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Hydrogen generation by the hydrolysis of magnesium–aluminum–iron material in aqueous solutions
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Hydrogen generation by the hydrolysis of magnesiumealuminumeiron material in aqueous solutions Cuiping Wang a,b, Tao Yang a, Yuheng Liu a, Jingjing Ruan a, Shuiyuan Yang a,b, Xingjun Liu a,b,* a b
Department of Materials Science and Engineering, College of Materials, Xiamen University, 361005, PR China Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005, PR China
article info
abstract
Article history:
Highly activated MgeAleFe materials are prepared from powder by mechanical ball milling
Received 17 March 2014
method for hydrogen generation. The hydrolysis characteristics of MgeAleFe materials in
Received in revised form
aqueous solutions under different experimental conditions are carefully investigated. The
6 May 2014
results show that the hydrolysis reactivity of MgeAleFe material can be significantly
Accepted 9 May 2014
improved by increasing the ball milling time and Fe content. The increase of NaCl solution
Available online 11 June 2014
concentration and initial temperature is also found to promote the hydrogen generation reaction. At 25 C, the Mg60eAl30eFe10 (wt%) material ball-milled for 4 h shows the best
Keywords:
performance in 0.6 mol L1 NaCl solution, and the reaction can produce 1013.33 ml g1
Hydrogen generation
hydrogen with a maximum hydrogen generation rate of 499.50 ml min1 g1. In compar-
Magnesiumealuminumeiron mate-
ison to NaCl solution, natural seawater is found to have an inhibiting effect on the hy-
rial
drolysis of MgeAleFe material. Especially, the presence of Mg2þ and Ca2þ in seawater can
Ball milling
can decrease the hydrogen greatly reduce the hydrogen conversion yield, and the SO2 4
Hydrolysis reaction
generation rate. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is widely considered to be a promising alternative to traditional energy in the future because of its high calorific value and clean combustion. It is an ideal energy source for fuel cells, which can convert the chemical energy of hydrogen into electricity without pollution [1e3]. However, hydrogen, as a secondary energy, must be produced from other energy sources. Currently, most industrial hydrogen production
methods are based on fossil fuels, such as steam reforming of natural gas [4e6]. However, there are some limiting factors deter their applications, such as low conversion, high cost and environmental pollution. Additionally, the use of hydrogen energy is largely restricted by the safety and low efficient hydrogen storage. Therefore, the development of new methods for compact and convenient hydrogen sources is becoming increasingly important to the large-scale commercialization of fuel cells [7,8].
* Corresponding author. Department of Materials Science and Engineering, College of Materials, Xiamen University, 361005, PR China. Tel.: þ86 592 2187888; fax: þ86 592 2187966. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijhydene.2014.05.047 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Many studies have been focused on the hydrogen production from the hydrolysis reaction of light metals. In this regard, Mg and Al alloys are primarily considered for reasons of reactivity, economy and "greenness" [9,10]. Metals, such as Mg and Al, are so active thermodynamically that they readily corrode in water with hydrogen evolution. Furthermore, in this process, the hydrogen storage is no longer needed and the corresponding cost will be reduced. Thus, Mg and Al alloys have great potential to provide on-board hydrogen for fuel cells. The hydrolysis reactions of Mg and Al with water can be generally described as follows [11,12]: Mg þ 2H2 O/MgðOHÞ2 þ H2
(1)
2Al þ 6H2 O/2AlðOHÞ3 þ 3H2
(2)
In the present work, Fe was selected as the catalyst to activate MgeAl-based material, which has very cheap price and plentiful reserve on the earth. A new hydrogen generation material, MgeAleFe material, was successfully prepared by a ball milling method. The main purpose of this study was to improve the reactivity of MgeAleFe materials, and to study the reactive properties of these materials in aqueous solutions. Keeping in mind the salinity of seawater (35 g dm3), the 0.6 mol L1 NaCl solution in a similar concentration of seawater was chosen as the starting solution for hydrogen production experiments.
Experimental
However, under mild conditions, the reaction of Mg and Al with water is limited and the production of hydrogen is therefore only to a negligible extent. This is because that the passive layers (the metal oxide or hydroxide) formed on the metal surface can prevent the contact between metal and water. As a result, the hydrogen generation reaction is blocked. Therefore, in order to achieve a high hydrogen production, much effort has been devoted to overcome this problem for many years. Ball milling process has been demonstrated to be an effective method to improve the reactivity of Mg and Al alloys. Mge10 wt% Ni material after ball milling for 0.5 h were found to react completely in 1 M KCl aqueous within an hour reaction [12]. Mg-based powders ball-milled with chlorides (KCl, LiCl, NaCl, AlCl3, and MgCl2) were investigated, and the 6 h milled Mge3 mol% AlCl3 showed the best performance with a hydrogen yield of 93.86% and an initial hydrogen generation rate of 455.9 ml min1 g1 [13]. Al-based powders mechanical alloyed with low melting point metals [14e16], active metals [17e19] and inorganic additives [20e23] have also been reported to display higher hydrolysis reactivity than un-milled Al powder. Furthermore, hydro-reactive material of MgeAleCoeBi alloy was successfully prepared using highenergy ball milling [24]. The hydrolysis reaction is immediately noticeable in seawater that can generate 397 ml min1 g1 of hydrogen in the first two minutes with a hydrogen conversion yield of 97.1%. This type of MgeAl-based alloy is believed to have great value in the production of hydrogen. However, alloying with high-priced metals Co and Bi in the preparation process not only increases the cost but also make the recycling more difficult for complex hydrolysis product. Moreover, reports on the hydrolysis characteristics of MgeAl-based alloy are still insufficient. Therefore, a lot of work is still needed to be done for this material. According to the research of Wang et al. [25], the 0.5 h milled Mge10 wt% FeCl3 material was found to have outstanding hydrolysis properties. This is attributed to the effective micro-galvanic cells formed between Mg and Fe elements. In the hydrolysis of AleFe alloy, Fe was also reported to accelerate the hydrogen generation by inducing both intergranular and galvanic corrosion simultaneously [26]. Therefore, Fe is expected to be an effective additive to promote the hydrogen generation of MgeAl-based material in aqueous solution.
