Storage and permeation of hydrogen molecule,atom and ions (H+ and H−) through silicon carbide nanotube; a DFT approach

The barriers for the encapsulation and decapsulation of hydrogen ions (cationic hydrogen and hydride), atom, and molecule through silicon carbide nanotube are thoroughly studied. DFT method is selected to measure the kinetic barriers for the passage of hydrogen atom, ions and molecule through nanotube via scanning potential energy surface. The kinetic barriers for the passage (encapsulation and decapsulation) of hydrogen are very important to understand the mechanism of hydrogen storage and release. The barriers for the permeation of H, H⁺ and H^? across SiC nanosheet are lower compared to hydrogen molecule (H₂). The exohedral and endohedral adsorption of hydrogen ions (cation and anion), atom and exohedral hydrogen molecule on silicon carbide are exothermic in nature. Whereas the encapsulation of hydrogen molecule in silicon carbide is endothermic. Electronic properties are analyzed through measurement of energy gap between highest occupied and lowest unoccupied molecular orbitals gap (G_H-L) and the density of state (DOS) spectra. The G_H-L analysis reveals that endohedral complexes have more pronounced effect on electronic properties compared to exohedral complexes. The SiC nanotube has highly favorable properties for storage and release of hydrogen ions, and atom.     

Calcium-decorated graphdiyne as a high hydrogen storage medium: Evaluation of the structural and electronic properties

Graphdiyne (GDY) is a new member of carbon allotropes consisting of sp and sp² hybridized carbon atoms. In this work, the hydrogen adsorption on Calcium (Ca) decorated GDY and the influence of adatom on structural properties of GDY are investigated, using first principles plane wave calculations with Van der Waals corrections. The results show that similar to graphyne (GY) and unlike carbon nanotube (CNT), fullerene and graphene, clustering of Ca on GDY hinders due to the higher binding energy of the adatom to the carbon frame than the Ca cohesive energy. It can be seen that the Ca-decoration promotes hydrogen storage capacity of GDY, extremely (E_ads = ?0.266 and ?0.066 eV for Ca-decorated and pristine GDY, respectively). It is concluded that, the best site for the Ca trapping is 18-membered ring in which, Ca lies in-plane of GDY (E_ads = ?3.171 eV). Fourteen H₂ molecules (with the average adsorption energy of ～0.2 eV/H₂) can be adsorbed on the Ca atom from one side. The hydrogen storage capacity is estimated to be as high as 17.95 wt% for the both sides of GDY. So, the Ca-decorated GDY is offered as a promising candidate for hydrogen storage applications.     

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.     

Li-decorated porous graphene as a high-performance hydrogen storage material: A first-principles study

Development of novel carbon-based nanoporous materials with high reversible capacity and excellent cycling stability is a hot topic in the field of hydrogen delivery and storage. In this work, first-principles calculations are carried out to discuss the hydrogen storage properties of Li-decorated porous graphene (Li-PG). The binding energies, electronic structures, storage capacities of hydrogen on different sites are investigated in details. The computational results show that with the increase of lithium doping concentration, the electron concentration of donor atoms exceeds the N_c value, and as a consequence, the PG changes from the p-type semiconductor to the n-type degenerate semiconductor. The maximum hydrogen adsorption configurations of H1a-H'1b and H2a-H'2b systems are obtained, and the average binding energy of per H₂ molecule is 0.245 eV and 0.263 eV, respectively. Furthermore, ab inito MD simulation results show that the H1-H'1 and H2-H'2 systems can hold up to sixteen and fifteen H₂ molecules, which corresponds to a hydrogen storage capacity of 10.89 wt% and 10.79 wt% at T = 300 K (no external pressure), respectively.     

Comparison of hydrogen absorption in metallic and semiconductor single-walled Ge- and GeO2-doped carbon nanotubes

First-principles calculations were carried out to compare hydrogen absorption in pristine metallic and semiconductor carbon nanotubes (CNTs) with the situation in their Ge- and GeO₂-doped counterparts. We found out that the pristine carbon nanotubes have low absorption efficiency (?1.53 eV in the metallic, and ?2.06 eV in the semiconductor carbon nanotube). When Ge was doped into both carbon nanotubes, the hydrogen absorption was enhanced to ?5.29 eV in the metallic and ?3.99 eV in the semiconductor carbon nanotubes. Investigating the Partial density of states proved that there was considerable overlap between Ge 4p and hydrogen 1s orbitals in both CNTs. When CNTs were doped with GeO₂, hydrogen atoms were bound to oxygen atoms, due to high electronegativity of oxygen atom. The hydrogen absorption was found to be increased remarkably in the metallic carbon nanotube (?6.59 eV). In order to compare the binding energy of Ge and GeO₂ doped metallic and semiconductor carbon nanotubes, the partial density of states and the magnetization of the samples were studied.     

