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Electrical conductivity of PEMFC under loading
Journal of Power Sources 289 (2015) 160e167
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Electrical conductivity of PEMFC under loading M. Hamour a, b, J.C. Grandidier b, c, A. Ouibrahim a, *, S. Martemianov b, d Laboratoire d'Energ etique M ecanique et Mat eriaux e LEMM, Universit e Mouloud Mammeri, Tizi-Ouzou, Algeria Institut Pprimme UPR du CNRS 3346 e CNRS, Universit e de Poitiers, ENSMA, France c Departement Physique et M ecanique des Mat eriaux, ENSMA, Futuroscope, Poitiers, France d Departement Fluides, Thermique, Combustion, ENSIP, Poitiers, France a
b
h i g h l i g h t s Electrical conductivity of GDL in PEMFC under loading. Electrical conductivity of SSF of bipolar plates in PEMFC under loading. Effect of the number of layers of GDL and of SSF under loading on the electrical conductivity. Electrical conductivity of the combined system GDL þ SSF under loading. Efficiency of the compression on the rate of increase of the electrical conductivity of GDL, SSF and GDL þ SSF.
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Article history: Received 16 January 2015 Received in revised form 16 April 2015 Accepted 22 April 2015 Available online 12 May 2015
Conducting an experimental investigation, we show that the electrical conductivity s (Sm1) of the gas diffusion layer (GDL) and bipolar plates in stainless steel foam (SSF), as the heart of a Proton Exchange Membrane Fuel Cell (PEMFC), depends in fact strongly on mechanical compression arising in an operating system. Thus, this mechanical compression significantly affects the current density and s; while it is not so far introduced in modelling and performance analysis. By mean of a developed metrology for this purpose and using Van der Pauw method, we studied, upon applied mechanical loads by varying the compression p, the electrical properties of different layers of a carbon cloth for GDL, of SSF, and, as in operating fuel cell, of the combination GDL þ SSF in a sandwich form. A strong dependency is observed for each of these materials and their combination up to saturation for high enough p which is seen then to reduce the electrical resistance; while the number of layers has no influence. The obtained results are analysed in term of rate of increase of s with an interesting conclusion for their application. © 2015 Published by Elsevier B.V.
Keywords: Carbon cloth Electrical conductivity Gas diffusion layer Fuel cell Stainless steel foam Van der Pauw method
1. Introduction While the principles of fuel-cell technology are of course well known, but it remains that their real commercialization still needs important efforts of investigations [1,2] in order to reduce their costs, to extend their life time [1e3] and to overcome some problems such those lied to heat and water management [4e6]. Each single PEMFC comprises a membrane-electrode assembly (MEA), GDLs, and bipolar plates (BPs) with gas channels as the main components [7]. Among other components, such as GDL, which affect the performance of fuel cells, bipolar plates are one of their main key
* Corresponding author. E-mail address: [email protected] (A. Ouibrahim). http://dx.doi.org/10.1016/j.jpowsour.2015.04.145 0378-7753/© 2015 Published by Elsevier B.V.
components and constitute over 80% of the mass and almost all of the volume in a typical fuel cell stack [8]. They are used to distribute the gas (hydrogen, oxygen or air), to collect an electric current and to provide heat management [9]. They promotes the homogenization of flow in the right electrode, greatly increases transfer of the mass and heat, and provides electrical conduction in the dipole separator with a very weak volume resistance. Thus, physicochemical and electrical properties of GDLs and bipolar plates appear very important to optimize heat and water management. Such properties depend on the materials chosen and on the appropriate fabrication technology used [10]. In general, the material usually used in the design of the GDL is carbon paper or carbon cloth, which is made hydrophobic by adding polytetrafluorethylene (PTFE), commonly also known as Teflon, to facilitate liquid water removal [11]. In the case of bipolar plates, in order to overcome the
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disadvantage of corrosion difficulty with metallic bipolar plates [12], stainless steel is a candidate material for metal bipolar plates [9,12]. It has also attracted much attention because of its mechanical properties and a relatively low price [12]. Moreover, investigating morphology, mechanical properties and corrosion resistance of material foams, it was shown that stainless steel foam exhibits high compressive plateau stress, structural integrity and good deformability [13]. So that, extended to bipolar plates materials, it was observed that the performances of fuel cell with NieCr metal foam is improved for gas flow-field [14]. This effectiveness of the metal foam was also emphasized in Ref. [15], where using metal foams as a GDL in gas flow field, the authors consider that cellular metal foams possessing good structural and conductive properties can therefore replace bipolar plates in fuel cells. Then, in fine, stainless steel foam (SSF) which is found in general, as metal foams, to perform better than the conventional channel design flow-field and to improve then the cell performances, appears as a good material to replace the graphite bipolar plate's fuel cell existing [16,17]. Furthermore, although we have nowadays an effective knowledge on the electrical properties of the GDL and SSF, their behaviour with the applied mechanical pressure is however poorly known. Yet, such a characteristic plays an important role, at a first order, on the fuel cell efficiency, so that, in order to improve the performance and cost-effectiveness of PEMFC, several groups have studied the effect of compressive stresses on fuel cell performances [18e21]. In a PEMFC, all components are held together by high compression, mainly to prevent gas leakages but also to provide low contact resistances. This assembling procedure is known to cause large strains of fuel cell components, which results in significant changes in its mechanical, electrical and thermal properties. These changes affect the rates of mass, charge, and heat transport through the GDL, thus impacting fuel cell performance and lifetime [22,23]. Moreover, mechanical and thermal stresses are also induced during functioning of the fuel cells and they may vary significantly [22e25]. They are provided by the heat production, which may lead via dilatation to deterioration effects [24,25]. Generally, the mechanical stresses arising during cell assembly and during operating cell, because of the dilatation effect, affect the system in contradictory way. As a matter of fact, a sufficient pressure is necessary to ensure good electrical contact by reducing the contact resistances; while too high stresses lead to MEA damage. Then, a perfect knowledge of the coupling between the physical phenomena (heat transfer and electrical production) and the mechanical loading is a fundamental step. Thus, the knowledge of the properties of the materials used for GDL and SSF is essential to ensure the increase of the performances of the full cells as well as of their durability together with the minimization of the applied compressive stresses on the GDL and SSF [26]. Obviously then, correct predictions of the electrical performance, heat exchange and water flux in fuel-cell stack require to take into account the dependency of the electrical properties on mechanical and thermal loads. In this context, experimental determination of the influence of mechanical loading on the GDL and on the SSF electrical conductivity is conducted in this work. For this purpose, the Van der Pauw method [27,28], also called the method of four points, is used in order to avoid the problem related to the influence of the contact resistance on the measurement procedure. This method is often used to test the in-plane electrical conductivity of various thin materials [28]. It was recently used by Ref. [29] to determine the electrical conductivity of bipolar plates in PEMFCs. Samples with different thicknesses have been first tested in order to justify the use of the four points measurements in porous media and to estimate the possible errors related to the contact resistance. The conducted experiments are performed in the range of mechanical loading corresponding to stresses arising in an operating fuel cell.
