Effect of Resin Matrix Precursor on the Properties of Graphite

Energy & Fuels 2007, 21, 1681-1687

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Effect of Resin Matrix Precursor on the Properties of Graphite Composite Bipolar Plate for PEM Fuel Cell Biraj Kumar Kakati* and Dhanapati Deka Department of Energy, Tezpur UniVersity, Assam, Napaam, Dist: Sonitpur, Tezpur, 784028, India ReceiVed August 3, 2006. ReVised Manuscript ReceiVed January 30, 2007

A bipolar plate is one of the prime components of proton exchange membrane fuel cells, and advanced composite bipolar plates where a polymer is used as a binder and graphite is used as a major reinforcement are prepared by a compression molding technique. Study on the effect of different types of resin matrix on the properties of composite bipolar plates, such as bulk density, porosity, bulk conductivity, hardness, flexural strength, and so forth, shows that composites with different resin matrix precursors exhibit different physicomechanical properties. Moreover, in the case of resole- and novolak-based composites, a single-cell performance analysis shows variation in output power density. In this study, a novel concept of using triple continuous structure to provide graphite polymer blends with high electrical conductivity, high shore hardness, high flexural strength, less porosity, and low density has been proposed, and a study on the effect of different types of resins (epoxy resin, vinyl ester resin, and resole- and novolak-type phenolic resins) on the properties and performance of entire bipolar plates reveals that novolak-type powdered phenolic resin gives better mechanical properties than the other three types of resin. However, the resole-type phenolic resin-based composite has the highest electrical conductivity among the other three resin-based composites.

1. Introduction Among all the fuel cells developed so far, the polymer electrolyte membrane (PEM) fuel cell (FC) is one of the most promising power sources for residential and automotive applications due to its attractive features such as high power density, lower operating temperature, convenient fuel supply, longer life span, modular shape, and so forth.1,2 However, in practice, voltage losses or overpotential exists during the operation of PEM fuel cells and reduces the efficiency advantage. Three major types of losses are observed in PEM fuel cells:3(i) activation lossessslow reaction kinetics in the cathode catalyst layer, (ii) mass transport (reactant gas and protons) lossessfailure to transport sufficient reactant to the electrodes, and (iii) ohmic lossesslow protonic conduction through the thickness of the polymer electrolyte and also the bipolar plate. A bipolar plate is one of the key components (Figure 1) of fuel cells which consumes around 38% of the total costs and may consume up to 80% of the total weight of the fuel cell stacks. So, one of the biggest challenges for the development of PEM fuel cells is the reduction of both cost and weight of the bipolar plate without compromising the performance and efficiency.4,5 Bipolar plates should have the following properties to use in the fuel cell environment: (i) less porosity with high mechanical strength, (ii) high electronic conductivity (bulk) with low contact resistance, (iii) high thermal conductivity, (iv) * Corresponding author tel.: +919864182150; fax: +913712267006; e-mail: [email protected]. (1) Srinivasan, S. J. Electrochem. Soc. 1989, 136, 41c. (2) Lee, S. J.; Mukherjee, S. Electrochim Acta 1998, 43, 3693. (3) Larmine, J.; Dicks. A. Fuel Cell System Explained; John Wiley & Sons: New York, 2000. (4) Bar-On, I.; Kirchain, R.; Roth, R. J. Power Sources 2002, 109, 7175. (5) Kaiser, R.; Fritz, H. G.; Eisenbach, C. D. Proceedings of the 18th Stuttgarter Kunststoffkolloquium, March 19-20, 2003; Sprint Druck: Stuttgart, 2003; p 3v4.

Figure 1. Schematic diagram of PEM fuel cell stack showing use of bipolar plates.

integrated and uniformly distributed cooling channels, and (v) high corrosion resistance to the fuel cell environment. To achieve the above properties, different polymer matrix precursors with different reinforcing constituents are used for the development of composite bipolar plates. From the experimental analysis, it is found that the bipolar plates developed have superior electrical, mechanical, and thermal properties with a low porosity, which are the prime requirements of bipolar plates. 2. Experimental 2.1. Development of Composite Bipolar Plates. Nowadays, several types of materials are being used for the development of bipolar plates for PEM fuel cells8 like metal sheets, polymer-coated metal sheets, electrographite, flexible graphite, carbon-carbon (6) Mehta, V.; Cooper, J. S. J. Power Sources 2003, 114, 32-53. (7) Middleman, E.; Kout, W.; Vogelcar, B.; Lenssen, J.; de Wall, E. J. Power Sources 2003, 118, 44-46. (8) Hermann, A.; Chaudhury, T.; Spagnol, P. Int. J. Hydrogen Energy 2005, 30, 1297-1302.

