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NaBH4 system activated by CoCl2 solution
Renewable Energy 46 (2012) 203e209
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Controllable hydrogen generation and hydrolysis mechanism of AlLi/NaBH4 system activated by CoCl2 solution Mei-Qiang Fan*, Shu Liu, Wen Qiang Sun, Yong Fei, Hua Pan, Kang-Ying Shu* Department of Materials Science and Engineering, 258 Xueyuan Street, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2011 Accepted 18 March 2012 Available online 8 April 2012
Hydrogen generation performance of solid-state AlLi/NaBH4 mixture activated by CoCl2 solution was evaluated in the present work. Hydrogen generation performance can be regulated by AlLi/NaBH4 weight ratio, Li content, CoCl2 concentration, and hydrolysis temperature, among others. 1 g Al-20 wt% Li/NaBH4 mixture (weight ratio, 1:1) yields 1674 mL hydrogen with 89% efficiency in 2.5 wt%CoCl2 solution at 303 K. The relative hydrolysis mechanism and optimized composition design are reported and discussed through the analysis of XRD, SEM, IR, and particle size distribution. The hydrolysis byproducts LiOH and Co2B/Al(OH)3 are the major factors to improve the hydrolysis performance of an AlLi/NaBH4 mixture. Our experimental data show that the new method of hydrogen generation from AlLi/NaBH4 hydrolysis activated by CoCl2 solution may supply free CO2, portable hydrogen for proton exchange membrane fuel cell. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen generation AlLi alloy Sodium borohydride Cobalt dichloride Catalyst
1. Introduction There is an urgent demand to develop renewable energy with high efficiency and zero emission to address energy demand and environmental problems because of the increasing pressure of air pollution and the energy crisis caused by the large-scale consumption of fossil fuels. With its high calorific value, hydrogen has been recognized as an ideal energy carrier in fuel cells and internal combustion engines. However, the widespread use of hydrogen is hampered by the lack of cheap, safe, and efficient hydrogen storage methods [1,2]. There are no viable and established hydrogen storage methods which can provide reversible storage for hydrogen density >5 wt% at mild conditions. Typical complex hydrides such as light metal boron hydrides and amide have gained increasing attention because of their high hydrogen density. However, further research still has to be conducted before they can be practically applied. In the meantime, on-board hydrogen generation with off-board spent fuel regeneration promises to be a realistic solution for near-term hydrogen storage applications using chemical hydrides and metals. Sodium borohydride (NaBH4) is a good candidate for hydrogen generation and storage for fuel cell applications, offering high
* Corresponding authors. E-mail addresses: [email protected] (M.-Q. Fan), [email protected] (K.-Y. Shu). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2012.03.028
theoretical gravimetric hydrogen storage capacity of 10.8 wt%. NaBH4 hydrolysis can supply stable hydrogen for fuel cells and the hydrogen generation rate can be regulated by adding catalysts. NaBH4-based hydrolysis systems possess many advantages such as satisfactory reaction controllability, mild operation temperature, safe fuel storability, and so on, which make them a potential source of portable hydrogen. However, NaBH4-based hydrolysis systems cannot be widely applied because of low hydrogen density and high cost. The low solubility of NaBH4 and NaBO2$2H2O byproduct limits the maximum concentration to 20 wt% NaBH4 in aqueous solution, which only generates a hydrogen density of approximate 2e4 wt% [3]. In comparison with traditional hydrogen generation from aqueous NaBH4, the hydrolysis of solid-state NaBH4 in limited water amount can increase hydrogen density. Many interesting results have been achieved via optimization of NaBH4/H2O molar ratio, synthesis of high-performance catalyst, and ultra mixing of NaBH4/catalyst [4,5]. Liu [4] found that 6.7 wt% hydrogen densities could be obtained in a NaBH4/H2O/CoCl2 system. Moreover, if combined with another hydrogen generation material (aluminum and aluminum alloy), NaBH4 presents high hydrolysis performance because of the interaction of Al/NaBH4 hydrolysis. The byproduct Al(OH)3 from Al hydrolysis has a catalytic effect on the hydrolysis kinetic of NaBH4 [6]. Interestingly, numerous heat and alkaline byproducts from NaBH4 hydrolysis also stimulate Al hydrolysis. Shafirovich [7] found that the nano-Al/NaBH4 generated approximately 7 wt% hydrogen densities in limited water amount at 298 K. Catalysts such as transition metals (supported and non-supported)
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steel balls in an argon-filled glove box. The ball to mixture weight ratio was 26:1. The mixtures were then milled for 15 h in a QM3SPO4 planetary ball miller at 450 rpm under 0.2 MPa argon atmosphere unless noted otherwise.
1800 1600 1400 1200
2.2. Measurement of hydrogen evolution
1000 800 5wt% Li 10wt% Li 15wt% Li 20wt% Li
600 400 200 0
0
1
2
3
4
5
Fig. 1. Hydrogen generation from AlLi/NaBH4 mixture activated by CoCl2 solution with different Li content.
