奔驰CCM、SGL气体扩散层、Ballard石墨双极板的面内电导、垂直电导和接触电阻、湿度、载荷和PTFE对结果的影响[材料对标其一]

In-plane and through-plane electricalconductivities and contact resistances of a Mercedes-Benz catalyst-coatedmembrane, gas diffusion and micro-porous layers and a Ballard graphite bipolarplate: Impact of humidity, compressive load and polytetrafluoroethylene

Hamidreza Sadeghifar

Abstract

By precisely deconvoluting the bulk fromthe contact and residual resistances, the in-plane electrical conductivities ofvirgin (dry or as-is), wet and dried catalyst-coated membranes (CCMs), amicro-porous layer (MPL) and several dry and wet gas diffusion layers (GDLs)with different PTFE loading are accurately determined. The through-planeconductivities of the GDLs, MPL and a bipolar plate (BPP) are rigorouslymeasured using the two-thickness method. The contact resistances of the BPPwith the GDLs (3–25mΩcm2) and MPL (14–57mΩcm2) areprecisely deconvoluted as well. Moreover, the impact of pressure, PTFE andhumidity on the resistances are thoroughly investigated. This study revealsthat the in-plane electrical conductivity of a wet CCM (70Sm−1) is three times lowerthan its dry version (210Sm−1). It is found that the in-plane conductivity of GDLs (∼6000Sm−1) does not change with PTFE, MPL and humidityand is significantly higher than those of wet and dry CCMs. Thethrough-plane conductivities of BPP (1935Sm−1), untreated andPTFE-treated GDLs (300–1500Sm−1), and MPL (50–350Sm−1) are in descending order. The through-plane GDL conductivity increases with pressure anddecreases with PTFE, MPL and cyclic loading but it is not affected byhumidity. It is also found that MPL and high PTFE loading increase, butpressure decreases, the GDL-BPP contact resistance dramatically.

Table 1 Specifications of the samplestested in the present study (data from the manufacturers).

Fig. 1. (a) The electrical resistanceequipment and devices (including ohm-meter Micro Junior 2) used in the presentwork and (b) isometric and bottom views of the testbed for the in-plane electricalconductivity measurements showing the samples of the same materials with twodifferent lengths, and the GPPs arrangement.

Table 2 The instrument and devices used inthe present work for measuring the electrical resistances of fuel cellcomponents.

Fig. 2. (a): The so-called two-thicknessmethod for measuring the through-plane electrical conductivity (σS) ofmaterials and their electrical contact resistances (ECRGPP-S) in terms ofpressure using Eqs. (4) and (5). A minimum of two different thicknesses (tS1& tS2) of the same materials are required. (b)–(d) The through-planeelectrical testbed for measuring the through-plane electrical conductivityand/or contact resistances as functions of pressure P: (b) one GDL sample onthe top of the first two (bottom) gold-plated probes (GPPs); (c) the GDLsandwiched between the four GPPs at the pressure of P; and (d) the cables (clipleads) shown connect the four GPPs to the ohm-meter Raytech Micro Junior 2.

Fig. 3. Through-plane electrical resistancenetwork for measuring the through-plane electrical conductivities of (a)different GDLs and (b) a BPP and their contact resistances with GPPs(gold-plated probes) using the socalled two-thickness method. (c) Electricalresistance network for measuring the through-plane electrical conductivity ofMPL and its contact resistances with GPPs based on the indirect two-thicknessmethod. (d and e) Electrical resistance networks for measuring the contactresistance of BPP with GDLs and MPL, respectively. All parameters in gray arealready known and the other parameters, which are unknown at their firstappearance, are each assigned a non-gray color.

Table 3 Uncertainties in measuringdifferent geometric parameters and the in-plane (IP) and

through-plane (TP) total resistances in thepresent study.

