Thermogravimetric Measurement of Hydrogen Absorption in Alkali-Modified Carbon Materials

Recent advances in the functionalization, substitutional doping and applications of graphene/graphene composite nanomaterials

Recently, graphene and graphene-based nanomaterials have emerged as advanced carbon functional materials with specialized unique electronic, optical, mechanical, and chemical properties. These properties have made graphene an exceptional material for a wide range of promising applications in biological and non-biological fields. The present review illustrates the structural modifications of pristine graphene resulting in a wide variety of derivatives. The significance of substitutional doping with alkali-metals, alkaline earth metals, and III-VII group elements apart from the transition metals of the periodic table is discussed. The paper reviews various chemical and physical preparation routes of graphene, its derivatives and graphene-based nanocomposites at room and elevated temperatures in various solvents. The difficulty in dispersing it in water and organic solvents make it essential to functionalize graphene and its derivatives. Recent trends and advances are discussed at length. Controlled reduction reactions in the presence of various dopants leading to nanocomposites along with suitable surfactants essential to enhance its potential applications in the semiconductor industry and biological fields are discussed in detail.

Hydrogen production and storage through adsorption and dissociation of H2O on pristine and functionalized SWCNT: a DFT approach

CONTEXT

Adsorption of 1 and 2 H2O molecules for hydrogen production and storage on the surface of pristine, carbamic acid and 2-amino 3-acetylpyridine functionalized SWCNTs with the dimensionality of (2, 4), (5, 5), and (6, 0) at various positions, i.e., center and edges were investigated by using computational DFT calculations. Adsorption energies and structural and electronic parameters were determined for pristine and functionalized SWCNTs with four different H2O orientations. Functionalization of 2-amino 3-acetylpyridine resulted in more favorable adsorption energies for 1 and 2 H2O molecules splitting as compared to spitting on pristine and carbamic acid functionalized SWCNT. Calculated adsorption constant, Kad confirmed greater binding interactions of functionalized SWCNTs with 1 and 2 H2O molecules as compared to pristine SWCNT. Isosurface for the adsorption of 1 and 2 H2O molecules on pristine and functionalized SWCNTs elaborated altered electrophilic and nucleophilic character. Effect of H2O concentration was monitored to determine hydrogen storage capacity which was found to be 7.17 wt.% for thirty molecules. An important finding of study is production of Stone-Wales (SW) defect upon H2O adsorption leading to increase in hydrogen production and its storage capacity. The functionalization of topological defected SWCNTs provides distinctive applications of CNTs for gas storage purposes.t: METHODS: In the current study, First Principal Density Functional Theory (DFT) calculations were carried which provides greater computational efficiency as compared to many traditional quantum mechanical methods. SCME: ADF (2018) modeling suite software with in framework of DFT approach using exchange correlation (XC) LDA-GGA (Generalized Gradient Approximation) with PBE (Perdew, Burke and Ernzerhof) functional and DZ (Double beta) basis set was employed to investigate structural, energetic and electronic aspects of adsorption on the surface of pristine, carbamic acid and 2-amino 3-acetly pyridine functionalized SWCNTs. (2, 4), (5, 5) and (6, 0) SWCNTs were designed using Avogadro's software and were imported to SCM: ADF graphical interface and were optimized as adsorbent. Single point energy (SPE), geometry optimization and high accuracy frequency calculations were performed to determine energetic, electronic and thermodynamic characteristics and feasibility of adsorption using XC of GGA-PBE functional & DZ basis set.