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To conclude this investigation, we have also conducted experimental tests associating GDL and SSF together, in a sandwich form, as it has to be in an operating fuel cell; an important phase and information not considered so far.
2. Experimental apparatus and procedures 2.1. Preliminary on the Van der Pauw method A simple method for measuring the in-plane electrical conductivity of thin isotropic materials was developed by Vander Pauw (VDP) in 1958 [28]. Several aspects of this technique have been described by Refs. [27] and [28] and it has many applications in industry as well as in research. This method assumes that samples are semi-infinite and thin, but the experimental conditions do not always respect such assumptions. To determine the electrical conductivity of a sample, the VDP method uses four probes to measure the resistance in two orthogonal in-plane directions, as shown in Fig. 1. Two adjacent probes are used to apply a constant current while the other two probes are used to measure the electric potential. The VDP method allows testing samples of any size and shape. It requires that the probes be placed on the periphery of the sample and their contact areas on the sample must be as small as possible. They are usually placed in a straight line and equidistant. Furthermore, this method uses the ability to determine the electrical resistivity r which can be calculated from the following formula:
r ¼ G$DV=I
(1)
where DV is the potential difference, I is the applied current and G a geometrical factor having the dimension of a length. It depends on the geometry of the sample and of the sensor positioning. This geometry factor expresses the modified current flow, resulting in a change of the potential distribution. In ideal cases and for some not very complex geometry, G can be calculated analytically using the following relationship proposed by Ref. [30] for a thick layer:
G ¼ 2pdl
(2)
where p is the applied compression, l the dimensionless correction factor (see values in Tables 1 and 2) and d the distance between 2 successive points. Many of these cases have been reported in the literature [31]. Most of the correction factors l are determined by the “method of images”. In the literature, the real samples are identified to “ideal” cases, using the correction factor l. It is sufficient to choose the geometrical factor closest to the case studied in order to have a value of l which depends on the ratio d/e, where e is the thickness
Fig. 1. Schematic description of the method of 4 points.
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M. Hamour et al. / Journal of Power Sources 289 (2015) 160e167 Table 1 Characteristic parameters of the tested GDL. Thickness (mm) Diameter of fibbers (mm) Area (cm2) Bulk density (g cm3) Porosity by cm2 (%) Young's modulus (MPa) Poisson's ratio Specific heat Cp (J kg1 K) Dimensionless correction factor l
0.280 10 55 0.45 78 6.3 0.09 500 0.031
Table 2 Characteristic parameters of the tested SSF. Thickness (mm) Area (cm2) Bulk density (g cm3) Porosity by cm2 (%) Young's modulus (GPa) Poisson's ratio Thermal conductivity (W m1 K) Dimensionless correction factor l
6.35 66 0.55 24 200 0.3 16.3 0.291
Fig. 2. Four aligned straight electrodes.
of the sample. The electrical conductivity s is defined as the reciprocal of the electrical resistivity r given by the relation (1). Then, using (1) and (2), we have:
s¼
I 2pd$l$DV
(3)
Analytical calculations have however their limits, the four point's method can therefore not be used for certain studied “ideal” geometries. Fortunately, the use of digital computing has revived the possibility of calculating the correction factor in more complex cases.
2.2. Experimental device To investigate the dependence of GDL and SSF electrical properties on the applied mechanical load, a special device has been developed to measure the electrical conductivity of these materials subjected to various mechanical loads provided by pressure variations. The material GDL which we want to test the electrical conductivity, has been embedded between two plates made of epoxy and has been compressed mechanically. In these experiments, the pressure was varied up to 8 MPa and progressively applied step-bystep on the device containing the GDLs. The same procedure was also repeated for the SSF. Electrical in-plane conductivity of a film material can be measured with the VDP method. The measuring device developed consists of four aligned straight electrodes inserted inside epoxy plate. The electrode tip protrudes from the plate and is in contact with the material being tested (Fig. 2). A block diagram of the electrical measuring system is shown in Fig. 3 with the four aligned straight electrodes (Fig. 2). It is composed of the measuring device: an ampere meter, a voltmeter and a microcomputer which links and controls the two meters. A mechanical press with a load capacity of 50 KN, a load sensor with a measurement range of 0e50 kN and an accuracy of 0.1% was fixed to the material tester between two pressure plates to control GDL compression pressure. For all the experiments, the electrical current applied is fixed and indicated by the ampere meter. The distance between the voltage probes was kept constant during the course of the measurements. This was achieved by embedding the two probes in a rectangular epoxy body. The potential difference for
Fig. 3. Block diagram of the measuring system.
each load applied value is measured by the voltmeter. The automation of the data acquisition allows to record the response in terms of the tension variation DV which then leads to calculate the electrical conductivity of the medium by using the relation (3). 2.3. GDL carbon cloth The tested GDL material is used in fuel-cell applications because of its high porosity, good thermal and electrical properties [10]. This porous material has a fibber structure, Fig. 4. This carbon cloth was coated with 30% of mass of polytetrafluoroethylene (PTFE) and has the following characteristic parameters (physicalemechanicaleelectrical) listed in Table 1 [10]. 2.4. Bipolar plates The tested bipolar material is SSF commercial use. The porous
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Fig. 4. Carbon cloth: a) An overview picture of the carbon cloth, b) Microscope view after a magnification of 20.
