Hydrogen Storage

To enhance the dehydrogenation/rehydrogenation kinetic behavior of the LiBH₄–MgH₂ composite system, TiF₄ is used as an additive. The effect of this additive on the hydride composite system has been studied by means of laboratory and advanced synchrotron techniques. Investigations on the synthesis and mechanism upon hydrogen interaction show that the addition of TiF₄ to the LiBH₄–MgH₂ composite system during the milling procedure leads to the in situ formation of well-distributed nanosized TiB₂ particles. These TiB₂ nanoparticles act as nucleation agents for the formation of MgB₂ upon dehydrogenation process of the hydride composite system. The effect of TiB₂ nanoparticles is maintained upon cycling.

The system Ca(BH₄)₂–Mg₂NiH₄is used as a model to prove the unique possibility to fully reverse the borohydride decomposition process even in cases where the decomposition reaction leads to undesired stable boron containing species (boron sinks). The formation of MgNi_2.5B₂directly from Ca(BH₄)₂ or from CaB₁₂H₁₂ and amorphous boron allows an unexpectedly easy transfer of the boron atoms to reversibly form Ca(BH₄)₂ during rehydrogenation. In addition, to the best of our knowledge, the mutual destabilisation of the starting reactants is observed for the first time in Ca(BH₄)₂ based Reactive Hydride Composite (RHC) systems. A detailed account of dehydrogenation and rehydrogenation reaction mechanisms as the function of applied experimental conditions is given.

The pincer d⁸-monohydride complex RhH{xant(PⁱPr₂)₂} (xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) promotes the release of 1 equiv of hydrogen from H₃BNH₃ and H₃BNHMe₂ with TOF_50% values of 3150 and 1725 h^–1, to afford [BH₂NH₂]_n and [BH₂NMe₂]₂ and the tandem ammonia borane dehydrogenation–cyclohexene hydrogenation. DFT calculations on the ammonia borane dehydrogenation suggest that the process takes place by means of cis-κ²-PP-species, through four stages including: (i) Shimoi-type coordination of ammonia borane, (ii) homolytic addition of the coordinated H–B bond to afford a five-coordinate dihydride-boryl-rhodium(III) intermediate, (iii) reductive intramolecular proton transfer from the NH₃ group to one of the hydride ligands, and (iv) release of H₂ from the resulting square-planar hydride dihydrogen rhodium(I) intermediate.

In order to understand the role of AlCl₃ addition on the Li–N–H system, we have systematically investigated the hydrogen sorption kinetics and the reactions between LiNH₂–LiH and AlCl₃ additive with a multitechnique approach involving differential scanning calorimetry (DSC), hydrogen volumetric measurements, X-ray powder diffraction (XRPD), Fourier transform infrared analysis (FTIR) and solid-state nuclear magnetic resonance (NMR). Different interactions were identified as a function of the amount of added AlCl₃. For low AlCl₃ addition (0.03 mol), the Al³⁺ is incorporated into the interstitial sites by the LiNH₂ structure. When AlCl₃ amount increased (0.08 and 0.13 mol), the formation of new amide-chloride phases were detected by XRPD and indexed with cubic and hexagonal Li–Al–N–H–Cl geometries. Occurrence of such new phases was also confirmed by FTIR and NMR. The formation of these new Li–Al–N–H–Cl phases modifies the kinetics as well as the thermodynamic behavior of the original Li–N–H system. Interesting, in all AlCl₃-doped composites, hydrogen was stored reversibly with faster sorption kinetics than un-doped Li–N–H system and with a significant reduction of NH₃ emission. This improvement can be associated with the Al³⁺ incorporation into LiNH₂ that promotes the migration of Li⁺, while for high AlCl₃ doping, the formation of new phases Li–Al–N–H–Cl also weakens the N–H bond.

In the present work we focus the attention on the phase structural transformations occurring upon the desorption process of the LiBH₄ + LiAlH₄ system. This study is conducted by means of manometric–calorimetric, in situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD) and ex situ Solid State Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. The desorption reaction is characterized by two main dehydrogenation steps starting at 320 and 380 °C, respectively. The first step corresponds to the decomposition of LiAlH₄ into Al and H₂ via the formation of Li₃AlH₆ whereas the second one refers to the dehydrogenation of LiBH₄ (molten state). In the range 328–380 °C, the molten LiBH₄ reacts with metallic Al releasing hydrogen and forming an unidentified phase which appears to be an important intermediate for the desorption mechanism of LiBH₄–Al-based systems. Interestingly, NMR studies indicate that the unknown intermediate is stable up to 400 °C and it is mainly composed of Li, B, Al and H. In addition, the NMR measurements of the annealed powders (400 °C) confirm that the desorption reaction of the LiBH₄ + Al system proceeds via an amorphous boron compound.

