Study On Thickness Uniformity Of Graphite/Resin Composite Bipolar Plate By Hot Pressing

Proton exchange membrane fuel cell can convert the chemical energy into electrical energy,and it has higher energy conversion and less pollution,which has attracted more and more attention.Bipolar plate is one of the main components of proton exchange membrane fuel cells.Graphite / resin composite bipolar plate has attracted much attention due to its good comprehensive performance.But the composite bipolar plate formed by hot pressing the powder directly has the phenomenon of uneven thickness.The accumulation of this thickness error in the assembly of many bipolar plates seriously affects the assembly and even leads to the failure of the battery pack.In this paper,composite plate(bipolar plate after processing outlet)and bipolar plate are prepared by mixing graphite and polyphenylene sulfide powder.The causes of uneven thickness of composite plate and bipolar plate are analyzed,and the improvement methods are proposed.(1)The composite plate mold is designed and manufactured.In order to solve the problem that the forming surface has higher intermediate temperature and lower edge temperature,heating rods with different powers are used.During the experiment,the center and edge of the forming surface almost reach 340°C at the same time,ensuring the uniformity of the forming temperature.At the same time,the gap between the core and push plate is optimized,and the overflow is reduced.(2)The hot pressing experiment of the composite plate is carried out,and the influence of hot pressing parameters on thickness uniformity is studied.The optimal process parameters are obtained through the experiment: the powder mass is 219 g,the reserved gap between upper and lower half of mold is 2mm,the holding time of mold during the gap is1 min ～ 2 min,the mold temperature is 340°C,the mold opening temperature is 180°C,and the cooling method is water cooling.The maximum thickness difference can be reduced by about 30% by optimizing the process parameters.It is proved that melt flow is a reason for uneven thickness of composite plate.(3)The overall thickness of the composite plate is divided into convex,concave and wave shapes,and the reasons for the formation of the three shapes are summarized.The measurement results of thickness and groove depth show that the composite plate and bipolar plate have similar thickness inequality laws.Melt flow is a reason that affect the thickness inequality of composite bipolar plate.(4)In order to verify that the air in the powder will adversely affect the thickness uniformity of the composite bipolar plate,the powder vibration method is used to discharge the air.After vibration,the volume of the powder with the same mass can be reduced by about30%,and most of the air is discharged.Experiments show that the maximum thickness difference of the composite plate can be reduced by about 49% after vibration.It is proved that powder vibration can promote the thickness uniformity of composite bipolar plate,and the composite bipolar plate prepared by adding powder vibration does not affect the uniformity of groove depth.The thickness uniformity of composite plate and bipolar plate is improved by adding the reserved gap between upper and lower half of mold and powder vibration for the first time,which has important guiding significance for hot pressing qualified composite bipolar plate in the future.It is proved that the thickness uniformity of the composite bipolar plate can be improved by optimizing the mold structure,optimizing the hot pressing parameters and discharging the air in the powder.These favorable factors can be applied to the hot pressing of the composite bipolar plate at the same time,and the composite bipolar plate with more uniform thickness is expected to be obtained.