The Role of Cobalt Clusters (Co n , n = 1–5) Supported on Defective γ–Graphyne for Efficient Hydrogen Adsorption: A First Principles Study

In this theoretical work, density functional theory calculations show the effect of small cobalt clusters (Con, n = 1–5) adsorbed on pristine γ-graphyne (γ-GY), and modify N-doped γ-GY monolayers (GYNs-def). Different geometrical configurations are assessed with the adsorption energy, charge transfer, and density of states. The system with vacancy defects shows a large adsorption energy (19.96 eV) for the Co5 cluster. This behavior may be associated to the overlapping of the electronic state contributions between cobalt and carbon atoms in the valence states. This indicates that the Co5 cluster could be deposited on N-doped γ-GY monolayers (Con@GYNs-def). The lowest-energy systems are evaluated to estimate the strength of the interaction with hydrogen molecules (xH2, where x = 1–5). According to the adsorption energy values, the modified γ-GY monolayers are allowed to be a suitable support material to capture H2 molecules via the small Con clusters. The hydrogen retention capacity for the supported cobalt atoms corresponding to the lowest-energy configurations and larger systems are evaluated by using molecular dynamics simulations with the Born–Oppenheimer approximation. The role played by defects in the GYNs-def monolayers is important, since the Con clusters remain attached to the vacancy with the absence of surface diffusion. This study may represent a guide to tailor novel nanostructures based on small cobalt clusters supported on graphyne monolayers modified to be applied in H2 adsorption.