Alternative bottom-up approach to design and prepare hybrid magnesium nanostructures for solid state hydrogen storage

Hydrogen is described as a sustainable energy carrier, but its storage has been challenging. Solid state storage using magnesium (Mg) has been identified as a promising solution with high gravimetric and volumetric storage capacities. However, Mg suffers from poor hydrogen thermodynamics and kinetics, and requires a temperature close to 400 °C to release H2 from its hydride. Various methods have been investigated to lower the H2 release temperature such as alloying of Mg with transition metals (TMs) and nanosizing. Adding TMs by ball milling improved the sorption kinetics, but the thermodynamics remained relatively unchanged. Nanoconfinement of Mg in porous materials improved the thermodynamics but resulted in low gravimetric densities due to the weight of the host material. Reduction in H2 release temperature following Mg nanosizing was also reported, but further improvement was challenging because of the appearance of enthalpy-entropy compensations effects at the nanoscale. Whether such compensation effects are due to the synthetic methods used to generate Mg or intrinsic thermodynamic evolutions at the nanoscale remains uncertain. This work aims to investigate this dilemma by advancing synthetic methods to prepare Mg nanoparticles (NPs) with a controlled morphology in order to reveal any relationship between the H2 properties and size of nanostructures synthesised. In this respect, the thermal decomposition of Grignard reagents is an attractive approach but to date, di-n-butylmagnesium (bu2Mg) is the sole precursor suitable for the preparation of Mg. However, bu2Mg requires a high decomposition temperature relatively close to the melting of MgH2. Thus, the first step of this thesis focuses on identifying alternative Mg precursors with a low decomposition temperature to better control the synthesis and minimize surface contamination. Morphology control of the Mg NPs generated is also investigated by inducing steric/electrostatic stabilization through the use of surfactants and salts. The aim is to identify any potential particle size effect/hydrogen properties relationships that could reveal paths toward alternative thermodynamic properties. Local doping of Mg nanostructures with TMs is further examined as a way to alter enthalpy/entropy properties and lower the H2 release temperature. This work shows that a bottom-up approach using alternative Mg precursors is a suitable method to generate NPs with hydrogen storage properties improved to some extent