SSF AISI 316 (Fe/Cr18/Ni10/Mo3) was obtained from firm Good fellow [32]. This is still with high chromium (18%) and nickel (10%) content and with additions of molybdenum (3%). It can be used it in fuel-cell applications because of its high porosity, good thermal and electrical properties. It's a porous material as indicated in Fig. 5 and its characteristic parameters (physicalemechanicalethermal) are listed in Table 2. The electrical conductivity of a porous material is difficult to evaluate, it is the case for the SSF. The existing data and correlations are limited and usually do not take into account the dependence of the electrical conductivity on mechanical loading. 3. Experimental results 3.1. Carbon cloth Pressures were applied on the layers of carbon cloth having (6 cm 6 cm) dimensions. The tested GDL sample was positioned between two insulating plates of epoxy. The in-plane conductivity of the GDL samples was measured using the 4-probe method inside the mechanical device. During the experiments, the pressure was varied up to 8 MPa and progressively applied, step-by-step, on the device containing the GDLs. The mechanical load was applied by means of a power press. The obtained results for two and four GDL layers are presented in Fig. 6. All the experiments indicate that the GDL's electrical conductivity increases monotonically with increasing compression, whatever the number of layers. It can be noticed that the two curves associated to the two layers practically collapse for lower pressures, down to one MPa then slightly diverges for higher
Fig. 5. Microscope view of SSF before compression, magnified 20 times.
Fig. 6. Electrical conductivity versus pressure, for 2 and 4 layers of GDL.
pressures. More precise analysis shows that the measurements indicate a slight decrease of the electrical conductivity with the number of layers. This effect is related with the influence of the contact resistances. Nevertheless, this phenomenon is not significant with the VDP method. It can be admit that measured values are not influenced by the boundary conditions of GDL. 3.2. Stainless steel foam (SSF) The same tests have been conducted using SSF material under the same conditions as above to determine the electrical conductivity versus pressure. The in-plane conductivity was measured for several samples taken from the same material and the results obtained for one and two plates SSF are presented in Fig. 7. The same observation, as previously, shows that the SSF's electrical conductivity increases monotonically with the compression increase, whatever the number of the SSF plates, and two curves corresponding to the different number of SSF plates collapse on each other. Increasing the compression reduces the pore volume of the medium, and produces high-quality contact inside the material. The obtained results on the slight influence of the number of GDL layers and the SSF plates imply that contact resistances can be considered as negligible in our experiments. Experimental results obtained in this study are within the range of uncertainty of 4% for the carbon cloth and 6% for the SSF. Six experimental tests have been conducted for each, GDL and SSF, in order to check the reproducibility of the results at the same operating conditions.
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Fig. 9. Electrical conductivity versus pressure for 2 GDL layer between 2 SSF plates. Fig. 7. Electrical conductivity versus pressure for different SSF plates.
3.3. GDL and SSF combination We now consider the combination of GDL and SSF, associated in a sandwich form (Fig. 8) as it has to be in an operating fuel cell. The effect of the loading on the electrical conductivity of the combined system GDL þ SSF in a sandwich form is shown in Fig. 9. For comparison purpose, the effect of the loading is also reported for plates of SSF alone, not combined with GDL. The presence of GDL between the two plates affects the value of the electrical conductivity which then decreases but only by around 15%. The dependencies with pressure are not modify by presence of GDL.
we have a strong increase of s due to the compaction of the material, while in the second one, for large enough pressure and up to 8 MPa, this increase is quite less pronounced, involving then a fully compacted structure. That means while the material is more and more compacted, the volume of the pores decreases then, the air is evacuated and the contact greatly improves at the early pressures to reach thus an asymptotic behavior. Furthermore, as to the electrical conductivity resulting from the combination, in a sandwich form, of the GDL layer between the bipolar plates, it has been observed that the presence of GDL, which leads then to increase the contact resistance of the whole system, slightly affects the electrical conductivity of SSF the value of which is a hundred time larger to that of the GDL electrical conductivity.
4. Data analysis 4.1. Compression effect on the electrical conduction s Figs. 6 and 7 display the experimental variations of the electrical conductivity s of different layers of the carbon cloth and the SSF obtained for different pressures increasing from 0 to 8 MPa. The obtained curves can be separated in two regions. The first one, which concerns the weak compressions up to around 4 MPa,
4.2. Empirical equation of variation of electrical conductivity with compression From the obtained results on the evolution of the electrical conductivity s with the applied load p, it is then possible and specially interesting to deduce an empirical relationship between these two parameters in order to take into account this coupling in performance calculations. As a matter of fact, in the range 0 < p < 8 MPa, for the studied GDL material (Quintech carbon cloth), Fig. 10, The experimental fitted equation obtained for the GDL is:
sy3:103 logð1850p þ 49600Þ
(4)
In the case of the SSF, Fig. 11, the corresponding equation is similar, only parameters are different:
sy25:104 logð51000p þ 117000Þ
(5)
Recently in Ref. [33], the authors measured the through-plane and in-plane effective electrical conductivity of carbon paper (GDL) under various compression forces. They showed that the effective electrical conductivity is linearly dependent on the compressive force while (4) above indicates rather a logarithmic dependency of s on p. Now, it is also interesting to consider the electrical conductivity of the whole system when combining GDL þ SSF in a sandwich form as indicated in Fig. 9. In this case the correlation curve of Fig. 12 is represented by the corresponding equation:
sy23:104 logð50000p þ 15000Þ Fig. 8. Combination of GDL þ SSF in a loaded sandwich form.
(6)
M. Hamour et al. / Journal of Power Sources 289 (2015) 160e167
Fig. 10. Tendency Curve of the GDL electrical conductivity variation versus the applied pressure (from Fig. 6).
