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Energy & Fuels 2008, 22, 1200–1203
Performance Analysis of a Proton-Exchange Membrane Fuel Cell (PEMFC) with Various Hydrophobic Agents in a Gas Diffusion Layer Jui-Hsiang Lin,* Wei-Hung Chen, Yen-Ju Su, and Tse-Hao Ko Department of Materials Science and Engineering, Feng Chia UniVersity, Taichung, Taiwan 40724 ReceiVed NoVember 23, 2007. ReVised Manuscript ReceiVed January 22, 2008
The microporous layer (MPL) between the carbon cloth and catalyst layer is a significant part in a protonexchange membrane fuel cell (PEMFC). The water management of membrane electrode assembly (MEA) is a crucial technology for improving performance. It is now the aim of this study to present the effects of various hydrophobic agents on the performance in a PEMFC. MPL loading with FEP121A and LDW-40 provide optimization of the performance (1007.7 and 980.33 mA/cm2 at 0.5 V). The polarization curves of the fuel cell were plotted under similar operating conditions with different MPL. These results provided a wide and cheap choice of hydrophobic agents in the MEA.
1. Introduction
PEMFC. The GDL is typically treated by partially coating it with a hydrophobic polymer to prevent flooding of the electrodes by the produced water. Hydrophobic agents are usually treated with polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP). Studies have been performed to elucidate the role of MPL on the GDB that generally consists of carbon black and fluorocarbon polymer4 and the pore-size dispersion of MPL with various kinds and sizes of carbon black powders.5–7 In this study, the water management function was performed using a fluorocarbon polymer, which was dry-dispersed on a traditional GDL. The goal of this study was to present the effects of various hydrophobic agents on the performance of a PEMFC system. The polarization curves of the fuel cell were plotted under similar operating conditions with different MPL (with the same carbon black powder and various fluorocarbon polymers) coated on the GDL. The major driving forces of water transportation for the GDL were suggested by analyzing the characteristics of the polarization curves associated with fuel cell performance.
A proton-exchange membrane fuel cell (PEMFC) consists of a membrane electrode assembly (MEA), gas diffusion layers (GDLs), and bipolar plates with gas channels. The GDL have a gas diffusion backing (GDB) and a microporous layer (MPL). The protons migrate through the electrolyte and combine with oxygen from the air and electrons to produce water. During operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons and electrons. However, the GDLs have two major functions. First, the reaction gases can successfully diffuse into the catalyst layer and uniformly spread thereon because of the porous structure of the GDLs. Second, the electrons generated by the anode catalysis are drained from the anode and enter the external circuit. Therefore, the GDL should be a porous material and good electric conductor. Additionally, to prevent liquid water molecules from filling the pores of the GDLs and thereby impeding the delivery of the reaction gas, GDLs undergo in advance to hydrophobic treatment to deliver the reaction gases and the necessary water vapor to the catalyst layer. On the anode side of the PEMFC, the membrane tends to dry out. On the cathode side, the cathode tends to become flooded by both the produced water and the mass transport. Therefore, a method is required to transport water to the anode and away from the cathode. Appropriate water management is critical to ensuring the strong performance and long life of a PEMFC.1,2 Taniguchi et al.3 demonstrated that fibrous carbon substrate was treated with fluororesin and sintered at high temperature. The electrode is treated with sufficient waterresistant additive to enable it to be used as a cathode in a
2.1. Materials. GDLs were prepared using FCW1005 PANbased carbon-fiber cloths (Challenge Carbon Technology Co., Ltd.). The fluorocarbon polymers were 10 wt % by Teflon FEP 121A (Dupont Co., Ltd.) and 10 wt % LDW-40 (Daikin Industries, Ltd.) (diluted from 10 mL of hydrophobic agents solution and 90 mL of deionized water). Vulcan XC-72 (Cabot Co., Ltd.) was mixed with fluorocarbon polymer solution. In Table 1, there are typical property data for two kinds of hydrophobic agents. The performances were measured by FCED PD50 (Asia Pacific Fuel Cell Technologies,
* To whom correspondence should be addressed: Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan 40724. Telephone: +886-4-24517250, ext. 5303. Fax: +886-4-24518401. E-mail:
[email protected]. (1) Qi, Z.; Kaufman, A. J. Power Sources 2002, 109, 38–41. (2) Chen, J.; Matsuura, T.; Hori, M. J. Power Sources 2004, 131, 155– 161. (3) Taniguchi, S.; Hamada, A.; Miyake, Y.; Kaneko, M. U.S. Patent 6,083,638, 2000.
