Calcium-atom-modified boron phosphide (BP) biphenylene as an efficient hydrogen storage material

Porous nanosheets have attracted significant attention as viable options for energy storage materials because of their exceptionally large specific surface areas. A recent study (Int. J. Hydrogen Energy, 2024, 66, 33-39) has demonstrated that Li/Na-metalized inorganic BP-biphenylene (b-B₃P₃) and graphenylene (g-B₆P₆) analogues possess suitable functionalities for hydrogen (H₂) storage. Herein, we evaluate the H₂ storage performance of alkaline earth metal (AEM = Be, Mg, Ca)-decorated b-B₃P₃ and g-B₆P₆ structures based on first-principles density functional theory (DFT) calculations. Our investigations revealed that individual Be and Mg atoms are not stable on pure b-B₃P₃ and g-B₆P₆ sheets, and the formation of aggregates is favored due to their low binding energy to these surfaces. However, the binding energy improves for Ca-decorated b-B₃P₃ (b-B₃P₃(mCa)) and g-B₆P₆ (g-B₆P₆(nCa)) structures, forming stable and uniform mCa(nCa) (m and n stand for the numbers of Ca atom) coverages on both sides. Under maximum hydrogenation, the b-B₃P₃(8Ca) and g-B₆P₆(16Ca) structures exhibited the ability to adsorb up to 32H₂ and 48H₂ molecules with average adsorption energy (E _a) values of -0.23 eV per H₂ and -0.25 eV per H₂, respectively. Gravimetric H₂ uptakes of 7.28 wt% and 5.56 wt% were found for b-B₃P₃(8Ca)@32H₂ and g-B₆P₆(16Ca)@48H₂ systems, exceeding the target of 5.50 wt% set by the US Department of Energy (DOE) to be reached by 2025. Our findings indicate the importance of these b-B₃P₃ and g-B₆P₆ sheets for H₂ storage technologies.