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Energy & Fuels 2007, 21, 2294-2298
Hydrogen Generation by Hydrolysis Reaction of Ball-Milled Al-Bi Alloys Mei-Qiang Fan,†,‡ Fen Xu,*,† and Li-Xian Sun*,† Materials & Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ReceiVed January 9, 2007. ReVised Manuscript ReceiVed April 9, 2007
An investigation of hydrogen generation by the hydrolysis reaction of Al-Bi alloys in different media (pure water, 1 M NaCl solution, methanol, and ethanol) was performed at room temperature. The results show that the media have a great effect on the hydrolysis of Al-Bi alloys. In 1 M NaCl solution and in pure water, Al-16 wt % Bi alloy presents the maximum conversion yields at 92.75% (970 mL g-1) and 89.88% (930 mL g-1), as well as first 5 min hydrogen generation rates of 92 and 46 mL min-1 g-1, respectively. However, the hydrolysis of the ball-milled Al-Bi alloys in methanol and ethanol corresponds to only a little or even no hydrogen production. The hydrolysis mechanism of the Al-Bi alloys is based on the microgalvanic cell between the anode (Al) and cathode (Bi), so the high ionic conduction of the media plays an important role in improving the Al reactivity. Bi can activate Al, and a higher content of Bi metal can accelerate the hydrolysis rate of Al. The scanning electron microscopy images and energy dispersive X-ray results showed that the shape of Al particles changes from the initial round to platelet, and the mean size first increases markedly after being milled. Then, with a prolonging of the milling time, the Al platelet ruptures, and the mean size decreases. So, the Bi distribution remains uniform with increasing ball-milling time and results in the higher reactivity of the milled alloys.
Introduction Recently, the requirement for a safe, low-cost, and car-carried type of hydrogen gas generator has become stronger because the use of fuel cells is approaching practicality. Among many chemicals (such as hydrocarbons,1-5 metal,6,7 and NaBH48), Al hold promise as a hydrogen-generating material for fuel cells because the hydrolysis reaction of 1 g of Al produces 1245 mL of H2, and Al is also easily carried by a car. But the problem with the use of Al metal as a hydrogen source is its high inertness to water, especially the aluminum oxide formed on the surface. Several methods have been developed for the mechanical and chemical activation of Al metal. For example, Al melted together with other particular metals, such as Ga, Sn, In, Pb, Bi, Mg, or Ca, has a high activity of reacting with water to produce hydrogen at room temperature. Kravchenko et al.7 developed a new Al-based composite containing 5.3 wt % Ga, 2 wt % In, 5.4 wt % Sn, 7.3 wt % Zn, and 80 wt % Al * Corresponding author. Tel./fax: 0086-411-84379213 (L.-X.S.), 0086411-84379213 (F.X.). E-mail:
[email protected] (L.-X.S.),
[email protected] (F.X.). † Dalian Institute of Chemical Physics. ‡ Graduate School of the Chinese Academy of Sciences. (1) Toshihide, H.; Naoki, I.; Takanori, M.; Toshimitsu, S. Energy Fuels 2004, 18 (1), 122-126. (2) Agus, H.; Sandun, F.; Naveen, M.; Sushil, A. Energy Fuels 2005, 19 (5), 2098-2106. (3) Alexander, F.; Alexander, G. Energy Fuels 2006, 20 (3), 1242-1249. (4) Yuguo, W.; Naresh, S.; Frank, E. H.; Gerald, P. H. Energy Fuels 2006, 20, 2612-2615. (5) Sadashiv, M. S.; Martin, A. A. Energy Fuels 2006, 20, 2616-2622. (6) Grosjean, M. H.; Zidoune, M.; Roue´, L.; Huot, J. Y. Int. J. Hydrogen Energy 2006, 31, 109-119. (7) Kravchenko, O. V.; Semenenko, K. N.; Bulychev, B. M.; Kalmykov, K. B. J. Alloys Compd. 2005, 397, 58-62. (8) Xia, Z. T.; Chan, S. H. J. Power Sources 2005, 152, 46-49.
