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A novel AlBiOCl composite for hydrogen generation from water
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 5 5 e6 6 6 2
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A novel AleBiOCl composite for hydrogen generation from water Chong Zhao a, Fen Xu a,**, Lixian Sun a,*, Jun Chen a, Xiaolei Guo a, Erhu Yan a, Fang Yu a, Hailiang Chu a, Hongliang Peng a, Yongjin Zou a, Zongwen Liu b,***, Fuwei Li c a
Guangxi Key Laboratory of Information Materials & Guangxi Collaborative Innovation Centre of Structure and Property for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China b School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia c Lanzhou Institute of Chemical Physics, Lanzhou, PR China
article info
abstract
Article history:
In this paper, a novel AleBiOCl material has been prepared by milling Al powder and BiOCl
Received 30 August 2018
firstly. Experimental results show that BiOCl-doped can prevent an inert alumina film
Received in revised form
forming on the surface of Al particles and induce the rapid hydrogen generation as well as
20 December 2018
high conversion rate. SEM, XRD, EDS, TEM, XPS and calorimetric techniques are used for
Accepted 21 December 2018
the mechanism analysis of the samples. The results demonstrate the fresh surface of Al,
Available online 14 February 2019
AlCl3, Bi and Bi2O3 are produced in situ under ball milling Al and BiOCl, which play an important role in hydrolysis reaction of Al. The hydrogen yield of Al-15 wt% BiOCl rises to
Keywords:
1058.1 mL g1 in about 5 min, corresponding to the high conversion yield of 91.6% at room
Hydrogen generation
temperature. After doping additives (such as LiH, Bi or AlCl3), hydrogen generation per-
AleBiOCl composite
formances of AleBiOCl-additive are further improved. For example, the conversion yield
AleH2O reaction
and maximum hydrogen generation rate (MHGR) of AleBiOCleLiH can increase to 94.9%
Additives
and 3178.5 mL g1 min1, respectively. Therefore, the proposed materials in this paper are
Mechanism
expected to serve as a hydrogen generation material for the fuel cells. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction On-demand hydrogen produced by metals hydrolysis can overcome the difficulty of hydrogen storage and transportation. The research of hydrogen produced by Al hydrolysis has particularly attracted broad attention, because of its high theoretical yields and low cost [1e3]. However, a dense
oxide film on the surface of Al can prevent Al-water reaction in practice. One of the key techniques of producing H2 from AleH2O reaction is to destroy the inert film on the surface of Al [4e7]. For solving this problem, a number of methods were reported. For instance, metallic Al could react with water for releasing H2 in aqueous alkaline solutions, such as in Na2SnO3 or NaAlO2 solution [8,9]. The alkaline solution helps to destroy
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (F. Xu), [email protected] (L. Sun), [email protected] (Z. Liu). https://doi.org/10.1016/j.ijhydene.2018.12.165 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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the inert layer on the surface of Al and realize AleH2O continuous reaction at ambient condition. But the strong alkaline solutions have a certain degree of corrosion. Meanwhile, a lot of researches indicated doped metal hydrides (such as LiH, MgH2, CaH2, LiBH4, NaBH4, NaAlH3, LiAlH4 and Li3AlH6 [10e13]) into Al powder were effective approaches. The doped metal hydrides could break down the dense oxide film on the surface of Al particles and increase the released hydrogen amount of Al-based composite as they also produced H2 by reacting with water themselves. And alloying of Al with low melting point metals (such as Li, Ga, In, Sn, Bi, etc.) by an electric arc technique [14,15] or by high-energy ball milling method [16e18] was also effective method for this goal. Wang et al. [19] used the gas atomization to prepare the Al-10 wt%Bi-10 wt%Sn composite with core-shell structure. The conversion yield of the composite was 91.3%, but H2 yield for per g composite was not high due to the content of additives reached 20 wt%. The advantage of the composite is that it has a certain oxidation resistance. Meanwhile, the research results showed that the use of large amounts of additives like salts (such as NaCl, KCl, NiCl2, MgCl2, SnCl2 and BiCl3 [20e22] would also lead to a decrease in hydrogen emissions. In addition, some other methods like adding the oxides [23,24] could enhance the hydrogen generation performance of Ale H2O reaction. Dupiano et al. [24] found that hydrogen conversion yield of AleBi2O3 system could be close to 100% within about 34 min at 80 C. Experimental results showed that Bi2O3 had the best catalytic role in oxide additives reported, such as CaO, TiO2, Al2O3, MgO, MoO3, etc. [18,23] (summarized in appendix Table S1), which could be effective in promoting Ale H2O reaction. However, the releasing hydrogen rate of Ale Bi2O3 was still too slow. Based on the above analysis, another kind of bismuth compound (BiOCl), which is one of heterogeneous catalysts and unique layered structure powder [25], was chosen to enhance Al activity for AleH2O reaction. To the best of our knowledge, there is no report for H2 generation based on hydrolysis of AleBiOCl system. In this work, hydrogen generation performance of AleBiOCl has been studied. The effects of metallic elements (Bi, In, Ce, Co), hydrides (LiH, NaH, KH, CaH2) and inorganic salts (LiCl, NaCl, KCl, AlCl3) also have been systematically studied. The results demonstrate that the AleBiOCl material has good performances of hydrogen generation and is expected to supply hydrogen for mobile hydrogen sources.
Experimental section
samples oxidized, all operations were carried out under argon atmospheres. Al and BiOCl powders were weighed and put into special stainless pots in an argon-filled glove box. The stainless pots had some stainless-steel balls with 6 mm in diameter. The mass ratio of ball to powder was controlled as 45:1, 60: 1 or 90:1. The ball milling times was respectively set as 1, 3, 5 or 7 h. Then, the ball milling conditions were a rotational speed of 250 rpm and 0.2 MPa argon atmospheres without special emphasis. After ball milling, the special stainless was put into the argon-filled glove box; the milled powder was taken out and kept in the glove box.
Hydrogen generation and characterization The equipment used to evaluate hydrogen generation performance was described in our previously published report [12]. Firstly, 0.1 g AleBiOCl sample was put into a 250 mL reactor under argon atmosphere in the glove box. This reactor was placed into a thermostatic water bath to keep the temperature constant at the required temperature. After constant temperature, 10 mL of distilled water with the same temperature was added into the reactor. The releasing gas from Alwater reaction flowed through a condenser and dryer to absorb moisture, and then was collected in a container filled with water. The volume of H2 gas is measured by the water displacement method. The water displaced was measured by electronic balance (UX2200H, Shimadzu Corporation, Japan), which connected with a computer which could automatically record the mass change data every second. Finally, the hydrogen volume producing was obtained according the mass of water displaced and was converted into the amount at 298.15 K through measuring the actual temperature of the gas. In addition, the data of blank experiment was subtracted from the hydrogen volume. Each experiment was repeated three times at least; the error was lower than 5%. The micro morphology of prepared sample was observed by a scanning electron microscopy (SEM, JSM-6360LV, JEOL Ltd, Japan) and a transmission electron microscope (TEM, Talos F200X, FEI company, USA) and the corresponding elemental mapping of the sample was obtained using an energy dispersive spectrometer (EDS). The X-ray powder diffraction (XRD) pattern of the sample was recorded by using a D8 Advance diffractometer equipped with Cu/Ka radiation and irradiated with a scanning rate of 10 /min. The elemental analyses of samples (XPS) were carried out on an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher). The heat amount of Al-water reaction was measured on a C80 Calvet type calorimeter (Setaram, France) using a membrane mixing steel cell.
