A comparative study of the reversible hydrogen storage behavior in several metal decorated graphyne

In virtue of the first-principle calculations, the hydrogen storage behavior in several metal decorated graphyne was investigated. It is found that the hydrogen storage capacity can be as large as 18.6, 10.5, 9.9 and 9.5 wt% with average adsorption energy of about −0.27, −0.36, −0.76 and −0.70 eV/H₂ for Li, Ca, Sc, Ti decorated graphyne, respectively. The results suggest potential candidates for hydrogen storage at ambient condition. The adsorption mechanism for H₂ on metal coated graphyne was mainly attributed to the polarization induced by electrostatic field of metal atoms on graphyne and the hybridization between the metal atoms and hydrogen molecules. Furthermore, the formation of super-molecules of hydrogen can enhance the adsorption energy.     

Density functional theory study on hydrogen storage capacity of yttrium decorated graphyne nanotube

The hydrogen storage capacity of yttrium decorated graphyne nanotubes is calculated using spin polarized DFT method. The stabilities, electronic properties and the structures of Y attachment on graphyne tube are investigated. It is revealed that Y can be separately adsorbed on graphyne tube with the binding energy of 6.76 eV and the clustering of metal atoms is hindered. The geometry optimization shows that Y atoms decorated graphyne tube can capture 42H₂ molecules through Dewar-Kubas like interaction and the polarization under the electrostatic potential formed by Y and graphyne tubes. The weight percent capacity is 5.71 wt%, with an average hydrogen adsorption energy of −0.153 eV per H₂, indicating its potential application on hydrogen storage candidates.     

Potassium decorated γ-graphyne as hydrogen storage medium: Structural and electronic properties

Different sites for K adsorption in γ-graphyne were investigated using density functional theory (DFT) calculations and optical and structural properties of the structures were examined. For the most stable structures, we put one H₂ molecule in different directions on the various sites to evaluate the hydrogen adsorption capability of them. Then, one to nine H₂ molecules in sequence were added to the best structure. Results show that clustering of the K atoms is hindered on the graphyne surface and the most desirable adsorption site for K atom is the hollow site of 12-membered ring with adsorption energy of 5.86 eV. Also, this site is the best site for H₂ adsorption onto K-decorated graphyne with E_das of −0.212 eV. Adding of number of H₂ molecule on this site shows that K atom can bind nine H₂ molecules at one side of the graphyne with the average adsorption energy of 0.204 eV/H₂. Therefore, for one side ca. 8.95 wt % and for both sides of the graphyne with a K atom in each side ca. 13.95 wt % of the hydrogen storage capacity can be achieved. This study shows that K-decorated graphyne can be a promising candidate for the hydrogen storage applications.     

Li-decorated graphyne as high-capacity hydrogen storage media: First-principles plane wave calculations

Taking into account the van der Waals correction, the characteristics of the Li-decorated graphyne as the hydrogen storage medium have been explored using first-principles plane wave calculations. We find that Li atom can be adsorbed not only over the center of large hexagon (H_L site) but also over the center of small hexagon (H_S site). For double-side Li decorations, there are 14H₂ molecules can be adsorbed on Li-decorated graphyne primitive cell with the adsorption energy of 0.19 eV/H₂. As a result, the hydrogen storage capacity of 13.0 wt% can be obtained. This suggests that the Li-decorated graphyne system can serve as a high-capacity hydrogen storage medium.     

Enhanced reversible hydrogen storage performance of light metal-decorated boron-doped siligene: A DFT study

The use of nanomaterials for hydrogen storage could play a very important role in the large-scale utilization of hydrogen as an energy source. However, nowadays several potential hydrogen storage nanomaterials do not have a large gravimetric density and stability at room temperature. In this work, we have investigated the hydrogen storage performances of Na-, K- and Ca-decorated B-doped siligene monolayer (BSiGeML) using density functional theory calculations. The results show that boron doping improves the interaction between the metal adatom and the siligene monolayer (SiGeML). The K- and Ca-decorated BSiGeMLs can bind up to seven H₂ molecules per metal adatom, whereas Na-decorated BSiGeML only adsorb four H₂ molecules per adsorption site. The effect of temperature and pressure on the hydrogen storage capacity of BSiGeMLs was also evaluated. At room temperature, all the H₂ molecules adsorbed on Na-, and Ca-decorated BSiGeML are stable at mild pressure. The metal decoration of both sides of BSiGeML may lead to hydrogen gravimetric densities exceeding the target of 5.5 wt% proposed by DOE for the year 2025. K- and Ca-decorated BSiGeML could be efficient hydrogen molecular storage media compared to undoped SiGeML and other 2D pristine materials.     