A first principles study of hydrogen storage in lithium decorated defective phosphorene

In this study, we studied defect-engineering and lithium decoration of 2D phosphorene for effective hydrogen storage using density functional theory. Contrary to graphene, it is found that the presence of point-defects is not preferable for anchoring of H₂ molecules over defective phosphorene. According to previous research, strategies such as defect engineering, metal decoration, and doping enhance the hydrogen storage capacity of several 2D materials. Our DFT simulations show that point defects in phosphorene do not improve the hydrogen storage capacity compared to pristine phosphorene. However, selective lithium decoration over the defective site significantly improves the hydrogen adsorption capacity yielding a binding energy of as high as ?0.48 eV/H₂ in Li-decorated single vacancy phosphorene. Differential charge densities and projected density of states have been computed to understand the interactions and charge transfer among the constituent atoms. Strong polarization of the H₂ molecule is evidenced by the charge accumulation and depletion. The PDOS shows that the presence of Li leads to enhanced charge transfer. The maximum gravimetric density was investigated by sequentially adding H₂ molecules to the Li-decorated single vacancy defective phosphorene. The Li-decorated single vacancy phosphorene is found to possess a gravimetric density of around 5.3% for hydrogen storage.     

Hydrogen storage on Ti decorated SiC nanostructures: A first principles study

In the present study we report the hydrogen adsorption behavior of two SiC nanostructures; a planar sheet and a nanotube (10, 0) of 1 nm diameter decorated by Ti atoms on it. All calculations have been performed using a plane-wave based pseudopotential method. The lowest energy structure of the Ti adsorbed SiC sheet shows that Ti atom distorts the sheet in such a way that one of the Si atoms goes down the plane and the Ti atom bind with nearest three C atoms. The interaction of this Ti decorated sheet with hydrogen suggests that each Ti atom can bind up to four hydrogen molecules (all hydrogens are adsorbed in the molecular form) with an average binding energy of 0.37 eV. For SiC nanotube, the adsorption of Ti favors the hexagonal hollow site. Moreover, on interaction of this Ti decorated tube with hydrogen leads to dissociation of the first hydrogen molecule in the atomic form and thereafter adsorbs hydrogen in the molecular form. The average binding energy of hydrogen molecules on this Ti decorated tube is estimated to be 0.65 eV. Based on these results we infer that the Ti decorated SiC nanostructures moderately bind with hydrogen molecules (within the energy window for hydrogen storage materials) and therefore, can be considered as one of the potential hydrogen storage material.     

A novel hydrogen storage medium of Ca-coated B40: First principles study

Using first principles study, we have investigated the hydrogen storage capacity of Ca-coated B₄₀. Our result shows that Ca prefers to adsorb on the top hollow center of heptagonal ring of B₄₀ due to the large binding energy of ?2.820 eV. Bader charges calculation indicates that charges transfer from Ca to B₄₀ result in an induced electric field so that H₂ molecules are polarized and adsorbed onto the surface of B₄₀ without dissociation. The Ca₆B₄₀ complex can adsorb up to 30 H₂ molecules with average adsorption energy of ?0.177 eV/H₂ and the hydrogen storage gravimetric density reaches up to 8.11 wt.%, higher than the goal from DOE by the year 2020. These findings will suggest a new and potential structure for hydrogen storage in the future.     

