Stand-alone micro direct borohydride fuel cells using mixture of sodium borohydride and potassium hydroxide

Transcription

1 Journal of Mechanical Science and Technology 25 (4) (2011) 931~935 DOI /s z Stand-alone micro direct borohydride fuel cells using mixture of sodium borohydride and potassium hydroxide Jong Pil Choi, Young Ho Seo * and Byeong Hee Kim Medical & Bio-Material Research Center, Department of Mechatronics Engineering, Kangwon National University, Chuncheon, , Korea (Manuscript Received August 15, 2010; Revised December 20, 2010; Accepted January 17, 2011) Abstract This paper presents a simple and low-cost micro direct borohydride fuel cell (DBFC) using a mixture of a fuel (sodium borohydride, NaBH 4 ) and a liquid electrolyte (potassium hydroxide, KOH) in order to improve three-phase contact in micro fuel cells. It consists of an anode, cathode, and a fuel chamber for the liquid mixture of the fuel and the electrolyte. For anode catalysts, gold (Au) was sputtered on a Pyrex glass, and manganese dioxide supported on carbon (MnO 2 /C) was coated on nickel foam for cathode catalysts. The NaBH 4 and 1 M KOH were used as the hydrogen source and electrolyte respectively. The DBFC has a simple configuration without auxiliary devices and is cost effective because a platinum catalyst and ion exchange membranes were not used. The size of the DBFC was mm 3, and the active area was mm 2. The DBFC showed a maximum power of 1.18 mw at 9 V, and three serially connected cells illuminated two LEDs for one hour without refueling and any auxiliary devices. Keywords: Direct borohydride fuel cell; Nickel foam; Mixed fuel; Stand-alone Introduction Recently, there has been a growing demand for alternative power sources for portable electronic devices such as mobile phones, PDAs, notebook computers, and power chips for point of care testing (POCT). Micro fuel cells such as the polymer electrolyte fuel cell (PEMFC) and the direct methanol fuel cell (DMFC) are attractive candidates to replace batteries due to their high efficiency, high power density, short charging time, and ultra-low emission of environmental pollutants such as carbon monoxide and carbon dioxide. The main issue facing the micro fuel cell is the production of a compact and lightweight system [1, 2]. These micro fuel cells require very expensive ion exchange membranes and novel catalysts such as platinum (Pt) and also require strict operating conditions such as humidification, uniform contact, and temperature and water management. In addition, the hydrogen storage technology cannot meet the application requirements of micro PEMFCs [3, 4]. These issues cause tremendous problems for the commercial development of micro fuel cells. Moreover, two unresolved problems still plague DMFCs. First, their anode performance is poor compared with that in the PEMFC because of the larger overpotential of DMFC anode This paper was recommended for publication in revised form by Associate Editor Yong-Tae Kim * Corresponding author. Tel.: , Fax.: address: mems@kangwon.ac.kr KSME & Springer 2011 due to the complex reaction of methanol oxidation rather than that of PEMFC anode. Second, there is the problem of methanol crossover from the anode to the cathode side due to a gradient of methanol concentration. As a result, the DMFC has a lower power density and open circuit voltage (OCV) than the PEMFC [5-7]. Interest in DBFCs has increased lately because they require neither ion exchange membranes nor Pt catalysts. Compared with hydrogen or methanol PEMFC cells, direct borohydride fuel cells (DBFC) have higher theoretical voltage and faster anodic electro-oxidation rates. Besides, borohydride is, as a stable solid compound, easy to store and transport safely and clean to use with no hazardous and pollutant emissions [7]. In addition, it may be possible to make a small and simple system without the membrane and an auxiliary device. Especially, the cost of a DBFC can be lowered due to the use of nonnoble metals such as manganese dioxide as the cathode catalyst. Moreover, NaBH 4 is known to be a hydrogen generator with excellent efficiency of hydrogen storage. In a DBFC, eight electrons can be obtained from just one molecule of NaBH 4. The reaction in the DBFC is given by the following equations: Anode BH OH - BO H 2 O + 8e - E anode = -4V (1) Cathode 2O 2 +4H 2 O + 8e - 8OH - E cathode = 0V (2)

