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Experimental study on the effect of low melting point metal additives on hydrogen production in the aluminum–water reaction
Energy 88 (2015) 537e543
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Experimental study on the effect of low melting point metal additives on hydrogen production in the aluminumewater reaction Weijuan Yang*, Tianyou Zhang, Junhu Zhou, Wei Shi, Jianzhong Liu, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 13 April 2015 Accepted 22 May 2015 Available online 15 June 2015
Aluminum (Al) is a promising hydrogen carrier. Continuous reaction of pure Al and water (H2O) cannot proceed smoothly because Al particles are covered with a protective oxide layer. Thus, 20% Mg, Li, Zn, Bi, and Sn content were added as additives to AleH2O reaction at high temperature. Thermogravimetric experiments were conducted to determine the reactivity of pure Al and five other samples with additives in a vapor atmosphere. Experiments indicated that Mg and Li drove the AleH2O reaction, but Zn, Bi, and Sn had little effect. Thus, Mg and Li were selected as activators in the hydrogen generation of the AleH2O reaction conducted on a specially designed experimental facility. Hydrogen was monitored in the reaction of Al-based composites with H2O vapor in real time. Among them, Ale20%Li achieved the fastest hydrogen generation rate (309.74 ml s 1 g 1) and the largest hydrogen amount (1038.9 ml g 1). XRD (Xray diffraction), SEM (scanning electron microscopy), and TEM (transmission electron microscopy) were used for product analyses to identify the influence of adding Mg and Li. This method of Al energy utilization may be used in underwater propulsion systems. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Aluminum Low melting point metals Hydrogen generation Hydrolysis
1. Introduction Hydrogen, as a renewable and environmentally friendly fuel with high heating value, has gained extensive research attention [1,2]. This gas is in urgent demand because of the air pollution caused by the combustion of fossil fuels. However, hydrogen storage is one of the key challenges in developing a hydrogen economy [3]. Aluminum (Al) is recognized as one of the most suitable hydrogen carriers because of its high hydrogen capacity of 0.11 g/g [4]. Al and its alloys have been studied as sources of hydrogen [1,4e7]. In neutral conditions, the release of hydrogen was observed through cutting, drilling, or grinding of freshly exposed Al surfaces in water (H2O) [8,9]. However, this reaction soon stopped because of the rapid passivation of the Al surface [9]. The main problem in this method is the passive surface oxide film and its by-products [e.g., Al(OH)]. Removing the protective film is the key to overcoming this problem. Metal particles with small sizes and mechanical treatments, such as ball milling, are investigated to increase the specific surface area and pitting corrosion on Al
* Corresponding author. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.energy.2015.05.069 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
[4,10e13]. Hydroxide ions (OH 1) in alkaline solutions can destroy the protective oxide layer on the Al surface. The most commonly used hydroxide is sodium hydroxide (NaOH) [4]. Other hydroxides are also used as the reacting base, such as potassium hydroxide and calcium hydroxide [14,15]. Milling Al with different oxide modifiers, including Bi2O3, Cr2O3, MoO3, TiO2, and ZnO, has proven to be an effective method to improve reaction efficiency [16e18]. One of the most commonly used methods to promote AleH2O reaction is chemical activation through the modification of the composition of Al alloys. Gallium and its liquid alloys preclude the formation of a protective oxide film on Al and improve Al activity [19]. AleCa alloy was also used in hydrogen production, in which hydrogen yield was 47.87% when Ca content was 20% [20]. A small fraction of Li enables a spontaneous reaction of activated Al particles with H2O and results in an almost 100% yield of hydrogen generation [21]. The AleIneZnesalteH2O system can produce hydrogen at room temperature. The Ale5%Ine3%Zne2%NaCl mixture in H2O has the highest hydrogen yield of 1035 ml/g in 4 min when the ball milling time is 10 h [22]. Hydride is added to Al in the process of ball milling, which greatly improves the hydrolysis of Al powder [23]. The formation of LiAl2(OH)7$xH2O proves to be the key in enhancing the hydrogen generation of AleH2O reaction. Hydrogen generation from the reaction of AleBieNaCl and H2O is used in a portable hydrogen generator for fuel cell applications [24].
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Different metals, such as Zn, Bi, Mg, and Sn, are added to Al to react with pure H2O. AleBi alloy has a faster hydrolysis rate than other alloys at room temperature [25]. The reason behind the improved hydrolysis of Al through the addition of Bi is the micro-galvanic cell formed between the anode (Al) and the cathode (Bi). The reaction of AleH2O is exothermic (15.4 kJ/g), and this heat can hardly be utilized in aqueous solutions at room temperature. This study focused on the reaction of molten Al with H2O vapor at an elevated temperature. In this hydrogen generation method, the produced hydrogen and released heat can be utilized simultaneously. Thus, the whole energy utilization efficiency of the system will be improved. The different additives, such as Li, Mg, Zn, Bi, and Sn, were selected as activators because of their role in promoting the AleH2O reaction in the aqueous solutions of previous studies. These metals and their oxides are all nontoxic. Their properties are listed in Table 1. The experiments on thermogravimetric analyzer and special setup were conducted to study the effects of lowmelting-point metal on Al reactivity. XRD (X-ray diffraction), SEM (scanning electron microscopy), and TEM (transmission electron microscopy) were used to analyze the products and to explore the activation mechanism.