Hydrogen storage in Ca-decorated carbyne C10-ring on either Dnh or D(n/2)h symmetry. DFT study

We computationally investigate the hydrogen storage properties of carbyne C₁₀-ring structure on either D_nh or D_(n/2)h symmetry decorated with calcium (Ca) atoms adsorbed on its outer surface. The calculations are carried out on DFT-GGA-PW91 and DFT-GGA-PBE levels of theory as implemented in Biovia Materials Studio modeling and simulation software. To account for van der Waals interactions we also carried out calculations using DFT-D method of Grimme. Dmol³ is used to calculate total energies, HOMO-LUMO electronic charge density, Mulliken population analysis, and electrostatic potential fitting charges (ESP). Based on these results: i) the average binding energy of Ca atom doping to C₁₀-ring is ~2.3 eV (PW91) and ~2.1 eV (PBE). ii) Up to seven H₂ molecules per Ca atom can be physically adsorbed with an average energy of ~0.2 eV per H₂ molecule. iii) This physisorption leads to 8.09 wt percentage (wt. %) for the gravimetric storage capacity. According to these results, calcium-decorated carbyne C₁₀-ring structure is excellent candidate for hydrogen storage at ambient conditions with application to fuel cells.     

Hydrogen storage in coiled carbon nanotubes

We report a density functional calculation of the adsorption of molecular hydrogen on the external surface of coiled carbon nanotube (CCNT). Binding energies of single molecule have been studied as a function of three different orientations and at three different sites like hexagon, pentagon and heptagon. The binding energy values are larger than linear (5,5) armchair nanotube, which has approximately same diameter as that of coiled carbon nanotube. The curvature and topology of CCNT are responsible for this considerable enhancement. The system with full coverage is also studied. When the nanotube surface is fully covered with one molecule per graphitic hexagon, pentagon and heptagon gives the 6.8 wt% storage capacity. The binding energy per molecule decreases due to repulsive interactions between neighbor molecules. It gives good storage medium for hydrogen. Almost it meets the DOE target.     

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.     

Exploring the application of AlN graphyne in calcium ion batteries

Destiny functional theory (DFT) calculations are undertaken in order to scrutinize the electrochemical and calcium (Ca) storage characteristics of a graphyne-like aluminum nitride monolayer (G-AlNyen) as an electrode material for Ca-ion batteries (CIBs). The results show that the change in internal energy as well as the cell voltage values for the CIB with the G-AlNyen anode are comparable to others with two-dimensional 2D nano-materials. It is shown that Ca is adsorbed primarily onto the center of a hexagonal and triangular ring of G-AlNyen with absorption energies of ?2.06 and ?0.42 eV. After increasing the concentration of Ca atoms on G-AlNyen, the adsorption energy as well as the cell voltage decreases. Lower values of 0.15–0.32 eV related to the diffusion barrier confirm that the diffusion of Ca in the 2D nano-sheets is rapid. G-AlNyen shows a maximum theoretical capacity of approximately 869.23 mAh h g^?1. The results are evaluated in terms of charge transfer, structure, energy as well as electronic characteristics and provide insight into the construction of better anode materials with higher capacity for the CIB.     

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

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

A novel hydrogen-sensitive sensor based on Pd nanorings/TNTs composite structure

Hydrogen sensors with a novel composite structure comprised of Pd nanorings distributed on TiO₂ nanotube arrays were developed and tested. Effect of the TiO₂ nanotube diameter size, Pd nanorings thickness on the sensors' hydrogen response characteristics were investigated. Time dependence of resistance of the Pd nanorings/TNTs composite structure on various hydrogen concentrations was also carried out and demonstrated good room temperature hydrogen sensitive characteristics. Optimized experiments demonstrated that the hydrogen sensor composed of 25 nm-thickness Pd nanorings distributed on the 77 nm-diameter size TiO₂ nanotube showed a fast response time (3.8 s) and high sensitivity (92.05%) at 0.8 vol% H₂. A hydrogen sensitive characteristics model is proposed and the Pd nanorings' important role in the hydrogen sensitive mechanisms is described. The hydrogen sensor's excellent hydrogen sensitive characteristics is ascribed to the Pd nanorings' quick and continual formation and breakage of multiple passages due to absorption and desorption of hydrogen atoms.     

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 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.     

