Effects of Stoichiometry on the H2‐Storage Properties of Mg(NH2)2–LiH–LiBH4 Tri‐Component Systems

The hydrogen desorption pathways and storage properties of 2 Mg(NH₂)₂–3 LiH–x LiBH₄ samples (x =0, 1, 2, and 4) were investigated systematically by a combination of pressure composition isotherm (PCI), differential scanning calorimetric (DSC), and volumetric release methods. Experimental results showed that the desorption peak temperatures of 2 Mg(NH₂)₂–3 LiH–x LiBH₄ samples were approximately 10–15 °C lower than that of 2 Mg(NH₂)₂–3 LiH. The 2 Mg(NH₂)₂–3 LiH–4 LiBH₄ composite in particular began to release hydrogen at 90 °C, thereby exhibiting superior dehydrogenation performance. All of the LiBH₄‐doped samples could be fully dehydrogenated and re‐hydrogenated at a temperature of 143 °C. The high hydrogen pressure region (above 50 bar) of PCI curves for the LiBH₄‐doped samples rose as the amount of LiBH₄ increased. LiBH₄ changed the desorption pathway of the 2 Mg(NH₂)₂–3 LiH sample under a hydrogen pressure of 50 bar, thereby resulting in the formation of MgNH and molten [LiNH₂–2 LiBH₄]. That is different from the dehydrogenation pathway of 2 Mg(NH₂)₂–3 LiH sample without LiBH₄, which formed Li₂Mg₂N₃H₃ and LiNH₂, as reported previously. In addition, the results of DSC analyses showed that the doped samples exhibited two independent endothermic events, which might be related to two different desorption pathways.     

Metal‐Borohydride‐Modified Zr(BH4)4⋅8 NH3: Low‐Temperature Dehydrogenation Yielding Highly Pure Hydrogen

Due to its high hydrogen density (14.8 wt %) and low dehydrogenation peak temperature (130 °C), Zr(BH₄)₄ ? 8 NH₃ is considered to be one of the most promising hydrogen‐storage materials. To further decrease its dehydrogenation temperature and suppress its ammonia release, a strategy of introducing LiBH₄ and Mg(BH₄)₂ was applied to this system. Zr(BH₄)₄ ? 8 NH₃–4 LiBH₄ and Zr(BH₄)₄ ? 8 NH₃–2 Mg(BH₄)₂ composites showed main dehydrogenation peaks centered at 81 and 106 °C as well as high hydrogen purities of 99.3 and 99.8 mol % H₂, respectively. Isothermal measurements showed that 6.6 wt % (within 60 min) and 5.5 wt % (within 360 min) of hydrogen were released at 100 °C from Zr(BH₄)₄ ? 8 NH₃–4 LiBH₄ and Zr(BH₄)₄ ? 8 NH₃–2 Mg(BH₄)₂, respectively. The lower dehydrogenation temperatures and improved hydrogen purities could be attributed to the formation of the diammoniate of diborane for Zr(BH₄)₄ ? 8 NH₃–4 LiBH₄, and the partial transfer of NH₃ groups from Zr(BH₄)₄ ? 8 NH₃ to Mg(BH₄)₂ for Zr(BH₄)₄ ? 8 NH₃–2 Mg(BH₄)₂, which result in balanced numbers of BH₄ and NH₃ groups and a more active H^δ+ ??? ^?δH interaction. These advanced dehydrogenation properties make these two composites promising candidates as hydrogen‐storage materials.     

Exploring Ternary and Quaternary Mixtures in the LiBH4-NaBH4-KBH4-Mg(BH4)2-Ca(BH4)2 System

Binary combinations of borohydrides have been extensivly investigated evidencing the formation of eutectics, bimetallic compounds or solid solutions. In this paper, the investigation has been extended to ternary and quaternary systems in the LiBH₄-NaBH₄-KBH₄-Mg(BH₄)₂-Ca(BH₄)₂ system. Possible interactions among borohydrides in equimolar composition has been explored by mechanochemical treatment. The obtained phases were analysed by X-ray diffraction and the thermal behaviour of the mixtures were analysed by HP-DSC and DTA, defining temperature of transitions and decomposition reactions. The release of hydrogen was detected by MS, showing the role of the presence of solid solutions and multi-cation compounds on the hydrogen desorption reactions. The presence of LiBH₄ generally promotes the release of H₂ at about 200 °C, while KCa(BH₄)₃ promotes the release in a single-step reaction at higher temperatures.     

