Alkali/alkaline-earth metal functionalization of 8-16-4 graphyne: A DFT analysis of hydrogen storage efficiency

Abstract

Hydrogen fuel is critical for mitigating fossil fuel dependence. However, design of an optimized secure hydrogen storage system is still a significant challenge. This study computationally evaluates pristine and Li, Na, K, and Ca- decorated 8–16-4 graphyne (GY) for hydrogen storage using the DFT-D2 method. The GY substrate, featuring sp./sp 2 -hybridized carbons, Dirac points, and zero bandgap, exhibits unique electronic properties. To find the most stable metal-GY configurations, eleven different sites and various distances of metal atoms from the carbon frame were examined. The results show that the maximum binding energies of Li, Na, K, and Ca on the GY structure are − 4.327, − 3.763, − 4.004, and − 5.532 eV, respectively. Additionally, the first hydrogen adsorption energies on Li, Na, K, and Ca-decorated GY are − 0.243, − 0.413, − 0.108, and − 0.373 eV, respectively. The maximum number of adsorbed H2 molecules and capacities in mentioned conditions for Li, Na, and Ca-GY reached 6 (5.59 wt%), 9 (7.73 wt%), and 9 (7.20 wt%), respectively, so Na and Ca provide gravimetric capacities within 5–10 wt%, but at maximum loadings, their average adsorption energies fall below the 0.2–0.4 eV window; they are therefore promising at partial H₂ coverages but do not satisfy both criteria concurrently at full loading. In addition, K-GY failed to enhance H2 storage due to its poor performance. Ab initio molecular dynamics simulations confirmed the thermal stability of Li, Na, and Ca-GY structures at 300 K. These results improve the carbon substrate design for sustainable hydrogen storage applications.