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The hampered kinetics of reactive hydride composites (RHCs) in hydrogen storage and release, which limits their use for extensive applications in hydrogen storage S1and energy conversion, can be improved using additives. However, the mechanism of the kinetic restriction and the additive effect on promoting the kinetics have remained unclear. These uncertainties are addressed by utilizing versatile transmission electron microscopy (TEM) on the LiBH4-MgH2 composite under the influence of the 3TiCl3·AlCl3 additives. The formation of the MgB2 phase, as the rate-limiting step, is emphatically studied. According to the observations, the heterogeneous nucleation of MgB2 relies on different nucleation centers (Mg or TiB2 and AlB2). The varied nucleation and growth of MgB2 are related to the in-plane strain energy density at the interface, resulting from the atomic misfit between MgB2 and its nucleation centers. This leads to distinct MgB2 morphologies (bars and platelets) and different performances in the dehydrogenation kinetics of LiBH4-MgH2. It was found that the formation of numerous MgB2 platelets is regarded as the origin of the kinetic improvement. Therefore, to promote dehydrogenation kinetics in comparable RHC systems for hydrogen storage, it is suggested to select additives delivering a small atomic misfit. View Full-Text

Due to the considerable interest in vanadium niobium oxides as a lithium storage material, the kinetics and transformation processes of the V2O5–5Nb2O5 system have been investigated by in situ synchrotron powder X-ray diffraction. The diffraction data after the thermal treatments selected with a view on the most significant features were supplemented with specific ex situ experiments conducted using a laboratory rotating anode X-ray diffractometer. The morphological changes of the mixed powders assuming an amorphous and nanocrystalline solid solution structure as a function of the temperature were inspected by scanning electron microscopy observations. The structural solution of the powder diffraction pattern of the phase recorded in situ at a temperature of about 700 °C was compatible with an orthorhombic crystal structure with the space group Amm2. The obtained lattice parameters for this structure were a = 3.965 Å; b = 17.395 Å, c = 17.742 Å, and the cell composition was V4Nb20O60, Pearson symbol oA84, and density = 4.10 g cm−3. In this structure, while the niobium atoms may be four-, five-, and six-fold coordinated by oxygen atoms, the vanadium atoms were six-fold or seven-fold coordinated. At the temperature of 800 °C and just above, the selected 1 : 2 and 1 : 3 V2O5–Nb2O5 compositions, respectively, returned mostly a tetragonal VNb9O25 phase, in line with earlier observations conducted for determination of the stability phase diagram of such quasi-binary systems.

Trabajo presentado en el 2nd Congress of the Interdivisional Group on Chemistry for Renewable Energy of the Italian Chemical Society – SCI, celebrado en Padova (Italia), del 12 al 14 de febrero de 2020., BaCe0.65Zr0.20Y0.15O3-δ-Ce0.85Gd0.15O2-δ (BCZ20Y15-GDC15) dual-phase material is currently one of the most promising dense ceramic membrane for H2 separation and purification at T > 600 °C. Indeed, this composite material reached H2 permeability values among the highest ever reported for mixed protonic and electronic conductor (MIEC) membranes (≥ 0.3 mL·min-1 ·cm2 at 750°C). 1, 2 A selective H2 separation and purification is allowed through BCZ20Y15- GDC15 membrane by incorporating it into the crystal structure of its constituents as charge protonic defects and electrons/holes that are transported to the opposite side of the membrane under a H2 partial pressure gradient, thus providing a non-galvanic separation, i.e. without external power. 3 Besides the H2 permeability performances, it shows also a good chemical stability under harsh reducing conditions and under CO2- and H2S- containing atmospheres. 5, 6 This talk provides a comprehensive overview of the recent results reached by our group in such MIEC material for H2 separation. The preparation method, hydrogen permeability and longterm stability of the BCZ20Y15-GDC15 composite system under reducing harsh environments will be discussed. In-situ synchrotron XRD analyses under H2 atmospheres will be also shown., This work has been funded by the Italian National Research Council – Italian Ministry of Economic Development agreement 2016-2018 “Ricerca di Sistema Elettrico Nazionale”.

