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Hydrogen production from solid sodium borohydride with hydrogen peroxide decomposition reaction
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 8 7 0 e1 2 8 7 7
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen production from solid sodium borohydride with hydrogen peroxide decomposition reaction Taegyu Kim* Department of Aerospace Engineering, College of Engineering, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501-759, Republic of Korea
article info
abstract
Article history:
This study investigates hydrogen production from solid sodium borohydride with
Received 30 June 2010
hydrogen peroxide decomposition reaction for a fuel cell based air-independent propulsion
Received in revised form
system in space and underwater applications. Sodium borohydride in the solid state was
18 August 2010
used as a hydrogen source in the present study. Pure hydrogen could be generated by
Accepted 20 August 2010
a catalytic hydrolysis reaction in which the water source was obtained from the hydrogen
Available online 29 September 2010
peroxide decomposition. Hydrogen peroxide was selected as an oxidizer, being decomposed catalytically to generate oxygen and water. The pure oxygen was provided to a fuel
Keywords:
cell and the water was stored separately for the hydrolysis reaction. A fuel cell system was
Air-independent propulsion (AIP)
fabricated to validate the fuel cell based air-independent power system proposed in the
Fuel cell system
present study. Two catalytic reactors were prepared; one for the solid sodium borohydride
Sodium borohydride
hydrolysis reaction and the other for the hydrogen peroxide decomposition reaction. The
Hydrolysis reaction
hydrogen and oxygen generation rate were measured based on the various conditions. The
Hydrogen peroxide
performance evaluation of a fuel cell system proposed in the present study was carried out. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Air-independent propulsion (AIP) is defined as power generation independent of the air in the atmosphere. Initially, the AIP system was developed for submarines that use diesel fuel to increase underwater time [1]. The AIP technologies include Stirling engines, MESMA (Module Energie Sous Marin Autonome) engines, and CCD (closed-cycle diesel) engines [1,2]. However, they still contain problems such as the limitation of a safe submersible depth and lack of infrared stealth. In space applications, the power system also must be air-independent. The options for space power systems are presented in Fig. 1. The solar cell is the simplest way to retrieve energy from space; however, it cannot be used in the exploration of the dark sides of planets or in deep space missions. Primary batteries can be used as alternatives to the solar cell because it is not dependent on an illumination source. However, it is
unfeasible for long term space missions due to its low energy density. The radioisotope thermoelectric generator (RTG) is a good option for long term missions due to its high energy density. The RTG generates electric energy from heat released from radioactive decay using a thermoelectric effect. However, a serious concern is the radioactive contamination in manned space missions. The aforementioned problems can be resolved by the utilization of fuel cells, which provide ideal alternatives for existing propulsion and power systems. The fuel cell was first commercialized as a space power source by NASA (National Aeronautics and Space Administration) in the Gemini program, which is the second manned space project of the United States [3]. In underwater applications, the Class 212A and 214 submarines [4] from Germany, S-273 submarine [1] from Russia, and XDM program [2] from Canada have used a fuel cell as the AIP systems.
* Tel.: þ82 62 230 7123; fax: þ82 62 230 7729. E-mail address: [email protected]. 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.102
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Fig. 1 e The options for space power systems.
Fig. 2 shows the energy conversion paths of a chemical fuel. The reaction path of a fuel cell is basically the same as that of hydrogen combustion. However, the energy conversion path of a fuel cell differs from that of a conventional combustion process. A heat engine based on a combustion process requires a kinematical mechanism that converts thermal energy to a mechanical output. On the other hand, a fuel cell is an electrochemical conversion device in which chemical energy is converted directly to electric energy. The thermal efficiency of a fuel cell is higher than that of heat engines. Additionally, the energy density of a fuel cell is higher than that of existing batteries because fuel cells use chemical fuels, such as hydrogen [5]. There are several types of fuel cells, including the polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC), molten carbon fuel cell (MCFC), and the solid oxide fuel cell (SOFC) [5]. The PEMFC has currently attracted a great amount of attention within the last
few decades as a promising solution to the increasing demands for a new power source for electronics [6], automobiles, robots [7] and aircraft [8]. However, a major obstacle in the successful development of PEMFCs is the difficulties with storing and handling gaseous hydrogen. Hydrogen is often stored cryogenically for space missions. This approach, however, is not suitable for long durations of space applications due to the exceptionally high boiling-off rate of liquid hydrogen. Oxidizer storage for a fuel cell reaction with hydrogen is an important issue for space and underwater power applications where air independence is necessary. The fuel cells for NASA’s Gemini, Apollo, and Space Shuttle programs carried cryogenic liquid oxygen as the oxidizer. The liquid oxygen invariably evaporates over a few weeks, thus limiting the mission duration. Chemical storage in the liquid state at normal temperatures and pressures has a significantly higher energy density in comparison to the suggested technologies. It can be converted
Fig. 2 e Energy conversion paths of a chemical fuel.
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thermally or catalytically in order to generate hydrogen or oxidizer gas when needed. In the present study, hydrogen peroxide was selected as an oxidizer source. A catalytic reaction was used to decompose hydrogen peroxide, generating oxygen and water. The pure oxygen was provided to a fuel cell and the water byproduct was stored separately. Sodium borohydride in the solid state was used as a hydrogen source. Pure hydrogen can be generated by a catalytic hydrolysis reaction of sodium borohydride, in which the water source was obtained from the hydrogen peroxide decomposition. Solid hydrogen storage possesses a high energy density, which increases mission time. Additionally, solid hydrogen storage does not suffer the same problems as cryogenic hydrogen. Properties of sodium borohydride and hydrogen peroxide are presented in Table 1. A fuel cell system was fabricated to validate the proposed fuel cell based air-independent power system. Two catalytic reactors were prepared; one for sodium borohydride hydrolysis reaction and the other for the hydrogen peroxide decomposition reaction. A cobalt metal catalyst was synthesized for the sodium borohydride hydrolysis reaction while a platinum metal catalyst was synthesized for the hydrogen peroxide decomposition reaction. The performance of the fuel cell system was evaluated for various conditions.
