In the case of the Polymer-electrolyte fuel cells (PEFCs), the reaction sites exist on the platinum (Pt) surface covered with PFSI. Though PFSI membrane is used as an electrolyte of the PEFC, the membrane does not soak deeply into the electrodes as a liquid electrolyte does. Therefore, PFSI solution was impregnated into the catalyst layers to increase the contact areas between Pt and PFSI. In our previous work 1 we proposed a new preparation method of the M&E assembly which emphasized the colloid formation of the PFSI to optimize the network of PFSIs in the catalyst layer and also to simplify the fabrication process of the M&E assembly. Following this work, we focused on the microstructure of the catalyst layer. The importance of the morphological properties of the gas-diffusion electrodes on performance has been reported in several papers. The catalyst layer was claimed to have had two distinctive pore distributions with a boundary of ca. 0.1 pm. The smaller pore (primary pore) was identified with the space in and between the primary particles in the agglomerate of the carbon support and the larger one (secondary pore) was that between the agglomerates. 2 In our recent work,3 we reported that the PFSI was distributed only in the secondary pores, and the reaction sites were therefore limited to that location. The results indicated that the PEFC system required a particular design rather than a conventional one " for the fuel cells with liquid electrolytes. We proposed that novel structure and/or preparation methods of the catalyst layer were keys to higher utilization ofPt.

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