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 excha nge membrane fuel cells have been interpreted with the aid of a finite tran smission-line model in which the ionic conductivity of the catalyst layer d ecreases with distance from the membrane. By purging the cathode compartmen t of the cell with nitrogen during measurements, the impedance response bec omes dominated by charging of the catalyst's double layer through the layer 's ionic resistance. Fitting of simulated data to the experimental data pro vides a profile of the variation of the catalyst layer's ionic conductivity with distance from the membrane. The power of this approach is demonstrate d by comparing data for electrodes with and without impregnated ionomer (Na fion). The Nafion-containing electrode is shown to have a much higher ionic conductivity, and, consequently, has a larger active catalyst area and pro vides better fuel cell performance. Furthermore, its ionic conductivity dec reases with distance from the membrane, consistent with the nonuniform Nafi on impregnation expected with the immersion method used. (C) 1999 The Elect rochemical Society. S1099-0062(99)01-082-2. All rights reserved.