In Situ Cathode Reaction

The impedance spectra of the DMFC cathode electrodes are obtained by subtracting the anode impedance from the total cell impedance. The cell impedance, ZDMFC, was obtained from normal operation of the DMFC (i.e., the cathode side was fed with air or 02 and the anode side was fed with methanol solution). The anode impedance was measured by supplying H2 to the cathode compartment, which was used as a dynamic hydrogen reference electrode. Since the impedance of the H2 electrode is negligible, the measured impedance is considered to be the anode impedance, Zanode. The cathode impedance is therefore 

The arc in the medium-frequency range is due to the charge transfer of the ORR. The low-fiequency arc is due to mass transport, especially in the case of air. To access the catalytic sites, oxygen needs to diffuse through nitrogen and water, which leads to mass transport resistance. 

The charge-transfer resistances and double-layer capacitances of oxygen reduction and methanol oxidation can be simplified using the following equations  

In spite of the bulk electrolyte resistance cancelling out in Equation 6.68, small differences between the series resistance in the measurements of ZDMPC and Zanode also contribute to cathode impedance. Considering the mass transport resistance of 02 in the pores and the bulk electrolyte resistance, the cathode impedance is given by 

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

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