Theoretical study of hydrogen adsorption on the graphene quantum dots doped with various first row transition metals: Switch of spin state as a way to improve H2 adsorption.

A rapid increase of human population in the last years stimulates a search for alternative renewable and environmentally friendly energy resources. Hydrogen energy becomes one of the most promising candidates to replace the largely consumed fossil fuels. The first crucial step in the process of obtaining the hydrogen energy is an efficient storage of hydrogen gas. Modified graphene nanomaterials have already proved their capability as adsorbents of gas molecules. An affinity to bind, and subsequently store, hydrogen molecules is investigated using density functional theory (DFT) for the series of graphene quantum dots (GQDs) doped with different transition metals. Overall, considering the calculated binding energies and bonding distances, the Fe-doped GQD is the most promising material for H 2 capture, with storage capacity limited to three H 2 molecules on one Fe atom. Further potential candidates for effective H 2 adsorption are Mn-, and Cr-doped GQDs. Our calculations suggest that an induced change of their spin states may significantly enhance their H 2 adsorption ability. Such a change of spin state would be more easily accomplished in the case of Mn-doped GQD, having the doublet and quartet spin state energetically closer to each other (ca. 16.5 kJ mol−1). • H 2 adsorption ability of the GQDs doped with various transition metals is theoretically investigated. • The Fe-doped GQDs are the most promising materials for H 2 capture. • An induced change of the spin state may improve the H 2 adsorption ability of Mn-, and Cr-doped GQDs. • The storage capacity of one transition metal dopant atom is limited to three H 2 molecules. [ABSTRACT FROM AUTHOR]