Electrochemical Characterization of Phosphosilicate-modified Nafion Membranes
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Electrochemical Characterization of Phosphosilicate-modified Nafion Membranes L. C. Klein, Department of Ceramic and Materials Engineering, Rutgers, The State University of New Jersey, 607 Taylor Rd., Piscataway, NJ 08854-8065 M. Aparicio, CSIC, Instituto de Ceramic y Vidrio, Campus de la Universidad Autónoma de Madrid, Camino de Valdelatas, s/n, 28049 Cantoblanco (Madrid), SPAIN
ABSTRACT Hybrid membranes containing Nafion and phosphosilicate gels were prepared using infiltration and recasting methods. The hybrid membranes were investigated using complex impedance spectroscopy. Conductivities were determined as a function of relative humidity. Infiltrated membranes, which contained gel in more or less discrete particles, were compared with recast membranes where the gel was more evenly distributed. Both of the modified membranes were compared to unmodified Nafion. The impedance spectra were analyzed to distinguish electrode interface effects from bulk transport phenomena.
INTRODUCTION Proton-exchange membrane fuel cells (PEMFC) represent an alternative to internal combustion and diesel engines for cars, trucks and buses [1]. PEMFCs convert chemical energy directly to electrical energy using fixed electrodes and a proton-conducting electrolyte. A serious drawback in PEMFCs is water management. The proton conductivity of the electrolyte membrane increases linearly with the water content, with the highest conductivity corresponding to a fully hydrated membrane. While it is desirable to operate a fuel cell at a temperature above the boiling point of water for several reasons including minimizing CO poisoning of Pt catalysts, at T>100°C, the membranes dehydrate. To operate above 100°C and maintain hydration, the total pressure increases dramatically. Therefore, we have introduced hygroscopic gels into the membrane to increase water retention when operating above the boiling point of water [2-5]. By far the most common proton-exchange membrane is the polymer Nafion. Nafion is the trade mark (produced by DuPont) of a perfluorinated material composed of carbon-fluorine backbone chains and perfluoro side chains with sulfonic acid ion-exchange groups: (CF2-CF2)x
(CF-CF2) O (CF2-CF)y CF3
(CF2)z
-
+
SO3 H
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The favorable properties of this polymer lie in the combination of the high hydrophobicity of the perfluoronated polymer backbone and the high hydrophilicity of the sulfonic acid branch. The hydration water of the hydrophilic branches provides high proton conductivity in the presence of water. The activation enthalpy for water diffusion in hydrated Nafion membranes is nearly the same as that in pure water. The hydration water is composed of three water molecules per sulfonic acid group that form the primary hydration shell. The proton conductivity for Nafion roughly follows the water diffusion coefficient, i.e. the conductivity is provided by the diffusion of hydrated protons such as H3O+ [6,7]. The practical problem encountered in PEMFC is keeping the membrane wet under operating conditions. The p
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