Computational analysis of light metal decorated C₃N₂ monolayers for efficient hydrogen storage

Hydrogen (H₂) stands at the forefront of clean energy solutions due to its exceptional gravimetric energy density, environmental friendliness, and widespread availability. However, the development of safe, efficient, and high-capacity H₂ storage remains a critical bottleneck for its practical deployment. Due to the cost and safety limitations of conventional methods like liquefaction and high-pressure storage, automotive applications increasingly depend on material-based H₂ storage solutions. In this study, a detailed computational investigation of a novel two-dimensional (2D) carbon nitride (C₃N₂) monolayer (ML) as a promising H₂ storage material is performed. The interaction of pristine C₃N₂ with H₂ is weak. This is overcome by enhancing H₂ storage performance by functionalizing C₃N₂ with selected light metal dopants such as Mg, K, and Ca. In this paper, using a first-principles study, we show that C₃N₂ can accommodate up to four dopants, each exhibiting strong binding energies of −2.93, −2.92, and −4.22 eV/dopant for Mg, K, and Ca, respectively. Bader charge analysis further reveals substantial charge transfer from the dopants to the C₃N₂ monolayer, effectively transforming the dopants into cations. Thermal stability of metal-doped C₃N₂ systems is evaluated at 300 K using ab initio molecular dynamics (AIMD) simulations, which confirm robust structural integrity under ambient conditions. Each dopant adsorbs a maximum of five H₂ molecules with average adsorption energies within the desired range of −0.15 to −0.60 eV/H₂, suitable for ambient temperature operation. We find that 4Mg-, 4K-, and 4Ca-doped C₃N₂ systems achieve H₂ storage capacities of 9.47, 5.96, and 6.57 wt%, respectively, all surpassing the U.S. Department of Energy (DOE) 2025 target of 5.5 wt%. This study establishes metal-doped C₃N₂ as a promising 2D nanomaterial for next-generation H₂ storage and provides valuable design insights for developing practical solid-state H₂ carriers.