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Development and Characterization of Expanded Graphite-Based Nanocomposite as Bipolar Plate for Polymer Electrolyte Membrane Fuel Cells (PEMFCs) S. R. Dhakate,* S. Sharma, M. Borah, R. B. Mathur, and T. L. Dhami Carbon Technology Unit, Engineering Materials DiVision, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110 012, India, and Department of Energy, Tezpur UniVersity, Tezpur 784 028, Assam, India ReceiVed February 24, 2008. ReVised Manuscript ReceiVed April 29, 2008
Expanded graphite-based nanocomposites as bipolar plates for polymer electrolyte membrane fuel cells (PEMFCs) are developed by compression molding technique. The expanded graphite is prepared by chemical intercalation of 50 BS mesh particle size of natural graphite by strong acid. It is found that composite bipolar plate with a varying weight percent of resin and EG gives the high electrical conductivity, competitive mechanical properties with a bulk density of 1.5-1.55 g/cm-3, and air tightness. The EG-based composites with 40-45 wt % of resin are suitable for achieving desired properties of the bipolar plate as per the DOE advanced series target. The addition of 5 wt % of carbon black is helpful for improving the electrical conductivity without compromising other properties of the bipolar plate. The lower value of the modulus of expanded graphite (3-8 GPa) composite as compared to graphite-resin-based composite (>12 GPa) plates attributes that these plates are more flexible and able to withstand shock and vibration during mobile operation of the fuel cell stack.
Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are the most promising power sources for various portable electronic devices, stationary and mobile applications1 that operate at low temperatures between 70 and 80 °C. The main components of the PEMFCs are electrolyte membrane, gas diffusion layer, platinum catalyst, and bipolar plate. The bipolar plate is one of the most important components of PEMFC, which accounts for as much as 60-80% of the fuel cell stack weight2 and 40-50% of the total cost of the fuel cell stack. The U.S. Department of Energy (DOE) advanced a series of property requirements of the resin/graphite composite for the bipolar plate, in which the main properties are electrical conductivity (100 S/cm) and bending strength (25 MPa), respectively.3 The conventional materials for producing the bipolar plate are graphite or metal. Graphite plates are preferred over the metallic plates because of their high corrosion resistance and lower density.4 On the other hand, disadvantages are difficulties in machining and its brittleness.5,6 Because of this reason, a graphite-based bipolar plate requires a thickness of the order of several millimeters * To whom correspondence should be addressed. Fax: 0911125726938. E-mail:
[email protected]. (1) Shen, C. H.; Pan, M.; We, Q.; Yuan, R. Z. J. Power Sources 2006, 159, 1078–1083. (2) Mighri, F.; Huneault, M. A.; Champagne, M. F. Polym. Eng. Sci. 2004, 44, 1755–1765. (3) Website http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/ pdfs/fuelcells.pdf, 2007. (4) Mepsted, G. O.; Moore, J. M. Handbook of Fuel CellssFundamentals, Technology and Applications; John Wiley and Sons Ltd.: New York, 2003, pp 286-293. (5) Makkus, R. C.; Janssen, A. H. H.; De Brujin, F. A.; Mallant, R. K. A. M. Fuel Cell Bull. 2000, 3 (17), 5–9. (6) Wilson, M. S. Composites bipolar plate for electrochemical cells. WO00/25372, 2000.
