Grantee Research Project Results
Final Report: Nanostructured C6B: A Novel Boron Rich Carbon for H2 Storage
EPA Grant Number: R830420C010Subproject: this is subproject number 010 , established and managed by the Center Director under grant R830420
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: Center for Environmental and Energy Research (CEER)
Center Director: Earl, David A.
Title: Nanostructured C6B: A Novel Boron Rich Carbon for H2 Storage
Investigators: Jones, Linda E. , Cormack, Alastair , Shelby, James
Institution: Alfred University
EPA Project Officer: Aja, Hayley
Project Period: July 1, 2004 through December 31, 2005
RFA: Targeted Research Center (2004) Recipients Lists
Research Category: Hazardous Waste/Remediation , Targeted Research
Objective:
We proposed that a novel carbon, C6B, having a significantly large boron concentration (17 wt %) in the lattice, can be synthesized into novel carbon microstructures (keying on nanotubes). The unique nanostructure is one that is crenellated or puckered along the tube axis as a result of the presence of these large boron concentrations. Substitutional boron strains the lattice creating saddle points. The development of these saddle points, or folds, increases the total surface area of the nanostructured carbon. It was proposed that these fine structures can be incorporated with a regular and repeated periodicity, thereby enhancing active carbon area (area on which H2 is adsorbed). The net result is a nanotube or carbon nanostructure that has folds along the axis ultimately resulting in a significant enhancement of the surface area. In addition, this solid would be lighter than carbon and at a substitution of 17 wt %, a 2 percent increase in the specific energy of this H2 storage medium would be gained.
Furthermore, boron is expected to increase the adsorptive capacity of carbon on the basis of modifying carbon’s electronic structure. It is well established that boron modifies the electronic structure of both sp2 carbons (graphites) and boron-doped multiwalled nanotubes via the creation of defect sites in these solids. Boron thereby enables electron transfer, moving electrons away from regions of high electron density and distributing the electron population throughout the solid. In addition, boron is less electronegative than carbon, further contributing to a modification of the electron association with localized carbon sites. We do not now know how these two often-competing phenomena influence the adsorptive capacity of carbon, yet arguments can be made that increasing the defect site concentrations in these carbon solids will increase the numbers of active sites involved for adsorption and hence the total H2 adsorbed per weight of storage material.
Fundamental work is needed to understand the role of large concentrations of boron on the structure of carbons and carbon nanotubes and on electronic structure and therefore the adsorption of hydrogen. This study was undertaken to synthesize these novel carbons and evaluate their structures. We also proposed that, on the basis of this understanding, the synthesis of these novel carbons can be scaled to allow for a commercially viable and responsive H2 storage material.
The objective of this research project was to synthesize boron-rich nanotubes of composition C6B via chemical vapor deposition for the purpose of creating an H2 storage medium. We were building upon our experience with this novel form of carbon. The advantage of the C6B deposition process is that it enables high concentrations of boron to be placed in the solid throughout its growth process. Saddling, therefore, occurs in a continuous and periodic fashion. This material, when synthesized in a nanostructured form either as a nanotube or lower aspect-ratio possibly discontinuous form, would have extremely high active surface area for the adsorption of H2 as a result of the saddling created via the incorporation of 17 at.% boron into a hexagonal lattice.
Regarding carbon and phenomena of adsorption on carbons, we studied: (1) the role of boron on defect structure and concentrations; and (2) the nature of H2 adsorption on these defect sites.
Summary/Accomplishments (Outputs/Outcomes):
CXB Nanotubes: Microstructure and Boron Concentration
We used the catalytic chemical vapor deposition (Cat-CVD) method between benzene and BCl3 to produce boron-doped carbon nanotubes having a high concentration of boron substituted. The high boron content creates a large density of point defects. We obtained boron-doped nanotubes or filaments with large diameter compared to the carbon nanotube reference. We employed X-ray photoelectron spectroscopy to obtain the boron concentration.
The synthesis of carbon and boron-doped carbon samples was carried out in a flow system. The materials were deposited on fused quartz substrates coated with pure nickel film 1-3 nm thick. BCl3 and C6H6 were used as reactants. The deposits were observed by scanning electron microscopy. X-ray powder diffraction was used to characterize the crystalline structure of the products. X-ray photoelctron spectroscopy was employed to evaluate the compositions and chemical bondings of boron-doped carbon samples.
The results were strongly dependent on the applied reaction parameters.
Influence of Boron on Carbon Nanotube Structure
The major limitation of structural carbon materials is their poor oxidation resistance at high temperature. Substitution of boron for carbon in these materials inhibits the oxidation reaction and has been found to improve graphitization and increase longitudinal growth and diameter in nanotubes.
In this study, molecular mechanics and quantum mechanics computer modeling were carried out to explore how large substitutional boron concentration modifies the nanotube on the atomic, crystal, and micro levels. The influence of boron on the structure was investigated by substituting boron for carbon symmetrically and asymmetrically within a circumcorenen host molecule, C54H18.
Substituting boron symmetrically for carbon produces a distortion of the molecular structure. The molecule incurs an angular distortion that produces an out-of-plane buckling, having a deflection of δ = 0.7 Å. Among the boron substitution from 2 percent to 20 percent, the distortion maximum is δ = 0.7 Å. Substituting boron asymmetrically within the host molecule produced no buckling, δ = 0.
