Characterization of ionic conductivity profiles within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy

Experimental impedance data recorded for operating cathodes in proton exchange membrane fuel cells have been interpreted with the aid of a finite transmission-line model in which the ionic conductivity of the catalyst layer decreases with distance from the membrane. By purging the cathode compartment of the cell with nitrogen during measurements, the impedance response becomes dominated by charging of the catalyst's double layer through the layer's ionic resistance. Fitting of simulated data to the experimental data provides a profile of the variation of the catalyst layer's ionic conductivity with distance from the membrane. The power of this approach is demonstrated by comparing data for electrodes with and without impregnated ionomer (Nafion). The Nafion-containing electrode is shown to have a much higher ionic conductivity, and, consequently, has a larger active catalyst area and provides better fuel cell performance. Furthermore, its ionic conductivity decreases with distance from the membrane, consistent with the nonuniform Nafion impregnation expected with the immersion method used. (C) 1999 The Electrochemical Society. S1099-0062(99)01-082-2. All rights reserved.