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Energy & Fuels 2008, 22, 4092–4097
Effects of Fabricated Gas Diffusion Layers with Different Reinforce Materials in Proton Exchange Membrane Fuel Cell (PEMFC) Tse-Hao Ko, Yuan-Kai Liao,* and Ching-Han Liu Department of Material Science and Engineering, Feng Chia UniVersity, Taichung 40724, Taiwan ReceiVed June 3, 2008. ReVised Manuscript ReceiVed August 15, 2008
This study uses different carbon fiber reinforcing materials impregnated with different phenolic resin concentrations to produce composite materials for use as fuel cell gas diffusion layer (GDL) substrates. Because of the differing structures of various carbon fiber reinforcing materials, GDL substrates made from these materials display different characteristics. For instance, GDL substrates produced from oxidized carbon felt have a relatively loose structure and can absorb a large quantity of resin; the properties of this type of substrate may change significantly with resin content. In contrast, the weave structure of oxidized carbon cloth tends to lessen the influence of the impregnated resin content. With regard to fuel-cell performance, GDL substrates fabricated from oxidized fiber felt yield the best performance when resin content is 10 wt % and have a limiting current density of as high as 1677 mA/cm2. A resin content of 2 wt % yields optimal performance when oxidized carbon cloth is the raw material, and the limiting current density can achieve 2207 mA/cm2.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) are considered to be among the most promising sources of alternative energy.1 Because of the high cost of the materials used in these fuel cells, however, they are still not widely used in everyday applications. Many types of materials have consequently been employed in research in an effort to cut costs and boost performance.2-6 The constituent materials of fuel cell gas diffusion layers (GDLs) include a GDL substrate, a hydrophobic agent, and a microporous layer. Carbon paper and carbon cloth are both often used as GDL substrates in PEMFCs.7,8 Conventional methods of fabricating carbon paper employ the hightemperature graphitization of carbon fiber to produce highperformance graphite fibers and then mixing the graphite fibers with resin to produce sheets of paper.9 Because of the hightemperature graphitization step, however, this method of producing carbon paper is relatively costly and complex. The production of carbon cloth requires the spinning and weaving of carbon fibers, followed by graphitization to boost conductivity. When carbon cloth is used to fabricate GDL substrates, loose yarn and the softness of the cloth tend to make handling and processing difficult. However, thanks to good * To whom correspondence should be addressed: Department of Material Science and Engineering, Feng Chia University, Taichung 40724, Taiwan. Telephone: +886-4-24517250 ext. 5303. Fax: +886-4-24518401. E-mail:
[email protected]. (1) Litster, S; McLean, G. J. Power Sources 2004, 130, 61–77. (2) Chun, Y. G.; Kim, C. S.; Peck, D. H.; Shin, D. R. J. Power Sources 1998, 71, 174–178. (3) Ferna´ndez, R; Aparicio, P. F.; Daza, L. J. Power Sources 2005, 151, 18–24. (4) de Miguel, S. R.; Vilella, J. I.; Jablonski, E. L.; Scelza, O. A.; de Lecea, C. S. M.; Solano, A. L. Appl. Catal., A 2002, 232, 237–246. (5) Wang, B. J. Power Sources 2005, 152, 1–15. (6) Hentall, P. L.; Lakeman, J. B.; Mepsted, G. O.; Adcock, P. L.; Moore, J. M. J. Power Sources 1999, 80, 235–241. (7) Zhang, X.; Shen, Z. Fuel 2002, 81, 2199–2201. (8) Ko, T. H.; Liao, Y. K.; Liu, C. H. New Carbon Mater. 2007, 22, 97–101. (9) Walker, N. J. Carbon Fiber: Technology Uses and Prospects; The Plastics and Rubber Institute: Malaysia, 1986.
