- Email: [email protected]
A 10 kW class PEM fuel cell stack based on the catalyst-coated membrane (CCM) method
International Journal of Hydrogen Energy 31 (2006) 1010 – 1018 www.elsevier.com/locate/ijhydene
A 10 kW class PEM fuel cell stack based on the catalyst-coated membrane (CCM) method Mingruo Hu∗ , Sheng Sui, Xinjian Zhu, Qingchun Yu, Guangyi Cao, Xueying Hong, Hengyong Tu Institute of Fuel Cell, Shanghai Jiao Tong University, Shanghai, 200030, P.R. China Received 8 August 2005; received in revised form 30 October 2005; accepted 10 February 2006 Available online 30 March 2006
Abstract A 60-cell 10 kW class PEM fuel cell stack has been developed and tested. By using the catalyst-coated membrane (CCM) method, a power density of 0.36 W/cm2 was reached for a single cell with a 600 cm2 active area. The total Pt loading was 0.6 mg/cm2 for the anode and cathode catalyst layers. Cyclic voltammogram (CV) tests revealed that the CCM method had a higher Pt utilization when compared to the hydrophobic method. Furthermore, a maximum power of 10.9 kW was reached at an air utilization of 30%. A 400-h performance test showed a ±2 V fluctuation of the stack voltages. Coupling the 10 kW class stack with an external humidifier gave a relatively poor result of 6 kW. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Proton exchange membrane; Fuel cell; Stack; Catalyst-coated membrane (CCM); External humidifier
1. Introduction Proton exchange membrane (PEM) fuel cells offer several advantages as power sources. They can operate at low temperatures (50–80◦ ), start-up and shutdown rapidly, respond quickly to changing electric loads, and sustain unlimited thermal cycles. They are ideally suited for transportation, and other power applications. PEM fuel cell stacks operating on hydrogen can be 40–50% electrically efficient and 80% system efficient (if heat recovery is included). Recently, PEM fuel cell stacks based on different materials, structures and fabricating methods have been developed in several countries. Their power output varies from less than 100 W for portable power applications [1] ∗ Corresponding author. Tel.: +86 21 6293 3786 808; fax: +86 21 6293 2154. E-mail address: [email protected] (M. Hu).
to 1 kW or several kilowatts for residential use [2–6], to 20–75 kW for cars [7] and to 200–250 kW for stationary use [8]. In this paper, we describe the research and development of a 10 kW class PEM fuel cell based on our previous experience in developing a 1 kW class PEM fuel cell stack [9]. The design and fabrication of the MEA, the bipolar plate, the sealing method, the humidifier and the test bench will be presented. The related experimental results will also be discussed in detail. 2. R&D procedures for the 10 kW class PEM fuel cell The R&D procedures for this 10 kW class PEM fuel cell can be divided into five parts: the single cell fabrication, the design and manufacture of the bipolar plate, the sealing method, the design and construction of a test
0360-3199/$30.00 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.02.018
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1011
at 360 ◦ C for 25 min. A homogeneous suspension of 30% PTFE and Vulcan XC-72 carbon powder was then coated onto the carbon paper to make a micro-layer. The whole gas diffusion layer was sintered again at 360 ◦ C for 25 min.
Fig. 1. Hot-press machine.
bench for the 10 kW stack, and the development of an external humidifier. 2.1. Preparation of the single cell using the catalystcoated membrane (CCM) method 2.1.1. Membrane pretreatment Before it was used to prepare a single cell, the Nafion membrane was pretreated by boiling first in 5% aqueous H2 O2 solution and then in 1 M H2 SO4 solution to remove the organic and metallic contaminants. The decontaminated membrane was then stored in de-ionized water. 2.1.2. Preparation of the gas diffusion layer Carbon paper was first soaked in 30% PTFE aqueous solution. After air-drying, the carbon paper was sintered
2.1.3. Preparation of the catalyst coated membrane A catalyst-coated membrane (CCM) has several advantages over a catalyst-coated gas diffusion layer prepared by a hydrophobic method. A CCM has a lower Pt loading with a higher utilization efficiency, a closer contact between the membrane and catalyst layer to prevent delamination from swelling, and most importantly, a better performance [10,11]. As the Nafion membrane has a tendency to swell after the pretreatment process, the coating process should satisfy two considerations. The first one is to keep the periphery of the active area on a membrane stable enough to prevent wrinkling, and the second one is to elevate the temperature to evaporate the water in the membrane and the solvent in the catalyst ink. With these two points in mind, our equipment was designed to coat the catalyst ink, which contains Pt/C catalyst, Nafion solution and isopropyl alcohol, directly onto the membrane. 2.1.4. Hot-pressing of the single cell Hot-pressing was conducted at 140 ◦ C and 10 atm for 90 s. The cell thickness can be precisely controlled to within ±0.01 mm based on our newly designed hotpress machine as shown in Fig. 1. 2.1.5. Size chosen for the single cell in the stack In order to keep a shorter stack size to make more uniform gas distributions in the manifolds, a large active
Table 1 Design project of the 10 kW PEM fuel cell stack Item
Design project
Active area of a single cell Total Pt loading (anode plus cathode) Membrane Carbon paper Cell number Material of the bipolar plate Size of the bipolar plate Flow field Manifold direction Cooling method Direction of the air and hydrogen flow Stack size (including two end plates) Stack weight (including two end plates)
600 cm2 0.6 mg/cm2 (Johnson Matthey 4100) Nafion 1135 (Dupont) Toray 120 (Toray) 60 Graphite 960 cm2 Serpentine U-typea Water Co-flow 47 × 30.5 × 42 cm2 55 kg
a U-type means the inlets and outlets of manifolds exist in the same end plate.
