Cucurbiturils as promising hydrogen storage materials: a case study of cucurbit[7]uril

Abstract

We have assessed the hydrogen storage capability of cucurbiturils that are experimentally available. For this purpose, first we have investigated the hydrogen binding ability of the repeating unit and prompted by an encouraging result, we have studied the hydrogen storage capacity of cucurbit[7]uril, as a representative of the cucurbituril family, at the ωB97X-D/6-31G(d,p) level of theory. Cucurbit[7]uril is found to interact with hydrogen in both exohedral and endohedral fashion. A total of 52 hydrogen molecules are found to be stored in cucurbit[7]uril, in which five hydrogens remain in the cavity of the cage and the remaining hydrogens prefer to bind exohedrally, leading to 8.3 gravimetric wt% of hydrogen. The N and O centers act as the active sites for the exohedral hydrogen binding. Each hydrogen in 52H₂@cucurbit[7]uril interacts with cucurbit[7]uril having average binding energy value of 7.8 kJ mol⁻¹. BSSE correction reduces the binding energy to some extent. The variation of binding energy per H₂ molecule is also explored when H₂ molecules are adsorbed in a sequence. All probable hydrogen binding processes are found to be exothermic in nature. The effect of an external electric field in improving binding energy and its consequence on structures and different bonding parameters are explored.