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.     

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.     

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.     

Li decorated heteroborospherene C4B32 as high capacity and reversible hydrogen storage media: A DFT study

In this work, adsorption of H₂ molecules on heteroborospherene C_2v C₄B₃₂ decorated by alkali atoms (Li) is studied by density functional theory calculations. The interaction between Li atoms and C₄B₃₂ is found to be strong, so that it prevents agglomeration of the former. An introduced hydrogen molecule tilts toward the Li atoms and is stably adsorbed on C₄B₃₂. It is obtained that Li₄C₄B₃₂ can store up to 12H₂ molecules with hydrogen uptake capacity of 5.425 wt% and average adsorption energy of ?0.240 eV per H₂. Dynamics simulation results show that 6H₂ molecules can be successfully released at 300 K. Obtained results demonstrate that Li decorated C₄B₃₂ is a promising material for reversible hydrogen storage.     

Defect and interstitial B synergistically promote the stability and H2 dissociation of Pd6 supported by graphene

As a potential hydrogenation catalyst, palladium nanoparticles supported by graphene encounter three major problems: transition metal agglomeration, interstitial H atom, and the competition between desorption of H₂ and Pd_n-H_x complex from graphene sheet. In this paper, defects and interstitial B are used to promote the stability and H₂ dissociation of Pd₆ supported by graphene. The introduction of defects increases the binding energy of Pd, Pd₆ and Pd₆B to graphene by a factor of 5–7 and 7–9 before and after hydrogen adsorption, respectively. It indicates that defects can effectively avoid the desorption competition between Pd_nH_x and hydrogen molecules. Moreover, the energy barrier of dissociation for the first hydrogen molecule on Pd₆B/C₄₉ is 0.49 eV, which is lower than 0.75 eV on Pd₆/C₄₉ and 0.69 eV on Ti₆/C₄₉.     

Density functional theory (DFT) study on the effects of Li doping on electronic states of graphene

The electronic structures of a lithium ion (Li⁺) doped-graphene at the ground and low-lying excited states have been investigated by means of density functional theory (DFT) method. A graphene composed of 19 benzene rings was used as a model of graphene, while the edge carbon atom was terminated by hydrogen atom (expressed by C₅₄H₁₈). The geometry optimization showed that the Li⁺ ion binds to a hexagonal site where six carbon atoms interact equivalently to the Li⁺ ion. When the Li⁺ ion interacts with the graphene surface, the electronic configuration of the Li⁺ ion is changed from (1s)²(2s)⁰ to (1s)²(2s)^0.01(2p)⁰(3p)^0.02, suggesting that the sp-hybridization of lithium ion is important in the adsorption to the graphene surface. The band gap of graphene is slightly red-shifted by the doping of Li⁺ ion due to the interaction with the sp-hybrid orbital. The effects of Li⁺ on both the ground and excited electronic states of graphene were discussed on the basis of theoretical results.     

The enhancement of hydrogen storage capacity in Li,Na and Mg-decorated BC3 graphene by CLICH and RICH algorithms

New hydrogen adsorption states on Li, Na, and Mg-decorated graphene-type BC₃ sheet have been investigated by first-principles calculations. The structural, electronic and binding properties, metal binding and nH₂ (n = 1–10) adsorption states of these systems are studied in detail with the Mulliken analysis, charge density differences, and partial density of states. To enhance the number of the adsorbed H₂ molecules per metal atom, and also to generate the better initial coordinates for reducing the simulation time, we present two masthematical algorithms (CLICH and RICH). The tested results on BC₃ sheet and boron-doped graphene (C₃₀B₂) show that these algorithms can increase the number of adsorbed hydrogen molecules by minimizing the computational time. It is seen that nH₂ (n = 1–10) adsorbed Li,/Na and/Mg-decorated BC₃ single- and double-sided systems are industrial materials for hydrogen storage technology, their adsorption energies fall into the acceptable adsorption energy range (0.20–0.60 eV/H₂). It is concluded from the optimized geometries and charge density differences for the higher number of H₂ adsorbed systems that not only decorated metal atom but also the sheet plays an important role in hydrogen storage process, because the boron atoms in the sheet expand the induced electric field between the adatoms and BC₃ sheet. From Mulliken analysis, there is a charge transfer mechanism between H₂ molecules and metal atoms. Moreover, the resonant peaks for the sheet, metal atoms and H₂ molecules in partial density of states curves indicate the possible hybridizations. Additionally, these adsorption processes are supported by charge density difference plots.     

