Optimizing the hydrogen storage in boron nitride nanotubes by defect engineering

We use ab initio density functional theory calculations to study the interaction of hydrogen with vacancies in boron nitride nanotubes to optimize the hydrogen storage capacity through defect engineering. The vacancies reconstruct by forming B–B and N–N bonds across the defect site, which are not as favorable as heteronuclear B–N bonds. Our total energy and structure optimization results indicate that the hydrogen cleaves these reconstructing bonds to form more stable atomic structures. The hydrogenated defects offer smaller charge densities that allow hydrogen molecule to pass through the nanotube wall for storing hydrogen inside the nanotubes. Our optimum reaction pathway search revealed that hydrogen molecules could indeed go through a hydrogenated defect site with relatively small energy barriers compared to the pristine nanotube wall. The calculated activation energies for different diameters suggest a preferential diameter range for optimum hydrogen storage in defective boron nitride nanotubes.     

ZnCr LDH nanosheets modified graphitic carbon nitride for enhanced photocatalytic hydrogen production

ZnCr layered double hydroxides (ZnCr LDH) nanosheets modified graphitic carbon nitride (g-C₃N₄) nanohybrids were fabricated via a self-assembly procedure through electrostatic interaction between these two components. Such 2D-2D inorganic-organic hybrid material was employed for photocatalytic hydrogen production under visible light for the first time. The physical and photophysical properties of the hybrid nanocomposites were investigated to reveal the effect of ZnCr LDH nanosheets on the photocatalytic activities of g-C₃N₄. It was found that 1 wt% ZnCr LDH nanosheets modified g-C₃N₄ was optimal for the formation of intimate interfacial contact. The visible light photocatalytic H₂ production activity over g-C₃N₄ was enhanced about 2.8 times after ZnCr LDH nanosheets modification. The significant enhancement in photocatalytic performance for ZnCr LDH/g-C₃N₄ heterojunction should be attributed to the promoted charge transfer and separation efficiency, resulting from the intimate interfacial contact and Type II band alignment between ZnCr LDH and g-C₃N₄.     

Investigation of hydrogen storage capacity of various carbon materials

Hydrogen storage capacity of various carbon materials, including activated carbon (AC), single-walled carbon nanohorn, single-walled carbon nanotubes, and graphitic carbon nanofibers, was investigated at 303 and 77 K, respectively. The results showed that hydrogen storage capacity of carbon materials was less than 1 wt% at 303 K, and a super activated carbon, Maxsorb, had the highest capacity (0.67 wt%). By lowering adsorption temperature to 77 K, hydrogen storage capacity of carbon materials increased significantly and Maxsorb could store a large amount of hydrogen (5.7 wt%) at a relatively low pressure of 3 MPa. Hydrogen storage capacity of carbon materials was proportional to their specific surface area and the volume of micropores, and the narrow micropores was preferred to adsorption of hydrogen, indicating that all carbon materials adsorbed hydrogen gas through physical adsorption on the surface.     

Highly dispersed Pd-MnOx nanoparticles supported on graphitic carbon nitride for hydrogen generation from formic acid-formate mixtures

Highly dispersed Pd and MnO_x nanoparticles supported on the graphitic carbon nitride with different composition have been prepared by a simple liquid deposition-reduction method and used as an efficient FA decomposition catalyst for hydrogen generation. The catalytic activity depends on the Pd-MnO_x composition of catalyst, the FA/SF ratio and the reaction temperature. The Pd-MnO_x/CN-1 catalyst exhibited excellent catalytic activity for hydrogen generation with the initial TOF of 465 h^?1 from formic acid-sodium formate mixture aqueous solution with a FA/SF ratio of 1:8 at 348 K. The synergetic effect between Pd nanoparticles and the carbon nitride support, the Mn/Pd ratios, the good dispersion of nanoparticles and the nature of the carbon nitride support were suggested to be responsible for the efficient catalytic performance of the Pd-MnO_x/CN nanocatalysts.     

