116 - Journal of The Electrochemical Society 151 3...

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Liquid Water Transport in Gas Diffusion Layer of Polymer Electrolyte Fuel Cells Ugur Pasaogullari* and C. Y. Wang** ,z Electrochemical Engine Center and Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA High-current-density performance of polymer electrolyte fuel cells ~ PEFCs ! is known to be limited by transport of reactants and products. In addition, at high current densities, excessive amount of water is generated and condenses, filling the pores of electrodes with liquid water, and hence limiting the reactant transport to active catalyst. This phenomenon known as ‘‘flooding’’ is an important limiting factor of PEFC performance. In this work, the governing physics of water transport in both hydrophilic and hydrophobic diffusion media is described along with one-dimensional analytical solutions of related transport processes. It is found that liquid water transport across the gas diffusion layer ~ GDL ! is controlled by capillary forces resulting from the gradient in phase saturation. A one-dimensional analytical solution of liquid water transport across the GDL is derived, and liquid saturation in excess of 10% is predicted for a local current density of 1.4 A/cm 2 . Effect of GDL wettability on liquid water transport is explored in detail for the first time. Furthermore, the effect of flooding on oxygen transport and cell performance is investigated and it is seen that flooding diminishes the cell performance as a result of decreased oxygen transport and surface coverage of active catalyst by liquid water. © 2004 The Electrochemical Society. @ DOI: 10.1149/1.1646148 # All rights reserved. Manuscript submitted June 26, 2003; revised manuscript received September 22, 2003. Available electronically February 5, 2004. Polymer electrolyte fuel cells ~ PEFCs ! have drawn much atten- tion in the last decade as a promising candidate for high-efficiency, low-emission power sources. High-current-density operation of PEFCs, of special interest to vehicle applications, is prone to liquid water formation due to excessive water generation at the cathode. The ensuing two-phase transport of reactant and product species becomes a limiting mechanism for cell performance, particularly at high current densities, i.e. , . 1 A/cm 2 . Therefore, a fundamental un- derstanding of two-phase transport in porous gas diffusion layers ~ GDLs ! of PEFCs is essential in order to improve performance. The importance of water management to PEFC performance is repeatedly expressed in the literature. 1-8 A vast majority of currently available polymer electrolytes requires hydration in order to provide higher proton conductivity. 1 When the gas phase is saturated with water vapor, water condensation takes place and resulting liquid water starts to fill the open pores of the GDL and cover the catalyst particles, rendering them electrochemically inactive. This conflicting requirement of membrane hydration and electrode flooding avoid- ance must be met simultaneously in order to achieve higher perfor- mance. Flooding of electrodes is generally linked to high-current-
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This note was uploaded on 10/28/2010 for the course EE 89 taught by Professor Asgarian during the Fall '10 term at Amirkabir University of Technology.

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116 - Journal of The Electrochemical Society 151 3...

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