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.     

First principles study on hydrogen storage in yttrium doped graphyne: Role of acetylene linkage in enhancing hydrogen storage

Through Density Functional Theory Simulations we predict that a Ytrrium atom attached on graphyne surface can adsorb up to a maximum of 9 molecular hydrogens (H₂), with a uniform binding energy of ∼0.3 eV/H₂ and an average desorption temperature of around 400 K (ideal for fuel cell applications), leading to 10 wt% of hydrogen, substantially higher than the requirement by DoE. The higher hydrogen wt% in Y doped graphyne compared to Y doped Single Walled Carbon Nanotubes (SWNT) and graphene is due to the presence of sp hybridized C atoms (in the acetylene linkage) supplying additional in-plane p_x-p_y orbitals leading to π (π*) bonding (antibonding) states. Charge transfer from metal to carbon nanostructure results in a redistribution of s, p, d orbitals of the metal leading to a non - spin polarized ground state in Y doped graphyne, due to the presence of the acetylene linkage, whereas Y doped SWNT and graphene remain magnetic like the isolated metal atom. In the non-magnetic graphyne + Y system, the net charge transfer from Y to successive H₂ molecules is less than in magnetic Y + graphene and Y + SWNT systems, enabling Y + graphyne to store a larger number of H₂ molecules. Furthermore, our ab initio MD simulations show that the system is stable even at room temperature and there is no dissociation of H₂ molecules, enabling the system to achieve 100% desorption. So Y doped graphyne is found to be a promising hydrogen storage device with high wt%, 100% recyclability and desirable desorption temperature.     

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.     

Graphitic carbon nitride (g-C3N4) decorated with Yttrium as potential hydrogen storage material: Acumen from quantum simulations

With the aid of the state-of-the-art Density Functional Theory simulations, triazine-like graphitic carbon nitride or g-C₃N₄ (abbreviated as gCN hereafter) nanosheet decorated with Y has been explored for reversible hydrogen storage applications in light fuel cell vehicles. The Y atom is found to bind strongly with gCN (binding energy ～ ?6.85 eV), can reversibly store 9 H₂ with an average adsorption energy of ?0.331 eV/H₂, an average desorption temperature of 384.24 K, and a storage capacity of 8.55% by weight, optimum for fuel cell application as prescribed by the Department of Energy. The bonding of Y on gCN involves a charge transfer from Y 4d orbitals to C and N 2p orbitals, whereas the adsorption of H₂ is due to Kubas interactions involving net charge transfer from Y 4d orbital to H 1s orbital. We have computed the diffusion energy barrier for Y atoms as 3.07 eV, which may prevent metal-metal clustering. Further, ab-initio molecular dynamics simulation has been performed to check the structural stability of the present system. The system is found to be stable at 500 K with different concentrations of Y doping. The present system with the appropriate average adsorption energy per H₂, suitable desorption temperature, and structural stability at higher temperatures is promising for onboard light fuel cell applications.     

Hydrogen storage in scandium decorated triazine based g-C3N4: Insights from DFT simulations

Hydrogen is being considered a ‘fuel of the future,’ a viable alternative to fossil fuels in fuel cell vehicles. Using Density Functional Theory simulations, reversible, onboard hydrogen storage in Sc-decorated triazine-based graphitic carbon nitride (g-C₃N₄) has been explored. Sc atom binds strongly on the g-C₃N₄ structure with a binding energy of ?7.13 eV. Each Sc atom can reversibly bind 7 molecules of hydrogen, giving a net gravimetric storage capacity of 8.55 wt%, an average binding energy of ?0.394 eV per H₂, and a corresponding desorption temperature of 458.28 K, fulfilling the criteria prescribed by the US Department of Energy. The issue of transition metal clustering has been investigated by computing the diffusion energy barrier (2.79 eV), which may be large enough to hinder the clustering tendencies. The structural integrity of Sc-g-C₃N₄ has been verified through ab-initio Molecular Dynamics simulations. The interaction mechanism of Sc over g-C₃N₄ and H₂ over Sc-g-C₃N₄ has been explored using density of states and charge transfer analysis. A flow of charge from valence 3d orbitals of Sc towards vacant orbitals of g-C₃N₄ during the binding of Sc over g-C₃N₄ is observed. The binding of H₂ on Sc-g-C₃N₄ may be via Kubas type of interactions which is stronger than physisorption due to net charge gain by H 1s orbital from Sc 3d orbital. Our systematic investigations indicate that Sc-decorated g-C₃N₄ may be a high-performance material for reversible hydrogen storage applications.     

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.     

