Electrochemical Impedance Spectroscopy for Direct Methanol Fuel Cell Diagnostics

Electrochemical impedance spectroscopy (EIS) was used as a diagnostics tool for direct methanol fuel cell (DMFC) single cells and stacks, capable of separating individual contributions to the overall polarization of fuel cells under load. Anode impedance spectra were interpreted assuming porous electrode model and a reaction mechanism involving one adsorbed intermediate. No evidence of methanol transport limitations was found at the anode under operating conditions tailored for portable applications of DMFCs. Anode experiments revealed substantial poisoning of the electrode by methanol-derived surface species (CO) and negative order of methanol oxidation in methanol concentration. Because of the clear indication of the presence of adsorbed species at the cathode, the cathode process was assumed to be a combination of oxygen reduction reaction and methanol oxidation occurring in parallel. The porosity and oxygen transport effects were included for the cathode in order to adequately describe the impedance spectra. In addition to divulging methanol crossover, the cathode spectra provided an indication of nonequipotentiality of the cathode, flooding of the cathode backing, flooding/dry-out of the cathode catalyst layer, and hindering of oxygen transport in the cathode backing by crossover methanol. The ability of EIS to reveal these phenomena proved to be highly useful in the identification of performance issues in individual cells of a six-cell DMFC stack.