Fig. 4. In-plane electrical conductivity ofthe dry and wet CCMs, several GDLs with different PTFE loading and an SGL MPL.The impact of humidity on the CCM in-plane conductivity is significant. Pdenotes the pressure applied on the probes as shown in Fig. 1.

Fig. 5. Effect of humidity on the in-planetotal electrical resistance of a CCM over time in two straight days. P is thepressure applied on the probes as shown in Fig. 1.

这是一个比较有趣的现象，文章对它的解释是：

water affects the CCM structure. The membranein the CCM swells as a result of absorbing water and this swelling causesseveral cracks inside the CCM. These cracks disconnect or detach a considerableproportion of the carbon particles or chains from each other. This phenomenoncan be considered the main reason for a significant reduction in the in-planeelectrical conductivity of wet CCMs.

Fig. 6. (a) Through-plane electricalconductivities of a BPP, an MPL and several GDLs with different PTFE loadingversus pressure and (b) effect of humidity on the through-plane electricalresistance of GDL SGL 24AA: Through-plane electrical conductivity of GDLsdecreases with PTFE and MPL but increases with compressive pressure.Practically speaking, GDL electrical resistance does not change with humidity.

Fig. 7. Electrical contact resistance ofdifferent GDLs (a) with gold-plated probes (GPPs) and (b) with BPP: Effect ofPTFE and MPL over a range of pressure (compressive load). Both PTFE and MPLincrease the GDL contact resistance; however, the impact of MPL on both GDL-GPPand GDL-BPP contact resistances is significant. The error bars are not visiblefor the GDLs data in (a) as the errors are very small for the GDL-GPP contactresistances.

Fig. 8. Impact of cyclic loading on thethrough-plane total resistance of a wet GDL 24AA.

Summary and conclusions

The present work showed how the in-planeelectrical conductivities of CCMs and GDLs change with humidity, PTFE and MPL:

The in-plane electrical conductivity ofGDLs does not change with humidity while the in-plane conductivity of a wet CCM(70 Sm−1) is three times as low as its dry version (210Sm−1).

Practically speaking, dry and wet GDLs havethe same in-plane electrical conductivities.

The in-plane conductivity of GDLs (∼6000 Sm−1) is one order of magnitude higher than that ofa dry CCM (210 Sm−1).

GDLs have in-plane conductivities twoorders of magnitude higher than wet CCMs’.

The in-plane conductivity of GDLs does notchange with PTFE, regardless of the PTFE content.

MPL does not affect the in-plane GDL(substrate) conductivity.

Through several sets of systematicelectrical tests, the present study also revealed the true impact of PTFE, MPL,humidity and pressure on the through-plane electrical conductivity andresistance of GDLs. The variations of BPP electrical conductivity with pressurewere also studied:

The through-plane electrical conductivityof GDLs increases with pressure.

Through-plane GDL conductivity decreaseswith PTFE, irrespective of the PTFE loading.

The through-plane electrical resistance ofGDLs is practically independent of humidity and exponentially reduces withpressure.

Loading and unloading cycles reduce thetotal resistances of GDLs.

The through-plane conductivity of MPL(50–350 Sm−1) is considerably smaller than that of GDLs(300–1500 Sm−1) and increases with pressure.

MPL reduces the through-plane conductivityof GDLs significantly.

The through-plane conductivity of BPP,which is independent of pressure, is considerably higher than those of GDLs andMPLs (σBPP > σGDL withoutPTFE > σGDL withPTFE > σMPL).

This work also shed light on the truebehaviour of the contact resistances of GDLs, MPLs and BPPs. The effect ofpressure, PTFE and MPL on the BPP-GDL contact resistance were also thoroughlyinvestigated:

The contact resistance of GDLs with bothgold-plated probes (GPPs) and BPP increases with PTFE.

MPL increases the electrical contactresistance of GDLs dramatically.

Electrical contact resistances of GDLs withboth GPPs and BPP show exponential functionalities with the pressure.