Comparison of the Hydrogen Release Properties of Zn(BH 4 ) 2 -Added MgH 2 Alloy and Zn(BH 4 ) 2 and Ni-Added MgH 2 Alloy

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1 [Research Paper] 대한금속 재료학회지 (Korean J. Met. Mater.), Vol. 56, No. 3 (2018) pp DOI: /KJMM Comparison of the Hydrogen Release Properties of Zn(BH 4 -Added MgH 2 Alloy and Zn(BH 4 and Ni-Added MgH 2 Alloy Myoung Youp Song* and Young Jun Kwak Division of Advanced Materials Engineering, Hydrogen & Fuel Cell Research Center, Engineering Research Institute, Chonbuk National University, Jeonju 54896, Republic of Korea Abstract: Samples with compositions of 99 w/o MgH 2 +1 w/o Zn(BH 4 (designated MgH 2-1Zn(BH 4 ) and 96 w/o MgH 2 +2 w/o Zn(BH 4 +2 w/o Ni (designated MgH 2-2Zn(BH 4-2Ni) were prepared by milling in a planetary ball mill in a hydrogen atmosphere (reaction-involving milling). The hydrogen release properties of MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni were compared. A larger quantity of additives and the addition of Ni decreased the temperature at which MgH 2 decomposes in the as-milled samples. Activation processes were not required for these two samples. A larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased the quantity of hydrogen absorbed in 60 min (U (60 min)), the initial hydrogen release rate, and the quantity of hydrogen released in 60 min (R (60 min)). MgH 2-2Zn(BH 4-2Ni had an effective hydrogen storage capacity of about 5.5 w/o at the cycle number, CN, of one (CN=1). A ZnH 2 phase, which has not been reported to be formed, was found in the MgH 2-1Zn(BH 4 sample and the Zn(BH 4 and Ni-added MgH 2 sample after hydrogen uptake-release cycling. Mg 2 Ni was formed in the Zn(BH 4 and Niadded MgH 2 sample after hydrogen uptake-release cycling. The rapid hydrogen release rate of the Mg 2 NiH 4 formed in MgH 2-2Zn(BH 4-2Ni is believed to increase the initial hydrogen release rate of MgH 2-2Zn(BH 4-2Ni. The Mg 2 Ni decomposed from Mg 2 NiH 4 might facilitate the nucleation of a Mg-H solid solution in the MgH 2-2Zn(BH 4-2Ni sample. There is also a slight possibility that the contraction due to the relatively rapid hydrogen release of the Mg 2 NiH 4 provides passages for the hydrogen released from neighboring MgH 2. (Received December 20, 2017; Accepted January 9, 2018) Keywords: hydrogen absorbing materials; mechanical milling; hydrogen; microstructure; Zn(BH 4 and/or Ni-added MgH 2 alloy. 1. INTRODUCTION Magnesium hydride (MgH 2 ) has a certain number of advantages compared with other hydrides: magnesium (Mg) exists in abundance, its price is less elevated, and Mg is light. However, Mg has the following two important weaknesses in potential applications: MgH 2 releases hydrogen at relatively high temperature and its formation and decomposition rates are low. The complex metal hydride zinc borohydride (Zn(BH 4 ), one of the metal borohydrides, has attracted the interest of numerous researchers due to its high hydrogen density (8.4 w/o) [1] and its low *Corresponding Author: Myoung Youp Song [Tel: , songmy@jbnu.ac.kr ] Copyright c The Korean Institute of Metals and Materials decomposition temperature ( K). Nakagawa et al. [1] synthesized Zn(BH 4 by grinding zinc chloride (ZnCl 2 ) and sodium borohydride (NaBH 4 ) together. The preparation of the Zn(BH 4 was accompanied by the formation of sodium chloride (NaCl). Nakamori et al. [2,3] prepared the metal borohydrides M(BH 4 ) n (M = Ca, Sc, Ti, V, Cr, Mn, Zn (fourth period in the periodic table), and Al; n = 2 4) using a mechanical grinding process. They reported that the hydrogen desorption temperature of the M(BH 4 ) n decreased as the Pauling electronegativity of the M increased. In order to improve the hydrogen sorption properties of Mg, Ni [4-6], Ti [7,8], and Ti and/or Ni [9] were added. Orimo et al. [6] synthesized the alloys Mg-x a/o Ni (x = 33, 38, 43, and 50) with different nanometerscale structures by mechanically grinding the magnesium nickel (Mg 2 Ni) mixed with various amounts of nickel

