Study of the controllable reactivity of aluminum alloys and their promising application for hydrogen generation

Energy Conversion and Management 51 (2010) 594–599

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Study of the controllable reactivity of aluminum alloys and their promising application for hydrogen generation Mei-qiang Fan a,b, Li-xian Sun b,*, Fen Xu b a b

Department of Materials Science and Engineering, China Jiliang University, 258 Xueyuan Street, Hangzhou 310018, PR China Materials and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China

a r t i c l e

i n f o

Article history: Received 8 April 2009 Accepted 7 November 2009 Available online 2 December 2009 Keywords: Hydrolysis performance Aluminum alloys The additives

a b s t r a c t The hydrolysis performances of two aluminum alloys are investigated as their reactivity can be controlled via the different additives. The additive of NaCl has the positive effect to improve the hydrolysis properties of the aluminum alloys with quicker hydrolysis kinetic and lower hydrolysis temperature. For examples, in 6 min of hydrolysis reaction, the Al–5 wt%Hg–5 wt%NaCl can produce 971 mL g1 hydrogen, higher than 917 mL g1 hydrogen from Al–10 wt%Hg alloy. The Al–In–NaCl alloy has lower hydrolysis temperature about 10 K than that of Al–In alloy. Meanwhile, the reactivity of Al alloys can be improved or reduced via the additive metals. It can be found that the additive cadmium can reduce the reactivity of Al–Hg alloy. The Al–Hg–Cd alloys can keep good stability at the moist atmosphere below 343 K and have excellent hydrolysis performance around 343–373 K. The debased reactivity of Al–Hg–Cd composite comes from the formation of CdHg2 compounds in the milling process. But the additive Zn and Ga doped into the Al–In–NaCl alloys can quickly increase the reactivity of the alloy which can quickly react with water at room temperature and have high hydrogen yield up to the theoretic value. Therefore, it is a promising possibility that the controllable reactivity of aluminum alloys can be obtained through the different additive according to the practical request, and the Al alloys can produce pure hydrogen for the fuel cell via the hydrolysis reaction. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The development of H2 fuel cells for vehicles, stationary and mobile applications has been an effective approach to satisfy the requirements of the fossil fuel shortcoming and the environment protecting as the clean electricity energy is generated in the fuel cell via the consume of hydrogen energy and oxygen. The hydrogen source is one of the key technologies to the application of the fuel cell, but there is a big problem for large-scale utilization of hydrogen as no efficient hydrogen storage material can be practically applied. Nowadays, many focuses pay attention on finding a cheap and safe hydrogen storage material containing high hydrogen capacity such as chemical hydrides [1–4] and metals [5–12]. The reaction of the materials with water is an attractive method of producing pure hydrogen for fuel cells as they cannot only storage hydrogen, but also supply hydrogen where it is needed and when it is needed. Especially that the hydrolysis of aluminum and aluminum alloys with aqueous alkaline solutions are a feasible alterna-

* Corresponding author. Address: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China. Fax: +86 411 84379213. E-mail address: [email protected] (L.-X. Sun). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.11.005

tive for hydrogen production to supply fuel cells [6]. Comparing to other materials such as NaBH4 [3] and BH3NH3 [4], the price of aluminum powder is only 10–20 times lower than the price of sodium borohydride [7,8]. In addition, the aluminum hydroxide of hydrolysis production can be cheaply cycled to prepare the aluminum in the Hall–Heroult process [11] again. It had also been found that the aluminum alloys activated by some metals [9] such as bismuth, mercury, tin and so on, had high reactivity to react with water at room temperature. In our previous works [10], the activated aluminum alloys had high hydrogen yield and hydrogen generation rate, while they were stored three months in the Ar atmosphere or airtight container. The hydrolysis of aluminum alloys was based on the work of micro-galvanic cell between aluminum anode and other metal cathode in the water. However, the activated aluminum alloys were unstable in moist condition and accordingly had the safety problem in the store process. Therefore, finding a new aluminum alloy only having high reactivity at appropriate temperature is necessary. The aim of this work is to explore the additive effect on the reactivity of aluminum alloys and prepare the controlled reactivity of aluminum alloy according to the practical request, which can keep stable at mild conditions.