Crumpled Graphene-Storage Media for Hydrogen and Metal Nanoclusters

Understanding the structural behavior of graphene flake, which is the structural unit of bulk crumpled graphene, is of high importance, especially when it is in contact with the other types of atoms. In the present work, crumpled graphene is considered as storage media for two types of nanoclusters-nickel and hydrogen. Crumpled graphene consists of crumpled graphene flakes bonded by weak van der Waals forces and can be considered an excellent container for different atoms. Molecular dynamics simulation is used to study the behavior of the graphene flake filled with the nickel nanocluster or hydrogen molecules. The simulation results reveal that graphene flake can be considered a perfect container for metal nanocluster since graphene can easily cover it. Hydrogen molecules can be stored on graphene flake at 77 K, however, the amount of hydrogen is low. Thus, additional treatment is required to increase the amount of stored hydrogen. Remarkably, the size dependence of the structural behavior of the graphene flake filled with both nickel and hydrogen atoms is found. The size of the filling cluster should be chosen in comparison with the specific surface area of graphene flake.

Li-fluorine codoped electrospun carbon nanofibers for enhanced hydrogen storage

Carbon materials have attracted increasing attention for hydrogen storage due to their great specific surface areas, low weights, and excellent mechanical properties. However, the performance of carbon materials for hydrogen absorption is hindered by weak physisorption. To improve the hydrogen absorption performance of carbon materials, nanoporous structures, doped heteroatoms, and decorated metal nanoparticles, among other strategies, are adopted to increase the specific surface area, number of hydrogen storage sites, and metal catalytic activity. Herein, Li–fluorine codoped porous carbon nanofibers (Li–F–PCNFs) were synthesized to enhance hydrogen storage performance. Especially, perfluorinated sulfonic acid (PFSA) polymers not only served as a fluorine precursor, but also inhibited the agglomeration of lithium nanoparticles during the carbonization process. Li–F–PCNFs showed an excellent hydrogen storage capacity, up to 2.4 wt% at 0 °C and 10 MPa, which is almost 24 times higher than that of the pure porous carbon nanofibers. It is noted that the high electronegativity gap between fluorine and lithium facilitates the electrons of the hydrogen molecules being attracted to the PCNFs, which enhanced the hydrogen adsorption capacity. In addition, Li–F–PCNFs may have huge potential for application in fuel cells.

We developed a facile, yet general, approach for preparing Li–fluorine codoped porous carbon nanofiber (Li–F–PCNF) composites, which showed excellent hydrogen storage performance.

Computationally Predicted High-Throughput Free-Energy Phase Diagrams for the Discovery of Solid-State Hydrogen Storage Reactions

The design of multinary solid-state material systems that undergo reversible phase changes via changes in temperature and pressure provides a potential means of safely storing hydrogen. However, fully mapping the stabilities of known or newly targeted compounds relative to competing phases at reaction conditions has previously required many stringent experiments or computationally demanding calculations of each compound's change in Gibbs energy with respect to temperature, G(T). In this work, we have extended the approach of constructing chemical potential phase diagrams based on ΔG_f(T) to enable the analysis of phase stability at non-zero temperatures. We first performed density functional theory calculations to compute the formation enthalpies of binary, ternary, and quaternary compounds within several compositional spaces of current interest for solid-state hydrogen storage. Temperature effects on solid compound stability were then accounted for using our recently introduced machine learned descriptor for the temperature-dependent contribution G^δ(T) to the Gibbs energy G(T). From these Gibbs energies, we evaluated each compound's stability relative to competing compounds over a wide range of conditions and show using chemical potential and composition phase diagrams that the predicted stable phases and H₂ release reactions are consistent with experimental observations. This demonstrates that our approach rapidly computes the thermochemistry of hydrogen release reactions for compounds at sufficiently high accuracy relative to experiment to provide a powerful framework for analyzing hydrogen storage materials. This framework based on G(T) enables the accelerated discovery of active materials for a variety of technologies that rely on solid-state reactions involving these materials.

