Hydrogen storage in Li-doped charged single-walled carbon nanotubes

Density functional calculations have been carried out to investigate the interaction of hydrogen molecules with Li-doped charged single-walled carbon nanotubes (SWNTs). The results show that binding of H₂ on positively charged Li-doped SWNTs is enhanced compared with that on uncharged systems. As the charge of the Li-doped SWNTs increases, the binding energy of H₂ increases with the largest binding energy of 0.26 eV. The reason for the increase in H₂ binding energy is that the positively charged tube improves the charge transfer from Li atom to the tube and make the Li atom more ionized, which strengthens the polarization interaction between H₂ and Li atom.     

Hydrogen storage in platinum loaded single-walled carbon nanotubes

To study the hydrogen storage capacity, platinum (Pt) nanoparticles were deposited on single-walled carbon nanotubes (SWNT) using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as a precursor. To verify Pt deposition on the surface of the SWNT, a Transmission Electron Microscope (TEM) was used to obtain surface morphology. The TEM images show that Pt nanoparticles were homogeneously distributed on the surface of SWNT. Commercial SWNT were also used to compare the results. Thermal Gravimetric Analysis at heating rate of 5 °C/min is measured for pure SWNT and Pt loaded SWNT. Before hydrogen storage measurements these samples were reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 hour. Hydrogen storage capacity of these SWNT was investigated under 25 bar pressure and room temperature as well as liquid nitrogen temperature.     

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.     

Hydrogen storage behaviors of platinum-supported multi-walled carbon nanotubes

In this work, the hydrogen storage behaviors of multi-walled carbon nanotubes (MWNTs) loaded by crystalline platinum (Pt) particles were studied. The microstructure of the Pt/MWNTs was characterized by X-ray diffraction and transmission electron microscopy. The pore structure and total pore volumes of the Pt/MWNTs were analyzed by N₂/77 K adsorption isotherms. The hydrogen storage capacity of the Pt/MWNTs was evaluated at 298 K and 100 bar. From the experimental results, it was found that Pt particles were homogeneously distributed on the MWNT surfaces. The amount of hydrogen storage capacity increased in proportion to the Pt content, with Pt-5/MWNTs exhibiting the largest hydrogen storage capacity. The superior amount of hydrogen storage was linked to an increase in the number of active sites and the optimum-controlled micropore volume for hydrogen adsorption due to the well-dispersed Pt particles. Therefore, it can be concluded that Pt particles play an important role in hydrogen storage characteristics due to the hydrogen spillover effect.     

Hydrogen storage capacity at high pressure of raw and purified single wall carbon nanotubes produced with a solar reactor

Samples of single wall carbon nanotubes (SWNTs) were prepared using a solar reactor. Graphite targets containing different catalysts (Ni/Co, Ni/Y, Ni/Ce) allowed the synthesis of SWNTs soot in which nanotubes had different diameter distributions. Several consecutive stages of HCl treatment and thermal oxidation in air (HCl protocol) purified the samples. Another protocol involving HNO₃ treatment and H₂O₂ oxidation (HNO₃ protocol) was also used. Isotherms of hydrogen adsorption were volumetrically measured at 253 K under pressures below 6 MPa on raw and treated samples. The highest adsorption capacity (0.7  wt%) was measured on raw soot. HCl protocol clearly increases the BET surface area (_SBET)$(S_{\mathrm{BET}})$ and the microporous volume (_W0(_N2))$(W_0({\mathrm{N}}_2))$ measured by N₂ at 77 K of the treated samples with respect to the as-produced materials, whereas HNO₃ protocol decreases them. A correlation between textural properties and hydrogen storage capacities is discussed.     

Topological defects embedded large-sized single-walled carbon nanotubes for hydrogen storage: A molecular dynamics study