Hydrogen storage in coiled carbon nanotubes

We report a density functional calculation of the adsorption of molecular hydrogen on the external surface of coiled carbon nanotube (CCNT). Binding energies of single molecule have been studied as a function of three different orientations and at three different sites like hexagon, pentagon and heptagon. The binding energy values are larger than linear (5,5) armchair nanotube, which has approximately same diameter as that of coiled carbon nanotube. The curvature and topology of CCNT are responsible for this considerable enhancement. The system with full coverage is also studied. When the nanotube surface is fully covered with one molecule per graphitic hexagon, pentagon and heptagon gives the 6.8 wt% storage capacity. The binding energy per molecule decreases due to repulsive interactions between neighbor molecules. It gives good storage medium for hydrogen. Almost it meets the DOE target.     

Facile synthesis and hydrogen storage application of nitrogen-doped carbon nanotubes with bamboo-like structure

This paper reports a facile method for the preparation of nitrogen-doped carbon nanotubes (N-doped CNTs) that shows enhanced hydrogen storage capacity. The synthesis method involves simple pyrolysis of melamine using FeCl₃ as catalyst in tube furnace. The materials were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, elemental analysis, Raman spectroscopy, and nitrogen adsorption–desorption analysis. The results indicated that the prepared N-doped CNTs have a bamboo-like structure with thin compartment layers. The nitrogen doping concentration, specific surface area, and total pore volume of the N-doped CNTs were determined to be 1.5 at%, 135 m²/g, and 0.38 cm³/g, respectively. The hydrogen adsorption measurements at 77 K showed that the N-doped CNTs exhibits gravimetric hydrogen uptake of 0.21 wt% at 1 bar and 1.21 wt% at 7 bar. At room temperature, hydrogen uptake as high as 0.17 wt% at 298 K and 19 bar is achieved, which is among the highest data reported for the N-doped carbon materials under the same condition.     

An efficient exfoliation method to obtain graphitic carbon nitride nanosheets with superior visible-light photocatalytic activity

Graphitic carbon nitride (g-C₃N₄) has a promising application in the photocatalytic field due to its large aspect ratio and the favorable band gap energy. Herein, g-C₃N₄ nanosheets (g-C₃N₄ NS) with high photoactivity are obtained with the aid of isopropanol (IPA) in the synthesis process. The introduced IPA causes a more intense oxidation in the exfoliation process and the obtained g-C₃N₄ NS owns its unique properties of a broaden absorption range of visible light, an enlarged surface area and the irregular surface. As a result, the g-C₃N₄ NS has good photocatalytic activity in the degradation of organic pollutant. Moreover, the photocatalytic hydrogen evolution rate of g-C₃N₄ NS is three times as that of g-C₃N₄ NS* synthesized without IPA using the same method.     

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.     

The synthesized nickel-doped multi-walled carbon nanotubes for hydrogen storage under moderate pressures

Hydrogen adsorption capacity of Multiwalled carbon nanotubes (MWCNTs) decorated with Nickel (Ni) nanoparticles has been presented at room temperature and under moderate pressures of 4–20 bar. The functionalization of carbon nanotubes was carried by H₂SO₄-HNO₃ reducing agents and the Ni supported MWCNTs (Ni-MWCNTs) were prepared by wet chemical method. The structure and morphology characterization of samples were performed by XRD, TEM, EDX and SEM analyses. These nanotubes then subjected to hydrogenation step by using Sievert's-like apparatus. The hydrogenation of the Ni-MWCNTs was performed at 298 K and moderate hydrogen pressures of 4–20 bar. The obtained results show that there is a correlation between hydrogen storage capacity and hydrogen pressure that; as the pressure was increased, hydrogen uptake capacity enhanced due to physisorption. In addition, maximum hydrogen storage capacity of Ni-MWCNTs was found to be 0.298 wt % at room temperature and under pressure of 20 bar.     

Effects of cryomilling on the structures and hydrogen storage characteristics of multi-walled carbon nanotubes

Cryomilling was performed on multi-walled carbon nanotubes (MWCNTs) to investigate the effect of cyomilling on the structures and hydrogen storage characteristics of MWCNTs. Two milling speeds (300 and 700 rpm) and two milling times (2 and 6h) were applied in the cryomilling process. The results showed that the agglomeration of MWCNTs was significantly reduced, their lengths were shortened and the crystalline structure became amorphous at higher milling speed, whereas the milling time had no significant effect on the dispersibilities or structures of the MWCNTs. The hydrogen adsorption capacities of cryomilled MWCNTs at 700 rpm were improved by approximately 22% compared to that of unmilled MWCNTs due to the improvements in the specific surface area (17.4%) and pore volume (34.9%). Cryomilling is an effective method for increasing the surface area and pore volume, and macropores were transformed into mesopores, thereby enhancing the hydrogen storage capacity of the MWCNT surface. These results were confirmed by scanning and transmission electron microscopies, X-ray diffraction and Raman spectroscopic analysis.     