Borophene and Boron Fullerene Materials in Hydrogen Storage: Opportunities and Challenges

Two-dimensional materials have led to a leap forward in materials science research, especially in the fields of energy conversion and storage. Borophene and its spherical counterpart boron fullerene represent emerging materials that have attracted much attention in the whole area of advanced energy materials and technologies. Owing to their prominent features, such as electronic environment and geometry, borophene and boron fullerene have been used in versatile applications, such as supercapacitors, superconductors, anode materials for photochemical water splitting, and biosensors. Herein, one of the most promising applications/areas of hydrogen storage is discussed. Boron fullerenes have been considered and discussed for hydrogen-storage applications, and recently borophene was also included in the list of materials with promising hydrogen-storage properties. Studies focus mainly on doped borophene systems over pristine borophene due to enhanced features available upon decoration with metal atoms. This Review introduces very recent progress and novel paradigms with respect to both borophene derivatives and boron fullerene based systems reported for hydrogen storage, with a focus on the synthesis, physiochemical properties, hydrogen-storage mechanism, and practical applications.

Multiple Li+- and Mg2+-decorated PAHs: potential systems for reversible hydrogen storage

Computational exploration of hydrogen storage efficiency of multiple metal ion (Li⁺/Mg²⁺)-decorated PAHs.

Interactions of hydrogen molecules with complexes of lithium cation and aromatic nitrogen-containing heterocyclic anions

Highly stable salt functional groups consisting of lithium cation and aromatic anions (C n H n N5-n -Li) are studied for hydrogen storage using ab initio calculations, force field development, and grand canonical Monte Carlo simulations. Second-order Møller-Plesset perturbation theory with the resolution of identity approximation calculations are calibrated at the CCSD(T)/complete basis set (CBS) level of theory. The calibrations on different types of binding sites are different, but can be used to correct the van der Waals interactions systematically. The anion and salt functional groups provide multiple binding sites. With increased number of nitrogen atoms in the aromatic anion, the number of binding sites increases but the average binding energy decreases. Among the functional groups considered, CHN4-Li exhibits the largest number of binding sites (14) and a weak average binding energy of 5.7 kJ mol(-1) with CCSD(T)/CBS correction. The calculated adsorption isotherms demonstrate that the introduction of the functional group significantly enhances hydrogen uptake despite relatively weak average binding energy. Therefore, it is concluded that searching for functional groups with the larger number of binding sites is another key factor for enhancing the hydrogen storage capacity, given that other conditions such as free volume and surface area are fixed.

Incorporating magnesium and calcium cations in porous organic frameworks for high-capacity hydrogen storage

We propose incorporating a bi-functional group consisting of magnesium or calcium cations and a 1,2,4,5-benzenetetroxide anion (C6H2O4(4-)) in porous materials to enhance the hydrogen storage capacity. The C6H2O4M2 bifunctional group is highly stable and polarized, and each group provides 18 (M = Mg) or 22 (M = Ca) binding sites for hydrogen molecules with an average binding energy of ca. 10 kJ mol(-1) per hydrogen molecule based on RIMP2/ TZVPP calculations. Two porous materials (PAF-Mg or PAF-Ca) constructed with the bi-functional groups show remarkable improvement in hydrogen uptakes at normal ambient conditions. At 233 K and 10 MPa, the predicted gravimetric uptakes are 6.8 and 6.4 wt% for PAF-Mg and PAF-Ca respectively. This work reveals that fabricating materials with large numbers of binding sites and relatively low binding energies is a promising approach to achieve high capacity for on-board storage of hydrogen.

A prospect for LiBH4 as on-board hydrogen storage

In contrast to the traditional metal hydrides, in which hydrogen storage involves the reversible hydrogen entering/exiting of the host hydride lattice, LiBH4 releases hydrogen via decomposition that produces segregated LiH and amorphous B phases. This is obviously the reason why lithium borohydride applications in fuel cells so far meet only one requirement — high hydrogen storage capacity. Nevertheless, its thermodynamics and kinetics studies are very active today and efficient ways to meet fuel cell requirements might be done through lowering the temperature for hydrogenation/dehydrogenation and suitable catalyst. Some improvements are expected to enable LiBH4 to be used in on-board hydrogen storage.

