Theoretical And Experimental Study On Cold Start And Ice Formation Mechanism Of Metal Foam Pem Fuel Cell

Proton exchange membrane fuel cell(PEMFC)is recognized as a popular power plant in the near future,due to its low emission and high energy conversion efficiency.As one major issue in the commercial adoption,the cold start of PEMFC has been attracting considerable attention in recent years.Ice formation mechanism has top priority in the cold start research of PEMFC.The current conventional gas flow field with channel-land structure leads to inefficient mass and heat transfer in the fuel cell.Under subfreezing environment,water accumulates under the land,resulting in serious ice formation inside the electrode and operating performance degradation.Metal foam material is regarded as a promising alternative of the conventional flow field,due to its high porous structure and the resultant improved mass,heat and charge transport in PEMFC.In this paper,the cold start issue of PEMFC with novel metal foam flow field is studied,and the heat and mass transfer process in PEMFC under zero environment is further explored.The methods and strategies to improve the cold start performance of fuel cell are detailed discussed based on theoretical method,experimental tests,fuel cell design.The main work of this paper includes:1.Rebuilt of water generation and phase transition mechanism in the cold start process of PEMFC.According to the present studies in the literature,water is divided into four states in PEMFC under subzero temperature,including water vapor,ice,membrane water and frozen membrane water.In this work,theoretical analysis is carried out based on an analytical model for PEMFC cold start.With consideration of supercooled water,the water generation and phase transition mechanism are rebuilt.The water is further summarized into supercooled water,vapor,ice,membrane water and frozen membrane water.Based on experimental validation and physical analysis,this work proposed that water generated in reaction should be main membrane water.2.Experimental investigation on PEMFC cold start behavior with porous metal foam as cathode flow distributor.Based on the experimental tests,the results indicate that compared with the parallel channel case,the limiting current density of metal foam case is increased by 29.4%,and maximum net power is increased by 47.4% at 80oC.For cold start tests,metal foam performs superior to the conventional parallel flow channel under galvanostatic control,benefiting from its extremely porous structure and the resultant uniform reactant gas and heat distribution.3.Establishment of the steady/transient three-dimensional multiphase model for PEMFC under normal conditions.A three-dimensional multiphase model is built to study the steady performance and dynamic response of PEMFCs with metal foam and parallel flow channel as cathode flow field,respectively.The results indicate that the metal foam flow field is able to replace partial function of gas diffusion layer(GDL)in the uniform distribution of reactant gases.The cell performance also benefits from the better water removal of metal foam flow field.Besides,under transient conditions,larger flow area between metal foam and GDL makes PEMFC more sensitive to the step change of operating parameters.4.Development of three-dimensional multiphase cold start model for PEMFC.A three-dimensional numerical model for PEMFC cold start is established.The coupled transport processes of heat,mass and charge in PEMFC under various subzero temperatures and startup modes are simulated.The results show that ice formation is slower in PEMFC with metal foam flow field,due to the superior performance of metal foam in water removal and uniform distribution of reactant gas,leading to better startup performance.However,the excellent thermal conductivity of metal foam results in faster heat loss of PEMFC.This work aims to provide a new theoretical basis for the multi-environmental applications of PEMFC with metal foam flow field.