Table 2 Samples used for the experiments. Reagents
Supplier
Mean particle size
Purity (%)
Al AleLi alloy Mg
Aladdin Zhoushan Copper Sinopharm Chemical Reagent Corp Alfa Aesar Sinopharm Chemical Reagent Corp Shanghai HUSHI
10 mm 45 mm 45 mm
99.9 99.0 99.9
45 mm 45 mm
99.0 99.5
45 mm
99.5
Zn Bi Sn
2. Experimental All the reagents are listed in Table 2, including the suppliers, particle sizes, and chemical purity. Deionized water was used in the experiments. Different composites were made through hand mixing, except for the AleLi mixture. Different Li contents were prepared through mixing Al and AleLi alloy (Ale20%Li). Experiments were conducted with a high-temperature thermogravimetric analyzer (CAHN THERMAX500, Thermo Electron Corporation, USA) to investigate the reactivity of different Al-based samples at a heating rate of 25 K/min up to 1000 C. The samples (10 mg) were placed in a crucible, which was in the argon atmosphere. The flux of argon and H2O vapor were 500 and 100 ml/min respectively. The samples with relatively high activities were used to generate hydrogen in a PDEF (particularly designed experimental facility, Fig. 1). The stainless steel reactor was cylindrical with an inner diameter of 5 mm and a height of 10 mm. The reactor was kept sealed, and the argon was sent into the reactor to exclude air. Samples (1.5 g) were placed in the reactor and heated to 700 C to ensure they were in a molten state. A K-type thermocouple that was anchored to the reactor and inserted in the middle of the sample monitored the temperature inside the reactor. To avoid oxidation and hydrolysis of the sample, only argon was fed into the reactor when heating the sample. Once the temperature rose to 700 C, H2O (5 ml/min) was pumped into the reactor using an injection pump. The flux of water was controlled through the linear velocity of injector with an error less than 0.5%. H2O and argon were heated to approximately 200 C after they mixed, and then flowed into the reactor. Upon being exposed to H2O vapor, samples reacted with H2O immediately. Hydrogen, H2O vapor, and argon were removed from the reactor, and then the gas was cooled, dried, and measured using a gas analyzer with a precision of 1%. The output
Fig. 1. Schematic diagram of the experimental setup.
data of hydrogen were automatically recorded in a notebook computer every second. The fluxes of H2O and argon were 5 and 500 ml/min respectively for the whole experiment. The precision of the injection pump was 0.05%, and the flow meter that measured the argon flow was 1%. After the test, the final weight was obtained through an electronic balance with the precision of 0.001 g. The products were collected after the test ended and then measured using XRD (X'Pert PRO, Netherlands), SEM equipped with an energy Dispersive X-ray Spectrometer (SEM-EDX, SU-70, Japan), and TEM (Tecnai G2 F20 S-TWIN, America). 3. Results 3.1. Effect of metal elements Fig. 2 shows the thermogravimetric results of six samples: pure Al, Ale20%Li, Ale20%Mg, Ale20%Zn, Ale20%Bi, and Ale20%Sn. The sample of pure Al presented the least final WG (weight gain) and the reaction was weak below 900 C. WG is defined as the final weight to the initial weight. The Ale20%Li sample demonstrated the best reactivity and gained the highest DTG (derivative thermogravimetric) peaks. Second to Ale20%Li, Ale20%Mg had a WG of 140% and a DTG peak at approximately 600 C. Except for the samples of Ale20%Li and Ale20%Mg, the others all obtained the fastest WG rate at the temperature of 1000 C, which is the highest temperature limit. For all these samples, the WG process
Table 1 Physical properties of metals. Metal
Melting point ( C)
Boiling point ( C)
Melting point of oxide ( C)
H2 capacity (ml g
Al Li Mg Zn Bi Sn
660 180 648 420 271 231.9
2519 1342 1107 907 1564 2602
2054 1205 2825 1973 824 1630
1244 1600 933.3 344.6 160.8 377.7
1
)
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Fig. 2. TG (thermogravimetric) and DTG curves of the samples: (a) Al, (b) Ale20%Li, (c) Ale20%Mg, (d) Ale20%Zn, (e) Ale20%Bi, and (f) Ale20%Sn.
underwent two phases. The Ale20%Zn had another DTG peak between the two phases as phase I and II in Fig. 2. To facilitate the comparison of different samples, the WG of this section was added to the WG of phase I for Ale20%Zn. For pure Al, the first WG phase Table 3 WG and efficiency of samples with different compositions. Samples
Total WG (m/m0, %)
WG of phase I
WG of phase II
Efficiency (%)
Al Ale20wt%Mg Ale20wt%Li Ale20wt%Zn Ale20wt%Bi Ale20wt%Sn
112.01 143.64 182.1 119.6 115.45 124.3
102.9 121.2 119.6 103.4 103.2 103.1
114.3 118.3 161.2 116.1 112.1 120.8
13.52 50.51 87.37 25.77 21.05 31.74
occurred at 660 C, which is the melting point of Al. A similar situation was found in the other five samples. Significant differences appeared in phase II of WG. For the Ale20%Li sample, the WG of phases I and II occurred in succession, which implied the effect of Li on the AleH2O reaction. For the other five samples, the second WG phase occurred after 820 C. The reaction was weak between phases I and II. As summarized in Table 3, Zn, Bi, and Sn barely promoted the AleH2O reaction with respect to the pure Al that only considered the WG. Ale20%Zn, Ale20%Bi, and Ale20%Sn achieved almost the same WG as pure Al. The reaction of Mg and Li with H2O drove the AleH2O reaction based on the WG of phase I. The WG of Ale20%Li in the phase II obtained the largest WG value among the other samples. Efficiency was defined as the final WG to the theoretical WG, which was calculated according to the generated stable oxide
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products. The efficiency of the pure AleH2O reaction was 13.52%. The efficiencies of Ale20%Zn, Ale20%Bi, and Ale20%Sn were relatively low. For Ale20%Mg, Mg brought the AleH2O reaction along moderately, and the efficiency rose up to 50.51%. In comparison, Li exerted the best influence on the AleH2O reaction and had an efficiency of 87.37%. Single curve integral method was adopted to calculate kinetic parameter based on thermogravimetric results. The activation energy (Ea) of pure Al was 399.05 kJ/mol, but Ea of Ale20%Li was only 32.17 kJ/mol which was the smallest one among the six samples. Lower Ea presented easier condition of reaction startup. The reaction of AleH2O started up over 700 C while Ale20%Li with water below 200 C. Reaction rate constant (K) was achieved and depicted in Fig. 3. Ale20%Li had the largest reaction rate constant below 600 C and the value was one or two orders of magnitude bigger than the other. The second one was Ale20%Mg sample. Based on the results of thermogravimetric experiments, the addition of Zn, Bi, and Sn had little impact on the activation of Al, and the reaction of these three metals with H2O was weak. Thus, Li and Mg were selected to be the additives for pure Al particles in the next section. 3.2. Hydrogen generation of Al and the effect of additives Fig. 4 illustrates the results of hydrogen generation using the special experimental facility of five different compositions: Al, Ale10%Li, Ale20%Li, Ale10%Mg, and Ale20%Mg. Table 4 lists the maximum and mean H2 generation rate and yield of H2. Mean H2 generation rate was calculated as the average of the range from 10% to 90% of the maximum H2 generation rate. Yield ratio was defined as the volume of generated H2 to the theoretical H2 generation. From the curves of hydrogen evolution in Fig. 4, the hydrogen generated increased sharply to the climax at the beginning. Ale10% Li, Ale20%Li, Ale10%Mg, and Ale20%Mg could produce more than 460 ml H2 in approximately 2 min when in contact with H2O. After the peak, the generated H2 dropped rapidly and the reaction ended. The H2 amount of Ale20%Li reached 1038.85 ml g 1, which was the highest among these five samples. The Ale10%Li case ranked only second to this measurement. The effect of Mg was worse than that of Li. However, the addition of Mg and Li enhanced the
Fig. 4. Hydrogen generation rate of different compositions (Al, Ale10%Li, Ale20%Li, Ale10%Mg, and Ale20%Mg).
