PRESSURE-DEPENDENCE OF THE OXYGEN REDUCTION REACTION AT THE PLATINUM MICROELECTRODE NAFION INTERFACE - ELECTRODE-KINETICS AND MASS-TRANSPORT

The investigation of oxygen reduction kinetics at the platinum/Nafion(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 Nafion 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/Nafion interface.