Investigation of structural, phonon, thermodynamic, electronic, and mechanical properties of non-toxic XZnH3 (X = Li, Na, K) perovskites for solid-state hydrogen storage: A DFT and AIMD approach

Due to its abundance on earth, clean, and non-toxicity, hydrogen is one of the best choices to replace fossil fuels. However, finding efficient storage methods remains a significant challenge. Perovskite hydrides have gained attention as potential solid-state storage materials due to their safety and higher density. In this paper, we conducted a comprehensive investigation into the stability, electronic, mechanical, and thermodynamic properties of the non-toxic XZnH3 (where X = Li, Na, K) compounds as potential candidates for solid-state hydrogen storage using Density functional theory. The structural characteristics and XRD patterns demonstrate that these materials exhibit a cubic phase with KZnH3 has the higher volume due it is higher lattice constant of 3.83 & Aring;, compared to LiZnH3 (3.58 & Aring;) and NaZnH3 (3.68 & Aring;). The phonon dispersion curves revealed the absence of negative frequencies, confirming the dynamical stability of these materials. Additionally, the formation energy results validated their synthesizability and thermodynamic stability, and the elastic constants (Cij) satisfied the Born criteria, ensuring that these materials are mechanically stable. Moreover, the AIMD simulations were performed to investigate the thermal stability of these compounds. Analysis of the electronic band structure and density of states verified the metallic nature of all XZnH3 perovskites, highlighting their excellent electrical conductivity which enhances the charge transfer, and improves the adsorption and desorption of hydrogen atoms. In addition, the thermodynamic properties including free energy, enthalpy, zero-point energy, entropy, and specific heat capacity were analyzed at different temperatures. The mechanical properties of XZnH3 (X = Li, Na, K) hydrides, including high bulk modulus, Debye temperature, and melting point, suggest their structural resilience and significant resistance to deformation. Their favorable B/G ratios and Cauchy pressures indicate ductility, aligning with their enhanced mechanical performance. The calculated gravimetric hydrogen storage capacities reveal that LiZnH3 exhibits the highest capacity of 4.01 wt%, followed by NaZnH3 at 3.31 wt%, and KZnH3 with the lowest value at 2.81 wt%, which are close to the benchmark established by the US Department of Energy (DOE) for on-board applications. These results position Zn-based perovskite hydrides as promising nontoxic candidates for hydrogen storage systems.