Structural, hydrogen storage capacity, electronic and optical properties of Rb₂LiXH₆ (X=Si, Ge, Sn) hydrogen storage materials from first-principles investigation

Double perovskites exhibit great potential in solid-state hydrogen storage. However, the regulatory mechanism of B-site elements—particularly those from Group IVA (Si, Ge, Sn)—in Rb-based double perovskites and their quantitative impact on hydrogen storage performance remain unclear, which limits the rational design of high-efficiency storage materials. This study employs first-principles calculations based on density functional theory (DFT) to systematically investigate Rb₂LiXH₆ (X=Si, Ge, Sn), focusing on the structural, electronic, optical, and hydrogen storage properties modulated by B-site (X) substitution. Results show their lattice parameters are 8.31 Å, 8.29 Å, and 8.53 Å, with formation enthalpies of −0.94 eV, −0.35 eV, and −0.35 eV, respectively. Only Rb₂LiGeH₆ and Rb₂LiSnH₆ are kinetically stable, possessing gravimetric hydrogen storage capacities of 2.34 wt% and 1.98 wt%, as well as dehydrogenation temperatures of 257.6 K and 259.82 K—values that meet the 2025 hydrogen storage targets issued by the U.S. Department of Energy (DOE). These two materials exhibit metallic characteristics and strong low-energy ultraviolet absorption, with absorption intensity enhanced by the increasing atomic radius of X. This work clarifies the structure-property relationships of Rb-based double perovskites regulated by B-site elements, provides a reliable strategy for tuning light-responsive hydrogen storage via IVA-group doping, and offers valuable theoretical insights for the development of high-performance solid-state hydrogen storage materials, thereby advancing the practical application of hydrogen energy systems.