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Effect of addition of water-soluble salts on the hydrogen generation of aluminum in reaction with hot water
Journal of Alloys and Compounds 679 (2016) 364e374
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Effect of addition of water-soluble salts on the hydrogen generation of aluminum in reaction with hot water S.S. Razavi-Tousi*, J.A. Szpunar Department of Mechanical Engineering, University of Saskatchewan, S7N 5A9 Saskatoon, Saskatchewan, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 19 March 2016 Accepted 4 April 2016 Available online 7 April 2016
Aluminum powder was ball milled for different durations of time with different weight percentages of water-soluble salts (NaCl and KCl). The hydrogen generation of each mixture in reaction with hot water was measured. A scanning electron microscope (SEM) as well as energy-dispersive spectroscopy (EDS) were used to investigate the morphology, surfaces and cross sections of the produced particles. The results show that the presence of salts in the microstructure of the aluminum considerably increases the hydrogen generation rate. At shorter milling times, the salt covers the aluminum particles and becomes embedded in layers within the aluminum matrix. At higher milling durations, salt and aluminum phases form composite particles. A higher percentage of the second phase significantly decreases the milling time needed for activation of the aluminum particles. Based on the EDS results from cross sections of the milled particles, a mechanism for improvement of the hydrogen generation rate in the presence of salts is suggested. © 2016 Elsevier B.V. All rights reserved.
Keywords: Microstructure Mechanical milling Hydrogen Aluminum matrix Corrosion
1. Introduction Researchers have adopted different approaches to improving the kinetics of the reaction of aluminum with water for the purpose of hydrogen generation. Adding low melting point metals to aluminum, using basic solutions, applying supercritical water conditions, and ball milling in the presence of oxides or watersoluble salts are among the techniques used to improve the hydrogen generation rate [1e18]. Although most of these methods have been effective in improving the reaction, some reservations remain about their potential for large-scale application. For example, low melting point metals are expensive and most of them are toxic, the supercritical condition is not hardly applicable for onboard hydrogen generation, and the use of basic solutions raises safety and environmental concerns. Among different methods, ball milling of aluminum powder in the presence of water-soluble salts is considered effective and safe [9,19e25]. Water-soluble salts (NaCl, KCl, etc.) are neither expensive nor toxic and their presence significantly shortens the completion time of the reaction. The effects of water-soluble salts during milling on the reaction
* Corresponding author. E-mail addresses: [email protected], [email protected] (S.S. RazaviTousi). http://dx.doi.org/10.1016/j.jallcom.2016.04.038 0925-8388/© 2016 Elsevier B.V. All rights reserved.
kinetics and consequently on hydrogen generation have been explained in two ways: Firstly, some researchers point out that the reaction is promoted because of the Cl ions released by dissolving salts in water [3]. These Cl ions can deteriorate the protectiveness of the hydroxide layer on the surface of the particles [26e28], and thus the aluminum substrate is corroded more effectively. Secondly, the presence of salt particles can modify the structure of aluminum particles. Alinejad et al. consider that salt particles modify the structure of aluminum particles by providing more surfaces for the reaction. They suggest a schematic model to illustrate how the presence of salt particles promotes the reaction [23]. In this model, they assume that salt particles cover the aluminum particles during ball milling. As the mixture is exposed to water, salt particles are dissolved and a fresh surface of aluminum is exposed to water. In spite of the number of works that have used water-soluble salts to activate aluminum, no study has attempted to distinguish whether their addition affects the chemical reaction or the structure of particles. The structural and chemical effects can be separated by introducing a particular amount of a salt during ball milling or adding it to the hot water during the reaction and then measuring the hydrogen production rate for either case. In this study, the hydrogen generation of pure aluminum was firstly compared with that of an aluminum-salt mixture. Then, the microstructure of the aluminum-salt particles was studied after
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different milling times and the results correlated to hydrogen generation. Finally, the effects of the different salt types (NaCl or KCl) were examined.
2. Experimental procedure 2.1. Ball milling Alfa Aeasar Company provided aluminum powder of 99.8% purity (MFCD00134029, average particle size of 227 mm). After mixing 50 wt% of sodium chloride (purity: þ99%, average particle size of 281 mm) with the aluminum powder, the mixture was ball milled. High-energy ball milling was done in a planetary ball mill (Torrey Hills- ND2L) with stainless steel cups (285 ml capacity) and balls (28 of 16 mm and 6 of 18 mm diameter) in an argon atmosphere. The ball to powder ratio was 30:1 and the mill speed was maintained at 200 RPM for durations of 0.25, 0.5, 1, 2, 4, 7, 11 and 19 h. In order to evaluate the effect of the weight percentage and type of salts, mixtures of 25 wt% NaCl- 75 wt% Al, 75 wt% NaCl- 25 wt% Al and 50 wt% KCl- 50 wt% Al were also ball milled for durations of 2, 7 and 19 h. The KCl powder, with a commercial purity, was refined from Potash Corporation of Saskatchewan products.
2.2. Hydrogen measurement
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3. Results and discussion 3.1. Effect of milling time and presence of salt 3.1.1. Hydrogen generation Fig. 1(a) shows the effect of milling time on the hydrogen generation of aluminum-salt mixtures through their reaction with hot water. Hydrogen generation was not observed for the mixture of aluminum-salt that was not ball milled, thus it is not reported in Fig. 1(a). Milling the mixture for durations as short as 15 or 30 min resulted in measurable hydrogen generation. Increasing milling time from 1 h up to 7 h significantly enhanced the hydrogen generation rate. However, further milling from 7 h up to 19 h did not appreciably increase the amount of generated hydrogen as the amount for the 7 h sample was already close to the theoretical limit. Fig. 1(b) presents the hydrogen generation results from the pure aluminum powder milled for the same durations, which are published in Ref. [18]. Since the starting material, milling, and the hydrogen measurement conditions were the same for both experiments, the comparison of Fig. 1(a) and (b) illustrates the effect of the presence of sodium chloride on the hydrogen generation rate. Clearly, the aluminum milled in the presence of salt reacted significantly faster than the pure aluminum; the maximum amount of produced hydrogen in one hour increased from 230 ml to 1200 ml. Nevertheless, it is significant that the presence of salt without milling was ineffective, as neither pure aluminum nor an aluminum-salt mixture could generate hydrogen in the as-received
The details of the hydrogen measurement procedure are similar to those described in Ref. [18]. We added powder mixtures with the equivalent of one g of aluminum in to an Erlenmeyer flask containing 200 ml of distilled water at 80 ± 3 C with a constant stirring rate of 120 rpm. The produced hydrogen gas passed through a desiccant (CoCl2) to absorb moisture, and then an ADM2000 flowmeter to measure its flow with an accuracy of 0.1 ml/min. The flowmeter was connected to a computer running ADM Trend software to acquire the data. A baseline curve was obtained by measuring the flow from 200 ml of distilled water at 80 C with a constant stirring rate of 120 rpm with no powder added. This was then subtracted from the data obtained from the reactions to ensure that the measurements did not include any contribution from water moisture or expansion of the air in the flask during heating.
2.3. Scanning electron microscopy A Hitachi SU6600 field emission scanning electron microscope (SEM) was employed to examine the surfaces and cross sections of the powders. A small amount of each powder was added to a conductive carbon resin powder, mixed and mounted. The mounted samples were polished using abrasive grinding papers from 600 to 2000 grit followed by additional grinding with diamond pastes of 3 and 0.04 mm. Particles sizes of the powders were calculated using software written in Visual Basic using several SEM images. The weighted average (Eq. (1)) was calculated and reported as the average particle size:
P i ni Di D¼ P i ni
(1)
where Di is the diameter of the particle i, ni is the number of particles with size Di, and D is the weighted average.
Fig. 1. Effect of milling time on hydrogen generation of (a) the aluminum-salt mixture, and (b) pure aluminum during the reaction with hot water.