2 Myoung Youp Song and Young Jun Kwak 245 (Ni). As the Ni content increased, the total hydrogen content increased from 1.7 w/o to 2.2 w/o and the hydrogen releasing temperature decreased from 440 K to 373 K. Lu et al. [9] synthesized core-shell structured binary Mg-Ti and ternary Mg-Ti-Ni composites using an arc plasma method followed by electroless plating in solutions. They reported that hydrogenated composites with core shell structures containing MgH 2 core and Ti or Mg-Ni hydrides shells were observed and the coeffect of TiH 2 and Mg 2 Ni mainly improved the hydrogen sorption properties of Mg. Srinivasan et al. [10] prepared zinc borohydride, Zn(BH 4, by solid-state mechanochemical process. Various catalysts such as TiCl 3, TiF 3, nano Ni, nano Fe, Ti, nano Ti, and Zn were added to the borohydride to lower the decomposition temperature in the range of K without significantly decreasing the hydrogen storage capacity of the sample. They reported that among the different catalysts, a 1.5 m/o nano Ni added sample had the optimum behaviour in terms of fast kinetics and decreasing the melting and decomposition temperature of Zn(BH 4 [11]. In this work Zn(BH 4 and Ni were picked as the dopants to promote the hydrogen uptake and release rates of magnesium. In the present work, Zn(BH 4 and/or Ni were doped to MgH 2 to improve the hydrogen uptake and release features. Samples with compositions of 99 w/o MgH 2 +1 w/o Zn(BH 4 and 96 w/o MgH 2 +2 w/o Zn(BH 4 +2 w/o Ni were prepared by milling in a planetary ball mill in a hydrogen atmosphere (reaction-involving milling). The percentages of the additives were less than 4 w/o to increase hydrogen uptake and release rates without a major sacrifice of the hydrogen storage capacity. The samples were designated MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni, respectively. The hydrogen uptake and release features of the prepared samples were examined and in particular, the hydrogen release properties of MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni were compared. prepared in the authors previous study [12], and Ni (Alfa Aesar, average particle size μm, 99.9% (metals basis), C typically < 0.1%) were utilized as starting materials without further purification. Grinding was carried out under a hydrogen pressure of approximately 12 bar at a rotational speed of 400 rpm for 2 h in a planetary ball mill (Planetary Mono Mill; Pulverisette 6, Fritsch), as explained in the authors previous study [13-15]. Mixtures with planned compositions were mixed with balls whose weight was 45 times the sample weight [ball to powder ratio=45/ 1]. Sample handling was conducted in a glove box filled with argon (Ar). The quantities of hydrogen absorbed by and released from samples as reaction time advanced were measured using a Sieverts type hydrogen uptake and release apparatus, as presented previously [16]. Hydrogen pressures were maintained nearly constant by dosing an appropriate amount of hydrogen from the standard volume to the reactor during the hydrogen uptake reaction and by removing an appropriate amount of hydrogen