A theoretical first principles computational investigation into the potential of aluminum-doped boron nitride nanotubes for hydrogen storage

Hydrogen storage remains a largely unsolved problem facing the green energy revolution. One approach is physisorption on very high surface area materials incorporating metal atoms. Boron nitride nanotubes (BNNTs) are a promising material for this application as their behaviour is largely independent of the nanoscopic physical features providing a greater degree of tolerance in their synthesis. Aluminum doping has been shown to be a promising approach for carbon nanotubes but has been underexplored for BNNTs. Using first principles density functional theory, the energetics, electronics and structural impacts of aluminum adsorption to both zigzag and armchair polymorphs of BNNTs was investigated along with their potential capacity to adsorb hydrogen. The fine atomic structural and electronic details of these interactions is discussed. We predicted that in an ideal situation, highly aluminum-doped armchair and zigzag BNNTs could adsorb up to 9.4 and 8.6 wt percent hydrogen, well above the United States Department of Energy targets marking these as promising materials worthy of further study.     

Thermal buckling of temperature-dependent composite super elliptical plates reinforced with carbon nanotubes

In this work, the problem of thermal buckling of composite plates reinforced with carbon nanotubes (CNTs) is investigated. Distribution of CNTs as reinforcements through the thickness direction of the plate is assumed to be either uniform or functionally graded (FG). Properties of the reinforcement and matrix are both temperature dependent. Properties of the composite media are obtained according to a refined rule of mixture approach where the e?ciency parameters are introduced. The plate is in a super elliptical shape where the simple elliptical shape and rectangular shapes are obtained as especial cases. In these types of plates due to the round corners, stress concentration phenomenon is eliminated. Based on the Ritz method where the shape functions are of the polynomial type, the governing equations are obtained. These equations are solved using an iterative eigenvalue problem since the properties are temperature dependent. Numerical results are validated for the simple case of an isotropic plate. Novel numerical results are provided for plates reinforced with CNTs in different shapes, various volume fractions and different patterns of CNT distribution. It is shown that FG-X pattern of CNTs in matrix results in the maximum critical buckling temperature.     

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.     

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.     

Structural,electronic and optical properties of titania nanotubes

Abstract

This review paper describes primarily recent theoretical calculations with some supporting experimental findings on titania nanotubes. Nanotubes with different types and sizes are discussed in detail in terms of existing theoretical and experimental achievements. Both classical and quantum mechanical simulations are focused on. The properties of these nanotubes have been treated within first principle density functional electronic structure simulation methods. In this paper, we pay particular attention to computational aspects, but when appropriate, relationships with experimental results on titania nanostructures will be mentioned. First, the structural properties of titania nanotubes are reviewed, focusing from experimental growth mechanism to possible theoretical stable structure and orientation. Second, the electronic structure of nanotubes is discussed in terms of band gap modifications of titania and photocatalytic efficiencies in photoelectrochemical devices. Finally, current computational limitations and future directions are described with respect to the performances of nanotube titania based photosensitive devices.     

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.     

Synthesis of boron and nitrogen co-doped carbon nanotubes and their application in hydrogen storage

Boron and nitrogen codoped carbon nanotubes (B,N-CNTs) were synthesized by floating catalyst chemical vapor deposition (FCCVD) using ethanol, ferrocene, boric acid and imidazole as carbon source, catalyst, boron and nitrogen precursors, respectively. The samples were analyzed using transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoemission spectroscopy. 1.5 at% B and 1.34 at% N could be doped in the resultant structure, which has higher length (few μm) with higher thermal stability (621 °C). At pressure 16 bar, hydrogen adsorption for B,N-CNTs was found to be 1.96 and 0.35 wt% at 77 K and 303 K, respectively. Hydrogen storage as function of time was also reported for both the cases. The adsorption process follow pseudo second order kinetics. The present study reveals that the codoping of CNTs aid in tuning properties of CNTs for hydrogen storage application.     

