Mechanisms of dopants influence on hydrogen uptake in COF-108: A first principles study

Mechanisms of dopants (Li, Na, Mg, and Al) influence on hydrogen uptake in COF-108 were investigated by means of first principles. The binding energy of dopants in COF-108 was estimated from the first principles total energy calculations. All doped systems are shown positive binding energies with the metallic state of the dopant as the reference. The lowest binding energy of 0.518 eV appeared in the Na-doped system while a large amount of energy (2.692 eV) is required for Al to dope into COF-108. Electronic structure analysis shows that dopants Li and Na move the conduction band crossing the Fermi energy level and introduce weakly bonded electrons near the Fermi energy, which may polarize the hydrogen molecules. It is expectable that interaction between hydrogen molecule and the host COF-108 could be enhanced by the polarization of hydrogen molecule. Therefore the hydrogen uptake will be improved in the doped systems. Dopant Mg slightly reduces the band gap between the valence and conduction bands, but is hard to build chemical bonds with the host atoms owing to the less overlaps between the bond peaks of Mg and the COF-108. It hardly affects the electron distributions of the COF-108 and therefore weakly changes the chemical interactions between atoms in COF-108.     

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.     

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.     

Computational evaluation of Mg-decorated g-CN as clean energy gas storage media

Ab initio studies were conducted to evaluate the performance of hydrogen storage by Mg-decorated graphite carbon nitride (g-CN, heptazine structure). In our calculations, we found that each unit of this material can accommodate one Mg atom. Partial charges from Mg were transferred to the pristine material, making itself more electropositive. This is favorable for hydrogen storage, as the adsorbed H₂ molecules can be easily polarized, and the electrostatic interactions can be enhanced. The configurations of the Mg-decorated g-CN with multiple adsorbed H₂ molecules were presented in this study, and the related adsorption mechanisms were also discussed in details. Each unit can adsorb at most 7 H₂ molecules with adsorption energies ranging from −0.276 eV to −0.130 eV. In addition, besides Mg, we also noticed that the nitrogen atoms also perform well in hydrogen adsorption. For this novel material, its highest capacity of hydrogen storage can reach to 7.8 wt%, highly surpassing the target value of 5.5 wt% set by the U.S. department of energy (DOE)[1]. The computational results provided in this study indicates a promising prospect for alkali metal functionalized 2D materials in energy storage; and through decent explorations, the performance of this class of materials can be largely improved.     

An investigation of Li-decorated N-doped penta-graphene for hydrogen storage

By making use of first principles calculations, lithium-decorated (Li-decorated) and nitrogen-doped (N-doped) penta-graphene (PG) was investigated as a potential material for hydrogen storage. The geometric and electronic structures of two types of N-doped PG were studied, and the band gaps were 1.86 eV and 2.06 eV, respectively, depending on the positions of the substitution. The probable adsorption sites for Li atoms on topside and downside were calculated. Hydrogen molecules were added one by one to Li-decorated N-doped PG to research the maximum hydrogen gravimetric density. It is found that up to 5 hydrogen molecules on topside and 8 hydrogen molecules on downside can be adsorbed around a Li atom, and the average adsorption energies are in the range of physical adsorption processes (0.1–0.4 eV). The gravimetric densities can reach 7.88 wt% for N-doped PG with Li decoration. Our results suggest that Li-decorated N-doped PG is a significantly promising material for hydrogen storage.     

Boron‐ and nitrogen‐doped penta‐graphene as a promising material for hydrogen storage: A computational study

We fulfill a comprehensive study based on density functional theory (DFT) computations to cast insight into the dissociation mechanism of hydrogen molecule on pristine, B‐, and N‐doped penta‐graphene. The doping effect has been also illustrated by varying the concentration of dopant from 4.2 at% (one doping atom in 24 host atoms) to 8.3 at% (two doping atoms in 24 host atoms) and by contemplating different doping sites. Our theoretical investigation shows that the adsorption energy of H₂ molecule and H atom on the substrate can be substantially enhanced by incorporating boron or nitrogen into penta‐graphene sheet. The B‐ and N‐doped penta‐graphene can effectively decompose H₂ molecule into two H atoms. Our results demonstrate that activation energies for H₂ dissociation and H diffusion on the B‐ and N‐doped penta‐graphene are much smaller than the pristine penta‐graphene. Further investigation of increasing concentration dopants of the penta‐graphene sheet gives sufficiently low activation barrier for H₂ dissociation process. This investigation reveals that the boron and nitrogen dopants can act as effective active site for H₂ dissociation and storage.     

