Studies On Multi-phase Heat And Mass Transfer Of Proton Exchange Membrane Fuel Cell Under Various Cold Start Modes

Proton exchange membrane fuel cell(PEMFC)has been considered as one of the most promising energy conversion devices in automotive applications.One of the major technical challenges for the commercialization of PEMFC is cold start.Start-up mode,initial temperature,initial water content in membrane/CL and assisted cold start are essential conditions which determine the failure/success of cold start.Few previous researches relate to PEMFC start-up mode and assisted cold start,even fewer for the start-up modes of constant power,maximum power,maximum power with hydrogen-oxygen catalytic reactions.In this study,numerical simulation and experimental methods are used to investigate the multiphase heat and mass transfer of PEMFC under different cold start-up modes.The work and innovation can be summarized as follows:(1)The polarization curve of self-assembled PEMFC is measured.The current density and the inlet pressure are compared from different initial temperatures of-7℃、-10℃ and-20℃ under the 0.3V constant voltage cold start-up mode.(2)Set up one-dimensional multi-phase PEMFC stack cold start model.The model is highly efficient,keeping the integrity of main heat and mass transfer process by considering membrane absorption/desorption,water freezing/melting in membrane and pores,water evaporation/condensation and electrochemical reaction.The model is compatible for constant power cold start,maximum power cold start,constant current density cold start and constant voltage cold start mode.(3)The differences between the constant power high current density cold start,the constant power low current density cold start,constant voltage cold start and constant current density cold start are compared.The constant power cold start mode is elucidated in detail based on the different start-up power,initial water content,initial temperature and the ionomer volume fraction in catalyst layer.(4)The maximum power cold start mode is investigated in details and compared with other cold start modes based on one-dimensional multi-phase stack model.The effects on the performance of the stack from initial water content and start-up temperature are analyzed.The maximum power cold start mode could better balance the heat generation and ice formation,leading to improved cold start survivability than that in the constant voltage and constant current density modes.(5)The effects of anode and cathode hydrogen-oxygen catalytic reaction on the maximum power cold start are investigated.The hydrogen-oxygen catalytic reaction assisted cold start method is developed and analyzed in the study.The interactions between this anode assisted method and various start-up modes are discussed.Rapid start-up from-40℃,effect of design parameters,rapid start-up without ice formation and super-cooling phenomenon are analyzed.The study revealed the principle of improving the stack cold start ability by anode hydrogen-oxygen catalytic reaction during cold start process.It is proved that anode hydrogen-oxygen catalytic reaction can improve the stack power output and inherient heat generation.The possibility of cold start from-20℃ without ice formation is testified.(6)Anode hydrogen-oxygen catalytic reaction assisted cold start is analyzed under the 0.3 V constant voltage cold start-up mode by conducting three-dimensional multi-phase simulation.The evolutions of different parameters such as current density,volume averaged ice volume fractions in the cathode catalyst layer,water contents,cell average temperature,various heat generation rate and heat loss rate from different start-up temperatures are analyzed with different anode air percents in the inlet mixture.Transient distributions of different parameters such as ice volume fractions and cell average temperature in the middle cross section of the cathode catalyst layer are investigated with different air percent in the inlet mixture.