Determination of time constants of diffusion and electrochemical processes in Polymer Electrolyte Membrane Fuel Cells

Abstract

This work presents an experimental analysis of the time constants associated to diffusion and electro chemical processes in a 50 cm2 Polymer Electrolyte Membrane (PEM) fuel cell. The experimental tech niques and results include polarization curves and Electrochemical Impedance Spectroscopy (EIS) analysis of the fuel cell, where the time constants are determined from the analysis of the Distribution of Relaxation Times (DRT). EIS results are also used to determine the cell ohmic resistance, where High Frequency Resistance (HFR) values are calculated from the Nyquist plots. A wide range of operating conditions of the fuel cell are analysed, including back pressure (0.5 bare1 bar), cell temperature (55 C, 65 C, 75 C), reactant gases relative humidity (30%, 60%, 90%), cathode stoichiometry (lc 2.5e3.5), and oxygen concentration (air and pure oxygen). The effect of the operating conditions on the time constants are discussed, and Damkohler number is introduced and discussed.