State of Water in Nafion 117 Proton Exchange Membranes Studied by Dielectric Relaxation Spectroscopy
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State of Water in Nafion 117 Proton Exchange Membranes Studied by Dielectric Relaxation Spectroscopy Georgios Polizos, Zijie Lu, Digby D. Macdonald, and Evangelos Manias Materials Sci & Eng, Penn State University, University Park, PA, 16802
ABSTRACT The dynamics and the nature of water environments in a fuel cell proton exchange membrane are studied experimentally. Specifically, the dynamics of water in Nafion 117 membranes, in the acid form, were investigated at two hydration levels and several temperatures by means of dielectric relaxation spectroscopy; two different dielectric spectroscopy experimental setups were employed for low (10-2-107 Hz, 25 to –80 oC) and microwave (0.04526 GHz, 35 oC) frequency ranges. Three states of water were clearly identified: (a) water strongly bound to the sulfonic groups (in quantitative agreement with previous investigations) defining the first hydration layer; (b) loosely bound water, surrounding the first layer, and (c) free water, having similar dynamics as in the bulk/liquid water. This is the first time that the dynamics of loosely bound water are experimentally observed. INTRODUCTION Nafion® is the commercial name of the perfluorinated ionomers manufactured by E.I. DuPont, designed with high proton conductivity so as to be used as electrolytes in fuel cell applications [1]. The conductivity mechanism is generally believed to be controlled by the proton diffusion through the membrane’s water network and, as a result, several studies have focused on exploring the nature of this water [2,3]. Infrared spectroscopy (IR) [4,5] and differential scanning calorimetry (DSC) [6] can detect distinct water types: bulk-like water, at high very high hydration levels; water bound –strongly or loosely– to the SO3- groups, via IR signatures; and hydrophobic water, entrapped in the fluorocarbon polymer chains under certain sample preparation conditions. In addition, solid state nuclear magnetic resonance (NMR) indicates a distribution of water environments [7]. Dielectric relaxation spectroscopy (DRS) can be used to detect all these water dynamics directly, however to date it has been employed mainly in the low frequency region (< 10 MHz) and focused on conductivity studies. There, and at low temperatures, a change in the activation energy of the conductivity mechanism was observed, due to water “freezing” at about –58 oC [8]; in addition, a relaxation attributed to the rotation of SO3-/water complexes (first hydration layer) has also been reported [9]. Here we report a systematic DRS study, combining both low and microwave frequencies, where all three water relaxation processes with distinct dynamics are measured and identified. EXPERIMENTAL DETAILS Nafion 117 membrane was purchased, purified as per ref. [10], and subsequently equilibrated at controlled relative humidity (by using saturated NaCl solutions in sealed vessels);
The membrane’s water content λ was set to the desired value by controlling the relative humidity in an environmental vessel and equilibrating for 3 days
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