Reagents and preparation of Al based composite Aluminium powders (Angang Group Aluminium Powder Co., Ltd., mean size: 10 mm, 99% purity), Bismuth (III) chloride oxide powders (Alfa Aesar, 99.95%, metals basis) were used as the starting materials. Bi (99.99%), In (99.99%), Ce (99.9%), Co (99.5%), LiCl (99.9%), NaCl (99.99%), KCl (99.95%), AlCl3 (99.99%), LiH (99.94%), NaH (60% in paraffin oil), KH (30% in paraffin oil) and CaH2 (>92%) were purchased from Alfa Aesar. AleBiOCl composite was prepared by ball milled Al with BiOCl in a planetary ball mill (Retsch PM-400 MA). To avoid
Results and discussion Effects of BiOCl content In this part, the hydrogen generation performances of Al-x wt.% BiOCl (x ¼ 5, 10, 15, 20 or 25) at 298.15 K were characterized and presented in Fig. 1. As can be seen from Fig. 1a, pure Al cannot basically react with water. For the most Al-x wt.% BiOCl composites (except for 5 wt% and 10 wt% BiOCl),
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Fig. 1 e Curves of hydrogen conversion yield (a) and hydrogen generation rate (b) of AleBiOCl with different amounts of BiOCl at 298.15 K.
start times of hydrogen release are very short when deionized water is added into the reactor (see illustration of Fig. 1a). The start time of hydrogen release is a induction time according to the three reaction stages mechanism stated by Dupiano et al. [22]. The induction time of hydrogen production was strongly affected by environment temperature, the additive type and the grain size of the ground powder mixture. It generally exceeds several minutes or hours. From the inset in Fig. 1a, except for induction time of Al-10 wt% BiOCl is in about 40 s and Al-5 wt.% BiOCl almost does not react, others (Al-x wt.% BiOCl, x ¼ 15, 20 or 25) are all in 15 s. This indicates that Al powder has been more effectively activated by milling with BiOCl. After induction period, they enter a period of “Fast” reaction. In the period of “Fast” reaction, the conversion yield and maximum hydrogen generation rate (MHGR) all augment with increasing the content of BiOCl before 90 s (see Fig. 1a and b). It further indicates that the conversion yield of Al-25 wt% BiOCl is the highest and the MHGRs of samples doped 25 wt%, 20 wt% and 15 wt% BiOCl are 905.4, 839.5 and 491.4 mL g1 min1, respectively, at the period of “Fast” reaction (see Fig. 1b). After entering the period of “slow” reaction (after about 200 s), the increase of conversion yield of Al-25 wt% BiOCl becomes very slow, and the conversion yields of Al-15 wt% BiOCl and Al-20 wt % BiOCl are preferable. Finally, the peak area of Al-15 wt% BiOCl is the largest (see Fig. 1b). It implies that Al-15 wt% BiOCl has the highest hydrogen release. It can be found that the conversion yields of Al-15 wt% BiOCl and Al-20 wt% BiOCl are 91.6% (corresponding to hydrogen yield of 1058.1 mL g1) and 90.9% (corresponding to hydrogen yield of 988.2 mL g1), respectively (see Fig. 1a). But the conversion yield and hydrogen yield of Al25 wt% BiOCl are only 86.0% and 876.5 mL g1. This is ascribed to that BiOCl cannot produce hydrogen itself and excess BiOCl would reduce the conversion yield. It demostrates that AleBiOCl system has the advantages of large amount of hydrogen release, fast reaction speed and low reaction temperature comparing to that of AleBi2O3 [24]. Hereby, Al-15 wt% BiOCl is chosen for subsequent studies.
Effects of the ball-to-powder ratio and ball milling time The mass ratio of balls to the powder is an important factor in the milling process. The mass ratio of ball to AleBiOCl
powders has a great effect on the hydrogen production performance of the composite. From Fig. S1 and Table 1, the MHGR of AleBiOCl increases with increasing the mass ratio of ball to powder; that is the MHGR rises from 412.2 to 890.1 mL g1 min1 for the mass ratio from 45: 1 to 90: 1. This ascribes that increasing the mass ratio can increase the number of collisions per unit time, inducing to Al particles have more active surface and promote the initial hydrogen generation rate of AleBiOCl. However, all conversion yields are basically overlap in the end (see Fig. S1). The results suggest the ball-to-powder ratio only affects on hydrogen generation rate and not on the conversion yield for AleBiOCl. Fig. 2 shows the SEM images of Al-15 wt% BiOCl with different milling times at the optimal ratio of ball to powder of 60:1. It is observed that from Fig. 2, the shapes of Al particles change from the initial spherical (the as-received Al powder without ball milling, Fig. 2e) to platelet, and its mean size changes markedly from 100 mm to 10 mm (Fig. 2aed). When milled for 1 h, the particles of AleBiOCl composites tend to flatten with some cracks caused by cold welding and fracture, and form large particles of laminar structure. Prolonging milling time to 5 h, there is a tendency that the flake-like particles shrink into small blocks. Continuing to extend the milling time to 7 h, the size of particle is decreased gradually. Generally, the smaller the size of particle is, the larger it's specific surface area and the more defects or cracks the particle has. Namely, the particles hold more active sites, which help to improve the hydrolysis reaction speed of Al. The
Table 1 e Hydrogen generation performances of the composites with different conditions of ball-milling. The conditions of ball-milling The mass ratio of 45: 1 ball to powder 60: 1 90: 1 Ball milling time 1 h 3h 5h 7h
MHGR mL Hydrogen Conversion g1 min1 yield mL g1 yield % 412.2 491.4 890.1 156.0 278.0 491.4 903.0
1045.3 1058.1 1030.2 596.0 1053.4 1058.1 865.1
90.5 91.6 89.2 51.6 91.2 91.6 74.9
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Table 2 e Hydrogen generation performances of Ale BiOCl with different hydrolysis temperatures. Water temperature K
MHGR mL g1 Hydrogen yield Conversion mL g1 yield % min1
273.15 288.15 298.15 303.15 318.15
37.7 105.5 508.2 713.9 1404.6
355.9 932.2 1058.1 1072.4 1087.9
30.8 80.7 91.6 92.8 94.2
1087.9 mL g1, indicating that the increasing water temperature improves the kinetic of hydrolysis of AleBiOCl composite. According to the Arrhenius equation: k ¼ A expð Ea =RTÞ
Fig. 2 e SEM images of Al-15 wt% BiOCl milled for 1 h (a), 3 h (b), 5 h (c), 7 h (d) and raw material (e) at the ratio of ball to powder of 60:1.
performances of hydrogen generation of Al-15 wt% BiOCl with different ball milling time are also presented in Table 1. As can be seen from Table 1, the 1 h milled sample already possesses better hydrolysis properties such as its conversion yield of 51.6% under the room temperature while it can be further improved to 91.2% after being milled for 3 h. As the ball milling time prolongs to 5 h, the conversion yield runs up to the highest value (91.6%). However, continuing to prolong the ball-milling time to 7 h, it causes the decrease of the conversion yield (74.9%). But the hydrogen generation rates of all samples rise with increasing the ball milling time. Al powders immediately react with water at the fastest speed after ball milled for 7 h (see Fig.S2). The MHGR rises from 508.2 to 903.0 mL g1 min1 corresponding to ball-milling time from 5 h to 7 h. However, the sample milled 7 h shows obviously lower H2 yield than one of the 5 h. It maybe belongs to the initial speed of Al-water reaction is too fast to timely remove the excessive hydroxide by-product on the Al particles surface for 7 h milled sample, leading to difficulty for Al to keep in touch with water. Hence, the optimal milling time for Al-15 wt % BiOCl sample is 5 h.
where k is the maximum chemical reaction rate (here is MHGR, mL g1 min1), A is a pre-exponential factor, R is the gas constant (8.314 J mol1 K1), and T is the absolute temperature (K). A plot of lnk vs the reciprocal absolute temperature (1/T) is presented in Fig. 3. The apparent activation energy (Ea) of the reaction of AleBiOCl with water is calculated as 26.9 kJ mol1. As well known, a low activation energy generally is beneficial to make a high reaction reactivity. This result indicates that doping BiOCl into Al powder obviously reduces the apparent activation energy of the AleH2O reaction (which is 42.5e68.4 kJ mol1 in NaOH solution [24]). Hence, it further demonstrates doped BiOCl can enhance the reactivity of Al. The reaction heat of the AleBiOCl with water was measured by a C80 calorimeter (see Fig. S3) and the measured value is 14.5 kJ g1. It implies Al almost reacts completely with water for Al-15 wt% BiOCl composite, due to the reaction heat of pure Al with water is 16.45 kJ g1 [26,27]. This result is consistent with the conversion rate (91.6%) of Al-15 wt% BiOCl (see Section Effects of BIOCl content).