2 932 J. P. Choi et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 931~935 Total reaction BH O 2 BO H 2 O E total = 4V (3) where E anode is the standard anode potential, E cathode is the standard cathode potential, and E total is the theoretical OCV of the DBFC. The Etotal of 4 V is remarkably higher than that obtained from the PEMFC and the DMFC. In addition, the NaBH4 can be easily stored and distributed, and is chemically stable in high alkaline media [8, 9]. Generally, pure hydrogen feeding in micro fuel cells is relatively difficult due to storage problems because of its small size, and it is difficult to generate a good triple phase boundary (i.e., the area of contact between the reactant gas, electrolyte, and electrode catalyst for the electrochemical reaction). Therefore, ths paper examined a micro DBFC with a mixture of a fuel and a liquid electrolyte. NaBH 4 and 1 M KOH were used as the hydrogen source and electrolyte respectively. Pt, Pd, Au, Ag, Ni and etc were used as anode materials of DBFC, and Pt and MnO2 were generally used as cathode materials [5]. - There were two main problems in DBFC including BH 4 crossover and hydrolysis of BH - 4 at anodic metal surface [7]. Since MnO 2 has sufficient electro-catalytic activity for oxygen reduction and has not any catalytic activity for the - electro-oxidation and hydrolysis of BH 4 ion, this type of DBFC cells may avoid not only the degradation of the cathode performances arising from BH - 4 crossover but also do not need to use expensive ion exchange membranes and Pt catalyst in the cathode [7]. To reduce the manufacturing cost and degradation of catalyst of the DBFC, we used a non-noble metal (MnO 2 ) as the cathode catalyst and Au deposited on a glass as the anode catalyst. The bonding process in the micro DBFC is a key technology in the assembly of its components and in preventing leakage of the liquid mixture. Ultraviolet (UV) adhesive and adhesive tape were used to bond the glass and the Ni-foam, respectively, to the fuel chamber. Characteristic evaluation and performance tests were conducted according to the amount of catalyst loading and the mixture ratio between the MnO 2 and C for the cathode. 2. Fabrication Fig. 1 shows the schematic of the proposed micro DBFC. Its simple configuration and small size are due to the lack of an ion exchange membrane and any auxiliary devices. It consists of an anode, cathode, and a fuel chamber containing the fuel and electrolyte mixture. In addition, both electrodes acted simultaneously as current collectors to reduce the volume of the DBFC. To prepare the cathode catalyst, MnO 2 powder (E-TEK Co.) was mixed with Nafion solution (5 wt.%, Dupont Co.) to form a paste, and carbon powder (Vulcan XC-72, E-TEK Co.) was added to increase the surface area of the electrochemical reaction region and improve the three-phase contact. The role of Nafion was binder of MnO 2 and carbon powder. R.X. Feng et al. [7] was used polytetrafluoroethylene (PTFE) Table 1. Various compositions of the cathode catalyst. Reference Number Amount of loading (g/cm 2 ) Nafion (wt%) MnO 2 /C (wt%) Ratio of MnO 2 :C : : : : : :30 Fuel chamber (fuel + liquid electrolyte) Anode (Sputtered Au on Glass) 14mm 14mm 10mm Cathode (MnO 2 /C on Ni-foam) 3mm 5mm Refueling hole Fig. 1. Schematic diagram of proposed DBFC consisting of anode, cathode, and fuel chamber; total volume is mm 3. Fig. 2. SEM image of MnO 2 /C catalyst loaded on Ni-foam for the cathode. as binder. Next, the paste was brushed on one side of a mm 2 piece of Ni-foam (thickness = 1 mm, porosity > 95%). Air can be easily supplied to the cathode side by free convection due to its high porosity and its three-dimensional network structure, allowing it to load a greater amount of the catalyst paste and combine strongly with the catalyst. Ni foam and MnO 2 -C- Nafion mixture were hydrophobic materials, thus mixedreactant can t get out through Ni foam. Table 1 shows the detailed composition of the cathode catalyst for several types of catalysts. The scanning electron microscopy (SEM) image of the catalyst-loaded Ni-foam is shown in Fig. 2. On the other hand, air should not be directly supplied to the anode due to the increase of oxidation selectivity on the anode side. There-

