Stabilized Li-decoration and enhanced hydrogen storage on reduced graphene oxides

Hydrogen storage properties of Li-decorated graphene oxides containing epoxy and hydroxyl groups are studied by using density functional theory. The Li atoms form Li₄O/Li₃OH clusters and are anchored strongly on the graphene surface with binding energies of −3.20 and −2.84 eV. The clusters transfer electrons to the graphene substrate, and the Li atoms exist as Li⁺ cations with strong adsorption ability for H₂ molecules. Each Li atom can adsorb at least 2H₂ molecules with adsorption energies greater than −0.20 eV/H₂. The hydrogen storage properties of Li-decorated graphene at different oxidation degrees are studied. The computations show that the adsorption energy of H₂ is −0.22 eV/H₂ and the hydrogen storage capacity is 6.04 wt% at the oxidation ratio O/C = 1/16. When the O/C ratio is 1:8, the storage capacity reaches 10.26 wt% and the adsorption energy is −0.15 eV/H₂. These results suggest that reversible hydrogen storage with high recycling capacities at ambient temperature can be realized through light-metal decoration on reduced graphene oxides.     

A density functional theory and grand canonical Monte Carlo simulations study the hydrogen storage on the Li-decorated net-τ

To find ideal hydrogen storage media, hydrogen storage performance of Li decorated net-τ has been investigated by first-principles calculations. Maximum 6 Li atoms are adsorbed on net-τ, with the average binding energy of 2.15 eV for per Li atom. Based on 6Li-decorated net-τ, up to twenty H₂ molecules are adsorbed, with a high H₂ storage capacity of 12.52 wt% and an appropriate adsorption energy of 0.21 eV/H₂. Finally, H₂ uptake performance is measured by GCMC simulations. Our results suggest that Li-decorated net-τ may be a promising hydrogen storage medium under realistic conditions.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.     

Li decorated penta-silicene as a high capacity hydrogen storage material: A density functional theory study

The H₂ adsorption characteristics of Li decorated single-sided and double-sided penta-silicene are predicted via density functional theory (DFT). The orbital hybridization results in Li atom strongly bind onto the surface of the penta-silicene with a large binding energy and it keeps the decorated Li atoms from aggregation. Moreover, Li decorated double-sided penta-silicene can store up to 12H₂ molecules with the average hydrogen adsorption energy of ?0.220 eV/H₂ and hydrogen uptake capacity of 6.42 wt%, respectively. The ab initio molecular dynamics (AIMD) simulations demonstrate the H₂ molecules are released gradually from the substrate material with the increasing simulation time and the calculated desorption temperature T_D is 281 K in the suitable operating temperature range. Our explorations confirm that Li decorated penta-silicene can be regarded as a promising hydrogen storage candidate for hydrogen storage applications.     

Hydrogen adsorption on Li decorated graphyne-like carbon nanosheet: A density functional theory study

Motivated by novel graphyne-like carbon nanostructure C₆₈-GY, spin-polarized DFT calculations with dispersion-correction were performed to investigate the hydrogen adsorption capacity of Li decorated C₆₈-GY nanosheet. The binding energy between Li and C₆₈-GY was larger than the cohesive energy of bulk metal, indicating Li atoms would prefer to separately attached on C₆₈-GY. The ab initio molecular dynamics simulation has been performed to confirm the stability of Li/C complex. When five Li atoms decorated on C₆₈-GY, 14H₂ molecules were captured. The maximum hydrogen storage density was 8.04 wt% with an average hydrogen adsorption energy of −0.227 eV per H₂. The positively charged Li atoms aroused electrostatic field and induced the polarization of H₂. It was notable to observe strong hybridization between the main peak of H-1s orbitals with Li below Fermi level, which was responsible for the enhancement of hydrogen binding energy, indicating its potential application on hydrogen storage.     

