A theoretical research of dihydrogen storage in ScxNy (x+y=4) compounds

Abstract

The dihydrogen storage capacity of ScxNy (x + y = 4) compounds have been theoretically investigated at different levels. At B3LYP-D3/6-311G(3df,3pd) level, ScN3 has multiple isomers with similar energies, which is an interference of hydrogen storage research. Sc2N2 and Sc3N has four and three isomers, respectively. For both systems, the lowest-lying isomers are planar Sc2N201 and Sc3N 01, which are energetically much low-lying by at least 20 kcal/mol than the other isomers, respectively. Sc3N 01 can adsorb 8H2 with gravimetric uptake capacity of 9.77 wt %. It satisfies the target specified by US DOE, however, some hydrogen molecules will dissociate and bond atomically on scandium atoms. The strong binding energy (0.66 eV/H2) exceeds the reversible adsorption range (0.1–0.4 eV/H2), which will cause high operating temperature to desorb hydrogen during the application process. Sc2N201 can adsorb 9H2 in the molecular form. The H2 gravimetric uptake capacity of Sc2N201 (9H2) (13.33 wt %) exceeds the target set by US Department of Energy, moreover, its average adsorption energy (0.32 eV/H2) is in the reversible adsorption range. The interaction of Sc2N201 with H2 molecules is considered by means of the bond critical points (bcp) in the quantum theory of atoms in molecules (QTAIM). The Gibbs free energy corrected adsorption energy points that the adsorption of Sc2N201(9H2) is energetically favorable below 240 K. Therefore, in ScxNy (x + y = 4), the planar compound Sc2N201 is more suitable to be a dihydrogen adsorption material.