First-principles prediction of hydrogen storage capabilities in Pd-based alkali metal hydride X2PdH4 (X = Na, K, Rb, and Cs)

As an emerging hydrogen storage material, metal hydride provides a safe storage solution for future large-scale application of hydrogen energy due to its excellent reliability and safety. Using first-principles predictions, the structural, mechanical, electronic, thermodynamic, optical, and hydrogen storage properties of potential solidstate hydrogen storage materials, palladium-based alkali metal hydride X2PdH4 (X = Na, K, Rb and Cs) were assessed. The formation enthalpies, mechanical properties, and phononic characteristics indicate that these four hydrides exhibit thermodynamic, mechanical, and dynamic stability. The B/G ratio indicates that X2PdH4 compounds are ductile materials. The electronic structure and Poisson's ratio confirm that the bonding in these four hydrides is ionic, and details of the band structure reveal that they are all semiconductors. Na2PdH4 has the smallest band gap, facilitating electronic transitions between the valence and conduction bands. The optical properties indicate that palladium-based alkali metal hydride X2PdH4 (X = Na, K, Rb and Cs) are materials with high refractive indices. Additionally, thermodynamic properties were derived using post-processed phonon data. Notably, the hydrogen storage properties indicate that Na2PdH4 has the highest hydrogen storage capacity by weight. The excellent mechanical, electronic, and hydrogen storage properties of Na2PdH4 make it the most promising of these four hydrides as a potential future solid-state hydrogen storage material.