Density functional theory study of reversible hydrogen storage in monolayer beryllium hydride by decoration with boron and lithium

The hydrogen storage capacity of M-decorated (M = Li and B) 2D beryllium hydride is investigated using first-principles calculations based on density functional theory. The Li and B atoms were calculated to be successfully and chemically decorated on the Surface of the α-BeH₂ monolayer with a large binding energy of 2.41 and 4.45eV/atom. The absolute value was higher than the cohesive energy of Li and B bulk (1.68, 5.81eV/atom). Hence, the Li and B atoms are strongly bound on the beryllium hydride monolayer without clustering. Our findings show that the hydrogen molecule interacted weakly with B/α-BeH₂(B-decorated beryllium hydride monolayer) with a low adsorption energy of only 0.0226 eV/H₂ but was strongly adsorbed on the introduced active site of the Li atom in the decorated BeH₂ with an improved adsorption energy of 0.472 eV/H₂. Based on density functional theory, the gravimetric density of 28H₂/8li/α-BeH₂) could reach 14.5 wt.% higher than DOE's target of 6.5 wt. % (the criteria of the United States Department of Energy). Therefore, our research indicates that the Li-decorated beryllium hydride monolayer could be a candidate for further investigation as an alternative material for hydrogen storage.     

Computational insights of alkali metal (Li / Na / K) atom decorated buckled bismuthene for hydrogen storage

The mechanism of hydrogen molecule adsorption on 2D buckled bismuthene (b-Bi) monolayer decorated with alkali metal atoms was studied using density functional theory based first principles calculations. The decorated atoms Li, Na and K exhibited distribution on surface of b-Bi monolayer with increasing binding energy of 2.6 eV, 2.9 eV and 3.6 eV respectively. The adsorption of H₂ molecule on the slabs appeared stable which was further improved upon inclusion of van der Waals interactions. The adsorption behaviour of H₂ molecules on the decorated slabs is physisorption whereas the slabs were able to bind up to five H₂ molecules. The average adsorption energy per H₂ molecules are in range of 0.1–0.2 eV which is good for practical applications. The molecular dynamics simulation also confirmed the thermodynamic stabilities of five H₂ molecules adsorbed on the decorated slabs. The storage capacity values are found 2.24 wt %, 2.1 wt %, and 2 wt %, for respective cases of Li, Na and K atoms decorated b-Bi. The analysis of the adsorbed cases pointed to electrostatic interaction of Li and H₂ molecule. The adsorption energies, binding energies, charge analysis, structural stability, density of states, and hydrogen adsorption percentage specifies that the decorated b-Bi may serve as an efficient hydrogen storage material and could be an effective medium to interact with hydrogen molecules at room temperature.     

A DFT study of H2 adsorption on lithium decorated 3D hybrid Boron-Nitride-Carbon frameworks

Based on density functional theory (DFT) and first-principles molecular dynamics (MD),a new 3D hybrid Boron-Nitride-Carbon–interconnected frameworks (BNCIFs) consisting of organic linkers with Li decoration is created and optimized. Firstly, Li adsorption behaviors on several BNC_xcomplexes are investigated and compared systematically. The results indicate C substitution of N atom in pure BN layer could improve the metal binding energy effectively. Secondly, the BNC layer (BNC_NN) is chosen to model the frameworks of BNCIFs. The average binding energy of adsorbed Li atoms on BNCIFs is 3.6 eV which is much higher than the cohesive energy of bulk Li and avoids the Li clustering problem. Finally, we study the H₂ adsorptions on the Li decorated BNCIFs by DFT. Every Li atom could adsorb four H₂ molecules with an average binding energy of 0.24 eV. The corresponding gravimetric and volumetric storage capacities are 14.09 wt% and 126.2 g/L respectively overpassing the published 2020 DOE target. The excellent thermal stability of 160H₂@40Li@BNCIFs is also proved by MD. This nanostructure could be served as a promising hydrogen storage medium at ambient conditions.     

