reference25 - Ionic Conductivity of PEMFC Electrodes Effect...

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Unformatted text preview: Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading Guangchun Li and Peter G. Pickup* ,z Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7 The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell ~ PEMFC ! electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization, determined by cyclic voltam- metry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed ~ 55-76% ! cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder. © 2003 The Electrochemical Society. @ DOI: 10.1149/1.1611493 # All rights reserved. Manuscript submitted January 13, 2003; revised manuscript received May 2, 2003. Available electronically September 15, 2003. Proton exchange membrane fuel cells ~ PEMFCs ! have attracted huge interest in recent years due to their high efficiency and friend- liness to the environment. Significant improvement of cell perfor- mance has been achieved by using Nafion membranes as the elec- trolyte and mixing the catalyst with solubilized Nafion. 1-3 However, the high costs of platinum-based catalysts and the Nafion electrolyte are still major barriers to the commercialization of the PEMFC. Generally, there are two approaches to address this problem: one is to try to find more active and cheaper catalysts and membranes; 4 another is to increase utilization of platinum and Nafion in the cata- lyst layer without lowering the cell performance. 5-7 In order to ac- complish these goals, a deeper knowledge of the electrochemical processes, and the structure, composition, and morphology of elec- trodes is prerequisite. Unfortunately, the structure of the electrodes and mass transport properties in the electrodes are still not very clear. 8 Oxygen reduction at the cathode of a PEMFC cell requires both electron and proton transport as shown in Eq. 1 O 2 1 4e 1 4H 1 5 2H 2 O @ 1 # Electron transport is provided by the carbon support, while Nafion incorporated into the catalyst layer transports protons to the active Pt sites. Actually, oxygen reduction in the catalyst layer is a quite com- plex process, taking place at three-phase interfaces of the catalyst, gas, and electrolyte....
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reference25 - Ionic Conductivity of PEMFC Electrodes Effect...

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