1012
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
area of 600 cm2 was chosen for the single cell in our 10 kW class PEM fuel cell stack. 2.2. Design and manufacture of the bipolar plate Serpentine flow fields were designed for the flow of air, hydrogen and cooling water. The number of channels at the cathode side is three times that of the anode
side. Furthermore, graphite plates were grooved with a depth precision of ±0.02 mm. The size of the bipolar plate is 960 cm2 and a 62.5% utilization of the bipolar plate was reached. 2.3. Sealing method The sealing line was formed from liquid silicon rubber and dried in a vacuum oven at 50 ◦ C. 2.4. Summary of the stack design and difficulties in fabrication The design of the 10 kW class PEM fuel cell is summarized in Table 1. Furthermore, difficulties encountered in fabrication are presented as follows:
Fig. 2. The 10 kW PEM fuel cell stack.
(1) The Nafion membrane wrinkles easily. It is important to keep the periphery of the active area on the membrane as flat as possible when making a CCM and when hot-pressing a single cell in order to make a high-quality gasket. (2) In order to make a good contact with the bipolar plate, the thickness of a gas diffusion electrode must be precisely controlled over a large region. (3) It is also important to precisely control the depth for the gas channel and sealing groove in order to
Fig. 3. Schematic diagram of the test bench for the 10 kW class PEM fuel cell stack.
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1013
Table 2 Basic operating parameters for the 10 kW class PEM fuel cell stack Item
Parameter
Anode/cathode reaction gas H2 utilization Operating pressure at anode/cathode Cooling water temperature out of the stack Humidification temperature for anode/cathode
99.7% H2 (by volume)/air 80% Atmospheric pressure for both sides 60 ◦ C 65/55 ◦ C
improve the gas distribution in the flow fields and to make a good seal. (4) The graphite bipolar plate is very fragile and it is important to develop a technique that can assemble PEM fuel cell stack with flexible gas diffusion layers and sealing lines. After solving the above difficulties, a 10 kW class PEM fuel cell stack was assembled as shown in Fig. 2. 2.5. Test bench and operating procedures for the 10 kW class PEM fuel cell Schematic diagram of the test bench and basic operating parameters for the 10 kW class PEM fuel cell stack are shown in Fig. 3 and Table 2. As shown in Table 2, our 10 kW class PEM fuel cell stack was operated at atmospheric pressure. The general operating procedures for testing the stack are as follows: (1) Elevate the anode/cathode humidification temperature 5/6 to 65/55 ◦ C. (2) Adjust Valve 2 and Valve 4 to the desired inlet pressures. Adjust Flowmeter 15 and Flowmeter 16 to control the exhaust flux. Valve 13 and Valve 14 are always fully opened to keep atmospheric pressures at 7 and 8. (3) Open water Pump 9 as needed to control the cooling water temperature 10 out of the stack. (4) Adjust the electronics load, and record the I –V data after the stack output becomes stable. (5) After testing, open Valve 1 and Valve 3 to purge the pipelines and the stack with N2.
2.6. Development of an external humidifier An external humidifier was also developed in order to compact the PEM fuel cell stack system and to reduce the parasitic power needed for evaporating the de-ionized water in a humidification tank. The basic principles for the external humidifier are as follows:
Fig. 4. The external humidifier.
cooling water first enters the PEM fuel cell stack to absorb the waste heat. Then, it is distributed into each cell of the external humidifier at an elevated temperature, where it supplies water to the air and the hydrogen through a water permeable membrane, e.g. a Nafion membrane. The appearance of this external humidifier is shown in Fig. 4.
3. Results and discussions 3.1. Performance evaluation for small-scale PEM fuel cells Prior to the construction of the full stack, we constructed two test cells of 25 cm2 with the same total Pt loading of 0.6 mg/cm2 to compare the CCM method to our former hydrophobic method. Other materials for these two cells are shown in Table 1. Evaluations of the performances and cyclic voltammogram (CV) tests for these two single cells are shown in Figs. 5 and 6 separately.
1014
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
Fig. 5. Performance of single cells using different methods for the fabrication of the catalyst layer.
Fig. 6. Cyclic voltammograms (scan rate of 50 mV/s) of single cells using different methods for the fabrication of the catalyst layer.
