Article abstract
Nature Materials 7, 75 - 83 (2008)
Published online: 9 December 2007 | doi:10.1038/nmat2074
Subject Categories: Materials for energy | Computation, modelling and theory
Parallel cylindrical water nanochannels in Nafion fuel-cell membranes
Klaus Schmidt-Rohr & Qiang Chen
Abstract
The structure of the Nafion ionomer used in proton-exchange membranes of H2/O2 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 (
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 (5 nm)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.
- Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA
Correspondence to: Klaus Schmidt-Rohr e-mail: srohr@iastate.edu
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