Computational studies on hydrogen storage in aluminum nitride nanowires/tubes

One-dimensional AlN nanowires/tubes were exploited as hydrogen storage media. The adsorption of atomic and molecular hydrogen on AlN nanowires was investigated by using density functional theory computations. Hydrogen atoms prefer to adsorb on top of neighboring threefold-coordinated N and Al atoms in pairs. A hydrogen molecule, however, prefers to adsorb on top of threefold-coordinated Al atoms in the nanowire surface, with an adsorption energy of 0.21 eV. H(2) dissociation is exothermic in the surface of AlN nanowires, and the dissociation barrier is rather low (0.76 eV), indicating that chemisorption is a feasible route for hydrogen storage in AlN nanowires/tubes. A maximum 3.66 wt% of molecular and 2.44 wt% of atomic hydrogen can be stored in AlN nanowires/tubes.

High surface area microporous carbon materials for cryogenic hydrogen storage synthesized using new template-based and activation-based approaches

High surface area microporous carbon materials were synthesized using new, simple, and innovative approaches based on traditional template and chemical activation methods. The resulting surface area and porosity were characterized using Brunauer-Emmett-Teller (BET)-type measurements and analysis, and the hydrogen storage capacity was determined using excess hydrogen adsorption measurements at 77 K and up to 40 bar hydrogen pressure. For our direct one-step aerosol-assisted template-based synthesis method of mixing the template precursor and carbon precursor solutions, a specific surface area value of up to nearly 2000 m(2) g(-1) and an excess hydrogen storage capacity of 4.2 wt% was observed. For our chemical activation-based synthesis method of homogeneously mixing the chemical activation reagent into the carbon precursor solution, a specific surface area value of nearly 3000 m(2) g(-1) and an excess hydrogen adsorption capacity of nearly 5.8 wt% were observed. The surface area and hydrogen uptake results varied systematically with the synthesis parameters, and we observed a strong correlation between the BET values of the specific surface area and the excess hydrogen adsorption capacity.

Controllable synthesis of hierarchical nanostructured hollow core/mesopore shell carbon for electrochemical hydrogen storage

Hierarchical nanostructured hollow core/mesopore shell carbon (HN-HCMSC) represents an innovative concept in electrochemical hydrogen storage. This work deals with physical characteristics and electrochemical hydrogen storage behavior of the HN-HCMSCs, produced by a replica technique using solid core/mesopore shell (SCMS) silica as template. HN-HCMSCs with various core sizes and/or shell thicknesses have been fabricated through the independent control of the core sizes and/or shell thicknesses of the SCMS silica templates. The superb structural characteristics of the HN-HCMSCs including large specific surface area and micropore volume, and particularly well-developed three-dimensionally interconnected hierarchical nanostructure (hollow macroporous core in combination with meso-/microporous shell), provide them with great potential for electrochemical hydrogen storage. A discharge capacity up to 586 mAh/g, corresponding to 2.17 wt % hydrogen uptake, has been demonstrated in 6 M KOH for the HN-HCMSC with a core size of 180 nm and a shell thickness of 40 nm at a discharge rate of 25 mA/g. Furthermore, the HN-HCMSC also possesses excellent cycling capacity retainability and rate capability.

Adsorption of gases in carbon nanotubes: are defect interstitial sites important?

Molecular simulations are used to shed light on an ongoing controversy over where gases adsorb on single walled carbon nanotube bundles. We have performed simulations using models of carbon nanotube bundles composed of tubes of all the same diameter (homogeneous) and tubes of different diameters (heterogeneous). Simulation data are compared with experimental data in an effort to identify the best model for describing experimental data. Adsorption isotherms, isosteric heats of adsorption, and specific surface areas have been computed for Ar, CH 4, and Xe on closed, open, and partially opened homogeneous and heterogeneous nanotube bundles. Experimental data from nanotubes prepared from two different methods, electric arc and HiPco, were examined. Experimental adsorption isotherms and isosteric heats for nanotubes prepared by the electric arc method are in best agreement with simulations for heterogeneous bundles of closed nanotubes. Models including adsorption in defect interstitial channels are required to achieve good agreement with experiments. Experimental isosteric heats and specific surface areas on HiPco nanotubes are best described by a model consisting of heterogeneous bundles with approximately 11% of the nanotubes opened.

