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.     

Effect of boron substitution on hydrogen storage in Ca/DCV graphene: A first-principle study

By using density functional calculations, the effects of boron are investigated in the new hydrogen storage systems, which are formed by substituting different numbers of boron atoms to the first (BDDCV-F) and the second (BDDCV-S) neighbor of double carbon-vacancy (DCV). The layered host systems of boron-substituted DCV graphene are decorated with Ca metal to increase the number of adsorbed H₂ molecules. Storing of H₂ applications are performed by using two coordinate algorithms as CLICH (Cap-Like Initial Conditions for Hydrogens) and RICH (Rotational Initial Conditions for Hydrogens). The adsorption properties of (1–14) H₂ molecules on the constructed systems are examined. The results for BDDCV-F and BDDCV-S boron-doped systems are compared with each other and those of the pure-DCV graphene. To compare the stabilities of BDDCV-systems, the formation energies are calculated. It is concluded from Mulliken charge analysis, the partial density of states and electron density differences that boron substitution process to different locations of the DCV graphene plays a crucial role on the charge transfer between Ca atom the layered host system, ionic nature and the binding properties of the systems. The herringbone-like anisotropic electron density is transformed to isotropic density with the substitution of the boron atoms. Then, the electric field, which is induced by ionic interactions and governs H₂ adsorption processes, is changed and intensified along with the sheet. In this way, it can be achieved more effective H₂ adsorption. It is seen from the adsorption energies of single- and double-side Ca-decorated systems that the processes of boron-substitution and Ca-decoration can considerably improve the hydrogen storage capability of pure-DCV graphene system, thus (8 and 10)H₂ can be adsorbed per Ca-atom in these-type systems. The high gravimetric density of 5.80% is calculated, although larger cell and empty surface states. Moreover, the average desorption temperatures are calculated by using van't Hoff equation, and it is seen that the DCV including boron-substituted systems have closer desorption temperatures to the room temperature than pure-DCV. To check the H₂ desorption of the systems, molecular dynamics simulations are performed at 200 K and 300 K temperatures.     

A study on the hydrogen storage capacity of Ni-plated porous carbon nanofibers

In this study, we prepared highly porous carbon-nanofiber-supported nickel nanoparticles as a promising material for hydrogen storage. The porous carbons were activated at 1050 °C, and the nickel nanoparticles were loaded by an electroless metal-plating method. The textural properties of the porous carbon nanofibers were analyzed using N₂/77 K adsorption isotherms. The hydrogen storage capacity of the carbons was evaluated at 298 K and 100 bar. It was found that the amount of hydrogen stored was enhanced by increasing nickel content, showing 2.2 wt.% in the PCNF-Ni-40 sample (5.1 wt.% and 6.4% of nickel content and dispersion rate, respectively) owing to the effects of the spill-over of hydrogen molecules onto the metal–carbon interfaces. This result clearly indicates that the presence of highly dispersed nickel particles can enhance high-capacity hydrogen storage.     

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.     

Improving the mechanical processing of titanium by hydrogen doping: A first-principles study

Solute H in Ti alloys has an important effect on their deformation and ductility, therefore changing the mechanical properties and improving the mechanical processing. We have performed first-principles calculations to investigate the effects of H on the mechanical properties of the hexagonal close packed α-Ti and the body centered cubic β-Ti, and make an attempt to understand how the solute H improves the mechanical processing. We compute the structural parameters, elastic properties, and the generalized stacking fault energies for α- and β-Ti with and without the H addition. We find H decreases the shear moduli and the unstable stacking fault energies of α-Ti, enhancing the deformation tendency, while these quantities are increased by H in β-Ti. We predict the H effect on the ductility using different criteria, and find H makes α-Ti more ductile, but raises the brittleness of β-Ti. Our results indicate H may have a favorable effect on improving the mechanical processing of Ti alloys.     

