Effects of Assembly Torque on a Proton ExchangeMembrane Fuel Cell with Stamped Metallic Bipolar Plates

Chen－Yu Chen

Sheng－Chun Su

Abstract

In this work， a proton exchange membranefuel cell with stamped 304 stainless steel bipolar plates is successfullyassembled． The total weight of thebipolar plates， membrane electrode assembly and gaskets in the fuel cell isreduced by 37％ when replacing milled graphite plates by the stamped 304stainless steel plates． Moreover， thegas leaking rates and electrochemical characteristics of the fuel cell atdifferent assembly torques are studied． It is found that increasing thetorque increases the sealing performance． However， an excess torque （＞6 N－m） decreases the power due to a decrease inthe porosity and hydrophobicity of the gas diffusion layer， and a deformationof the flow channels in the metallic plate． The pressure loss rate declines by 56％ as the torque increases from2 N－m to 7 N－m． The best performance is obtained at 6 N－m． The cellperformance increases by about 21％ as the torque increases from 2 N－m to 6 N－m．However， the performance decreases by about 11％ as the torque further increasesfrom 6 N－m to 7 N－m． The suspected reason for the above phenomenon is verifiedvia the electrochemical impedance spectroscopies and the microscopic images ofthe gas diffusion layer．

文章中的pressure loss不是指流体阻力而是指用压力下降测量气密性能。

气体扩散层的孔隙率下降、疏水性下降、金属板流道变形都是基于EIS的推测，均无实测数据支持。

Figure 1 The exploded view of the PEMFC  with stamped SS304 bipolar plates used in this work

Figure 2 The schematic diagram of the  combinaiton of the stamped metallic pibolar plates and the silicon rubber  gasket

Figure 3 Photos of the stamped SS304  bipolar plate and RTV silicon rubber gaskets

Table 1 The specification of the PEMFC  with stamped metallic bipolar plates used in this study

Figure 4 The  rates of pressure loss of the PEMFC with the stamped SS304 bipolar plates at different  torques

Figure 5 The polarization curves of the  PEMFC with the stamped SS304 bipolar plates at different assembly torques  （Tcell ＝65C， Td，air ＝70C， Td，H2＝70C， λair ＝3．0， λH2＝1．5 ）

Figure 6 The EIS results of the PEMFC  with the stamped SS304 bipolar plates at different torques at （a） 3 A ； （b） 8  A ； （c） 13 A． （Tcell ＝65C， Td，air ＝70C， Td，H2＝70C， λair ＝3．0， λH2＝1．5 ）

Figure 7 The ohmic resistance of the  PEMFC as a function of assembly torque at 3 A， 8 A and 13A

Figure 8 Rct＋Rmt of the PEMFC as a  function of assembly torque at 3 A， 8 A and 13 A

Figure 9 The microscopic images of the  GDL after fuel cell operation at a magnification of 100

times at （a） 2 N－m； （b） 4 N－m； （c） 7 N－m．

4． Conclusion

In this work， the PEMFC with stamped  SS304 bipolar plates and two layers of RTV silicon rubber gasket is  successfully developed． In comparison to the traditional PEMFC， the total  weight of the bipolar plates， MEA and gaskets of our PEMFC can be reduced by  37％． The rate of pressure loss declines by 56％ as the torque increases from 2  N－m to 7 N－m． The best sealing performance is obtained with the RTV silicon  rubber gasket at the torque of 7 N－m， but the best power generation performance  is obtained at 6 N－m． Increasing the assembly torque increases the sealing  performance． However， an excess torque decreases the power output due to a  decrease in GDL porosity and hydrophobicity， and the deformation of the flow  channel in the stamped metallic bipolar plate． The performance of the PEMFC  increases by about 21％ as the torque rises from 2 N－m to 6 N－m． However， the  performance decreases by about 11％ as the torque further increases from 6 N－m  to 7N－m． This is due to the decreased ohmic resistance of the PEMFC at  elevated torques and the increased Rct＋Rmt at high currents with the torque  higher than 6 N－m．