Enhancement in hydrogen storage in carbon nanotubes under modified conditions

We investigate the hydrogen adsorbing characteristics of single-walled carbon nanotubes (CNTs) through fundamental molecular dynamics simulations that characterize the role of ambient pressure and temperature, the presence of surface charges on the CNTs, inclusion of metal ion interconnects, and nanocapillary effects. While the literature suggests that hydrogen spillover due to the presence of metallic contaminants enhances storage on and inside the nanotubes, we find this to be significant for alkali and not transition metals. Charging the CNT surfaces does not significantly enhance hydrogen storage. We find that the bulk of the hydrogen storage occurs inside CNTs due to their nanocapillarity effect. Storage is much more dependent on external thermodynamic conditions such as the temperature and the pressure than on these facets of the CNT structure. The dependence of storage on the external thermodynamic conditions is analyzed and the optimal range of operating conditions is identified.

Hydrogen physisorption on the organic linker in metal organic frameworks: ab initio computational study

Research for materials offering efficient hydrogen storage and transport has recently received increased attention. Metal organic frameworks (MOFs) provide one promising group of materials where several recent advances were reported in this direction. In this computational study ab initio methods are employed to study the physisorption of hydrogen on conjugated systems. These systems are used as models for the organic linker within MOFs. Here, we focus on the adsorption sites related to the organic linker with special attention to the edge site, which was only recently reported to exist as the weakest adsorbing site in MOFs. We also investigate chemically modified models of the organic connector that result in enforcing this adsorption site. This may be crucial for improving the uptake properties of these materials to the goal defined by DOE for efficient hydrogen transport materials.

Carbon nanoscrolls: a promising material for hydrogen storage

A multiscale theoretical approach was used for the investigation of hydrogen storage in the recently synthesized carbon nanoscrolls. First, ab initio calculations at the density functional level of theory (DFT) were performed in order to (a) calculate the binding energy of H2 molecules at the walls of nanoscrolls and (b) fit the parameters of the interatomic potential used in Monte Carlo simulations. Second, classical Monte Carlo simulations were performed for estimating the H2 storage capacity of "experimental size" nanoscrolls containing thousands of atoms. Our results show that pure carbon nanoscrolls cannot accumulate hydrogen because the interlayer distance is too small. However, an opening of the spiral structure to approximately 7 A followed by alkali doping can make them very promising materials for hydrogen storage application, reaching 3 wt % at ambient temperature and pressure.

Using first principles calculations to identify new destabilized metal hydride reactions for reversible hydrogen storage

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage, but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through mixing metal hydrides with other compounds. A very large number of possible destabilized metal hydride reaction schemes exist, but the thermodynamic data required to assess the enthalpies of these reactions are not available in many cases. We have used density functional theory calculations to predict the reaction enthalpies for more than 300 destabilization reactions that have not previously been reported. The large majority of these reactions are predicted not to be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low, and hence these reactions need not be investigated experimentally. Our calculations also identify multiple promising reactions that have large enough hydrogen storage capacities to be useful in practical applications and have reaction thermodynamics that appear to be suitable for use in fuel cell vehicles and are therefore promising candidates for experimental work.