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2. Experimental The starting materials were aluminum powder, Indium powder, zinc powder, cadmium powder and mercury. The compositions were milled in a special stainless steel pots filled with an argon atmosphere for 5 h in a planetary QM-ISP ball miller. The milling parameters were kept constant and the ball-to powder weight ratio was 60:1 at 450 rp min1. All samples were handled into the stainless pots in the air. Then the pots were vacuumed and filled Ar atmosphere. The hydrolysis reaction of the mixture (0.2 g) in 50 mL pure water was carried out in a stainless steel chamber attached to a gas burette graduated in 0.1 mL increments at different temperature. The gas produced was flowed through a condenser and drierite to remove water vapor before measurement of H2 volume. The hydrogen volume was measured by the water trap method [6] and the hydrogen generation rate was calculated from the amount evolved from the beginning of the test. The hydrogen yield was defined as the volume of produced hydrogen over the theoretical volume of hydrogen that should be released assuming that all material was hydrolyzed. Powder X-ray diffraction (XRD) studies were carried out on PANalytical X-ray diffractometer (crystalline silicon is the internal standard).

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lot of conductive Na+ and Cl. In the dissolution process, the dissolution enthalpy of NaCl is also produced accordingly. Therefore, the great temperature and free-moved ions concentration [5] favor to the work of micro-galvanic cell in micrometric zones. (2) The produced Cl can improve the negative potential of aluminum alloy. McCafferty [13] found that the pitting potential for pure aluminum and aluminum alloys varied linearly with the logarithm of the chloride concentration in the range from 0.073 to 0.12 V. (3) The equal mix of aluminum and mercy. The NaCl has brittleness properties and produces many scrappy particles in the milling process. The particles are distributed into the aluminum surfaces and can prevent the combinations of Al–Al efficiently. Therefore, the milled alloys present equally powder and have uniformly high reactivity. Fig. 2 shows the XRD of the Al–10 wt%Hg alloy before and after hydrolysis reaction. The Hg peaks almost disappear, reflecting that the Hg composition has been dispersed equally in the Al matrix. The peaks of Al and AlOOH (Bohmite) were dominated in Fig. 2 (curves a and b). Obviously, AlOOH (Bohmite) come from the reaction of Al and water as Al–Hg alloy has high reactivity with the potential of 1900 mV [14] and formed the micro-galvanic cell between Al(anode) and Hg(cathode) in water. Fig. 3 shows the effects of milling time on the reactivity of Al– 5 wt%Hg–5 wt%NaCl. The liquid Hg easily disperses in the alloy

3. Results and discussion 3.1. Hydrolysis properties of Al–Hg alloy

Hydrogen generation (mL g-1)

3.1.1. Effect of NaCl on the reactivity of Al–Hg alloy The milled Al–Hg alloys in our experiments present the quick hydrolysis reaction to produce hydrogen in Fig. 1, especially that Al–Hg–NaCl alloy has higher reactivity than Al–Hg alloy. The hydrolysis of the Al–10 wt%Hg alloy generates 917 mL g1 in 2 min of hydrolysis reaction, corresponding to 82% hydrogen yield. While the Al–Hg–NaCl alloy can produce 956 mL g1, 961 mL g1, 971 mL g1 with 1 wt%, 3 wt%, 5 wt% of the additive NaCl, respectively, in the same condition. Furthermore, the hydrogen generation rate increases with the additive NaCl content increasing. The aluminum alloys can react with water completely in 1 h and the hydrogen is no more produced in case of the alkali solution entering. The effect of NaCl on the reactivity of the Al–Hg alloy has several following roles. (1) Accelerating the work of micro-galvanic cell. The NaCl in the alloys dissolves into water and generated a

Fig. 2. XRD patterns of Al–Hg alloy before (a) and after (b) hydrolysis reaction.

1000

800

600

Al-10wt% Hg Al-9wt% Hg-1wt% NaCl Al-7wt% Hg-3wt% NaCl Al-5wt% Hg-5wt% NaCl

400

200

0 0

1

2

3

4

5

6

Time (min) Fig. 1. Hydrogen generation of the hydrolysis of Al–Hg–NaCl alloy in pure water at room temperature.

Fig. 3. Hydrogen generation of the hydrolysis of Al–Hg–NaCl alloys in pure water at different milling time.

