Computational Exploration of APH6 (A = Na, K) Hydrides as High-Performance Materials for Hydrogen Storage and Photocatalytic Fuel Cell Applications using ab-initio and AIMD approach

Hydrogen is widely recognized as a clean and sustainable energy carrier, driving the urgent need for safe and efficient solid-state storage materials. In this study, the structural, thermodynamic, electronic, optical, and photocatalytic properties of APH6 (A = Na, K) perovskite hydrides are comprehensively investigated using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Both compounds exhibit dynamic and thermal stability at 300 K, as confirmed by AIMD and quasi-harmonic analyses. The predicted hydrogen storage capacities of NaPH6 (10.08 wt%) and KPH6 (7.95 wt%) surpass the U.S. DOE target, confirming their potential for solid-state hydrogen storage applications. Electronic structure calculations reveal that NaPH6 is a direct band gap semiconductor (1.32 eV), while KPH6 possesses an indirect band gap (2.68 eV), making both suitable for visible-light-responsive optoelectronic devices. The band-edge alignment analysis indicates that both hydrides exhibit appropriate redox potentials for visible-light-driven photocatalysis, with NaPH6 favoring enhanced absorption and KPH6 demonstrating higher oxidative strength. Optical investigations show dielectric constants, refractive indices, and strong visible–UV absorption, confirming excellent light–matter interaction behavior. The high optical conductivity and broad absorption spectra further endorse their use in photonic, photovoltaic, and photoelectrochemical systems. Elastic constant evaluations satisfy Born stability criteria, with KPH6 exhibiting higher stiffness than NaPH6. Overall, the favorable hydrogen storage capacity, robust stability, semiconducting nature, and multifunctional optoelectronic characteristics of APH6 (A = Na, K) hydrides underscore their potential as next-generation materials for integrated hydrogen storage and solar energy conversion technologies, inspiring future exploration of perovskite hydrides in sustainable energy applications.