H2 physisorption on covalent organic framework linkers and metalated linkers: a strategy to enhance binding strength

Abstract

Hydrogen (H₂) is deemed as an attractive energy carrier alternative to fossil fuels, and it is required to be stored for many applications. Physisorption is one of the promising ways to store H₂ for its practical applications. Covalent organic frameworks (COFs) are promising candidates for H₂ storage due to their high porosity, large surface area and tunable characteristics. To improve hydrogen physisorption in the COFs, the chelation of transition metals (TMs) in the building blocks of the framework, i.e., organic linkers of the COF materials, was studied by using the first principles-based quantum mechanical dispersion-corrected density functional theory (DFT-D) method. Here, we report a total number of 96 H₂ complexes made up of six different COF linkers and chelated with the Sc, Ti and V transition metal (TM) atoms interacting with up to 4 H₂ molecules. The molecular interactions between the physisorbed H₂ molecules and these Sc-, Ti- and V-chelated linkers were explored in detail by employing the DFT-D method. The binding enthalpy (ΔH) of most of the complexes is higher than ∼10 kJ mol⁻¹, which is the basic requirement for practical H₂ storage. In the total interaction energy (between the physisorbed H₂ molecules and the chelated linkers), the dispersion and electrostatic interactions are dominant. This study is essential in finding out more efficient COF linkers for practical H₂ storage. It can also help to improve the uptake properties of existing porous materials for effective H₂ storage. The present study paves the way for designing transition metal chelated COFs for an effective H₂ storage and the knowledge gained from this study is expected to provide some inspiration for developing the corresponding experiments.