- Email: [email protected]
Design and fabrication of carbon nanotube-based microfuel cell and fuel cell stack coupled with hydrogen storage device
International Journal of Hydrogen Energy 32 (2007) 4272 – 4278 www.elsevier.com/locate/ijhydene
Design and fabrication of carbon nanotube-based microfuel cell and fuel cell stack coupled with hydrogen storage device A. Leela Mohana Reddy, S. Ramaprabhu ∗ Alternative Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India Received 28 April 2007; received in revised form 3 June 2007; accepted 5 June 2007 Available online 13 August 2007
Abstract A metal hydride-based hydrogen storage device capable of storing 70 l of hydrogen reversibly at ambient conditions with 500 g of ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 has been designed and developed. Good quality of multiwalled carbon nanotubes (MWNTs) have been synthesized by pyrolysis of acetylene over hydrogen decrepitated alloy hydride catalyst. Pt-supported MWNTs (Pt/MWNTs) electrocatalysts have been prepared by chemical reduction method using functionalized MWNTs. Fabrications of a fuel cell stack and a planar configured microfuel cell have been done using membrane electrode assembly prepared with Pt/MWNTs electrocatalyst and Nafion 1135 membrane. Performance studies of fuel cell stack and microfuel cell were done by coupling them to hydrogen storage device followed by demonstration of the working of electronic devices. 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Metal hydrides; Hydrogen storage; Carbon nanotubes; Fuel cell
1. Introduction Fuel cells, the energy-converting devices with a high efficiency and low/zero emission, are attracting increasing attention in recent decades due to high energy demands, fossil fuel depletion and environmental pollution. Among different types of fuel cell, polymer electrolyte membrane fuel cell (PEMFC) has attracted much attention for vehicles and portable electronic devices due to its advantageous features such as a low-operating temperature, sustained operation at high current density, low weight, compactness, long stack life, fast start-up and suitability during discontinuous operation [1–5]. The four main areas of present day fuel cell development is focused on (1) design and development of bipolar plates, (2) development of electrode materials, (3) safe storage of fuel and (4) development of membranes. Vehicular applications require a fuel cell stack working at high current and high voltage. However, portable electronic devices require a fuel cell with low current and high voltage,
∗ Corresponding author. Tel.: +91 44 22574862; fax: +91 44 22570509.
E-mail address: [email protected] (S. Ramaprabhu).
currently referred as microfuel cell. Bipolar plates which are channeled on both sides for the uniform supply of oxygen and hydrogen gases on either side are generally used for the development of fuel cell stack. The cell stack has received the greatest amount of research and development attention because within the cell stack is the “heart” of the fuel cell, i.e., the electrochemical process. The design of planar microfuel cell is based on the use of two printed circuit boards (PCB) having a thickness of 1 mm each with the required number of cells on the PCB. It is generally believed that the large amount of depleting platinum required as a catalyst in PEMFC is one of the main reasons why fuel cells are excluded from commercialization. In the past two decades, continuous efforts have been devoted to increase the utilization of Pt and reduce the amount of Pt used in PEMFC. Electrocatalysts with small size and high dispersion result in high electrocatalytic activity [6]. This suggests that it is highly desirable to have good Pt supporting materials with high surface area, which will enhance the Pt dispersion and hence reduce the catalyst loading, thereby improving the fuel cell performance [7–10]. Carbon nanotubes (CNTs) are attractive materials for catalyst support in PEMFC due to their morphology and interesting properties such as nanometer size, high accessible surface area, corrosion
0360-3199/$ - see front matter 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2007.06.013
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
resistance, good electronic conductivity and high stability [11]. One of the major problems in hydrogen-fueled fuel cells is the safety storage of hydrogen. There are few major ways of storing hydrogen, such as gaseous storage, liquid hydrogen storage, glass microsphere storage, underground storage and metal hydride storage [12]. Due to deficiencies such as less volume, weight and losses involved in these storage modes, metal hydrides can be considered as the best way of storing atomic hydrogen and solving the present perennial problems regarding pollution and depletion caused by fossil fuels. Hydrogen forms metal hydrides with some metals and alloys, leading to solid-state storage under moderate temperature and pressure that gives them the important safety advantage over the gas and liquid-storage methods. Metal hydrides have higher hydrogenstorage density (6.5 H atoms/cm3 for MgH2 ) than hydrogen gas (0.99 H atoms/cm3 ) or liquid hydrogen (4.2 H atoms/cm3 ) [13–15]. Hence, metal hydride storage is a safe, volumeefficient storage method for on-board vehicle applications. Storing it as metal hydrides seems to be the most cost-efficient way [12,16–19]. From our earlier studies on hydrogen-storage materials, we have found that ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 possesses good hydrogen-storage properties [20–23]. Since large storage capacity and fast kinetics of hydrogen absorption are the most important characteristics for a good storage material, a hydrogen-storage device has been designed and fabricated with ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 as the storage material and its performance has been studied. Hence, the present work is aimed at designing and fabricating CNT-based
4273
fuel cell stack and micro fuel cell and studying the performance by coupling to in-house fabricated metal hydride-based hydrogen-storage device. 2. Experimental 2.1. Design and development of hydrogen-storage device A schematic of the hydrogen-storage device is shown in Fig. 1(a). The device is made up of special stainless steel (SS) materials, which can withstand high pressure. The inlet/outlet of the unit has a needle valve to control the flow of incoming/outgoing hydrogen gas. The pressure gauge (0–50 bar) measures the gas pressure inside the unit. The porous sintered SS filter (pore size 5 m) acts as a barrier for alloy powder through which hydrogen is distributed or collected from the alloy. The alloy is filled in the annular space. The photograph of the storage device developed is shown in Fig. 1(b). To study the performance of the hydrogen-storage device, 500 g of ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 alloy has been used. In the first charging test, the alloy in the device was kept under vacuum (10−3 torr) for 12 h. After completing the evacuation process, the inlet of the device has been connected to a high-pressure hydrogen cylinder through a regulator. Hydrogen was introduced to a pressure of 20 bar at 30 ◦ C through the inlet. As the hydrogen absorption kinetics experiments in ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 show that [20] the equilibrium pressure can be reached in 20 min, the hydrogen flow was
Fig. 1. (a) The design details of the hydrogen-storage device and (b) photograph of hydrogen-storage device.