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Fig. 12. Tendency Curve of the electrical conductivity variation versus the applied pressure for the combination GDL þ SSF (from Fig. 9).
layers and 48% for 2 layers (a difference around 4%), as shown in Fig. 13, while in the case of the SSF a maximum of 44% for 1 plate and 38% for 2 plates (a difference of 6%). Such an observation of the effect of the numbers of layers and plates was not evident from the experimental curves Figs. 6 and 7, (s versus P). - 90% of the increase of Ds is reached for just a half of the 8 MPa exerted maximum pressure roughly for both GDL and SSF. That is to say only 4 MPa are sufficient to reach the 90% of the maximum rate of Ds for GDL (2 and 4 layers) and SSF (1 and 2 plates). This suggests that there is no need to reach pressure as high as 8Mpa to gain a maximum of electrical conduction. Of course, the same observation, as above, also holds in Fig. 15 for the combination GDL þ SSF in a sandwich form. Moreover and interesting to observe in Fig. 15 is that this representation, in term of rate of increase, allows to show the difference between the values of the two electrical conductivities (SSF without and with GDL) at small and large loading pressures. Indeed we observe that the difference between the 2 curves, in term of values, decreases as the pressure increases, that is to say the sandwich GDL þ SSF tend to the electrical conductivity of SSF alone; in other words the Fig. 11. Tendency Curve of the SSF electrical conductivity variation versus the applied pressure (from Fig. 7).
5. Rate of increase of Ds Using the experimental curves (Fig. 6 for GDL, Fig. 7 for SSF plates and Fig. 9 for combination of GDL þ SSF), issued from direct measurements (electrical conductivity s versus exerted pressure p), we determine the gap Ds ¼ s sm as the difference between s and its minimum sm (value of s before exerting any pressure, i.e. p ¼ 0), and we then consider the rate Ds/sm versus the exerted pressure p. Thus, the obtained curves, Figs. 13e15, allow the four following observations: - For an exerted pressure as large as 8 MPa, the maximum gain of Ds reached is around 50% for the GDL (Fig. 13), around 44% for the SSF (Fig. 14) and around 43% for GDL þ SSF (Fig. 15) - With this analysis (rate of Ds versus P), the rate of increase of Ds is clearly shown to be clearly different with the number of layers, although relatively small. It increases with the increase of the layers, reaching, in the case of GDL, a maximum of 52% for 4
Fig. 13. Rate of increase of the electrical conductivity versus pressure for 2 and 4 layers of GDL.
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evaluate the possible influence of the contact resistances. The inplane electrical conductivities of GDL, SSF and GDL þ SSF have been determined using the Van der Pauw method. The experimental uncertainty of the provided experiments is estimated at about 4% for GDL, 6% for SSF and 7% for GDL þ SSF. All the tests thus conducted led to the following results:
Fig. 14. Rate of increase of the electrical conductivity versus exerted pressure for different SSF plates.
Fig. 15. Rate of increase of the electrical conductivity versus exerted pressure for the combination GDL þ SSF in a sandwich form.
contact resistance between these two combined materials decreases.
6. Conclusion The mechanical, electrical and thermal performances of a fuel cell greatly depend on the associated properties of the materials which have to be chosen to constitute its main components, as bipolar plates together with GDL. For this purpose we have here considered the study, under mechanical loading situations, of the electrical conductivity s of Quintech carbon clothes (as GDL), of SSF foam (as bipolar plates) and in final, of the combination of both GDL and SSF (GDL þ SSF) in a sandwich form, as in operating fuel cell situation. A measuring procedure and an apparatus have been developed for such a study, important for a reliable characterization of electrical properties of GDL, SSF and their association. The loading experiments were conducted for several layers of GDL as well as of SSF and GDL þ SSF, increasingly compressed, at each test, up to 8 MPa. The mechanical loading experiments as well as the tests for different layers for GDL and SSF have the objective to
1. A strong dependence of the medium electrical conductivity s on the applied mechanical load p has been observed for the tested materials. Thus, the electrical conductivity of GDL, SSF and GDL þ SSF increases with the increase of the compression p up to 8 MPa. 2. But for high enough pressure, we observe some saturation in the increase of the electrical conductivity. Such a situation corresponds to a compacted structure of the medium. During the compression, the volume of the pores is reduced as the air is evacuated and then the contact between the fibbers is improved, increasing then the electrical conductivity. That means the effect of the compression is to reduce the electrical resistance, as was expected. 3. The evolution of s with p is logarithmic and the associated equation of evolution s (p) is consequently determined for each GDL, SSF and GDL þ SSF. 4. The value of the electrical conductivity does not depend on the number of layers for each GDL, SSF and GDL þ SSF. Such a result is of course of great interest in term of application. 5. Moreover, an analysis of the experimental results insight the rate of increase of the electrical conductivity allows two interesting practical remarks: a. A maximum of 50% of increase of s can be reached for pressures as large as 8 MPa, b. Only around a half of this maximum pressure is needed to gain 90% of the 50% maximum increase of s. c. Finally, such an analysis reveals in fact an evident influence of the numbers of layers, although small to be considered, within the range of uncertainty. d. The same observation, as above, also holds for the combination GDL þ SSF in a sandwich form. e. Moreover the representation in term of rate of increase, allows to clearly show the difference between the values of the two electrical conductivities (SSF without and with GDL) at small and at large loading pressures. Indeed we observe that the difference, in term of values, between the 2 curves decreases as the pressure increases, that is to say the sandwich GDL þ SSF tend to the electrical conductivity of SSF alone. We have observed that mechanical stresses in fuel cells are important and strongly depend on the operating conditions. That is why thermo/electro/mechanical coupling is an essential element. The results here presented can be used for modelling of heat transfer processes in fuel cells and electrolyzers. Finally, to achieve improvements of these results, it is essential to have a deep insight into the processes occurring inside the cell and its components. Experimental measurements involving operating fuel cell are underway in order to determine the electrical conductivity and its variation in operating fuel cell compression situation. These results, describing the balance between compression and performance, will provide vital information for system fabrication and for industrial applications.
