Hydrogen storage capacity on vanadium decorated graphene

We explored the molecular hydrogen storage capacity of complex system with Hafnium doped graphene (Gr-Hf) using the Density Functional Theory (DFT) method. We efficiently adsorbed molecular six hydrogens on the Hf doped graphene surface. The quantum chemically calculated adsorption energy is found negatively in the range of - 344.433 Ry to -333.836 Ry this implies as increasing the adsorbed hydrogen molecule on hafnium doped graphene (Gr-Hf) sheet the adsorption energy decreases continuously. The binding energy of after adsorbing second hydrogen molecule too much larger than the next adsorbed H2 molecules i.e., the binding energy per hydrogen molecule highly decreases when we increase adsorbed atom (2.197 Ry in 2H2 to 1.048 Ry in 3H2) then small decreases for next adsorbed H2 molecules. The extracted binding energy found in the range 2.197 Ry to 2.120 Ry, fermi energy found minimum for 1H2 shows the minimum electron occupancy at the different energy levels. The fermi energy increases accordingly, the electron occupancy also increases and evaluates higher electron occupancy with fermi energy 2.850 eV for 6H2 and density of states (DOS) confirm the weak interaction between σ bonding electrons of H2 molecule with Hf doped graphene complex system. The proposed system opens a new insight for hydrogen storage-based devices.

A new theoretical model for inelastic tunneling in realistic systems : comparing STM simulations with experiments

How do defects affect hydrogen spillover on graphene-supported Pt? A DFT study

The adsorptions of hydrogen both on square-packed single-walled carbon nanotube (SWCNT) arrays and on isolated nanotubes were investigated by a combination of a classical potential and density functional theory (DFT) method. Excess adsorption of hydrogen on the SWCNT with diameters of 1.225, 2.04, and 2.719 nm at 77 K and at ambient temperature, T = 300 K, has been calculated. DFT calculations indicate that the excess gravimetric storage capacity of exohedral adsorption of hydrogen on SWCNT array is as much as 30% of that of endohedral adsorption when the van der Walls (VDW) gap was fixed at 0.335 nm, while excess adsorption of hydrogen outside the nanotubes is close to or exceeds the excess endohedral adsorption for isolated nanotubes. The total excess gravimetric storage capacity (including endohedral and exohedral adsorptions) of hydrogen on open nanotube arrays of 2.719 nm is 7.1 wt % at 77 K and 4 MPa, while the total excess adsorption of hydrogen on open isolated nanotubes of 2.719 nm reaches 9.5 wt...

Effects of pressure, temperature, and geometric structure of pillared graphene on hydrogen storage capacity

Hydrogen adsorption on graphene sheets doped with group 8B transition metal: A DFT investigation

First-principles study on hydrogen storage by graphitic carbon nitride nanotubes

Hydrogen adsorption on Ru-decorated (8,0) zigzag single-walled carbon nanotube (SWCNT) was studied using density functional theory (DFT). Several decoration sites on the CNT surface were investigated before atomic or molecular hydrogen adsorption. The most stable location for a single Ru atom is above the hollow site, with an adsorption energy of Eads(Ru) = −2.133 eV. Ru decoration increases hydrogen adsorption energy nearly 46% compared to pristine CNT. When a hydrogen molecule is considered on Ru/SWCNT its adsorption is dissociative with an Eads(H2) = −0.697 eV. The Ru-decorated SWCNT systems exhibit magnetic properties. Density of states (DOS) and overlap population density of states (OPDOS) were computed in order to study the evolution of the chemical bonding. C–C bonds interact with Ru and are weakened after adsorption. Strong Ru–H bonds are formed during hydrogen adsorption process at expenses of C–Ru bonds. The mains interactions include the Ru 5pz and 4dz2 and C 2pz bands.

A class of high-surface-area carbon hypothetical structures has been investigated that goes beyond the traditional model of parallel graphene sheets hosting layers of physisorbed hydrogen in slit-shaped pores of variable width. The investigation focuses on structures with locally planar units (unbounded or bounded fragments of graphene sheets), and variable ratios of in-plane to edge atoms. Adsorption of molecular hydrogen on these structures was studied by performing grand canonical Monte Carlo simulations with appropriately chosen adsorbent-adsorbate interaction potentials. The interaction models were tested by comparing simulated adsorption isotherms with experimental isotherms on a high-performance activated carbon with well-defined pore structure (approximately bimodal pore-size distribution), and remarkable agreement between computed and experimental isotherms was obtained, both for gravimetric excess adsorption and for gravimetric storage capacity. From this analysis and the simulations performed on the new structures, a rich spectrum of relationships between structural characteristics of carbons and ensuing hydrogen adsorption (structure-function relationships) emerges: (i) Storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets, which creates surface areas exceeding of 2600 m(2)/g, the maximum surface area for infinite graphene sheets, carried mainly by edge sites; we call the resulting structures open carbon frameworks (OCF). (ii) For OCFs with a ratio of in-plane to edge sites ≈1 and surface areas 3800-6500 m(2)/g, we found record maximum excess adsorption of 75-85 g of H(2)/kg of C at 77 K and record storage capacity of 100-260 g of H(2)/kg of C at 77 K and 100 bar. (iii) The adsorption in structures having large specific surface area built from small polycyclic aromatic hydrocarbons cannot be further increased because their energy of adsorption is low. (iv) Additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. We conclude that OCF structures, if synthesized, will give hydrogen uptake at the level required for mobile applications. The conclusions define the physical limits of hydrogen adsorption in carbon-based porous structures.