(4) Passalacqua, E.; Lufrano, F.; Squadrito, G.; Patti, A.; Giorgi, L. Electrochim. Acta 1998, 43, 3665–3673. (5) Park, G. G.; Sohn, Y. J.; Yim, S. D.; Yang, T. H.; Yoon, Y. G.; Lee, W. Y.; Eguchi, K.; Kim, C. S. J. Power Sources 2006, 163, 113–118. (6) Wang, X. L.; Zhang, H. M.; Zhang, J. L.; Xu, H. F.; Tian, Z. Q.; Chen, J.; Zhong, H. X.; Liang, Y. M.; Yi, B. L. Electrochim. Acta 2006, 51, 4909–4915. (7) Kannan, A. M.; Munukutla, L. J. Power Sources 2007, 167, 330– 335.
2. Experimental Section
10.1021/ef7007024 CCC: $40.75 2008 American Chemical Society Published on Web 02/14/2008
Performance Analysis of a PEMFC
Energy & Fuels, Vol. 22, No. 2, 2008 1201
Table 1. Typical Property Data of Two Kinds of Hydrophobic Agents (FEP121A and LDW-40)a materials code FEP121A LDW-40 a
source
resin solid (%)
melting temperature (°C)
particle size (nm)
viscosity of dispersion at 25 °C (cP)
DuPont Daikin
54 40
260 330
180 180
25 8
This table comes from the publish website of DuPont and Daikin.
Ltd., Taiwan). The MEA was fabricated with an activated area of 25 cm2, and the membrane was obtained from DuPont (Nafion NRE211). 2.2. Fluorocarbon Polymer Coating and Sintering. A square 36 cm2 piece of FCW1005 carbon cloth was cut out. The carbon cloths were clean and dust-free and then coated with mixed solutions. One of the MPL sprayed solutions was mixed with FEP121A and XC-72, and the other was mixed with LDW-40 and XC-72. The treated carbon cloths were dried at 70 °C for 30 min. Carbon cloths of uncoating, coating MPL with FEP121A, and coating MPL with LDW-40 mixed solutions were designated A, B, and C, respectively. Sample B was treated at 280 °C for 30 min to evaporate all remaining glycerol and then at 350 °C for 30 min to uniformly distribute fluorocarbon polymer attachment and throughout the GDL. Sample B was treated 280 °C for 30 min and then at 380 °C for 30 min with the same functions. The various treated temperatures were stood on the different fluorocarbon polymers. 2.3. Characteristic Analysis. Surface resistance was measured from a series GDLs. The GDL test area was 25 cm2. At least 30 samples were measured, and the average value was calculated. The in-plane resistance analysis was performed using an Elemental Vario EL III. The through-plane resistance was measured by 2-point, circular (10 mm in diameter). The Cu-plated contacts were under various pressure loadings. Measurements were made a minimum of 5 points on a GDL, and the average value was calculated for 5 pieces of GDL. Measurement of Gurley porosity was performed in a Gurley-type porosimeter (ASTM D726-58), with the specimen fixed on the instrument cylinder and fastened among sealing plates. The cylinder was then lowered slowly. Gas permeability characteristics of the various GDLs were evaluated directly with a Gurley (Model 4110) apparatus, whose cylinder with a 1.0 in.2 opening was positioned at several locations of the cut GDL surface. Gurley porosity values were acquired as an average of several time (in seconds) determinations for 300 cm3 and weight of 5 oz. of displaced air; the length of time required for a GDL was positively correlated with the closed-cell character of GDL samples. The surface morphology of the pure fluorocarbon polymer solutions and GDLs coating MPL were investigated visually via high-resolution scanning electron microscopy (HRSEM; HITACHI S-4800, Japan).