Materials Mg powder (99.8 wt% purity), Al powder (99.9 wt% purity), and Fe powder (99.9 wt% purity) were used as the starting materials. Stearic acid (99.9 wt% purity) was used as the millingassisted agent and 1 wt% of it was mixed with the metal powders in an argon-filled glove box prior to milling process. Ball milling was performed in a planetary ball mill (QM-1SP-2, Nanjing University Instrument Plant, China) under argon atmosphere, equipped with stainless steel jars 100 ml and steel balls 6 mm in diameter. Ball to powder mass ratio was kept 20:1 and milling speed was maintained at 450 rpm. Distilled water was used to prepare the aqueous solutions. Seawater was directly sampled in “Baicheng coast” of Xiamen and then kept at room temperature. The tested solutions were freshly prepared before performing the H2 generation experiments.
Hydrogen measurement The method used for the hydrogen measurement was described in previous study [22]. The hydrolysis reaction was performed in a 250 ml flask reactor that has two openings: a water inlet and a gas outlet. 0.3 g powders and 100 ml selected aqueous solution were used for each test. Hydrogen measurement started immediately when the powders came into contact with the solution. The conversion yield (%) was defined as the volume of generated hydrogen over the theoretical H2 yield at 25 C and 1 atm. The hydrogen generation rate was calculated by differentiating the measured hydrogen volume over reaction time. The theoretical hydrogen generation amount of 1 g Al (or Mg) is 1360 ml (or 1019 ml) at 298 K and 1 atm.
Analysis and characterization methods X-ray diffraction (XRD) studies were carried out in an X-ray diffractometer (D/MAX-Ultima IV, RIGAKU Corporation, Japan). Microstructure studies were performed using the scanning electron microscopy (SEM, SU-70, HITACHI, Japan), equipped with AZTEC energy dispersive X-ray spectroscopy (EDS) measurements. The solid hydrolysis product in the reactor was collected by removing solution through a filter and then dried under vacuum at 50 C for 24 h after being rinsed with distilled water for three times. The temperature of
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Fig. 1 e SEM images of Mg55eAl30eFe15 (wt%) materials ball-milled for different time (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, and the EDS mapping of Mg55eAl30eFe15 (wt%) material ball-milled for 4 h (e) Mg, (f) Al and (g) Fe.
solution was automatically measured using a thermocouple connected with an online recorder.
Results and discussion Effect of ball milling time Fig. 1 shows the SEM images of Mg55eAl30eFe15 (wt%) material ball-milled for different time from 0 h to 4 h. It can be observed that considerable change in the microstructure of the materials occurred with the increase of ball milling time. As can be seen in Fig. 1(a), for the as-received sample without ball milling, the most part of powder is spherical. After ball milling for 1 h, as shown in Fig. 1(b), the powders are seldom affected by cold welding and fracture but only flattened by deformation. When the ball milling time is prolonged to 2 h, due to the serious deformation and cold welding, the materials turn to the bigger flake-like particles (Fig. 1(c)) with some smaller metal fragments welded on the surface. As the milling time increases to 4 h, the powders are further fractured and squashed, and a laminar structure is formed by the welding or
re-welding of the cracked metal flakes, as shown in Fig. 1(d). During the ball milling process, the impacts of ball-to-powder create many fresh active surfaces and defects, which will increase the reactivity of material to generate hydrogen [10,12]. Fig. 1(e)e(g) show the EDS mapping of the particle designated in Fig. 1(d) for Mg, Al, and Fe elements respectively. It can be seen that the Mg, Al and Fe are embedded together to form irregular particle after ball milling for 4 h. Such a structure is favorable to generate a large number of galvanic cells between different elements and therefore promote the hydrolysis of material in NaCl solution to produce hydrogen [24]. Fig. 2 shows the XRD patterns of Mg55eAl30eFe15 (wt%) materials with different ball milling time. Only the diffraction peaks of Mg, Al and Fe can be clearly identified in the patterns. This indicates that no phase transformation occurred during ball milling. Moreover, peak broadening are observed after ball milling. It implies the decrease of crystallite size and the accumulation of microstrains during ball milling process. As the consequence of these, the hydrolysis reactivity of material can be improved for generating hydrogen [13,21]. Furthermore, the change in the relative intensity of the diffraction peaks can also be observed from Fig. 2, indicating that a
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Fig. 2 e XRD patterns of Mg55eAl30eFe15 (wt%) materials ball-milled for different time (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h.
preferred orientation occurred in the ball milling process. For instance, a texture of Mg (0002) preferably appears with the increase of milling time as shown in Fig. 2. Precious studies have demonstrated that the Mg (0002) plane is more susceptible to pitting corrosion [27,28]. Therefore, the formation of Mg (0002) planes on the surface may facilitate the hydrolysis reaction for hydrogen generation. Fig. 3(a) shows the hydrogen generation curves of Mg55eAl30eFe15 (wt%) material ball-milled for different time reacting with 0.6 mol L1 NaCl solution at 25 C. From Fig. 3(a), it is found that the as-received powders have almost no reactivity and barely react in NaCl solution to generate hydrogen. The maximum conversion yield reaches only 1.97% after 1 h of reaction. However, the increase of ball milling time results in significant improvement for the hydrolysis reactivity of Mg55eAl30eFe15 (wt%) material. When the milling time further increases from 1 h to 4 h, the maximum conversion yield gradually increases from 33.84% to 99.66%, the maximum hydrogen generation rate increases dramatically from 16.60 ml min1 g1 to 687.60 ml min1 g1. In addition, the effect of ball-milling time on hydrogen generation of pure Mg, pure Al and pure Fe materials is also investigated. Similarly, as shown in Fig. 3(b), an increase of ball milling time can promote the hydrolysis of pure Mg in 0.6 mol L1 NaCl solution at 25 C. But no hydrogen can be obtained from pure Al and pure Fe ball-milled for different time (0~4 h) under the same conditions (not shown). As can be seen in Fig. 4(a), the hydrolysis product has a porous structure composed of many nano-sheets. Unlike the dense surface, this kind of porous structure may enable the inner parts of particle continuously react with solution to produce hydrogen. XRD pattern of the product is shown in
Fig. 4(b), Firstly, no characteristic peaks of Mg and Al are detected in this pattern. Combining with the data of hydrogen generation in Fig. 3, it can be concluded that the 4 h-milled Mg55eAl30eFe15 (wt%) material can react completely to generate hydrogen. The characteristic peaks of Fe are still clearly measured. It suggests that Fe does not react with NaCl solution to generate hydrogen. Secondly, Mg6Al2(OH)18$4.5H2O and Al(OH)3 are clearly identified in the reaction product. The present result differs from the previous one where Mg(OH)2 was observed instead of Al(OH)3 [24]. This can be attributed to the different reaction solutions adopted (seawater in previous report and NaCl solution in ours) as well as the difference in materials composition. Furthermore, the solid products after reaction can be easily recycled. Fe can be directly separated from the hydrolysis product by magnet, and the residual metal hydroxides can be reused as a flame retardant [29].