Composition‐Dependent Reaction Pathways and Hydrogen Storage Properties of LiBH4/Mg(AlH4)2 Composites

Herein, an initial attempt to understand the relationships between hydrogen storage properties, reaction pathways, and material compositions in LiBH₄–x Mg(AlH₄)₂ composites is demonstrated. The hydrogen storage properties and the reaction pathways for hydrogen release from LiBH₄–x Mg(AlH₄)₂ composites with x=1/6, 1/4, and 1/2 were systematically investigated. All of the composites exhibit a four‐step dehydrogenation event upon heating, but the pathways for hydrogen desorption/absorption are varied with decreasing LiBH₄/Mg(AlH₄)₂ molar ratios. Thermodynamic and kinetic investigations reveal that different x values lead to different enthalpy changes for the third and fourth dehydrogenation steps and varied apparent activation energies for the first, second, and third dehydrogenation steps. Thermodynamic and kinetic destabilization caused by the presence of Mg(AlH₄)₂ is likely to be responsible for the different hydrogen desorption/absorption performances of the LiBH₄–x Mg(AlH₄)₂ composites.     

LiAlH4与LiNH2的相互作用机理及储氢特性

将LiAlH4和LiNH2按摩尔比1：2进行球磨复合,随后将复合物进行加热放氢特性研究,然后对其完全放氢后的产物进行再吸氢特性研究。通过X射线衍射分析(XRD)、热分析(DSC)和红外 (FTIR)分析等测试手段对其反应过程进行了系统分析研究。研究结果表明,LiAlH4/2LiNH2加热放氢分为3个反应阶段,放氢后生成Li3AlN2,总放氢量达到8.65wt%。放氢生成的Li3AlN2在10MPaH2压力和400℃条件下,可以可逆吸氢5.0wt%,吸氢后的产物为 LiNH2 、AlN和LiH,而不能再生成LiAlH4。本文对LiAlH4/2LiNH2复合物放氢/再氢化过程机理进行了分析。     

ChemInform Abstract: Crystal Structure and Spectroscopic Studies of LiNH4(H2PO4)2 — A New Solid Acid in the LiH2PO4—NH4H2PO4 System.

LiNH₄(H₂PO₄)₂ is prepared from an equimolar aqueous mixture of NH₄H₂PO₄ and LiH₂PO₄ by slow evaporation at 25 °C.     

MAlH4(M=Li,Na)储氢材料*

氢能是一种新型的清洁能源,有望替代碳经济,而氢的储存是氢能应用的关键。近年来,研究集中在具有储氢容量高和可逆性好等优点的固态储氢材料上。许多新型储氢材料不断出现,其中以MAlH₄(M=Li, Na)为代表的金属复合氢化物体系被认为是最有前景的储氢材料之一。本文综述了MAlH₄(M=Li, Na)作为可逆储氢材料的研究现状,主要从吸放氢反应、储氢性能、反应机理、理论计算和存在的问题等方面进行了讨论,并指出其相关发展趋势。     

Synergetic Effects of In Situ Formed CaH2 and LiBH4 on Hydrogen Storage Properties of the Li–Mg–N–H System

