气体扩散层SGL 10BA的应力应变、热导、热接触电阻特性的测量和文献对比

Thermal Conductivity and Contact Resistanceof Compressed Gas Diffusion Layer of PEM Fuel Cell

I． Nitta

O． Himanen

M． Mikkola

Abstract

This paper discusses the effect of compression pressure on the mechanical and thermalproperties of gas diffusion layers （GDL）． The stress–strain curve of theGDL revealed one nonlinear and two piecewise linear regions within the compression pressure range of0–5．5 MPa． The thermal conductivityof the compressed GDL seems to be independent of the compression pressureand was determined to be 1．18 ± 0．11 W m–1 K–1at room temperature． The thermal contact resistance between the GDL andgraphite was evaluated by augmenting experiments with computer modelling． The thermal contact resistance decreasednonlinearly with increasing compression pressure． According to the resultshere， the thermal bulk resistance of theGDL is comparable to the thermal contact resistance between the GDL andgraphite． A simple one－dimensional model predicted a temperature drop of 1．7–4．4 °C across the GDL and catalyst layerdepending on compression pressures．

Fig． 1 Cross－sectional view of the GDL  and flow field plate taken by optical microscope （Olympus PMG3）． The GDL is  compressed under the rib and partially intrudes into the channel．

Fig． 2 Schematic of experimental setup for  measuring the stress–strain behaviour of GDL．

The stress–strain behaviour of the GDL （SGL SIGRACET® 10 BA） was measured  with the experimental setup illustrated in Figure 2．

It  was found that the more GDLs stacked the longer the interval required to achieve  the steady－state conditions after the weights were loaded． Therefore， the load was increased at 5–20 min intervals，  depending on the number of GDL samples under test．

Fig． 3 Schematic of thermal properties  measurement system．

Fig． 4 Stress–strain curve of the GDL measured  with different number of GDLs．

文献燃料电池气体扩散层的可压缩性：基材中PTFE含量、MPL中PTFE及含量、气体扩散层和密封复合、试验机刚度的影响［设计因素其六］的数据SGL 10BA 323 ± 10um，这篇文献中The thickness  of uncompressed GDL， which was 380 um reported by the manufacturer， was  determined to be 370 ± 10 um in a separate measurement with a low compression  pressure．这个厚度差异很吓人。

但是应力应变两篇文献压缩30％应力1．5MPa，压缩20％ 应力有一定差异，这篇0．8MPa，文献燃料电池气体扩散层的可压缩性：基材中PTFE含量、MPL中PTFE及含量、气体扩散层和密封复合、试验机刚度的影响［设计因素其六］0．3－0．5MPa。

Although  the curves obtained with different number of GDLs indicated almost identical  compressive behaviour， the strain of each GDL decreased as more GDLs were  stacked．

However，  the properties of the bulk GDL or interface between the GDL and graphite do  not depend on the number of the stacked GDLs．

Fig． 5 Temperature drops between the points  ‘B’ and ‘C’ as a function of number of stacked GDL．

Fig． 6 Thermal conductivity of GDL as a  function of compressed GDL thickness．

这张图是比较费解的。

作者的解释：One possible reason for this is the fact that the heat flux can be  transferred through the air inside the GDL but electrons cannot．

the thermal conductivity of air （0．026 W  m–1 K–1） is orders of magnitude smaller than that of

PTFE （11．7 W m–1 K–1） and that of carbon  fibre （129 W m–1 K–1） 电导相差1个数量级

A second possible reason for the  different behavior between electric and thermal conductivity under the  compression pressure is the significant difference between the electric  conductivity of carbon fibres （1 × 10＾5 S m–1 ［58］） and PTFE （2．7 × 10＾–15 S  m–1 ［59］）．

电导相差20个数量级

The kGDL（h） obtained here is  approximately four times higher than the reported value （0．3 W m–1 K–1 at  approximatel 2 MPa）．

文献中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］大致也是在后面这个状态0．6 W m–1 K–1 at approximatel 1．4 MPa。要么存在测量问题，要么是材料特性不同。

用同一种供应商的不同种材料不进行测量，直接引用进行认知是非常危险的。难得白嫖一次数据，竟然数据趋变化趋势和数值差异这么大。

SGL 10BA、SGL 24BA除了厚度似乎差异不大

SGL 10BA长这样

The 10BA
sample showed the most marked anisotropy in permeability with the higher value coinciding with the distinct “machine direction”．

SGL 24BA长这样

The 24BA samples consisted of fibers randomly oriented in two dimensions and accordingly do not exhibit significant anisotropy in the plane．

不是一家人。

Fig． 7 Thermal contact resistance between  GDL and graphite as a function of compressed GDL thickness．

Fig． 8 Modelled temperature profile and  measured temperature （a） region around temperature measured point， and （b）  enlarged in the vicinity of GDL．

Table 1 Cell design parameters and  material properties．

Fig． 9 Calculated temperature drop inside  the cell as a function of compressed GDL thickness．

Summary and Conclusion

The purpose of this study was to evaluate  the mechanical and thermal properties of the GDL experimentally as a function  function of compression．

The stress–strain curve of the GDL revealed  one nonlinear and two linear regions as the compression pressure was  increased up to 5．5 MPa． This was probably because of the nature of the GDL，  i．e． a rough surface and void volume consisting of pores with two different pore  diameter ranges．

The dependence of the thermal  conductivity of GDL on the compression pressure could not be

clearly observed． The obtained value of the thermal conductivity of GDL， 1．18 ± 0．11 W m–1  K–1， was approximately four times larger than those reported in the literature．

The thermal contact resistance between  GDL and graphite decreased nonlinearly with increasing compression pressure． This  may be attributed to the increase in actual contact area at the interface as  compression pressure was increased．

The  thermal bulk resistance of the GDL was comparable to the thermal contact  resistance between the GDL and graphite， suggesting  both bulk and interface of the GDL should be considered properly in modelling  studies．

A simple one－dimensional model of a fuel  cell employing the evaluated thermal parameters showed that the temperature drop  inside the fuel cell decreased as the GDL was compressed more． The predicted  temperature drop across the GDL and CL ranged from 1．7 to 4．4 °C for the  compressed GDL thickness from 129 to 328 um correspondingly． Uneven temperature  distribution may cause local overheating and lead to the degradation of the  cell components and thus， fuel cell has to be carefully designed to minimise  the harmful effects of inhomogeneous compression．