and causes the fuel cell stack to be massive and voluminous.7 These considerations have motivated researchers to develop alternative materials. Graphite-based polymer composite bipolar plates have the potential to replace graphite plates; they offer the advantages of greater ease of manufacturing than graphite plates.8 However, polymer composites are associated with problems of electrical conductivity that remain to be solved; therefore, excessive graphite fillers have to be incorporated to the composites to meet the minimum requirement of electrical conductivity. High graphite loading in the polymer composite substantially reduces the strength and ductility of polymer composites, and this issue may be considered. There are some studies reported in the literature on bipolar plate made from expanded graphite (EG) for PEMFCs,9–13 and more studies are available on the improvement of physical and electrical properties of various polymers14–20 by incorporating a few percentage of EG. Heo et al.9 reported the development of bipolar plate made from EG by preform molding technique. The electrical (7) Bisaria, M. K. Injection moldable conductive aromatic thermoplastic liquid crystalline polymer compositions. WO00/44005, 2000. (8) Cunningham, B. D.; Baird, D. G. J. Power Sources 2007, 168, 418– 425. (9) Heo, S. I.; Oh, K. S.; Yun, J. C.; Jung, S. H.; Yang, Y. C. J. Power Sources 2007, 171, 396–403. (10) Xiao, M.; Lu, Y.; Wang, S. J.; Zhao, Y. F.; Meng, Y. Z. J. Power Sources 2006, 160, 165–174. (11) Bhattacharya, A.; Hazra, A.; Chatterjee, S.; Sen, P.; Laha, S.; Basumallick, I. J. Power Sources 2004, 136, 208–210. (12) Yan, X.; Hou, M.; Zhabg, H.; Jing, F.; Ming, P.; Yi, B. J. Power Sources 2006, 160, 252–257. (13) Kalaitzidou, K.; Fukushima, H.; Drzal, L. T. Composites, Part A 2007, 38, 1675–1682. (14) Chen, G.; Wu, C.; Weng, W.; Wu, D.; Yan, W. Polymer 2003, 44, 1781–1784. (15) Debelack, B.; Lafdi, K. Carbon 2007, 45, 1727–1734. (16) Sheng, W.; Wong, S. C. Compos. Sci. Technol. 2003, 63, 225235.
10.1021/ef800135f CCC: $40.75 2008 American Chemical Society Published on Web 07/04/2008
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Figure 1. (a) SEM of NG flakes. (b) SEM of NG flakes at higher magnification. (c) SEM of EG. (d) SEM of EG at higher magnification.
conductivity and flexural strength increase as the stamping pressure and curing time increase and reach the maximum value. Xiao et al.10 reported the properties of bipolar plate made from poly(arylene disulfide)/graphite nanosheets. In this study, graphite nanosheet composites are fabricated using ultrasonic irradiation that possesses a flexural strength of 32.05 MPa and an electrical conductivity of 158.7 S/cm. Bhattacharya et al.11 studied the EG as an electrode material for an alcohol fuel cell and found that the EG electrode has efficient electro-catalytic activity than an unexpanded graphite electrode because of the nanochannel formed after expansion. Yan et al.12 used EG bipolar plates is in PEMFC stack, and it was found that the performance of the stack was excellent. Kalaitzidou et al.13 studied morphology and mechanical properties of exfoliated graphite nanoplates-polypropylene nanocomposites fabricated by melt mixing and injection molding. Therefore, in the present investigation, efforts are paid to use the reasonable amount of EG instead of natural graphite (NG) in the polymer to obtain the requisite properties. As compared to conventional NG, EG has unique properties because it is produced from graphite flakes intercalated with highly concentrated acid, which can be expanded up to a few hundred times their initial volume; the expansion leads to the separation of the graphite sheet into nanoplates with a very high aspect ratio. (17) Zheng, W.; Wong, S. C.; Sue, H. J. Polymer 2000, 73, 6767–6773. (18) Pan, Y. X.; Yu, Z. Z.; Ou, Y. C.; Hu, G. H. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1626–1633. (19) Shen, J. W.; Chen, X. M.; Huang, W. Y. J. Appl. Polym. Sci. 2003, 88, 1864–1869. (20) Yasmin, A.; Luo, J. J.; Daniel, I. M. Compos. Sci. Technol. 2006, 66, 1179–1186.
These have many unique properties, such as excellent electrical and thermal conductivity because of the layered structure. Meanwhile, because of porous structure, it is possible for the polymer to intercalate to produce EG/polymer nanocomposites. Experimental Section Synthesis of EG. The NG flake of size -50 + 100 BS mesh was dried at 75 °C in a vacuum oven for 5 h to remove the moisture content. It was mixed and saturated with acid consisting of concentrated sulfuric acid and nitric acid in a ratio of 3:1 for 10-15 h to form the graphite-intercalated compound (GIC). The mixture was stirred from time to time to obtain the uniform intercalation of each flake. The mixture was carefully washed and filtrated with water until the pH level of the solution reached 7. After washing, the acid-treated flakes dried at 60 °C in a vacuum oven. GIC was rapidly expanded at temperatures between 800 and 900 °C for 10-20 s in a muffle furnace to form EG. Preparation of Composite. Owing to its good heat resistance, electrical insulation, dimensional stability, flame resistance, and chemical resistance, phenolic resin has been widely used. Novolactype phenolic resin in powder form is used as a matrix binder. EG was used as a major reinforcement. The novolac phenolic resin/EG composites of size 60 × 20 × 4 mm were prepared by the compression molding technique.21 The preform mixture with different weight percentage (wt %) of EG and novolac phenolic resin powder was mixed, and mixture was run in the grinder for 30-60 min to obtain a uniform EG/resin compound. The mixture was pored in the cavity of a three-piece die mold. The mold was placed into a hot press. At temperatures (21) Dhakate, S. R.; Mathur, R. B.; Kakati, B. K.; Dhami, T. L. Int. J. Hydrogen Energy 2007, 32, 4537–4543.
