Alloying ratio versus cluster size for reversible hydrogen storage in 3d transition metal doped small Mg clusters: Dispersion corrected DFT study

Dispersion corrected density functional theory (& omega;B97X-D DFT) method is used to study the hydrogen storage behaviour of 3d TM doped small Mg clusters. Initially, hydrogenation of TMMgn clusters with TM: Mg alloying ratio of 1: 1, 1: 2 and 1:3 have been tested, which showed the maximum hydrogen storage capacity for the 1:1 ratio. Hydrogen gravimetric density values of the corresponding VMg, FeMg and NiMg dimers are 11.8 wt%, 9.1 wt% and 6.8 wt%, respectively. Geometry also plays an important role here as isomers with sufficiently exposed TM atoms adsorb the maximum number of H2 molecules. In extending the cluster size keeping the TM:Mg ratio fixed at 1:1, the gravimetric density of molecular hydrogen is found to remain the same. The nature of molecular hydrogen interaction with the clusters has been confirmed to be in between physisorption and chemisorption as per requirement of DOE targets for efficient hydrogen storage materials. Based on the statistical analysis of hydrogen occupation at different thermodynamic conditions, calculation of desorption temperature and dissociation barrier along with molecular dynamics simulation results have established selectivity based on size, geometry and alloying ratio for 3d TM doped small Mg clusters in hydrogen storage performance. Accordingly, the linear/planar isomers of VnMgn (n = 1-2) and FenMgn (n = 2-3) clusters have been outlined for the possible use as efficient reversible hydrogen storage materials.