First-principles study of H2 adsorption on the pristine and oxidized (8,0) carbon nanotube

The effect of oxygen, hydrogen, and (oxygen + hydrogen) molecules adsorption on the structural and electrical properties of (8,0) carbon nanotube (CNT) are investigated through density functional theory. The obtained results indicate endothermical chemisorption of O₂ on the nanotube surface with a large binding energy of about 598 meV and a significant charge transfer of about 0.43 e per molecule. It is discussed that the O₂ chemisorption creates hole carries in the (8,0) carbon nanotube and thus increases the work function of the system. In the case of hydrogen molecule, a weak physisorption on the surface of CNT (∼−5 meV) is identified. The adsorption of H₂ on CNT is also accompanied by hole doping and increment of the work function of the CNT, while the charge transfer between CNT and H₂ is negligible. The band offsets in the H₂-CNT junction are calculated to examine and describe the observed hole doping in this system. The effect of oxygenation of CNT on hydrogen adsorption is also investigated and the most favorable adsorption configuration is found and the related adsorption energy is calculated. It is argued that the oxygenation of CNT enhances the physisorption of hydrogen molecules. It is shown that hydrogen molecule adsorption on the oxidized CNT cancels hole doping and hence decreases the work function of the system.     

First principle study of reversible hydrogen storage in Sc grafted Calix[4]arene and Octamethylcalix[4]arene

The use of hydrogen as a sustainable clean energy source has several benefits, such as reduction in dependency on petroleum fuel and emission of green house gases, and enhanced energy security. The H₂ storage properties of Sc grafted calix[4]arene (CX) and octamethylcalix[4]arene (MCX) are investigated by using density functional theory with M06/6-311G(d,p) level of theory. It is observed that Sc strongly binds with benzene rings of CX and MCX through Dewar coordination with average Sc binding energy of 1.09 and 1.25 eV, respectively for CXSc₄ and MCXSc₄. Each Sc atom adsorbs 4 H₂ molecules on both the Sc grafted systems and H₂ molecules are bound by Kubas interaction with H₂ interaction energy in the range of 0.2–0.5 eV. The calculated conceptual reactivity index shows the stability of the systems increases with number of hydrogen molecules. Hirshfeld charge analysis shows the charge transfer mechanism during H₂ adsorption. Born-Oppenheimer molecular dynamics simulations of CXSc₄-16H₂ and MCXSc₄-16H₂ systems, show that these systems are stable up to 273 K and all the adsorbed H₂ releases at 373 K. The hydrogen storage capacity of Sc grafted CX system is found to be 8.9 wt % and for MCX system is 9.7 wt %. The energy and storage capacity meets the US Department of Energy target, which makes them a propitious hydrogen storage material.     

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.     

High capacity reversible hydrogen storage in zirconium doped 2D-covalent triazine frameworks: Density Functional Theory investigations

Employing the state-of-the art Density Functional Theory (DFT) Simulations, we have investigated hydrogen storage capability in zirconium doped novel 2D heterostructures, Covalent Triazine Frameworks (CTFs), specifically CTF-1, rich in nitrogen functionalities. Zirconium atom is strongly bonded to the triazine framework with a -3.61 eV binding energy, and each Zr atom was found to adsorb 7 H₂ molecules reversibly with binding energy ?0.38 eV per H₂ on an average giving a gravimetric storage capacity of 7.1% which accomplishes the US D.o.E. targets for suitable hydrogen storage substrates. The system stability at ambient and higher temperatures as verified using ab initio Molecular Dynamics simulations as well as existence of sufficient diffusion energy barrier preventing metal-metal clustering certifies the practical viability of the system as a high capacity H₂ storage device. The mechanism of interaction of Zr on 2D CTF-1 and H₂ molecules on Zr+CTF-1 have been analyzed by partial density of states, charge density distribution plot and Bader Charge Analysis. Charge transfer from Zr 4d orbital to 2p orbital of triazine ring was observed, whereas bonding of H₂ is through Kubas interaction which involves the charge donation from the filled σ orbitals of hydrogen molecules to the vacant metal d orbitals, and the subsequent back donation of charge from the occupied metal d orbitals to the vacant σ1 orbital of hydrogen molecules. As the system is stable, can hold high H₂ wt% (7.1%) with suitable desorption temperature (442 K at ambient), we propose that Zr doped 2D CTF-1 can act as a potential H₂ storage device.     

