Most effective way to improve the hydrogen storage abilities of Na-decorated BN sheets: applying external biaxial strain and an electric field

Abstract

Density functional calculations were used to investigate the hydrogen storage abilities of Na-atoms-decorated BN sheets under both external biaxial strain and a vertical electric field. The Na atom generally has the weakest binding strength to a given substrate compared with the other elements in the periodic table [PANS, 2016, 113, 3735]. Consequently, it is understudied in comparison to other elements and there are few reports about the hydrogen storage abilities of Na-decorated nanomaterials. We calculated that the average binding energy (E_b) of Na atoms to the pure BN sheet is 1.08 eV, which is smaller than the cohesive energy of bulk Na (1.11 eV). However, the E_b can be increased to 1.15 eV under 15% biaxial strain, and further up to 1.53 eV with the control of both 15% biaxial strain and a 5.14 V nm⁻¹ electric field (E-field). Therefore, the application of biaxial strain and an external upward E-field can prevent clustering of the Na atoms on the surface of a BN sheet, which is crucial for the hydrogen storage. Each Na atom on the surface of a BN sheet can adsorb only one H₂ molecule when no strain or E-field is applied; however, the absorption increases to five H₂ molecules under 15% biaxial strain and six H₂ molecules under both 15% biaxial strain combined with a 5.14 V nm⁻¹E-field. The average adsorption energies for H₂ of BN-(Na-mH₂) (m = 1–6) are within the range of practical applications (0.2–0.6 eV). The hydrogen gravimetric density of the periodic BN-(Na-6H₂)₄ structure is 9 wt%, which exceeds the 5.5 wt% value that should be met by 2017 as specified by the US Department of Energy. On the other side, removal of the biaxial strain and E-field can help to desorb the H₂ molecule. These findings suggest a new route to design hydrogen storage materials under near-ambient conditions.