hydrogen generation rate and H2 yield. The rate of hydrogen and amount of hydrogen generated increased with the content increase of Mg or Li, as expected. Moreover, a greater fraction of Li broadened the time span of hydrogen generation. Though some USA patents [28,29] demonstrated some systems which generated hydrogen through the reaction of molten aluminum-rich alloy with water, no information on reaction rates or efficiencies was available. Here the comparison was implemented with the researches on the systems producing hydrogen via immerging Al alloy in aqueous solution (shown in Table 5). Our testes of molten Al with Li/Mg addition showed obvious advantage in the mean H2 generation rate, as well as the same level of H2 yield efficiency. The mean H2 generation rate reached 150.7 ml min 1 g 1 in the Ale20%Li case, and it was over half bigger than anyone listed in Table 5. Moreover, 309.74 ml min 1 g 1 of the maximum H2 generation rate in our tests was two times over that by Rosenband et al. [21].
4. Discussions
Fig. 3. Reaction rate constant of samples.
Based on previous studies [4,26,27], forming oxide film in the oxidation of Al is easy. Considering the fact that the addition of Li or Mg promoted the AleH2O reaction, they might have some influence in preventing the formation of oxide film. The reaction products after thermogravimetric experiments were measured using XRD for the crystalline species. The reaction products of the three typical samples (Al, Ale20%Li, and Ale20%Mg) via the PDEF were analyzed through SEM and TEM for surface morphology. Fig. 5 presents XRD patterns for the reaction products collected after the experiments. Great differences occurred in the XRD patterns after Li or Mg was added into Al. Peaks of Al, Al2O3, and Al2.667O4 were present in the products of pure Al with H2O reaction. For the Ale20%Li sample, XRD peaks included Al, Al2O3, and LiAlO2. Nevertheless, no lithium oxide or hydroxide was found. Similarly, only Al, Al2O3, and MgAl2O4 peaks were observed in Ale20%Mg. As shown in Fig. 6, Al, Al2O3, Bi, and Bi2O3 peaks were detected. However, no compound of Bi and Al, such as BiAlOx, was found. Ale20%Sn and Ale20%Zn were the same. As discussed above, the generation of LiAlO2 and MgAl2O4 testified that the oxidation products of Li or Mg later reacted with Al2O3. This reaction explained the promotion of AleH2O reaction after the addition of Li and Mg.
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Table 4 Hydrogen generation of samples with different compositions. Samples
Maximum H2 generation rate (ml min
Al Ale10%Li Ale20%Li Ale10%Mg Ale20%Mg
30.03 290.01 309.74 163.31 268.86
1
g
1
)
Mean H2 generation rate (ml min
1
g
1
)
14.2 144.1 150.7 80.5 135.9
Time(s)
Amount of H2 (ml g
340 243 537 283 308
142.32 959.25 1038.85 460.94 709.2
1
)
Efficiency (%) 11.41 74.98 78.93 38.0 60.0
Table 5 Published experimental results on hydrogen generation by Al alloy in aqueous solutions. Metal 80AleGaeSneZneIn 80AleBieGaeZn 88AleSi 97.5AleLi 40AleTiO2 a b
Treatment Melting Ball milling Alloy Mixing Hand mixing
Mean H2 generation rate (ml min a
40 100a 33a 103a 0.98a
1
g
1
)
Hydrogen yield (ml g 1000 510 320 1030 1021
1
)
Efficiency (%)
Refs
80.38b 49.61b 29.23b 82.20b 82.07b
[12] [25] [7] [21] [17]
Calculated based on the data in figures of corresponding papers. Calculated based on the hydrogen yield and reaction equations.