which produces 1000 mL g-1 of H2 at 82 °C in pure water. Obviously, the hydrolysis reaction of Al is affected by the addition of some metal additives. The addition of a small amount of certain metals, such as Ga,9 Sn,10 In,11 and Zn,12 is known to activate Al in NaCl solution. It has also been reported that Bi can accelerate anodic dissolution of Al when Bi occurs in a solid Al solution. Reding13 reported that Bi leads Al to a more anodic potential than pure Al. The melting Al-Bi alloys have a high reactivity as they are easily corroded in water. So far, there is no study of the hydrolysis reaction of ball-milled Al-Bi alloys to produce hydrogen. In the present work, hydrogen production was obtained from the hydrolysis of milled Al-Bi alloys in water or other media. Then, the microcosmic structure of the alloys was examined by scanning electron microscopy (SEM). It is expected that the chloride ion in solution could be favorable to destroy the passive Al(OH)3 layer on particles,14 thus leading to the extensive reaction between a freshly exposed Al surface and water. Experimental Section The starting materials were elemental powders of pure Al and Bi. The composites were mixed in an argon-filled glove box. Then, milling was performed by a QM-1SP planetary ball miller under a (9) Flamini, D. O.; Saidman, S. B.; Bessone, J. B. Corros. Sci. 2006, 48, 1413-1425. (10) Kyung, K. L.; Kwang, B. K. Corros. Sci. 2001, 43, 561-575. (11) Despic, A. R.; Drazic, D. M.; Purenovic, M. M.; Cikovic, N. J. Appl. Electrochem. 1976, 6, 527. (12) Yongen, T.; Lingbin, L.; Herbert, W. R.; Laiwen W. J. Power Sources 2004, 138, 313-318. (13) Reding, J. T. J. Mater. Protect. 1966, 5, 15. (14) Joong, K.; Yun, S. P. Electrochim. Acta 1995, 40, 1863-1869.
10.1021/ef0700127 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
Hydrolysis Reaction of Ball-Milled Al-Bi Alloys
Figure 1. Hydrogen generation of the hydrolysis of Al-Bi alloys in pure water.
0.2∼0.3 MPa argon atmosphere with a ball-to-powder ratio of 16: 1. The hydrolysis reactions (2Al + 6H2O ) 2Al(OH)3 + 3H2) of the Al-Bi alloys (0.1 g) with 20 mL of distilled water, a 1 M NaCl solution, and other media were carried out in a stainless steel chamber attached to a gas burette graduated in 0.1 mL increments at room temperature. The water vapor in produced H2 flowed through a condenser and then was removed by drierite prior to measurement of the H2 volume. The produced hydrogen volume was measured by the water trap method, and the corresponding generation rate was calculated. Hydrogen production is expressed as a conversion yield, defined as the volume of produced hydrogen over the theoretical volume of hydrogen that should be released assuming that all material is hydrolyzed. Microstructure studies (energy dispersive X-ray, EDX, analysis) were performed on a CAMEBAX-microbeam electron microprobe equipped with a KEVEX energy-dispersive analyzer. A differential thermal analysis of alloy samples was carried out on a Setaram setsys 16/18 apparatus (Setaram Company, France) at a temperature range from 30 to 800 °C.
