Optimization of MEA Structure

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

As can be seen in these figures, the low-frequency arc, which is dependent on electrode potential, is the well-known kinetic arc. But close examination of the high-frequency region (1-20 kHz) in the AC spectra reveals a certain distortion of the kinetic arc from a perfect semicircle, which is the result of superimposition of a small potential-independent impedance branch on the kinetic arc. This behaviour was observed for all types of membranes, experimental conditions, and electrodes employed in Paganin et al. s [4] work. 

The electrodes have 20 wt% Pt/C a 0.4 mg Pt/cm2 and 1.1 mg Nafion /cm2 b 0.1 mg Pt/cm2 and 0.28 mg Nafion /cm2 [4], (Reprinted from Electrochimica Acta, 43(24), Paganin VA, Oliveira CLF, Ticianelli EA, Springer TE, Gonzalez ER. Modelistic interpretation of the impedance response of a polymer electrolyte fuel cell, 3761-6, 1998, with permission from Elsevier and the authors.) 

Nafion content in the catalyst layer plays an important role in electrode performance. Incorporation of Nafion ionomer into carbon-supported catalyst particles to form the catalyst layer for the gas diffusion electrode can establish a three-dimensional reaction zone, which has been proven by cyclic voltammetric measurements. An optimal Nafion content in the catalyst layer of the electrode may minimize the performance loss that arises from ohmic resistance and mass transport limitations of the electrode [6], 

The function of a proton-conducting ionomer such as Nafion in the catalyst layer is to provide an ionic path for proton migration from the membrane to the reaction site at the catalyst surface. Therefore, the content of the proton-conducting ionomer in the catalyst layer will greatly influence the transport of protons to the catalyst sites. The impedance spectra of fuel cells with different Nafion loadings in the catalyst layers of both the cathode and the anode at OCV were compared by 

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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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