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a Hydrogen generation (mL g-1)

Hydrogen generation (mL g-1)

1200

1000

800

Al90Hg8Cd2 Al87Hg10Cd3

363K

600

Al85Hg10Cd5 Al80Hg10Cd10

400

200

1200 1000 800 363 K 358 K 353 K 348 K 343 K

600 400 200

Al87Hg10Cd3

0 0 0

1

2

3

4

0

5

1

2

and the alloy of milled 1 h presents high reactivity to produce 953 mL g1 hydrogen in 9 min of hydrolysis reaction. With prolonging time to 2 h, the alloys can generate 981 mL g1 hydrogen in the same conditions. However, the reactivity of the Alloy becomes worse if milling time further prolongs to 5 h even to 10 h. The reason may come from that the higher reactivity of Al–Hg alloy easily react with oxygen in air and results in lower hydrogen yields in the experiments. 3.1.2. Effect of cadmium on the reactivity of Al–Hg alloy However, the reactivity of the Al–Hg alloys reduced rapidly with the additive cadmium. Fig. 4 shows the hydrogen generation curves of the Al alloys in 363 K. The Al80Hg10Cd10 alloy has lower reactivity at 363 K and its hydrolysis only produces 168 mL g1 hydrogen in 5 min of hydrolysis reaction. The additive cadmium presents enormous poisons on the reactivity of Al–Hg alloys. And with the decrease of cadmium content, the Al alloys have higher reactivity to produce 992 mL g1 hydrogen for Al90Hg8Cd2 alloys, 1072 mL g1 hydrogen for Al87Hg10Cd3 alloys, 1047 mL g1 hydrogen for Al85Hg10Cd5 alloys at 363 K in 2 min of hydrolysis reaction, respectively. But if the cadmium content is too less, the Al alloys such as Al90Hg8Cd2 become active at room temperature again. The effect of cadmium on the reactivity of the Al–Hg alloys is further investigated in Fig. 5 which presents the hydrogen generation of the Al alloys at different temperature. The Al87Hg10Cd3 and Al85Hg10Cd3 alloys have no reactivity and cannot react with water at 343 K. But with increasing temperature from 348–363 K, the reactivity of the alloys become higher and they react quicker to produce hydrogen. For example, the Al87Hg10Cd3 alloys produced 809 mL g1 at 348 K, 926 mL g1 at 353 K, 1065 mL g1 at 358 K and 1106 mL g1 at 363 K in the 2 min of hydrolysis reactions, respectively. The same results also appear in the Al85Hg10Cd3 alloy. Obviously, the alloy keeps stable at lower temperature and the hydrolysis kinetic is evidently improved with higher temperature. From the XRD patterns of the Al–Hg–Cd alloys in Fig. 6, it can be found that the peaks of cadmium existed in the XRD patterns of Al80Hg10Cd10 alloy, and with cadmium content decreasing to 5 wt%, the peaks of cadmium disappear and the peaks of CdHg2 appear. And the XRD patterns have no change with the cadmium content further decreasing. According to the binary Alloy phase diagrams, the addition of cadmium has immiscibility with Al, but form the solid solution or metallic compound with Hg, therefore, the additive cadmium decrease the combination of Hg and Al accordingly. That explains why the reactivity of Al–Hg alloy is reduced. However, the CdHg2 at the temperature above 348 K

b Hydrogen generation (mL g-1)

Fig. 4. Hydrogen generation of the hydrolysis of Al–Hg–Cd alloys at 363 K.

3

4

5

4

5

Time (min)

Time (min) 1200 1000 800

363 K 358 K 353 K 348 K 343 K

600 400 200

Al85Hg10Cd5

0 0

1

2

3

Time (min) Fig. 5. Hydrogen generation of the hydrolysis of Al–Hg–Cd alloys ((a) Al87Hg10Cd5 and (b) Al85Hg10Cd3) at different temperature.

Fig. 6. XRD patterns of Al–Hg–Cd alloy (a, Al80Hg10Cd10; b, Al87Hg10Cd5; c, Al85Hg10Cd3; d, Al90Hg8Cd2).

decomposes Cd–Hg solid solution and free-moved Hg, which forms the micro-galvanic cell with Al in the water. As to Hg–Cd solid solution, it needs higher temperature to produce free-moved Hg in the following equation. D

