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Study of hydrogen production and storage based on aluminum–water 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 9 ( 2 0 1 4 ) 6 3 2 8 e6 3 3 4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Study of hydrogen production and storage based on aluminumewater reaction Shani Elitzur*, Valery Rosenband, Alon Gany Faculty of Aerospace Engineering, Technion e Israel Institute of Technology, Haifa 32000, Israel
article info
abstract
Article history:
The rate and yield of hydrogen production from the reaction between activated aluminum
Received 12 December 2013
and water has been investigated. The effect of different parameters such as water
Received in revised form
ealuminum ratio, water temperature and aluminum particle size and shape was studied
3 February 2014
experimentally. The aluminum activation method developed in-house involves 1%e2.5% of
Accepted 6 February 2014
lithium-based activator which is diffused into the aluminum particles, enabling sustained
Available online 1 March 2014
reaction with tap water or sea water at room temperature. Hydrogen production rates in the range of 200e600 ml/min/g Al, at a yield of about 90%, depending on operating pa-
Keywords:
rameters, were demonstrated. The work further studied the application in proton ex-
Hydrogen
change membrane (PEM) fuel cells in order to generate green electric energy,
Hydrogen storage
demonstrating theoretical specific electric energy storage that can exceed batteries by 10
Aluminumewater reaction
e20 folds.
Aluminum activation
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Fuel cell
Introduction In recent years much effort has been made in order to reduce the world dependence on fossil fuels and to meet the increasing energy needs using alternative sources of energy. Hydrogen is the most attractive alternative fuel because of its outstanding reaction energy (with air), three times higher than for hydrocarbon fuels, and its environmental friendly products, water or water vapor. The main disadvantages associated with the use of hydrogen are difficulty of storage and transportation due to its very low density, and safety problem because of its very high reactivity. Overcoming those challenges of hydrogen storage, transportation, and safety is essential for a wide use of hydrogen energy. Safe and compact hydrogen storage and in-situ hydrogen production from aluminumewater reaction can be achieved
using a novel, in-house, thermo-chemical treatment of aluminum particles. The method involves a small fraction of a lithium-based activator (typically 2.5%) which diffuses into the aluminum particles and modifies the hydroxide/oxide protective film on their surface, allowing a spontaneous and sustained chemical reaction between aluminum and water which produces hydrogen [1]. The stoichiometric reaction (Eq. (1)) yields theoretically 11% hydrogen mass compared to the aluminum mass (equivalent to over 1.2 l of hydrogen per gram of aluminum), making the concept very efficient for hydrogen storage. 3 Al þ 3H2 O/AlðOHÞ3 þ H2 2
(1)
Different attempts and approaches have been applied in the world to activate aluminum. It has been suggested to use alloys of aluminum with different metals such as Ga, In, Zn,
* Corresponding author. Tel.: þ972 547439066. E-mail addresses: [email protected] (S. Elitzur), [email protected] (V. Rosenband), [email protected] (A. Gany). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.02.037
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 9 ( 2 0 1 4 ) 6 3 2 8 e6 3 3 4
Sn and Bi [2e7]. Kravchenko et al. [3] and Ziebarth et al. [4] created Al alloys by melting aluminum with various metals and investigated the effect of the concentration of individual components in the alloy on the hydrogen yield. The doping procedure requires high temperatures for melting all the components (700e900 C), and results in relatively slow reaction with water, even at elevated temperatures. Wang et al. [5], Ilyukhina et al. [6], and Fan et al. [7] created different aluminum alloys using mechanical alloying method. The investigations were typically conducted at atmospheric pressure and room temperature, resulting in a range of hydrogen production yields from 20% to over 90%, where quaternary aluminum alloys (e.g., AleGaeIneSn), particularly with higher contents of Ga and Sn, exhibited the higher yield range. Activation of aluminum by mechanical treatment of the metal using cutting or ball milling which exposes a fresh and reactive surface of the aluminum particles while increasing the surface area and mixing with additives has also been investigated [2,8e11]. In some experiments [8,10], salt was used as nano-miller and cover; when the aluminumsalt powder comes in contact with water the salt particles are washed away and the aluminum reacts with water. This method gives high hydrogen yields when nano-particles of aluminum (which are very expensive) and high temperatures are used. Razavi-Tousi et al. [11] investigated the dependence of the reaction kinetics on the microstructure of the aluminum powder in order to optimize the milling time. Another method uses alkaline solutions, mainly sodium hydroxide, in order to destroy the protective oxide layer on the aluminum surface. The reaction is relatively slow [2,12,13], and caution is required for working with strong alkaline solutions. Yet another approach is to use water at very high temperatures (under high pressure) using aluminum powder without additives. Yavor et al. [14] got 60e80% hydrogen yield using 6 mm aluminum powder at water temperature of 120e150 C, and close to 100% at 200 C [15]. Vlaskin et al. [16] obtained 70e90% hydrogen yield using 4e7 mm aluminum powder with water temperature of 230e300 C. Both researches obtained better results for smaller particles and higher temperatures. This paper presents a parametric investigation of the chemical reaction between activated aluminum and water to produce hydrogen in-situ, safely and compactly. The activated aluminum has been prepared via a novel method developed in-house enabling the diffusion of a small fraction of lithium into the aluminum particles. It further studies the application of the hydrogen produced in proton exchange membrane (PEM) fuel cells to generate green electric energy.
