Effect of Li Adsorption on the Electronic and Hydrogen Storage Properties of Acenes: A Dispersion-Corrected TAO-DFT Study

Abstract

Due to the presence of strong static correlation effects and noncovalent interactions, accurate prediction of the electronic and hydrogen storage properties of Li-adsorbed acenes with n linearly fused benzene rings (n = 3-8) has been very challenging for conventional electronic structure methods. To meet the challenge, we study these properties using our recently developed thermally-assisted-occupation density functional theory (TAO-DFT) with dispersion corrections. In contrast to pure acenes, the binding energies of H2 molecules on Li-adsorbed acenes are in the ideal binding energy range (about 20 to 40 kJ/mol per H2). Besides, the H2 gravimetric storage capacities of Li-adsorbed acenes are in the range of 9.9 to 10.7 wt%, satisfying the United States Department of Energy (USDOE) ultimate target of 7.5 wt%. On the basis of our results, Li-adsorbed acenes can be high-capacity hydrogen storage materials for reversible hydrogen uptake and release at ambient conditions.

Figure 1

Structures of (a) pure 5-acene, (b) Li-adsorbed 5-acene, (c) Li-adsorbed 5-acene with one H₂ molecule adsorbed on each Li adatom, (d) Li-adsorbed 5-acene with two H₂ molecules adsorbed on each Li adatom, and (e) Li-adsorbed 5-acene with three H₂ molecules adsorbed on each Li adatom. Here, grey, white, and purple balls represent C, H, and Li atoms, respectively.

Figure 2. Singlet-triplet energy (ST) gap of pure/Li-adsorbed n-acene as a function of the chain length, calculated using TAO-BLYP-D.

The inset shows a close-up view for the ST gap of Li-adsorbed n-acene.

Figure 3. Li binding energy on n-acene as a function of the chain length, calculated using TAO-BLYP-D.

Figure 4. Vertical ionization potential for the lowest singlet state of pure/Li-adsorbed n-acene as a function of the chain length, calculated using TAO-BLYP-D.

Figure 5. Vertical electron affinity for the lowest singlet state of pure/Li-adsorbed n-acene as a function of the chain length, calculated using TAO-BLYP-D.

Figure 6. Fundamental gap for the lowest singlet state of pure/Li-adsorbed n-acene as a function of the chain length, calculated using TAO-BLYP-D.

Figure 7. Symmetrized von Neumann entropy for the lowest singlet state of pure/Li-adsorbed n-acene as a function of the chain length, calculated using TAO-BLYP-D.

Figure 8. Average H₂ binding energy on Li-adsorbed n-acene (n = 3–8) as a function of the number of H₂ molecules adsorbed on each Li adatom, calculated using TAO-BLYP-D.

Figure 9. Binding energy of the y^th H₂ molecule (y = 1–3) on Li-adsorbed n-acene (n = 3–8), calculated using TAO-BLYP-D.