conductivity and water absorption, carbon cloth is used on a large scale in direct methanol fuel cells. This study used an improved composite material production method to fabricate GDL substrate. Oxidized fiber felt and oxidized carbon cloth were used as the raw materials in conjunction with phenolic resin to bind the carbon fibers together. The production of conventional carbon/carbon composite materials generally attempts to achieve a highly compact material. To prevent the contraction of phenolic resin substrate and carbon fiber reinforcing material during high-temperature carbonization, repeated impregnation is used to eliminate internal voids.10,11 Because a GDL serves as a channel for gas flow and electron conduction and appropriate voids will facilitate gas flow, there is no need for this manufacturing process when producing GDL substrate for fuel cells.12 This study therefore used simplified procedures for producing composites and reduced the ratio of resin in the material. The study investigated the effect of different carbon fiber reinforcing materials and different amounts of impregnating resin and discussed the resulting characteristics when the material was used as a fuel cell GDL substrate. 2. Experimental Section This study investigated the characteristics of different GDL substrates and employed an improved composite material fabrication method to produce GDL substrate. The oxidized fiber felt was manufactured by the Kou Toong Carpet Co., Ltd. This felt had a base weight of 190 g/cm2, consisted of oxidized PAN fibers, and had a thickness of 1 mm. The oxidized carbon cloth was manufactured by the CeTech Co., Ltd. This cloth had a thickness of 0.65 mm, consisted of oxidized PAN fibers, and was produced by spinning and weaving. The substrate consisted of phenolic resin (type PF650) in a methanol solution provided by the Chang Chun Plastics Co., Ltd. (10) Tzeng, S. S.; Pan, J. H. Mater. Sci. Eng., A 2001, 316, 127–134. (11) Hatta, H.; Suzuki, K.; Shigei, T.; Somiya, S.; Sawada, Y. Carbon 2001, 39, 83–90. (12) Neergat, M.; Shukla, A. K. J. Power Sources 2002, 104, 289–294.
10.1021/ef800426r CCC: $40.75 2008 American Chemical Society Published on Web 09/30/2008
GDLs with Different Reinforce Materials in PEMFC
Figure 1. Impregnation ratios of different types of oxidized fiber fabrics impregnated with different resin concentrations: (9) oxidized fiber felt and (b) oxidized carbon cloth.
Before producing the composite material, the oxidized fiber felt and oxidized carbon cloth were both carbonized in a protective gas (nitrogen) at 1000 °C. According to Ko et al., an increased carbonization temperature will reduce the number of surface functional groups on the material, which will decrease the adhesion between the carbon fiber and the resin and make the composite material less brittle.13 Because the properties of the desired GDL substrate are different from those of ordinary composites, we employed a carbonization temperature of 1000 °C to ensure that the resulting GDL substrate would not be too brittle. Before producing the composite material, we diluted the phenolic resin with methanol, so that the percentages of resin in solution were 0, 2, 5, 8, and 10 wt %. After carbonization at 1000 °C, pieces of oxidized fiber felt and oxidized carbon cloth were impregnated in solutions with the foregoing resin concentrations. The test pieces were then placed in a constant pressure oven to dry for 15 min at 70 °C and remove excess methanol solvent. After drying, the pieces were placed in a pressing mold and a constant pressure of 10 kg/cm2 was applied at a temperature of 170 °C. Hot pressing was performed for 15 min, after which the test pieces were subjected to a second carbonization process. The second carbonization step consists of graphitization of the reinforcing material (carbon fiber felt and carbon fiber cloth) and phenolic resin at a temperature of 1300 °C in a protective gas (nitrogen). The finished test pieces resulting from this step consisted of oxidized fiber felt and oxidized carbon cloth impregnated with 0, 2, 5, 8, and 10 wt % phenolic resin. The GDL substrate produced from oxidized fiber felt is termed “carbon paper” below, and the GDL substrate produced from oxidized carbon cloth is termed “improved carbon fiber cloth”.
