Washing Experiment of the Gas Diffusion Layer in a Proton-Exchange

May 8, 2008 - Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan. Energy Fuels , 2008, 22 (4), pp 2533–25...
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Energy & Fuels 2008, 22, 2533–2538

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Washing Experiment of the Gas Diffusion Layer in a Proton-Exchange Membrane Fuel Cell Jui-Hsiang Lin,* Wei-Hung Chen, Shih-Hsuan Su, Yen-Ju Su, and Tse-Hao Ko Department of Materials Science and Engineering, Feng Chia UniVersity, Taichung 40724, Taiwan ReceiVed February 17, 2008. ReVised Manuscript ReceiVed March 27, 2008

This study exhibits a novel and simple process to test durability of the gas diffusion backing (GDB) with a microporous layer (MPL) and simulates operating a proton-exchange membrane fuel cell (PEMFC) that produces liquid water. One of the major concerns of the gas diffusion layer (GDL) inside a PEMFC is durability. From the analysis of morphology, resistance, and Gurley porosity, the characteristics of each GDL are compared. In long time water washing, the in-plane resistance of the GDL reduces from 29 to 23 mΩ/cm2 over 200 h. The GDL shows a higher performance (1227 mA/cm2 at 0.3 V) in long time water washing (200 h) than GDB at a low voltage region (1093 mA/cm2 at 0.3 V). The high GDL degradation rate causes the migration of the site of MPL during water washing. The inside MPL is weaker than the outward on performance in a PEMFC, but it keeps performance better than GDB.

1. Introduction Nearly every major automaker in the world has announced or demonstrated a fuel-cell-powered concept vehicle and/or prototype, including Toyota, Honda, Nissan, Mazda, Renault, Hyundai, Fiat, Peugeot, and Volkswagen. One exception is BMW, which has focused its development efforts on hydrogenpowered internal combustion engines (ICEs), using fuel cells for auxiliary power only. Fuel cells are used on transportation extensively, such as golf carts, forklifts, mining vehicles, cranes, bikes, scooters, water taxis, and boats. Fuel cells are also being considered for locomotives, trucks, and marine vessels and as auxiliary power in cars and trucks. The durability of proton-exchange membrane fuel cells (PEMFCs) is major energy for stationary and transportation power applications on vehicle commercialization.1,2 Durability is difficult to quantify and improve basically because of testing required for quantity and duration (i.e., up to several thousand hours or more). However, durability impacts are seen in structural changes, collapse of anode/cathode layers, and the mitigation of failures related to reactant gas starvation, stop/ start operation, poor water management, and freeze start are well-established in many new fuel cell systems. Early studies have always focused on the durability and degradation of the catalyst layer and membrane. Iojoiu et al.1 found enhancement stability and durability of the protonexchange membranes is critical to the lifetime and commercial viability of a PEMFC. Borup et al.2 examined the loss of the active catalyst surface area during fuel cell testing and developed offline potential cycling accelerated testing. The U.S. Depart-

* To whom correspondence should be addressed: Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan. Telephone: +886-4-24517250 ext. 5303. Fax: +886-4-24518401. E-mail: [email protected]. (1) Iojoiu, C.; Chabert, F.; Marechal, M.; Kissi, N. El. J. Power Sources 2006, 153, 198–209. (2) Borup, R. L.; Davey, J. R.; Garzon, F. H.; Wood, D. L.; Inbody, M. A. J. Power Sources 2006, 163, 76–81.

