Jong Hyun Jang

Complex Capacitance Analysis of Ionic Resistance and Interfacial Capacitance within Cathode Layers of PEMFC and DMFC Electrodes

In PEMFCs (polymer electrolyte membrane fuel cells) and DMFCs (direct methanol fuel cells), the catalyst layers are prepared together with a Nafion ionomer to extend reaction sites throughout the bulk electrodes. In order to achieve high performances with the large TPB (three-phase boundary) area, the catalyst layers should have well-developed networks for electron conduction (catalyst particles), proton transport (Nafion ionomer particles), and reactant gas supply (pores). Therefore, characterization of the microstructures of catalyst layers, as well as performance measurements, is very important to optimize various preparation factors. Also, the microstructure characterization is required to elucidate degradation mechanisms for practical fuel cell operation. In this study, a complex capacitance analysis of impedance data was developed to evaluate the catalyst/ionomer interfacial capacitance and ionic resistance of ionomer networks, without non-linear data fitting. Firstly, assuming no Faradaic reactions, equivalent circuits for the catalyst layers were suggested, which are similar to EDLC systems with porous carbon electrodes. Then, with the simulated complex capacitances, it was confirmed that the plots of the real and imaginary parts as a function of ac frequency are determined by the catalyst/ionomer interfacial capacitances and RC time constants, which are important characteristics for high fuel cell performances. Experimentally, the condition of no Faradaic reactions was realized by supplying nitrogen or water to the cathodes instead of air and fixing the dc potential at 0.4 V during the impedance measurements. By analyzing the real and imaginary capacitance plots of experimental impedance data, the catalyst/ionomer interfacial area and proton conductivity were graphically estimated. Also, the interfacial capacitances and ionomer resistances could be quantitatively determined from the real capacitance at low frequency and peak frequency in the imaginary capacitance plots. Here, the experimental impedance and transformed complex capacitances could be well described by equivalent circuits that contain the transmission line model. From the complex capacitance analysis of impedance data measured at 0.4 V under a N2/H2 atmosphere for PEMFC MEAs, it was found that the ionic resistance within cathode layers was higher for the MEA with lower ionomer content (20 wt.%), especially at low relative humidity. In single cell testing, the MEA with 20 wt.% ionomer showed larger activation overpotential, probably due to the limited utilization of active sites with high ionic resistances. 0 1 2 3 0 1 2