Effect of Ni nanoparticle distribution on hydrogen uptake in carbon nanotubes

Ni decoration on carbon nanotubes (CNTs) performed by electroless nickel (EN) deposition is investigated. The effect of Ni particle distribution on hydrogen uptake of CNTs is also studied. The chemical composition, crystal structure and microstructure of the CNTs with or without Ni loading are characterized using an inductively coupled plasma spectrometer (ICP), X-ray diffraction meter (XRD) and transmission electron microscope (TEM) coupled with an energy dispersive spectroscope (EDS). The hydrogen uptake in CNTs with or without Ni loading is measured using a high-pressure microbalance at room temperature under a hydrogen pressure of 6.89 MPa. The experimental results show that fine and well-dispersed metallic Ni nanoparticles can be obtained by EN. The density and particle distribution depend on deposition temperature and time. An enhanced hydrogen storage capacity of CNTs can be obtained by Ni decoration, which provided a spillover reaction. The hydrogen storage capacity of the as-received CNTs was 0.39 wt.%. As much as 1.27 wt.% of hydrogen can be stored when uniformly distributed nano-sized Ni particles are formed on the surface of the CNTs. However, the beneficial effect is lost when the active sites for either physical or chemical adsorption are blocked by excessive Ni loading.     

Carbon nanotubes reinforced proton exchange membranes in fuel cells: An overview

The proton exchange membrane (PEM) is the core component in a fuel cell. In this review, recent progress and developments on per-fluorinated and non-fluorinated membranes with carbon nanotubes (CNTs) as reinforced fillers have been summarized on many key topics. Topics reviewed stem from correlating the mechanical stability, thermal stability, water retention capacity and proton conductivity of various membranes across different functionalized CNTs. In addition, topics such as the preparation strategies of membrane matrix and CNTs filler, the reinforced mechanism of CNTs in membrane are presented. Throughout, the impact of interactions between CNTs and various types of PEM is also discussed to present a deeper perspective. Finally, the strategy for improving the performance of PEM and the challenges of CNTs-based membranes are analyzed for prospects.     

Hydrogen storage on silicon,carbon, and silicon carbide nanotubes: A combined quantum mechanics and grand canonical Monte Carlo simulation study

Grand canonical Monte Carlo (GCMC) simulation combined with ab initio quantum mechanics calculations were employed to study hydrogen storage in homogeneous armchair open-ended single walled silicon nanotubes (SWSiNTs), single walled carbon nanotubes (SWCNTs), and single walled silicon carbide nanotubes (SWSiCNTs) in triangular arrays. Two different groups of nanotubes were studied: the first were (12,12) SiNTs, (19,19) CNTs, and (15,15) SiCNTs and the second were (7,7) SiNTs, (11,11) CNTs, and (9,9) SiCNTs with the diameters of ∼26 and ∼15 Å for the first and second groups, respectively. The simulations were carried out for different thermodynamic states. The potential energy functions (PEFs) were calculated using ab initio quantum mechanics and then fitted with (12,6) Lennard-Jones (LJ) potential model as a bridge between first principles calculations and GCMC simulations. The absolute, excess, and delivery adsorption isotherms of hydrogen were calculated for two groups of nanotubes. The isosteric heat of adsorption and the radial distribution functions (RDFs) for the adsorbed molecules on different nanotubes were also computed. Different isotherms were fitted with the simulation adsorption data and the model parameters were correlated. According to the results, the hydrogen uptake values in (19,19) CNT array exceeded the US DOE (Department of Energy) target of 6.0 wt% (FY 2010) at 77 K and 1.0 and 2.0 MPa for absolute and excess uptakes, respectively. The results also show that SiNTs and SiCNTs are not more useful materials compared with corresponding CNTs for hydrogen storage.     

Adsorption of hydrogen atoms onto the exterior wall of carbon nanotubes and their thermodynamics properties

In the present work, we present a systematic analysis of the chemisorption process pathway of hydrogen atoms onto the exterior wall of (5,5) carbon nanotubes using the ONIOM2 (B3LYP(6–31+G(d,p):UFF)) scheme, and we avoid the gross assumption of fixing any of the carbon atoms during the simulation. It is shown that the adsorption of hydrogen atoms onto the sidewall of CNTs are energetically favorable and the most stable state is to form two H–C σ-bonds while the original σ-bond between the carbon atoms is totally severed. In particular, we examined the molecular thermodynamics properties for the reaction at a range of temperatures from 77 K to 1000 K, and the results suggests that the reaction is possible at ambient temperature, but it is less favorable than that at lower temperatures.     

