Unveiling the Potential of Ternary Hydrides: An Ab Initio Study on Rb2CaH4 and Cs2CaH4 for Solid-State Hydrogen Storage

Efficient solid-state hydrogen storage remains a pivotal challenge for sustainable energy systems. This study presents a comprehensive first-principles investigation into the structural, electronic, mechanical, and thermodynamic properties of ternary hydrides Rb2CaH4 and Cs2CaH4 using density functional theory. Both compounds are found to crystallize in a thermodynamically stable tetragonal I4/mmm structure. Electronic structure analysis reveals an insulating character with wide band gaps of 3.32 eV and 3.03 eV, respectively, ensuring stability during hydrogen desorption cycles. Mechanical calculations indicate that these hydrides exhibit brittle behavior characterized by mixed ionic-covalent bonding. Crucially, the gravimetric hydrogen storage capacities are determined as 1.88 wt% for Rb2CaH4 and 1.30 wt% Cs2CaH4. While gravimetric values are modest, volumetric capacities (~40-44 g H2/L) align with DOE 2025 targets. Thermodynamic modeling predicts desorption temperatures above 600 K, suggesting these materials are particularly viable candidates for stationary high-temperature thermal energy storage applications rather than mobile uses