Y-decorated MoS2 monolayer for promising hydrogen storage: A DFT study

The potential hydrogen storage performance of the constructed Y-decorated MoS₂ was investigated via first-principles density functional theory (DFT) calculations. The Y could be stably decorated on the MoS₂ monolayer with adsorption energy being ?4.82 eV, the absolute value of which was higher than the cohesive energy of bulk Y. The introduced H₂ interacted strongly with the Y-decorated MoS₂ with an elongated bond length and reasonable adsorption energy being 0.792 Å and ?0.904 eV, respectively. There would be four H₂ in maximum adsorbed and stored on the Y-decorated MoS₂ with average adsorption energy being ?0.387 eV. Moreover, the hydrogen gravimetric capacity of the MoS₂ with full Y coverage on each side could be improved to be 4.56 wt% with average adsorption energy being ?0.295 eV. Our study revealed that the MoS₂ decorated with Y could be a potential material to effectively store H₂ with promising gravimetric density.     

Density functional theory study on hydrogen storage capacity of yttrium decorated graphyne nanotube

The hydrogen storage capacity of yttrium decorated graphyne nanotubes is calculated using spin polarized DFT method. The stabilities, electronic properties and the structures of Y attachment on graphyne tube are investigated. It is revealed that Y can be separately adsorbed on graphyne tube with the binding energy of 6.76 eV and the clustering of metal atoms is hindered. The geometry optimization shows that Y atoms decorated graphyne tube can capture 42H₂ molecules through Dewar-Kubas like interaction and the polarization under the electrostatic potential formed by Y and graphyne tubes. The weight percent capacity is 5.71 wt%, with an average hydrogen adsorption energy of −0.153 eV per H₂, indicating its potential application on hydrogen storage candidates.     

A comparative study of the reversible hydrogen storage behavior in several metal decorated graphyne

In virtue of the first-principle calculations, the hydrogen storage behavior in several metal decorated graphyne was investigated. It is found that the hydrogen storage capacity can be as large as 18.6, 10.5, 9.9 and 9.5 wt% with average adsorption energy of about −0.27, −0.36, −0.76 and −0.70 eV/H₂ for Li, Ca, Sc, Ti decorated graphyne, respectively. The results suggest potential candidates for hydrogen storage at ambient condition. The adsorption mechanism for H₂ on metal coated graphyne was mainly attributed to the polarization induced by electrostatic field of metal atoms on graphyne and the hybridization between the metal atoms and hydrogen molecules. Furthermore, the formation of super-molecules of hydrogen can enhance the adsorption energy.     

Li decorated 6,6,12-graphyne: A new star for hydrogen storage material

Based on ab initio calculations, we investigated the hydrogen storage capacity of Li decorated 6,6,12-graphyne (Li@GY). Due to the unique sp hybridization in GY, Li atoms can strongly bind to carbon atoms to avoid the formation of Li clusters on the surface of GY. It is found that the hydrogen storage capacity of Li@GY is high up to 19.3 wt% with the average adsorption energy of −0.230 eV which lying in the ideal adsorption energy range for practical application of hydrogen economy. The density of states and charge density difference demonstrated that the adsorption mechanism mainly depended on the electrostatic field produced by the Li ions on GY. Moreover, the formation of super hydrogen molecules induced by the electrostatic field around Li ions can further enhance the hydrogen absorption energy. Our results indicated that Li decorated 6,6,12-graphyne would be a potential material for hydrogen storage.     

Enhancement of hydrogen storage properties of metal-organic framework-5 by substitution (Zn,Cd and Mg) and decoration (Li,Be and Na)

Two strategies of decoration by three elements Z = Li, Be and Na in cyclic site, and substitution of Zn by Mg and Cd in unit cell were used in the framework of functional density theory to tune the hydrogen storage properties of metal-organic framework-5 (MOF-5) based on Zn whose decomposition temperature and initial gravimetric capacity are 300 K and 1.57 wt% respectively.Based on the adsorption of hydrogen molecules in the crystal surface at three different adsorption sites with three orientations of H₂, we show that our system may reach a maximum gravimetric storage capacity of 4.09 wt% for multiple hydrogen molecules. Moreover, the functionalization of Z combined to the substitution, shows an exceptional improvement of hydrogen storage properties. For example, Mg-MOF-5 decorated with Li₂ has a capacity up to 5.41 wt% and 513 K as desorption temperature.     

