Electronic and Hydrogen Storage Properties of Li-Terminated Linear Boron Chains Studied by TAO-DFT

Abstract

It has been extremely difficult for conventional computational approaches to reliably predict the properties of multi-reference systems (i.e., systems possessing radical character) at the nanoscale. To resolve this, we employ thermally-assisted-occupation density functional theory (TAO-DFT) to predict the electronic and hydrogen storage properties of Li-terminated linear boron chains (Li₂B_n), with n boron atoms (n = 6, 8, …, and 16). From our TAO-DFT results, Li₂B_n, which possess radical character, can bind up to 4 H₂ molecules per Li, with the binding energies in the desirable regime (between 20 and 40 kJ/mol per H₂). The hydrogen gravimetric storage capacities of Li₂B_n range from 7.9 to 17.0 wt%, achieving the ultimate goal of the United States Department of Energy. Accordingly, Li₂B_n could be promising media for storing and releasing H₂ at temperatures much higher than the boiling point of liquid nitrogen.

Figure 1

Structures of (a) linear boron chain (B₆), (b) Li-terminated linear boron chain (Li₂B₆), and (c–g) Li₂B₆ with x H₂ molecules (x = 1–5) adsorbed on each Li, obtained with TAO-BLYP-D. Here, pink, purple, and white balls represent B, Li, and H atoms, respectively. For the longer Li₂B_n (n = 8, 10, …, and 16), the H₂ adsorption patterns remain similar.

Figure 2

Singlet-triplet energy gap of Li₂B_n, obtained with TAO-BLYP-D.

Figure 3

Li binding energy on B_n, obtained with TAO-BLYP-D.

Figure 4

(a) Vertical ionization potential, (b) vertical electron affinity, and (c) fundamental gap for the ground state of Li₂B_n, obtained with TAO-BLYP-D.

Figure 5

Symmetrized von Neumann entropy for the ground state of Li₂B_n, obtained with TAO-BLYP-D.

Figure 6

Active orbital occupation numbers (H−5, …, H−1, H, L, L+1, …, and L+5) for the ground state of Li₂B_n, obtained with TAO-BLYP-D. For brevity, HOMO and LUMO are denoted as H and L, respectively.

Figure 7

Average hydrogen binding energy on Li₂B_n (n = 6, 8, …, and 16) with x H₂ molecules (x = 1–5) adsorbed on each Li, obtained with TAO-BLYP-D.

Figure 8

Binding energy of the y^th H₂ molecule (y = 1–5) on Li₂B_n (n = 6, 8, …, and 16), obtained with TAO-BLYP-D.

Figure 9

Li atomic charge for Li₂B_n (n = 6, 8, …, and 16) with x H₂ molecules (x = 0–5) adsorbed on each Li, obtained with TAO-BLYP-D. Here, the CHELPG scheme is employed to calculate the Li atomic charge.

Figure 10

Isosurfaces of charge density (the isovalue is 0.02 e/ų) for (a) B₆ and (b–g) Li₂B₆ with x H₂ molecules (x = 0–5) adsorbed on each Li, obtained with TAO-BLYP-D. Here, pink, purple, and white balls represent B, Li, and H atoms, respectively.