Parallel cylindrical water nanochannels in Nafion fuel-cell membranes

TitleParallel cylindrical water nanochannels in Nafion fuel-cell membranes
Publication TypeJournal Article
Year of Publication2008
AuthorsSchmidt-Rohr K, Chen Q
Journal TitleNature Materials
Volume7
Pages75-83
Date PublishedJan
Type of ArticleArticle
ISBN Number1476-1122
Accession NumberISI:000252117700024
KeywordsANGLE X-RAY, CONDUCTIVITY, diffusion, ION-EXCHANGE MEMBRANES, MEMBRANES, NEUTRON-SCATTERING DATA, PERFLUORINATED IONOMER MEMBRANES, POLYMER ELECTROLYTE, proton, SAXS, temperature, TRANSPORT
Abstract

The structure of the Nafion ionomer used in proton-exchange membranes of H-2/O-2 fuel cells has long been contentious. Using a recently introduced algorithm, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic 'ionomer peak' arises from long parallel but otherwise randomly packed water channels surrounded by partially hydrophilic side branches, forming inverted-micelle cylinders. At 20 vol% water, the water channels have diameters of between 1.8 and 3.5 nm, with an average of 2.4 nm. Nafion crystallites (similar to 10 vol%), which form physical crosslinks that are crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross-sections of similar to(5nm)(2). Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The new model can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures.

DOI10.1038/nmat2074
Alternate JournalNat. Mater.