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

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 equivalent to 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 H-2 molecules on both materials (e.g., 32H(2)@B2P2 and 24H(2)@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 32H(2)@B2P2) and 0.20 eV (24H(2)@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 H-2 storage.