On the Determination of PEM Fuel Cell Catalyst Layer Resistance from Impedance Measurement in H-2/N-2 Cells

The commonly employed the H-2/N-2 cell impedance method for the determination of PEMFC catalyst layer ionic resistance often results in significant deviations from the predicted idealized homogeneous catalyst layer properties. In this study, the effect of the distribution of resistance and capacitance within the thickness of the CL is modeled to examine whether it explains the observed deviations. It is found that uniformly and non-uniformly distributed CLs show limiting real impedance (vertical line on Nyquist plot) at low frequency, as long as the impedance has no faradaic contribution. However, using this value of real impedance for obtaining the ionic resistance of the catalyst layer leads to significant errors if the system is non-uniformly distributed. It is shown that Nyquist plots with non-vertical mid-frequency regions, resembling experimentally measured H-2/N-2 cell response, can be generated with a nested transmission line circuit (agglomerate model) if there is a significant difference in resistance in different agglomerates. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.007301jes] All rights reserved.