Synthesis, structure and properties of Al-based borohydrides for hydrogen storage

This thesis is dedicated to chemistry and hydrogen storage properties of novel complex hydrides. The main efforts were focused on synthesis and characterization of new Al-based borohydrides and amidoboranes. Somewhat different investigation on the hydrolysis of KBH4 in the atmosphere of CO2 was also performed. The series of mixed-cation M[Al(BH4)4] (M = Li+, Na+, K+, NH4+, Rb+, Cs+) were successfully obtained by a reaction of the corresponding MBH4 with Al(BH4)3. This method provides a high theoretical hydrogen capacity and enables to store highly reactive and unstable Al(BH4)3 in a solid-state form. A good quality of the obtained samples allowed to solve their crystal structures using variable temperature synchrotron X-ray powder diffraction (XRPD). The thermal decomposition of M[Al(BH4)4] shows various pathways: Al(BH4)3 is released below 100 °C for M = Li+ and Na+, while heavier derivatives evolve hydrogen and diborane at about 150 °C. Li[Al(BH4)4] firstly decomposes into Li4Al3(BH4)13 at ~60 °C, desorbing Al(BH4)3, and the latter decomposes at ~90 °C releasing the rest of the starting borohydrides. NH4[Al(BH4)4] occupies a special position, as it contains protic and hydridic hydrogens, recombining into hydrogen already at 35 °C. In general, the experimental decomposition temperatures of metal borohydrides linearly correlate with the square root of the ionic potential of metal atoms calculated from dynamical charges on cations. Al(BH4)3 reacts with ammonia borane, NH3BH3, producing mononuclear Al(BH4)3·NH3BH3 complex. It releases at ~70 °C two equivalents of hydrogen, showing considerably lower decomposition temperature, compared to pure NH3BH3. The striking property of this system is an endothermic dehydrogenation on the first decomposition step, compared to the exothermic one for ammonia borane and its other known complexes. This opens a possibility for its direct rehydrogenation. The main drawback of this system is the low stability of the dehydrogenation intermediate, Al(BH4)3·NHBH, decomposition above 100 °C. In order to suppress its decomposition, we investigated N-substituted derivatives of ammonia borane, obtaining a more promising complex, Al(BH4)3·CH3NH2BH3. Al(BH4)3 serves as a template in the potentially reversible ammonia borane dehydrogenation, with Al atom coordinating both the starting and the final products. Aiming to substitute highly reactive Al(BH4)3 by a halide salt, and using N-substituted NH3BH3, we obtained an ionic form of AlCl3·CH3NH2BH3, namely [Al(CH3NH2BH3)2Cl2][AlCl4]. The further analysis of these compounds will help to define a system for the reversible storage of hydrogen in ammonia borane. A different way to obtain hydrogen using new Al-based reactive hydride composite (RHC) was also explored in this work. In particular, NaAlH4–4NH3BH3 system showed fascinating properties of releasing high purity H2 and low hydrogen release temperature of 70 °C and formation of Na[Al(NH2BH3)4]. The properties of the first Al-based amidoborane look promising. This compound decomposes in two steps with the formation of NaBH4, 8 equivalents of pure hydrogen and an amorphous product AlN4B3H(0÷1.8). The latter reversibly reabsorb about 27% of released hydrogen. The example of Na[Al(NH2BH3)4] decomposition to AlN4B3H(0÷1.8) requires further in-depth studies, viz. its chemical structure and an optimization of the rehydrogenation process. The interest in Al-based complex hydrides has been growing during the last years. Any knowledge about new Al-containing complex hydrides is very important for the future solid-state hydrogen storage. Data obtained in this work might be important in the further predictions of potential RHC systems, applying the computational methods. On the practical side, the combination of any of the described compounds with other hydrides may open avenues to new RHCs with enhanced hydrogen storage properties.