According to the results of XRD, no Mg or Li simple substance was found in their respective products. Considered Mg and Li were oxidized completely, the mass fraction of MgO or Li2O could be obtained, and then the ratio of Al oxidation could be calculated. Table 6 depicted the mass and energy analysis of our testes in TGA (thermal gravimetric analysis) and PDEF. uMxOy was defined as the mass fraction of MgO or Li2O in the products, and uAl2O3 was the mass fraction of Al2O3. When calculating uMxOy and uAl2O3, oxides in the form of LiAlO2 and MgAl2O4 were also included. The oxidation degree of Al was named aAl. The results in TGA and PDEF coincided with each other well, and the Ale20%Li case presented a higher aAl and a higher heat release ratio. uMxOy kept about 23%, and uAl2O3 showed great diversity. aAl of Ale20%Li reached up to 83.31%, while aAl of Ale20%Mg was 40% or so. Though adding Mg decreased the theoretical heat of the sample, the addition of Mg or Li both increased heat release greatly by promoting aAl. The case of Ale20%Li demonstrated a heat release ratio of 88% under 1000 C, and this result was satisfying. Fig. 7 shows SEM and TEM images of the products collected after reaction. For the Al case, the original Al particle surfaces were unbroken and dense. The oxide layer was clearly visible in TEM
photos, and the particles were generally spherical with few pittings. This result verified the fact that the main product of the AleH2O reaction is Al2O3, and the oxide film is the main factor hindering the continuance of the reaction. From the SEM and TEM images of the product for Ale20%Mg, some needles were found in the upper left, and fine oxide grains covered the surface. The needles were predicted to be MgAl2O4 based on the corresponding XRD pattern. Adding Mg to Al promoted the crack of Al oxide film, thus facilitating the AleH2O reaction. As seen in Fig. 7(c), the surface appearance of the Ale20%Li product was markedly different from that of pure Al. Hexagonal crystal systems were clearly observed and needles could be seen as well. In contrast to the corresponding XRD pattern, the crystalline hexagonal grain with a size of 20e45 mm was LiAlO2, and solid oxide film was not formed. The crystals were not compact, and interspaces between crystals were visible. The appearance of LiAlO2 and MgAl2O4 showed a different structure from Al2O3, which contributes to the promotion of AleH2O reactivity. In conclusion, Li is an effective activator that drives the AleH2O reaction.
Fig. 5. XRD patterns of the condensed products after reaction (1-Al, 2-Al2O3, 3-MgO, 4MgAl2O4, 5-Al2.667O4, and 6-LiAlO2).
Fig. 6. XRD patterns of Ale20%Zn, Ale20%Bi, and Ale20%Sn (*-Al, C-Al2O3, +-ZnO, B-Al2.667O4, --Bi, △-Bi2O3, :-Sn, and A-SnO2).
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Table 6 The mass and energy analysis of the testes in TGA and PDEF. Cases
a
Al Alb Ale20%Lia Ale20%Lib Ale20%Mga Ale20%Mgb a b
Mass analysis of the products
Energy analysis
uMxOy (%)
uAl2O3 (%)
aAl (%)
Theoretical heat (kJ/Kg)
Released heat (kJ/Kg)
Ratio (%)
0 0 23.53 23.80 23.20 22.12
22.69 32.58 69.13 67.50 44.83 52.65
13.51 17.25 83.31 80.44 41.62 43.25
16,950 16,950 19,406 19,406 16,544 16,544
2290.0 2923.87 17142.84 16753.66 8627.67 10,103
13.15 17.25 88.33 86.33 52.15 61.07
In TGA. In PDEF.
5. Conclusions Li, Mg, Zn, Bi, and Sn, were added into Al as additives to investigate the effect on AleH2O reactivity by thermogravimetric analyzer. The samples with the additives presented two-phase reacting processes, which were distinct from Al. Except Li and Mg, the additives showed little effects on AleH2O reactivity. The activation energy of Ale20%Li was the smallest, followed by Ale20%Mg. The reaction rate constants of Ale20%Li and Ale20%Mg were obviously bigger than the others below 660 C. Li and Mg were selected as the additives to investigate hydrogen production
in a specially designed reactor. The sample of Ale20%Li gained the fastest H2 production rate of 309.74 ml g 1 s 1 and the highest H2 yield of 1038 ml g 1. In the Ale20%Mg case, 60% of the H2 yield ratio was achieved. The sample with less fractions of Li or Mg presented poor hydrogen production performance but better than that for pure Al. Besides a satisfying H2 yield efficiency, the addition of Li or Mg resulted in significant H2 generation rate which was much greater than those tested in aqueous solution. Based on XRD, SEM, and TEM results, LiAlO2 or MgAl2O4 was generated with Li or Mg addition. LiAlO2 and MgAL2O4 changed the surface appearance, and new crystalline grains were formed and
Fig. 7. SEM and TEM micrographs of products: (a) Al, (b) Ale20%Mg, (c) Ale20%Li.
W. Yang et al. / Energy 88 (2015) 537e543
solid oxide film was not evidently visible on the surface. This result partly explained the reason for promoting the AleH2O reaction because of Li or Mg. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51376160) and the Fundamental Research Funds for the Central Universities (2015FZA4010). References [1] Shkolnikov E, Zhuk A, Vlaskin M. Aluminum as energy carrier: feasibility analysis and current technologies overview. Renew Sustain Energy Rev 2011;15(9):4611e23. [2] Dunn S. Hydrogen futures: toward a sustainable energy system. Int J Hydrogen Energy 2002;27(3):235e64. [3] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32(9):1121e40. [4] Wang H, Leung D, Leung M, Ni M. A review on hydrogen production using aluminum and aluminum alloys. Renew Sustain Energy Rev 2009;13(4): 845e53. [5] Miller TF, Walter JL, Kiely DH. A next-generation AUV energy system based on aluminum-seawater combustion. In: Symposium on Autonomous Underwater Vehicle Technology; 2002. p. 111e9. [6] Elitzur S, Rosenband V, Gany A. Study of hydrogen production and storage based on aluminum-water reaction. Int J Hydrogen Energ 2014;39(12): 6328e34. s J, Munoz M, Casado J. Aluminum and aluminum alloys as [7] Soler L, Macana sources of hydrogen for fuel cell applications. J Power Sources 2007;169(1): 144e9. [8] Hofmann DC, Vecchio KS. Submerged friction stir processing (SFSP): an improved method for creating ultra-fine-grained bulk materials. Mater Sci Eng A 2005;402(1):234e41. [9] Uehara K, Takeshita H, Kotaka H. Hydrogen gas generation in the wet cutting of aluminum and its alloys. J Mater Process Tech 2002;127(2):174e7. [10] Dupiano P, Stamatis D, Dreizin EL. Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders. Int J Hydrogen Energ 2011;36(8):4781e91. [11] Czech E, Troczynski T. Hydrogen generation through massive corrosion of deformed aluminum in water. Int J Hydrogen Energ 2010;35(3):1029e37. [12] Kravchenko O, Semenenko K, Bulychev B, Kalmykov K. Activation of aluminum metal and its reaction with water. J Alloy Compd 2005;397(1): 58e62.