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state. This result implies that in the time span of these experiments (1 h of reaction), the effect of Cl ions on improving the corrosion of aluminum by hot water was negligible. Moreover, the presented results establish that ball milling cannot be sufficiently effective without the presence of a water-soluble salt nor can the presence of the salt be effective without milling. Thus, a strong synergy exists between the milling process and the presence of a salt. 3.1.2. Microstructural characterization In order to realize the mechanism behind the improvement of the reaction rate by milling, SEM was used to examine the microstructures of the milled powders. Fig. 2 shows secondary electron SEM images (100) of the as-received mixture and the mixture milled for durations of 15 min, 30 min, 1hr, 2hr, 4hr, 7hr, 11hr and 19 h. The as-received powder was a mixture of cubic shaped salt crystals and irregularly shaped aluminum particles. Milling for 15 min significantly changed the morphology of the mixture; salt crystals were crushed into much smaller particles while aluminum particles were deformed and flattened without a noticeable change in their size (Daverage, 15 min ¼ 224 mm). Milling from 30 min (Daverage, 30 min ¼ 208 mm) up to 1 h, 2 h and 4 h gradually decreased the aluminum particles' sizes. The particle sizes for the 1 h, 2 h and 4 h milled powders were 76 mm, 50 mm and 23 mm, respectively. For the mixtures milled for 15 min or 30 min, it was still possible to distinguish between the salt and aluminum particles and the measured particle size corresponded to Al particles. This was not
the case for the mixtures milled for more than 1 h where the measured particle sizes were obtained from the Al-salt composite particles. As Fig. 2(f)e(i) show, milling from 4 h up to 19 h did not result in any significant change in the sizes of particles. The particle sizes for the 7 h, 11 h and 19 h milled powders were 22 mm, 22 mm and 21 mm, respectively. The trend observed in Fig. 2 for reduction of the particle size is not noticeably different from that of the pure aluminum milled for the same durations and reported as Fig. 1 in Ref. [18]. Particularly for the powders milled more than 4 h, the shapes and sizes of the particles were similar for the pure aluminum and the aluminumsalt mixture. On the other hand, comparison of the hydrogen generation of the 7 h, 11 h and 19 h milled powders (Fig. 1) shows the hydrogen generation rate markedly increased in the presence of the salt during milling. The increased hydrogen rate, in spite of the rather similar particle sizes, implies there were other factors affecting hydrogen generation in addition to the size of particles. Comparison of Fig. 1(a) and (b) reveals an obvious difference between the effect of milling time on hydrogen production in the presence and absence of salt. The 7 h milled sample produced the largest amount of hydrogen among the milled pure aluminum powders. Milling more or less than 7 h caused a reduction in the produced hydrogen. In our previous work, this phenomenon was explained thoroughly based on the microstructural evolution of aluminum powder during milling [18]. The microstructure of the 7 h milled particles was laminated, which increased the surface
Fig. 2. Secondary electron SEM images ( 100) of (a) the as received mixture and the mixtures milled for (b) 15 min, (c) 30 min, (d) 1 h, (e) 2 h, (f) 4 h, (g) 7 h, (h) 11 h and (i) 19 h.
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Fig. 3. (a) The higher magnification image of structure within the rectangle marked in Fig. 2(b), (b) the higher magnification image of structure within the rectangle marked in Fig. 2(c), (c) the higher magnification image of structure within the rectangle marked in Fig. 2(e), (d) the higher magnification image of the structure marked in Fig. 3(c). Fig. 3(e), 3(f) and 3(g) high magnification images of the particles milled for 7 h, 11 h and 19 h, respectively. Fig. 3(h) The 40 K magnification micrograph from the surface of the particle in Fig. 3(g).
Fig. 4. XRD patterns of the as received mixture and the mixture milled for durations of 15 min, 30 min, 1 h, 2 h, 4 h, 7 h, 11 h and 19 h.
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area of the powder, and in turn this microstructure improved the kinetics of the reaction. This phenomenon was not the case when a salt phase was present during milling. Fig. 1(a) shows that longer millings generally improved the kinetics of the reaction and there was no milling time following which the hydrogen production declined. Nevertheless, further milling after 7 h did not have a remarkable effect on hydrogen production, implying that the microstructural evolution resulted by milling reached a steady state
after 7 h and further milling was not beneficial to the reaction. This comparison reveals that the optimum milling times for the pure aluminum and the aluminum-salt mixtures are based on different paths of microstructural evolution. Fig. 3 shows high magnification images of the milled mixtures. Fig. 3(a) shows the higher magnification image of the rectangle marked in Fig. 2(b) for the 15 min milled sample. Such a short milling time initially crushed the salt crystals and formed
Fig. 5. SEM images and the corresponding element distribution maps obtained by EDS from the surface of the particles milled for (a) 30 min, (b) 1 h, (c) 2 h, (d) 7 h and (e) 11 h. The red and green colors represent the presence of aluminum and chlorine, respectively. Please note the change in the scale bar. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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aggregates composed of much smaller particles. This significant reduction in particle size is explained by the brittle nature of salt, while aluminum particles were deformed but not fractured. Fig. 3(b) shows the higher magnification image of structure within the rectangle marked in Fig. 2(c) for a particle milled for 30 min. Fine salt particles can be seen covering the surface of the aluminum
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particle. Fig. 3(c) shows the higher magnification image of structure within the rectangle marked in Fig. 2(e) for a particle milled for 2 h Fig. 3(d) shows the higher magnification image of structure marked in Fig. 3(c). The sizes of the salt particles covering the aluminum particles were in the range of 100e500 nm. The as-received salt particles were 200e300 mm, so milling for 2 h reduced their size by
Fig. 6. SEM images and the corresponding element distribution maps obtained by EDS from the cross section of the particles milled for (a) 15 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h and (f) 7 h. Please note the change in the scale bar.
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almost three orders of magnitude. Such a remarkable size reduction is also reflected in X-ray diffraction patterns. Fig. 4 shows the XRD patterns of the as-received mixture and the mixture milled for durations of 15 min, 30 min, 1hr, 2hr, 4hr, 7hr, 11hr and 19 h. The graph shows that ball milling from 15 min up to 19 h did not cause any reaction or formation of new phases. Nevertheless, a significant line broadening occurred for the salt phase during the first 1e2 h of milling. This line broadening is explained by the crystallite size reduction. As Fig. 3(a)e(d) show, milling initially crushed the salt particles, which then covered the aluminum particles after further milling. Coating of aluminum particles by smaller salt particles was first noted by Alinejad et al. [23]. They discuss that the presence of salt at the surface of aluminum particles prevents formation of an oxide layer. As the mixture is exposed to water, the salt is dissolved and fresh aluminum surface reacts with water, which explains the improvement of the hydrogen generation in the presence of a water-soluble salt. Their argument is correct, but it cannot be the only mechanism explaining the role of salt. SEM examinations revealed that the aluminum particles in all mixtures milled for 15 min, 30 min, 1 h and 2 h were covered by salt but they performed differently in hydrogen generation. Moreover, as Fig. 3(e)e(g) show, the concept of salt coating cannot accurately be the case for the mixtures milled for longer durations. That is because, based on SEM images presented in Fig. 3, a salt layer did not cover an aluminum core for the 7 h, 11 h and 19 h milled particles, but the particles are a mixture of the aluminum and salt phases. Fig. 3 (h) shows the typical microstructure observed from the higher magnification micrographs of the particles milled for longer periods (7 h, 11 h and 19 h). It shows that each particle itself is formed by agglomeration of smaller particles with a size of 0.1e3 mm. Such a fine microstructure explains the improved kinetics and high affiliation of milled particles to react with water. In order to reveal the mechanisms explaining the effect of salt on the reaction rate, we analyzed the surfaces and cross sections of the milled mixtures by EDS. Fig. 5 shows SEM images and the corresponding element distribution maps obtained by EDS from the surface of the particles milled for 30 min, 1 h, 2 h, 7 h and 19 h. The red and green colors represent the presence of aluminum and chlorine, respectively. Fig. 5(a) shows that a layer of salt covered the aluminum particle from the mixture milled for 30 min. The layer of salt was thick enough to prevent detecting the presence of aluminum except just at the edges of the particle where the salt was thinner or absent. The map also shows that the small particles around the bigger one were salt particles. Fig. 5(b) shows a similar result for the particle milled for 1 h; there was a significant amount of salt on the aluminum particle. Nevertheless, EDS from the particle milled for 2 h shows a different map of elements (Fig. 5(c)). Although the presence of salt was still strong, a marked presence of aluminum was also detected. The presence of aluminum in EDS maps implies that the salt layer did not cover the particle completely or the layer was thin enough to allow detection of aluminum. Further milling up to 7 h and 19 h resulted in the uniform presence of salt and aluminum at the surface of the particles (Fig. 5(d) and (e)). Fig. 6 shows SEM images and the corresponding element distribution maps obtained by EDS from the cross sections of the particles milled for 15 min, 30 min, 1 h, 2 h, 4 h and 7 h. The results from 11 h to 19 h milled particles are not reported here as they are similar to those of 4 h and 7 h milled particles. The red and green colors represent the presence of aluminum and chlorine, respectively. The blue color represents the presence of carbon from the carbon resin used for mounting the particles. Fig. 6(a) shows that after 15 min milling, thick layers of salt covered the deformed aluminum particles. Further milling up to 30 min and 1 h (Fig. 6(b)
and (c)) resulted in production of particles formed by layers of aluminum and salt, which are clearly distinguishable by the resolution of the EDS. Fig. 6(d) shows that milling up to 2 h combined the aluminum and salt and produced composite particles with a fine distribution of phases. Prolonged milling up to 4 h and 7 h (Fig. 6(e) and (f)) generated a very fine distribution of salt and
Fig. 7. EDS line-scans from the cross section of particles milled for (a) 15 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h, (f) 7 h, (g) 11 h and (h) 19 h. Please note the change in the scale bar.
