A Chemical Modification Strategy for Hydrogen Storage in Covalent Organic Frameworks

Abstract

We developed a two-step doping strategy for chemical modification of covalent organic frameworks (COFs) as hydrogen storage materials. The first step was the boron (B) doping of organic building units, suppressing the clustering of subsequent metal dopants on frameworks. The second step was the doping of metal atoms, forming trapping centers for H2 molecules on B-doped COFs. Using ab initio calculations, we explored the doping processes and studied the dependence of the structural stability and electronic properties of doped building units on the B concentration. In organic building units of COF-1, two-B para substitution was energetically more favorable than other B substitutions. The clustering of Sc, Ti, and Ca was suppressed on such B-doped COF-1. Sc and Ti preferred the double-sided adsorption, while Ca only favored the single-sided adsorption, due to their different interactions with the substrates. Each Sc and Ti atom bound four H2 molecules through the Kubas interaction, while each Ca atom could adsorb six H2 molecules via the polarization of H2 molecules under the electric field surrounding the Ca. A hydrogen storage gravimetric capacity larger than 6.5 wt % was achieved in these modified frameworks. Our work indicates that COFs modified by the two-step doping are promising for hydrogen storage applications in fuel cell-powered vehicles.