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

Abstract

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 Ead 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.