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aluminum phases in each particle. The 11 h and 19 h milled mixtures showed similar results. In each composite particle milled for more than 4 h, the size of salt and aluminum constituents was small enough that the resolution of the EDS maps cannot distinguish between the different phases. Fig. 7 shows the EDS line-scans from the cross sections of particles milled for 15 min, 30 min, 1 h, 2 h, 4 h, 7 h, 11 h and 19 h. The red and green lines represent the presence of aluminum and chlorine, respectively. Each scan was obtained from the cross section of one particle. Thick layers of aluminum and salt are visible in each scan of particles milled for shorter times. As milling continued, the layers became thinner and a greater presence of salt and aluminum phases is visible along the scanned line at the cross sections of the particles. Considering the change in the scale, it is clear that the thickness of salt and aluminum phases changed from around 100 mm for the 15 min milled particle to 1e2 mm for the 19 h milled particle. These line scans confirm the results obtained by EDS maps, which reveal that increasing the milling time results in a homogeneous distribution of very fine salt and aluminum segments. Considering Figs. 5e7, the mechanism behind the synergic effect of milling and the presence of salt on the hydrogen generation rate can now be explained. Ball milling produced composite particles formed by the aggregation of salt and aluminum phases. As the composite particles were exposed to hot water, the salt was dissolved and aluminum particles remained with many voids and tunnels left because of the dissolution of salt. That porous structure of aluminum particles provided a significant quantity of fresh surfaces for reaction with hot water. The longer the milling time, the finer the aluminum and salt phases were, which resulted in a higher specific surface area of aluminum phase after dissolution of the salt. In order to confirm this observation, the 19 h milled mixture was treated with glycerin. A small amount of the 19hr milled powder was added to a liquid carbon paste, which was dried by hot air trapping many particles into its surface. The mounted particles on the resin were treated in 100 ml of glycerin for 1 h and then the remaining powder was washed with ethanol and dried. Glycerin dissolves salt but does not react with aluminum; thus the microstructure of the aluminum phase without salt could be revealed. The reason for using the carbon mount was to hold together the loose structure of the particles after the salt was removed from it. Fig. 8(a) shows a few 19 h milled particles after the treatment with glycerin. Fig. 8(b), a higher magnification of the marked rectangle in Fig. 8(a), shows that the remaining aluminum structure was formed by the aggregation of small particles with a noticeable amount of porosity in the microstructure. Fig. 8(c) shows an aluminum particle milled for 19 h without the addition of any salt. The
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comparison establishes that the presence of salt during milling can change an almost pore-free microstructure to a significantly porous structure. These pores provide space for water to penetrate into a particle and result in a significant increase in the hydrogen generation rate.
3.2. Effect of weight percentage and type of the water-soluble salt Fig. 9(a) and (b) show the hydrogen generation of the 25 wt% and 75 wt% salt-aluminum mixtures milled for 2hr, 7hr and 19 h, respectively. All results are normalized for reaction of 1 g of aluminum. Clearly, the hydrogen generation of the 75 wt% saltaluminum mixture was considerably faster compared to the 50 wt% salt mixture (Fig. 1(a)). For the 19 h milled sample with 75 wt% of salt, the reaction was completed in only 5 min. However, the 25 wt% salt-aluminum mixture reacted slower with water: In one hour, around half of the reaction (650 ml compared to the 1270 ml for the full reaction) was completed. This suggests that the higher amount of salt in the mixture promoted the reaction rate significantly. This effect is explained by a higher quantity of pores in the microstructure of the aluminum particles as the salt phase dissolved in hot water. Accordingly, a higher specific surface area was obtained at a higher salt percentage, which improved the kinetics of hydrogen generation. Potash powder (KCl) with a ratio of 50:50 was also added to aluminum during ball milling, and the hydrogen generation of the mixture in reaction with hot water was measured. Fig. 9(c) shows the hydrogen generation of the aluminum-potash mixture milled for 2 h, 7 h and 19 h. The data from 50 wt% salt mixture was also added as the dotted lines for comparison. As Fig. 9(c) shows, 7 h of milling was enough to allow the reaction to complete in 4 min, and ball milling over 7 h had no effect on the hydrogen generation rate. It is clear that KCl had a noticeably better effect on hydrogen generation compared to NaCl. Indeed, adding 50 wt% of KCl to aluminum worked even better than adding 75 wt% of NaCl. Considering that NaCl and KCl are both relatively affordable and abundant materials, KCl has an obvious advantage over NaCl. That is because a shorter period of milling is needed for the activation, which in turn saves energy and reduces the final cost of the produced hydrogen. Moreover, for a given amount of the produced hydrogen, a lower amount of the water-soluble salt in the mixture (50% for KCl compared to 75% for NaCl) means a lighter hydrogen fuel for vehicles. SEM was used to examine the milled mixtures in order to study the effect of weight percentage and type of water-soluble salts on the morphology of particles. Fig. 10 shows the SEM images of the AleNaCl (25 wt% of salt), AleNaCl (75 wt% of salt) and AleKCl (50 wt%) mixtures milled for 2 h, 7 h and 19 h. The AleNaCl (25 wt
Fig. 8. (a) 19 h milled particles after the treatment with glycerin, (b) the higher magnification of the marked rectangular in Fig. 8(a), (c) an aluminum particle milled for 19 h without addition of salt.
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measured particle sizes for the 2 h, 7 h and 19 h milled powders were 22 mm, 17 mm and 16 mm, respectively. Comparing the mixture of 25 wt% salt to 75 wt%, a higher amount of salt clearly had a marked effect on the particle sizes. This difference is explained by the effect of salt in preventing the cold welding of aluminum particles. Unlike aluminum particles that are ductile and can be cold welded by impacts [29,30], salt is a brittle phase and promotes fracture rather than cold welding during milling. Accordingly, as Fig. 10 shows, an increase in salt to 75 wt% resulted in a reduction in the particle sizes of the powders during milling. A smaller particle size in turn meant a higher specific surface area, which improved the reaction kinetics. On the other hand, the effect of 50 wt% KCl compared with 75 wt% of NaCl produced a negligible difference on the morphology of the powders in the mixtures. That means the effect of 50 wt% of KCl on the reduction in particle size is similar to that of 75 wt% of NaCl. The improvement related to the use ofKClis also evident from the results of hydrogen generation (Fig. 9). One reason for the better performance of KCl might be its higher hardness compared to NaCl (KCl:2.5, NaCl:2, on the Mohs scale). The higher hardness means KCl is more brittle and fractures to smaller segments by impacts, which can result in finer particles after milling and consequently a higher hydrogen generation rate. Nevertheless, further investigation is required to establish the explanation for the superiority of KCl over NaCl in facilitating the aluminum-water reaction. The AleKCl composition milled for 7 h or 19 h achieved a complete yield after 6e9 min, which demonstrates a fast reaction compared to the other methods of activation. For example, Soler et al. measured the hydrogen yield of Al/Co alloy in 1 M KOH solution at room temperature and found that 80% of the reaction was completed in 20 min [22]. They also found that the Al/Si alloy in hot saturated Ca(OH)2 solution yielded 80% after 100 min, and aluminum powder with a 0.075 M Na2SnO3solution measured 80% of yield after 80 min [31]. Fan et al. milled aluminum with both MgCl2 and Bi, and observed an almost complete reaction after 8 min [2]. Wang et al. obtained 100% yield between 5 and 100 min for the aluminum alloyed with over 50 atom% of In/Ga/Sn metals [32], while Parmuzina observed 100% yield after 150e500 min [33]. The above mentioned works prove that milling aluminum powder with water-soluble salts is not only a more effective activation method than others but also is safer and more affordable.
4. Conclusion
Fig. 9. Hydrogen generation of the (a) 25 wt% salt-aluminum, (b) 75 wt% saltaluminum, and (c) 50 wt% aluminum-potash mixture milled for 2 h, 7 h and 19 h. The dotted lines in Fig. 9(c) represent the 50 wt% salt results.