2.
Fuel cell system
2.1.
System description
Fig. 3 shows the proposed fuel cell system. The system consists of solid sodium borohydride, hydrogen peroxide, and the fuel cell. Hydrogen peroxide generates the oxygen and water by a decomposition reaction. The oxygen is used as an oxidizer for the fuel cell and the water is stored separately. Hydrogen is generated by a hydrolysis reaction of sodium borohydride with the water produced by hydrogen peroxide decomposition reaction. The hydrogen and oxygen generated by the above reactions are supplied to the anode and cathode of the fuel cell, respectively. The fuel cell then generates the electricity.
2.2.
Hydrogen source
Chemical hydrides have garnered a great amount of attention for being a new method of hydrogen storage [9]. Typically,
Table 1 e Properties of sodium borohydride and hydrogen peroxide.
Phase state Concentration (wt.%) Storage density (kg/m3) Storage temperature ( C) Vapor pressure (bar) Freezing point ( C) Toxicity
NaBH4
H2O2
Solid 98 1074 <314a e 505 Non-toxic
Liquid 90 1347 7 to 38 0.00345 @ 20 C 12 Non-toxic (skin burned)
a Decomposition temperature.
Fig. 3 e Concept of a fuel cell system proposed in the present study.
they include sodium borohydride (NaBH4), lithium aluminum hydride (LiBH4), ammonia borane (H3NBH3), and so on. The sodium borohydride is stored in a solid or liquid phase at atmospheric pressure and temperature. It is a stable and nonflammable hydrogen source. In addition, it contains relatively high hydrogen content, and is renewable and environmentally friendly. It is easy to control the hydrogen generation rate; pure hydrogen can be obtained by the catalytic hydrolysis reaction given below [10].
NaBH4 þ 2H2O / NaBO2 þ 4H2
(1)
Hydrogen is the only gaseous product in the reaction; therefore, pure hydrogen can be obtained after separating the borate vapor. The borate can be recycled into sodium borohydride. No heat input is required because the sodium borohydride hydrolysis is an exothermic reaction. The sodium borohydride hydrolysis reaction can be accelerated by a catalyst such as platinum [11] and ruthenium [12]. These noble metal catalysts are highly active in the sodium borohydride hydrolysis reaction; however, these catalysts are very costly. Cobalt [13,17], nickel [14], and its borides [15,18] have been studied as alternatives to noble metal catalysts for a number of years. Sodium borohydride is used in the form of an aqueous solution for general use as a hydrogen source. The sodium borohydride hydrolysis depends strongly on pH and temperature [9]. As an aqueous solution, sodium borohydride maintains an alkaline state higher than pH 12 to inhibit self-hydrolysis in the absence of a catalyst. Generally, the alkaline solution can be made by adding sodium hydroxide
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(NaOH) to the sodium borohydride aqueous solution. The sodium borohydride alkaline solution can be hydrolyzed using a catalyst when the hydrogen is needed. The maximum concentration of sodium borohydride in the alkaline solution is limited by the solubility of sodium borohydride [10]. The concentration of sodium borohydride solution would be 51.2 wt% for a stoichiometric hydrolysis reaction. However, the concentration is limited to 34.2 wt% due to its solubility. Considering the solubility of sodium borate that is a byproduct of the hydrolysis reaction, the concentration is severely limited to 12.1 wt%. The hydrogen storage density depends strongly on the concentration of sodium borohydride. The sodium borohydride with a concentration higher than 20 wt% is required to meet the demands of the hydrogen storage density. As a result, borate crystals form, which causes a clogging problem and the degradation of the catalyst. Therefore, sodium borohydride in the solid state was used in the present study.
2.3.
Oxygen source
Hydrogen peroxide has been widely used as an oxidizer alternative to LOX (liquid oxygen) for submarines and rockets. In the present study, hydrogen peroxide was decomposed to supply oxygen and water to the fuel cell [16] and to the sodium borohydride hydrolysis reactor. The hydrogen peroxide generates oxygen and water from a catalyst given below.
H2O2 / H2O þ 0.5O2
(2)
The oxygen storage density depends on the concentration of hydrogen peroxide. The hydrogen peroxide with 90 wt% concentration gives 595 g of oxygen per 1 L hydrogen peroxide. The amount of heat generated by the hydrogen peroxide decomposition also depends on the concentration of hydrogen peroxide. Although the hydrogen peroxide decomposition is an exothermic reaction, the resultant heat is consumed for heating the water included in the concentrated hydrogen peroxide. The heat generation has a positive value when the hydrogen peroxide concentration is higher than 67 wt%. Thus the hydrogen peroxide concentration for monopropellant rockets is generally higher than 90 wt%. Therefore, with the above reasons, a high-concentration hydrogen peroxide was used in the present study.
3.
Experimental
3.1.
Hydrogen generation
An experiment on hydrogen generation from sodium borohydride in the solid state was performed. Water stored in a separate vessel was injected into the solid sodium borohydride, subsequently generating hydrogen through the hydrolysis reaction. The water can be obtained from either the hydrogen peroxide decomposition reaction or the cathode of a fuel cell in which water is the resultant product of a fuel cell reaction. Because of the solid state storage of sodium borohydride, the sodium hydroxide is not required to control
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self-hydrolysis, and the sodium borohydride reacts with water for a stoichiometric hydrolysis reaction to occur. Fig. 4 shows the experimental setup for hydrogen generation from the solid state sodium borohydride. The pellets of sodium borohydride were placed in a glass flask. The water was supplied through a tube using a syringe pump. A thermocouple, located inside the flask, was used to measure the reaction temperature. Cobalt nitrate, Co(NO3)2, was mixed with the water as a catalyst to accelerate the hydrolysis reaction. The amount of solution injected to the sodium borohydride was 2 ml with 0.1 M concentration of cobalt nitrate. 1.6 g of sodium borohydride was used in this experiment.