Results and Discussion Reactivity of Milled Al-Bi Alloys in Water. Figure 1 shows the hydrogen production profiles for the reaction of pure water and different Al-Bi alloys. The results obtained from Figure 1 demonstrate that Bi content has a great effect on the hydrolysis of Al alloys in water. The hydrolysis reaction rate increases with Bi content augmentation from 10 to 23 wt % in the first 5 min. However, the hydrolysis reaction rate decreases when Bi content increases from 23 to 30 wt %. The positive effect of Bi on the hydrolysis reaction is ascribed to three reasons. First, the Al-Bi alloy had an open circuit potential of -1.85 V, which is lower than that of pure Al of -1.67 V. Second, the numerous defects and fresh surfaces (as shown in Figure 2b) obtained in the milling process result in Al corrosion. It can be seen that the unmilled Al sample shows a small rotund grain with an alumina layer on the surface. But the fresh milled Al-16 wt % Bi alloy presents platelets with a mean size larger than 20 um, while unmilled Al indicates small granular particles with a mean diameter of 13 um as shown in Figure 2. Furthermore, the Bi is relatively uniformly distributed into the Al, resulting in the creation of more microgalvanic cells between Al and dispersed Bi elements. During the hydrolysis reaction, Bi can also prevent the formation of the passive Al precipitate of the Al(OH)3 layer on the Al powder surface.13 But the Al-Bi alloys with the same weight produced less hydrogen if the Bi percentage increased beyond 16 wt %. At last, the hydrolysis of the Al-Bi alloy is
Energy & Fuels, Vol. 21, No. 4, 2007 2295
based on the formation of the micogalvanic cell formed between the anode (Al) and cathode (Bi), similar to the Mg-Ni alloy.6 The value of pH for the hydrolysis residue of the Al-10 wt % Bi alloy is 10.37, while the pH of the residues of the Al-16 wt % Bi, Al-23 wt % Bi, and Al-30 wt % Bi alloys correspond to 9.72, 9.64, and 9.67. The local pH at the powder/solution interface may be greater than 10.37. The local alkalization is due to the formation of hydroxide ions (H2O + e- ) OH - + 1/ H ) on the cathode component (Bi) and takes part in the 2 2 formation of Al(OH)3 (Al + 3OH- ) Al(OH)3 + 3e-) on the anode component (Al). Because Al(OH)3 is a hermaphroditic material and easily forms soluble Al(OH)4- with hydroxide ions, therefore, the bare Al can continue to react with hydroxide ions.14 Bi displays a low hydrogen overpotential and can be used as an appropriate cathode material to induce severe galvanic corrosion of the Al. Thus, Al-Bi alloys were elaborated to increase the yield and the kinetics of the hydrolysis reaction. The hydrolysis reaction of Al-16 wt % Bi alloy with water steam in the temperature range of 30-800 °C in Ar is shown in Figure 3. With the addition of Bi, the Al-16 wt % Bi alloy has a larger endothermic peak at 525 °C, which shows that alloys react with water steam drastically. And there is also a small endothermic peak at 660 °C that is the phase-change peak of pure Al. Therefore, the kinetics of the hydrolysis reaction may be improved with an increase in the temperature. The hydrolysis reaction can release an amount of heat15 (-444.1 KJ mol-1) which results in an increase of 1 °C in temperature for 100 mg of Al-16 wt % Bi alloy immersed in 20 mL of water in the experiments. The reaction performs favorably, so it is controllable and available for use as the hydrogen source for fuel cell applications. As shown in Figure 4, which illustrates the heat flow curves of Al-Bi alloys in Ar, the alloys have two endothermic peaks in the temperature range of 264 ( 10 °C and 655 ( 20 °C, which are slightly lower than the melting points of Bi (271 °C) and Al (660 °C), respectively. The temperatures of the observed endothermic effect agree well with the melting point of a eutectic (rich Bi phase and rich Al phase) in the Al-Bi alloy as the Al-Bi alloy belongs to immiscible alloys. Less than 0.03 atom % of the Bi dissolves in Al at the monotectic temperature,16 so most of the Bi is dissociated around the Al particle. Figure 4 confirms that Bi still keeps a low atom % in Al after being milled due to the slight change of phase transition temperature. Therefore, it is the Bi dissociated around the Al particle that leads to the improving Al reactivity as the hydrolysis rate results show that the hydrolysis rate of the Al-Bi alloy increases with increasing Bi content. That is why increasing Bi content results in the high hydrolysis rate. The Hydrolysis of Al-Bi Alloys in a 1 M NaCl Solution. When hydrolysis is performed in a 1 M NaCl aqueous solution, the reaction is more drastic than that in pure water. The conversion yields (%), total H2 volume (mL H2 g-1) after 2 h of hydrolysis, and hydrogen generation rate (mL H2 min-1 g-1) at 0.5 h of the hydrolysis reaction tested are presented in Tables 1 and 2. The media have a marked effect on the hydrolysis performance of Al-Bi alloys in contrast to that observed previously in pure water. The Al-Bi alloy displays a higher conversion yield in a 1 M NaCl solution than that in pure water. When the several Al-Bi alloys are compared, the hydrogen yield increases (15) Kunio, Ueharaa, Hideo, Takeshitaa, Hiromi, Kotaka. J. Mater. Proc. Technol. 2002, 127, 174. (16) Wilder, T. C.; Elliott, J. F. J. Electrochem. Soc. 1964, 111, 352.
