reference30 - Temperature Dependence of the Electrode...

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Temperature Dependence of the Electrode Kinetics of Oxygen Reduction at the Platinum/Nation | Interface--A Microelectrode Investigation Arvind Parthasarathy,* Supramaniam Srinivasan,** and A. John Appleby** Center for Electrochemical Systems and Hydrogen Research, Texas Engineering Experiment Station, Texas A&M University, College Station, Texas 77843-3402 Charles R. Martin** Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 ABSTRACT A knowledge of the temperature dependence of the electrode-kinetic parameters for oxygen reduction at the platinum/ proton exchange membrane (PEM) interface and of mass-transport parameters of oxygen in the PEM is of vital importance in analyzing the performance of proton exchange-membrane fuel cells. The microelectrode technique which was previously developed to determine these parameters at the platinum/Nation | interface at 25~ was used in the present investigation at the same electrode/electrolyte interface. This study was carried out in the temperature range of 30-80~ and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the | activation energy for oxygen reduction at the platinum/Nation interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid (TFMSA) interface but less than values obtained at the Pt/H3PO4 and Pt/HC104 interfaces. The diffusion coefficient of oxygen in Nation increases with temperature while its solubility decreases with temperature. These parameters also depend on the water content of the membrane. The conductivity of the membrane increases with temperature until it reaches a plateau at a temperature of 80~ the dependence of the conductivity on temperature was correlated with the variation of water content of Nation with temperature. Oxygen reduction at the platinum/Nafion interface has recently been the subject of active research. I-4 Such funda- mental investigations arc immediately relevant to an un- derstanding of the operation of the oxygen electrode of proton-exchange-membrane fuel cells. 5 Quantitative data regarding oxygen reduction kinetics and mass transport of oxygen in Nation at the platinum microelectrode/Nafion interface were recently obtained. I These investigations were carried out under ambient conditions (25~ and Iatm 02 pressure). Analogous data at conditions that closely mimic typical fuel cell operating conditions are not avail- able. Such data would be useful in comparing the elec- trode-kinetic behavior at smooth electrodes and the porous gas-diffusion electrodes used in fuel cells. The proton-exchange-membrane fuel cell (PEMFC) is typically operated at temperatures in the range of ca. 80- 90~ and at pressures of 5 aim oxygen and hydrogen. These conditions result in minimizing activation, ohmic, and mass-transport overpotentials even at current densities as high as 2 A/cm 2. s-8 We have recently conducted half-cell
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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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reference30 - Temperature Dependence of the Electrode...

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