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

Recently synthesized two-dimensional (2D) borophene possesses unique structural, mechanical, electrical and optical properties. Herein, we present a comprehensive study of H(2) storage in alkali metal decorated and defect containing 2D borophene using density functional theory calculations. While the adsorption of H(2) over pristine borophene was found to be weak with a binding energy of −0.045 eV per H(2), metal decoration and point defects enhanced the adsorption strength significantly. Interestingly, the magnitudes of binding energy for a single H(2) molecule over Li, Na and Ca decorated borophene were found to increase up to −0.36, −0.34, and −0.12 eV per H(2), respectively. On the other hand, while the binding energy of one H(2) molecule over the borophene substrate containing a single vacancy (SV) was only −0.063 eV per H(2), similar to that of phosphorene, the binding energy increased to an enormous −0.69 eV per H(2) over borophene containing a double vacancy (DV). To gain further insight into the H(2) 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 H(2) 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 H(2) molecules. These results suggest that borophene can be an effective substrate for H(2) storage by carefully engineering it with metal decoration and point defects.