4274
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
allowed for 30 min. The amount of stored hydrogen in 500 g of the ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 is calculated from the storage capacity (1.6 wt%) as 80 l. Then the inlet of the device has Table 1 Operating conditions of the hydrogen-storage device Hydrogen charging pressure Hydrogen charging time at 30 ◦ C Amount of stored hydrogen Hydrogen desorbing pressure Hydrogen desorbing temperature (maximum) Hydrogen flow rate Amount of released hydrogen
20 bar 30 min 80 l (in 500 g of alloy) 1 bar 80 ◦ C 0.2 l/min 70 l
been disconnected from the hydrogen cylinder by closing the inlet valve for studying the discharging characteristics. The volume of desorbed hydrogen from the storage device has been measured by water displacement method. A gas flow meter with a flow rate of 0.2 l/min was employed for measuring the flow rate. In the initial stage of discharge test, hydrogen was discharged at 30 ◦ C by bringing the pressure from 20 to 1 bar. At the end of the time period, the hydrogen flow rate was found to decrease. By increasing the temperature of the device to a maximum of 80 ◦ C, the remaining hydrogen in the material was further discharged. The total amount of hydrogen released was approximately 70 l. The operational conditions of the device are given in Table 1.
Fig. 2. Photographs of (a) bipolar graphite plates for development of fuel cell stack, (b) planar configuration of cathode and anode channeled plates for the development of six cells microfuel cell and (c) cathode of planar microfuel cell having provision for air breathing.
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
4275
Fig. 3. SEM and TEM images of (a and b) purified MWNTs and (c and d) Pt-loaded MWNTs.
2.2. Preparation of MWNTs Multiwalled carbon nanotubes (MWNTs) have been synthesized by catalytic chemical vapor deposition of acetylene over Mm-based AB3 alloy hydride catalyst using single furnace technique [24,25]. The Mischmetal (Bharat Rare Earths Metals, India) used has the following composition: Ce 50%, La 35%, Pr 8%, Nd 5%, Fe 0.5% and other rare earth elements 1.5%. Alloy hydride catalyst has been obtained by hydrogen decrepitation technique using high-pressure hydrogen absorption/desorption facility. About 250 mg of the hydride powder was placed in a quartz boat and then introduced into the flow reactor (quartz tube with an inner diameter of 30 mm and a length of 100 mm) for the synthesis of MWNTs. The alloy hydride powders were heated at 500 ◦ C in hydrogen flow (50 ml/min) for 1 h in order to reduce any surface-oxidized catalyst particles. The hydrogen gas flow was stopped and temperature of the furnace was raised to 700 ◦ C for the production of MWNTs. Acetylene was then allowed to flow (50 ml/min) for 30 min. The deposition was carried out at atmospheric pressure and in an argon flow. The purification of the as-grown sample was carried out by air oxidation at 500 ◦ C for 2 h to remove the amorphous carbon and to open the ends of CNT. The
above air-oxidized CNTs were then refluxed with concentrated HNO3 for 24 h, followed by washing with de-ionized water several times and then the sample was dried in air for 30 min at 100 ◦ C. 2.3. Preparation of Pt/MWNTs electrocatalysts Purified MWNTs were ultrasonicated in aqua regia solution for 3 h. After the sonication procedure, they were washed with de-ionized water several times and dried in air for 30 min at 100 ◦ C. The dried sample was ultrasonicated in 10 ml of acetone for 1 h, and then 0.075 M H2 PtCl6 solution was added slowly during stirring. After 12 h, the mixture was reduced by adding a reducing solution containing 0.1 M NaBH4 and 1 M NaOH. After the completion of reaction, the solution was washed with de-ionized water, filtered and dried by vacuum filtration using a filter. The recovered Pt-loaded MWNTs were dried at 80 ◦ C for 3 h [26]. Morphological characteristics of CNTs were obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM measurements were carried out by ultrasonicating the sample in acetone for 1 h for good dispersion and then deposited onto copper grids.
4276
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
Fig. 4. In-house developed fuel cell stack having (a) three cell and (b) six cell PEM microfuel cell which operates with H2 /O2 and (c) six cell PEM microfuel cell which operates with H2 /air.
2.4. Preparation of membrane electrode assembly The membrane electrode assembly (MEA) was obtained by sandwiching a pretreated Nafion 1135 (Nafion R) membrane between the anode and the cathode. Both the anode and the cathode layers consisted of a backing layer (carbon paper, Torray, USA), a gas diffusion layer and a catalyst layer. Carbon black (Vulcan Xc-72) was treated at 600 ◦ C for 3 h to remove organic matter. Next, the pretreated carbon was mixed with polytetrafluoroethylene (PTFE) and isopropyl alcohol in a supersonic mixer. Finally, this mixture was applied on wetproofed carbon cloth and dried at 300 ◦ C. To form a hydrophilic layer, glycerol was added to the mixture and sprayed on the electrode. The uncatalyzed carbon electrode was cut to a proper size and installed in a sample holder for coating catalyst layer. To prepare the catalyst layer, the required amount of catalyst was suspended in de-ionized water and ultrasonicated by adding 5 wt% Nafion solution. The amount of Nafion loading was controlled to 1.2 mg/cm2 . The suspension was spread uniformly over a carbon electrode by spin coating technique.
