Combined Density Functional Theory And Kinetic Monte Carlo Study Of Hydrogen Spillover On Fluorine-Decorating Covalent Organic Frameworks

How to reduce the H migration barrier in hydrogen spillover is one of the crucial points for hydrogen storage. In this study, we use COF-8 as a representative to study the effect of F decoration for hydrogen spillover on the covalent organic framework (COF). We have found an effective way to promote the hydrogen spillover process: F-doping method. In this method, the F atoms are doped on the top of C atoms of hexahydroxytriphenylene (HHTP) or 4-phenylboronic acid (BTPA) linker of COF-8. Due to its extremely strong electronegativity, F atoms extract electrons of COF-8 and change the charge distribution on the surface of COF-8. It predicts that the maximum hydrogen storage capacity of F-doped COF-8 can reach 4.9 wt %. The doping of F atoms not only enhances the atomic H adsorption energy on COF-8 but also improves the contact between the Pt cluster and COF-8.With the F atoms doping, the H migration barrier from the Pt-6 cluster to HHTP and BTPA linkers can be lowered from 2.1 eV to 0.67 or 0.25 eV, respectively. The kinetic Monte Carlo simulations show that the optimal reaction temperatures of H atoms migrating from the Pt-6 cluster to HHTP and BTPA linkers are 257.0 and 95.5 K, respectively. The second hydrogen atom can migrate from the Pt cluster to the F-doping COF-8 surface. The calculated migration barriers on COF-8_FD_A and COF- 8_FD_B are 0.89 and 0.51 eV, respectively, and the optimal reaction temperatures are 337 and 201 K, respectively, which are slightly larger than those of the first hydrogen atom. The results reveal that the F-doping method can promote the H spillover process, and this method can also be applied on COF-5. The H-2 desorption in F-doped COFs can be realized by the Eley-Rideal recombination reaction. Because the Pt cluster and F atoms are doped on two opposite sides of COF-8, further works need to be done to investigate the controllable deposition method.