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Evaluation of the metal-added Mg hydrogen storage material and comparison with the oxide-added Mg
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JIEC-1939; No. of Pages 9 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Evaluation of the metal-added Mg hydrogen storage material and comparison with the oxide-added Mg Myoung Youp Song a, Hye Ryoung Park b, Sung Nam Kwon a,* a
Division of Advanced Materials Engineering, Research Center of Advanced Materials Development, Engineering Research Institute, Chonbuk National University, 567 Baekje-daero Deokjin-gu, Jeonju, 561-756, South Korea b School of Applied Chemical Engineering, Chonnam National University, 77 Yongbong-ro Buk-gu, Gwangju, 500-757, South Korea
A R T I C L E I N F O
Article history: Received 17 October 2013 Received in revised form 20 January 2014 Accepted 25 February 2014 Available online xxx
A B S T R A C T
A Mg–5 wt% Ni–2.5 wt% Fe–2.5 wt% Ti (named Mg–5Ni–2.5Fe–2.5Ti) hydrogen storage material was prepared by reactive mechanical grinding, and hydrogen absorption and desorption properties were then investigated by a Sievert’s type volumetric apparatus. The activated Mg–5Ni–2.5Fe–2.5Ti sample absorbed 5.67 wt% H for 60 min at 573K under 12 bar H2 and desorbed 4.72 wt% H for 60 min at 573K under 1.0 bar H2. Mg–5Ni–2.5Fe–2.5Ti had a higher initial hydriding rate at 593K under 12 bar H2 and a higher dehydriding rate at 593K under 1.0 bar H2 than oxide-added Mg samples with Mg–xFe2O3–yNi compositions. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has definite advantages, namely, that it is a clean energy (near-zero emission of pollutants and greenhouse gases), is non-toxic, and is the most abundant element in the universe (roughly 75% of the universe’s baryonic mass). Furthermore, hydrogen is the lightest fuel and thus has high density energy per unit mass (about 142 kJ/g of energy density per unit mass) [1]. So, nowadays, the use of hydrogen as the promising alternative energy carrier of the future is suggested. Hydrogen energy is a secondary energy which can be obtained from the energy source of fossil fuels, nuclear and renewable energy. However, because hydrogen is present in the gaseous state at room temperature and is highly flammable, there is a risk in handling. For that reason, in order to use hydrogen as energy carrier, many scientific and technical issues must be resolved. For instance, hydrogen storage systems based on compressed and liquefied hydrogen have to overcome several drawbacks associated with safety concerns and technical problems (e.g., system volume and weight, capacity of hydrogen storage, etc.). Magnesium, one of the prospective hydrogen storage materials, is able to store and release hydrogen. It has a high hydrogen storage capacity of about 7.6 wt% and is abundant in the earth’s crust.
* Corresponding author. Tel.: +82 63 270 2379; fax: +82 63 270 2386. E-mail address: [email protected] (S.N. Kwon).
However, its hydriding and dehydriding rates are very low, and its hydriding and dehydriding reactions occur at quite high temperatures. Several factors remarkably disturb the hydrogenation of magnesium. One of them is the oxidation of the magnesium surface and/or formation of magnesium hydroxide [2]. The magnesium oxide layer, on the magnesium surface, prevents hydrogen molecules from penetrating into the material [3]. Therefore, a high-temperature activation process is needed to eliminate and/or crack the magnesium oxide layer [4,5]. Another factor is the limited dissociation rate of hydrogen molecules on the metal surface [6]. The pure magnesium surface is not active for the dissociation of the hydrogen molecules [7]. Song [8] reviewed the kinetic studies of the hydriding and dehydriding reactions of magnesium; the hydriding and dehydriding reactions of magnesium are nucleation-controlled under hydrogen pressures which are not much higher or lower than the equilibrium plateau pressure at a given temperature, and progress by a mechanism of nucleation and growth, and the hydriding rates of magnesium are controlled by the diffusion of hydrogen through the growing magnesium-hydride layer. The hydriding and dehydriding kinetics of Mg can be improved, therefore, by a treatment such as mechanical alloying [9] which can facilitate nucleation by creating many defects on the surface and/or in the interior of Mg, or by the additive acting as active sites for the nucleation, and shorten the diffusion distances by reducing the effective particle sizes of Mg.
http://dx.doi.org/10.1016/j.jiec.2014.02.048 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Other studies reported different mechanisms; the growth of magnesium hydride is controlled by the slow migration of the interface between the hydride and magnesium [10,11], and hydrogen diffusion along the hydride–metal interface rather than throughout the hydride layer [12]. Many studies have been conducted in an attempt to improve the reaction kinetics of magnesium with hydrogen by adding some catalytic materials and performing mechanical treatment and/or alloying [13–20]. The dissociation rate of hydrogen molecules, which indicates the degree of surface reactivity, can be improved by adding catalytic metals to reduce the activation energy. For example, elements Pd [21], Co, Ni or Fe [22,23] and graphite [24,25] were added. Nucleation can be facilitated by creating active nucleation sites and defects; these are made by mechanical treatment and/or alloying with additives [26]. The diffusion distance of hydrogen, which influences the growth rate of the magnesium hydride, can be decreased by the mechanical treatment and/or alloying of Mg with additives, thereby reducing the magnesium particle size [2]. In addition, the hydrogen mobility can be improved by additives that create microscopic paths of hydrogen [2]. Consequently, a rough surface of magnesium possessing many cracks and defects is considered more advantageous for hydrogen absorption [27]. Some researchers enhanced the hydriding and dehydriding rates of Mg by grinding MgH2 mechanically with oxides, such as V2O5, Cr2O3, Nb2O5, and MgO, and VN, VC, or high-purity V, etc. [28–35], or through the addition of transition metal fluorides such as FeF3 [36]. Barkhordarian et al. [30–32] reported that Nb2O5 has the best catalytic effects among all the previously investigated catalysts for the hydrogen sorption reaction of magnesium. In our previous work, Fe2O3 was added to increase the hydriding and dehydriding rates of magnesium [37]. Song et al. [38,19] studied the hydrogen storage properties of the sample Mg–xFe2O3–yNi prepared by grinding Mg mechanically under H2 with ultrafine Fe2O3 particles and Ni particles. In this work, Ni, Fe and Ti were selected to improve hydriding and dehydriding rates of magnesium. Ni is known to form Mg2NiH4 which has higher hydriding and dehydriding rates than magnesium. Fe is cheap as compared with Ni, and may act active site for the dissociative chemisorption of H2 [39]. Ti is believed to increase the hydriding and dehydriding rates when it was added. Mg–5 wt% Ni–2.5 wt% Fe–2.5 wt% Ti powder was prepared by mechanical grinding under H2 atmosphere (reactive mechanical grinding) using a planetary ball mill, which can lead to the formation of a powder with small particles (with high surface reactivity) and many defects. Its hydrogen-storage properties were then investigated. We named this sample Mg–5Ni–2.5Fe–2.5Ti. Finally, the hydrogen-storage properties of the Mg–5Ni–2.5Fe–2.5Ti sample were compared with those of oxide-added Mg samples such as Mg–x wt% Fe2O3–y wt% Ni samples (named Mg–xFe2O3–yNi). 2. Experimental procedures The starting materials were pure Mg (particle size 75–150 mm, purity 99.6%), Ni (average particle size 2.2–3.0 mm, purity 99.9%), Fe (particle size <10 mm, purity 99.9%), and Ti (particle size <45 mm, purity 99.98%) powders. Mg, Ni, and Fe powders were provided by Alfa Aesar GmbH (Germany), and Ti powder was provided by Sigma-Aldrich (USA). These powders were mixed to obtain the composition Mg– 5 wt% Ni–2.5 wt% Fe–2.5 wt% Ti. The total weight of the mixture was 8 g. The mixture was put into a stainless steel container with 105 hardened steel balls (total weight: 360 g), and the container with a volume of 250 ml was then sealed hermetically. All handling was performed in a glove box filled with Ar to prevent oxidation. Mechanical grinding using a planetary ball mill was performed at a