To measure the hydrogen evolution, 0.4 g of the AleLi/NaBH4 mixture was mixed with water (4 mL). The weight ratio of AleLi/ NaBH4 mixture was 1:1 and the CoCl2 concentration was 2.5 wt%, unless otherwise indicated. The mixture was pressed into a tablet in a 10 mm diameter stainless steel mold under 5 ton pressures before hydrolysis. Hydrolysis of the AleLi/NaBH4 mixture was carried out in CoCl2 solution at 303 K and 1 atm in a sealed reactor attached to a condenser. The generated hydrogen was collected at 298 K and 1 atm and measured from the water level change in the cylinder, i.e. water displacement. The hydrogen generation rate was calculated from the first bubble evolved from the beginning of the test. 2.3. Microstructure analysis
and metal salts [8e10] have been reported to increase the decomposition rate of NaBH4. They can also accelerate Al hydrolysis via functioning as a cathode of a micro-galvanic cell with aluminum. The AlCo/NaBH4 [11] and Al/NaBH4/CoCl2 systems [6] had better hydrolysis performance compared to Al/NaBH4. Thus far, the hydrolysis of the Al/NaBH4 system has to be performed either in an alkaline solution or by using nanoscale Al powder because of the thin aluminum oxide layer preventing sustainable Al hydrolysis. Unfortunately, strong alkaline solutions are difficult to handle and nanoscale Al powder is expensive. In the present paper, a new composition of milled AlLi alloy and NaBH4 powder activated in CoCl2 solution is designed to obtain high hydrogen density which can meet the demand of U.S. Department of Energy [3]. Good hydrogen generation performance of AlLi/NaBH4 powder can be obtained in neutral solution. The hydrogen generation amount and rate can be regulated by Li content, CoCl2 concentration and hydrolysis temperature. The composition design and relative hydrolysis mechanism are reported and discussed via hydrogen generation experiments and microstructure analysis. The aim of this work is to find the composition with highest hydrogen yield and elaborate the possible application of AlLi/NaBH4 activated by CoCl2 solution for hydrogen generation. 2. Experimental 2.1. Materials Li flakes (f * 0.5 mm, 99.9% purity), aluminum powder (mean size, 10 mm, common grade, 99.9% purity) (Beijing Xingry Technology Company Ltd.), NaBH4 (solid, 98.0% purity) (Tianjin Delan Chemical Company), and CoCl2$6H2O (98.0% AR) were used as the starting materials. All reagents were used as received. All reagents were weighed and placed in 50 mL stainless steel jars with stainless
Powder X-ray diffraction (XRD) patterns of the as-prepared samples were obtained via X-ray diffractometer (Thermo ARL, Switzerland, model ARL X’TRA) over a range of diffraction angles (q) from 2q ¼ 10 to 2q ¼ 80 with Cu Ka radiation filtered by a monochromator. Scanning electron microscopy (SEM) observations were performed using the model JSM-5610LV from JEOL Company which was equipped with INCA energy dispersive X-ray spectroscopy (EDS) measurements. Particle size distribution of the as-prepared samples was measured by a particulate size description analyzer (Dandong Better size, China, specification, BT2003). Surface area was collected by a surface area and porosity analyzer (Micromeritics, USA, model ASAP2020). The solid hydrolysis byproduct in the reactor was filtered using a vacuum pump and then dried in an oven at 313 K. The Fourier transform infrared (FTIR) spectra were recorded from KBr pellets in the range of 4000400 cm1 on a Nicolet 5700 spectrometer. 3. Results and discussion 3.1. Hydrogen generation performance Fig. 1 and Table 1 show the effect of lithium content on hydrogen generation of the AlLi/NaBH4 mixture in 2.5 wt% CoCl2 solution at 303 K. The mixtures exhibited high hydrogen generation rate, and most of the hydrogen were generated within 2 min of the reaction. As the Li content in AlLi alloy increased from 5 to 20 wt%, hydrogen generation volume increased from 924 to 1674 mL g1. The maximum hydrogen generation rate increased from 788 to 3137 mL g1 min1. Increasing Li content improved the hydrolytic performance of the Al/NaBH4 mixture, particularly Al, which resulted in an increase in conversion efficiency from 49% to 89%. Aluminum has low reactivity in a neutral solution because of an alumina layer covering its surface which prevents its reaction with water [12], so most of the hydrogen generated from Al-5 wt%Li/
Table 1 Hydrogen generation volume and maximum rate of AlLi/NaBH4 mixture activated i 2.5 wt% CoCl2 solution (milling time 15 h, maq/mmixture ¼ 10, Troom ¼ 298 K, pressure 5t). Materials
Maximum hydrogen generation rate (mL g1 min1)
Hydrogen generation volume (mL g1)
Efficiency (%)
Median diameter (mm)
Special surface area (m2 kg1)
Al-5wt% Li/NaBH4 Al-10wt% Li/NaBH4 Al-15wt% Li/NaBH4 Al-20wt% Li/NaBH4
788 1652 2166 3137
924 1167 1532 1674
49 62 81 89
9.84 9.76 9.71 8.77
285.84 290.72 301.31 323.69
M.-Q. Fan et al. / Renewable Energy 46 (2012) 203e209
Fig. 2. XRD patterns of different milled AlLi alloy.
NaBH4 mixture is due to the NaBH4 hydrolysis activated by CoCl2 solution. Although the hydrolytic byproduct NaBO2$2H2O provides alkaline which may stimulate Al hydrolysis, the effect is insignificant based on Fig. 1. Aluminum reactivity is gradually improved in stronger alkaline from Li hydrolysis due to higher Li content in AlLi alloy. Therefore, the hydrogen evolution from Al hydrolysis is increased. The milled AlLi particle size has been reported to decrease with increased Li content [13] due to the formation of AlLi alloy. The AlLi alloy has low enthalpy of formation (41.763 kJ/mol) [14], meaning it can be easily formed in the milling process. The peaks of AlLi phase (Fig. 2) became distinct and broadened when Li content increased from 5 to 20 wt%, reflecting that the formation of AlLi phase contributes to the gradual reduction of grain size. Correspondingly, Fig. 3 shows SEM micrographs of milled AlLi alloy; more small and irregular grains were found with increased Li content. The formation of AlLi phase prevents the formation of AleAl and LieLi atoms. More defects and dislocations are also generated due to repeated grain breakup, cold welding, and rewelding in the milling process. The median diameters of AlLi alloy decrease gradually, leading to an increase in the surface area (Table 1). This favors more contact between Al and water, resulting in the improved kinetic of AlLi/NaBH4 hydrolysis. The detailed particle size distribution is shown in Fig. 4. The particle size distribution shifts smaller with increasing Li content in AlLi alloy. Fig. 5 shows hydrogen generation of Al-20 wt%Li/NaBH4 mixture with different weight ratios in 2.5 wt% CoCl2 solution at 303 K. With Al-20 wt% Li/NaBH4 weight ratio changing from 9:1 to 1:3, the maximum hydrogen generation rate increases from 1751 to 3775 mL g1 min1, and the accumulated hydrogen generation
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volume increases from 1294 to 1913 mL g1. The hydrolysis reaction is ended in 1e2 min, with the conversion efficiency above 85%. Released heat and protons are generated in the hydrolysis process of AlLi/NaBH4 which stimulates the hydrolysis kinetic. The enthalpies of Al/Li/NaBH4 hydrolysis are 444, 204, and 216 kJ mol1 at 298 K [15], respectively. As the Al/Li/NaBH4 hydrolysis proceeds, the solution temperature increases and the largest value can be up to 373 K in the limited water amount. The increased solution temperature improves the hydrolysis kinetic of Al/Li/NaBH4. Fig. 6 shows the hydrogen generation curves of Al-20 wt% Li/NaBH4 mixture at different temperatures. As expected, maximum hydrogen generation rate increases from 2973 to 3235 mL g1min1 as solution temperature increases from 298 to 313 K. In the hydrolysis process, the hydrolysis byproduct Co2B generated from the reaction of NaBH4 and CoCl2 has dual catalytic ability on AlLi/NaBH4 hydrolysis. First, it may cover the Al surface and act as a cathode of a micro-galvanic couple [6]. The protons from AlLi/NaBH4 hydrolysis accelerate the corrosion rate of the AlCo2B micro-galvanic fuel. Second, Co2B is a good promoter for NaBH4 hydrolysis [10,11]. The released heat and hydrolysis byproducts Al(OH)3, CoB2 and NaBO2.2H2O have large effect on Al/ NaBH4 hydrolysis. Fig. 7 shows the hydrogen generation of Al-20 wt% Li/NaBH4 mixture in different concentrations of CoCl2 solution. The Al-20 wt % Li/NaBH4 mixture in pure water only yields 1258 mL g1 with 1186 mL g1 min1 maximum hydrogen generation rate. The same mixture in 0.5 wt% CoCl2 solution yields 1652 mL g1 hydrogen and 1221 mL g1 min1 maximum hydrogen generation rate. With CoCl2 concentration further increased to 2.5 wt%, the maximum hydrogen generation amount and rate are increased to 1674 mL g1 and 1751 mL g1 min1. The hydrogen generation amount and rate are proportional to CoCl2 concentration to some degree: higher CoCl2 concentration results in producing more Co2B amount, thus stimulating Al/NaBH4 hydrolysis. 3.2. Hydrolysis mechanism Many papers [4,6,7] have demonstrated the hydrolysis mechanism of Al/NaBH4, according to reactions (1, 2, 3). The reactions (1, 2) cannot occur continuously due to the hydrolysis byproduct Al(OH)3 and NaBO2$2H2O. The Al(OH)3 deposited on Al surface and prevented the contact of aluminum and water. The NaBO2.2H2O had alkaline and the alkaline reduced self-hydrolysis of NaBH4. Therefore, the sustainable hydrolysis reaction of Al/NaBH4 and water has to be performed under the help of catalyst. The alkaline such as NaOH, KOH dissolved Al(OH)3 and exposed Al surface directly to water [11]. Many metals and their borides such as Co, Co2B [4e6] often acted as a good promoter for NaBH4 hydrolysis. So there are many different hydrolysis processes in the reaction of
Fig. 3. SEM micrographs of different milled AlLi alloy: (a) 5 wt% Li and (b) 20 wt% Li.