High-capacity hydrogen storage in zirconium decorated zeolite templated carbon: Predictions from DFT simulations

The hydrogen (H₂) storage capacity of Zirconium (Zr) decorated zeolite templated carbon (ZTC) has been investigated using sophisticated density functional theory (DFT) simulations. The analysis shows that the Zr atom gets bonded with ZTC strongly with binding energy (BE) of ?3.92 eV due to electron transfer from Zr 4d orbital to C 2p orbital of ZTC. Each Zr atom on ZTC can attach 7H₂ molecules with average binding energy of ?0.433 eV/H₂ providing gravimetric wt% of 9.24, substantially above the limit of 6.5 wt% set by the DoE of the United States of America. The H₂ molecules are involved via Kubas interaction with Zr atom, which involves the charge transfer between Zr 4d orbital and H 1s orbital with interaction energy higher than physisorption but lower than chemisorption. The structural integrity of the system is confirmed via molecular dynamics (MD) simulations at room temperature and at highest desorption temperature of 500 K. We have investigated the chances of metal clustering by computing diffusion energy (E_D) barrier for the movement of Zr atom, and we obtained via calculation, we can infer that the presence of E_D barrier of ～2.36 eV may prevent the possibility. As the system ZTC has been synthesized, Zr doped ZTC is stable, existence of sufficient diffusion barrier prevents the clustering and adsorption energy and wt% of H₂ are within the range prescribed by DoE, we feel that Zr decorated ZTC can be fabricated as promising hydrogen storage material for fuel cell applications.     

Effect of nitrogen induced defects in Li dispersed graphene on hydrogen storage

As a candidate for hydrogen storage medium, Li decorated graphene with experimentally realizable nitrogen defects was investigated for geometric stability and hydrogen capacity using density functional theory (DFT) calculations. Among the three types of defective structures, it is expected that Li metal atoms are well dispersed on the graphene sheets with pyridinic and pyrrolic defects without clustering as the bond strength of Li on pyridinic and pyrrolic N-doped graphene layers is higher than the cohesive energy of the Li metal bulk. The two stable structures were found to exhibit hydrogen uptake ability up to three H₂ per Li atom. The binding energies of the hydrogen molecules for these structures were in the range of 0.12–0.20 eV/H₂. These results demonstrate that a Li/N-doped graphene system could be used as a hydrogen storage material.     

Al-decorated carbon nanotube as the molecular hydrogen storage medium

Al-decorated, single-walled carbon nanotube has been investigated for hydrogen storage applications by using Density Functional Theory (DFT) based calculations. Single Al atom-decorated on (8,0) CNT adsorbs upto six H₂ molecules with a binding energy of 0.201 eV/H₂. Uniform decoration of Al atom is considered for hydrogen adsorption. The first Al atom has a binding energy of 1.98 eV on (8,0) CNT and it decreases to 1.33 eV/Al and 0.922 eV/Al respectively, when the number of Al atoms is increased to four and eight. Each Al atom in (8,0) CNT-8Al adsorbs four H₂ molecules, without clustering of Al atoms, and the storage capacity reaches to 6.15 wt%. This gravimetric storage capacity is higher than the revised 2015 target of U.S Department of Energy (DOE). The average adsorption binding energy of H₂ in (8,0) CNT-8(Al+4H₂), i.e. 0.214 eV/H₂, lies between 0.20 and 0.60 eV/H₂ which is required for adsorbing and desorbing H₂ molecules at near ambient conditions. Thus, Al-decorated (8,0) CNT is proposed as a good hydrogen storage medium which could be useful for onboard automobile applications, at near ambient conditions.     