As shown in Fig. 5, the CCM method offers a large performance increase over the hydrophobic method based on an air utilization of 30% and a H2 utilization of 80%. This is because the H2 desorption peak of the CCM method is higher than that of the hydrophobic method as shown in Fig. 6, as a result, a higher reaction area can be reached [12,13]. 3.2. Performance of a single cell for the 10 kW class PEM fuel cell stack Based on the results of the two small-scale test cells, a single cell of 600 cm2 was tested in order to confirm the number of cells needed in a 10 kW stack. As shown in Fig. 7, when keeping an air utilization of 30% and a H2 utilization of 80%, a maximum power
density of 0.36 W/cm2 (215 W power output for a single cell) could be reached at 666.67 mA/cm2 (i.e. 400 A). Therefore, 60 cells were chosen for the 10 kW class PEM fuel cell stack. The decrease in maximum power density from 0.42 to 0.36 W/cm2 may be explained by the non-uniformity in oxygen distribution over the larger active area. At the same current density, the current will be quite larger for a 600 cm2 active area than that for a 25 cm2 active area. As a result, more water will be produced. If the channel width and the rib width are kept constant, the gas channel is much longer for a 600 cm2 flow field than that of a 25 cm2 flow field, and more serpentine flow patterns exist. Therefore, the oxygen transport resistance in the 600 cm2 flow field will be higher than that in the 25 cm2 flow field. This results in greater non-uniformity in the oxygen distribution for the 600 cm2 flow field. The greater non-uniformity of oxygen distribution in turn causes more over-potential [14] for the single cell with a 600 cm2 active area than that with a 25 cm2 active area. 3.3. Stack performance at different air utilizations Air utilization is one of the most important indexes to evaluate the performance of a PEM fuel cell system. The higher the air utilization, the smaller the parasitic power is needed. According to Faraday’s law, the theoretical consumption of H2 , O2 and air for the 10 kW class stack at different currents can be calculated by Eqs. (1)–(3), and the results are shown in Table 3. Theoretical H2 consumption VHT2 at different currents is I × G × N × 60 VHT2 = (L/ min), (1) 2×F
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1015
Fig. 7. Performance of the 600 cm2 single cell. Table 3 Theoretical consumptions of H2 , O2 and air at different currents Current I (A)
H2 consumption (L/min)
O2 consumption (m3 /h)
Air consumption (m3 /h)
25 50 75 100 125 150 175 200 225 250 275 300 325 350
10.45 20.90 30.35 41.80 52.25 62.70 73.15 83.60 94.05 104.50 114.95 125.40 135.85 146.30
0.31 0.63 0.94 1.25 1.56 1.88 2.19 2.50 2.81 3.13 3.44 3.75 4.06 4.37
1.49 2.98 4.46 5.95 7.44 8.93 10.42 11.90 13.39 14.88 16.37 17.86 19.35 20.84
where I, G, N and F represent current, gas volume per mole, cell number and Faraday constant, and have the units or values of A, 22.4 L/mol at 20 ◦ C and 1 atm, 60 cells for this 10 kW stack and 96485.3 C/mol. (The last constant “60” in Eq. (1) is 60 s/min.) Theoretical O2 consumption VOT2 at different currents is VOT2 =
I × G × N × 3600 4 × F × 1000
(m3 /h),
(2)
where the constant “3600” means 3600 s/h, and the constant “1000” means 1000 L/m3 . T at different currTheoretical air consumption Vair ents is T = Vair
VOT2 0.21
(m3 /h).
(3)
As a result, the actual gas consumption V A can be calculated by VA =
VT ,
(4)
where V T and represent the theoretical gas consumption and the gas utilization. Based on the above calculations, the 10 kW class PEM fuel cell stack was tested at three different air utilizations of 50%, 40% and 30%, and the other operating parameters are shown in Table 2. The testing bench and the operating procedures are referred in Section 2.5. The resulting performance is shown in Fig. 8. As shown in Fig. 8, the performance of the 10 kW class PEM fuel cell stack increases with decreasing air utilization. For a large air utilization of 50%, the maximum output power is only 6.2 kW at 200 A; however,
1016
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
Fig. 8. Performance of the 10 kW class PEM fuel cell stack at different air utilizations.
Fig. 9. Voltage distributions of 60 cells in the 10 kW class PEM fuel cell stack.
after decreasing the air utilization to 30%, a maximum power of 10.9 kW can be reached at 325 A, corresponding to a power density of 0.3 W/cm2 . As shown in Eq. (4), the decrease of air utilization increases the actual air amount entering the stack, and as a result, the concentration of O2 at reaction area is increased [15]. Fig. 9 shows the voltage distributions of the 60 cells in the 10 kW class PEM fuel cell stack operating at
different currents of 0, 125 and 325 A with an air utilization of 30% and a H2 utilization of 80%. As shown in Fig. 9, the stack has an even voltage distribution with a maximum fluctuation of ±0.05 V at 325 A. However, a slightly decrease in voltage is found at 325 A. This can be explained by considering the flow patterns. For a U-type flow pattern, the pressure differences between an inlet manifold and an outlet manifold are different for different cells. The pressure differences for the cells near the end plate containing the inlets and outlets of manifolds are higher than the pressure difference for high cell numbers. As a consequence, more gas enters into the cells near the end plate containing the inlets and outlets of the manifolds [16]. That is why the voltages for high cell numbers are a little bit worse than those for low cell numbers at a higher current. As a result, a Z-type manifold, i.e. the inlets and outlets of manifolds exist in the different end plates of a stack, has been considered to be superior to a U-type manifold [17]. However, it is easier to assemble a PEM fuel cell stack with a U-type design and so U-type manifolds are chosen in this 10 kW stack. Fig. 10 demonstrates the power output from the 10 kW class PEM fuel cell stack. Ninety-six electrical bulbs, with rated powers of 100 W each are assembled on a white board. As a result, a nominal 10 kW electrical power is outputted in this demonstration.
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1017
Fig. 10. Demonstration of the power output from the 10 kW class PEM fuel cell stack.
Fig. 12. Performance of the 10 kW class PEM fuel cell stack with an external humidifier.
Fig. 11. 400-h performance test of the 10 kW class PEM fuel cell stack.
A constant air utilization of 30% was kept during the testing, while the other operating parameters are shown in Table 2. The humidifier was first assembled with 30 humidification cells, each of which has the same working area of 600 cm2 . The resulting performance of the PEM fuel cell stack was 5.8 kW. After that, 10 more humidification cells were further assembled into the humidifier. However, only 6 kW electrical power was outputted as shown in Fig. 12. As only a 3.45% increase in the electrical power output was obtained after 33.33% more humidification cells were added, no more humidification cells were further assembled into the external humidifier. Furthermore, two humidity sensors were placed at the outlets of the humidifier. Relative humidity of 40–50% was found in the air and the hydrogen out of the external humidifier. When compared with the humidification method shown in Fig. 3, which uses humidification tanks and usually presents relative humidity around 100%, relative humidity of 40–50% explains the lower performance as shown in Fig. 12. Although Nafion membrane would have been preferred for the humidification cell, because of its high price another kind of water-permeable membrane with a thickness of 0.5 mm was used in this external humidifier. Based on the above analysis, we conclude that the thickness of our membrane is the main reason for the relative poor performance of the humidifier.