In this paper, we investigate the performance of large-sized single-walled carbon nanotubes (SWCNTs) incorporated with mono vacancy (MV), double vacancy (DV), and Stone-Wales (SW) topological defects as a medium for hydrogen adsorption using molecular dynamics (MD) simulations. A novel potential energy distribution (PED) method is employed with MD simulations to determine the adsorbed hydrogen molecules and associated binding energy. In addition, we extended our work to bundles of defected SWCNT (D-SWCNT) that provided the most prominent adsorption capacity subjected to temperature and pressure variations. In particular, four representative (8,8), (13,13), (19,19), and (33,0) SWCNTs are simulated under various thermodynamic conditions, and collected adsorption isotherms data reveals higher gravimetric density for large-sized SWCNT. At 77 K and 100 bar, the maximum hydrogen uptake in pristine SWCNTs is 6.88–7.73 wt%, depending on the size of the nanotubes. In contrast, the binding energy decreases as the nanotube size increases. At 77 K, (8,8) and (19,19) SWCNTs have average binding energies of 0.043 and 0.021 eV, respectively. Meanwhile, (19,19) SWCNT incorporated with 1% DV defects having 5–8 rings (DV1) and MV defects yields the maximum storage capacity of 9.07 wt% and 8.62 wt%, respectively, at 77 K. Furthermore, the increment of about 43.29% in wt.% is obtained for DV1 defected nanotube relative to pristine SWCNT at 300 K and 100 bar. Moreover, our results indicate the maximum hydrogen uptake of 8.65, 7.15, 2.57, and 1.33 wt% in the square array of DV1 defect embedded SWCNTs at 77, 100, 200, and 300 K, respectively, compared to 9.07, 6.65, 2.24, and 1.11 wt% in the isolated D-SWCNT at identical conditions. As a result, the D-SWCNT bundles are better suited for hydrogen storage at high temperatures than the isolated D-SWCNT. Our present study paves the way to progress toward the efficient usage of D-SWCNTs with few chemical alterations for scaled-up applications.     

Hydrogen storage in MgH2 coated single walled carbon nanotubes

We present a density functional theory (DFT) study on the hydrogen storage capacity of (5,5) arm-chair single walled carbon nanotubes (SWCNTs) functionalized with magnesium hydride (MgH₂). Being lightweight and rich in hydrogen, MgH₂ adsorbs H₂ molecules in the vicinity of carbon nanotubes. The H₂ molecules are adsorbed dissociatively on SWCNT + MgH₂ complex. The H-H distance gets increased by more than ten times of the initial bond length 0.74 Å of the H₂ molecule. The hydrogen storage capacity of three configurations namely C1MgH₂, C5MgH₂ and C10MgH₂ is reported. The density of states is computed for all the systems. The average binding energies of C5MgH₂ and C10MgH₂ when H₂ molecule is adsorbed are 1.86 eV/H₂ and 1.96 eV/H₂, which are approximately equal. Thus, increasing the number of MgH₂ molecule does not vary the binding energy of H₂ adsorption. The corresponding temperature, in which desorption will take place, is 2285 K and 2457 K for C5MgH₂ and C10MgH₂ systems respectively, which are much above the room temperature.     

Hydrogen storage in microwave-treated multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) treated by microwave and heat treatment were used for hydrogen storage. Their storage capacity was measured using a quadruple quartz crystal microbalance in a moisture-free chamber at room temperature and at relatively low pressure (0.5 MPa). Deuterium was also used to monitor the presence of moisture. The hydrogen storage capacity of the microwave-treated MWCNTs was increased to nearly 0.35 wt% over 0.1 wt% for the pristine sample and increased further to 0.4 wt%, with improved stability after subsequent heat-treatment. The increase in the storage capacity by the microwave treatment was mostly attributed to the introduction of micropore surfaces, while the stability improvement after the subsequent heat treatment was related to the removal of functional groups. We also propose a measurement method that eliminates the moisture effect by measuring the storage capacity with hydrogen and deuterium gas.     

Hydrogen transport in single-walled carbon nanotubes encapsulated by palladium

Hydrogen transport and loading into single-walled carbon nanotubes (SWCNT) encapsulated by thin Pd layers onto a massive Pd substrate were studied using a complex of vacuum thermal desorption, cyclic voltammetry and ESR methods. By adding SWCNT the hydrogen capacity of the Pd–SWCNT composite under electrochemical loading increases as much as 25% relative to Palladium metal alone. This provides moderate growth in the gravimetric capacity of the total composite based on a massive Pd substrate. The hydrogen binding energy in the SWCNT (eH = 0.075 eV/H-atom), estimated by studies of hydrogen transport in the Pd–SWCNT composite was lower than predicted for the Pd–SWCNT complex, but higher than the physisorption on the bare SWCNT. Using ESR we established that the Pd–C_x e-complexes formed at the wall of nanotube could be considered as hydrogen adsorption site, providing both high net gravimetric capacity and low hydrogen binding energy in the Pd encapsulated SWCNT. The results obtained provide an opportunity to probe a condensed hydrogen phase of nanometer scale confined in SWCNT, encapsulated by transition metals.     