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Electrical conductivity of PEMFC under loading M. Hamour a, b, J.C. Grandidier b, c, A. Ouibrahim a, *, S. Martemianov b, d Laboratoire d'Energ etique M ecanique et Mat eriaux e LEMM, Universit e Mouloud Mammeri, Tizi-Ouzou, Algeria Institut Pprimme UPR du CNRS 3346 e CNRS, Universit e de Poitiers, ENSMA, France c Departement Physique et M ecanique des Mat eriaux, ENSMA, Futuroscope, Poitiers, France d Departement Fluides, Thermique, Combustion, ENSIP, Poitiers, France a
b
h i g h l i g h t s Electrical conductivity of GDL in PEMFC under loading. Electrical conductivity of SSF of bipolar plates in PEMFC under loading. Effect of the number of layers of GDL and of SSF under loading on the electrical conductivity. Electrical conductivity of the combined system GDL þ SSF under loading. Efficiency of the compression on the rate of increase of the electrical conductivity of GDL, SSF and GDL þ SSF.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2015 Received in revised form 16 April 2015 Accepted 22 April 2015 Available online 12 May 2015
Conducting an experimental investigation, we show that the electrical conductivity s (Sm1) of the gas diffusion layer (GDL) and bipolar plates in stainless steel foam (SSF), as the heart of a Proton Exchange Membrane Fuel Cell (PEMFC), depends in fact strongly on mechanical compression arising in an operating system. Thus, this mechanical compression significantly affects the current density and s; while it is not so far introduced in modelling and performance analysis. By mean of a developed metrology for this purpose and using Van der Pauw method, we studied, upon applied mechanical loads by varying the compression p, the electrical properties of different layers of a carbon cloth for GDL, of SSF, and, as in operating fuel cell, of the combination GDL þ SSF in a sandwich form. A strong dependency is observed for each of these materials and their combination up to saturation for high enough p which is seen then to reduce the electrical resistance; while the number of layers has no influence. The obtained results are analysed in term of rate of increase of s with an interesting conclusion for their application. © 2015 Published by Elsevier B.V.
Keywords: Carbon cloth Electrical conductivity Gas diffusion layer Fuel cell Stainless steel foam Van der Pauw method
1. Introduction While the principles of fuel-cell technology are of course well known, but it remains that their real commercialization still needs important efforts of investigations [1,2] in order to reduce their costs, to extend their life time [1e3] and to overcome some problems such those lied to heat and water management [4e6]. Each single PEMFC comprises a membrane-electrode assembly (MEA), GDLs, and bipolar plates (BPs) with gas channels as the main components [7]. Among other components, such as GDL, which affect the performance of fuel cells, bipolar plates are one of their main key
* Corresponding author. E-mail address: [email protected] (A. Ouibrahim). http://dx.doi.org/10.1016/j.jpowsour.2015.04.145 0378-7753/© 2015 Published by Elsevier B.V.
components and constitute over 80% of the mass and almost all of the volume in a typical fuel cell stack [8]. They are used to distribute the gas (hydrogen, oxygen or air), to collect an electric current and to provide heat management [9]. They promotes the homogenization of flow in the right electrode, greatly increases transfer of the mass and heat, and provides electrical conduction in the dipole separator with a very weak volume resistance. Thus, physicochemical and electrical properties of GDLs and bipolar plates appear very important to optimize heat and water management. Such properties depend on the materials chosen and on the appropriate fabrication technology used [10]. In general, the material usually used in the design of the GDL is carbon paper or carbon cloth, which is made hydrophobic by adding polytetrafluorethylene (PTFE), commonly also known as Teflon, to facilitate liquid water removal [11]. In the case of bipolar plates, in order to overcome the
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disadvantage of corrosion difficulty with metallic bipolar plates [12], stainless steel is a candidate material for metal bipolar plates [9,12]. It has also attracted much attention because of its mechanical properties and a relatively low price [12]. Moreover, investigating morphology, mechanical properties and corrosion resistance of material foams, it was shown that stainless steel foam exhibits high compressive plateau stress, structural integrity and good deformability [13]. So that, extended to bipolar plates materials, it was observed that the performances of fuel cell with NieCr metal foam is improved for gas flow-field [14]. This effectiveness of the metal foam was also emphasized in Ref. [15], where using metal foams as a GDL in gas flow field, the authors consider that cellular metal foams possessing good structural and conductive properties can therefore replace bipolar plates in fuel cells. Then, in fine, stainless steel foam (SSF) which is found in general, as metal foams, to perform better than the conventional channel design flow-field and to improve then the cell performances, appears as a good material to replace the graphite bipolar plate's fuel cell existing [16,17]. Furthermore, although we have nowadays an effective knowledge on the electrical properties of the GDL and SSF, their behaviour with the applied mechanical pressure is however poorly known. Yet, such a characteristic plays an important role, at a first order, on the fuel cell efficiency, so that, in order to improve the performance and cost-effectiveness of PEMFC, several groups have studied the effect of compressive stresses on fuel cell performances [18e21]. In a PEMFC, all components are held together by high compression, mainly to prevent gas leakages but also to provide low contact resistances. This assembling procedure is known to cause large strains of fuel cell components, which results in significant changes in its mechanical, electrical and thermal properties. These changes affect the rates of mass, charge, and heat transport through the GDL, thus impacting fuel cell performance and lifetime [22,23]. Moreover, mechanical and thermal stresses are also induced during functioning of the fuel cells and they may vary significantly [22e25]. They are provided by the heat production, which may lead via dilatation to deterioration effects [24,25]. Generally, the mechanical stresses arising during cell assembly and during operating cell, because of the dilatation effect, affect the system in contradictory way. As a matter of fact, a sufficient pressure is necessary to ensure good electrical contact by reducing the contact resistances; while too high stresses lead to MEA damage. Then, a perfect knowledge of the coupling between the physical phenomena (heat transfer and electrical production) and the mechanical loading is a fundamental step. Thus, the knowledge of the properties of the materials used for GDL and SSF is essential to ensure the increase of the performances of the full cells as well as of their durability together with the minimization of the applied compressive stresses on the GDL and SSF [26]. Obviously then, correct predictions of the electrical performance, heat exchange and water flux in fuel-cell stack require to take into account the dependency of the electrical properties on mechanical and thermal loads. In this context, experimental determination of the influence of mechanical loading on the GDL and on the SSF electrical conductivity is conducted in this work. For this purpose, the Van der Pauw method [27,28], also called the method of four points, is used in order to avoid the problem related to the influence of the contact resistance on the measurement procedure. This method is often used to test the in-plane electrical conductivity of various thin materials [28]. It was recently used by Ref. [29] to determine the electrical conductivity of bipolar plates in PEMFCs. Samples with different thicknesses have been first tested in order to justify the use of the four points measurements in porous media and to estimate the possible errors related to the contact resistance. The conducted experiments are performed in the range of mechanical loading corresponding to stresses arising in an operating fuel cell.