Lithium decoration characteristics for hydrogen storage enhancement in novel periodic porous graphene frameworks

Hydrogen storage in porous materials by physical adsorption is being discussed to provide widespread usage of hydrogen energy systems. One of the recent hydrogen storage media that store hydrogen physically is Porous Graphene Frameworks (PGFs). In the study, three different PGFs were constructed by using Benzene-1,3,5-tricarboxylic acid (BTC), 4,40,400-Benzene-1,3,5-triyltribenzoate (BTB) and 4,40,400-(benzene-1,3,5triyl-tris (benzene-4,1-diyl))tribenzoate (BBC) organic linkers. The geometries of the structures were optimized and lithium atoms were dispersed inside. Then, thirty-three different structures were derived. Finally, hydrogen storage capacities and surface areas of each structure were computed. It was found out that 160 lithium dispersed Graphene-BBC structure has the highest hydrogen storage capacity with 4.26 wt % at 298K and 100 bars while 70 lithium dispersed graphene-BTB structure store 9.81 wt % hydrogen at 77K and 4 bars, and lithium free graphene-BBC structure store 20,68 wt % hydrogen at 77K and 100 bars. Lithium dispersion enabled extra surfaces for Graphene-BTB and Graphene-BBC structures to the limits. But surface area of Graphene-BTC structure decreased with lithium dispersion. The number of limits for Graphene-BTB and Graphene-BBC named structures were 70 and 200 lithium atoms, respectively. At the final it is pointed out that constructed novel PGFs could store comparable and relatively high hydrogen in various conditions. The existence of lithium atoms played a minor role to enhance hydrogen storage capacity but the limits are critically important to reach maximum capacity.     

Exfoliated MoS2 with porous graphene nanosheets for enhanced electrochemical hydrogen evolution

Porous graphene (P-rGO) was synthesized from graphene oxide (GO) via a one-pot calcination method with CO₂ as an activation agent at 800 °C. Due to the special porous structure, the surface area of P-rGO can be increased to ～759 m²/g. The P-rGO was then used as a support to incorporate with chemical exfoliated molybdenum disulfide (MoS₂) for the fabrication of MoS₂/P-rGO composite. Compared to bulk MoS₂, the exfoliated MoS₂ is in the 1T phase with a metallic property and smaller charge transfer resistance, thus has a better activity in electrochemical hydrogen evolution reaction (HER). The HER activity of 1T MoS₂ could be further increased after the combination with P-rGO. The overpotential of 1T MoS₂/P-rGO was only ～130 mV vs. RHE, and the corresponding Tafel slope was ～75 mV Dec^?1. The special porous structure and good electric conductivity of P-rGO decrease the charge transfer resistance of the composite without sheltering too many active sites of MoS₂, thus leading to the enhanced HER activity. As an efficient noble metal free HER catalyst, the 1T MoS₂/P-rGO has great potential for large-scale hydrogen production.     

Enhanced hydrogen storage performances of LiBH4 modified with three-dimensional porous fluorinated graphene

Novel fluorinated graphene (FG) nano-sheets with three-dimensional (3D) porous structure were synthesized by one-pot hydrothermal reaction, and then ball milled with LiBH₄ to prepare the hydrogen storage composite material. The LiBH₄ with 20 wt.% FG composite begins to release hydrogen at 204 °C, 120 °C lower than that of pure LiBH₄. Moreover, it can release 3.45 wt.% hydrogen at 400 °C within 1000 s, which is 2.57 times faster than pure LiBH₄. The reversibility of the LiBH₄–FG composite also has been enhanced, its absorption capacity still reaches 78.6% of initial hydrogen uptake at the 4th cycle. According to the phase composition analyses, F^– can partially substitute the anionic H in LiBH₄ or LiH, resulting in a favorable thermodynamic modification. Additionally, the activation energy (E_a) of hydrogen desorption of LiBH₄ is reduced from 181.80 kJ/mol to 130.87 kJ/mol. The remarkably improved hydrogen storage performances of LiBH₄ are largely attributed to the combined effects of the nano-modifying and the function of F anion of the FG.     

A first-principles study of hydrogen storage of high entropy alloy TiZrVMoNb

In this study, density functional theory calculations have been carried out to study the hydrogen storage properties of high entropy alloy (HEA) TiZrVMoNb. It reveals that a BCC→FCC phase transformation occurs when the hydrogen content reaches 1.5 wt% during hydrogenation process, and octahedral and tetrahedral interstitial sites are preferable for hydrogen occupation before and after phase transformation, respectively. Further energetic analyses show that different hydrogen occupations in HEAs play an important role in the thermal stability of hydrides. The maximum hydrogen storage capacity for TiZrVMoNb is predicted to be 2.65 wt%, which is comparable to the largest value of 2.7 wt% for TiZrVHfNb and larger than that of other reported HEA hydrogen storage materials reported in the literature. As compared with the previously reported HEA TiZrHfMoNb with the change of only one principal element, the TiZrVMoNb not only has much higher hydrogen storage capacity, but also has more moderate hydrogen desorption temperature. The difference in hydrogen storage properties between these two HEAs is mainly attributed to the atomic weight, site occupation, lattice distortion and chemical effect of metal elements. The present study thus suggests that the TiZrVMoNb HEA has great potential as hydrogen storage materials and proposes a strategy to enhance the hydrogen storage properties of HEAs.     

