Hydrogen adsorption characteristics of metal and halogen intercalated graphite: a density functional theory study

While most studies on graphite intercalation compounds (GICs) as hydrogen storage materials use molecular dynamics and first principles approaches, few focus on the detailed hydrogen adsorption characteristics of the intercalators themselves. Usually, intercalators are divided into metals, halogens, and compounds. However, the hydrogen adsorption mechanisms of the common intercalators such as alkali metals Li and Na, halogen elements F, and compounds FeCl₃ have not yet been revealed. Therefore, we studied the microscopic interactions between Li, Na, F, FeCl₃ intercalators, and H₂, including charges, potentials, intermolecular forces, and molecular orbital mixing, using density functional theory (DFT). Our study results show that compared to current hydrogen storage materials like planar graphite, GICs have better hydrogen storage capacity due to their interlayer hydrogen adsorption properties. Each Li, Na, and F atom can adsorb 6 H₂, while 8 H₂ was adsorbed by the FeCl₃ molecule. Using Li and Na atoms as intercalators, GICs adsorb hydrogen through van der Waals forces with adsorption energy values of 0.15 eV and 0.16 eV, respectively, exhibiting a physical adsorption form. Using F atoms as intercalators, the adsorption energy is similar to alkali metals, and the adsorption form is also similar. However, F atoms gain charge from H₂ when adsorbing, which is opposite to the alkali metals losing charge characteristic. Using FeCl₃ as an intercalator, GICs have reached a maximum interlayer spacing distance of 9.40 Å, with an adsorption energy value of 1.06 eV, exhibiting a slight polarisation phenomenon. The adsorption form is a type of physical-chemical adsorption similar to metal dihydrogen complexes caused by Kubas coordination. By comparison, we found that FeCl₃ intercalators have the highest hydrogen adsorption energy and demonstrate considerable stability, making them the most promising intercalators for hydrogen adsorption among the four. In addition, compared to the strong chemical adsorption of H₂ by transition metals loaded at the boundary of carbon nanomaterials, FeCl₃ in the interlayer space evenly adsorbs each H₂ through physical-chemical adsorption, which helps to dissociate and release H₂.

The GIC structure in this study was constructed and optimised using the Dmol3 module based on the GGA-PBE method from the Materials Studio 2020 software package. The hydrogen adsorption system was calculated using the Gaussian 09W software package based on the B3LYP functional with 6-31G * basis set, and metal elements were calculated using the SDD basis set. The charge transfer, electrostatic potential, and independent gradient model based on Hirshfeld partition and density of states are processed using the Multiwfn 3.8 package. All images are rendered using the VMD 1.91 package.