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10 9
100
12
a
8
10
80
4
40
3 2
Differential (%)
Differential (%)
5
8 6 5
40
4 3
20
20
2
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0 0.00
1.74 3.05
8.95
26.68
57.35
0 80.00
13
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c
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d
11 80
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9 60
7 6 40
5 4 3
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Differential (%)
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7
Cumulation (%)
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Cumulation (%)
6
0 0.00
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1 1.39
3.05
8.95
0 80.00
21.42
0 0.00
1.74
8.95
3.05
0 80.00
19.23
Fig. 4. Particle size distribution of different AlLi/NaBH4 mixture: (a) 5 wt% Li, (b) 10 wt% Li, (c) 15 wt% Li, and (d) 20 wt% Li.
1800 1600 1400 1200 1000 800
9:1 3:1 1:1 1:3
600 400 200 0 0
1
2
3
4
t (min) Fig. 5. Hydrogen generation of Al-20 wt%Li/NaBH4 mixture with different weight ratios activated by 2.5 wt% CoCl2 solution.
(1)
DH ¼ 426.5 kJ mol1 at 298 K NaBH4 þ 4H2O / 4H2 þ NaBO2$2H2O
(2)
DH ¼ 216.7 kJ mol1 at 298 K 2Li þ 2H2O / 2LiOH þ H2
Accumulated hydrogen volume (mL g )
2000
Al þ 3H2O / Al(OH)3 þ 3H2
-1
Accumulated hydrogen volume (mL g-1)
AlLi/NaBH4 mixture and water. The composites of AlLi/NaBH4 have high reactivity and easily react with water at 298 K. Hydrolysis byproduct LiOH in reaction (3) is a good catalyst for Al hydrolysis but deteriorates NaBH4 self-hydrolysis. However, good hydrolysis of NaBH4 in alkaline solution can be obtained in CoCl2 solution due to the catalytic effect of Co2B [10,11], which is generated in reaction (4). Thus, the addition of Li and CoCl2 is important in improving the hydrolysis kinetics of Al/NaBH4 and altering the hydrolysis mechanism via the formation of new catalyst in the hydrolysis process.
(3)
1600 1400 1200 1000 298 K 303 K 308 K 313 K
800 600 400 200 0 0
1
2
3
4
t (min) Fig. 6. Hydrogen generation of Al-20 wt% Li/NaBH4 mixture activated by 2.5 wt% CoCl2 solution at different temperatures.
Accumulated hydrogen volume (mL g-1)
M.-Q. Fan et al. / Renewable Energy 46 (2012) 203e209
1800 1600 1400 1200 1000 800
2.5wt% CoCl2aq
600
1.0wt% CoCl2aq 0.5wt% CoCl2aq
400
0.2wt% CoCl2aq
200
water
0 0
1
2
3
4
5
t (min) Fig. 7. Hydrogen generation of Al-20 wt%Li/NaBH4 mixture activated by different concentrations of CoCl2 solution.
DH ¼ 204.5 kJ mol1 at 298 K 2CoCl2 þ 4NaBH4 þ 6H2O / Co2B þ 3HBO2 þ 12.5H2
(4)
Fig. 8 shows the XRD patterns of hydrolysis byproducts of AlLi/ NaBH4 mixture at different hydrolysis conditions. The peaks of Al are identified in the XRD patterns of the hydrolysis byproducts of Al-20 wt%Li alloy, reflecting that part of initial aluminum has not completely reacted with water. With 5 wt% NaBH4 substituting Al, the peaks of Al disappear and the peaks of LiAl2(OH)7$xH2O and Bayerite Al(OH)3 become conspicuous. With 50 wt% NaBH4 further substituting Al, the peaks become broadened and the particle size decreases based on the Scherrer equation ðDhkl ¼ ðklÞ=ðbcosqhkl ÞÞ. The peaks of NaBO2$2H2O, Co2B, and Na2B4O7 can be identified. The formation of Na2B4O7 comes from the reaction of NaBO2 and CO2 [4]. The initial material NaBH4 is observed in the hydrolysis byproducts of the AlLi/NaBH4 mixture because of its high stability in alkaline solution. But it is not identified in XRD patterns of hydrolysis byproduct of AlLi/NaBH4 mixture activated by CoCl2 solution. Fig. 9 shows SEM micrographs of hydrolysis byproducts of different Al-20 wt%Li/NaBH4 mixtures, small plate LiAl2(OH)7$xH2O and flocculent solid Al(OH)3 were found in the of Al-20 wt%Li alloy. With more addition of NaBH4, the particle size of plate
207
LiAl2(OH)7$xH2O decreases smaller and the Al(OH)3 becomes looser. It can be reflected that NaBH4 hydrolysis has some effect on AlLi hydrolysis. Fig. 10 shows the IR absorption curve of hydrolysis byproducts of different AlLi/NaBH4 mixtures activated by 2.5 wt% CoCl2 solution with different Li content. The detailed results are listed in the following: IR peaks (KBr, cm1) of hydrolysis byproduct of Al-20 wt% Li/NaBH4 mixture, 3462 (vs), 1649 (s), 1386 (vs), 1254 (w), 1130 (w), 1001 (s), 948 (m), 825(w), 742(s), and 536 (vs); IR peaks (KBr, cm1) of hydrolysis byproducts of Al-5 wt% Li/NaBH4 mixture: 3448 (w), 1646 (m), 1386 (m), 1132 (w), 999 (m), 948 (m), 825 (m), 708 (w), and 536 (m). The wide band identified in the 3000e3600 cm1 region was stimulated by OeH stretching and the band at approximately 