To investigate the boron influence on nanotubes, we started to construct the two-dimensional (2D) lattice. A unit cell of 18 carbon atoms was used to facilitate the construction of C5B lattice. The lattice parameter calculation used the Cambridge Serial Total Energy Package (CASTEP), the quantum mechanical method. The C5B lattice was constructed by uniformly substituting three carbon by boron atoms along the diagonal of the unit cell. With the substitution of 17 percent boron into the graphite layer, the dimensions and angle of the unit cell are both modified: the area of the unit cell is expanded by 4 percent and the angle is twisted by 0.5˚. The angle between the nanotube vector (n,0) and (n,n) is changed to 30.3˚.
Based on the 2D lattice, carbon (9,0) nanotube and C5B (9,0) nanotube are simulated using CASTEP. There is 4 percent strain on length and 3 percent strain on diameter of the unit cell with the substitution of 17 percent boron symmetrically distributed. As a result of the twisted 2D lattice, a chirality of 2.4˚ is present in the C5B (9,0) nanotube.
Snapshots of the distortion of four different 17 percent boron-doped (9,0) nanotubes along the tube axis were simulated using molecular mechanics. One tube (A) was uniformly boron-doped, the remaining three (B, C, D) were randomly disordered tubes. From the snapshots of the cross section of the tubes, irregular buckling happens on the wall of the three disordered boron-doped nanotubes. Furthermore, according to the calculation, tube A has a ground state energy of 7,262 kJ/mol, and disordered tubes have an energy range from 7,200 to 7,540 kJ/mol. As we addressed before, the ground state energy is related to the location of boron atoms in the lattice. Therefore, it is expected that the boron-doped nanotubes grown in the vapor phase likely are to have disordered boron distribution.
Carbon nanotubes were grown on 0.5 nm thick nickel coated fused silica substrate. The nanotubes are multiwalled, having a diameter within the range 20-60 nm. The energy dispersive spectrograph (EDS) spectrum gives the elements of carbon, nickel, oxygen, and silicon, where the latter three are from the coated substrate. We used the same synthesis condition, except that BCl3 was added into the reaction chamber to grow the boron-doped product. There is big difference from the pure carbon product: filaments having diameter of 300 nm grow instead of tubes. Filaments are shorter, grown perpendicular to the substrate surface. The EDS spectrum reveals that boron, carbon, and oxygen are present in the filaments.
Conclusions:
CXB Nanotubes: Microstructure and Boron Concentration
- Microstructure of products is a function of synthesis conditions, such as temperature and time. Higher temperature and longer time lower the tubular curvature.
- Boron concentration is 5-9 wt % in the filaments, instead of 17 wt % under equilibrium reaction.
- Substitutional boron lower the Fermi level of carbon host, therefore, the C1s peak position of samples shifts to lower binding energy.
Influence of Boron on Carbon Nanotube Structure
The influence of substitutional boron into carbon were investigated using molecular and quantum mechanics computer modeling. A circumcoronen (C54H18) molecule, 2D graphite layer, and (9,0) carbon nanotubes were chosen as the host carbon lattices. With the substitution of 2-20 at.% boron into the circumcoronen molecule, the molecule incurs an angular distortion that produces an out-of-plane buckling, having a maximum deflection of δ = 0.7 Å at 11 wt % boron.
With the substitution of 17 percent boron into the graphite layer, the dimensions and angle of the unit cell are both modified: the area of the unit cell is expanded by 4 percent and the angle is twisted by 0.5˚. There is 4 percent strain on length and 3 percent strain on diameter of (9,0) nanotubes with the substitution of 17 percent boron symmetrically distributed. As a result of the twisted 2D lattice, a chirality of 2.4˚ is present in the C5B (9,0) nanotube. Irregular buckling happens on the wall of disordered boron-doped nanotubes. According to the ground state energy calculation, it is expected that boron-doped nanotubes grown in the vapor phase likely are to have disordered boron distribution.
Boron-doped carbon filaments were catalytically synthesized via a CVD reaction using benzene and BCl3. Compared to pure carbon nanotubes grown under the same condition, boron-doped carbon filaments have larger diameters of 300 nm.
Supplemental Keywords:
nanostructure, boron-doped carbon, hydrogen storage, carbon microstructures, nanotubes, CxB, nanotechnology, clean technologies, hydrogen economy, fuel cell technology, energy efficiency, alternative energy source,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, TREATMENT/CONTROL, Ecosystem Protection/Environmental Exposure & Risk, Sustainable Industry/Business, POLLUTION PREVENTION, Aquatic Ecosystems & Estuarine Research, Sustainable Environment, Energy, Technology, Aquatic Ecosystem, Technology for Sustainable Environment, Environmental Engineering, clean energy, energy conservation, clean technologies, cleaner production, sustainable development, environmental conscious construction, green building design, nanotechnology, clean manufacturing, energy efficiency, energy technology, nanomaterials, alternative energy source, water quality, environmentally conscious design, ceramic materialsRelevant Websites:
Progress and Final Reports:
Original AbstractMain Center Abstract and Reports:
R830420 Center for Environmental and Energy Research (CEER) Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.