Ballard石墨双极板材料的热导和气体扩散层之间的接触热阻：压缩、PTFE、微孔层、不平整性和压缩滞回效应［材料对标其二］

Thermal conductivity of a graphite bipolarplate （BPP） and its thermal contact resistance with fuel cell gas diffusion layers： Effect of compression， PTFE， micro porous layer （MPL）， BPP out－of－flatness and cyclic load

Hamidreza Sadeghifar

Ned Djilali

Majid Bahrami

Abstract

This paper reports on measurements ofthermal conductivity of a graphite bipolar plate （BPP） as a function oftemperature and its thermal contact resistance （TCR） with treated and untreatedgas diffusion layers （GDLs）． The thermal conductivity of the BPP decreaseswith temperature and its thermal contact resistance with GDLs， which hasbeen overlooked in the literature， is found to be dominant over a relativelywide range of compression． The effects of PTFE loading， micro porous layer（MPL）， compression， and BPP out－of－flatness are also investigatedexperimentally． It is found that high PTFE loadings， MPL and even small BPPout－of－flatness increase the BPP－GDL thermal contact resistance dramatically．The paper also presents the effect of cyclic load on the total resistance of aGDL－BPP assembly， which sheds light on the behavior of these materials underoperating conditions in polymer electrolyte membrane fuel cells．

Fig． 1． （a） Main components of a PEMFC and（b） all the main thermal resistances inside a cell．

Fig． 2． Testbed of the TCR machine used forthermal resistance measurement in this study．

Fig． 3． Three types of experiments to beperformed by the TCR machine for measuring the GDL－BPP TCR （the cylindricalfluxmeters and the thermocouples placed inside them were not labeled）．

Fig． 4． Measuring deviations in theflatness of the BPP sample surfaces with a dial test indicator．

Table 1 Maximum deviations in the flatnessof the studied samples＇ surfaces

Fig． 5． Thermal conductivity of thegraphite BPP （kBPP） at different temperatures and compression （obtained fromrepeated tests）： RBBP＝tBBP／kBPPA．

Fig．6． Thermal conductivity of the graphiteBPP （kBPP） as a function of temperature．

Fig． 7． TCR between the graphite BPP andthe Armco－iron fluxmeters at different temperatures and compression （obtainedfrom repeated tests）．

Fig． 8． Thermal conductivity of Sigracetuntreated and treated GDLs （kGDL） as a function of compression： RGDL＝tGDL／kGDLA．

Fig． 9． Thermal contact resistances ofSigracet untreated and treated GDLs with the Armco－iron fluxmeters （FM） as afunction of compression： TCRGDL－FM ＆ TCRMPL－FM．

Fig． 10． Experimental data of TCR betweenthe graphite BPP and 14 different SGL GDLs （TCRGDL－BPP） at an averagetemperature of 55 C．

Fig． 11． Contribution of the TCR betweenthe graphite BPP and different SGL GDLs into the total resistance of thestudied BPP－GDL assemblies at an average temperature of 55 C．

Fig． 12． Effect of the BPP out－of－flatnesson TCRBPP－GDL （for comparison， the data of SGL 24 already shown in Fig． 11 hasbeen duplicated in this figure）．

Fig． 13． Effect of load cycles on the totalresistance of SGL 24BA－BPP 5．84 assembly （including the contact resistance ofthe GDL with the two fluxmeters）．

Fig． 14． Effect of load cycles on the totalresistance of SGL 24DA－BPP 5．84 assembly （including the contact resistance ofthe GDL with the two fluxmeters）．

Summary and conclusion

Thermal conductivity of a graphite BPP wasmeasured under different temperatures and pressures， with the following key results：

Thermal conductivity of the graphite BPP and its thermal contact resistance with theArmco－iron fluxmeters decrease with increasing temperature．

Thevariation of the BPP thermal conductivity in terms of temperature can beconveniently represented in a compact form suitable for thermal analysis andmodeling．

The TCR between the BPP and GDLs withdifferent PTFE loadings and／or MPL were also measured in terms of compression．The effect of compression， PTFE， MPL， out－of－flatness， and cyclic loads on the BPP－GDLTCR were investigated thoroughly：

TheTCR between BPP and GDL increases with both MPL and PTFE， regardless of thePTFE loading．

HighPTFE loading， MPL， and the BPP out－of－flatness increase the GDL－BPP TCRdramatically．

TheBPP－GDL TCR can be the dominant resistance in GDL－BPP assembly， as itscontribution can increase to almost 60％ and 40％ at the compression of 1 and 5bar， respectively．

Load cycling reduces the total thermal resistance of BPP－GDL assemblyconsiderably．

Thereduction effect of load cycling on the thermal resistance of BPP－GDL assemblyis more pronounced for GDLs with lower PTFE loading．