Amidoboranes and hydrazinidoboranes: State of the art, potential for hydrogen storage, and other prospects

Following the seminal BeNeH compound ammonia borane NH3BH3, a series of derivatives have been developed as possible chemical hydrogen carriers. First, alkali and alkaline earth derivatives of ammonia borane, i.e. amidoboranes M(NH2BH3)(n) with n = 1 or 2, emerged. Then, hydrazine borane, which is actually a derivative of ammonia borane, was rediscovered and considered as a precursor of alkali and alkaline earth derivatives, i.e. hydrazinidoboranes M(N2H3BH3)(n). A number of B-N-H ionic salts were, in this way, synthesized and reported. The present review provides the general background (e.g. syntheses, crystallographic structures, thermal stability, dehydrogenation properties, and decomposition mechanisms) relating to these alkali and alkaline earth amidoboranes and hydrazinidoboranes, and carries on two objectives. The first objective is to discuss the potential of these materials (for the application for which they have been primarily developed over the past decades, namely hydrogen storage) and the challenges they are facing. It is concluded that the light alkali amidoboranes, namely the lithium, sodium and potassium amidoboranes) should be further developed owing to better dehydrogenation properties in terms of the onset temperature of dehydrogenation, the extent of dehydrogenation, and the purity of the hydrogen released. They should, however, be further developed with the aim of upscaling, something that has not yet been achieved. Achieving high technological readiness levels is thereby the main challenge ahead. The second objective of the present review is to explore alternative uses because, as things stand, none of the developed materials have the expected features for hydrogen storage. We suggest that any of the discussed derivatives are potentially solid-state reducing agents, energetic materials, and/or precursors of boron nitride-based ceramics, bearing in mind that for this last use, if the targeted material is porous, it could be regarded as a reversible hydrogen sorbent. It is thus important to maintain efforts in the development of the current derivatives as well as in the discovery of novel ones. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.