3.2.
Hydrogen peroxide decomposition
Platinum was selected as a catalyst for hydrogen peroxide decomposition. The g-alumina (g-Al2O3) was used as a support to increase the surface area that provides an active site for catalytic reactions. The used g-alumina has 255 m2/g of surface area and 1.14 cc/g of total pore volume. A wet impregnation method was used to load the platinum onto the g-alumina. The prepared g-alumina powder was immersed in 1 M aqueous solution of platinum chloride, H2PtCl6, for 12 h with vigorous stirring. The moisture was removed by drying the catalyst loaded g-alumina in a convection oven at 120 C for 2 h. A calcination procedure followed in a furnace at 350 C for 2 h. After completion of coating, the catalyst surface was activated by reduction in a steady flowing hydrogen environment at 350 C for 5 h. The prepared platinum catalyst, Pt/g-Al2O3, was packed into a tubular reactor and trapped with glass wool filters. The inner diameter and length of the reactor were 1.3 cm and 8.7 cm, respectively. The fixed platinum catalyst was 3.2 cm in length. The glass wool filter at the inlet of reactor acted as a flow distributor of hydrogen peroxide. Hydrogen peroxide was decomposed to generate steam and heated oxygen on the platinum catalyst. A syringe pump supplied hydrogen peroxide to the reactor at a controlled rate. The concentration of hydrogen peroxide was measured using a refractometer (PR-50HO, ATAGO) with a small quantity of sample. The temperature of the reactor was recorded by a thermocouple that was positioned in the middle of platinum catalyst.
Fig. 4 e Experimental setup for hydrogen generation from sodium borohydride in the solid state.
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Fig. 5 e Diagram of the fuel cell system; H2O2 cartridge, water catch vessel, solid NaBH4 reactor, fuel cell stack were inserted in an airtight container.
The hydrogen peroxide with a concentration of 90 wt% was used in this experiment. The flow rate of hydrogen peroxide was 150 ml/h considering the oxygen consumption for a fuel cell reaction. The hydrogen peroxide decomposition was carried out for conditions in which the reactor was both cooled and not cooled.
3.3.
Integrated test with a fuel cell
Fig. 5 shows a diagram of the fuel cell system; H2O2 cartridge, water catch vessel, solid NaBH4 reactor, fuel cell stack were
inserted in an airtight container. Fig. 6 is the photograph of the fuel cell system. The temperatures of the fuel cell stack and the H2O2 decomposition reactor were measured by two temperature sensors. The pressure inside the airtight container was measured; when the pressure increased abnormally, it was programmed in the controller that the venting valve was open for safety. Performance evaluations of the fuel cell system were conducted for two conditions. The first condition comprises the operation of a fuel cell with the hydrogen generated by the sodium borohydride hydrolysis reaction. This was compared
Fig. 6 e Photograph of the fuel cell system.
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to the performance of fuel cell using pure hydrogen. In this case, the oxygen was supplied from the air. The second condition is the operation of a fuel cell with the oxygen generated by hydrogen peroxide decomposition reaction. This was then compared to the performance of an air-fed fuel cell. The hydrogen was provided from the sodium borohydride hydrolysis reaction. A commercially available fuel cell stack was prepared for the performance evaluation. The nominal power output of the fuel cell stack was 100 W.
4.
Results and discussion
4.1.
Hydrogen generation from solid sodium borohydride
Fig. 7 shows the hydrogen generation rate and reaction temperature as a function of time. The hydrogen generation rate slowly increased with increasing reaction temperature after the catalytic solution was injected into the sodium borohydride hydrolysis reactor. After 17 min, the hydrogen generation rate suddenly increased followed by the rapid increase of reaction temperature. The sodium borohydride was dissolved by the injected water with the elapsed reaction time. The reaction temperature increased due to the heat generated by the sodium borohydride hydrolysis, which is an exothermic reaction. The reaction temperature reached 50 C, resulting in the sudden increase of reaction rate. The maximum hydrogen generation rate and maximum temperature were 913.5 ml/min and 84.8 C, respectively. The accumulated hydrogen amount and hydrogen yield as a function of time are shown in Fig. 8. The total amount of the generated hydrogen was 4154 ml for 30 min and a hydrogen yield of 100% was accomplished.
4.2. Oxygen generation by hydrogen peroxide decomposition The oxygen generation rate and reactor temperature of the hydrogen peroxide decomposition reactor as a function of reaction time are shown in Fig. 9. The oxygen generation rate and reactor temperature sharply increased after the initiation of the reaction. The oxygen generation rate reached 1100 ml/min and remained constant over the duration of the reaction. The hydrogen peroxide was decomposed completely at the exit of the reactor and a maximum temperature reached 595.6 C after 11 min elapsed after the initiation of the reaction. This operation at the aforementioned maximum temperature will require either thermal isolation or a cooling mechanism for system packaging, which increases the volume and weight of the system. The reactor was cooled to a temperature lower than 80 C. Fig. 10 shows the results of the hydrogen peroxide decomposition under a cooling condition. The oxygen generation rate was intensely unstable in comparison to the result obtained from the hydrogen peroxide decomposition without cooling. This can be explained by the heat transfer limitations that exist between reactants and catalysts as the reactor was cooled. The average generation rate of oxygen was 950.4 ml/min, which is 13.6% lower than the oxygen generation rate without cooling. Therefore, it can be observed that the
Fig. 7 e Hydrogen generation rate and reaction temperature as a function of time.
prepared catalyst has a high decomposition performance at low temperature. The temperature of oxygen gas flow at the reactor exit was also measured, which was found to be near room temperature. The generated oxygen gas could be provided to a fuel cell without additional cooling. The accumulated oxygen amount and oxygen yield as a function of time for conditions with and without cooling are shown in Fig. 11. These results were calculated for 6 min from the steady state of oxygen generation rate. The reactor cooling produced a decrease of oxygen yield of approximately 15% due to the low reaction temperature. Thus, a longer reactor is required for the reactor cooling situation.
