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 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.