ELECTRODE-KINETICS OF OXYGEN REDUCTION AT CARBON-SUPPORTED AND UNSUPPORTED PLATINUM MICROCRYSTALLITE NAFION(R) INTERFACES

The electrode kinetics of oxygen reduction at the platinum microcrystallite/Nafion(R) interface was investigated as a function of temperature and pressure. These studies were conducted using porous gas-diffusion electrodes, containing unsupported platinum (10 mg cm-2) or carbon-supported platinum (0.4 mg cm-2), in proton exchange membrane fuel cells, using H-2 and O2 as reactants. The effects of platinum loading and of Nafion impregnation on the electrode kinetic parameters were elucidated. Over the range of current densities from 1 to 1000 mA cm-2 (geometric), the Tafel slope was ca. -60 mV per decade and was practically independent of temperature, O2 pressure, and type of electrode. The platinum electrode was still in the potential regime where oxygenated species are present on the surface (Temkin conditions). The reaction order with respect to O2 is unity and the activation energy for O2-reduction was ca. 80 kJ mol-1. These electrode kinetic parameters are in good agreement with those obtained al the platinum microelectrode/Nafion interface, as previously determined in our laboratories.