from the reactor to the standard volume during the hydrogen release reaction. The quantity of hydrogen absorbed by or released from the samples as a function of time was calculated from the variation in the hydrogen pressure in the known standard volume. Microstructures were observed by a scanning electron microscope (SEM, JEOL JSM-5900) at diverse magnifications for the samples milled in a hydrogen atmosphere and for those dehydrogenated after hydrogen uptake-release cycling. X-ray diffraction (XRD) patterns for the samples dehydrogenated after hydrogen uptake-release cycling were obtained using a powder diffractometer (Rigaku D/MAX 2500) with Cu Kα radiation (diffraction angle range 10-80, scan speed 4 /min). 3. RESULTS AND DISCUSSION 2. EXPERIMENTAL DETAILS MgH 2 (Aldrich, hydrogen-storage grade), Zn(BH 4 Figure 1 shows the microstructure observed by SEM of MgH 2-1Zn(BH 4 after reaction-involving milling. Particle sizes are not homogeneous; some particles are

3 246 대한금속 재료학회지 제56권 제3호 (2018년 3월) Fig. 1. Microstructure observed by SEM of MgH2-1Zn(BH4)2 after reaction-involving milling. Fig. 2. XRD pattern of MgH2-1Zn(BH4)2 after reaction-involving milling. large and some are fine. The surfaces of particles are quite flat. The XRD pattern of MgH2-1Zn(BH4)2 after reaction- Fig. 3. Variations in the released hydrogen quantity, R, with temperature for (a) as-milled MgH2-1Zn(BH4)2 heated from room temperature to 683 K and (b) as-milled MgH2-2Zn(BH4)2-2Ni heated from room temperature to 673 K with a heating rate of 5 K/ min in 1.0 bar H2. The quantity of released hydrogen, R, is defined as the percentage of released hydrogen on the basis of sample weight. involving milling is presented in Fig. 2. The XRD Figure 3 shows the variations in the released pattern reveals β-mgh 2, and small amounts of γ- hydrogen quantity, R, with temperature for as-milled MgH2, MgO, NaCl, and Zn(BH4)2. The γ-mgh2 phase, MgH2-1Zn(BH4)2 heated from room temperature to 683 which was not contained in the purchased MgH2, is K and as-milled MgH2-2Zn(BH4 )2-2Ni heated from formed during reaction-involved milling even in a low room temperature to 673 K with a heating rate of 5 hydrogen pressure of about 12 bar. MgO is believed K/min in 1.0 bar H2. The value of R for the as- to be formed by the reaction of Mg and oxygen milled MgH2-1Zn(BH4)2 increases very slowly from adsorbed on the surfaces of particles. NaCl is from room temperature to about 635 K and very rapidly the starting material Zn(BH4)2; NaCl is a by-product from about 635 K to 683 K. This shows that the formed during milling for preparation of Zn(BH4)2. MgH2 phase in the as-milled MgH2-1Zn(BH4)2 begins

4 Myoung Youp Song and Young Jun Kwak 247 to decompose at about 635 K. The values of R for the as-milled MgH 2-1Zn(BH 4 are 0.28 w/o at 523 K, 0.76 w/o at 633 K, 2.47 w/o at 653 K, and 4.34 w/ o at 683 K. The value of R for the as-milled MgH 2-2Zn(BH 4-2Ni increases very slowly from room temperature to about 624 K and rapidly from about 624 K to about 638 K. This shows that the MgH 2 phase in the as-milled MgH 2-2Zn(BH 4-2Ni begins to decompose at about 624 K. The values of R for the as-milled MgH 2-2Zn(BH 4-2Ni are 0.28 w/o at 613 K, 3.81 w/o at 623 K, and 4.26 w/o at 673 K. The very slight decrease in temperature from 627 K to 613 K is thought to be due to a larger decease in temperature produced by the absorption of heat with the release of hydrogen than the increase in temperature caused by heating the sample. Comparison of the curves in Fig. 3(a) and Fig. 3 (b) indicates that