| The Micro-Porous layer (MPL) made up of carbon powder (VXC-72) and hydrophobic agent (PTFE) sandwiched between catalyst layer and gas diffusion layer (GDL) of Proton Exchange Membrane Fuel Cell (PEMFC) can deal with water flood well and improve the performance of PEMFC. In this paper, MPL on wetproof carbon paper were preparation by screen printer technology. The hydrophobicity of MPL was characterized by contact angle. Water expelled capacity was evaluated by the performance of assembled single cell. The morphological characteristics were measured using scanning electron microscopy (SEM). The pore structure was tested using mercury-intrusion porosimetry. The effect of PTFE content, pore-former content and pore shape in the MPL on the performance of PEMFC were investigated respectively. The static resistance of assembled cell was analyzed. Conclusions had been summarized as following:(l)Comparing with cell without MPL, the internal resistance of single cells with MPL is much lower. As the current density increase, the internal resistances of single cells decrease until single cells reach the highest performance, then internal resistance increase. The performance of single cell with 30%PTFE MPL is the best one at the higher density(≥700mA/cm2), the higher current density, the bigger advantage that MPL with 30%PTFE shows. The pore-size distribution of MPL is between 0.01 u and 10μ , there is two main pore, one is 0.08~10 μ , in charge of expelling water, the other is 0.01~0.08 μ , in charge of transferring gas, the more pore volume between 0.08 μ and 10 μ is, the more excellent water expelled capacity is, the electrode flood reduce. In order to keep enough gas channels, pore volume between 0.01 μ and 0.08 μ need to keep a certain volume, the performance of single cell can be kept.(2)NFL;CL is an effective pore-former. An appropriate amount of NH4CL content in the MPL can improve the performance of single cell, especially at high current density (≥900mA/cm2). NH4CL almost doesn't weaken the hydrophobicity of MPL. In the contrary, the hydrophobicity can be improved slightly. As NH4CL content increase, the porosity of MPL increase, the internal resistance of single cell decrease except MPL with 70% NH4CL. Superabundant NH4CL (70%) result rough surface of MPL, although the porosity of MPL can be improved, the internal resistance of single cell increases.(3)The MPL with gradient pore (GP) is designed. MPL with GP keeps high porosity, gas and water channels are enough. The existences of water in porous media were analyzed theoretically. Water expelling force were compared between the straight pore (SP) and GP, water in the MPL with GP increase a capillary force caused by its pore shape, water expelled capacity in the MPL with GP is more easy. The performance of assembled single cell shows that the MPL with GP is more excellent than MPL with SP. The highest power density of MPL with GP increase by about 5% than that MPL with optimal pore-former (50%NH4CL).(4)The static resistance of cells was measured by Electrochemical Impedance Spectroscopy (EIS). Results show that the internal resistance of cells decrease as the pressure of cells increase, when the pressures of cell reach 2MPa, the resistances keep a fixed value. The internal resistance of cell with MPL is much lower than that without MPL, resistance increase slightly as PTFE content increase, and as NH4CL in the MPL increase, resistance increased slightly. The resistance of cell using MPL with GP is a little lower than that using MPL with SP. |
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