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Fig. 1 e Schematic illustration of the experimental facility.
Batch type experiments were conducted in a glass reactor. The aluminum powder was put in the reactor first, then water was added and a spontaneous reaction started. Since hydrogen has low solubility in water the amount of hydrogen produced from the reaction was measured by water displacement. The hydrogen produced from the reaction was channeled to a glass container filled with water driving the water out from the container to a cup placed on an electronic balance. In some experiments the measurements were conducted with a digital mass flow meter (Aalborg, USA). The temperature of the reaction was measured by a digital thermometer. Schematic illustration of the experimental facility is presented in Fig. 1. The influence of pressure on the reaction was tested using a closed vessel, as described in Fig. 2. The tests applied either 9 mm aluminum particles or 20 mm wide, 0.2 mm thick Al flakes (average values), manufactured in China and supplied by MCT-Materials Ltd Israel. Regularly, the experiments were conducted with 2 g of an Al powder containing 2.5 wt% of activator. The high pressure tests used 4 g of Al, whereas the effect of activator fraction was studied with aluminum powders containing 1% and 5% of activator. The amount of hydrogen produced in the experiments was compared to the theoretical amount of the stoichiometric reaction (Eq. (1)), which produces about 1.24 l of hydrogen per 1 g of aluminum at standard conditions (1 atm, 273 K). For the calculation of the actual yield the hydrogen temperature and pressure at the experiment were
Experimental approach and installation A parametric experimental study has been conducted in order to find the influence of different operating factors and conditions on the activated aluminumewater reaction, measuring the reaction rate and efficiency. Effects of water/aluminum mass ratio, fraction of activator used, water temperature, pressure, the size and shape of the aluminum particles, and the type of water (i.e., tap water, sea water, and distilled water) have been investigated.
Fig. 2 e Schematic illustration of the pressure experiment installation.
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Fig. 3 e Hydrogen production rate vs. time for different water/aluminum mass ratios (2 g Al, 2.5% activator).
Fig. 5 e Hydrogen yield vs. time for different water/ aluminum mass ratios (2 g Al, 2.5% activator).
accounted for. Water vapor generated at the reactor condenses while cooling down to room temperature during flowing in the pipe and staying in the water displacement vessel. Hence, its maximum volumetric fraction (at saturation) would not exceed 2.5% at atmospheric experiments and 0.3% at high pressure tests.
Figs. 3 and 4 present the hydrogen production rate vs. time for different water/aluminum mass ratios and different fractions of activator, respectively. As can be seen, the reaction is faster when the water/aluminum mass ratio is lower (mainly due to the effect of temperature increase during the exothermic reaction, which is accelerated for a smaller heated mass) and when the activator fraction in the aluminum powder is bigger. The latter effect should be due to the fact, that for a too low activator fraction, its capability of destructing the protective properties of the oxide or hydroxide layer on the aluminum particle deteriorates. The yield of hydrogen produced is presented in Figs. 5 and 6. The maximum attainable hydrogen volume was calculated for the experiment conditions, assuming that the gas entering the water vessel was at room temperature and atmospheric pressure. In-test temperature variations of the hydrogen produced imply possible 2% error in determining the reaction yield. As can be seen, when the water/aluminum mass ratio is lower, the volume of hydrogen produced reaches its maximum value faster. In the range of activator fraction tested (1, 2.5 wt %), the reaction exhibited higher hydrogen production rate and yield for higher activator fraction, whereas the water/ aluminum mass ratio in the tested range had a little effect on the yield. In the tests with 2.5 wt% of activator the reaction yield was in the range of 86e91% (Figs. 5 and 7).
Results Waterealuminum mass ratio The influence of the mass ratio between water and activated aluminum powder on the reaction rate and the hydrogen volume produced was tested with 2 g of aluminum powder and different amounts of tap water (15, 20, 25, 30 ml) at room temperature (21e23 C). The aluminum hydroxide product of the reaction may absorb water (forming hydrate or gel). In addition, at low waterealuminum ratios boiling and evaporation of water take place. Hence, excess water beyond the stoichiometric ratio may be required for complete reaction. Aluminum powders, treated by either 2.5 wt% or 1 wt% of activator, were tested. In order to compare results of experiments conducted with different amounts of aluminum powders; the results are presented per unit mass (1 g) of aluminum.
Fig. 4 e Hydrogen production rate vs. time for different water/aluminum mass ratios (2 g Al, 1% activator).
Fig. 6 e Hydrogen yield vs. time for different water/ aluminum mass ratios (2 g Al, 1% activator).
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 9 ( 2 0 1 4 ) 6 3 2 8 e6 3 3 4
Fig. 7 e Overall reaction time and hydrogen yield vs. water/ aluminum mass ratio (2 g Al, 2.5% activator).
