Density functional theory study of light metal (Li/Na/Ca) functionalized borophosphene for reversible hydrogen storage

Abstract

Borophosphene was investigated for hydrogen storage performance by density functional theory (DFT) calculations. At first, the parameters of the borophosphene structure were calculated and validated with literature. Owing to the poor hydrogen storage performance of pristine borophosphene, decoration by Li, Na and Ca was adopted. DFT results showed that decoration enhanced the binding energy by an order of magnitude from −0.047 eV/$H_2$ over pristine material to −0.20–−0.42 eV/$H_2$ over decorated ones. PDOS and Bader charge analysis elucidated the role of adatom decoration in charge transfer and better binding. Up to 10, 12 and 20 $H_2$ molecules could be adsorbed over a single Li, Na and Ca adatom, respectively, in supercell of 32 atoms. The desorption temperature was calculated from the binding energy using von’t Hoff equation. A complete discharge of the stored molecules from decorated borophosphene could be realized in the temperature range of 125–531 K at 1 atm pressure. Further, $H_2$ storage by functionalization at multiple sites of the substrate was also performed to evaluate the theoretical gravimetric density. With Li and Na overloading, gravimetric densities of 6.22%, and 5.34% were obtained. However, the reversible hydrogen storage for the best case of Li decorated borophosphene under practical conditions was obtained as 2.9%. The metal–metal clustering aspect was elucidated using Nudge Elastic Band (NEB) calculations. NEB results showed that inter-site energy barriers of the adatoms were larger than their thermal energy by an order indicating lower possibility of clustering.