First-principles study on the dehydrogenation of Li4BN3H10 modified by Co

The dehydrogenation of Li₄BN₃H₁₀ modified by Co is investigated by first-principles calculations using GGA-PW91 method within CASTEP. The calculation results show that Co modification may result in favorable H-desorption of Li₄BN₃H₁₀ by reducing the hydrogen dissociation energy, due to the weaker B–H and N–H interactions, the formation of Co–B bond and the appearance of semiconducting nature in Li₄BN₃H₁₀–Co. In addition, the B–N bond formed upon energetically favorable dehydrogenation may favor for Li₄BN₃H₁₀ dehydrogenation. Although Co is a good catalyst for Li₄BN₃H₁₀ dehydrogenation, Co doping in Li₄BN₃H₁₀ bulk is energetically unfavorable with the occupation energy of 3.818 eV. The energy cost for Co dopant should be reduced for Co-doped Li₄BN₃H₁₀ application.     

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.     

Effect of temperature on activated carbon nanotubes for hydrogen storage behaviors

In this work, activated multi-walled carbon nanotubes (Acti-MWNTs) with well-developed pore structures, a highly specific surface area, and higher hydrogen adsorption capacities due to CO₂ activation were prepared. The activation was performed at activation temperatures in the range of 500–1100 °C. The microstructure and crystallinity of the Acti-MWNTs were evaluated with a transmission electron microscope (TEM) and an FT-Raman spectrometer, respectively. The textural properties of the Acti-MWNTs were investigated by using a nitrogen gas sorption analyzer at 77 K. The hydrogen storage capacities of the Acti-MWNTs were investigated by BEL-HP at 298 K/100 bar. The hydrogen storage capacities of the Acti-MWNTs were enhanced to 0.78 wt.% by increasing activation temperatures to 900 °C, which resulted in the formation of a defective structure in the Acti-MWNTs. This result indicated that the CO₂ activation was one of the most effective methods to develop the textural properties, as well as to enhance the hydrogen storage capacities of MWNTs.     

Cobalt sulfide modified graphitic carbon nitride semiconductor for solar hydrogen production

Mesoporous graphitic carbon nitride (mpg-CN) was modified with cobalt sulfide (CoS) by using an impregnation-sulfidation approach. The gas-phase sulfidation of CoS in nanoporous networks at certain temperature allows for the formation of intimate contact between the host carbon nitride and CoS nanoparticle, establishing noble-metal free heterojunctions for charge separation and collection at material interface. The resultant CoS/mpg-CN was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂-sorption, X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy, photoluminescence (PL) spectroscopy and electrochemical measurement. On the basis of the physicochemical and electrocatalytic characterization results of CoS/mpg-CN, it was revealed that CoS functioned as a cocatalyst to promote the migration of excited electron from mpg-CN toward CoS as well as to provide reactive sites for H₂ production at lower overpotentials. As a result, the rate of H₂ evolution over mpg-CN under visible light illumination (λ > 420 nm) was significantly improved after loading with CoS and the optimal loading amount of CoS was found to be 1.0 at. %.     

Modification of single wall carbon nanotubes (SWNT) for hydrogen storage

Due to unique structural, mechanical and electrical properties of single wall carbon nanotubes, SWNTs, they have been proposed as promising hydrogen storage materials especially in automotive industries. This research deals with investing of CNT’s and some activated carbons hydrogen storage capacity. The CNT’s were prepared through natural gas decomposition at a temperature of 900?C over cobalt-molybdenum nanoparticles supported by nanoporous magnesium oxide (Co–Mo/MgO) during a chemical vapor deposition (CVD) process. The effects of purity of CNT (80–95%wt.) on hydrogen storage were investigated here. The results showed an improvement in the hydrogen adsorption capacity with increasing the purity of CNT’s. Maximum adsorption capacity was 0.8%wt. in case of CNT’s with 95% purity and it may be raised up with some purification to 1%wt. which was far less than the target specified by DOE (6.5%wt.). Also some activated carbons were manufactured and the results compared to CNTs. There were no considerable H₂-storage for carbon nanotubes and activated carbons at room-temperature due to insufficient binding between H₂ molecules carbon nanostructures. Therefore, hydrogen must be adsorbed via interaction of atomic hydrogen with the storage environment in order to achieve DOE target, because the H atoms have a very stronger interaction with carbon nanostructures.     