D

CdHg2 ! CdHg solid solution þ Hg ! Cd þ 2Hg

ð1Þ

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3.2. Hydrolysis properties of Al–In alloys As Al–In alloy has 1.1 V potential [15], higher than 1.29 V potential of water decomposition, the Al–In alloy cannot hydrolysis in the water at room temperature, even hardly reacts with hot water at 363 K in Fig. 7. However, the alloys become active in the hot electrolyte solution. In the saturated NaNO3 solution at 363 K, the Al–In alloy can produces 704 mL g1 in 5 min of the hydrolysis reaction. And the alloy can generate 1068 mL g1 in the same conditions in the saturated NaCl solution. After the milling process, the Indium composite is dispersed in the surface of Al alloy and can form micro-galvanic cell in the water while there are free-ion transfer. Therefore, many free-mobile ions in the electrolyte solution favor to the work of micro-galvanic cell and results in the faster kinetic. Furthermore, the chloride concentration penetrated through the oxide film and localized dissolution of aluminum at the metal/oxide interface, resulting in the more negative potential of Al alloy [13]. 3.2.1. Effect of NaCl on the reactivity of Al–In alloys Meanwhile, a little additive NaCl doped into the Al–In alloy via the milling process shows the same excellent improvement to that of Al–Hg alloy. Fig. 8 shows the hydrolysis performance of Al– 10 wt%In–1 wt%NaCl alloy at different temperature. It can be seen that the alloy has good stability in water at 343 K, but with temperature increasing, the alloy has higher reactivity to produce a lot of H2 in the water. Obviously, the additive NaCl reduces the hydrolysis temperature of Al–In alloy. In the 3 min of hydrolysis reaction, the produced H2 is 878 mL g1, 1000 mL g1 and 1108 mL g1at 353 K, 358 K and 363 K, respectively. Furthermore, the alloy presents fast hydrolysis performance when the additive NaCl content increases. In Fig. 9, which presents the hydrogen performance of Al–10 wt%In–(1, 5, 10) wt%NaCl alloy at 353 K. In the earlier 2 min of hydrolysis reaction, 1 g Al–10 wt%In–10 wt%NaCl alloy re-

1000 800 600 343 K 353 K 358 K 363 K

400 200 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time (min) Fig. 8. Hydrogen generation of the hydrolysis of Al–10 wt%In–1 wt%NaCl alloys at different temperature.

Fig. 9. Hydrogen generation of the hydrolysis of Al–In–NaCl alloys (a, Al–10 wt%In– 1 wt%NaCl alloy; b, Al–10 wt%In–5 wt%NaCl alloy; c, Al–10 wt%In–10 wt%NaCl alloy) at 353 K.

acts with water to produce 895 mL hydrogen, comparing to 812 mL hydrogen from 1 g Al–10 wt%In–5 wt%NaCl alloy and 560 mL from 1 g Al–10 wt%In–1 wt% NaCl alloy at the same condition. However, the high NaCl content results in that the hydrogen production finally decreases accordingly because the activated Al content decreases.

1200

Hydrogen generation (mL g-1)

1200

Hydrogen generation (mL g-1)

Evidently, the formation of Cd–Hg alloy is responsible to the reduced reactivity of Al alloys and the free-moved Hg breaks away from the Cd–Hg alloy activates the Al alloys again in the hot water. It is a good path to control the reactivity of Al–Hg alloy via the additive cadmium. The Al alloy can keep stable in air at room temperature, even in the moist air at 343 K. But as to some other Al alloy such as Al–In alloy, which has lower reactivity and cannot react with water even at high temperature. So it needs improving its reactivity via the additives.

1000

800

600

363k Al-15wt% In alloy in water Al-15wt% In alloy in saturated NaNO3 solution

400

Al-15wt% In alloy in saturated NaCl solution 200

0 0

2

4

6

8

10

Time (min) Fig. 7. Hydrogen generation of the hydrolysis of Al–10 wt%In alloy in the saturated electrolyte solution at 363 K.

3.2.2. Effects of some metals on the reactivity of Al–In alloys The higher reactivity of Al–In alloy is obtained by the additive some metals such as Zn and Ga. It can be confirmed from the Fig. 10. Al–10 wt%In–5 wt%Ga–5 wt%NaCl alloy can quickly react with water and produces 909 mL g1 hydrogen in the 8 min of hydrolysis reaction. Al–In–Zn–NaCl alloy has higher reactivity to produces 987 mL g1 hydrogen in 4 min of the hydrolysis reaction at room temperature. The distinct improvement of Al reactivity obviously comes from the additives (Zn, Ga). Shayeb [16] found that the additive Zn could increase the defects and cracking of the protective alumina layer, which facilitated the diffusion of the deposited In into the surface layers and improved Al reactivity. The additive Zn also made the negative potential of Al–In alloy transferring from 1.1 V to 1.5 V [17].

