Energy & Fuels 2008, 22, 3351–3354
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Performance of a Polymer Electrolyte Membrane Fuel Cell with Fabricated Carbon Fiber Cloth Electrode Yuan-Kai Liao,* Tse-Hao Ko, and Ching-Han Liu Department of Material Science and Engineering, Feng Chia UniVersity, Taichung, Taiwan ReceiVed April 3, 2008. ReVised Manuscript ReceiVed May 24, 2008
Thanks to its excellent conductivity, gas permeability, and corrosion resistance, carbon fiber cloth is widely used in many applications. The carbon electrodes used in polymer electrolyte membrane fuel cells consist of gas diffusion and catalyst layers. While most gas diffusion layers are made from carbon fiber paper or carbon fiber cloth, the use of conventional carbon fiber cloth to produce fuel cell electrodes may result in peeling fibers and material that is excessively soft and difficult to assemble. This study employs PAN-based carbon fiber cloth and uses phenolic resin to improve the structure and characteristics of the gas diffusion layer (GDL). This study focused on the material used in the GDL. Results indicate that an improved carbon fiber cloth can eliminate assembly difficulties. Investigation of the effect of different resin contents revealed that GDL carbon fiber cloth with a relatively low resin content exhibits good through-plane resistance and thus offers good cell performance.
1. Introduction The carbon electrodes used in fuel cells are composed of gas diffusion, catalyst, and microlayers. Because carbon fiber cloth possesses directionality and because it may lose fibers during assembly, most studies currently use carbon fiber paper to fabricate GDLs.1–4 This study developed a new carbon fiber cloth production process. Carbon fiber cloth was impregnated with phenolic resin in order to bond the fibers. Large amounts of phenolic resin are used in the production of carbon/carbon composite materials, and yield carbon with a graphite structure after high-temperature carbonization. This study attempted to improve conventional methods of preparing carbon/carbon composites by adding small quantities of phenolic resin as a means of improving the structure of the carbon fiber cloth. Past studies of carbon/carbon composites indicated that the higher the carbonization temperature of carbon fiber cloth, the weaker the strength of the interface between resin and cloth.5 It was therefore thought that the use of carbon fiber cloth with a 1000 °C carbonization temperature to fabricate an improved GDL could retain some of the material’s original softness. The study relied on control of pressure and temperature at the time of hot pressing to determine the carbon fiber cloth’s thickness, and a continuous carbonization process was developed to simulate industrial production; this process has been patented. Past research has shown that the carbon fiber graphitization temperature has considerable influence on the resulting microstructure and characteristics. The conductivity of carbon fiber rises significantly as the graphitization temperature is increased, and the reduction in surface functional groups brings about partial * To whom correspondence should be addressed. TEL: +886-4-24517250 ext. 5303. FAX: +886-4-24518401. E-mail:
[email protected]. (1) Litster, S.; McLean, G. J. Power Sources 2004, 130, 61–67. (2) Xuejun, Z.; Zengmin, S. Fuel 2003, 81, 2199–2201. (3) Williams, M. V.; Begg, E.; Bonville, L.; Kunz, H. R.; Fenton, J. M. J. Electrochem. Soc. 2004, 8, 1173–1180. (4) Scherer, G. G. Solid State Ionics 1997, 94, 249–257. (5) Ko, T. H. Polym. Compos. 1993, 14, 247–256.