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Figure 2. SEM images and EDX results of multicomponent Al-based materials with four selected regions. (a) The unmilled Al powders; (b) the fresh milled Al-16 wt % Bi alloy; (c) the Al-16 wt % Bi alloy immerged in 1 M NaCl solution. Compositions in designed fields (mass%): (a) 1. O-1.06, Al 98.94; 2. O-0, Al-100; 3. O-0, Al-100; 4. O-0, Al-100. (b) 1. Al-85.52, Bi-14.48, O-0; 2. Al-81.51, Bi-14.58, O-3.91; 3. Al-80.94, Bi-15.05, O-3.99; 4. Al-80.64, Bi-16.09, O-3.27. (c) 1. Al-56.18, Bi-0, O-29.32, Na-3.97 (3.94 atomic %), Cl-10.52 (6.77 atomic %); 2. Al-49.88, Bi-9.17, O-33.67, Na-6.12 (5.9 atomic %), Cl-10.34 (6.47 atomic %); 3. Al-45.41, Bi-9.17, O-30.66, Na-5.78 (5.78 atomic %), Cl-8.97 (6.10 atomic %); 4. Al-51.19, Bi-0, O-37.32, Na-3.06 (2.89 atomic%), Cl-8.43 (5.17 atomic%).
Figure 4. Heat flow curves of Al-Bi alloys in Ar: (a) Al-16 wt % Bi alloy, (b) Al-23 wt % Bi alloy, and (c) Al-30 wt % Bi alloy. Figure 3. Heat flow curves of the reaction of Al-16 wt % Bi alloy and water steam in Ar. Table 1. Conversion Yield (%), H2 Volume (mL H2 g-1) after 2 h of Hydrolysis, and Hydrogen Generation Rates (mL H2 g-1) at 0.5 h of Hydrolysis Reaction Tested in Pure Water at Room Temperature
Table 2. Conversion Yield (%), H2 Volume (mL H2 g-1) after 2 h of Hydrolysis, and Hydrogen Generation Rates (mL H2 g-1) at 0.5 h of Hydrolysis Reaction Tested in 1 M NaCl Solution at Room Temperature hydrogen generation rate (mL min-1 g-1)
hydrogen generation rate (mL min-1 g-1)
sample
0-5 min
5-15 min
15-30 min
hydrogen generation mL g-1
Al-10 wt % Bi Al-16 wt % Bi Al-23 wt % Bi Al-30 wt % Bi
14 46 72 50
12 32 35 30
10.7 146 3.7 7.3
920 940 790 720
conversion yield % 82.00 89.88 82.41 82.61
from 82.00% to 90.14% for Al-10 wt % Bi alloy, 89.88% to 92.75% for Al-16 wt % Bi alloy, 82.41% to 91.27% for Al23 wt % Bi alloy, and 82.61% to 88.35% for Al-30 wt % Bi alloy. The hydrogen generation rate increases obviously in the
sample
0-5 min
5-15 min
15-30 min
hydrogen generation mL g-1
conversion yield %
Al-10 wt % Bi Al-16 wt % Bi Al-23 wt % Bi Al-30 wt % Bi
24 92 102 64
35 16 24.5 30.5
20.6 8.7 3.3 7
1010 970 875 770
90.14 92.75 91.27 88.35
first 5 min, from 14 to 24 mL min-1 g-1 for Al-10 wt % Bi alloy, from 46 to 92 mL min-1 g-1 for Al-16 wt % Bi alloy, from 72 to 102 mL min-1 g-1 for Al-23 wt % Bi alloy, and from 50 to 64 mL min-1 g-1 for Al-30 wt % Bi alloy. From Figure 5, it can be found that the hydrolysis curves for milled Al-Bi alloys in a 1 M NaCl solution present three distinctive
Hydrolysis Reaction of Ball-Milled Al-Bi Alloys
Figure 5. Hydrogen generation of the hydrolysis of Al-Bi alloys in 1 M NaCl solution.