Water temperature The reaction was tested using 20 ml of tap water in different initial temperatures (23, 35, 55, 70 C) reacting with 2 g Al powder which contains 2.5 wt% of activator. The aluminumewater reaction is highly exothermic, increasing the water temperature during the reaction process. Fig. 8 presents the hydrogen production rate during the aluminumewater reaction for different initial water temperatures. It can be seen that when the initial temperature of the water is higher the reaction delay is shorter and the maximum hydrogen production rate is higher, as could be expected. The cumulative hydrogen yield is presented in Fig. 9. This figure shows clearly that although the reaction is faster when the initial water temperature is higher, the overall yield of hydrogen produced is similar for all the tested temperatures. The overall hydrogen yield presented in Fig. 10, was similar in all the experiments, 87e91%, almost indifferent with the initial water temperature.
Water type Tests were conducted with different types of water: tap water, sea water (Mediterranean sea, about 4.2% salt concentration), and distilled water, revealing that hydrogen production at a similar good yield could be obtained with all types of water. This result is important in particular for marine and
Fig. 8 e Hydrogen production rate vs. time for different initial water temperatures (2 g Al D 20 ml water, 2.5% activator).
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Fig. 9 e Hydrogen yield vs. time for different initial water temperatures (2 g Al D 20 ml water, 2.5% activator).
underwater vessel applications, which should carry only activated aluminum powder, using the water in the surroundings to produce hydrogen and generate electricity. Fig. 11 describes the hydrogen generation rate per unit mass of Al produced from water of different types.
Shape and size of aluminum particles For comparison with the aluminum particles used so far (9 mm mean diameter) the hydrogen flow rate and the reaction efficiency were tested also for aluminum flakes, 20 mm wide and 0.2 mm thick (average values). Fig. 12 presents SEM photographs of aluminum powder and aluminum flakes used in the experiments. Fig. 13 shows the test results of hydrogen generation rate vs. time for different water/Al mass ratios and for aluminum flakes with two values of activator fractions: 2.5% and 5%. From Fig. 13 one can see higher rate of hydrogen generation compared to 9 mm Al particles (Fig. 3), though under similar conditions the flakes exhibited somewhat longer reaction delay times. Also shown in Fig. 13, that higher activator fractions in flakes as well as smaller water/Al ratios, imply higher reaction rate and shorter delay time. The higher reaction rates obtained using aluminum flakes is apparently due to their much larger specific surface compared to the 9 mm particles (6.46 m2/g compared to 0.25 m2/g, respectively). Fig. 14 presents the reaction efficiency, namely, hydrogen generation yield, vs. water/Al mass ratio for aluminum flakes
Fig. 10 e Hydrogen yield vs. initial water temperature (2 g Al D 20 ml water, 2.5% activator).
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Fig. 11 e Hydrogen production rate vs. time for water of different types (2 g Al D 20 ml water, 2.5% activator).
containing different fractions of activator. It was found that the increase in activator fraction from 2.5 wt% to 5 wt% increased the hydrogen generation yield by approximately 15%. Furthermore, at the same activator fraction, the water/Al mass ratio had only a minor effect on the yield. Nevertheless, the overall reaction yield for flakes was found lower than for Al particles by about 18% (for 2.5 wt% of activator). One may attribute the lower reaction yield of the flakes mainly to the much larger initial fraction of aluminum oxide contained in the flakes (estimated by about 10% on the basis of nano aluminum powders with a similar specific surface) due to their much larger surface area.
High pressure experiments Tests at high pressure conditions have also been conducted. A glass reactor containing 4 g of aluminum powder was sealed in a closed steel vessel (0.4 l), and water was added, as can be seen in Fig. 2. The water inlet was then immediately closed and the reaction proceeded, causing an increase in pressure and temperature. At the end of the reaction, after cooling of the gas, a valve allowing the hydrogen to exit the vessel and pass through a digital mass flow meter was opened. The pressure and temperature histories in the vessel are presented in Figs. 15 and 16, respectively. The pressure rises according to the reaction rate (volume of hydrogen produced in the close vessel) and the gas temperature. Fig. 15 describes the partial
Fig. 13 e Hydrogen production rate vs. time (2 g Al flakes).
pressure evolution of the gaseous reaction products (pure hydrogen and some water vapor). The pressure rises quickly and decreases gradually due to gradual cooling of the gases in the vessel, whose temperature history is shown in Fig. 16. In general, the reaction yield was found to be similar to that of the atmospheric pressure experiments. As discussed before, at too low waterealuminum ratio, the reaction yield is somewhat lower, apparently due to absorption of water by the aluminum hydroxide product and water evaporation.
Application to fuel cell One of the best applications for hydrogen generation using the activated aluminumewater reaction is in proton exchange membrane (PEM) fuel cells. PEM fuel cells produce electricity from hydrogen and oxygen, and the only byproduct of this procedure is water or water vapor. Experiments have been conducted where the hydrogen produced via the activated aluminumewater reaction has been channeled to a fuel cell (Horizon H-30), rated at 30 W power, demonstrating stable electricity generation. The fuel cell current, voltage, and power were measured for different loads, in the range of 1.2 Ue14 U, and compared to those published by the manufacturer. A good agreement was found, as shown in Figs. 17 and 18. In fact, the actual output of the fuel cell was higher than the nominal manufacturer data, as can be expected in a new fuel cell.
