Pore-Scale Investigation On Two-Phase Flow And Electrochemical Reaction In Porous Electrode Of Fuel Cells

With the increasing consumption of energy in human society,the greenhouse effect is becoming more and more serious due to the large use of fossil energy.Hydrogen,as an efficient and clean energy,is considered to be the key to achieving global "carbon neutralization".Proton Exchange Membrane fuel cells(PEMFC),which convert chemical energy in hydrogen into electrical energy through electrochemical reaction,are the most promising energy conversion devices at present.Because fuel cells generate water during operation and flooding at high current density results in reduced performance,it is necessary to increase power density while maintaining operational stability.To achieve this goal,the electrode components of fuel cells need to be optimized to gain a deeper understanding of the two-phase flow and electrochemical reaction processes in the porous electrode microstructure.In this paper,a pore-scale simulation platform for investigating the two-phase flow and electrochemical reaction in PEMFC was established,and the strategies for optimizing the porous electrode structure were discussed based on the platform.The numerical reconstruction algorithm is used to generate the computational domains of the microstructures of Gas Diffusion Layer(GDL),Micro Porous Layer(MPL)and Catalyst Layer(CL).The reconstructed electrode structure takes full account of various components and micro-pore,while the parameters of each component can be flexibly adjusted and precisely controlled.Firstly,by comparing the transmission resistance and performance of different porous electrode structures under water flooding,the effects of porosity and cracks on electrode performance in GDL and MPL were explored,and the structure was optimized.The simulation results show that the gradient porosity GDL is helpful to increase the reaction area and average oxygen concentration in CL under water flooding.MPL shows more effective in-plane transmission capacity and contributes to the uniform distribution of oxygen.Cracks and perforated structures can promote water transport in GDL / MPL components.The perforation at the breakthrough point can further guide the accumulation of liquid water to the breakthrough,reduce the loss of oxygen concentration caused by flooding,and the average current density after flooding is increased by 23%.The systematic perforation design shows the best performance under water flooding conditions by separating the transmission of liquid water and oxygen,and the average current density is increased by 46%.Then the coupling process of complex two-phase transmission and reaction in CL is explored.It is found that liquid water is first generated in small pores,then accumulates in large pores and looks for breakthrough paths.This phenomenon can not be observed in the previous study of directly giving the inlet of liquid water.When water flooding occurs,the main pores in CL are occupied by liquid water,and oxygen cannot penetrate into the depth of Cl,so that the reaction in CL is mainly concentrated near the MPL side,the effective reaction depth(starting from MPL side)is only hundreds of nanometers,and the deeper reaction sites in CL are inactivated.The results show that large contact angle is conducive to accelerate the breakthrough of liquid water in Cl and increase the effective reaction depth.Compared with CL with contact angle of 110 °,the total reaction rate of CL with contact angle of 140 ° increased by 121%.In addition,the large contact angle increases the gas-liquid interface area in Cl,which is conducive to the evaporation of liquid water.Based on the research results,this paper proposes the structural optimization of gradient platinum(Pt)distribution and perforation in the traditional particle stacking CL.It is found that under the same platinum loading,the gradient distribution of Pt can increase the utilization of Pt,so as to increase the total reaction rate after flooding by 30%.The uniformly distributed perforation with a diameter of 80 nm improved the distribution of liquid water,increased the effective reaction depth,and increased the total reaction rate by 24%.Finally,the effects of the new columnar array CL structure on the two-phase transmission and electrochemical reaction were investigated.The simulation results show that the columnar array structure with porosity of 0.63 improves the liquid water distribution and increases the effective reaction depth.Compared with the CL with traditional particle stacking structure,the total reaction rate after flooding is increased by 172%.In this paper,a cylindrical array CL structure with designed distribution is proposed.The simulation results show that under the same conditions,this structure can further optimize the distribution of liquid water in Cl,and the total reaction rate after flooding is 287% higher than that of traditional CL.