surface hydrophobicity.6–9 This study examined the characteristics of improved carbon fiber cloth when used in fuel cell GDLs. The improved carbon fiber cloth and assembled fuel cells were tested to investigate the effect of resin content on performance and compare performance with that of commercial products. 2. Experiment Carbon fiber cloth was used as the raw material in the production of improved carbon fiber cloth. The PAN-based carbon fiber cloth was manufactured by Challenge Carbon Technology Co., Ltd., and had an original thickness of 0.7 mm. This carbon fiber cloth had been produced via high-temperature carbonization at 1000 °C. Type PF-650 phenolic resin was provided by Chang Chun Plastics Co., Ltd. After being cut to an appropriate size, the carbon fiber cloth was impregnated with various ratios of phenolic resin. The phenolic resin to carbon fiber ratios were respectively 0.51 wt %, 2.69 wt %, 5.06 wt %, and 7.08 wt %. After impregnation, the test pieces were first dried for 15 min at 70 °C and then subjected to hot pressing in a hot press machine. Hot pressing was performed for 15 min at 170 °C and a pressure of 10 kg/cm2 to yield semifinished carbon fiber cloth. The continuous carbon fabric carbonization process employed in this study used a high-temperature tube furnace. The furnace’s central heating region had a length of 10 cm and a temperature of 1,300 °C; the temperature gradually decreased with distance from the center. The ceramic furnace tube had a length of 100 cm; continuous carbonization time totaled 20 min. The carbonized carbon fiber cloth was washed thoroughly with acetone and deionized water to remove surface residue and dried in preparation for subsequent procedures. An Accupyc 1330 pycnometer was used to determine the density of the test pieces. The ASTM D570 standard test method for water (6) Li, T.; Zheng, X. Carbon 1995, 33, 469–472. (7) Chae, H. G.; Minus, M. L.; Rasheed, A.; Kumar, S. Polymer 2007, 48, 3781–3789. (8) Rahaman, M. S. A.; Ismail, A. F.; Mustafa, A. Polym. Degrad. Stab. 2007, 92, 1421–1432. (9) Kuo, H. H.; Chern Lin, J. H.; Ju, C. P. Carbon 2005, 43, 229–239.
10.1021/ef800235n CCC: $40.75 2008 American Chemical Society Published on Web 07/04/2008
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Table 1. Properties of PAN-Based Carbon Papers basis water resin weight uptake porosity hydrophobicity content thickness density (%) (deg) (wt %) (mm) (g · cm-3) (g · cm-2) (%) 0.51 2.69 5.06 7.08
0.49 0.50 0.49 0.48
1.6664 1.7125 1.6179 1.7385
218.2 240.3 242.6 247.9
116.5 114 101.9 93.1
82.3 84.6 83.4 83.6
124.3 126.7 125.6 126.5
absorption was used to test water content. In-plane resistivity was measured using a Loresta GP MCP-T600 m following the standard JIS K 7194 test method. The test pieces were assembled as single cells in order to study the characteristics of the improved carbon fiber cloth GDLs. The anodes and cathodes both consisted of carbon fiber cloth, and the proton exchange membrane consisted of a solid catalyst-coated electrolyte membrane (Gore three-layer CCM MEA with a thickness of 35 µm; catalyst deposition: cathode, 0.6 mg Pt/cm2; anode, 0.45 mg Pt-alloy/cm2) in the form of a membrane electrode assembly. The bipolar plate consisted of a flexible, grooved graphite plate. Final assembly of each single cell fuel cell was performed using a stainless steel plate and a PTFE gasket. All test cells had an area of 25 cm2 and were tested using dry gas passed through a humidifier at a temperature of 40 °C. Gas (H2) at the anode end had a velocity of 200 cc/min; gas (O2) at the cathode end likewise had a velocity of 200 cc/min. Gas pressure was 1 kg/cm2 in both cases. We focused on the material used for the GDL in this study, and plan to study spraying or coating a microlayer and PTFE on the GDL in the future. We did not use a microlayer on the carbon fiber cloth in this study, nor did we bond the CCM and carbon fiber cloth together by hot-pressing, but only used 40 kgf/cm torsion to ensure close contact between the layers.
