| Metal borohydride ammoniates,with chemical formula M?BH4?n·nNH3?n denotes the valence of M?,have recently atrtracted considerable attention as promising candidates for solid-state hydrogen storage because of their high gravimetric and volumetric hydrogen densities.Metal borohydride ammoniate molecules contain both BH4 and NH3 groups,which creates a H?+-H?-coexistence environment,thereby facilitating hydrogen release at lower temperatures.Unfortunately,hydrogen release from metal borohydride amoniates are still exothermic in nature,and often suffers from contamination by ammonia,which results in limited reversibility for hydrogen storage in these materials,consequently prohibiting their use in practical on-board applications.In this paper,the effects of composing with metal hydrides,nanosizing and forming derivatives on dehydrogenation properties of Mg?BH4?n·nNH3 were systematically investigated.Besides,the corresponding hydrogen desorption and absorption mechanisms were also revealed.LiH was combined with Mg?BH4?2·6NH3 as an ammonia inhibitor,and a reactive composite with compositions of Mg?BH4?2·6NH3-xLiH were prepared and investigated.Hydrogen release from the Mg?BH4?2-6NH3-xLiH composites started at 80?,and amounts to 12.3 wt%at 400 C with a three-step dehydrogenation process.Upon heating,the coexistence of H?+ in NH3 groups and H?-in LiH facilitates hydrogen release at lower temperatures and suppresses the release of NH3 by the local combination of H?+ and H?-.Hydrogenation experiment revealed that the sample dehydrogenated at 210? took up?2.2 wt%of hydrogen under 100 bar of hydrogen.pressure at 175? due to the reversible hydrogenation of Li2Mg?NH?2.This is superior to the pristine Mg?BH4?2.6NH3 since no hydrogen absorption was detected for dehydrogenated sample under the same conditions.A reactive composite with several compositions of Mg?BH4?2·2NH3-xNaAlIH4 were prepared and investigated.It shows that combining NaAlH4 with Mg?BH4?2·2NH3 significantly reduces the operating dehydrogenation temperatures and effectively suppresses the emission of NH3 by-products.The dehydrogenation onset temperature of the Mg?BH4?2·2NH3-2NaAlH4 system is lowered to ca.70 ?,which is much lower than the onset temperatures of either Mg?BH4?2·2NH3 or NaAlH4.In addition,ammonia emission from Mg?BH4?2·2NH3 is thoroughly suppressed by the addition of NaAlH4,leading to approximately 11.3 wt%hydrogen released upon heating to 570?.Hydrogen desorption from the Mg?BH4?2·2NH3-2NaAlH4 sample is endothermic in nature,and the dehydrogenated product took up approximately 3.5 wt%of hydrogen under 100 bar of hydrogen at 450?,which shows improvement on hydrogen absorption performance,because hydrogen storage,in Mg?BH4?2·2NH3 is nearly irreversible.The reversible hydrogen storage mainly originated from the hydrogenation of Al3Mg2,Na,MgAlB4 and Al0.95Mg0.05.We have successfully synthesized Mg?BH4?2·6NH3 nanoparticles measuring 20-40 nm in diameter with uniform morphologies by an ultrasound-assisted wet-chemistry approach.The Mg?BH4?2·6NH3 nanoparticles display a quite different thermal decomposition behaviour with respect to the bulk sample.Upon heating,hydrogen is the primary decomposition product of the Mg?BH4?2·6NH3 nanoparticles,which starts releasing hydrogen even below 30? and peaks at 135?,representing more than 95 and 80? reductions in the onset and peak dehydrogenation temperature relative to the bulk sample.Mechanistic investigation indicated that during the decomposition of Mg?BH4?2· 6NH3 nanoparticles,BN and a novel Mg-B-N compound were formed rather than Mg and BN.First-principle calculations revealed that the formation energy and the reaction barrier of H2 at the surface are considerably lower than that in the bulk and also lower than those of NH3.This property is the most important reason for the changed dehydrogenation behaviour of the Mg?BH?4·6NH3 nanoparticles because their surface properties are dominant.Mg2K?BH4?5·4NH3,a new ammine borohydride,has been synthesized via simply ball-milling Mg?BH?4·2NH3 and KBH4.Structural analyses reveal that Mg2K?BH4?5·4NH3,crystalize in a tetragonal structure and lattice parameters a=5.129 A,b=7.168 A,c=8.398 A,?=?=?=90.0° and V=308.8 A3.After further investigating the dehydrogenation properties,it reveals that Mg2K?BH4?5.4NH3 started to liberate H2 at around 64? and is able to release?13.6 wt%H2.Upon heating,Mg2K?BH4?5-4NH3 first decomposed to form MgBNH4,BN and KBH4 with the release of hydrogen.Then,the reaction between the newly form MgBNH4 and KBH4 resulted in the release of hydrogen and the production of KH,BN,B and Mg.In the meanwhile,it is observed that the fully dehydrogenated products can be partly rehydrogenated under 100 bar hydrogen pressure,which was ascribed to the formation of MgH2.Further hydrogen uptake analysis indicated that approximately 2 wt%of hydrogen was recharged into the fully dehydrogenated Li2Mg?BH4?2?NH2?2 sample at 390? and 100 bar due to the reversible hydrogenation of Mg,LiH and B.A novel dual-cation/anion complex hydrides,which contains a theoretical hydrogen capacity of 12.1 wt%,and low decomposition temperature was successfully synthesized by ball milling a mixture consisting of Li2BH4NH2 and MgBH4NH2.The structure of Li2Mg?BH4?2?NH2?2 is determined by high-resolution powder X-ray diffraction,and this material crystallizes in a triclinic structure with the following lattice parameters:a=5.270 A,b=4.070 A,c=5.957 A,?=76.00,?=64.8°,?=69.2° and V=78.1 A3.Upon heating,the as-prepared Li2Mg?BH4?2?HN2?2 decomposes to release approximately 8.7 wt%hydrogen in a three-step reaction at 100-450?.In addition,a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product.This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity.Further investigation shows that Li2Mg?BH4?2?NH2?2 first decomposes to LiMgBN2,LiBH4,BN,LiH and MgBNH8 at 100-250?,and then,the newly formed MgBNH8 reacts with LiH to form Mg,LiH and BN at 250-340?.Finally,the decomposition of LiBH4 releases hydrogen and generates LiH and B at 340-450?. |
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