Hydrogen storage properties and mechanisms of the Ca(BH₄)₂‐doped Mg(NH₂)₂–2 LiH system are systematically investigated. It is found that a metathesis reaction between Ca(BH₄)₂ and LiH readily occurs to yield CaH₂ and LiBH₄ during ball milling. The Mg(NH₂)₂–2 LiH–0.1 Ca(BH₄)₂ composite exhibits optimal hydrogen storage properties as it can reversibly store more than 4.5 wt % of H₂ with an onset temperature of about 90 °C for dehydrogenation and 60 °C for rehydrogenation. Isothermal measurements show that approximately 4.0 wt % of H₂ is rapidly desorbed from the Mg(NH₂)₂–2 LiH–0.1 Ca(BH₄)₂ composite within 100 minutes at 140 °C, and rehydrogenation can be completed within 140 minutes at 105 °C and 100 bar H₂. In comparison with the pristine sample, the apparent activation energy and the reaction enthalpy change for dehydrogenation of the Mg(NH₂)₂–2 LiH–0.1 Ca(BH₄)₂ composite are decreased by about 16.5 % and 28.1 %, respectively, and thus are responsible for the lower operating temperature and the faster dehydrogenation/hydrogenation kinetics. The fact that the hydrogen storage performances of the Ca(BH₄)₂‐doped sample are superior to the individually CaH₂‐ or LiBH₄‐doped samples suggests that the in situ formed CaH₂ and LiBH₄ provide a synergetic effect on improving the hydrogen storage properties of the Mg(NH₂)₂–2 LiH system.     

储氢研究进展

氢能是21世纪主要的新能源之一。作为一种新型的清洁能源,氢的廉价制取、安全高效储存与输送及规模应用是当今研究的重点课题,而氢的储存是氢能应用的关键。储氢材料能可逆地大量吸放氢,在氢的储存与输送过程中是一种重要载体。本文综述了目前所采用或正在研究的主要储氢材料与技术,如高压气态储氢、低温液态储氢、金属氢化物储氢、化学氢化物储氢、吸附储氢、金属有机骨架储氢等,比较了各种储氢的优缺点,并指出其相关发展趋势。     

储氢研究进展

氢能是21世纪主要的新能源之一。作为一种新型的清洁能源,氢的廉价制取、安全高效储存与输送及规模应用是当今研究的重点课题,而氢的储存是氢能应用的关键。储氢材料能可逆地大量吸放氢,在氢的储存与输送过程中是一种重要载体。本文综述了目前所采用或正在研究的主要储氢材料与技术,如高压气态储氢、低温液态储氢、金属氢化物储氢、化学氢化物储氢、吸附储氢、金属有机骨架储氢等,比较了各种储氢的优缺点,并指出其相关发展趋势。     

硼氢化锂储氢材料研究*

车载储氢是推进氢燃料车规模化商业应用的“瓶颈”环节,开发高性能车载储氢材料/技术成为当前能源及材料领域关注的热点。近年来,随着储氢材料领域的不断拓展,以硼氢化锂（LiBH₄）为典型代表的高储量配位金属氢化物日渐成为新兴的储氢材料研究热点。本文从体系成分/反应路径调整、纳米结构调制、阴/阳离子替代及催化体系构建等方面概述了改善LiBH4综合储/放氢性能的最新研究进展,旨在明确配位硼氢化物储氢材料研究中的关键问题及可能的解决途径。     

Preparation and Catalytic Activity of a Novel Nanocrystalline ZrO2@C Composite for Hydrogen Storage in NaAlH4

Sodium alanate (NaAlH₄) has attracted intense interest as a prototypical high‐density hydrogen‐storage material. However, poor reversibility and slow kinetics limit its practical applications. Herein, a nanocrystalline ZrO₂@C catalyst was synthesized by using Uio‐66(Zr) as a precursor and furfuryl alcohol (FA) as a carbon source. The as‐synthesized ZrO₂@C exhibits good catalytic activity for the dehydrogenation and hydrogenation of NaAlH₄. The NaAlH₄‐7 wt % ZrO₂@C sample released hydrogen starting from 126 °C and reabsorbed it starting from 54 °C, and these temperatures are lower by 71 and 36 °C, respectively, relative to pristine NaAlH₄. At 160 °C, approximately 5.0 wt % of hydrogen was released from the NaAlH₄‐7 wt % ZrO₂@C sample within 250 min, and the dehydrogenation product reabsorbed approximately 4.9 wt % within 35 min at 140 °C and 100 bar of hydrogen. The catalytic function of the Zr‐based active species is believed to contribute to the significantly reduced operating temperatures and enhanced kinetics.     

