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Materials for high-capacity hydrogen storage

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 425395402
 
Final Report Year 2024

Final Report Abstract

Among the methods available to store hydrogen, solid-state storage is attractive from safety and volumetric energy density points of view. Among these, Reactive Hydride Composite systems (RHCs) offer high gravimetric hydrogen storage properties on surplus, being interesting for mobile usage. But, their sluggish hydrogenation/de-hydrogenation kinetics is hampering this class of material suitable for real applications. Additives were found that improve the kinetics, however, their role in the process was unknown at the start of this project. The project investigated hydrogenation/de-hydrogenation processes in RHCs via combination of transition electron microscopy (TEM) and X-ray based techniques, also under operative conditions (i.e. in situ). The goal was to gain understanding of the factors that play a key role in nucleation, growth and diffusion processes in RHCs. The chosen promising RHC systems, LiBH4-MgH2 and NaBH4-MgH2, were successfully prepared at the Helmholtz-Zentrum Hereon. The different states of these materials were categorized as milled, incompletely desorbed, and completely desorbed, depending on the material type. The chosen additives, 3TiCl3·AlCl3, were selected for their ready availability, cost-effectiveness, and notable enhancement of the kinetics in RHC systems. Their kinetics was determined by employing different kinetic models, for the different stages. Detailed material characterization was performed at KIT, Karlsruhe, using various transmission electron microscopy (TEM) techniques at the Karlsruhe Nano- and MicroFacility KNMFi. In this project we successfully discovered the role of the additives in the kinetics improvement. A first evidence-based crystallographic-chemical model of nucleation and growth in the RHC systems was developed and the diffusion and mass transport kinetics in the RHC systems was modeled. Compared to the pristine system, the material doped with 3TiCl3·AlCl3 exhibits superior dehydrogenation kinetics which relates to a reduced MgB2 nucleation time. This reduction is accompanied by a MgB2 morphology change from bar-shape to platelet-shape. Detailed TEM investigations verify the presence of TiB2/AlB2 particles that form during the milling process, and, further, remaining MgB2 after cycling. These reaction products offer a small (or no) interatomic and interplanar mismatch towards the MgB2, less growth restrictions and are suggested here to provide a kinetically favored path for the nucleation. The suitable interatomic and interplanar mismatches between MgB2 and TiB2/AlB2, the refined microstructure, and the homogenous distribution of nuclei sites enhance the interphase nucleation, avoiding the interphase-controlled mechanism. First-principles calculations, including surface energies, interfacial energy, work of adhesion, and electronic structure of the interfaces of MgB2 and TiB2, reveal an interface of B- terminated MgB2 (0001) planes and Ti-terminated TiB2 (0001) planes having highest stability. Our results show that the TiB2 particles possess good nucleation potency for MgB2 particles from the thermodynamic perspective. Based on our results, nucleation and growth of product phases in RHCs are suggested to be enhanced by a low strain energy density between RHCs and additivecomponents containing particles thereby improving the desorption kinetics. Another result to consider is the experimental reaction temperature, which always surpassed the calculated thermodynamic desorption temperature. This was attributed to the need for the liquid phase of LiBH4 or NaBH4, allowing for suitable species mobility. This aspect goes beyond the scope of this project and requires further investigations.

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