Computational Investigation of Hydrogen Storage and Photocatalytic Potential of Double-Hydride Perovskites Rb2LiXH6 (X = Al, B)

Abstract

In this study, we have investigated the structural and physical properties of Rb₂LiXH₆ (where X = Al, B) by using density functional theory, with particular focus on its photocatalytic, optoelectronic, phonon, and thermodynamic features, highlighting its potential for hydrogen storage applications. The results reveal that both substances, Rb₂LiAlH₆ and Rb₂LiBH₆, exhibit a cubic crystal structure, with computed lattice parameters of 5.98 and 5.74 Å, respectively. Moreover, the calculations of formation and cohesive energies for the studied structures confirm that both materials are feasible to synthesize and exhibit thermodynamic stability. Hydrogen storage properties, including gravimetric and volumetric capacities and desorption temperatures, are evaluated. The gravimetric capacities are reported as 2.86% for Rb₂LiAlH₆ and 3.11% for Rb₂LiBH₆. In this research, the photocatalytic activity was also assessed, indicating that the Rb₂LiAlH₆ compound exhibits improved photocatalytic performance, supporting its suitability for photocatalytic water splitting. The electronic band structure shows that Rb₂LiAlH₆ and Rb₂LiBH₆ compounds exhibit indirect band-gap semiconducting behavior, with GGA band gaps of 1.809 and 0.096 eV and HSE-03 band gaps of 2.603 and 0.383 eV, respectively. Both double perovskite hydrides exhibit  mechanical stability and demonstrate brittle properties, as revealed by Poisson's and Pugh's ratios. Thermodynamic parameters, including heat capacity, enthalpy, entropy, Debye temperature, melting temperature, lattice parameters, and minimum thermal conductivity, are analyzed to assess stability at elevated temperatures and suitability for industrial applications. The optical properties indicate that Rb₂LiAlH₆ and Rb₂LiBH₆ exhibit enhanced absorption coefficients in the order of (10⁴ cm⁻¹) and hold potential for optoelectronic applications in the UV and visible regions. The overall analysis reveals Rb₂LiXH₆ (where X = Al, B) as a promising applicant for future H₂ retention due to its favorable gravimetric capacity and remarkable resilience.