3. Results and Discussion 3.1. Structure and Characteristic. Sample B and C were present in the structural advantages associated with the binding capacity of the FEP121A and LDW-40. Parts a and b of Figure 1 show the fluorocarbon polymer solutions of FEP121A and LDW-40, respectively. The percentages of FEP resin solid in FEP121A and LDW-40 were 54 and 39%; the respective single carbon powder sizes were about 200 and 150 nm. LDW-40 solution exhibited less dispersion (the single powders usually conglomerated to large particles about 5 µm) and easy to deposit and congregate (Figure 1c). Therefore, the LDW-40 mixed slurry was difficult to spray on the GDL surface uniformly. In this spraying of MPL, the electrolyte was deposited of a mixture of water, carbon black powder, and fluorocarbon polymer. This mixture is then sprayed onto carbon cloth (parts a and b of Figure 2). Table 2 compares Gurley porosity coefficients of sample A, B, and C. According to carbon powder loadings, the MPL becomes entrenched in carbon-fiber paper during spray
Figure 1. HRSEM images of various hydrophobic agents. (a) FEP121A particles by 80 000× magnification, (b) LDW-40 particles by 30 000× magnification, and (c) LDW-40 particles by 5000× magnification.
deposition of carbon slurry. Sample B and C were loading 0.17 and 0.14 mm of carbon slurry, respectively. The different thicknesses of GDL were caused by various hydrophobic agents. FEP121A was a dispersive solution and caught more carbon black particles on the surface of GDL, but the LDW-40 was not. These loadings caused gas permeability to vary. Although the dispersion of MPL was clearly discovered, the Gurley porosities of samples B and C were similar (23.51 and 22.39 cm3 cm-2 s-1). The Gurley porosity of sample A was (33.11 cm3 cm-2 s-1) higher than the coated MPL samples (B and C); the result indicates that Gurley porosity of GDL were decreased with MPL loading. The carbon black powder and fluorocarbon polymer were occupied most of gas channels inner GDL structure and cause the decline in Gurley porosity of GDL. Figure 3 plots the relationship among the types of the hydrophobic agents and the in- and through-plane resistances in GDLs. The in-plane resistances of samples A, B, and C were 0.41, 0.43, and 0.40 Ω/sq., respectively. Sample A at pressures of 1, 3, 5, 10, and 25 kg/cm2 had through-plane resistances of 1807.8, 1059.1, 775.7, 556.5, and 364.8 mΩ cm, respectively. Those of sample B were 6210.2, 2803.0, 1796.3, 1023.1, and
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Lin et al.
Figure 4. Cell polarization curves of sample A, B, and C, respectively, in a single PEMFC. The measurements were performed in H2/O2 at 0.5/0.5 SLPM for sample A, B, and C. (9) A, raw carbon cloth; (b) B, MPL with FEP121A; and (2) C, MPL with LDW-40. MPL ) microporous layer.
Figure 2. HRSEM images of carbon cloth loading MPL with various hydrophobic agents. (a) MPL with FEP121A and (b) MPL with LDW40. MPL ) microporous layer. Table 2. Physical Properties of Sample A, B, and Ca in-plane Gurley base porosity weight MPL loading thickness resistance (mg/cm2) (mm) (Ω/sq.) (cm3 cm-2 s-1) sample (g/m2) A B C
253.17 264.79 259.62
2.58 2.01
0.53 0.70 0.67
0.41 0.43 0.40
33.11 23.51 22.39
a A, raw carbon cloth; B, MPL with FEP121A; and C, MPL with LDW-40. MPL ) microporous layer.
Figure 3. Curves of the pressure loadings versus through- and in-plane resistances of sample A, B, and C, respectively. (9 and 0) A, raw carbon cloth; (b and O) B, MPL with FEP121A; and (2 and 4) C, MPL with LDW-40. MPL ) microporous layer. sq. ) square of testing area.
545.5 mΩcm, respectively. Those of sample C were 4630.4, 2266.3, 1570.7, 954.4, and 533.7 mΩcm, respectively. Increasing pressure loading influenced the through-plane resistances, caused interlocking of the electrical transport path, and increased carbon-fiber density. The effect of high pressure on electrical
conductivity was stronger than that of weak pressurization. In the high-pressure region, an important property in the design and optimization of PEMFC was obtained. Theoretically, the compression of GDL increased the density of carbon fiber. However, excessive compression damages the carbon fibers in GDL.8 In fact, coated MPL GDLs were less conductive than the raw GDL, which fact is attributable to the insulating hydrophobic agent with high resistances. The Vulcan XC-72 is a high electrical conductivity carbon material