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To conclude this investigation, we have also conducted experimental tests associating GDL and SSF together, in a sandwich form, as it has to be in an operating fuel cell; an important phase and information not considered so far.
2. Experimental apparatus and procedures 2.1. Preliminary on the Van der Pauw method A simple method for measuring the in-plane electrical conductivity of thin isotropic materials was developed by Vander Pauw (VDP) in 1958 [28]. Several aspects of this technique have been described by Refs. [27] and [28] and it has many applications in industry as well as in research. This method assumes that samples are semi-infinite and thin, but the experimental conditions do not always respect such assumptions. To determine the electrical conductivity of a sample, the VDP method uses four probes to measure the resistance in two orthogonal in-plane directions, as shown in Fig. 1. Two adjacent probes are used to apply a constant current while the other two probes are used to measure the electric potential. The VDP method allows testing samples of any size and shape. It requires that the probes be placed on the periphery of the sample and their contact areas on the sample must be as small as possible. They are usually placed in a straight line and equidistant. Furthermore, this method uses the ability to determine the electrical resistivity r which can be calculated from the following formula:
r ¼ G$DV=I
(1)
where DV is the potential difference, I is the applied current and G a geometrical factor having the dimension of a length. It depends on the geometry of the sample and of the sensor positioning. This geometry factor expresses the modified current flow, resulting in a change of the potential distribution. In ideal cases and for some not very complex geometry, G can be calculated analytically using the following relationship proposed by Ref. [30] for a thick layer:
G ¼ 2pdl
(2)
where p is the applied compression, l the dimensionless correction factor (see values in Tables 1 and 2) and d the distance between 2 successive points. Many of these cases have been reported in the literature [31]. Most of the correction factors l are determined by the “method of images”. In the literature, the real samples are identified to “ideal” cases, using the correction factor l. It is sufficient to choose the geometrical factor closest to the case studied in order to have a value of l which depends on the ratio d/e, where e is the thickness
Fig. 1. Schematic description of the method of 4 points.
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M. Hamour et al. / Journal of Power Sources 289 (2015) 160e167 Table 1 Characteristic parameters of the tested GDL. Thickness (mm) Diameter of fibbers (mm) Area (cm2) Bulk density (g cm3) Porosity by cm2 (%) Young's modulus (MPa) Poisson's ratio Specific heat Cp (J kg1 K) Dimensionless correction factor l
0.280 10 55 0.45 78 6.3 0.09 500 0.031
Table 2 Characteristic parameters of the tested SSF. Thickness (mm) Area (cm2) Bulk density (g cm3) Porosity by cm2 (%) Young's modulus (GPa) Poisson's ratio Thermal conductivity (W m1 K) Dimensionless correction factor l
6.35 66 0.55 24 200 0.3 16.3 0.291
Fig. 2. Four aligned straight electrodes.
of the sample. The electrical conductivity s is defined as the reciprocal of the electrical resistivity r given by the relation (1). Then, using (1) and (2), we have:
s¼
I 2pd$l$DV
(3)
Analytical calculations have however their limits, the four point's method can therefore not be used for certain studied “ideal” geometries. Fortunately, the use of digital computing has revived the possibility of calculating the correction factor in more complex cases.
2.2. Experimental device To investigate the dependence of GDL and SSF electrical properties on the applied mechanical load, a special device has been developed to measure the electrical conductivity of these materials subjected to various mechanical loads provided by pressure variations. The material GDL which we want to test the electrical conductivity, has been embedded between two plates made of epoxy and has been compressed mechanically. In these experiments, the pressure was varied up to 8 MPa and progressively applied step-bystep on the device containing the GDLs. The same procedure was also repeated for the SSF. Electrical in-plane conductivity of a film material can be measured with the VDP method. The measuring device developed consists of four aligned straight electrodes inserted inside epoxy plate. The electrode tip protrudes from the plate and is in contact with the material being tested (Fig. 2). A block diagram of the electrical measuring system is shown in Fig. 3 with the four aligned straight electrodes (Fig. 2). It is composed of the measuring device: an ampere meter, a voltmeter and a microcomputer which links and controls the two meters. A mechanical press with a load capacity of 50 KN, a load sensor with a measurement range of 0e50 kN and an accuracy of 0.1% was fixed to the material tester between two pressure plates to control GDL compression pressure. For all the experiments, the electrical current applied is fixed and indicated by the ampere meter. The distance between the voltage probes was kept constant during the course of the measurements. This was achieved by embedding the two probes in a rectangular epoxy body. The potential difference for
Fig. 3. Block diagram of the measuring system.
each load applied value is measured by the voltmeter. The automation of the data acquisition allows to record the response in terms of the tension variation DV which then leads to calculate the electrical conductivity of the medium by using the relation (3). 2.3. GDL carbon cloth The tested GDL material is used in fuel-cell applications because of its high porosity, good thermal and electrical properties [10]. This porous material has a fibber structure, Fig. 4. This carbon cloth was coated with 30% of mass of polytetrafluoroethylene (PTFE) and has the following characteristic parameters (physicalemechanicaleelectrical) listed in Table 1 [10]. 2.4. Bipolar plates The tested bipolar material is SSF commercial use. The porous
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Fig. 4. Carbon cloth: a) An overview picture of the carbon cloth, b) Microscope view after a magnification of 20.