10.1021/ef0603582 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007

1682 Energy & Fuels, Vol. 21, No. 3, 2007 composites, graphite-polymer composites, and so forth. For the development of composite bipolar plates, researchers are using different kinds of reinforcing constituents with different polymer matrixes so as to achieve the optimum properties. So as to achieve the required properties, the bipolar plates were developed by compression moulding technique using a triple continuous network of natural graphite (NG), carbon black (CB; acetylene black), and carbon fiber (CF; T-300) with epoxy resin, vinyl ester resin, and resole- and novolak-type phenolic resins. Except for the novolak-type phenolic resin-based composite, preforms were developed for the resin-based composites for compression moulding. For epoxy-based composites, required amounts of the three components were mixed well with the help of a grinder, and then it was mixed with a resin matrix. During mixing, a stipulated amount (14% of resin) of hardener (Mat Hard) was used with the resin. The mixer was then allowed to dry at a temperature of around 70 °C.9 The completely dried mixture was again ground to powder form and used for compression moulding. The same process was also done in the case of vinyl ester resinbased composites, with the only difference being that an accelerator (1.5% cobalt octate) and a catalyst (1.5% methyl ethyl ketone peroxide, MEKP) were used instead of a hardener.10 Whereas, in the case of the resole-type phenolic resin-based composite, there is no need for either a hardener or an accelerator or a catalyst. The curing temperature for epoxy and vinyl ester resins is 85 °C, and they are post-cured at temperatures of 150 and 180 °C. Whereas, for the resole-type phenolic resin, the curing temperature was almost 95 °C, and it was post-cured at 150 °C. Again, in the case of the novolak-type phenolic resin-based composite, the uniform mixture of powder resin with the other reinforcing constituents was cured at a temperature of 105 °C and post-cured at a temperature of 180 °C.11 2.2. Characterization of Composites. All of the composite samples with different compositions were characterized for bulk density, electrical resistivity, shore hardness, and flexural strength. Some of the samples were also used to measure porosity as per ASTM standards (ASTM D792). To measure porosity, kerosene density and bulk density of the samples were first measured. The bulk density (Bd) and kerosene density (Kd) of the samples were measured with the help of a highly precise digital balance (model ME 40290) applying the Archimedes principle. Porosity of the sample can be calculated with the help of the following formula: Porosity ) (1 - Bd /Kd) × 100%

Kakati and Deka

Figure 2. Schematic diagram of electrical conductivity measurement setup.

DC886). It is a dynamic indentation hardness test that drops a diamond-tipped hammer vertically from a fixed height onto the surface of the material under test. The height of the rebound of the hammer can be noted with the help of the scale attached to the apparatus and is a measure of the hardness of the material. Flexural strength was measured with the help of the three-point method using an Instron Universal testing machine (model 4411, series IX automated material testing system 1.38) as per ASTM standards (ASTM D790). The span length for the testing was kept to 10 cm, and samples of 50 × 10 × 5 mm3 were used, and reported values were averages of a minimum of four to eight test samples. The crosshead speed was maintained at 0.5 mm/min. Microstructures of a few composite samples were studied by optical microscopy (Zeiss, model MC80 DX). Microstructures of a few composite samples were studied by scanning electron microscopy (SEM; model LEO 440) as well as optical microscopy. For single-cell i-V performance analysis, bipolar plates of size (10 × 10 × 0.4 cm3) were developed.

3. Results and Analysis 3.1. Effect of Resin Content on the Properties of GraphiteResin Composites. So as to determine the optimum properties with optimum resin content and other optimum values, samples were prepared first with resole-type phenolic resin. From Figure 3, it can be summarized that the bulk density of the composite

