Hydration and Interfacial Water in Nafion Membrane Probed by Transmission Infrared Spectroscopy Rukma Basnayake, Geneva R. Peterson, Dominick J. Casadonte, Jr., and Carol Korzeniewski* Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409-1061 ReceiVed: June 30, 2006; In Final Form: September 9, 2006 Infrared spectroscopy was applied to probe water inside pores and channels of Nafion membrane exchanged with either proton (H + ) or sodium ions (Na + ). Transmission measurements were performed on freestanding Nafion 112 (∼50 μm thickness) in a cell that enabled adjustment of the relative humidity. Experiments that employed Na + -exchanged Nafion focused on relative humidity environments at or below about 32% generated through the use of humectants. Under these conditions, narrow features in the O-H stretching spectral region near 3650 - 3720 cm -1 , previously attributed to interfacial water, were detected and matched to bands in vibrational sum frequency (VSF) spectra of water/air, water/organic, and salt-solution/air interfaces. The features correspond to the stretching mode of the “free” OH group of water oriented with one hydrogen atom toward other water molecules and interacting through hydrogen bonding and the other straddling the interface extending into fluorocarbon-rich regions (∼3668 cm -1 ) or air-filled segments (∼3700 cm -1 ) in the membrane. For membrane exchanged with H + , -SO 3 - groups were easily shifted to -SO 3 H as water was removed upon exposure to a few Torr of vacuum at 95 °C. In contrast, residual water was retained by membrane exchanged with Na + after exposure to these conditions for up to 72 h. The permeation of methanol and acetone into Na + -exchanged Nafion 112 was also examined. The C-H and O-H stretching modes of methanol were perturbed in a manner that suggests the polymer disrupts hydrogen bonding interactions within the solvent, similar to the effect it exerts on pure water. For acetone, the C-H stretching modes were not shifted appreciably compared to those of the bulk liquid. However, the carbonyl band was affected, indicating the likely importance of dipolar interactions between solvent molecules and polar groups on the polymer. Control experiments performed with poly(hexafluoropropylene-co-tetrafluoroethylene) (FEP) membrane did not show evidence for water or methanol permeation, which demonstrates the critical role played by the ion-filled channels and pores in facilitating solvent transport within Nafion membrane. Introduction The perfluorosulfonated ion exchange material Nafion 1,2 has played an important role in practical areas of electrochemistry, including fuel cells, water electrolyzers, chloralkali cells, and chemical sensors. 1,3 Nafion contains a poly(tetrafluoroethylene) backbone and perfluoroether side chains that terminate in a sulfonate group. 1 A general structure is shown in Scheme 1. The performance of Nafion as an ion transport material is influenced by the interaction of water molecules with ion- exchange groups (-SO 3 - ) on the polymer and free charge- compensating cations (e.g., H + , Na + , etc.) within the pores and channels of the polymer. 2-5 Molecular level interactions that influence the state of water in Nafion materials have been investigated by spectroscopic techniques. 1 The sensitivity of infrared spectroscopy toward water has made it valuable for the study of hydration. 6-18 Perturbations to the O-H stretching and H-O-H bending modes have been detected and ascribed to water molecules present in different environments inside Nafion, such as in nanometer-scale clusters, 16,19 at polymer interfaces, 7,9,16,19 and within proton solvent shells (i.e., (H 2 O) n H + , where n can vary from 1 to large values). 11,14,18 This paper presents results of infrared spectroscopy experi- ments that shed further light on properties of interfacial water within Nafion membrane. Transmission infrared measurements were performed on freestanding Nafion 112. A cell was used that enabled the humidity of the gas surrounding the membrane to be adjusted. 11,14,17 The thickness of Nafion 112 is ∼50 μm. Thus, the strong vibrational modes of the polymer backbone, which involve mainly -CF 2 and -SO 3 - group motion, become saturated in transmission infrared measurements. 11,15,16,18 How- ever, the fundamental vibrational modes of Nafion are below 1500 cm -1 . The region above 1500 cm -1 is free of bands from the strong fundamental vibrational modes of the polymer, enabling the observation of features for permeating molecules (i.e., vapor of water and other types of solvents). 7,9,11,14,16,17 The work described herein focuses on perturbations to the O-H stretching modes of water in the spectral region between 3000 and 4000 cm -1 . In addition to the broad features typical of modes for bulk liquid water, also observed are sharper features in the range of 3650-3720 cm -1 characteristic of the “free” OH stretching mode for interfacial water. 7,9,16,20-22 Measurements were performed on Nafion membrane in H + - and Na + -exchanged forms in different hydration states at ambient temperature and pressure. Insights into the environment for water * Corresponding author. Telephone: (806) 742-4181. Fax: (806) 742- 1289. E-mail: carol.korzeniewski@ttu.edu. SCHEME 1: Structure of Nafion Polymer 23938 J. Phys. Chem. B 2006, 110, 23938-23943 10.1021/jp064121i CCC: $33.50 © 2006 American Chemical Society Published on Web 11/04/2006