Abstract In PEMFCs, one of the most important components is a gas diffusion layer (GDL). The GDL performs the essential functions such as passage for reactant gases and electrons transports, heat and water removal, mechanical support to the membrane electrode assembly (MEA), and protection of the catalyst layer from corrosion or erosion caused by flows or other factors. The GDL usually consists of a micro porous layer (MPL), which is composed of carbon powder and hydrophobic agent such as Teflon (PTFE), and a macro porous substrate such as felt, paper, or cloth carbon fiber. In this study, the new instrument was designed that measured electrical resistance and permeability of the gas diffusion layer with or without MPL. The power point of this instrument is measuring these properties synchronically at various applied compresses. Physical aspects such as electrical resistance and permeability of various kind carbon substrates such as EP40 (carbon paper with MPL) made by Ballard Co., TGP-H-060T (carbon paper without MPL) made by Toray Co. and LT2500 W (carbon cloth with MPL) made by E-TEK Co. have been investigated by the designed instrument. Increasing the applied compress leads to reduced permeability which is because of decreasing the size of pores. Also the results showed that Toray carbon paper have higher permeability than other substrates because it is without micro porous layer. A homogeneous suspension of Teflon and Vulcan XC-72 carbon powder was then coated onto the Toray carbon paper to form a micro porous layer. The of homogeneity of MPL inks have been provided by the following four methods which three of them used ultrasound waves for dispersing the particles including pulse ultrasonic, continuous ultrasonic and bath ultrasonic, and the other used mechanical force provided by a magnetic stirrer as dispersant. Painting these MPL inks on four same carbon substrates (Toray TGP-H-060T) provided four types of GDL. Electrical resistance and permeability of these GDLs were measured and the effects of micro porous layer on the physical properties were investigated. The results indicated that the GDL made by bath ultrasonic method have lesser electrical resistance and more permeability than samples with gas diffusion layer made by different techniques. The last portion of the project was dedicated to studying the effects of compressing the GDL on the fuel cell performance. So three electrodes which had diffusion layers made by bath ultrasonic technique were compressed up to 12%, 21% and 30% then they were coated with catalyst layer. The MEAs consisted of Nafion 112 membranes, catalyst layers and Toray carbon papers with micro porous layers made by bath ultrasonic as gas diffusion layer and compressed up to 12, 21 and 30% were eva‎luated in three single test cells in same operational conditions. The performance of the cells investigated at different relative humidity of cathode 33, 50 and 100%, relative humidity of anode 100% and the cell temperature and pressure of 75°C and 15 psi. The polarization curves were obtained showing that the performance of the cell containing diffusion layer compressed up to 12% is better than those of containing diffusion layer compressed up to 21 and 30%. Electrochemical impedance spectroscopy confirmed the results that obtained from polarization curves. Keywords: Carbon substrate, electrical resistivity, permeability, compressibility, PEM fuel cell.

محمد ژياني , يوسف غايب

بيژن نجفي , كيوان رئيسي