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.     

Hydrogen storage in boron-doped carbon nanotubes: Effect of dopant concentration

Boron-doped carbon nanotubes (BCNTs) with varying B content (0–8 at%) were prepared by thermo-catalytic decomposition of ethanol in presence of boric acid at 1073 K. It was observed that hydrogen adsorption capacity improved to a critical B content of 3.86 at% and then decreased. Maximum hydrogen adsorption was found to be 0.497 wt% at 273 K and 16 bar with 3.86 at% of boron doping in CNTs. With the help of transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, it was found that in addition to dopant concentration, dopant bonding with carbon structures, crystallinity and defects play pivotal roles in determining the extent of hydrogen adsorption by BCNTs. The thermogravimetric studies revealed the oxidation stability of the BCNTs. The hydrogen adsorption kinetics was found to follow the pseudo-second-order model. The rate constant value was minimum for the BCNT with the highest hydrogen storage capacity.     

Hydrogen storage in cobalt-embedded ordered mesoporous carbon

Ordered mesoporous carbons (OMCs) were synthesized by using ordered mesoporous silica as a template, and chitosan as carbon precursors. A novel process of pre-impregnation is proposed to prepare cobalt-embedded OMC. This process is based on using cobalt chelated chitosan as carbon precursor. The surface functional groups and metal contents were determined by X-ray photoelectron spectroscopy. The bulk cobalt contents in the cobalt-embedded OMCs were measured by an atomic absorption spectrometer. The morphology of the OMCs was observed by small angle X-ray scattering analysis and transmission electron microscope. The OMC texture characteristics were determined by using nitrogen adsorption analysis. Hydrogen capacities of the OMCs were obtained by a volumetric method. The cobalt-embedded OMCs possess obviously higher hydrogen adsorption capacity than that of pure OMC. At 298 K and under 5.5 MPa, the hydrogen capacities of the OMC and OMC–Co-5 are 0.2 and 0.45 wt%, respectively. The H₂/Co ratio of the hydrogen adsorbed on the OMC–Co-5 is 1.54 indicating a Kubas-type interaction between Co and H₂. In addition, the hydrogen spillover effect might occur in parallel.     

H2 storage on single- and multi-walled carbon nanotubes

The adsorption of hydrogen on single-walled and multi-walled carbon nanotubes (CNTs) was investigated at 77 and 298 K, in the pressure range of 0–1000 Torr. The adsorption isotherms indicate that adsorption follows the Langmuir model. Hydrogen uptakes were found to depend strongly on the nature of the CNTs. Single-walled CNTs adsorb significantly higher quantities of hydrogen per unit mass of the solid, while the opposite is true on a per unit surface area basis. This observation implies that adsorption takes place selectively on specific sites on the surface. The hydrogen uptake capacity of CNTs was also found to be affected by the purity of the materials, increasing with increasing purity. Temperature programmed desorption indicated that relatively strong adsorption bonds develop between adsorbent and adsorbate and that a single type of adsorption site exists on the solid surface.     

Hydrogen storage properties of sumanene

Numerous studies associated with carbon-based materials have shown excellent results for the adsorption of important molecules. Bearing in mind that hydrogen is important as an energy source in this paper we investigated the adsorption properties of sumanene toward hydrogen molecules. We used a theoretical and computational approach in the framework of density functional theory. Frontier molecular orbitals, HOMO and LUMO, are visualized and molecular electrostatic potential surfaces are created, in order to locate adsorption places. We determined H₂ adsorption binding energies, for which we obtained the applicable results. The adsorption properties of sumanene molecules toward hydrogen molecules were discussed through analysis of the density of states, partial density of states and overlap population density of states. Our results indicate that the sumanene can be very useful in the practical application for storage of hydrogen, which is the basis for its successful energy implementation.     

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.     

