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.     

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.     

Remarkable improvement in hydrogen storage capacities of two-dimensional carbon nitride (g-C3N4) nanosheets under selected transition metal doping

We have performed DFT simulations to quest for an optimal material for onboard hydrogen (H₂) storage applications. Using first-principles calculations, we established that the selected transition metals (M: Sc, Ti, Ni, V) decorated two-dimensional (2D) g-C₃N₄ sheets as optimal materials with reversible and significantly high H₂ gravimetric densities. By effectively avoiding metal-metal (M-M) clustering effect in case of mono doping, up to four molecules of H₂ per dopant could be adsorbed with an average binding energy of around 0.30–0.6 eV/H₂, which is ideal for practical applications. Decorating the g-C₃N₄ sheet with (M-M) dimers, the systems are found to be even more efficient for H₂ binding than single dopant decoration. The stability of these M decorated g-C₃N₄ sheets have been confirmed with ab-initio molecular dynamics simulations. We have further calculated the H₂ desorption temperatures of metal decorated g-C₃N₄ sheets, which confirms the practical application of these metal decorated sheets at ambient working conditions.     

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.     

High-capacity reversible hydrogen storage in scandium decorated holey graphyne: Theoretical perspectives

We have investigated the hydrogen storage capabilities of scandium decorated holey graphyne, a recently synthesized carbon allotrope, by applying density functional theory and molecular dynamics simulations. We have observed that one unit cell of holey graphyne can adsorb 6 Sc atoms, and each Sc atom can adsorb up to 5H₂ molecules with an average binding energy and average desorption temperature of ?0.36 eV/H₂ and 464 K, respectively. The gravimetric weight percentage of hydrogen is 9.80%, which is considerably higher than the Department of Energy, United-States requirements of 6.5%. We have found that a total amount of 1.9e charge transfers from the 3d and 4s orbitals of Sc atom to the C-2p orbitals of holey graphyne by performing the Bader charge analysis. Hydrogen molecules are bonded with the scandium atom by Kubas interactions. The ab-initio molecular dynamics simulations confirm the structural integrity of scandium decorated holey graphyne system at the high desorption temperatures. The presence of sufficient diffusion energy barriers for the Sc atom ensure the avoidance of metal-metal clustering in the system.     

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.     

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.     

Selective decorating of BC3 and C3N nanosheets with single metal atom for hydrogen storage

Employing first-principles calculations, we have studied the structure, stability and hydrogen storage efficiency of pristine and defective BC₃ and C₃N monolayer functionalized by a variety of single metal adatoms. It is found that single Sc adatom, acting as an optimal dopant on perfect BC₃ monolayer, is able to adsorb up to nine H₂ molecules as strongly as around 0.24 eV/H₂, which allows for a hydrogen storage capacity of 7.19 wt% for Sc atoms stably adsorbing on double sides of BC₃ monolayer with eighteen H₂ molecules (18H₂@2Sc/BC₃). Moreover, the desorption temperature and thermodynamical stability of multiple H₂ adsorbed Sc-decorated BC₃ sheet have been addressed and the saturate configuration of 18H₂@2Sc/BC₃ is predicted to be stable at mild temperatures and pressures, i.e. less than 250 K at 1 bar, or larger than 24 bar at room temperature. This study indicates that the Sc-decorated BC₃ monolayer could be a potential H₂ storage candidate, and provides an instructive guidance for designing metal-functionalized carbon-based sheets in hydrogen storage.     

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.     

Revealing the interfacial phenomenon of metal-hydride formation and spillover effect on C48H16 sheet by platinum metal catalyst (Pt (n=1-4)) for hydrogen storage enhancement

Hydrogen is a worldwide green energy carrier, however due its low storage capacity, it has yet to be widely used as an energy carrier. Therefore, the quantum chemical method is being employed in this investigation for better understand the hydrogen storage behaviour on Pt _(n = 1-4) cluster decorated C₄₈H₁₆ sheet. The Pt_(n = 1-4) clusters are strongly bonded on the surface of C₄₈H₁₆ sheet with binding energies of ?3.06, ?4.56, ?3.37, and ?4.03 eV respectively, while the charge transfer from Pt_(n = 1-4) to C₄₈H₁₆ leaves an empty orbital in Pt atom, which will be crucial for H₂ adsorption. Initially, the molecular hydrogen is adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet through the Kubas interaction with adsorption energies of ?0.85, ?0.66, ?0.72, and ?0.57 eV respectively, while H–H bond is elongated due to the transfer of electron from σ (HH) orbital to unfilled d orbital of the Pt atom, resulting in a Kubas metal-dihydrogen complexes. Furthermore, the dissociative hydrogen atoms adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet have adsorption energies of ?1.14 eV, ?1.02 eV, ?0.95 eV, and ?1.08 eV, which are greater than the molecular hydrogen adsorption on Pt_(n = 1-4) cluster supported C₄₈H₁₆ sheet with lower activation energy of 0.007, 0.109, 0.046, and 0.081 eV respectively. To enhance the dissociative hydrogen adsorption energy, positive and negative external electric fields are applied in the charge transfer direction. Increasing the positive electric field makes H–H bond elongation and good adsorption, whereas increasing the negative electric field results H–H bond contraction and poor adsorption. Thus, by applying a sufficient electric field, the H₂ adsorption and desorption processes are can be easily tailored.     

Sc/Ti-decorated and B-substituted defective C60 as efficient materials for hydrogen storage

Doping heteroatoms and producing defects are perfect methods to improve the hydrogen storage property of TM-decorated carbon materials. In this view, four novel Sc/Ti-decorated and B- substituted defective C₆₀ fullerenes (B₂₄C₂₄) are explored. The special stability, large specific surface, uniform distribution of the metal and positively charged states make these four fullerenes have high hydrogen storage capacities. Especially, each Sc atom in Sc₆B₂₄C₂₄(B₄) can adsorb up to five H₂ molecules with a storage capacity of 6.80 wt %. The adsorbed H₂ molecules in Sc₆B₂₄C₂₄(B₄)–30H₂ begin to relax at 190 K and are 100% released at 290 K. Moreover, a comparative study is carried out for hydrogen storage properties of Sc-decorated B₄, C₄, or N₄ coordination environments. These results provide a new focus on the nature of B-, and N-substituted defective carbon nanomaterials.     

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.     

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.     

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.     

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.     

Study on the hydrogen storage performance of graphene(N)–Sc–graphene(N) structure

The electronic properties of a sandwich graphene(N)–Sc–graphene(N) structure and its average adsorption energies after the adsorption of 1, 3, 5, 7, 10, and 14H₂ molecules were investigated by first principles. The average binding energies and adsorption distances of Sc atoms at different adsorption sites in N-doped bilayer graphene (N–BLG) were calculated. It was found that Sc atoms located at different adsorption sites of BLG generated metal clusters. The binding energy of the Sc atom located at the TT position of N–BLG (5.19 eV) was higher than the experimental cohesion energy (3.90 eV), and eliminated the impact of metal clusters on adsorption properties. It was found that the G(N)–Sc–G(N) system could stably adsorb 10 hydrogen molecules with an average adsorption energy of 0.24 eV. Therefore, it can be speculated that G(N)–Sc–G(N) is an excellent hydrogen storage material.     

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.     

Computational exploration of magnesium-decorated carbon nitride (g-C3N4) monolayer as advanced energy storage materials

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C₃N₄, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C₃N₄ monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from ?0.25 eV to ?0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C₃N₄ is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.