%) mixture had the biggest particle size for all milling durations. The measured particle sizes for the 2 h, 7 h and 19 h milled AleNaCl (25 wt% of salt) powders were 77 mm, 24 mm and 23 mm, respectively. For the AleNaCl (75 wt% of salt) powder, particle sizes of 20 mm, 20 mm and 18 mm were measured for the 2 h, 7 h and 19 h milled powders, respectively. Regarding the AleKCl mixtures, the
The presence of salt during ball milling had a significant role in improving the hydrogen generation rate of aluminum in reaction with hot water. Considering that the salt had no effect on hydrogen generation without ball milling, the improvement is explained by the changes in the structure of particles rather than the presence of Cl ions. SEM examination shows that the refinement of the powder during milling was improved by the presence of salt particles. EDS results establish that during ball milling, salt particles were embedded inside aluminum particles. Salt particles dissolve upon immersion in hot water, leaving many voids/tunnels for water to penetrate inside aluminum particles, which consequently improved the kinetics of the oxidation reaction. This explanation is also supported by the fact that a higher weight percentage of the salt in the aluminum matrix resulted in higher hydrogen production. Finally, measured data show that using different water-soluble salts (KCl or NaCl) resulted indifferent rates of hydrogen production. An explanation for the effect of different water-soluble salts on the hydrogen generation rate merits further study.
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Fig. 10. SEM images of the AleNaCl (25 wt% of salt) powder milled for (a) 2 h, (b) 7 h, (c) 19 h, and the AleNaCl (75 wt% of salt) powder milled for (d) 2 h, (e) 7 h, (f) 19 h, and AleKCl (50 wt%) powder milled for (g) 2 h, (h) 7 h and (i) 19 h at 100 magnification.
References [1] M.Q. Fan, L.X. Sun, F. Xu, Study of the controllable reactivity of aluminum alloys and their promising application for hydrogen generation, Energy Convers. Manage 51 (2010) 594e599. [2] M.Q. Fan, F. Xu, L.X. Sun, Studies on hydrogen generation characteristics of hydrolysis of the ball milling Al-based materials in pure water, Int. J. Hydrogen Energy 32 (2007) 2809e2815. [3] M.Q. Fan, F. Xu, L.X. Sun, Hydrogen generation by hydrolysis reaction of ballmilled Al-Bi alloys, Energy Fuel 21 (2007) 2294e2298. [4] M.-q. Fan, L.-x. Sun, F. Xu, Feasibility study of hydrogen production for micro fuel cell from activated AleIn mixture in water, Energy 35 (2010) 1333e1337. [5] A.V. Ilyukhina, O.V. Kravchenko, B.M. Bulychev, E.I. Shkolnikov, Mechanochemical activation of aluminum with gallams for hydrogen evolution from water, Int. J. Hydrogen Energy 35 (2010) 1905e1910. [6] A.V. Parmuzina, O.V. Kravchenko, B.M. Bulychev, E.I. Shkolnikov, A.G. Burlakova, Oxidation of activated aluminum with water as a method for hydrogen generation, Russ. Chem. Bull. 58 (2009) 493e498. [7] P. Dupiano, D. Stamatis, E.L. Dreizin, Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders, Int. J. Hydrogen Energy 36 (2011) 4781e4791. [8] Z.Y. Deng, Y.B. Tang, L.L. Zhu, Y. Sakka, J.H. Ye, Effect of different modification agents on hydrogen-generation by the reaction of Al with water, Int. J. Hydrogen Energy 35 (2010) 9561e9568. [9] E. Czech, T. Troczynski, Hydrogen generation through massive corrosion of deformed aluminum in water, Int. J. Hydrogen Energy 35 (2010) 1029e1037. [10] Z.Y. Deng, J.M.F. Ferreiraw, Y. Tanaka, J.H. Ye, Physicochemical mechanism for the continuous reaction of gamma-Al2O3-modified aluminum powder with water, J. Am. Ceram. Soc. 90 (2007) 1521e1526. [11] Z.-Y. Deng, Y.-F. Liu, Y. Tanaka, J. Ye, Y. Sakka, Modification of Al particle surfaces by c-Al2O3 and its effect on the corrosion behavior of Al, J. Am. Ceram. Soc. 88 (2005) 977e979. [12] A.C.D. Chaklader, Hydrogen generation from water split reaction, U.S. Patent,
6,582,676 B2 (2003). [13] A.N. Streletskii, I.V. Kolbanev, A.B. Borunova, P.Y. Butyagin, Mechanochemical activation of aluminum: 3. Kinetics of interaction between aluminum and water, Colloid J. 67 (2005) 631e637. [14] H.W. Wang, H.W. Chung, H.T. Teng, G.Z. Cao, Generation of hydrogen from aluminum and water - effect of metal oxide nanocrystals and water quality, Int. J. Hydrogen Energy 36 (2011) 15136e15144. [15] X.-N. Huang, C.-J. Lv, Y. Wang, H.-Y. Shen, D. Chen, Y.-X. Huang, Hydrogen generation from hydrolysis of aluminum/graphite composites with a coreeshell structure, Int. J. Hydrogen Energy 37 (2012) 7457e7463. [16] X.N. Huang, C.J. Lv, Y. Wang, H.Y. Shen, D. Chen, Y.X. Huang, Hydrogen generation from hydrolysis of aluminum/graphite composites with a core-shell structure, Int. J. Hydrogen Energy 37 (2012) 7457e7463. [17] S.S. Razavi-Tousi, J.A. Szpunar, Role of ball milling of aluminum powders in promotion of aluminum-water reaction to generate hydrogen, Metall. Mater. Trans. E 1 (2014) 247e256. [18] S.S. Razavi-Tousi, J.A. Szpunar, Effect of structural evolution of aluminum powder during ball milling on hydrogen generation in aluminumewater reaction, Int. J. Hydrogen Energy 38 (2013) 795e806. [19] M.-Q. Fan, F. Xu, L.-X. Sun, J.-N. Zhao, T. Jiang, W.-X. Li, Hydrolysis of ball milling AleBiehydride and AleBiesalt mixture for hydrogen generation, J. Alloy Compd. 460 (2008) 125e129. [20] K. Mahmoodi, B. Alinejad, Enhancement of hydrogen generation rate in reaction of aluminum with water, Int. J. Hydrogen Energy 35 (2010) 5227e5232. [21] J. Skrovan, A. Alfantazi, T. Troczynski, Enhancing aluminum corrosion in water, J. Appl. Electrochem. 39 (2009) 1695e1702. [22] L. Soler, J. Macanas, M. Munoz, J. Casado, Aluminum and aluminum alloys as sources of hydrogen for fuel cell applications, J. Power Sources 169 (2007) 144e149. [23] B. Alinejad, K. Mahmoodi, A novel method for generating hydrogen by hydrolysis of highly activated aluminum nanoparticles in pure water, Int. J. Hydrogen Energy 34 (2009) 7934e7938. [24] B. Alinejad, K. Mahmoodi, K. Ahmadi, A new route to mass production of
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[25] [26]
[27] [28] [29]
S.S. Razavi-Tousi, J.A. Szpunar / Journal of Alloys and Compounds 679 (2016) 364e374 metal hydroxide/oxide hydroxide nanoparticles, Mater Chem. Phys. 118 (2009) 473e476. K. Mahmoodi, B. Alinejad, Fast and facile synthesis of boehmite nanofibers, Powder Technol. 199 (2010) 289e292. E. Van Gheem, J. Vereecken, C. Le Pen, Influence of different anions on the behaviour of aluminium in aqueous solutions, J. Appl. Electrochem. 32 (2002) 1193e1200. Z. Szklarska-Smialowska, Pitting corrosion of aluminum, Corros. Sci. 41 (1999) 1743e1767. E. McCafferty, Sequence of steps in the pitting of aluminum by chloride ions, Corros. Sci. 45 (2003) 1421e1438. S.S. Razavi Tousi, R. Yazdani Rad, E. Salahi, I. Mobasherpour, M. Razavi, Production of Ale20 wt.% Al2O3 composite powder using high energy milling,
Powder. Technol. 192 (2009) 346e351. [30] S.S. Razavi Tousi, R. Yazdani Rad, E. Salahi, M. Razavi, Effect of milling time and addition of alumina powder on the structural properties and fracture surface of nanocrystalline Al, Mater. Sci. Pol. 27 (2009) 875e884. [31] L. Soler, A.M. Candela, J. Macanas, M. Munoz, J. Casado, Hydrogen generation from water and aluminum promoted by sodium stannate, Int. J. Hydrogen Energy 35 (2010) 1038e1048. [32] W. Wang, D.M. Chen, K. Yang, Investigation on microstructure and hydrogen generation performance of Al-rich alloys, Int. J. Hydrogen Energy 35 (2010) 12011e12019. [33] A.V. Parmuzina, O.V. Kravchenko, Activation of aluminium metal to evolve hydrogen from water, Int. J. Hydrogen Energy 33 (2008) 3073e3076.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of addition of water-soluble salts on the hydrogen generation of aluminum in reaction with hot water S.S. Razavi-Tousi*, J.A. Szpunar Department of Mechanical Engineering, University of Saskatchewan, S7N 5A9 Saskatoon, Saskatchewan, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 19 March 2016 Accepted 4 April 2016 Available online 7 April 2016
Aluminum powder was ball milled for different durations of time with different weight percentages of water-soluble salts (NaCl and KCl). The hydrogen generation of each mixture in reaction with hot water was measured. A scanning electron microscope (SEM) as well as energy-dispersive spectroscopy (EDS) were used to investigate the morphology, surfaces and cross sections of the produced particles. The results show that the presence of salts in the microstructure of the aluminum considerably increases the hydrogen generation rate. At shorter milling times, the salt covers the aluminum particles and becomes embedded in layers within the aluminum matrix. At higher milling durations, salt and aluminum phases form composite particles. A higher percentage of the second phase significantly decreases the milling time needed for activation of the aluminum particles. Based on the EDS results from cross sections of the milled particles, a mechanism for improvement of the hydrogen generation rate in the presence of salts is suggested. © 2016 Elsevier B.V. All rights reserved.