3.4. 400-h performance test of the 10 kW class PEM fuel cell stack A 400-h performance test of the 10 kW class PEM fuel cell stack has been performed since the stack was assembled. A current of 125 A was kept during the test with a constant air utilization of 30%, and other operating parameters are shown in Table 2. As shown in Fig. 11, a ±2 V fluctuation of the stack voltage can be seen during the 400 h, which is attributed to the water accumulation in the stack; after dry N2 is supplied to the anode and cathode sides to carry liquid water out of the stack, the performance of the stack was restored.
4. Conclusions and future plans 3.5. Performance test of the 10 kW class PEM fuel cell stack with an external humidifier The 10 kW class PEM fuel cell stack was also tested with an external humidifier as shown in Fig. 4.
4.1. Conclusions A 10 kW class PEM fuel cell stack, which is comprised of 60 cells, has been developed and tested. By using the CCM method for the catalyst layer
1018
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
fabrication, a power density of 0.36 W/cm2 was reached for a 600 cm2 active area with a total Pt loading of 0.6 mg/cm2 . CV tests revealed a higher Pt utilization for the CCM method when compared with the hydrophobic method. Furthermore, the 10 kW class PEM fuel cell stack was tested at three different air utilizations of 50%, 40% and 30%, while the H2 utilization was kept constant at 80%. A maximum power of 10.9 kW was reached at 325 A, corresponding to a power density of 0.3 W/cm2 , after the air utilization was reduced to 30%. Although, the voltages from the first cell to the last in the stack had a declining trend at 325 A, the stack still showed overall even voltage distributions with a maximum fluctuation of ±0.05 V at 325 A. A 400-h performance testing showed only ±2 V fluctuation of the stack voltage, which was attributed to the water accumulation in the stack. At last, coupling the 10 kW class stack with an external humidifier gave a relatively poor result of 6 kW. 4.2. Future plans Based on the above conclusions, our future plans are as follows: (1) Optimize the flow field design and material selection (e.g. replace Toray 120 with Toray 60, a thinner carbon paper) to increase the stack air utilization and the performance of an external humidifier. (2) Develop a method and/or equipment for efficient water and thermal management to decrease the performance fluctuation in a PEM fuel cell stack. (3) Develop automatic equipment that will continuously coat the proton exchange membrane. Acknowledgments The authors would like to thank Shanghai Jiao Tong University and Biaozheng New Energy High-Tech Co. Ltd. (Shanghai) for supporting this research work. The authors would like to give their special thanks to CHINO Corporation, Japan for donating the FC5100 PEM fuel cell test bench for the performance testing of two smallscale PEM fuel cells of 25 cm2 .
References [1] Morner SO, Klein SA. Experimental evaluation of the dynamic behavior of an air-breathing fuel cell stack. J Solar Energy Eng, Trans ASME 2001;123:225–31. [2] Giddey S, Ciacchi FT, Badwal SPS. Design, assembly and operation of polymer electrolyte membrane fuel cell stacks to 1 kWe capacity. J Power Sources 2004;125:155–65. [3] Tsai MH, Yan YY, Chu HS, Shyu RJ, Tsau F. Analysis of a 3 kW PEM fuel cell stack developed by ITRI. Fuel Cell Sci Eng Technol 2004;201–4. [4] Scholta J, Berq N, Wilde P, Jorissen L, Garche J. Development and performance of a 10 kW PEMFC stack. J Power Sources 2004;127:206–12. [5] Cho EA, Jeon U-S, Hong S-A, Oh I-H, Kang S-G. Performance of a 1 kW-class PEMFC stack using TiN-coated 316 stainless steel bipolar plates. J Power Sources 2005;142:177–83. [6] Santis M, Schmid D, Ruge M, Freunberger S, Buchi FN. Modular stack-internal air humidification concept-verification in a kW stack. Fuel Cells 2004;4:214–8. [7] Barbir F, Fuchs M, Husar A, Neutzler J. Design and operational characteristics of automotive PEM fuel cell stacks. Fuel cell power for transportation, SAE SP-1505, SAE, Warrendale, PA; 2000. p. 63–9. [8] Pokojski M. First demonstration of the 250-kW polymer electrolyte fuel cell for stationary application (Berlin). J Power Sources 2000;86:140–4. [9] Hu M, Zhu X, Cao G, Gu A. Experimental study of a PEM (proton exchange membrane) fuel cell stack of 1 kW class (in Chinese). J Eng Thermal Energy Power 2004;19:119–21. [10] US Patent 5,211,984. [11] Litster S, Mclean G. PEM fuel cell electrodes. J Power Sources 2004;130:61–76. [12] Cheng X, Yi B, Han M, Zhang J, Qiao Y, Yu J. Investigation of platinum utilization and morphology in catalyst layer of polymer electrolyte fuel cells. J Power Sources 1999;79: 75–81. [13] Ticianelli EA, Derouin CR, Srinivasan S. Localization of platinum in low catalyst loading electrodes to attain high power densities in SPE fuel cells. J Electroanal Chem 1988;251: 275–95. [14] Nguyen TV, White RE. A water and heat management model for proton-exchange-membrane fuel cell. J Electrochem Soc 1993;140:2178–86. [15] Berning T, Djilali N. Three-dimensional computational analysis of transport phenomena in a PEM fuel cell—a parametric study. J Power Sources 2003;124:440–52. [16] Costamagna P, Arato E, Achenbach E. et al. Fluid dynamic study of fuel cell devices: simulation and experimental validation. J Power Sources 1994;52:243–9. [17] Thirumalai D, White RE. Mathematical modeling of protonexchange-membrane fuel–cell stack. J Electrochem Soc 1997;144:1717–23.