disk revolution speed of 250 rpm under 12 bar H2 for 15 min and it was paused for 5 min. After repeating milling and pausing four times, the container was refilled with hydrogen up to of 12 bar. This process (milling, pausing, and refilling of hydrogen) was repeated, resulting in the total milling time of 4 h. The quantity of absorbed or desorbed hydrogen was measured as a function of time by a volumetric method, using a Sievert’s type hydriding and dehydriding apparatus described previously [40]. The decrease in the hydrogen pressure in the reactor due to the hydriding reaction was compensated for by supplying hydrogen from a known standard volume. The variation of the hydrogen pressure in this known standard volume enabled us to calculate the absorbed hydrogen quantity as a function of time. The increase in the hydrogen pressure in the reactor due to the dehydriding reaction was decreased by transferring hydrogen from the reactor to the known standard volume. The variation of the hydrogen pressure in this known standard volume enabled us to calculate the desorbed hydrogen quantity as a function of time. These methods enable us to measure absorbed or desorbed hydrogen quantity as a function of time under nearly constant hydrogen pressures. 0.5 g of the samples was used for these measurements. After hydriding measurement for 1 h, dehydriding measurement was performed for 1 h, and the sample was then dehydrided in vacuum for 2 h. The powders were characterized by X-ray diffraction (XRD) with Cu Ka radiation, using a Rigaku D/MAX 2500 powder diffractometer. The microstructures of the powders were observed by a JSM-6400 scanning electron microscope (SEM) operated at 20 kV. 3. Results Fig. 1 shows the microstructure, observed by SEM, of Mg–5Ni– 2.5Fe–2.5Ti after reactive mechanical grinding. The sample exhibits small and large particles with rounded edges. The particle size is not homogeneous. The XRD pattern of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding is shown in Fig. 2. The XRD pattern was analyzed by an XRD analysis program MDI JADE 6.5. The sample contains Mg, MgH2, and very small amounts of Ni, Fe, TiH1.924, and MgO, showing that MgH2, TiH1.924, and MgO are formed during reactive mechanical grinding. MgH2 and TiH1.924 are formed by the reaction of Mg and Ti, respectively, with hydrogen. MgO is formed by the reaction of Mg with oxygen impurity in hydrogen. In order to calculate the crystallite size and strain of Mg in Mg– 5Ni–2.5Fe–2.5Ti after reactive mechanical grinding, the Williamson-Hall method [41] is applied in which the following equation was used: B cosu ¼ K l=t þ 4 e sin u
(1)
where, B is full width at half maximum (FWHM), K shape factor(0.9), l wavelength (1.54056 A˚), t crystallite size, and e strain. A Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding is exhibited in Fig. 3. The crystallite size of Mg calculated using the Williamson-Hall equation is 64.6 (4.2) nm. The strain of Mg crystallite is 0.177 (0.0105)%. Fig. 4 shows the variation of the stored hydrogen quantity vs. time curve of the Mg–5Ni–2.5Fe–2.5Ti sample at 573K under 12 bar H2 with the number of hydriding–dehydriding cycles, n. The quantity of stored hydrogen is expressed as the percentage of the absorbed hydrogen with respect to the sample weight. As the number of hydriding cycles increases from n = 1 to 3, the hydriding rate increases. However, as the number of hydriding cycles increases from n = 3 to 4, the hydriding rate decreases. At the 3rd cycle, the Mg–5Ni–2.5Fe–2.5Ti sample absorbs 5.21 wt% H for
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JIEC-1939; No. of Pages 9 M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Fig. 1. Microstructure, observed by SEM, of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding.
5 min, 5.50 wt% H for 10 min, 5.56 wt% H for 20 min, and 5.67 wt% H for 60 min at 573K under 12 bar H2. Approximately 91.9% of the hydrogen absorbed for 60 min was absorbed in the first 5 min. An effective hydrogen-storage capacity is defined as the quantity of
hydrogen absorbed for 60 min. The prepared Mg–5Ni–2.5Fe–2.5Ti sample has an effective hydrogen-storage capacity near 5.7 wt% H. The variation of the released hydrogen quantity vs. time curve for the Mg–5Ni–2.5Fe–2.5Ti sample at 573K under 1.0 bar H2 with
Fig. 2. XRD pattern of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding.
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Fig. 3. Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding.
the number of hydriding–dehydriding cycles, n is presented in Fig. 5. The quantity of released hydrogen is also expressed as the percentage of the released hydrogen with respect to the sample weight. As the number of hydriding–dehydriding cycles increases from n = 1 to 4, the dehydriding rate increases. The dehydriding rate at n = 4 is very similar to that at n = 3. This shows that the activation of the Mg–5Ni– 2.5Fe–2.5Ti sample is completed after three hydriding–dehydriding cycles. At the 3rd cycle, the Mg–5Ni–2.5Fe–2.5Ti sample desorbs 0.52 wt% H for 5 min, 1.09 wt% H for 10 min, 2.49 wt% H for 20 min, 3.63 wt% H for 30 min, and 4.72 wt% H for 60 min. Table 1 presents stored and released hydrogen quantities (SHQ and RHQ) of the Mg–5Ni–2.5Fe–2.5Ti sample at various numbers of hydriding–dehydriding cycles, n, after 5, 10, 20, 30, and 60 min for absorption at 573K under 12 bar H2, and for desorption at 573K under 1.0 bar H2. Fig. 6 shows the variation of the stored hydrogen quantity vs. time curve under 12 bar H2 with temperature for the activated Mg–5Ni–2.5Fe–2.5Ti sample. The initial hydrogen absorption rate decreases as the temperature increases from 523K to 623K. The initial hydrogen absorption rates at 523K, 553K, 573K, 593K,
and 623K are 1.62, 1.55, 1.09, 0.82, and 0.55 wt% H/min, respectively. The variation of the released hydrogen quantity vs. time curve under 1.0 bar H2 with temperature for the activated Mg–5Ni– 2.5Fe–2.5Ti sample is presented in Fig. 7. The initial hydrogen desorption rate increases rapidly with the increase in temperature from 523K to 623K. The initial hydrogen desorption rates at 523K, 553K, 573K, 593K, and 623K are 0.001, 0.005, 0.128, 0.439, and 0.616 wt% H/min, respectively. Table 2 provides the stored and released hydrogen quantities (SHQ/RHQ) of the activated Mg–5Ni–2.5Fe–2.5Ti sample at various temperatures after 5, 10, 30, and 60 min for absorption under 12 bar H2, and for desorption under 1.0 bar H2. The activated Mg– 5Ni–2.5Fe–2.5Ti sample absorbs 5.21 wt% H for 5 min, 5.50 wt% H for 10 min, 5.56 wt% H for 20 min, 5.62 wt% H for 30 min, and 5.67 wt% H for 60 min at 573K under 12 bar H2. The activated Mg–5Ni– 2.5Fe–2.5Ti sample desorbs 0.61 wt% H for 5 min, 1.18 wt% H for 10 min, 2.58 wt% H for 20 min, 3.67 wt% H for 30 min, and 4.68 wt% H for 60 min at 573K under 1.0 bar H2. The microstructure, observed by SEM, of Mg–5Ni–2.5Fe–2.5Ti dehydrided after eight hydriding–dehydriding cycles is shown in Fig. 8. The sample exhibits small and large agglomerates. The
Fig. 4. Variation of the stored hydrogen quantity vs. time curve of the Mg–5Ni– 2.5Fe–2.5Ti sample at 573K under 12 bar H2 with the number of hydriding– dehydriding cycles, n.
Fig. 5. Variation of the released hydrogen quantity vs. time curve of the Mg–5Ni– 2.5Fe–2.5Ti sample at 573K under 1.0 bar H2 with the number of hydriding– dehydriding cycles, n.