3 J. P. Choi et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 931~ fore, the anode used in this paper was prepared by sputtering Au on a Pyrex glass substrate. This also reduced the total manufacturing process. The thicknesses of the sputtered Au and Pyrex glass substrate were 200 nm and 500 µm respectively. Au was deposited by DC magnetron sputter coater (Sunicoat-564, Sunic system Inc.) at 2 mbar for 130 sec. The supplied power was 500W, and the plasma gas was argon of 50 sccm. Grain size of the deposited Au layer was less than 5nm, thus dense gold layer was formed on the Pyrex glass. Generally stainless steel, gold coated stainless steel and graphite were used as current collect material. In this work, the sputtered Au layer worked as both anode catalyst and current collector. Pyrex glass was just supporting materials. Au was not consumed by electrochemical reaction. So, thickness of Au was just constrained by electrical function. Thickness of 200nm was generally used in the integrated circuit (IC) device, thus the thickness of gold was decided as 200nm. However optimum thickness of gold in the electrochemical reaction should be investigated. The mixture of fuel and electrolyte was prepared by dissolving NaBH 4 in an alkaline solution of KOH. The selfhydrolysis rate of NaBH 4 solutions depends on the ph and temperature of the liquid electrolyte. The half-life of such solutions has been shown empirically to follow the equation below [10, 11] 3. Experimental results Fig. 3 shows the fabricated components and assembly of the micro DBFC. The fuel chamber was made of acrylic, with a volume of 0.3 ml ( mm 3 ). UV adhesive and waterproof adhesive tape (467MP, 3M Co.) were used to bond both electrodes to the fuel chamber. The dimensions of the micro DBFC were mm 3, and the active area of both electrodes was smaller than mm 2 due to the machining limitation of the fuel chamber. After the assembly, the fuel chamber was filled with a fuel-electrolyte mixture of 4 wt.% NaBH 4 in 1M KOH. The micro DBFC was operated in fully passive conditions without any auxiliary devices. From our fabricated micro DBFC, we characterized the performance of the micro DBFC by a polarization curve. In addition, a long operating test was performed. Because no electronic loader exists to measure in the microampere range, the polarization curves were measured by scanning the various resistance steps ranging from 100 kω to 1 Ω and simultaneously measuring the cell voltage of the micro DBFC. Each current step was kept stabilized for 20 seconds at each measurement step before the next step. Fig. 4 illustrates the cell performance according to the mixture ratios of MnO 2 and carbon powder when the total amount of the catalyst loading was 5 mg/cm 2. With increasing the amount of MnO 2, OCV was increased from 1.1 V to V, and the cell performance was improved due to an increase of the reduction reaction rate of the MnO 2 by increasing ionic conductivity. However, in the cell with 100 wt.% MnO 2, the cell voltage in the low current density region was dramatically decreased because of the low electronic conductivity. A maximum power density of 1.18 mw/cm 2 at 9 V was obtained for the micro DBFC with mixture ratios of reference number 3 (70 wt.% MnO 2 and 30 wt.% C), shown in Table 1. OCV should not be related with amount of catalyst. But very small current passed through the measurement system, this causes a voltage drop in OCV. In additions, power density should not depend on the size of active area. However, power density increased as active area increased. In addition, thin gold layer was used as both anode catalyst and current collector. Small active area of the proposed DBFC (10 10 mm 2 ) and thin layer of gold as current collector were main causes of low power density. ( ) log t = ph (34T 1.92), (4) 10 1/ 2 where t 1/2 is the time it takes an NaBH 4 solution to decompose in minutes and T is temperature in Kelvin. In this paper, we used 1 M KOH (ph = 14) and 4 wt.% NaBH 4 solution. All experiments were carried out at room temperature (around 293 K) and in ambient conditions. Fig. 3. Photographs of components and assembly of the fabricated micro DBFC. Dimensions of micro DBFC are mm 3 ; active area is mm Ref.No.1 : 5g/cm 2 of MnO 2 /C (30wt%/70wt%) Ref.No.2 : 5g/cm 2 of MnO 2 /C (50wt%/50wt%) Ref.No.3 : 5g/cm 2 of MnO 2 Ref.No.4 : 5g/cm 2 of MnO 2 /C (100wt%/0wt%) Current (ma) 1.18 mw/cm 2 at 9V Fig. 4. Polarization curve of DBFCs with respect to mixture ratios of MnO 2 and carbon in the cathode catalyst (amount of MnO 2 /C loading was fixed at 5g/cm 2 ).