Density functional theory study of reversible hydrogen storage in monolayer beryllium hydride by decoration with boron and lithium

The hydrogen storage capacity of M-decorated (M = Li and B) 2D beryllium hydride is investigated using first-principles calculations based on density functional theory. The Li and B atoms were calculated to be successfully and chemically decorated on the Surface of the α-BeH₂ monolayer with a large binding energy of 2.41 and 4.45eV/atom. The absolute value was higher than the cohesive energy of Li and B bulk (1.68, 5.81eV/atom). Hence, the Li and B atoms are strongly bound on the beryllium hydride monolayer without clustering. Our findings show that the hydrogen molecule interacted weakly with B/α-BeH₂(B-decorated beryllium hydride monolayer) with a low adsorption energy of only 0.0226 eV/H₂ but was strongly adsorbed on the introduced active site of the Li atom in the decorated BeH₂ with an improved adsorption energy of 0.472 eV/H₂. Based on density functional theory, the gravimetric density of 28H₂/8li/α-BeH₂) could reach 14.5 wt.% higher than DOE's target of 6.5 wt. % (the criteria of the United States Department of Energy). Therefore, our research indicates that the Li-decorated beryllium hydride monolayer could be a candidate for further investigation as an alternative material for hydrogen storage.     

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.     

Effects of charging,strain, and doping on the interaction between H2 and nitrogen-rich Penta-CN2 sheet

The development of advanced materials for the safety and efficiency of hydrogen storage media is necessary. We computationally explored the hydrogen storage properties of penta-CN₂ sheet. The hydrogen adsorption properties of neutral, negatively charged, externally strained, and metal-doped penta-CN₂ sheets were investigated in detail. Here, for the first time, the effect of the strain of two-dimensional nonmetallic materials on hydrogen adsorption is investigated. We found that the hydrogen binding energy increases to ?0.20 eV and achieves storage capacities up to 9.00 wt % on the negatively charged substrate, and to ?0.14 eV at 18% stretching. Moreover, metal doping causes hydrogen adsorption energy to increase to ?0.25–0.82 eV. The hydrogen storage capacity of Li-doped defective CN₂ sheet is up to 10.90 wt%. Our study may provide new insights into the search for advanced materials for reversible hydrogen storage.     

DFT study of hydrogen storage on the metallic decoration of boron substitution on zeolite templated carbon vacancy

This work reports DFT calculations for the assessment of metallic decoration of boron substitution Zeolite Templated Carbon vacancy for hydrogen adsorption. The boron substitution on Zeolite Templated Carbon vacancy is characterized by the formation of pentagonal and heptagonal rings. Moreover, the boron substitution can be considered as a promising way for hydrogen storage, this way boron substitution is used on Zeolite Templated Carbon vacancy in order to create an active site for metallic decoration. Once that we develop a Boron substitution on Zeolite Templated Carbon vacancy, the decoration with Lithium, Sodium, and Calcium atoms is also carried out. The analysis reveals that the Na decoration has the best performance for hydrogen storage. The results show that boron substitution on Zeolite Templated Carbon vacancy decorated with 3 Sodium atoms can adsorb up to fifteen hydrogen molecules (5 hydrogen molecules per Sodium atom), this gives a gravimetric storage capacity of 6.55 % wt., which is enough for meeting DOE gravimetric targets. In addition, the average binding energies and adsorption energies are calculated in the range 0.2298–0.2144 eV/H₂, which constitute desirable energies for hydrogen adsorption. Besides, the hydrogen adsorption process is carried out by electrostatic interaction between the Na cation and the induced H₂ dipole. The calculation performed in this work reveals that the boron substitution on Zeolite Templated Carbon vacancy decorated with Na atoms is a good candidate as a medium for 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.     

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.     

Li decorated 6,6,12-graphyne: A new star for hydrogen storage material

Based on ab initio calculations, we investigated the hydrogen storage capacity of Li decorated 6,6,12-graphyne (Li@GY). Due to the unique sp hybridization in GY, Li atoms can strongly bind to carbon atoms to avoid the formation of Li clusters on the surface of GY. It is found that the hydrogen storage capacity of Li@GY is high up to 19.3 wt% with the average adsorption energy of −0.230 eV which lying in the ideal adsorption energy range for practical application of hydrogen economy. The density of states and charge density difference demonstrated that the adsorption mechanism mainly depended on the electrostatic field produced by the Li ions on GY. Moreover, the formation of super hydrogen molecules induced by the electrostatic field around Li ions can further enhance the hydrogen absorption energy. Our results indicated that Li decorated 6,6,12-graphyne would be a potential material for hydrogen storage.     

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.     

Hydrogen adsorption and storage behaviors of Li-decorated PdS2 monolayer: An extended tight-binding study based on DFT

The hydrogen adsorption and storage of the lithium-decorated PdS₂ monolayer at nano-size has been investigated by using extended tight-binding (GFN1-xTB) based on density functional theory (DFT). The calculation results demonstrate that the average adsorption energies of 1–5H₂ change in 0.47–0.20 eV/H₂ range which decreases with increasing of adsorbed hydrogen molecule number. The gravimetric density for hydrogen storage adsorption with 12Li atom and 60H₂ molecules of Li-decorated PdS₂ monolayer is about 6.98 wt% considered as possible application in hydrogen storage. The examination of the hydrogen store mechanism between the monolayer and hydrogen molecules is presented by polarization between Pd and H₂, which can be effect on the adsorption behavior.     