Effect of Fe content Fig. 5 shows the hydrogen generation curves of MgeAleFe materials ball-milled for 4 h with different Fe contents in 0.6 mol L1 NaCl solution at 25 C. As can be seen in Fig. 5, the addition of Fe has some favorable effect on the improvement for the hydrolysis performance of MgeAleFe material. When the content of Fe increases from 0 wt% to 15 wt%, the maximum conversion yield raises from 89.60% to 99.66%, the maximum hydrogen generation rate significantly increases from 141.03 ml min1 g1 to 687.60 ml min1 g1. It is mainly because that Fe has a more positive standard electrode potential of 0.44 V compared to Mg (2.37 V) and Al (1.66 V). Thus, the addition of Fe can promote the hydrolysis rates of Mg and Al by inducing severe galvanic corrosions. From the
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Fig. 3 e Hydrogen generation curves of (a) Mg55eAl30eFe15 (wt%) materials and (b) pure Mg ball-milled for different time reacting with 0.6 mol L¡1 NaCl solution.
experimental results discussed above, it can be concluded that Fe is an effective catalyst to accelerate the hydrogen generation of MgeAleFe material in NaCl solution. It is also noteworthy that the Mg70eAl30 (wt%) material ball-milled for 4 h can also enable a conversion yield of 89.60%. This result can be ascribed to the formation of galvanic cells generated between Mg and Al. As shown in Fig. 6, the result of SEM-EDS mapping reveals that Mg and Al are cold welded together after ball milling. Due to the difference of electrode potential, effective galvanic corrosion can be initiated between Mg and Al in NaCl solution. As an anode material, Mg preferentially corrodes, whereas Al can be cathodically protected in theory. Assuming that Mg reacts completely, however, the maximum contribution of Mg to hydrogen conversion yield can be calculated to be 63.61%. Therefore, the experimental conversion yield of 89.60% could not be only due to the hydrolysis of Mg. It is reasonable to conclude that Al also suffer from hydrolysis in this process. Furthermore, no hydrogen generation is observed from 4 h-milled pure Al under the same conditions. Based on that, the hydrolysis of Al can be explained in terms of the alkalization effect [30]. With the anodic dissolution of Mg, bubbles of hydrogen are generated at cathode (Al), which can result in the formation of OH ions on the surface of Al. The existence of OH ions will
promote the dissolution of Al oxide as well as Al, and then facilitate the precipitation of Al(OH)3. Simultaneously, a great amount of heat generated from the corrosion of Mg can also accelerate the hydrolysis of Al. The hydrolysis reactions of Al in alkaline environment can be described as follows [31e33]: 2H2 O þ 2e /H2 þ 2OH
(3)
Al2 O3 þ 3H2 O þ 2OH /2AlðOHÞ 4
(4)
Al þ 4OH /AlðOHÞ 4 þ 3e
(5)
AlðOHÞ 4 4AlðOHÞ3 þ OH
(6)
However, Fe itself is unable to react with 0.6 mol L1 NaCl solution to generate hydrogen. Therefore, too much Fe addition will reduce the final hydrogen volume generated from unit mass of material. When the content of Fe increases from 10 wt% to 15 wt%, the maximum hydrogen conversion yields are kept almost the same to be nearly 100%. Nevertheless, the total amount of hydrogen (965.17 ml g1) generated from Mg55eAl30eFe15 (wt%) material is lower than that (1013.33 ml g1) obtained from Mg60eAl30eFe10 (wt%) material. Therefore, in order to obtain more hydrogen with
Fig. 4 e (a) SEM image and (b) XRD pattern of hydrolysis product of Mg55eAl30eFe15 (wt%) material ball-milled for 4 h reacting with 0.6 mol L¡1 NaCl solution.
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Fig. 5 e Hydrogen generation curves of MgeAleFe materials ball-milled for 4 h with different Fe contents reacting with 0.6 mol L¡1 NaCl solution.
Fig. 7 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with NaCl solutions of different concentrations.
ensuring the high conversion yield, 10 wt% Fe is considered as the optimized composition under current conditions.
0.01 mol L1 NaCl solution, the conversion yield can reach 45.66% with a maximum hydrogen generation rate of 56.70 ml min1 g1. This indicates that the Mg60eAl30eFe10 (wt%) material possesses a high hydrolysis activity after 4 h ball milling and is extremely vulnerable to the solution containing NaCl. Furthermore, when the NaCl concentration increases from 0.05 mol L1 to 0.6 mol L1, the maximum conversion yield is further promoted from 84.74% to 99.40%, and the maximum hydrogen generation rate also increases gradually from 180.47 ml min1 g1 to 499.50 ml min1 g1.
Effect of NaCl concentration Fig. 7 shows the hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with NaCl solutions of different concentrations at 25 C. As can be seen in Fig. 7, almost no hydrogen can be generated in distilled water. But when the material is exposed to
Fig. 6 e (a) & (b) SEM image of Mg70eAl30 (wt%) material ball-milled for 4 h, and the corresponding EDS mapping for (c) Mg, (d) Al.
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The effect of NaCl on the hydrolysis of MgeAleFe material can be explained as: (1) the addition of NaCl can increase the conductivity of solution and promote the work of microgalvanic cells, which eventually enhances the hydrolysis of MgeAleFe material. (2) the increase of Cl concentration leads to the shift of potential of metal towards negative, and thus, accelerates the hydrolysis of metal to generate hydrogen [34,35]. (3) in the hydrolysis process, Cl could substitute OH to form the soluble chloride compounds, which contributes to the local dissolution of the surface film, thereby exposing active metal to renewed corrosion [36]. Therefore, the increase of NaCl concentration can effectively promote the hydrolysis of MgeAleFe material for hydrogen generation.
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generally believed that lowering the hydrogen generation temperature is important both for portable fuel cell designer and for end users. Therefore, this type of material is considered to have a broad prospect of application to provide hydrogen for fuel cells.
