Feasibility study of hydrogen production for micro fuel cell from activated Al–In mixture in water

Energy 35 (2010) 1333–1337

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Feasibility study of hydrogen production for micro fuel cell from activated Al–In mixture in water Mei-qiang Fan a, b, Li-xian Sun a, *, Fen Xu c, * a

Materials & Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China Department of Materials Science and Engineering, China Jiliang University, Hangzhou, 310018, PR China c Chemistry and Chemical Engineering College, Liaoning Normal University, 850 Huanghe Road, Dalian 116029, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 August 2009 Received in revised form 13 November 2009 Accepted 17 November 2009 Available online 4 December 2009

A safe and environmental-friendly method of hydrogen production from milled Al–In–Zn–salt mixture in water was proposed in this paper. The 10 hdmilled Al–In–Zn–salt mixture had high reactivity and produced hydrogen in water at room temperature. Its improved reactivity came from that the additive Zn and salts facilitate to the negative shift of Al–In alloy and benefited the combination of Al, In and Zn in the milling process. Optimized the composition content, 1 g of 10 hdmilled Ald5 wt%Ind3 wt%Znd2 wt%NaCl mixture had highest hydrogen yield of 1035 mL hydrogen/1 g Al in 4 min of hydrolysis reaction in water, corresponding to 9.21 wt% hydrogen (excluding water mass). Hydrogen supplying from milled Al–In–Zn–salt mixture was performed for micro fuel cell and 0.96 W was produced with the stable hydrogen supply rate. Therefore, the milled Al–In–Zn–salt mixture could be a feasible alternative for providing a source of CO2 free hydrogen production to supply micro fuel cell. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Hydrolysis Al In alloy Zn Salts

1. Introduction The development of H2 fuel cells for vehicles and mobile applications is very important to reduce greenhouse gas emission and pollution from the burning of fossil fuels because hydrogen has many attractive advantages such as high power density, non-polluted reaction products and abundant nature sources. Hydrogen is an attractive energy source for fuel cells where the energy obtained in the reaction of hydrogen and oxygen is converted to electric energy. There are different mature methods for hydrogen production from fossil fuel, nature gas and methanol [1]. Although the cost of these methods is cheap, there still exist a few parts per million carbon monoxides in the production which can cause the catalyst poisoning in proton exchange membrane fuel cells [2]. Hydrogen production from two-step solar water splitting with metal oxides is one of the most promising alternatives with higher efficiencies and lower costs. Several redox materials (Fe3O4/FeO [3,4], ZnO/Zn [5], Mn3O4 [6]) have been evaluated for such applications, but their high splitting temperature (above 800  C) limits their practical applications on board hydrogen production for vehicle fuel cell. Hydrogen storage and transportation are currently viewed as the greatest challenge to be resolved [7]. On board hydrogen production via the hydrolysis of NaBH4 [8], BH3NH3 [9], Mg-based

* Corresponding authors. Fax: þ86 411 84379213. E-mail addresses: [email protected] (L.-x. Sun), [email protected] (F. Xu). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.11.016

materials [10–12] and Al-based materials [13–16] can resolve the problem of hydrogen storage as they can produce hydrogen and supply hydrogen where it is needed and when it is needed. Among them, Al holds a promising alternative as it has lower cost (approximately 3 dollar Kg1) and shows excellent hydrolysis properties of theoretic 1245 mL hydrogen/1 g Al. Al can produce hydrogen via hydrolysis in the alkali or acids. It is also possible that Al alloys [15] could react with water at 82  C when Al alloys were activated by melting with some metals (Sn, Ga, Bi, Zn, In et al). Their hydrolysis reaction in water is based on the micro-galvanic cell between Al (anode) and other metals (cathode) [16]. Meanwhile, the byproduct aluminum hydroxide can be directly recycled to produce aluminum via the Bayer-Heroult-Hall process [17] and the cost of recycled aluminum is approximately 1.8 dollar Kg1. In this work, we have investigated the hydrolysis properties of milled Al–In–Zn–salt mixture in water or alkali solution. The aim is to reasonably explain the activation mechanism of Al alloys and find an effective process to improve Al reactivity through ball milling.

