Effect of Li Termination on the Electronic and Hydrogen Storage Properties of Linear Carbon Chains: A TAO-DFT Study

Accurate prediction of the electronic and hydrogen storage properties of linear carbon chains (C _n ) and Li-terminated linear carbon chains (Li₂C _n ), with n carbon atoms (n = 5-10), has been very challenging for traditional electronic structure methods, due to the presence of strong static correlation effects. To meet the challenge, we study these properties using our newly developed thermally-assisted-occupation density functional theory (TAO-DFT), a very efficient electronic structure method for the study of large systems with strong static correlation effects. Owing to the alteration of the reactivity of C _n and Li₂C _n with n, odd-even oscillations in their electronic properties are found. In contrast to C _n , the binding energies of H₂ molecules on Li₂C _n are in (or close to) the ideal binding energy range (about 20 to 40 kJ/mol per H₂). In addition, the H₂ gravimetric storage capacities of Li₂C _n are in the range of 10.7 to 17.9 wt%, satisfying the United States Department of Energy (USDOE) ultimate target of 7.5 wt%. On the basis of our results, Li₂C _n can be high-capacity hydrogen storage materials that can uptake and release hydrogen at temperatures well above the easily achieved temperature of liquid nitrogen.