Fig. 12 e SEM photograph of aluminum powder (left), mean particle size 9 mm, and aluminum flakes (right), thickness 0.2 mm, width 20 mm.
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Fig. 14 e Hydrogen yield vs. water/aluminum mass ratio and different activator fractions (2 g Al flakes). Fig. 17 e Fuel cell voltage vs. current.
Fig. 15 e Pressure history for different water/aluminum mass ratios (4 g Al, 2.5% activator).
Fig. 19 describes the voltage and current provided by the fuel cell during hydrogen generation experiment for a constant resistance of 1.8 U. The change in values during the experiment is due to the change in hydrogen production rate from the aluminumewater reaction. As the reaction proceeds and the hydrogen flow rate decreases, the voltage, current, and power of the fuel cell (Figs. 19 and 20) decrease as well. The overall specific energy (high heating value, HHV) of the hydrogen generated by the aluminumewater reaction is 15.8 kJ/kg Al (¼4.4 kWh/kg Al). With a typical fuel cell efficiency of 50% one obtains 2.2 kWh/kg Al of electric energy. On the basis of the aluminum mass it is about 18 times higher than the specific energy of a chargeable lithium-ion battery, which is about 120 Wh/kg.
Fig. 16 e Temperature history for different water/ aluminum mass ratios (4 g Al, 2.5% activator).
Fig. 18 e Fuel cell power vs. current.
There are many applications for this method of green electric energy generation. One of the best niches seems for marine and underwater propulsion, where there is no need to carry the water. Nevertheless, it can also be applied in electric cars or as battery replacement for electricity supply in remote communication posts, etc. In those applications the hydrogen generation and storage presented here becomes a method for electric energy generation and storage.
Conclusions Presented here is a parametric study on hydrogen generation from the reaction between activated aluminum powder and
Fig. 19 e Voltage and current vs. time provided by a Horizon H-30 fuel cell with a 1.8 U resistance load during a hydrogen production experiment.
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references
Fig. 20 e Power vs. time provided by a Horizon H-30 fuel cell with a 1.8 U resistance load during a hydrogen production experiment.
water. The activated aluminum which reacts spontaneously with water has been produced via an original process involving small fraction of lithium-based activator. Aluminum powder and water are simple and safe to store, hence providing a convenient way of hydrogen production insitu, avoiding the complications of hydrogen storage. In addition, the reaction between aluminum and water produces only environmental friendly products. High hydrogen production rates, 200e600 ml/min/g Al, have been achieved, and high efficiency of hydrogen production has been demonstrated, about 90% in most experiments. The main use of this hydrogen generation method should be to generate green energy via fuel cells. This is an environmental friendly method, as the only chemical product of the fuel cell is water (in liquid or vapor state). A clear advantage of it can be seen in marine and underwater propulsion, because there is no need to carry the water. Nevertheless, it can also be applied in electric cars or as battery replacement for electricity supply in remote communication posts, etc. The application of the aluminumewater reaction in fuel cells represents an efficient and compact electric energy storage means. Based on the aluminum mass, the theoretical specific electric energy (energy per unit mass) that can be obtained using this method may be almost 20 fold higher than that of lithium-ion batteries. In practice, when accounting for the fuel cell and peripherally equipment mass, as well as for the water mass (depending on the application), the current technology may offer 5e10 fold increase in specific electric energy storage.
Acknowledgment The authors greatly appreciate the generous financial support of Rutledge Global (Singapore) in the research.
[1] Rosenband V, Gany A. Application of activated aluminum powder for generation of hydrogen from water. Int J Hydrogen Energy 2010;35:10898e904. [2] Wang HZ, Leung DYC, Leung MKH, Ni M. A review on hydrogen production using aluminum and aluminum alloys. Renew Sustain Energy Rev 2009;13(4):845e53. [3] Kravchenko OV, Semenenko KN, Bulychev BM, Kalmykov KB. Activation of aluminum metal and its reaction with water. J Alloys Compd 2005;397(1e2):58e62. [4] Ziebarth JT, Woodall JM, Kramer RA, Choi G. Liquid phaseenabled reaction of AleGa and AleGaeIneSn alloys with water. Int J Hydrogen Energy 2011;36:5271e9. [5] Wang H, Chang Y, Dong S, Lei Z, Zhu Q, Luo P, et al. Investigation of hydrogen production using multicomponent aluminum alloys at mild conditions and its mechanism. Int J Hydrogen Energy 2013;38:1236e43. [6] Ilyukhina AV, Ilyukhin AS, Shkolnikov EI. Hydrogen generation from water by means of activated aluminum. Int J Hydrogen Energy 2012;37:16382e7. [7] Fan MQ, Xu F, Sun LX. Studies on hydrogen generation characteristics of hydrolysis of the ball milling Al-based materials in pure water. Int J Hydrogen Energy 2007;32:2809e15. [8] Mahmoody K, Alinejad B. Enhancement of hydrogen generation rate in reaction of aluminum with water. Int J Hydrogen Energy 2010;35:5227e32. [9] Uehara K, Takeshita H, Kotaka H. Hydrogen gas generation in the wet cutting of aluminum and its alloys. J Mater Process Technol 2002;127(2):174e7. [10] Alinejad B, Mahmoodi K. A novel method for generating hydrogen by hydrolysis of highly activated aluminum nanoparticles in pure water. Int J Hydrogen Energy 2009;34(19):7934e8. [11] Razavi-Tousi SS, Szpunar JA. Effect of structural evolution