On enhanced hydrogen adsorption on alkali (Cesium) doped $C_{60}$ and effects of the quantum nature of the H2 molecule on physisorption energies

摘要

Hydrogen storage by physisorption in carbon based materials is hindered bylow adsorption energies. In the last decade doping of carbon materials withalkali, earth alkali or other metal atoms was proposed as a means to enhanceadsorption energies, and some experiments have shown promising results. Weinvestigate the upper bounds of hydrogen storage capacities of $C_{60}Cs$clusters grown in ultracold helium nanodroplets by analyzing anomalies in theion abundance that indicate shell closure of hydrogen adsorption shells. Onbare $C_{60}^{+}$, a commensurate phase with 32 $H_2$ molecules was identifiedin previous experiments. Doping $C_{60}$ with a single cesium atom leads to anincrease in relative ion abundance for the first 10 $H_2$ molecules, and theclosure of the commensurate phase is shifted from 32 to 42 $H_2$ molecules.Density functional theory calculations indicate that thirteen energeticallyenhanced adsorption sites exist, where six of them fill the groove between Csand $C_{60}$ and 7 are located at the cesium atom. We emphasize the largeeffect of the quantum nature of the hydrogen molecule on the adsorptionenergies, i.e. the adsorption energies are decreased by around 50% for$(H_2)C_{60}Cs$ and up to 80% for $(H_2)C_{60}$ by harmonic zero-pointcorrections, which represent an upper bound to corrections for dissociationenergies ($D_e$ to $D_0$) by the vibrational ground states. Five normal modesof libration and vibration of $H_2$ physisorbed on the substrate contributeprimarily to this large decrease in adsorption energies. A similar effect canbe found for H2 physisorbed on benzene and is expected to be found for anyother weakly $H_2$-binding substrate.