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Figure 2. XRD spectra of (a) NG and (b) EG.
Figure 3. Bulk density and porosity with an increasing resin content in composites.
Figure 5. Variation in the bending strength and modulus with an increasing resin content in composites.
Figure 6. Variation of shore hardness with an increasing resin content in composites.
Figure 4. Electrical conductivity with an increasing resin content in composites.
between 90 and 100 °C, the pressure was slowly applied on the die mold and then kept for isothermal heating at 150 °C. In another course of investigation, additional filler carbon black (CB) was added in the EG/resin compound mixture with keeping the resin content 45 wt % constant. The CB content varies between 2 and 10 wt %, and composites were prepared as reported above. CB is a material composed of essentially elemental carbon in the form of near spherical particles of colloidal sizes, coalesced mainly
into particle aggregates obtained by partial combustion or thermal decomposition of hydrocarbons. The surface is one of the most important characteristics of CB, particularly because the particles are very small and the specific surface area, therefore, is very large. Characterization of Composites. The bulk density of the composite was measured as per the American Society for Testing and Materials (ASTM) standard (ASTM C559). Open porosity was measured by the kerosene pick-up method as per Archimedes’s principle. The morphology of EG and EG-based composites was studied by scanning electron microscopy (SEM, model LEO 440). The EG-resin polymer composites were characterized for mechanical and electrical properties. The bending strength and modulus of composites were measured on an INSTRON testing
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Figure 7. Electrical conductivity with an increasing CB content in composites.
Figure 8. Variation in the bending strength and modulus with an increasing CB content in composites. Figure 10. (a) SEM of EG-resin composites and (b) SEM of an EG-resin composite with CB. Table 1. Comparative Properties of the Bipolar Plate (DOE Target and EG) composites with DOE 2010 composites with 50 wt % EG target 55 wt % EG plus 5 wt % CB
properties bulk density (g cm-3) flexural strength (MPa) modulus (GPa) electrical conductivitya (S/cm) air permeability (MPa) shore hardness
Figure 9. Variation in shore hardness with an increasing CB content in composites.
machine model 4411 as per ASTM. The composites of size 60 × 20 × 4 mm were used for the bending strength measurements. Surface (shore) hardness of the bipolar plate sample was measured by means of a scleroscopic hardness tester obtained from “Coats Machine Tool Co. Ltd., London”. A metal hammer is allowed to strike the surface of the sample, and correspondingly, the hardness is read on the vertical scale to which the ball bounces back. The electrical resistivity of the composite bipolar plate was measured using the four-probe technique, and then the electrical conductivity was calculated. A Kiethley 224 programmable current source was used for providing constant current (I) supply, and voltage drop (V) between two pinpoints with a span of 1.2 cm was measured by Keithley 197A auto-ranging microvolt DMM. Gas permeability of the plates was determined using the permeability measurement apparatus, in which the composite sample was place between two hollow metal discs and sealed with the help of metal screws and rubber o-rings. One face of the sample holder
a
100
1.55 56 8 250
1.50 52 6 285
0.5 50
no leak 0.78 46
no leak 0.78 45
25
Including contact resistance.
is connected to the gas cylinder via a pressure gauge, while water is poured on the other face of the sample. The line pressure of the gas is increased gradually until the bubbling in water above the sample surface is noted. The sample is thus treated as impermeable up to that pressure.
