Density functional studies on the hydrogen storage capacity of boranes and alanes based cages

Abstract

The hydrogen storage (H-storage) capacity of various boranes and alanes have been investigated using density functional theory (DFT) based M05-2X method employing 6–31+G** basis set. The changes in the H-storage capacities of borane and alane upon substitution of antipodal atoms in the cages by C, Si, and N have also been investigated. It is found from the calculations that a maximum of 20 H₂ molecules can be adsorbed on the deltahedron faces of these cages. The maximum gravimetric density has been observed for boranes when compared to alanes. The H-storage capacity of closo-borane dianion [B₁₂H₁₂]^2-, monocarborane [CB₁₁H₁₂]^1-, dicarborane [C₂B₁₀H₁₂], and closo-azaborane [NB₁₁H₁₂] cages is almost similar (∼22 wt.%). Among these cages, B_BB dianion show higher binding energy (BE) and BE per H₂ molecule (BE/nH₂) which are 181.06 and 9.03 kJ/mol, respectively. In the case of alanes, dicarbalane [C₂A₁₀H₁₂] has maximum H-storage capacity of 11.6 wt.%. Based on these findings, a new MOF with carborane (MOF-5_CC) as linker has been designed. The calculation on the new MOF-5B_CC reveals that it has H-storage capacity of 6.4 wt.% with BE/nH₂ of 3.02 kJ/mol.