Theoretical study of hydrogen storage by spillover on porous carbon materials

Hydrogen storage by spillover in porous carbon material (PCM) has achieved great success in experiments. During the past 20 years, a large number of theoretical works have been performed to explore the hydrogen spillover mechanism, look for high-performance hydrogen storage materials and high-efficiency catalysts. In this paper, we summarize and analyze the results of the past researches, and draw the following conclusions: (1) In PCM surface, the stability of chemisorbed H can be reached through phase nucleation process, which can be initiated in the vicinity of surface impurities or defects. (2) To achieve the 2020 U.S. Department of Energy (DOE) target, the PCM material used for hydrogen storage by spillover should have a sp² carbon ratio greater than 0.43 and a surface area less than 3500 m²/g, which gives us an inspiration for exploring hydrogen spillover materials. (3) Due to a high barrier, the hydrogen spillover almost can not be initiated on pure PCM substrate at room temperature. By introducing the defects or impurities (e.g. holes, carbon bridges, oxygen functional groups, boron atoms and fluorine atoms), the spillover barriers can be reduced to a reasonable range. In addition, hydrogen atoms may also migrate in a gas phase. (4) According to our previous results of kinetic Monte Carlo simulations, there is a linear relationship between the reaction temperature and the migration barrier. The optimal barrier for the hydrogen spillover should be in the range of 0.60–0.88 eV. (5) Once the hydrogen atoms are chemically adsorbed on the carbon substrate, it is difficult to diffuse again due to the strong strength of C–H bond. Several theoretical diffusion mechanisms have been proposed. For example, the H atoms in physisorption state can diffuse freely on carbon surfaces with high mobility, using the shuttle gases (e.g. BH⁻₄, H₂O, HF and NH₃) to make the migration thermodynamically possible and decrease the migration barrier, the H atoms diffuse inside the interlayer space of the bi- and tetralayer graphene, and introducing the impurities on the surface to facilitate the hydrogen diffusion. (6) The H desorption through the directly recombination or the reverse spillover is unlikely to occur at normal temperature. The Eley-Rideal reaction may be the only possible mechanism for desorption of the adsorbed H atoms in carbon substrate. Finally, we have made a prospect for further research works on hydrogen storage by spillover.     

Enhanced HER catalysis based on MXene/N-doped graphene heterostructures: A first-principles study

Based on the first-principles density functional theory, we investigated the structural and electronic properties and hydrogen evolution reaction (HER) activity of the heterostructures composed of different MXene and N-doped graphene (NDG). Our results show noticeable electron transfer occurring between the interfaces of the heterostructure, and the addition of MXene modifies the electronic structure of the NDG surface. Furthermore, it was observed that the heterostructure enhanced the adsorption of H on NDG surface and improved HER activity. The effects of heterostructure types and H coverage rate on HER activity were also investigated. This study suggests that appropriate design of MXene/NDG heterostructure can make it a potential HER catalyst.     

A DFT study on enhanced adsorption of H2 on Be-decorated porous graphene nanosheet and the effects of applied electrical fields

The adsorption of the hydrogen molecule on the pure porous graphene nanosheet (P-G) or the one decorated with Be atom (Be-G) was investigated by the first-principle DFT calculations. The Be atom was adsorbed on the P-G with a binding energy of ?1.287 eV to successfully establish the reasonable Be-G. The P-G was a poor substrate to interact weakly with the H₂, whereas the Be-G showed a high affinity to the adsorbed H₂ with an enhanced adsorption energy and transferred electrons of ?0.741 eV and 0.11 e, respectively. A molecular dynamics simulation showed that the H₂ could also be adsorbed on the Be-G at room temperature with a reasonable adsorption energy of ?0.707 eV. The interaction between the adsorbed H₂ and the Be-G was further enhanced with the external electrical fields. The applied electrical field of ?0.4 V/Å was found to be the most effective to enhance the adsorption of H₂ on the Be-G with the modified adsorption energy and the improved transferred electrons being ?0.708 eV and 0.17 e, respectively. Our study shows that the Be-G is a promising substrate to interact strongly with the H₂ and could be applied as a high-performance hydrogen gas sensor, especially under the external electrical field.     