4.3.
Fuel cell performances
Fig. 12 shows the performance curve of a fuel cell under both hydrogen supply conditions; the solid line represents the pure hydrogen supply and the dashed line represents the hydrogen generated by the sodium borohydride hydrolysis reaction. The
Fig. 8 e The accumulated hydrogen amount and hydrogen yield as a function of time.
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Fig. 9 e The oxygen generation rate and reactor temperature of hydrogen peroxide decomposition reactor as a function of reaction time.
Fig. 11 e The accumulated oxygen amount and oxygen yield as a function of time at the conditions without and with cooling for 6 min from the steady state of oxygen generation rate.
performance of the fuel cell with the generated hydrogen from the sodium borohydride was comparable to that with the pure hydrogen. This leads to the conclusion that the pure hydrogen was generated from the sodium borohydride hydrolysis reaction. Fig. 13 shows the performance curve of a fuel cell with an oxygen supply generated by the hydrogen peroxide decomposition (solid line). The performance curve of an air breathing fuel cell is also plotted on Fig. 13 comparison (dashed line). The oxygen generated by the hydrogen peroxide decomposition was cooled down to the same temperature as the air in order to make the comparison under the same temperature condition. The voltage and output power of the fuel cell with the hydrogen peroxide decomposition were 11% and 12% higher than that of the air breathing fuel cell at the current
load of 6 A, respectively. This is because the nitrogen in the air has a negative effect on the fuel cell reaction in terms of oxygen concentration. The oxygen concentration was 100% in the case of the hydrogen peroxide decomposition. A very small amount of hydrogen peroxide, which is not decomposed in the reactor, can be supplied as a vapor form with the oxygen. However, there was no observed effect with the hydrogen peroxide vapor form because the vapor decomposed completely on the fuel cell catalyst. The fuel cell used platinum as a catalyst, which is the same catalyst used with the hydrogen peroxide decomposition. The sodium borohydride can be stored with the density of 1074 kg/m3. It is converted to hydrogen storage density of 227 kg/m3, which is 3 times higher than that of conventional
Fig. 10 e The oxygen generation rate and reactor temperature of hydrogen peroxide decomposition reactor as a function of reaction time with the cooling condition.
Fig. 12 e The performance curves of a fuel cell in both hydrogen supply conditions (Solid line is for the pure hydrogen supply and the dashed one is for the hydrogen generated by the sodium borohydride hydrolysis reaction).
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4. The performance of fuel cell was improved with the oxygen generated by the hydrogen peroxide decomposition reaction.
references
Fig. 13 e The performance curves of a fuel cell with an oxygen supply generated by the hydrogen peroxide decomposition (solid line) and an air breathing fuel cell (dashed line).
liquid hydrogen. The storage density of hydrogen peroxide is 1347 kg/m3, in which the oxygen of 44.1% is included. Thus the oxygen storage density is 595 kg/m3, which is less than that of liquid oxygen. However, the chemical storage is safer, cheaper, and easier for maintenance than liquid storage. In the current fuel cell system, pumps are needed to deliver the liquid fuels, which consume 2 W for operation of the 100 W fuel cell stack. It is less than 1% that the decrease of system efficiency by the energy consumption of pumps. Considering the considerable energy is required for hydrogen and oxygen to maintain the liquid state for long time, the proposed fuel cell system will be able to a new alternative AIP system with a high energy density in space and underwater applications.
5.
Conclusion
The preliminary experiments were conducted to develop the fuel cell based air-independent power system for space and underwater applications. 1. The characteristics of hydrogen generation from the sodium borohydride in the solid state by injecting the catalyst solution were investigated. 2. The hydrogen peroxide was decomposed to generate oxygen for an oxidizer of a fuel cell and the oxygen generation rate was measured. There was no large performance decline under the cooling condition. 3. There is no difference of performance in either feeding conditions of the pure hydrogen and the hydrogen generated by the sodium borohydride hydrolysis reaction.
