Pore confined synthesis of magnesium boron hydride nanoparticles

Abstract

Nanostructured materials based on light elements such as Li, Mg, and Na are essential for energy storage and conversion applications, but often difficult to prepare with control over size and structure. We report a new strategy that is illustrated for the formation of magnesium boron hydrides, relevant compounds for instance for reversible solid state hydrogen storage. We started with small (5-10 nm) MgH2 nanoparticles inside the ∼10 nm pores of a carbon scaffold, and larger MgH2 crystallites on the exterior surface of the scaffold. The large difference in reactivity between the two types of MgH2 is used to selective react the small MgH2 particles inside the pores with B2H6 to form magnesium boron hydrides under mild conditions. In this way pore-confined magnesium boron hydrides are formed with MgB12H12 as the major phase. Hydrogen release from the confined magnesium boron hydrides starts just above the synthesis temperature of 120 °C. The addition of Ni brings about the reaction to proceed readily at temperatures as low as 30 °C. Furthermore, by Ni addition and tuning the synthesis temperature, the product distribution can be steered toward Mg(BH4)2 and other magnesium boron hydrides. This shows the suitability of our method to selectively form pore-confined complex metal hydrides, such as MgB12H12 for which no synthesis strategy had been reported until now. This strategy might also be relevant for other novel energy storage and conversion materials, which are difficult to nanostructure in a controlled manner by conventional methods.