3. Results and Discussion Table 1 shows the characteristics of carbon fiber cloth GDLs made from carbon fiber cloth impregnated with different ratios of phenolic resin. Following impregnation and hot pressing, the carbon fiber cloth pieces had phenolic resin ratios of 0.51 wt %, 2.69 wt %, 5.06 wt %, and 7.08 wt % respectively. The original thickness of the carbon fiber cloth was 0.7 mm, and thickness after impregnation and hot pressing was roughly 0.5 mm. This reduction in thickness was mainly due to mutual adhesion of the fibers caused by the resin. Past research has indicated that the thickness of carbon fiber cloth depends on the fiber diameter, yarn fineness, and the weave structure.10,11 Carbon fiber cloth woven from fibers with different diameters may still have the same thickness when graphitization temperature is different, and this phenomenon occurred with the improved carbon fiber cloth. Because the weave of carbon fiber cloth determines thickness, different resin contents do not result in thickness changes. Table 1 also shows the base weights of carbon fiber cloth GDLs with different resin ratios. It can be seen that the base weight rises as resin content increases. While the carbon fiber cloth and phenolic resin had weight loss during the carbonization process,12,13 the base weight was closely correlated only with the amount of impregnated resin. In contrast with the Archimedes method, this pycnometer determines the true density of an object by measuring the pressure change when helium is displaced from a sealed chamber. Past research has shown that changes in the density of carbon fiber are mainly caused by release of noncarbon (10) Ko, T. H. J. Polym. Eng. Sci. 1991, 31, 1618–1626. (11) Ko, T. H.; Liao, Y. K.; Liu, C. H. New Carbon Mater. 2007, 22, 97–101. (12) Ko, T. H.; Huang, L. C. J. Appl. Polym. Sci. 1998, 70, 2409–2415. (13) Ko, T. H.; Hone, K. W. SAMPE J. 1992, 28, 17–23.
Figure 1. Variation of resistivity of carbon fiber cloth with different resin content: (b) in-plane resistivity; (9) through-plane resistivity.
elements at the graphitization temperature.14 The carbon fiber cloth GDLs tested in this study were prepared with the same graphitization temperature (1300 °C) and had similar graphite structure. Because the resin causes adhesion only between the carbon fibers, it has little effect on true density. The relationship between the porosity and phenolic resin ratio is shown in table 1. Porosity is calculated by true density and academically density. With the same graphitization temperature, the true density does not show great variation and therefore the porosity maintain at the approach value. Carbon fiber cloth GDLs have a porous structure and adsorb large amounts of water. Increasing the amount of added resin reduces the volume of pores in the cloth, which lessens the amount of adsorbed water. The result of the analysis of the hydrophobicity of carbon fiber cloth is shown in Table 1. Varied carbon fiber cloths used in this study were basic gas diffusion base materials and without Teflon coating treatment. The chief reason to determine the hydrophobic properties is the functional group on the surface of the carbon fiber. With the same graphitization temperature, the functional groups on the surface of carbon fiber are nearly at 1300 °C. Directional resistivity of the carbon fiber cloth was measured both in-plane and through-plane; resistivity measurement results are shown in Figure 1. Because electrons are transmitted in the direction of the carbon fibers,15 and since increasing resin content causes increased contact between individual fibers, forming a tighter configuration, higher resin content tends to reduce resistivity. Through-plane resistivity is typically different. Resistivity data clearly indicates that through-plane resistivity trends are opposite to in-plane trends. Because electrons are largely transmitted along the carbon fibers, conductivity is significantly lower perpendicular to the surface. Moreover, the greater the amount of resin adhering to the surface of the carbon fibers, the transmission of electrons from fiber to fiber is hindered, so resistivity is greater in the through-plane direction. Since we wished to investigate the fuel cell performance resulting from choice of materials in this study, the test GDLs did not receive hydrophobic treatment and did not have microlayers. Figure 2a,b shows testing results for single fuel cells. Improved carbon fiber cloth with a 0.51% resin content yielded significantly better performance than carbon cloth with (14) Watt, W.; Green, J. Confernce On Carbon Fibers, their Composites and Applications; Plastic Institute: London, 1971; p 23. (15) Ko, T. H.; Huang, L. C. J. Polym. Sci. 1998, 70, 2409–2415.