Effects of additives In this section, part of BiOCl in the AleBiOCl composite was replaced by additives such as hydride, metal element and
Effects of initial water temperature Elevating temperature is helpful for starting the Al-water reaction. Table 2 shows the hydrogen releasing properties of Al15 wt% BiOCl composite at different initial water temperatures. The results show that both conversion yields and the MHGR increase with increasing temperature from 273.15 to 318.15 K. The hydrogen yields also rises from 355.9 to
(1)
Fig. 3 e A curve of ln k vs 1000/T.
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chloride salt, and AleBiOCl-additives composites are prepared by ball milling Al powder with BiOCl and the above additives under the same preparation conditions as that of AleBiOCl. Table 3 and Fig. S3 show the hydrogen generation performances of AleBiOCl-additives (such as hydride, metal element and chloride salt). According to Table 3 and Fig. S3, all Al-based ternary composites with hydrides (such as LiH, NaH, KH and CaH2) exhibit the rapidly hydrolysis rate and relatively high conversion yield except for doped CaH2. Especially, the conversion yield of Al-10 wt% BiOCl-5 wt.% LiH reaches to 94.9% and its MHGR is the highest (3178 mL g1 min1), which is obviously superior to the Al-10 mol%LiH-10 mol% KCl (19.4% conversion rate in 25 C) [10]. For Al-based ternary composites with pure metals (such as Bi, In, Co and Ce), the results indicate these metal can greatly promote conversion rate of the composite, but the effects on dynamics of the reaction are different. For example, the conversion yields of AleBiOClMetals (Co, Ce or Bi) are all above 93%; but Co-doped or Cedoped make the MHGR of AleBiOCl reduce, In-doped has little effect and only Bi-doped makes its MHGR rise to 2566.5 mL g1 min1. For Al-based ternary composite with chloride salts (such as LiCl, KCl, NaCl and AlCl3), the four salts are all helpful to improve the hydrogen generation performances of AleBiOCl composite. This is attributed to the dissolution enthalpy of these salts in water (37.03 kJ mol-1 for LiCl, 3.88 kJ mol1 for NaCl, 17.22 kJ mol1 for KCl and 54.6 kJ mol1 for AlCl3) [16], which can heighten the temperature of the aqueous solution as they contact with water, resulting in the rate of AleH2O reaction will be greatly boosted. But the MHGR of AleBiOCleNaCl is minimum, which is only 573.0 mL g1 min1. It ascribes to the Mohs hardness of
Table 3 e A comparison of hydrolysis performance of the AleBiOCl system using different additives. Samples Al-15 wt% BiOCl Al-10 wt% BiOCl-5 wt.% LiH Al-10 wt% BiOCl-5 wt.% NaH Al-10 wt% BiOCl-5 wt.% KH Al-10 wt% BiOCl-5 wt.% CaH2 Al-10 wt% BiOCl-5 wt.% Bi Al-10 wt% BiOCl-5 wt.% In Al-10 wt% BiOCl-5 wt.% Ce Al-10 wt% BiOCl-5 wt.% Co Al-10 wt% BiOCl-5 wt.% LiCl Al-10 wt% BiOCl-5 wt.% NaCl Al-10 wt% BiOCl-5 wt.% KCl Al-10 wt% BiOCl-5 wt.% AlCl3
MHGR mL g1 min1
Hydrogen yield mL g1
Conversion yield %
491.4 3178.5
1058.1 1241.0
91.6 94.9
2158.5
911.1
75.6
2420.0
945.9
79.8
314.1
1002.2
82.6
2566.5
1153.4
99.9
582.0
1109.5
96.1
120.3
1080.4
93.6
201.0
1100.1
95.3
2231.2
1096.7
95.0
573.0
1146.5
99.3
2328.0
1108.2
96.0
2442.0
1154.2
99.9
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NaCl with the smallest value among them [20]. From Table 3, it can also be found that Al-10 wt% BiOCl-5 wt.% AlCl3 reaches 99.9% of conversion yield and 2442.0 mL g1 min1 of the MHGR. Obviously, the MHGR of Al-10 wt% BiOCl-5 wt.% AlCl3 is higher than that of Al-10 wt% Bi-10 wt% AlCl3 (900 ml g1 in 5 min) [28] and Al-30 wt% Bi-10 wt% C (270 mL g1 min1) [29]. In a word, hydrides have obvious effect on the rate of hydrogen evolution for AleBiOCl. But metallic elements and salts are mainly to enhance the conversion yield of AleBiOCl composite. According to above analysis, respectively doped LiH, Bi or AlCl3 is preferable choice for the AleBiOCl system.
Mechanism analysis of AleBiOCl composite To explore the hydrogen generation mechanism of AleBiOCl, knowing its chemical composition is of great importance. Fig. 4 shows the XRD patterns of AleBiOCl before and after milled for 5 h. From Fig. 4, it can be found that the composite mainly consists of Al and BiOCl before ball milling (see “a” curve in Fig. 4). However, the new peaks, which belonging to Bi and Bi2O3, appear on the “b” curve after ball milling. The result indicates that BiOCl becomes to Bi and Bi2O3 during the process of ball milling. The SEM, TEM images and the corresponding element distribution maps obtained by EDS of AleBiOCl are presented in Fig. 5 and 6, respectively. From Fig. 5, it can be observed that Al particles produce a lot of defects. The elements distribution of the selected area clearly indicates that Bi, O and Cl are homogenously dispersed around the Al particles. This point can also be verified by Fig. 6a. From Fig. 6, it can still be found that elementals Bi, Cl and O distribute on the surface of Al particles before hydrolysis (Fig. 6a); and the by-produces are floc and the elemental Bi and Cl are covered by elementals Al and O after hydrolysis (Fig. 6b). Xu et al. [22] certified that AlCl3 and Sn were produced in situ in the process of ball milling Al with SnCl2. Combining the above analysis, AlCl3 served as the oxidation product to balance the anions (Cl) in the AleBiOCl system. Although, there is no observed AlCl3 trace in XRD, which maybe owing to the amount of AlCl3 is very small or it is an amorphous. Therefore, the solid state reaction occurs in the ball milling process and can be explained as follows:
Fig. 4 e XRD patterns of AleBiOCl before (a) and after (b) milled for 5 h.
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Fig. 5 e SEM-EDS pictures of Al-15 wt% BiOCl milled for 5 h.
Fig. 6 e TEM-EDS pictures of Al-15 wt% BiOCl before (a) and after (b) hydrolysis.
