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

Abstract

Accurate prediction of the electronic and hydrogen storage properties of linear carbon chains (Cn) and Li-terminated linear carbon chains (Li2Cn), 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 Cn and Li2Cn with n, odd-even oscillations in their electronic properties are found. In contrast to Cn, the binding energies of H2 molecules on Li2Cn 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 Li2Cn 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, Li2Cn can be high-capacity hydrogen storage materials that can uptake and release hydrogen at temperatures well above the easily achieved temperature of liquid nitrogen.

Fully numerical calculations on atoms with fractional occupations and range-separated exchange functionals

“…Fractional occupations have been shown to yield better results for strongly correlated systems [19][20][21][22][23]. However, fractional occupations can only be justified at the Fermi level [24], and more recently it has been shown that energy minimization naturally leads to integer occupations below the Fermi level, and possible fractional occupations at the Fermi level for independent particle models like HF and DFT [25].…”

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

“…As presented in Fig. 2 , Li 2 B n ( n = 6, 8, …, and 16) has a singlet ground state (i.e., similar to Li 2 C n 57 ). As n increases, E ST changes drastically, implying that the electronic properties of Li 2 B n can be properly tuned by changing the length of Li 2 B n .…”

“…To begin with, we perform spin-unrestricted TAO-BLYP-D calculations to obtain the lowest singlet and lowest triplet states of Li 2 B n ( n = 6, 8, …, and 16), with the respective geometries being fully relaxed 57 . Subsequently, we calculate the singlet-triplet energy gap of Li 2 B n as where E S and E T are the lowest singlet and lowest triplet energies, respectively, of Li 2 B n .…”

“…Although Li 2 C n and Li 2 B n look similar in structure, their electronic and hydrogen storage properties are distinctly different. In particular, Li 2 C n were found to exhibit oscillatory diradical behavior with increasing chain length 57 , while Li 2 B n exhibit increasing polyradical character with the increase of chain length (as will be seen below). Owing to its reasonable accuracy in predicting the properties of multi-reference systems at the nanoscale, we employ TAO-DFT to predict the electronic and hydrogen storage properties of Li 2 B n ( n = 6, 8, …, and 16) in the present study.…”

“…Moreover, TAO-DFT reduces to KS-DFT when the static correlation energy of a system is insignificant, enabling a well-balanced description for both systems possessing non-radical character and systems possessing radical character 53 – 57 . In our previous TAO-DFT studies, Li-adsorbed acenes 55 and Li-terminated linear carbon chains (Li 2 C n ) 57 were found to be promising HSMs at near-ambient conditions, showing that the search for promising HSMs can be extended to large systems possessing radical character. Although Li 2 C n and Li 2 B n look similar in structure, their electronic and hydrogen storage properties are distinctly different.…”