Effect of seawater
Fig. 8 shows the hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with 0.6 mol L1 NaCl solutions at different initial temperatures. From Fig. 8, it can be seen that the increase of initial temperature of solution can promote the hydrogen generation. In the solutions at 0 C, 10 C, 25 C, 40 C, the maximum conversion yield reaches 79.93%, 94.08%, 99.40% and 99.88%, respectively. Accordingly, the maximum hydrogen generation rate also reaches 138.10 ml min1 g1, 228.77 ml min1 g1, 499.50 ml min1 g1 and 778.50 ml min1 g1, respectively. In fact, it's worth mentioning that the hydrolysis reaction (1) and (2) are exothermic process. A large amount of heat can be generated accompanied by hydrogen production, and the solution temperature will increased gradually, especially at the local region near the powderesolution interface, in which the ratio of solution to powder is small. For instance, the reaction heat generated from 0.3 g Mg60eAl30eFe10 (wt%) material in 0.6 mol L1 NaCl solution can rapidly heat up 10 ml solution from 10 C to 68.6 C in the first three minutes. Therefore, even at a low initial temperature, the proceeding of hydrolysis reaction can be impelled and a relatively high conversion yield can also be obtained, as shown in Fig. 8. It is
Seawater is salty and corrosive that can speed up the metal corrosion in many occasions. Recently, metal/seawater systems have been widely studied to provide hydrogen for fuel cells [10,24,37]. Therefore, the hydrogen generation reaction in seawater is tested to investigate the effect of natural seawater on the hydrolysis of MgeAleFe material. Fig. 9 shows the comparison between 0.6 mol L1 NaCl solution and seawater at 25 C for the hydrogen generation of Mg60eAl30eFe10 (wt%) ball-milled for 4 h. It can be seen from Fig. 10 that the maximum conversion yield in seawater is lower than that in 0.6 mol L1 NaCl solution, as well as the hydrogen generation rate. It indicates the seawater has an inhibiting effect on the hydrolysis of Mg60eAl30eFe10 (wt%) material compared to NaCl solution. Fig. 10 shows the XRD pattern of hydrolysis product after reacting with seawater. No peaks from Mg are detected. However, strong peaks of Al are clearly identified. It reveals that seawater mainly inhibit the hydrolysis of Al in reaction process. This phenomenon can be related to the difference in solution compositions, which have been reported to exert some influence on the hydrolysis of Al [38]. It is well known that seawater as a multi-component electrolyte contains many kinds of soluble salts, besides the common salt of NaCl. The major dissolved salts in seawater have a remarkably constant composition, which can be determined to be NaCl, MgCl2, Na2SO4 and CaCl2 [39,40]. The effects of the major sea salts on the hydrogen generation are presented in Fig. 11. It can be seen that 4 h ball-milled Mg60eAl30eFe10 (wt%) material can react with different salt solutions to produce hydrogen at 25 C. But some differences can still be observed in different solutions.
Fig. 8 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with 0.6 mol L¡1 NaCl solutions at different initial temperatures.
Fig. 9 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with 0.6 mol L¡1 NaCl solution and seawater.
Effect of initial solution temperature
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and weaken the alkalization effect on the hydrolysis of Al. The insoluble hydroxide will also cover on the surface of Al, thereby preventing the corrosion of Al. Additionally, PH buffering effect of seawater caused by 2 CO2eHCO 3 eCO3 equilibrium has also been reported to limit the PH fluctuations to a certain extent, which can prevent the corrosion of Al-based material in seawater [43]. Through the above analysis, it is reasonable to believe that the specific composition of seawater is an important factor in hindering the hydrolysis of MgeAleFe material. Nonetheless, due to the high complexity of component in seawater, the chemical composition alone can not account for the inhibiting effect of seawater. Thus, further study is needed in order to obtain a more comprehensive understanding of the inhibiting effect on the hydrolysis of MgeAleFe material in seawater.
Fig. 10 e XRD pattern of hydrolysis product of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with seawater.
In the NaCl and Na2SO4 solution, the MgeAleFe material can almost completely react within 1 h and the conversion yields can both reach almost 100%. However, the maximum hydrogen generation rate in 0.6 mol L1 NaCl solution (499.50 ml min1 g1) is much higher than that in 0.6 mol L1 Na2SO4 solution (185.87 ml min1 g1). This is because that the Cl is more aggressive than SO2 4 and can easily destroy the compact passive film and result in a relatively high hydrogen generation rate [41,42]. In the MgCl2 solution, the hydrogen generation is hindered greatly, and the conversion yield is reduced to 66.78%, which is significantly lower than that in the NaCl and Na2SO4 solution. Similar inhibiting effect can also be observed in CaCl2 solution. This can be explained that the Mg2þ and Ca2þ ions have strong affinity to OH ions, and can easily combine with OH ions to form the insoluble hydroxide (Ca(OH)2 and Mg(OH)2) [22]. It can therefore reduce the amount of OH ions
Conclusion In the present work, the highly activated MgeAleFe materials were successfully prepared by ball milling. The effects of different experimental parameters on the hydrolysis of the MgeAleFe materials in aqueous solutions were investigated. The results show that the increase of ball milling time can decrease the crystallite size of Mg and Al and form more micro-galvanic cells, which is beneficial to improve the hydrolysis reactivity of MgeAleFe material. Under the present milling conditions, increase in milling time up to 4 h results in an improved hydrogen production. The addition of Fe is also found to accelerate the hydrogen generation. The optimal content of Fe is determined to be 10 wt% under current conditions. At 25 C, the best performance for hydrogen generation has been obtained from the 4 h ball-milled Mg60eAl30eFe10 (wt%) material in 0.6 mol L1 NaCl solution, leading to a hydrogen yield of 99.40% (1013.33 ml g1) and a maximum rate of 499.50 ml min1 g1. In addition, an increased hydrolysis performance can be obtained by increasing the concentration and initial temperature of NaCl solution. A considerable reaction heat generated in the hydrolysis process can raise the solution temperature and promote hydrogen generation at low temperatures. High solution temperature can improve the hydrolysis kinetics effectively. The results obtained from various salt solutions further show that seawater exhibits an inhibiting effect on the hydrolysis of MgeAleFe material compared to NaCl solution. The presence of Mg2þ, Ca2þ and SO42 ions is found to hinder the hydrolysis of MgeAleFe materials at different levels. All in all, this MgeAleFe material with ball milling can be used to provide the low-cost production of hydrogen for fuel cells.
Acknowledgments
Fig. 11 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with different salt solutions.
This work was financially supported by the Science and Technology Bureau of Xiamen City (Project no. 3502Z20131153). Ping Liu is also acknowledged for helpful discussions and comments on the manuscript.