Al þ 3BiOCl / AlCl3 þ Bi þ Bi2O3
(2)
To further demonstrate that above the reaction occurred in the ball milling process, XPS analysis of AleBiOCl before and after ball-milled were performed, and the results are presented in Fig. 7. From XPS spectra of AleBiOCl after ball milling (see Fig. 7a), the AleBiOCl composite is made of elementals Al, Bi, Cl and O, which is the same as Figs. 5 and 6. Fig. 7b is the XPS spectra of Bi 4f7/2 and 4f5/2 for AleBiOCl composite before ball milled. It can be found that the peaks with binding energy of 159.22 eV and 164.53 should belong to Bi 4f7/2 and Bi 4f5/2 region for BiOCl [30,31], respectively. After ball milling, the XPS spectra of Bi 4f7/2 and 4f5/2 (Fig. 7c) show that there are four peaks. According to literature [32], the peaks at 159.13 eV and 164.44 eV should be assigned to Bi 4f7/2 and Bi 4f5/2 region for Bi2O3; and the peaks at 156.65 eV and 161.99 eV attribute to Bi 4f7/2 and Bi 4f5/2 region for Bi0. The result further demonstrates that the BiOCl changes to Bi and Bi2O3 after ball milling. It suggested that the reaction (see Eq. (2)) did happen under the ball milling process. In a word, the AleBiOCl composite consists of a large
amount of Al, a small amount of Bi, Bi2O3 and AlCl3 after ball milled Al powder with BiOCl. In addition, Al particles are activated through being repeatedly flattened, cold weld, fractured and rewelding in ball milling process [33]. At the same time, deformations as well as some cracks of Al particles are formed in the interior of metal crystal. The fresh surface of Al therefore is created by a huge shear force, leading to Al react with water for releasing H2. As the consequence, the mechanism of BiOCl activating Al is due to produce of fresh surface of Al, AlCl3, Bi and Bi2O3 in the ball milling process. Initially, hydrogen and local high temperature are produced by the reaction on the fresh surface of Al as meeting with water. Furthermore, some other fresh surfaces are created again by the dissolution of AlCl3 on the basis of its physicochemical properties. Here, AlCl3 is similar to the role of “salt gates” [34]. Meanwhile, part of the AlCl3 can also hydrolyze to releasing HCl. And the Hþ from the generated HCl can further react with Al in the local area to release H2 (△Н of the reaction is 491 kJ mol1 calculated by the way described at https://www.materialsproject.org/) and the Cl is beneficial for the removal of Al(OH)3 layer on the surface of Al
Fig. 7 e XPS spectra of AleBiOCl.
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through a pit corrosion process [28]. At the same time, the dissolution of AlCl3 can release a lot of instantaneous heat to accelerate hydrogen generation rate for the reaction of Ale H2O and to improve the kinetic of reaction of AleH2O. Additionally, the produced Bi plays a much more important role in promoting the hydrolysis of Al. It has been discovered that Al/ Bi can work as micro-galvanic cell which intensifies the corrosion of Al [16]. Finally, the active site of Bi2O3 can also promote the hydrolysis of Al [24], because the surface of Bi2O3 can absorb water to increase the chance of Al contacting water. The synergistic effect of these factors leads to improve the hydrogen generation performance of AleBiOCl composite.
Conclusions In summary, we proposed a new Al-based hydrogen generation material which was synthesized via milling Al powders with BiOCl. The study results showed that the apparent activation energy (Ea) of reaction AleH2O reduced to 26.9 kJ mol1 by doping BiOCl. Therefore, hydrogen generation performance of the material was enhanced; leading to it reacts with water at room temperature. The hydrogen yield and conversion yield of Al-15 wt% BiOCl composite were 1058.1 mL g1 and 91.6%, respectively. In addition, the effects of metal elements, hydrides and inorganic salts on the hydrogen production of Ale BiOCl were respectively investigated. The results showed that Bi, LiH and AlCl3 can speed the hydrolysis of AleBiOCl. The hydrogen conversion yields of AleBiOCleBi, AleBiOCleLiH and AleBiOCleAlCl3 were closed to 100%. Their mHGR and hydrogen yield were 2566.5 mL g1 min1 and 1153.4 mL g1 (for AleBiOCleBi), 3178.5 mL g1 min1 and 1241.0 mL g1 (for Ale BiOCleLiH), 2442.0 mL g1 min1 and 1154.2 mL g1 (for Ale BiOCleAlCl3), respectively. The doped LiH showed the optimal performance due to LiH itself could also react with water to release hydrogen, induced to enhance the hydrogen capacity of the system. The mechanism study of hydrogen generation from the reaction of AleBiOCl with water showed that a solid-solid reaction occurred, and Bi, Bi2O3 and AlCl3 had been produced in situ during ball milling process. The synergistic action of Bi, Bi2O3, AlCl3 and the fresh surface of Al markedly improved the performances of hydrogen generation of AleBiOCl composite. Experimental results demonstrate the proposed materials in this paper are expected to serve as a portable hydrogen generation material for the fuel cells.
Acknowledgements This work was supported by the National Natural Science Foundation of China (U1501242, 51671062 and 51871065), the Guangxi Collaborative Innovation Centre of Structure and Property for New Energy and Material (2012GXNSFGA06002), Guangxi Science and Technology Project (AD17195073 CE), Guangxi Major Science and Technology Special Project (AA17202030-1) and the Guangxi Key Laboratory of Information Laboratory Foundation (161002-Z, 161002-K and 161003-K). We would like to thank Mr. Yujie Sun from School of Material Science and Engineering, China
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University of Geosciences (Beijing) for his XRD analysis and valuable discussion.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.165.
references
[1] Zhong H, Ouyang LZ, Ye JS, Liu JW, Wang H, Yao XD, Zhu M. An one-step approach towards hydrogen production and storage through regeneration of NaBH4. Energy Storage Mater 2017;7:222e8. [2] Wang Y, Leung DYC. A high-performance aluminum-feed microfluidic fuel cell stack. J Power Sources 2016;336:427e36. [3] Xiao R, Chen J, Fu K, Zheng X, Wang T, Zheng J, Li X. Hydrolysis batteries: generating electrical energy during hydrogen absorption. Angew Chem Int Ed 2018;57:1e6. [4] Wang YQ, Gai WZ, Zhang XY, Pan HY, Cheng Z, Xu P, Deng ZY. Effect of storage environment on hydrogen generation by the reaction of Al with water. RSC Adv 2017;7:2103e9. [5] Meng HX, Wang N, Dong YM, Jia ZL, Gao LJ, et al. Influence of M-B (M ¼ Fe, Co, Ni) on aluminum-water reaction. J Power Sources 2014;268:550e6. [6] Li P, Xu F, Sun LX, Xia YP, Chen J, et al. Hydrogen generation performance of novel Al-LiH-metal oxides. Inorg Chem Front 2018;5:1700e6. [7] Ma Y, Dong X, Wang R, Bin D, Wang Y, et al. Combining water reduction and liquid fuel oxidization by nickel hydroxide for flexible hydrogen production. Energy Storage Mater 2018;11:260e6. [8] Soler L, Maria Candela A, Macanas J, Munoz M, Casado J. Hydrogen generation from water and aluminum promoted by sodium stannate. Int J Hydrogen Energy 2010;35:1038e48. [9] Soler L, Maria Candela A, Macanas J, Munoz M, Casado J. In situ generation of hydrogen from water by aluminum corrosion in solutions of sodium aluminate. J Power Sources 2009;192:21e6. [10] Liu Y, Wang X, Liu H, Dong Z, Li S, Ge H, et al. Effect of salts addition on the hydrogen generation of Al-LiH composite elaborated by ball milling. Energy 2015;89:907e13. [11] Wu T, Xu F, Sun LX, Cao Z, Chu HL, Sun YJ, et al. Al-Li3AlH6: a novel composite with high activity for hydrogen generation. Int J Hydrogen Energy 2014;39:10392e8. [12] Xu F, Zhang X, Sun L, Yu F, Li P. Hydrogen generation of a novel Al-NaMgH3 composite reaction with water. Int J Hydrogen Energy 2017;42:30535e42. [13] Lluis S, Jorge M, Maria M, Juan C. Synergistic hydrogen generation from aluminum, aluminum alloys and sodium borohydride in aqueous solutions. Int J Hydrogen Energy 2007;32:4702e10. [14] Wei C, Liu D, Xu S, Cui T, An Q, et al. Effects of Cu additives on the hydrogen generation performance of Al-rich alloys. J Alloy Compd 2018;738:105e10. [15] Ilyukhina AV, Kravchenko OV, Bulychev BM. Studies on microstructure of activated aluminum and its hydrogen generation properties in aluminum/water reaction. J Alloy Compd 2017;690:321e9. [16] Fan MQ, Xu F, Sun LX. Studies on hydrogen generation characteristics of hydrolysis of the ball milling Al-based materials in pure water. Int J Hydrogen Energy 2007;32:2809e15. [17] Preez SD, Bessarabov DG. Hydrogen generation by the hydrolysis of mechanochemically activated aluminum-tin-
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indium composites in pure water. Int J Hydrogen Energy 2018;43:21398e413. Chen X, Zhao Z, Liu X, Hao M, Chen A, et al. Hydrogen generation by the hydrolysis reaction of ball-milled aluminum lithium alloys. J Power Sources 2014;254:345e52. Wang CP, Liu YH, Liu HX, Yang T, Chen XR, Yang SY, Liu XJ. A novel self-assembling Al-based composite powder with high hydrogen generation efficiency. Sci Rep 2015;5:17428. Czech E, Troczynski T. Hydrogen generation through massive corrosion of deformed aluminum in water. Int J Hydrogen Energy 2010;35:1029e37. Rong S, Zhong BZ, Lan SY. The effect of NiCl2 and Na2CO3 on hydrogen production by Al/H2O system. Int J Hydrogen Energy 2017;42:3586e92. Xu F, Sun LX, Lan X, Chu H, Sun Y, Zhou H, et al. Mechanism of fast hydrogen generation from pure water using Al-SnCl2 and Bi-doped Al-SnCl2 composites. Int J Hydrogen Energy 2014;39:5514e21. Gai WZ, Fang CS, Deng ZY. Hydrogen generation by the reaction of Al with water using oxides as catalysts. Int J Energy Res 2013;38:918e25. Dupiano P, Stamatis D, Dreizin EL. Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders. Int J Hydrogen Energy 2011;36:4781e91. Hu J, Wu X, Huang C, Fan W, Qiu X. Visible light photocatalytic activity induced by Rh(III) modification on the surface of BiOCl. Appl Surf Sci 2016;387:45e50.
