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.