parts, that is, the first rapid hydrolysis (region I in Figure 5), the slow hydrolysis rate in the following minutes (region II), and a stop at the end (region III). The rapid hydrolysis was easily defined in the timespan of 5-20 min. The duration of the second period is related to the Bi content (20-50 min for Al-23 wt % Bi alloy and Al-30 wt % Bi alloy). But the duration became longer beyond 60 min with a decrease in the Bi content. The yield of hydrogen and the mechanism of its evolution during the hydrolysis reaction of Al-Bi alloys in a 1 M NaCl solution are different from that in pure water. The Cl- substitutes the fractional OH- ion to participate in the hydrolysis.14 The increase of the hydrolysis rate of the Al-Bi alloys in NaCl solution is associated with the penetration of chloride ions through the oxide film and the localized dissolution of Al at the metal/oxide interface.17 The corrosion pit propagation leads to the formation of blisters beneath the oxide film, which subsequently ruptures due to the formation of hydrogen gas in the occluded corrosion cell.18,19 Therefore, the passive layer on the particles was broken down, and the exposed Al continued to react with water. Figure 2c shows the SEM image and EDX of Al-16 wt % Bi alloy immersed in a 1 M NaCl solution. There are many pits where Bi is depleted completely, and the Cl atomic content is obviously higher than that of Na, in comparison to other places where the Bi content is higher and the atomic contents of Cl and Na are relatively close. The results also confirm that the Cl- accelerates the Al hydrolysis, and the increase of the H2 evolution rate with milled Al-Bi powders may reflect their high susceptibility to pitting corrosion in NaCl solution. The Hydrolysis of Al-Bi Alloys in Alcohol. The hydrolysis of Al-Bi alloys in methanol solution can be regarded as a corrosion process where the anodic reaction is the oxidation of Al into Al3+ and the cathode reaction is the reduction of CH3OH into CH3O-. But as shown in Figure 6, the milled Al-16 wt % Bi alloy reactivity in methanol is very low (maximum conversion yield ≈ 11.2%), and the other Al-Bi alloys have less reactivity in methanol. Such a result may be related to the lower dissociation constant20 (KCH3OH ) 10-17 < KH2O ) 10-14) (17) Mccafferty, E. Corros. Sci. 2003, 45, 301-308. (18) Natishan, P. M.; Mccafferty, E. J. Electrochem. Soc. 1989, 136, 53. (19) Ryan, R. L.; Mccafferty, E. J. Electrochem. Soc. 1995, 142, 2594. (20) Zhu, Y.-b; Shen, Z.-s.; Zhang, C.-f. Handbook of Electrochemistry Data; 1984.
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Figure 6. Hydrogen production profiles of the hydrolysis of the Al16 wt % Bi alloy in methanol and ethanol.