[26] Wang HZ, Leung DYC, Leung MKH, Ni M. A review on hydrogen production using aluminum and aluminum alloys. Renew Sustain Energy Rev 2009;13:845e53. [27] Uehara K, Takeshita H, Kotaka H. Hydrogen gas generation in the wet cutting of aluminum and its alloys. J Mater Process Technol 2002;127:174e7. [28] Fan MQ, Xu F, Sun LX, Zhao JN, Jiang T, Li WX. Hydrolysis of ball milling Al-Bi-hydride and Al-Bi-salt mixture for hydrogen generation. J Alloy Compd 2008;460:125e9. [29] Zhang F, Edalati K, Arita M, Horita Z. Fast hydrolysis and hydrogen generation on Al-Bi alloys and Al-Bi-C composites synthesized by high pressure. Int J Hydrogen Energy 2017;42:29121e30. [30] Morgan WE, Stec WJ, Vanwazer JR. Inner-oribital bindingenergy shifts of antimony and bismuth compounds. Inorg Chem 1973;12:953e5. [31] Wang CH, Shao CL, Liu YC, Zhang LN. Photocatalytic properties BiOCl and Bi2O3, nanofibers prepared by electrospinning. Scripta Mater 2008;59:332e5. [32] Dharmadhikari VS. Characterisation of thin films of bismuth oxide by X-ray photoelectron spectroscopy. J Electron Spectrosc Relat Phenom 1982;25:181e9. [33] Suryanarayana C. Mechanical alloying and milling. Prog Mater Sci 2001;46:1e184. [34] Alinejad B, Mahmoodi K. A novel method for generating hydrogen by hydrolysis of highly activated aluminum nanoparticles in pure water. Int J Hydrogen Energy 2009;34:7934e8.
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A novel AleBiOCl composite for hydrogen generation from water Chong Zhao a, Fen Xu a,**, Lixian Sun a,*, Jun Chen a, Xiaolei Guo a, Erhu Yan a, Fang Yu a, Hailiang Chu a, Hongliang Peng a, Yongjin Zou a, Zongwen Liu b,***, Fuwei Li c a
Guangxi Key Laboratory of Information Materials & Guangxi Collaborative Innovation Centre of Structure and Property for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China b School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia c Lanzhou Institute of Chemical Physics, Lanzhou, PR China
article info
abstract
Article history:
In this paper, a novel AleBiOCl material has been prepared by milling Al powder and BiOCl
Received 30 August 2018
firstly. Experimental results show that BiOCl-doped can prevent an inert alumina film
Received in revised form
forming on the surface of Al particles and induce the rapid hydrogen generation as well as
20 December 2018
high conversion rate. SEM, XRD, EDS, TEM, XPS and calorimetric techniques are used for
Accepted 21 December 2018
the mechanism analysis of the samples. The results demonstrate the fresh surface of Al,
Available online 14 February 2019
AlCl3, Bi and Bi2O3 are produced in situ under ball milling Al and BiOCl, which play an important role in hydrolysis reaction of Al. The hydrogen yield of Al-15 wt% BiOCl rises to
Keywords:
1058.1 mL g1 in about 5 min, corresponding to the high conversion yield of 91.6% at room
Hydrogen generation
temperature. After doping additives (such as LiH, Bi or AlCl3), hydrogen generation per-
AleBiOCl composite
formances of AleBiOCl-additive are further improved. For example, the conversion yield
AleH2O reaction
and maximum hydrogen generation rate (MHGR) of AleBiOCleLiH can increase to 94.9%
Additives
and 3178.5 mL g1 min1, respectively. Therefore, the proposed materials in this paper are
Mechanism
expected to serve as a hydrogen generation material for the fuel cells. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction On-demand hydrogen produced by metals hydrolysis can overcome the difficulty of hydrogen storage and transportation. The research of hydrogen produced by Al hydrolysis has particularly attracted broad attention, because of its high theoretical yields and low cost [1e3]. However, a dense
oxide film on the surface of Al can prevent Al-water reaction in practice. One of the key techniques of producing H2 from AleH2O reaction is to destroy the inert film on the surface of Al [4e7]. For solving this problem, a number of methods were reported. For instance, metallic Al could react with water for releasing H2 in aqueous alkaline solutions, such as in Na2SnO3 or NaAlO2 solution [8,9]. The alkaline solution helps to destroy
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (F. Xu), [email protected] (L. Sun), [email protected] (Z. Liu). https://doi.org/10.1016/j.ijhydene.2018.12.165 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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the inert layer on the surface of Al and realize AleH2O continuous reaction at ambient condition. But the strong alkaline solutions have a certain degree of corrosion. Meanwhile, a lot of researches indicated doped metal hydrides (such as LiH, MgH2, CaH2, LiBH4, NaBH4, NaAlH3, LiAlH4 and Li3AlH6 [10e13]) into Al powder were effective approaches. The doped metal hydrides could break down the dense oxide film on the surface of Al particles and increase the released hydrogen amount of Al-based composite as they also produced H2 by reacting with water themselves. And alloying of Al with low melting point metals (such as Li, Ga, In, Sn, Bi, etc.) by an electric arc technique [14,15] or by high-energy ball milling method [16e18] was also effective method for this goal. Wang et al. [19] used the gas atomization to prepare the Al-10 wt%Bi-10 wt%Sn composite with core-shell structure. The conversion yield of the composite was 91.3%, but H2 yield for per g composite was not high due to the content of additives reached 20 wt%. The advantage of the composite is that it has a certain oxidation resistance. Meanwhile, the research results showed that the use of large amounts of additives like salts (such as NaCl, KCl, NiCl2, MgCl2, SnCl2 and BiCl3 [20e22] would also lead to a decrease in hydrogen emissions. In addition, some other methods like adding the oxides [23,24] could enhance the hydrogen generation performance of Ale H2O reaction. Dupiano et al. [24] found that hydrogen conversion yield of AleBi2O3 system could be close to 100% within about 34 min at 80 C. Experimental results showed that Bi2O3 had the best catalytic role in oxide additives reported, such as CaO, TiO2, Al2O3, MgO, MoO3, etc. [18,23] (summarized in appendix Table S1), which could be effective in promoting Ale H2O reaction. However, the releasing hydrogen rate of Ale Bi2O3 was still too slow. Based on the above analysis, another kind of bismuth compound (BiOCl), which is one of heterogeneous catalysts and unique layered structure powder [25], was chosen to enhance Al activity for AleH2O reaction. To the best of our knowledge, there is no report for H2 generation based on hydrolysis of AleBiOCl system. In this work, hydrogen generation performance of AleBiOCl has been studied. The effects of metallic elements (Bi, In, Ce, Co), hydrides (LiH, NaH, KH, CaH2) and inorganic salts (LiCl, NaCl, KCl, AlCl3) also have been systematically studied. The results demonstrate that the AleBiOCl material has good performances of hydrogen generation and is expected to supply hydrogen for mobile hydrogen sources.