Li decorated penta-silicene as a high capacity hydrogen storage material: A density functional theory study

The H₂ adsorption characteristics of Li decorated single-sided and double-sided penta-silicene are predicted via density functional theory (DFT). The orbital hybridization results in Li atom strongly bind onto the surface of the penta-silicene with a large binding energy and it keeps the decorated Li atoms from aggregation. Moreover, Li decorated double-sided penta-silicene can store up to 12H₂ molecules with the average hydrogen adsorption energy of ?0.220 eV/H₂ and hydrogen uptake capacity of 6.42 wt%, respectively. The ab initio molecular dynamics (AIMD) simulations demonstrate the H₂ molecules are released gradually from the substrate material with the increasing simulation time and the calculated desorption temperature T_D is 281 K in the suitable operating temperature range. Our explorations confirm that Li decorated penta-silicene can be regarded as a promising hydrogen storage candidate for hydrogen storage applications.     

First principles study on hydrogen storage in yttrium doped graphyne: Role of acetylene linkage in enhancing hydrogen storage

Through Density Functional Theory Simulations we predict that a Ytrrium atom attached on graphyne surface can adsorb up to a maximum of 9 molecular hydrogens (H₂), with a uniform binding energy of ∼0.3 eV/H₂ and an average desorption temperature of around 400 K (ideal for fuel cell applications), leading to 10 wt% of hydrogen, substantially higher than the requirement by DoE. The higher hydrogen wt% in Y doped graphyne compared to Y doped Single Walled Carbon Nanotubes (SWNT) and graphene is due to the presence of sp hybridized C atoms (in the acetylene linkage) supplying additional in-plane p_x-p_y orbitals leading to π (π*) bonding (antibonding) states. Charge transfer from metal to carbon nanostructure results in a redistribution of s, p, d orbitals of the metal leading to a non - spin polarized ground state in Y doped graphyne, due to the presence of the acetylene linkage, whereas Y doped SWNT and graphene remain magnetic like the isolated metal atom. In the non-magnetic graphyne + Y system, the net charge transfer from Y to successive H₂ molecules is less than in magnetic Y + graphene and Y + SWNT systems, enabling Y + graphyne to store a larger number of H₂ molecules. Furthermore, our ab initio MD simulations show that the system is stable even at room temperature and there is no dissociation of H₂ molecules, enabling the system to achieve 100% desorption. So Y doped graphyne is found to be a promising hydrogen storage device with high wt%, 100% recyclability and desirable desorption temperature.     

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.     

Fundamentals of hydrogen storage processes over Ru/SiO2 and Ru/Vulcan

Hydrogen adsorption and desorption over Ru/SiO₂ and Ru/Vulcan are investigated in terms of hydrogen storage and release characteristics by both dynamic and static experiments. Ru particle dispersions as a function of metal loading were determined by HR-TEM and volumetric chemisorption experiments. Vulcan was more accommodating for spillover hydrogen than SiO₂. High Ru dispersions, i.e., small particle sizes, favored the amount of hydrogen spillover to Vulcan, as revealed by temperature programmed desorption (TPD) of hydrogen. TPD of hydrogen under He flow experiments over Ru/SiO₂ and Ru/Vulcan materials revealed a low temperature process (up to 200 °C) attributed to desorption of weakly bound hydrogen from Ru metal surface. A high temperature process (above 450 °C) was attributed to diffusion of hydrogen from the support to the Ru particle and desorption at the Ru sites. Hydrogen adsorbs strongly on Ru metal, as indicated by the initial heats of H₂ adsorption measured as 100 kJ/mol over 1 wt% Ru/Vulcan by adsorption calorimetry. At higher coverages, heat of adsorption of hydrogen was measured as 10 kJ/mol. Low heat of adsorption of hydrogen at high coverages indicate multilayer weak adsorption of hydrogen over the storage material, which can desorb at lower temperatures.     