Comparison of hydrogen absorption in metallic and semiconductor single-walled Ge- and GeO2-doped carbon nanotubes

First-principles calculations were carried out to compare hydrogen absorption in pristine metallic and semiconductor carbon nanotubes (CNTs) with the situation in their Ge- and GeO₂-doped counterparts. We found out that the pristine carbon nanotubes have low absorption efficiency (?1.53 eV in the metallic, and ?2.06 eV in the semiconductor carbon nanotube). When Ge was doped into both carbon nanotubes, the hydrogen absorption was enhanced to ?5.29 eV in the metallic and ?3.99 eV in the semiconductor carbon nanotubes. Investigating the Partial density of states proved that there was considerable overlap between Ge 4p and hydrogen 1s orbitals in both CNTs. When CNTs were doped with GeO₂, hydrogen atoms were bound to oxygen atoms, due to high electronegativity of oxygen atom. The hydrogen absorption was found to be increased remarkably in the metallic carbon nanotube (?6.59 eV). In order to compare the binding energy of Ge and GeO₂ doped metallic and semiconductor carbon nanotubes, the partial density of states and the magnetization of the samples were studied.     

Molecular and atomic adsorptions of hydrogen,oxygen, and nitrogen on defective carbon nanotubes: A first-principles study

To investigate the application properties of defective carbon nanotubes (CNTs) with gas adsorbates, structural and electronic properties after physical and chemical adsorptions of molecular and atomic hydrogen, oxygen and nitrogen on vacancy defects for capped single-walled carbon nanotubes (SWNTs) were simulated. The defective CNT shows the half-metal properties, but molecular chemisorption could convert the electronic property back to semiconductor. For the physisorptions of gas molecules, H₂ is unstable for the endothermic process. If gas molecules approach to the defect in angstrom range, the chemisorption could happen, and oxygen molecule gains nearly 2 electrons. CNT work function drops with chemisorptions of molecular or atomic hydrogen, while oxygen behaves opposite. The weaker nitrogen chemisorption is able to keep oxygen away from adsorbing as the protective agent. The adsorption of atomic nitrogen may fill in the missing carbon atom to “repair” the defect and reduce the work function by 0.83 eV.     

CFD modeling and experimental study of multi-walled carbon nanotubes production by fluidized bed catalytic chemical vapor deposition

A parametric study investigating the impact of temperature, gas velocity, and composition of the gaseous phase on the catalytic growth of multi-walled carbon nanotubes (CNT) has been performed. CNTs have been produced by catalytic chemical vapor deposition from methane decomposition over Co-Mo/MgO with average diameter of 188 μm with spherical shape in a fluidized bed reactor. The computational fluid dynamics (CFD) method was used for simulating the hydrodynamics of the reactor and investigating the operational and best velocity for producing high quality CNTs by this system. The operational and best velocities obtained by simulation were 0.015 to 0.05 m/s and near 0.015 m/s. Then the results used in the experiments with different temperature and gas compositions. CNTs products were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results showed that temperature of 900 °C, methane to hydrogen volume ratio 1:4 and 0.02 m/s are the best quantities of the parameters for CNTs growth.     

Boron nitride nanotubes (BNNTs) decorated Pd-ternary alloy (Pd63·2Ni34·3Co2.5) for H2 sensing

The micro-electro-mechanical system (MEMS)-based field effect transistor (FET) sensor for hydrogen detection was fabricated by modifying the gate electrode with boron nitride nanotubes (BNNTs) decorated Pd-ternary alloy (Pd_63·2Ni_34·3Co_2.5) as a hydrogen sensing layer Electro-thermal properties of the micro-heater embedded under sensor membrane were analyzed by a finite element method (FEM) simulation. The structural and morphological properties of the gate electrode were studied by Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). A variation in gate potential is observed due to the H₂ atmosphere that leads to the variation in the depletion region, therefore, changing the current in the channel (BNNTs decorated Pd-ternary alloy). The BNNTs-decorated Pd ternary alloy displayed high sensing response, fast response and recovery time for H₂ gas, low power consumption, long-term stability, and wide detection range from 1 to 5000 ppm H₂. The drain current of the H₂ FET sensor varied significantly at hydrogen gas exposure and increased with H₂ concentration. As proposed H₂ FET sensor can be utilized to the H₂ leak detection system for safe applications.     