In the past decades, complex hydrides and complex hydrides-based materials have been thoroughly investigated as materials for energy storage, owing to their very high gravimetric and volumetric hydrogen capacities and interesting cation and hydrogen diffusion properties. Concerning hydrogen storage, the main limitations of this class of materials are the high working temperatures and pressures, the low hydrogen absorption and desorption rates and the poor cyclability. In the past years, research in this field has been focused on understanding the hydrogen release and uptake mechanism of the pristine and catalyzed materials and on the characterization of the thermodynamic aspects, in order to rationally choose the composition and the stoichiometry of the systems in terms of hydrogen active phases and catalysts/destabilizing agents. Moreover, new materials have been discovered and characterized in an attempt to find systems with properties suitable for practical on-board and stationary applications. A significant part of this rich and productive activity has been performed by the research groups led by the Experts of the International Energy Agreement Task 32, often in collaborative research projects. The most recent findings of these joint activities and other noteworthy recent results in the field are reported in this paper.

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.

To improve the packing efficiency in tank scale, hydrides have been compacted into pellet form; however, poor hydrogen permeability through the pellets results in sluggish kinetics. In this work, the hydrogen sorption properties of compacted 2LiBH4single bondMgH2 doped with 30 wt % activated carbon nanofibers (ACNF) are investigated. After doping with ACNF, onset dehydrogenation temperature of compacted 2LiBH4single bondMgH2 decreases from 350 to 300 °C and hydrogen released content enhances from 55 to 87% of the theoretical capacity. The sample containing ACNF releases hydrogen following a two-step mechanism with reversible hydrogen storage capacities up to 4.5 wt % H2 and 41.8 gH2/L, whereas the sample without ACNF shows a single-step decomposition mainly from MgH2 with only 1.8 wt % H2 and 15.4 gH2/L. Significant kinetic improvement observed in the doped system is due to the enhancement of both hydrogen permeability and heat transfer through the pellet.

Mg2FeD6 is successfully synthesised with various degrees of purity using reactive ball milling and annealing under low pressure deuterium conditions to a maximum of 10 bar. The deuteride of the low cost ternary metal hydride Mg2FeH6, is synthesised to enable further characterisation studies such as isotopic exchange behaviour. Both on laboratory and industrial scales, keeping the pressure low reduces the need for expensive compression systems and also minimises the quantity of gas necessary for use; therefore it is important to assess synthesis under these cost effective conditions. This is especially the case when using a specialised gas such as high purity deuterium. The maximum pressure chosen is 10 bar, to comply with the High Pressure Safety Act in Japan. This Safety Act limits the use of any gas including hydrogen and deuterium to 10 bar eliminating the use of traditional synthesis methods for Mg2FeH6 or Mg2FeD6 synthesis at high pressure (120 bar). Ball milling parameters such as milling times, ball to powder ratios as well as sintering times were altered to achieve improved Mg2FeD6 yields under these low pressure conditions.

Combinations of complex metal borohydrides ball milled with the transition metal complex hydride, Mg2FeH6, are analysed and compared. Initially, the Reactive Hydride Composite (RHC) of Mg2+ cation mixtures of Mg2FeH6 and γ-Mg(BH4)2 is combined in a range of molar ratios and heated to a maximum of 450 °C. For the molar ratio of 6 Mg2FeH6 + Mg(BH4)2, simultaneous desorption of the two hydrides occurred, which resulted in a single event of hydrogen release. This single step desorption occurred at temperatures between those of Mg2FeH6 and γ-Mg(BH4)2. Keeping this anionic ratio constant, the desorption behavior of four other borohydrides, Li-, Na-, K-, and Ca-borohydrides was studied by using materials ball milled with Mg2FeH6 applying the same milling parameters. The mixtures containing Mg-, Li-, and Ca-borohydrides also released hydrogen in a single event. The Mass Spectrometry (MS) results show a double step reaction within a narrow temperature range for both the Na- and K-borohydride mixtures. This phenomenon, observed for the RHC systems at the same anionic ratio with all five light metal borohydride mixtures, can be described as simultaneous hydrogen desorption within a narrow temperature range centered around 300 °C.

Die Speicherung intermittierend zur Verfügung stehender erneuerbarer Energien (Wind, Sonne) über die Wasserstoff-route stellt eine Möglichkeit dar, diese Energie im MWh Maßstab bedarfsgerecht zur Verfügung zu stellen. Hierzu be-darf es effizienter und kostengünstig Elektrolyseure oder auch direkter photoelektrischer oder -katalytischer Verfahren zur Wasserstofferzeugung. Insbesondere aber muss dieser Wasserstoff vor der Rückverstromung oder stofflichen Nut-zung energieeffizient und kostengünstig zwischengespeichert werden. Der vorliegende Beitrag diskutiert den Stand der Technik bei der Speicherung von Wasserstoff in Metallhydriden und zeigt Vor- und Nachteile dieser Technologie ge-genüber der Druck- und Flüssigwasserstoffspeicherung auf.