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JIEC-1939; No. of Pages 9 M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 1 Stored and released hydrogen quantities (SHQ and RHQ) of the Mg–5Ni–2.5Fe–2.5Ti sample at various numbers of hydriding–dehydriding cycles, n, after 5, 10, 20, 30, and 60 min for absorption at 573K under 12 bar H2, and for desorption at 573K under 1.0 bar H2. Cycle
n=1 n=2 n=3 n=4 n=9
SHQ/RHQ (wt%) 5 min
10 min
20 min
30 min
60 min
1.91/0.52 5.15/0.48 5.21/0.52 4.90/0.61 4.94/0.48
3.57/1.09 5.42/1.09 5.50/1.09 5.10/1.18 5.15/0.96
4.41/2.36 5.52/2.36 5.56/2.49 5.16/2.58 5.28/2.22
4.73/3.24 5.56/3.45 5.62/3.63 5.19/3.67 5.33/3.41
5.02/4.20 5.60/4.50 5.67/4.72 5.30/4.68 5.46/4.28
agglomerate size is quite homogeneous. The agglomerates are consisting of fine particles and have cracks. The particles are smaller than those after reactive mechanical grinding. Fig. 9 shows the XRD pattern of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles. The sample contains Mg, MgH2, H0.3Mg2Ni, Fe, TiH1.924, and MgO, showing that MgH2 remains even after dehydriding reaction, and H0.3Mg2Ni, MgO and TiH1.924 are formed. The Mg2Ni hydride is formed during hydriding–dehydriding cycling, and H0.3Mg2Ni is formed by dehydriding reaction. MgO is formed by the reaction of Mg with oxygen impurity in hydrogen. The titanium hydride, TiH1.924, does not decompose even after dehydriding reaction. A Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe– 2.5Ti dehydrided after nine hydriding–dehydriding cycles is exhibited in Fig. 10. The crystallite size of Mg calculated using the Williamson-Hall equation is 40.0 (2.0) nm. The strain of Mg crystallite is 0.034 (0.0128)%. The crystallite size of Mg after nine hydriding–dehydriding cycles is smaller than that after reactive mechanical grinding [64.6 (4.2) nm]. The strain of Mg crystallite after nine hydriding–dehydriding cycles is smaller than that after reactive mechanical grinding [0.177 (0.0105)%]. The absorbed hydrogen quantity Ha vs. time t curves at 593K under 12 bar H2 for the activated Mg–xFe2O3–yNi [37,38,19,42] and Mg–5Ni–2.5Fe–2.5Ti samples are shown in Fig. 11. The Mg– xFe2O3–yNi samples were prepared by reactive mechanical grinding under conditions similar to those used for the preparation of the Mg–10Ni–5Fe–5Ti sample. The hydriding rate is very high in the beginning until approximately 5 min. The hydriding rates of all
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the samples except the Mg–10Fe2O3 sample then decrease rapidly, and after approximately 10 min the hydriding rate is very low. The hydriding rate of the Mg–10Fe2O3 sample decreases gradually after about 5 min. In the initial stage, the Mg–5Ni–2.5Fe–2.5Ti sample has the highest hydriding rate, followed, in descending order, by the Mg–10Fe2O3, Mg–7.5Fe2O3–7.5Ni, Mg–10Fe2O3–5Ni, and Mg– 5Fe2O3–5Ni samples. After approximately 20 min, the Mg– 10Fe2O3 sample has a higher hydriding rate than the Mg–5Ni– 2.5Fe–2.5Ti sample. The Mg–5Ni–2.5Fe–2.5Ti sample absorbs 4.57 wt% H for 5 min, 5.01 wt% H for 10 min, 5.12 wt% H for 20 min, 5.18 wt% H for 30 min, and 5.22 wt% H for 60 min at 593K under 12 bar H2. Fig. 12 presents the desorbed hydrogen quantity Hd vs. time t curves at 593K under 1.0 bar H2 for the activated Mg–xFe2O3–yNi [37,38,19,42] and Mg–5Ni–2.5Fe–2.5Ti samples. The Mg–5Ni– 2.5Fe–2.5Ti sample has the highest dehydriding rate, which is much higher than those of the other samples, followed, in descending order, by the Mg–7.5Fe2O3–7.5Ni, Mg–10Fe2O3–5Ni, Mg–5Fe2O3–5Ni, and Mg–10Fe2O3 samples. All the samples containing Ni have higher dehydriding rates than the sample without Ni, Mg–10Fe2O3. The Mg–10Fe2O3 sample with quite a high hydriding rate has the lowest dehydriding rate among all the samples. The Hd vs. t curves for the samples containing Ni exhibit a high dehydriding rate for 2.5 min, and then the dehydriding rate decreases gradually, except for the Mg–5Ni–2.5Fe–2.5Ti sample. For the first 2.5 min, it is considered that the hydride of Mg2Ni decomposes. The addition of Ni to Mg via reactive mechanical grinding increases the dehydriding rate of Mg by the formation of Mg2Ni, which has a higher dehydriding rate than Mg. The Mg–5Ni– 2.5Fe–2.5Ti sample has the highest value of Hd after 60 min, followed, in descending order, by the Mg–7.5Fe2O3–7.5Ni, Mg– 10Fe2O3–5Ni, Mg–5Fe2O3–5Ni, and Mg–10Fe2O3 samples. The activated Mg–5Ni–2.5Fe–2.5Ti sample desorbs 1.75 wt% H for 5 min, 4.32 wt% H for 10 min, 5.29 wt% H for 20 min, 5.37 wt% H for 30 min, and 5.42 wt% H for 60 min. 4. Discussion The Mg–5Ni–2.5Fe–2.5V sample has a high reactivity with hydrogen, and exhibits quite high hydriding and dehydriding kinetics even at n = 1, as shown in Figs. 4 and 5. At the first cycle, the Mg–5Ni–2.5Fe–2.5Ti sample absorbs 1.91 wt% H for 5 min,
Fig. 6. Variation of the stored hydrogen quantity vs. time curve under 12 bar H2 with temperature for the activated Mg–5Ni–2.5Fe–2.5Ti sample.
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M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 2 Stored and released hydrogen quantities (SHQ and RHQ) of the activated Mg–5Ni– 2.5Fe–2.5Ti sample at various temperatures after 5, 10, 20, 30, and 60 min for absorption under 12 bar H2, and for desorption under 1.0 bar H2. Temperature (K)
523 553 573 593 623
Fig. 7. Variation of the released hydrogen quantity vs. time curve under 1.0 bar H2 with temperature for the activated Mg–5Ni–2.5Fe–2.5Ti sample.
3.57 wt% H for 10 min, 4.41 wt% H for 20 min, and 5.02 wt% H for 60 min at 573K under 12 bar H2. The Mg–5Ni–2.5Fe–2.5Ti sample desorbs 0.52 wt% H for 5 min, 1.09 wt% H for 10 min, 2.36 wt% H for 20 min, 3.24 wt% H for 30 min, and 4.20 wt% H for 60 min at 573K under 1.0 bar H2 at the first cycle. Fig. 6 shows that the initial hydrogen absorption rate decreases with increasing temperature. The decrease in the initial hydrogen absorption rate with increasing temperature is because the difference between the applied hydrogen pressure (12 bar H2) and the equilibrium plateau pressures, which is the driving force for the hydriding reaction, is reduced as the temperature increases. Fig. 7 shows that the initial hydrogen desorption rate increases with increasing temperature. As the temperature increases, the driving force for the dehydriding reaction increases. This effect and the effect of temperature increase are believed to increase the initial hydrogen desorption rate.