3. Results and Discussion The test pieces resulting from impregnation of different reinforcing materials with solution containing five different phenolic resin concentrations had significantly different ratios of resin/reinforcing material. Figure 1 is a schematic diagram showing resin concentration and resin impregnation ratios. It can be seen that the carbon paper absorbed far more resin than the improved carbon fiber cloth at all resin concentrations. As the concentration of the resin in solution increased, the ratio of resin in the test pieces also increased. When carbon paper was impregnated with 10 wt % resin solution, the resulting resin content was 26.9 wt % but the resulting improved carbon fiber (13) Ko, T. H. Polym. Compos. 1993, 14, 247–256.
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cloth had a resin content of only 7.08%. The large difference in absorption between the two materials is mainly attributable to their different weave structures. Felt consists of nonwoven fibers, and the large amounts of space between fibers promote the absorption of resin. On the other hand, cloth is produced by spinning and weaving, which results in a tighter structure. No pressure was applied during the impregnation process, and the experiment relied on the absorbency of the materials to impregnate them with resins. As a result, the looser structure of the oxidized fiber felt allowed it to absorb a greater amount of resin. Figure 2 shows the scanning electron microscopy (SEM) images of different carbon fiber reinforce materials in plane and cross-section. Figure 2a shows the SEM image of carbon cloth reinforce material in plane. Figure 2b shows the SEM image of carbon cloth reinforce material in cross-section. Figure 2c shows the SEM image of carbon felt reinforce material in plane. Figure 2d shows the SEM image of carbon felt reinforce material in cross-section. It can easily been seem that carbon cloth reinforce material has a regulation and tight structure relative to carbon felt reinforce material. Figure 3 shows the densities of the two types of textiles after impregnation with different resin concentrations. An Accupyc 1330 pycnometer was used to determine the density of the test pieces. 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. It can be seen from Figure 3 that an increasing resin concentration causes the density of the carbon paper and improved carbon fiber cloth to increase. In the case of oxidized fiber felt, the density increased to 1.52 g/cm3 from the 1.46 g/cm3 of the original material and the density of the oxidized carbon cloth increased to 1.74 g/cm3 from an original 1.62 g/cm3. Past research has shown that changes in the density of carbon fiber are mainly caused by the release of non-carbon elements at the graphitization temperature. The final graphitization temperature of the GDL substrate prepared in this study was 1300 °C in all cases, which yielded substrates with a similar graphite structure. Density variations were largely attributable to changes in resin content. The relationship between the density and impregnation ratio in Figure 1 is only a general trend, however, and the resin underwent major weight loss during the carbonization process. Density therefore cannot be increased effectively by merely raising the resin content. The relationship between the thickness of the carbon paper and improved carbon fiber cloth and resin concentration is shown in Figure 4. The thickness of the carbon paper decreased with the resin concentration and fell from the original 0.82 to 0.49 mm, which represented a compression ratio of 59.8%. In contrast, the thickness of the improved carbon fiber cloth stayed nearly constant at 0.5 mm and did not change with an increasing resin concentration. Those are attributable to the differing structures of oxidized fiber felt and oxidized carbon cloth. Oxidized fiber felt has a chaotic three-dimensional fiber structure and is highly compressible. In contrast, the thickness of oxidized carbon cloth is determined by the coarseness of the yarn and the fabric pick count, and there is usually little space for compression. The American Society for Testing and Materials (ASTM) D570 standard test method for water absorption was used to test water content. This testing method is commonly used to test conventional carbon/carbon composites. Water content varied as shown in Figure 5. The data trends reveal that the internal voids in the materials facilitate the absorption of water.
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Figure 2. (a) SEM of carbon cloth reinforce material in plane. (b) SEM of carbon cloth reinforce material in cross-section. (c) SEM of carbon felt reinforce material in plane. (d) SEM of carbon felt reinforce material in cross-section.
Figure 3. Density of different oxidized fiber fabrics impregnated with different resin concentrations: (9) oxidized fiber felt and (b) oxidized carbon cloth.