1646 cm1 was caused by HeOeH deformation bands. Bands characteristic of AleO (1386 cm1), BeO, and HeOeB vibrators were also observed. Peaks at 3462 (OH stretching), 1386 (AlO stretching), and 536 cm1 (OH stretching) became stronger with increased Li content. Higher Li content creates more LiAl2(OH)7$xH2O groups, which corresponds to the XRD analysis in Fig. 8. The characteristic BeH stretching band of NaBH4 appears in hydrolysis byproducts of Al-5 wt% Li/NaBH4 mixture, but is not observed in those of Al-20 wt% Li/NaBH4 mixture, showing that AlLi hydrolysis has some effect on NaBH4 hydrolysis on the contrary. The possible mechanism can be depicted in Fig. 11 to describe the hydrolysis behavior of AlLi/NaBH4 in CoCl2 solution. The hydrolysis process can be divided into three periods: 1) The initial period: the reactions (1), (2), (3), and (4) can occur simultaneously in water, but reactions (1) and (2) do not proceed sustainably due to the hydrolysis byproducts Al(OH)3 and NaBO2. 2) The second period: the hydrolysis byproducts LiOH and Co2B act as good catalysts and simulate the Al/NaBH4 hydrolysis in reactions (1) and (2). Co2B has dual catalytic effects on Al/ NaBH4 hydrolysis. Co2B is a good promoter for NaBH4 hydrolysis, its catalytic ability is improved as Al(OH)3 acts as a carrier. Many previous works [11,16] have demonstrated that the homogeneous distribution of Co2B in Al(OH)3 is favorable for NaBH4 hydrolysis. Co2B also functions as a cathode and forms a micro-galvanic cell with anode Al in reactions (5) and (6): At the anode: Al þ 3H2O e 3e / Al(OH)3 þ 3Hþ
(5)
At the cathode: 3Hþ (Co2B) þ 3e / 1.5H2
(6)
Fig. 8. XRD patterns of hydrolysis byproducts of AlLi/NaBH4 mixtures. (a) Al-20 wt% Li alloy in water, (b) Al-20 wt% Li/NaBH4 with weight ratio of 95:5 in water, (c) Al-20 wt% Li/ NaBH4 with weight ratio of 1:1 in water, (d) Al-20 wt% Li/NaBH4 with weight ratio of 1:1 activated by 2.5 wt%CoCl2 solution, and (e) Al-5 wt% Li/NaBH4 with weight ratio of 1:1 activated by 2.5 wt%CoCl2 solution.
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Fig. 9. SEM micrographs of different AlLi/NaBH4 mixtures: (a) Al-20 wt% Li alloy in water, (b) Al-20 wt% Li/NaBH4 with weight ratio of 95:5 in water, (c) Al-20 wt% Li/NaBH4 with weight ratio of 1:1 in water, (d) Al-20 wt% Li/NaBH4 with weight ratio of 1:1 in 2.5 wt% CoCl2 solution.
% Transmittance (a.u.)
The hydrolysis byproducts LiOH, NaBO2, Al(OH)3, etc, can alter the solution conductivity and improve the electrochemical corrosion of Al. The solution presents alkaline and accelerates the reaction of Al and H2O. Fig. 12 shows successive hydrolysis performance of Al-20 wt%Li/NaBH4 at 303 K. Al-20 wt%Li/NaBH4mixture yields 1772 mL hydrogen g1 within 30 min in the hydrolysis byproducts of Al-20 wt%Li/NaBH4 activated by 2.5 wt% CoCl2 solution. It confirms that the hydrolysis byproducts of LiAl2(OH)7$xH2O, Bayerite Al(OH)3, NaBO2$2H2O, and Co2B still have high ability to accelerate AlLi/NaBH4 hydrolysis. But its maximum hydrogen generation rate is lower than that of Al-20 wt%Li/NaBH4 in 2.5 wt%
1253.56
CoCl2 solution and that of Al-20 wt%Li/NaBH4 in pure water because LiOH concentration is decreased in the followed period. 3) The end period: most of side reactions (7) and (8) occur between hydrolysis byproducts in the period. The hydrolysis byproduct presents alkaline and its pH can be kept at a stable value (approximately 11e13). The Al(OH)3 on the surface of Al can be eliminated and the corrosion of Al proceeds sustainably. LiOH þ 2Al(OH)3 þ xH2O 4 LiAl2(OH)7$xH2O
(7)
Al(OH)3 þ NaBO2 4 NaAlO2 þ H3BO3
(8)
b 1648.87
1130.13 825.42 948.42 741.53 1000.92
1386.14 536.14
3461.76
717.78 536.14 825.42
a
1646.46 1386.34
500
1000
1500
3448.27
2000
2500
3000
3500
4000
-1
cm
Fig. 10. IR curves of AlLi/NaBH4 mixture activated by 2.5 wt%CoCl2 solution after hydrolysis with different Li content: (a) 5 wt% Li and (b) 20 wt% Li.
Fig. 11. Schematic hydrolysis mechanism of AlLi/NaBH4 mixture in CoCl2 solution.
M.-Q. Fan et al. / Renewable Energy 46 (2012) 203e209
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Acknowledgments
1800
This work is financially supported by the National Science Foundation of China (Project Nos. 21003112, 11175169) and the Zhejiang Basic Research Program of China (No. Y4090507).
1600 1400 1200
References
1000 800 600 400 200 00
0.4g Al-20wt% Li/ NaBH4 +4ml 2.5wt % CoCl2 solution 0.4g Al-20wt% Li/ NaBH4+4mlH2O 0.4g Al-20wt% Li/ NaBH4+hydrolysis byproduct in shot 1
5
10
15
20
25
30
Fig. 12. Hydrogen generation of Al-20 wt% Li/NaBH4 in consecutive runs.