SHQ/RHQ (wt%) 5 min
10 min
20 min
30 min
60 min
4.48/0.04 4.66/0.04 5.21/0.52 4.57/1.75 3.16/3.72
4.67/0.04 4.79/0.13 5.50/1.09 5.01/4.32 4.91/5.24
4.79/0.09 4.96/0.22 5.56/2.49 5.12/5.29 5.05/5.29
4.87/0.04 5.03/0.22 5.62/3.63 5.18/5.37 5.06/5.29
5.00/0.09 5.13/0.39 5.67/4.72 5.22/5.42 5.14/5.29
Added Ti forms titanium hydride, TiH1.924, after reactive mechanical grinding, and TiH1.924 remains undecomposed in the sample dehydrided after hydriding–dehydriding cycling. H0.3Mg2Ni is formed in the sample dehydrided after hydriding– dehydriding cycling. Fig. 1 shows that the sample after reactive mechanical grinding exhibits small and large particles with rounded edges. The particle size is not homogeneous but more homogeneous than those of the Mg–xFe2O3–yNi samples [37,38,19,42]. The Mg–xFe2O3–yNi samples after reactive mechanical grinding have long and large particles with flat surfaces, together with small particles. The particles of the Mg–xFe2O3–yNi samples are much larger than those of the Mg–5Ni–2.5Fe–2.5Ti sample. The addition of Ni, Fe, and Ti by reactive mechanical grinding is believed to decrease the particle size of Mg and create defects. Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding contains Ti hydride, which is brittle. The titanium hydride is considered to be pulverized during reactive mechanical grinding and this pulverized titanium hydride is believed to help Mg pulverized into finer particles effectively by being pulverized itself. Fig. 8 shows that the agglomerates of Mg–5Ni–2.5Fe–2.5Ti after hydriding–dehydriding cycles consist of fine particles. Expansion and contraction of the particles due to the hydriding and dehydriding reactions of Mg and H0.3Mg2Ni is considered to form
Fig. 8. Microstructure, observed by SEM, of Mg–5Ni–2.5Fe–2.5Ti dehydrided after eight hydriding–dehydriding cycles.
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Fig. 9. XRD pattern of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles.
defects and cracks on the particles. Expansion and contraction will also propagate the cracks and cause the particles to become finer. The stress produced by the expansion and contraction of the particles with the hydriding and dehydriding reactions is likely concentrated on the additives and other phases included in the sample such as Fe, TiH1.924, and MgO. This also causes cracks to be formed on the particles. These cracks will grow with the
hydriding–dehydriding cycling, and finally the particles will be broken up into fine particles. The Mg–xFe2O3–yNi samples after hydriding–dehydriding cycling have long and large particles with flat surfaces, together with small particles [37,38,19,42]. The particles of the Mg–xFe2O3–yNi samples after hydriding–dehydriding cycling are much larger than those of the Mg–5Ni–2.5Fe– 2.5Ti sample after hydriding–dehydriding cycling and are not
Fig. 10. Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles.
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8
6
5
Ha (wt%)
4
3
Mg-5Ni-2.5Fe-2.5Ti 2
Mg-10Fe2O3 Mg-7.5Fe2O3-7.5Ni
1
Mg-10Fe2O3-5Ni Mg-5Fe2O3-5Ni
0 0
10
20
30
40
50
60
Time (min) Fig. 11. Absorbed hydrogen quantity Ha vs. time t curves at 593K under 12 bar H2 for the activated Mg–xFe2O3–yNi and Mg–5Ni–2.5Fe–2.5Ti samples.
homogeneous in size. The fine particles forming the agglomerates of the Mg–5Ni–2.5Fe–2.5Ti sample after hydriding–dehydriding cycling are quite homogeneous. The effects of hydriding–dehydriding cycling are considered to be stronger in smaller particles than in larger particles. This led to smaller particles and more defects in the Mg–5Ni–2.5Fe–2.5Ti sample than in the Mg–xFe2O3– yNi samples. As mentioned above, Song [8] reviewed the kinetic studies of the hydriding and the dehydriding reactions of Mg. Many studies do not agree on the rate-controlling step(s) for the hydriding or dehydriding of magnesium. However, most studies do agree that the hydriding and dehydriding reactions of Mg are nucleationcontrolled under certain conditions and progress by a mechanism of nucleation and growth, and that the hydriding rates of Mg are controlled by the diffusion of hydrogen through a growing Mg hydride layer. The reactive mechanical grinding of Mg with Ni, Fe, and Ti is thought to facilitate nucleation by creating many defects on the surface and in the interior of Mg, or by the additives acting as active
Mg-5Ni-2.5Fe-2.5Ti
Mg-5Fe2O3-5Ni
Mg-7.5Fe2O3-7.5Ni
Mg-10Fe2O3
Mg-10Fe2O3-5Ni
6
5
Hd (wt%)
4
sites for the nucleation, and shorten the diffusion distances of hydrogen atoms by reducing the particle size of Mg. The XRD pattern of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles shows that the sample contains H0.3Mg2Ni phase which has higher hydriding and dehydriding rates than magnesium. The addition of Ni is believed to increase the hydriding and dehydriding rates of Mg by forming H0.3Mg2Ni. Created defects and cracks, and fragmentation into fine particles, due to the expansion and contraction of the hydrideforming materials (Mg and Mg2Ni) with the hydriding and dehydriding reactions, are also considered to increase the hydriding and dehydriding rates of the alloy. Mg–5Ni–2.5Fe–2.5Ti contains Fe and Ti, but Mg–xFe2O3–yNi does not contain them. In our previous work [42], 84 wt% Mg–14 wt% Ni–6 wt% Ti showed much higher hydriding and dehydriding rate than 84 wt% Mg–14 wt% Ni–6 wt% Fe2O3. Difference in the Ti content is considered to make the difference between them. Transition elements such as Ni, Ti, and Fe are known to act active sites for dissociative and associative chemisorptions of hydrogen [42]. However, since the hydriding reaction rate of Mg is controlled by nucleation of Mg hydride and diffusion hydrogen atoms through growing hydride layer [8], it is believed that addition of Ti or Fe contributes more strongly to the increase of effects of reactive mechanical grinding and hydriding–dehydriding cycling (creation of defects and diminution of particle size) rather than acts as active sites for dissociative and associative chemisorptions. The addition Fe and Ti is believed to make Mg–5Ni–2.5Fe–2.5Ti have higher hydriding and dehydriding rates than Mg–xFe2O3–yNi. 5. Conclusions A nanocrystalline Mg–5Ni–2.5Fe–2.5Ti sample was prepared by reactive mechanical grinding and hydriding–dehydriding cycling. The crystallite size of Mg after nine hydriding–dehydriding cycles was 40.0 (2.0) nm and its strain was 0.034 (0.0128)%. The activation process of Mg–5Ni–2.5Fe–2.5Ti was completed after three hydriding–dehydriding cycles. The prepared Mg–5Ni–2.5Fe–2.5Ti sample had an effective hydrogen-storage capacity near 5.7 wt% H. The activated Mg–5Ni–2.5Fe–2.5Ti sample absorbed 5.21 wt% H for 5 min, 5.50 wt% H for 10 min, 5.56 wt% H for 20 min, and 5.67 wt% H for 60 min at 573K under 12 bar H2. The activated Mg–5Ni–2.5Fe– 2.5Ti sample desorbed 0.52 wt% H for 5 min, 1.09 wt% H for 10 min, 2.49 wt% H for 20 min, 3.63 wt% H for 30 min, and 4.72 wt% H for 60 min at 573K under 1.0 bar H2. The hydrogen absorption and desorption rates of the activated Mg–5Ni–2.5Fe–2.5Ti showed strong dependence on temperature. The initial hydrogen absorption rate decreased, but the initial hydrogen desorption rate increased rapidly with the increases in the temperature from 523K to 623K. The hydrogen-storage properties of the Mg–5Ni–2.5Fe–2.5Ti sample were compared with those of oxide-added Mg samples. Mg–5Ni–2.5Fe– 2.5Ti had a higher initial hydriding rate at 593K under 12 bar H2 and a higher dehydriding rate at 593K under 1.0 bar H2 than oxide-added Mg samples with Mg–xFe2O3–yNi compositions.
3
Acknowledgments 2
This work was supported for a research-oriented professor (M.Y. Song) of Chonbuk National University in 2013. This paper was supported by research funds of Chonbuk National University for S.N. Kwon.
1
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Time (min) Fig. 12. Desorbed hydrogen quantity Hd vs. time t curves at 593K under 1.0 bar H2 for the activated Mg–xFe2O3–yNi and Mg–5Ni–2.5Fe–2.5Ti samples.
