Humidity dependent structure of water at the interfaces between perfluorosulfonated ionomer thin film and Pt and HOPG studied by sum frequency generation spectroscopy

Humidity-dependent water structures at the interfaces between perfluorosulfonate ionomer (PFSI = Nafion) thin film and Pt and HOPG were determined at room temperature by using sum frequency generation (SFG) spectroscopy. At low relative humidity (RH) condition, a peak centered around 3600 cm− 1 corresponding to water molecules interacting with sulfonic acid group in proton channel at PFSI surface was observed at both interfaces. However, while this peak increased with RH at PFSI/Pt interface, that at PFSI/HOPG did not increase. These results suggest that the structure of water molecules, especially the water molecules at the proton channel of PFSI surface, is strongly affected by the interaction of PFSI with Pt and HOPG.

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► Water at Pt/PFSI and HOPG/PFSI interfaces were studied by SFG spectroscopy.
► Water mainly exists at sulfonic acid site on the surface of proton channel at low RH.
► Water starts to adsorb even on hydrophobic fluorocarbon carbon site at high RH.
► Ordered water structure was observed at Pt/HOPG interface.
► Disordered water structure was found at HOPG/PFSI interface at high RH.