In-situ nitrogen doping in carbon nanotubes using a fluidized bed reactor and hydrogen storage behavior of the doped nanotubes

Nitrogen atoms were successfully doped into the lattice of carbon nanotubes (hereafter called N-CNT) by chemical vapor deposition in a fluidized bed reactor using a solid precursor Imidazole. X-ray Photoelectron Spectroscopy estimated that ～5.4 atom% of nitrogen was doped in the synthesized CNTs. The successful formation of N-CNTs was also confirmed by Fourier Transforms Infrared Spectroscopy. Transmission electron microscopy revealed the morphological features of N-CNTs and their changes with temperature. The use of fluidized bed resulted in the formation of uniform nature of N-CNTs. In-depth studies of influence of nitrogen doping in CNTs for hydrogen storage was studied by comparing with bare CNTs, also synthesized by a the same fluidized bed reactor. The hydrogen storage capacity for N-CNTs (0.8 wt.%) was enhanced significantly at very low temperature (163 K), compared to bare CNTs (0.28 wt.%).     

Fast mass and charge transport through electrically aligned CNT/polymer nanocomposite membranes

This work represents the properties of electrically aligned carbon nanotubes (CNT)/polycarbonate (PC) nanocomposites towards the development of hydrogen gas separation membranes. A fraction (0.1 weight %) of CNTs synthesized by chemical vapour deposition method have been dispersed homogeneously throughout PC matrix by ultrasonication. The alignment of CNT in PC matrix has been accomplished by applying an external electric field of 1250 V/cm during solution casting. These nanocomposites have been studied by gas permeation, electrical, and dielectric constant measurements. Gas permeability measurements obtained here that electrically aligned nanocomposite membranes can be used as good hydrogen separating media. I–V characteristics and dielectric constant shows the enhancement in conductivity and permittivity of these nanocomposites. Overall experimental results exhibit here that alignment of CNTs in polymer matrix shows the dramatic improvement in mass and charge transport properties. Copyright © 2016 John Wiley & Sons, Ltd.     

Carbon nanotubes/aluminum composite as a hydrogen source for PEMFC

Al matrix composites reinforced with 0–5 vol. % carbon nanotubes (CNTs) were fabricated by spark plasma sintering (SPS) to examine their hydrogen generation properties from the hydrolysis of Al in 10 wt. % NaOH solution at room temperature. The 5 vol. % CNTs/Al composite exhibits a maximum hydrogen generation rate of 120 ml/min g, which is about 6 times higher than that of Al without CNTs due to the synergetic effects of the porous Al matrix, which has a large reaction area and galvanic corrosion between the Al matrix and the CNTs. The hydrogen gas generated from the hydrolysis of the CNTs/Al composite has high purity without any production of undesirable CO. PEMFC produced electricity at 10 A and 0.73 V for 13 min, with hydrogen generated from the hydrolysis of 3.5 g–5 vol. % CNTs/Al composite. The CNTs/Al composite was effectively used as a hydrogen source for PEMFC.     

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.     

Thermophysical Properties of Zirconia Toughened Alumina Ceramics with Boron Nitride Nanotubes Addition

Abstract

The demand for high thermal conductivity substrates with electrically insulating materials are increasing with the emerging markets in power electronics and mobile telecommunication device packages. Effective heat transfer in those packages is important to provide high performance and reliability of the product. This paper mainly presents the thermophysical properties of zirconia toughened alumina ceramics with the addition of small amount of boron nitride nanotubes (BNNTs). The effects of the boron nanotubes addition on the sintering behavior, the microstructure and the thermal properties of the yttria-stabilized zirconia toughened alumina (YZTA), nanocomposite ceramics are investigated. The addition of 0.3?wt% boron nitride nanotubes into the YZTA matrix enhanced the thermal diffusivity as well as a mechanical strength. Above all, the addition of boron nitride nanotubes greatly decreased the coefficient of thermal expansion (CTE) of the composites in which the CTE of pure alumina increases with increasing temperatures. Moreover, the BNNTs added YZTA composites revealed a drastic decrease in CTE at high temperature range, 400–800?°C. This enhanced thermal stability of YZTA–BNNT composites may have a potential application to the high temperature structural ceramics and high power semiconductor packaging substrate.