Keywords: Microstructure Mechanical milling Hydrogen Aluminum matrix Corrosion
1. Introduction Researchers have adopted different approaches to improving the kinetics of the reaction of aluminum with water for the purpose of hydrogen generation. Adding low melting point metals to aluminum, using basic solutions, applying supercritical water conditions, and ball milling in the presence of oxides or watersoluble salts are among the techniques used to improve the hydrogen generation rate [1e18]. Although most of these methods have been effective in improving the reaction, some reservations remain about their potential for large-scale application. For example, low melting point metals are expensive and most of them are toxic, the supercritical condition is not hardly applicable for onboard hydrogen generation, and the use of basic solutions raises safety and environmental concerns. Among different methods, ball milling of aluminum powder in the presence of water-soluble salts is considered effective and safe [9,19e25]. Water-soluble salts (NaCl, KCl, etc.) are neither expensive nor toxic and their presence significantly shortens the completion time of the reaction. The effects of water-soluble salts during milling on the reaction
* Corresponding author. E-mail addresses: [email protected], [email protected] (S.S. RazaviTousi). http://dx.doi.org/10.1016/j.jallcom.2016.04.038 0925-8388/© 2016 Elsevier B.V. All rights reserved.
kinetics and consequently on hydrogen generation have been explained in two ways: Firstly, some researchers point out that the reaction is promoted because of the Cl ions released by dissolving salts in water [3]. These Cl ions can deteriorate the protectiveness of the hydroxide layer on the surface of the particles [26e28], and thus the aluminum substrate is corroded more effectively. Secondly, the presence of salt particles can modify the structure of aluminum particles. Alinejad et al. consider that salt particles modify the structure of aluminum particles by providing more surfaces for the reaction. They suggest a schematic model to illustrate how the presence of salt particles promotes the reaction [23]. In this model, they assume that salt particles cover the aluminum particles during ball milling. As the mixture is exposed to water, salt particles are dissolved and a fresh surface of aluminum is exposed to water. In spite of the number of works that have used water-soluble salts to activate aluminum, no study has attempted to distinguish whether their addition affects the chemical reaction or the structure of particles. The structural and chemical effects can be separated by introducing a particular amount of a salt during ball milling or adding it to the hot water during the reaction and then measuring the hydrogen production rate for either case. In this study, the hydrogen generation of pure aluminum was firstly compared with that of an aluminum-salt mixture. Then, the microstructure of the aluminum-salt particles was studied after
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different milling times and the results correlated to hydrogen generation. Finally, the effects of the different salt types (NaCl or KCl) were examined.
2. Experimental procedure 2.1. Ball milling Alfa Aeasar Company provided aluminum powder of 99.8% purity (MFCD00134029, average particle size of 227 mm). After mixing 50 wt% of sodium chloride (purity: þ99%, average particle size of 281 mm) with the aluminum powder, the mixture was ball milled. High-energy ball milling was done in a planetary ball mill (Torrey Hills- ND2L) with stainless steel cups (285 ml capacity) and balls (28 of 16 mm and 6 of 18 mm diameter) in an argon atmosphere. The ball to powder ratio was 30:1 and the mill speed was maintained at 200 RPM for durations of 0.25, 0.5, 1, 2, 4, 7, 11 and 19 h. In order to evaluate the effect of the weight percentage and type of salts, mixtures of 25 wt% NaCl- 75 wt% Al, 75 wt% NaCl- 25 wt% Al and 50 wt% KCl- 50 wt% Al were also ball milled for durations of 2, 7 and 19 h. The KCl powder, with a commercial purity, was refined from Potash Corporation of Saskatchewan products.
2.2. Hydrogen measurement
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3. Results and discussion 3.1. Effect of milling time and presence of salt 3.1.1. Hydrogen generation Fig. 1(a) shows the effect of milling time on the hydrogen generation of aluminum-salt mixtures through their reaction with hot water. Hydrogen generation was not observed for the mixture of aluminum-salt that was not ball milled, thus it is not reported in Fig. 1(a). Milling the mixture for durations as short as 15 or 30 min resulted in measurable hydrogen generation. Increasing milling time from 1 h up to 7 h significantly enhanced the hydrogen generation rate. However, further milling from 7 h up to 19 h did not appreciably increase the amount of generated hydrogen as the amount for the 7 h sample was already close to the theoretical limit. Fig. 1(b) presents the hydrogen generation results from the pure aluminum powder milled for the same durations, which are published in Ref. [18]. Since the starting material, milling, and the hydrogen measurement conditions were the same for both experiments, the comparison of Fig. 1(a) and (b) illustrates the effect of the presence of sodium chloride on the hydrogen generation rate. Clearly, the aluminum milled in the presence of salt reacted significantly faster than the pure aluminum; the maximum amount of produced hydrogen in one hour increased from 230 ml to 1200 ml. Nevertheless, it is significant that the presence of salt without milling was ineffective, as neither pure aluminum nor an aluminum-salt mixture could generate hydrogen in the as-received
The details of the hydrogen measurement procedure are similar to those described in Ref. [18]. We added powder mixtures with the equivalent of one g of aluminum in to an Erlenmeyer flask containing 200 ml of distilled water at 80 ± 3 C with a constant stirring rate of 120 rpm. The produced hydrogen gas passed through a desiccant (CoCl2) to absorb moisture, and then an ADM2000 flowmeter to measure its flow with an accuracy of 0.1 ml/min. The flowmeter was connected to a computer running ADM Trend software to acquire the data. A baseline curve was obtained by measuring the flow from 200 ml of distilled water at 80 C with a constant stirring rate of 120 rpm with no powder added. This was then subtracted from the data obtained from the reactions to ensure that the measurements did not include any contribution from water moisture or expansion of the air in the flask during heating.
2.3. Scanning electron microscopy A Hitachi SU6600 field emission scanning electron microscope (SEM) was employed to examine the surfaces and cross sections of the powders. A small amount of each powder was added to a conductive carbon resin powder, mixed and mounted. The mounted samples were polished using abrasive grinding papers from 600 to 2000 grit followed by additional grinding with diamond pastes of 3 and 0.04 mm. Particles sizes of the powders were calculated using software written in Visual Basic using several SEM images. The weighted average (Eq. (1)) was calculated and reported as the average particle size:
P i ni Di D¼ P i ni
(1)
where Di is the diameter of the particle i, ni is the number of particles with size Di, and D is the weighted average.
Fig. 1. Effect of milling time on hydrogen generation of (a) the aluminum-salt mixture, and (b) pure aluminum during the reaction with hot water.