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Hydrogen generation by the hydrolysis of magnesiumealuminumeiron material in aqueous solutions Cuiping Wang a,b, Tao Yang a, Yuheng Liu a, Jingjing Ruan a, Shuiyuan Yang a,b, Xingjun Liu a,b,* a b
Department of Materials Science and Engineering, College of Materials, Xiamen University, 361005, PR China Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005, PR China
article info
abstract
Article history:
Highly activated MgeAleFe materials are prepared from powder by mechanical ball milling
Received 17 March 2014
method for hydrogen generation. The hydrolysis characteristics of MgeAleFe materials in
Received in revised form
aqueous solutions under different experimental conditions are carefully investigated. The
6 May 2014
results show that the hydrolysis reactivity of MgeAleFe material can be significantly
Accepted 9 May 2014
improved by increasing the ball milling time and Fe content. The increase of NaCl solution
Available online 11 June 2014
concentration and initial temperature is also found to promote the hydrogen generation reaction. At 25 C, the Mg60eAl30eFe10 (wt%) material ball-milled for 4 h shows the best
Keywords:
performance in 0.6 mol L1 NaCl solution, and the reaction can produce 1013.33 ml g1
Hydrogen generation
hydrogen with a maximum hydrogen generation rate of 499.50 ml min1 g1. In compar-
Magnesiumealuminumeiron mate-
ison to NaCl solution, natural seawater is found to have an inhibiting effect on the hy-
rial
drolysis of MgeAleFe material. Especially, the presence of Mg2þ and Ca2þ in seawater can
Ball milling
can decrease the hydrogen greatly reduce the hydrogen conversion yield, and the SO2 4
Hydrolysis reaction
generation rate. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is widely considered to be a promising alternative to traditional energy in the future because of its high calorific value and clean combustion. It is an ideal energy source for fuel cells, which can convert the chemical energy of hydrogen into electricity without pollution [1e3]. However, hydrogen, as a secondary energy, must be produced from other energy sources. Currently, most industrial hydrogen production
methods are based on fossil fuels, such as steam reforming of natural gas [4e6]. However, there are some limiting factors deter their applications, such as low conversion, high cost and environmental pollution. Additionally, the use of hydrogen energy is largely restricted by the safety and low efficient hydrogen storage. Therefore, the development of new methods for compact and convenient hydrogen sources is becoming increasingly important to the large-scale commercialization of fuel cells [7,8].
* Corresponding author. Department of Materials Science and Engineering, College of Materials, Xiamen University, 361005, PR China. Tel.: þ86 592 2187888; fax: þ86 592 2187966. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijhydene.2014.05.047 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Many studies have been focused on the hydrogen production from the hydrolysis reaction of light metals. In this regard, Mg and Al alloys are primarily considered for reasons of reactivity, economy and "greenness" [9,10]. Metals, such as Mg and Al, are so active thermodynamically that they readily corrode in water with hydrogen evolution. Furthermore, in this process, the hydrogen storage is no longer needed and the corresponding cost will be reduced. Thus, Mg and Al alloys have great potential to provide on-board hydrogen for fuel cells. The hydrolysis reactions of Mg and Al with water can be generally described as follows [11,12]: Mg þ 2H2 O/MgðOHÞ2 þ H2
(1)
2Al þ 6H2 O/2AlðOHÞ3 þ 3H2
(2)
In the present work, Fe was selected as the catalyst to activate MgeAl-based material, which has very cheap price and plentiful reserve on the earth. A new hydrogen generation material, MgeAleFe material, was successfully prepared by a ball milling method. The main purpose of this study was to improve the reactivity of MgeAleFe materials, and to study the reactive properties of these materials in aqueous solutions. Keeping in mind the salinity of seawater (35 g dm3), the 0.6 mol L1 NaCl solution in a similar concentration of seawater was chosen as the starting solution for hydrogen production experiments.
Experimental
However, under mild conditions, the reaction of Mg and Al with water is limited and the production of hydrogen is therefore only to a negligible extent. This is because that the passive layers (the metal oxide or hydroxide) formed on the metal surface can prevent the contact between metal and water. As a result, the hydrogen generation reaction is blocked. Therefore, in order to achieve a high hydrogen production, much effort has been devoted to overcome this problem for many years. Ball milling process has been demonstrated to be an effective method to improve the reactivity of Mg and Al alloys. Mge10 wt% Ni material after ball milling for 0.5 h were found to react completely in 1 M KCl aqueous within an hour reaction [12]. Mg-based powders ball-milled with chlorides (KCl, LiCl, NaCl, AlCl3, and MgCl2) were investigated, and the 6 h milled Mge3 mol% AlCl3 showed the best performance with a hydrogen yield of 93.86% and an initial hydrogen generation rate of 455.9 ml min1 g1 [13]. Al-based powders mechanical alloyed with low melting point metals [14e16], active metals [17e19] and inorganic additives [20e23] have also been reported to display higher hydrolysis reactivity than un-milled Al powder. Furthermore, hydro-reactive material of MgeAleCoeBi alloy was successfully prepared using highenergy ball milling [24]. The hydrolysis reaction is immediately noticeable in seawater that can generate 397 ml min1 g1 of hydrogen in the first two minutes with a hydrogen conversion yield of 97.1%. This type of MgeAl-based alloy is believed to have great value in the production of hydrogen. However, alloying with high-priced metals Co and Bi in the preparation process not only increases the cost but also make the recycling more difficult for complex hydrolysis product. Moreover, reports on the hydrolysis characteristics of MgeAl-based alloy are still insufficient. Therefore, a lot of work is still needed to be done for this material. According to the research of Wang et al. [25], the 0.5 h milled Mge10 wt% FeCl3 material was found to have outstanding hydrolysis properties. This is attributed to the effective micro-galvanic cells formed between Mg and Fe elements. In the hydrolysis of AleFe alloy, Fe was also reported to accelerate the hydrogen generation by inducing both intergranular and galvanic corrosion simultaneously [26]. Therefore, Fe is expected to be an effective additive to promote the hydrogen generation of MgeAl-based material in aqueous solution.
