Be2+ and Mg2+-decorated sulflower: Potential systems for molecular hydrogen storage

Abstract

The hydrogen binding efficiency of multiple metal-ion (Be2+, Mg2+)-decorated “First Generation” Sulflower (C16S8) systems has been investigated for the first time using density functional ω-B97XD method and 6311++G(d,p) basis set. Our calculations show that the central ring of the aforesaid system can be decorated by a single di-positive metal ion, followed by favorable decoration of at most one pair of metal ions (di-positive each) on the peripheral five-membered rings, both on same and opposite faces with certain preferences. All of the metal ion-decorated complexes are capable of efficient hydrogen binding. Be2+ and Mg2+-decked single ion complexes effectively bind six and four H2 molecules respectively. Moreover, each of the double ion-decorated systems can adsorb ten H2 molecules irrespective of the facial orientation of the metal ions. The average interaction energy (ΔE) between sulflower and metal ions (single and double ions) as well as the average binding energy (ΔBE) per molecular hydrogen of the concerned metal-ion-decorated complexes is found to be much higher for Be2+-decked systems. The nature of interaction between hydrogen molecules and metal ions is explicated by the topological analysis (AIM Analysis) and NBO formalisms. In case of Be2+-decked systems, the amount of charge transfer from H2 bonding orbital to metal anti-bonding orbital is much higher than analogous Mg2+-decorated systems. The Natural Population Analysis (NPA) evaluates the charge variation on the acceptor metal ions due to hydrogen adsorption. In short, our theoretical study gives a comprehensive account of the relationship between the metal ion-decorated sulflower systems and hydrogen molecules, which will further motivate researchers in the field of efficient hydrogen storage materials.