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state. This result implies that in the time span of these experiments (1 h of reaction), the effect of Cl ions on improving the corrosion of aluminum by hot water was negligible. Moreover, the presented results establish that ball milling cannot be sufficiently effective without the presence of a water-soluble salt nor can the presence of the salt be effective without milling. Thus, a strong synergy exists between the milling process and the presence of a salt. 3.1.2. Microstructural characterization In order to realize the mechanism behind the improvement of the reaction rate by milling, SEM was used to examine the microstructures of the milled powders. Fig. 2 shows secondary electron SEM images (100) of the as-received mixture and the mixture milled for durations of 15 min, 30 min, 1hr, 2hr, 4hr, 7hr, 11hr and 19 h. The as-received powder was a mixture of cubic shaped salt crystals and irregularly shaped aluminum particles. Milling for 15 min significantly changed the morphology of the mixture; salt crystals were crushed into much smaller particles while aluminum particles were deformed and flattened without a noticeable change in their size (Daverage, 15 min ¼ 224 mm). Milling from 30 min (Daverage, 30 min ¼ 208 mm) up to 1 h, 2 h and 4 h gradually decreased the aluminum particles' sizes. The particle sizes for the 1 h, 2 h and 4 h milled powders were 76 mm, 50 mm and 23 mm, respectively. For the mixtures milled for 15 min or 30 min, it was still possible to distinguish between the salt and aluminum particles and the measured particle size corresponded to Al particles. This was not
the case for the mixtures milled for more than 1 h where the measured particle sizes were obtained from the Al-salt composite particles. As Fig. 2(f)e(i) show, milling from 4 h up to 19 h did not result in any significant change in the sizes of particles. The particle sizes for the 7 h, 11 h and 19 h milled powders were 22 mm, 22 mm and 21 mm, respectively. The trend observed in Fig. 2 for reduction of the particle size is not noticeably different from that of the pure aluminum milled for the same durations and reported as Fig. 1 in Ref. [18]. Particularly for the powders milled more than 4 h, the shapes and sizes of the particles were similar for the pure aluminum and the aluminumsalt mixture. On the other hand, comparison of the hydrogen generation of the 7 h, 11 h and 19 h milled powders (Fig. 1) shows the hydrogen generation rate markedly increased in the presence of the salt during milling. The increased hydrogen rate, in spite of the rather similar particle sizes, implies there were other factors affecting hydrogen generation in addition to the size of particles. Comparison of Fig. 1(a) and (b) reveals an obvious difference between the effect of milling time on hydrogen production in the presence and absence of salt. The 7 h milled sample produced the largest amount of hydrogen among the milled pure aluminum powders. Milling more or less than 7 h caused a reduction in the produced hydrogen. In our previous work, this phenomenon was explained thoroughly based on the microstructural evolution of aluminum powder during milling [18]. The microstructure of the 7 h milled particles was laminated, which increased the surface
Fig. 2. Secondary electron SEM images ( 100) of (a) the as received mixture and the mixtures milled for (b) 15 min, (c) 30 min, (d) 1 h, (e) 2 h, (f) 4 h, (g) 7 h, (h) 11 h and (i) 19 h.
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Fig. 3. (a) The higher magnification image of structure within the rectangle marked in Fig. 2(b), (b) the higher magnification image of structure within the rectangle marked in Fig. 2(c), (c) the higher magnification image of structure within the rectangle marked in Fig. 2(e), (d) the higher magnification image of the structure marked in Fig. 3(c). Fig. 3(e), 3(f) and 3(g) high magnification images of the particles milled for 7 h, 11 h and 19 h, respectively. Fig. 3(h) The 40 K magnification micrograph from the surface of the particle in Fig. 3(g).
Fig. 4. XRD patterns of the as received mixture and the mixture milled for durations of 15 min, 30 min, 1 h, 2 h, 4 h, 7 h, 11 h and 19 h.
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area of the powder, and in turn this microstructure improved the kinetics of the reaction. This phenomenon was not the case when a salt phase was present during milling. Fig. 1(a) shows that longer millings generally improved the kinetics of the reaction and there was no milling time following which the hydrogen production declined. Nevertheless, further milling after 7 h did not have a remarkable effect on hydrogen production, implying that the microstructural evolution resulted by milling reached a steady state
after 7 h and further milling was not beneficial to the reaction. This comparison reveals that the optimum milling times for the pure aluminum and the aluminum-salt mixtures are based on different paths of microstructural evolution. Fig. 3 shows high magnification images of the milled mixtures. Fig. 3(a) shows the higher magnification image of the rectangle marked in Fig. 2(b) for the 15 min milled sample. Such a short milling time initially crushed the salt crystals and formed
Fig. 5. SEM images and the corresponding element distribution maps obtained by EDS from the surface of the particles milled for (a) 30 min, (b) 1 h, (c) 2 h, (d) 7 h and (e) 11 h. The red and green colors represent the presence of aluminum and chlorine, respectively. Please note the change in the scale bar. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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aggregates composed of much smaller particles. This significant reduction in particle size is explained by the brittle nature of salt, while aluminum particles were deformed but not fractured. Fig. 3(b) shows the higher magnification image of structure within the rectangle marked in Fig. 2(c) for a particle milled for 30 min. Fine salt particles can be seen covering the surface of the aluminum
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particle. Fig. 3(c) shows the higher magnification image of structure within the rectangle marked in Fig. 2(e) for a particle milled for 2 h Fig. 3(d) shows the higher magnification image of structure marked in Fig. 3(c). The sizes of the salt particles covering the aluminum particles were in the range of 100e500 nm. The as-received salt particles were 200e300 mm, so milling for 2 h reduced their size by
Fig. 6. SEM images and the corresponding element distribution maps obtained by EDS from the cross section of the particles milled for (a) 15 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h and (f) 7 h. Please note the change in the scale bar.
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almost three orders of magnitude. Such a remarkable size reduction is also reflected in X-ray diffraction patterns. Fig. 4 shows the XRD patterns of the as-received mixture and the mixture milled for durations of 15 min, 30 min, 1hr, 2hr, 4hr, 7hr, 11hr and 19 h. The graph shows that ball milling from 15 min up to 19 h did not cause any reaction or formation of new phases. Nevertheless, a significant line broadening occurred for the salt phase during the first 1e2 h of milling. This line broadening is explained by the crystallite size reduction. As Fig. 3(a)e(d) show, milling initially crushed the salt particles, which then covered the aluminum particles after further milling. Coating of aluminum particles by smaller salt particles was first noted by Alinejad et al. [23]. They discuss that the presence of salt at the surface of aluminum particles prevents formation of an oxide layer. As the mixture is exposed to water, the salt is dissolved and fresh aluminum surface reacts with water, which explains the improvement of the hydrogen generation in the presence of a water-soluble salt. Their argument is correct, but it cannot be the only mechanism explaining the role of salt. SEM examinations revealed that the aluminum particles in all mixtures milled for 15 min, 30 min, 1 h and 2 h were covered by salt but they performed differently in hydrogen generation. Moreover, as Fig. 3(e)e(g) show, the concept of salt coating cannot accurately be the case for the mixtures milled for longer durations. That is because, based on SEM images presented in Fig. 3, a salt layer did not cover an aluminum core for the 7 h, 11 h and 19 h milled particles, but the particles are a mixture of the aluminum and salt phases. Fig. 3 (h) shows the typical microstructure observed from the higher magnification micrographs of the particles milled for longer periods (7 h, 11 h and 19 h). It shows that each particle itself is formed by agglomeration of smaller particles with a size of 0.1e3 mm. Such a fine microstructure explains the improved kinetics and high affiliation of milled particles to react with water. In order to reveal the mechanisms explaining the effect of salt on the reaction rate, we analyzed the surfaces and cross sections of the milled mixtures by EDS. Fig. 5 shows SEM images and the corresponding element distribution maps obtained by EDS from the surface of the particles milled for 30 min, 1 h, 2 h, 7 h and 19 h. The red and green colors represent the presence of aluminum and chlorine, respectively. Fig. 5(a) shows that a layer of salt covered the aluminum particle from the mixture milled for 30 min. The layer of salt was thick enough to prevent detecting the presence of aluminum except just at the edges of the particle where the salt was thinner or absent. The map also shows that the small particles around the bigger one were salt particles. Fig. 5(b) shows a similar result for the particle milled for 1 h; there was a significant amount of salt on the aluminum particle. Nevertheless, EDS from the particle milled for 2 h shows a different map of elements (Fig. 5(c)). Although the presence of salt was still strong, a marked presence of aluminum was also detected. The presence of aluminum in EDS maps implies that the salt layer did not cover the particle completely or the layer was thin enough to allow detection of aluminum. Further milling up to 7 h and 19 h resulted in the uniform presence of salt and aluminum at the surface of the particles (Fig. 5(d) and (e)). Fig. 6 shows SEM images and the corresponding element distribution maps obtained by EDS from the cross sections of the particles milled for 15 min, 30 min, 1 h, 2 h, 4 h and 7 h. The results from 11 h to 19 h milled particles are not reported here as they are similar to those of 4 h and 7 h milled particles. The red and green colors represent the presence of aluminum and chlorine, respectively. The blue color represents the presence of carbon from the carbon resin used for mounting the particles. Fig. 6(a) shows that after 15 min milling, thick layers of salt covered the deformed aluminum particles. Further milling up to 30 min and 1 h (Fig. 6(b)
and (c)) resulted in production of particles formed by layers of aluminum and salt, which are clearly distinguishable by the resolution of the EDS. Fig. 6(d) shows that milling up to 2 h combined the aluminum and salt and produced composite particles with a fine distribution of phases. Prolonged milling up to 4 h and 7 h (Fig. 6(e) and (f)) generated a very fine distribution of salt and
Fig. 7. EDS line-scans from the cross section of particles milled for (a) 15 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h, (f) 7 h, (g) 11 h and (h) 19 h. Please note the change in the scale bar.
