Multiscale Study of Hydrogen Adsorption, Diffusion, and Desorption on Li-Doped Phthalocyanine Covalent Organic Frameworks

In this paper, we performed a multiscale study on the hydrogen storage capacity of Li-doped phthalocyanine covalent organic frameworks (Li-doped Pc-PBBA COF). We combine the first-principles studies of hydrogen adsorption and migration energies with the kinetic Monte Carlo simulations of hydrogen adsorption, diffusion, and desorption processes in Li-doped Pc-PBBA COP. The first-principles calculations revealed that the Li atoms can be doped on the surface of the channel of Pc-PBBA COF with a binding energy of 1.08 eV. Each Li cation can bind three H-2 molecules with an average adsorption energy of 0.11 eV. At most, 24 H-2 molecules can be adsorbed in one formula unit, corresponding to a maximum of gravimetric density of 5.3 wt % and volumetric uptake of 45.2 g/L. The diffusion barriers of H-2 between different Li-adsorption sites are in the range 0.027-0.053 eV. The KMC simulations have predicted that the optimum conditions of hydrogen storage for Li-doped Pc-PBBA COF are at T = 250 K and P = 100 bar, with a gravimetric density of 4.70 wt % and volumetric uptake of 40.23 g/L. At T = 300 K and P = 1 bar, the adsorbed H-2 molecules have fast desorption kinetics, and 97% hydrogen can be released from the adsorbed phase to the gas phase. A two-step modification method (the B-substitution is first) was also advanced to suppress the Li clustering behavior and further improve the binding energy of H-2 molecules to doped Li atoms.