Electrochemical Impedance Study of Electrode‐Membrane Assemblies in PEM Fuel Cells: I. Electro‐oxidation of H 2 and H 2 / CO Mixtures on Pt‐Based Gas‐Diffusion Electrodes

Abstract

In situ EIS data are presented on the anodic process in proton exchange membrane (PEM) fuel cells and the influence of CO poisoning of the Pt gas‐diffusion electrodes (GDE) is examined. A characterization of the effects of interfacial kinetics in determining polarization losses in the presence of CO is performed by comparing impedance patterns obtained for cells of the type with . The impedance spectrum of the poisoned electrode is strongly dependent on potential and on the CO concentration in the gas feed. In the range between 0 and 0.3 V the impedance increases rapidly with the potential, while at potentials higher than 0.3 V a “pseudoinductive” behavior is observed, in which a positive loop at higher frequencies is accompanied by a low frequency (LF) loop in the fourth quadrant. The latter was assigned to a new rate‐determining process, the oxidation of by . As a critical potential is attained, the diameters of the two loops become almost equal and the LF limit of the impedance approaches the value for unpoisoned electrode, showing that the activity of the electrode activity has been restored. The value of is 0.43 and 0.58 V for electrodes poisoned with 100 ppm and 2% CO, respectively. At very high potentials, where the oxidizing species are , the impedance pattern is reversed to the second and third quadrants. Stripping voltammetry and polarization curves recorded in situ, are used to support the conclusions obtained from impedance measurements. © 1999 The Electrochemical Society. All rights reserved.