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aluminum phases in each particle. The 11 h and 19 h milled mixtures showed similar results. In each composite particle milled for more than 4 h, the size of salt and aluminum constituents was small enough that the resolution of the EDS maps cannot distinguish between the different phases. Fig. 7 shows the EDS line-scans from the cross sections of particles milled for 15 min, 30 min, 1 h, 2 h, 4 h, 7 h, 11 h and 19 h. The red and green lines represent the presence of aluminum and chlorine, respectively. Each scan was obtained from the cross section of one particle. Thick layers of aluminum and salt are visible in each scan of particles milled for shorter times. As milling continued, the layers became thinner and a greater presence of salt and aluminum phases is visible along the scanned line at the cross sections of the particles. Considering the change in the scale, it is clear that the thickness of salt and aluminum phases changed from around 100 mm for the 15 min milled particle to 1e2 mm for the 19 h milled particle. These line scans confirm the results obtained by EDS maps, which reveal that increasing the milling time results in a homogeneous distribution of very fine salt and aluminum segments. Considering Figs. 5e7, the mechanism behind the synergic effect of milling and the presence of salt on the hydrogen generation rate can now be explained. Ball milling produced composite particles formed by the aggregation of salt and aluminum phases. As the composite particles were exposed to hot water, the salt was dissolved and aluminum particles remained with many voids and tunnels left because of the dissolution of salt. That porous structure of aluminum particles provided a significant quantity of fresh surfaces for reaction with hot water. The longer the milling time, the finer the aluminum and salt phases were, which resulted in a higher specific surface area of aluminum phase after dissolution of the salt. In order to confirm this observation, the 19 h milled mixture was treated with glycerin. A small amount of the 19hr milled powder was added to a liquid carbon paste, which was dried by hot air trapping many particles into its surface. The mounted particles on the resin were treated in 100 ml of glycerin for 1 h and then the remaining powder was washed with ethanol and dried. Glycerin dissolves salt but does not react with aluminum; thus the microstructure of the aluminum phase without salt could be revealed. The reason for using the carbon mount was to hold together the loose structure of the particles after the salt was removed from it. Fig. 8(a) shows a few 19 h milled particles after the treatment with glycerin. Fig. 8(b), a higher magnification of the marked rectangle in Fig. 8(a), shows that the remaining aluminum structure was formed by the aggregation of small particles with a noticeable amount of porosity in the microstructure. Fig. 8(c) shows an aluminum particle milled for 19 h without the addition of any salt. The
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comparison establishes that the presence of salt during milling can change an almost pore-free microstructure to a significantly porous structure. These pores provide space for water to penetrate into a particle and result in a significant increase in the hydrogen generation rate.
3.2. Effect of weight percentage and type of the water-soluble salt Fig. 9(a) and (b) show the hydrogen generation of the 25 wt% and 75 wt% salt-aluminum mixtures milled for 2hr, 7hr and 19 h, respectively. All results are normalized for reaction of 1 g of aluminum. Clearly, the hydrogen generation of the 75 wt% saltaluminum mixture was considerably faster compared to the 50 wt% salt mixture (Fig. 1(a)). For the 19 h milled sample with 75 wt% of salt, the reaction was completed in only 5 min. However, the 25 wt% salt-aluminum mixture reacted slower with water: In one hour, around half of the reaction (650 ml compared to the 1270 ml for the full reaction) was completed. This suggests that the higher amount of salt in the mixture promoted the reaction rate significantly. This effect is explained by a higher quantity of pores in the microstructure of the aluminum particles as the salt phase dissolved in hot water. Accordingly, a higher specific surface area was obtained at a higher salt percentage, which improved the kinetics of hydrogen generation. Potash powder (KCl) with a ratio of 50:50 was also added to aluminum during ball milling, and the hydrogen generation of the mixture in reaction with hot water was measured. Fig. 9(c) shows the hydrogen generation of the aluminum-potash mixture milled for 2 h, 7 h and 19 h. The data from 50 wt% salt mixture was also added as the dotted lines for comparison. As Fig. 9(c) shows, 7 h of milling was enough to allow the reaction to complete in 4 min, and ball milling over 7 h had no effect on the hydrogen generation rate. It is clear that KCl had a noticeably better effect on hydrogen generation compared to NaCl. Indeed, adding 50 wt% of KCl to aluminum worked even better than adding 75 wt% of NaCl. Considering that NaCl and KCl are both relatively affordable and abundant materials, KCl has an obvious advantage over NaCl. That is because a shorter period of milling is needed for the activation, which in turn saves energy and reduces the final cost of the produced hydrogen. Moreover, for a given amount of the produced hydrogen, a lower amount of the water-soluble salt in the mixture (50% for KCl compared to 75% for NaCl) means a lighter hydrogen fuel for vehicles. SEM was used to examine the milled mixtures in order to study the effect of weight percentage and type of water-soluble salts on the morphology of particles. Fig. 10 shows the SEM images of the AleNaCl (25 wt% of salt), AleNaCl (75 wt% of salt) and AleKCl (50 wt%) mixtures milled for 2 h, 7 h and 19 h. The AleNaCl (25 wt
Fig. 8. (a) 19 h milled particles after the treatment with glycerin, (b) the higher magnification of the marked rectangular in Fig. 8(a), (c) an aluminum particle milled for 19 h without addition of salt.
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measured particle sizes for the 2 h, 7 h and 19 h milled powders were 22 mm, 17 mm and 16 mm, respectively. Comparing the mixture of 25 wt% salt to 75 wt%, a higher amount of salt clearly had a marked effect on the particle sizes. This difference is explained by the effect of salt in preventing the cold welding of aluminum particles. Unlike aluminum particles that are ductile and can be cold welded by impacts [29,30], salt is a brittle phase and promotes fracture rather than cold welding during milling. Accordingly, as Fig. 10 shows, an increase in salt to 75 wt% resulted in a reduction in the particle sizes of the powders during milling. A smaller particle size in turn meant a higher specific surface area, which improved the reaction kinetics. On the other hand, the effect of 50 wt% KCl compared with 75 wt% of NaCl produced a negligible difference on the morphology of the powders in the mixtures. That means the effect of 50 wt% of KCl on the reduction in particle size is similar to that of 75 wt% of NaCl. The improvement related to the use ofKClis also evident from the results of hydrogen generation (Fig. 9). One reason for the better performance of KCl might be its higher hardness compared to NaCl (KCl:2.5, NaCl:2, on the Mohs scale). The higher hardness means KCl is more brittle and fractures to smaller segments by impacts, which can result in finer particles after milling and consequently a higher hydrogen generation rate. Nevertheless, further investigation is required to establish the explanation for the superiority of KCl over NaCl in facilitating the aluminum-water reaction. The AleKCl composition milled for 7 h or 19 h achieved a complete yield after 6e9 min, which demonstrates a fast reaction compared to the other methods of activation. For example, Soler et al. measured the hydrogen yield of Al/Co alloy in 1 M KOH solution at room temperature and found that 80% of the reaction was completed in 20 min [22]. They also found that the Al/Si alloy in hot saturated Ca(OH)2 solution yielded 80% after 100 min, and aluminum powder with a 0.075 M Na2SnO3solution measured 80% of yield after 80 min [31]. Fan et al. milled aluminum with both MgCl2 and Bi, and observed an almost complete reaction after 8 min [2]. Wang et al. obtained 100% yield between 5 and 100 min for the aluminum alloyed with over 50 atom% of In/Ga/Sn metals [32], while Parmuzina observed 100% yield after 150e500 min [33]. The above mentioned works prove that milling aluminum powder with water-soluble salts is not only a more effective activation method than others but also is safer and more affordable.
4. Conclusion
Fig. 9. Hydrogen generation of the (a) 25 wt% salt-aluminum, (b) 75 wt% saltaluminum, and (c) 50 wt% aluminum-potash mixture milled for 2 h, 7 h and 19 h. The dotted lines in Fig. 9(c) represent the 50 wt% salt results.