2.5. Design and development of fuel cell stack and microfuel cell The three-cell fuel cell stack was assembled using three MEAs, two monopolar, two bipolar graphite plates with gas channels machined with a serpentine geometry (Fig. 2(a)), Teflon gaskets and two aluminum end plates. The electrodes were of 11.56 cm2 area. The anode was a 3.4 × 3.4 cm2 20% Pt/C electrode (E-TEk), with a platinum loading of 0.25 mg/cm2 . The cathode was prepared from a suspension containing a mixture of Pt/MWNTs (Pt content of 20 wt%), with a platinum loading of 0.5 mg/cm2 . Three MEAs were made by sandwiching a pretreated Nafion 1135 membrane between the anode and the cathode. In the design of planar microfuel cell, two PCB having a thickness of 1 mm each were taken. Six cells on the PCB were designed using marker ink pen and then copper present at unwanted area of PCB was selectively etched out using FeCl3 solution. Gas flow channels were machined with serpentine flow geometry as shown in Fig. 2(b). MEA was made by
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
3.0
500
2.0
400
1.5
300
1.0
200
0.5
100
0
50
100
150
200
250
300
350
Power density (mW/cm2)
2.5
0.0
0 400
Current density (mA/cm2) Fig. 5. Polarization curves of fuel cell stack at room temperature.
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
140 Planar micro fuel cell (6 cells) H2/O2
120 100 80 60 40 20
0
10
20
30
40
50
60
70
80
Power density (mW/cm2)
Voltage (V)
3. Results and discussions
0 90 100
Current density (mA/cm2) Fig. 6. Polarization curves of six cells PEM microfuel cell with O2 and H2 at room temperature.
80
Planar micro fuel cell (6 cells) H2/air
4.5
70
4.0
60
3.5 3.0
50
2.5
40
2.0
30
1.5
20
1.0
Power density (mW/cm2)
5.0
Voltage (V)
Fig. 3(a and b) shows the SEM and TEM images of purified MWNTs, respectively. From Fig. 3 it is clear that good-quality MWNTs have been obtained by CCVD technique using Mmbased AB3 alloy hydride catalyst. Fig. 3(c and d) shows the SEM and TEM images of Pt/MWNTs. The TEM images of Pt/MWNTs show a more or less uniform distribution of noble metal particles of size of about 3–5 nm on the MWNTs. The energy dispersive analysis (EDAX) shows that the amount of Pt loaded on the CNT support with reference to carbon can be evaluated qualitatively as 20%. Photographs of fuel cell stack and planar configured microfuel cells are in Fig. 4. Polarization studies were performed by connecting the fuel cell stack and microfuel cell to an in-house fabricated metal hydride-based hydrogen-storage device. Prior to polarization studies, the electrodes were activated between open-circuit potential and high current densities. The activation cycle is necessary to activate the catalyst for the oxygen reduction reaction. In the low current density region, rapid voltage drop in the potential–current curve, generally known as activation polarization, reflects the sluggish kinetics intrinsic to the oxygen reduction reaction at the cathode surface. As the current density increases, a mild drop in voltage is observed because of the cell resistance. Mass transport limitations account for the rapid drop seen at higher current densities. An open circuit potential of 2.72 V has been observed for three-cell PEMFC stack, and after drawing current, a maximum power density of 500 mW/cm2 at 1.75 V has been obtained (Fig. 5). Planar microfuel cell having serial connection six cells shows an open circuit potential of 5.52 V, a maximum power density of 125 mW/cm2 has been obtained with H2 and O2 as feed gases (Fig. 6). Whereas microfuel cell with novel air breathing technique gives an OCV of 4.92 V and a maximum power density of 70 mW/cm2 at 2 V (Fig. 7). The applicability of the presented approach has beendemonstrated by coupling the storage, microfuel cell system to power several electronic devices.
600 Polarisation graph for a 3 cell stack at RT
Voltage (V)
sandwiching a pretreated Nafion 1135 membrane between six anodes and cathodes. Both the anode and the cathode layer consisted of a backing layer, a diffusion layer and a catalyst layer. Pt/C-20% (SGL carbon) with a Pt loading of 0.25 mg/cm2 was used as the anode catalyst, which was kept identical in all the electrodes. Pt/CNT electrocatalysts were used for the oxygen reduction reaction at the cathode, with a Pt loading of 0.5 mg/cm2 . A single membrane of size 10 cm × 15 cm and six anode and cathode electrodes each of size 2 cm × 2 cm were used. The design of microfuel cell was further simplified by using novel air breathing technique. For this, at cathode side of cell, holes were made as shown in Fig. 2(c); this design helps in the utilization of atmospheric air at the cathode side. Whereas at the anode side, for the supply of hydrogen, metal hydridebased hydrogen storage device is connected. The design and development of hydrogen-storage devices is described in the abovesection.
4277
10
0.5
0
0.0 0
10
20
30
40
50
60
Current density (mA/cm2) Fig. 7. Polarization curves of six cells PEM microfuel cell with air and H2 at room temperature.
