Reversible hydrogen storage of light transition metal-functionalized C9N4 monolayers under ambient conditions

Driven by the potential of hydrogen (H₂) as a sustainable alternative to conventional energy sources, we have conducted spin-polarized density functional theory (DFT) calculations to examine the viability of a twodimensional porous C₉N₄ monolayer as an efficient H₂ storage material. Our findings reveal that the adsorption energy of H₂ molecules on the pristine C₉N₄ is insufficient for effective storage. However, when the C₉N₄ monolayer is decorated with selected light transition metals (Sc, Ti, V), the adsorption energy improves significantly. We find that a 2 × 2 supercell of C₉N₄ can accommodate a maximum of four dopants of Sc, Ti and V. The resulting TMs-decorated C₉N₄ structure (TMs@C₉N₄) can adsorb up to 28 H₂ molecules, with average adsorption energies of − 0.245, − 0.337, and − 0.320 eV of the systems 4Sc@C₉N₄, 4Ti@C₉N₄, and 4 V@C₉N₄, respectively, satisfying the targets set by the US Department of Energy (DOE). Additionally, the gravimetric H₂ densities reach 9.93, 9.72 and 9.52 wt% for 4Sc@C₉N₄, 4Ti@C₉N₄, and 4 V@C₉N₄, respectively. Furthermore, electronic and magnetic analyses indicate that TMs@C₉N₄ has the potential to serve as a superior candidate for energy storage applications. Finally, we explore the H₂ storage at practical conditions of pressure and temperature using the Langmuir-adsorption model.