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.     

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.     

Enhancement of hydrogen storage capacity on co-functionalized GaS monolayer under external electric field

Hydrogen storage properties of co-functionalized 2D GaS monolayer have been systematically investigated by first-principles calculations. The strength of the binding energy of hydrogen (H₂) molecules to the pristine GaS surface shows the physisorption interactions. Co-functionalized GaS sheet by Li, Na, K and Ca atoms enhanced the capacity of binding energies of hydrogen and strength of hydrogen storage considerably. Besides, DFT calculations show that there is no structural deformation during H₂ desorption from co-functionalized GaS surface. The binding energies of per H₂ molecules is found to be 0.077 eV for pristine GaS surface and 0.064 eV–0.37 eV with the co-functionalization of GaS surface. Additionally, in the presence of applied external electric field enhanced the strength of binding energies and it is found to be 0.09 eV/H₂ for pristine GaS case and 0.19 eV/H₂ to 0.38 eV/H₂ for co-functionalized GaS surface. Among the studied GaS monolayer is found to be the superior candidate for hydrogen storage purposes. The theoretical studies suggest that the electronic properties of the 2D GaS monolayer show the electrostatic behavior of hydrogen molecules which confirms by the interactions between adatoms and hydrogen molecules before and after hydrogen adsorption.     

Novel permeable material “yttrium decorated zeolite templated carbon” for hydrogen storage: Perspectives from density functional theory

The hydrogen storage capacity of a novel permeable material viz Yttrium (Y) decorated zeolite templated carbon (ZTC) has been investigated using ab-initio DFT based simulations. The study reveals that each Y atom bonded on ZTC can attach at the most of 7H₂ molecules with average binding energy of ?0.35 eV/H_2. The gravimetric hydrogen storage capacity of ZTC with full decoration of Y atom comes about to 8.61 wt% which is sufficiently higher than the limit of 6.5 wt% set by the energy department of the United States of America. The desorption temperature of the system is 437 K. The stability of the structure over such an elevated temperature has been ensured via molecular dynamics (MD) simulations. The stability of the structure at room temperature and presence of sufficient energy barrier for the diffusion of Y atom signifies that the chances of metal-metal clustering are negligible. It has been discerned that it is the Kubas interaction which plays the key role in the interaction between Y and H₂ molecules. The outcomes show that ZTC adorned with Y is a capable material for hydrogen storage which will inspire the instrumentalists to fabricate ZTC based fuel cell device.     

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.     

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.     

Superior hydrogen storage capacity of Vanadium decorated biphenylene (Bi+V): A DFT study

Herein, the hydrogen storage competency of vanadium-decorated biphenylene (Bi+V) has been investigated using Density Functional Theory simulations. The metal atom interacts with biphenylene with a binding energy value of −2.49 eV because of charge transfer between V 3d and C 2p orbitals. The structure and electronic properties are studied in terms of adsorption energy values, the spin-polarized partial density of states (PDOS), band structure plots, and charge transfer analysis. The Kubas-type interactions lead to average hydrogen adsorption energy values of −0.51 eV/H₂ which fulfills DOE-US criteria (0.2–0.7 eV/H₂). The diffusion energy barrier value of 1.75 eV lowers the chances of metal clustering. The complex binds 5H₂ on each V-atom resulting in a storage capacity of 7.52 wt% with an average desorption temperature of 595.96 K. The ab-initio molecular dynamics (AIMD) and phonon dispersions validates structural integrity at higher temperatures suggesting the excellent storage properties of this material at room temperature.     