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

Hydrogen storage capacity of Al, Ca, Mg, Ni, and Zn decorated phosphorus-doped graphene: Insight from theoretical calculations

Hydrogen adsorption on Ni doped carbon nanocone

We have employed density functional theory (DFT) calculations to investigate the adsorption of molecular oxygen and hydrogen on 3d transition metal (TM) surfaces with varying ordered magnetic structures in the bulk, namely ferromagnetic Fe(110), Co(0001), Ni(111) and diamagnetic Cu(111). The trend observed in the energies of adsorption was compared with the magnetic moment of the cell using the d-band centre model of chemisorption and the Stoner model of magnetic energy. As the gap between the d-band centre and the Fermi level of the TM decreases, more antibonding orbitals are present above the Fermi level and thus unoccupied, leading to stronger binding. Correspondingly, the shift in the d-band centre decreases the density of states (DOS) at the Fermi level giving rise to the ordered magnetic structure. Keywords: d-Band centre, chemisorption, Hedvall effect, magnetism.

The adsorption of molecular hydrogen on few-layer graphene (FLG) structures is studied using molecular dynamics simulations. The interaction between graphene and hydrogen molecules is described by the Lennard-Jones potential. The effects of pressure, temperature, number of layers in a FLG, and FLG interlayer spacing are evaluated in terms of molecular trajectories, binding energy, binding force, and gravimetric hydrogen storage capacity (HSC). The simulation results show that the effects of temperature and pressure can offset each other to improve HSC. An insufficient interlayer spacing (0.35 nm) largely limits the HSC of FLG because hydrogen adsorbed at the edges of the graphene prevents more hydrogen from entering the structure. A low temperature (77 K), a high pressure, a large number of layers in a FLG, and a large FLG interlayer spacing maximize the HSC.

Graphenes are emerging electrode materials used in many technologies such as electronics, sensors, as well as energy conversion and storage. The pristine graphenes, due to the elimination of density of states (DOS) at/near the Dirac point (Fermi level), have limited activities for charge transfer and storage. Fortunately, introduction of structural defects and disorders in graphenes can increase the DOS near the Fermi level by forming mid-gap and/or defect states, similar to that in silicon and other semiconducting materials. This improvement significantly promoted the application of pristine graphenes in electrochemical sensing and energy devices. In addition, defects related electronic structure variations in graphenes provides novel opportunity in studying the electrochemical structure-property relationship [1]. However, the delicate control over the defect-related electronic structures and precise correlation between the electrochemical activity and defect density remain challenging. Researchers led by Prof. Dongping Zhan and Prof. Bin Ren in Xiamen University have made significant advance in this field. In their recent work published in J. Am. Chem. Soc. [2], the authors demonstrated that the density of vacancy defects in a graphene sheet can be precisely controlled through Ar irradiation. They prepared patterns of different defect densities on a single-layer graphene sheet, and simultaneously mapped the defect density and the heterogeneous electron transfer (ET) rate of hydroxymethylferrocene (FcMeOH) oxidation over the same patterned graphene sheet (Figure 1), using Raman spectroscopy and the so-called scanning electrochemical microscopy. This new approach allowed them to quantitatively correlate the defect density in graphenes with the electrochemical activity for the heterogeneous ET reactions. The experimental results showed that the ET activity of graphene can be improved by increasing the density of vacancy defects. In addition, they obtained an optimal ET rate of FcMeOH oxidation over a graphene sheet with a moderate defect density. At such an optimal state, a balance between the increase of Fermi DOS and the decrease of sheet conductivity due to increasing the defect density was achieved, so that the whole graphene sheet not only becomes electrochemically activated, but also maintains structural integrity. According to their density functional theory calculations, the vacancy defect can induce mid-gap states in graphene. Because multiple electronic levels and states are involved in the heterogeneous ET between a solid electrode and redox molecules, the corresponding ET rates become proportional to the electronic overlap integral between the electrode and the redox molecules [1]. The introduction of mid-gap states should be able to enlarge the overlap between the DOS distribution of graphene and the redox molecules. The work by Zhan and coworkers represents an elegant example in tailoring the structure and property of graphene through defect density engineering.