Macro Mass Transfer In The Cathode Of HT-PEM Fuel Cells

High temperature proton exchange membrane fuel cell(HT-PEMFC)is a efficient and environmental protection electrical energy generating device which can convert hydrogen to electric energy.It has the advantages of small volume,quiet operation,quick start and high reliability,etc.PEMFC not only can be used in small mobile power,standby power and distributed power station,but also can provide the impetus for the common vehicles,such as,cars,aircrafts,ships,trains,etc.The catalyst layer and diffusion layer are important parts of HT-PEMFC,which have great effect upon the mass transfer performance of fuel cell.This thesis studied on the HT-PEMFC through simulating the oxygen concentration and water concentration distribution in the diffusion layer and catalyst layer,the results had important influences on PEMFC structure parameters optimization.Based on reading massive literature materials,this thesis summarized the development history and research status of fuel cells,introduced the working principle of PEMFC in detail,described the characteristics and the uses of PEMFC,emphasized the structure and key components of PEMFC.At the same time,this thesis analyzed the working state of PEMFC according to the electrochemical thermodynamics and kinetics theory.On the basis of previous studies,this thesis established a two-dimensional,single phase,steady state mathematical model to study oxygen and water diffusion laws in the HT-PEMFC cathode.The model proposed basic hypotheses and coupled with the mass equation,momentum equation,energy equation,component equation,charge transport equation in the proton exchange membrane as well as Butler-Volmer equation of the relationship between current and voltage in the catalyst layer.The boundary conditions were given.The numbers of mesh were obtained according to the simulation,program charts were drew.The volt ampere characteristics of HT-PEMFC were obtained by simulation,contrasting them with the experimental characteristics of the test,the study found that the simulation results had a high degree of agreement with the test results,which proving the effectiveness of mathematical model and simulation method.According to the established mathematical model,the characteristics of the HT-PEMFC,the distribution of cathode reactant and product were obtained by simulation.The thesis discussed the effects of diffusion layer porosity,diffusion layer thickness,channel depth,channel width and fuel cell temperature on the distribution of oxygen concentration and water vapor concentration in the cathode.The study found that electrochemical reaction consumed oxygen along the flow direction of gas,which resulted in oxygen concentration decreased,at the same time,electrochemical reaction produced water,so the water concentration in HT-PEMFC cathode increased gradually;the oxygen concentration increased along the direction of the catalyst layer to the diffusion layer,while the water concentration decreased;oxygen input increased rapidly and water discharge more smoothly by decreasing the thickness of the diffusion layer or raising diffusion layer porosity properly,meanwhile,electrochemical reaction rate went up could speed up oxygen consumption and water production in the cathode of HT-PEMFC,increasing the channel depth or channel width in the cathode of HT-PEMFC accelerated oxygen consumption and water production as well;elevating the cell temperature properly not only could push the proton transfer speed in proton exchange membrane and material transfer speed in electrode,but also could raise the electrochemical reaction rate in the catalyst layer,which caused oxygen concentration decline and water concentration rise in the cathode.Research results have great significance on optimizing the structure parameters of HT-PEMFC and applying HT-PEMFC popularly.