4. Conclusions The hydrogen evolution characteristics of AlLi/NaBH4 mixtures activated by CoCl2 solution are evaluated. For the given mixture composition, regulating AlLi/NaBH4 weight ratio, Li amount and CoCl2 concentration can improve hydrogen generation performance of the system. Hydrogen generation amount and rate of 1 g Al-20 wt%Li/NaBH4 (weight ratio 1:1) was up to 1674 mL and 3137 mL min1 g1 with 89% efficiency. Using hydrolysis experiments, XRD, SEM, etc., the effects of Li and CoCl2 are elaborated as good promoters for Al/NaBH4 hydrolysis. Their hydrolysis products LiOH and Co2B are good promoters for Al/NaBH4 hydrolysis. Their catalytic reactivity can be improved with the interaction of Al/ NaBH4 hydrolysis. Co2B loaded in Al(OH)3 carrier accelerate NaBH4 hydrolysis. NaBO2 presents alkaline and simulates Al hydrolysis. Co2B functions as a cathode and forms micro-galvanic cell which stimulates electrochemical corrosion of Al. The experimental data may lay a foundation for designing a practical hydrogen generator for portable application.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Controllable hydrogen generation and hydrolysis mechanism of AlLi/NaBH4 system activated by CoCl2 solution Mei-Qiang Fan*, Shu Liu, Wen Qiang Sun, Yong Fei, Hua Pan, Kang-Ying Shu* Department of Materials Science and Engineering, 258 Xueyuan Street, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2011 Accepted 18 March 2012 Available online 8 April 2012
Hydrogen generation performance of solid-state AlLi/NaBH4 mixture activated by CoCl2 solution was evaluated in the present work. Hydrogen generation performance can be regulated by AlLi/NaBH4 weight ratio, Li content, CoCl2 concentration, and hydrolysis temperature, among others. 1 g Al-20 wt% Li/NaBH4 mixture (weight ratio, 1:1) yields 1674 mL hydrogen with 89% efficiency in 2.5 wt%CoCl2 solution at 303 K. The relative hydrolysis mechanism and optimized composition design are reported and discussed through the analysis of XRD, SEM, IR, and particle size distribution. The hydrolysis byproducts LiOH and Co2B/Al(OH)3 are the major factors to improve the hydrolysis performance of an AlLi/NaBH4 mixture. Our experimental data show that the new method of hydrogen generation from AlLi/NaBH4 hydrolysis activated by CoCl2 solution may supply free CO2, portable hydrogen for proton exchange membrane fuel cell. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen generation AlLi alloy Sodium borohydride Cobalt dichloride Catalyst
1. Introduction There is an urgent demand to develop renewable energy with high efficiency and zero emission to address energy demand and environmental problems because of the increasing pressure of air pollution and the energy crisis caused by the large-scale consumption of fossil fuels. With its high calorific value, hydrogen has been recognized as an ideal energy carrier in fuel cells and internal combustion engines. However, the widespread use of hydrogen is hampered by the lack of cheap, safe, and efficient hydrogen storage methods [1,2]. There are no viable and established hydrogen storage methods which can provide reversible storage for hydrogen density >5 wt% at mild conditions. Typical complex hydrides such as light metal boron hydrides and amide have gained increasing attention because of their high hydrogen density. However, further research still has to be conducted before they can be practically applied. In the meantime, on-board hydrogen generation with off-board spent fuel regeneration promises to be a realistic solution for near-term hydrogen storage applications using chemical hydrides and metals. Sodium borohydride (NaBH4) is a good candidate for hydrogen generation and storage for fuel cell applications, offering high
* Corresponding authors. E-mail addresses: [email protected] (M.-Q. Fan), [email protected] (K.-Y. Shu). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2012.03.028
theoretical gravimetric hydrogen storage capacity of 10.8 wt%. NaBH4 hydrolysis can supply stable hydrogen for fuel cells and the hydrogen generation rate can be regulated by adding catalysts. NaBH4-based hydrolysis systems possess many advantages such as satisfactory reaction controllability, mild operation temperature, safe fuel storability, and so on, which make them a potential source of portable hydrogen. However, NaBH4-based hydrolysis systems cannot be widely applied because of low hydrogen density and high cost. The low solubility of NaBH4 and NaBO2$2H2O byproduct limits the maximum concentration to 20 wt% NaBH4 in aqueous solution, which only generates a hydrogen density of approximate 2e4 wt% [3]. In comparison with traditional hydrogen generation from aqueous NaBH4, the hydrolysis of solid-state NaBH4 in limited water amount can increase hydrogen density. Many interesting results have been achieved via optimization of NaBH4/H2O molar ratio, synthesis of high-performance catalyst, and ultra mixing of NaBH4/catalyst [4,5]. Liu [4] found that 6.7 wt% hydrogen densities could be obtained in a NaBH4/H2O/CoCl2 system. Moreover, if combined with another hydrogen generation material (aluminum and aluminum alloy), NaBH4 presents high hydrolysis performance because of the interaction of Al/NaBH4 hydrolysis. The byproduct Al(OH)3 from Al hydrolysis has a catalytic effect on the hydrolysis kinetic of NaBH4 [6]. Interestingly, numerous heat and alkaline byproducts from NaBH4 hydrolysis also stimulate Al hydrolysis. Shafirovich [7] found that the nano-Al/NaBH4 generated approximately 7 wt% hydrogen densities in limited water amount at 298 K. Catalysts such as transition metals (supported and non-supported)
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steel balls in an argon-filled glove box. The ball to mixture weight ratio was 26:1. The mixtures were then milled for 15 h in a QM3SPO4 planetary ball miller at 450 rpm under 0.2 MPa argon atmosphere unless noted otherwise.
1800 1600 1400 1200
2.2. Measurement of hydrogen evolution
1000 800 5wt% Li 10wt% Li 15wt% Li 20wt% Li
600 400 200 0
0
1
2
3
4
5
Fig. 1. Hydrogen generation from AlLi/NaBH4 mixture activated by CoCl2 solution with different Li content.
To measure the hydrogen evolution, 0.4 g of the AleLi/NaBH4 mixture was mixed with water (4 mL). The weight ratio of AleLi/ NaBH4 mixture was 1:1 and the CoCl2 concentration was 2.5 wt%, unless otherwise indicated. The mixture was pressed into a tablet in a 10 mm diameter stainless steel mold under 5 ton pressures before hydrolysis. Hydrolysis of the AleLi/NaBH4 mixture was carried out in CoCl2 solution at 303 K and 1 atm in a sealed reactor attached to a condenser. The generated hydrogen was collected at 298 K and 1 atm and measured from the water level change in the cylinder, i.e. water displacement. The hydrogen generation rate was calculated from the first bubble evolved from the beginning of the test. 2.3. Microstructure analysis
and metal salts [8e10] have been reported to increase the decomposition rate of NaBH4. They can also accelerate Al hydrolysis via functioning as a cathode of a micro-galvanic cell with aluminum. The AlCo/NaBH4 [11] and Al/NaBH4/CoCl2 systems [6] had better hydrolysis performance compared to Al/NaBH4. Thus far, the hydrolysis of the Al/NaBH4 system has to be performed either in an alkaline solution or by using nanoscale Al powder because of the thin aluminum oxide layer preventing sustainable Al hydrolysis. Unfortunately, strong alkaline solutions are difficult to handle and nanoscale Al powder is expensive. In the present paper, a new composition of milled AlLi alloy and NaBH4 powder activated in CoCl2 solution is designed to obtain high hydrogen density which can meet the demand of U.S. Department of Energy [3]. Good hydrogen generation performance of AlLi/NaBH4 powder can be obtained in neutral solution. The hydrogen generation amount and rate can be regulated by Li content, CoCl2 concentration and hydrolysis temperature. The composition design and relative hydrolysis mechanism are reported and discussed via hydrogen generation experiments and microstructure analysis. The aim of this work is to find the composition with highest hydrogen yield and elaborate the possible application of AlLi/NaBH4 activated by CoCl2 solution for hydrogen generation. 2. Experimental 2.1. Materials Li flakes (f * 0.5 mm, 99.9% purity), aluminum powder (mean size, 10 mm, common grade, 99.9% purity) (Beijing Xingry Technology Company Ltd.), NaBH4 (solid, 98.0% purity) (Tianjin Delan Chemical Company), and CoCl2$6H2O (98.0% AR) were used as the starting materials. All reagents were used as received. All reagents were weighed and placed in 50 mL stainless steel jars with stainless
