Theoretical investigation of hydrogen bonding and ion pair formation in Nafion membrane models



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The structural, energetic, and infrared spectral properties of water clusters interacting with model perfluorosulfonate (Nafion) ionomer segments have been investigated by means of quantum chemical calculations. Density functional theory (DFT) calculations were conducted in order to model interfacial water condensed in pores and channels of Nafion and help shed light on the structural and vibrational properties of hydrated Nafion membranes. The computational models examine the orientations of the water molecules in the vicinity of the two phases of the ionomer, and were applied to understand the appearance of O-H stretching bands that deviated from the typical bulk water values, as well as the appearance of multiple free O-H stretching bands. The computational models consist of a sodium counter ion, the Nafion ionomer, and a cluster of water molecules positioned in three different regions of the membrane. In region 1 the waters are placed close to the SO3- group and the counter ion. In region 2 the waters are located between the SO3- group and the fluorinated segment and in region 3 the waters are interacting with the fluorinated groups. The results reveal insight into the structure, orientation, and energetics of Nafion and water clusters in and around the membrane interface; the results show that hydrogen atoms at the water-ionomer interface form strong hydrogen bonds to the hydrophilic sulfonate group as well as weak hydrogen bond interactions with the hydrophobic fluorinated backbone. They also suggest that hydrogen bonding stabilizes the types of water-ionomer environments that can lead to multiple free O-H stretching vibrational features in experimental spectra.



Nafion, Hydrogen bonding