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.     

Theoretical studies on hydrogen adsorption properties of lithium decorated diborene (B2H4Li2) and diboryne (B2H2Li2)

Using ab initio based quantum chemical calculations, we have studied the structure, stability and hydrogen adsorption properties of different boron hydrides decorated with lithium, examples of the corresponding anions being dihydrodiborate dianion, B₂H₂²⁻ and tetrahydrodiborate dianion, B₂H₄²⁻ which can be considered to be analogues and isoelectronic to acetylene (C₂H₂) and ethelene (C₂H₄) respectively. It is shown that there exists a B-B double bond in B₂H₄Li₂ and a B-B triple bond in B₂H₂Li₂. In both the complexes, the lithium sites are found to be cationic in nature and the calculated lithium ion binding energies are found to be very high. The cationic sites in these complexes are found to interact with molecular hydrogen through ion-quadrupole and ion-induced dipole interactions. In both the complexes, each lithium site is found to bind a maximum of three hydrogen molecules which corresponds to a gravimetric density of ∼23 wt% in B₂H₄Li₂ and ∼24 wt% in B₂H₂Li₂. We have also studied the hydrogen adsorption in a model one-dimensional nanowire with C₆H₄B₂Li₂ as the repeating unit and found that it can adsorb hydrogen to the extent 9.68 wt% and the adsorption energy is found to be −2.34 kcal/mol per molecular hydrogen.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.     

A DFT-D3 investigation on Li,Na, and K decorated C6O6Li6 cluster as a new promising hydrogen storage system

Because of the increasing demand for energy sources, searching for reversible and high-capacity hydrogen storage materials plays a vital role in the extensively utilizing of hydrogen as a clean energy source. In this study, dispersion-corrected density functional theory (DFT-D3) calculations are utilized to examine the possibility of storing H₂ molecules on Li, Na, and K alkali metals decorated C₆O₆Li₆ cluster. To evaluate H₂ adsorption capability, the adsorption energies, electron density difference iso-surfaces, and charge-transfers are calculated and discussed. The results indicate that a hydrogen molecule is physisorbed on the Li@C₆O₆Li₆, Na@C₆O₆Li₆, and K@C₆O₆Li₆ with average adsorption energies of −0.264, −0.150, and −0.109 eV, respectively. Double-sided alkali metal atoms decoration can lead to the maximum gravimetric density of 15.68, 14.49, and 13.79 wt% for 2Li@C₆O₆Li₆–8H₂, 2Na@C₆O₆Li₆–10H₂, and 2K@C₆O₆Li₆–12H₂ complexes, respectively. Finally, desorption temperatures reveal that the systems can operate as reversible hydrogen storage materials.     

Reconnoitring the nature of interaction and effect of electric field on Pd/Pt/Ni decorated 5-8-5/55–77 defected graphene sheet for hydrogen storage

