110 - M athematical Model of a Gas Diffusion Electrode...

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Mathematical Model of a Gas Diffusion Electrode Bonded to a Polymer Electrolyte Dawn M. Bernardi and Mark W. Verbrugge Physical Chemistry Dept., General Motors Research Laboratories, Warren, MI 48090 A mathematical model for an ion-exchange membrane attached to a gas-fedporous electrode is derived and discussed. The model is applied to simulate the oxygen electrode of apolymer-electrolyte fuel cell. Our discussion focuses on cellpolarization characteristics, water transport, and catalyst utilization-all of which must be con- sidered for fuel-cell design. Calculated polarization behavior is shown to compare favorably with published experimental data. Our results indicate that if the membrane maintains full saturation, its contribution to the total cell resistance is most significant at higher operating current densities (greater than 200 mA/cm2). Polarization re- sistance due to the oxygen reduction reaction appears to be important for allpractical current densities. Water transport, driven by pressure and electric-potential forces, is shown to be a complicated function the cell operating conditions. The utilization and distribution of noble-metal catalyst is discussed. Introduction It has been projected that in the not too distant future, the world population will double its current value and the global consumption of energy will have increased by a factor of five (Lindstrom, 1988). Such predictions offer motivation for the development of efficient power generators. In addition, it is desirable to design our future power systems so that only benign waste products are generated, modular construction is af- forded, and quiet operation is observed. The recent develop- ment of perfluorosulfonic-acid (PSA) polymer-electrolyte fuel cells (McElroy and Nuttall, 1982; Nuttall and McElroy, 1983; Watkins, 1988; Ticianelli et al., 1988a,b) presents a promising approach to addressing our expanding energy requirements for stationary and vehicular applications. [The polymer electrolyte in these fuel cells is also referred to as an ion-exchange mem- brane (IEM), a proton-exchange membrane (PEM), or a solid polymer electrolyte.] The premier advantage associated with fuel cells is that they are not limited by Carnot efficiency. No gases are heated to create explosive reactants, and no moving parts similar to pistons and crankshafts are required to convert thermal energy into mechanical energy. The energy released from the inter- atomic bonds of the reactants is converted more efficiently into usable electrical energy relative to traditional power-gen- erating devices. Because no mobile electrolyte is employed, Correspondence concerning this article should be addressed to D. M. Bernardi corrosion problems in the polymer-electrolyte fuel cell are re- duced and cell construction is simplified. Operation of poly- mer-electrolyte cells, however, is complicated by the electro- osmotic flow of water through the PSA membrane pores-the conductivity of the membrane depends strongly on its water content, and during cell operation the membrane can become
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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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110 - M athematical Model of a Gas Diffusion Electrode...

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