4 934 J. P. Choi et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 931~ (a) Ref.No.5 : 3g/cm 2 of MnO 2 Ref.No.3 : 5g/cm 2 of MnO 2 Ref.No.6 : 7g/cm 2 of MnO Current (ma) (b) Fig. 5. Effect of the micro DBFC according to the amount of MnO 2 /C loading: (a) Ni-foam images according to amount of MnO 2 /C loading, (b) Polarization curve with respect to the loading (ratio of MnO 2 /C was fixed at 70wt% / 30wt%). Ref.No.3 : 5mg/cm 2 of MnO 2 /C (70wt% / 30wt%) Voltage Power Time (min) Fig. 6. Stability test of the micro DBFC (ref. no. 3) with discharge of 1 ma/cm 2 for 1 hour. : (amount of fuel and electrolyte was 0.3 ml). - In order to optimize the amount of the MnO 2 /C, three different weights of the catalyst loading (3 g, 5 g, and 7 g) were prepared, and their polarization curves were measured. For the three cases, the mixture ratios of MnO 2 /C were fixed at ref. no. 3 of 70 wt.% : 30 wt.%. Fig. 5(a) shows photographs of the Ni-foam according to the amount of loading, and Fig. 5(b) shows their respective polarization curves. For the different amounts of loading, the micro DBFC with a catalyst of 5 g exhibited the best performance of 1.17 mw/cm 2 at 6 V. The higher catalyst loading could facilitate the electrochemical reaction by increasing the active area. However, if loading is excessive, the mass transport of air is impeded because the pores in the Ni-foam are blocked. Performance is dramatically decreased in the case of 7 g. From Fig. 4 and Fig. 5, we found the best composition of the cathode catalyst was 70 wt.% MnO 2 and 30 wt.% C, and the optimal amount of MnO 2 /C was 5 g/cm 2, the specimen of reference number 3 in Table 1. Long-term and stability tests of the micro DBFC (reference number 3) were conducted by monitoring cell voltage during its operation. Fig. 6 shows the behavior of the cell voltage and power at a constant current of 1 ma. The cell power gradually decreased to about 2 mw after one hour of operation. The reason for degradation was the consumption of the fuel in the limited chamber and the reaction of the hydroxide ion (OH - ) with the carbon dioxide (CO) in the air, forming carbonate (CO 3 ). As a result, the ion conductivity of the electrolyte is gradually reduced, which increases the ohmic losses. Although the fabricated DBFCs in this paper have low power density and short operating time, they can make small and cheaply and apply to the lab on a chip (LOC) and point-ofcare (POC) chips. The DBFC could be used as power sources of biochip which have several characteristic of disposable, ondemand, and short-time operation. To verify the feasibility as the power sources of the bio chips such as diagnostic devices, glucose sensors, and DNA chips, a stacking test was carried out using the fabricated DBFCs in three series after single cell tests. As can be seen in Fig. 7, the DBFCs successfully turn on two LEDs (2.5mW at 2.0V, Bright LED Electronics Co.) at 2.4 V. The operation time was about one hour due to the limited amount of fuel and the degradation of the cell performance. In this work, liquid electrolyte of KOH was used. Since alkaline solution might cause carbonate problem by CO 2 in air, alkaline solution of electrolyte could be replaced by pollutantfree solution. 4. Conclusion (a) Fig. 7. LEDs lighting test using serially connected micro DBFCs: (a) OFF state, (b) ON state. (b) This paper presented a micro DBFC with the simple configuration of an anode, cathode, and a fuel chamber that was designed and fabricated. Au deposited on a glass and MnO 2 /C on Ni-foam were used as the anode and cathode catalysts respectively. NaBH 4 and KOH were also used as the hydrogen source and electrolyte respectively. The performance tests of the fabricated micro DBFCs were conducted with point-by-