Polymer Electrolyte Membrane Fuel Cell
Figure 2. (a, b) Effect of resin contents of carbon fiber cloth on current density of PEMFC: (b) current density and power density of carbon fiber cloth with 0.51 wt % resin content; (9) current density and power density of carbon fiber cloth with 5.06 wt % resin content.
a 5.06% resin content. Past research suggests that fuel cell polarization curves can be analyzed in terms of three regions.16 The first of these is the activation polarization region: Because the catalyst coated membranes (CCMs) used in the fuel cells were all produced by the Gore Company, we believed that the initial reaction performance should be similar. The next part is the ohmic polarization region: Due to differences in the conductivity of the carbon cloth, carbon cloth with relatively low resistivity should perform better in the ohmic polarization region. But because the difference in in-plane resistivity between the two types of carbon fiber cloth was not great, the results could largely be attributed to the difference in through-plane resistance. The final part is the mass transfer polarization region: There is no significant mass transfer effect when GDLs are made using improved carbon fiber cloth, so the polarization curve does not decrease rapidly. This provides ample margin for the fuel cell to operate at low voltage and high current density. Figure 3a,b compares the performance of improved carbon fiber cloth GDLs with that of commercial GDLs. The commercial GDLs employed E-TEK B1/A carbon fiber cloth and Toray TGPH120 carbon fiber paper respectively. Both commercial products were used as-is, were not impregnated with (16) Larminie, J.; Dicks, A. Fuel cell systems explained; John Wiley & Sons: West Sussex, 1999; p 37.
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Figure 3. (a, b) Performance of different gas diffusion layers in PEMFCs: (b) current density and power density of E-TEK B1/A; (9) current density and power density of Toray TGPH-120; (2) current density and power density of carbon fiber cloth with 0.51 wt % resin content.
PTFE, nor were microporous layers added. Three-layer MEAs produced by Gore were used as the CCMs, which allowed the performance of the materials to be compared on a level basis. The figure shows that the GDLs employing E-TEK B1/A carbon fiber cloth and Toray TGPH120 carbon fiber paper yielded the very similar current densities of 1201 mA/cm2 and 1156 mA/ cm2 respectively when the load was 0.4 V. The improved carbon fiber cloth yielding the best performance contained 0.51 wt % phenolic resin; its current density of 2207 mA/cm2 at a load of 0.4 V was higher than that of fuel cells with GDLs employing commercial carbon fiber cloth and carbon fiber paper. Thickness of commercial products E-TEK B1/A and Toray TGPH 120 are 0.34 mm and 0.39 mm. The electric conductivity is base on the basis weight of carbon fabric. The thickness of carbon fiber clothes we used in this study are approach to 0.5 mm. We assume the fabricated carbon fiber clothes will have better electric conductivity. Furthermore, the gasket used in the single cell is 0.4 mm and applicable pressure is help for the cell performance. In this study, thickness of gasket may not fit to the commercial gas diffusion layer E-TEK B1/A and Toray TGPH. The commercial GDLs used in fuel cell performance testing were used as-is, and this study did not investigate water management or service life, but only performed very basic
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performance testing of different GDL materials. Subsequent impregnation of the optimal material with PTFE and addition of a microlayer should significantly improve fuel cell performance. 4. Conclusions Comparison of improved carbon fiber cloth GDLs with conventional carbon cloth GDLs indicates that the former offer the advantages of easy assembly and relatively high performance. The thickness of the improved carbon fiber cloth was controlled by the weave structure of the original cloth and did not change significantly with resin content. Due to the identical final carbonization temperatures, the density of the carbon fiber cloth was roughly constant. Increasing resin content reduced
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internal space in the improved carbon fiber cloth, which reduced water content. The effect of different resin contents in improved carbon fiber cloth is chiefly expressed in terms of through-plane resistance. As the phenolic resin content increased, the throughplane resistivity of the improved carbon fiber cloth increased and the in-plane resistivity decreased, but the decrease in the in-plane resistivity was proportionally far less than the increase in through-plane resistivity. Carbon fiber cloth containing relatively low resin content possesses low through-plane resistance, which yields optimal performance, and this performance exceeds that of ordinary carbon fiber cloth and carbon fiber paper GDLs. EF800235N