2. Experimental details 2.1. Synthesize and hydrolysis of Al–In mixtures The starting mixtures were elemental powders of pure metals (Al, In, Zn), salts (KCl, NaCl, SnCl2 and MgCl2). The Al powder size

M.-q. Fan et al. / Energy 35 (2010) 1333–1337

2.2. Hydrogen production for fuel cell system The hydrogen supplying for micro fuel cell was performed in an Al–H2O reactor attached a water tank, a micro pump, a hydrogen pressure gauge and a micro flow meter. The schematic diagram of fuel cell system with the auxiliary instruments were described in our previous works [18]. The hydrogen reactor was stainless steel with internal 125 mL volume in the experiment and the reactor could endure the maximum hydrogen pressure of 10 MPa. The amounts of Al alloy and water were 2 g and 15 mL in our experiments, respectively. The water flow rate was 15 mL/min. The hydrogen production rate was measured by the hydrogen pressure gauge (0–6 MPa) and hydrogen volume was calculated in Eq.(1).

VH2 ¼

 P0  125  VAl

alloy

 Vwater

P1  T0

 (1)

where: P0: 1 atm; T0: 298 K; T1: the experimental temperature; P1: the experimental pressure; V2g Al alloy: 1.2 mL; Vwater: 15 mL. The temperatures of hydrolysis residual production and hydrogen gas were measured near the bottom or the top of the reactor through two thermocouples (Ni–Cd alloy) and the temperature read were shown from temperature measuring instrument from xia men yu dian company of China, respectively. Hydrogen supplying rate was controlled as 100 mL min1 by a hydrogen reductor (the reductor is an apparatus which can adjust the hydrogen pressure) and detected by a micro flow meter (0–500 mL/min). The membrane electrode assemblies and working conditions of the fuel cell were according to the experiments by Liang et al. [19].

Amount of generated H2 ( mL/1g Al)

1000

800

600

400 Al-10wt Al-10wt Al-10wt Al-10wt Al-10wt

200

0 0

2

4

In-5wt In-5wt In-5wt In-5wt In-10wt

6

Zn-5wt Zn-5wt Zn-5wt Zn-5wt Zn

NaCl KCl SnCl2 MgCl2

8

Time (min) Fig. 1. Hydrogen production from the hydrolysis of the Ald10 wt%Ind5 wt% Znd5 wt%salt mixture.

wt%Ind10 wt%Zn alloy only produces 484 mL hydrogen/1 g Al in 8 min of hydrolysis reaction. But with the additive salts, Ald10 wt%Ind5 wt%Znd5 wt%salt mixtures produce 749w987 mL hydrogen/1 g Al, about 300–500 mL hydrogen/1 g Al higher than that of Ald10 wt%Ind10 wt%Zn alloy at the same conditions. The role of salts is attributed to three factors. Firstly, the enthalpy of dissolution of salts in water [11] greatly favors the hydrolysis reaction as the extra-released heat of the exothermic salt-dissolution leads to increase the kinetic of the hydrolysis reaction. Secondly, the salts act as electrolyte in water for galvanic cell between Al and In (or Zn). Furthermore, the produced chloride ion from the salt-dissolution can penetrate through the oxide film and localize dissolution of Al at the metal/oxide interface when the Al hydrolysis happens. The chloride ion concentration in the solid/ water interface also leads to more negative shift of Al alloy potential [20] and accelerates the Al hydrolysis accordingly. Thirdly, the composition of Al–In–Zn alloy easily forms a mass in the milling process and results to the unequally distribution. But the above problem can be prevented under the help of the additive salts. 3.2. Effect of mass radio and NaOH concentration Fig. 2 shows the effect of mass radio (mAl–In–Zn–NaCl mixture/mH2O) on the hydrolysis reaction of Ald10 wt%Ind5 wt%Znd5 wt%NaCl

1000

800

( mL/1g Al)

was approximately 13 mm and the powder size of other materials were confined in 100–300 mum. The composites were mixed in an argon–filled glove box. Then ball milling was performed by QM-1SP planetary ball miller under a 0.2w0.3 MPa argon atmosphere. Ballto-powder mass ratio corresponded to 60:1. The milling time was usually 10 h and the rotate speed was 450 r/min (if not specially noted). The hydrolysis reaction of the mixture (0.2 g) in 100 mL pure water was carried out in a stainless steel chamber attached to a gas burette graduated in 0.1 mL increments at room temperature. The produced gas was flowed through a condenser and a desiccator to remove water vapor before hydrogen volume measurement. The generated hydrogen volume was measured by the water displacement method [13] and the generated hydrogen rate was calculated from the amount evolved from the beginning of the test. The hydrolysis experiments repeated two times and had good data reproducibility with a relative error not exceeding 3%. Powder Xray diffraction (XRD) studies were carried out on PANalytical X-ray Diffractometer (Crystalline silicon is the internal standard).

