Water Transport Research On Non-humidifying Proton Exchange Membrane Fuel Cell

The proton exchange membrane fuel cell (PEMFC) is considered as one of the most promising alternative automotive power sources to compete with internal combustion engines due to its higher efficiency, lower overall emission and eco-friendliness. Water management of PEMFC is one of the hotspots in recent research, and the way of humidification is a key technology in it. Water movement inside a PEMFC can be researched to taking the humidification from the cells, which is called non-humidifying PEMFC. Based on the mathematical simulation for the PEMFC, this thesis studies water transport and distribution inside the cell, and designs a water-protect layer which is used for non-humidifying PEMFC.Firstly of all, the two-phase, isothermal, two-dimension and steady-state mathematical model of PEMFC is developed with the characteristics of mass transfer and electrochemical. In the model, momentum conservation, mass transportation and charge balance, as well as phase change in gas diffusion layers (GDL) and chemical reaction in the catalyst layers are considered. The model parameters are obtained according to the experiment. Results of simulation are compared with the experimental results. The effect of current density and relative humidity to water inside transportation and distribution is studied by using the model, as well as the effect to water protect capability. The results are as following:(1) As the current density increasing, the diffusion and electro-osmosis actions in membrane are strengthened, as well as the net transfer from the anode to the cathode due to the faster enhance of electro-osmosis actions. Water content in the membrane is increasing at the cathode side, while the anode side is reducing. Liquid water saturation in the cathode side of GDL is increasing, while the anode side is reducing, especially, there is little liquid water in the anode side of GDL when the current density is high. (2) Reducing the relative humidity in the anode side can degrade the performance of cell by keeping the cathode side saturated. Reducing the relative humidity in the cathode side to some extent (RH=75%) can upgrade the performance of cell by keeping the anode side saturated. Humidifying the gas into the cell is necessary in order to make the PEMFC works better.The effect of structure and parameters in GDL to water inside transportation and distribution is studied by using the model, as well as the effect to water protect capability. The results are as following:(1) Under the condition of gas saturated at both sides, the performance of cell is upgraded with larger porosity, smaller thickness, smaller mean aperture, stronger hydrophobicity in the GDL, especially at the high current density. The Micro-Porous layer (MPL) plays a role of ejecting water, giving better performance to the cell. (2) Under the condition of low humidification, the water protect capability is better with smaller porosity, larger thickness, smaller mean aperture, weaker hydrophobicity in the GDL.Based on the research of all above, in order to hold water generated in the catalyst layer and keep the membrane wet in condition of non-humidity, a water-protect layer is placed between the MPL and the GDL. The ideal of Orthogonal experimental design and modeling study have been used to confirm the best scheme. The water transportation and distribution inside the PEMFC with water protect layer are analyzed and compared. The results are as following:(1) Under low humidification, the performance is best when the water-protect layer's porosity is 0.4, thickness is 100um, mean aperture is 10um and the rate of hydrophile hole is 60%. (2) The water-protect layer can hold the water generated in reaction, which is very important to the wetness of membrane under low humidification. (3) Water-protect layer gives a better performance to cell when the cathode gas is lowly humidified.