Reversible Hydrogen Storage Media by g-CN Monolayer Decorated with NLi₄: A First-Principles Study

A two-dimensional graphene-like carbon nitride (g-CN) monolayer decorated with the superatomic cluster NLi₄ was studied for reversible hydrogen storage by first-principles calculations. Molecular dynamics simulations show that the g-CN monolayer has good thermal stability at room temperature. The NLi₄ is firmly anchored on the g-CN monolayer with a binding energy of -6.35 eV. Electronic charges are transferred from the Li atoms of NLi₄ to the g-CN monolayer, mainly due to the hybridization of Li(2s), C(2p), and N(2p) orbitals. Consequently, a spatial local electrostatic field is formed around NLi₄, leading to polarization of the adsorbed hydrogen molecules and further enhancing the electrostatic interactions between the Li atoms and hydrogen. Each NLi₄ can adsorb nine hydrogen molecules with average adsorption energies between -0.152 eV/H₂ and -0.237 eV/H₂. This range is within the reversible hydrogen storage energy window. Moreover, the highest achieved gravimetric capacity is up to 9.2 wt%, which is superior to the 5.5 wt% target set by the U.S. Department of Energy. This study shows that g-CN monolayers decorated with NLi₄ are a good candidate for reversible hydrogen storage.