Mechanisms of dopants influence on hydrogen uptake in COF-108: A first principles study

Mechanisms of dopants (Li, Na, Mg, and Al) influence on hydrogen uptake in COF-108 were investigated by means of first principles. The binding energy of dopants in COF-108 was estimated from the first principles total energy calculations. All doped systems are shown positive binding energies with the metallic state of the dopant as the reference. The lowest binding energy of 0.518 eV appeared in the Na-doped system while a large amount of energy (2.692 eV) is required for Al to dope into COF-108. Electronic structure analysis shows that dopants Li and Na move the conduction band crossing the Fermi energy level and introduce weakly bonded electrons near the Fermi energy, which may polarize the hydrogen molecules. It is expectable that interaction between hydrogen molecule and the host COF-108 could be enhanced by the polarization of hydrogen molecule. Therefore the hydrogen uptake will be improved in the doped systems. Dopant Mg slightly reduces the band gap between the valence and conduction bands, but is hard to build chemical bonds with the host atoms owing to the less overlaps between the bond peaks of Mg and the COF-108. It hardly affects the electron distributions of the COF-108 and therefore weakly changes the chemical interactions between atoms in COF-108.