Experimental section
samples oxidized, all operations were carried out under argon atmospheres. Al and BiOCl powders were weighed and put into special stainless pots in an argon-filled glove box. The stainless pots had some stainless-steel balls with 6 mm in diameter. The mass ratio of ball to powder was controlled as 45:1, 60: 1 or 90:1. The ball milling times was respectively set as 1, 3, 5 or 7 h. Then, the ball milling conditions were a rotational speed of 250 rpm and 0.2 MPa argon atmospheres without special emphasis. After ball milling, the special stainless was put into the argon-filled glove box; the milled powder was taken out and kept in the glove box.
Hydrogen generation and characterization The equipment used to evaluate hydrogen generation performance was described in our previously published report [12]. Firstly, 0.1 g AleBiOCl sample was put into a 250 mL reactor under argon atmosphere in the glove box. This reactor was placed into a thermostatic water bath to keep the temperature constant at the required temperature. After constant temperature, 10 mL of distilled water with the same temperature was added into the reactor. The releasing gas from Alwater reaction flowed through a condenser and dryer to absorb moisture, and then was collected in a container filled with water. The volume of H2 gas is measured by the water displacement method. The water displaced was measured by electronic balance (UX2200H, Shimadzu Corporation, Japan), which connected with a computer which could automatically record the mass change data every second. Finally, the hydrogen volume producing was obtained according the mass of water displaced and was converted into the amount at 298.15 K through measuring the actual temperature of the gas. In addition, the data of blank experiment was subtracted from the hydrogen volume. Each experiment was repeated three times at least; the error was lower than 5%. The micro morphology of prepared sample was observed by a scanning electron microscopy (SEM, JSM-6360LV, JEOL Ltd, Japan) and a transmission electron microscope (TEM, Talos F200X, FEI company, USA) and the corresponding elemental mapping of the sample was obtained using an energy dispersive spectrometer (EDS). The X-ray powder diffraction (XRD) pattern of the sample was recorded by using a D8 Advance diffractometer equipped with Cu/Ka radiation and irradiated with a scanning rate of 10 /min. The elemental analyses of samples (XPS) were carried out on an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher). The heat amount of Al-water reaction was measured on a C80 Calvet type calorimeter (Setaram, France) using a membrane mixing steel cell.
Reagents and preparation of Al based composite Aluminium powders (Angang Group Aluminium Powder Co., Ltd., mean size: 10 mm, 99% purity), Bismuth (III) chloride oxide powders (Alfa Aesar, 99.95%, metals basis) were used as the starting materials. Bi (99.99%), In (99.99%), Ce (99.9%), Co (99.5%), LiCl (99.9%), NaCl (99.99%), KCl (99.95%), AlCl3 (99.99%), LiH (99.94%), NaH (60% in paraffin oil), KH (30% in paraffin oil) and CaH2 (>92%) were purchased from Alfa Aesar. AleBiOCl composite was prepared by ball milled Al with BiOCl in a planetary ball mill (Retsch PM-400 MA). To avoid
Results and discussion Effects of BiOCl content In this part, the hydrogen generation performances of Al-x wt.% BiOCl (x ¼ 5, 10, 15, 20 or 25) at 298.15 K were characterized and presented in Fig. 1. As can be seen from Fig. 1a, pure Al cannot basically react with water. For the most Al-x wt.% BiOCl composites (except for 5 wt% and 10 wt% BiOCl),
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Fig. 1 e Curves of hydrogen conversion yield (a) and hydrogen generation rate (b) of AleBiOCl with different amounts of BiOCl at 298.15 K.
start times of hydrogen release are very short when deionized water is added into the reactor (see illustration of Fig. 1a). The start time of hydrogen release is a induction time according to the three reaction stages mechanism stated by Dupiano et al. [22]. The induction time of hydrogen production was strongly affected by environment temperature, the additive type and the grain size of the ground powder mixture. It generally exceeds several minutes or hours. From the inset in Fig. 1a, except for induction time of Al-10 wt% BiOCl is in about 40 s and Al-5 wt.% BiOCl almost does not react, others (Al-x wt.% BiOCl, x ¼ 15, 20 or 25) are all in 15 s. This indicates that Al powder has been more effectively activated by milling with BiOCl. After induction period, they enter a period of “Fast” reaction. In the period of “Fast” reaction, the conversion yield and maximum hydrogen generation rate (MHGR) all augment with increasing the content of BiOCl before 90 s (see Fig. 1a and b). It further indicates that the conversion yield of Al-25 wt% BiOCl is the highest and the MHGRs of samples doped 25 wt%, 20 wt% and 15 wt% BiOCl are 905.4, 839.5 and 491.4 mL g1 min1, respectively, at the period of “Fast” reaction (see Fig. 1b). After entering the period of “slow” reaction (after about 200 s), the increase of conversion yield of Al-25 wt% BiOCl becomes very slow, and the conversion yields of Al-15 wt% BiOCl and Al-20 wt % BiOCl are preferable. Finally, the peak area of Al-15 wt% BiOCl is the largest (see Fig. 1b). It implies that Al-15 wt% BiOCl has the highest hydrogen release. It can be found that the conversion yields of Al-15 wt% BiOCl and Al-20 wt% BiOCl are 91.6% (corresponding to hydrogen yield of 1058.1 mL g1) and 90.9% (corresponding to hydrogen yield of 988.2 mL g1), respectively (see Fig. 1a). But the conversion yield and hydrogen yield of Al25 wt% BiOCl are only 86.0% and 876.5 mL g1. This is ascribed to that BiOCl cannot produce hydrogen itself and excess BiOCl would reduce the conversion yield. It demostrates that AleBiOCl system has the advantages of large amount of hydrogen release, fast reaction speed and low reaction temperature comparing to that of AleBi2O3 [24]. Hereby, Al-15 wt% BiOCl is chosen for subsequent studies.