An ab-initio study of hydrogen storage performance of Si2BN nanotubes decorated with group 8B transition metals

Binding of group 8B transition metals (TMs) on Si₂BN nanotubes and the adsorption of H₂ molecules on TM-decorated Si₂BN nanotubes are investigated in the framework of first principles-based density functional theory (DFT). Our results show the adsorption energy of H₂ molecule on TM-decorated Si₂BN nanotube (−2.57 eV) to be greater than those of the other reported adsorbents. The enhancement of the adsorption property is attributed to the structural deformation and the induced charge transfer between TM–decorated Si₂BN nanotube and the hydrogen molecule. Our findings reveal the TM–decorated Si₂BN nanotubes to be highly sensitive to the presence of H₂ molecules. Additionally, we also investigated the adsorption process of multiple H₂ molecules on TM-decorated Si₂BN nanotubes. Our observations lead us to surmise that the maximum storage number of H₂ molecules adsorbed over the TM-decorated Si₂BN nanotubes is four. These results suggest useful potential for the TM-decorated Si₂BN nanotube to be considered as an appropriate medium for hydrogen storage.     

A DFT study of H2 adsorption on lithium decorated 3D hybrid Boron-Nitride-Carbon frameworks

Based on density functional theory (DFT) and first-principles molecular dynamics (MD),a new 3D hybrid Boron-Nitride-Carbon–interconnected frameworks (BNCIFs) consisting of organic linkers with Li decoration is created and optimized. Firstly, Li adsorption behaviors on several BNC_xcomplexes are investigated and compared systematically. The results indicate C substitution of N atom in pure BN layer could improve the metal binding energy effectively. Secondly, the BNC layer (BNC_NN) is chosen to model the frameworks of BNCIFs. The average binding energy of adsorbed Li atoms on BNCIFs is 3.6 eV which is much higher than the cohesive energy of bulk Li and avoids the Li clustering problem. Finally, we study the H₂ adsorptions on the Li decorated BNCIFs by DFT. Every Li atom could adsorb four H₂ molecules with an average binding energy of 0.24 eV. The corresponding gravimetric and volumetric storage capacities are 14.09 wt% and 126.2 g/L respectively overpassing the published 2020 DOE target. The excellent thermal stability of 160H₂@40Li@BNCIFs is also proved by MD. This nanostructure could be served as a promising hydrogen storage medium at ambient conditions.     

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.     

Optimization of the pore structure of nickel/graphite hybrid materials for hydrogen storage

Nickel/graphite hybrid materials were prepared by mixed acid treatment of graphite flakes, following metal nanoparticle deposition. The textural properties were studied by BET surface area measurement and t-plot methods with N₂/77 K adsorption isotherms. The hydrogen storage characteristics of the nickel/graphite at 298 K and 10 MPa were studied using a pressure-composition-temperature apparatus. The pore structure of the materials was studied as a function of processing conditions. In the optimum material, the hydrogen storage capacity was as high as 4.48 wt.%. The total amount of storage was not proportional to the specific surface area or metal content of the adsorbate. A dipole-induced model on nickel/carbon surfaces is proposed for the hydrogen storage mechanism.     

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

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

Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content

20 activated carbons (ACs) were prepared by activation of four coals of different ranks (bituminous, low-ash bituminous and sub-bituminous coals, and one anthracite) with potassium hydroxide, in order to evaluate their hydrogen storage capacities at −196 °C. The effect of surface area and oxygen content on hydrogen storage was examined. Oxygen content was determined by temperature-programmed desorption. The significance of oxygen content on hydrogen storage capacity was evaluated by Analysis of Variance (ANOVA). Apparent surface areas higher than 3000 m² g⁻¹ and hydrogen adsorption as high as 6.8 wt.% were obtained. The best results were obtained with ACs from bituminous coals. No significant effect of oxygen content on hydrogen adsorption was observed. We concluded that surface area controls hydrogen storage capacity at −196 °C.     