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JIEC-1939; No. of Pages 9 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Evaluation of the metal-added Mg hydrogen storage material and comparison with the oxide-added Mg Myoung Youp Song a, Hye Ryoung Park b, Sung Nam Kwon a,* a
Division of Advanced Materials Engineering, Research Center of Advanced Materials Development, Engineering Research Institute, Chonbuk National University, 567 Baekje-daero Deokjin-gu, Jeonju, 561-756, South Korea b School of Applied Chemical Engineering, Chonnam National University, 77 Yongbong-ro Buk-gu, Gwangju, 500-757, South Korea
A R T I C L E I N F O
Article history: Received 17 October 2013 Received in revised form 20 January 2014 Accepted 25 February 2014 Available online xxx
A B S T R A C T
A Mg–5 wt% Ni–2.5 wt% Fe–2.5 wt% Ti (named Mg–5Ni–2.5Fe–2.5Ti) hydrogen storage material was prepared by reactive mechanical grinding, and hydrogen absorption and desorption properties were then investigated by a Sievert’s type volumetric apparatus. The activated Mg–5Ni–2.5Fe–2.5Ti sample absorbed 5.67 wt% H for 60 min at 573K under 12 bar H2 and desorbed 4.72 wt% H for 60 min at 573K under 1.0 bar H2. Mg–5Ni–2.5Fe–2.5Ti had a higher initial hydriding rate at 593K under 12 bar H2 and a higher dehydriding rate at 593K under 1.0 bar H2 than oxide-added Mg samples with Mg–xFe2O3–yNi compositions. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has definite advantages, namely, that it is a clean energy (near-zero emission of pollutants and greenhouse gases), is non-toxic, and is the most abundant element in the universe (roughly 75% of the universe’s baryonic mass). Furthermore, hydrogen is the lightest fuel and thus has high density energy per unit mass (about 142 kJ/g of energy density per unit mass) [1]. So, nowadays, the use of hydrogen as the promising alternative energy carrier of the future is suggested. Hydrogen energy is a secondary energy which can be obtained from the energy source of fossil fuels, nuclear and renewable energy. However, because hydrogen is present in the gaseous state at room temperature and is highly flammable, there is a risk in handling. For that reason, in order to use hydrogen as energy carrier, many scientific and technical issues must be resolved. For instance, hydrogen storage systems based on compressed and liquefied hydrogen have to overcome several drawbacks associated with safety concerns and technical problems (e.g., system volume and weight, capacity of hydrogen storage, etc.). Magnesium, one of the prospective hydrogen storage materials, is able to store and release hydrogen. It has a high hydrogen storage capacity of about 7.6 wt% and is abundant in the earth’s crust.
* Corresponding author. Tel.: +82 63 270 2379; fax: +82 63 270 2386. E-mail address: [email protected] (S.N. Kwon).
However, its hydriding and dehydriding rates are very low, and its hydriding and dehydriding reactions occur at quite high temperatures. Several factors remarkably disturb the hydrogenation of magnesium. One of them is the oxidation of the magnesium surface and/or formation of magnesium hydroxide [2]. The magnesium oxide layer, on the magnesium surface, prevents hydrogen molecules from penetrating into the material [3]. Therefore, a high-temperature activation process is needed to eliminate and/or crack the magnesium oxide layer [4,5]. Another factor is the limited dissociation rate of hydrogen molecules on the metal surface [6]. The pure magnesium surface is not active for the dissociation of the hydrogen molecules [7]. Song [8] reviewed the kinetic studies of the hydriding and dehydriding reactions of magnesium; the hydriding and dehydriding reactions of magnesium are nucleation-controlled under hydrogen pressures which are not much higher or lower than the equilibrium plateau pressure at a given temperature, and progress by a mechanism of nucleation and growth, and the hydriding rates of magnesium are controlled by the diffusion of hydrogen through the growing magnesium-hydride layer. The hydriding and dehydriding kinetics of Mg can be improved, therefore, by a treatment such as mechanical alloying [9] which can facilitate nucleation by creating many defects on the surface and/or in the interior of Mg, or by the additive acting as active sites for the nucleation, and shorten the diffusion distances by reducing the effective particle sizes of Mg.
http://dx.doi.org/10.1016/j.jiec.2014.02.048 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Other studies reported different mechanisms; the growth of magnesium hydride is controlled by the slow migration of the interface between the hydride and magnesium [10,11], and hydrogen diffusion along the hydride–metal interface rather than throughout the hydride layer [12]. Many studies have been conducted in an attempt to improve the reaction kinetics of magnesium with hydrogen by adding some catalytic materials and performing mechanical treatment and/or alloying [13–20]. The dissociation rate of hydrogen molecules, which indicates the degree of surface reactivity, can be improved by adding catalytic metals to reduce the activation energy. For example, elements Pd [21], Co, Ni or Fe [22,23] and graphite [24,25] were added. Nucleation can be facilitated by creating active nucleation sites and defects; these are made by mechanical treatment and/or alloying with additives [26]. The diffusion distance of hydrogen, which influences the growth rate of the magnesium hydride, can be decreased by the mechanical treatment and/or alloying of Mg with additives, thereby reducing the magnesium particle size [2]. In addition, the hydrogen mobility can be improved by additives that create microscopic paths of hydrogen [2]. Consequently, a rough surface of magnesium possessing many cracks and defects is considered more advantageous for hydrogen absorption [27]. Some researchers enhanced the hydriding and dehydriding rates of Mg by grinding MgH2 mechanically with oxides, such as V2O5, Cr2O3, Nb2O5, and MgO, and VN, VC, or high-purity V, etc. [28–35], or through the addition of transition metal fluorides such as FeF3 [36]. Barkhordarian et al. [30–32] reported that Nb2O5 has the best catalytic effects among all the previously investigated catalysts for the hydrogen sorption reaction of magnesium. In our previous work, Fe2O3 was added to increase the hydriding and dehydriding rates of magnesium [37]. Song et al. [38,19] studied the hydrogen storage properties of the sample Mg–xFe2O3–yNi prepared by grinding Mg mechanically under H2 with ultrafine Fe2O3 particles and Ni particles. In this work, Ni, Fe and Ti were selected to improve hydriding and dehydriding rates of magnesium. Ni is known to form Mg2NiH4 which has higher hydriding and dehydriding rates than magnesium. Fe is cheap as compared with Ni, and may act active site for the dissociative chemisorption of H2 [39]. Ti is believed to increase the hydriding and dehydriding rates when it was added. Mg–5 wt% Ni–2.5 wt% Fe–2.5 wt% Ti powder was prepared by mechanical grinding under H2 atmosphere (reactive mechanical grinding) using a planetary ball mill, which can lead to the formation of a powder with small particles (with high surface reactivity) and many defects. Its hydrogen-storage properties were then investigated. We named this sample Mg–5Ni–2.5Fe–2.5Ti. Finally, the hydrogen-storage properties of the Mg–5Ni–2.5Fe–2.5Ti sample were compared with those of oxide-added Mg samples such as Mg–x wt% Fe2O3–y wt% Ni samples (named Mg–xFe2O3–yNi). 2. Experimental procedures The starting materials were pure Mg (particle size 75–150 mm, purity 99.6%), Ni (average particle size 2.2–3.0 mm, purity 99.9%), Fe (particle size <10 mm, purity 99.9%), and Ti (particle size <45 mm, purity 99.98%) powders. Mg, Ni, and Fe powders were provided by Alfa Aesar GmbH (Germany), and Ti powder was provided by Sigma-Aldrich (USA). These powders were mixed to obtain the composition Mg– 5 wt% Ni–2.5 wt% Fe–2.5 wt% Ti. The total weight of the mixture was 8 g. The mixture was put into a stainless steel container with 105 hardened steel balls (total weight: 360 g), and the container with a volume of 250 ml was then sealed hermetically. All handling was performed in a glove box filled with Ar to prevent oxidation. Mechanical grinding using a planetary ball mill was performed at a