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Experimental study on the effect of low melting point metal additives on hydrogen production in the aluminumewater reaction Weijuan Yang*, Tianyou Zhang, Junhu Zhou, Wei Shi, Jianzhong Liu, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 13 April 2015 Accepted 22 May 2015 Available online 15 June 2015
Aluminum (Al) is a promising hydrogen carrier. Continuous reaction of pure Al and water (H2O) cannot proceed smoothly because Al particles are covered with a protective oxide layer. Thus, 20% Mg, Li, Zn, Bi, and Sn content were added as additives to AleH2O reaction at high temperature. Thermogravimetric experiments were conducted to determine the reactivity of pure Al and five other samples with additives in a vapor atmosphere. Experiments indicated that Mg and Li drove the AleH2O reaction, but Zn, Bi, and Sn had little effect. Thus, Mg and Li were selected as activators in the hydrogen generation of the AleH2O reaction conducted on a specially designed experimental facility. Hydrogen was monitored in the reaction of Al-based composites with H2O vapor in real time. Among them, Ale20%Li achieved the fastest hydrogen generation rate (309.74 ml s 1 g 1) and the largest hydrogen amount (1038.9 ml g 1). XRD (Xray diffraction), SEM (scanning electron microscopy), and TEM (transmission electron microscopy) were used for product analyses to identify the influence of adding Mg and Li. This method of Al energy utilization may be used in underwater propulsion systems. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Aluminum Low melting point metals Hydrogen generation Hydrolysis
1. Introduction Hydrogen, as a renewable and environmentally friendly fuel with high heating value, has gained extensive research attention [1,2]. This gas is in urgent demand because of the air pollution caused by the combustion of fossil fuels. However, hydrogen storage is one of the key challenges in developing a hydrogen economy [3]. Aluminum (Al) is recognized as one of the most suitable hydrogen carriers because of its high hydrogen capacity of 0.11 g/g [4]. Al and its alloys have been studied as sources of hydrogen [1,4e7]. In neutral conditions, the release of hydrogen was observed through cutting, drilling, or grinding of freshly exposed Al surfaces in water (H2O) [8,9]. However, this reaction soon stopped because of the rapid passivation of the Al surface [9]. The main problem in this method is the passive surface oxide film and its by-products [e.g., Al(OH)]. Removing the protective film is the key to overcoming this problem. Metal particles with small sizes and mechanical treatments, such as ball milling, are investigated to increase the specific surface area and pitting corrosion on Al
* Corresponding author. E-mail address: [email protected] (W. Yang). http://dx.doi.org/10.1016/j.energy.2015.05.069 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
[4,10e13]. Hydroxide ions (OH 1) in alkaline solutions can destroy the protective oxide layer on the Al surface. The most commonly used hydroxide is sodium hydroxide (NaOH) [4]. Other hydroxides are also used as the reacting base, such as potassium hydroxide and calcium hydroxide [14,15]. Milling Al with different oxide modifiers, including Bi2O3, Cr2O3, MoO3, TiO2, and ZnO, has proven to be an effective method to improve reaction efficiency [16e18]. One of the most commonly used methods to promote AleH2O reaction is chemical activation through the modification of the composition of Al alloys. Gallium and its liquid alloys preclude the formation of a protective oxide film on Al and improve Al activity [19]. AleCa alloy was also used in hydrogen production, in which hydrogen yield was 47.87% when Ca content was 20% [20]. A small fraction of Li enables a spontaneous reaction of activated Al particles with H2O and results in an almost 100% yield of hydrogen generation [21]. The AleIneZnesalteH2O system can produce hydrogen at room temperature. The Ale5%Ine3%Zne2%NaCl mixture in H2O has the highest hydrogen yield of 1035 ml/g in 4 min when the ball milling time is 10 h [22]. Hydride is added to Al in the process of ball milling, which greatly improves the hydrolysis of Al powder [23]. The formation of LiAl2(OH)7$xH2O proves to be the key in enhancing the hydrogen generation of AleH2O reaction. Hydrogen generation from the reaction of AleBieNaCl and H2O is used in a portable hydrogen generator for fuel cell applications [24].
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Different metals, such as Zn, Bi, Mg, and Sn, are added to Al to react with pure H2O. AleBi alloy has a faster hydrolysis rate than other alloys at room temperature [25]. The reason behind the improved hydrolysis of Al through the addition of Bi is the micro-galvanic cell formed between the anode (Al) and the cathode (Bi). The reaction of AleH2O is exothermic (15.4 kJ/g), and this heat can hardly be utilized in aqueous solutions at room temperature. This study focused on the reaction of molten Al with H2O vapor at an elevated temperature. In this hydrogen generation method, the produced hydrogen and released heat can be utilized simultaneously. Thus, the whole energy utilization efficiency of the system will be improved. The different additives, such as Li, Mg, Zn, Bi, and Sn, were selected as activators because of their role in promoting the AleH2O reaction in the aqueous solutions of previous studies. These metals and their oxides are all nontoxic. Their properties are listed in Table 1. The experiments on thermogravimetric analyzer and special setup were conducted to study the effects of lowmelting-point metal on Al reactivity. XRD (X-ray diffraction), SEM (scanning electron microscopy), and TEM (transmission electron microscopy) were used to analyze the products and to explore the activation mechanism.
