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

Abstract

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 COF. 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 H2 molecules with an average adsorption energy of 0.11 eV. At most, 24 H2 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 H2 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 H2 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 H2 molecules to doped Li atoms.

Tunable Water and CO2 Sorption Properties in Isostructural Azine-Based Covalent Organic Frameworks through Polarity Engineering

“…In addition, we chemically modified the channels by means of metal-doping, which had been predicted earlier to enhance gas sorption capacities and selectivities. [22][23][24][25] Therefore, AB-COF and ATFG-COF were loaded with either lithium or zinc acetate (LiOAc × 2 H 2 O and Zn(OAc) 2 × 2 H 2 O) by dissolving the salts in methanol and then adding the COF (see ESI for details). After soaking and washing, Li@AB-COF/Li@ATFG-COF and Zn@AB-COF/Zn@ATFG-COF were obtained, which showed metal loadings of 8% (with respect to all available coordination sites, see ESI for details) for Zn@ATFG-COF, 1% for Li@ATFG-COF, 0.6% for Zn@AB-COF, and 0.8% for Li@AB-COF.…”

“…However, metal ions were never successfully integrated in COFs postsynthetically. [22][23][24][25][26] It has been shown that doping with metal salts in MOFs can have a significant impact on gas uptakes, especially for CO 2 and H 2 sorption. [27][28][29][30] Here, we present two isostructural microporous COFsan azine-benzene-COF (AB-COF) and azine-triformylphloroglucinol-COF (ATFG-COF) based on a water-stable azine linkage -and develop a versatile COF platform combining the key features for both CO 2 capture and water sorption, namely (i) high CO 2 uptake capacity, (ii) tunable polarity, and (iii) gated water-uptake at low relative humidity.…”

New Li-doped fullerene-intercalated phthalocyanine covalent organic frameworks designed for hydrogen storage

“…Our previous work has shown that the hydrogen uptake in Pc-PBBA COF can be enhanced by 43% by lithium doping. 38 The lithium doping plays a better role in fullerene-intercalated Pc-PBBA COFs due to the increasing of pore volume and surface area, which provide more resident places for hydrogen adsorption. At T = 298 K and P = 100 bar, for these four Li-doped C n -Pc-PBBA COFs, the gravimetric uptakes of H 2 reach 4.2 wt%, and the volumetric uptakes of H 2 reach 18.2 g L À1 .…”

Covalent Organic Frameworks: Synthesis, Properties and Applications—An Overview

“…Simulations for the adsorption of COFs impregnated with metal ions, mainly lithium [ 35 , 36 , 37 ], sodium, potassium [ 38 ] and transition metals [ 39 ] have also been carried out. It was observed that the interactions between H 2 molecules and metal ions trapped within COF pores are more favorable and lead to adsorption capacities several times higher than those reported for metal-free COFs, at similar temperature and pressure conditions.…”