Study On Vibration Characteristics Of Water And Optimization Analysis Of Its Transmission Processin In Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell(PEMFC)is considered to be one of the most potential vehicle power devices with the advantages of zero pollution,high specific power and compact structure.The water management of PEMFC is a difficult,especially in the actual road conditions of the vehicle,the vibration increase the difficulty of fuel cell water management.Therefore,effective water management under actual vehicle conditions not only benefits the stable operation of PEMFC,but also has great significance for improving the durability of PEMFC for vehicles.This paper focuses on the key issues about the PEMFC water management under the actual working conditions of the vehicle.The main research contents of the thesis include:(1)Study on the vibration characteristics of the droplet on the surface of gas diffusion layer GDL.In view of the lack of research on the impact of vibration factors on the management of fuel cell water,the dynamic behavior of the droplet vibration on the surface of the GDL is observed by high-speed camera technology,and the change of the Wenzel-Cassie state of the water droplet on the GDL surface under the horizontal and vertical vibration conditions is studied.When the vertical amplitude is 2.5mm,the frequency is greater than 15Hz or the amplitude is 2mm,and the frequency is more than 20Hz,the droplets are separated from the GDL surface,and the Wenzel-Cassie state to the Cassie state is changed,and the droplet contact diameter is less than the initial diameter for a long time in the vibration period,making the gas more easy to discharge the droplets from the fuel cells.On this basis,the motion of the droplets on the GDL surface under the interaction of vibration and gas is further analyzed.It is found that the combination of auto vibration and gas blowing can accelerate the discharge of water droplets on the GDL surface from the fuel cells.(2)Study on the durability of PEMFC under vibration condiatin.The 300 hour durability accelerated test of the PEMFC under vibration and static environment respectively shows that the performance degradation of the PEMFC under the vibration environment is far greater than that under the static environment.Under.the vibration,the PEMFC was operated for 300 h,the voltage dropped from 0.643 V to 0.612 V,and the voltage decay rate reached 103 ?Vh-1.In a stationary environment,the voltage is reduced from 0.658V to 0.642V and the voltage decay rate is approximately 53?Vh-1.A variety of electrochemical and physical characterization methods were used for analysis.It was found that the root cause of the degradation of the PEMFC under vibration environment was the loosening of the connecting bolts and the acceleration of the Pt particle shedding and migration under the vibration environment,.(3)Study on the enhancement of liquid water transmission by perforated GDL.According to the experiment,it is found that the droplets are first appeared in the largest hole in GDL,and the idea of separating the GDL gas and liquid transmission channels is proposed.The study shows that the transmission resistance of water can be reduced significantly after the hole drilling,and the water flooding phenomenon between the GDL and the catalytic layer(CL)can be significantly reduced.The maximum breakthrough pressure of liquid water is 700Pa,2000Pa and 5700Pa under punching a diameter of 200 ?m,100 pm and without drilling conditions.Based on the Level Set method,a dynamic droplet model was established.The droplet growth,splitting and merging of droplets under the different punching diameter and spacing were simulated,combining fuel cell polarization curve and ac impedance test,the optimum the punching diameter 100 ?m and spacing 2mm were obtained.Through laser drilling GDL material to guide the transmission of water from CL to gas flow path,the flooding of PEMFC is alleviated.The idea and experimental results of gas liquid transport channel separation can provide theoretical basis and design parameters for GDL material structure design.(4)Study on the characteristics of water transmission by hydrogen recycle.The effect of hydrogen cycle on the performance of the stack under the conditions of different humidity,temperature and pressure is studied by building a test platform of 1kW PEMFC system.The dynamic model of a multi parameter coupled hydrogen recycle PEMFC stack system was established,and the accuracy of the model is verified by comparing with the experimental data.The different operating parameters(temperature,the cathode humidity,and hydrogen cycle ratio)and the thickness of the membrane on the water transfer process and nitrogen diffusion were analyzed.It is found that pressure has an important influence on nitrogen diffusion.With the increase of the inlet pressure,the molar coefficient of hydrogen and water decrease,and the molar coefficient of nitrogen increases gradually.The greater the pressure,the shorter the nitrogen diffusion reaches equilibrium.The equilibrium time is 800s,1200s and 1400s respectively under the pressure 60kPa,40kPa and 20kPa.When the nitrogen diffusion reaches equilibrium,the molar coefficients are 50%,40%and 20%respectively.On this basis,the method of improving the hydrogen circulation system is proposed,the anode humidifier is cancelled,the anode flooding and the membrane dry phenomenon are relieved,and the stability of the stack is improved.