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 (Na3Al1-xBexH6), Si (Na3Al1-xSixH6), and Fe (Na3Al1-xFexH6) 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 (Na3Al0.75Be0.25H6) and 􀀀 35.02 kJ/mol.H₂ for (Na3Al0.5Si0.5H6), values that closely align with those suggested by the U.S. Department of Energy (DOE). The decomposition temperature (Td) dropped from 632.76 K for the unsubstituted hydride Na3AlH6 to 392.21 K for (Na3Al0.5Fe0.5H6), 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 Na3Al0.5Be0.5H6, 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.