of aluminum powder during ball milling on hydrogen generation in aluminumewater reaction. Int J Hydrogen Energy 2013;38:795e806. [12] Soler L, Macana´s J, Mun˜oz M, Casado J. Aluminum and aluminum alloys as sources of hydrogen for fuel cell applications. J Power Sources 2007;169(1):144e9. [13] Soler L, Macana´s J, Mun˜oz M, Casado J. Synergistic hydrogen generation from aluminum, aluminum alloys and sodium borohydride in aqueous solutions. Int J Hydrogen Energy 2007;32:4702e10. [14] Yavor Y, Goroshin S, Bergthorson J, Frost D, Stowe R, Ringuette S. Aluminumewater reaction kinetics at temperatures up to 150 C. In: 9th international symposium on special topics in chemical propulsion (9-ISICP), Quebec, Canada; July 9e13, 2012. [15] Yavor Y, Goroshin S, Bergthorson J, Frost D, Stowe R, Ringuette S. Enhanced hydrogen generation from aluminumewater reactions. Int J Hydrogen Energy 2013;38:14992e5002. [16] Vlaskin MS, Shkolnikov EI, Bersh AV. Oxidation kinetics of micron-sized aluminum powder in hightemperature boiling water. Int J Hydrogen Energy 2011;36:6484e95.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Study of hydrogen production and storage based on aluminumewater reaction Shani Elitzur*, Valery Rosenband, Alon Gany Faculty of Aerospace Engineering, Technion e Israel Institute of Technology, Haifa 32000, Israel
article info
abstract
Article history:
The rate and yield of hydrogen production from the reaction between activated aluminum
Received 12 December 2013
and water has been investigated. The effect of different parameters such as water
Received in revised form
ealuminum ratio, water temperature and aluminum particle size and shape was studied
3 February 2014
experimentally. The aluminum activation method developed in-house involves 1%e2.5% of
Accepted 6 February 2014
lithium-based activator which is diffused into the aluminum particles, enabling sustained
Available online 1 March 2014
reaction with tap water or sea water at room temperature. Hydrogen production rates in the range of 200e600 ml/min/g Al, at a yield of about 90%, depending on operating pa-
Keywords:
rameters, were demonstrated. The work further studied the application in proton ex-
Hydrogen
change membrane (PEM) fuel cells in order to generate green electric energy,
Hydrogen storage
demonstrating theoretical specific electric energy storage that can exceed batteries by 10
Aluminumewater reaction
e20 folds.
Aluminum activation
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Fuel cell
Introduction In recent years much effort has been made in order to reduce the world dependence on fossil fuels and to meet the increasing energy needs using alternative sources of energy. Hydrogen is the most attractive alternative fuel because of its outstanding reaction energy (with air), three times higher than for hydrocarbon fuels, and its environmental friendly products, water or water vapor. The main disadvantages associated with the use of hydrogen are difficulty of storage and transportation due to its very low density, and safety problem because of its very high reactivity. Overcoming those challenges of hydrogen storage, transportation, and safety is essential for a wide use of hydrogen energy. Safe and compact hydrogen storage and in-situ hydrogen production from aluminumewater reaction can be achieved
using a novel, in-house, thermo-chemical treatment of aluminum particles. The method involves a small fraction of a lithium-based activator (typically 2.5%) which diffuses into the aluminum particles and modifies the hydroxide/oxide protective film on their surface, allowing a spontaneous and sustained chemical reaction between aluminum and water which produces hydrogen [1]. The stoichiometric reaction (Eq. (1)) yields theoretically 11% hydrogen mass compared to the aluminum mass (equivalent to over 1.2 l of hydrogen per gram of aluminum), making the concept very efficient for hydrogen storage. 3 Al þ 3H2 O/AlðOHÞ3 þ H2 2
(1)
Different attempts and approaches have been applied in the world to activate aluminum. It has been suggested to use alloys of aluminum with different metals such as Ga, In, Zn,
* Corresponding author. Tel.: þ972 547439066. E-mail addresses: [email protected] (S. Elitzur), [email protected] (V. Rosenband), [email protected] (A. Gany). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.02.037
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 9 ( 2 0 1 4 ) 6 3 2 8 e6 3 3 4
Sn and Bi [2e7]. Kravchenko et al. [3] and Ziebarth et al. [4] created Al alloys by melting aluminum with various metals and investigated the effect of the concentration of individual components in the alloy on the hydrogen yield. The doping procedure requires high temperatures for melting all the components (700e900 C), and results in relatively slow reaction with water, even at elevated temperatures. Wang et al. [5], Ilyukhina et al. [6], and Fan et al. [7] created different aluminum alloys using mechanical alloying method. The investigations were typically conducted at atmospheric pressure and room temperature, resulting in a range of hydrogen production yields from 20% to over 90%, where quaternary aluminum alloys (e.g., AleGaeIneSn), particularly with higher contents of Ga and Sn, exhibited the higher yield range. Activation of aluminum by mechanical treatment of the metal using cutting or ball milling which exposes a fresh and reactive surface of the aluminum particles while increasing the surface area and mixing with additives has also been investigated [2,8e11]. In some experiments [8,10], salt was used as nano-miller and cover; when the aluminumsalt powder comes in contact with water the salt particles are washed away and the aluminum reacts with water. This method gives