a larger quantity of additives and the addition of Ni decrease the temperature at which MgH 2 decomposes. The quantity of absorbed hydrogen, U, is also defined as the percentage of absorbed hydrogen on the basis of sample weight. As the cycle number, CN, increased, the initial hydrogen uptake rates, the quantities of hydrogen absorbed for 60 min, U (60 min), the initial hydrogen release rates, and the quantities of hydrogen released for 60 min, R (60 min), of MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni decreased. This means that the activation processes of MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni were not required. The U versus t curves at 593 K in 12 bar H 2 for MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni at the cycle number, CN, of one (CN=1) are shown in Fig. 4. Both samples have quite high initial hydrogen uptake rates and quite large quantities of hydrogen absorbed after 60 min, U (60 min). MgH 2-2Zn(BH 4-2Ni has a slightly lower initial hydrogen uptake rate but higher quantity of hydrogen absorbed for 60 min, U (60 Fig. 4. U versus t curves at 593 K in 12 bar H 2 for MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni at CN=1. min), than MgH 2-1Zn(BH 4, showing that a larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased the U (60 min). MgH 2-1Zn(BH 4 absorbs 3.97 w/o H in 5 min and 4.83 w/o H in 60 min. MgH 2-2Zn(BH 4-2Ni absorbs 3.84 w/o H in 5 min and 5.47 w/o H in 60 min. MgH 2-2Zn(BH 4-2Ni has an effective hydrogen storage capacity (the quantity of hydrogen absorbed for 60 min) of about 5.5 w/o. Table 1 shows the changes in the U with the time t at 593 K in 12 bar hydrogen at the first cycle (CN=1) for MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni. Figure 5 shows the R versus t curves at 623 K in 1.0 bar H 2 for MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni at CN=1. Both samples have quite high initial hydrogen release rates and quite large quantities of hydrogen released after 60 min, R (60 min), showing that a larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased both the initial hydrogen release rate and the R (60 min). MgH 2-2Zn(BH 4-2Ni has a higher initial hydrogen release rate and larger R (60 min) than Table 1. Changes in U (w/o H) with t at 593 K in 12 bar H 2 at CN=1 for MgH 2-1Zn(BH 4 and MgH 2-2Zn(BH 4-2Ni. 2.5 min 5 min 10 min 30 min 60 min MgH 2-1Zn(BH MgH 2-2Zn(BH 4-2Ni

5 248 대한금속 재료학회지 제56권 제3호 (2018년 3월) Fig. 5. R versus t curves at 623 K in 1.0 bar H2 for MgH21Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni at CN=1. Fig. 7. Microstructure observed by SEM of MgH2-1Zn(BH4)2 dehydrogenated at the 6th hydrogen uptake-release cycle. from the start to 20 min for MgH2-1Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni at CN=1 are shown in Fig. 6. MgH 2-2Zn(BH4 )2-2Ni has a higher initial hydrogen release rate and larger R (60 min) than MgH 2 1Zn(BH4)2. MgH2-1Zn(BH4)2 releases 0.56 w/o H in 2.5 min and 4.47 w/o H in 20 min. MgH2-2Zn(BH4)2-2Ni releases 1.60 w/o H in 2.5 min and 5.25 w/o H in 20 min. Figure 7 shows the microstructure observed by SEM of MgH 2-1Zn(BH 4 dehydrogenated at the 6 t h hydrogen uptake-release cycle. Particles are larger than those after reaction-involving milling. Particle sizes are not homogeneous. Some particles are agglomerated. Fig. 6. R versus t curves at 623 K in 1.0 bar H2 from the start to 20 min for MgH2-1Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni at CN=1. Maintaining the samples at relatively high temperatures of K is believed to make the