For polymer electrolyte membrane fuel cell ͑PEMFC͒ and direct methanol fuel cell ͑DMFC͒ catalyst layers ͑CLs͒, a complex capacitance analysis of impedance data was developed to evaluate the catalyst/ionomer interfacial capacitance and ionic resistance of ionomer networks without nonlinear data fitting. First, assuming no faradaic reactions, equivalent circuits for the CLs were suggested, which are similar to electric double-layer capacitor systems with porous carbon electrodes. Then, with the

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... mulated 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 the ionic resistances 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 by fixing the dc potential at 0.4 V during the impedance measurements. By performing a complex capacitance analysis, the interfacial capacitances and ionic resistances of PEMFC membrane electrode assemblies ͑MEAs͒ can be obtained at various relative humidities, proving that the catalytic activity in fuel cell operation depends on ionic resistances as well as on the catalyst/ionomer interfacial area. The effects of various MEA preparation methods on the ionomer distributions and DMFC performances were analyzed by a complex capacitance analysis. Theory Electrochemical processes are utilized to operate fuel cells, including fuel oxidation at anodes, ion transport through membranes, and oxygen reduction at cathodes, where the generated electrons flow through external wires that connect the anodes, the electric load, and the cathodes. Therefore, the general equivalent circuit for z