High capacity reversible hydrogen storage in titanium doped 2D carbon allotrope Ψ-graphene: Density Functional Theory investigations

Using the state-of-the art Density Functional Theory simulations, here we report the hydrogen storage capability in titanium decorated ?- Graphene, an advanced 2D allotrope of carbon which is made of hexagonal, pentagonal and heptagonal ring of carbon and metallic in nature. Titanium is strongly bonded on the surface of ?- Graphene and each Ti can bind maximum of 9H₂ having average adsorption energy of ?0.30 eV and average desorption temperature of 387 K yielding gravimetric H₂ uptake of 13.14 wt%, much higher than the prescribed limit of 6.5 wt % by DoE's. The interaction of Ti on ?- Graphene have been presented by electronic density of states analysis, charge transfer and plot for spatial distribution of charge. There is orbital interaction between Ti 3d and C 2p of ?- Graphene involving transfer of charge whereas bonding of hydrogen molecules is through Kubas type of interactions involving charge donation from σ orbitals of hydrogen molecules to the vacant 3d orbital of Ti and the subsequent back donation to σ1 orbital of hydrogen from filled 3d orbital of Ti. The structural stability of the system at temperatures corresponding to the highest temperature at which H₂ desorbs was verified using ab-initio Molecular Dynamics calculations and presence of sufficient energy barrier for diffusion which prevents clustering between metal atoms assures the practical viability of the system as high capacity H₂ adsorbing material. Overall, found that Ti doped Ψ-Graphene is stable, 100% recyclable and has high hydrogen storage capacity with suitable desorption temperature. As a result of our findings, we are confident that Ti doped Ψ-Graphene may be used as a potential hydrogen adsorbing material in the upcoming clean, green, hydrogen economy.     

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.     

Sc and Ti-functionalized 4-tert-butylcalix[4]arene as reversible hydrogen storage material

Using the idea of metal functionalized material for H₂ storage, 4-tert-butylcalix[4]arene (CA) functionalized with Sc and Ti atoms are explored. The first principles density functional theory (DFT) with M06 functional and 6-311G(d,p) basis set is used to explore the hydrogen storage properties of metal functionalized CA. Sc and Ti strongly binds with CA by Dewar coordination with high binding energy. It is found that maximum four hydrogen molecules are adsorbed on each metal site in Sc and Ti functionalized CA. Hydrogen molecules are adsorbed on metals by Kubas and Niu-Rao-Jena mechanism. In Sc functionalized CA system all 4 hydrogen molecules on each Sc bind in molecular fashion while on each Ti in Ti functionalized CA, the first hydrogen molecule binds in dissociative fashion and remaining three hydrogen molecules bind in a molecular form. The stability of Sc and Ti functionalized CA is studied by computing conceptual DFT parameters, which obeys maximum hardness and minimum electrophilicity principle. Hirshfeld charge analysis and electrostatic potential map explore the charge transfer mechanism during the hydrogen adsorption. Born-Oppenheimer molecular dynamics simulations are performed at temperature range 200–473 K to study the stability of the system and the reversibility of adsorbed hydrogen from the system. The calculated H wt% is found to be 10.3 and 10.1, respectively for Sc and Ti functionalized CA systems on complete H₂ saturation. This study explores that Sc and Ti functionalized CA systems are efficient reversible hydrogen storage material.     

200 NL H2 hydrogen storage tank using MgH2–TiH2–C nanocomposite as H storage material

MgH₂-based hydrogen storage materials are promising candidates for solid-state hydrogen storage allowing efficient thermal management in energy systems integrating metal hydride hydrogen store with a solid oxide fuel cell (SOFC) providing dissipated heat at temperatures between 400 and 600 °C. Recently, we have shown that graphite-modified composite of TiH₂ and MgH₂ prepared by high-energy reactive ball milling in hydrogen (HRBM), demonstrates a high reversible gravimetric H storage capacity exceeding 5 wt % H, fast hydrogenation/dehydrogenation kinetics and excellent cycle stability. In present study, 0.9 MgH₂ + 0.1 TiH₂ +5 wt %C nanocomposite with a maximum hydrogen storage capacity of 6.3 wt% H was prepared by HRBM preceded by a short homogenizing pre-milling in inert gas. 300 g of the composite was loaded into a storage tank accommodating an air-heated stainless steel metal hydride (MH) container equipped with transversal internal (copper) and external (aluminium) fins. Tests of the tank were carried out in a temperature range from 150 °C (H₂ absorption) to 370 °C (H₂ desorption) and showed its ability to deliver up to 185 NL H₂ corresponding to a reversible H storage capacity of the MH material of appr. 5 wt% H. No significant deterioration of the reversible H storage capacity was observed during 20 heating/cooling H₂ discharge/charge cycles. It was found that H₂ desorption performance can be tailored by selecting appropriate thermal management conditions and an optimal operational regime has been proposed.     

