Abstract One of the most basic step in the commercialization of porotone exchange membrane fuell cell technology, is increasing the power density membrane-electrode assembly.The membrane-electrode assembly including membrane and anode and cathode gas diffusive electrode's that are penetration into a set together and formation The membrane -electrode assembly.Gas diffusion electrode is formed from carbon substrate, diffusion layer and catalyst layer. Catalyst layer of platinum nanoparticles formed Nafion ionomer that in the film type with few nanometers thickness are placed along catalyst particles. Nafion ionomer in the catalyst layer helps to retain moisture and prevent membrane dehydration, especially at high current densities. An optimum Nafion content in the catalyst layer is necessary for good performance .Very low Nafion results in poor contact of the electrolyte with the catalyst and hence poor electrode performance. Too high a concentration of Nafion ionomer causes reduction of electrode performance due to blocking of the catalyst sites, blocking of the electrode pores, reduction of gas permeability and increased mass transfer overpotential.ThusThe control of Nafion ionomer content has a decisive role in the microstructure of the catalyst layer and performance of membrane-electrode assembly. Therefore in this research project, cathode electrodes have been made by non homogeneous distribution of Nafion ionomer within the electrode. Electrodes ionomer content was varied between (27%-35% Wt) and equal amount of loading of platinum nanoparticles on carbon substrate,LT2500W(carbon cloth with Micro Porse Layer) made by E-TEK company, Prepared cathodes in porotone exchange membrane fuell cell were eva‎luated in different operating condition including varius temperatures of cell, anode and cathode and different pressures of oxygen and hydrogen reactive gases by fuell cell set.The performance of these electrodes were investigated using polarization curves. The morphology of catalyst layer microstructure was studied by Scanning electron microscopy. The results showed that change of amount Nafion ionomer in the catalyst layer has caused in the change of microstructure and micro prouses were smaller showed with increasing percentages of Nafion and less porosity was considered. Also, the results of polarization curves showed the performance of the electrode made by homogenous distributed ionomer is lower than the electrode made by non homogeneous distributed ionomer secondly, the performance of the electrodes made by non homogeneous distributed ionomer strongly depend on cathode relative humidity. Cathode made by lower ionomer content needs higher cathode relative humidity and the cathode made by higher ionomer content works better in low cathode relative humidity and electrode with 32% Nafion in the cathode catalyst layer has the highest performance. This can related with increasing three phase boundary. At low Nafion content proton exchange ionomer is not available of the number of catalyst and many active sites cannot participate in oxygen reduction reaction. At high Nafion content, the film thickness formed on catalyst particles increased and decreased the performance of cell due to blocked catalyst sites. Keywords: PEMFC, MEA, Nano structure of catalyst layer, Optimum Nafion content

محمد ژياني

احمد ساعتچي , يوسف غايب