Morphology of water transport channels and hydrophobic clusters in Nafion from high spatial resolution AFM-IR spectroscopy and imaging

The nanoscale distribution of distinctly different water molecules within a perfluorinated sulfonic acid polymer Nafion film has been chemically imaged for the first time. The visualization was achieved using the newly developed AFM-IR microspectroscopy method, which enabled the chemical imaging of water domains to be separated based upon their characteristic water molecular OH-stretching vibration. Chemical imaging results have revealed ionic bound water molecules, iH2O, (3211 cm− 1) that were clustered in domains, while the hydrated free bulk-like water molecules, bH2O, (3482 cm− 1) were found within transportable channels that linked these clusters. These results give invaluable experimental chemical evidence required to test the numerous cluster-network models that have been proposed so far. A direct visual insight of the Nafion water structure given by this method will enlighten the understanding of the present proton exchange mechanism that is essential for the development of more efficient fuel cells.

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► Distinct water molecules within a Nafion film have been imaged using AFM-IR.
► Bound water is clustered in domains, while bulk-like water resides within channels.
► AFM-IR characterizes the nanoscale nature of water transport channels in Nafion.