Development of magnesium-based multilayer PVD coatings for hydrogen storage applications

On the long list of solid-state hydrogen storage materials, magnesium hydride stands out for its relatively high hydrogen storage capacity of 7.7 wt%, combined with the low cost and abundance of magnesium. For practical applications however, issues such as the slow kinetics and the high stability of magnesium hydride must be resolved in order to reduce the potential operating temperatures of a magnesium-based solid-state hydrogen storage system. Catalysis has been widely used to improve the hydrogen storage kinetics and thin film techniques have been used to explore novel structures and combinations of materials in order to improve both the kinetics and thermodynamics of hydrogen storage in magnesium. The original contribution to knowledge of this work lies in the study and understanding of the evolution of a range of novel thin film multilayer coatings and the effect of the structure, structural evolution and materials on the hydrogen storage properties of these materials, each consisting of 150 layers of magnesium, < 20 nm thick, separated by < 3 nm thick layers of a nickel-rich, iron-based transition metal mix, chromium and vanadium. The samples, as well as a non-catalysed control sample, were produced by means of magnetron-assisted physical vapour deposition and delaminated from the substrate for volumetric, gravimetric and calorimetric hydrogen cycling measurements. The coatings were analysed both before and after hydrogen cycling to understand the structural evolution of the coatings from highly structured thin film multilayers to flaky thin film particles containing finely distributed nano-crystalline catalyst particles. The formation of the intermetallic Mg2Ni in one of the samples was found to be beneficial for the hydrogenation kinetics, whilst the dehydrogenation kinetics were found to be affected mostly by the nano-crystalline transition metal phases that formed in the catalysed samples during hydrogen cycling. This resulted in hydrogenation and dehydrogenation of magnesium hydride in less than 4 and 13 minutes at 250°C with activation energies as low as 60.6 ± 2.5 kJ mol-1