A 10 kW class PEM fuel cell stack based on the catalyst-coated membrane (CCM) method Mingruo Hu∗ , Sheng Sui, Xinjian Zhu, Qingchun Yu, Guangyi Cao, Xueying Hong, Hengyong Tu Institute of Fuel Cell, Shanghai Jiao Tong University, Shanghai, 200030, P.R. China Received 8 August 2005; received in revised form 30 October 2005; accepted 10 February 2006 Available online 30 March 2006
Abstract A 60-cell 10 kW class PEM fuel cell stack has been developed and tested. By using the catalyst-coated membrane (CCM) method, a power density of 0.36 W/cm2 was reached for a single cell with a 600 cm2 active area. The total Pt loading was 0.6 mg/cm2 for the anode and cathode catalyst layers. Cyclic voltammogram (CV) tests revealed that the CCM method had a higher Pt utilization when compared to the hydrophobic method. Furthermore, a maximum power of 10.9 kW was reached at an air utilization of 30%. A 400-h performance test showed a ±2 V fluctuation of the stack voltages. Coupling the 10 kW class stack with an external humidifier gave a relatively poor result of 6 kW. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Proton exchange membrane; Fuel cell; Stack; Catalyst-coated membrane (CCM); External humidifier
1. Introduction Proton exchange membrane (PEM) fuel cells offer several advantages as power sources. They can operate at low temperatures (50–80◦ ), start-up and shutdown rapidly, respond quickly to changing electric loads, and sustain unlimited thermal cycles. They are ideally suited for transportation, and other power applications. PEM fuel cell stacks operating on hydrogen can be 40–50% electrically efficient and 80% system efficient (if heat recovery is included). Recently, PEM fuel cell stacks based on different materials, structures and fabricating methods have been developed in several countries. Their power output varies from less than 100 W for portable power applications [1] ∗ Corresponding author. Tel.: +86 21 6293 3786 808; fax: +86 21 6293 2154. E-mail address: [email protected] (M. Hu).
to 1 kW or several kilowatts for residential use [2–6], to 20–75 kW for cars [7] and to 200–250 kW for stationary use [8]. In this paper, we describe the research and development of a 10 kW class PEM fuel cell based on our previous experience in developing a 1 kW class PEM fuel cell stack [9]. The design and fabrication of the MEA, the bipolar plate, the sealing method, the humidifier and the test bench will be presented. The related experimental results will also be discussed in detail. 2. R&D procedures for the 10 kW class PEM fuel cell The R&D procedures for this 10 kW class PEM fuel cell can be divided into five parts: the single cell fabrication, the design and manufacture of the bipolar plate, the sealing method, the design and construction of a test
0360-3199/$30.00 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.02.018
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1011
at 360 ◦ C for 25 min. A homogeneous suspension of 30% PTFE and Vulcan XC-72 carbon powder was then coated onto the carbon paper to make a micro-layer. The whole gas diffusion layer was sintered again at 360 ◦ C for 25 min.
Fig. 1. Hot-press machine.
bench for the 10 kW stack, and the development of an external humidifier. 2.1. Preparation of the single cell using the catalystcoated membrane (CCM) method 2.1.1. Membrane pretreatment Before it was used to prepare a single cell, the Nafion membrane was pretreated by boiling first in 5% aqueous H2 O2 solution and then in 1 M H2 SO4 solution to remove the organic and metallic contaminants. The decontaminated membrane was then stored in de-ionized water. 2.1.2. Preparation of the gas diffusion layer Carbon paper was first soaked in 30% PTFE aqueous solution. After air-drying, the carbon paper was sintered
2.1.3. Preparation of the catalyst coated membrane A catalyst-coated membrane (CCM) has several advantages over a catalyst-coated gas diffusion layer prepared by a hydrophobic method. A CCM has a lower Pt loading with a higher utilization efficiency, a closer contact between the membrane and catalyst layer to prevent delamination from swelling, and most importantly, a better performance [10,11]. As the Nafion membrane has a tendency to swell after the pretreatment process, the coating process should satisfy two considerations. The first one is to keep the periphery of the active area on a membrane stable enough to prevent wrinkling, and the second one is to elevate the temperature to evaporate the water in the membrane and the solvent in the catalyst ink. With these two points in mind, our equipment was designed to coat the catalyst ink, which contains Pt/C catalyst, Nafion solution and isopropyl alcohol, directly onto the membrane. 2.1.4. Hot-pressing of the single cell Hot-pressing was conducted at 140 ◦ C and 10 atm for 90 s. The cell thickness can be precisely controlled to within ±0.01 mm based on our newly designed hotpress machine as shown in Fig. 1. 2.1.5. Size chosen for the single cell in the stack In order to keep a shorter stack size to make more uniform gas distributions in the manifolds, a large active
Table 1 Design project of the 10 kW PEM fuel cell stack Item
Design project
Active area of a single cell Total Pt loading (anode plus cathode) Membrane Carbon paper Cell number Material of the bipolar plate Size of the bipolar plate Flow field Manifold direction Cooling method Direction of the air and hydrogen flow Stack size (including two end plates) Stack weight (including two end plates)
600 cm2 0.6 mg/cm2 (Johnson Matthey 4100) Nafion 1135 (Dupont) Toray 120 (Toray) 60 Graphite 960 cm2 Serpentine U-typea Water Co-flow 47 × 30.5 × 42 cm2 55 kg
a U-type means the inlets and outlets of manifolds exist in the same end plate.