Enhanced photoelectrochemical water splitting using zinc selenide/graphitic carbon nitride type-II heterojunction interface

The objective of this research is to construct a type-II heterojunction interface for effective photoelectrochemical (PEC) water splitting for hydrogen generation. A series of ZnSe/g-C₃N₄ heterojunctions is prepared by ultrasonication procedure and tested for PEC water splitting for the first time. The successful formation of ZnSe/g-C₃N₄ is confirmed by phase, morphological and optical analysis. Linear sweep voltammetry of 0.05 ZG (0.05% ZnSe/g-C₃N₄) showed a six-fold higher photocurrent density of 500 μA than g-C₃N₄. These results are supported by the Tafel slopes and PL (photoluminescence spectroscopy) studies by showing the smallest slope and lesser electron-hole recombination for 0.05 ZG. Increased lifetime of 107 ms and a higher donor density of 3.6 × 10¹⁹ cm^?3 for 0.05 ZG is observed. The smallest semicircle for 0.05 ZG in EIS implies the least charge transfer resistance among the prepared heterojunctions. All the results comply with each other showing the successful formation of type-II heterojunction for enhanced PEC water splitting.     

Grand canonical Monte Carlo simulation of hydrogen physisorption in single-walled boron nitride nanotubes

The properties of hydrogen physisorption in single-walled boron nitride nanotubes (SWBNNTs) and single-walled carbon nanotubes (SWCNTs) are investigated in detail by the grand canonical Monte Carlo simulations. A great deal of our computational results show that the hydrogen storage capacity of SWBNNTs is slightly larger than the capacity of SWCNTs at any time when their diameters were equal and in the same conditions, and indicate that the hydrogen storage capacity of SWBNNTs at 293 K and 10 MPa with a diameter of more than 30 nm or at 293 K and 15 MPa with a diameter of more than 25 nm could exceed the 2010 goal of 6 wt%, which is presented by the U.S. Department of Energy. In addition, these results are discussed in theory.     

Synthesis of graphitic carbon nitride by directly heating sulfuric acid treated melamine for enhanced photocatalytic H2 production from water under visible light

Herein we report the synthesis of graphitic carbon nitride (g-C₃N₄) by directly heating sulfuric acid treated melamine precursor. Thermoanalytical methods (TG-DSC) in combination with XRD, XPS and elemental analysis were used to characterize the condensation steps of the precursor. The TG-DSC curves clearly show significant difference in thermal behavior between the treated and untreated melamine. The sublimation of melamine during condensation was significantly suppressed by treating melamine with sulfuric acid. The decomposition of melamine sulfuric acid and the condensation of melamine occur simultaneously. The N/C ratio of the prepared carbon nitride (1.53) is slight higher than that of the ideal crystal g-C₃N₄ (1.33), indicating the incomplete condensation of amino groups in the material. The XPS and elemental analysis show that there is no sulfur residue in the final product. The sample synthesized from sulfuric acid treated melamine shows relatively higher BET surface area. The photocatalytic performance of the as prepared carbon nitride was evaluated under visible light irradiation (λ > 420 nm). The photocatalytic H₂ production rate on sample synthesized from sulfuric acid treated melamine is 2 times higher than that on sample synthesized from untreated melamine.     

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.     

Experimental investigation of hydrogen storage in single walled carbon nanotubes functionalized with borane

Single walled carbon nanotubes (SWCNTs) dispersed in 2-propanol are deposited on the alumina substrate using drop cast method. The deposited SWCNTs are characterized using the techniques SEM, EDS and FTIR. Then the SWCNTs are functionalized with BH₃ using LiBH₄ as the precursor. FTIR, XPS and CHNS techniques are used to confirm the functionalization. The functional groups are identified from FTIR studies. The various elements present in the functionalized SWCNTs are identified from XPS and CHNS studies. The functionalized samples are hydrogenated and the hydrogen storage capacity of these samples is estimated using CHNS studies.