Oscillations in the Gas Channels - The Forgotten Player in Impedance Spectroscopy in Polymer Electrolyte Fuel Cells B. Modeling the Wave

Abstract

A two-dimensional, steady-state impedance model is presented, rigorously derived from continuum mechanics and conservation laws. The model will reflect unique features of the local impedance response of air-fed PEFCs. Oxygen concentration oscillations originatingfrom the modulated ac current are not confined to the GDL by allowing for a non-zero amplitude at the GDL-channel interface. Therefore, the propagation of oxygen concentration oscillations within the cathodic gas flow becomes possible. To describe the oscillating oxygen concentration field within air-fed PEFC channels during impedance spectroscopy measurements, the fluid dynamics within the channel is approximated by a laminar flow profile. For the sake of simplicity, electrode reactions are assumed to obey Tafel's law. The concentration wave within the channels will be characterized, and the bearings on measurable impedances will be analyzed.