There is plenty of graphene based Hydrogen storage technologies and studies still few questions like ‘what kind of interaction present between Metal-Metal, Metal-Graphene, Metal-Hydrogen and Graphene-Metal and a possible way of controlling it to enhance H₂ adsorption’ are not revealed properly. Similarly, the chosen metal atoms Pd, Pt and Ni are widely reported as a promising catalyst yet there is no conclusive evidence to show the best among three atoms. Thus, in this present work 5-8-5 and 55–77 defected graphene is decorated with the Pd, Pt and Ni metal atoms to adsorb Hydrogen molecules. The obtained results have shown that the better adsorption of H₂ molecule depends on Metal-Metal and Metal-Graphene interaction. Similarly, the adhesive force between Pt and 5-8-5/55–77 sheets are slightly higher than the Pd and Ni atoms. Pd–Pd (−0.47 eV) and Pt–Pt (−1.99 eV) interaction values on the surface of 5-8-5 sheets are slightly lesser in magnitude than the Pd–C (−1.14 eV, −1.19 eV) and Pt–C (−2.42 eV, −2.55 eV) interactions. The topological analysis results exhibit the partially covalent nature of interaction and it confirms that the adhesive force between Metal-Graphene is higher than the cohesive force between Metal-Metal on 5-8-5 and 55–77 sheets. The electrophilicity results of Pd, Pt and Ni decorated sheets show that the two Pt decorated 5-8-5 sheet has higher electrophilicity value of 16.782 eV which is considerably higher than other sheets and this particular 5-8-5-Pt₂ system has higher H₂ adsorption energy value of −1.696 eV. The overall pattern of H₂ adsorption on chosen three metals are Pt > Ni > Pd and our results show that both strong Metal-Metal and Metal-Graphene interactions lead to poor adsorption activity. The metals are strongly polarizing the H₂ molecules which lead to good adsorption. Further, the results confirm that the π orbitals of Metal and Graphene play a major role in the adsorption of excessive H₂ molecules. In order to enhance and control the H₂ adsorption energy, a positive electric field is applied to the system. This applied electric field enhances the polarization which leads to H–H bond elongation and strong adsorption. From the obtained results, it is conclusive that the 5-8-5-Pt system has shown good response for the supplied electric field with the maximum adsorption energy value of −5.23 eV. Comparatively, the 5-8-5 systems are responding well for the applied electric field by increasing the adsorption energy than 55–77 systems.     

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.     

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

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

Light and stable LinB14(n=1–5) clusters for high capacity hydrogen storage at room temperature: A DFT study

In this article, we have explored the hydrogen (H₂) storage capacity of the Li doped B clusters Li_nB₁₄(n = 1–5) using density functional theory (DFT). The geometrical and Bader's topological parameters indicate that the clusters adsorb H₂ in the molecular form. The Li atom polarises the H₂ molecules for their effective adsorption on the clusters. The Li_nB₁₄ (n = 1–5) clusters are found to be stable even after H₂ adsorption at room temperature. The average adsorption energy is found to be in the range of 0.12–0.14 eV/H₂. Among the various clusters, the Li₅B₁₄ shows maximum H₂ storage capacity (13.89 wt%) at room temperature. The ADMP simulation reveals that within few femtoseconds (fs), the H₂ molecules begin to move away from the clusters and within 400 fs most of the H₂ molecules moved away from the clusters.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

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%).     

Analysis of hydrogen storage mechanism in bilayer double-vacancy defective graphene modified using transition metals: Insights from Ti-BDVG(Ti)-Ti

In this study, using the first principles calculation and analysis, we found that the B-doping in double-vacancy defective graphene could effectively increase the binding energy of Ti atoms in each adsorption site, especially in the H2 adsorption site with a maximum binding energy of 8.3 eV. However, N-doped bilayer graphene (N-BLG) reduced the binding energy of Ti atoms by 88% of the adsorption sites. Given these two findings, a B- and N-doped bilayer double-vacancy-defective graphene (Ti-BDVG(Ti)-Ti) was constructed. Our findings also showed that the Ti-BDVG(Ti)-Ti outer surface and inner surface could adsorb 32 and 12H₂ molecules, respectively, of which 22, 20 and 2H₂ molecules are adsorbed by Kubas, electrostatic interactions and chemisorption, respectively. The hydrogen storage mechanism of Ti-BDVG(Ti)-Ti involves multiple adsorption modes, and this hydrogen storage mechanism provides a theoretical basis for the rational design of hydrogen storage materials with maximum effective hydrogen storage capacity.     

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.