Hydrogen is recognized as a possible future energy carrier, which can be produced from renewable energy and water. A major challenge in a future ‘hydrogen economy’ is the development of safe, compact, robust, and efficient means of hydrogen storage, in particular for mobile applications. The present review focuses on light metal boron based hydrides, for which the general interest has expanded significantly during the past few years. Synthesis methods, physical, chemical and structural properties of novel boron based hydrides are reviewed along with new approaches for improving kinetic and thermodynamic properties: (i) anion substitution, (ii) reactive hydride composites and (iii) nanoconfinement of hydrides and chemical reactions. The light metal borohydrides reveal a fascinating structural chemistry and have the potential for storing large amounts of hydrogen. A combination of the different approaches may provide a new route to a wide range of interesting energy storage materials in the future.

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Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H À , substituting for the oxide anion in BaTiO 3 . Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical tetrahydroborate anion BH 4 À and new synthesis protocols involving rare-earth elements. Photophysical, electronic and hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH 4 -specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 Â the unit cell volume in AB(BH 4 ) 3 . In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing BH 4 À 2X À (X À ¼ Cl À , Br À , I À ) provides a link to the known ABX 3 halides.

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Mechanochemical synthesis using CeCl 3 -MBH 4 (M = Li, Na or K) mixtures are investigated and produced a new compound, LiCe(BH 4 ) 3 Cl, which crystallizes in a cubic space group I4̅ 3m, a = 11.7204(2) Å. The structure contains isolated tetranuclear anionic clusters [Ce 4 Cl 4 (BH 4 ) 12 ] 4− with a distorted cubane Ce 4 Cl 4 core, charge-balanced by Li + cations. Each Ce atom coordinates three chloride ions and three borohydride groups via the η 3 −BH 3 faces, thus completing the coordination environment to an octahedron. Combination of synchrotron radiation powder X-ray diffraction (SR-PXD), powder neutron diffraction and density functional theory (DFT) optimization show that Li cations are disordered, occupying 2/3 of the 12d Wyckoff site. DFT calculation indicates that LiCe(BH 4 ) 3 Cl is stabilized by higher entropy rather than lower enthalpy, in accord with the disorder in Li positions. The structural model also agrees well with the very high lithium ion conductivity measured for LiCe(BH 4 ) 3 Cl of 1 × 10 −4 Scm −1 at T = 20°C. In situ SR-PXD reveals that the decomposition products consist of LiCl, CeB 6 and CeH 2 . The Sieverts measurements show that 4.7 wt % H 2 is released during heating to 500°C. After rehydrogenation at 400°C and p(H 2 ) = 100 bar for 24 h an amount of 1.8 wt % H 2 is released in the second dehydrogenation. The 11 B MAS NMR spectra of the central and satellite transitions for LiCe(B(D/H) 4 ) 3 Cl reveal highly asymmetric manifolds of spinning sidebands from a single 11 B site, reflecting dipolar couplings of the 11 B nuclear spin with the paramagnetic electron spin of the Ce 3+ ions.

A series of monometallic borohydrides and borohydride eutectic mixtures have been investigated during thermal ramping by mass spectroscopy, differential scanning calorimetry, and photography. Mixtures of LiBH4-NaBH4, LiBH4-KBH4, LiBH4-Mg(BH4)2, LiBH4-Ca(BH4)2, LiBH4-Mn(BH4)2, NaBH4-KBH4, and LiBH4-NaBH4-KBH4 all displayed melting behaviour below that of the monometallic phases (up to 167 °C lower). Generally, each system behaves differently with respect to their physical behaviour upon melting. The molten phases can exhibit colour changes, bubbling and in some cases frothing, or even liquid-solid phase transitions during hydrogen release. Remarkably, the eutectic melt can also allow for hydrogen release at temperatures lower than that of the individual components. Some systems display decomposition of the borohydride in the solid-state before melting and certain hydrogen release events have also been linked to the adverse reaction of samples with impurities, usually within the starting reagents, and these may also be coupled with bubbling or frothing of the ionic melt.