Table 2 Samples used for the experiments. Reagents
Supplier
Mean particle size
Purity (%)
Al AleLi alloy Mg
Aladdin Zhoushan Copper Sinopharm Chemical Reagent Corp Alfa Aesar Sinopharm Chemical Reagent Corp Shanghai HUSHI
10 mm 45 mm 45 mm
99.9 99.0 99.9
45 mm 45 mm
99.0 99.5
45 mm
99.5
Zn Bi Sn
2. Experimental All the reagents are listed in Table 2, including the suppliers, particle sizes, and chemical purity. Deionized water was used in the experiments. Different composites were made through hand mixing, except for the AleLi mixture. Different Li contents were prepared through mixing Al and AleLi alloy (Ale20%Li). Experiments were conducted with a high-temperature thermogravimetric analyzer (CAHN THERMAX500, Thermo Electron Corporation, USA) to investigate the reactivity of different Al-based samples at a heating rate of 25 K/min up to 1000 C. The samples (10 mg) were placed in a crucible, which was in the argon atmosphere. The flux of argon and H2O vapor were 500 and 100 ml/min respectively. The samples with relatively high activities were used to generate hydrogen in a PDEF (particularly designed experimental facility, Fig. 1). The stainless steel reactor was cylindrical with an inner diameter of 5 mm and a height of 10 mm. The reactor was kept sealed, and the argon was sent into the reactor to exclude air. Samples (1.5 g) were placed in the reactor and heated to 700 C to ensure they were in a molten state. A K-type thermocouple that was anchored to the reactor and inserted in the middle of the sample monitored the temperature inside the reactor. To avoid oxidation and hydrolysis of the sample, only argon was fed into the reactor when heating the sample. Once the temperature rose to 700 C, H2O (5 ml/min) was pumped into the reactor using an injection pump. The flux of water was controlled through the linear velocity of injector with an error less than 0.5%. H2O and argon were heated to approximately 200 C after they mixed, and then flowed into the reactor. Upon being exposed to H2O vapor, samples reacted with H2O immediately. Hydrogen, H2O vapor, and argon were removed from the reactor, and then the gas was cooled, dried, and measured using a gas analyzer with a precision of 1%. The output
Fig. 1. Schematic diagram of the experimental setup.
data of hydrogen were automatically recorded in a notebook computer every second. The fluxes of H2O and argon were 5 and 500 ml/min respectively for the whole experiment. The precision of the injection pump was 0.05%, and the flow meter that measured the argon flow was 1%. After the test, the final weight was obtained through an electronic balance with the precision of 0.001 g. The products were collected after the test ended and then measured using XRD (X'Pert PRO, Netherlands), SEM equipped with an energy Dispersive X-ray Spectrometer (SEM-EDX, SU-70, Japan), and TEM (Tecnai G2 F20 S-TWIN, America). 3. Results 3.1. Effect of metal elements Fig. 2 shows the thermogravimetric results of six samples: pure Al, Ale20%Li, Ale20%Mg, Ale20%Zn, Ale20%Bi, and Ale20%Sn. The sample of pure Al presented the least final WG (weight gain) and the reaction was weak below 900 C. WG is defined as the final weight to the initial weight. The Ale20%Li sample demonstrated the best reactivity and gained the highest DTG (derivative thermogravimetric) peaks. Second to Ale20%Li, Ale20%Mg had a WG of 140% and a DTG peak at approximately 600 C. Except for the samples of Ale20%Li and Ale20%Mg, the others all obtained the fastest WG rate at the temperature of 1000 C, which is the highest temperature limit. For all these samples, the WG process
Table 1 Physical properties of metals. Metal
Melting point ( C)
Boiling point ( C)
Melting point of oxide ( C)
H2 capacity (ml g
Al Li Mg Zn Bi Sn
660 180 648 420 271 231.9
2519 1342 1107 907 1564 2602
2054 1205 2825 1973 824 1630
1244 1600 933.3 344.6 160.8 377.7
1
)
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Fig. 2. TG (thermogravimetric) and DTG curves of the samples: (a) Al, (b) Ale20%Li, (c) Ale20%Mg, (d) Ale20%Zn, (e) Ale20%Bi, and (f) Ale20%Sn.
underwent two phases. The Ale20%Zn had another DTG peak between the two phases as phase I and II in Fig. 2. To facilitate the comparison of different samples, the WG of this section was added to the WG of phase I for Ale20%Zn. For pure Al, the first WG phase Table 3 WG and efficiency of samples with different compositions. Samples
Total WG (m/m0, %)
WG of phase I
WG of phase II
Efficiency (%)
Al Ale20wt%Mg Ale20wt%Li Ale20wt%Zn Ale20wt%Bi Ale20wt%Sn
112.01 143.64 182.1 119.6 115.45 124.3
102.9 121.2 119.6 103.4 103.2 103.1
114.3 118.3 161.2 116.1 112.1 120.8
13.52 50.51 87.37 25.77 21.05 31.74
occurred at 660 C, which is the melting point of Al. A similar situation was found in the other five samples. Significant differences appeared in phase II of WG. For the Ale20%Li sample, the WG of phases I and II occurred in succession, which implied the effect of Li on the AleH2O reaction. For the other five samples, the second WG phase occurred after 820 C. The reaction was weak between phases I and II. As summarized in Table 3, Zn, Bi, and Sn barely promoted the AleH2O reaction with respect to the pure Al that only considered the WG. Ale20%Zn, Ale20%Bi, and Ale20%Sn achieved almost the same WG as pure Al. The reaction of Mg and Li with H2O drove the AleH2O reaction based on the WG of phase I. The WG of Ale20%Li in the phase II obtained the largest WG value among the other samples. Efficiency was defined as the final WG to the theoretical WG, which was calculated according to the generated stable oxide
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products. The efficiency of the pure AleH2O reaction was 13.52%. The efficiencies of Ale20%Zn, Ale20%Bi, and Ale20%Sn were relatively low. For Ale20%Mg, Mg brought the AleH2O reaction along moderately, and the efficiency rose up to 50.51%. In comparison, Li exerted the best influence on the AleH2O reaction and had an efficiency of 87.37%. Single curve integral method was adopted to calculate kinetic parameter based on thermogravimetric results. The activation energy (Ea) of pure Al was 399.05 kJ/mol, but Ea of Ale20%Li was only 32.17 kJ/mol which was the smallest one among the six samples. Lower Ea presented easier condition of reaction startup. The reaction of AleH2O started up over 700 C while Ale20%Li with water below 200 C. Reaction rate constant (K) was achieved and depicted in Fig. 3. Ale20%Li had the largest reaction rate constant below 600 C and the value was one or two orders of magnitude bigger than the other. The second one was Ale20%Mg sample. Based on the results of thermogravimetric experiments, the addition of Zn, Bi, and Sn had little impact on the activation of Al, and the reaction of these three metals with H2O was weak. Thus, Li and Mg were selected to be the additives for pure Al particles in the next section. 3.2. Hydrogen generation of Al and the effect of additives Fig. 4 illustrates the results of hydrogen generation using the special experimental facility of five different compositions: Al, Ale10%Li, Ale20%Li, Ale10%Mg, and Ale20%Mg. Table 4 lists the maximum and mean H2 generation rate and yield of H2. Mean H2 generation rate was calculated as the average of the range from 10% to 90% of the maximum H2 generation rate. Yield ratio was defined as the volume of generated H2 to the theoretical H2 generation. From the curves of hydrogen evolution in Fig. 4, the hydrogen generated increased sharply to the climax at the beginning. Ale10% Li, Ale20%Li, Ale10%Mg, and Ale20%Mg could produce more than 460 ml H2 in approximately 2 min when in contact with H2O. After the peak, the generated H2 dropped rapidly and the reaction ended. The H2 amount of Ale20%Li reached 1038.85 ml g 1, which was the highest among these five samples. The Ale10%Li case ranked only second to this measurement. The effect of Mg was worse than that of Li. However, the addition of Mg and Li enhanced the
Fig. 4. Hydrogen generation rate of different compositions (Al, Ale10%Li, Ale20%Li, Ale10%Mg, and Ale20%Mg).
