Research On Consistency And Gas Purge Process Of Proton Exchange Membrane Fuel Cell Based On Impedance

Enhancing the start-up and stable operation capability at sub-zero temperature is a major challenge for commercial applications of proton exchange membrane fuel cell(PEMFC)systems.When the fuel cell stack is operated at sub-zero temperature,the internal water and heat management is difficult,and the inconsistency between the single cells is more likely to cause the deterioration of the overall performance of the stack.The initial water content of the membrane will affect the storage space of liquid water and heat production rate of the fuel cell,which is crucial to the success of sub-zero start-up.Based on AC impedance technology,the consistency analysis method and gas purge process are studied in this thesis,which can provide guidance for improving the stability of fuel cell stack at sub-zero temperature and the membrane initial water content control.In the aspect of consistency analysis,a parameter identification method combining genetic algorithm and non-linear least squares based on AC impedance model is proposed,which effectively identifies the internal parameters of fuel cell.Then,the consistency of fuel cell stack under different operating conditions is analyzed synthetically by combining fitting parameters,cell voltages and EIS.The water and heat management and gas transfer status in the cells are analyzed by using fitting parameters,and the reasons for the difference of single cell performance under different operating conditions are analyzed.In the gas purge experiment,the high frequency resistance(HFR)was used as the characterization of membrane water content.Firstly,the gas purge mode of cathode with humid air and anode with humid hydrogen is selected by theoretical calculation and gas purge pre-experiment.Then,the effects of purge gas humidity,purge gas flow rate and stack temperature on purge performance are investigated experimentally.The experimental results show that the higher the humidity of purge gas is,the longer it takes for the HFR to reach stability,and the smaller the stabile HFR is.The higher the purge gas flow rate is,the faster the HFR of fuel cell rises and the greater HFR is at equilibrium.The higher the stack temperature is,the faster the HFR increases during the purging process,and the smaller the HFR is when the purging reaches equilibrium.On the basis of the existing three-stage purge model and the experiment data,an improved gas purge model is proposed,which explains the reason why the HFR rises rapidly at the beginning of the gas purge process and the overshoot phenomenon exists in the HFR change process.Then the validity of the model is verified by comparing and analyzing the simulation curve of the model with the HFR curve of typical purging process.Finally,some internal parameters of fuel cell during purging process are studied by using the model.The consistency analysis method and gas purge process of fuel cell stack have been studied in this thesis,which will provide guidance for improving the consistency of fuel cell in low temperature environment and controlling the initial membrane water content.The research achievements have considerable significance on improving the capability of sub-zero start-up and stable operation of fuel cells system.