high hydrogen yields when nano-particles of aluminum (which are very expensive) and high temperatures are used. Razavi-Tousi et al. [11] investigated the dependence of the reaction kinetics on the microstructure of the aluminum powder in order to optimize the milling time. Another method uses alkaline solutions, mainly sodium hydroxide, in order to destroy the protective oxide layer on the aluminum surface. The reaction is relatively slow [2,12,13], and caution is required for working with strong alkaline solutions. Yet another approach is to use water at very high temperatures (under high pressure) using aluminum powder without additives. Yavor et al. [14] got 60e80% hydrogen yield using 6 mm aluminum powder at water temperature of 120e150 C, and close to 100% at 200 C [15]. Vlaskin et al. [16] obtained 70e90% hydrogen yield using 4e7 mm aluminum powder with water temperature of 230e300 C. Both researches obtained better results for smaller particles and higher temperatures. This paper presents a parametric investigation of the chemical reaction between activated aluminum and water to produce hydrogen in-situ, safely and compactly. The activated aluminum has been prepared via a novel method developed in-house enabling the diffusion of a small fraction of lithium into the aluminum particles. It further studies the application of the hydrogen produced in proton exchange membrane (PEM) fuel cells to generate green electric energy.
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Fig. 1 e Schematic illustration of the experimental facility.
Batch type experiments were conducted in a glass reactor. The aluminum powder was put in the reactor first, then water was added and a spontaneous reaction started. Since hydrogen has low solubility in water the amount of hydrogen produced from the reaction was measured by water displacement. The hydrogen produced from the reaction was channeled to a glass container filled with water driving the water out from the container to a cup placed on an electronic balance. In some experiments the measurements were conducted with a digital mass flow meter (Aalborg, USA). The temperature of the reaction was measured by a digital thermometer. Schematic illustration of the experimental facility is presented in Fig. 1. The influence of pressure on the reaction was tested using a closed vessel, as described in Fig. 2. The tests applied either 9 mm aluminum particles or 20 mm wide, 0.2 mm thick Al flakes (average values), manufactured in China and supplied by MCT-Materials Ltd Israel. Regularly, the experiments were conducted with 2 g of an Al powder containing 2.5 wt% of activator. The high pressure tests used 4 g of Al, whereas the effect of activator fraction was studied with aluminum powders containing 1% and 5% of activator. The amount of hydrogen produced in the experiments was compared to the theoretical amount of the stoichiometric reaction (Eq. (1)), which produces about 1.24 l of hydrogen per 1 g of aluminum at standard conditions (1 atm, 273 K). For the calculation of the actual yield the hydrogen temperature and pressure at the experiment were
Experimental approach and installation A parametric experimental study has been conducted in order to find the influence of different operating factors and conditions on the activated aluminumewater reaction, measuring the reaction rate and efficiency. Effects of water/aluminum mass ratio, fraction of activator used, water temperature, pressure, the size and shape of the aluminum particles, and the type of water (i.e., tap water, sea water, and distilled water) have been investigated.
Fig. 2 e Schematic illustration of the pressure experiment installation.
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Fig. 3 e Hydrogen production rate vs. time for different water/aluminum mass ratios (2 g Al, 2.5% activator).
Fig. 5 e Hydrogen yield vs. time for different water/ aluminum mass ratios (2 g Al, 2.5% activator).
accounted for. Water vapor generated at the reactor condenses while cooling down to room temperature during flowing in the pipe and staying in the water displacement vessel. Hence, its maximum volumetric fraction (at saturation) would not exceed 2.5% at atmospheric experiments and 0.3% at high pressure tests.
Figs. 3 and 4 present the hydrogen production rate vs. time for different water/aluminum mass ratios and different fractions of activator, respectively. As can be seen, the reaction is faster when the water/aluminum mass ratio is lower (mainly due to the effect of temperature increase during the exothermic reaction, which is accelerated for a smaller heated mass) and when the activator fraction in the aluminum powder is bigger. The latter effect should be due to the fact, that for a too low activator fraction, its capability of destructing the protective properties of the oxide or hydroxide layer on the aluminum particle deteriorates. The yield of hydrogen produced is presented in Figs. 5 and 6. The maximum attainable hydrogen volume was calculated for the experiment conditions, assuming that the gas entering the water vessel was at room temperature and atmospheric pressure. In-test temperature variations of the hydrogen produced imply possible 2% error in determining the reaction yield. As can be seen, when the water/aluminum mass ratio is lower, the volume of hydrogen produced reaches its maximum value faster. In the range of activator fraction tested (1, 2.5 wt %), the reaction exhibited higher hydrogen production rate and yield for higher activator fraction, whereas the water/ aluminum mass ratio in the tested range had a little effect on the yield. In the tests with 2.5 wt% of activator the reaction yield was in the range of 86e91% (Figs. 5 and 7).