ment of Energy3 showed the requirements for commercial applications: PEMFC are required to demonstrate durability of about 6000 h under normal operating conditions. However, the durability issue of the gas diffusion layer (GDL) has not attracted extensive attention in recent studies. In this study, we examine the durability of a microporous layer (MPL) on the GDL and the effects that fuel cell operating conditions have on the GDL durability by measuring the loss of MPL. 2. Experimental Section 2.1. Manufacture of GDL. The GDL can be divided into the gas diffusion backing (GDB) and MPL. The GDB mainly consists of carbon fiber paper or carbon fiber cloth, each one of which is a normal type of GDB. These GDBs were manufactured from carbon fiber and produced according to Taiwan patent I261639 and U.S. application publication 2005/10124246. The MPL has a crucial role in achieving high performance for PEMFC. The MPL consists of carbon-black powder and a hydrophobic agent, both of which are applied to the GDB surface. In addition to the Vulcan XC-72, many other carbon blacks have been applied to fabricate MPL.4–8 In this study, we used to prepare GDB by oxidized fiber felt (Kuo Tung Felt Co., Ltd.) and phenolic resin (Chang Chun Plastics Co., Ltd.) as raw materials. The oxidized fiber felt was first carbonized to produce carbon fiber felt at 1000 °C. The carbon fiber felt was impregnated with the phenolic resin mixture and baked at 70 °C for 15 min. The hot pressing at 170 °C and pressure of 10 kg/cm2 was performed to change the composite material to the form of carbon fiber paper. Then, carbonization was performed at high temperature again. The Vulcan XC-72 (Cabot Corp., Boston, MS) (3) U.S. Department of Energy. Multi-Year Research, Department, and Demonstration Plan: Planned Program Activities for 2003-2010. (4) Antolini, E.; Passos, R. P.; Ticianelli, E. A. J. Power Sources 2002, 109, 477–482. (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.; Zhu, X. B.; Chen, J. A.; Yi, B. L. J. Power Sources 2006, 162, 474–479. (7) 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. (8) Jordan, L. R.; Shukla, A. K.; Behrsing, T.; Avery, N. R.; Muddle, B. C.; Forsyth, M. J. Power Sources 2000, 86, 250–254.

10.1021/ef800116c CCC: $40.75  2008 American Chemical Society Published on Web 05/08/2008

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Lin et al. resolution scanning electron microscopy (HRSEM, HITACHI S-4800, Japan). 2.4. Electrochemical Characterization of GDLs in a PEMFC. Single-cell voltage and current density were measured simultaneously, using a FCED PD50 (Asia Pacific Fuel Cell Technologies, Ltd., Taiwan). The activated area in each cell was 25 cm2, and the bipolar plates consisted of serpentine-type grooved graphite plates, which were made of highly compact graphite. Polarization (voltage versus current density) of a single PEMFC was obtained under specific operating conditions: cell temperature, 40 °C; pure H2 fuel gas for the anode, 500 c.c./min; pure O2 fuel gas for the cathode, 500 c.c./min; both gases have a relative humidity (RH) of 95%.

Figure 1. Schematic illustration of GDL washing with pure water.

was mixed with 10% FEP solution (diluted from 10 mL of Dupont FEP 121A solution and 90 mL of deionized water) and then stirred for 5 min at room temperature. The GDB was sprayed with this mixed solution to form a one-side precursor of the MPL. The sprayed GDB was baked at 70 °C for 15 min to dry, baked at 240 °C for 30 min, and sintered at 350 °C for 30 min. 2.2. GDL Washing of a Simulated Operating Fuel Cell. U.S. Patent 7,063,9139 discloses a porous carbon substrate pretreated with a hydrophobic polymer, which was dried to obtain a hydrophobic carbon substrate. Then, the hydrophobic carbon substrate was coated with a layer of a fluorocarbon polymer and carbon particle mixture. The substrate was finally subjected to a heat treatment. To prevent the performance degradation of the fuel cell because of water flooding, a hydrophobic treatment was typically performed on the carbon substrate of the GDL, so that the excessive water may be readily drained away, thereby prolonging the service life of the fuel cell. Furthermore, because the rough surfaces of carbon cloth and carbon paper may affect reaction and efficiency of the catalyst on the catalyst layers, a leveling treatment on such surfaces was typically needed for the hydrophobic layer. In this study, a cyclic spout of the water experiment was introduced to investigate the endurance of GDL. The experiments were performed by two cutting tubes, and the two ends of the GDL were clamped with them (Figure 1). Then, the samples were washed in pure water (500 cm3/min) on the MPL surface. The washing times were 1, 5, 10, 50, 100, and 200 h and named GDL-1HR, GDL5HR, GDL-10HR, GDL-50HR, GDL-100HR, and GDL-200HR, respectively. 2.3. Characterization of GDLs. Measurement of Gurley porosity was performed in a Gurley-type porosimeter (ASTM D72658), with the specimen fixed on the instrument cylinder and fastened among sealing plates. The cylinder was then lowered slowly. The automatic chronometer, joined to a photocell, was used to record the time (in seconds) needed to discharge from the cylinder air volume of 300 mL through the GDL, indicated as a Gurley number. Gurley porosity characteristics of the various GDLs were evaluated directly with a Gurley (model 4110) apparatus, whose cylinder with a 6.45 cm2 opening was positioned at several locations of the cutting GDL surface. The weight less was measured in 1, 5, 10, 50, 100, and 200 h, respectively. The washing area was 19.6 cm2. The true densities of carbon paper were tested with an Accupyc 1330 pycnometer. The sample was placed in the instrument cell and given helium gas. Measurements were made 90 times, with the last 10 cycles calculated for an average value. Surface resistance was measured at various GDLs. The GDL test area was 25 cm2. At least 30 m samples were measured, and the average value was calculated. Elemental analysis was performed using an Elemental Vario EL III. The through-plane resistance was measured by two points (10 mm in diameter) and various forces. Measurements were made a minimum of five points on a GDL, and the average value was calculated for five pieces of GDL. The surface morphology of the GDL was investigated visually via high(9) Ji, C.; O’Hara, J. E.; Mathias, M. F. U.S. Patent 7,063,913, 2006.