disk revolution speed of 250 rpm under 12 bar H2 for 15 min and it was paused for 5 min. After repeating milling and pausing four times, the container was refilled with hydrogen up to of 12 bar. This process (milling, pausing, and refilling of hydrogen) was repeated, resulting in the total milling time of 4 h. The quantity of absorbed or desorbed hydrogen was measured as a function of time by a volumetric method, using a Sievert’s type hydriding and dehydriding apparatus described previously [40]. The decrease in the hydrogen pressure in the reactor due to the hydriding reaction was compensated for by supplying hydrogen from a known standard volume. The variation of the hydrogen pressure in this known standard volume enabled us to calculate the absorbed hydrogen quantity as a function of time. The increase in the hydrogen pressure in the reactor due to the dehydriding reaction was decreased by transferring hydrogen from the reactor to the known standard volume. The variation of the hydrogen pressure in this known standard volume enabled us to calculate the desorbed hydrogen quantity as a function of time. These methods enable us to measure absorbed or desorbed hydrogen quantity as a function of time under nearly constant hydrogen pressures. 0.5 g of the samples was used for these measurements. After hydriding measurement for 1 h, dehydriding measurement was performed for 1 h, and the sample was then dehydrided in vacuum for 2 h. The powders were characterized by X-ray diffraction (XRD) with Cu Ka radiation, using a Rigaku D/MAX 2500 powder diffractometer. The microstructures of the powders were observed by a JSM-6400 scanning electron microscope (SEM) operated at 20 kV. 3. Results Fig. 1 shows the microstructure, observed by SEM, of Mg–5Ni– 2.5Fe–2.5Ti after reactive mechanical grinding. The sample exhibits small and large particles with rounded edges. The particle size is not homogeneous. The XRD pattern of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding is shown in Fig. 2. The XRD pattern was analyzed by an XRD analysis program MDI JADE 6.5. The sample contains Mg, MgH2, and very small amounts of Ni, Fe, TiH1.924, and MgO, showing that MgH2, TiH1.924, and MgO are formed during reactive mechanical grinding. MgH2 and TiH1.924 are formed by the reaction of Mg and Ti, respectively, with hydrogen. MgO is formed by the reaction of Mg with oxygen impurity in hydrogen. In order to calculate the crystallite size and strain of Mg in Mg– 5Ni–2.5Fe–2.5Ti after reactive mechanical grinding, the Williamson-Hall method [41] is applied in which the following equation was used: B cosu ¼ K l=t þ 4 e sin u
(1)
where, B is full width at half maximum (FWHM), K shape factor(0.9), l wavelength (1.54056 A˚), t crystallite size, and e strain. A Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding is exhibited in Fig. 3. The crystallite size of Mg calculated using the Williamson-Hall equation is 64.6 (4.2) nm. The strain of Mg crystallite is 0.177 (0.0105)%. Fig. 4 shows the variation of the stored hydrogen quantity vs. time curve of the Mg–5Ni–2.5Fe–2.5Ti sample at 573K under 12 bar H2 with the number of hydriding–dehydriding cycles, n. The quantity of stored hydrogen is expressed as the percentage of the absorbed hydrogen with respect to the sample weight. As the number of hydriding cycles increases from n = 1 to 3, the hydriding rate increases. However, as the number of hydriding cycles increases from n = 3 to 4, the hydriding rate decreases. At the 3rd cycle, the Mg–5Ni–2.5Fe–2.5Ti sample absorbs 5.21 wt% H for
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Fig. 1. Microstructure, observed by SEM, of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding.
5 min, 5.50 wt% H for 10 min, 5.56 wt% H for 20 min, and 5.67 wt% H for 60 min at 573K under 12 bar H2. Approximately 91.9% of the hydrogen absorbed for 60 min was absorbed in the first 5 min. An effective hydrogen-storage capacity is defined as the quantity of
hydrogen absorbed for 60 min. The prepared Mg–5Ni–2.5Fe–2.5Ti sample has an effective hydrogen-storage capacity near 5.7 wt% H. The variation of the released hydrogen quantity vs. time curve for the Mg–5Ni–2.5Fe–2.5Ti sample at 573K under 1.0 bar H2 with
Fig. 2. XRD pattern of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding.
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Fig. 3. Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding.
the number of hydriding–dehydriding cycles, n is presented in Fig. 5. The quantity of released hydrogen is also expressed as the percentage of the released hydrogen with respect to the sample weight. As the number of hydriding–dehydriding cycles increases from n = 1 to 4, the dehydriding rate increases. The dehydriding rate at n = 4 is very similar to that at n = 3. This shows that the activation of the Mg–5Ni– 2.5Fe–2.5Ti sample is completed after three hydriding–dehydriding cycles. At the 3rd cycle, the Mg–5Ni–2.5Fe–2.5Ti sample desorbs 0.52 wt% H for 5 min, 1.09 wt% H for 10 min, 2.49 wt% H for 20 min, 3.63 wt% H for 30 min, and 4.72 wt% H for 60 min. Table 1 presents stored and released hydrogen quantities (SHQ and RHQ) of the Mg–5Ni–2.5Fe–2.5Ti sample at various numbers of hydriding–dehydriding cycles, n, after 5, 10, 20, 30, and 60 min for absorption at 573K under 12 bar H2, and for desorption at 573K under 1.0 bar H2. Fig. 6 shows the variation of the stored hydrogen quantity vs. time curve under 12 bar H2 with temperature for the activated Mg–5Ni–2.5Fe–2.5Ti sample. The initial hydrogen absorption rate decreases as the temperature increases from 523K to 623K. The initial hydrogen absorption rates at 523K, 553K, 573K, 593K,
and 623K are 1.62, 1.55, 1.09, 0.82, and 0.55 wt% H/min, respectively. The variation of the released hydrogen quantity vs. time curve under 1.0 bar H2 with temperature for the activated Mg–5Ni– 2.5Fe–2.5Ti sample is presented in Fig. 7. The initial hydrogen desorption rate increases rapidly with the increase in temperature from 523K to 623K. The initial hydrogen desorption rates at 523K, 553K, 573K, 593K, and 623K are 0.001, 0.005, 0.128, 0.439, and 0.616 wt% H/min, respectively. Table 2 provides the stored and released hydrogen quantities (SHQ/RHQ) of the activated Mg–5Ni–2.5Fe–2.5Ti sample at various temperatures after 5, 10, 30, and 60 min for absorption under 12 bar H2, and for desorption under 1.0 bar H2. The activated Mg– 5Ni–2.5Fe–2.5Ti sample absorbs 5.21 wt% H for 5 min, 5.50 wt% H for 10 min, 5.56 wt% H for 20 min, 5.62 wt% H for 30 min, and 5.67 wt% H for 60 min at 573K under 12 bar H2. The activated Mg–5Ni– 2.5Fe–2.5Ti sample desorbs 0.61 wt% H for 5 min, 1.18 wt% H for 10 min, 2.58 wt% H for 20 min, 3.67 wt% H for 30 min, and 4.68 wt% H for 60 min at 573K under 1.0 bar H2. The microstructure, observed by SEM, of Mg–5Ni–2.5Fe–2.5Ti dehydrided after eight hydriding–dehydriding cycles is shown in Fig. 8. The sample exhibits small and large agglomerates. The
Fig. 4. Variation of the stored hydrogen quantity vs. time curve of the Mg–5Ni– 2.5Fe–2.5Ti sample at 573K under 12 bar H2 with the number of hydriding– dehydriding cycles, n.
Fig. 5. Variation of the released hydrogen quantity vs. time curve of the Mg–5Ni– 2.5Fe–2.5Ti sample at 573K under 1.0 bar H2 with the number of hydriding– dehydriding cycles, n.
