Mechanism Of Water Transport In Gas Diffusion Layer Of Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell has a progressive prospect in the field of transportation, mobile power, distributed power plant and aerospace, et al. In the present paper, water transport in the gas diffusion layer, one of the most important component in PEMFC, has been fully studied via experimental, modeling and theoretical methods. The influence of GDL key parameters on the process of water transport inside of GDL has been shown out of the results obtained from the ex-situ experiments, the modeling studies and the theoretical analysis.An ex-situ visualization setup is built to study the liquid water breakthrough process in GDL, the process is visualized by a high speed camera and the breakthrough pressure is detected at different temperature. The results show that there is a fixed breakthrough point at the GDL surface, and the breakthrough pressure for one sample decreased when the temperature increased. The water droplet emerging from the gas diffusion layer should be purged in time and the serpentine flow plate shows good capability for liquid water removing.The water vapor condensation process is observed via the ex-situ setup built in-house. It is found that water vapor will condense in GDL when it flows through it, and the condensate disperses in the whole GDL. With the condensation goes on, the separated liquid droplet will merge together and breakthrough as the liquid water breakthrough introduced above. Meanwhile, effects of various PTFE loading and MPL on the water vapor condensation rate are also probed. The results show that the increasing of PTFE loading and adding MPL to the substrate will restrain the condensing of water vapor and then weaken the transportation polarization in GDL.A fiber-layer model and a pore network model are introduced to simulate the liquid water breakthrough in GDL, both of the two modeling show good fit for the study of breakthrough behavior. The ununiform wettability in gas diffusion layer will cause the increasing of the liquid water breakthrough pressure, and more large pores in the gas diffusion layer will make the liquid water breakthrough pressure decrease and the saturation increase. Moreover, the pore network model is utilized in the investigation of water vapor condensing in GDL. The parameter study results show that the water vapor condensation rate increases with increasing of porosity, mean pore diameter, GDL inlet temperature and the temperature difference between the inlet and outlet of GDL. Additionally, the influence of condensation heat on the temperature distribution in GDL can be ignored.Theoretical analysis for the generation of water in the catalyst layer shows that the water that flows into GDL is a mixture of liquid water and water vapor. It is found that the water vapor condensation rate is influenced by the defect in the carbon fiber, the carbon powder size, and the GDL thermal conductivity, which has a significant effect on the condensation rate. In addition, liquid water transport process in GDL is controlled by the capillary force, and the liquid water flow rate is affected by the carbon fiber size and the thickness of the substrate and MPL. Furthermore, the cracks in MPL are unfavorable for the drainage of liquid water. Finally, the parameter investigation on the GDL key parameters is conducted to explore their effects on water transport in GDL, then the optimization GDL properties is obtained from it.