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H₂. When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H₂ which shows that desorption would take place at ambient conditions.     

Small Pd cluster adsorbed double vacancy defect graphene sheet for hydrogen storage: A first-principles study

Stability and electronic properties of small Pd_n clusters (n = 1–5), adsorbed on different types of double vacancy (DV) defect graphene sheets are thoroughly investigated by both density functional theory (DFT) and molecular dynamics (MD). Defect bridge sites of DV(555-777) defect graphene sheet are identified to be the most favorable for Pd₄ cluster adsorption. MD calculations, performed using a canonical ensemble, showed this system to be highly stable up to 800 K. Much better hybridization between C 2p and Pd 4d and 5s orbitals near Fermi level as well as higher charge transfer to graphene sheet was found to be the governing reason for enhanced stability of Pd₄ cluster on DV(555-777) defect site. Comparative analysis of H₂ storage on Pd₄ cluster adsorbed pristine and DV(555-777) defect graphene sheet showed, while adsorption energy/H₂ molecule for both cases lie well within desirable energy window for a hydrogen storage media, the later is much more efficient energetically as distorted in plane sp² hybridization reduces the saturations of C–C bonds in the defect regions, making more electron density available for bonding; which leads to higher net charge gain of Pd₄ cluster and higher charge sharing with H₂ molecule.     

Si-doping effect on the enhanced hydrogen storage of single walled carbon nanotubes and graphene

We identified several parameters that correlate with the hydrogen physisorption energy and physicochemical properties of heteronuclear bonding in single-walled carbon nanotubes (SWCNT) and graphene. These parameters were used to find the most promising heteronuclear doping agents for SWCNTs and graphene for enhanced hydrogen storage capacity. Si-doping was showed to increase the amount of physisorbed hydrogen on such surfaces. Grand Canonical Ensemble Monte Carlo (GCMC) simulations showed that the hydrogen storage capacity of 10 at% Si-doped SWCNT (Si-CNT10) could reach a maximum of 2.5 wt%, almost twice the storage capacity of undoped SWCNTs, which were showed to reach a maximum capacity of 1.4 wt% at room temperature. To achieve this capacity, debundling effects of the uneven surfaces of Si-doped SWCNTs were found to be necessary. Similarly, 10 at% Si-doping on graphene (Si-GR10) was showed to increase the hydrogen storage capacity from 0.8 to 2.4 wt%.     

Melaleuca bark based porous carbons for hydrogen storage

Typical porous carbons were obtained from waster biomass, melaleuca bark activated by potassium hydroxide (KOH), and characterized by XRD, SEM, TEM, FTIR, XPS and N₂-sorption. The different samples with tunable morphologies and texture were prepared by controlling synthesis reaction parameters. The resulting samples demonstrate both high surface area (up to 3170 m² g⁻¹) and large hydrogen storage capacity (4.08 wt% at 77 K and 10 bar), implying their great potential as hydrogen storage materials.     

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.     

The effect of hydrogen on the mechanical properties of FeTi for hydrogen storage applications

In this paper, the density functional theory (DFT) within the generalized gradient approximation (GGA) was used. The single crystal elastic constants for the intermetallic FeTi and its hydrides FeTiH and FeTiH₂ are successfully obtained from the stress–strain relationship calculations and the strain energy-strain curves calculations, respectively. The shear modulus, Young's modulus, Poisson's ratio and shear anisotropic factors are also calculated. The bulk moduli derived from the elastic constants calculations of the cubic FeTi, orthorhombic P222₁ FeTiH and Cmmm FeTiH₂ are calculated. For cubic FeTi compound, the bulk modulus is in a good agreement with both theoretical results and experimental data available in the literature. More importantly, it is found that, the insertion of hydrogen into the FeTi crystal structure causes an increase in the bulk modulus. From the analysis of shear-to-bulk modulus ratio, it is found that FeTi compound and its hydrides are ductile and that this ductibility, changes with changing the concentration of hydrogen.     

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.