Amount of generated H2

1334

600 mAl-5 wt

In-5 wt Zn-5 wt 1:250 1:500 1:1000

2

4

400

NaCl mixture/mH2O

200

3. Results and discuss 3.1. Effect of salts Fig. 1 shows the effect of salts (KCl, NaCl, SnCl2 and MgCl2) on the hydrolysis reactivity of Ald10 wt%Ind5 wt%Znd5 wt%salt mixture in pure water. The results confirm that the additional salt has a great effect in improving the mixture reactivity. The Ald10

0 0

6

8

Time (min) Fig. 2. Effect of different mass ratio (m Ald10 wt%Ind5 wt%Znd5 wt%NaCl mixture/ mH2O) on hydrogen production of Ald10 wt %Ind5 wt%Znd5 wt%NaCl mixture in pure water.

M.-q. Fan et al. / Energy 35 (2010) 1333–1337

1000

Amount of generated H2 ( mL/1g Al)

1000

Amount of generated H2 ( mL/1g Al)

1335

800

600 in pure water in 0.1 MNaOH solution in 0.2 MNaOH solution

400

800

600

400

Al-10wt Al-10wt Al-10wt Al-10wt

200

200

In-5wt Zn-5wt NaCl In-5wt Ga-5wt NaCl In-10-5wt NaCl In-5wt Co(Ni,Cu,Si)-5wt

NaCl

0 0 0

2

4

6

8

0

2

Time (min)

4

6

8

Time (min)

Fig. 3. Hydrogen production of the hydrolysis of Ald10 wt%Ind5 wt%Znd5 wt%NaCl mixture in different media. a: pure water; b, 0.1 M NaOH solution; c, 0.2 M NaOH solution.

mixture in pure water. The hydrolysis reaction ceases in 1–3 min and produces about 982, 987 and 1003 mL hydrogen/1 g Al when the mass ratio is 1:1000, 1:500 and 1:250, respectively. The decreased water volume has some effects on the hydrolysis reaction as the higher global temperature with the mass radio increasing is produced from 444.4 kJ mol1 heat-released in the hydrolysis of 1 g Al and improves the hydrolysis kinetic. Fig. 3 shows hydrogen production of hydrolysis of Ald10 wt%Ind5 wt%Znd5 wt%NaCl mixture in different media. The results show that NaOH concentration has also some effects on the hydrogen production rate. The hydrolysis mechanism of Al alloys has been discussed from the XRD patterns of the hydrolysis residual production of Al–In– Zn–NaCl mixture in different media which are shown in Fig. 4. The peaks of AlOOH (Boehmite) and a few peaks of In and Zn are only observed in curves a, reflecting that only Al among metals (Al, In and Zn) reacts with water. The hydrolysis mechanism of Al alloys in water is based on the work of micro-galvanic cell, according to the described mechanism (Eq. (2)–(5)). The reactions (2–5) can take place spontaneously with the following factors. Firstly, the Al–In– Zn alloy had 1.5 V potential [21,22] which was lower than 1.29 V of the H2O decomposition potential. Secondly, the electron can

Fig. 5. Hydrogen production of the hydrolysis of Ald10 wt%Ind5 wt%X (X: Zn, Ga, Co, Si, N and Co)d5 wt%NaCl mixtures in pure water.

transfer from anode (Al) to cathode (In or Zn) under the help of ions in water. So the combination of Al–In–Zn alloy and ions in media favor to improving the hydrolysis kinetic correspondingly. That is why more milling time and higher NaOH concentration favor to higher hydrolysis kinetic.