The electrical conductivity was measured by the conventional fourprobe method at a constant current supply (100 mA). First, the electrical resistivity of the composite bipolar plate samples was measured using the four-probe technique. A Kiethley 224 programmable current source was used for providing a constant current (i) supply of 100 mA, while the voltage drop (V) in between two pin points with a span of 1.2 cm was measured by a Keithley 197A autoranging microvolt meter DMM. If l and b are the length and breadth of the contact surface to the current probe and d is the distance between the probes, then the conductivity of the sample is as follows: Y ) (i × d)/(V × l × b) The schematic diagram of the experimental setup for the determination of electrical conductivity is shown in Figure 2. The main advantage of this method is that it eliminates contact resistance. The shore hardness of the samples was measured with the help of a scleroscopic hardness tester as per ASTM standards (ASTM (9) Technical Data Sheet, System MAT-RES MR-691 with MAT-HARD MH-758; MAT-SOL Industries, Dehli, India. (10) Technical Data Sheet, Mechster 5310 (N); Mechamo Industries, Mumbai 400013, India. (11) Technical Data Sheet, Phenolic Resin 9624; Pheno Organic Limited: New Dehli 110033, India.

Figure 3. Variation in density and porosity with increasing resin volume fraction.

decreases with an increase in volume fraction of the phenolic resin. This is because resin has a lower density (≈ 1.2 g cm-3 of cured resin) and is replaced by NG of density 2.2 g cm-3. On the other hand, the porosity of the samples increases up to 50% volume fraction of the resin matrix and starts decreasing thereafter. The figure concludes that porosity of the bipolar plate is less at a resin volume fraction of about 35%, but the density is significantly high. Keeping in mind the acceptable density

Effect of Resin Matrix Precursor

as well as porosity of bipolar plates, later bipolar plate samples are developed with a 35% resin volume fraction. Figure 4 shows the variation in hardness and conductivity of composite plates with an increasing resin volume fraction. As the resin volume fraction was increased, the hardness of the

Figure 4. Variation in hardness and conductivity with increasing resin volume fraction.

composite bipolar plate was also found to be increasing almost smoothly. This is due to the higher shore hardness of cured resin than that of NG. However, conductivity of the composite bipolar plate showed a reverse trend with an increasing resin volume fraction. Due to very high resistivity of the resin matrix, the conductivity of the sample decreased sharply as the resin volume fraction was increased from 20 to 30% and decreased slowly with an increase of the resin content from 40% to 60%. From Figures 3 and 4, the optimum volume fraction of the resin for the next series of experiments was taken as 35% for all the series of experiments, and bipolar plates were developed at proper conditions as explained in section 2.1. 3.2. Effect of Matrix Precursors on Densities of Composite Bipolar Plates. The optimum properties achieved by the previous series of samples were not found to be satisfactory enough for application as bipolar plates in PEMFCs. So as to enhance the overall properties of bipolar plates, bipolar plates were developed with different reinforcing components and the effect of different polymer matrixes was studied. The preforms

Figure 5. Variation in density and comparison among different resinbased composites.

of the composites with compositions of CB, CF, and NG with resin were compacted by a compression moulding technique at different curing and post-curing temperatures with the application of a moulding pressure of about ∼100 kg cm-2. The densities of bipolar plates with different polymer matrix precursors were found to be different as different resin matrixes

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have different curing mechanisms. The highest density of a bipolar plate without CB is found to be in the range 1.8101.920 g cm-3 (Figure 5). 3.3. Effect of Matrix Precursors on Conductivity of Composite Bipolar Plates. The resin matrix does not only play the binder role but also plays a significant role in conductivity also.12 The effect of the same on conductivity was studied with different volume percentages of CB. Figure 6 interprets the effect of CB on the conductivity of the composite bipolar plates with different resin matrix precursors. From the figure, it is clearly visible that the highest conductivity can be achieved by resole-type phenolic resin (∼334 S cm-1) followed by novolaktype phenolic resin (∼263 S cm-1). The dotted line in the figure interprets the peak percolation of CB for each resin system, which is 20% for epoxy resin and 25% for the other three resin systems. The highest conductivity of bipolar plates with resoletype phenolic resin samples was found due to the large number of polar OH groups present in the resole-type phenolic resin,12 which helps in electronic conductivity and enhances the bulk conductivity of the bipolar plate.

Figure 6. Variation in conductivity and comparison among different resin-based composites.

