Light metal boronitrides: potential hydrogen storage materials

Abstract

The light metal boronitride systems Li-B-N, Mg-B-N and Li-Mg-B-N are potential hydrogen storage materials that have been selected for their similarity of structure and bonding to known hydrogen storage materials Li3N, NH3BH3 and MgH2. Further, their potential hydrogen uptake fulfils the 9wt% storage target set by the US DoE to be an efficient hydrogen storage material for use in portable energy devices in the transport sector. Syntheses within these three systems have been investigated to prepare phase pure Li3BN2, Mg3BN3 and LiMgBN2. It was found that closed system reactions using a Ni foil capsule improved the product quality compared with open system reactions undertaken in nitrogen for all phases. For the material Li3BN2, a new synthesis route from Li3N and B has been proposed with an intermediate Li metal phase formed. There is evidence that this reaction path also occurs for Mg3BN3. The formation of an Li-BN intercalate phase has been observed and characterised using X-ray Diffraction and 11B Solid State MAS NMR techniques. Two phases in the system Li-Mg-B-N were synthesised: LiMgBN2 and Li3Mg3B2N5. 11B solid state MAS NMR spectra have been measured and interpreted for a number of materials in the Li-Mg-B-N system, including first time observations of the spectra for amorphous boron, Li3BN2, Mg3BN3, LiMgBN2 and Li3Mg3B2N5. The MAS NMR technique has been used to monitor the nitridation of amorphous boron to form hexagonal BN.