Materials Mg powder (99.8 wt% purity), Al powder (99.9 wt% purity), and Fe powder (99.9 wt% purity) were used as the starting materials. Stearic acid (99.9 wt% purity) was used as the millingassisted agent and 1 wt% of it was mixed with the metal powders in an argon-filled glove box prior to milling process. Ball milling was performed in a planetary ball mill (QM-1SP-2, Nanjing University Instrument Plant, China) under argon atmosphere, equipped with stainless steel jars 100 ml and steel balls 6 mm in diameter. Ball to powder mass ratio was kept 20:1 and milling speed was maintained at 450 rpm. Distilled water was used to prepare the aqueous solutions. Seawater was directly sampled in “Baicheng coast” of Xiamen and then kept at room temperature. The tested solutions were freshly prepared before performing the H2 generation experiments.
Hydrogen measurement The method used for the hydrogen measurement was described in previous study [22]. The hydrolysis reaction was performed in a 250 ml flask reactor that has two openings: a water inlet and a gas outlet. 0.3 g powders and 100 ml selected aqueous solution were used for each test. Hydrogen measurement started immediately when the powders came into contact with the solution. The conversion yield (%) was defined as the volume of generated hydrogen over the theoretical H2 yield at 25 C and 1 atm. The hydrogen generation rate was calculated by differentiating the measured hydrogen volume over reaction time. The theoretical hydrogen generation amount of 1 g Al (or Mg) is 1360 ml (or 1019 ml) at 298 K and 1 atm.
Analysis and characterization methods X-ray diffraction (XRD) studies were carried out in an X-ray diffractometer (D/MAX-Ultima IV, RIGAKU Corporation, Japan). Microstructure studies were performed using the scanning electron microscopy (SEM, SU-70, HITACHI, Japan), equipped with AZTEC energy dispersive X-ray spectroscopy (EDS) measurements. The solid hydrolysis product in the reactor was collected by removing solution through a filter and then dried under vacuum at 50 C for 24 h after being rinsed with distilled water for three times. The temperature of
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Fig. 1 e SEM images of Mg55eAl30eFe15 (wt%) materials ball-milled for different time (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, and the EDS mapping of Mg55eAl30eFe15 (wt%) material ball-milled for 4 h (e) Mg, (f) Al and (g) Fe.
solution was automatically measured using a thermocouple connected with an online recorder.
Results and discussion Effect of ball milling time Fig. 1 shows the SEM images of Mg55eAl30eFe15 (wt%) material ball-milled for different time from 0 h to 4 h. It can be observed that considerable change in the microstructure of the materials occurred with the increase of ball milling time. As can be seen in Fig. 1(a), for the as-received sample without ball milling, the most part of powder is spherical. After ball milling for 1 h, as shown in Fig. 1(b), the powders are seldom affected by cold welding and fracture but only flattened by deformation. When the ball milling time is prolonged to 2 h, due to the serious deformation and cold welding, the materials turn to the bigger flake-like particles (Fig. 1(c)) with some smaller metal fragments welded on the surface. As the milling time increases to 4 h, the powders are further fractured and squashed, and a laminar structure is formed by the welding or
re-welding of the cracked metal flakes, as shown in Fig. 1(d). During the ball milling process, the impacts of ball-to-powder create many fresh active surfaces and defects, which will increase the reactivity of material to generate hydrogen [10,12]. Fig. 1(e)e(g) show the EDS mapping of the particle designated in Fig. 1(d) for Mg, Al, and Fe elements respectively. It can be seen that the Mg, Al and Fe are embedded together to form irregular particle after ball milling for 4 h. Such a structure is favorable to generate a large number of galvanic cells between different elements and therefore promote the hydrolysis of material in NaCl solution to produce hydrogen [24]. Fig. 2 shows the XRD patterns of Mg55eAl30eFe15 (wt%) materials with different ball milling time. Only the diffraction peaks of Mg, Al and Fe can be clearly identified in the patterns. This indicates that no phase transformation occurred during ball milling. Moreover, peak broadening are observed after ball milling. It implies the decrease of crystallite size and the accumulation of microstrains during ball milling process. As the consequence of these, the hydrolysis reactivity of material can be improved for generating hydrogen [13,21]. Furthermore, the change in the relative intensity of the diffraction peaks can also be observed from Fig. 2, indicating that a
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Fig. 2 e XRD patterns of Mg55eAl30eFe15 (wt%) materials ball-milled for different time (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h.
preferred orientation occurred in the ball milling process. For instance, a texture of Mg (0002) preferably appears with the increase of milling time as shown in Fig. 2. Precious studies have demonstrated that the Mg (0002) plane is more susceptible to pitting corrosion [27,28]. Therefore, the formation of Mg (0002) planes on the surface may facilitate the hydrolysis reaction for hydrogen generation. Fig. 3(a) shows the hydrogen generation curves of Mg55eAl30eFe15 (wt%) material ball-milled for different time reacting with 0.6 mol L1 NaCl solution at 25 C. From Fig. 3(a), it is found that the as-received powders have almost no reactivity and barely react in NaCl solution to generate hydrogen. The maximum conversion yield reaches only 1.97% after 1 h of reaction. However, the increase of ball milling time results in significant improvement for the hydrolysis reactivity of Mg55eAl30eFe15 (wt%) material. When the milling time further increases from 1 h to 4 h, the maximum conversion yield gradually increases from 33.84% to 99.66%, the maximum hydrogen generation rate increases dramatically from 16.60 ml min1 g1 to 687.60 ml min1 g1. In addition, the effect of ball-milling time on hydrogen generation of pure Mg, pure Al and pure Fe materials is also investigated. Similarly, as shown in Fig. 3(b), an increase of ball milling time can promote the hydrolysis of pure Mg in 0.6 mol L1 NaCl solution at 25 C. But no hydrogen can be obtained from pure Al and pure Fe ball-milled for different time (0~4 h) under the same conditions (not shown). As can be seen in Fig. 4(a), the hydrolysis product has a porous structure composed of many nano-sheets. Unlike the dense surface, this kind of porous structure may enable the inner parts of particle continuously react with solution to produce hydrogen. XRD pattern of the product is shown in
Fig. 4(b), Firstly, no characteristic peaks of Mg and Al are detected in this pattern. Combining with the data of hydrogen generation in Fig. 3, it can be concluded that the 4 h-milled Mg55eAl30eFe15 (wt%) material can react completely to generate hydrogen. The characteristic peaks of Fe are still clearly measured. It suggests that Fe does not react with NaCl solution to generate hydrogen. Secondly, Mg6Al2(OH)18$4.5H2O and Al(OH)3 are clearly identified in the reaction product. The present result differs from the previous one where Mg(OH)2 was observed instead of Al(OH)3 [24]. This can be attributed to the different reaction solutions adopted (seawater in previous report and NaCl solution in ours) as well as the difference in materials composition. Furthermore, the solid products after reaction can be easily recycled. Fe can be directly separated from the hydrolysis product by magnet, and the residual metal hydroxides can be reused as a flame retardant [29].