particles larger and agglomerated. MgH2-1Zn(BH4)2. MgH2-1Zn(BH4)2 releases 1.40 w/o H The XRD pattern of MgH2-1Zn(BH4)2 dehydrogenated in 5 min and 4.75 w/o H in 60 min. MgH 2 - at 623 K in 1.0 bar H2 for 60 min at 6th hydrogen 2Zn(BH4)2-2Ni releases 2.62 w/o H in 5 min and 5.32 uptake-release cycle is shown in Fig. 8. The XRD w/o H in 60 min. Table 2 shows the changes in the pattern reveals Mg, and small amounts of β-mgh2, R with the time t at 623 K in 1.0 bar hydrogen at MgO, NaCl, and ZnH2. This shows that ZnH2 phase, CN=1 for MgH2-1Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni. which has not been reported to be formed, is formed The R versus t curves at 623 K in 1.0 bar H2 during hydrogen uptake-release cycling. MgO is Table 2. Changes in R (w/o H) with t at 623 K in 1.0 bar H2 at CN=1 for MgH2-1Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni. MgH2-1Zn(BH4)2 MgH2-2Zn(BH4)2-2Ni 2.5 min min min min min

6 Myoung Youp Song and Young Jun Kwak 249 uptake-release cycle. The sample contains Mg, and small amounts of β-mgh 2, MgO, ZnH 2, Mg 2 Ni, and NaCl. This shows that the ZnH 2 phase, which has not been reported to be formed, and Mg 2 Ni phase were formed during the hydrogen uptake-release cycling of MgH 2-2.5Zn(BH 4-2.5Ni. Nakagawa et al. [1] reported that Zn(BH 4 releases hydrogen with toxic diborane (B 2 H 6 ) after it melts with an increase in the temperature, and this reaction can be described as follows: Zn(BH 4 Zn + B 2 H 6 + H 2. (1) Fig. 8. XRD pattern of MgH 2-1Zn(BH 4 dehydrogenated at the 6 th hydrogen uptake-release cycle. For the measurements of the hydrogen release properties of the as-milled MgH 2-1Zn(BH 4 sample (Fig. 3(a)), the sample was heated to a maximum of 683 K and the gases were released by the action of a vacuum pump. It is thought that, during this time, the NaCl remains unreacted, and the following reaction occurs in MgH 2-1Zn(BH 4 : Zn(BH 4 +vmgh 2 ZnH 2 +B 2 H 6 +vmg+vh 2. (2) Fig. 9. XRD pattern of MgH 2-2.5Zn(BH 4-2.5Ni dehydrogenated at the 4 th hydrogen uptake-release cycle. believed to be formed by the reaction of Mg and oxygen adsorbed on the surfaces of particles. NaCl is from the starting material Zn(BH 4 ; NaCl is a byproduct formed during milling for preparation of Zn(BH 4. The γ-mgh 2 phase, which appeared in the MgH 2-1Zn(BH 4 after reaction-involving milling, disappeared. The XRD pattern of a Zn(BH 4 and Ni-added MgH 2 sample, MgH 2-2.5Zn(BH 4-2.5Ni, after hydrogen uptake-release cycling was examined. Figure 9 shows the XRD pattern of MgH 2-2.5Zn(BH 4-2.5Ni dehydrogenated at 623 K in 1.0 bar H 2 for 60 min at 4 th hydrogen This indicates that a very small amount of B 2 H 6 is contained in the gas released in the experimental process for the Fig. 3(a). During the subsequent hydrogen uptake-release cycling of the MgH 2-1Zn(BH 4 sample, the NaCl, ZnH 2, and MgO remain un-reacted. Thus, during the subsequent hydrogen uptake-release cycling of the MgH 2-1Zn(BH 4 sample, the following reaction occurs between the Mg and H 2 : Mg + H 2 MgH 2 (3) For the measurements of the hydrogen release properties of the as-milled MgH 2-2Zn(BH 4-2Ni sample (Fig. 3(b)), the sample was heated to a maximum of 673 K and the following reaction occurs in MgH 2-2Zn(BH 4-2Ni: Zn(BH 4 + vmgh 2 + wni ZnH 2 + B 2 H 6 + wmg 2 Ni + (v-2w)mg + vh 2. (4) This also indicates that a very small amount of B 2 H 6 is contained in the gas released in the experimental process for the Fig. 3(b). During the subsequent hydrogen uptake-release cycling of the MgH 2-2Zn(BH 4-2Ni sample, the NaCl, ZnH 2, and MgO