Powder X-ray diffraction (XRD) patterns of the as-prepared samples were obtained via X-ray diffractometer (Thermo ARL, Switzerland, model ARL X’TRA) over a range of diffraction angles (q) from 2q ¼ 10 to 2q ¼ 80 with Cu Ka radiation filtered by a monochromator. Scanning electron microscopy (SEM) observations were performed using the model JSM-5610LV from JEOL Company which was equipped with INCA energy dispersive X-ray spectroscopy (EDS) measurements. Particle size distribution of the as-prepared samples was measured by a particulate size description analyzer (Dandong Better size, China, specification, BT2003). Surface area was collected by a surface area and porosity analyzer (Micromeritics, USA, model ASAP2020). The solid hydrolysis byproduct in the reactor was filtered using a vacuum pump and then dried in an oven at 313 K. The Fourier transform infrared (FTIR) spectra were recorded from KBr pellets in the range of 4000400 cm1 on a Nicolet 5700 spectrometer. 3. Results and discussion 3.1. Hydrogen generation performance Fig. 1 and Table 1 show the effect of lithium content on hydrogen generation of the AlLi/NaBH4 mixture in 2.5 wt% CoCl2 solution at 303 K. The mixtures exhibited high hydrogen generation rate, and most of the hydrogen were generated within 2 min of the reaction. As the Li content in AlLi alloy increased from 5 to 20 wt%, hydrogen generation volume increased from 924 to 1674 mL g1. The maximum hydrogen generation rate increased from 788 to 3137 mL g1 min1. Increasing Li content improved the hydrolytic performance of the Al/NaBH4 mixture, particularly Al, which resulted in an increase in conversion efficiency from 49% to 89%. Aluminum has low reactivity in a neutral solution because of an alumina layer covering its surface which prevents its reaction with water [12], so most of the hydrogen generated from Al-5 wt%Li/
Table 1 Hydrogen generation volume and maximum rate of AlLi/NaBH4 mixture activated i 2.5 wt% CoCl2 solution (milling time 15 h, maq/mmixture ¼ 10, Troom ¼ 298 K, pressure 5t). Materials
Maximum hydrogen generation rate (mL g1 min1)
Hydrogen generation volume (mL g1)
Efficiency (%)
Median diameter (mm)
Special surface area (m2 kg1)
Al-5wt% Li/NaBH4 Al-10wt% Li/NaBH4 Al-15wt% Li/NaBH4 Al-20wt% Li/NaBH4
788 1652 2166 3137
924 1167 1532 1674
49 62 81 89
9.84 9.76 9.71 8.77
285.84 290.72 301.31 323.69
M.-Q. Fan et al. / Renewable Energy 46 (2012) 203e209
Fig. 2. XRD patterns of different milled AlLi alloy.
NaBH4 mixture is due to the NaBH4 hydrolysis activated by CoCl2 solution. Although the hydrolytic byproduct NaBO2$2H2O provides alkaline which may stimulate Al hydrolysis, the effect is insignificant based on Fig. 1. Aluminum reactivity is gradually improved in stronger alkaline from Li hydrolysis due to higher Li content in AlLi alloy. Therefore, the hydrogen evolution from Al hydrolysis is increased. The milled AlLi particle size has been reported to decrease with increased Li content [13] due to the formation of AlLi alloy. The AlLi alloy has low enthalpy of formation (41.763 kJ/mol) [14], meaning it can be easily formed in the milling process. The peaks of AlLi phase (Fig. 2) became distinct and broadened when Li content increased from 5 to 20 wt%, reflecting that the formation of AlLi phase contributes to the gradual reduction of grain size. Correspondingly, Fig. 3 shows SEM micrographs of milled AlLi alloy; more small and irregular grains were found with increased Li content. The formation of AlLi phase prevents the formation of AleAl and LieLi atoms. More defects and dislocations are also generated due to repeated grain breakup, cold welding, and rewelding in the milling process. The median diameters of AlLi alloy decrease gradually, leading to an increase in the surface area (Table 1). This favors more contact between Al and water, resulting in the improved kinetic of AlLi/NaBH4 hydrolysis. The detailed particle size distribution is shown in Fig. 4. The particle size distribution shifts smaller with increasing Li content in AlLi alloy. Fig. 5 shows hydrogen generation of Al-20 wt%Li/NaBH4 mixture with different weight ratios in 2.5 wt% CoCl2 solution at 303 K. With Al-20 wt% Li/NaBH4 weight ratio changing from 9:1 to 1:3, the maximum hydrogen generation rate increases from 1751 to 3775 mL g1 min1, and the accumulated hydrogen generation
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volume increases from 1294 to 1913 mL g1. The hydrolysis reaction is ended in 1e2 min, with the conversion efficiency above 85%. Released heat and protons are generated in the hydrolysis process of AlLi/NaBH4 which stimulates the hydrolysis kinetic. The enthalpies of Al/Li/NaBH4 hydrolysis are 444, 204, and 216 kJ mol1 at 298 K [15], respectively. As the Al/Li/NaBH4 hydrolysis proceeds, the solution temperature increases and the largest value can be up to 373 K in the limited water amount. The increased solution temperature improves the hydrolysis kinetic of Al/Li/NaBH4. Fig. 6 shows the hydrogen generation curves of Al-20 wt% Li/NaBH4 mixture at different temperatures. As expected, maximum hydrogen generation rate increases from 2973 to 3235 mL g1min1 as solution temperature increases from 298 to 313 K. In the hydrolysis process, the hydrolysis byproduct Co2B generated from the reaction of NaBH4 and CoCl2 has dual catalytic ability on AlLi/NaBH4 hydrolysis. First, it may cover the Al surface and act as a cathode of a micro-galvanic couple [6]. The protons from AlLi/NaBH4 hydrolysis accelerate the corrosion rate of the AlCo2B micro-galvanic fuel. Second, Co2B is a good promoter for NaBH4 hydrolysis [10,11]. The released heat and hydrolysis byproducts Al(OH)3, CoB2 and NaBO2.2H2O have large effect on Al/ NaBH4 hydrolysis. Fig. 7 shows the hydrogen generation of Al-20 wt% Li/NaBH4 mixture in different concentrations of CoCl2 solution. The Al-20 wt % Li/NaBH4 mixture in pure water only yields 1258 mL g1 with 1186 mL g1 min1 maximum hydrogen generation rate. The same mixture in 0.5 wt% CoCl2 solution yields 1652 mL g1 hydrogen and 1221 mL g1 min1 maximum hydrogen generation rate. With CoCl2 concentration further increased to 2.5 wt%, the maximum hydrogen generation amount and rate are increased to 1674 mL g1 and 1751 mL g1 min1. The hydrogen generation amount and rate are proportional to CoCl2 concentration to some degree: higher CoCl2 concentration results in producing more Co2B amount, thus stimulating Al/NaBH4 hydrolysis. 3.2. Hydrolysis mechanism Many papers [4,6,7] have demonstrated the hydrolysis mechanism of Al/NaBH4, according to reactions (1, 2, 3). The reactions (1, 2) cannot occur continuously due to the hydrolysis byproduct Al(OH)3 and NaBO2$2H2O. The Al(OH)3 deposited on Al surface and prevented the contact of aluminum and water. The NaBO2.2H2O had alkaline and the alkaline reduced self-hydrolysis of NaBH4. Therefore, the sustainable hydrolysis reaction of Al/NaBH4 and water has to be performed under the help of catalyst. The alkaline such as NaOH, KOH dissolved Al(OH)3 and exposed Al surface directly to water [11]. Many metals and their borides such as Co, Co2B [4e6] often acted as a good promoter for NaBH4 hydrolysis. So there are many different hydrolysis processes in the reaction of
Fig. 3. SEM micrographs of different milled AlLi alloy: (a) 5 wt% Li and (b) 20 wt% Li.