AlðIn or ZnÞ þ H2 O0AlOOHðBoehmiteÞ þ H2 þ Inðor ZnÞ DH ¼ 444:4kj=mol (2) 1 H2 O  e0 H2 þ OH 2

(3)

Al þ OH  e0AlOH

(4)

AlOH þ H2 O0AlOOHðBoehmiteÞ þ H2

(5)

2Al þ 2NaOH þ 6H2 O02NaAlðOHÞ4 þ3H2

(6)

2NaAlðOHÞ4 02NaOH þ 2AlðOHÞ3

(7)

However, Al–In–Zn alloy also produces Al(OH)3 (Bayerite) in the following Eq. (6,7) except the above mechanism (Eq.(2)–(5)) when hydrolysis reaction occurs in NaOH solution. It can be

Δ:Al(OH)3 Bayerite Π:ΑlOOH Boehmite

Δ

Δ

Π

a

Π

Π

Amount of generated H2 ( mL/1g Al)

Internsity(a.u.)

Π

1000

Δ

Δ

Δ

b

In Al Zn

10

20

30

Zn

Al

Al In

Zn In 40

50

60

Al

600

400 Al-10wt In-5wt Zn-5wt NaCl Al-10wt In-3wt Zn-2wt NaCl Al-5wt In-3wt Zn-2wt NaCl Al-5wt In-1.5wt Zn-1.5wt NaCl

200

In In 70

800

80

2θ(deg) Fig. 4. XRD patterns of the hydrolysis residual of the Ald10wt%Ind5 wt%Znd5 wt%NaCl mixture. a, in pure water; b, in 0.2 M NaOH.

0 0

2

4

6

8

Fig. 6. Hydrogen production from the hydrolysis of different Al–In–Zn–NaCl mixtures in pure water.

1336

M.-q. Fan et al. / Energy 35 (2010) 1333–1337 Table 1 The crystallite sizes of Ald10wt%Ind5wt%Znd5wt%NaCl alloy at different milling time.

1000

Amount of generated H2 ( mL/1g Al)

800

Milling time (h)

600

400

1 5 10 20

1h 5h 10h 20h

200

0 0

2

4

6

8

Time (min) Fig. 7. Hydrogen production from the hydrolysis of the Ald10 wt%Ind5 wt%Znd5 wt%NaCl mixture with different milling time.

confirmed from curve b in Fig. 4 where the peaks of AlOOH (Boehmite) and Al(OH)3 (Bayerite) are found in the XRD patterns of the hydrolysis residual production of Al–In–Zn–NaCl mixture in 0.2 M NaOH. 3.3. Effect of additive metals (Zn, Ga, Cu, Co, Ni and Si) Fig. 5 shows hydrogen production from the milled Al–In–X– NaCl mixtures(x: Zn, Ga, Cu, Co, Ni and Si) in water at room temperature. The results present that Ald10 wt%Ind10 wt%NaCl mixture and Al–In–Ni (Co, Cu, Si)–NaCl mixture almost does not react with water at room temperature. However, Ald10 wt%Ind5 wt%Gad5 wt%NaCl mixture and Ald10 wt%Ind5 wt%Znd5 wt%NaCl mixture can quickly react with water to produce 909 mL hydrogen/1 g Al and 987 mL hydrogen/1 g Al in 8 min of hydrolysis reaction, respectively. The distinct improvement of Al reactivity obviously comes from the additives (Zn, Ga). Shayeb [23] 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 from 1.1 V of Al–In alloy to 1.5 V of Al–In–Zn alloy. The same results were also obtained with the additive Ga metal [24]. Fig. 6 shows hydrogen production of Al–In–Zn–NaCl mixture with different compositions in water. With Al content increase from

Crystallite sizes of Ald10wt%Ind5wt%Znd5wt%NaCl alloy (nm) Al (111)

In (111)

42 38 37 36

35 32 30 31

80 to 90 wt%, the hydrogen production rate becomes slower, but the generated hydrogen increases from 987 to 1034 mL hydrogen/1 g Al. But with Al content further increase from 90 to 92 wt%, the hydrogen production rate become slower and the conversion efficiency will also decrease. Obviously, there is a tradeoff issue, so the rational Al content should be pursued. 3.4. Effect of preparation condition Fig. 7 shows hydrogen production of the hydrolysis of Al–In–Zn– NaCl mixture with different milling time in pure water. It can be seen that the Al reactivity is proportional to the milling time. The 1 h-milled mixture already shows high reactivity and can produce 695 mL hydrogen/1 g Al hydrogen in 4 min of hydrolysis reaction in water at room temperature. The 5 h-milled mixture has higher reactivity and can produce 853 mL hydrogen/1 g Al at the same condition. Furthermore, it has higher reactivity with increase of milling time and achieves to the highest value in 10 h milling time. The higher reactivity of Al–In–Zn–NaCl mixture is attributed to the dispersed In–Zn on the Al surface in the longer milling process, where the XRD patterns of the milled Al–In–Zn–NaCl mixture are shown in Fig. 8. The diffraction peaks of the composite become much weaker and wider with prolonging milling time, reflecting the gradual reduction of the crystallite size and the accumulation of microstrains. It results from existing random deformation and the texture breakage as the considerable energy accumulates into the power in the long milling process. The crystallite sizes of the mixture are reduced with prolonging the milling time from 1 h to 10 h in Table 1, which benefit the combination of Al, In and Zn. But with longer milling time, the crystallite sizes of the mixture decrease smaller and the average particle size of In (111) becomes