Comparative Study of Hydrogen Adsorption on Carbon and BN Nanotubes

The physisorption and chemisorption of hydrogen in BN nanotubes, investigated by density functional theory (DFT), were compared with carbon nanotubes. The physisorption of H2 on BN nanotubes is less favorable energetically than on carbon nanotubes; BN nanotubes cannot adsorb hydrogen molecules effectively in this manner. Chemisorption of H2 molecules on pristine BN nanotubes is endothermic. Consequently, perfect BN nanotubes are not good candidates for hydrogen storage by either mechanism. Other strategies must be utilized if BN nanotubes are to be employed as hydrogen storage media such as utilizing them as supporting media for hydrogen-absorbing metal nanoclusters.

Computational studies of molecular hydrogen binding affinities: the role of dispersion forces, electrostatics, and orbital interactions

Intermolecular interactions between H2 and ligands, metals, and metal-ligand complexes determine the binding affinities of potential hydrogen storage materials (HSM), and thus their extent of potential for practical use. A brief survey of current activity on HSM is given. The key issue of binding strengths is examined from a basic perspective by surveying the distinct classes of interactions (dispersion, electrostatics, orbital interactions) in first a general way, and then in the context of calculated binding affinities for a range of model systems.

Hydrogen Release from Mixtures of Lithium Borohydride and Lithium Amide: A Phase Diagram Study

We recently reported the synthesis of a new quaternary hydride in the lithium-boron-nitrogen-hydrogen quaternary phase diagram with the approximate composition LiB0.33N0.67H2.67 having a theoretical hydrogen content of 11.9 wt %. This new compound forms by the reaction of appropriate amounts of lithium amide (LiNH2) and lithium borohydride (LiBH4) and releases greater than 10 wt % hydrogen when heated. A small amount of ammonia, 2-3 mol % of the generated gas, is also released. We now report a study of hydrogen and ammonia release from the series of reactant mixtures (LiNH2)x(LiBH4)1-x, where x=0.667 corresponds to the composition LiB0.33N0.67H2.67. We measured hydrogen and ammonia release amounts as a function of composition and found that maximum hydrogen and minimum ammonia release do occur for x=0.667. We also present evidence for an additional new quaternary phase and for two possibly metastable phases in this system.

New Insights into the Interaction of Hydrogen Atoms with Boron-Substituted Carbon

Boron substitution in carbon materials has been comprehensively investigated using the density functional theory method. It was found that there is a correlation between the stability of the graphene sheet, the distribution of pi electrons, the electrostatic potential, and the capability for hydrogen-atom adsorption. Boron substitution destabilizes the graphene structure, increases the density of the electron wave around the substitutional boron atoms, and lowers the electrostatic potential, thus improving the hydrogen adsorption energy on carbon. However, this improvement is only ca. 10-20% instead of a factor of 4 or 5. Our calculations also show that two substitutional boron atoms provide consistent and reliable results, but one substitutional boron results in contradictory conclusions. This is a warning to other computational chemists who work on boron substitution that the conclusion from one substitutional boron might not be reliable.

Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping

Density-functional calculations of the adsorption of molecular hydrogen on a planar graphene layer and on the external surface of a (4,4) carbon nanotube, undoped and doped with lithium, have been carried out. Hydrogen molecules are physisorbed on pure graphene and on the nanotube with binding energies about 80-90 meV/molecule. However, the binding energies increase to 160-180 meV/molecule for many adsorption configurations of the molecule near a Li atom in the doped systems. A charge-density analysis shows that the origin of the increase in binding energy is the electronic charge transfer from the Li atom to graphene and the nanotube. The results support and explain qualitatively the enhancement of the hydrogen storage capacity observed in some experiments of hydrogen adsorption on carbon nanotubes doped with alkali atoms.