4. Conclusion Metal hydride-based hydrogen-storage device is capable of storing 70 l of hydrogen reversibly at ambient conditions with
4278
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
500 g of ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 . Functionalization of MWNTs results in improved adhesion of Pt nanoparticles of around 3–5 nm diameter on to the MWNTs surface. Fabrications of MWNTs-based fuel cell stack and microfuel cell were done using MEAs, prechanneled bipolar plates and PCB, respectively. Performance studies of fuel cell stack and microfuel cell were done by coupling them to hydrogen-storage device followed by the demonstration of working of electronic devices. Serial configured fuel cell stack gives a maximum power density of 500 mW/cm2 at 1.75 V, whereas planar configured microfuel cell gives a maximum power density of 125 mW/cm2 at 2.0 V (H2 /O2 ) and 68 mW/cm2 at 2.0 V (H2 /air).
[10]
[11]
[12] [13] [14] [15]
Acknowledgments
[16]
Financial support from DRDO, RCI, DST, HEBL and IIT Madras for the present work is gratefully acknowledged.
[17]
References
[19]
[1] Costamagna P, Srinivasan S. Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000: Part I. Fundamental scientific aspects. J Power Sour 2001;102:242–52. [2] Haile SM. Fuel cell materials and components. Acta Materialia 2003;51: 5981–6000. [3] Ghenciu AF. Review of fuel processing catalysts for hydrogen production in PEM fuel cell systems. Curr Opin Solid State Mater 2002;6:389–99. [4] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sour 2003;114:32–53. [5] Costamagna P, Srinivasan S S. Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000: Part II. Engineering, technology development and application aspects. J Power Sour 2001;102:253–69. [6] Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA. Shape-controlled synthesis of colloidal platinum nanoparticles. Science 1996;272:1924. [7] Serp P, Corrias M, Kalck P. Carbon nanotubes and nanofibers in catalysis. Appl Catal A 2003;253:337–58. [8] Che GL, Lakshmi BB, Fisher ER, Martin CR. Carbon nanotubule membranes for electrochemical energy storage and production. Nature 1998;393:346. [9] Rajesh B, Thampi KR, Bonard JM, Xanthopoulos N, Mathieu HJ, Viswanathan B. Carbon nanotubes generated from template carbonization
[18]
[20] [21]
[22]
[23]
[24]
[25]
[26]
of polyphenyl acetylene as the support for electrooxidation of methanol. J Phys Chem B 2003;107:2701–8. Mastragostino M, Missiroli A, Soavi F. Carbon supports for electrodeposited Pt–Ru catalysts for DMFCs. J Electrochem Soc 2004;151:A1919–24. Shao Y, Yin G, Gao Y, Shi P. Durability study of Pt/C and Pt/CNTs catalysts under simulated PEM fuel cell conditions. J Electrochem Soc 2006;153:A1093–7. Zuttel A. Materials for hydrogen storage. Materi Today 2003;6(9): 24–33. Weast RC, Astle MJ, Beyer WH. CRC handbook of chemistry and physics. 64th ed., Boca Raton, FL: CRC Press; 1983. Eberle U, Arnold G, Helmholt RV. Hydrogen storage in metal–hydrogen systems and their derivatives. J Power Sour. 2006;154(2):456–60. Zhou L, Zhou Y, Sun Y. Studies on the mechanism and capacity of hydrogen uptake by physisorption-based materials. Int J Hydrogen Energy 2006;31(2):259–64. Schlapbach L, editor. Hydrogen in intermetallic compounds (300)—I: topics in applied physics, vol. 63. Berlin: Springer; 1988. Schlapbach L, editor. Hydrogen in intermetallic compounds—II: topics in applied physics, vol. 67. Berlin: Springer; 1992. Grochala W, Edwards PP. Hydrides of the chemical elements for the storage and production of hydrogen. Chem Rev 2004;104:1283–315. Johnson SR, Anderson PA, Edwards PP, Gameson I, Prendergast JW, Al-Mamouri M. et al. Chemical activation of MgH2 : a new route to superior hydrogen storage materials. Chem Commun 2005;22:2823–5. Mani N, Ramaprabhu S. Hydriding properties of Ti-substituted nonstoichiometric AB2 alloys. J Alloys Compds 2004;363:275–91. Mani N, Ramaprabhu S. Effect of substitutional elements on hydrogen absorption properties in ZrMnFe0.5 Ni0.5 and ZrMnFe0.5 Co0.5 . Int J Hyd Energy 2005;30:53–67. Krishna Kumar M, Ramaprabhu S. Hydrogen absorption characteristics in Mmx Tb1−x Co2 (x = 0, 0.05, 0.1, 0.15, 0.2). Int J Hyd Energy 2006, in Press. 10.1016/j.ijhydene.2006.09.016. Leela Mohana Reddy A, Ramaprabhu S. Structural and hydrogen absorption kinetics studies of polymer dispersed and boron added Zr-based AB2 alloy. Int J Hyd Energy 2006;31(7):867–76. Leela Mohana Reddy A, Shaijumon MM, Ramaprabhu S. Alloy hydride catalyst route of synthesis of single-walled carbon nanotubes, multiwalled carbon nanotubes and magnetic metal nanowire encapsulated multi-walled carbon nanotubes. Nanotechnology 2006;17:5299–305. Shaijumon MM, Ramaprabhu S. Synthesis of carbon nanotubes by pyrolysis of acetylene using alloy hydride materials as catalysts and their hydrogen adsorption studies. Chem Phys Lett 2003;374:513–20. Shaijumon MM, Rajalakshmi N, Ramaprabhu S. Platinum/multiwalled carbon nanotubes-platinum/carbon composites as electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cell. Appl Phys Lett 2006;88:253105–7.