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10 9
100
12
a
8
10
80
4
40
3 2
Differential (%)
Differential (%)
5
8 6 5
40
4 3
20
20
2
1
1
0 0.00
1.74 3.05
8.95
26.68
57.35
0 80.00
13
0 0.00
1.01
26.68
8.95
3.05
0 80.00
13
100
c
12
11
100
d
11 80
10
80
10
9 60
7 6 40
5 4 3
20
2
8
60
7 6
40
5 4 3
20
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1
Cumulation (%)
8
Differential (%)
9
Cumulation (%)
Differential (%)
60
7
Cumulation (%)
60
Cumulation (%)
6
0 0.00
80
9
7
12
100
b
11
1 1.39
3.05
8.95
0 80.00
21.42
0 0.00
1.74
8.95
3.05
0 80.00
19.23
Fig. 4. Particle size distribution of different AlLi/NaBH4 mixture: (a) 5 wt% Li, (b) 10 wt% Li, (c) 15 wt% Li, and (d) 20 wt% Li.
1800 1600 1400 1200 1000 800
9:1 3:1 1:1 1:3
600 400 200 0 0
1
2
3
4
t (min) Fig. 5. Hydrogen generation of Al-20 wt%Li/NaBH4 mixture with different weight ratios activated by 2.5 wt% CoCl2 solution.
(1)
DH ¼ 426.5 kJ mol1 at 298 K NaBH4 þ 4H2O / 4H2 þ NaBO2$2H2O
(2)
DH ¼ 216.7 kJ mol1 at 298 K 2Li þ 2H2O / 2LiOH þ H2
Accumulated hydrogen volume (mL g )
2000
Al þ 3H2O / Al(OH)3 þ 3H2
-1
Accumulated hydrogen volume (mL g-1)
AlLi/NaBH4 mixture and water. The composites of AlLi/NaBH4 have high reactivity and easily react with water at 298 K. Hydrolysis byproduct LiOH in reaction (3) is a good catalyst for Al hydrolysis but deteriorates NaBH4 self-hydrolysis. However, good hydrolysis of NaBH4 in alkaline solution can be obtained in CoCl2 solution due to the catalytic effect of Co2B [10,11], which is generated in reaction (4). Thus, the addition of Li and CoCl2 is important in improving the hydrolysis kinetics of Al/NaBH4 and altering the hydrolysis mechanism via the formation of new catalyst in the hydrolysis process.
(3)
1600 1400 1200 1000 298 K 303 K 308 K 313 K
800 600 400 200 0 0
1
2
3
4
t (min) Fig. 6. Hydrogen generation of Al-20 wt% Li/NaBH4 mixture activated by 2.5 wt% CoCl2 solution at different temperatures.
Accumulated hydrogen volume (mL g-1)
M.-Q. Fan et al. / Renewable Energy 46 (2012) 203e209
1800 1600 1400 1200 1000 800
2.5wt% CoCl2aq
600
1.0wt% CoCl2aq 0.5wt% CoCl2aq
400
0.2wt% CoCl2aq
200
water
0 0
1
2
3
4
5
t (min) Fig. 7. Hydrogen generation of Al-20 wt%Li/NaBH4 mixture activated by different concentrations of CoCl2 solution.
DH ¼ 204.5 kJ mol1 at 298 K 2CoCl2 þ 4NaBH4 þ 6H2O / Co2B þ 3HBO2 þ 12.5H2
(4)
Fig. 8 shows the XRD patterns of hydrolysis byproducts of AlLi/ NaBH4 mixture at different hydrolysis conditions. The peaks of Al are identified in the XRD patterns of the hydrolysis byproducts of Al-20 wt%Li alloy, reflecting that part of initial aluminum has not completely reacted with water. With 5 wt% NaBH4 substituting Al, the peaks of Al disappear and the peaks of LiAl2(OH)7$xH2O and Bayerite Al(OH)3 become conspicuous. With 50 wt% NaBH4 further substituting Al, the peaks become broadened and the particle size decreases based on the Scherrer equation ðDhkl ¼ ðklÞ=ðbcosqhkl ÞÞ. The peaks of NaBO2$2H2O, Co2B, and Na2B4O7 can be identified. The formation of Na2B4O7 comes from the reaction of NaBO2 and CO2 [4]. The initial material NaBH4 is observed in the hydrolysis byproducts of the AlLi/NaBH4 mixture because of its high stability in alkaline solution. But it is not identified in XRD patterns of hydrolysis byproduct of AlLi/NaBH4 mixture activated by CoCl2 solution. Fig. 9 shows SEM micrographs of hydrolysis byproducts of different Al-20 wt%Li/NaBH4 mixtures, small plate LiAl2(OH)7$xH2O and flocculent solid Al(OH)3 were found in the of Al-20 wt%Li alloy. With more addition of NaBH4, the particle size of plate
207
LiAl2(OH)7$xH2O decreases smaller and the Al(OH)3 becomes looser. It can be reflected that NaBH4 hydrolysis has some effect on AlLi hydrolysis. Fig. 10 shows the IR absorption curve of hydrolysis byproducts of different AlLi/NaBH4 mixtures activated by 2.5 wt% CoCl2 solution with different Li content. The detailed results are listed in the following: IR peaks (KBr, cm1) of hydrolysis byproduct of Al-20 wt% Li/NaBH4 mixture, 3462 (vs), 1649 (s), 1386 (vs), 1254 (w), 1130 (w), 1001 (s), 948 (m), 825(w), 742(s), and 536 (vs); IR peaks (KBr, cm1) of hydrolysis byproducts of Al-5 wt% Li/NaBH4 mixture: 3448 (w), 1646 (m), 1386 (m), 1132 (w), 999 (m), 948 (m), 825 (m), 708 (w), and 536 (m). The wide band identified in the 3000e3600 cm1 region was stimulated by OeH stretching and the band at approximately 1646 cm1 was caused by HeOeH deformation bands. Bands characteristic of AleO (1386 cm1), BeO, and HeOeB vibrators were also observed. Peaks at 3462 (OH stretching), 1386 (AlO stretching), and 536 cm1 (OH stretching) became stronger with increased Li content. Higher Li content creates more LiAl2(OH)7$xH2O groups, which corresponds to the XRD analysis in Fig. 8. The characteristic BeH stretching band of NaBH4 appears in hydrolysis byproducts of Al-5 wt% Li/NaBH4 mixture, but is not observed in those of Al-20 wt% Li/NaBH4 mixture, showing that AlLi hydrolysis has some effect on NaBH4 hydrolysis on the contrary. The possible mechanism can be depicted in Fig. 11 to describe the hydrolysis behavior of AlLi/NaBH4 in CoCl2 solution. The hydrolysis process can be divided into three periods: 1) The initial period: the reactions (1), (2), (3), and (4) can occur simultaneously in water, but reactions (1) and (2) do not proceed sustainably due to the hydrolysis byproducts Al(OH)3 and NaBO2. 