Results and Discussions Figure 1 shows the SEM micrograph of NG and EG. The NG micrograph shows different shape of flake particles (Figure 1a), and Figure 1b is a micrograph of flake particles at a higher magnification. Figure 1c is a micrograph of the EG flake, vermiform in shape with a loose and porous structure that is due to the opening of planer carbon networks wedged at the edge surface of crystallite by surface groups. Figure 1d is a micrograph of the loose structure of EG at a larger magnification. In EG, the expansion process causes destruction of the graphite crystal structure, which results in an expansion in
Expanded Graphite-Based Nanocomposite for PEMFCs
the c direction of about hundred times and an enormous increase in volume. After expansion, the EG emerges as a loose structure and porous material consisting of numerous graphite sheets of nanometers in thickness and micrometers in diameter. This structure endows that EG has a high surface area. Parts a and b of Figure 2 show the X-ray diffraction (XRD) spectra of NG and EG. It is found that, after expansion, the diffraction peak of graphite is broad and shifted to a lower angle. Therefore, after expansion of NG, the value of interlayer spacing increasing from 3.361 to 3.405 Å is due to the expansion of the graphite layer along the c axis as a consequence of oxidation and expansion at high temperatures. Figure 3 shows the bulk density and open porosity with an increase of the resin content in EG-based bipolar plate composites. From Figure 3, it is found that there is direct relationship of the bulk density of the composite with the resin content. The bulk density decreasing with an increase of the resin content is due to the increase in the ultimate weight loss during the curing process. On the other hand, an open porosity trend is different; upon an increasing resin content, porosity decreases up to the 40 wt % of resin, and later, porosity increases up to 60 wt % of resin. The porosity of composite plates decreases with an increasing resin content because, at lower contents of resin, nanosheets cannot be fully wetted by the matrix when the content of resin is lower than the critical value. The lot of EG sheets accumulated and makes it more difficult for the polymer to penetrate into the galleries between or inside these nanosheets, resulting in a higher value of porosity. The increase in the porosity at higher resin content is due to the fact that the polymer used develops the closed porosity during the curing cycle. The lower content of the resin is not sufficient for the surface interaction with each EG, and as a result, there is no uniform percolation in the bulk composites. The in-plane conductivity is largely influenced by the total filler content as well as the filler type and the degree of dispersion in the polymer matrix. Figure 4 shows the in-plane conductivity of EG resin composites with increasing resin contents. With an increase of the resin content, resistivity increases and conductivity decreases. This is due to the increases in the insulating resin phase in the composites. However, as per the target of the DOE3 for conductivity, the resin content between 40 and 50 wt % is sufficient to obtain the conductivity value >100 S/cm and requisite mechanical properties. Figure 5 shows the variation in the bending strength and modulus of composites with an increasing resin content. With an increase of the resin content, the bending strength and modulus increases up to 50 wt % of the resin. Above 50 wt % of resin, the properties are dominated by resin content and it is found that both strength and modulus decrease. At lower content of resin in composites, properties of composites mostly dominated by the EG content, in this case, composites are fragile and, in some cases, can be fractured easily with the application of a small load because the resin content is not sufficient to impregnate each EG particle by resin. However, above 40 wt % of resin, properties are improved. At a higher content of resin above the percolation threshold, resin induces voids in the composites.22 Therefore, the stress concentration effect is due to the voids and also increases while the bonding between EG particles and resin increases. Thus, the importance of the stress concentration increases, which changes the mode of the fracture behavior of composites. An increase of the resin content causes a decrease of the flexural strength. This explanation is supported indirectly by the decrease (22) Kuan, H. C.; Ma, C. C. M.; Chen, K. H.; Chen, S. M. J. Power Sources 2004, 134, 7–17.
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in the value of bulk density (Figure 3). The shore hardness of composite bipolar plates increases with the increase of the resin content as shown in Figure 6. An increase in the resin content in the composite gives a stronger and hard cross-link network and, as a consequence, increases the surface hardness of composites. The lower value of the modulus of EG (3-5 GPa) composites as compared to graphite-based composite (>10 GPa) plates suggests that these plates are more flexible and are able to withstand shock and vibration during mobile operation of the fuel cell stack. The gas permeability test was performed on all the samples, and it shows that the bipolar plates remain impermeable to oxygen gas even at pressures of 0.86 MPa as well as hydrogen. The reason could be the strong bonding between nanoflakes of the EG and polymer matrix. The porosity generated is of a close type because the polymer used has a tendency to develop closed porosity during the curing cycle. By considering this fact, the resin content between 40 and 50 wt % is sufficient to achieve the desired mechanical and electrical properties of composite bipolar plates. Further, in