Expanding the approach of core-shell nanostructures for reversible hydrogen storage across the Li, Na, Mg and Ca borohydrides family

Light weight metal borohydrides (i.e. LiBH4, NaBH4, Mg(BH4)2 and Ca(BH4)2) are believed to be promising hydrogen storage materials due to their high volumetric and gravimetric hydrogen density. However, their extreme hydrogen storage condition and irreversibility make them inappropriate for practical uses. Despite the focus on the addition of a second hydroxide and catalyst to improve the thermodynamic or kinetic of the borohydrides, nanoconfinement has shown more promising enhancement on their hydrogen release temperature and reversibility. However, the conventional carbon nanostructures are often reactive towards borohydride and create “dead mass” to the system, resulting in low and degrading hydrogen capacity upon cycling. Herein, this thesis explores different ways to restrict and stabilise the borohydride particles in nanoscale to reduce the hydrogen release temperature and improve their reversibility at moderate conditions. It was found the stabilisation of borohydride with capping agents alone was not sufficient to modify the hydrogen storage properties of the borohydrides. In order to optimise the stabilisation, borohydrides were doped with TiCl3 via a wet chemistry method in order to suppress their melting. With precise control of the synthesis process, Ti-doped Mg(BH4)2 nanoparticles were further confined within a uniform Ni shell, which led to a shortened hydrogen diffusion distance and closer reaction of the decomposition products. Remarkably, Mg(BH4)2 was able to release hydrogen as low as 45 C and could reversibly store 2.6 mass% of hydrogen at 250 C under 5 MPa hydrogen pressure with fast absorption and desorption kinetics. This work demonstrated the influence of efficient nanoconfinement of borohydrides on their hydrogen storage properties and opened up the path towards the design and synthesis of a practical hydride storage material