Hydrogen uptake of manganese oxide-multiwalled carbon nanotube composites

Hydrogen uptake of pristine multi-walled carbon nanotubes is increased more than three-fold at 298 K and hydrogen pressure of 4.0 MPa, upon addition of hydrogen spillover catalyst manganese oxide, from 0.26 to 0.94 wt%. Simple and convenient in situ reduction method is used to prepare Mn-oxide/MWCNTs composite. XRD, FESEM, and TEM demonstrates nanostructural characterization of pristine MWCNTs and composite. TGA analysis of Mn-oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage capacity of composite is attributed to spillover mechanism owing to decoration of Mn-oxide nanoparticles on outer surface of MWCNTs. Hydrogen uptake follows monotonous dependence on hydrogen pressure. Oxide-MWCNTs composite not only shows high hydrogen storage capacity as compared to pristine, but also exhibit significant cyclic stability upon successive adsorption–desorption cycles.     

Preparation,characterization and hydrogen storage studies of carbon nanotubes and their composites: A review

Current challenge for researchers worldwide is to construct a reliable, efficient, and affordable medium that can store hydrogen reversibly at ambient temperature and pressure for on-board applications. Carbon nanotubes (CNTs) and their composites are considered as leading source of solid-state reversible hydrogen storage medium owing to its unique characteristics including high surface area, nanoporous structure, tuneable properties, low mass density, cage like structure, chemical stability, dissociation of hydrogen molecule, and easy synthesis method. Nanocrystalline metal or metal oxide or hydride is doped/embedded into pristine CNTs via in-situ reduction, wetness impregnation, high-energy ball milling and sputtering method. Characterization techniques of pristine and composites are utilized to study morphological, thermal, qualitative, quantative, and elemental analysis. Nanocomposite hydrogen uptake capacity is frequently measured by volumetric and gravimetric methods. Multifold enhancement of hydrogen storage of composites compared to pristine CNTs is attributed to activation, acidification, purification, ball milling and spillover of physisorbed hydrogen by metal catalyst onto CNTs via spillover mechanism. Hydrogen uptake of CNTs and composites follow monotonous dependence on hydrogen pressure. Composites not only present high hydrogen uptake as compared to pristine CNTs but also shows significant cyclic stability upon successive adsorption–desorption cycles.     

Hydrogen uptake of cobalt and copper oxide-multiwalled carbon nanotube composites

Hydrogen storage capacity of a pristine multi-walled carbon nanotubes is increased 10-fold at 298 K and an equilibrium hydrogen pressure of ～23 atm, upon addition of a hydrogen spillover catalyst cobalt- and copper oxide, from 0.09 to 0.9 wt.%. In situ reduction method is utilized to synthesize Co-oxide/MWCNTs and Cu-oxide/MWCNTs composite. Blocking of channels and pores of MWCNTs by oxide nanoparticles during preparation method is responsible for low BET specific surface area of composites compared to pristine sample. X-ray diffraction, scanning, and transmission electron microscopy demonstrates nanostructural characterization of MWCNTs and composites. Thermogravimetric analysis of two oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage of both composites is attributed to the spillover mechanism due to decoration of Co and Cu-oxide nanoparticles on the outer surface of MWCNTs.     

Hydrogen storage of nanostructured TiO2-impregnated carbon nanotubes

Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO₂-nanorods and nanotubes has been performed at room temperature and moderate hydrogen pressures of 8–18 atm. Under hydrothermal synthesis conditions, nanorods (NRs) and nanoparticles (NPs) are found to form either of the two polymorphic phases, i.e., nanorods are formed of predominantly anatase phase while nanoparticles are formed of rutile phase. NRs and NPs are introduced into the CNT matrix via the wetness-impregnation method. These composites store up to 0.40 wt.% of hydrogen at 298 K and 18 atm, which is nearly five times higher the hydrogen uptake of pristine CNTs. The excess amount of hydrogen stored in TiO₂-impregnated CNTs is determined from the amount of TiO₂ in the sample and the measured hydrogen uptake of TiO₂ nanoparticles. Higher hydrogen uptake of NP-impregnated CNTs when compared pristine CNTs is accounted for by considering initial binding of hydrogen on TiO₂ and subsequent spillover in CNT–TiO₂-NPs.     

Hydrogen storage capability of carbon nanotube Be@C120

PM3 (RHF) type semiempirical quantum chemical calculations have been carried out on (nH₂+Be)@C₁₂₀ systems where C₁₂₀ is a capped tube and n15. The results indicate that all these systems are stable but endothermic in nature. (7H₂+Be)@C₁₂₀ system has the lowest heat of formation value.     

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.     