Design and fabrication of carbon nanotube-based microfuel cell and fuel cell stack coupled with hydrogen storage device A. Leela Mohana Reddy, S. Ramaprabhu ∗ Alternative Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India Received 28 April 2007; received in revised form 3 June 2007; accepted 5 June 2007 Available online 13 August 2007
Abstract A metal hydride-based hydrogen storage device capable of storing 70 l of hydrogen reversibly at ambient conditions with 500 g of ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 has been designed and developed. Good quality of multiwalled carbon nanotubes (MWNTs) have been synthesized by pyrolysis of acetylene over hydrogen decrepitated alloy hydride catalyst. Pt-supported MWNTs (Pt/MWNTs) electrocatalysts have been prepared by chemical reduction method using functionalized MWNTs. Fabrications of a fuel cell stack and a planar configured microfuel cell have been done using membrane electrode assembly prepared with Pt/MWNTs electrocatalyst and Nafion 1135 membrane. Performance studies of fuel cell stack and microfuel cell were done by coupling them to hydrogen storage device followed by demonstration of the working of electronic devices. 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Metal hydrides; Hydrogen storage; Carbon nanotubes; Fuel cell
1. Introduction Fuel cells, the energy-converting devices with a high efficiency and low/zero emission, are attracting increasing attention in recent decades due to high energy demands, fossil fuel depletion and environmental pollution. Among different types of fuel cell, polymer electrolyte membrane fuel cell (PEMFC) has attracted much attention for vehicles and portable electronic devices due to its advantageous features such as a low-operating temperature, sustained operation at high current density, low weight, compactness, long stack life, fast start-up and suitability during discontinuous operation [1–5]. The four main areas of present day fuel cell development is focused on (1) design and development of bipolar plates, (2) development of electrode materials, (3) safe storage of fuel and (4) development of membranes. Vehicular applications require a fuel cell stack working at high current and high voltage. However, portable electronic devices require a fuel cell with low current and high voltage,
∗ Corresponding author. Tel.: +91 44 22574862; fax: +91 44 22570509.
E-mail address: [email protected] (S. Ramaprabhu).
currently referred as microfuel cell. Bipolar plates which are channeled on both sides for the uniform supply of oxygen and hydrogen gases on either side are generally used for the development of fuel cell stack. The cell stack has received the greatest amount of research and development attention because within the cell stack is the “heart” of the fuel cell, i.e., the electrochemical process. The design of planar microfuel cell is based on the use of two printed circuit boards (PCB) having a thickness of 1 mm each with the required number of cells on the PCB. It is generally believed that the large amount of depleting platinum required as a catalyst in PEMFC is one of the main reasons why fuel cells are excluded from commercialization. In the past two decades, continuous efforts have been devoted to increase the utilization of Pt and reduce the amount of Pt used in PEMFC. Electrocatalysts with small size and high dispersion result in high electrocatalytic activity [6]. This suggests that it is highly desirable to have good Pt supporting materials with high surface area, which will enhance the Pt dispersion and hence reduce the catalyst loading, thereby improving the fuel cell performance [7–10]. Carbon nanotubes (CNTs) are attractive materials for catalyst support in PEMFC due to their morphology and interesting properties such as nanometer size, high accessible surface area, corrosion
0360-3199/$ - see front matter 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2007.06.013
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
resistance, good electronic conductivity and high stability [11]. One of the major problems in hydrogen-fueled fuel cells is the safety storage of hydrogen. There are few major ways of storing hydrogen, such as gaseous storage, liquid hydrogen storage, glass microsphere storage, underground storage and metal hydride storage [12]. Due to deficiencies such as less volume, weight and losses involved in these storage modes, metal hydrides can be considered as the best way of storing atomic hydrogen and solving the present perennial problems regarding pollution and depletion caused by fossil fuels. Hydrogen forms metal hydrides with some metals and alloys, leading to solid-state storage under moderate temperature and pressure that gives them the important safety advantage over the gas and liquid-storage methods. Metal hydrides have higher hydrogenstorage density (6.5 H atoms/cm3 for MgH2 ) than hydrogen gas (0.99 H atoms/cm3 ) or liquid hydrogen (4.2 H atoms/cm3 ) [13–15]. Hence, metal hydride storage is a safe, volumeefficient storage method for on-board vehicle applications. Storing it as metal hydrides seems to be the most cost-efficient way [12,16–19]. From our earlier studies on hydrogen-storage materials, we have found that ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 possesses good hydrogen-storage properties [20–23]. Since large storage capacity and fast kinetics of hydrogen absorption are the most important characteristics for a good storage material, a hydrogen-storage device has been designed and fabricated with ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 as the storage material and its performance has been studied. Hence, the present work is aimed at designing and fabricating CNT-based
4273
fuel cell stack and micro fuel cell and studying the performance by coupling to in-house fabricated metal hydride-based hydrogen-storage device. 2. Experimental 2.1. Design and development of hydrogen-storage device A schematic of the hydrogen-storage device is shown in Fig. 1(a). The device is made up of special stainless steel (SS) materials, which can withstand high pressure. The inlet/outlet of the unit has a needle valve to control the flow of incoming/outgoing hydrogen gas. The pressure gauge (0–50 bar) measures the gas pressure inside the unit. The porous sintered SS filter (pore size 5 m) acts as a barrier for alloy powder through which hydrogen is distributed or collected from the alloy. The alloy is filled in the annular space. The photograph of the storage device developed is shown in Fig. 1(b). To study the performance of the hydrogen-storage device, 500 g of ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 alloy has been used. In the first charging test, the alloy in the device was kept under vacuum (10−3 torr) for 12 h. After completing the evacuation process, the inlet of the device has been connected to a high-pressure hydrogen cylinder through a regulator. Hydrogen was introduced to a pressure of 20 bar at 30 ◦ C through the inlet. As the hydrogen absorption kinetics experiments in ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 show that [20] the equilibrium pressure can be reached in 20 min, the hydrogen flow was
Fig. 1. (a) The design details of the hydrogen-storage device and (b) photograph of hydrogen-storage device.
