Mechanistic insights into the thermal decomposition of ammonia borane, a material studied for chemical hydrogen storage

Abstract

Though ammonia borane NH₃BH₃ (AB) was discovered in the 1950s, it is fair to state that AB as a potential chemical hydrogen storage material was discovered more recently, in the 2000s. Unlike the isoelectronic ethane CH₃CH₃, AB is polar; three of its hydrogens are protic (NH₃ group) and the other three are hydridic (BH₃ group); the material is solid at ambient conditions owing to dihydrogen N–H^δ+⋯H^δ−–B interactions; and AB decomposes from 90 °C under thermogravimetric conditions. With such properties, AB has attracted much attention, even though AB in neat form is not suitable for the application mentioned above because it decomposes more than it dehydrogenates. Hence, strategies (based on solubilization, catalysis, chemical doping and nanosizing) aiming at destabilizing AB to make it release pure H₂ at <100 °C have been developed. Beyond the performance targeted for hydrogen storage, this provided us with better understanding of the mechanisms of decomposition. Indeed, studies on thermal decomposition of neat AB have revealed just how complex the mechanisms are (due to the involvement of two possible key intermediates initiating the decomposition, the formation of various volatile products, the existence of counterintuitive homopolar reactions, and the formation of polymeric residues of complex composition, for example). Studies on destabilized AB have provided insights into several mechanistic aspects including the reaction intermediates, the decomposition pathways, and the nature of the residue forming upon the release of 1 and ≥2 equiv. H₂. We presently have a fairly good understanding of the mechanisms of decomposition of AB, which is discussed in more detail below. In that respect, this review focuses firstly on the complexity of thermal decomposition of neat AB, secondly on what we know with regard to thermal decomposition of destabilized AB, and thirdly on all outstanding questions. It is very important to have an excellent knowledge of the reaction mechanisms if technological progress is to be made with AB as a chemical hydrogen storage material.