| A commercially available hydrogen storage technology is one of the major obstaclesto the widespread use of fuel cells for transportation. In addition to safety and reliability,high volumetric and gravimetric storage densities are also needed. System-baseddensities of at least 62 kg/m3 and 6.5 wt％hydrogen are desired according to the U.S.Department of Energy. The stored hydrogen should be recoverable and preferablyrechargeable on-board at temperatures of＜100-200℃with pressures belowapproximately 100 bar. Magnesium due to its high theoretical hydrogen storage capacity(7.6 wt％), resource abundance and low cost, has excellent potential forhydrogen-related applications. However, its hydrogenation/dehydrogenation occurs onlyat high temperature (＞300℃) and the reaction rate is too slow to form the practical basisfor hydrogen storage. Mechanical milling has been used by many researchers forpreparing composites of magnesium with various additives for improving kinetics,however, long ball milling times are often necessary (＞10 h) to form a nanocrystallinemicrostructure. This is energy consuming, especially for batch-milling of large quantitiesof materials. This work aims to improve hydrogen absorption/desorption kinetics ofmagnesium through ball milling with various additives, including LiBH4 and LiNH2.XRD (X-ray diffraction), and PCT (Pressure Component Temperature) measurementswere used to study the hydrogen absorption/desorption performance of Mg-basedhydrogen storage materials.A special milling pot including a tank and a lid was designed and fabricated which can work well under high pressure (5 MPa) or in vacuum.The effect of LiBH4 on the hydrogen absorption/desorption performance ofmagnesium was investigated. In the case of LiBH4/Mg ball milled in argon, it was foundthat the hydrogen storage properties of LiBH4/Mg mixtures exhibit dramatic improvementcompared with plain magnesium powder. For example, at 250℃, a LiBH4/Mg (1:4)composite can absorb 6.7 wt％hydrogen in 60 min, while only less than 1 wt.％hydrogenwas absorbed by pure magnesium in the same period under similar conditions. Meanwhile,the average desorption rates at 350℃were 2.85×10-4 wt％s-1 and 1.01×10-4 wt％s-1 forLiBH4/Mg and Mg, respectively (the equilibrium time for each point was 30 seconds).These results suggest that LiBH4 also plays an important role in improving the hydrogendesorption kinetic properties of MgH2. In the case of LiBHjMg ball milled in hydrogen,highly activated magnesium hydride was synthesized directly by ball milling LiBH4/Mgmixtures under high hydrogen pressure. The synthesized magnesium hydride exhibitssuperior kinetics, absorbing 5.70 wt％of hydrogen at 200℃within 80 min.The effect of LiNH2 on the hydrogen absorption/desorption performance ofmagnesium was investigated. In the case of Mg+5wt%LiNH2 ball milled in argon, at 200℃, a LiNH2/Mg composite can absorb 2.65 wt％and 5.95 wt％hydrogen in 2 hour and12 hour respectively, while hardly no hydrogen was absorbed by pure magnesium in thesame period under similar conditions. In the case of Mg+5wt%LiNH2 ball milled inhydrogen, highly activated magnesium hydride was synthesized directly by ball millingLiNH2/Mg mixtures under high hydrogen pressure. The synthesized magnesium hydrideexhibits superior kinetics, absorbing 5.19 wt％of hydrogen at 200℃within 2 hour.The above results suggest that both LiBH4 and LiNH2 play an important role inimproving the kinetic properties of magnesium.The milled LiBH4/LiNH2 in argon gas was investigated. The XRD results showedthat LiBH4 and LiNH2 (1:1,1:2,1:3) have a complete reaction after 1 hour of milling;between 1:2 and 1:3 molar ratio of LiBH4 and LiNH2 have almost an entirely reaction toform the new phase.
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