5 J. P. Choi et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 931~ point and continuous polarization scans. The best performance shown by the MnO 2 /C cathode catalyst of 5 g/cm 2 in the composition of 70 wt.% MnO 2 and 30 wt.% C resulted from the increased electrochemical reaction, in turn caused by increasing the active area and the smooth mass transport of the air. The maximum power density obtained for the micro DBFC was 1.18 mw/cm 2 at 9 V. A stability test was conducted under the constant current of 1 ma/cm 2 using only single filling of fuel and electrolyte of 0.3ml. The cell power gradually decreased to 2 mw after one hour, and we were able to keep two LEDs illuminated for one hour using the fabricated micro DBFC three-cell module. Acknowledgment In the memory of the late Dr. Jong Pil Choi This work was supported by the Regional Core Research Program funded by the Korea Ministry of Education, Science and Technology (Medical & Bio-Material Research Center). This work, also, was partly supported by the 2 nd stage of BK21 project funded by the Korea Ministry of Education, Science and Technology, Republic of Korea. Nomenclature E anode : Standard anode potential E cathode : Standard cathode potential E total : Theoretical open circuit voltage t : Time T : Temperature in Kelvin References [1] K. Shah and R. S. Besser, Novel microfabrication approaches for directly patterning PEM fuel cell membranes, J. Power Sources, 123 (2003) [2] S. Aravamudhan and S. Bhansali, Porous silicon based orientation independent, self-priming micro direct ethanol fuel cell, Sensors and Actuators A, (2005) [3] M. A. Priestnall, V. P. Kotzeva, D. J. Fish and E. M Nilsson, Compact mixed-reactant fuel cells, J. Power Sources, 106 (2002) [4] R. Zeng and P. K. Shen, Selective membrane electrode assemblies for bipolar plate-free mixed-reactant fuel cells, J. Power Sources, 170 (2007) [5] B. H. Liu, Z. P. Li and S. Suda, Development of highperformance planar borohydride fuel cell modules for portable applications, J. Power Sources, 175 (2008) [6] S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo and M. Binder, A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst, Int. J. Hydrogen Energy, 25 (2000) [7] R. X. Feng, H. Dong, Y. D. Wang, X. P. Ai, Y. L. Cao and H. X. Yang, A simple and high efficient direct borohydride fuel cell with MnO 2 -catalyzed cathode, Electrochemistry Comm., 7 (2005) [8] S. U. Jeong, R. K. Kim, E. A. Cho, H. J. Kim, S. W. Nam, I. H. Oh, S. A. Hong and S. H. Kim, A study on hydrogen generation from NaBH 4 solution using the high-performance Co-B catalyst, J. Power Sources, 144 (2005) [9] G. Y. Moon, S. S. Lee, K. Y. Lee, S. H. Kim and K. H. Song, Behavior of hydrogen evolution of aqueous sodium borohydride solutions, J. Ind. and Eng. Chemistry, 14 (2008) [10] A. Verma and S. Basu, Development of the direct borohydride fuel cell, J. Alloys and Compounds, (2005) Young Ho Seo received the Ph.D. degree from the Korea Advanced Institute of Science and Technology (KAIST), for his MEMS-based fuel cell development completed in August, During , he worked as a Senior Researcher of Nanomechanical Research Center at Korea Institute of Machinery and Materials (KIMM). In September 2006, Dr. Seo moved to Kangwon National University, where he is currently an Assistant Professor in the Departments of Mechanical and Mechatronics Engineering. Dr. Seo's research interests are focused on the fabrication of micro-nano hybrid patterns for optical and bio-medical applications. Byeong Hee Kim received his Ph.D. degree from the mechanical engineering department of Seoul National University, Korea, in the fields of precision machining and numerical control of the machine tools. He is currently a professor of the department of mechanical and mechatronics engineering in Kangwon National University, Korea, and the CEO of For CAST Co.. He has been involved and is carrying out the several research projects related to the nano and bio-medical technologies such as AAO-based surface nano structuring for optical and biomedical applications; micropumps; biochips; and M/NEMS based medical implants. And he is also interested in fuel cells and other green technologies.