[1] Polmar N, Moore KJ. Cold war submarines: the design and construction of U.S. and Soviet submarines. Brassey’s Inc.; 2004. 45. [2] Zimmerman S. Submarine technology for the 21st Century. Trafford; 2000. p. 68. [3] Sipiera DM, Sipiera PP. Project Gemini. Scholastic Library Pub; 1998. p. 23. [4] McConnell VP. Now, voyager? The increasing marine use of fuel cells. Fuel Cells Bull 2010;2010(5):12e7. [5] O’Hayre R, Cha S, Colella W, Prinz F. Fuel cell Fundamentals. New York: John Wiley & Sons, Inc.; 2006. pp. 10e11. [6] Kim T. Micro Methanol reformer combined with a catalytic combustor for a PEM fuel cell. Int J Hydrogen Energy 2009;34: 6790e8. [7] Joh HI, Ha TJ, Hwang SY, Kim JH, Chae SH, Cho JH, et al. A direct methanol fuel cell system to power a humanoid robot. J Power Sources 2010;195(1):293e8. [8] Bradley TH, Moffitt BA, Mavris DN, Parekh DE. Development and experimental characterization of a fuel cell powered aircraft. J Power Sources 2007;171(2):793e801. [9] Schlesinger HI, Brown HC, Finholt AE, Gilbreath JR, Hockstra HR, Hyde EK. Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen. J Am Chem Soc 1953;75:215. [10] Kojima Y, Suzuki KI, Fukumoto K, Sasaki M, Yamamoto T, Kawai Y, et al. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27:1029e34. [11] Krishnan P, Yang TH, Lee WY, Kim CS. PtRueLiCO2-an efficient catalyst for hydrogen generation from sodium borohydride solutions. J Power Sources 2005;147:17e23. [12] Amendola SC, Sharp-Goldman SL, Saleem Janjua M, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969e75. [13] Ye W, Zhang H, Xu D, Ma L, Yi B. Hydrogen generation utilizing alkaline sodium borohydride solution and supported cobalt catalyst. J Power Sources 2007;164:544e8. [14] Kim JH, Kim KT, Kang YM, Kim HS, Song MS, Lee YJ, et al. Studyon degradation of filamentrary Ni catalyst on hydrolysis of sodium borohydride. J Alloys Compounds 2004; 379:222e7. [15] Fernandes R, Patel N, Miotello A. Efficient catalytic properties of CoeNiePeB catalyst powders for hydrogen generation by hydrolysis of alkaline solution of NaBH4. Int J Hydrogen Energy 2009;34:2893e900. [16] Kim T, Hwang JS, Kwon S. A MEMS methanol reformer heated by decomposition of hydrogen peroxide. Lab Chip 2007;7(7):836e47. [17] Chamoun R, Demirci UB, Zaatar Y, Khoury A, Miele P. A CoeaAl2O3eCu as shaped catalyst in NaBH4 hydrolysis. Int J Hydrogen Energy 2010;35:6583e91. [18] Zhao J, Ma H, Chen J. Improved hydrogen generation from alkaline NaBH4 solution using carbon-supported CoeB as catalysts. Int J Hydrogen Energy 2007;32:4711e6.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen production from solid sodium borohydride with hydrogen peroxide decomposition reaction Taegyu Kim* Department of Aerospace Engineering, College of Engineering, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501-759, Republic of Korea
article info
abstract
Article history:
This study investigates hydrogen production from solid sodium borohydride with
Received 30 June 2010
hydrogen peroxide decomposition reaction for a fuel cell based air-independent propulsion
Received in revised form
system in space and underwater applications. Sodium borohydride in the solid state was
18 August 2010
used as a hydrogen source in the present study. Pure hydrogen could be generated by
Accepted 20 August 2010
a catalytic hydrolysis reaction in which the water source was obtained from the hydrogen
Available online 29 September 2010
peroxide decomposition. Hydrogen peroxide was selected as an oxidizer, being decomposed catalytically to generate oxygen and water. The pure oxygen was provided to a fuel
Keywords:
cell and the water was stored separately for the hydrolysis reaction. A fuel cell system was
Air-independent propulsion (AIP)
fabricated to validate the fuel cell based air-independent power system proposed in the
Fuel cell system
present study. Two catalytic reactors were prepared; one for the solid sodium borohydride
Sodium borohydride
hydrolysis reaction and the other for the hydrogen peroxide decomposition reaction. The
Hydrolysis reaction
hydrogen and oxygen generation rate were measured based on the various conditions. The
Hydrogen peroxide
performance evaluation of a fuel cell system proposed in the present study was carried out. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Air-independent propulsion (AIP) is defined as power generation independent of the air in the atmosphere. Initially, the AIP system was developed for submarines that use diesel fuel to increase underwater time [1]. The AIP technologies include Stirling engines, MESMA (Module Energie Sous Marin Autonome) engines, and CCD (closed-cycle diesel) engines [1,2]. However, they still contain problems such as the limitation of a safe submersible depth and lack of infrared stealth. In space applications, the power system also must be air-independent. The options for space power systems are presented in Fig. 1. The solar cell is the simplest way to retrieve energy from space; however, it cannot be used in the exploration of the dark sides of planets or in deep space missions. Primary batteries can be used as alternatives to the solar cell because it is not dependent on an illumination source. However, it is
unfeasible for long term space missions due to its low energy density. The radioisotope thermoelectric generator (RTG) is a good option for long term missions due to its high energy density. The RTG generates electric energy from heat released from radioactive decay using a thermoelectric effect. However, a serious concern is the radioactive contamination in manned space missions. The aforementioned problems can be resolved by the utilization of fuel cells, which provide ideal alternatives for existing propulsion and power systems. The fuel cell was first commercialized as a space power source by NASA (National Aeronautics and Space Administration) in the Gemini program, which is the second manned space project of the United States [3]. In underwater applications, the Class 212A and 214 submarines [4] from Germany, S-273 submarine [1] from Russia, and XDM program [2] from Canada have used a fuel cell as the AIP systems.
* Tel.: þ82 62 230 7123; fax: þ82 62 230 7729. E-mail address: [email protected]. 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.102
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Fig. 1 e The options for space power systems.
Fig. 2 shows the energy conversion paths of a chemical fuel. The reaction path of a fuel cell is basically the same as that of hydrogen combustion. However, the energy conversion path of a fuel cell differs from that of a conventional combustion process. A heat engine based on a combustion process requires a kinematical mechanism that converts thermal energy to a mechanical output. On the other hand, a fuel cell is an electrochemical conversion device in which chemical energy is converted directly to electric energy. The thermal efficiency of a fuel cell is higher than that of heat engines. Additionally, the energy density of a fuel cell is higher than that of existing batteries because fuel cells use chemical fuels, such as hydrogen [5]. There are several types of fuel cells, including the polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC), molten carbon fuel cell (MCFC), and the solid oxide fuel cell (SOFC) [5]. The PEMFC has currently attracted a great amount of attention within the last
few decades as a promising solution to the increasing demands for a new power source for electronics [6], automobiles, robots [7] and aircraft [8]. However, a major obstacle in the successful development of PEMFCs is the difficulties with storing and handling gaseous hydrogen. Hydrogen is often stored cryogenically for space missions. This approach, however, is not suitable for long durations of space applications due to the exceptionally high boiling-off rate of liquid hydrogen. Oxidizer storage for a fuel cell reaction with hydrogen is an important issue for space and underwater power applications where air independence is necessary. The fuel cells for NASA’s Gemini, Apollo, and Space Shuttle programs carried cryogenic liquid oxygen as the oxidizer. The liquid oxygen invariably evaporates over a few weeks, thus limiting the mission duration. Chemical storage in the liquid state at normal temperatures and pressures has a significantly higher energy density in comparison to the suggested technologies. It can be converted
Fig. 2 e Energy conversion paths of a chemical fuel.