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water at room temperature as they have more negative potential. Meanwhile, the EH2 O decreases with the temperature increasing [18,19] through Eqs. (6) and (7). That explains the improvements of hydrolysis kinetics in higher temperature.

DE ¼ EH2 O  EAlalloy P 0 RT ðcH2 PH2 ÞðcO2 PO2 Þ In 2F ax

ð6Þ

E0H2 O ð25  C;1 bar;H2 Oliq Þ ¼ 1:229 V

ð7Þ

EH2 O ¼ E0H2 O 

Fig. 10. Hydrogen generation of the hydrolysis of Al alloys (a, Al–10 wt%In– 5 wt%Zn–5 wt%NaCl alloy; b, Al–10 wt%In–5 wt%Ga–5 wt%NaCl) at room temperature.

ð5Þ 1 2

Therefore, the controllable reactivity of aluminum alloys in water can be summarized in several factors. (1) Reducing or enhancing reactivity of Al alloys via the additives, for example, the additive cadmium can reduce the reactivity of Al–Hg alloys and the additive NaCl has the opposite effect. (2) Change the reactivity of water through the variety of temperature. (3) Improving the pathway of electron transfer via increasing the conductive ions concentrations. And in micro area of interface of Al alloys and water, the three factors are attributed to the fast hydrolysis reaction.

4. Conclusion

In

Π:ΑlOOH(Bohmite) Al

Internsity

Al

Π Zn

Π

Zn Π

In

In

In Zn Al

10

20

30

40

Al

Zn

In

50

60

In

In

b

In In

a

70

80

2θ Fig. 11. XRD patterns of Al–10 wt%In–5 wt%Zn–5 wt%NaCl alloy before (a) and after (b) hydrolysis reaction.

In summary, the effects of NaCl and some metals on the reactivity of Al–Hg and Al–In alloys are studied. The additives are doped to the Al alloys via the milling method. The results show that the additive NaCl has the positive effect to improve the reactivity of Al alloys which have better hydrolysis performance. As to the metals, it can be found that the additive cadmium can reduce the reactivity of Al–Hg alloy and the additive Zn can enhance the reactivity of Al–In alloy. The Al–Hg–Cd alloys can keep good stability at the moist atmosphere below 343 K and has excellent hydrolysis performance around 343–373 K. The controllable reactivity of Al–Hg–Cd comes from the formation of CdHg2 compounds. The additive Zn and Ga dipped into the Al–In–NaCl alloys can quickly increase the reactivity of the Al alloys which can quickly react with water at room temperature and have high hydrogen yield up to the theoretic value. Therefore, the controllable reactivity of Al alloys can be obtained through the different additive according to the practical request. Acknowledgements

From the XRD patterns of Al–10 wt%In–5 wt%Zn–5 wt%NaCl alloy before and after hydrolysis in Fig. 11, the peaks of In and Zn have no change, reflecting that In and Zn are preserved after the hydrolysis reaction. However, the peaks of AlOOH (Bohmite) appear and displace those of aluminum after hydrolysis reaction. Obviously, the hydrolysis reaction can be described as the following formula:

AlðIn and ZnÞ þ H2 O ) AlOOHðBohmiteÞ þ H2 þ In þ Zn

The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (Nos. 2083309, 20873148, 50671098 and U0734005), the National High Technology Research and Development Program of China (2007AA05Z115 and 2007AA05Z102), the National Basic Research program (973 program) of China (2010CB631303 and the zhejiang Basic Research Program of China (Y4090507).

ð2Þ

The hydrogen yield and the hydrolysis mechanism during cathodic polarization of pure Al and Al–Sn alloys were explained earlier [17,18]. One hydrogen molecule is produced for a couple of electrons introduced from the external source.

3H2 O þ 3e ) 3H þ 3OH

ð3Þ

3 Al þ 3H þ 2H2 O ) AlOOH ðBohmiteÞ þ H2 2

ð4Þ

When the Al–Hg and Al–In alloys contact with water, there exists the DE between EAlalloy and EH2 O . The hydrolysis reactions (3) and (4) are stimulated by the electron transfer if DE P 0. Therefore, Al–Hg alloy and Al–In–Zn–NaCl has high reactivity to react with

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