Effects of the ball-to-powder ratio and ball milling time The mass ratio of balls to the powder is an important factor in the milling process. The mass ratio of ball to AleBiOCl
powders has a great effect on the hydrogen production performance of the composite. From Fig. S1 and Table 1, the MHGR of AleBiOCl increases with increasing the mass ratio of ball to powder; that is the MHGR rises from 412.2 to 890.1 mL g1 min1 for the mass ratio from 45: 1 to 90: 1. This ascribes that increasing the mass ratio can increase the number of collisions per unit time, inducing to Al particles have more active surface and promote the initial hydrogen generation rate of AleBiOCl. However, all conversion yields are basically overlap in the end (see Fig. S1). The results suggest the ball-to-powder ratio only affects on hydrogen generation rate and not on the conversion yield for AleBiOCl. Fig. 2 shows the SEM images of Al-15 wt% BiOCl with different milling times at the optimal ratio of ball to powder of 60:1. It is observed that from Fig. 2, the shapes of Al particles change from the initial spherical (the as-received Al powder without ball milling, Fig. 2e) to platelet, and its mean size changes markedly from 100 mm to 10 mm (Fig. 2aed). When milled for 1 h, the particles of AleBiOCl composites tend to flatten with some cracks caused by cold welding and fracture, and form large particles of laminar structure. Prolonging milling time to 5 h, there is a tendency that the flake-like particles shrink into small blocks. Continuing to extend the milling time to 7 h, the size of particle is decreased gradually. Generally, the smaller the size of particle is, the larger it's specific surface area and the more defects or cracks the particle has. Namely, the particles hold more active sites, which help to improve the hydrolysis reaction speed of Al. The
Table 1 e Hydrogen generation performances of the composites with different conditions of ball-milling. The conditions of ball-milling The mass ratio of 45: 1 ball to powder 60: 1 90: 1 Ball milling time 1 h 3h 5h 7h
MHGR mL Hydrogen Conversion g1 min1 yield mL g1 yield % 412.2 491.4 890.1 156.0 278.0 491.4 903.0
1045.3 1058.1 1030.2 596.0 1053.4 1058.1 865.1
90.5 91.6 89.2 51.6 91.2 91.6 74.9
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Table 2 e Hydrogen generation performances of Ale BiOCl with different hydrolysis temperatures. Water temperature K
MHGR mL g1 Hydrogen yield Conversion mL g1 yield % min1
273.15 288.15 298.15 303.15 318.15
37.7 105.5 508.2 713.9 1404.6
355.9 932.2 1058.1 1072.4 1087.9
30.8 80.7 91.6 92.8 94.2
1087.9 mL g1, indicating that the increasing water temperature improves the kinetic of hydrolysis of AleBiOCl composite. According to the Arrhenius equation: k ¼ A expð Ea =RTÞ
Fig. 2 e SEM images of Al-15 wt% BiOCl milled for 1 h (a), 3 h (b), 5 h (c), 7 h (d) and raw material (e) at the ratio of ball to powder of 60:1.
performances of hydrogen generation of Al-15 wt% BiOCl with different ball milling time are also presented in Table 1. As can be seen from Table 1, the 1 h milled sample already possesses better hydrolysis properties such as its conversion yield of 51.6% under the room temperature while it can be further improved to 91.2% after being milled for 3 h. As the ball milling time prolongs to 5 h, the conversion yield runs up to the highest value (91.6%). However, continuing to prolong the ball-milling time to 7 h, it causes the decrease of the conversion yield (74.9%). But the hydrogen generation rates of all samples rise with increasing the ball milling time. Al powders immediately react with water at the fastest speed after ball milled for 7 h (see Fig.S2). The MHGR rises from 508.2 to 903.0 mL g1 min1 corresponding to ball-milling time from 5 h to 7 h. However, the sample milled 7 h shows obviously lower H2 yield than one of the 5 h. It maybe belongs to the initial speed of Al-water reaction is too fast to timely remove the excessive hydroxide by-product on the Al particles surface for 7 h milled sample, leading to difficulty for Al to keep in touch with water. Hence, the optimal milling time for Al-15 wt % BiOCl sample is 5 h.
where k is the maximum chemical reaction rate (here is MHGR, mL g1 min1), A is a pre-exponential factor, R is the gas constant (8.314 J mol1 K1), and T is the absolute temperature (K). A plot of lnk vs the reciprocal absolute temperature (1/T) is presented in Fig. 3. The apparent activation energy (Ea) of the reaction of AleBiOCl with water is calculated as 26.9 kJ mol1. As well known, a low activation energy generally is beneficial to make a high reaction reactivity. This result indicates that doping BiOCl into Al powder obviously reduces the apparent activation energy of the AleH2O reaction (which is 42.5e68.4 kJ mol1 in NaOH solution [24]). Hence, it further demonstrates doped BiOCl can enhance the reactivity of Al. The reaction heat of the AleBiOCl with water was measured by a C80 calorimeter (see Fig. S3) and the measured value is 14.5 kJ g1. It implies Al almost reacts completely with water for Al-15 wt% BiOCl composite, due to the reaction heat of pure Al with water is 16.45 kJ g1 [26,27]. This result is consistent with the conversion rate (91.6%) of Al-15 wt% BiOCl (see Section Effects of BIOCl content).
Effects of additives In this section, part of BiOCl in the AleBiOCl composite was replaced by additives such as hydride, metal element and
Effects of initial water temperature Elevating temperature is helpful for starting the Al-water reaction. Table 2 shows the hydrogen releasing properties of Al15 wt% BiOCl composite at different initial water temperatures. The results show that both conversion yields and the MHGR increase with increasing temperature from 273.15 to 318.15 K. The hydrogen yields also rises from 355.9 to
(1)
Fig. 3 e A curve of ln k vs 1000/T.
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chloride salt, and AleBiOCl-additives composites are prepared by ball milling Al powder with BiOCl and the above additives under the same preparation conditions as that of AleBiOCl. Table 3 and Fig. S3 show the hydrogen generation performances of AleBiOCl-additives (such as hydride, metal element and chloride salt). According to Table 3 and Fig. S3, all Al-based ternary composites with hydrides (such as LiH, NaH, KH and CaH2) exhibit the rapidly hydrolysis rate and relatively high conversion yield except for doped CaH2. Especially, the conversion yield of Al-10 wt% BiOCl-5 wt.% LiH reaches to 94.9% and its MHGR is the highest (3178 mL g1 min1), which is obviously superior to the Al-10 mol%LiH-10 mol% KCl (19.4% conversion rate in 25 C) [10]. For Al-based ternary composites with pure metals (such as Bi, In, Co and Ce), the results indicate these metal can greatly promote conversion rate of the composite, but the effects on dynamics of the reaction are different. For example, the conversion yields of AleBiOClMetals (Co, Ce or Bi) are all above 93%; but Co-doped or Cedoped make the MHGR of AleBiOCl reduce, In-doped has little effect and only Bi-doped makes its MHGR rise to 2566.5 mL g1 min1. For Al-based ternary composite with chloride salts (such as LiCl, KCl, NaCl and AlCl3), the four salts are all helpful to improve the hydrogen generation performances of AleBiOCl composite. This is attributed to the dissolution enthalpy of these salts in water (37.03 kJ mol-1 for LiCl, 3.88 kJ mol1 for NaCl, 17.22 kJ mol1 for KCl and 54.6 kJ mol1 for AlCl3) [16], which can heighten the temperature of the aqueous solution as they contact with water, resulting in the rate of AleH2O reaction will be greatly boosted. But the MHGR of AleBiOCleNaCl is minimum, which is only 573.0 mL g1 min1. It ascribes to the Mohs hardness of
Table 3 e A comparison of hydrolysis performance of the AleBiOCl system using different additives. Samples Al-15 wt% BiOCl Al-10 wt% BiOCl-5 wt.% LiH Al-10 wt% BiOCl-5 wt.% NaH Al-10 wt% BiOCl-5 wt.% KH Al-10 wt% BiOCl-5 wt.% CaH2 Al-10 wt% BiOCl-5 wt.% Bi Al-10 wt% BiOCl-5 wt.% In Al-10 wt% BiOCl-5 wt.% Ce Al-10 wt% BiOCl-5 wt.% Co Al-10 wt% BiOCl-5 wt.% LiCl Al-10 wt% BiOCl-5 wt.% NaCl Al-10 wt% BiOCl-5 wt.% KCl Al-10 wt% BiOCl-5 wt.% AlCl3
MHGR mL g1 min1
Hydrogen yield mL g1
Conversion yield %
491.4 3178.5
1058.1 1241.0
91.6 94.9
2158.5
911.1
75.6
2420.0
945.9
79.8
314.1
1002.2
82.6
2566.5
1153.4
99.9
582.0
1109.5
96.1
120.3
1080.4
93.6
201.0
1100.1
95.3
2231.2
1096.7
95.0
573.0
1146.5
99.3
2328.0
1108.2
96.0
2442.0
1154.2
99.9
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NaCl with the smallest value among them [20]. From Table 3, it can also be found that Al-10 wt% BiOCl-5 wt.% AlCl3 reaches 99.9% of conversion yield and 2442.0 mL g1 min1 of the MHGR. Obviously, the MHGR of Al-10 wt% BiOCl-5 wt.% AlCl3 is higher than that of Al-10 wt% Bi-10 wt% AlCl3 (900 ml g1 in 5 min) [28] and Al-30 wt% Bi-10 wt% C (270 mL g1 min1) [29]. In a word, hydrides have obvious effect on the rate of hydrogen evolution for AleBiOCl. But metallic elements and salts are mainly to enhance the conversion yield of AleBiOCl composite. According to above analysis, respectively doped LiH, Bi or AlCl3 is preferable choice for the AleBiOCl system.
