Electronic Structure and Vibrational Mode Study of Nafion Membrane Interfacial Water Interactions

Abstract

The structural, energetic, and mid-infrared spectral properties of water clusters interacting with a model perfluorosulfonate (Nafion) ionomer segment have been investigated by means of quantum chemical calculations. Density functional theory calculations were conducted to model interfacial water condensed in pores and channels of Nafion and helped shedding light on the structural and vibrational properties of hydrated Nafion membranes. The computational models consisted of a sodium counterion, the Nafion ionomer, and a cluster of water molecules positioned in three different regions of the ionomer. The orientations of the water molecules in the vicinity of the ionomer were examined 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 calculations revealed insights 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 hydrogen bond to the hydrophilic sulfonate group as well as to the hydrophobic fluorinated backbone.

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion

“…35 Although very weak, the O–H stretching band arising from non-H-bonded H 2 O, 15,16,38−42 that is, OH groups without a donating H-bond, was identified at 3693 cm –1 for temperature above 80 °C. 38,43 It was observed that O–H stretching and O–H bending band intensities increased significantly with increasing RH and the bands at 1410 and 914 cm –1 assigned to S=O and S–OH, respectively, 18 of SO 3 H groups diminished with increasing hydration; however, the band at 625 cm –1 remained constant for all RHs. For O–H stretching, appearance of a new shoulder peak at around 3223 cm –1 was noticed with increasing RH.…”

“…The typical spectra of hydrated Nafion showed a broad absorption band at 3448 cm –1 corresponding to O–H stretching, 1721 and 1633 cm –1 corresponding to O–H bending, 1311 corresponding to CF 3 stretching, 1193 cm –1 corresponding to asymmetrical C–F stretching, 1142 cm –1 corresponding to symmetrical C–F stretching, 1055 cm –1 corresponding to symmetrical S–O stretching with CCO bending, 967 cm –1 corresponding to symmetrical S–O stretching with C–S stretching, and 625 cm –1 corresponding to the CF 2 rocking mode . Although very weak, the O–H stretching band arising from non-H-bonded H 2 O, ,,− that is, OH groups without a donating H-bond, was identified at 3693 cm –1 for temperature above 80 °C. , It was observed that O–H stretching and O–H bending band intensities increased significantly with increasing RH and the bands at 1410 and 914 cm –1 assigned to SO and S–OH, respectively, of SO 3 H groups diminished with increasing hydration; however, the band at 625 cm –1 remained constant for all RHs. For O–H stretching, appearance of a new shoulder peak at around 3223 cm –1 was noticed with increasing RH.…”

Ab Initio Molecular Dynamics Simulations on the Hydrated Structures of Na+–Nafion Models

“…The value obtained agrees very well with the predicted bending frequencies of water molecules of several cluster Na + −Nafion−H 2 O models (1590−1640 cm −1 ) calculated by means the B3LYP/SVP approach. 13 In the experimental IR spectra the peaks of the bending modes have a more complicated structure usually composed of two peaks at ∼1630 and ∼1730 cm −1 , respectively. 20,63 The band with lower frequency perfectly fits with our predicted number and experimentally was assigned to the bending modes of water molecules.…”

“…Further, the work also explains the differences in the . The basic Nafion model used in this work is similar to that used in previous work 12,13 and is shown in Scheme 1.…”

Theoretical analyses on water cluster structures in polymer electrolyte membrane by using dissipative particle dynamics simulations with fragment molecular orbital based effective parameters

“… 26–28 Kawakami and Shigemoto also used MD simulation to verify the diffusion mechanism of proton. 29 In addition, using density functional theory (DFT) calculation, water cluster interacting with sulfonic acid group, 30 Nafion/Pt interface, 31 and degradation of Nafion 32 have been also reported.…”