Figure 7. Hydrogen generation of the hydrolysis of Al-16 wt % Bi alloy in water at different milling durations.
of CH3OH, which results in less free-mobile ions in methanol solution. So the electron-transfer reaction must be seriously limited, and thus no significant hydrolysis occurs. This judgment is further confirmed by the result of the Al hydrolysis in ethanol20 (KCH3CH2OH ) 10-21) or propanol solutions. No hydrogen release is observed in ethanol in contrast to that observed (a little) in methanol solution. For ethanol and propanol, due to the inductive effect of the alkyl groups, which induces a stronger RO-H bonding formation as compared with methanol, this results in the direct reaction of a metal M with an alcohol ROH becoming slower.21 Effect of Milling Time on the Hydrolysis of the Al-Bi Alloys. The relationship of hydrogen release and milling time was studied for the ball-milled Al-16 wt % Bi alloy. The results are shown in Figure 7. The milling time has some effect on the hydrolysis rate, and the reacted fraction is proportional to the milling time within the studied time range. As the milling time increase, the hydrolysis rate becomes faster. The highest hydrogen yield was obtained with 10 h of milling, which displays a conversion of 84.20% (880 mL g-1) in the 60 min of hydrolysis reaction. Figure 8 shows the SEM image of Al16 wt % Bi alloy with different milling times. It can be seen that the shape of Al particles changes from the initial round to a platelet and the mean size increases markedly after milling (21) Bradley, D. C.; Mehrotva, R. C.; Gauv, D. P. Metal Alkoxides; Academic Press: London, 1979.
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Figure 8. SEM images and EDX results of Al-16 wt % Bi alloy with different milling times. (a) Milled 2 h; (b) milled 5 h; (c) milled 10 h; (d) milled 20 h. Compositions in designed fields (mass %): (a) 1. Al-75.34, Bi-22.87, O-1.79; 2. Al-70.17, Bi-27.48, O-2.36; 3. Al-75.3, Bi-21.63, O-3.07; 4. Al-76.49, Bi-21.71, O-1.8. (b) 1. Al-76.69, Bi-15.13, O-8.18; 2. Al-74.29, Bi-17.28, O-8.43; 3. Al-73.98, Bi-15.33, O-10.68; 4. Al73.34, Bi-14.07, O-12.59. (c) 1. Al-74.63, Bi-14.48, O-8.49; 2. Al-81.51, Bi-14.58, O-6.25; 3. Al-69.52, Bi-27.78, O-2.7; 4. Al-70.68, Bi-26.56, O-2.76. (d) 1. Al-80.64, Bi-16.09, O-3.27; 2. Al-80.94, Bi-15.07, O-3.99; 3. Al-81.51, Bi-14.58, O-3.91; 4. Al-85.52, Bi-14.48, O-0.
for 2 h. But with prolonging of the milling time, the Al platelet ruptures and the mean size decreases. Meanwhile, the Bi distribution becomes more uniform at longer milling times. Therefore, the alloy prepared with a longer milling time has a faster hydrolysis rate and higher hydrogen yield because of the creation of more microgalvanic cells between the anode (Al) and the cathode (Bi). Summary We have demonstrated a new approach to produce hydrogen from the hydrolysis of the ball-milled Al-Bi alloys in various media at room temperature. The best performance for hydrogen evolution has been obtained with the ball-milling of Al-16 wt % Bi alloy, leading to a maximum conversion of 92.75% (970 mL g-1) in 1 M NaCl and 89.88% (940 mL g-1) in pure water,
which corresponds to a hydrogen yield of 8.63 wt % [wt% hydrogen ) 2(VH2/22.4 L/mol)] and 7.65 wt % (where the water weight is not included in the calculation and VH2 stands for the volume of hydrogen produced from hydrolysis of the Al-Bi alloy), respectively. The hydrolysis mechanism of the Al-Bi alloy is based on the microgalvanic cell formed between the anode (Al), cathode (Bi), and high ionic conductivity in the media. The hydrolysis reactivity of the alloy reduces with a decrease in the dissociation constant (i.e., 1 M NaCl solution > pure water > methanol > ethanol). Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20473091, 50671098, and 20573112). EF0700127