Hydrogen adsorption studies of TiFe surfaces via 3-d transition metal substitution

Hydrogen adsorption over TiFe surface and doped TiFe surface is investigated within density functional theory. Surface energy calculations confirm that TiFe (111) surface has the minimum value among three low index crystallographic surfaces, (100), (111) and (110). The (111) TiFe surface has two different terminations one with Fe and the other with Ti. Here both the (111) surfaces with different terminations are considered for doping with all the 3-d transition metal atoms from Sc to Zn. Furthermore, the molecular hydrogen adsorption over all the doped surfaces is investigated. V was found to be the most suitable element for doping in Fe terminated (111) surface. V doping in Fe terminated surface enhanced E_ads by 0.6 eV from ?3.30 eV (undoped) to ?3.90 eV after doping. Whereas in case of Ti terminated surface Co was found to be the best element for doping as it enhanced E_ads by ～0.5 eV from ?2.64 eV (undoped) to ?3.15 eV after doping. A significant decrease in d-band width from 1.95 eV to 1.22 eV in case of Co substitution in Ti terminated surface and from 2.42 eV to 1.33 eV in case of V substitution in Fe terminated surface enhances the hydrogen adsorption in TiFe (111) surface. Thus, even using a very small amount of dopant can influence the hydrogen adsorption properties of TiFe alloy.     

Density functional theory calculations of hydrogen adsorption on Ti-, Zn-, Zr-, Al-, and N-doped and intrinsic graphene sheets

The effect of different doped atoms on the interactions between graphene sheets and hydrogen molecules were investigated by density functional theory calculations. The interactions between graphene sheets and hydrogen molecules can be adjusted by doped atoms. The Ti-doped graphene sheet had the largest interaction energy with the hydrogen molecule (approximately −0.299 eV), followed by the Zn-doped graphene sheet (about −0.294 eV) and then the Al-doped graphene sheet (approximately −0.13 eV). The doped N atom did not improve the interactions between the N-doped graphene sheet and the hydrogen molecule. Our results may serve as a basis for the development of hydrogen storage materials.     

A study on the hydrogen storage performance of graphene–Pd(T)–graphene structure

The 1–6 H₂ molecule adsorption energy and electronic properties of sandwich graphene–Pd(T)–Graphene (G–Pd(T)–G) structure were studied by the first-principle analysis. The binding energies, adsorption energies, and adsorption distances of Pd atoms-modified single-layer graphene and bilayer graphene with H₂ molecules at B, H, T adsorption sites were calculated. In bilayer graphene, the adsorption properties at T sites were found to be more stable than those at B and H sites. The binding energy of Pd atoms (4.16 eV) on bilayer graphene was higher than the experimental cohesion energy of Pd atoms (3.89 eV), and this phenomenon eliminated the impact of metal clusters on adsorption properties. It was found that three H₂ molecules were stably adsorbed on the G–Pd(T)–G structure with an average adsorption energy of 0.22 eV. Therefore, it can be speculated that G–Pd(T)–G is an excellent hydrogen storage material.     

First-principles study of single atom adsorption on capped single-walled carbon nanotubes

The effects of hydrogen, oxygen, and nitrogen atomic chemisorption on capped armchair (5, 5) single-walled carbon nanotubes (SWCNT) are investigated by first-principles calculations based on the density functional theory aimed at the CNT based fuel cell applications. O or N chemisorption could break C–C bond to form doping type structure. C–C bonds are weakened from H chemisorption, favoring hydrogen storage. Both C–adatom and related C–C bond lengths fluctuate from the cap top to the tube for each type of adsorbate. There is a total amount of about 1.0 e charge transfer between N or O atom and the carbon atoms, and the catalytic activity is expected to be higher with N adsorption around the cap top. The adsorption energies and work functions also vary with the adsorption at different sites. Atomic chemisorptions are more stable on the cap than on the tube due to smaller local curvature radius. The work functions increase to above 5.0 eV with the adsorption of N and O, and drop below 4.8 eV for H adsorption, comparing with 4.89 eV for the clean tube. DOS study reveals orbital information for electrons of adatom contributed to the valence bands and the conduction bands.     

Phosphorene: A promising candidate for H2 storage at room temperature

Owing to the existence of periodic channels in phosphorene, this 2D material can be a good candidate for room temperature reversible hydrogen storage. The density functional theory calculations (DFT), including van der Waals interactions (vdW-DF2) coupled with the cooper exchange functional (C09), has been applied to study the potential of phosphorene as a new 2D material for hydrogen storage. Our results show that the adsorption energy (−292 to −277 meV) of H₂ on phosphorene is appropriate for storage. The analysis of diffusion pathways between different physisorbed states on phosphorene shows that a single hydrogen molecule diffuses very easily along the open channel (less than 1 meV along the zigzag direction), as compared to 14 meV for diffusion across the channels (along the armchair direction). The potential energy surfaces for the dissociative chemisorption of H₂ was computed on highly symmetric sites of phosphorene and the highest activation barrier was found to be 2.77 eV. The very large dissociation energy coupled with a strong physisorption of H₂ on phosphorene and facile diffusion, makes this 2D material a promising candidate for H₂ storage at room temperature.