hydrogen generation rate and H2 yield. The rate of hydrogen and amount of hydrogen generated increased with the content increase of Mg or Li, as expected. Moreover, a greater fraction of Li broadened the time span of hydrogen generation. Though some USA patents [28,29] demonstrated some systems which generated hydrogen through the reaction of molten aluminum-rich alloy with water, no information on reaction rates or efficiencies was available. Here the comparison was implemented with the researches on the systems producing hydrogen via immerging Al alloy in aqueous solution (shown in Table 5). Our testes of molten Al with Li/Mg addition showed obvious advantage in the mean H2 generation rate, as well as the same level of H2 yield efficiency. The mean H2 generation rate reached 150.7 ml min 1 g 1 in the Ale20%Li case, and it was over half bigger than anyone listed in Table 5. Moreover, 309.74 ml min 1 g 1 of the maximum H2 generation rate in our tests was two times over that by Rosenband et al. [21].
4. Discussions
Fig. 3. Reaction rate constant of samples.
Based on previous studies [4,26,27], forming oxide film in the oxidation of Al is easy. Considering the fact that the addition of Li or Mg promoted the AleH2O reaction, they might have some influence in preventing the formation of oxide film. The reaction products after thermogravimetric experiments were measured using XRD for the crystalline species. The reaction products of the three typical samples (Al, Ale20%Li, and Ale20%Mg) via the PDEF were analyzed through SEM and TEM for surface morphology. Fig. 5 presents XRD patterns for the reaction products collected after the experiments. Great differences occurred in the XRD patterns after Li or Mg was added into Al. Peaks of Al, Al2O3, and Al2.667O4 were present in the products of pure Al with H2O reaction. For the Ale20%Li sample, XRD peaks included Al, Al2O3, and LiAlO2. Nevertheless, no lithium oxide or hydroxide was found. Similarly, only Al, Al2O3, and MgAl2O4 peaks were observed in Ale20%Mg. As shown in Fig. 6, Al, Al2O3, Bi, and Bi2O3 peaks were detected. However, no compound of Bi and Al, such as BiAlOx, was found. Ale20%Sn and Ale20%Zn were the same. As discussed above, the generation of LiAlO2 and MgAl2O4 testified that the oxidation products of Li or Mg later reacted with Al2O3. This reaction explained the promotion of AleH2O reaction after the addition of Li and Mg.
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Table 4 Hydrogen generation of samples with different compositions. Samples
Maximum H2 generation rate (ml min
Al Ale10%Li Ale20%Li Ale10%Mg Ale20%Mg
30.03 290.01 309.74 163.31 268.86
1
g
1
)
Mean H2 generation rate (ml min
1
g
1
)
14.2 144.1 150.7 80.5 135.9
Time(s)
Amount of H2 (ml g
340 243 537 283 308
142.32 959.25 1038.85 460.94 709.2
1
)
Efficiency (%) 11.41 74.98 78.93 38.0 60.0
Table 5 Published experimental results on hydrogen generation by Al alloy in aqueous solutions. Metal 80AleGaeSneZneIn 80AleBieGaeZn 88AleSi 97.5AleLi 40AleTiO2 a b
Treatment Melting Ball milling Alloy Mixing Hand mixing
Mean H2 generation rate (ml min a
40 100a 33a 103a 0.98a
1
g
1
)
Hydrogen yield (ml g 1000 510 320 1030 1021
1
)
Efficiency (%)
Refs
80.38b 49.61b 29.23b 82.20b 82.07b
[12] [25] [7] [21] [17]
Calculated based on the data in figures of corresponding papers. Calculated based on the hydrogen yield and reaction equations.