3. Results and Discussion 3.1. Characterization Analysis of GDL. In this study, GDLs exhibited infinite diversity of micrographs in various degree of washing. Figure 2 compared the surface of GDL to various washing times, which corresponded to the degree of performance in a PEMFC. Parts a and b of Figure 2 showed the raw GDB and GDL (spraying MPL), respectively. The MPL was made with FEP121A solution and Vulcan XC-72 loadings. The images of water, washing on the GDL surface at different times, are shown in parts c-h of Figure 2, which present water washing in 1, 5, 10, 50, 100, and 200 h with 500 mL/min, respectively. The GDL surface, which blanketed with MPL and carbon fibers, showed few cracks on the GDL surface (Figure 2b). However, after 1 h washing, extensive cracks appeared on the surface of GDL. As the washing time increased, the cracks became serious. In 10 h washing, few carbon fibers appeared on the GDL surface and parts of MPL disappeared by washing. With the washing time over 50 h, more carbon fibers appeared on the GDL surface than short washing time samples. In 200 h washing, the carbon fibers were clearly exhibited on the GDL surface and most of MPL had disappeared by washing. The in-plane resistance values of GDLs, which under different washing times, are shown in Figure 3. The resistance of GDB was also plotted for comparison. The in-plane resistance of GDB was lower than GDL slightly, because the FEP was not conductive, thereby the in-plane resistances of the samples increased with the deposition. The in-plane resistance of GDL10HR is also lower than GDL, GDL-1HR, and GDL-5HR slightly, because of washing. According to Figure 2, in 10 h washing, parts of carbon fibers were bared on the GDL surface gradually. As the washing time exceeds 10 h, the in-plane resistance was equal to GDB approximately. The GDL-10HR, GDL-50HR, GDL-100HR, and GDL-200HR result in a similar resistance with GDB. Micrographs of GDB and GDLs (Figure 2) were revealed that MPL was the most significant factor of in-plane resistance. Before water washing, the GDL surface was blanketed with MPL and carbon fibers. After water washing, the GDL surface appeared with bared carbon fibers gradually and most of the MPL disappeared. It caused in-plane resistance to decrease. Figure 4 presented a comparison of the different pressure and through-plane electrical resistance curved with GDB and various washing time of GDLs. These samples, at 0, 5, 10, 15, 20, 25, 30, 40, 50, and 60 pounds of pressure, exhibited through-plane resistances on the decrease. The GDL was using mixed Vulcan XC-72 powder and FEP121A solution to form a precursor of MPL. The Vulcan XC-72 was a high electrical conductivity carbon material, and the FEP121A solution was an insulator material. A MPL contained a quantity of Vulcan XC-72 powder and FEP121A solution; moreover, the most Vulcan XC-72 was coated on the GDB surface and the FEP121A solution was

