First-principles predictions of potential hydrogen storage materials: Novel sandwich-type ethylene dimetallocene complexes

The hydrogen storage capacities of a sandwich-type ethylene dimetallocene complex (Cp₂Ti₂C₂H₄) are studied using first-principles calculations. It is found that the TiC₂H₄Ti molecule can intercalate into the two cyclopentadienyl (Cp) rings and form a stable sandwich-type complex. Each Ti atom can adsorb a maximum of three H₂ molecules, which corresponds to a gravimetric storage capacity of 4.73 wt%. This hydrogen storage capacity is close to the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, the Cp₂Ti₂C₂H₄ molecule proposed in this paper is favorable for both adsorption and desorption of hydrogen molecules at room temperature and ambient pressure because its average binding energy of 0.34 eV/H₂.     

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.     

Vibrational spectra of Ti:C2H4(nH2) and Ti:C2H4(nD2) (n = 1-5) complexes and the equilibrium isotope effect: Calculations and experiment

Calculations of the ability of titanium-ethylene complexes of the type, Ti:C₂H₄, to absorb molecular hydrogen have been performed using density functional theory. A maximum of 5H₂ molecules can be adsorbed on Ti:C₂H₄ thereby giving an uptake capacity of 11.72 wt%, in excellent agreement with previous experimental results reported by two of us (Phys. Rev. Lett., 100, 105505, 2008). Calculations of the vibrational frequencies in such complexes with both H₂ and D₂, Ti:C₂H₄(nH₂) and Ti:C₂H₄(nD₂), n = 1-5, have also been performed and the values obtained used to find the Equilibrium Isotope Effect (EIE). Measurements of the EIE are also reported and these are in excellent agreement with the EIE calculated for 5H₂ molecules adsorbed in the complex.     

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.     

Ti decorated heterocyclic rings for hydrogen storage

This study uses first-principles calculations to investigate and compare the hydrogen storage properties of Ti doped benzene (C₆H₆Ti) and Ti doped borazine (B₃N₃H₆Ti) complexes. C₆H₆Ti and B₃N₃H₆Ti complex each can adsorb four H₂ molecules, but the former has a 0.11 wt% higher H₂ uptake capacity than the latter. Ti atoms bind to C₆H₆ more strongly than B₃N₃H₆. The hydrogen adsorption energies with Gibbs free energy correction for C₆H₆Ti and B₃N₃H₆Ti complexes are 0.17 and 0.45 eV, respectively, indicating reversible hydrogen adsorption. The hydrogen adsorption properties of C₆H₆Ti have also been studied after boron (B) and nitrogen (N) atom substitutions. Several B and N substituted structures between C₆H₆Ti and B₃N₃H₆Ti with different boron and nitrogen concentration and at different positions were considered. Initially, one boron and one nitrogen atom is substituted for two carbon atoms of benzene at three different positions and three different structures are obtained. Seven structures are possible when four carbon atoms of benzene are replaced by two boron and two nitrogen atoms at different positions. The hydrogen storage capacity of the C₆H₆Ti complex increases as boron and nitrogen atom concentrations increases. The positions of substituted boron and nitrogen atoms have less impact on H₂ uptake capacity for the same B and N concentration. The position and concentration of B and N affects the H₂ adsorption energy as well as the temperature and pressure range for thermodynamically favorable H₂ adsorption. The H₂ desorption temperature for all the complexes is found to be higher than 250 K indicates the stronger binding of H₂ molecules with these complexes.     

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H₂. When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H₂ which shows that desorption would take place at ambient conditions.     

Functionalized tetrahedral silsesquioxane cages for hydrogen storage

The hydrogen storage capacity of functionalized Tetrahedral Silsesquioxane (H₄Si₄O₆) cages is obtained using density functional theory (M062X) and second order Møller-Plesset (MP2) method with 6-311++G7 basis set. We labelled Tetrahedral Silsesquioxane (H₄Si₄O₆) as ‘TS’. We replaced four hydrogen in TS one by one with C₂HBe or C₂HTi group and labelled as TSR₁M_1, TSR₂M₂ TSR₃M₃ and TSR₄M₄ where RM can be either C₂HBe or C₂HTi. In TSRM when one hydrogen in a cage is replaced by C₂HBe or C₂HTi maximum of two and five hydrogen molecules, get adsorbed per Be and Ti atom respectively with respective H₂ capacity of 1.61 and 3.42 wt %. H₂ uptake capacity of TSR_mM_m (m = 1, 2, 3 and 4) has increased extensively when all the hydrogen in cage are replaced either C₂HBe or C₂HTi. TSR₄M₄ with RM = C₂HTi can adsorbs maximum of 20H₂ molecules with highest H₂ uptake of 7.46 wt % among all the studied complexes. Calculated Gibbs free energy corrected H₂ adsorption energies show that adsorption of H₂ molecules on all the complexes is thermodynamically favourable. The desorption temperature for the complexes were calculated by using the van't Hoff equation. Calculated interaction energies show that H₂ molecules interact strongly with Be atom than Ti atom. The molecular dynamics (MD) simulations have also been performed using atom centered density matrix propagation (ADMP) at ambient conditions. Interaction of hydrogen molecules and the metal atom is confirmed through the density of states (DOS) plot.     

