LBM Study On The Effect Of Structure And Degradation On Gas-liquid Two-phase Transport In The GDL Of PEMFC

Proton Exchange Membrane Fuel Cell(PEMFC)is a power generation device that differs from traditional fossil fuels.When operating at high operating current density,excessive liquid water generated by electrochemical reactions can block the pores of the Gas Diffusion Layer(GDL).In addition,prolonged operation can also cause excessive liquid water to corrode carbon fibers and hydrophobic agents,causing damage to the GDL structure,thereby hindering the diffusion of reactants,ultimately reducing the overall performance of fuel cells.Therefore,it is necessary to optimize the structure design of GDL for the study of fuel cell water and gas management,and to improve the drainage performance,gas diffusion performance and aging resistance performance.In order to improve the water management ability of the GDL,in this study,the two-dimensional microstructure of the representative GDL with the true distribution characteristics of polytetrafluoroethylene(PTFE)was reconstructed by using Matlab random algorithm,and the content gradient structure of polytetrafluoroethylene was proposed.Subsequently,by implementing the multiphase lattice Boltzmann method(LBM),the effects of polytetrafluoroethylene content and gradient distribution on the transport behavior of liquid water were studied.The results support the following findings: an increase in polytetrafluoroethylene content in traditional GDLs is beneficial for liquid removal,but excessive content can significantly reduce the corresponding effective porosity of the GDL,thereby weakening fuel cell performance,The optimal PTFE content of the obtained traditional GDL is 10 wt.%.For the GDL with gradient distribution,when the polytetrafluoroethylene content in the entrance area is high,it will be beneficial to the water management of the GDL.Compared with the traditional GDL with 10 wt.% polytetrafluoroethylene content,the optimal bi-gradient and tri-gradient GDLs with polytetrafluoroethylene content show lower liquid saturation,better effective porosity,and shorter liquid water and gas steady-state time,The corresponding effective porosity increased by 4.2% and 5.8%,respectively.The above study can provide guidance for the design of high-performance fuel cells with gradient GDLs.To further improve the water management ability and aging resistance performance of the GDL,the liquid water transport characteristics in the traditional GDL with different degrees of aging and the gradient design GDL were compared.The results support the discovery that as the traditional GDL ages,the water removal performance of the GDL significantly decreases,and a large amount of liquid water remains inside the GDL,which can even lead to water flooding.Compared to the degradation of carbon fibers,the degradation of polytetrafluoroethylene leads to a more hydrophilic GDL,which reduces local capillary pressure and forms multiple paths for liquid water transport,leading to more serious water flooding problems.Properly increasing the content of polytetrafluoroethylene is beneficial to the aging resistance performance of the GDL,but too high content will occupy the pores and reduce the effective porosity,thus reducing the performance of the fuel cell.The simulation results show that the GDL shows good aging resistance performance and gas diffusion ability when the content of polytetrafluoroethylene is 10 wt.%.In addition,the high content of polytetrafluoroethylene distributed at the entrance of the bi-gradient GDL structure is conducive to water management.After aging,the entrance area also has a high capillary pressure to control the breakthrough of liquid water.The GDL with the best bi-gradient design of polytetrafluoroethylene(14 wt.% in the entrance area and 6 wt.% in the exit area)has increased the liquid saturation by 8.9% and reduced the effective porosity by3.3% before and after aging.The GDL with tri-gradient design of polytetrafluoroethylene content also has higher drainage performance and aging resistance performance than the traditional GDL.At the same time,compared with the bi-gradient structure,the tri-gradient GDL has better capillary pressure gradient distribution,and the gentle capillary pressure is more conducive to the drainage of liquid water,the optimal PTFE tri-gradient design GDL(14 wt.% in the inlet area,10 wt.% in the middle area,and 6 wt.% in the outlet area)increased liquid saturation by 8.6% before and after aging,and decreased effective porosity by 3.0%.This study provides an accurate assessment of the liquid drainage performance of GDLs with different degrees of aging,and can guide the design of GDLs with high aging resistance performance and high drainage performance.