Hydrogen storage in Li, Na and Ca decorated and defective borophene: a first principles study.

Abstract

Recently synthesized two-dimensional (2D) borophene possesses unique structural, mechanical, electrical and optical properties. Herein, we present a comprehensive study of H2 storage in alkali metal decorated and defect containing 2D borophene using density functional theory calculations. While the adsorption of H2 over pristine borophene was found to be weak with a binding energy of −0.045 eV per H2, metal decoration and point defects enhanced the adsorption strength significantly. Interestingly, the magnitudes of binding energy for a single H2 molecule over Li, Na and Ca decorated borophene were found to increase up to −0.36, −0.34, and −0.12 eV per H2, respectively. On the other hand, while the binding energy of one H2 molecule over the borophene substrate containing a single vacancy (SV) was only −0.063 eV per H2, similar to that of phosphorene, the binding energy increased to an enormous −0.69 eV per H2 over borophene containing a double vacancy (DV). To gain further insight into the H2 adsorption process and identify sources of charge transfer, differential charge densities and projected density of states were calculated. Significant charge accumulation and depletion caused strong polarization of the H2 molecules. Finally, Na, Li and Ca decorated borophene yielded the gravimetric densities 9.0%, 6.8%, and 7.6%, respectively. The gravimetric density of the borophene containing a DV was found to be the highest, a staggering 9.2%, owing to increased interactions between DV borophene and the H2 molecules. These results suggest that borophene can be an effective substrate for H2 storage by carefully engineering it with metal decoration and point defects.