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Figure 2. Comparison of surface HRSEM of GDB and GDLs in washing with various times. (a) GDB (carbon fiber paper), (b) GDL (MPL coating on GDB), (c) GDL-1HR (in 1 h water washing), (d) GDL-5HR (in 5 h water washing), (e) GDL-10HR (in 10 h water washing), (f) GDL-50HR (in 50 h water washing), (g) GDL-100HR (in 100 h water washing), (e) GDL-200HR (in 200 h water washing). GDB, gas diffusion backing; GDL, gas diffusion layer; MPL, microporous layer.

dipped into all of the GDB. Therefore, the through-plane resistance of GDB was lower than GDL slightly. Increasing the pressure caused carbon fibers to interlock; more compression of GDL on electrical conductivity was stronger than without compression. Theoretically, the compression of GDL increased the bulk density of GDL and the electron pass paths. These samples, before and after washing, exhibited the similar throughplane resistance at high pressure. The result shows that MPL was not washed out from GDL but washed into GDL.

Graphs such as Figures 5 and 6 can already provide a quite good and simple visual representation of MPL, which was washed into GDL and not washed out. Figure 5 plotted the Gurley porosity and less weight of GDL in various washing times. The Gurley porosity values are obtained as an average of several determinations time (in seconds): GDB, GDL, GDL1HR, GDL-5HR, GDL-10HR, GDL-50HR, GDL-100HR, and GDL-200HR shown as 77, 56, 52, 51, 49, 51, 50, and 52 cm3 cm-2 s-1, respectively. It accounted for nearly 30% reduction

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Figure 3. In-plane resistance of GDB and GDL as a function of the washing time. (9) GDB (carbon fiber paper) and (0) GDL (MPL coating on GDB).

Figure 4. Effect of various pressures on the through-plane resistance of GDB and GDLs in washing with various times. (9) GDB (carbon fiber paper), (b) GDL (MPL coating on GDB), (2) GDL-1HR (in 1 h water washing), (1) GDL-5HR (in 5 h water washing), ([) GDL-10HR (in 10 h water washing), (″left solid triangle″) GDL-50HR (in 50 h water washing), (″right solid triangle″) GDL-100HR (in 100 h water washing), and (f) GDL-200HR (in 200 h water washing).

in relative Gurley porosity. This phenomenon was from the GDB surface of the blanketed MPL and blocked the path of gas (Figure 2). In this case, if MPL would wash out from GDL, the gas permeability should nearly revert to the initial state (77 cm3 cm-2 s-1). In fact, the gas permeability of washed GDLs is an invariant value. Therefore, in the relative permeability measurement, MPL was washed through GDL. During washing, weight less was measured in 1, 5, 10, 50, 100, and 200 h, respectively. The weight less of the above treatment was shown as 0.2, 6, 2.6, 2.3, 3.1, and 1.5 mg (Figure 5). In 200 h, weight less was summed to 15.7 mg (0.8 mg/cm2) for the washing treatment. It accounted for nearly 4.5% reduction in weight (GDL was about 0.3557 g). This result proved that MPL treatment peeled slightly by washing and most of MPL washed into GDL. The decline in gas permeability of GDL reduced the distances of conductive carbon fibers, improved the contacts of the carbon fibers, and affected the performance of a PEMFC. In early

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Figure 5. Curve of the Gurley porosity and weight less (the weight after washing minus the initial weight) of GDB and GDLs in various washing times. (9) GDB (carbon fiber paper) and (0) GDL (MPL coating on GDB).