7 250 대한금속 재료학회지제 56 권제 3 호 (2018 년 3 월 ) remain un-reacted. Figure 9 shows that ZnH 2 and Mg 2 Ni were formed during the hydrogen uptake-release cycling of the Zn(BH 4 and Ni-added MgH 2 sample. Thus, during the subsequent hydrogen uptake-release cycling of the MgH 2-2Zn(BH 4-2Ni sample, the following reaction occurs among the Mg, Mg 2 Ni, and H 2 : xmg + ymg 2 Ni + (x+2y)h 2 xmgh 2 + ymg 2 NiH 4.(5) Milling of MgH 2 with Zn(BH 4 and/or Ni in hydrogen atmosphere is thought to create defects, create cracks and clean surfaces, and decrease the particle size. Song et al. [17] reported that the rate-controlling steps of the hydrogen release reaction of Mg 2 Ni hydride were the Knudsen flow and bulk flow of the hydrogen molecules through pores, interparticle channels or cracks. For the hydrogen release reaction at K in bar H 2 of an activated, mechanically alloyed mixture of 90 w/o Mg + 10 w/o Ni, the rate-controlling steps were analyzed as both the bulk and Knudsen flows in the ranges higher than 0.5 < R 0.1. The hydrogen release rate was considered to be controlled mainly by the Knudsen flow as the ranges of R become higher [18]. The contraction due to the relatively rapid hydrogen release of Mg 2 NiH 4 in the samples is believed to provide passages for the hydrogen released from neighboring MgH 2, facilitating the hydrogen release of MgH 2. Comparison of the curves in Fig. 3(a) and Fig. 3 (b) indicates that a larger quantity of additives and the addition of Ni decreases the temperature at which MgH 2 decomposes. Figure 5 shows that MgH 2-2Zn(BH 4-2Ni has a higher initial hydrogen release rate and a larger R (60 min) than MgH 2-1Zn(BH 4. Mg 2 Ni and Mg are known to absorb and release hydrogen under similar temperature and hydrogen pressure conditions [19,20]. From the results of Stampfer et al. [21], the variation in the equilibrium plateau pressure P eq with temperature was calculated and can be expressed by ln P eq (bar) = 9,348/T (6) The equilibrium plateau pressures at 593 K are 2.73 bar for Mg-H 2 system [21] and 4.99 bar for Mg 2 Ni- H 2 system [22]. Mg 2 Ni is known to have higher hydrogen uptake and release rates than Mg [19,20]. Summarizing, a larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increase the U (60 min), the initial hydrogen release rate, and the R (60 min). Milling of MgH 2 with Zn(BH 4 and/or Ni in a hydrogen atmosphere and hydrogen uptake-release cycling increase the hydrogen release rate by creating defects, making cracks and clean surfaces, and decreasing the particle size. The rapid hydrogen release rate of the Mg 2 NiH 4 formed in MgH 2-2Zn(BH 4-2Ni is believed to increase the initial hydrogen release rate of MgH 2-2Zn(BH 4-2Ni. The Mg 2 Ni decomposed from Mg 2 NiH 4 might facilitate the nucleation of a Mg-H solid solution in the MgH 2-2Zn(BH 4-2Ni sample. There is also a slight possibility that the contraction due to the relatively rapid hydrogen release of Mg 2 NiH 4 provides passages for the hydrogen released from neighboring MgH 2. The formed Mg 2 Ni phase is believed to contribute more strongly to the increases in the initial hydrogen release rates and the R (60 min) than the ZnH 2 that is formed and remains un-decomposed during the hydrogen uptake-release cycling. 4. CONCLUSIONS Samples with compositions of 99 w/o MgH 2 +1 w/o Zn(BH 4 (named MgH 2-1Zn(BH 4 ) and 96 w/o MgH 2 +2 w/o Zn(BH 4 +2 w/o Ni (named MgH 2-2Zn(BH 4-2Ni) were prepared by grinding in a planetary ball mill in a hydrogen atmosphere. A larger quantity of additives and the addition of Ni decreased the temperature at which MgH 2 decomposes in the asmilled samples. Activation processes were not required for these two samples. A larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased the U (60 min), the initial hydrogen release rate, and the R (60 min). MgH 2-2Zn(BH 4-2Ni had an effective hydrogen storage capacity of about 5.5 w/o at CN=1. ZnH 2 phase, which has not been reported to be formed, was found

