Hydrogen Adsorption on Alkali-Metal-Decorated g-SiC5 Siligraphene: A First-Principles Study

Abstract In this study, density functional theory calculations were used to evaluate pristine and alkali-metal-decorated g-SiC5 as molecular hydrogen adsorbents. Pristine g-SiC5 is dynamically stable along the sampled phonon path but binds H2 weakly, with an adsorption energy of −0.136 eV. Li, Na, and K decoration creates positively polarized adsorption centers that strengthen H2 binding without dissociation. Among the single-metal systems, Li@g-SiC5 provides the most consistent combination of metal binding, compact metal–H2 geometry, and moderate average adsorption energies. Each Li center accommodates up to four H2 molecules in the optimized configuration. Increasing the Li coverage produces optimized 8H2@2Li@g-SiC5, 12H2@3Li@g-SiC5, and 16H2@4Li@g-SiC5 configurations with average adsorption energies of −0.28, −0.25, and −0.24 eV/H2 and theoretical gravimetric capacities of 4.22, 6.08, and 7.82 wt %, respectively. Charge-density-difference and reduced-density-gradient analyses support polarization-assisted physisorption. The AIMD simulations performed over 5 ps indicate substantial retention of the adsorbed H2 molecules at 77 K and partial release at 300 K, while the g-SiC5 framework remains structurally intact at both temperatures. The thermodynamic model predicts a strong dependence of H2 uptake on temperature and pressure. For the fully loaded system, the characteristic van’t Hoff temperature increases from 306.95 K at 1 bar to 422.72 K at 12 bar, while the grand canonical independent-site model predicts 3.09–5.84 H2 molecules per cell at 300 K and 5–12 bar. Thus, the 7.82 wt % capacity represents the maximum optimized loading rather than the equilibrium uptake under all operating conditions.