Novel synthesis of porous Mg scaffold as a reactive containment vessel for LiBH4

Abstract

A novel porous Mg scaffold was synthesised and melt-infiltrated with LiBH₄ to simultaneously act as both a confining framework and a destabilising agent for H₂ release from LiBH₄. This porous Mg scaffold was synthesised by sintering a pellet of NaMgH₃ at 450 °C under dynamic vacuum. During the sintering process the multi-metal hydride, decomposed to Mg metal and molten Na. The vacuum applied in combination with the applied sintering temperature, created the ideal conditions for the Na to vaporise and to gradually exit the pellet. The pores of the scaffold were created by the removal of the H₂ and Na from the body of the NaMgH₃ pellet. The specific surface area of the porous Mg scaffold was determined by the Brunauer–Emmett–Teller (BET) method and from Small-Angle X-ray Scattering (SAXS) measurements, which was 26(1) and 39(5) m² g⁻¹ respectively. The pore size distribution was analysed using the Barrett–Joyner–Halenda (BJH) method which revealed that the majority of the pores were macropores, with only a small amount of mesopores present in the scaffold. The melt-infiltrated LiBH₄ was highly dispersed in the porous scaffold according to the morphological observation carried out by a Scanning Electron Microscope (SEM) and also catalysed the formation of MgH₂ as seen from the X-ray diffraction (XRD) patterns of the samples after the infiltration process. Temperature Programmed Desorption (TPD) experiments, which were conducted under various H₂ backpressures, revealed that the melt-infiltrated LiBH₄ samples exhibited a H₂ desorption onset temperature (T_des) at 100 °C which is 250 °C lower than the bulk LiBH₄ and 330 °C lower than the bulk 2LiBH₄/MgH₂ composite. Moreover, the LiH formed during the decomposition of the LiBH₄ was itself observed to fully decompose at 550 °C. The as-synthesised porous Mg scaffold acted as a reactive containment vessel for LiBH₄ which not only confined the complex metal hydride but also destabilised it by significantly reducing the H₂ desorption temperature down to 100 °C.