another course of experiments, the resin content is kept constant at 45 wt % and additional filler CB is added to see its effect on the overall properties of the bipolar plate. Figure 7 shows the variation in electrical conductivity with an increase of the CB content. It is found that, upon addition of CB, up to 5 wt % electrical conductivity increases and is 290 S/cm as compared to without CB at 250 S/cm. Upon further increases of CB, conductivity decreases up to 170 S/cm for 10 wt % of CB. It seems that composites with 5 wt % of CB provide higher conductivity than without CB in composites. This indicates that CB particles effectively formed an additional conductive network in composites. This is counter-intuitive, because the intrinsic conductivity of EG is much higher than that of CB and the presence of CB particle should reduce the overall conductivity instead of increasing it. This can be interpreted in terms of a positive synergistic effect between EG and CB upon electrical conductivity.23 The positive synergistic effect can be explained on the basis of an additional conductive pathway formed in the presence of very small CB particles between the EG. The intrinsic conductivity of EG is around 105 S/cm, and the intrinsic conductivity of CB is between 10 and 100 S/cm. The CB particles between the EG act like a bridge for electrons, which leads to the increase in electrical conductivity. The CB can impart electrical conductivity because of its high area and very small particle size. On the other hand, upon an increase of the CB content, the conductivity of the composite bipolar plate decreases because of a percolation threshold.24 The CB is generally found in agglomerated form because of its high surface area, and a higher content causes a decrease in the conduction path of electrons because of the multiple network of CB between EG and CB has a much lower electrical conductivity than EG. This infers that the percolation threshold also depends upon the type of fillers. Figure 8 shows the variation in the bending strength and modulus of the composite with CB content. Upon CB addition, the bending strength decreases up to the 5 wt %, and later, the strength increases up to 10 wt % of CB. The value of the bending strength is less than that without CB-added composites. The decrease in the strength is due to the change in the fracture behavior of composites because CB addition increases particle to particle interactions that are attributed to its high surface area and very small size of CB particles. The same trend is noticed in the case of the shore hardness data. The shore hardness is
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almost the same up to the CB content of 5 wt % and, thereafter, increases up to 10 wt % of CB (Figure 9). Parts a and b of Figure 10 show the microstructure of composite plates observed by SEM. The micrograph shows the layered and network structure of the EG (Figure 10a). As discussed above, the EG of multipore properties have good affinity to polymers. The polymers are absorbed in the pores and galleries of EG, which gives the nanocomposites without distinguishing the individual phase, i.e., EG and resin polymer. However, in the SEM micrographs of EG/resin/CB composites, it clearly shows the CB particles uniformly distributed in the composite and many CB particles are present between the EG layers (Figure 10b). This imparts the high in-plane conductivity between adjacent EG layers of the composites. Table 1 summaries properties of the EG composite with the DOE target of the bipolar plate.3 It is found that all the properties are satisfied between 40 and 50 wt % of resin with only the bulk density of ∼1.55 g/cm3 as compared to 1.9 g/cm3 of graphite-based composites. The addition of CB is helpful in improving the electrical conductivity. This infers that the lower value of density with competitive properties is really helpful in reducing the weight and volume of the fuel cell assembly. However, with a higher content of CB, electrical conductivity decreases but the bending strength and hardness increases.
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of only 1.55 g/cm3 as compared 1.90 g/cm3 of conventional NG-polymer composites. The mechanical properties with bulk density of 1.5-1.55 g/cm3 are competitive as per DOE advanced series of property requirements. The composites with 40-45 wt % of polymer resin are suitable to achieve desired properties of a bipolar plate. The addition 5 wt % of CB is helpful for further improving the electrical conductivity without compromising other properties of the bipolar plate. The lower value of the modulus of EG composite (3-8 GPa) as compared to graphite-based composite (>12 GPa) plates suggest that these plates are more flexible and are able to withstand shock and vibration during mobile operation of the fuel cell stack. These lightweight bipolar plates reduced the volume and weight of the ultimate fuel cell stack and are helpful for improving the performance of the fuel cell. Acknowledgment. The authors are highly grateful to Dr. Vikram Kumar, Director of NPL, for his kind permission to publish the results and to Dr. A. K. Gupta, Head of Engineering Materials Division, for his encouragement throughout this investigation. Also one of the authors (Munu Borah, Department of Energy, Tezpur University) is highly grateful to the Director of NPL for giving her the opportunity to work in NPL for her 1 year M. Tech project. EF800135F
Conclusions In this investigation, EG-based polymer nanocomposites are developed as bipolar plate for the PEM fuel cell with a density
(23) Clingerman, M. L.; Weber, E. H.; King, J. A.; Schulz, K. H. Polym. Compos. 2002, 23, 911–924. (24) Yi, X. S.; Wu, G.; Ma, D. J. Appl. Polym. Sci. 1998, 67, 131–138.
