Interaction of H2 and He with metal atoms, clusters, and ions

Lithium-grafted Si-doped γ-graphyne as a reversible hydrogen storage host material

In recent years, hydrogen (H2) has become the most sought-after sustainable energy carrier by virtue of its high energy content and carbon-free emission. The practical implementation of hydrogen as an alternative fuel calls for an efficient and secure storage medium. Within this framework, we have investigated Li-grafted Si-doped γ-graphyne for H2 storage applications by implementing the cutting-edge density functional theory (DFT). A dynamically and thermally stable Si-doped γ-graphyne (SiG) monolayer is functionalized with Li metal atoms that augmented the hydrogen binding strength of the nanolayer by almost three times, owing to the polarization effect of the Li atoms. The Li metal atoms get adsorbed over the monolayer, allowing a binding energy of -2.73 eV that is greater than the Li cohesive energy (-1.63 eV), which eliminates the metal-metal clustering probability. The reliability of the Li-functionalized SiG monolayer (Li8SiG) at elevated temperature has been further substantiated by performing and analyzing ab initio molecular dynamics (AIMD) simulations at 400 K. It is noteworthy that a total of four H2 molecules are held up by each Li atom with an average adsorption energy of -0.32 eV and a maximum gravimetric capacity of 8.48 wt%, which remarkably follows the US-DOE parameters. Partial density of states and Hirshfeld charge analysis are utilized to recognize the interaction channel which reveals the Kubas and Niu-Rao-Jena-like bonding among the metal atoms and loaded hydrogen molecules. The hydrogen occupancy calculated at different temperatures and pressures indicates that hydrogen molecules can be reversibly stored over the Li8SiG system, and it is noted that adsorbed H2 begins to desorb at 280 K, with complete desorption at 400 K and 20 atm (or lower). AIMD simulations are further performed to authenticate the H2 desorption at various temperatures, which agrees well with the occupation number analysis. All the outcomes advocate for efficient reversible hydrogen storage over the proposed host material.

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

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.

Electronic Structure Calculations of Hydrogen Storage in Lithium-Decorated Metal-Graphyne Framework

Porous metal-graphyne framework (MGF) made up of graphyne linker decorated with lithium has been investigated for hydrogen storage. Applying density functional theory spin-polarized generalized gradient approximation with the Perdew-Burke-Ernzerhof functional containing Grimme's diffusion parameter with double numeric polarization basis set, the structural stability, and physicochemical properties have been analyzed. Each linker binds two Li atoms over the surface of the graphyne linker forming MGF-Li₈ by Dewar coordination. On saturation with hydrogen, each Li atom physisorbs three H₂ molecules resulting in MGF-Li₈-H₂₄. H₂ and Li interact by charge polarization mechanism leading to elongation in average H-H bond length indicating physisorption. Sorption energy decreases gradually from ≈0.4 to 0.20 eV on H₂ loading. Molecular dynamics simulations and computed sorption energy range indicate the high reversibility of H₂ in the MGF-Li₈ framework with the hydrogen storage capacity of 6.4 wt %. The calculated thermodynamic practical hydrogen storage at room temperature makes the Li-decorated MGF system a promising hydrogen storage material.

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.

Hydrogen sorption efficiency of titanium decorated calix[4]pyrroles

Ti decorated calix[4]pyrrole and octamethylcalix[4]pyrrole is explored as a potential H₂storage material.

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

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.

Theoretical study of the interaction between molecular hydrogen and [MC60]+ complexes

In this work we present a density functional theory study of the interaction between a positively charged exohedral metallofullerene and several hydrogen molecules.

A conceptual DFT study of the hydrogen trapping efficiency in metal functionalized BN system

We investigate the hydrogen trapping efficiency of various metals functionalized on BN systems for potential hydrogen storage applications using conceptual DFT’s stability and reactivity descriptors.

Theoretical Study of Hydrogen Storage in Ca-Coated Fullerenes

First principles calculations based on gradient corrected density functional theory and molecular dynamics simulations of Ca decorated fullerene yield some novel results: (1) C60 fullerene decorated with 32 Ca atoms on each of its 20 hexagonal and 12 pentagonal faces is extremely stable. Unlike transition metal atoms that tend to cluster on a fullerene surface, Ca atoms remain isolated even at high temperatures. (2) C60Ca32 can absorb up to 62 H2 molecules in two layers. The first 30 H2 molecules dissociate and bind atomically on the 60 triangular faces of the fullerene with an average binding energy of 0.45 eV/H, while the remaining 32 H2 molecules bind on the second layer quasi-molecularly with an average binding energy of 0.11 eV/H2. These binding energies are ideal for Ca coated C60 to operate as a hydrogen storage material at near ambient temperatures with fast kinetics. (3) The gravimetric density of this hydrogen storage material can reach 6.2 wt %. Simple model calculations show that this density is the limiting value for higher fullerenes.