Hydrogensulfate mit fehlgeordneten Wasserstoffatomen – Synthese und Struktur von Li[H(HSO4)2](H2SO4)2 sowie Verfeinerung der Struktur von α-NaHSO4

Hydrogen Sulfates with Disordered Hydrogen Atoms – Synthesis and Structure of Li[H(HSO₄)₂](H₂SO₄)₂ and Refinement of the Structure of α-NaHSO₄ The structure of Li[H(HSO₄)₂](H₂SO₄)₂ has been determined for the first time whereas the structure of α-NaHSO₄ has been refined, so that direct determination of the hydrogen positions was possible. Both compounds crystallize triclinic in the space group P1 with the lattice constants a = 6.708(2), b = 6.995(1), c = 7.114(1) Å, α = 75.53(1), β = 84.09(2) and γ = 87.57(2)° (Z = 4) for α-NaHSO₄ and a = 4.915(1), b = 7.313(1), c = 8.346(2) Å, α = 82.42(3), β = 86.10(3) and γ = 80.93(3)° (Z = 1) for Li[H(HSO₄)₂](H₂SO₄)₂. In both compounds there are disordered hydrogen positions. In the structure of α-NaHSO₄ there are two crystallographically different HSO₄^? tetrahedra and two different coordinated Na atoms. The system of hydrogen bonds can be described by chains in [0–11] direction. The disordering of the H atoms reduces the differences between the S? O and S? OH distances (1.45 and 1.50 Å) while in the ordered HSO₄ unit “regular” bond lengths are observed (1.45 und 1,57 Å). In the structure of Li[H(HSO₄)₂](H₂SO₄)₂ there are two crystallographically different SO₄-tetrahedra. The first one belongs to the [H(HSO₄)₂]^? unit while the second one represents H₂SO₄ molecules. The H atom which is located nearby the symmetry centre and connects two HSO₄ units by a short O…?O distance of 2.44 Å. Li is located on a symmetry centre and is slightly distorted octahedrally coordinated by oxygen atoms of six different SO₄ tetrahedra. The system of hydrogen bonds can be regarded as consisting of double layers parallel to the xy-plane.     

Strategies for the Improvement of the Hydrogen Storage Properties of Metal Hydride Materials

Metal hydrides are an important family of materials that can potentially be used for safe, efficient and reversible on‐board hydrogen storage. Light‐weight metal hydrides in particular have attracted intense interest due to their high hydrogen density. However, most of these hydrides have rather slow absorption kinetics, relatively high thermal stability, and/or problems with the reversibility of hydrogen absorption/desorption cycling. This paper discusses a number of different approaches for the improvement of the hydrogen storage properties of these materials, with emphasis on recent research on tuning the ionic mobility in mixed hydrides. This concept opens a promising pathway to accelerate hydrogenation kinetics, reduce the activation energy for hydrogen release, and minimize deleterious possible by‐products often associated with complex hydride systems.     

Transformation Kinetics of LiBH4–MgH2 for Hydrogen Storage

The reactive hydride composite (RHC) LiBH₄–MgH₂ is regarded as one of the most promising materials for hydrogen storage. Its extensive application is so far limited by its poor dehydrogenation kinetics, due to the hampered nucleation and growth process of MgB₂. Nevertheless, the poor kinetics can be improved by additives. This work studied the growth process of MgB₂ with varying contents of 3TiCl₃·AlCl₃ as an additive, and combined kinetic measurements, X-ray diffraction (XRD), and advanced transmission electron microscopy (TEM) to develop a structural understanding. It was found that the formation of MgB₂ preferentially occurs on TiB₂ nanoparticles. The major reason for this is that the elastic strain energy density can be reduced to ~4.7 × 10⁷ J/m³ by creating an interface between MgB₂ and TiB₂, as opposed to ~2.9 × 10⁸ J/m³ at the original interface between MgB₂ and Mg. The kinetics of the MgB₂ growth was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, describing the kinetics better than other kinetic models. It is suggested that the MgB₂ growth rate-controlling step is changed from interface- to diffusion-controlled when the nucleation center changes from Mg to TiB₂. This transition is also reflected in the change of the MgB₂ morphology from bar- to platelet-like. Based on our observations, we suggest that an additive content between 2.5 and 5 mol% 3TiCl₃·AlCl₃ results in the best enhancement of the dehydrogenation kinetics.