Grand canonical Monte Carlo study on the hydrogen adsorption behaviour of four graphite intercalation compound clusters

Due to their interlayer space adsorption characteristics, graphite intercalation compounds (GICs) are considered potential hydrogen storage materials. Still, due to their diverse types and complex structures, there are few studies on the detailed hydrogen storage capacity of different types of intercalators, especially for small cluster structures. This study uses the grand canonical Monte Carlo approach to investigate the hydrogen storage capacity of four metal and halogen intercalator GIC cluster structures under room temperature from 1 MPa to 70 MPa. The study results show that the GICs using alkali metals Li as intercalators can achieve the highest hydrogen loading of about 89 at 70 MPa, more than other GICs. The Li-based GIC achieved the goal of the hydrogen storage rate of 6.5 wt% at the pressure of 21 MPa, indicating that lighter alkali metals positively affect GIC hydrogen storage. The GICs using halogen elements F as intercalators can achieve the shortest hydrogen adsorption distance of 0.11 Å at 70 MPa, and show the strongest hydrogen compression capability. The GICs using transition metal compounds, FeCl₃ as intercalators, can release the highest adsorption heat with 1.25 kcal/mol, indicating that the adsorption process is achieved through physicochemical adsorption. However, the FeCl₃-based GICs meet the hydrogen storage goal at a pressure of 48 MPa, indicating that the intercalator FeCl₃ has the heaviest mass, which increases the mass proportion of the adsorption unit and decreases the hydrogen storage rate. Thus, it is suggested that the GICs using alkali metals as intercalators are suited for high hydrogen storage situations. The GICs using transition metals as intercalators are suited for more stable hydrogen storage situations.