4274
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
allowed for 30 min. The amount of stored hydrogen in 500 g of the ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 is calculated from the storage capacity (1.6 wt%) as 80 l. Then the inlet of the device has Table 1 Operating conditions of the hydrogen-storage device Hydrogen charging pressure Hydrogen charging time at 30 ◦ C Amount of stored hydrogen Hydrogen desorbing pressure Hydrogen desorbing temperature (maximum) Hydrogen flow rate Amount of released hydrogen
20 bar 30 min 80 l (in 500 g of alloy) 1 bar 80 ◦ C 0.2 l/min 70 l
been disconnected from the hydrogen cylinder by closing the inlet valve for studying the discharging characteristics. The volume of desorbed hydrogen from the storage device has been measured by water displacement method. A gas flow meter with a flow rate of 0.2 l/min was employed for measuring the flow rate. In the initial stage of discharge test, hydrogen was discharged at 30 ◦ C by bringing the pressure from 20 to 1 bar. At the end of the time period, the hydrogen flow rate was found to decrease. By increasing the temperature of the device to a maximum of 80 ◦ C, the remaining hydrogen in the material was further discharged. The total amount of hydrogen released was approximately 70 l. The operational conditions of the device are given in Table 1.
Fig. 2. Photographs of (a) bipolar graphite plates for development of fuel cell stack, (b) planar configuration of cathode and anode channeled plates for the development of six cells microfuel cell and (c) cathode of planar microfuel cell having provision for air breathing.
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
4275
Fig. 3. SEM and TEM images of (a and b) purified MWNTs and (c and d) Pt-loaded MWNTs.
2.2. Preparation of MWNTs Multiwalled carbon nanotubes (MWNTs) have been synthesized by catalytic chemical vapor deposition of acetylene over Mm-based AB3 alloy hydride catalyst using single furnace technique [24,25]. The Mischmetal (Bharat Rare Earths Metals, India) used has the following composition: Ce 50%, La 35%, Pr 8%, Nd 5%, Fe 0.5% and other rare earth elements 1.5%. Alloy hydride catalyst has been obtained by hydrogen decrepitation technique using high-pressure hydrogen absorption/desorption facility. About 250 mg of the hydride powder was placed in a quartz boat and then introduced into the flow reactor (quartz tube with an inner diameter of 30 mm and a length of 100 mm) for the synthesis of MWNTs. The alloy hydride powders were heated at 500 ◦ C in hydrogen flow (50 ml/min) for 1 h in order to reduce any surface-oxidized catalyst particles. The hydrogen gas flow was stopped and temperature of the furnace was raised to 700 ◦ C for the production of MWNTs. Acetylene was then allowed to flow (50 ml/min) for 30 min. The deposition was carried out at atmospheric pressure and in an argon flow. The purification of the as-grown sample was carried out by air oxidation at 500 ◦ C for 2 h to remove the amorphous carbon and to open the ends of CNT. The
above air-oxidized CNTs were then refluxed with concentrated HNO3 for 24 h, followed by washing with de-ionized water several times and then the sample was dried in air for 30 min at 100 ◦ C. 2.3. Preparation of Pt/MWNTs electrocatalysts Purified MWNTs were ultrasonicated in aqua regia solution for 3 h. After the sonication procedure, they were washed with de-ionized water several times and dried in air for 30 min at 100 ◦ C. The dried sample was ultrasonicated in 10 ml of acetone for 1 h, and then 0.075 M H2 PtCl6 solution was added slowly during stirring. After 12 h, the mixture was reduced by adding a reducing solution containing 0.1 M NaBH4 and 1 M NaOH. After the completion of reaction, the solution was washed with de-ionized water, filtered and dried by vacuum filtration using a filter. The recovered Pt-loaded MWNTs were dried at 80 ◦ C for 3 h [26]. Morphological characteristics of CNTs were obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM measurements were carried out by ultrasonicating the sample in acetone for 1 h for good dispersion and then deposited onto copper grids.
4276
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
Fig. 4. In-house developed fuel cell stack having (a) three cell and (b) six cell PEM microfuel cell which operates with H2 /O2 and (c) six cell PEM microfuel cell which operates with H2 /air.
2.4. Preparation of membrane electrode assembly The membrane electrode assembly (MEA) was obtained by sandwiching a pretreated Nafion 1135 (Nafion R) membrane between the anode and the cathode. Both the anode and the cathode layers consisted of a backing layer (carbon paper, Torray, USA), a gas diffusion layer and a catalyst layer. Carbon black (Vulcan Xc-72) was treated at 600 ◦ C for 3 h to remove organic matter. Next, the pretreated carbon was mixed with polytetrafluoroethylene (PTFE) and isopropyl alcohol in a supersonic mixer. Finally, this mixture was applied on wetproofed carbon cloth and dried at 300 ◦ C. To form a hydrophilic layer, glycerol was added to the mixture and sprayed on the electrode. The uncatalyzed carbon electrode was cut to a proper size and installed in a sample holder for coating catalyst layer. To prepare the catalyst layer, the required amount of catalyst was suspended in de-ionized water and ultrasonicated by adding 5 wt% Nafion solution. The amount of Nafion loading was controlled to 1.2 mg/cm2 . The suspension was spread uniformly over a carbon electrode by spin coating technique.