Hydrogen trapping efficiency of Li decorated porous boron fullerene B38: The first-principles study

The demand for clean renewable energy is urgent in current. The hydrogen application is difficult mainly due to the ratively low capacity in the storage medium. In this work, the adsorption and desorption of the hydrogen molecules by the Li atoms decorated B₃₈ cage are studied by the density functional theory. The calculated largest binding energy of one Li atom (2.68 eV and 2.58 eV) is upon the hexagonal hole of the B₃₈ cage, which is much larger than the experimental cohesive energy of bulk Li (1.63 eV). Each Li atom in the outside of the B₃₈ cage can adsorb up to four H₂ molecules. The E_ad of B₃₈(Li-nH₂)₄ decreases from the 0.22 eV for n = 1 to the 0.11 eV for n = 4. The B₃₈(Li–4H₂)₄ structure achieves the 6.85 wt% hydrogen gravimetric density, which is higher than the goal of 5.5 wt% before 2017 set by the United States Department of Energy. The almost the same partial density of states for the fifth H₂ molecule as that of the isolated H₂ molecule, the longer 4.5 Å distance between the fifth H₂ molecule and the Li atom, together with the small NBO charges all reveal the weak electronic field around the Li⁺, which can interpret the weak H₂ adsorption mechanism. Finally, the B₃₈Li₄ structure can easily release 9H₂ molecules at 373 K known from the molecular dynamic simulation and practically trap about 1.08H₂ molecules at 373 K/3 atom condition calculated by the grand partition function. Thus, its reversible practical HGD of B₃₈Li₄-14.34H₂ is 6.18 wt%, which is almost the same value as the theoretical 6.85 wt% for B₃₈(Li–4H₂)₄. Our studies will be the strong theory basis for the future application in hydrogen storage material development.     

First-principles study of superior hydrogen storage performance of Li-decorated Be2N6 monolayer

The potential application of pristine Be₂N₆ monolayer and Li-decorated Be₂N₆ monolayer for hydrogen storage is researched by using periodic DFT calculations. Based on the obtained results, the Be₂N₆ monolayer gets adsorb up to seven H₂ molecules with an average binding energy of 0.099 eV/H₂ which is close to the threshold energy of 0.1 eV required for practical applications. Decoration of the Be₂N₆ monolayer with lithium atom significantly improves the hydrogen storage ability of the desired monolayer compared to that of the pristine Be₂N₆ monolayer. This can be attributed to the polarization of H₂ molecules induced by the charge transfer from Li atoms to the Be₂N₆ monolayer. Decoration of Be₂N₆ monolayer with two lithium atoms gives a promising medium that can hold up to eight H₂ molecules with average adsorption energy of 0.198 eV/H₂ and hydrogen uptake capacities of 12.12 wt%. The obtained hydrogen uptake capacity of 2Li/Be₂N₆ monolayer is much higher than the target set by the U.S. Department of Energy (5.5 wt% by 2020). Based on the van't Hoff equation, it is inferred that hydrogen desorption can occur at T_D = 254 K for 2Li/Be₂N₆ (8H₂) system which is close to ambient conditions. This is a remarkable result indicating important practical applications of 2Li/Be₂N₆ medium for hydrogen storage purposes.     

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.     

Hydrogen storage in MgH2 coated single walled carbon nanotubes

We present a density functional theory (DFT) study on the hydrogen storage capacity of (5,5) arm-chair single walled carbon nanotubes (SWCNTs) functionalized with magnesium hydride (MgH₂). Being lightweight and rich in hydrogen, MgH₂ adsorbs H₂ molecules in the vicinity of carbon nanotubes. The H₂ molecules are adsorbed dissociatively on SWCNT + MgH₂ complex. The H-H distance gets increased by more than ten times of the initial bond length 0.74 Å of the H₂ molecule. The hydrogen storage capacity of three configurations namely C1MgH₂, C5MgH₂ and C10MgH₂ is reported. The density of states is computed for all the systems. The average binding energies of C5MgH₂ and C10MgH₂ when H₂ molecule is adsorbed are 1.86 eV/H₂ and 1.96 eV/H₂, which are approximately equal. Thus, increasing the number of MgH₂ molecule does not vary the binding energy of H₂ adsorption. The corresponding temperature, in which desorption will take place, is 2285 K and 2457 K for C5MgH₂ and C10MgH₂ systems respectively, which are much above the room temperature.     