Mechanism analysis of AleBiOCl composite To explore the hydrogen generation mechanism of AleBiOCl, knowing its chemical composition is of great importance. Fig. 4 shows the XRD patterns of AleBiOCl before and after milled for 5 h. From Fig. 4, it can be found that the composite mainly consists of Al and BiOCl before ball milling (see “a” curve in Fig. 4). However, the new peaks, which belonging to Bi and Bi2O3, appear on the “b” curve after ball milling. The result indicates that BiOCl becomes to Bi and Bi2O3 during the process of ball milling. The SEM, TEM images and the corresponding element distribution maps obtained by EDS of AleBiOCl are presented in Fig. 5 and 6, respectively. From Fig. 5, it can be observed that Al particles produce a lot of defects. The elements distribution of the selected area clearly indicates that Bi, O and Cl are homogenously dispersed around the Al particles. This point can also be verified by Fig. 6a. From Fig. 6, it can still be found that elementals Bi, Cl and O distribute on the surface of Al particles before hydrolysis (Fig. 6a); and the by-produces are floc and the elemental Bi and Cl are covered by elementals Al and O after hydrolysis (Fig. 6b). Xu et al. [22] certified that AlCl3 and Sn were produced in situ in the process of ball milling Al with SnCl2. Combining the above analysis, AlCl3 served as the oxidation product to balance the anions (Cl) in the AleBiOCl system. Although, there is no observed AlCl3 trace in XRD, which maybe owing to the amount of AlCl3 is very small or it is an amorphous. Therefore, the solid state reaction occurs in the ball milling process and can be explained as follows:
Fig. 4 e XRD patterns of AleBiOCl before (a) and after (b) milled for 5 h.
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Fig. 5 e SEM-EDS pictures of Al-15 wt% BiOCl milled for 5 h.
Fig. 6 e TEM-EDS pictures of Al-15 wt% BiOCl before (a) and after (b) hydrolysis.
Al þ 3BiOCl / AlCl3 þ Bi þ Bi2O3
(2)
To further demonstrate that above the reaction occurred in the ball milling process, XPS analysis of AleBiOCl before and after ball-milled were performed, and the results are presented in Fig. 7. From XPS spectra of AleBiOCl after ball milling (see Fig. 7a), the AleBiOCl composite is made of elementals Al, Bi, Cl and O, which is the same as Figs. 5 and 6. Fig. 7b is the XPS spectra of Bi 4f7/2 and 4f5/2 for AleBiOCl composite before ball milled. It can be found that the peaks with binding energy of 159.22 eV and 164.53 should belong to Bi 4f7/2 and Bi 4f5/2 region for BiOCl [30,31], respectively. After ball milling, the XPS spectra of Bi 4f7/2 and 4f5/2 (Fig. 7c) show that there are four peaks. According to literature [32], the peaks at 159.13 eV and 164.44 eV should be assigned to Bi 4f7/2 and Bi 4f5/2 region for Bi2O3; and the peaks at 156.65 eV and 161.99 eV attribute to Bi 4f7/2 and Bi 4f5/2 region for Bi0. The result further demonstrates that the BiOCl changes to Bi and Bi2O3 after ball milling. It suggested that the reaction (see Eq. (2)) did happen under the ball milling process. In a word, the AleBiOCl composite consists of a large
amount of Al, a small amount of Bi, Bi2O3 and AlCl3 after ball milled Al powder with BiOCl. In addition, Al particles are activated through being repeatedly flattened, cold weld, fractured and rewelding in ball milling process [33]. At the same time, deformations as well as some cracks of Al particles are formed in the interior of metal crystal. The fresh surface of Al therefore is created by a huge shear force, leading to Al react with water for releasing H2. As the consequence, the mechanism of BiOCl activating Al is due to produce of fresh surface of Al, AlCl3, Bi and Bi2O3 in the ball milling process. Initially, hydrogen and local high temperature are produced by the reaction on the fresh surface of Al as meeting with water. Furthermore, some other fresh surfaces are created again by the dissolution of AlCl3 on the basis of its physicochemical properties. Here, AlCl3 is similar to the role of “salt gates” [34]. Meanwhile, part of the AlCl3 can also hydrolyze to releasing HCl. And the Hþ from the generated HCl can further react with Al in the local area to release H2 (△Н of the reaction is 491 kJ mol1 calculated by the way described at https://www.materialsproject.org/) and the Cl is beneficial for the removal of Al(OH)3 layer on the surface of Al
Fig. 7 e XPS spectra of AleBiOCl.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 5 5 e6 6 6 2
through a pit corrosion process [28]. At the same time, the dissolution of AlCl3 can release a lot of instantaneous heat to accelerate hydrogen generation rate for the reaction of Ale H2O and to improve the kinetic of reaction of AleH2O. Additionally, the produced Bi plays a much more important role in promoting the hydrolysis of Al. It has been discovered that Al/ Bi can work as micro-galvanic cell which intensifies the corrosion of Al [16]. Finally, the active site of Bi2O3 can also promote the hydrolysis of Al [24], because the surface of Bi2O3 can absorb water to increase the chance of Al contacting water. The synergistic effect of these factors leads to improve the hydrogen generation performance of AleBiOCl composite.
Conclusions In summary, we proposed a new Al-based hydrogen generation material which was synthesized via milling Al powders with BiOCl. The study results showed that the apparent activation energy (Ea) of reaction AleH2O reduced to 26.9 kJ mol1 by doping BiOCl. Therefore, hydrogen generation performance of the material was enhanced; leading to it reacts with water at room temperature. The hydrogen yield and conversion yield of Al-15 wt% BiOCl composite were 1058.1 mL g1 and 91.6%, respectively. In addition, the effects of metal elements, hydrides and inorganic salts on the hydrogen production of Ale BiOCl were respectively investigated. The results showed that Bi, LiH and AlCl3 can speed the hydrolysis of AleBiOCl. The hydrogen conversion yields of AleBiOCleBi, AleBiOCleLiH and AleBiOCleAlCl3 were closed to 100%. Their mHGR and hydrogen yield were 2566.5 mL g1 min1 and 1153.4 mL g1 (for AleBiOCleBi), 3178.5 mL g1 min1 and 1241.0 mL g1 (for Ale BiOCleLiH), 2442.0 mL g1 min1 and 1154.2 mL g1 (for Ale BiOCleAlCl3), respectively. The doped LiH showed the optimal performance due to LiH itself could also react with water to release hydrogen, induced to enhance the hydrogen capacity of the system. The mechanism study of hydrogen generation from the reaction of AleBiOCl with water showed that a solid-solid reaction occurred, and Bi, Bi2O3 and AlCl3 had been produced in situ during ball milling process. The synergistic action of Bi, Bi2O3, AlCl3 and the fresh surface of Al markedly improved the performances of hydrogen generation of AleBiOCl composite. Experimental results demonstrate the proposed materials in this paper are expected to serve as a portable hydrogen generation material for the fuel cells.
Acknowledgements This work was supported by the National Natural Science Foundation of China (U1501242, 51671062 and 51871065), the Guangxi Collaborative Innovation Centre of Structure and Property for New Energy and Material (2012GXNSFGA06002), Guangxi Science and Technology Project (AD17195073 CE), Guangxi Major Science and Technology Special Project (AA17202030-1) and the Guangxi Key Laboratory of Information Laboratory Foundation (161002-Z, 161002-K and 161003-K). We would like to thank Mr. Yujie Sun from School of Material Science and Engineering, China
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University of Geosciences (Beijing) for his XRD analysis and valuable discussion.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.165.
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