2.5. Design and development of fuel cell stack and microfuel cell The three-cell fuel cell stack was assembled using three MEAs, two monopolar, two bipolar graphite plates with gas channels machined with a serpentine geometry (Fig. 2(a)), Teflon gaskets and two aluminum end plates. The electrodes were of 11.56 cm2 area. The anode was a 3.4 × 3.4 cm2 20% Pt/C electrode (E-TEk), with a platinum loading of 0.25 mg/cm2 . The cathode was prepared from a suspension containing a mixture of Pt/MWNTs (Pt content of 20 wt%), with a platinum loading of 0.5 mg/cm2 . Three MEAs were made by sandwiching a pretreated Nafion 1135 membrane between the anode and the cathode. In the design of planar microfuel cell, two PCB having a thickness of 1 mm each were taken. Six cells on the PCB were designed using marker ink pen and then copper present at unwanted area of PCB was selectively etched out using FeCl3 solution. Gas flow channels were machined with serpentine flow geometry as shown in Fig. 2(b). MEA was made by
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
3.0
500
2.0
400
1.5
300
1.0
200
0.5
100
0
50
100
150
200
250
300
350
Power density (mW/cm2)
2.5
0.0
0 400
Current density (mA/cm2) Fig. 5. Polarization curves of fuel cell stack at room temperature.
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
140 Planar micro fuel cell (6 cells) H2/O2
120 100 80 60 40 20
0
10
20
30
40
50
60
70
80
Power density (mW/cm2)
Voltage (V)
3. Results and discussions
0 90 100
Current density (mA/cm2) Fig. 6. Polarization curves of six cells PEM microfuel cell with O2 and H2 at room temperature.
80
Planar micro fuel cell (6 cells) H2/air
4.5
70
4.0
60
3.5 3.0
50
2.5
40
2.0
30
1.5
20
1.0
Power density (mW/cm2)
5.0
Voltage (V)
Fig. 3(a and b) shows the SEM and TEM images of purified MWNTs, respectively. From Fig. 3 it is clear that good-quality MWNTs have been obtained by CCVD technique using Mmbased AB3 alloy hydride catalyst. Fig. 3(c and d) shows the SEM and TEM images of Pt/MWNTs. The TEM images of Pt/MWNTs show a more or less uniform distribution of noble metal particles of size of about 3–5 nm on the MWNTs. The energy dispersive analysis (EDAX) shows that the amount of Pt loaded on the CNT support with reference to carbon can be evaluated qualitatively as 20%. Photographs of fuel cell stack and planar configured microfuel cells are in Fig. 4. Polarization studies were performed by connecting the fuel cell stack and microfuel cell to an in-house fabricated metal hydride-based hydrogen-storage device. Prior to polarization studies, the electrodes were activated between open-circuit potential and high current densities. The activation cycle is necessary to activate the catalyst for the oxygen reduction reaction. In the low current density region, rapid voltage drop in the potential–current curve, generally known as activation polarization, reflects the sluggish kinetics intrinsic to the oxygen reduction reaction at the cathode surface. As the current density increases, a mild drop in voltage is observed because of the cell resistance. Mass transport limitations account for the rapid drop seen at higher current densities. An open circuit potential of 2.72 V has been observed for three-cell PEMFC stack, and after drawing current, a maximum power density of 500 mW/cm2 at 1.75 V has been obtained (Fig. 5). Planar microfuel cell having serial connection six cells shows an open circuit potential of 5.52 V, a maximum power density of 125 mW/cm2 has been obtained with H2 and O2 as feed gases (Fig. 6). Whereas microfuel cell with novel air breathing technique gives an OCV of 4.92 V and a maximum power density of 70 mW/cm2 at 2 V (Fig. 7). The applicability of the presented approach has beendemonstrated by coupling the storage, microfuel cell system to power several electronic devices.
600 Polarisation graph for a 3 cell stack at RT
Voltage (V)
sandwiching a pretreated Nafion 1135 membrane between six anodes and cathodes. Both the anode and the cathode layer consisted of a backing layer, a diffusion layer and a catalyst layer. Pt/C-20% (SGL carbon) with a Pt loading of 0.25 mg/cm2 was used as the anode catalyst, which was kept identical in all the electrodes. Pt/CNT electrocatalysts were used for the oxygen reduction reaction at the cathode, with a Pt loading of 0.5 mg/cm2 . A single membrane of size 10 cm × 15 cm and six anode and cathode electrodes each of size 2 cm × 2 cm were used. The design of microfuel cell was further simplified by using novel air breathing technique. For this, at cathode side of cell, holes were made as shown in Fig. 2(c); this design helps in the utilization of atmospheric air at the cathode side. Whereas at the anode side, for the supply of hydrogen, metal hydridebased hydrogen storage device is connected. The design and development of hydrogen-storage devices is described in the abovesection.
4277
10
0.5
0
0.0 0
10
20
30
40
50
60
Current density (mA/cm2) Fig. 7. Polarization curves of six cells PEM microfuel cell with air and H2 at room temperature.
