On the development of proton conducting polymer membranes for hydrogen andmethanol fuel cells

The transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones are explained in terms of distinct differences on the microstructures and in the pK(a) of the acidic functional groups. T he less pronounced hydrophobic/hydrophilic separation of sulfonated polyeth erketones compared to NAFION corresponds to narrower, less connected hydrop hilic channels and to larger separations between less acidic sulfonic acid functional groups. At high water contents, this is shown to significantly r educe electroosmotic drag and water permeation whilst maintaining high prot on conductivity. Blending of sulfonated polyetherketones with other polyary ls even further reduces the solvent permeation (a factor of 20 compared to NAFION), increases the membrane flexibility in the dry state and leads to a n improved swelling behaviour. Therefore, polymers based on sulfonated poly etherketones are not only interesting low-cost alternative membrane materia l for hydrogen fuel cell applications, they may also help to reduce the pro blems associated with high water drag and high methanol cross-over in direc t liquid methanol fuel cells (DMFC). The relatively high conductivities obs erved for oligomers containing imidazole as functional groups may be exploi ted in fully polymeric proton conducting systems with no volatile proton so lvent operating at temperatures significantly beyond 100 degreesC, where me thanol vapour may be used as a fuel in DMFCs. (C) 2001 Elsevier Science B.V . All rights reserved.