Effective hydrogen storage in Na2(Be/Mg)H4 hydrides: Perspective from density functional theory

Abstract

The utilization of hydrogen energy has garnered significant attention as a viable and environmentally friendly energy alternative. However, there exist some technological challenges pertaining to the storage of hydrogen. In this context, the present study utilizes first-principles calculations to examine the physical characteristics of Na2(Be/Mg)H4 hydrides to better understand their potential for use in hydrogen storage applications. The structure stability of Na2BeH4 and Na2MgH4 hydrides is assessed by formation enthalpy and phonon dispersion calculations. Elastic constants have been utilized to determine the elastic and mechanical stabilities of the studied hydrides. Both hydrides satisfy the established Born stability criteria, suggesting that both Na2BeH4 and Na2MgH4 are mechanically stable compounds. The assessment of electronic properties reveals that Na2BeH4 and Na2MgH4 have semiconducting characteristics with direct and indirect band gaps of 2.22 and 3.10 eV, respectively. Furthermore, various optical characteristics have been evaluated and compared. The computed values of gravimetric and volumetric hydrogen storage capacities for Na2BeH4 (Na2MgH4) hydrides are 6.78 (5.38) wt% and 85.11 (62.78) gH2l−1, respectively, fulfilling the standard set by US-DOE for 2025. This study suggests that both Na2BeH4 and Na2MgH4 hydrides have the potential to be an incredibly efficient choice for solid-state hydrogen storage.