Hydrogen adsorption on calcium,potassium, and magnesium-decorations aluminene using density functional theory

A low-cost hydrogen storage with high capacity is still a bottleneck to achieve a hydrogen economy for a sustainable clean fuel cell vehicle. Aluminene has been identified as a potential hydrogen storage material due to its high surface area. In this work, calcium, potassium, and magnesium were introduced at low concentrations as interstitial dopants to planar aluminene to determine its effects on hydrogen adsorption using density functional theory. Results showed that these impurities can easily be chemisorped with absolute binding energies ranging from 0.95 eV to 3.50 eV on the top, bridge, and hollow sites of aluminene in ascending order. This chemisorption is validated by the overlapping of sp orbitals between the dopant atoms and aluminum as shown in the density of states. Electron transfer from the aluminum to the dopant atoms were observed in the charge density difference allowing reactivity of the hydrogen atoms to the dopants. These materials have zero magnetization and remained metallic. Furthermore, hydrogen molecules were physisorped near the dopants with absolute adsorption energies ranging from 23 meV to 81 meV which would be suitable as a storage material near room temperature. Finally, the calculated gravimetric densities show that aluminene with impurities at low concentrations can still be potential hydrogen storage materials.     

A first-principles study of Li and Na co-decorated T4,4,4-graphyne for hydrogen storage

To obtain high hydrogen storage performance, Li and Na co-decorated T4,4,4-graphyne have been studied by the method of first-principles calculations in this paper. Li and Na atoms are bound on hexagonal ring and acetylenic ring included in T4,4,4-graphyne, with the average adsorption energy of 1.73 and 2.38 eV, respectively. Our calculations show that the maximum gravimetric density of H₂ uptake is 10.46 wt%, and an appropriate adsorption energy is reached. Moreover, by plotting charge density differences, it is found that the induced electric field between Li/Na and T4,4,4-graphyne can enhance the adsorption for hydrogen molecule. Furthermore, this complex is thermodynamic stable at room temperature, which is certificated by molecule dynamics simulation. Our results demonstrate that Li and Na co-decorated T4,4,4-graphyne is an alternative material for hydrogen storage.     

Hydrogen storage in Li decorated and defective pentaoctite phosphorene: A density functional theory study

Hydrogen storage in 2D pentaoctite phosphorene was investigated by density functional theory (DFT) calculations. Defect engineering and Li decoration were adopted to evaluate their effects on the hydrogen storage. The formation energies for two types of point defects, single vacancy (SV) and double vacancy (DV) were calculated. The DFT results showed that pristine pentaoctite had a very weak binding with H₂ molecule. With the defect formation energies in the order of black phosphorene, the point defects marginally improved the binding energy of H₂ molecule. However, Li decoration over pristine and defective substrates enhanced the binding energy of H₂ molecule by 5–10 fold improving from around ?0.03 eV/H₂ to ?0.25 eV/H₂, thereby, resulting a better H₂ storage capacity. PDOS calculation evidenced the charge transfer from Li atom as its key attribute. In addition, multiple Li adatoms were decorated over the substrate at the favorable sites. In Li decorated pristine, SV, and DV defective substrates, up to 5, 6, and 3 H₂ molecules could be absorbed at each Li adatom. The diffusion energy barrier of Li from one favorable site to another was calculated to be an order of magnitude higher that its thermal energy causing an impedance to clustering.     

Density functional study of adsorption and desorption dynamics of hydrogen in zirconium doped aluminium clusters

To reduce the greenhouse effect and as a fuel alternatives hydrogen is used as a secure and clean energy. But there are some challenges in the storage of hydrogen energy, the present accurate dynamics of adsorption and release of hydrogen. We report the adsorption and desorption of hydrogen atoms (up to eight) on the ZrAl_m (m = 3 to 7) clusters using density functional theory within B3PW91/LANL2DZ basis sets in the GAUSSIAN 09 package. Adsorption energy for per hydrogen atom in all clusters is found in the range of ?2.5 eV to ?3.3 eV, which shows the chemisorption of hydrogen on the ZrAl_m clusters. Change in the DOS with number of hydrogen is attributed to the charge transfer between the ZrAl_m clusters and hydrogen atoms. Highest HOMO-LUMO gap of 3.055 eV is found for the ZrAl₃H₇ cluster which indicates that the ZrAl₃H₇ cluster is chemically more stable. Work function values for hydrogen doped clusters are in the range of 3.5 eV–4.4 eV which suggests the low optical absorption in considered clusters. The calculated enthalpy difference further confirms the chemisorption of hydrogen on ZrAl_m clusters ZrAl₃H_n and ZrAl₇H_n(for n = 1 to 5 and 7) are more exothermic than other considered clusters. Desorption energies per hydrogen for these clusters are found lower than the some of the best catalytic clusters Pd₁₃ and Pt₁₃ indicating more catalytic active nature.