3.4. Effect of Matrix Precursors on Shore Hardness of Composite Bipolar Plates. A plot of the shore hardness of the composite bipolar plates with different volume fractions of CB shows an almost linear trend with different types of resin matrixes (Figure 7). The maximum shore hardness was found to be 75 for novolak-type phenolic resin-based composite bipolar plates. This can be explained by the better gap-filling properties of novolak-type phenolic resins than those of the other three resin matrix precursors. At higher concentrations of CB, the differences between the shore hardness of bipolar plates with different resins was found to be negligible because, at higher concentrations, CB played a dominating role over other reinforcing components. 3.5. Effect of Matrix Precursors on Flexural Properties of Composite Bipolar Plates. The effect of CB concentration with different polymer matrixes exhibited almost the same trend as with electrical conductivity. From Figure 8, it can be summarized that the flexural strength of the novolak-type phenolic resin-based composite is highest. The higher shore hardness for the same is due to the better gap-filling properties and better crosslinking of novolak-type phenolic resin. The same (12) Zhang, Jie; Zou, Yan-wen; He, Jun. J. of Zhejiang UniV, Sci. 2005 6A(10): 1080-1083.

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Figure 9. SEM micrograph of epoxy resin-based composite bipolar plate. Figure 7. Variation in shore hardness and comparison among different resin matrix precursor-based composite bipolar plates.

Figure 10. SEM micrograph of vinyl ester resin-based composite bipolar plate.

Figure 8. Effect of resin matrix precursors on the flexural strength composite bipolar plates.

properties are also responsible for enhancing the bulk electrical conductivity. The optimum value of flexural strength for novolak-type phenolic resin was nearly 75 MPa. The flexural strength of the composite plates decreases above a 25% CB concentration due to the fact that properties of the bipolar plates change from ductile to brittle. The rate of decrease in flexural strength above the peak value was found to be more in the case of novolak-type phenolic resin than the other three resin-based composites. Whereas, the peak percolation was found to be 20% for epoxy resin-based composite bipolar plates. 3.6. SEM Micrograph Analysis. The SEM micrographs of the composite bipolar plates (cross-section) are shown in Figure 9-12. It is to be noted that Figure 12 is at a magnification of 2000×, while the other figures are at a magnification of 1000×. From the figures, it is observed that the CB particles are trying to accommodate the intergranular space between various flakes of natural graphite. Moreover, they act like an electrical bridge between graphite-graphite, fiber-fiber, or graphite-fiber contacts, which helps in increasing the bulk conductivity of the composite bipolar plate. In the case of Figure 12 (of novolaktype phenolic resin), the distribution of CB particles is found to be more uniform than in the cases of other composite bipolar plates. Also, the CB particles highlighted show the better gapfilling properties of novolak-type phenolic resin. Due to this

Figure 11. SEM micrograph of resole-type phenolic resin-based composite bipolar plate.

property, the mechanical properties are found to be more pronounced for composite bipolar plates with novolak-type phenolic resin. But due to the more polar OH group present in resole-type phenolic resin, the electrical conductivity was found to be more than that of the other three composites. From the SEM micrographs, it is observed that the natural graphite flakes are aligned in a direction parallel to the plane of the plates, which helps in increasing the electrical conductivity of the plates.13 3.7. i-V Analysis for Resole- and NoWolak-Type Phenolic Resin-Based Composites. The flow channels were machined (13) Mighri, F.; Huneault, M. A.; Champagne, M. F. Polym. Eng. Sci. 2004, 44 (9), 1755-1765.

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Figure 15. Network formation and bonding behavior of epoxy resin.

Figure 12. SEM micrograph of novolak-type phenolic resin-based composite bipolar plate.

Figure 13. i-V performance of resole-type phenolic resin-based composites. Figure 16. Bonding behavior and network formation of vinyl ester resin.

Figure 17. Network formation and bonding behavior of resole-type phenolic resin.

Figure 14. i-V performance of novolak-type phenolic resin-based composites.

on the bipolar plates using a numerical control machine tool of Beijing Carving Technological Co. Ltd. The single-cell i-V performance of a 100 cm2 composite bipolar plate with 50 cm2 active area was tested for samples with resole-type and novolaktype phenolic resin-based composite bipolar plates, as they had better electrical as well as mechanical properties in comparison to the other resin-based bipolar plates. For resole-type phenolic resin-based bipolar plates, the peak power density was found to be around 480 mA cm-2, while it was 400 mA cm-2 for

novolak-based composites. This can be explained with the help of a greater number of polar atomic OH groups being present in the resole-type phenolic resin than in the novolak-type phenolic resin. But the responses to voltage as well as power density with the current density are found to be more uniform in the case of novolak-type phenolic resin composites. From Figures 13 and 14, it is observed that the concentration polarization starts at 1400 mA cm-2 for resole-type phenolic resin composites, while the same is found to be more than 1400 mA cm-2 for novolak-type phenolic resin-based composites. This is due to the greater mechanical properties as well as lesser porosity of the samples with novolak-based composite bipolar plates. 4. Curing Behavior of Resins The curing behavior of the resins helps in analyzing the electrical as well as mechanical properties. The curing of epoxy

