Pore Network Simulations For Transport Characteristic In Gas Diffusion Layer Of Proton Exchange Membrane Fuel Cell

In recent years,proton exchange membrane fuel cell(PEMFC)has become an attracted new energy,because it directly converts clean hydrogen fuel into electricity which people work life rely on through chemical reactions.The conversion process for energy is not limited by Carnot cycle,thus it has high conversion efficiency.In addition,PEMFC also has other advantages such as high energy density,no greenhouse gas emissions,excellent start-up characteristics at low temperature and high reliability.Proton exchange membrane fuel cells are promising power sources for portable equipment,vehicles,dispersed power stations and their successful applications play a significant role in reducing the environmental degradation and promoting energy revolution.However,there are still various technical barriers for the commercialization of PEMFC,including water management issues.Reasonable water management can not only improve the performance of PEMFC,but also have a positive impact on its durability,therefore,the water management has been one of the research hotspots for fuel cells.The gas diffusion layer,usually a carbon-fiber based paper,is one of main porous components in a PEMFC stack and plays a critical role in water and reactants transport.Furthermore,the modifications and designs of the gas diffusion layer are beneficial to optimize water management and improve the performance of fuel cell.Macroscopic models based on governing equations are difficult to capture microscopic information,so mesoscopic models are attractive for exploring the transport characteristics in gas diffusion layer.Pore network model is one of the mesoscopic model that successfully apply to the simulation of transport phenomenon in porous materials.In this thesis,the impact of pore-scale design in gas diffusion layer on water and oxygen transport is studied by the regular pore network model which is based on invasion percolation algorithm.1.Liquid water and oxygen transport characteristic in gas diffusion layer with uniform distribution of hydrophilic pores are studied.We also reveal the effect of hydrophilic degree,inlet coverage,injection point size,and the pore size distribution on two-phase transport in mixed-wet gas diffusion layer.The simulation results show that exist an optimal hydrophilic pore fraction(f=0.4)leading to minimum liquid water saturation and maximum limiting current density,and the value of optimum hydrophilic pore fraction is independent of inlet coverage and injection point size.Besides,there is no optimum hydrophilic pore fraction in GDL with very wide pore size distribution.2.The transport characteristics in four types of gas diffusion layer with gradient distribution of hydrophilic pores are studied,and the effect of gradient in each wettability configuration is considered.The results reveal that oxygen transport not only depends on total liquid water saturation,but also depends on water distribution.In each wettability configuration,there is an optimal hydrophilic pore fraction,and its value is related to the distribution and the gradient of hydrophilic pore.The optimum hydrophilic pore fraction is decreased with the gradient increasing.3.Local pore design in gas diffusion layer,such as groove,is also applied to improve two phase transport,the effect of groove position,depth and width on water permeability and oxygen diffusivity is studied.Analysis indicates that the groove changes the distribution of capillary pressure and optimizes the water distribution in GDL,thus improving the performance of oxygen diffusion and liquid penetration to some extent.When the groove penetrates GDL and the width is 150μm,the GDL has the best comprehensive oxygen and liquid water transport performance.