Figure 6. True density of GDB and GDL as a function of the washing time. (9) GDB (carbon fiber paper) and (0) GDL (MPL coating on GDB).

studies, Pharoah10 found that adding a MPL decreased the permeability by several orders of magnitude; however, this reduction was not relevant to convective transport. Williams et al.11 obtained different results, indicating that GDL with high gas permeability has a high limiting current; increased permeability may also increase the effective diffusion coefficient and enhance convection. The effect of performance with reduced paths and gas permeability is discussed in our future study. The MPL, which was washed into GDL, was the true density curve with various washing times on the other evidence (Figure 6). The true density of GDB, GDL, GDL-1HR, GDL-5HR, GDL-10HR, GDL-50HR, GDL-100HR, and GDL-200HR were 1.35, 1.60, 1.61, 1.62, 1.61, 1.62, 1.60, and 1.62 g/cm3, respectively. All of the samples with MPL exceeded the true density of GDB. The result was due to the Vulcan XC-72, which had 1.965 g/cm3 in true density. The MPL included both FEP121A solution and Vulcan XC-72 powder. In Figure 6, the increasing Vulcan XC-72 powder induced the carbon-black powder to enter the void carbon fibers and increased the true density of GDL. In a long time of water washing, the true density kept about 1.6 g/cm3, which included MPL. In fact, the (10) Pharoah, J. G. J. Power Sources 2005, 144, 77–82. (11) Williams, M. V.; Kunz, H. R.; Fenton, J. M. J. Electrochem. Soc. 2004, 151, A1617–A1627.

Washing Experiment of GDL in a PEMFC

Figure 7. Polarization curves of GDB and GDL. (9) GDB (carbon fiber paper) and (0) GDL (MPL coating on GDB).

surface shape of 200 h washed GDL (Figure 2h) showed few MPL on the GDL surface. The invariant true density of long time washed GDL demonstrated that the high density of MPL did not wash out GDL. According to Figures 5 and 6, the results are enough to provide that MPL was washed into GDL and not washed out. 3.2. Electrochemical Performance of GDL in a PEMFC. Early studies12–14 focused on MPL components, such as carbon black and hydrophobic agents, and optimized MPL content and thickness. They demonstrated that thin MPL was generated by using the effects on performance by enhancing gas permeability and electrical conductivity while simultaneously maintaining the ability of water management. To investigate effects of MPL in a GDL on polarization characteristics of a PEMFC, membrane electrode assembly (MEA) is fabricated in-house with bared GDB and GDL. The GDB is carbon fiber paper, and the GDL is carbon fiber paper plus a MPL (MPL, 10 wt % FEP plus Vulcan XC-72 powder, loading about 1 mg/cm2). Figure 7 showed the effect of GDB and GDL on the polarization behaviors of MEA under an operating condition of 40 °C cell temperature, 95% RH of anode/cathode at 40 °C, and 0 psig back pressure. In this operating condition, the cathode was expected to be in an oversaturated environment. The MEA made of GDB showed a current density of 415 mA/cm2 at 0.6 V, whereas the MEA of GDL records a current density of 654 mA/cm2 at 0.6 V. This is because pores inside GDB may fill up with liquid water compared to GDL, where hydrophobic pores allowed gases to pass through to the catalyst layer. However, at higher current density, it appeared that MPL imposed an additional diffusion resistance to oxygen transport into the catalyst layer, making the mass transport limiting current smaller. As properly designed and fabricated, MPL should also be beneficial at high current densities. Figure 8 compared the performance with various GDL, which corresponded to the degree of water washing. The operating conditions were cell temperature at 40 °C with 0 psig back pressure, using H2 on the anode and O2 on the cathode. The gas flow was 0.5/0.5 SLPM, and the gas humidification was (12) Lim, C.; Wang, C. Y. Electrochim. Acta 2004, 49, 4149–4156. (13) Moreira, J.; Ocampo, A. L.; Sebastian, P. J.; Smit, M. A.; Salazar, M. D.; del Angel, P.; Montoya, J. A.; Pe´rez, R.; Martı´nez, L. Int. J. Hydrogen Energy 2003, 28, 625–627. (14) Park, S.; Lee, J. W.; Popov, B. N. J. Power Sources 2006, 163, 357–363.

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Figure 8. Polarization curves of GDLs in various washing times. (9) GDL (MPL coating on GDB), (b) GDL-1HR (in 1 h water washing), (2) GDL-5HR (in 5 h water washing), (1) GDL-10HR (in 10 h water washing), ([) GDL-50HR (in 50 h water washing), (″left solid triangle″) GDL-100HR (in 100 h water washing), and (″right solid triangle″) GDL-200HR (in 200 h water washing).