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JIEC-1939; No. of Pages 9 M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 1 Stored and released hydrogen quantities (SHQ and RHQ) of the Mg–5Ni–2.5Fe–2.5Ti sample at various numbers of hydriding–dehydriding cycles, n, after 5, 10, 20, 30, and 60 min for absorption at 573K under 12 bar H2, and for desorption at 573K under 1.0 bar H2. Cycle
n=1 n=2 n=3 n=4 n=9
SHQ/RHQ (wt%) 5 min
10 min
20 min
30 min
60 min
1.91/0.52 5.15/0.48 5.21/0.52 4.90/0.61 4.94/0.48
3.57/1.09 5.42/1.09 5.50/1.09 5.10/1.18 5.15/0.96
4.41/2.36 5.52/2.36 5.56/2.49 5.16/2.58 5.28/2.22
4.73/3.24 5.56/3.45 5.62/3.63 5.19/3.67 5.33/3.41
5.02/4.20 5.60/4.50 5.67/4.72 5.30/4.68 5.46/4.28
agglomerate size is quite homogeneous. The agglomerates are consisting of fine particles and have cracks. The particles are smaller than those after reactive mechanical grinding. Fig. 9 shows the XRD pattern of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles. The sample contains Mg, MgH2, H0.3Mg2Ni, Fe, TiH1.924, and MgO, showing that MgH2 remains even after dehydriding reaction, and H0.3Mg2Ni, MgO and TiH1.924 are formed. The Mg2Ni hydride is formed during hydriding–dehydriding cycling, and H0.3Mg2Ni is formed by dehydriding reaction. MgO is formed by the reaction of Mg with oxygen impurity in hydrogen. The titanium hydride, TiH1.924, does not decompose even after dehydriding reaction. A Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe– 2.5Ti dehydrided after nine hydriding–dehydriding cycles is exhibited in Fig. 10. The crystallite size of Mg calculated using the Williamson-Hall equation is 40.0 (2.0) nm. The strain of Mg crystallite is 0.034 (0.0128)%. The crystallite size of Mg after nine hydriding–dehydriding cycles is smaller than that after reactive mechanical grinding [64.6 (4.2) nm]. The strain of Mg crystallite after nine hydriding–dehydriding cycles is smaller than that after reactive mechanical grinding [0.177 (0.0105)%]. The absorbed hydrogen quantity Ha vs. time t curves at 593K under 12 bar H2 for the activated Mg–xFe2O3–yNi [37,38,19,42] and Mg–5Ni–2.5Fe–2.5Ti samples are shown in Fig. 11. The Mg– xFe2O3–yNi samples were prepared by reactive mechanical grinding under conditions similar to those used for the preparation of the Mg–10Ni–5Fe–5Ti sample. The hydriding rate is very high in the beginning until approximately 5 min. The hydriding rates of all
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the samples except the Mg–10Fe2O3 sample then decrease rapidly, and after approximately 10 min the hydriding rate is very low. The hydriding rate of the Mg–10Fe2O3 sample decreases gradually after about 5 min. In the initial stage, the Mg–5Ni–2.5Fe–2.5Ti sample has the highest hydriding rate, followed, in descending order, by the Mg–10Fe2O3, Mg–7.5Fe2O3–7.5Ni, Mg–10Fe2O3–5Ni, and Mg– 5Fe2O3–5Ni samples. After approximately 20 min, the Mg– 10Fe2O3 sample has a higher hydriding rate than the Mg–5Ni– 2.5Fe–2.5Ti sample. The Mg–5Ni–2.5Fe–2.5Ti sample absorbs 4.57 wt% H for 5 min, 5.01 wt% H for 10 min, 5.12 wt% H for 20 min, 5.18 wt% H for 30 min, and 5.22 wt% H for 60 min at 593K under 12 bar H2. Fig. 12 presents the desorbed hydrogen quantity Hd vs. time t curves at 593K under 1.0 bar H2 for the activated Mg–xFe2O3–yNi [37,38,19,42] and Mg–5Ni–2.5Fe–2.5Ti samples. The Mg–5Ni– 2.5Fe–2.5Ti sample has the highest dehydriding rate, which is much higher than those of the other samples, followed, in descending order, by the Mg–7.5Fe2O3–7.5Ni, Mg–10Fe2O3–5Ni, Mg–5Fe2O3–5Ni, and Mg–10Fe2O3 samples. All the samples containing Ni have higher dehydriding rates than the sample without Ni, Mg–10Fe2O3. The Mg–10Fe2O3 sample with quite a high hydriding rate has the lowest dehydriding rate among all the samples. The Hd vs. t curves for the samples containing Ni exhibit a high dehydriding rate for 2.5 min, and then the dehydriding rate decreases gradually, except for the Mg–5Ni–2.5Fe–2.5Ti sample. For the first 2.5 min, it is considered that the hydride of Mg2Ni decomposes. The addition of Ni to Mg via reactive mechanical grinding increases the dehydriding rate of Mg by the formation of Mg2Ni, which has a higher dehydriding rate than Mg. The Mg–5Ni– 2.5Fe–2.5Ti sample has the highest value of Hd after 60 min, followed, in descending order, by the Mg–7.5Fe2O3–7.5Ni, Mg– 10Fe2O3–5Ni, Mg–5Fe2O3–5Ni, and Mg–10Fe2O3 samples. The activated Mg–5Ni–2.5Fe–2.5Ti sample desorbs 1.75 wt% H for 5 min, 4.32 wt% H for 10 min, 5.29 wt% H for 20 min, 5.37 wt% H for 30 min, and 5.42 wt% H for 60 min. 4. Discussion The Mg–5Ni–2.5Fe–2.5V sample has a high reactivity with hydrogen, and exhibits quite high hydriding and dehydriding kinetics even at n = 1, as shown in Figs. 4 and 5. At the first cycle, the Mg–5Ni–2.5Fe–2.5Ti sample absorbs 1.91 wt% H for 5 min,
Fig. 6. Variation of the stored hydrogen quantity vs. time curve under 12 bar H2 with temperature for the activated Mg–5Ni–2.5Fe–2.5Ti sample.
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JIEC-1939; No. of Pages 9 6
M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 2 Stored and released hydrogen quantities (SHQ and RHQ) of the activated Mg–5Ni– 2.5Fe–2.5Ti sample at various temperatures after 5, 10, 20, 30, and 60 min for absorption under 12 bar H2, and for desorption under 1.0 bar H2. Temperature (K)
523 553 573 593 623
Fig. 7. Variation of the released hydrogen quantity vs. time curve under 1.0 bar H2 with temperature for the activated Mg–5Ni–2.5Fe–2.5Ti sample.
3.57 wt% H for 10 min, 4.41 wt% H for 20 min, and 5.02 wt% H for 60 min at 573K under 12 bar H2. The Mg–5Ni–2.5Fe–2.5Ti sample desorbs 0.52 wt% H for 5 min, 1.09 wt% H for 10 min, 2.36 wt% H for 20 min, 3.24 wt% H for 30 min, and 4.20 wt% H for 60 min at 573K under 1.0 bar H2 at the first cycle. Fig. 6 shows that the initial hydrogen absorption rate decreases with increasing temperature. The decrease in the initial hydrogen absorption rate with increasing temperature is because the difference between the applied hydrogen pressure (12 bar H2) and the equilibrium plateau pressures, which is the driving force for the hydriding reaction, is reduced as the temperature increases. Fig. 7 shows that the initial hydrogen desorption rate increases with increasing temperature. As the temperature increases, the driving force for the dehydriding reaction increases. This effect and the effect of temperature increase are believed to increase the initial hydrogen desorption rate.
