Design Of Flow Channel And Research On Fluid Management In Proton Exchange Membrane Fuel Cells

Proton Exchange Membrane Fuel Cell(PEMFC)is a powerful system that directly converts the chemical energy of a fuel into electricity.Due to the high power density,high energy conversion efficiency,no pollution,low noise,fast startup and low operating temperature,PEMFC has become one of the most promising power sources.Flow channel on bipolar plate has an important effect on cell performance.They provide the pathways for gas reactants to distribute over the surface and diffuse into the micro-channels of the membrane electrode assembly,and then to participate in electrochemical reactions.In addition,liquid water at cathode side generated by the electrochemical reactions migrates outward through the gas diffusion layer and drains out also through the flow channel.Therefore,the flow channel has the function of evenly distributing the reactants and draining excess liquid water away.This work reviews domestic and international research progresses about flow fields in two aspects:gas distribution and water management.Aiming at the gas distribution and mass transfer of the flow field,pressure-establishing parallel flow field and convection-enhanced multi-serpentine flow lield were designed.The theory of flow distribution in the parallel flow field and mechanism of uniformity improvement in the pressure-establishing parallel flow field were investigated;the mechanism of convection enhancement under ribs in designed multi-serpentine flow field was also investigated.Aiming at the water removal performance,the comers of single-serpentine flow field and multi-serpentine flow field were improved,and the diverting straight channel was designed.The transport mechanisms of liquid water in the three newly designed flow channels were studied.The main research work in this paper is as follows:(1)Method and theory for enhancement of gas distribution uniformity in pressure-establishing parallel flow field with micro-channelsThrough theoretical calculations,it is found that increasing the mean pressure drop in sub-channels or reducing the difference of pressure drops between sub-channels can improve the uniformity of gas distribution in the parallel flow field.According to the first method,the pressure-establishing parallel flow field was designed.Numerical simulation was conducted to study its effect on mass transfer and electrical properties of PEMFC.The results show that the pressure-establishing parallel flow field can significantly improve the gas distribution uniformity and thus the output performance when it is located at cathode side;the smaller the size of the micro-channel of the pressure-establishing parallel flow field,the stronger the pressure-establishing capacity,the higher the uniformity of gas distribution,and the more obvious the improvement of cell performance.When the micro-channel size is 0.2 mm,the maximum power density of the cell with tlie pressure-establishing parallel flow field is 22.8%higher than that with the conventional one and about 10%lower than that with the serpentine one.However,the pressure loss is only 3%of the serpentine flow field.(2)Research on method of convection enhancement under ribs and mechanism of convective mass transferConvection under ribs can help remove the trapped liquid water in the gas diffusion layer and improve the output performance of a PEMFC.Based on the multi-serpentine flow field,two convection-enhanced multi-serpentine flow fields were designed.The mass transfer mechanism was studied by numerical simulation,and the results show that there are significantly higher pressure gradients in the adjacent flow channels of the two convection-enhanced multi-serpentine flow fields compared with the conventional one,which greatly enhances the eonvection under ribs and improves the drainage performance of the gas diffusion layer.As a result,the output performance is improved.(3)Construction of testing system for single PEMFC and experimental study on cell performanceTesting system for single PEMFC was constructed and single PEMFCs with different flow fields were assembled.The improvement of cell performance by the pressure-establishing parallel flow field and convection-enhanced multi-serpentine flow fields were verified through experimental test.Besides,mass flow rates of hydrogen and oxygen significantly affect the cell performance,thus they were also studied.The results show that the cell performance is enhanced and gradually reaches stable when increasing the mass flow rates of hydrogen or oxygen.(4)Design of flow channels for water removal from the gas diffusion layer and the research of water transport mechanism in the designed flow channelsAiming at the liquid water removal from the gas diffusion layer and water transport efficiency in the flow channel,two diverting corners suitable for single-serpentine and multi-serpentine flow fields,respectively,and a diverting straight channel suitable for most flow fields were designed.Simulation of Computational Fluid Dynamics with the volume-of-fluid(VOF)method were conducted to investigate the water removal and transport mechanism in the three flow channels.The simulation results show that all the three newly designed flow channels can effectively remove the liquid water on the surface of the gas diffusion layer,thereby increasing the efficiency of reactants diffusing into the gas diffusion layer.The hydrophobic walls of the three flow channels weaken the viscous effect,and thus improve the water transport efficiency in the flow channel.In addition,the effect of the structure size of the flow channel,the wall contact angle and air flow velocity on water behaviors were partially analyzed.