40/40 °C dew point on the anode and cathode, respectively. The result shows that the performance was decreasing gradually with an increasing degree of water washing. The performance of GDL, GDL-1HR, GDL-5HR, GDL-10HR, GDL-50HR, GDL-100HR, and GDL-200HR showed the current densities of 654, 610, 543, 473, 419, 423, and 403 mA/cm2, respectively, at 0.6 V. An explanation of these experimental results can derive the analysis above about the characteristics of each GDL. Some early studies15–17 have recently considered the effects of electrical resistance of GDL and prove that, under certain conditions, electrical resistance of GDL is sufficient to alter characteristics of current density distributions under gas channels and land areas. Meng et al.15 showed that current distribution is determined by two factors: oxygen supply and lateral electronic resistance in GDL. At a high cell voltage, the lateral electronic resistance dictated the current distribution. Sivertsen et al.16 showed that changing conductivity radically alters the current distribution by changing the relative influence of resistance to activation overpotentials. According to Figure 2, the MPL moved off from the GDL surface in long time water washing. It caused the decrease in resistance with the washing time increasing (Figure 3). The MPL was washed into GDL and affected performance in a PEMFC slightly. A crucial function of GDL is to provide electrons through conduction and serve as a bridge between CL and the bipolar plate (currentcollecting land). The effect of water washing on performance at 0.7, 0.5, and 0.3 Vages in a PEMFC is shown in Figure 9. The MEA made of GDB showed current densities of 209, 644, and 1093 mA/ cm2 at 0.7, 0.5, and 0.3 V, respectively. The MEA made of GDL showed current densities of 327, 1025, 1742 mA/cm2 at 0.7, 0.5, and 0.3 V, respectively. The MEA made of GDL200HR showed current densities of 176, 682, 1227 mA/cm2 at 0.7, 0.5, and 0.3 V, respectively. This result indicates that the use of the MPL augmented the effect on fuel cell performance and is highly dependent upon MPL loading. The performance in a PEMFC reduced with the washing time (15) Meng, H.; Wang, C. Y. J. Electrochem. Soc. 2004, 151, A358– A367. (16) Sivertsen, B. R.; Djilali, N. J. Power Sources 2005, 141, 65–78. (17) Zhou, T.; Liu, H. J. Power Sources 2006, 161, 444–453.

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kept invariant, thus causing the performance to be reduced and exhibited a higher current density than GDB at a low-voltage region. The effect of inside MPL was weaker than outward MPL on performance in a PEMFC; even so, it kept the hydrophobic poring and was better than GDB. 4. Conclusions

Figure 9. Current densities at 0.3, 0.5, and 0.7 V as a function of GDL in various washing times. (0) 0.7 V, (O) 0.5 V, and (4) 0.3 V.

increasing. The GDL-10HR, GDL-50HR, GDL-100HR, and GDL-200HR showed the similar performance and greater than GDB at various voltage. To better explain this observation, Figures 2–6 showed a crucial characterization of GDL in long time washing, in which MPL was washed into GDL and not washed out. When MPL kept blanketing outward the GDL, the performance exhibited the highest current density. When MPL washed into GDL, the resistance reduced and the porosity was

The MPL coated on GDB makes the water distribution in the MEA more uniform and effectively avoids flooding of the cathode, resulting in a high PEMFC performance. The performances of GDB and GDL are 415 and 654 mA/cm2, respectively, at 0.6 V. In long time washing, the MPL forms cracks and falls into the inside of GDL. Indeed, these facts cause decreasing of resistance and performance. However, GDL in long time water washing (200 h) shows a similar performance with GDB at a high-voltage region (402 and 415 mA/cm2 at 0.6 V) and higher performance than GDB at a low-voltage region (1227 and 1093 mA/cm2 at 0.3 V). The effect of inside MPL is weaker than outward MPL on performance in a PEMFC, but it keeps the performance better than GDB. In this work, we proved that the MPL on the GDL surface has a greater performance than MPL into GDL. Acknowledgment. The authors thank the Office of Technology Licensing and Precision Instrument Support Center of Feng Chia University for this study. EF800116C