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thermally or catalytically in order to generate hydrogen or oxidizer gas when needed. In the present study, hydrogen peroxide was selected as an oxidizer source. A catalytic reaction was used to decompose hydrogen peroxide, generating oxygen and water. The pure oxygen was provided to a fuel cell and the water byproduct was stored separately. Sodium borohydride in the solid state was used as a hydrogen source. Pure hydrogen can be generated by a catalytic hydrolysis reaction of sodium borohydride, in which the water source was obtained from the hydrogen peroxide decomposition. Solid hydrogen storage possesses a high energy density, which increases mission time. Additionally, solid hydrogen storage does not suffer the same problems as cryogenic hydrogen. Properties of sodium borohydride and hydrogen peroxide are presented in Table 1. A fuel cell system was fabricated to validate the proposed fuel cell based air-independent power system. Two catalytic reactors were prepared; one for sodium borohydride hydrolysis reaction and the other for the hydrogen peroxide decomposition reaction. A cobalt metal catalyst was synthesized for the sodium borohydride hydrolysis reaction while a platinum metal catalyst was synthesized for the hydrogen peroxide decomposition reaction. The performance of the fuel cell system was evaluated for various conditions.
2.
Fuel cell system
2.1.
System description
Fig. 3 shows the proposed fuel cell system. The system consists of solid sodium borohydride, hydrogen peroxide, and the fuel cell. Hydrogen peroxide generates the oxygen and water by a decomposition reaction. The oxygen is used as an oxidizer for the fuel cell and the water is stored separately. Hydrogen is generated by a hydrolysis reaction of sodium borohydride with the water produced by hydrogen peroxide decomposition reaction. The hydrogen and oxygen generated by the above reactions are supplied to the anode and cathode of the fuel cell, respectively. The fuel cell then generates the electricity.
2.2.
Hydrogen source
Chemical hydrides have garnered a great amount of attention for being a new method of hydrogen storage [9]. Typically,
Table 1 e Properties of sodium borohydride and hydrogen peroxide.
Phase state Concentration (wt.%) Storage density (kg/m3) Storage temperature ( C) Vapor pressure (bar) Freezing point ( C) Toxicity
NaBH4
H2O2
Solid 98 1074 <314a e 505 Non-toxic
Liquid 90 1347 7 to 38 0.00345 @ 20 C 12 Non-toxic (skin burned)
a Decomposition temperature.
Fig. 3 e Concept of a fuel cell system proposed in the present study.
they include sodium borohydride (NaBH4), lithium aluminum hydride (LiBH4), ammonia borane (H3NBH3), and so on. The sodium borohydride is stored in a solid or liquid phase at atmospheric pressure and temperature. It is a stable and nonflammable hydrogen source. In addition, it contains relatively high hydrogen content, and is renewable and environmentally friendly. It is easy to control the hydrogen generation rate; pure hydrogen can be obtained by the catalytic hydrolysis reaction given below [10].
NaBH4 þ 2H2O / NaBO2 þ 4H2
(1)
Hydrogen is the only gaseous product in the reaction; therefore, pure hydrogen can be obtained after separating the borate vapor. The borate can be recycled into sodium borohydride. No heat input is required because the sodium borohydride hydrolysis is an exothermic reaction. The sodium borohydride hydrolysis reaction can be accelerated by a catalyst such as platinum [11] and ruthenium [12]. These noble metal catalysts are highly active in the sodium borohydride hydrolysis reaction; however, these catalysts are very costly. Cobalt [13,17], nickel [14], and its borides [15,18] have been studied as alternatives to noble metal catalysts for a number of years. Sodium borohydride is used in the form of an aqueous solution for general use as a hydrogen source. The sodium borohydride hydrolysis depends strongly on pH and temperature [9]. As an aqueous solution, sodium borohydride maintains an alkaline state higher than pH 12 to inhibit self-hydrolysis in the absence of a catalyst. Generally, the alkaline solution can be made by adding sodium hydroxide
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(NaOH) to the sodium borohydride aqueous solution. The sodium borohydride alkaline solution can be hydrolyzed using a catalyst when the hydrogen is needed. The maximum concentration of sodium borohydride in the alkaline solution is limited by the solubility of sodium borohydride [10]. The concentration of sodium borohydride solution would be 51.2 wt% for a stoichiometric hydrolysis reaction. However, the concentration is limited to 34.2 wt% due to its solubility. Considering the solubility of sodium borate that is a byproduct of the hydrolysis reaction, the concentration is severely limited to 12.1 wt%. The hydrogen storage density depends strongly on the concentration of sodium borohydride. The sodium borohydride with a concentration higher than 20 wt% is required to meet the demands of the hydrogen storage density. As a result, borate crystals form, which causes a clogging problem and the degradation of the catalyst. Therefore, sodium borohydride in the solid state was used in the present study.
2.3.
Oxygen source
Hydrogen peroxide has been widely used as an oxidizer alternative to LOX (liquid oxygen) for submarines and rockets. In the present study, hydrogen peroxide was decomposed to supply oxygen and water to the fuel cell [16] and to the sodium borohydride hydrolysis reactor. The hydrogen peroxide generates oxygen and water from a catalyst given below.
