Li-functionalized hydroxy cycloparaphenylene nanohoops for reversible H₂ physisorption: A DFT study

This work presents a density functional theory investigation of H₂ adsorption on lithium-decorated hydroxyl-functionalized cycloparaphenylene (CPP-OH) nanohoops. The CPP-OH system was first optimized and subsequently coordinated with five and ten Li atoms, yielding [CPP-OH]5Li and [CP-OH]10Li complexes capable of adsorbing up to 15 and 30H₂ molecules, respectively, with adsorption energies ranging from -3.05 to -5.52 kcal/mol, consistent with physisorption-type interactions suitable for reversible storage. The corresponding gravimetric hydrogen storage capacities span 2.04-3.92 wt%, placing these Li-decorated CPP-OH systems within the range reported for other Li-functionalized carbon-based adsorbents. Natural bond orbital analysis reveals charge transfer from the σ(H-H) orbitals of H₂ toward acceptor orbitals on Li, confirming a donor-acceptor interaction between hydrogen molecules and lithium centers. In addition, very short desorption (recovery) times estimated from the computed energy barriers suggest fast adsorption/desorption kinetics, indicating that Li-decorated CPP-OH nanohoops are promising candidates not only as model hydrogen storage materials but also as reusable hydrogen gas-sensing platforms.