1012
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
area of 600 cm2 was chosen for the single cell in our 10 kW class PEM fuel cell stack. 2.2. Design and manufacture of the bipolar plate Serpentine flow fields were designed for the flow of air, hydrogen and cooling water. The number of channels at the cathode side is three times that of the anode
side. Furthermore, graphite plates were grooved with a depth precision of ±0.02 mm. The size of the bipolar plate is 960 cm2 and a 62.5% utilization of the bipolar plate was reached. 2.3. Sealing method The sealing line was formed from liquid silicon rubber and dried in a vacuum oven at 50 ◦ C. 2.4. Summary of the stack design and difficulties in fabrication The design of the 10 kW class PEM fuel cell is summarized in Table 1. Furthermore, difficulties encountered in fabrication are presented as follows:
Fig. 2. The 10 kW PEM fuel cell stack.
(1) The Nafion membrane wrinkles easily. It is important to keep the periphery of the active area on the membrane as flat as possible when making a CCM and when hot-pressing a single cell in order to make a high-quality gasket. (2) In order to make a good contact with the bipolar plate, the thickness of a gas diffusion electrode must be precisely controlled over a large region. (3) It is also important to precisely control the depth for the gas channel and sealing groove in order to
Fig. 3. Schematic diagram of the test bench for the 10 kW class PEM fuel cell stack.
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1013
Table 2 Basic operating parameters for the 10 kW class PEM fuel cell stack Item
Parameter
Anode/cathode reaction gas H2 utilization Operating pressure at anode/cathode Cooling water temperature out of the stack Humidification temperature for anode/cathode
99.7% H2 (by volume)/air 80% Atmospheric pressure for both sides 60 ◦ C 65/55 ◦ C
improve the gas distribution in the flow fields and to make a good seal. (4) The graphite bipolar plate is very fragile and it is important to develop a technique that can assemble PEM fuel cell stack with flexible gas diffusion layers and sealing lines. After solving the above difficulties, a 10 kW class PEM fuel cell stack was assembled as shown in Fig. 2. 2.5. Test bench and operating procedures for the 10 kW class PEM fuel cell Schematic diagram of the test bench and basic operating parameters for the 10 kW class PEM fuel cell stack are shown in Fig. 3 and Table 2. As shown in Table 2, our 10 kW class PEM fuel cell stack was operated at atmospheric pressure. The general operating procedures for testing the stack are as follows: (1) Elevate the anode/cathode humidification temperature 5/6 to 65/55 ◦ C. (2) Adjust Valve 2 and Valve 4 to the desired inlet pressures. Adjust Flowmeter 15 and Flowmeter 16 to control the exhaust flux. Valve 13 and Valve 14 are always fully opened to keep atmospheric pressures at 7 and 8. (3) Open water Pump 9 as needed to control the cooling water temperature 10 out of the stack. (4) Adjust the electronics load, and record the I –V data after the stack output becomes stable. (5) After testing, open Valve 1 and Valve 3 to purge the pipelines and the stack with N2.
2.6. Development of an external humidifier An external humidifier was also developed in order to compact the PEM fuel cell stack system and to reduce the parasitic power needed for evaporating the de-ionized water in a humidification tank. The basic principles for the external humidifier are as follows:
Fig. 4. The external humidifier.
cooling water first enters the PEM fuel cell stack to absorb the waste heat. Then, it is distributed into each cell of the external humidifier at an elevated temperature, where it supplies water to the air and the hydrogen through a water permeable membrane, e.g. a Nafion membrane. The appearance of this external humidifier is shown in Fig. 4.
3. Results and discussions 3.1. Performance evaluation for small-scale PEM fuel cells Prior to the construction of the full stack, we constructed two test cells of 25 cm2 with the same total Pt loading of 0.6 mg/cm2 to compare the CCM method to our former hydrophobic method. Other materials for these two cells are shown in Table 1. Evaluations of the performances and cyclic voltammogram (CV) tests for these two single cells are shown in Figs. 5 and 6 separately.
1014
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
Fig. 5. Performance of single cells using different methods for the fabrication of the catalyst layer.
Fig. 6. Cyclic voltammograms (scan rate of 50 mV/s) of single cells using different methods for the fabrication of the catalyst layer.
