Ac Impedance Spectroscopy Study of Modified Proton-Exchange Membrane Nanocomposites
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Ac Impedance Spectroscopy Study of Modified Proton-Exchange Membrane Nanocomposites
Françoise Damay and Lisa C. Klein Department of Ceramic and Materials Engineering, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, NJ 08854-8065, U.S.A.
ABSTRACT The dielectric and electrical behaviors of a NafionTM membrane, a NafionTM/7SiO2-2P2O5ZrO2 gel composite, and a NafionTM/ZrP particle composite were investigated using ac impedance spectroscopy at different relative humidities and temperatures. Only NafionTM/7SiO22P2O5-ZrO2 showed a slightly higher conductivity than NafionTM at high temperature. For NafionTM/ZrP, proton mobility was significantly lower, particularly at high relative humidity. The results suggest that improvement in fuel cell performance, previously measured on composite membranes, does not merely result from increased hydrophilic properties.
INTRODUCTION One limitation of the current Proton Exchange Membrane Fuel Cell (PEMFC) operating at about 80°C is that 5 to 10 ppm of CO in the H2 feed gas is enough to poison the Pt anode electrocatalyst. One way to increase the CO tolerance level is to increase the operating temperature, since the free energy of adsorption of CO on Pt has a larger positive temperature dependence than that of H2 [1]. However, because the mechanism of the proton conduction in the perfluorosulfonated polymer NafionTM membrane commonly used in PEMFC relies on the presence of water, operating a fuel cell at a temperature above the boiling point of water leads to poor performance, when the membrane dehydrates. Since increasing the operating pressure of the fuel cell is not an energetically efficient solution, it is better to modify the properties of the proton-exchange membrane and to enhance its water retention. For that purpose, incorporation of hydrophilic oxides has been pursued, with sol-gel techniques permitting oxides to enter the nanopores of the polymer [2]. Our recent work [3] focused on NafionTM membranes infiltrated by a sol-gel process with a phosphosilicate glass 7SiO2-2P2O5-ZrO2 (SPZ). Phosphosilicate glasses are well known for their fast-proton conductivity, attributed to strong hydrogen bonds between water molecules and POH groups of the glass surface. Fuel cell measurements carried out on this composite showed that PEMFC performance at 140oC, using humidified H2 and O2 at 3 atm pressure, was enhanced, compared to the original NafionTM membrane. A similar observation was made on NafionTM infiltrated with hydrophilic ZrP particles [4]. These results were attributed to improved water retention in both NafionTM/SPZ and NafionTM/ZrP composite membranes. To understand more accurately the role of the added phase in the composites, we have performed ac impedance
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spectroscopy measurements at various relative h
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