Hydrogen trapping efficiency of Li decorated porous boron fullerene B38: The first-principles study

The demand for clean renewable energy is urgent in current. The hydrogen application is difficult mainly due to the ratively low capacity in the storage medium. In this work, the adsorption and desorption of the hydrogen molecules by the Li atoms decorated B₃₈ cage are studied by the density functional theory. The calculated largest binding energy of one Li atom (2.68 eV and 2.58 eV) is upon the hexagonal hole of the B₃₈ cage, which is much larger than the experimental cohesive energy of bulk Li (1.63 eV). Each Li atom in the outside of the B₃₈ cage can adsorb up to four H₂ molecules. The E_ad of B₃₈(Li-nH₂)₄ decreases from the 0.22 eV for n = 1 to the 0.11 eV for n = 4. The B₃₈(Li–4H₂)₄ structure achieves the 6.85 wt% hydrogen gravimetric density, which is higher than the goal of 5.5 wt% before 2017 set by the United States Department of Energy. The almost the same partial density of states for the fifth H₂ molecule as that of the isolated H₂ molecule, the longer 4.5 Å distance between the fifth H₂ molecule and the Li atom, together with the small NBO charges all reveal the weak electronic field around the Li⁺, which can interpret the weak H₂ adsorption mechanism. Finally, the B₃₈Li₄ structure can easily release 9H₂ molecules at 373 K known from the molecular dynamic simulation and practically trap about 1.08H₂ molecules at 373 K/3 atom condition calculated by the grand partition function. Thus, its reversible practical HGD of B₃₈Li₄-14.34H₂ is 6.18 wt%, which is almost the same value as the theoretical 6.85 wt% for B₃₈(Li–4H₂)₄. Our studies will be the strong theory basis for the future application in hydrogen storage material development.