Study of the reversible hydrogen storage performance of Ti-decorated hexagonal B36 by DFT calculations with van der Waals corrections

In this work, we report on the study of the hydrogen storage capability of titanium (Ti) decorated B₃₆ nanosheets using density functional theory (DFT) calculations with van der Waals corrections. Ti atoms are strongly bonded to the surface of B₃₆ with a binding energy of 6.23 eV, which exceeds the bulk cohesive energy of crystalline Ti. Ti-decorated B₃₆ (2Ti@B₃₆) can reversibly adsorb up to 12 H₂ molecules with a hydrogen storage capacity of 4.75 wt % and average adsorption energy between 0.361 and 0.674 eV/H₂. The values of desorption temperature and the results of molecular dynamics simulations enable to conclude that 2Ti@B₃₆ is a perspective reversible material for hydrogen storage under real conditions.     

High capacity and reversible hydrogen storage on two dimensional C2N monolayer membrane

Searching advanced materials with high capacity and efficient reversibility for hydrogen storage is a key issue for the development of hydrogen as a clean energy. Here, we have explored the potential application of C₂N monolayer using as a promising material for hydrogen storage through a comprehensive density functional theory (DFT) investigation. Our calculational results indicate that hydrogen molecule can only form weak interaction on neutral C₂N monolayer with the adsorption energy of 0.06 eV. However, if extra charges (5 e^?) are introduced to the system, the adsorption energy of hydrogen molecule on C₂N will be dramatically enhanced to 0.27 eV. Moreover, once the extra charges are moved from the system, the adsorbed hydrogen molecule will be spontaneously released from C₂N monolayer without any barrier. Interestingly, the average adsorption energy for each of the 48 absorbed H₂ molecules is 0.28 eV with the charge injection (8 e^?). This adsorption energy meets the criterion of the Department of Energy (DOE) for hydrogen storage (0.2–0.6 eV). Moreover, C₂N has a high hydrogen storage capacity of 10.5 wt %. Overall, this investigation demonstrates that the new fabricated C₂N can be used as an efficient material for hydrogen storage with high capacity and reversibility by modifying the charges that it carried. The narrow band gap (1.70 eV) of C₂N also ensures the electrochemical methods can be easily realized in experiment.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

Selective decoration of nitrogenated holey graphene (C2N) with titanium clusters for enhanced hydrogen storage application

For an envisioned hydrogen (H₂) economy, the design of new multifunctional two-dimensional (2D) materials have been a subject of intense research for the last several decades. Here, we report the thriving H₂ storage capacity of 2D nitrogenated holey graphene (C₂N), functionalized with Ti_n (n = 1–5) clusters. By using spin polarized density functional theory (DFT) calculations implemented with the van der Waals corrections, the most favourable adsorption site for the Ti_n clusters on C₂N has been revealed. With the monomer Ti, the functionalization was evenly covered on C₂N having 5% doping concentration (C₂N–Ti). For C₂N–Ti sheet, Ti binds to C₂N with a strong binding energy of ~6 eV per Ti which is robust enough to hinder any Ti–Ti clustering. Bader charge analysis reveals that the Ti_n clusters donate significant charges to C₂N sheet and become cationic to polarize the H₂ molecules, thus act as efficient anchoring agents to adhere multiple H₂ molecules. Each Ti in C₂N–Ti could adsorb a maximum of 10H₂ molecules, with the adsorption energies in the range of ?0.2 to ?0.4 eV per H₂ molecule, resulting into a high H₂ storage capacity of 6.8 wt%, which is promising for practical H₂ storage applications at room temperature. Furthermore, Ti_m (m = 2, 3, 4, 5) clusters have been selectively decorated over C₂N. However, with Ti_m functionalization H₂ storage capacities fall short of the desirable range due to large molecular weights of the systems. In addition, the H₂ desorption mechanism at different conditions of pressure and temperature were also studied by means of thermodynamic analysis that further reinforce the potential of C₂N–Ti as an efficient H₂ storage material.     

Li decorated heteroborospherene C4B32 as high capacity and reversible hydrogen storage media: A DFT study

In this work, adsorption of H₂ molecules on heteroborospherene C_2v C₄B₃₂ decorated by alkali atoms (Li) is studied by density functional theory calculations. The interaction between Li atoms and C₄B₃₂ is found to be strong, so that it prevents agglomeration of the former. An introduced hydrogen molecule tilts toward the Li atoms and is stably adsorbed on C₄B₃₂. It is obtained that Li₄C₄B₃₂ can store up to 12H₂ molecules with hydrogen uptake capacity of 5.425 wt% and average adsorption energy of ?0.240 eV per H₂. Dynamics simulation results show that 6H₂ molecules can be successfully released at 300 K. Obtained results demonstrate that Li decorated C₄B₃₂ is a promising material for reversible hydrogen storage.     

