Unlocking reversible hydrogen storage in penta-hexa-hepta graphene through lithium functionalization

A newly proposed two-dimensional carbon allotrope, penta–hexa–hepta graphene (PHH-Gr), is systematically investigated as a potential hydrogen storage medium using first-principles calculations. The thermodynamic, dynamical, and thermal robustness of PHH-Gr are verified through cohesive energy evaluation, phonon spectrum analysis, and ab initio molecular dynamics (AIMD) simulations. Lithium decoration is found to be energetically favorable, with Li atoms strongly anchored at heptagonal hollow sites and minimal clustering tendency due to large Li–Li separations and high diffusion barriers. Electronic structure analysis demonstrates substantial charge transfer from Li to the PHH-Gr substrate while preserving metallic characteristics. Hydrogen adsorption on pristine PHH-Gr is weak, whereas Li functionalization markedly enhances H 2 binding through polarization and charge-transfer effects. Each Li species can adsorb up to four hydrogen molecules, yielding a maximum gravimetric and volumetric hydrogen storage capacities of 10.75 wt% and 91.2 g L −1 , respectively. Thermodynamic analysis using differential adsorption energies reveals that while the maximum storage capacity requires cryogenic temperatures or elevated pressures, a practical working capacity of approximately 6–8 wt% is achievable under reversible cycling conditions (298 K, 30 bar for uptake; 373 K, 3 bar for release). AIMD simulations also verify that the weakly bound fourth H 2 molecule per Li site desorbs at room temperature under ambient pressure. These results highlight Li-decorated PHH-Gr as a robust and efficient material for reversible hydrogen storage applications with a working capacity exceeding DOE targets.