Storage of Hydrogen Adsorbed on Alkali Metal Doped Single-Layer All-Carbon Materials

Abstract

Controversial experimental results have suggested that the presence of alkali metals in graphitic adsorbents would intensify hydrogen adsorption. Mainly by use of density functional theory methods, several authors have confirmed this intensification, which has been explained by a significant charge transfer from dopand, located on top of the center of a ring (the hollow configuration), to substrate. However, in this work, it was found that, for each distance of the hollow configuration of the Li−graphene interaction, depending on the initial guess, not only one but two qualitatively different self-consistent field solutions would be possible. One of them would concentrate charge in the region between lithium and graphene. The other one would be just the opposite. At fully correlated second-order Möller−Plesset level, for each distance, the latter was found to be significantly more stable than the former. So, it seems that the charge transfer from lithium to graphene described by several authors would not take place. The set of results reported here would provide evidence that solutions for the interaction of alkali metals with single-layer all-carbon materials would be extremely sensitive to initial guesses and correlation treatments. It is concluded that, in the case of alkali metal doping being capable of significantly intensifying physisorption of molecular hydrogen on single-layer all-carbon materials, the postulated charge transfer from dopand (located on top of the center of a ring) to substrate would not be the mechanism.