Results Waterealuminum mass ratio The influence of the mass ratio between water and activated aluminum powder on the reaction rate and the hydrogen volume produced was tested with 2 g of aluminum powder and different amounts of tap water (15, 20, 25, 30 ml) at room temperature (21e23 C). The aluminum hydroxide product of the reaction may absorb water (forming hydrate or gel). In addition, at low waterealuminum ratios boiling and evaporation of water take place. Hence, excess water beyond the stoichiometric ratio may be required for complete reaction. Aluminum powders, treated by either 2.5 wt% or 1 wt% of activator, were tested. In order to compare results of experiments conducted with different amounts of aluminum powders; the results are presented per unit mass (1 g) of aluminum.
Fig. 4 e Hydrogen production rate vs. time for different water/aluminum mass ratios (2 g Al, 1% activator).
Fig. 6 e Hydrogen yield vs. time for different water/ aluminum mass ratios (2 g Al, 1% activator).
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Fig. 7 e Overall reaction time and hydrogen yield vs. water/ aluminum mass ratio (2 g Al, 2.5% activator).
Water temperature The reaction was tested using 20 ml of tap water in different initial temperatures (23, 35, 55, 70 C) reacting with 2 g Al powder which contains 2.5 wt% of activator. The aluminumewater reaction is highly exothermic, increasing the water temperature during the reaction process. Fig. 8 presents the hydrogen production rate during the aluminumewater reaction for different initial water temperatures. It can be seen that when the initial temperature of the water is higher the reaction delay is shorter and the maximum hydrogen production rate is higher, as could be expected. The cumulative hydrogen yield is presented in Fig. 9. This figure shows clearly that although the reaction is faster when the initial water temperature is higher, the overall yield of hydrogen produced is similar for all the tested temperatures. The overall hydrogen yield presented in Fig. 10, was similar in all the experiments, 87e91%, almost indifferent with the initial water temperature.
Water type Tests were conducted with different types of water: tap water, sea water (Mediterranean sea, about 4.2% salt concentration), and distilled water, revealing that hydrogen production at a similar good yield could be obtained with all types of water. This result is important in particular for marine and
Fig. 8 e Hydrogen production rate vs. time for different initial water temperatures (2 g Al D 20 ml water, 2.5% activator).
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Fig. 9 e Hydrogen yield vs. time for different initial water temperatures (2 g Al D 20 ml water, 2.5% activator).
underwater vessel applications, which should carry only activated aluminum powder, using the water in the surroundings to produce hydrogen and generate electricity. Fig. 11 describes the hydrogen generation rate per unit mass of Al produced from water of different types.
Shape and size of aluminum particles For comparison with the aluminum particles used so far (9 mm mean diameter) the hydrogen flow rate and the reaction efficiency were tested also for aluminum flakes, 20 mm wide and 0.2 mm thick (average values). Fig. 12 presents SEM photographs of aluminum powder and aluminum flakes used in the experiments. Fig. 13 shows the test results of hydrogen generation rate vs. time for different water/Al mass ratios and for aluminum flakes with two values of activator fractions: 2.5% and 5%. From Fig. 13 one can see higher rate of hydrogen generation compared to 9 mm Al particles (Fig. 3), though under similar conditions the flakes exhibited somewhat longer reaction delay times. Also shown in Fig. 13, that higher activator fractions in flakes as well as smaller water/Al ratios, imply higher reaction rate and shorter delay time. The higher reaction rates obtained using aluminum flakes is apparently due to their much larger specific surface compared to the 9 mm particles (6.46 m2/g compared to 0.25 m2/g, respectively). Fig. 14 presents the reaction efficiency, namely, hydrogen generation yield, vs. water/Al mass ratio for aluminum flakes
Fig. 10 e Hydrogen yield vs. initial water temperature (2 g Al D 20 ml water, 2.5% activator).
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Fig. 11 e Hydrogen production rate vs. time for water of different types (2 g Al D 20 ml water, 2.5% activator).
containing different fractions of activator. It was found that the increase in activator fraction from 2.5 wt% to 5 wt% increased the hydrogen generation yield by approximately 15%. Furthermore, at the same activator fraction, the water/Al mass ratio had only a minor effect on the yield. Nevertheless, the overall reaction yield for flakes was found lower than for Al particles by about 18% (for 2.5 wt% of activator). One may attribute the lower reaction yield of the flakes mainly to the much larger initial fraction of aluminum oxide contained in the flakes (estimated by about 10% on the basis of nano aluminum powders with a similar specific surface) due to their much larger surface area.