Chemical processing of double-walled carbon nanotubes for enhanced hydrogen storage

Double-walled carbon nanotubes (DWCNTs) were modified for enhanced hydrogen storage by employing a combination of two techniques: KOH activation for the formation of defects on DWCNT surfaces and loading of the DWCNTs with nanocrystalline Pd. The physical properties of the pristine DWCNTs and chemically modified DWCNTs were systematically characterised by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and Brunauer–Emmett–Teller (BET) surface area measurements. The amounts of hydrogen storage capacity were measured at ambient temperature and found to be 1.7, 2.0, 3.7, and 2.8 wt% for pristine DWCNTS, 2 wt% Pd DWCNTs, activated DWCNTs, and 2 wt% Pd activated DWCNTs, respectively. Hydrogen molecules could be adsorbed on defect sites created by chemical activation in DWCNTs through van der Waals forces. For Pd nanoparticle loaded DWCNTs, H₂ molecules could be dissociated into atomic hydrogen and adsorbed on defect sites. We found that the hydrogen storage capacity of DWCNTs can be significantly enhanced by chemical activation or loading with Pd nanoparticles.     

Mechanical response of hydrogen-filled single-walled carbon nanotubes under torsion

Molecular dynamics simulations are performed to investigate the torsional buckling behavior of single-walled carbon nanotubes (SWCNTs) filled with hydrogen gas. The simulation model accounts for both the mechanical deformation of the SWCNT and the interactions among the hydrogen and carbon atoms. It is found that the critical torsional moment and stiffness of the SWCNT are both significantly dependent on the hydrogen molecule storage density. Importantly, the change in torsional stiffness differs from that of conventional linear elastic materials as a result of the nonlinear oscillatory response due to nonlinear mechanical effects. It is shown that under large deformations, the SWCNT switches reversibly between different morphological patterns. Each change in pattern corresponds to an abrupt release of the strain energy and a singularity in the stress–strain curve. It is shown that at higher hydrogen storage densities, the hydrogen molecules exert a stabilizing effect on the SWCNT. The degree of torsional stability is determined principally by the distribution of the hydrogen molecules within the SWCNT. Finally, it is shown that the torsional deformation of the SWCNT is characterized by a stick-slip phenomenon.     

Hydrogen sorption hysteresis and superior storage capacity of silicon-carbide nanotubes over their carbon counterparts

We report for the first time the results of an extensive experimental study of hydrogen sorption in silicon-carbide nanotubes (SiCNTs), which were synthesized using the reaction between SiO vapor and carbon nanotubes (CNTs) in an argon atmosphere in the temperature range 1200 °C–1500 °C. The as-synthesized SiCNTs were then purified using a sodium hydroxide solution in order to remove the side products of the synthesis reaction. The hydrogen sorption characteristics of the as-synthesized SiCNTs, as well as those of the purified SiCNTs were then measured at 25 °C and for pressures of up to 100 bars. The results reveal hysteresis between the adsorption and desorption isotherms, which we attribute to the presence of metal impurities and/or the multilayer structure of the nanotubes. The hydrogen storage capacity of the as-synthesized SiCNTs is similar to that of the CNTs, whereas for the purified SiCNTs it is 50% higher than that of the CNTs, in agreement with the results of molecular simulations reported previously. In addition, the hydrogen uptake rate in the SiCNTs is about five times faster than that in the CNTs and, in contrast with the CNTs, its desorption from SiCNTs is completely reversible under vacuum.     

Effect of different reductants for palladium loading on hydrogen storage capacity of double-walled carbon nanotubes

The effects of different reductants for palladium loading on the hydrogen sorption characteristics of double-walled carbon nanotubes (DWCNTs) have been investigated. Pd nanoparticles were loaded on DWCNT surfaces for dissociation of H₂ into atomic hydrogen, which spills over to the defect sites on the DWCNTs. When we use different reductants, the reduction capabilities and other effects of the different reductants are different, which affects the hydrogen storage capacity of the DWCNTs. In this work, the amount of hydrogen storage capacity was determined (by AMC Gas Reactor Controller) to be 1.7, 2.0, 2.55, and 3.0 wt% for pristine DWCNTS and for 2.0%Pd/DWCNTs using H₂, l-ascorbic acid, and NaBH₄ as reductants, respectively. We found that the hydrogen storage capacity can be enhanced by loading with 2% Pd nanoparticles and selecting a suitable reductant. Furthermore, the sorption can be attributed to the chemical reaction between atomic hydrogen and the dangling bonds of the DWCNTs.