In the case of carbon paper, the original material and test pieces impregnated with 2 wt % phenolic resin had water contents of 712 and 709%, respectively. After this, water content quickly fell to 351% and finally to 274% when the test pieces were impregnated with 10 wt % resin. The improved carbon fiber cloth displayed no significant decreasing trend but fell from 130% for the original material to 93% for test pieces impreg-
Figure 4. Thickness of different oxidized fiber fabrics impregnated with different resin concentrations: (9) oxidized fiber felt and (b) oxidized carbon cloth.
nated with 10 wt % phenolic resin. There is a close relationship between internal voids and the amount of absorbed water. Because the felt contains many voids, it can absorb far more water than the cloth. The rapid drop in water content in the carbon paper is due to mutual adhesion between the fibers after impregnation with resin, which reduces the amount of internal space. It can be clearly seen that the contraction of the carbon paper results from the shrinkage in thickness, and water content
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Figure 5. Water content of different oxidized fiber fabrics impregnated with different resin concentrations: (9) oxidized fiber felt and (b) oxidized carbon cloth.
Figure 6. In-plane resistivity of different oxidized fiber fabrics impregnated with different resin concentrations: (9) oxidized fiber felt and (b) oxidized carbon cloth.
Table 1 percentage of resin hydrophobicity of felt hydrophobicity of cloth
2 wt %
5 wt %
8 wt %
10 wt %
124.3° 122.6°
126.7° 124.7°
125.6° 124.3°
126.5° 127.2°
quickly dropped with a decrease in thickness. However, because the water content of the improved carbon fiber cloth is controlled by its structure and relatively fixed thickness, it did not decrease significantly. Wetting properties of different carbon reinforce materials are shown in Table 1. Varied carbon fiber reinforce materials used in this study were basic gas diffusion base materials and without Teflon-coating treatment. We use the contact angle to determine hydrophobic properties of different carbon fiber reinforce materials. The most important factor to determine the hydrophobic properties is the functional group on the surface of the carbon fiber. With the same graphitization temperature (1300 °C), the functional groups on the surface of carbon cloth and felt are similar. For this reason, the contact angles of different reinforce materials are approximate. Figures 6 and 7 respectively show the relationship between the resin concentration and the in-plane resistivity and throughplane resistivity of the materials. The in-plane resistivity was measured using a Loresta GP MCP-T600 m following the standard JIS K 7194 test method. The original resistivity of carbon paper was 94.9 m Ωcm, and the resistivity of carbon paper impregnated with 2 wt % resin was 93.6 m Ωcm; however, resistivity fell rapidly after that point. Carbon paper impregnated with 10 wt % had a resistivity of 26.5 m Ωcm, which was 72.1% lower than the peak value. The improved carbon fiber cloth did not display a major decrease in resistivity, falling only 10.1% from 15.9 m Ωcm for the original material to 14.3 m Ωcm. Because electrons are transmitted in the direction of the carbon fibers and increasing resin content causes increased contact between individual fibers, forming a tighter configuration, higher resin content tends to reduce resistivity. Because the thickness and compaction of carbon paper increase with resin content, the resistivity falls by a greater degree than in the case of improved carbon fiber cloth. Through-plane resistivity is typically different from in-plane resistivity. Resistivity data indicate that through-plane resistivity trends are opposite to in-plane trends. Because electrons are largely transmitted along the carbon fibers, conductivity is significantly
Figure 7. Through-plane resistivity of different oxidized fiber fabrics impregnated with different resin concentrations: (9) oxidized fiber felt and (b) oxidized carbon cloth.
lower perpendicular to the surface. Moreover, the transmission of electrons from fiber to fiber is hindered when there is more resin adhering to the surface of the fibers; therefore, resistivity is higher in the through-plane direction. The through-plane resistivity of carbon paper increased from the value of 17.9 Ωcm for the original material to 127.3 Ωcm. The through-plane resistivity of carbon paper displayed a smaller increase and rose from 28.2 Ωcm for the original material to 62.9 Ωcm. The fibers in carbon felt have a chaotic three-dimensional arrangement that is unlike the orderly, compact arrangement in carbon paper, which provides many routes for the transmission of electrons. As a consequence, because the resin enters voids in the loose material when carbon felt is impregnated, the higher the resin content, the lower the conductivity. The test pieces were assembled as single cells to study the characteristics of the carbon paper and improved carbon fiber cloth. The anodes and cathodes both consisted of GDL substrate, and the proton exchange membrane consisted of a solid catalystcoated electrolyte membrane (Gore three-layer CCM MEA with a thickness of 35 µm; catalyst deposition: cathode, 0.6 mg of Pt/cm2; anode, 0.45 mg of 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-
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Figure 8. (a) Voltage versus current density for carbon paper GDL substrates impregnated with different resin concentrations. (b) Power density versus current density for carbon paper GDL substrates impregnated with different resin concentrations.
