Dehydrogenation of ammonia borane confined by low-density porous aromatic framework

Ammonia borane (AB) has been considered as an outstanding candidate material for on-board hydrogen storage due to its high stoichiometric hydrogen content (19.6 wt %) and moderate dehydrogenation temperature. However, slow dehydrogenation kinetics below 100 C and release of volatile byproducts (ammonia, borazine, and diborane) limited its practical applications. In this work, low-density and highly porous aromatic framework (PAF-1; BET, 4657 cm ² g^-1; pore volume, 2.55 cm³ g^-1) was utilized as a template for the first time to nanoconfine AB molecules. The dehydrogenation behavior of the confined AB was studied by temperature- programmed desorption mass spectrometry (TPD-MS) and pressure-composition- temperature (PCT) analyses. It was found that the AB molecules can be fully confined within the nanopores when the weight ratio of AB/PAF-1 is around 1:1. More importantly, AB started to dehydrogenate at very low temperature (around 50 C) with the peak of 77 C in the absence of any volatile byproducts such as ammonia, borazine, or diborane. Furthermore, about 4 wt % of hydrogen was evolved in the first 25 min at 75 C which is 27 times higher than the pristine AB, displaying higher kinetics at low temperatures. Compared with other porous supports such as MOFs, the PAF-1 has a very low framework density because it is built up only by light C and H elements. This could significantly improve the hydrogen systemic gravimetric capacity of the AB-confined system and thus increase feasibility in practical applications.