Ab initio and periodic DFT investigation of hydrogen storage on light metal-decorated MOF-5

The effect of light metal (M = Li, Be, Mg, and Al) decoration on the stability of metal organic framework MOF-5 and its hydrogen adsorption is investigated by ab initio and periodic density functional theory (DFT) calculations by employing models of the form BDC:M₂:nH₂ and MOF-5:M₂:nH₂, where BDC stands for the benzenedicarboxylate organic linker and MOF-5 represents the primitive unit cell. The suitability of the periodic DFT method employing the GGA-PBE functional is tested against MP2/6-311 + G* and MP2/cc-pVTZ molecular calculations. A correlation between the charge transfer and interaction energies is revealed. The metal-MOF-5 interactions are analyzed using the frontier molecular orbital approach. Difference charge density plots show that H₂ molecules get polarized due to the charge generated on the metal atom adsorbed over the BDC linker, resulting in electrostatic guest-host interactions.Our solid state results show that amongst the four metal atoms, Mg and Be decoration does not stabilize the MOF-5 to any significant extent. Li and Al decoration strengthened the H₂-MOF-5 interactions relative to the pure MOF-5 exhibited by the enhanced binding energies. The hydrogen binding energies for the Li- and Al-decorated MOF-5 were found to be sensible for allowing reversible hydrogen storage at ambient temperatures. A high hydrogen uptake of 4.3 wt.% and 3.9 wt.% is also predicted for the Li- and Al-decorated MOF-5, respectively.     

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.     

Hydrogen storage in lithium-decorated benzene complexes

Based on first-principles calculations, we find Li-decorated benzene complexes are promising materials for high-capacity hydrogen storage. Lithium atoms in the complexes are always positively charged, and each one can bind at most four H₂ molecules by a polarization mechanism. Therefore, a hydrogen uptake of 8.6 wt% and 14.8 wt% can be achieved in isolated C₆H₆–Li and Li–C₆H₆–Li complexes, respectively. The binding energy in the two cases is 0.22 eV/H₂ and 0.29 eV/H₂, respectively, suitable for reversible hydrogen storage. Various dimers may form, but the hydrogen storage capacity remains high or uninfluenced. This study provides not only a promising hydrogen storage medium but also comprehensions to other hydrogen storage materials containing six-carbon rings.     

Yttrium-decorated C48B12 as hydrogen storage media: A DFT study

We report a density functional theory calculation dedicated to analyze the behavior of hydrogen adsorption on Yttrium-decorated C₄₈B₁₂. Electron deficient C₄₈B₁₂ is found to promote charge transfer between Y atom and substrate leading to an enhanced local electric field which can significantly improve the hydrogen adsorption. The analysis shows that Y atoms can be individually adsorbed on the pentagonal sites without clustering of the metal atoms, and each Y atom can bind up to six H₂. molecules with an average binding energy of −0.46 eV/H₂, which is suitable for ambient condition hydrogen storage. The Y atoms are found to trap H₂ molecules through well-known “Kubas-type” interaction. Our simulations not only clarify the mechanism of the reaction among C₄₈. B₁₂, Y atoms and H₂ molecules, but also predict a promising candidate for hydrogen storage application with high gravimetric density (7.51%).     

Hydrogen storage on superalkali NLi4 decorated β12-borophene: A first principles insights

The hydrogen storage properties of superalkali NLi₄ decorated β₁₂-borophene have been comprehensively investigated based on first-principles density functional calculations (DFT). It is found that the NLi₄ cluster can be stably anchored on the surface of β₁₂-borophene because of its large binding energy. The calculated Bader charge population indicates that the charges are transferred from Li atoms to the original monolayer and causes the NLi₄ steady adsorbs onto the surface of β₁₂-borophene. For H₂ storage, two sides of NLi₄ decorated β₁₂-borophene can adsorb up to 24H₂ molecules with an ideal H₂ adsorption energy of ?0.176 eV/H_2. Meanwhile, the hydrogen uptake density achieves 7.66 wt% and surpasses the target of 6.5 wt% from U.S. Department of Energy (DOE). In addition, the adsorption reasons of H₂ molecules include the orbital hybridization between H₂ and β₁₂-borophene from the calculated projected density of states (PDOS) and the polarization effect of electrostatic field from the calculated charge density difference. We hope this theoretical study can encourage the experimental fabrication for hydrogen storage applications in the near future.