According to the results of XRD, no Mg or Li simple substance was found in their respective products. Considered Mg and Li were oxidized completely, the mass fraction of MgO or Li2O could be obtained, and then the ratio of Al oxidation could be calculated. Table 6 depicted the mass and energy analysis of our testes in TGA (thermal gravimetric analysis) and PDEF. uMxOy was defined as the mass fraction of MgO or Li2O in the products, and uAl2O3 was the mass fraction of Al2O3. When calculating uMxOy and uAl2O3, oxides in the form of LiAlO2 and MgAl2O4 were also included. The oxidation degree of Al was named aAl. The results in TGA and PDEF coincided with each other well, and the Ale20%Li case presented a higher aAl and a higher heat release ratio. uMxOy kept about 23%, and uAl2O3 showed great diversity. aAl of Ale20%Li reached up to 83.31%, while aAl of Ale20%Mg was 40% or so. Though adding Mg decreased the theoretical heat of the sample, the addition of Mg or Li both increased heat release greatly by promoting aAl. The case of Ale20%Li demonstrated a heat release ratio of 88% under 1000 C, and this result was satisfying. Fig. 7 shows SEM and TEM images of the products collected after reaction. For the Al case, the original Al particle surfaces were unbroken and dense. The oxide layer was clearly visible in TEM
photos, and the particles were generally spherical with few pittings. This result verified the fact that the main product of the AleH2O reaction is Al2O3, and the oxide film is the main factor hindering the continuance of the reaction. From the SEM and TEM images of the product for Ale20%Mg, some needles were found in the upper left, and fine oxide grains covered the surface. The needles were predicted to be MgAl2O4 based on the corresponding XRD pattern. Adding Mg to Al promoted the crack of Al oxide film, thus facilitating the AleH2O reaction. As seen in Fig. 7(c), the surface appearance of the Ale20%Li product was markedly different from that of pure Al. Hexagonal crystal systems were clearly observed and needles could be seen as well. In contrast to the corresponding XRD pattern, the crystalline hexagonal grain with a size of 20e45 mm was LiAlO2, and solid oxide film was not formed. The crystals were not compact, and interspaces between crystals were visible. The appearance of LiAlO2 and MgAl2O4 showed a different structure from Al2O3, which contributes to the promotion of AleH2O reactivity. In conclusion, Li is an effective activator that drives the AleH2O reaction.
Fig. 5. XRD patterns of the condensed products after reaction (1-Al, 2-Al2O3, 3-MgO, 4MgAl2O4, 5-Al2.667O4, and 6-LiAlO2).
Fig. 6. XRD patterns of Ale20%Zn, Ale20%Bi, and Ale20%Sn (*-Al, C-Al2O3, +-ZnO, B-Al2.667O4, --Bi, △-Bi2O3, :-Sn, and A-SnO2).
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Table 6 The mass and energy analysis of the testes in TGA and PDEF. Cases
a
Al Alb Ale20%Lia Ale20%Lib Ale20%Mga Ale20%Mgb a b
Mass analysis of the products
Energy analysis
uMxOy (%)
uAl2O3 (%)
aAl (%)
Theoretical heat (kJ/Kg)
Released heat (kJ/Kg)
Ratio (%)
0 0 23.53 23.80 23.20 22.12
22.69 32.58 69.13 67.50 44.83 52.65
13.51 17.25 83.31 80.44 41.62 43.25
16,950 16,950 19,406 19,406 16,544 16,544
2290.0 2923.87 17142.84 16753.66 8627.67 10,103
13.15 17.25 88.33 86.33 52.15 61.07
In TGA. In PDEF.
5. Conclusions Li, Mg, Zn, Bi, and Sn, were added into Al as additives to investigate the effect on AleH2O reactivity by thermogravimetric analyzer. The samples with the additives presented two-phase reacting processes, which were distinct from Al. Except Li and Mg, the additives showed little effects on AleH2O reactivity. The activation energy of Ale20%Li was the smallest, followed by Ale20%Mg. The reaction rate constants of Ale20%Li and Ale20%Mg were obviously bigger than the others below 660 C. Li and Mg were selected as the additives to investigate hydrogen production
in a specially designed reactor. The sample of Ale20%Li gained the fastest H2 production rate of 309.74 ml g 1 s 1 and the highest H2 yield of 1038 ml g 1. In the Ale20%Mg case, 60% of the H2 yield ratio was achieved. The sample with less fractions of Li or Mg presented poor hydrogen production performance but better than that for pure Al. Besides a satisfying H2 yield efficiency, the addition of Li or Mg resulted in significant H2 generation rate which was much greater than those tested in aqueous solution. Based on XRD, SEM, and TEM results, LiAlO2 or MgAl2O4 was generated with Li or Mg addition. LiAlO2 and MgAL2O4 changed the surface appearance, and new crystalline grains were formed and
Fig. 7. SEM and TEM micrographs of products: (a) Al, (b) Ale20%Mg, (c) Ale20%Li.
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solid oxide film was not evidently visible on the surface. This result partly explained the reason for promoting the AleH2O reaction because of Li or Mg. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51376160) and the Fundamental Research Funds for the Central Universities (2015FZA4010). References [1] Shkolnikov E, Zhuk A, Vlaskin M. Aluminum as energy carrier: feasibility analysis and current technologies overview. Renew Sustain Energy Rev 2011;15(9):4611e23. [2] Dunn S. Hydrogen futures: toward a sustainable energy system. Int J Hydrogen Energy 2002;27(3):235e64. [3] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32(9):1121e40. [4] Wang H, Leung D, Leung M, Ni M. A review on hydrogen production using aluminum and aluminum alloys. Renew Sustain Energy Rev 2009;13(4): 845e53. [5] Miller TF, Walter JL, Kiely DH. A next-generation AUV energy system based on aluminum-seawater combustion. In: Symposium on Autonomous Underwater Vehicle Technology; 2002. p. 111e9. [6] Elitzur S, Rosenband V, Gany A. Study of hydrogen production and storage based on aluminum-water reaction. Int J Hydrogen Energ 2014;39(12): 6328e34. s J, Munoz M, Casado J. Aluminum and aluminum alloys as [7] Soler L, Macana sources of hydrogen for fuel cell applications. J Power Sources 2007;169(1): 144e9. [8] Hofmann DC, Vecchio KS. Submerged friction stir processing (SFSP): an improved method for creating ultra-fine-grained bulk materials. Mater Sci Eng A 2005;402(1):234e41. [9] Uehara K, Takeshita H, Kotaka H. Hydrogen gas generation in the wet cutting of aluminum and its alloys. J Mater Process Tech 2002;127(2):174e7. [10] Dupiano P, Stamatis D, Dreizin EL. Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders. Int J Hydrogen Energ 2011;36(8):4781e91. [11] Czech E, Troczynski T. Hydrogen generation through massive corrosion of deformed aluminum in water. Int J Hydrogen Energ 2010;35(3):1029e37. [12] Kravchenko O, Semenenko K, Bulychev B, Kalmykov K. Activation of aluminum metal and its reaction with water. J Alloy Compd 2005;397(1): 58e62.
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