Numerical Simulation Of Proton Exchange Membrane Fuel Cell At High Temperature And High Pressure

As an electrochemical generator,Proton Exchange Membrane Fuel Cell(PEMFC)is widely used in the field of transportation,power generation and military.PEMFC shows many advantages like high efficiency,eco-friendly and high reliability.The operating temperature of traditional PEMFC is around 60℃,the battery faces many problems such as difficulty of water management and low efficiency of cooling system.Improving the operating temperature can effectively solve these problems.Many PEMFC for vehicles starts to use the "high temperature and high pressure" technology gradually.Thus,it is important to study the performance of batteries at high temperature and high pressure.In this paper,a complete three-dimensional mathematical model was introduced for PEMFC,and the numerical calculation was carried out by using Fluent platform.Firstly,the influence of humidity on the steady state performance of the battery was analyzed.The main results are as follows: When operating under high temperature and high pressure operation,the condensation process of water vapor is delayed,and the water flooding has less effect on battery.At this time,the main parameter affecting the output voltage of the battery is the water content in the membrane.Under the condition of low cathode humidity,the water content in the membrane is very low.Even under high current,the battery still faces the threat of dry membrane,the polarization loss of the battery increases.In general,when the humidity of both anode and cathode reaches 100%,the battery shows the best output performance,reducing the anode humidity has no obvious effect on the battery performance,but reducing the cathode humidity significantly reduces the output voltage.The response of the battery output characteristics when the external current load changed stepwise was also discussed.The results are as follow: During the current loading and unloading process,the output power shows overshoot behavior.The overshoot of output power at high temperature and high pressure is less than that at low temperature and low pressure,but the response time is slightly longer than that at low temperature and low pressure,the response time of all working conditions is within 0.25 s.A three-dimensional geometric model of PEMFC with parallel flow fields andinterdigited flow fields was established,and the influence of flow field structure on battery performance at high temperature and pressure was discussed.When the load is changed,the response time of the output voltage is about 0.1 s,when the load variation range is the largest the response time reaches 0.5s.The response time of oxygen mass fraction range from 0.5 s to 3 s,and the response time of water content in the membrane and liquid water content in the catalyst layer is about 180 s which shows serious hysteresis.During the variable load operation,oxygen and liquid water mass transfer shows no overshoot behavior,while the battery voltage and water content in the membrane shows significant overshoot behavior.As the amplitude of the current change increases,the overshoot amplitude increases.By comparing the two flow fields,it can be found that the interdigitated flow field responds more quickly under variable load,the voltage undershoot is not obvious under loading,and the oxygen mass transfer can reach a new steady state within 0.5 s,while it needs 3 s to reach steady state in the parallel flow field.And the interdigitated flow field shows better comprehensive performance during variable load operation,less voltage loss,lower liquid water content in the catalyst layer,and higher oxygen mass fraction.The reason is that the end of the gas flow channel in the interdigitated flow field is closed,which makes the mass transfer mode of reactants and by-products change from free diffusion to forced convection.The gas mass transfer is smoother,and the removal effect of liquid water beneath land areas is better.It is also observed that the current distribution,water distribution and oxygen distribution in the catalyst layer are quite different due to the different geometry of the two flow fields.The current density distribution of the interdigitated flow field is even along the gas flow direction,while the current density of the parallel flow field increases continuously along the direction of gas flow.The current near the inlet areas is low and the electrochemical reaction is weak.Therefore,the interdigitated flow field is more suitable for batteries with long flow channel design.The above conclusions can provide a reference for the selection of the flow field structure of the battery during high temperature and high pressure operation.