Inorganic nanotube composites based on polyaniline: Potential room-temperature hydrogen storage materials

Inorganic nanotubes as a support material for polyaniline were used for hydrogen storage. To this end, a solid-state preparation method has been developed for controlling the hydrogen storage capacity of these nanocomposites. The hydrogen storage capacities have measured at room temperature and at a low pressure of 0.5 MPa using the precise quadrupole quartz crystal microbalance technique in a chamber free of moisture. The optimum nanocomposite shows an enhanced hydrogen storage capacity of 0.78 wt.% with excellent reversibility when compared to less than 0.019 wt.% capacity of the pristine nanotubes and 0.05 wt.% of polyaniline. This large increase in the hydrogen capacity can be attributed to the chemisorption hydrogen uptake, which was enhanced by the sorption sites created through the milling process of polyaniline with the nanotubes. This is in addition to the hydrogen adsorption contribution by a controlled lumen size that is suitable for a maximum hydrogen adsorption through inserting polyaniline chains into the nanotubes.     

Hydrogen storage in Ni-B nanoalloy-doped 2D graphene

Two-dimensional graphene material is doped with Ni-B nanoalloys via a chemical reduction method, and shows that the optimal graphene doped with Ni (0.14 wt.%) and B (0.63 wt.%) has a hydrogen capacity of 2.81 wt.% at 77 K and 106 kPa, which is more than twice of that of the pristine graphene. The measured adsorption isotherms of hydrogen and nitrogen suggest that the Ni-B nanoalloys function as catalytic centers to induce the dissociative adsorption of hydrogen (spillover) on the graphene. The Ni-B nanoalloys without using any noble metal may be a promising catalyst for hydrogen storage application.     

Hydrogen adsorption in nanotube and cylindrical pore: A grand canonical Monte Carlo simulation study

Physisorption of targeted amount of hydrogen within carbonaceous material is a formidable task. Even though at 80 K adsorption is satisfactory but at 298 K storing desirable amount of hydrogen is difficult. Here we report grand canonical monte carlo simulation of hydrogen adsorption within two different cylindrical pores in the temperature range 60–298 K and in the pressure range 1–500 bar. In one we construct a cylindrical pore (CP) of ≈2.0 nm diameter by removing carbon atoms from the center of stacked graphene sheets. In the other single walled carbon nanotube (SWCNT) of similar diameter is used for the adsorption. In all of our simulations intermolecular hydrogen interactions are modeled using the classical Silvera-Goldman potential, which contains both Lennard-Jones and electrostatic sites. Total amount of adsorbed hydrogen is always greater in SWCNT (adsorbed both inside and outside the wall) than in CPs, however amount of hydrogen adsorbed inside SWCNT only is always smaller than that inside CP. Surface defects created during removal of carbon atoms in CP results in almost 2 wt% increase in uptake compared to SWCNT.     

Hydrogen adsorption on modified activated carbon

The aim of this work is to investigate hydrogen adsorption on prepared super activated carbon (AC). Litchi trunk was activated by potassium hydroxide under N₂ or CO₂ atmosphere. Nanoparticles of palladium were impregnated in the prepared-AC. Hydrogen adsorption was accurately measured by a volumetric adsorption apparatus at 77, 87, 90 and 303 K, up to 5 MPa. Experimental results revealed that specific surface area of the prepared-AC increased according to KOH/char ratio. The maximum specific surface area reached up to 3400 m²/g and total pore volume of 1.79 cm³/g. The maximum hydrogen adsorption capacity of 2.89 wt.% at 77 K and under 0.1 MPa, was obtained on these materials. The hydrogen adsorption capacity of the 10 wt.% Pd-AC was determined as 0.53 wt.% at 303 K and under 6 MPa. This amount is higher than that on the pristine AC (0.41 wt.%) under the same conditions.     

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.     

Hydrogen storage performance in palladium-doped graphene/carbon composites

In this study a two-dimensional graphene sheet (GS) doped with palladium (Pd) nanoparticles was physically mixed with a superactivated carbon (AC) receptor and used as a hydrogen adsorbent. The hydrogen adsorption/desorption isotherm of the Pd-doped GS catalyst/AC composite (Pd-GS/AC) is determined using a static volumetric measurement at room temperature (RT) and pressure up to 8 MPa. The experiments show that the H₂ uptake capacity of 0.82wt.% for Pd-GS/AC is obviously enhanced, measuring 49% more than the 0.55wt.% for Pd-free GS/AC at RT and 8 MPa. Highly reversible behavior of Pd-GS/AC is also observed. Moreover, the isosteric heat of adsorption for Pd-GS/AC (−14 to −10 kJ/mol) is higher than that for pristine AC (−8 kJ/mol). An increase in H₂ uptake in the Pd-GS/AC suggests the occurrence of a relatively strong interaction between the spilt-over H and the receptor sites due to the spillover effect.     

