methodology, PSUs carrying di- and trisulfonated aromatic side chains were successfully synthesized. In particular, membranes based on a PSU main chain carrying disulfonated napthoxybenzoyl side chains exhibited a distinct phase separation between the hydrophobic polymer main chain and the hydrophilic sulfonated side chains, and formed a well-defined and efficient network of water-filled nanopores. The latter resulted in excellent proton conductivity at controlled levels of water uptake in contrast to conventional sulfonated aromatic polymers. The investigation of alternative acidic moieties is also of great interest since desulfonation may become a critical issue at high temperatures. This has motivated the search for ionomers based on phosphonic acid units, which generally have a higher hydrothermal stability than sulfonic acid units. Phosphonated PSU was successfully prepared by reacting lithiated PSU with chlorophosphonic acid esters. However, as underlined in a recent literature review, the acidity of phosphonic acid units directly attached to aromatic rings was too low to result in reasonable levels of water uptake. An original approach was therefore developed in which PSU with pendant iodinated benzoyl side chains were prepared via lithiation chemistry. The latter polymer was then further modified to yield the more acidic ?CF2PO3H2 units located on aromatic side chains. Membranes based on ionomers having 0.90 mmol of phosphonic acid units/g of dry polymer took up 6 wt% water when immersed at room temperature, and levels of conductivity comparable to those reached by a membrane based on a sulfonated polysulfone having 0.86 mmol of sulfonic acid/g of dry polymer were recorded.