H2O2 / H2O þ 0.5O2
(2)
The oxygen storage density depends on the concentration of hydrogen peroxide. The hydrogen peroxide with 90 wt% concentration gives 595 g of oxygen per 1 L hydrogen peroxide. The amount of heat generated by the hydrogen peroxide decomposition also depends on the concentration of hydrogen peroxide. Although the hydrogen peroxide decomposition is an exothermic reaction, the resultant heat is consumed for heating the water included in the concentrated hydrogen peroxide. The heat generation has a positive value when the hydrogen peroxide concentration is higher than 67 wt%. Thus the hydrogen peroxide concentration for monopropellant rockets is generally higher than 90 wt%. Therefore, with the above reasons, a high-concentration hydrogen peroxide was used in the present study.
3.
Experimental
3.1.
Hydrogen generation
An experiment on hydrogen generation from sodium borohydride in the solid state was performed. Water stored in a separate vessel was injected into the solid sodium borohydride, subsequently generating hydrogen through the hydrolysis reaction. The water can be obtained from either the hydrogen peroxide decomposition reaction or the cathode of a fuel cell in which water is the resultant product of a fuel cell reaction. Because of the solid state storage of sodium borohydride, the sodium hydroxide is not required to control
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self-hydrolysis, and the sodium borohydride reacts with water for a stoichiometric hydrolysis reaction to occur. Fig. 4 shows the experimental setup for hydrogen generation from the solid state sodium borohydride. The pellets of sodium borohydride were placed in a glass flask. The water was supplied through a tube using a syringe pump. A thermocouple, located inside the flask, was used to measure the reaction temperature. Cobalt nitrate, Co(NO3)2, was mixed with the water as a catalyst to accelerate the hydrolysis reaction. The amount of solution injected to the sodium borohydride was 2 ml with 0.1 M concentration of cobalt nitrate. 1.6 g of sodium borohydride was used in this experiment.
3.2.
Hydrogen peroxide decomposition
Platinum was selected as a catalyst for hydrogen peroxide decomposition. The g-alumina (g-Al2O3) was used as a support to increase the surface area that provides an active site for catalytic reactions. The used g-alumina has 255 m2/g of surface area and 1.14 cc/g of total pore volume. A wet impregnation method was used to load the platinum onto the g-alumina. The prepared g-alumina powder was immersed in 1 M aqueous solution of platinum chloride, H2PtCl6, for 12 h with vigorous stirring. The moisture was removed by drying the catalyst loaded g-alumina in a convection oven at 120 C for 2 h. A calcination procedure followed in a furnace at 350 C for 2 h. After completion of coating, the catalyst surface was activated by reduction in a steady flowing hydrogen environment at 350 C for 5 h. The prepared platinum catalyst, Pt/g-Al2O3, was packed into a tubular reactor and trapped with glass wool filters. The inner diameter and length of the reactor were 1.3 cm and 8.7 cm, respectively. The fixed platinum catalyst was 3.2 cm in length. The glass wool filter at the inlet of reactor acted as a flow distributor of hydrogen peroxide. Hydrogen peroxide was decomposed to generate steam and heated oxygen on the platinum catalyst. A syringe pump supplied hydrogen peroxide to the reactor at a controlled rate. The concentration of hydrogen peroxide was measured using a refractometer (PR-50HO, ATAGO) with a small quantity of sample. The temperature of the reactor was recorded by a thermocouple that was positioned in the middle of platinum catalyst.
Fig. 4 e Experimental setup for hydrogen generation from sodium borohydride in the solid state.
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Fig. 5 e Diagram of the fuel cell system; H2O2 cartridge, water catch vessel, solid NaBH4 reactor, fuel cell stack were inserted in an airtight container.
The hydrogen peroxide with a concentration of 90 wt% was used in this experiment. The flow rate of hydrogen peroxide was 150 ml/h considering the oxygen consumption for a fuel cell reaction. The hydrogen peroxide decomposition was carried out for conditions in which the reactor was both cooled and not cooled.
3.3.
Integrated test with a fuel cell
Fig. 5 shows a diagram of the fuel cell system; H2O2 cartridge, water catch vessel, solid NaBH4 reactor, fuel cell stack were
inserted in an airtight container. Fig. 6 is the photograph of the fuel cell system. The temperatures of the fuel cell stack and the H2O2 decomposition reactor were measured by two temperature sensors. The pressure inside the airtight container was measured; when the pressure increased abnormally, it was programmed in the controller that the venting valve was open for safety. Performance evaluations of the fuel cell system were conducted for two conditions. The first condition comprises the operation of a fuel cell with the hydrogen generated by the sodium borohydride hydrolysis reaction. This was compared
Fig. 6 e Photograph of the fuel cell system.
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to the performance of fuel cell using pure hydrogen. In this case, the oxygen was supplied from the air. The second condition is the operation of a fuel cell with the oxygen generated by hydrogen peroxide decomposition reaction. This was then compared to the performance of an air-fed fuel cell. The hydrogen was provided from the sodium borohydride hydrolysis reaction. A commercially available fuel cell stack was prepared for the performance evaluation. The nominal power output of the fuel cell stack was 100 W.
4.
Results and discussion
4.1.
Hydrogen generation from solid sodium borohydride
Fig. 7 shows the hydrogen generation rate and reaction temperature as a function of time. The hydrogen generation rate slowly increased with increasing reaction temperature after the catalytic solution was injected into the sodium borohydride hydrolysis reactor. After 17 min, the hydrogen generation rate suddenly increased followed by the rapid increase of reaction temperature. The sodium borohydride was dissolved by the injected water with the elapsed reaction time. The reaction temperature increased due to the heat generated by the sodium borohydride hydrolysis, which is an exothermic reaction. The reaction temperature reached 50 C, resulting in the sudden increase of reaction rate. The maximum hydrogen generation rate and maximum temperature were 913.5 ml/min and 84.8 C, respectively. The accumulated hydrogen amount and hydrogen yield as a function of time are shown in Fig. 8. The total amount of the generated hydrogen was 4154 ml for 30 min and a hydrogen yield of 100% was accomplished.
