The role of Al substitution in Na3AlH6 hydrides: Structural and thermodynamic insights for hydrogen storage technologies

Abstract

In this work, the structural, thermodynamic, and electronic properties, as well as the diffusion kinetics and volumetric and gravimetric capacities of sodium and aluminum hydride Na₃AlH₆, were evaluated and enhanced by substituting the aluminum element with Be (Na₃Al_1-xBexH₆), Si (Na₃Al_1-xSi_xH₆), and Fe (Na₃Al_1-xFe_xH₆) with x = 0.25 and x = 0.5. All calculations were performed according to density functional theory (DFT), using the generalized gradient approximation (GGA) developed by Perdew, Burke, and Ernzerhof for solids (PBEsol). The results show an improvement in the thermodynamic properties. For instance, the formation enthalpy decreased from −82.25 kJ/mol.H₂ for the unsubstituted hydride Na₃AlH₆ to −34.24 kJ/mol.H₂ for (Na₃Al_0.75Be_0.25H₆) and −35.02 kJ/mol.H₂ for (Na₃Al_0.5Si_0.5H₆), values that closely align with those suggested by the U.S. Department of Energy (DOE). The decomposition temperature (T_d) dropped from 632.76 K for the unsubstituted hydride Na₃AlH₆ to 392.21 K for (Na₃Al_0.5Fe_0.5H₆), corresponding to the operational temperature range of hydrogen fuel cells (PEM) from 289 to 393 K. Furthermore, the gravimetric capacity of hydrogen increased from 5.93 wt% for the unsubstituted hydride Na₃AlH₆ to 6.40 wt% for Na₃Al_0.5Be_0.5H_6, in line with the DOE's recommended value of 6 wt%. Analysis of the density of states of Na₃AlH₆ revealed that the bandgap is 2.98 eV, indicating that the hydride Na₃AlH₆ is insulating. The activation energy of hydride Na₃AlH₆ varies between 0.8 and 3.05 eV.