First-Principles Screening of 3d, 4d, and 5d Transition-Metal-Decorated g-C3N4 for Hydrogen Storage

Transition-metal-decorated g-C3N4 is a promising hydrogen storage material. Using DFT calculations, we systematically evaluated the hydrogen storage performance of TM@g-C3N4 composites anchored with 27 transition metals (3d/4d/5d). A four-step screening framework was established: adsorption energy selection, H-H bond length and ICOHP analysis, structural symmetry evaluation, and single-atom dispersion verification. Five candidates (TM = Sc, Ti, Mn, Y, and Hf) were identified, among which Ti@g-C3N4 exhibits the best performance, with a theoretical maximum gravimetric density of 9.38 wt %. Electronic structure analysis reveals that Ti atoms significantly activate H2 molecules through Kubas interactions, enabling high-capacity reversible hydrogen storage. Thermodynamic analysis shows that within the Department of Energy (DOE) operating window (233–358 K, 5–12 atm), Ti@g-C3N4 maintains an effective gravimetric density of 8.53–9.38 wt %, exceeding the DOE target of 6.5 wt %, and hydrogen release can be effectively triggered by heating or depressurization. This study provides a theoretical reference for the design of two-dimensional hydrogen storage systems.