Theoretical study on low-lying electronic states of NiH2

The low-lying electronic states of the NiH2 molecule were investigated by using the MCQDPT2 method. In order to accurately describe the strong correlation derived from the nickel 3d9 super-configuration, a set of diffuse secondary 3d' orbitals were included in the active space, yielding a large active space of 12 electrons in 13 orbitals. It is shown that the absolute minimum energy configuration of NiH2 is bent, in agreement with the experimental observation. The global ground state is 1A1 (or A1 in the spin-orbit coupling case), whereas the lowest linear state is 3Deltag (or 3g). Some other cheaper single-configurational and multi-configurational methods were also used to study both states, and their shortcomings are discussed. Our theoretical results suggest that the arrangement of the experimental frequencies of NiH2 and NiD2 may be incorrect.

Hydrogen storage and the 18-electron rule

We show that the 18-electron rule can be used to design new organometallic systems that can store hydrogen with large gravimetric density. In particular, Ti containing organic molecules such as C(4)H(4), C(5)H(5), and C(8)H(8) can store up to 9 wt % hydrogen, which meets the Department of Energy target for the year 2015. More importantly, hydrogen in these materials is stored in molecular form with an average binding energy of about 0.55 eV /H(2) molecule, which is ideal for fast kinetics. Using molecular orbitals we have analyzed the maximum number of H(2) molecules that can be adsorbed as well as the nature of their bonding and orientation. The charge transfer from the H(2) bonding orbital to the empty d(xy) and d(x(2)-y(2) ) orbitals of Ti has been found to be singularly responsible for the observed binding of the hydrogen molecule. It is argued that early transition metals are better suited for optimal adsorption/desorption of hydrogen.

Computational studies of molecular hydrogen binding affinities: the role of dispersion forces, electrostatics, and orbital interactions

Intermolecular interactions between H2 and ligands, metals, and metal-ligand complexes determine the binding affinities of potential hydrogen storage materials (HSM), and thus their extent of potential for practical use. A brief survey of current activity on HSM is given. The key issue of binding strengths is examined from a basic perspective by surveying the distinct classes of interactions (dispersion, electrostatics, orbital interactions) in first a general way, and then in the context of calculated binding affinities for a range of model systems.

The electronic structure of transition metal dihelide dications

Multi-reference configuration interaction (MRCI) calculations have been employed to characterize the low-lying states of first-row transition metal dihelide dications, He(2)TM(2+) (TM = Sc-Cu). The most important state-ordering principles were determined to be the occupation of the 4s orbital and orientation of the occupied 3d orbital. The ground states of all species are predicted to be of D(infinityh) symmetry arising from a 3d(n+1) electronic configuration. For excited states with singly occupied 4s or doubly occupied 3d(sigma) orbitals, bending to C(2v) symmetry typically lowers the energy and shortens the He-TM bond length. Coupled cluster singles and doubles with a perturbative treatment of triple excitations (CCSD(T)) results for ground state spectroscopic properties are in agreement with the MRCI predicted trends.

Electronic structure, reactivity, and spectroscopy of dihydrides of group-IB metals

Atomic pseudopotentials and highly correlated wave functions, including spin-orbit interactions, have been used to evaluate the electronic structure, stability, and spectroscopy of triatomic molecule MH(2), with a metal M belonging to group IB (Cu, Ag, and Au). CuH(2) and AuH(2) have been recently observed by IR spectroscopy in solid hydrogen and bending anharmonic wave numbers have been assigned to these two systems. The AgH(2) molecule has not been detected nor experimentally characterized, despite several theoretical works arguing on its stability. Our results confirm that the MH(2) radicals have a metastable bent ground state separated from the dissociation into [M+H(2)] ground state by barriers which have been evaluated to 1.43, 0.78, and 0.80 eV, for Cu, Ag, and Au compounds, respectively. These barriers are calculated smaller than in previous determinations but still large enough to stabilize the MH(2) systems. Spectroscopic data are calculated for these radicals.

Infrared spectra and structures of the stable CuH(2)(-), AgH(2)(-), AuH(2)(-), and AuH(4)(-) anions and the AuH(2) molecule

Gold is noble, but excited gold is reactive. Reactions of laser-ablated copper, silver, and gold with H(2) in excess argon, neon, and pure hydrogen during condensation at 3.5 K give the MH molecules and the (H(2))MH complexes as major products and the MH(2)(-) and AuH(4)(-) anions as minor products. These new molecular anions are identified from matrix infrared spectra with isotopic substitution (HD, D(2), and H(2) + D(2)) and comparison to frequencies calculated by density functional theory. The stable linear MH(2)(-) anions are unique in that their corresponding neutral MH(2) molecules are higher in energy than M + H(2) and thus unstable to M + H(2) decomposition. Infrared spectra are observed for the bending modes of AuH(2), AuHD, and AuD(2) in solid H(2), HD, and D(2), respectively. The observation of square-planar AuH(4)(-) attests the stability of the higher Au(III) oxidation state for gold. The synthesis of CuH(2)(-) in solid compounds has potential for use in hydrogen storage.