The Platinum Microelectrode/Nafion Interface: An Electrochemical Impedance Spectroscopic Analysis of Oxygen Reduction Kinetics and Nafion Characteristics

Abstract

The kinetics of oxygen reduction at the platinum/proton‐exchange‐membrane interface is of direct importance in solid‐polymer‐electrolyte fuel‐cell research and development. Previous studies at a platinum microelectrode/Nafion® interface yielded electrode‐kinetic parameters for this reaction. The objectives of this study were to use electrochemical impedance spectroscopy (EIS) to study the oxygen‐reduction reaction under lower humidification conditions than previously studied. The EIS technique permits the discrimination of electrode kinetics of oxygen reduction, mass transport of in the membrane, and the electrical characteristics of the membrane. Electrode‐kinetic parameters for the oxygen‐reduction reaction, corrosion current densities for Pt, and double‐layer capacitances were calculated. The production of water due to electrochemical reduction of oxygen greatly influenced the EIS response and the electrode kinetics at the Pt/Nafion interface. From the finite‐length Warburg behavior, a measure of the diffusion coefficient of oxygen in Nafion and diffusion‐layer thickness was obtained. An analysis of the EIS data in the high‐frequency domain yielded membrane and interfacial characteristics such as ionic conductivity of the membrane, membrane grain‐boundary capacitance and resistance, and the uncompensated resistance.