A comprehensive first principles investigation of A$_2$BH$_6$ type (A= Li,Na, and K; B= Al, and Si) double perovskite hydrides for high capacity hydrogen storage

TL;DR

This study uses first principles calculations to explore the stability, electronic properties, and hydrogen storage potential of A$_2$BH$_6$ double perovskite hydrides, identifying promising candidates for energy storage applications.

Contribution

It provides a comprehensive first-principles analysis of A$_2$BH$_6$ hydrides, revealing their stability, electronic behavior, optical properties, and suitability for hydrogen storage, which was not previously detailed.

Findings

All studied hydrides are thermodynamically and mechanically stable.

Si-based hydrides are semiconducting, Al-based are metallic.

Li$_2$AlH$_6$ and Li$_2$SiH$_6$ show high hydrogen capacity and suitable desorption temperatures.

Abstract

Recent breakthroughs in vacancy-ordered double perovskite hydride materials have underscored their significant potential for integration into next-generation high-capacity hydrogen energy storage systems. We perform extensive first principles calculations leveraging both the GGA and hybrid-HSE06 functionals to systematically explore the intrinsic properties of A$_2$BH$_6$ complex hydrides. Thermodynamic stability for each hydride is demonstrated and confirmed by negative formation energies, determined by both the GGA and HSE06 formalisms. Additionally, mechanical stability is validated through compliance with Born's stability criteria. Electronic properties analysis reveals a semiconducting behavior in Si based hydrides (A2SiH6 ), whereas Al based (A$_2$AlH$_6$) display metallic nature, regardless of the A site atoms and functionals adopted. For the semiconducting hydrides, we have observed higher optical absorption peak greater than 106 (1/cm) in the UV regime indicating potential application in UV-optoelectronic devices. Furthermore, all studied compounds adhere to Debye's low-temperature specific heat behavior and converge to the classical Dulong-Petit limit at elevated temperatures, in accordance with fundamental thermodynamic principles. For hydrogen storage applications, both Al- and Si-based hydrides. meet key benchmarks set by the U.S. Department of Energy (DOE), achieving gravimetric hydrogen capacities (C$_{wt}$) exceeding 5.5 percent when A = Li or Na, and exhibiting volumetric hydrogen densities greater than 40 g.H2/L. Among all studied hydrides, Li$_2$AlH$_6$ and Li$_2$SiH$_6$ emerge as the two most promising candidates due to their outstanding Cwt greater than 12.0 percent , elevated density greater than 140 g.H$_2$/L, and favorable hydrogen desorption temperature ranges TD = 450 to 650 K.