In-situ diagnostics of metal hydride composites during cyclic hydrogenation by neutron radiography and tomography

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Hydrogen from renewable energy sources will play an important role as a future energy carrier. Therefore, hydrogen storage technologies are crucial in a hydrogen-based energy cycle. Compared to high pressure and cryogenic hydrogen storage, metal hydrides (MH) offer the possibility to store hydrogen with a high volumetric storage density (up to 150 g-H2/l, cf. 700 bar pressure vessels with max. 39 g-H2/l) under low pressure (2-30 bar) at moderate temperatures (room temperature) without boil-off losses.For high dynamic MH-based storage solutions MH-composites (MHC) based on a hydride forming alloy and a highly heat conducting secondary phase are advantageous. The latter is utilized to transport the reaction heat during hydrogenation and dehydrogenation, since the MH reaction zone offers only poor thermal conductivity (< 1 W/(mK)) which is not suitable for fast loading and unloading dynamics. In-situ diagnostics is needed to understand the processes inside MHC-based hydrogen storage systems under realistic operation conditions. For this purpose, neutron imaging is well suited because of the large cross-section of the hydrogen nuclei towards neutron radiation. With neutron imaging it is possible to visualize hydrogen concentrations in a non-destructive, in-situ and quantitative way. Loading and unloading can be investigated in real-time with a few seconds temporal resolution combined with a spatial resolution at micrometer-scale. Neutron tomography is the only method to resolve the three-dimensional distribution of hydrogen within a MHC quantitatively.In this DFG project it is planned to investigate for the first time the microscopic processes of the phase formation during hydrogenation-dehydrogenation of light-metal-based MHC (Mg- and Ti-base alloys) and its effect on the morphology and the microstructure of the MHC with in-situ imaging methods (neutron radiography and tomography) in real time. For the second phase a set of different highly heat conducting graphite modifications will be used. The influence of the MHC phase composition (dispersity, anisotropy, pore size distribution), the parameters of preparation and the gas and heat transfer properties on the hydriding/dehydriding kinetics and the long-term behavior will be investigated by a systematic variation of the parameters. From the structural changes of the MHC during the hydrogen-solid reactions valuable conclusions for efficient and long-term stable solid-state hydrogen storage systems can be deduced.