Multiple reaction mixtures with different composition ratios of MCl 3 −LiBH 4 (M = La, Gd) were studied by mechano-chemical synthesis, yielding two new bimetallic borohydride chlorides, LiM(BH 4 ) 3 Cl (M = La, Gd). The Gdcontaining phase was obtained only after annealing the ball-milled mixture. Additionally, a solvent extracted sample of Gd(BH 4 ) 3 was studied to gain insight into the transformation from Gd(BH 4 ) 3 to LiGd(BH 4 ) 3 Cl. The novel compounds were investigated using in situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectroscopy, Sieverts measurements, Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The two new compounds, LiLa(BH 4 ) 3 Cl and LiGd(BH 4 ) 3 Cl, have high lithium ion conductivities of 2.3 × 10 −4 and 3.5 × 10 −4 S·cm −1 (T = 20°C) and high hydrogen densities of ρ m = 5.36 and 4.95 wt % H 2 , and both compounds crystallize in the cubic crystal system (space group I-43m) with unit cell parameter a = 11.7955(1) and a = 11.5627(1) Å, respectively. The structures contain isolated tetranuclear anionic clusters [M 4 Cl 4 (BH 4 ) 12 ] 4− with distorted cubane M 4 Cl 4 cores M = La or Gd. Each lanthanide atom coordinates three chloride ions and three borohydride groups, thus completing the coordination environment to an octahedron. The Li + ions are disordered on 2/3 of the 12d Wyckoff site, which agrees well with the very high lithium ion conductivities. The conductivity is purely ionic, as electronic conductivities were measured to only 1.4 × 10 −8 and 9 × 10 −8 S·cm −1 at T = 20°C for LiLa(BH 4 ) 3 Cl and LiGd(BH 4 ) 3 Cl, respectively. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) reveals that the decomposition products at 300°C consist of LaB 6 /LaH 2 or GdB 4 /GdH 2 and LiCl. The size of the rare-earth metal atom is shown to be crucial for the formation and stability of the borohydride phases in MCl 3 −LiBH 4 systems. Figure 9. Sieverts measurement of LaCl 3 −LiBH 4 (1:3, s1, solid line), GdCl 3 −LiBH 4 (1:3, s2, dash), and solvent-extracted Gd(BH 4 ) 3 (s3, dot) showing the first desorption conducted in the temperature range RT to 450°C (ΔT/Δt = 2°C/min and p(H 2 ) = 1 bar).

Fourteen solvent- and halide-free ammine rare-earth metal borohydrides M(BH4)3·nNH3, M = Y, Gd, Dy, n = 7, 6, 5, 4, 2, and 1, have been synthesized by a new approach, and their structures as well as chemical and physical properties are characterized. Extensive series of coordination complexes with systematic variation in the number of ligands are presented, as prepared by combined mechanochemistry, solvent-based methods, solid-gas reactions, and thermal treatment. This new synthesis approach may have a significant impact within inorganic coordination chemistry. Halide-free metal borohydrides have been synthesized by solvent-based metathesis reactions of LiBH4 and MCl3 (3:1), followed by reactions of M(BH4)3 with an excess of NH3 gas, yielding M(BH4)3·7NH3 (M = Y, Gd, and Dy). Crystal structure models for M(BH4)3·nNH3 are derived from a combination of powder X-ray diffraction (PXD), (11)B magic-angle spinning NMR, and density functional theory (DFT) calculations. The structures vary from two-dimensional layers (n = 1), one-dimensional chains (n = 2), molecular compounds (n = 4 and 5), to contain complex ions (n = 6 and 7). NH3 coordinates to the metal in all compounds, while BH4(-) has a flexible coordination, i.e., either as a terminal or bridging ligand or as a counterion. M(BH4)3·7NH3 releases ammonia stepwise by thermal treatment producing M(BH4)3·nNH3 (6, 5, and 4), whereas hydrogen is released for n ≤ 4. Detailed analysis of the dihydrogen bonds reveals new insight about the hydrogen elimination mechanism, which contradicts current hypotheses. Overall, the present work provides new general knowledge toward rational materials design and preparation along with limitations of PXD and DFT for analysis of structures with a significant degree of dynamics in the structures.