High pressure experiments Tests at high pressure conditions have also been conducted. A glass reactor containing 4 g of aluminum powder was sealed in a closed steel vessel (0.4 l), and water was added, as can be seen in Fig. 2. The water inlet was then immediately closed and the reaction proceeded, causing an increase in pressure and temperature. At the end of the reaction, after cooling of the gas, a valve allowing the hydrogen to exit the vessel and pass through a digital mass flow meter was opened. The pressure and temperature histories in the vessel are presented in Figs. 15 and 16, respectively. The pressure rises according to the reaction rate (volume of hydrogen produced in the close vessel) and the gas temperature. Fig. 15 describes the partial
Fig. 13 e Hydrogen production rate vs. time (2 g Al flakes).
pressure evolution of the gaseous reaction products (pure hydrogen and some water vapor). The pressure rises quickly and decreases gradually due to gradual cooling of the gases in the vessel, whose temperature history is shown in Fig. 16. In general, the reaction yield was found to be similar to that of the atmospheric pressure experiments. As discussed before, at too low waterealuminum ratio, the reaction yield is somewhat lower, apparently due to absorption of water by the aluminum hydroxide product and water evaporation.
Application to fuel cell One of the best applications for hydrogen generation using the activated aluminumewater reaction is in proton exchange membrane (PEM) fuel cells. PEM fuel cells produce electricity from hydrogen and oxygen, and the only byproduct of this procedure is water or water vapor. Experiments have been conducted where the hydrogen produced via the activated aluminumewater reaction has been channeled to a fuel cell (Horizon H-30), rated at 30 W power, demonstrating stable electricity generation. The fuel cell current, voltage, and power were measured for different loads, in the range of 1.2 Ue14 U, and compared to those published by the manufacturer. A good agreement was found, as shown in Figs. 17 and 18. In fact, the actual output of the fuel cell was higher than the nominal manufacturer data, as can be expected in a new fuel cell.
Fig. 12 e SEM photograph of aluminum powder (left), mean particle size 9 mm, and aluminum flakes (right), thickness 0.2 mm, width 20 mm.
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Fig. 14 e Hydrogen yield vs. water/aluminum mass ratio and different activator fractions (2 g Al flakes). Fig. 17 e Fuel cell voltage vs. current.
Fig. 15 e Pressure history for different water/aluminum mass ratios (4 g Al, 2.5% activator).
Fig. 19 describes the voltage and current provided by the fuel cell during hydrogen generation experiment for a constant resistance of 1.8 U. The change in values during the experiment is due to the change in hydrogen production rate from the aluminumewater reaction. As the reaction proceeds and the hydrogen flow rate decreases, the voltage, current, and power of the fuel cell (Figs. 19 and 20) decrease as well. The overall specific energy (high heating value, HHV) of the hydrogen generated by the aluminumewater reaction is 15.8 kJ/kg Al (¼4.4 kWh/kg Al). With a typical fuel cell efficiency of 50% one obtains 2.2 kWh/kg Al of electric energy. On the basis of the aluminum mass it is about 18 times higher than the specific energy of a chargeable lithium-ion battery, which is about 120 Wh/kg.
Fig. 16 e Temperature history for different water/ aluminum mass ratios (4 g Al, 2.5% activator).
Fig. 18 e Fuel cell power vs. current.
There are many applications for this method of green electric energy generation. One of the best niches seems for marine and underwater propulsion, where there is no need to carry the water. Nevertheless, it can also be applied in electric cars or as battery replacement for electricity supply in remote communication posts, etc. In those applications the hydrogen generation and storage presented here becomes a method for electric energy generation and storage.
Conclusions Presented here is a parametric study on hydrogen generation from the reaction between activated aluminum powder and
Fig. 19 e Voltage and current vs. time provided by a Horizon H-30 fuel cell with a 1.8 U resistance load during a hydrogen production experiment.
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references
Fig. 20 e Power vs. time provided by a Horizon H-30 fuel cell with a 1.8 U resistance load during a hydrogen production experiment.
water. The activated aluminum which reacts spontaneously with water has been produced via an original process involving small fraction of lithium-based activator. Aluminum powder and water are simple and safe to store, hence providing a convenient way of hydrogen production insitu, avoiding the complications of hydrogen storage. In addition, the reaction between aluminum and water produces only environmental friendly products. High hydrogen production rates, 200e600 ml/min/g Al, have been achieved, and high efficiency of hydrogen production has been demonstrated, about 90% in most experiments. The main use of this hydrogen generation method should be to generate green energy via fuel cells. This is an environmental friendly method, as the only chemical product of the fuel cell is water (in liquid or vapor state). A clear advantage of it can be seen in marine and underwater propulsion, because there is no need to carry the water. Nevertheless, it can also be applied in electric cars or as battery replacement for electricity supply in remote communication posts, etc. The application of the aluminumewater reaction in fuel cells represents an efficient and compact electric energy storage means. Based on the aluminum mass, the theoretical specific electric energy (energy per unit mass) that can be obtained using this method may be almost 20 fold higher than that of lithium-ion batteries. In practice, when accounting for the fuel cell and peripherally equipment mass, as well as for the water mass (depending on the application), the current technology may offer 5e10 fold increase in specific electric energy storage.
Acknowledgment The authors greatly appreciate the generous financial support of Rutledge Global (Singapore) in the research.
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