Design Strategies for Complex Hydrides AMg 3 H 9 (A = Tc, Ru, Rh) for Hydrogen Storage: A DFT Investigation

In this study, their structural, electronic, mechanical, and hydrogen storage properties were systematically examined using density functional theory (DFT). The hydrides were structurally optimized with a lattice parameter of 4.569 Å, 4.596 Å and 4.627 Å for TcMg 3 H 9 , RuMg 3 H 9 and RhMg 3 H 9 respectively. The optimized cubic structure of the hydrides ensures thermodynamic stability with negative formation energies. The thermal stability of the hydrides has been evaluated by the Ab initio molecular dynamics simulations at 300 K. The study of electronic band structure and density of states of the hydrides ensures metallic behaviour under HSE06 calculations. Charge density and population analyses reveal that the bonding has a mixed ionic-covalent character, primarily driven by the transfer of electronic charge from the metal atoms to hydrogen. The mechanical durability of investigated hydrides is ensured by the mechanical properties and by meeting the born stability criteria of elastic constants. The mechanical analysis reveals that the studied hydrides exhibit ductile behaviour. The gravimetric hydrogen storage capacities of the hydrides TcMg 3 H 9 , RuMg 3 H 9 and RhMg 3 H 9 are 5.01 wt.%, 4.92 wt.% and 4.87wt.% respectively and their corresponding desorption temperatures are 202.35 K, 157.30 K and 151.64 K which suggests favourable hydrogen release under operating conditions. The above results reveal that AMg 3 H 9 hydrides ensure structural and mechanical stability, high hydrogen storage and moderate desorption temperatures which makes them suitable candidates for solid-state hydrogen storage applications.