Exploring promising KAH₃ (A = Ca, Sr and Ba) hydrides for solid-state hydrogen storage and thermoelectric applications

The implementation of hydrogen energy as a renewable and ecologically friendly energy carrier has generated significant interest, however, efficient storage remains a formidable obstacle. In order to address the above, scientists switched their focus towards hydride materials, which provide a secure and effective way of storing hydrogen. In this work, we have investigated the structural, mechanical, thermodynamical, thermoelectric and hydrogen storage properties of potassium-based KAH₃ (A = Ca, Sr and Ba) perovskite-type hydrides using density functional theory (DFT) computations. Structural stability of KAH₃ hydrides has been assessed by formation enthalpy calculations, which ensure that all the studied hydrides are stable and synthesizable. The most optimal lattice parameters of KCaH₃ KSrH₃ and KBaH₃ are found to be 4.47, 4.78 and 5.14 Å, respectively. Additionally, we have determined the elastic stiffness constants, which confirm the elastic stability of KCaH₃ hydrides as they fulfill the Born stability criteria. After a detailed study of electronic properties, it is found that KCaH₃ hydrides possess semiconducting characteristics. Furthermore, we have studied the thermodynamical characteristics, which show that KAH₃ (A = Ca, Sr, and Ba) hydrides obey Debye's specific heat law and the Dulong-Petit law at high temperatures, indicating their thermodynamical stability. In addition, the appropriateness of the under-study compounds for thermoelectric applications, various thermoelectric parameters such as Seebeck coefficient (S), electrical conductivity (σ/τ), electronic thermal conductivity (κ/τ) and thermoelectric power factor (PF) has been determined using BoltzTrap2 code and plotted as a function of chemical potential (μ). The thermoelectric properties indicate the suitability of KAH₃ compounds for thermoelectric applications. The calculated values of gravemetric storage capacities of KCaH₃, KSrH₃ and KBaH₃ hydrides are 3.68%, 2.33%, and 1.68% respectively. Additionally, the volumetric capacities of KCaH₃ and KSrH₃ compounds have achieved the target set by DOE for 2025. Furthermore, the computed desorption temperatures (T_d) for KAH₃, KSrH₃ and KBaH₃ are 399, 396 and 393 K, respectively. Interestingly, our computed T_d values of these hydrides are very close to the value (289–399 K) set by DOE for the year of 2025. In short, our findings suggest that the KAH₃ (A = Ca, Sr, and Ba) hydrides have the potential to be used in hydrogen storage and thermoelectric devices.