4.2. Oxygen generation by hydrogen peroxide decomposition The oxygen generation rate and reactor temperature of the hydrogen peroxide decomposition reactor as a function of reaction time are shown in Fig. 9. The oxygen generation rate and reactor temperature sharply increased after the initiation of the reaction. The oxygen generation rate reached 1100 ml/min and remained constant over the duration of the reaction. The hydrogen peroxide was decomposed completely at the exit of the reactor and a maximum temperature reached 595.6 C after 11 min elapsed after the initiation of the reaction. This operation at the aforementioned maximum temperature will require either thermal isolation or a cooling mechanism for system packaging, which increases the volume and weight of the system. The reactor was cooled to a temperature lower than 80 C. Fig. 10 shows the results of the hydrogen peroxide decomposition under a cooling condition. The oxygen generation rate was intensely unstable in comparison to the result obtained from the hydrogen peroxide decomposition without cooling. This can be explained by the heat transfer limitations that exist between reactants and catalysts as the reactor was cooled. The average generation rate of oxygen was 950.4 ml/min, which is 13.6% lower than the oxygen generation rate without cooling. Therefore, it can be observed that the
Fig. 7 e Hydrogen generation rate and reaction temperature as a function of time.
prepared catalyst has a high decomposition performance at low temperature. The temperature of oxygen gas flow at the reactor exit was also measured, which was found to be near room temperature. The generated oxygen gas could be provided to a fuel cell without additional cooling. The accumulated oxygen amount and oxygen yield as a function of time for conditions with and without cooling are shown in Fig. 11. These results were calculated for 6 min from the steady state of oxygen generation rate. The reactor cooling produced a decrease of oxygen yield of approximately 15% due to the low reaction temperature. Thus, a longer reactor is required for the reactor cooling situation.
4.3.
Fuel cell performances
Fig. 12 shows the performance curve of a fuel cell under both hydrogen supply conditions; the solid line represents the pure hydrogen supply and the dashed line represents the hydrogen generated by the sodium borohydride hydrolysis reaction. The
Fig. 8 e The accumulated hydrogen amount and hydrogen yield as a function of time.
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Fig. 9 e The oxygen generation rate and reactor temperature of hydrogen peroxide decomposition reactor as a function of reaction time.
Fig. 11 e The accumulated oxygen amount and oxygen yield as a function of time at the conditions without and with cooling for 6 min from the steady state of oxygen generation rate.
performance of the fuel cell with the generated hydrogen from the sodium borohydride was comparable to that with the pure hydrogen. This leads to the conclusion that the pure hydrogen was generated from the sodium borohydride hydrolysis reaction. Fig. 13 shows the performance curve of a fuel cell with an oxygen supply generated by the hydrogen peroxide decomposition (solid line). The performance curve of an air breathing fuel cell is also plotted on Fig. 13 comparison (dashed line). The oxygen generated by the hydrogen peroxide decomposition was cooled down to the same temperature as the air in order to make the comparison under the same temperature condition. The voltage and output power of the fuel cell with the hydrogen peroxide decomposition were 11% and 12% higher than that of the air breathing fuel cell at the current
load of 6 A, respectively. This is because the nitrogen in the air has a negative effect on the fuel cell reaction in terms of oxygen concentration. The oxygen concentration was 100% in the case of the hydrogen peroxide decomposition. A very small amount of hydrogen peroxide, which is not decomposed in the reactor, can be supplied as a vapor form with the oxygen. However, there was no observed effect with the hydrogen peroxide vapor form because the vapor decomposed completely on the fuel cell catalyst. The fuel cell used platinum as a catalyst, which is the same catalyst used with the hydrogen peroxide decomposition. The sodium borohydride can be stored with the density of 1074 kg/m3. It is converted to hydrogen storage density of 227 kg/m3, which is 3 times higher than that of conventional
Fig. 10 e The oxygen generation rate and reactor temperature of hydrogen peroxide decomposition reactor as a function of reaction time with the cooling condition.
Fig. 12 e The performance curves of a fuel cell in both hydrogen supply conditions (Solid line is for the pure hydrogen supply and the dashed one is for the hydrogen generated by the sodium borohydride hydrolysis reaction).
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4. The performance of fuel cell was improved with the oxygen generated by the hydrogen peroxide decomposition reaction.
references
Fig. 13 e The performance curves of a fuel cell with an oxygen supply generated by the hydrogen peroxide decomposition (solid line) and an air breathing fuel cell (dashed line).
liquid hydrogen. The storage density of hydrogen peroxide is 1347 kg/m3, in which the oxygen of 44.1% is included. Thus the oxygen storage density is 595 kg/m3, which is less than that of liquid oxygen. However, the chemical storage is safer, cheaper, and easier for maintenance than liquid storage. In the current fuel cell system, pumps are needed to deliver the liquid fuels, which consume 2 W for operation of the 100 W fuel cell stack. It is less than 1% that the decrease of system efficiency by the energy consumption of pumps. Considering the considerable energy is required for hydrogen and oxygen to maintain the liquid state for long time, the proposed fuel cell system will be able to a new alternative AIP system with a high energy density in space and underwater applications.
5.
Conclusion
The preliminary experiments were conducted to develop the fuel cell based air-independent power system for space and underwater applications. 1. The characteristics of hydrogen generation from the sodium borohydride in the solid state by injecting the catalyst solution were investigated. 2. The hydrogen peroxide was decomposed to generate oxygen for an oxidizer of a fuel cell and the oxygen generation rate was measured. There was no large performance decline under the cooling condition. 3. There is no difference of performance in either feeding conditions of the pure hydrogen and the hydrogen generated by the sodium borohydride hydrolysis reaction.
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