Humidity-dependent water structure on the Nafion thin film surface was determined at room temperature by using sum frequency generation spectroscopy (SFG). When the Nafion thin film was exposed to a low relative humidity (RH) environment, a peak at 3720 cm -1 due to the "dangling OH" or "free OH" of water molecules and a peak centered around 3600 cm -1, which was assigned to be due to water molecules interacting with sulfonate groups of the Nafion surface, were observed. The intensities of these peaks increased as RH was increased. A broad peak centered around 3200-3300 cm -1 due to water adsorbed on hydrophobic, i.e., fluorocarbon, sites of Nafion was observed when the Nafion thin film was exposed to a high RH environment (RH > 60%). The behavior of surface water is compared with that of water in bulk Nafion.

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