Study Of The Cold Start Characteristics And Start-up Methods Of Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cells(PEMFCs)are regarded as important tool to promote the development of a clean energy society,have become a research hotspot for nearly half a century due to their high efficiency,zero emissions.But there are still many technical barriers that need to be broken through before large-scale production of fuel cells.The difficulty of starting PEMFC at low temperature is one of the problems that need to be solved urgently.How to maximize the strengths and avoid weaknesses to improve the reaction heat of the battery as much as possible under the premise of low icing rate is the key to a successful cold start of PEMFC.The cold start model of PEMFC has been established in this paper to conduct a comprehensive study of the phenomenon of species migration and electrochemical reaction in PEMFC by change the type of channels,material properties of the porous layer and operating conditions,aiming at providing theoretical fundamentals for developing the PEMFC cold start strategy.Firstly,the 3D-PEMFC cold start model has been established to study the influence of the changed geometry of the channel on the cold-start process of the PEMFC.It is found that under the action of the serpentine channel,the material distribution is more uniform,and the phenomenon of excessive concentration of ice distribution is avoided,which is the lowest among the three flow fields.When the local pores of the single-channeled straight flow field PEMFC are completely blocked,the local maximum ice content of the single-channeled serpentine flow field PEMFC is only 0.493.In addition,the stress generated by the expansion of water freezing acts on the porous layer of the PEMFC,causing the corresponding deformation of the porous material.Compared with the single-channeled straight flow field,the maximum Von·Mises stress of the single-channel serpentine flow field porous layer and the three-channel serpentine flow field porous layer decreased by 62% and53%,respectively under the same conditions.Due to the complex structure of the serpentine channel,the pressure loss in the channel is serious,and the inlet pressure required by the serpentine channel is 2-3 times.Especially for the single serpentine channel,too long distance between the inlet and outlet will cause water vapor to accumulate in the middle of the channel.The three-serpentine channel can alleviate the flooding phenomenon in the channel by dispersing the water vapor into the three channels.The 1D-PEMFC cold-start model considering liquid water was used to study the effects of different material properties and operating conditions on the cold-start effect.The time for the ionomer to absorb water to reach saturation has been effectively prolonged by reducing the initial water content in the ionomer from 4 to 1.6,and the ice volume fraction decreased from 0.98 to 0.84 at 250 s.This is also the reason for increasing the ionomer content in catalyst layer(CL).While increasing the CL porosity enhanced the water removal ability of the PEMFC,thereby reducing the ice growth rate.The ice volume fraction at 250 s was reduced by 14% by increasing the CL porosity from 0.25 to 0.4.Although increasing the current load to start the PEMFC will shorten the time for the ionomer to absorb water to saturation and accelerate the formation of ice,it can effectively increase the temperature growth rate of the PEMFC.Ice within the pores of the PEMFC started at-2 °C under a current load with 15 A began to melt at 162 s.Finally,an adaptive strategy based on evaluation model is innovatively established to track the startup load of PEMFC.According to the results obtained from the cold start experiment,the output of the PEMFC under constant load at low temperature has a large change with time.First,the established TOPSIS method based on grey correlation analysis(GCA)has been established,the matrix corresponding to the 360 s obtained from the evaluation model is provided to the adaptive strategy as the learning target.By comparing the prediction results of the two learning models(four cases)on the learning target,the prediction result of the model closest to the learning target curve is27 A,and the PEMFC under this load can get the best start-up performance.The research work in this paper further explains the reasons for the difficulty of low temperature start-up of PEMFC.By studying the cold-start process of PEMFC,suggestions are put forward for how to improve the cold-start effect from three aspects of PEMFC channel geometry,material properties and operating conditions.