Numerical Simulation Of Functional Graded Design Of Membrane Electrodes Assembly And Novel Flow Fields In Proton Exchange Membrane Fuel Cells

The burning and mining of fossil fuels cause various environmental problems,such as air pollution,greenhouse gas emission,and ground subsidence.It is urgent to find clean new energy or sustainable energy conversion technoloties because fossil fuels are non-renewable energy sources.Fuel cells are energy conversion devices that transfer hydrogen energy to electricity without the combustion process.Fuel cells have beed rapidly developed for decades.Among them,proton exchange membrane?PEM?fuel cells are promising candidate for commerizliation.At the present,the main problems of PEM fuel cells are their high cost and poor durability,which attract great interests of researhers to design and optimize the flow channel structure or catalyst distribution.Experimental studies of PEM fuel cells suffered from a high cost.Numerial modeling of PEM fuel cells not only reduces the cost of experimental studies but also obtains the values of some parameters which are hardly obtained by experimental studies.Numerical model can effectively improve the accuracy of experimental results.This thesis starts from the development of a two-dimensional,two-phase flow,non-isothermal proton exchange membrane fuel cell model,which is used to study the effect of anisotropy of the physical parameters of the porous diffusion electrode on the cell performance.The modeling results are validated by experimental data,which confirms the accuracy of the model.Then,a novel flow field with controllable pressure gradients between adjacent channels is designed to investigate the effect of the pressure gradient between adjancent channels on reactant distribution and water management in the porous electrode of PEM fuel cells.Finally,the effect of the graded distributions of Pt loading and operating temperature on the cell performance is studied through a segmented fuel cell design integrated with a numerical model.The main research contents are shown as follows:Firstly,gas diffusion electrode exhibites strong anisotropy due to the spatial arrangement of carbon fiber within the GDL.Therefore,the transport of proton,electron,water,heat and gas through the gas diffusion electrode could be divided into two directions:in-plane and through-plane directions.To capture the anisotropic properties of the electrode and compare the anisotropy of different parameters on the cell performance,a two-phase flow,non-isothermal,proton exchange membrane fuel cell model was developed and experimentally validated.The results showed that the isotropic model over-predicted the fuel cell performance.The results of anisotripic model showed better consistence with the experimental data.At high current density,the anisotropic proton conductivity and gas diffusion coefficient had the most significant influence on the fuel cell performance.In the full range of current density,the anisotropies of gas permeability and thermal conductivity had limited effect on fuel cell performance,so that can be ignored.Finally,the sensitive index reflected that the anisotropy of capillary diffusivity was considerable importance to the cell performance due to the strong dependence of liquid water transport and capillary mechanism within the porous electrode.Furthermore,to investigate the effect of the pressure gradient of adjacent flow channels on the water removal and cell performance,a novel flow field with controllable pressure gradient between adjacent channels was manufractured and a two dimensional,across-the-channel,two-phase model was developed.The cathode side employed a novel flow field with three different channel/rib ratios(W_ch/W_rib=0.5,1.0,1.5),the anode side used a traditional parallel flow field as a reference.Two types of carbon papers with different thicknesses?200?m and 400?m?were used in experimental studies.The experimental data were compared with the modeling results to verify the accuracy of the model.The effect of the pressure difference between the adjacent flow channels on the oxygen concentration and water content in the porous electrode under the flow channel and the ribs was studied,and more attentions were paid on the mechanisms of mass transport and water removal rates under pressure gradients.The results revealed that oxygen concentration was increased,and the water saturation was reduced under the rib with a pressure gradient generated across the adjacent channels.The optimal pressure gradient is between 0.1 to 0.2 atm for the GDLs and channels typically used in practice.Finally,a segmented fuel cell device was manufactured,in which the design of graded distributions of Pt loading and temperature on the cathode electrode were adopted to achieve the purpose of simoutaneously reducing the Pt loading,homogenizing the current density,and improving the cell performance.A two-dimensional,across-channel,two-phase flow numerical model was verified by the experimental data.The results showed that the individual increase of Pt loading and higher temperature towards the cathode outlet improved the fuel cell performance,in which the effect of temperature was more obvious.However,the homogenity of current density became worse when Pt loading and tempertre were individually designed.A systematical design of the gradients of platinum loading and operating temperature from the inlet to the outlet was capable of reducing the usage of catalyst without deteriorating the performance and the homogeneity of current density.When the Pt loading is 0.4 mg cm^-2 at the cathode inlet,the Pt loading gradient is 0.01 mg cm^-3,and the operating temperature increase towards outlet from 30?to 65?.The usage of Pt catalyst is saved by 18.8%,the current density is increased by 69%,and the standard deviation of current density is 4.55%at 0.3V compared to uniform electrode.