Light Alkali Metal Functionalized Two‐Dimensional C 5 N Monolayers for Enhanced Hydrogen Storage

This work presents a comprehensive computational investigation of a novel two‐dimensional (2D) organic carbon nitride (C 5 N) monolayer for hydrogen (H 2 ) storage applications. Van der Waals‐corrected density functional theory calculations reveal that pristine C 5 N interacts weakly with H 2 molecules, thereby limiting its storage capability. To address this limitation, the C 5 N monolayer is functionalized with light alkali metals M (M = Li, Na, and K) to enhance its adsorption performance. Energetic analysis demonstrates that C 5 N can stably accommodate up to six metal dopants, with average binding energies of −2.88, −1.84, and −1.80 eV per dopant for Li, Na, and K, respectively. These values are significantly stronger than the corresponding cohesive energies of the dopants, thus preventing metal aggregation. The thermal stability of the functionalized systems is confirmed through ab initio molecular dynamics simulations at 300 K. Electronic structure and charge analysis, based on projected density of states and Bader charge calculations, indicate charge transfer values of 0.99, 0.99, and 0.85 electrons per dopant from Li, Na, and K to the C 5 N monolayer, respectively. This charge transfer leads to enhanced polarization, reduced bandgap, and improved H 2 adsorption performance. Each metal dopant can adsorb up to eight H 2 molecules, leading to a maximum storage capacity of 48 H 2 molecules per unit cell. The corresponding gravimetric densities are 9.42, 8.61, and 7.93 wt% for the Li‐, Na‐, and K‐functionalized C 5 N systems, respectively, with moderate average adsorption energies (−0.16 to −0.17 eV/H 2 ) that are suitable for reversible H 2 storage. Thermodynamic analysis under practical fuel‐cell operating temperature and pressure conditions further confirms the favorable H 2 adsorption/desorption behavior of the functionalized C 5 N systems. The reversibility of the designed materials is evaluated through desorption temperature and recovery time calculations, indicating efficient H 2 release kinetics. In addition, volumetric storage performance is assessed through density‐to‐volume ratio analysis. Overall, these results highlight alkali metal‐functionalized C 5 N monolayers as promising candidates for efficient and reversible H 2 storage applications.