4. Conclusion Metal hydride-based hydrogen-storage device is capable of storing 70 l of hydrogen reversibly at ambient conditions with
4278
A.L.M. Reddy, S. Ramaprabhu / International Journal of Hydrogen Energy 32 (2007) 4272 – 4278
500 g of ZrMn0.85 Cr 0.1 V0.05 Fe0.5 Ni0.5 . Functionalization of MWNTs results in improved adhesion of Pt nanoparticles of around 3–5 nm diameter on to the MWNTs surface. Fabrications of MWNTs-based fuel cell stack and microfuel cell were done using MEAs, prechanneled bipolar plates and PCB, respectively. Performance studies of fuel cell stack and microfuel cell were done by coupling them to hydrogen-storage device followed by the demonstration of working of electronic devices. Serial configured fuel cell stack gives a maximum power density of 500 mW/cm2 at 1.75 V, whereas planar configured microfuel cell gives a maximum power density of 125 mW/cm2 at 2.0 V (H2 /O2 ) and 68 mW/cm2 at 2.0 V (H2 /air).
[10]
[11]
[12] [13] [14] [15]
Acknowledgments
[16]
Financial support from DRDO, RCI, DST, HEBL and IIT Madras for the present work is gratefully acknowledged.
[17]
References
[19]
[1] Costamagna P, Srinivasan S. Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000: Part I. Fundamental scientific aspects. J Power Sour 2001;102:242–52. [2] Haile SM. Fuel cell materials and components. Acta Materialia 2003;51: 5981–6000. [3] Ghenciu AF. Review of fuel processing catalysts for hydrogen production in PEM fuel cell systems. Curr Opin Solid State Mater 2002;6:389–99. [4] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sour 2003;114:32–53. [5] Costamagna P, Srinivasan S S. Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000: Part II. Engineering, technology development and application aspects. J Power Sour 2001;102:253–69. [6] Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA. Shape-controlled synthesis of colloidal platinum nanoparticles. Science 1996;272:1924. [7] Serp P, Corrias M, Kalck P. Carbon nanotubes and nanofibers in catalysis. Appl Catal A 2003;253:337–58. [8] Che GL, Lakshmi BB, Fisher ER, Martin CR. Carbon nanotubule membranes for electrochemical energy storage and production. Nature 1998;393:346. [9] Rajesh B, Thampi KR, Bonard JM, Xanthopoulos N, Mathieu HJ, Viswanathan B. Carbon nanotubes generated from template carbonization
[18]
[20] [21]
[22]
[23]
[24]
[25]
[26]
of polyphenyl acetylene as the support for electrooxidation of methanol. J Phys Chem B 2003;107:2701–8. Mastragostino M, Missiroli A, Soavi F. Carbon supports for electrodeposited Pt–Ru catalysts for DMFCs. J Electrochem Soc 2004;151:A1919–24. Shao Y, Yin G, Gao Y, Shi P. Durability study of Pt/C and Pt/CNTs catalysts under simulated PEM fuel cell conditions. J Electrochem Soc 2006;153:A1093–7. Zuttel A. Materials for hydrogen storage. Materi Today 2003;6(9): 24–33. Weast RC, Astle MJ, Beyer WH. CRC handbook of chemistry and physics. 64th ed., Boca Raton, FL: CRC Press; 1983. Eberle U, Arnold G, Helmholt RV. Hydrogen storage in metal–hydrogen systems and their derivatives. J Power Sour. 2006;154(2):456–60. Zhou L, Zhou Y, Sun Y. Studies on the mechanism and capacity of hydrogen uptake by physisorption-based materials. Int J Hydrogen Energy 2006;31(2):259–64. Schlapbach L, editor. Hydrogen in intermetallic compounds (300)—I: topics in applied physics, vol. 63. Berlin: Springer; 1988. Schlapbach L, editor. Hydrogen in intermetallic compounds—II: topics in applied physics, vol. 67. Berlin: Springer; 1992. Grochala W, Edwards PP. Hydrides of the chemical elements for the storage and production of hydrogen. Chem Rev 2004;104:1283–315. Johnson SR, Anderson PA, Edwards PP, Gameson I, Prendergast JW, Al-Mamouri M. et al. Chemical activation of MgH2 : a new route to superior hydrogen storage materials. Chem Commun 2005;22:2823–5. Mani N, Ramaprabhu S. Hydriding properties of Ti-substituted nonstoichiometric AB2 alloys. J Alloys Compds 2004;363:275–91. Mani N, Ramaprabhu S. Effect of substitutional elements on hydrogen absorption properties in ZrMnFe0.5 Ni0.5 and ZrMnFe0.5 Co0.5 . Int J Hyd Energy 2005;30:53–67. Krishna Kumar M, Ramaprabhu S. Hydrogen absorption characteristics in Mmx Tb1−x Co2 (x = 0, 0.05, 0.1, 0.15, 0.2). Int J Hyd Energy 2006, in Press. 10.1016/j.ijhydene.2006.09.016. Leela Mohana Reddy A, Ramaprabhu S. Structural and hydrogen absorption kinetics studies of polymer dispersed and boron added Zr-based AB2 alloy. Int J Hyd Energy 2006;31(7):867–76. Leela Mohana Reddy A, Shaijumon MM, Ramaprabhu S. Alloy hydride catalyst route of synthesis of single-walled carbon nanotubes, multiwalled carbon nanotubes and magnetic metal nanowire encapsulated multi-walled carbon nanotubes. Nanotechnology 2006;17:5299–305. Shaijumon MM, Ramaprabhu S. Synthesis of carbon nanotubes by pyrolysis of acetylene using alloy hydride materials as catalysts and their hydrogen adsorption studies. Chem Phys Lett 2003;374:513–20. Shaijumon MM, Rajalakshmi N, Ramaprabhu S. Platinum/multiwalled carbon nanotubes-platinum/carbon composites as electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cell. Appl Phys Lett 2006;88:253105–7.