TEMPERATURE-DEPENDENCE OF THE ELECTRODE-KINETICS OF OXYGEN REDUCTION AT THE PLATINUM NAFION(R) INTERFACE - A MICROELECTRODE INVESTIGATION

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/Nafion(R) interface at 25-degrees-C was used in the present investigation at the same electrode/electrolyte interface. This study was carried out in the temperature range of 30-80-degrees-C 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/Nafion(R) 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/HClO4 interfaces. The diffusion coefficient of oxygen in Nafion 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-degrees-C; the dependence of the conductivity on temperature was correlated with the variation of water content of Nafion with temperature.