Insights into Hydrogen Atom Adsorption on and the Electrochemical Properties of Nitrogen-Substituted Carbon Materials

The nitrogen substitution in carbon materials is investigated theoretically using the density functional theory method. Our calculations show that nitrogen substitution decreases the hydrogen adsorption energy if hydrogen atoms are adsorbed on both nitrogen atoms and the neighboring carbon atoms. On the contrary, the hydrogen adsorption energy can be increased if hydrogen atoms are adsorbed only on the neighboring carbon atoms. The reason can be explained by the electronic structures analysis of N-substituted graphene sheets. Nitrogen substitution reduces the pi electron conjugation and increases the HOMO energy of a graphene sheet, and the nitrogen atom is not stable due to its 3-valent character. This raises an interesting research topic on the optimization of the N-substitution degree, and is important to many applications such as hydrogen storage and the tokamaks device. The electronic structure studies also explain well why nitrogen substitution increases the capacitance but decreases the electron conductivity of carbon electrodes as was experimentally observed in our experiments on the supercapacitor.

Monte Carlo simulations of hydrogen adsorption in alkali-doped single-walled carbon nanotubes

Monte Carlo simulations and Widom's test particle insertion method have been used to calculate the solubility coefficients (S) and the adsorption equilibrium constants (K) in single-walled (10,10) armchair carbon nanotubes including single nanotubes, and nanotube bundles with various configurations with and without alkali dopants. The hydrogen adsorption isotherms at room temperature were predicted by following the Langmuir adsorption model using the calculated constants S and K. The simulation results were in good agreement with experimental data as well as the grand canonical Monte Carlo simulation results reported in the literature. The simulations of nanotube bundle configurations suggest that the gravimetric hydrogen adsorption increases with internanotube gap size. It may be attributed to favorable hydrogen-nanotube interactions outside the nanotubes. The effect of alkali doping on hydrogen adsorption was studied by incorporating K+ or Li+ ions into nanotube arrays using a Monte Carlo simulation. The results on hydrogen adsorption isotherms indicate hydrogen adsorption of 3.95 wt% for K-doping, and 4.21 wt% for Li-doping, in reasonable agreement with the experimental results obtained at 100 atm and room temperature.

Effect of Expanded Graphite Lattice in Exfoliated Graphite Nanofibers on Hydrogen Storage

A graphite exfoliation technique, using intercalation of a concentrated sulfuric/nitric acid mixture followed by a thermal shock, has successfully exfoliated a herringbone graphite nanofiber (GNF). The exfoliated GNF retains the overall nanosized dimensions of the original GNF, with the exfoliation temperature determining the degree of induced defects, lattice expansion, and resulting microstructure. High-resolution transmission electron microscopy indicated that the fibers treated at an intermediate temperature of 700 degrees C for 2 min had dislocations in the graphitic structure and a 4% increase in graphitic lattice spacing to 3.5 A. The fibers treated at 1000 degrees C for 36 h were expanded along the fiber axis, with regular intervals of graphitic and amorphous regions ranging from 0.5 to >50 nm in width. The surface area of the starting material was increased from 47 m(2)/g to 67 m(2)/g for the 700- degrees C treatment and to 555 m(2)/g for the 1000- degrees C treatment. Hydrogen uptake measurements at 20 bar indicate that the overall hydrogen uptake and operative adsorption temperature are sensitive to the structural variations and graphitic spacing. The increased surface area after the 1000- degrees C treatment led to a 1.2% hydrogen uptake at 77 K and 20 bar, a 3-fold increase in hydrogen physisorption of the starting material. The uptake of the 700- degrees C-treated material had a 0.29% uptake at 300 K and 20 bar; although low, this was a 14-fold uptake over the starting material and higher than other commonly used pretreatment methods that were tested in parallel. These results suggest that selective exfoliation of a nanofiber is a means by which to control the relative binding energy of the hydrogen interaction with the carbon structure and thus vary the operative adsorption temperature.