Hydrogen storage capacity of alkali and alkaline earth metal ions doped carbon based materials: A DFT study

The hydrogen storage (H-storage) capacity of alkali (Li⁺, Na⁺ and K⁺) and alkaline earth metal ion (Mg²⁺ and Ca²⁺) doped cubane, cyclohexane and adamantane has been investigated using Density Functional Theory (DFT) based M05-2X functional employing 6-31+G∗∗ basis set. The adsorption of number of H₂ molecules on the metal ion doped complexes depends on ionic radii and charge of the metal ions. Among the 15 complexes investigated in this study, Mg²⁺ ion doped cubane, cyclohexane and adamantane complexes have higher H-storage capacity when compared to other complexes. The calculated binding energy (BE) of 5H₂@Cub-Mg²⁺ complex is 46.85 kcal/mol with binding energy per H₂ molecule (BE/nH₂) of 9.37 kcal/mol. The corresponding gravimetric density of the complexes is 7.3 wt%. In the case of 4H₂@Cyc-Mg²⁺ complex, the BE is 32.19 kcal/mol (BE/nH₂ is 8.05 kcal/mol with 6.9 wt% in gravimetric density). The Adm-Mg²⁺ complexes adsorb 4H₂ molecules with BE of 33.33 kcal/mol, the BE of per H₂ molecule is 8.33 kcal/mol. The corresponding gravimetric density of the complex is around 4.8 wt%, respectively. A new linker modified MOP-9 has been constructed based on the results and their H-storage capacity has also estimated.     

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%).     

Enhanced reversible hydrogen storage performance of NbCl5 doped 2LiH–MgB2 composite

2LiBH₄ + MgH₂ system is considered as an attractive candidate for reversible hydrogen storage with high capacity and favorable thermodynamics. However, its reaction kinetics has to be further improved for the practical application. In this work, we investigated the effect of NbCl₅ additive on the de/hydrogenation kinetics and microstructure refinement in 2LiH–MgB₂ composite systematically. The hydrogenation and dehydrogenation kinetics of 2LiH–MgB₂ composite can be significantly enhanced with the increase of NbCl₅ content. The 3 mol% NbCl₅ doped 2LiH–MgB₂ composite exhibits the superior reversible hydrogen storage performance, which requires 50 min to uptake 9.0 wt% H₂ at 350 °C and release 8.5 wt% H₂ at 400 °C, respectively. In contrast, the undoped 2LiH–MgB₂ sample uptakes 6.2 wt% H₂ and releases 3.1 wt% H₂ under identical measurement conditions. Moreover, the 3 mol% NbCl₅ doped 2LiH–MgB₂ composite can release more than 9.0 wt% H₂ within 300 min at 400 °C without obvious degradation of capacity over the first 10 cycles. Microstructure analyses clearly indicate that NbCl₅ additive first reacts with LiH to form Nb and LiCl during ball-milling process, and then NbH is formed after the first hydrogenation and stabilized upon further de/hydrogenation cycling. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system through facilitating hydrogen diffusion rapidly as well as prevent the particles from further growth in the subsequent hydrogenation and dehydrogenation processes.     

Li-decorated graphyne as high-capacity hydrogen storage media: First-principles plane wave calculations

Taking into account the van der Waals correction, the characteristics of the Li-decorated graphyne as the hydrogen storage medium have been explored using first-principles plane wave calculations. We find that Li atom can be adsorbed not only over the center of large hexagon (H_L site) but also over the center of small hexagon (H_S site). For double-side Li decorations, there are 14H₂ molecules can be adsorbed on Li-decorated graphyne primitive cell with the adsorption energy of 0.19 eV/H₂. As a result, the hydrogen storage capacity of 13.0 wt% can be obtained. This suggests that the Li-decorated graphyne system can serve as a high-capacity hydrogen storage medium.     

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.     

Ultra-high capacity hydrogen storage of B6Be2 and B8Be2 clusters

The B₆Be₂ and B₈Be₂ clusters, adopting fascinating inverse sandwich-like geometries, were recently predicted with quantum chemical calculations. Both systems exhibit the high stability and double aromaticity with 4σ/6π or 6σ/6π delocalized electrons. The hydrogen storage of two systems is studied in the present paper. Our calculations show that B₆Be₂ and B₈Be₂ clusters have the ultra-high capacity hydrogen storage, each Be site can bound up with seven H₂ molecules, corresponding to a gravimetric density of 25.3 wt percentage (wt%) for B₆Be₂ and 21.1 wt% for B₈Be₂, respectively, which far exceeds the target (5.5 wt%) proposed by the US department of energy (DOE) in 2017. The average absorption energies of 0.10–0.45 eV/H₂ for B₆Be₂ and 0.11–0.50 eV/H₂ for B₈Be₂ at the wB97XD level suggest that both systems are ideal for reversible hydrogen storage and release. The reversibility of H₂ molecules on B₆Be₂ and B₈Be₂ complexes are faithfully demonstrated with the Born-Oppenheimer molecular dynamics (BOMD) simulations. The Be-doped boron nanostructure is a promising candidate for ultra-high hydrogen storage materials.