Lithium decorated 2D orthorhombic (o)-B2X2 monolayers for hydrogen storage: first principles calculations

Abstract

Over the last few years, scientists and researchers have shown significant interest in the search for suitable two-dimensional (2D) materials that can store hydrogen efficiently, possess high gravimetric capacity, and demonstrate excellent physisorption properties for hydrogen molecules. In this regard, we have investigated, using density functional theory (DFT) calculations, lithium decorated 2D orthorhombic (o)-B₂X₂ monolayers (X ≡ P or N atoms) as possible solid-state and lightweight candidate materials for hydrogen storage. Our findings reveal that o-B₂P₂ exhibits semi-metallic behavior, while o-B₂N₂ exhibits a semi-conductor state with a band gap of 0.64 eV. During the lithium decoration process, lithium adatoms exhibited strong binding energies of −3.09 and −1.98 eV for o-B₂P₂ and o-B₂N₂, respectively. These energies are significantly higher than the cohesive energy of Li (−1.63 eV), suggesting the absence of lithium-cluster formation. Furthermore, the lithium decoration process effectively enhances the adsorption of H₂ molecules on both materials (e.g., 32H₂@B₂P₂ and 24H₂@B₂N₂), leading to high gravimetric hydrogen storage capacities of 8.18 and 9.7 wt%, respectively. With reference to an average hydrogen adsorption energy of 0.18 (for 32H₂@B₂P₂) and 0.20 eV (24H₂@B₂N₂), the corresponding desorption temperatures were found to be 126 and 148 K. Based on these results, it can be deduced that Li-decorated (o)-B₂P₂ and (o)-B₂N₂ hold great promise as highly effective substrates for H₂ storage.