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Figure 18. Network formation and bonding behavior of novolak-type phenolic resin.

resin takes place with the help of a hardener, while vinyl ester resin needs an accelerator as well as a catalyst for the same. Figure 15 shows the curing mechanism of epoxy resin (MATRES MR-691) in the presence of a hardener (MAT HARD MH-758).9 Hardeners are generally some anhydrides such as phthalic anhydride which first react with free hydroxyl groups on the chain, freeing a carboxylic group to react with another chain hydroxyl group or epoxide group, and help in crosslinking. Sometimes, polyfunctional amines are also used as curing agents. The curing behavior with epoxy resin in the presence of a polyfunctional amine hardener is shown below. In the case of vinyl ester resin, it cures with the help of an accelerator (cobalt octate) and a catalyst (MEKP).10 The schematic diagram of vinyl ester resin is shown in Figure 16a, while the curing mechanism is shown in Figure 16b. From the figure, it is observed that different vinyl ester molecules are bonded with each other with the help of styrene (S) monomers in the presence of an accelerator and catalyst. Due to the low viscosity of vinyl ester resin, it is mainly used as binder matrix for resin transfer moulding. But the thermogravimetric analysis (TGA) of the same exhibits that it is more degradable than novolak-type phenolic resin under heat treatment.

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The curing of resole- and novolak-type phenolic resin is found to be almost the same. Novolaks are permanently soluble and fusible, and a curing agent must be used with them. Common curing agents are hexamethylene tetramine and paraformaldehyde. From the curing behavior of the resole-type phenolic resin (Figure 17) and novolak-type phenolic resin (Figure 18b), it is clearly visible that the crosslinking is better in the case of novolak-type phenolic resin, and due to the higher number of polar OH groups present in cured resole-type phenolic resin, it has a high electrical conductivity. The TGA analysis of the composite bipolar plates were done for each type of resin matrix precursor (Figure 19). The TGA analysis concludes that the thermal degradation is higher for epoxy resin (almost 28.36%), while it is 10.07% for novolakbased composite bipolar plates. The large numbers of OH groups present and poor cross-linking properties are solely responsible for the higher thermal degradation of epoxy-based composites. Again, due to the higher thermal stability (500 °C) of novolakbased composites, it can be used for other types of fuel cells also (like alkaline FCs, AFCs). The temperatures were found to be 350, 400, and 450 °C for epoxy, vinyl ester, and resolebased composites, respectively. 5. Conclusion and Discussions When porosity is compared with density and also hardness with conductivity and an analysis is performed, it is found that the optimum resin concentration lies between 30% and 35%. In the case of conductivity, it can be concluded that resol-type phenolic resin gives a better result than the other three resin matrix precursors, provided that the optimum concentration

Figure 19. (a) TGA characteristic of epoxy resin-based composite bipolar plate. (b) TGA characteristic of vinyl ester resin-based composite bipolar plate. (c) TGA characteristic of resole-type phenolic resin-based composite bipolar plate. (d) TGA characteristic of novolak-type phenolic resin-based composite bipolar plate.

Effect of Resin Matrix Precursor

value is 20% CB for vinyl ester resin and 25% for the other three resin systems. The best ever mechanical properties can be achieved for novolak-type phenolic resin precursors. Again, the SEM analysis of the composites shows that the bonding in fiber-graphite, fiber-carbon black, or fiber-resin is better in the case of novolak-type phenolic resin, which may lead to better mechanical properties. It can be also explained with the help of the bonding behavior of resins. Moreover, novolak-type phenolic resin has a high char yield and good strength at high temperatures,14and also, from thermogravimetric analysis, it is clearly observed that novolak-type phenolic resin-based bipolar

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plates can be used at relatively higher temperatures (∼500 °C) also. From the above studies, it can be concluded that novolaktype phenolic resin-based composites give the overall best properties and can be used for other types of fuel cells also, like AFCs, phosphoric acid FCs, and so forth. EF0603582

(14) Yan, Y.; Shi, X.; liu, J.; Zhao, T.; Yu. Y. J. Appl. Polym. Sci. 2002, 83, 1651-1657.