| Proton exchange membrane fuel cell(PEMFC)is considered to be one of the potential substitutes for traditional power sources due to its high power density,fast dynamic response,low operating temperature,and environmental friendliness.From its runtime and cost perspectives,PEMFC vehicles have shown great market potential.However,as the main focus in recent years,the cold start and optimization of the flow field are still the major barriers for its commercialization.How to improve its selfstartup ability by means of optimizing the cell structure without using auxiliary heat source;and how to shorten the cold start time by adopting an effective startup strategy are the two key issues during the cold start operation.At present,the most critical problem associated with the traditional "channel-rib" flow channels is the uneven distribution of reactant gas,which seriously restricts the heat and mass transfer process inside the cell and limits the increase of cell power density.By virtue of its high porosity and lightweight,metal foam is considered to be a promising alternative to flow channels.However,the current applications of metal foam in fuel cells are still lacking understanding in material selection,flow field design,species transport in the foam,and especially for cold start performance.In this study,the electrode and flow field optimization are studied.Not only the influence of various structural parameters on the water-heat transfer in the cell under normal condition is explored,but also the ice-forming mechanism of the cell at subzero temperature is deeply understood and different cold start strategies are compared.For the experiment,electrochemical analysis is used to characterize the various impedances of the cell under normal conditions and high-frequency resistance during the process of purging,freezing,and startup process under cold start conditions.The in-situ and exsitu experiments are combined to explore the advantages and obstacles of the application of metal foam in fuel cells.The design and preparation of the metal foam flow field are investigated in detail.For the simulation,1D cold start and 3D steadystate models have been built to determine the heat and mass transfer mechanism and give supplements to the experimental results.This work can be concluded in the following four aspects:1.Electrode optimization experiment: due to the ionomer hydration effect and the positive role of the microporous layer(MPL)in the water management of cell,the effects of ionomer fraction(I/C ratio)and MPL insertion methods on the cell performance during normal and subzero conditions are compared by experiments.The conjecture that the core water product area in the catalyst layer(CL)moves towards the side of MPL as I/C ratio increases is put forward and verified.The influence of MPL on the ice-forming mechanism in the cell at different start-up temperatures and the phenomenon of voltage reversal during the cold start operation are explored.The results show that the high ionomer content is not conducive to the improvement of cold start performance.The results show that the increase of I/C ratio can reduce the dependence of the cell on the inlet humidity during normal conditions.The scanning electron microscope shows that the high I/C ratio will cause obviously agglomeration of CL,thus reducing the cell performance during normal and subzero conditions.Anode MPL has a positive effect on cold start performance at-7 and-10 °C.When hydrogen is lacking,voltage reversal due to carbon corrosion and water electrolysis occurs at the initial stage of startup.On the basis of theoretical analysis and experimental study,the freezing law during cold start is revealed,which provides theoretical support for MEA optimization.2.Cold start process simulation: Based on MATLAB,a 1D single-cell cold start model is built.The effects of operating parameters,CL structural parameters,and startup mode on the cell cold start performance are analyzed.The results show that the startup temperature,current density,and initial water content will affect the cell cold start performance,and thus corresponding startup strategies need to be matched.The optimization of CL structural parameters is contradictory between rapid heating below the freezing point with a rapid power increase in power above the freezing point.The galvanostatic,potentiostatic,maximum power and variable current startup modes have their own advantages and disadvantages according to the initial state of the cell.It is of positive significance to develop a hybrid startup mode based on the internal water content and temperature of the cell.3.Metal foam in-situ performance and ex-situ experiment and electrochemical characterization: Combined with electrochemical analysis,the cell performances are compared by using parallel flow channel and metal foam as cathode flow field during normal and subzero conditions.The effect of inlet humidity on normal output performance and effect of stratup temperature,current density and initial water content on cold start performance are investigated.Results show that the power density of metal foam PEMFC is three times higher than that of conventional flow field cells,but it also faces the problem that the cell is easily flooded in the cathode.Compared with parallel flow field fuel cell,metal foam fuel cell has advantages in output performance and ice storage.The metal foam is characterized by contact angle,permeability,stress-strain,and resistance.Besides,a manifold-type metal foam fuel cell is designed and manufactured.The effects of foam grade,compression ratio and PTFE treatment on the physical properties of the material and the effect of gravity and gas feed type on the cell performance of manifold-type metal foam fuel cells are investigated.The results of exsitu experiments show that the metal foam has good potential for flow field development and the new fuel cell design can effectively alleviate the flooding issue.4.Layered metal foam PEMFC design and 3D simulation: Based on the finite element software Fluent,the 3D multiphase-flow steady-state model is built,and the influence of different hydrophobicity,porosity,and permeability of metal foam on cell performance is compared.Low contact angles,porous and high permeability are more conducive to the storage of liquid water in the foam.It is proposed to use a more hydrophilic and lower porosity foam on the side away from the GDL.The results show the effect of capillary pressure can effectively alleviate flooding and improve the selfhumidification performance of the cell.It provides a new solution to reduce dependence on auxiliary humidifying devices in fuel cell applications. |
PDF Full Text Request