Confining light metal hydrides in micro- or mesoporous scaffolds is considered to be a promisingway to overcome the existing challenges for these materials, e.g. their application in hydrogenstorage. Different techniques exist which allow us to homogeneouslyfill pores of a host matrix withthe respective hydride, thus yielding well defined composite materials. For this report, the orderedmesoporous carbon CMK-3 was taken as a support for LiAlH4realized by a solution impregnationmethod to improve the hydrogen desorption behavior of LiAlH4by nanoconfinement effects. It isshown that upon heating, LiAlH4is unusually oxidized by coordinated tetrahydrofuran solventmolecules. The important result of the herein described work is thefinding of afinal compositecontaining nanoscale aluminum oxide inside the pores of the CMK-3 carbon host instead of a metal oralloy. This newly observed unusual oxidation behavior has major implications when applying thesecompounds for the targeted synthesis of homogeneous metal–carbon composite materials.

In this paper we performed a comprehensive investigation of the structural and sorption properties of a 40 wt. % NaAlH₄ confined in a ordered mesoporous carbon (OMC, i.e. CMK-3) by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), ²³Na{¹H} and ²⁷Al{¹H} solid-state magic angle spinning-nuclear magnetic resonance (MAS-NMR).

This study evidences a remarkable improvement of the sorption kinetics of NaAlH₄ due to its existence in nanometer size within the OMC. The pressure composition isotherm (PCI) analysis (for the re-absorption step) of the nanoconfined NaAlH₄ would suggest an alteration of its equilibrium thermodynamic properties.

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.

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.

In the present work we report the desorption pathway of the 2NaBH4+MgH2 system. Ex-situ X-ray powder diffraction (XRPD) and solid state magic anglespinning (MAS) nuclear magnetic resonance (NMR) measurements have beenperformed on samples heat treated up to 450°C for different times. Ex-situ X-raypowder diffraction experiments conducted on fully desorbed samples allowed us toidentify nanocrystalline MgB2and metallic Na as dehydrogenation products. 11B and 23Na NMR analyses have been also carried out in order to evaluate the structuralevolution of decomposed materials. Our measurements show that the localstructure of MgB2is influenced by replacement of Mg with Na atoms in the Mgsites. Moreover, amorphous Na2[B12H12] was detected in the partially desorbedsample and in thefinal products of the decomposition reaction. The presence of the[B12H12]2 anion was confirmed by both direct comparison with the 11B{1H} NMR spectrum of pure Na2[B12H12] and dynamic cross-polarization experiments.

A novel method to enhance the reaction kinetics of the reactive hydride composite 2NaBH₄ + MgH₂ is described. It has been discovered that short-term exposure to a moist atmosphere has a very beneficial effect on the desorption reaction of the 2NaBH₄ + MgH₂ mixture. By this procedure it is possible to achieve, after drying, both faster desorption kinetics and greater amounts of released hydrogen compared to ball-milled material without further treatment.

The hydrogen absorption mechanism of the 2NaH + MgB2 system has been investigated in detail. Depending on the applied hydrogen pressure, different intermediate phases are observed. In the case of absorption measurements performed under 50 bar of hydrogen pressure, NaBH4 is found not to be formed directly. Instead, first an unknown phase is formed, followed upon further heating by the formation of NaMgH3 and a NaH-NaBH4 molten salt mixture; only at the end after heating to 380 degrees C do the reflections of the crystalline NaBH4 appear. In contrast, measurements performed at lower hydrogen pressure (5 bar of H-2), but under the same temperature conditions, demonstrate that the synthesis of NaBH4 is possible without occurrence of the unknown phase and of NaMgH3. This indicates that the reaction path can be tuned by the applied hydrogen pressure. The formation of a NaH-NaBH4 molten salt mixture is observed also for the measurement performed under 5 bar of hydrogen pressure with the formation of free Mg. However, under this pressure condition the formation of crystalline NaBH4 is observed only during cooling at 367 degrees C. For none of the applied experimental conditions has it been possible to achieve the theoretical gravimetric hydrogen capacity of 7.8 wt %.

Due to its high hydrogen content and its favourable overall thermodynamics magnesium tetrahydroborate has been considered interesting for hydrogen storage applications. In this work we show that unsolvated amorphous magnesium tetrahydroborate can be obtained by reactive ball milling of commercial MgB₂ under 100 bar hydrogen atmosphere. The material was characterized by solid-state NMR which showed the characteristic features of Mg(BH₄)₂, together with those of higher borohydride species. High pressure DSC and TPD-MS showed thermal behaviour similar to that of Mg(BH₄)₂ but with broadened signals. In situ synchrotron X-ray powder diffraction confirmed the amorphous state of the material and showed the typical crystalline decomposition products of Mg(BH₄)₂ at elevated temperatures.