On enhanced hydrogen adsorption on alkali (cesium) doped C60 and effects of the quantum nature of the H2 molecule on physisorption energies

Abstract

Hydrogen storage by physisorption in carbon based materials is hindered by low adsorption energies. In the last decade doping of carbon materials with alkali, earth alkali or other metal atoms was proposed as a means to enhance adsorption energies, and some experiments have shown promising results. We investigate the upper bounds of hydrogen storage capacities of C60Cs clusters grown in ultracold helium nanodroplets by analyzing anomalies in the ion abundance that indicate shell closure of hydrogen adsorption shells. On bare C60 + , a commensurate phase with 32 H2 molecules was identified in previous experiments. Doping C60 with a single cesium atom leads to an increase in relative ion abundance for the first 10 H2 molecules, and the closure of the commensurate phase is shifted from 32 to 42 H2 molecules.Density functional theory calculations indicate that thirteen energetically enhanced adsorption sites exist, where six of them fill the groove between Cs and C60 and 7 are located at the cesium atom. We emphasize the large effect of the quantum nature of the hydrogen molecule on the adsorption energies, i.e. the adsorption energies are decreased by around 50% for (H2)C60Cs and up to 80% for (H2)C60 by harmonic zero-point corrections, which represent an upper bound to corrections for dissociation energies (De to D0) by the vibrational ground states. Five normal modes of libration and vibration of H2 physisorbed on the substrate contribute primarily to this large decrease in adsorption energies. A similar effect can be found for H2 physisorbed on benzene and is expected to be found for any other weakly H2-binding substrate. © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http:// creativecommons.org/licenses/by-nc-nd/4.0/

Cs+ Solvated in Hydrogen—Evidence for Several Distinct Solvation Shells

“…The excess energy would be well below the endothermicity of the reaction H2Cs + → HCs + + H (2) for which Kaiser et al have computed a value of 4.75 eV (4.48 eV if harmonic zero-point correction is included). 29 On the other hand, the solvation energy is more than sufficient to cause rapid evaporation of a few solvent molecules, thus leading to the observed anomalies in the ion abundance.…”

“…[26][27][28] A similar methodology has been applied previously for (H2)nC60Cs + . 29 We report here only the ground state equilibrium geometries and potential energies including harmonic zero-point corrections. A proper treatment of the nuclear quantum dynamics is beyond the scope of this article.…”

Adsorption of sodium and cesium on aggregates of C60

“…For additional insight we now turn to other reports. Theoretical work has been limited so far to the lighter alkalis (with the exception of jellium-type calculations for C 60 fully coated with one or more layers of cesium [4,13], and recent calculations restricted to Cs monomer and dimers [33,34]). These studies reveal a delicate balance between Coulomb repulsion between atomic ions favoring uniform coverage, and metallic bonding favoring island formation [35].…”

Reactivity of Cobalt‐Fullerene Complexes towards Deuterium

“…Because hydrogen is physically adsorbed on these materials, cryogenic cooling is needed to reach a volumetric energy density useful for practical applications. Computational studies indicate that the binding strength can be enhanced by decorating fullerenes with transition metals or alkali metals . Although the theoretical hydrogen weight percentages are impressive (almost 9 wt% for Sc and Ca), some of these findings are questioned by calculations that predict clustering of the transition metal atoms, which would drastically lower the maximal amount of adsorbed hydrogen.…”