%) mixture had the biggest particle size for all milling durations. The measured particle sizes for the 2 h, 7 h and 19 h milled AleNaCl (25 wt% of salt) powders were 77 mm, 24 mm and 23 mm, respectively. For the AleNaCl (75 wt% of salt) powder, particle sizes of 20 mm, 20 mm and 18 mm were measured for the 2 h, 7 h and 19 h milled powders, respectively. Regarding the AleKCl mixtures, the
The presence of salt during ball milling had a significant role in improving the hydrogen generation rate of aluminum in reaction with hot water. Considering that the salt had no effect on hydrogen generation without ball milling, the improvement is explained by the changes in the structure of particles rather than the presence of Cl ions. SEM examination shows that the refinement of the powder during milling was improved by the presence of salt particles. EDS results establish that during ball milling, salt particles were embedded inside aluminum particles. Salt particles dissolve upon immersion in hot water, leaving many voids/tunnels for water to penetrate inside aluminum particles, which consequently improved the kinetics of the oxidation reaction. This explanation is also supported by the fact that a higher weight percentage of the salt in the aluminum matrix resulted in higher hydrogen production. Finally, measured data show that using different water-soluble salts (KCl or NaCl) resulted indifferent rates of hydrogen production. An explanation for the effect of different water-soluble salts on the hydrogen generation rate merits further study.
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Fig. 10. SEM images of the AleNaCl (25 wt% of salt) powder milled for (a) 2 h, (b) 7 h, (c) 19 h, and the AleNaCl (75 wt% of salt) powder milled for (d) 2 h, (e) 7 h, (f) 19 h, and AleKCl (50 wt%) powder milled for (g) 2 h, (h) 7 h and (i) 19 h at 100 magnification.
References [1] M.Q. Fan, L.X. Sun, F. Xu, Study of the controllable reactivity of aluminum alloys and their promising application for hydrogen generation, Energy Convers. Manage 51 (2010) 594e599. [2] M.Q. Fan, F. Xu, L.X. Sun, Studies on hydrogen generation characteristics of hydrolysis of the ball milling Al-based materials in pure water, Int. J. Hydrogen Energy 32 (2007) 2809e2815. [3] M.Q. Fan, F. Xu, L.X. Sun, Hydrogen generation by hydrolysis reaction of ballmilled Al-Bi alloys, Energy Fuel 21 (2007) 2294e2298. [4] M.-q. Fan, L.-x. Sun, F. Xu, Feasibility study of hydrogen production for micro fuel cell from activated AleIn mixture in water, Energy 35 (2010) 1333e1337. [5] A.V. Ilyukhina, O.V. Kravchenko, B.M. Bulychev, E.I. Shkolnikov, Mechanochemical activation of aluminum with gallams for hydrogen evolution from water, Int. J. Hydrogen Energy 35 (2010) 1905e1910. [6] A.V. Parmuzina, O.V. Kravchenko, B.M. Bulychev, E.I. Shkolnikov, A.G. Burlakova, Oxidation of activated aluminum with water as a method for hydrogen generation, Russ. Chem. Bull. 58 (2009) 493e498. [7] P. Dupiano, D. Stamatis, E.L. Dreizin, Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders, Int. J. Hydrogen Energy 36 (2011) 4781e4791. [8] Z.Y. Deng, Y.B. Tang, L.L. Zhu, Y. Sakka, J.H. Ye, Effect of different modification agents on hydrogen-generation by the reaction of Al with water, Int. J. Hydrogen Energy 35 (2010) 9561e9568. [9] E. Czech, T. Troczynski, Hydrogen generation through massive corrosion of deformed aluminum in water, Int. J. Hydrogen Energy 35 (2010) 1029e1037. [10] Z.Y. Deng, J.M.F. Ferreiraw, Y. Tanaka, J.H. Ye, Physicochemical mechanism for the continuous reaction of gamma-Al2O3-modified aluminum powder with water, J. Am. Ceram. Soc. 90 (2007) 1521e1526. [11] Z.-Y. Deng, Y.-F. Liu, Y. Tanaka, J. Ye, Y. Sakka, Modification of Al particle surfaces by c-Al2O3 and its effect on the corrosion behavior of Al, J. Am. Ceram. Soc. 88 (2005) 977e979. [12] A.C.D. Chaklader, Hydrogen generation from water split reaction, U.S. Patent,
6,582,676 B2 (2003). [13] A.N. Streletskii, I.V. Kolbanev, A.B. Borunova, P.Y. Butyagin, Mechanochemical activation of aluminum: 3. Kinetics of interaction between aluminum and water, Colloid J. 67 (2005) 631e637. [14] H.W. Wang, H.W. Chung, H.T. Teng, G.Z. Cao, Generation of hydrogen from aluminum and water - effect of metal oxide nanocrystals and water quality, Int. J. Hydrogen Energy 36 (2011) 15136e15144. [15] X.-N. Huang, C.-J. Lv, Y. Wang, H.-Y. Shen, D. Chen, Y.-X. Huang, Hydrogen generation from hydrolysis of aluminum/graphite composites with a coreeshell structure, Int. J. Hydrogen Energy 37 (2012) 7457e7463. [16] X.N. Huang, C.J. Lv, Y. Wang, H.Y. Shen, D. Chen, Y.X. Huang, Hydrogen generation from hydrolysis of aluminum/graphite composites with a core-shell structure, Int. J. Hydrogen Energy 37 (2012) 7457e7463. [17] S.S. Razavi-Tousi, J.A. Szpunar, Role of ball milling of aluminum powders in promotion of aluminum-water reaction to generate hydrogen, Metall. Mater. Trans. E 1 (2014) 247e256. [18] S.S. Razavi-Tousi, J.A. Szpunar, Effect of structural evolution of aluminum powder during ball milling on hydrogen generation in aluminumewater reaction, Int. J. Hydrogen Energy 38 (2013) 795e806. [19] M.-Q. Fan, F. Xu, L.-X. Sun, J.-N. Zhao, T. Jiang, W.-X. Li, Hydrolysis of ball milling AleBiehydride and AleBiesalt mixture for hydrogen generation, J. Alloy Compd. 460 (2008) 125e129. [20] K. Mahmoodi, B. Alinejad, Enhancement of hydrogen generation rate in reaction of aluminum with water, Int. J. Hydrogen Energy 35 (2010) 5227e5232. [21] J. Skrovan, A. Alfantazi, T. Troczynski, Enhancing aluminum corrosion in water, J. Appl. Electrochem. 39 (2009) 1695e1702. [22] L. Soler, J. Macanas, M. Munoz, J. Casado, Aluminum and aluminum alloys as sources of hydrogen for fuel cell applications, J. Power Sources 169 (2007) 144e149. [23] B. Alinejad, K. Mahmoodi, A novel method for generating hydrogen by hydrolysis of highly activated aluminum nanoparticles in pure water, Int. J. Hydrogen Energy 34 (2009) 7934e7938. [24] B. Alinejad, K. Mahmoodi, K. Ahmadi, A new route to mass production of
374
[25] [26]
[27] [28] [29]
S.S. Razavi-Tousi, J.A. Szpunar / Journal of Alloys and Compounds 679 (2016) 364e374 metal hydroxide/oxide hydroxide nanoparticles, Mater Chem. Phys. 118 (2009) 473e476. K. Mahmoodi, B. Alinejad, Fast and facile synthesis of boehmite nanofibers, Powder Technol. 199 (2010) 289e292. E. Van Gheem, J. Vereecken, C. Le Pen, Influence of different anions on the behaviour of aluminium in aqueous solutions, J. Appl. Electrochem. 32 (2002) 1193e1200. Z. Szklarska-Smialowska, Pitting corrosion of aluminum, Corros. Sci. 41 (1999) 1743e1767. E. McCafferty, Sequence of steps in the pitting of aluminum by chloride ions, Corros. Sci. 45 (2003) 1421e1438. S.S. Razavi Tousi, R. Yazdani Rad, E. Salahi, I. Mobasherpour, M. Razavi, Production of Ale20 wt.% Al2O3 composite powder using high energy milling,
Powder. Technol. 192 (2009) 346e351. [30] S.S. Razavi Tousi, R. Yazdani Rad, E. Salahi, M. Razavi, Effect of milling time and addition of alumina powder on the structural properties and fracture surface of nanocrystalline Al, Mater. Sci. Pol. 27 (2009) 875e884. [31] L. Soler, A.M. Candela, J. Macanas, M. Munoz, J. Casado, Hydrogen generation from water and aluminum promoted by sodium stannate, Int. J. Hydrogen Energy 35 (2010) 1038e1048. [32] W. Wang, D.M. Chen, K. Yang, Investigation on microstructure and hydrogen generation performance of Al-rich alloys, Int. J. Hydrogen Energy 35 (2010) 12011e12019. [33] A.V. Parmuzina, O.V. Kravchenko, Activation of aluminium metal to evolve hydrogen from water, Int. J. Hydrogen Energy 33 (2008) 3073e3076.