Effect of Fe content Fig. 5 shows the hydrogen generation curves of MgeAleFe materials ball-milled for 4 h with different Fe contents in 0.6 mol L1 NaCl solution at 25 C. As can be seen in Fig. 5, the addition of Fe has some favorable effect on the improvement for the hydrolysis performance of MgeAleFe material. When the content of Fe increases from 0 wt% to 15 wt%, the maximum conversion yield raises from 89.60% to 99.66%, the maximum hydrogen generation rate significantly increases from 141.03 ml min1 g1 to 687.60 ml min1 g1. It is mainly because that Fe has a more positive standard electrode potential of 0.44 V compared to Mg (2.37 V) and Al (1.66 V). Thus, the addition of Fe can promote the hydrolysis rates of Mg and Al by inducing severe galvanic corrosions. From the
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Fig. 3 e Hydrogen generation curves of (a) Mg55eAl30eFe15 (wt%) materials and (b) pure Mg ball-milled for different time reacting with 0.6 mol L¡1 NaCl solution.
experimental results discussed above, it can be concluded that Fe is an effective catalyst to accelerate the hydrogen generation of MgeAleFe material in NaCl solution. It is also noteworthy that the Mg70eAl30 (wt%) material ball-milled for 4 h can also enable a conversion yield of 89.60%. This result can be ascribed to the formation of galvanic cells generated between Mg and Al. As shown in Fig. 6, the result of SEM-EDS mapping reveals that Mg and Al are cold welded together after ball milling. Due to the difference of electrode potential, effective galvanic corrosion can be initiated between Mg and Al in NaCl solution. As an anode material, Mg preferentially corrodes, whereas Al can be cathodically protected in theory. Assuming that Mg reacts completely, however, the maximum contribution of Mg to hydrogen conversion yield can be calculated to be 63.61%. Therefore, the experimental conversion yield of 89.60% could not be only due to the hydrolysis of Mg. It is reasonable to conclude that Al also suffer from hydrolysis in this process. Furthermore, no hydrogen generation is observed from 4 h-milled pure Al under the same conditions. Based on that, the hydrolysis of Al can be explained in terms of the alkalization effect [30]. With the anodic dissolution of Mg, bubbles of hydrogen are generated at cathode (Al), which can result in the formation of OH ions on the surface of Al. The existence of OH ions will
promote the dissolution of Al oxide as well as Al, and then facilitate the precipitation of Al(OH)3. Simultaneously, a great amount of heat generated from the corrosion of Mg can also accelerate the hydrolysis of Al. The hydrolysis reactions of Al in alkaline environment can be described as follows [31e33]: 2H2 O þ 2e /H2 þ 2OH
(3)
Al2 O3 þ 3H2 O þ 2OH /2AlðOHÞ 4
(4)
Al þ 4OH /AlðOHÞ 4 þ 3e
(5)
AlðOHÞ 4 4AlðOHÞ3 þ OH
(6)
However, Fe itself is unable to react with 0.6 mol L1 NaCl solution to generate hydrogen. Therefore, too much Fe addition will reduce the final hydrogen volume generated from unit mass of material. When the content of Fe increases from 10 wt% to 15 wt%, the maximum hydrogen conversion yields are kept almost the same to be nearly 100%. Nevertheless, the total amount of hydrogen (965.17 ml g1) generated from Mg55eAl30eFe15 (wt%) material is lower than that (1013.33 ml g1) obtained from Mg60eAl30eFe10 (wt%) material. Therefore, in order to obtain more hydrogen with
Fig. 4 e (a) SEM image and (b) XRD pattern of hydrolysis product of Mg55eAl30eFe15 (wt%) material ball-milled for 4 h reacting with 0.6 mol L¡1 NaCl solution.
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Fig. 5 e Hydrogen generation curves of MgeAleFe materials ball-milled for 4 h with different Fe contents reacting with 0.6 mol L¡1 NaCl solution.
Fig. 7 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with NaCl solutions of different concentrations.
ensuring the high conversion yield, 10 wt% Fe is considered as the optimized composition under current conditions.
0.01 mol L1 NaCl solution, the conversion yield can reach 45.66% with a maximum hydrogen generation rate of 56.70 ml min1 g1. This indicates that the Mg60eAl30eFe10 (wt%) material possesses a high hydrolysis activity after 4 h ball milling and is extremely vulnerable to the solution containing NaCl. Furthermore, when the NaCl concentration increases from 0.05 mol L1 to 0.6 mol L1, the maximum conversion yield is further promoted from 84.74% to 99.40%, and the maximum hydrogen generation rate also increases gradually from 180.47 ml min1 g1 to 499.50 ml min1 g1.
Effect of NaCl concentration Fig. 7 shows the hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with NaCl solutions of different concentrations at 25 C. As can be seen in Fig. 7, almost no hydrogen can be generated in distilled water. But when the material is exposed to
Fig. 6 e (a) & (b) SEM image of Mg70eAl30 (wt%) material ball-milled for 4 h, and the corresponding EDS mapping for (c) Mg, (d) Al.
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The effect of NaCl on the hydrolysis of MgeAleFe material can be explained as: (1) the addition of NaCl can increase the conductivity of solution and promote the work of microgalvanic cells, which eventually enhances the hydrolysis of MgeAleFe material. (2) the increase of Cl concentration leads to the shift of potential of metal towards negative, and thus, accelerates the hydrolysis of metal to generate hydrogen [34,35]. (3) in the hydrolysis process, Cl could substitute OH to form the soluble chloride compounds, which contributes to the local dissolution of the surface film, thereby exposing active metal to renewed corrosion [36]. Therefore, the increase of NaCl concentration can effectively promote the hydrolysis of MgeAleFe material for hydrogen generation.
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generally believed that lowering the hydrogen generation temperature is important both for portable fuel cell designer and for end users. Therefore, this type of material is considered to have a broad prospect of application to provide hydrogen for fuel cells.
