Exploring structural, electronic, mechanical, and hydrogen storage properties of Mg3ZH8 (Z = Fe, Co): a density functional theory study

Abstract

The development of efficient hydrogen storage materials is crucial for practical hydrogen energy utilization. This study uses first-principles density functional theory (DFT) to examine the structural, electronic, mechanical, optical, and hydrogen storage properties of Mg₃ZH₈ (Z = Fe, Co) complex hydrides. Both compounds are thermodynamically and dynamically stable, as confirmed by negative formation energies and phonon spectra free of imaginary modes. Mg₃FeH₈ exhibits a ferromagnetic metallic ground state, while Mg₃CoH₈ shows non-magnetic metallic behavior, with transition-metal d states playing a key role in their electronic and magnetic properties. The calculated gravimetric hydrogen storage capacities of 5.90 wt% for Mg₃FeH₈ and 5.76 wt% for Mg₃CoH₈, along with high volumetric densities of 195.6 and 186.6 g H₂ per L, respectively, surpass current U.S. Department of Energy targets. Dehydrogenation thermodynamics show moderate desorption temperatures, with Mg₃FeH₈ demonstrating better hydrogen release behavior. Mechanical analysis indicates ductility with positive Cauchy pressure, high Pugh's ratios, and moderate Vickers hardness, suggesting resistance to hydrogen embrittlement. Optical and transport properties reveal metallic conductivity, which enhances hydrogen desorption kinetics. These findings position Mg₃ZH₈ (Z = Fe, Co) as a promising hydrogen storage material and provide a solid theoretical basis for future experimental work.