The Ca(BH4)2-MgH2 composite system represents a promising candidate for mobile hydrogen storage due to a 10.5 wt % theoretical hydrogen storage capacity and an estimated equilibrium temperature lower than 160 °C. For this system, the reversibility was achieved without further addition of additives. In this study, the decomposition path of the Ca(BH4)2 + MgH2 composite system is investigated in detail by in situ synchrotron radiation powder X-ray diffraction and differential scanning calorimetry combined with thermogravimetry. The sorption properties are analyzed by volumetric measurements. 11B{1H} solid state magic angle spinning-nuclear magnetic resonance was employed for the characterization of the final amorphous or nanocrystalline boron-based decomposition products. This study shows that the intermediate formation of Ca4Mg3H14 upon dehydrogenation of the Ca(BH4)2-MgH2 composite system is not a necessary step, and its presence can be adjusted modifying the preparation procedure. Moreover, the d-value mismatch calculated for the {111}CaB6/{1011}Mg plane pair is the lowest among the other plane pairs considered in the system. The mismatch in the third direction between CaB6 and Mg is also extremely good. These findings propose Mg as a supporter of the heterogeneous nucleation of CaB6 during decomposition of the Ca(BH4)2 + MgH2 composite system.

The present investigation focuses on a new synthesis route of NaBH4 starting from the 2NaH + MgB2 system subjected to mechanochemical activation under reactive hydrogen atmosphere. The milling process was carried out under two different hydrogen pressures (1 and 120 bar) with two different rotation speeds (300 and 550 rpm). The reaction products were characterized by ex-situ solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR), ex-situ X-ray powder diffraction (XRPD) and Infrared Spectroscopy (IR). From the results of these analyses, it can be concluded that milling in all the considered conditions led to the formation of NaBH4 (cubic-Fm-3m). In particular, a reaction yield of 5 and 14 wt% is obtained after 20 h of milling at 120 bar of H2 for the tests performed at 300 rpm and 550 rpm, respectively. The presence of MgH2 is also detected among the final products on the as milled powders. The influence of the milling conditions and the evaluation of the parameters related the mechanochemical process are here discussed.

Light metal tetrahydroborates are regarded as promising materials for solid state hydrogen storage. Due to both a high gravimetric hydrogen capacity of 11.5 wt % and an ideal dehydrogenation enthalpy of 32 kJ mol-1 H2, Ca(BH4)2 is considered to be one of the most interesting compounds in this class of materials. In this work, a comprehensive investigation of the effect of different selected additives (TiF4, NbF5, Ti-isopropoxide, and CaF2) on the reversible hydrogenation reaction of calcium borohydride is presented combining different investigation techniques. The chemical state of the Nb- and Ti-based additives is studied by X-ray absorption spectroscopy (e.g., XANES). Transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDX) was used to show the local structure, size, and distribution of the additive/catalyst. 11B{1H} solid state magic angle spinning-nuclear magnetic resonance (MAS NMR) was carried out to detect possible amorphous phases. The formation of TiB2 and NbB2 nanoparticles was observed after milling or upon sorption reactions of the Nb- and Ti-based Ca(BH4)2 doped systems. The formation of transition-metal boride nanoparticles is proposed to support the heterogeneous nucleation of CaB6. The {111}CaB6/{1011}NbB2, {111}CaB6/{1010}NbB2, as well as {111}CaB6/{1011}TiB2 plane pairs have the potential to be the matching planes because the d-value mismatch is well below the d-critical mismatch value (6%). Transition-metal boride nanoparticles act as heterogeneous nucleation sites for CaB6, refine the microstructure thus improving the sorption kinetics, and, as a consequence, lead to the reversible formation of Ca(BH4)2.

The structure of an unknown crystalline phase observed during the hydrogen absorption reaction of the powder mixtures 2NaH + MgB2 at high pressure has been studied by ab-initio structure determination from powder diffraction. The sequence of un-overlapped peaks extracted from the X-ray powder diffraction pattern could be indexed with a primitive cubic cell with lattice parameter a = 7.319 Å. The diffraction patterns of the peaks are matched with the Pa-3 space group. The stoichiometry of the hydrogen absorption reaction suggests the presence of a high-boron content phase in the compound under investigation. Assuming this phase to be composed only by boron atoms and therefore having a density similar to that found for boron polymorphs, the solution with a space group of Pa-3 leads to reasonable B–B interatomic distances.