Porosity Distribution Optimization Of Gas Diffusion Layer In Pem Fuel Cells Based On The Variational Principle

Proton exchange membrane fuel cell(PEMFC)is a clean energy device that uses hydrogen and oxygen as fuel and converts chemical energy into electricity directly without combustion.The main by-product of the fuel cell reaction is water,which has the advantages of high efficiency,low noise,and no pollution.PEMFCs can be divided into two categories according to the different operating temperature: high-temperature proton exchange membrane fuel cell(HT-PEMFC),which operates at 120℃-160℃,and low-temperature proton exchange membrane fuel cell(LT-PEMFC),which operates at 50℃-80℃.The cathode gas diffusion layer(GDL)is one of the important components of the membrane electrode assembly,and its porosity is a key factor affecting the transfer of reaction gases,electrons,liquid water and heat in the fuel cell.Considering that the difference in operating temperature makes the existence of water in HT-PEMFC and LT-PEMFC different,this thesis established three-dimensional numerical models for these two PEMFCs and carried out corresponding model verification.Subsequently,based on the variational principle,the performance optimizations of these two fuel cells were realized from the perspective of the porosity optimization of the cathode gas diffusion layer.For HT-PEMCF,a three-dimensional steady-state isothermal numerical calculation model was established,and a generalized function was constructed with the average porosity of the cathode GDL as the constraint and the oxygen transfer mass as the optimization objective.The optimal porosity distribution governing equation corresponding to the maximum oxygen transfer mass in the cathode GDL was obtained by combining the variational principle.The optimization of the porosity distribution of the HT-PEMFC cathode GDL was achieved theoretically and numerically by solving with the numerical software FLUENT.Considering the feasibility of the design,seven design schemes of in-plane nonuniform arrangement and linear gradient arrangement along the thickness direction were proposed to further verify the validity of the 3D porosity obtained from the variational optimization.For LT-PEMFC,a three-dimensional steady-state non-isothermal numerical calculation model was established,and a generalized function was constructed with the water saturation in the GDL as the optimization objective while keeping the average porosity of the cathode GDL constant.Based on the variational method,the governing equations that the porosity needs to satisfy when the water content in the cathode GDL is minimum were derived,and the relevant equations were solved to achieve the theoretical optimization of the porosity distribution of the LT-PEMFC cathode GDL.The optimization results were compared with the linear gradient distribution along the thickness direction,and the reliability of the optimization model was verified.