Innovative Materials and Systems for Solid State Hydrogen Storage

The research presented in this doctoral thesis concerns with the development of novel materials and systems for solid state hydrogen storage. The first group of works presented is on alkaline and alkaline-earth borohydrides. The possibility to enhance their properties with the help of nanosupports has been widely explored. An attempt to improve the dehydrogenation kinetics of lithium borohydride has been made dispersing this material on the surface of modified nanotubes and graphite. The resulting nanoconfined material displayed a decreased decomposition temperature in comparison with pure material and further decreasing was observed when the surface area of the supports was increased. An analogous experiment was performed to investigate this effect in combination with the assets of a reactive hydride composite, where two materials are mixed to obtain a compound with a lower decomposition enthalpy. The effect of the mixture was beneficial in presence of the support, due to lower temperature melting. For calcium borohydride an ordered mesoporous carbon was used after chemical activation. The increased properties of this support resulted in lower decomposition temperature and improved reversibility for a number of cycles at different pressure values. The second research line is focused on magnesium hydride. To improve its kinetic properties a zirconium-nickel alloy was investigated to evaluate its influence on the reaction rate, both in absorption and desorption. The degradation observed in experimental reactors, of different magnesium hydride powders catalyzed with a transition metal oxide, motivated the fabrication of pellets with the addition of a binding agent, to obtain mechanical resistance, still allowing hydrogen diffusion. Each pellet was supposed to behave as an independent system, so they were also tested in a small reactor. Several hydrogen absorption/desorption cycles were performed to compare the behaviour of the small reactor with the laboratory data obtained on smaller quantity of powdered and pelletized specimens. Finally, the feasibility of a vehicular hydrogen tank system was investigated using an interstitial metal hydride as storage material. Apart from material basic characterization, two different kinds of experiment were performed. Static tests (measurements with automatic flow control and constant settings) were used to evaluate wether the requirements for desorption are met by the tank set-up. Then, dynamic tests were designed and applied on the tank, where the hydrogen flow was fluctuating following a hypothetical on-road trial. It was possible to underline the heat management issues of high-demanding performances and to analyze some solutions for that. Different cycles were carried out on the tank to find the ideal setting for high average and peak flows in a realistic experiment.