SSF AISI 316 (Fe/Cr18/Ni10/Mo3) was obtained from firm Good fellow [32]. This is still with high chromium (18%) and nickel (10%) content and with additions of molybdenum (3%). It can be used it in fuel-cell applications because of its high porosity, good thermal and electrical properties. It's a porous material as indicated in Fig. 5 and its characteristic parameters (physicalemechanicalethermal) are listed in Table 2. The electrical conductivity of a porous material is difficult to evaluate, it is the case for the SSF. The existing data and correlations are limited and usually do not take into account the dependence of the electrical conductivity on mechanical loading. 3. Experimental results 3.1. Carbon cloth Pressures were applied on the layers of carbon cloth having (6 cm 6 cm) dimensions. The tested GDL sample was positioned between two insulating plates of epoxy. The in-plane conductivity of the GDL samples was measured using the 4-probe method inside the mechanical device. During the experiments, the pressure was varied up to 8 MPa and progressively applied, step-by-step, on the device containing the GDLs. The mechanical load was applied by means of a power press. The obtained results for two and four GDL layers are presented in Fig. 6. All the experiments indicate that the GDL's electrical conductivity increases monotonically with increasing compression, whatever the number of layers. It can be noticed that the two curves associated to the two layers practically collapse for lower pressures, down to one MPa then slightly diverges for higher
Fig. 5. Microscope view of SSF before compression, magnified 20 times.
Fig. 6. Electrical conductivity versus pressure, for 2 and 4 layers of GDL.
pressures. More precise analysis shows that the measurements indicate a slight decrease of the electrical conductivity with the number of layers. This effect is related with the influence of the contact resistances. Nevertheless, this phenomenon is not significant with the VDP method. It can be admit that measured values are not influenced by the boundary conditions of GDL. 3.2. Stainless steel foam (SSF) The same tests have been conducted using SSF material under the same conditions as above to determine the electrical conductivity versus pressure. The in-plane conductivity was measured for several samples taken from the same material and the results obtained for one and two plates SSF are presented in Fig. 7. The same observation, as previously, shows that the SSF's electrical conductivity increases monotonically with the compression increase, whatever the number of the SSF plates, and two curves corresponding to the different number of SSF plates collapse on each other. Increasing the compression reduces the pore volume of the medium, and produces high-quality contact inside the material. The obtained results on the slight influence of the number of GDL layers and the SSF plates imply that contact resistances can be considered as negligible in our experiments. Experimental results obtained in this study are within the range of uncertainty of 4% for the carbon cloth and 6% for the SSF. Six experimental tests have been conducted for each, GDL and SSF, in order to check the reproducibility of the results at the same operating conditions.
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Fig. 9. Electrical conductivity versus pressure for 2 GDL layer between 2 SSF plates. Fig. 7. Electrical conductivity versus pressure for different SSF plates.
3.3. GDL and SSF combination We now consider the combination of GDL and SSF, associated in a sandwich form (Fig. 8) as it has to be in an operating fuel cell. The effect of the loading on the electrical conductivity of the combined system GDL þ SSF in a sandwich form is shown in Fig. 9. For comparison purpose, the effect of the loading is also reported for plates of SSF alone, not combined with GDL. The presence of GDL between the two plates affects the value of the electrical conductivity which then decreases but only by around 15%. The dependencies with pressure are not modify by presence of GDL.
we have a strong increase of s due to the compaction of the material, while in the second one, for large enough pressure and up to 8 MPa, this increase is quite less pronounced, involving then a fully compacted structure. That means while the material is more and more compacted, the volume of the pores decreases then, the air is evacuated and the contact greatly improves at the early pressures to reach thus an asymptotic behavior. Furthermore, as to the electrical conductivity resulting from the combination, in a sandwich form, of the GDL layer between the bipolar plates, it has been observed that the presence of GDL, which leads then to increase the contact resistance of the whole system, slightly affects the electrical conductivity of SSF the value of which is a hundred time larger to that of the GDL electrical conductivity.
4. Data analysis 4.1. Compression effect on the electrical conduction s Figs. 6 and 7 display the experimental variations of the electrical conductivity s of different layers of the carbon cloth and the SSF obtained for different pressures increasing from 0 to 8 MPa. The obtained curves can be separated in two regions. The first one, which concerns the weak compressions up to around 4 MPa,
4.2. Empirical equation of variation of electrical conductivity with compression From the obtained results on the evolution of the electrical conductivity s with the applied load p, it is then possible and specially interesting to deduce an empirical relationship between these two parameters in order to take into account this coupling in performance calculations. As a matter of fact, in the range 0 < p < 8 MPa, for the studied GDL material (Quintech carbon cloth), Fig. 10, The experimental fitted equation obtained for the GDL is:
sy3:103 logð1850p þ 49600Þ
(4)
In the case of the SSF, Fig. 11, the corresponding equation is similar, only parameters are different:
sy25:104 logð51000p þ 117000Þ
(5)
Recently in Ref. [33], the authors measured the through-plane and in-plane effective electrical conductivity of carbon paper (GDL) under various compression forces. They showed that the effective electrical conductivity is linearly dependent on the compressive force while (4) above indicates rather a logarithmic dependency of s on p. Now, it is also interesting to consider the electrical conductivity of the whole system when combining GDL þ SSF in a sandwich form as indicated in Fig. 9. In this case the correlation curve of Fig. 12 is represented by the corresponding equation:
sy23:104 logð50000p þ 15000Þ Fig. 8. Combination of GDL þ SSF in a loaded sandwich form.
(6)
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Fig. 10. Tendency Curve of the GDL electrical conductivity variation versus the applied pressure (from Fig. 6).