2) The second period: the hydrolysis byproducts LiOH and Co2B act as good catalysts and simulate the Al/NaBH4 hydrolysis in reactions (1) and (2). Co2B has dual catalytic effects on Al/ NaBH4 hydrolysis. Co2B is a good promoter for NaBH4 hydrolysis, its catalytic ability is improved as Al(OH)3 acts as a carrier. Many previous works [11,16] have demonstrated that the homogeneous distribution of Co2B in Al(OH)3 is favorable for NaBH4 hydrolysis. Co2B also functions as a cathode and forms a micro-galvanic cell with anode Al in reactions (5) and (6): At the anode: Al þ 3H2O e 3e / Al(OH)3 þ 3Hþ
(5)
At the cathode: 3Hþ (Co2B) þ 3e / 1.5H2
(6)
Fig. 8. XRD patterns of hydrolysis byproducts of AlLi/NaBH4 mixtures. (a) Al-20 wt% Li alloy in water, (b) Al-20 wt% Li/NaBH4 with weight ratio of 95:5 in water, (c) Al-20 wt% Li/ NaBH4 with weight ratio of 1:1 in water, (d) Al-20 wt% Li/NaBH4 with weight ratio of 1:1 activated by 2.5 wt%CoCl2 solution, and (e) Al-5 wt% Li/NaBH4 with weight ratio of 1:1 activated by 2.5 wt%CoCl2 solution.
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Fig. 9. SEM micrographs of different AlLi/NaBH4 mixtures: (a) Al-20 wt% Li alloy in water, (b) Al-20 wt% Li/NaBH4 with weight ratio of 95:5 in water, (c) Al-20 wt% Li/NaBH4 with weight ratio of 1:1 in water, (d) Al-20 wt% Li/NaBH4 with weight ratio of 1:1 in 2.5 wt% CoCl2 solution.
% Transmittance (a.u.)
The hydrolysis byproducts LiOH, NaBO2, Al(OH)3, etc, can alter the solution conductivity and improve the electrochemical corrosion of Al. The solution presents alkaline and accelerates the reaction of Al and H2O. Fig. 12 shows successive hydrolysis performance of Al-20 wt%Li/NaBH4 at 303 K. Al-20 wt%Li/NaBH4mixture yields 1772 mL hydrogen g1 within 30 min in the hydrolysis byproducts of Al-20 wt%Li/NaBH4 activated by 2.5 wt% CoCl2 solution. It confirms that the hydrolysis byproducts of LiAl2(OH)7$xH2O, Bayerite Al(OH)3, NaBO2$2H2O, and Co2B still have high ability to accelerate AlLi/NaBH4 hydrolysis. But its maximum hydrogen generation rate is lower than that of Al-20 wt%Li/NaBH4 in 2.5 wt%
1253.56
CoCl2 solution and that of Al-20 wt%Li/NaBH4 in pure water because LiOH concentration is decreased in the followed period. 3) The end period: most of side reactions (7) and (8) occur between hydrolysis byproducts in the period. The hydrolysis byproduct presents alkaline and its pH can be kept at a stable value (approximately 11e13). The Al(OH)3 on the surface of Al can be eliminated and the corrosion of Al proceeds sustainably. LiOH þ 2Al(OH)3 þ xH2O 4 LiAl2(OH)7$xH2O
(7)
Al(OH)3 þ NaBO2 4 NaAlO2 þ H3BO3
(8)
b 1648.87
1130.13 825.42 948.42 741.53 1000.92
1386.14 536.14
3461.76
717.78 536.14 825.42
a
1646.46 1386.34
500
1000
1500
3448.27
2000
2500
3000
3500
4000
-1
cm
Fig. 10. IR curves of AlLi/NaBH4 mixture activated by 2.5 wt%CoCl2 solution after hydrolysis with different Li content: (a) 5 wt% Li and (b) 20 wt% Li.
Fig. 11. Schematic hydrolysis mechanism of AlLi/NaBH4 mixture in CoCl2 solution.
M.-Q. Fan et al. / Renewable Energy 46 (2012) 203e209
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Acknowledgments
1800
This work is financially supported by the National Science Foundation of China (Project Nos. 21003112, 11175169) and the Zhejiang Basic Research Program of China (No. Y4090507).
1600 1400 1200
References
1000 800 600 400 200 00
0.4g Al-20wt% Li/ NaBH4 +4ml 2.5wt % CoCl2 solution 0.4g Al-20wt% Li/ NaBH4+4mlH2O 0.4g Al-20wt% Li/ NaBH4+hydrolysis byproduct in shot 1
5
10
15
20
25
30
Fig. 12. Hydrogen generation of Al-20 wt% Li/NaBH4 in consecutive runs.
4. Conclusions The hydrogen evolution characteristics of AlLi/NaBH4 mixtures activated by CoCl2 solution are evaluated. For the given mixture composition, regulating AlLi/NaBH4 weight ratio, Li amount and CoCl2 concentration can improve hydrogen generation performance of the system. Hydrogen generation amount and rate of 1 g Al-20 wt%Li/NaBH4 (weight ratio 1:1) was up to 1674 mL and 3137 mL min1 g1 with 89% efficiency. Using hydrolysis experiments, XRD, SEM, etc., the effects of Li and CoCl2 are elaborated as good promoters for Al/NaBH4 hydrolysis. Their hydrolysis products LiOH and Co2B are good promoters for Al/NaBH4 hydrolysis. Their catalytic reactivity can be improved with the interaction of Al/ NaBH4 hydrolysis. Co2B loaded in Al(OH)3 carrier accelerate NaBH4 hydrolysis. NaBO2 presents alkaline and simulates Al hydrolysis. Co2B functions as a cathode and forms micro-galvanic cell which stimulates electrochemical corrosion of Al. The experimental data may lay a foundation for designing a practical hydrogen generator for portable application.
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