Metal-Decoration-Free Li3C2 Monolayer with Heptacoordinate Carbons as a Promising Hydrogen Storage Medium

Two-dimensional (2D) materials have been intensively studied as potential hydrogen storage mediums, because of the chemical stability, large specific surface area, and light weight. Here, a combination of crystal structure prediction and first-principles calculations predicts a metal-decoration-free Li3C2 monolayer that exhibits a hydrogen storage capacity of 8.2 wt % and high average hydrogen desorption temperature of 286 K. Li3C2 monolayer is constructed by two hexagonal Li layers sandwiched with Li-C layers containing a C dimer. Interestingly, each C atom binds with six neighboring Li atoms and one C atom, forming a quasi-planar heptacoordinate carbon moiety. Li3C2 monolayer can adsorb up 2 H-2 molecules with the average adsorption energy of 0.22 eV/H-2, suitable for reversible hydrogen storage. Analysis suggests that the hybridization of Li sp and H s orbitals plays a key role in hydrogen adsorption. Moreover, the exploration of the effect of temperature and pressure on hydrogen storage performance of Li3C2 monolayer shows that, at room temperature, all adsorbed hydrogen structures are stable at pressure of 40 bar. The calculated van't Hoff desorption temperatures and molecular dynamics simulations demonstrate the realization of hydrogen desorption at near room temperature and the effectiveness of hydrogen storage/release cycles. In addition, we provide a possible route to experimentally grow Li3C2 monolayer on suitable substrates, such as the Cu(111) surface and graphene.