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.

A surprisingly high degree of structural and compositional dynamics is observed in the system LiBH 4 -LiCl as a function of temperature and time. Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data reveals that Cl -readily substitutes for BH 4 -in the structure of LiBH 4 . Prolonged heating a sample of LiBH 4 -LiCl (1:1 molar ratio) above the phase transition temperature and below the melting point (108 < T < 275°C) can produce highly chloride substituted hexagonal lithium borohydride, h-Li(BH 4 ) 1-x Cl x , e.g., x ∼ 0.42, after heating from room temperature (RT) to 224°C at 2.5°C/min. LiCl has a higher solubility in h-LiBH 4 as compared to orthorhombic lithium borohydride, o-LiBH 4 , which is illustrated by a LiBH 4 -LiCl (1:1) sample equilibrated at 245°C for 24 days and left at RT for another 13 months. Rietveld refinement reveals that this sample contains o-Li(BH 4 ) 0.91 Cl 0.09 and LiCl. This illustrates a significantly faster dissolution of LiCl in h-LiBH 4 as compared to a slower segregation of LiCl from o-LiBH 4 , which is also demonstrated by in situ SR-PXD from three cycles of heating and cooling of the same Li(BH 4 ) 0.91 Cl 0.09 sample. The substitution of the smaller Cl -for the larger BH 4 -ion is clearly observed as a reduction in the unit cell volume as a function of time and temperature. A significant stabilization of h-LiBH 4 is found to depend on the degree of anion substitution. Variable temperature solid-state magic-angle spinning (MAS) 7 Li and 11 B NMR experiments on pure LiBH 4 show an increase in full width at half maximum (fwhm) when approaching the phase transition from o-to h-LiBH 4 , which indicates an increase of the relaxation rate (i.e., T 2 decreases). A less pronounced effect is observed for ion-substituted Li(BH 4 ) 1-x Cl x , 0.09 < x < 0.42. The MAS NMR experiments also demonstrate a higher degree of motion in the hexagonal phase, i.e., fwhm is reduced by more than a factor of 10 at the o-to h-LiBH 4 phase transition.

a b s t r a c tThe new concept, anion substitution, is explored for possible improvement of hydrogen storage properties in the system LiBH 4 -LiI. The structural chemistry and the substitution mechanism are analyzed using Rietveld refinement of in situ synchrotron radiation powder X-ray diffraction (SR-PXD) data, attenuated total reflectance infrared spectroscopy (ATR-IR), differential scanning calorimetry (DSC) and Sieverts measurements. Anion substitution is observed as formation of two solid solutions of Li(BH 4 ) 1−x I x , which merge into one upon heating. The solid solutions have hexagonal structures (space group P6 3 mc) similar to the structures of h-LiBH 4 and ␤-LiI. The solid solutions have iodide contents in the range ∼0-62 mol% and are stable from below room temperature to the melting point at 330 • C. Thus the stability of the solid solutions is higher as compared to that of the orthorhombic and hexagonal polymorphs of LiBH 4 and ␣-and ␤-LiI. Furthermore, the rehydrogenation properties of the iodide substituted solid solution Li(BH 4 ) 1−x I x , measured by the Sieverts method, are improved as compared to those of LiBH 4 . After four cycles of hydrogen release and uptake the Li(BH 4 ) 1−x I x solid solution maintains 68% of the calculated hydrogen storage capacity in contrast to LiBH 4 , which maintains only 25% of the storage capacity after two cycles under identical conditions.

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