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Fig. 12. Tendency Curve of the electrical conductivity variation versus the applied pressure for the combination GDL þ SSF (from Fig. 9).
layers and 48% for 2 layers (a difference around 4%), as shown in Fig. 13, while in the case of the SSF a maximum of 44% for 1 plate and 38% for 2 plates (a difference of 6%). Such an observation of the effect of the numbers of layers and plates was not evident from the experimental curves Figs. 6 and 7, (s versus P). - 90% of the increase of Ds is reached for just a half of the 8 MPa exerted maximum pressure roughly for both GDL and SSF. That is to say only 4 MPa are sufficient to reach the 90% of the maximum rate of Ds for GDL (2 and 4 layers) and SSF (1 and 2 plates). This suggests that there is no need to reach pressure as high as 8Mpa to gain a maximum of electrical conduction. Of course, the same observation, as above, also holds in Fig. 15 for the combination GDL þ SSF in a sandwich form. Moreover and interesting to observe in Fig. 15 is that this representation, in term of rate of increase, allows to show the difference between the values of the two electrical conductivities (SSF without and with GDL) at small and large loading pressures. Indeed we observe that the difference between the 2 curves, in term of values, decreases as the pressure increases, that is to say the sandwich GDL þ SSF tend to the electrical conductivity of SSF alone; in other words the Fig. 11. Tendency Curve of the SSF electrical conductivity variation versus the applied pressure (from Fig. 7).
5. Rate of increase of Ds Using the experimental curves (Fig. 6 for GDL, Fig. 7 for SSF plates and Fig. 9 for combination of GDL þ SSF), issued from direct measurements (electrical conductivity s versus exerted pressure p), we determine the gap Ds ¼ s sm as the difference between s and its minimum sm (value of s before exerting any pressure, i.e. p ¼ 0), and we then consider the rate Ds/sm versus the exerted pressure p. Thus, the obtained curves, Figs. 13e15, allow the four following observations: - For an exerted pressure as large as 8 MPa, the maximum gain of Ds reached is around 50% for the GDL (Fig. 13), around 44% for the SSF (Fig. 14) and around 43% for GDL þ SSF (Fig. 15) - With this analysis (rate of Ds versus P), the rate of increase of Ds is clearly shown to be clearly different with the number of layers, although relatively small. It increases with the increase of the layers, reaching, in the case of GDL, a maximum of 52% for 4
Fig. 13. Rate of increase of the electrical conductivity versus pressure for 2 and 4 layers of GDL.
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evaluate the possible influence of the contact resistances. The inplane electrical conductivities of GDL, SSF and GDL þ SSF have been determined using the Van der Pauw method. The experimental uncertainty of the provided experiments is estimated at about 4% for GDL, 6% for SSF and 7% for GDL þ SSF. All the tests thus conducted led to the following results:
Fig. 14. Rate of increase of the electrical conductivity versus exerted pressure for different SSF plates.
Fig. 15. Rate of increase of the electrical conductivity versus exerted pressure for the combination GDL þ SSF in a sandwich form.
contact resistance between these two combined materials decreases.
6. Conclusion The mechanical, electrical and thermal performances of a fuel cell greatly depend on the associated properties of the materials which have to be chosen to constitute its main components, as bipolar plates together with GDL. For this purpose we have here considered the study, under mechanical loading situations, of the electrical conductivity s of Quintech carbon clothes (as GDL), of SSF foam (as bipolar plates) and in final, of the combination of both GDL and SSF (GDL þ SSF) in a sandwich form, as in operating fuel cell situation. A measuring procedure and an apparatus have been developed for such a study, important for a reliable characterization of electrical properties of GDL, SSF and their association. The loading experiments were conducted for several layers of GDL as well as of SSF and GDL þ SSF, increasingly compressed, at each test, up to 8 MPa. The mechanical loading experiments as well as the tests for different layers for GDL and SSF have the objective to
1. A strong dependence of the medium electrical conductivity s on the applied mechanical load p has been observed for the tested materials. Thus, the electrical conductivity of GDL, SSF and GDL þ SSF increases with the increase of the compression p up to 8 MPa. 2. But for high enough pressure, we observe some saturation in the increase of the electrical conductivity. Such a situation corresponds to a compacted structure of the medium. During the compression, the volume of the pores is reduced as the air is evacuated and then the contact between the fibbers is improved, increasing then the electrical conductivity. That means the effect of the compression is to reduce the electrical resistance, as was expected. 3. The evolution of s with p is logarithmic and the associated equation of evolution s (p) is consequently determined for each GDL, SSF and GDL þ SSF. 4. The value of the electrical conductivity does not depend on the number of layers for each GDL, SSF and GDL þ SSF. Such a result is of course of great interest in term of application. 5. Moreover, an analysis of the experimental results insight the rate of increase of the electrical conductivity allows two interesting practical remarks: a. A maximum of 50% of increase of s can be reached for pressures as large as 8 MPa, b. Only around a half of this maximum pressure is needed to gain 90% of the 50% maximum increase of s. c. Finally, such an analysis reveals in fact an evident influence of the numbers of layers, although small to be considered, within the range of uncertainty. d. The same observation, as above, also holds for the combination GDL þ SSF in a sandwich form. e. Moreover the representation in term of rate of increase, allows to clearly show the difference between the values of the two electrical conductivities (SSF without and with GDL) at small and at large loading pressures. Indeed we observe that the difference, in term of values, between the 2 curves decreases as the pressure increases, that is to say the sandwich GDL þ SSF tend to the electrical conductivity of SSF alone. We have observed that mechanical stresses in fuel cells are important and strongly depend on the operating conditions. That is why thermo/electro/mechanical coupling is an essential element. The results here presented can be used for modelling of heat transfer processes in fuel cells and electrolyzers. Finally, to achieve improvements of these results, it is essential to have a deep insight into the processes occurring inside the cell and its components. Experimental measurements involving operating fuel cell are underway in order to determine the electrical conductivity and its variation in operating fuel cell compression situation. These results, describing the balance between compression and performance, will provide vital information for system fabrication and for industrial applications.
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