2000 100

Al

Al-water mixture

Al

Internsity (a.u.)

NaCl

NaCl

NaCl

Zn

milled 20h

Zn

Zn

Zn In

Zn

In In

milled 10h

Zn In

Zn

Zn

In

Zn

In

In NaCl

Zn

Zn

Zn In

30

40

Al

50

In

60

milled 5h

80

1500

60

1000

Hydrogen gas

40

500

Amount of generated H2 ( mL/1g Al)

Al

Temperature (°C)

In

20

milled 1h

0 70

80

2θ(deg) Fig. 8. XRD patterns of the Ald10 wt%Ind5 wt%Znd5 wt%NaCl mixture with different milling time.

0

2

4

6

8

10

Time (min) Fig. 9. Temperature curve and hydrogen production curve in the hydrogen reactor (water: 15 mL, MAld5 wt%Ind3 wt%Znd2 wt%NaCl mixture: 2 g).

M.-q. Fan et al. / Energy 35 (2010) 1333–1337

2 0.4

Current (A)

0.6

Cell voltage (V)

Acknowledgements

3

0.8

1337

b

The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 2083309, 20873148, 20903095, 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).

1 0.2

References

0

0.0 0

4

8

12

16

Time (min) Fig. 10. Power generation of the fuel cell at 1.6 A.

larger reversely. That may be explained why 10 hdmilled mixture has higher reactivity than that of 20 h-milled mixture. 3.5. Hydrogen supplying for micro fuel cell Fig. 9 shows temperature curve and hydrogen production curve in the hydrogen reactor (water: 15 mL, MAld5 wt %Ind3 wt%Znd2  wt%NaCl mixture: 2 g and 25 C). The temperature of the hydrolysis residual production quickly increase from room temperature to 100  C while the temperature of the hydrogen has small change, accompanied by the soaring of hydrogen production. The results reflect that the exothermic reaction happens and become violently with temperature increasing accordingly when the Al–In–Zn–NaCl mixture contacts with water. The amount of produced hydrogen is up to approximately 2000 mL in 4 min of hydrolysis reaction, nearly to the expected value. Fig. 10 shows power generation of the fuel cell at 1.6 A. The hydrogen supply rate can meet the normal work of fuel cell and 0.96 W is continuously produced for 16 min when the hydrogen supply rate is 100 mL/min. 4. Conclusions The milled Al–In–Zn–salt mixtures have high reactivity and react quickly with water at room temperature. The improved reactivity of Al is benefited by the additive salts and Zn(or Ga). The role of salts attributes to the enthalpy of dissolution, the more negative potential of Al obtained by the chloride ion concentration in the solid/water interface and the milled powdery prevented from forming a mass. As to the additive Zn(Ga), it improves the negative potential of Al–In alloy from 1.1 V to 1.5 V except that the additive Zn(Ga) facilitates the diffusion of the deposited In into the surface layers because the formation of Al–Zn alloys can create defects and cracking of the protective alumina layer. The Al–In–Zn– NaCl mixture has high reactivity and its hydrolysis properties have been effected by the mass ratio, NaOH solution and milling time. The optimized Al–In–Zn–NaCl mixture has highest hydrogen yield of 1035 mL hydrogen/1 g Al in water, corresponding to 9.21 wt% (excluding water mass). Furthermore, the hydrogen supplying from Al mixture is performed for micro fuel cell and the 0.96 W is produced with the stable hydrogen supply rate. Therefore, the milled Al–In–Zn–salt mixture is potentially applicable in devices that can provide a source of CO2 free hydrogen to fuel cell requirements.

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