Li decorated C9N4 monolayer as a potential material for hydrogen storage

Based on first−principles calculations, we investigate the possibility of the two-dimensional porous C₉N₄ material as for hydrogen storage, and find that the adsorption energy of H₂ molecules on the pristine C₉N₄ is too weak to meet the requirements of hydrogen storage, whereas the adsorption on the Li−decorated sheet is relatively moderate. Each C₉N₄ unit cell can incorporate 6 Li atoms, of which 3 Li atoms are located above the intrinsic hole and the others are below. The unit cell can hold 14 hydrogen molecules with an average adsorption energy of −0.12 eV, which meets the reversible storage condition of hydrogen, and the gravity density reaches 7.04 wt%. Particularly, 6Li@C₉N₄ maintains excellent H₂ storage performance under a tensile strain within 2%. The ab initio MD simulations performed at 300 K show that all 14 H₂ molecules remained on the double sides of 6Li@C₉N₄ in the absence and presence of strain. Therefore, we predict that Li−modified C₉N₄ could be a potential material with excellent ductility for hydrogen storage at room temperature.     

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.     

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.     

Hydrogen storage of dual-Ti-doped single-walled carbon nanotubes

The hydrogen adsorption capacity of dual-Ti-doped (7, 7) single-walled carbon nanotube (Ti-SWCNTs) has been studied by the first principles calculations. Ti atoms show different characters at different locations due to local doping environment and patterns. The dual-Ti-doped SWCNTs can stably adsorb up to six H₂ molecules through Kubas interaction at the Ti₂ active center. The intrinsic curvature and the different doping pattern of Ti-SWCNTs induce charge discrepancy between these two Ti atoms, and result in different hydrogen adsorption capacity. Particularly, eight H₂ molecules can be adsorbed on both sides of the dual-Ti decorated SWCNT with ideal adsorption energy of 0.198 eV/H₂, and the physisorption H₂ on the inside Ti atom has desirable adsorption energy of 0.107 eV/H₂, ideal for efficient reversible storage of hydrogen. The synergistic effect of Ti atoms with different doping patterns enhances the hydrogen adsorption capacity 4.5H₂s/Ti of the Ti-doped SWCNT (VIII), and this awaits experimental trial.     

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.     

Enhancement of hydrogen binding affinity with low ionization energy Li2F coating on C60 to improve hydrogen storage capacity

The capacity of hydrogen storage for solid sorbents depends strongly on the binding affinity between hydrogen molecules and solid sorbents. By coating C₆₀ with a low ionization energy material (Li₂F), we obtained an enhanced binding energy and an improved electron transfer between H₂ and hosts. With the first-principles calculations and charge analysis, we found that the orbital interactions play a dominant role in this system and eventually 68H₂ molecules can be stably stored by a C₆₀(Li₂F)₁₂ cluster with a binding energy of 0.12 eV/H₂. The resulting gravimetric and volumetric density of H₂ stored on C₆₀(Li₂F)₁₂ are 10.86 wt% and the 59 g/L through calculations. Our investigation indicates that metals or metal clusters with lower ionization energies would be beneficial to enhance interactions between hydrogen and hosts, and thus, the hydrogen storage capacities for solid sorbents can be greatly improved.     

Complex interaction of hydrogen with the monolayer TiS2 decorated with Li and Li2O clusters: an ab initio random structure searching approach

We have applied ab initio random structure searching to study the structure, stability and hydrogen storage properties of monolayer TiS₂ coated with Li and small Li₂O clusters. For the low Li covered system we found a complex adsorption mechanism: some hydrogen molecules were adsorbed due to polarization with Li, others due to polarization with S near the surface of TiS₂. The peculiarities of the interaction of the H₂ molecules with each other and the preferred adsorption sites allowed us to formulate a series of recommendations that can be useful when selecting the material for the most effective support. Moreover, the findings also show that the storage capacity of this system can reach up to 9.63 wt%, presenting a good potential as hydrogen storage material. As for the Li₂O clusters supported on TiS₂, we found that the polarization of the Li–O bond increases upon the adsorption of the Li₂O nanocluster. Moreover, the polarized Li–S bonds appear in addition to the already existing Li–O bonds. All this is possible due to the extraction of 1.46 electrons from the S atom of the substrate by O atom of the cluster, and should contribute to an increase in both the adsorption energy and the maximum capacity. The adsorption energies of H₂ for the systems studied here are within 0.11–0.16 eV/H₂ which is a recommended range for reversible hydrogen physisorption under standard test conditions. This study may stimulate experimental efforts to check the claims of high-capacity, stable and reversible hydrogen adsorption reported here.     

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.