As shown in Fig. 5, the CCM method offers a large performance increase over the hydrophobic method based on an air utilization of 30% and a H2 utilization of 80%. This is because the H2 desorption peak of the CCM method is higher than that of the hydrophobic method as shown in Fig. 6, as a result, a higher reaction area can be reached [12,13]. 3.2. Performance of a single cell for the 10 kW class PEM fuel cell stack Based on the results of the two small-scale test cells, a single cell of 600 cm2 was tested in order to confirm the number of cells needed in a 10 kW stack. As shown in Fig. 7, when keeping an air utilization of 30% and a H2 utilization of 80%, a maximum power
density of 0.36 W/cm2 (215 W power output for a single cell) could be reached at 666.67 mA/cm2 (i.e. 400 A). Therefore, 60 cells were chosen for the 10 kW class PEM fuel cell stack. The decrease in maximum power density from 0.42 to 0.36 W/cm2 may be explained by the non-uniformity in oxygen distribution over the larger active area. At the same current density, the current will be quite larger for a 600 cm2 active area than that for a 25 cm2 active area. As a result, more water will be produced. If the channel width and the rib width are kept constant, the gas channel is much longer for a 600 cm2 flow field than that of a 25 cm2 flow field, and more serpentine flow patterns exist. Therefore, the oxygen transport resistance in the 600 cm2 flow field will be higher than that in the 25 cm2 flow field. This results in greater non-uniformity in the oxygen distribution for the 600 cm2 flow field. The greater non-uniformity of oxygen distribution in turn causes more over-potential [14] for the single cell with a 600 cm2 active area than that with a 25 cm2 active area. 3.3. Stack performance at different air utilizations Air utilization is one of the most important indexes to evaluate the performance of a PEM fuel cell system. The higher the air utilization, the smaller the parasitic power is needed. According to Faraday’s law, the theoretical consumption of H2 , O2 and air for the 10 kW class stack at different currents can be calculated by Eqs. (1)–(3), and the results are shown in Table 3. Theoretical H2 consumption VHT2 at different currents is I × G × N × 60 VHT2 = (L/ min), (1) 2×F
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1015
Fig. 7. Performance of the 600 cm2 single cell. Table 3 Theoretical consumptions of H2 , O2 and air at different currents Current I (A)
H2 consumption (L/min)
O2 consumption (m3 /h)
Air consumption (m3 /h)
25 50 75 100 125 150 175 200 225 250 275 300 325 350
10.45 20.90 30.35 41.80 52.25 62.70 73.15 83.60 94.05 104.50 114.95 125.40 135.85 146.30
0.31 0.63 0.94 1.25 1.56 1.88 2.19 2.50 2.81 3.13 3.44 3.75 4.06 4.37
1.49 2.98 4.46 5.95 7.44 8.93 10.42 11.90 13.39 14.88 16.37 17.86 19.35 20.84
where I, G, N and F represent current, gas volume per mole, cell number and Faraday constant, and have the units or values of A, 22.4 L/mol at 20 ◦ C and 1 atm, 60 cells for this 10 kW stack and 96485.3 C/mol. (The last constant “60” in Eq. (1) is 60 s/min.) Theoretical O2 consumption VOT2 at different currents is VOT2 =
I × G × N × 3600 4 × F × 1000
(m3 /h),
(2)
where the constant “3600” means 3600 s/h, and the constant “1000” means 1000 L/m3 . T at different currTheoretical air consumption Vair ents is T = Vair
VOT2 0.21
(m3 /h).
(3)
As a result, the actual gas consumption V A can be calculated by VA =
VT ,
(4)
where V T and represent the theoretical gas consumption and the gas utilization. Based on the above calculations, the 10 kW class PEM fuel cell stack was tested at three different air utilizations of 50%, 40% and 30%, and the other operating parameters are shown in Table 2. The testing bench and the operating procedures are referred in Section 2.5. The resulting performance is shown in Fig. 8. As shown in Fig. 8, the performance of the 10 kW class PEM fuel cell stack increases with decreasing air utilization. For a large air utilization of 50%, the maximum output power is only 6.2 kW at 200 A; however,
1016
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
Fig. 8. Performance of the 10 kW class PEM fuel cell stack at different air utilizations.
Fig. 9. Voltage distributions of 60 cells in the 10 kW class PEM fuel cell stack.
after decreasing the air utilization to 30%, a maximum power of 10.9 kW can be reached at 325 A, corresponding to a power density of 0.3 W/cm2 . As shown in Eq. (4), the decrease of air utilization increases the actual air amount entering the stack, and as a result, the concentration of O2 at reaction area is increased [15]. Fig. 9 shows the voltage distributions of the 60 cells in the 10 kW class PEM fuel cell stack operating at
different currents of 0, 125 and 325 A with an air utilization of 30% and a H2 utilization of 80%. As shown in Fig. 9, the stack has an even voltage distribution with a maximum fluctuation of ±0.05 V at 325 A. However, a slightly decrease in voltage is found at 325 A. This can be explained by considering the flow patterns. For a U-type flow pattern, the pressure differences between an inlet manifold and an outlet manifold are different for different cells. The pressure differences for the cells near the end plate containing the inlets and outlets of manifolds are higher than the pressure difference for high cell numbers. As a consequence, more gas enters into the cells near the end plate containing the inlets and outlets of the manifolds [16]. That is why the voltages for high cell numbers are a little bit worse than those for low cell numbers at a higher current. As a result, a Z-type manifold, i.e. the inlets and outlets of manifolds exist in the different end plates of a stack, has been considered to be superior to a U-type manifold [17]. However, it is easier to assemble a PEM fuel cell stack with a U-type design and so U-type manifolds are chosen in this 10 kW stack. Fig. 10 demonstrates the power output from the 10 kW class PEM fuel cell stack. Ninety-six electrical bulbs, with rated powers of 100 W each are assembled on a white board. As a result, a nominal 10 kW electrical power is outputted in this demonstration.
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
1017
Fig. 10. Demonstration of the power output from the 10 kW class PEM fuel cell stack.
Fig. 12. Performance of the 10 kW class PEM fuel cell stack with an external humidifier.
Fig. 11. 400-h performance test of the 10 kW class PEM fuel cell stack.
A constant air utilization of 30% was kept during the testing, while the other operating parameters are shown in Table 2. The humidifier was first assembled with 30 humidification cells, each of which has the same working area of 600 cm2 . The resulting performance of the PEM fuel cell stack was 5.8 kW. After that, 10 more humidification cells were further assembled into the humidifier. However, only 6 kW electrical power was outputted as shown in Fig. 12. As only a 3.45% increase in the electrical power output was obtained after 33.33% more humidification cells were added, no more humidification cells were further assembled into the external humidifier. Furthermore, two humidity sensors were placed at the outlets of the humidifier. Relative humidity of 40–50% was found in the air and the hydrogen out of the external humidifier. When compared with the humidification method shown in Fig. 3, which uses humidification tanks and usually presents relative humidity around 100%, relative humidity of 40–50% explains the lower performance as shown in Fig. 12. Although Nafion membrane would have been preferred for the humidification cell, because of its high price another kind of water-permeable membrane with a thickness of 0.5 mm was used in this external humidifier. Based on the above analysis, we conclude that the thickness of our membrane is the main reason for the relative poor performance of the humidifier.
