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

Sonai Seenithurai,Jeng-Da Chai

doi:10.1038/s41598-017-05202-6

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.

Highlights

To meet the challenge, we study these properties using our newly developed thermally-assistedoccupation density functional theory (TAO-DFT), a very efficient electronic structure method for the study of large systems with strong static correlation effects
In 2015, the United States Department of Energy (USDOE) set the ultimate target of 7.5 wt% for the gravimetric storage capacities of onboard hydrogen storage materials for light-duty vehicles[5]
To obtain the ground state of Cn/Li2Cn (n = 5–10), spin-unrestricted TAO-BLYP-D calculations are performed for the lowest singlet and triplet energies of Cn/Li2Cn on the respective geometries that were fully optimized at the same level of theory

Summary

A TAO-DFT Study

We study these properties using our newly developed thermally-assistedoccupation density functional theory (TAO-DFT), a very efficient electronic structure method for the study of large systems with strong static correlation effects. To circumvent the formidable computational expense of high-level ab initio multi-reference methods, we have newly developed thermally-assisted-occupation density functional theory (TAO-DFT)[49,50,51] for the study of large ground-state systems (e.g., containing up to a few thousand electrons) with strong static correlation effects. Due to its computational efficiency and reasonable accuracy for large systems with strong static correlation, TAO-DFT has been successfully applied to the study of several strongly correlated electron systems at the nanoscale[17, 52,53,54], which are typically regarded as “challenging systems” for traditional electronic structure methods (e.g., KS-DFT with conventional XC density functionals and single-reference ab initio methods)[47].

Results and Discussion

H2 2 H2 3 H2 4 H2 1 H2 2 H2 3 H2 4 H2

Conclusions