Destabilization of ammonia borane via dual confinement in a cobalt-based Prussian blue analogue

Abstract

Ammonia borane (NH₃BH₃, referred to as AB) contains three protic and three hydridic hydrogen atoms, making it a promising candidate for solid-state hydrogen storage. However, in its pristine form, its practical application is limited by dehydrogenation temperatures exceeding 100 °C and the formation of byproducts with complex compositions. To overcome these limitations, we focused on destabilizing AB through confinement, made possible by using a Prussian blue analogue (PBA) as an oxygen-free host material. We selected the lacunar Co^II[Co^III(CN)₆]_2/3□_1/3 (referred to as CoCo) PBA due to its high thermal stability and the presence of coordinatively unsaturated Co²⁺ sites (CUS), which offer advantageous features for this purpose. Our results show that CoCo effectively confines AB, likely through a dual mechanism, with both chemisorption (where AB would coordinate to Co²⁺ CUS) and physisorption (where AB would be retained within the vacancies of the porous structure). Specifically, we found that approximately two-thirds of the AB would be chemisorbed, while one-third would be physisorbed. These findings highlight the crucial role of Co²⁺ CUS (of an oxygen-free host material) in AB confinement as well as in its isothermal dehydrogenation, likely involving homopolar B–H⋯H–B interactions.