SHQ/RHQ (wt%) 5 min
10 min
20 min
30 min
60 min
4.48/0.04 4.66/0.04 5.21/0.52 4.57/1.75 3.16/3.72
4.67/0.04 4.79/0.13 5.50/1.09 5.01/4.32 4.91/5.24
4.79/0.09 4.96/0.22 5.56/2.49 5.12/5.29 5.05/5.29
4.87/0.04 5.03/0.22 5.62/3.63 5.18/5.37 5.06/5.29
5.00/0.09 5.13/0.39 5.67/4.72 5.22/5.42 5.14/5.29
Added Ti forms titanium hydride, TiH1.924, after reactive mechanical grinding, and TiH1.924 remains undecomposed in the sample dehydrided after hydriding–dehydriding cycling. H0.3Mg2Ni is formed in the sample dehydrided after hydriding– dehydriding cycling. Fig. 1 shows that the sample after reactive mechanical grinding exhibits small and large particles with rounded edges. The particle size is not homogeneous but more homogeneous than those of the Mg–xFe2O3–yNi samples [37,38,19,42]. The Mg–xFe2O3–yNi samples after reactive mechanical grinding have long and large particles with flat surfaces, together with small particles. The particles of the Mg–xFe2O3–yNi samples are much larger than those of the Mg–5Ni–2.5Fe–2.5Ti sample. The addition of Ni, Fe, and Ti by reactive mechanical grinding is believed to decrease the particle size of Mg and create defects. Mg–5Ni–2.5Fe–2.5Ti after reactive mechanical grinding contains Ti hydride, which is brittle. The titanium hydride is considered to be pulverized during reactive mechanical grinding and this pulverized titanium hydride is believed to help Mg pulverized into finer particles effectively by being pulverized itself. Fig. 8 shows that the agglomerates of Mg–5Ni–2.5Fe–2.5Ti after hydriding–dehydriding cycles consist of fine particles. Expansion and contraction of the particles due to the hydriding and dehydriding reactions of Mg and H0.3Mg2Ni is considered to form
Fig. 8. Microstructure, observed by SEM, of Mg–5Ni–2.5Fe–2.5Ti dehydrided after eight hydriding–dehydriding cycles.
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JIEC-1939; No. of Pages 9 M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Fig. 9. XRD pattern of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles.
defects and cracks on the particles. Expansion and contraction will also propagate the cracks and cause the particles to become finer. The stress produced by the expansion and contraction of the particles with the hydriding and dehydriding reactions is likely concentrated on the additives and other phases included in the sample such as Fe, TiH1.924, and MgO. This also causes cracks to be formed on the particles. These cracks will grow with the
hydriding–dehydriding cycling, and finally the particles will be broken up into fine particles. The Mg–xFe2O3–yNi samples after hydriding–dehydriding cycling have long and large particles with flat surfaces, together with small particles [37,38,19,42]. The particles of the Mg–xFe2O3–yNi samples after hydriding–dehydriding cycling are much larger than those of the Mg–5Ni–2.5Fe– 2.5Ti sample after hydriding–dehydriding cycling and are not
Fig. 10. Williamson-Hall diagram for Mg peaks of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles.
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JIEC-1939; No. of Pages 9 M.Y. Song et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
8
6
5
Ha (wt%)
4
3
Mg-5Ni-2.5Fe-2.5Ti 2
Mg-10Fe2O3 Mg-7.5Fe2O3-7.5Ni
1
Mg-10Fe2O3-5Ni Mg-5Fe2O3-5Ni
0 0
10
20
30
40
50
60
Time (min) Fig. 11. Absorbed hydrogen quantity Ha vs. time t curves at 593K under 12 bar H2 for the activated Mg–xFe2O3–yNi and Mg–5Ni–2.5Fe–2.5Ti samples.
homogeneous in size. The fine particles forming the agglomerates of the Mg–5Ni–2.5Fe–2.5Ti sample after hydriding–dehydriding cycling are quite homogeneous. The effects of hydriding–dehydriding cycling are considered to be stronger in smaller particles than in larger particles. This led to smaller particles and more defects in the Mg–5Ni–2.5Fe–2.5Ti sample than in the Mg–xFe2O3– yNi samples. As mentioned above, Song [8] reviewed the kinetic studies of the hydriding and the dehydriding reactions of Mg. Many studies do not agree on the rate-controlling step(s) for the hydriding or dehydriding of magnesium. However, most studies do agree that the hydriding and dehydriding reactions of Mg are nucleationcontrolled under certain conditions and progress by a mechanism of nucleation and growth, and that the hydriding rates of Mg are controlled by the diffusion of hydrogen through a growing Mg hydride layer. The reactive mechanical grinding of Mg with Ni, Fe, and Ti is thought to facilitate nucleation by creating many defects on the surface and in the interior of Mg, or by the additives acting as active
Mg-5Ni-2.5Fe-2.5Ti
Mg-5Fe2O3-5Ni
Mg-7.5Fe2O3-7.5Ni
Mg-10Fe2O3
Mg-10Fe2O3-5Ni
6
5
Hd (wt%)
4
sites for the nucleation, and shorten the diffusion distances of hydrogen atoms by reducing the particle size of Mg. The XRD pattern of Mg–5Ni–2.5Fe–2.5Ti dehydrided after nine hydriding–dehydriding cycles shows that the sample contains H0.3Mg2Ni phase which has higher hydriding and dehydriding rates than magnesium. The addition of Ni is believed to increase the hydriding and dehydriding rates of Mg by forming H0.3Mg2Ni. Created defects and cracks, and fragmentation into fine particles, due to the expansion and contraction of the hydrideforming materials (Mg and Mg2Ni) with the hydriding and dehydriding reactions, are also considered to increase the hydriding and dehydriding rates of the alloy. Mg–5Ni–2.5Fe–2.5Ti contains Fe and Ti, but Mg–xFe2O3–yNi does not contain them. In our previous work [42], 84 wt% Mg–14 wt% Ni–6 wt% Ti showed much higher hydriding and dehydriding rate than 84 wt% Mg–14 wt% Ni–6 wt% Fe2O3. Difference in the Ti content is considered to make the difference between them. Transition elements such as Ni, Ti, and Fe are known to act active sites for dissociative and associative chemisorptions of hydrogen [42]. However, since the hydriding reaction rate of Mg is controlled by nucleation of Mg hydride and diffusion hydrogen atoms through growing hydride layer [8], it is believed that addition of Ti or Fe contributes more strongly to the increase of effects of reactive mechanical grinding and hydriding–dehydriding cycling (creation of defects and diminution of particle size) rather than acts as active sites for dissociative and associative chemisorptions. The addition Fe and Ti is believed to make Mg–5Ni–2.5Fe–2.5Ti have higher hydriding and dehydriding rates than Mg–xFe2O3–yNi. 5. Conclusions A nanocrystalline Mg–5Ni–2.5Fe–2.5Ti sample was prepared by reactive mechanical grinding and hydriding–dehydriding cycling. The crystallite size of Mg after nine hydriding–dehydriding cycles was 40.0 (2.0) nm and its strain was 0.034 (0.0128)%. The activation process of Mg–5Ni–2.5Fe–2.5Ti was completed after three hydriding–dehydriding cycles. The prepared Mg–5Ni–2.5Fe–2.5Ti sample had an effective hydrogen-storage capacity near 5.7 wt% H. The activated Mg–5Ni–2.5Fe–2.5Ti sample absorbed 5.21 wt% H for 5 min, 5.50 wt% H for 10 min, 5.56 wt% H for 20 min, and 5.67 wt% H for 60 min at 573K under 12 bar H2. The activated Mg–5Ni–2.5Fe– 2.5Ti sample desorbed 0.52 wt% H for 5 min, 1.09 wt% H for 10 min, 2.49 wt% H for 20 min, 3.63 wt% H for 30 min, and 4.72 wt% H for 60 min at 573K under 1.0 bar H2. The hydrogen absorption and desorption rates of the activated Mg–5Ni–2.5Fe–2.5Ti showed strong dependence on temperature. The initial hydrogen absorption rate decreased, but the initial hydrogen desorption rate increased rapidly with the increases in the temperature from 523K to 623K. The hydrogen-storage properties of the Mg–5Ni–2.5Fe–2.5Ti sample were compared with those of oxide-added Mg samples. Mg–5Ni–2.5Fe– 2.5Ti had a higher initial hydriding rate at 593K under 12 bar H2 and a higher dehydriding rate at 593K under 1.0 bar H2 than oxide-added Mg samples with Mg–xFe2O3–yNi compositions.
3
Acknowledgments 2
This work was supported for a research-oriented professor (M.Y. Song) of Chonbuk National University in 2013. This paper was supported by research funds of Chonbuk National University for S.N. Kwon.
1
0 0
10
20
30
40
50
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Time (min) Fig. 12. Desorbed hydrogen quantity Hd vs. time t curves at 593K under 1.0 bar H2 for the activated Mg–xFe2O3–yNi and Mg–5Ni–2.5Fe–2.5Ti samples.
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