First principle study on transition metal ammine borohydrides with amphoteric hydrogen for hydrogen storage applications

Usually the ions of a particular element in solids either in positive or in negative oxidation states, which are influenced by their chemical circumstances. Thus, it is quiet interesting for an atom having both positive and negative oxidation states within the same structural framework in a particular compound. Our structural and chemical bonding analyses show that the hydrogen ions in the transition metal ammine borohydrides (TMABHs) with the chemical formula M(BH4)2(NH3)2 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) have both + and - oxidation states. Moreover, our detailed analyses show that the hydrogen present in these compounds have amphoteric behaviour with hydrogen closer to boron is in negative oxidation state and that closer to nitrogen is in the positive oxidation state. The spin-polarised van der Waals interaction included calculation show that all these materials except M = Zn are having finite magnetic moment at the transitions metal site with the anti-ferromagnetic ordering as ground state. The localised magnetic moment similar to those present in molecular magnet with large band gap value indicating that these transparent magnets may find application in novel devices. Our nudged elastic band calculation show that the migration barrier for ammonia diffusion in these materials are more than 1 eV and hence these compounds may be used for energy storage applications since they have high weight percentage of hydrogen. The confirmation of the presence of amphoteric behaviour of hydrogen in TMABHs has implication in designing volume efficient hydrides for hydrogen storage applications. & COPY; 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.