Effect of seawater
Fig. 8 shows the hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with 0.6 mol L1 NaCl solutions at different initial temperatures. From Fig. 8, it can be seen that the increase of initial temperature of solution can promote the hydrogen generation. In the solutions at 0 C, 10 C, 25 C, 40 C, the maximum conversion yield reaches 79.93%, 94.08%, 99.40% and 99.88%, respectively. Accordingly, the maximum hydrogen generation rate also reaches 138.10 ml min1 g1, 228.77 ml min1 g1, 499.50 ml min1 g1 and 778.50 ml min1 g1, respectively. In fact, it's worth mentioning that the hydrolysis reaction (1) and (2) are exothermic process. A large amount of heat can be generated accompanied by hydrogen production, and the solution temperature will increased gradually, especially at the local region near the powderesolution interface, in which the ratio of solution to powder is small. For instance, the reaction heat generated from 0.3 g Mg60eAl30eFe10 (wt%) material in 0.6 mol L1 NaCl solution can rapidly heat up 10 ml solution from 10 C to 68.6 C in the first three minutes. Therefore, even at a low initial temperature, the proceeding of hydrolysis reaction can be impelled and a relatively high conversion yield can also be obtained, as shown in Fig. 8. It is
Seawater is salty and corrosive that can speed up the metal corrosion in many occasions. Recently, metal/seawater systems have been widely studied to provide hydrogen for fuel cells [10,24,37]. Therefore, the hydrogen generation reaction in seawater is tested to investigate the effect of natural seawater on the hydrolysis of MgeAleFe material. Fig. 9 shows the comparison between 0.6 mol L1 NaCl solution and seawater at 25 C for the hydrogen generation of Mg60eAl30eFe10 (wt%) ball-milled for 4 h. It can be seen from Fig. 10 that the maximum conversion yield in seawater is lower than that in 0.6 mol L1 NaCl solution, as well as the hydrogen generation rate. It indicates the seawater has an inhibiting effect on the hydrolysis of Mg60eAl30eFe10 (wt%) material compared to NaCl solution. Fig. 10 shows the XRD pattern of hydrolysis product after reacting with seawater. No peaks from Mg are detected. However, strong peaks of Al are clearly identified. It reveals that seawater mainly inhibit the hydrolysis of Al in reaction process. This phenomenon can be related to the difference in solution compositions, which have been reported to exert some influence on the hydrolysis of Al [38]. It is well known that seawater as a multi-component electrolyte contains many kinds of soluble salts, besides the common salt of NaCl. The major dissolved salts in seawater have a remarkably constant composition, which can be determined to be NaCl, MgCl2, Na2SO4 and CaCl2 [39,40]. The effects of the major sea salts on the hydrogen generation are presented in Fig. 11. It can be seen that 4 h ball-milled Mg60eAl30eFe10 (wt%) material can react with different salt solutions to produce hydrogen at 25 C. But some differences can still be observed in different solutions.
Fig. 8 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with 0.6 mol L¡1 NaCl solutions at different initial temperatures.
Fig. 9 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with 0.6 mol L¡1 NaCl solution and seawater.
Effect of initial solution temperature
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and weaken the alkalization effect on the hydrolysis of Al. The insoluble hydroxide will also cover on the surface of Al, thereby preventing the corrosion of Al. Additionally, PH buffering effect of seawater caused by 2 CO2eHCO 3 eCO3 equilibrium has also been reported to limit the PH fluctuations to a certain extent, which can prevent the corrosion of Al-based material in seawater [43]. Through the above analysis, it is reasonable to believe that the specific composition of seawater is an important factor in hindering the hydrolysis of MgeAleFe material. Nonetheless, due to the high complexity of component in seawater, the chemical composition alone can not account for the inhibiting effect of seawater. Thus, further study is needed in order to obtain a more comprehensive understanding of the inhibiting effect on the hydrolysis of MgeAleFe material in seawater.
Fig. 10 e XRD pattern of hydrolysis product of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with seawater.
In the NaCl and Na2SO4 solution, the MgeAleFe material can almost completely react within 1 h and the conversion yields can both reach almost 100%. However, the maximum hydrogen generation rate in 0.6 mol L1 NaCl solution (499.50 ml min1 g1) is much higher than that in 0.6 mol L1 Na2SO4 solution (185.87 ml min1 g1). This is because that the Cl is more aggressive than SO2 4 and can easily destroy the compact passive film and result in a relatively high hydrogen generation rate [41,42]. In the MgCl2 solution, the hydrogen generation is hindered greatly, and the conversion yield is reduced to 66.78%, which is significantly lower than that in the NaCl and Na2SO4 solution. Similar inhibiting effect can also be observed in CaCl2 solution. This can be explained that the Mg2þ and Ca2þ ions have strong affinity to OH ions, and can easily combine with OH ions to form the insoluble hydroxide (Ca(OH)2 and Mg(OH)2) [22]. It can therefore reduce the amount of OH ions
Conclusion In the present work, the highly activated MgeAleFe materials were successfully prepared by ball milling. The effects of different experimental parameters on the hydrolysis of the MgeAleFe materials in aqueous solutions were investigated. The results show that the increase of ball milling time can decrease the crystallite size of Mg and Al and form more micro-galvanic cells, which is beneficial to improve the hydrolysis reactivity of MgeAleFe material. Under the present milling conditions, increase in milling time up to 4 h results in an improved hydrogen production. The addition of Fe is also found to accelerate the hydrogen generation. The optimal content of Fe is determined to be 10 wt% under current conditions. At 25 C, the best performance for hydrogen generation has been obtained from the 4 h ball-milled Mg60eAl30eFe10 (wt%) material in 0.6 mol L1 NaCl solution, leading to a hydrogen yield of 99.40% (1013.33 ml g1) and a maximum rate of 499.50 ml min1 g1. In addition, an increased hydrolysis performance can be obtained by increasing the concentration and initial temperature of NaCl solution. A considerable reaction heat generated in the hydrolysis process can raise the solution temperature and promote hydrogen generation at low temperatures. High solution temperature can improve the hydrolysis kinetics effectively. The results obtained from various salt solutions further show that seawater exhibits an inhibiting effect on the hydrolysis of MgeAleFe material compared to NaCl solution. The presence of Mg2þ, Ca2þ and SO42 ions is found to hinder the hydrolysis of MgeAleFe materials at different levels. All in all, this MgeAleFe material with ball milling can be used to provide the low-cost production of hydrogen for fuel cells.
Acknowledgments
Fig. 11 e Hydrogen generation curves of Mg60eAl30eFe10 (wt%) material ball-milled for 4 h reacting with different salt solutions.
This work was financially supported by the Science and Technology Bureau of Xiamen City (Project no. 3502Z20131153). Ping Liu is also acknowledged for helpful discussions and comments on the manuscript.
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