8 Myoung Youp Song and Young Jun Kwak 251 in the MgH 2-1Zn(BH 4 sample and the Zn(BH 4 and Ni-added MgH 2 sample after hydrogen uptake-release cycling. Mg 2 Ni was formed in the Zn(BH 4 and Niadded MgH 2 sample after hydrogen uptake-release cycling. Milling of MgH 2 with Zn(BH 4 and/or Ni in hydrogen atmosphere and hydrogen uptake-release cycling are believed to create defects, make cracks and clean surfaces, and decrease the particle size. The rapid hydrogen release rate of the Mg 2 NiH 4 formed in MgH 2-2Zn(BH 4-2Ni is believed to increase the initial hydrogen release rate of MgH 2-2Zn(BH 4-2Ni. The Mg 2 Ni decomposed from Mg 2 NiH 4 might facilitate the nucleation of Mg-H solid solution in the MgH 2-2Zn(BH 4-2Ni sample. There is also a slight possibility that the contraction due to the relatively rapid hydrogen release of the Mg 2 NiH 4 provides passages for the hydrogen released from neighboring MgH 2. ACKNOWLEDGEMENTS This research was supported by a grant (2013 Ha A17) from Jeonbuk Research & Development Program funded by Jeonbuk Province. REFERENCES 1. T. Nakagawa, T. Ichikawa, Y. Kojima, and H. Fujii, Mater. Trans. 48, 556 (2007). 2. Y. Nakamori, H.-W. Li, K. Kikuchi, M. Aoki, K. Miwa, S. Towata, and S. Orimo, J. Alloy. Compd , 296 (2007). 3. Y. Nakamori, H.-W. Li, M. Matsuo, K. Miwa, S. Towata, and S. Orimo, J. Phys. Chem. Solids 69, 2292 (2008). 4. Á. Révész, M. Gajdics, and T. Spassov, Int. J. Hydrogen Energy 38, 8342 (2013). 5. J. Zou, S. Long, X. Chen, X. Zeng, and W. Ding, Int. J. Hydrogen Energy 40, 1820 (2015). 6. S. Orimo, K. Ikeda, H. Fujii, Y. Fujikawa, Y. Kitano, and K. Yamamoto, Acta Mater. 45, 2271 (1997). 7. M. Calizzi, D. Chericoni, L. H. Jepsen, T. R. Jensen, and L. Pasquini, Int. J. Hydrogen Energy 41, (2016). 8. S. Phetsinorath, J.-X. Zou, X.-Q. Zeng, H.-Q. Sun, and W.- J. Ding, T. Nonferr. Metal. Soc. 22, 1849 (2012). 9. C. Lu, J. Zou, X. Shi, X. Zeng, and W. Ding, Int. J. Hydrogen Energy 42, 2239 (2017). 10. S. Srinivasan, D. Escobar, Y. Goswami, and E. Stefanakos, Int. J. Hydrogen Energy 33, 2268 (2008). 11. S. Srinivasan, D. Escobar, M. Jurczyk, Y. Goswami, and E. Stefanakos, J. Alloy. Compd. 462, 294 (2008). 12. Y. J. Kwak, S. N. Kwon, S. H. Lee, I. W. Park, and M. Y. Song, Korean J. Met. Mater. 53, 500 (2015). 13. S.-H. Hong and M. Y. Song, Met. Mater. Int. 22, 1121 (2016). 14. M. Y. Song, Y. J. Kwak, and H. R. Park, Korean J. Met. Mater. 54, 503 (2016). 15. H. R. Park, Y. J. Kwak, and M. Y. Song, Korean J. Met. Mater. 55, 656 (2017). 16. M. Y. Song, S. H. Baek, J.-L. Bobet, J. Song, and S.-H. Hong, Int. J. Hydrogen Energy 35, (2010). 17. M. Y. Song, B. Darriet, M. Pezat, J. Y. Lee, and P. Hagenmuller, J. Less-Common Met. 118, 235 (1986). 18. M. Y. Song, J.-P. Manaud, and B. Darriet, J. Alloy. Compd. 282, 243 (1999). 19. J. J. Reilly and R. H. Wiswall Jr., Inorg. Chem. 7, 2254 (1968). 20. M. Y. Song, E. Ivanov, B. Darriet, M. Pezat, and P. Hagenmuller, J. Less-Common Met. 131, 71 (1987). 21. J. F. Stampfer Jr., C. E. Holley Jr., and J. F. Suttle, J. Amer. Chem. Soc. 82, 3504 (1959). 22. M. Y. Song and H. R. Park, J. Alloy. Compd. 270, 164 (1998).