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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.

The investigation of oxygen reduction kinetics at the platinum/Nation b interface is of great importance in the advancement of proton-exchange-membrane (PEM) fuel-cell technology. Investigations of this type have been conducted in our laboratories using a microelectrode technique. In these studies, pseudosteady-state voltammetry and electrochemical impedance spectroscopy were used to determine the oxygen reduction kinetics at ambient conditions as well as at elevated temperatures. This study focuses on the dependence of the oxygen reduction kinetics on oxygen pressure. Conventional Tafel analysis of the data shows that the reaction order with respect to oxygen is unity at both high and low current densities. Chronoamperometric measurements of the transport parameters for oxygen in Nation show that oxygen dissolution follows Henry's isotherm. The diffusion coefficient of oxygen is invariant with pressure; however, the diffusion coefficient for oxygen is lower when air is used as the equilibrating gas as compared to when oxygen is used for equilibration. These results are of value in understanding the influence of O2 partial pressure on the performance of PEM fuel cells and also in elucidating the mechanism of oxygen reduction at the platinum/Nation interface.Research on proton-exchange-membrane (PEM) fuel cells is rapidly gaining momentum because of the prospects of attaining high energy efficiencies and high power densities. ~-5 Both of these factors are vitally necessary for space and terrestrial applications. 6 It is well known that the slow kinetics of the oxygen reduction reaction (ORR) limits the performance of hydrogen-oxygen fuel cells. ~ We have, therefore, been studying the kinetics of the ORR under conditions that closely mimic typical operating conditions of the PEM fuel cell. A novel solid-state electrochemical cell based on a platinum-disk microelectrode was used for these investigations. Electrode-kinetic parameters for the ORR and mass-transport parameters for oxygen in Nation were determined using this solid-state cell. I-3 Studies to date have focused on the effect of temperature on O2-reduetion kinetics and 02 transport in Nation. 2 In a similar microelectrode study conducted at the Pt/recast Nalion interface, the ORR was studied over a wide range of temperatures at atmospheric pressure. 8 These studies also showed that membrane drying effects were pronounced at temperatures above 50~This pressure-dependence study was, therefore, conducted at a constant temperature of 50~Oxygen pressure also affects the kinetics of 02 reduction in the PEM fuel cell. 4 We have, therefore, used the cell and methods described in our previous reports I-~ to investigate the effects of oxygen pressure on the kinetics of the ORR at the Pt/Nafion interface and on the rate of 02 transport in Nation. Experiments with both pressurized 02 and pressurized air were conducted. The results of these experiments are described here. The pressure dependence of the ORR at the Pt/Nation interface has been previously investigated at a...

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