Scientific Content of COSY
Previous work including work by the Network partners indicates that the thermodynamic properties of a complex hydride system can indeed be tuned by combining reactive hydrides with borohydrides (“heat management”), which stabilize the dehydrogenated state. Stabilizing the dehydrogenated state reduces the enthalpy for dehydrogenation, thereby increasing the equilibrium hydrogen pressure and thus circumventing the stability problem of the complex hydrides. These novel systems are called ‘Reactive Hydride Composites’ as all compounds are chemically reacting during the sorption process. Exemplarily, the reversibility of such complex reactions has been proven. However, the desorption temperature of the investigated systems is still higher than estimations based on thermodynamic considerations and therefore limited by kinetic constraints. These may be overcome using proper catalysts.
Likewise, a very similar hydrogen storage composite system consists of mixtures of (earth) alkali metal nitrides and (earth) alkali hydrides. E.g. a 2LiH/LiNH2 composite can theoretically release 11.5 mass% hydrogen. Actually, neither the reaction mechanism nor kinetic constraints of such reactions are clearly known. Therefore, it is important to analyze the origin of the kinetic barriers/rate-limiting steps. Insight into the reaction mechanisms may help in the selection of a suitable catalyst to enhance the kinetics and lower the operating temperatures.
However, what are possible rate-limiting steps of those complex reactions, which slow down the overall kinetics? In standard metal hydride systems, the dissociation of hydrogen, its diffusion and the nucleation/phase formation of the hydride are the fundamental steps. Additionally, in complex hydride systems, metal atom diffusion has to be considered. With an additional reaction coordinate in reactive hydride composites, the impact of metal atom diffusion and phase formation is reinforced. A fundamental understanding of these processes is still in the fledgling stages. Addition of a suitable catalyst might affect one of the various elementary steps of the solid state reaction: e.g. forming nucleation centres or an intermediate structure with a lower activation barrier, lowering interface- and surface energies or forming gateways for the hydrogen.
The main objective of the Network will be to reach a fundamental understanding of the sorption kinetics in Reactive Hydride Composites and related systems through three different strategies: empirical studies, model systems and a theoretical/modelling approach.