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 paper together by hot-pressing but only used 40 kgf/cm2 torsion to ensure close contact between the layers. Parts a and b of Figure 8 show the performance curves of fuel cells with carbon paper gas diffusion substrates. Fuel-cell performance is similar for the original material and carbon paper impregnated with 2 wt % resin, and current densities were 44.8 and 47.6 mA/cm2, respectively, when the load was 0.7 V. The limiting current densities were 547 and 576 mA/cm2. Performance increased slightly as resin content increased. The limiting current densities were 218 and 1677 mA/cm2, respectively, at a load of 0.7 V when the resin concentration was 10 wt %. We assume that resistivity is the chief factor affecting fuel-cell performance. Past research suggests that fuel-cell polarization curves can be analyzed in terms of three regions. Because the
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Figure 9. (a) Voltage versus current density for improved carbon fiber cloth GDL substrates impregnated with different resin concentrations. (b) Power density versus current density for improved carbon fiber cloth GDL substrates impregnated with different resin concentrations.
CCMs used in this study were all Gore products, the greatest differences between fuel cells were the different characteristics of the GDL substrate. As resin concentration increased, the carbon paper GDL substrate displayed a clear decrease in inplane resistivity and increase in through-plane resistivity. It is difficult to determine which of these factors had the largest influence on fuel-cell performance. Parts a and b of Figure 9 show performance curves of fuel cells with improved carbon fiber cloth GDL substrates. The performance of cells with improved carbon fiber cloth GDLs differed greatly from that of cells containing carbon paper GDLs. Fuel-cell performance dropped as the resin concentration increases. Because of the low compressibility of improved carbon fiber cloth, in-plane resistivity dropped only slightly (10.1%) as resin concentration increased but through-plane resistivity increased by a factor of 2.23 as a result of the effect of the added resin on electron transmission pathways. Because of this, fuel-cell performance decreased as resin concentration increases. The best performance was achieved when the material was impregnated with 2 wt % resin, at which time the limiting current densities were 366 and 2207 mA/cm2, respectively. 4. Conclusions In this study, we produced composite materials by using oxidized fiber felt and oxidized carbon cloth as reinforcing
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materials and added different concentrations of phenolic resin as a substrate. Because of the loose fiber structure of the carbon paper, impregnation with a greater resin concentration caused resin absorption and compression to increase. In this case, inplane resistivity decreased with an increasing resin concentration from 94.9 to 26.5 m Ωcm, while through-plane resistivity increased from 17.9 to 127.3 Ωcm. When improved carbon fiber cloth was used, the tight weave of the cloth limited resin absorption, with the result that the cloth absorbed less resin than did the carbon paper. The thickness of the improved carbon fiber cloth remained almost constant at 0.5 mm. The in-plane resistivity of the improved carbon fiber cloth fell from 15.9 to
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14.3 m Ωcm as resin concentration increased, while the throughplane resistivity increased from 28.2 to 62.9 Ωcm. As for fuelcell performance, carbon paper GDL substrate yielded the best performance when impregnated with 10 wt % phenolic resin solution and displayed limiting current densities of 218 and 1677 mA/cm2 at a load of 0.7 V. The improved carbon fiber cloth yielded the best performance when impregnated with 2 wt % phenolic resin solution. Limiting current densities were 366 and 2207 mA/cm2 with a 0.7 V load. EF800426R