Preparation and Electrochemical Hydrogen Storage of Boron Nitride Nanotubes

Boron nitride (BN) nanotubes were synthesized through chemical vapor deposition over a wafer made by a LaNi5/B mixture and nickel powder at 1473 K. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were performed to characterize the microstructure and composition of BN nanotubes. It was found that the obtained BN nanotubes were straight with a diameter of 30-50 nm and a length of up to several microns. We first verify that the BN nanotubes can storage hydrogen by means of an electrochemical method, though its capacity is low at present. The hydrogen desorption of nonelectrochemical recombination in cyclic voltammograms, which is considered as the slow reaction at BN nanotubes, suggests the possible existence of strong chemisorption of hydrogen, and it may lead to the lower discharge capacity of BN nanotubes. It is tentatively concluded that the improvement of the electrocatalytic activity by surface modification with metal or alloy would enhance the electrochemical hydrogen storage capacity of BN nanotubes.

Hydrogen Adsorption on Functionalized Nanoporous Activated Carbons

There is considerable interest in hydrogen adsorption on carbon nanotubes and porous carbons as a method of storage for transport and related energy applications. This investigation has involved a systematic investigation of the role of functional groups and porous structure characteristics in determining the hydrogen adsorption characteristics of porous carbons. Suites of carbons were prepared with a wide range of nitrogen and oxygen contents and types of functional groups to investigate their effect on hydrogen adsorption. The porous structures of the carbons were characterized by nitrogen (77 K) and carbon dioxide (273 K) adsorption methods. Hydrogen adsorption isotherms were studied at 77 K and pressure up to 100 kPa. All the isotherms were Type I in the IUPAC classification scheme. Hydrogen isobars indicated that the adsorption of hydrogen is very temperature dependent with little or no hydrogen adsorption above 195 K. The isosteric enthalpies of adsorption at zero surface coverage were obtained using a virial equation, while the values at various surface coverages were obtained from the van't Hoff isochore. The values were in the range 3.9-5.2 kJ mol(-1) for the carbons studied. The thermodynamics of the adsorption process are discussed in relation to temperature limitations for hydrogen storage applications. The maximum amounts of hydrogen adsorbed correlated with the micropore volume obtained from extrapolation of the Dubinin-Radushkevich equation for carbon dioxide adsorption. Functional groups have a small detrimental effect on hydrogen adsorption, and this is related to decreased adsorbate-adsorbent and increased adsorbate-adsorbate interactions.

Comparative study of Li, Na, and K adsorptions on graphite by using ab initio method

A comprehensive study has been conducted to compare the adsorptions of alkali metals (including Li, Na, and K) on the basal plane of graphite by using molecular orbital theory calculations. All three metal atoms prefer to be adsorbed on the "middle hollow site" above a hexagonal aromatic ring. A novel phenomenon was observed, that is, Na, instead of Li or K, is the weakest among the three types of metal atoms in adsorption. The reason is that the SOMO (single occupied molecular orbital) of the Na atom is exactly at the middle point between the HOMO and the LUMO of the graphite layer in energy level. As a result, the SOMO of Na cannot form a stable interaction with either the HOMO or the LUMO of the graphite. On the other hand, the SOMO of Li and K can form a relatively stable interaction with either the HOMO or the LUMO of graphite. Why Li has a relatively stronger adsorption than K on graphite has also been interpreted on the basis of their molecular-orbital energy levels.

New alkali doped pillared carbon materials designed to achieve practical reversible hydrogen storage for transportation

We propose a new generation of materials to maximize reversible H2 storage at room temperature and modest pressures (<20 bars). We test these materials using grand canonical Monte Carlo simulations with a first-principles-derived force field and find that the Li pillared graphene sheet system can take up 6.5 mass% of H2 (a density of 62.9 kg/m(3) at 20 bars and room temperature. This satisfies the DOE (Department of Energy) target of hydrogen-storage materials for transportation. We also suggest ways to synthesize these systems. In addition we show that Li-doped pillared single-wall nanotubes can lead to a hydrogen-storage capacity of 6.0 mass% and 61.7 kg/m(3) at 50 bars and room temperature storage, which is close to the DOE target.