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Parallel cylindrical water nanochannels in Nafion fuel-cell membranes

Nature Materials volume 7, pages 75–83 (2008)Cite this article

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.

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Figure 1: Known structural features of Nafion.
Figure 2: Parallel water-channel (inverted-micelle cylinder) model of Nafion.
Figure 3: Effect of hydration on the ionomer peak of Nafion and its simulation by the water-channel model.
Figure 4: Simulated small-angle scattering curves for spherical-cluster models (Gierke, Yeager and local-order models).
Figure 6: Simulated small-angle scattering curves for network models.
Figure 5: Simulations of small-angle scattering for models with elongated polymer structures.

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Acknowledgements

Work at the Ames Laboratory was supported by the Department of Energy-Basic Energy Sciences (Materials Chemistry and Biomolecular Materials Program) under Contract No. DE-AC02-07CH11358.

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  1. Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA

    Klaus Schmidt-Rohr & Qiang Chen

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Contributions

K.S.R. ran the simulations and wrote the paper. Q.C. identified various Nafion models in the literature and reprocessed literature data for Fig. 1.

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Correspondence to Klaus Schmidt-Rohr.

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Schmidt-Rohr, K., Chen, Q. Parallel cylindrical water nanochannels in Nafion fuel-cell membranes. Nature Mater 7, 75–83 (2008). https://doi.org/10.1038/nmat2074

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