Pore-Confined Light Metal Hydrides for Energy Storage and Catalysis

Abstract

Light metal hydrides have enjoyed several decades of attention in the field of hydrogen storage, but their applications have recently begun to diversify more and more into the broader field of energy storage. For example, light metal hydrides have shown great promise as battery materials, in sensors and as efficient catalysts for ammonia decomposition, which could enable ammonia to be used for indirect hydrogen storage. Additionally, Li-based compounds have demonstrated prowess as ammonia synthesis catalysts while NaAlH4 shows the ability to perform hydrogenations of C-C multiple bonds, both in conjunction with transition metals. Although these topics seem to be very different they all depend on similar physical phenomena. The bulk or macrocrystalline metal hydrides are usually not very active in most of these applications. However, nanostructuring has been shown to be effective in improving the properties of metal hydrides. With this reasoning the nano-sizing and confinement of light metal hydrides to the pores of a scaffold (carbon being the most favoured due to its relative inertness) can yield significant improvements in their performance for a range of applications. For example, smaller particles display faster kinetics of hydrogen sorption compared to larger particles as they have shorter hydrogen diffusion distances, this also means a higher active surface area for catalysis. It may also be necessary for the hydrogen release and uptake from the metal hydrides to be reversible for the material to behave catalytically and nanoconfinement is a very effective way of achieving this reversibility. This thesis describes the use of nano-confinement as an approach to improve the properties of light metal hydrides in reversible hydrogen storage applications and catalysis. The focus is on the preparation of nanocomposites of sodium and lithium based hydrides, specifically NaAlH4 and the Li-N-H system confined in carbon nanoscaffolds, and unravelling the effects of nanoconfinement on their ability to reversibly store hydrogen and/or catalyse different hydrogenation reactions.