Abstract:The prerequisite for widespread use of hydrogen as an energy carrier is the development of new materials that can safely store it at high gravimetric and volumetric densities. Metal borohydrides M(BH 4 ) n (n is the valence of metal M), in particular, have high hydrogen density, and are therefore regarded as one such potential hydrogen storage material. For fuel cell vehicles, the goal for on-board storage systems is to achieve reversible store at high density but moderate temperature and hydrogen pressure. To this end, a large amount of effort has been devoted to improvements in their thermodynamic and kinetic aspects. This review provides an overview of recent research activity on various M(BH 4 ) n , with a focus on the fundamental dehydrogenation and rehydrogenation properties and on providing guidance for material design in terms of tailoring thermodynamics and promoting kinetics for hydrogen storage.

Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery.

Magnesium batteries are considered promising solutions for future energy storage beyond the lithium-ion battery era. However, the development of magnesium batteries is hindered by the lack of suitable electrolytes. Here we present solid Mg 2+ electrolytes based on ammine magnesium borohydride composites, Mg(BH 4 ) 2 •xNH 3 , which have conductivities ca. three orders of magnitude higher than the parent compounds (x = 1, 2, 3, and 6). A nanocomposite formed by the Mg(BH 4 ) 2 •xNH 3 composite and MgO nanoparticles exhibits outstanding Mg 2+ conductivity of the order of 10 −5 S cm −1 at room temperature and around 10 −3 S cm −1 at moderate temperature (ca. 70 °C), with an activation energy for Mg 2+ conduction of E a ∼108 kJ/mol (1.12 eV) and high thermal stability (T dec = 120 °C). Characterization using solid-state nuclear magnetic resonance, powder X-ray diffraction, and transmission electron microscopy reveals that the high Mg 2+ conductivity is attributed to amorphization of Mg(BH 4 ) 2 •xNH 3 resulting in a highly dynamic state. This nanocomposite is compatible with a Mg metal anode and allows stable Mg plating/stripping (at least 100 cycles) in a symmetric cell. The results represent a major advancement of solid-state multivalent ion conductors here demonstrated for Mg 2+ .

Experimental and theoretical studies on Mg(BH4)2 were carried out from the viewpoint of the formation of the intermediate compound MgB12H12 with B12H12 cluster. The full dehydriding and partial rehydriding reactions of Mg(BH4)2 occurred according to the following multistep reaction: Mg(BH4)2 -->1/6MgB12H12 + 5/6MgH2 + 13/6H2 <--> MgH2 + 2B + 3H2 <--> Mg + 2B + 4H2. The dehydriding reaction of Mg(BH4)2 starts at approximately 520 K, and 14.4 mass% of hydrogen is released upon heating to 800 K. Furthermore, 6.1 mass% of hydrogen can be rehydrided through the formation of MgB12H12. The mechanism for the formation of MgB12H12 under the present rehydriding condition is also discussed.