Multi-scale Two-phase Flow And Reaction Kinetics Of Proton Exchange Membrane Fuel Cells

In this study,lattice Boltzmann(LB)models are developed to study the two-phase transport processes and electrochemical reaction inside flow channel,GDL,MPL and CL,respectively,with the reconstructed realistic microstructure.The work can be summarized as follows:(1)A multi-component LB model is proposed to study the two-phase gas-liquid flow with realistic fluid properties,and the importance of improved model is in simultaneously realizing the realistic fluid flow characteristics for multi-component two-phase system,including high density and viscosity ratios,good thermodynamic consistency,and independently tunable surface tension.(2)The proposed LB model is applied to simulate the droplet flow at smooth solid surface,and the effects of surface tension,gas flow velocity and wall wettability are investigated.With the reconstructed GDL microstructure,dynamic behaviors of droplet at GDL surface are investigated,and corresponding visualization experiments are conducted to validate the modeling results in static cases.The results show that degree and rate of deformation are determined by the difference between liquid and gas pressures inside and outside the droplet,while pressure values are dependent on the structural and operating parameters and dynamically vary during the movement process before the droplet flow reaching a stable stage.Effected by the pore structure,droplet contact angle is varied at different positions of GDL surface.As the droplet contact angle and GDL porosity increases,droplet motion evolved from along the surface to fast sliding.(3)The optimized stochastic model is developed to reconstruct the realistic 3D microstructure of GDL,MPL and CL,based on their respective characteristics of transport and structure.The innovative work includes: PTFE(Poly Tetra Fluoroethylene)is generated with considering the effect of different drying ways on PTFE distribution,appropriately simplified the MPL structure according to different functions of micro pore and macro crack,simultaneously reconstructed the microtopography of carbon support,ionomer and Pt particle in CL.(4)With considering GDL and MPL microstructure simultaneously,a 3D LB model is developed to study the role of MPL macro crack in liquid water transport and redistribution at GDL/MPL interface.The proposed model is validated with visualization experiment in literature,and then the effects of PTFE distribution and pressure difference are investigated.The results show that PTFE distribution under vacuum-drying is almost uniform inside GDL,and the continuous switching of wettability between carbon fiber and PTFE can accelerate the liquid water permeation.While PTFE under air-drying accumulates at one side of GDL and reduces the porosity in this region,the larger pressure difference or longer time is therefore needed to liquid water break-through.As the pressure difference increases,the retaining amount of liquid water inside the narrow pore is improved.Inserting of MPL only latch the liquid water in GDL region close to macro crack,which helps reduce the water flooding and improve the gas transport to active sites.(5)A 3D single-phase LB model is developed to study the oxygen transport process and electrochemical reaction in pore-scale,and the effects of ionomer amount,Pt loading and carbon black size are investigated with the reconstructed different CL microstructures.The results show pore size distribution and number of effective Pt particle(covered by ionomer)are two critical factors affecting the oxygen diffusion and consumption rate.Improving the mass ratio between ionomer and electrode and Pt loading are beneficial to produce more three-phase boundary,while oxygen diffusion becomes more difficult due to the reduced pore size,and thus cause the low catalyst utilization in thin oxygen regions.Increasing the carbon black diameter is beneficial to enhance both the catalyst utilization and oxygen diffusion.