Hydrogen storage in hybrid nanostructured carbon/palladium materials: Influence of particle size and surface chemistry

Hydrogen adsorption on porous materials is one of the possible methods proposed for hydrogen storage for transport applications. One way for increasing adsorption at room temperature is the inclusion of metal nanoparticles to increase hydrogen–surface interactions. In this study, ordered mesoporous carbon materials were synthesized by replication of nanostructured mesoporous SBA-15 silica. The combination of different carbon precursors allowed to tailor the textural, structural and chemical properties of the materials. These carbons were used for the synthesis of hybrid nanostructured carbon/palladium materials with different sizes of metal nanoparticles. The hydrogen sorption isotherms were measured at 77 K and 298 K between 0.1 and 8 MPa. Hydrogen storage capacities strongly correlate with the textural properties of the carbon at 77 K. At room temperature, Pd nanoparticles enhance hydrogen storage capacity by reversible formation of hydride PdH_x and through the spillover mechanism. The hydrogen uptake depends on the combined influences of metal particle size and of carbon chemical properties. Carbons obtained from sucrose precursors lead to the hybrid materials with the highest storage capacities since they exhibits a large microporous volume and a high density of oxygenated surface groups.     

Hydrogen sorption in transition metal modified mesoporous materials

Ni, Rh and Pd incorporated mesoporous MCM-41, MCM-48, HMS and SBA-15 samples were synthesized and were characterized using XRD, ICP/EDX and N₂ adsorption–desorption at 77.4 K. The hydrogen adsorption studies in the synthesized materials were performed at 77.4 K (up to 112 kPa) and 303 K (up to 4000 kPa). The hydrogen adsorption isotherms of pristine and transition metals incorporated mesoporous materials at 77.4 K were completely reversible reflecting physisorption of hydrogen in these materials. The hydrogen adsorption isotherms at 303 K were not reversible showing the chemisorption of hydrogen in these materials at 303 K. Hydrogen sorption studies showed that transition metal modification improved the hydrogen storage capacity of mesoporous materials at 303 K. The desorption of the adsorbed hydrogen by heating up to 500 K from the mesoporous materials were also carried out for studying the recovery of adsorbed hydrogen from transition metal incorporated mesoporous materials.     

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.     

Investigation of hydrogen storage capacity of multi-walled carbon nanotubes deposited with Pd or V

This work presents the synthesis and characterization of multi-walled carbon nanotubes (multi-walled CNTs) deposited with Pd or V and their hydrogen storage capacity measured by Sievert's volumetric apparatus. The CNTs were grown by the CVD method using LPG and LaNi₅ as the carbon source and catalyst, respectively. Pd was impregnated on the CNTs by the reflux method with hydrogen gas as a reducing agent, while V was embedded on the CNTs by the vapor deposition method. The average metal particle size deposited on the CNTs was around 5.8 nm for Pd and 3.6 nm for V. Hydrogen adsorption experiments were performed at room temperature and at −196 °C under a hydrogen pressure of 65 bar. At −196 °C, the treated CNTs had a maximum hydrogen uptake of 1.21 wt%, while the CNTs deposited with Pd (Pd-CNTs) and CNTs deposited with V (V-CNTs) possessed lower surface areas, inducing lower hydrogen adsorption capacities of 0.37 and 0.4 wt%, respectively. For hydrogen sorption at room temperature, the CNTs decorated with the metal nanoparticles had a higher hydrogen uptake compared to the treated CNTs. Hydrogen adsorption capacity was 0.125 and 0.1 wt% for the Pd-CNTs and V-CNTs, respectively, while the hydrogen uptake of the treated CNTs was <0.01 wt%. For the second cycle, only half of the first hydrogen uptake was obtained, and this was attributed to the re-crystallization of the defect sites on the carbon substrate after the first hydrogen desorption.     

Adsorption and compression contributions to hydrogen storage in activated anthracites

We prepared activated carbons (ACs) that are among the best adsorbents for hydrogen storage. These ACs were prepared from anthracites and have surface areas (S_BET) as high as 2772 m² g⁻¹. Anthracites activated with KOH presented the highest adsorption capacities with a maximum of 5.3 wt.% at 77 K and 4 MPa. Non-linearity between hydrogen uptake at 77 K and pore texture was confirmed, as soon as their S_BET exceeded the theoretical limiting value of (geometrical) surface area, i.e., S_BET > 2630 m² g⁻¹. We separated adsorption and compression contributions to total hydrogen storage. The amount of hydrogen stored is significantly increased by adsorption only at moderate pressure: 3 MPa and 0.15 MPa at 298 and 77 K, respectively. Hydrogen adsorption on ACs at high pressure, above 30 MPa at 298 K and 8 MPa at 77 K, has not interest because more gas can be stored by simply compression in the same tank volume.