Computational Investigation of Hydrogen Adsorption by Alkali‐Metal‐Doped Organic Molecules: Role of Aromaticity

Abstract

Hydrogen storage: Simple organic molecular systems (C(n)H(n), n=4, 5, 6, 8) are proposed for hydrogen storage purposes based on the concept of aromaticity. The adsorption of hydrogen is attributed to pronounced charge transfer from the sodium atom (green, see picture) to the organic systems and the electrostatic interaction between the ion and hydrogen molecules. Theoretical studies on hydrogen adsorption in small organic molecular systems, such as cyclobutadiene (C(4)H(4)), the cyclopentadienyl radical (C(5)H(5)), benzene (C(6)H(6)), and cyclooctatetraene (C(8)H(8)) and their metal-doped modifications, are carried out. Our results reveal that the simple van der Waals surfaces of pure organic molecules are not good enough for hydrogen adsorption due to the weak interaction between hydrogen molecules and the organic molecular surface. However, doping of alkali-metal atoms in the above organic molecular systems increases their hydrogen adsorption ability significantly, mainly due to electron transfer from the metal atom to the carbon surface. This charged surface created around the metal atom is found to enhance the hydrogen adsorption capacity of the complex considerably, both in terms of interaction energy and the number of adsorbed hydrogen molecules, with a hydrogen adsorption capacity ranging from 10 to 12 wt %. The role of aromaticity in such molecular systems is important in stabilizing these ionized organo-alkali-metal complexes.