Metal adatoms-decorated silicene as hydrogen storage media

Hydrogen storage properties of 10 different adatom decorated silicene are carried out using density functional theory calculations with long-range van der Waals dispersion correction. It is found that the binding energy between metal adatoms and the silicene is greater than the cohesive energy of bulk metal so that clustering of adatom will not occur once it is bonded with silicene. The adsorption of H₂ on Li, Na, K, Mg, Ca, and Au decorated silicene is a weak physisorption. Differently, a weaker chemisorption is responsible for the adsorption of H₂ on Be, Sc, Ti, and V decorated silicene. In particular, silicene with Na, K, Mg, and Ca decorating on both sides leads to 7.31–9.40 wt% hydrogen storage capacity with desirable adsorption energy, indicating that the metal-decorated silicene can serve as a high capacity hydrogen storage medium.     

Electronic structure and hydrogen storage properties of Li–decorated single layer blue phosphorus

Single layer blue phosphorus (SLBP) is a promising two–dimensional material for nanoelectronic devices, but the electronic structure and hydrogen storage property of modified SLBP received little attention. Li atoms can be strongly bonded on SLBP in a 1:1 Li/P ratio with a binding energy larger than the cohesive energy of bulk Li. The geometric structure of SLBP suggests the 3s3p orbitals of the P atom hybridize in sp³ manner. But our analyses show that the 3s and 3p orbitals form bonding and antibonding orbitals respectively. The 3s orbitals are fully occupied as they have much lower energies, and the bonding orbitals formed by P 3p are occupied in pure SLBP. The decorated Li atoms transfer their 2s electrons to the antibonding orbital formed by P 3p. The Li atoms exist as +1 cations and they are ionically bonded on SLBP. H₂ molecules adsorbed on the Li⁺ cations are strongly polarized and form strong adsorption. When two H₂ are adsorbed on each Li atom decorated at the 1:1 Li/P ratio, the hydrogen storage capacity reaches 9.52 wt% but the H₂ molecules are arranged in two layers with the adsorption energy ?0.168 eV/H₂. When the Li atoms are decorated alternatively on the two sides of the P₆ rings with a Li/P ratio of 1:2, each Li atom can absorb two H₂ molecules in a single–layer; the hydrogen storage capacity is 5.48 wt% and the adsorption energy reaches ?0.227 eV/H₂. These results mean the Li–decorated SLBP can work at ambient temperature with high reversible hydrogen storage capacity.     

High-capacity hydrogen storage in Li-decorated (AlN)n (n = 12, 24, 36) nanocages

The capability of Li-decorated (AlN)_n (n = 12, 24, 36) nanocages for hydrogen storage has been studied by using density functional theory (DFT) with the generalized gradient approximation (GGA). It is found that each Al atom is capable of binding one H₂ molecule up to a gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy of 0.189, 0.154, and 0.144 eV/H₂ in the pristine (AlN)_n (n = 12, 24, 36) nanocages, respectively. Further, we find that Li atoms can be preferentially decorated on the top of N atoms in (AlN)_n (n = 12, 24, 36) nanocages without clustering, and up to two H₂ molecules can bind to each Li atom with an average binding energy of 0.145, 0.154, 0.102 eV/H₂ in the Li_n(AlN)_n (n = 12, 24, 36) nanocages, respectively. Both the polarization of the H₂ molecules and the hybridization of the Li-2p orbitals with the H-s orbitals contribute to the H₂ adsorption on the Li atoms. Thus, the Li-decorated (AlN)_n (n = 12, 24, 36) nanocages can store hydrogen up to 7.7 wt%, approaching the U.S. Department of Energy (DOE) target of 9 wt% by the year 2015, and the average binding energies of H₂ molecules lying in the range of 0.1–0.2 eV/H₂ are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. It is also pointed out that when allowed to interact with each other, the agglomeration of Li-decorated (AlN)_n nanocages would lower the hydrogen storage capacity.