3.4. 400-h performance test of the 10 kW class PEM fuel cell stack A 400-h performance test of the 10 kW class PEM fuel cell stack has been performed since the stack was assembled. A current of 125 A was kept during the test with a constant air utilization of 30%, and other operating parameters are shown in Table 2. As shown in Fig. 11, a ±2 V fluctuation of the stack voltage can be seen during the 400 h, which is attributed to the water accumulation in the stack; after dry N2 is supplied to the anode and cathode sides to carry liquid water out of the stack, the performance of the stack was restored.
4. Conclusions and future plans 3.5. Performance test of the 10 kW class PEM fuel cell stack with an external humidifier The 10 kW class PEM fuel cell stack was also tested with an external humidifier as shown in Fig. 4.
4.1. Conclusions A 10 kW class PEM fuel cell stack, which is comprised of 60 cells, has been developed and tested. By using the CCM method for the catalyst layer
1018
M. Hu et al. / International Journal of Hydrogen Energy 31 (2006) 1010 – 1018
fabrication, a power density of 0.36 W/cm2 was reached for a 600 cm2 active area with a total Pt loading of 0.6 mg/cm2 . CV tests revealed a higher Pt utilization for the CCM method when compared with the hydrophobic method. Furthermore, the 10 kW class PEM fuel cell stack was tested at three different air utilizations of 50%, 40% and 30%, while the H2 utilization was kept constant at 80%. A maximum power of 10.9 kW was reached at 325 A, corresponding to a power density of 0.3 W/cm2 , after the air utilization was reduced to 30%. Although, the voltages from the first cell to the last in the stack had a declining trend at 325 A, the stack still showed overall even voltage distributions with a maximum fluctuation of ±0.05 V at 325 A. A 400-h performance testing showed only ±2 V fluctuation of the stack voltage, which was attributed to the water accumulation in the stack. At last, coupling the 10 kW class stack with an external humidifier gave a relatively poor result of 6 kW. 4.2. Future plans Based on the above conclusions, our future plans are as follows: (1) Optimize the flow field design and material selection (e.g. replace Toray 120 with Toray 60, a thinner carbon paper) to increase the stack air utilization and the performance of an external humidifier. (2) Develop a method and/or equipment for efficient water and thermal management to decrease the performance fluctuation in a PEM fuel cell stack. (3) Develop automatic equipment that will continuously coat the proton exchange membrane. Acknowledgments The authors would like to thank Shanghai Jiao Tong University and Biaozheng New Energy High-Tech Co. Ltd. (Shanghai) for supporting this research work. The authors would like to give their special thanks to CHINO Corporation, Japan for donating the FC5100 PEM fuel cell test bench for the performance testing of two smallscale PEM fuel cells of 25 cm2 .
References [1] Morner SO, Klein SA. Experimental evaluation of the dynamic behavior of an air-breathing fuel cell stack. J Solar Energy Eng, Trans ASME 2001;123:225–31. [2] Giddey S, Ciacchi FT, Badwal SPS. Design, assembly and operation of polymer electrolyte membrane fuel cell stacks to 1 kWe capacity. J Power Sources 2004;125:155–65. [3] Tsai MH, Yan YY, Chu HS, Shyu RJ, Tsau F. Analysis of a 3 kW PEM fuel cell stack developed by ITRI. Fuel Cell Sci Eng Technol 2004;201–4. [4] Scholta J, Berq N, Wilde P, Jorissen L, Garche J. Development and performance of a 10 kW PEMFC stack. J Power Sources 2004;127:206–12. [5] Cho EA, Jeon U-S, Hong S-A, Oh I-H, Kang S-G. Performance of a 1 kW-class PEMFC stack using TiN-coated 316 stainless steel bipolar plates. J Power Sources 2005;142:177–83. [6] Santis M, Schmid D, Ruge M, Freunberger S, Buchi FN. Modular stack-internal air humidification concept-verification in a kW stack. Fuel Cells 2004;4:214–8. [7] Barbir F, Fuchs M, Husar A, Neutzler J. Design and operational characteristics of automotive PEM fuel cell stacks. Fuel cell power for transportation, SAE SP-1505, SAE, Warrendale, PA; 2000. p. 63–9. [8] Pokojski M. First demonstration of the 250-kW polymer electrolyte fuel cell for stationary application (Berlin). J Power Sources 2000;86:140–4. [9] Hu M, Zhu X, Cao G, Gu A. Experimental study of a PEM (proton exchange membrane) fuel cell stack of 1 kW class (in Chinese). J Eng Thermal Energy Power 2004;19:119–21. [10] US Patent 5,211,984. [11] Litster S, Mclean G. PEM fuel cell electrodes. J Power Sources 2004;130:61–76. [12] Cheng X, Yi B, Han M, Zhang J, Qiao Y, Yu J. Investigation of platinum utilization and morphology in catalyst layer of polymer electrolyte fuel cells. J Power Sources 1999;79: 75–81. [13] Ticianelli EA, Derouin CR, Srinivasan S. Localization of platinum in low catalyst loading electrodes to attain high power densities in SPE fuel cells. J Electroanal Chem 1988;251: 275–95. [14] Nguyen TV, White RE. A water and heat management model for proton-exchange-membrane fuel cell. J Electrochem Soc 1993;140:2178–86. [15] Berning T, Djilali N. Three-dimensional computational analysis of transport phenomena in a PEM fuel cell—a parametric study. J Power Sources 2003;124:440–52. [16] Costamagna P, Arato E, Achenbach E. et al. Fluid dynamic study of fuel cell devices: simulation and experimental validation. J Power Sources 1994;52:243–9. [17] Thirumalai D, White RE. Mathematical modeling of protonexchange-membrane fuel–cell stack. J Electrochem Soc 1997;144:1717–23.