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 of dual-Ti-doped single-walled carbon nanotubes

The hydrogen adsorption capacity of dual-Ti-doped (7, 7) single-walled carbon nanotube (Ti-SWCNTs) has been studied by the first principles calculations. Ti atoms show different characters at different locations due to local doping environment and patterns. The dual-Ti-doped SWCNTs can stably adsorb up to six H₂ molecules through Kubas interaction at the Ti₂ active center. The intrinsic curvature and the different doping pattern of Ti-SWCNTs induce charge discrepancy between these two Ti atoms, and result in different hydrogen adsorption capacity. Particularly, eight H₂ molecules can be adsorbed on both sides of the dual-Ti decorated SWCNT with ideal adsorption energy of 0.198 eV/H₂, and the physisorption H₂ on the inside Ti atom has desirable adsorption energy of 0.107 eV/H₂, ideal for efficient reversible storage of hydrogen. The synergistic effect of Ti atoms with different doping patterns enhances the hydrogen adsorption capacity 4.5H₂s/Ti of the Ti-doped SWCNT (VIII), and this awaits experimental trial.     

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.     

Effects of different palladium content loading on the hydrogen storage capacity of double-walled carbon nanotubes

The effects of different amounts palladium loading on the hydrogen sorption characteristics of double-walled carbon nanotubes (DWCNTs) have been investigated. The physical properties of the pristine DWCNTs and Pd/DWCNTs were systematically characterized by X-ray diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller surface area measurements. Pd nanoparticles were loaded on DWCNT surfaces for the dissociation of H₂ into atomic hydrogen, which spills over to the defect sites on the DWCNTs. When we use different Pd content, the particle size and dispersion will be different, which affects the hydrogen storage capacity of the DWCNTs. In this work, the hydrogen storage capacities were measured at ambient temperature and found to be 1.7, 1.85, 3.0, and 2.0 wt% for pristine DWCNTS, 1.0 wt%Pd/DWCNTs, 2.0 wt%Pd/DWCNTs, and 3.0 wt%Pd/DWCNTs, respectively. We found that the hydrogen storage capacity can be enhanced by loading with Pd nanoparticles and selecting a suitable content. Furthermore, the sorption can be attributed to the chemical reaction between the atomic hydrogen and the dangling bonds of the DWCNTs.     

Hydrogen adsorption properties of Ag decorated TiO2 nanomaterials

In this work, we present the synthesis of Ag doped TiO₂ materials. The products are characterized by powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption, and hydrogen adsorption. The Ag/TiO₂ materials exhibit 3.65 times higher in hydrogen adsorption capability compared with the non-doped TiO₂ materials thank to the existence of Ti³⁺ species, which are Kubas-type hydrogen adsorption centers, and the Ag nanoparticles which provide spillover effects. We believe that this is the first time that both Kubas-type adsorption and spillover are exploited in the design of novel hydrogen storage materials.     

Hydrogen saturation limit of Ti-doped BN nanotube with B-N defects: An insight from DFT calculations

Ti-decorated (10,0) single-walled BN nanotubes (BNNTs) with B-N defects was fully examined by density functional theory (DFT). According to DFT formalisms, the HOMO-LUMO gap found for the Ti-BNNTs is small compared to that of a wide-gap semiconducting pristine BNNT. The Ti atom does not form any clusters and protrudes to the external surface of the sidewall. The calculations suggest that the Ti-BNNT assembly can attract small molecules and it has a good affinity towards H₂ molecules. Up to seven H₂ can partially attach to the system in quasi-molecular fashion due to the partially cationic character of the functionalized Ti and heteropolar bonds exhibited at the BNNT surface. The binding energies of H₂ with Ti-BNNTs are within the optimal range for H₂ storage. The unique electronic structure is barely perturbed upon adsorption and the (H₂)_7xTi_xBNNT systems hydrogen storage capacity is in compliance to the specifications mandated by the Department of Energy.     

Hydrogen sorption properties of Pd-doped carbon molecular sieves

Decoration with transition metal catalysts has been reported to enhance H₂ storage capacity of carbon materials at ambient temperature. Furthermore, it has been proposed that surface oxygen groups may improve the process. In this study, a carbon molecular sieve was subjected to controlled oxidation and consequent doping with Pd nanoparticles. The H₂ sorption performance of the pristine and oxidized, undoped and doped materials was examined at 298 K up to 20 bar. It was found that the non-oxidized carbon-Pd composite did not show any spillover based sorption increase. On the other hand the oxidized samples reveal a slight enhancement that could be attributed to a weak chemisorption process initiated by the so-called ‘‘spillover’’ effect. Overall, the contribution of spillover to the total hydrogen storage capacity of this system (under the conditions studied) was not found to be of great significance.