Mechanical and thermal properties of carbon nanotubes and boron nitride nanotubes for fuel cells and hydrogen storage applications: A comparative review of molecular dynamics studies

The molecular dynamics (MD) simulation method has been widely used to study mechanical and thermal properties of nanomaterials. However, a comprehensive review that compares the thermal and mechanical properties of carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) and their hybrid structures obtained using the MD simulation method is still lacking. In this paper, we review and document the contradictory results on the mechanical and thermal properties of CNTs and BNNTs published in the literature. We identify, a critical lack of discussion in the literature concerning the thermal and mechanical properties of BNNTs and the influence of encapsulated hydrogen on these properties. We hope that future work will address some of these contradictory data obtained using the MD simulations. We also anticipated that this work would provide important insights into the mechanical and thermal characterisations of these nanotubes for hydrogen storage and fuel cells applications.     

Effect of Environmental Temperatures on Elastic Properties of Single-Walled Carbon Nanotube

Based on a method of molecular structural mechanics (MSM), the effect of environmental temperature on elastic properties of armchair and zigzag single-walled carbon nanotubes is investigated. Single-walled carbon nanotubes with different chiral vectors are considered as a molecular structural mechanics model, which is composed of the discrete molecular structures through the carbon-to-carbon bonds. By considering the effect of environmental temperature on force constant values of the bonds stretching, bonds angle bending and torsional resistance, the corresponding basic parameters of a truss of the single-walled carbon nanotubes are obtained in different environmental temperatures, respectively. Nanoscale structural mechanics simulation for the elastic properties of single-walled carbon nanotubes in different environmental temperatures reveals that the elastic modulus of single-walled carbon nanotubes decreases significantly with the increase of environmental temperature. It is noted that the Young's modulus of single-walled carbon nanotubes is more sensitive to environmental temperature than the shear modulus.     

On the feasibility of developing hydrogen storages capable of adsorption hydrogen both in its molecular and atomic states

This review systemizes the results of research done over the last decade to study the feasibility of the construction and use of solid-state hydrogen accumulators. Consideration has been given to nanoporous carbon and metal-organic framework structures, complex hydrides and nanoporous materials that were created by the authors of this review using radiation-induced methods. It has been shown that the structures that adsorb hydrogen adhering to a mechanism either of physical or chemical adsorption can accumulate it in a sufficient amount, but are not capable of desorbing it in the temperature range required by DOE [1]. It is asserted that the prospects for the creation of hydrogen absorbers with performances acceptable for the practical application are viewed from the standpoint of the development of structures capable of adsorbing hydrogen both in its molecular and atomic states. The authors give the results of their recent research; these show that nanocrystalline materials containing nanopores in their intergranular junctions can adsorb huge amount of hydrogen adhering to the mechanism of both physical and chemical adsorption.     

Nanomaterials for on-board solid-state hydrogen storage applications

Hydrogen has the highest gravimetric density (energy density per unit mass) of any fuel. The combustion of hydrogen releases energy in the form of heat. When hydrogen reacts with oxygen in a fuel cell, the reaction releases energy in the form of electricity. Unlike hydrocarbon-based fuels, the generation of energy from either the combustion of hydrogen or the reaction of hydrogen with oxygen in a fuel cell is not accompanied by the emission of greenhouse gases. This makes hydrogen a promising solution to solve global warming issues. However, hydrogen has a low volumetric density (low energy density per unit volume) which makes storing or transporting hydrogen extremely difficult and expensive. To accelerate the utilization of hydrogen as an energy carrier, it is necessary to develop advanced hydrogen storage methods that have the potential to have a higher energy density.The hydrogen storage market is segmented by application into: (1) Stationary power: stored hydrogen is consumed for example in a fuel cell for use in backup power stations, refueling stations, power stations; (2) Portable power: hydrogen storage applications for electronic devices such as mobile phones, flash lights, and portable generators; and (3) Transportation: industries including automobiles, aerospace, unmanned aerial systems, and hydrogen tanks used throughout the hydrogen supply chain. The increasing development of light and heavy fuel cell vehicles is expected to drive the development of on-board solid-state hydrogen technologies.A large number of research groups worldwide for many years have been trying to develop materials having the right set of thermodynamic and kinetic properties, along with all of the physical properties (high gravimetric density, high volumetric density, etc.) to allow for low-pressure storage system in ambient conditions. However, to date, no material has been found that satisfies all the desired properties to be viably used in many applications. Even if we consider only three parameters namely gravimetric density, volumetric density, and system cost, no materials that can meet the ultimate targets of the U.S. Department of Energy (DOE) or the 2030 targets of the European Union's Fuel Cells and Hydrogen Joint Undertaking (FCH JU) and the New Energy and Industrial Technology Development Organization (NEDO) in Japan.The present article reviews advances in solid-state hydrogen storage technology and compares the opportunities and challenges of selected materials. The materials reviewed in this article have a wider spectrum than the materials reviewed in other existing articles, including carbon nanotubes (CNTs), metal–organic frameworks (MOFs), graphene, boron nitride (BN), fullerene, silicon, amorphous manganese hydride molecular sieve, and metal hydrides. Pioneering works, important breakthroughs, as well as the latest developments for promising materials are also reviewed.In addition, for the first time the targets set by several official regulatory agencies for solid-state hydrogen storage are summarized. Achievements in academic and industrial research are compared against these targets.The future prospects of promising materials are analyzed based on how its practical application can be implemented according to market needs.     

Different effects of temperature on supramolecular protein and non-protein materials in hydrogen storage

The storage of large quantities of hydrogen at ambient temperature is a key factor in establishing a hydrogen-based economy. One strategy for hydrogen storage is to exploit the interaction between H₂ and a solid material by physisorption of hydrogen on porous materials. However, physisorption materials containing MOF, porous carbons, zeolites, clathrates, and synthesized organic polymers physisorb only about 1 wt% of H₂ at ambient temperature. One approach to solving this problem is to prepare new classes of physisorption materials which exhibits a mechanism different from the reported materials in hydrogen storage. Here we report the synthesis of apo cross-linked ferritin supramolecules by disulfide bonds, and their holo form. Unlike non-protein adsorbents, the hydrogen storage capacity of these protein materials increases as a function of temperature over the range of 20–40 °C. The holo supramolecules enable the adsorption of hydrogen up to 3.51 wt% at 40 °C and 40 bar H₂. In contrast, non-protein physisorption materials such as activated carbon and nano Fe₂O₃ marginally adsorb hydrogen, and, as reported, their ability to adsorb hydrogen decreases with increasing temperature under the same experimental condition. These results demonstrate that protein materials have a unique hydrogen storage mechanism which offers new opportunities in exploration of physisorption materials at ambient temperature.     

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.     

Transition metal decorated covalent triazine-based frameworks as a capacity hydrogen storage medium

From ab initio density functional theory (DFT) calculations, the structural stability and hydrogen adsorption capacity of transition metal (TM, TM = Sc, Ti, V, Cr, Mn) decorated covalent triazine-based framework (CTF) are discussed. It is found that by calculation, these TM atoms can adsorb on the CTF sheet without clusters. The Sc, Ti, V, Cr and Mn decorated CTF are predicated to bind five, four, three, three and two of hydrogen molecules. We found that Sc and Ti decorated CTF are suitable candidates for effective reversible hydrogen storage at near ambient condition, whereas V, Cr and Mn decorated CTF are not promising materials due to too large average bind energies per hydrogen molecule.     

Phosphorene: A promising candidate for H2 storage at room temperature

Owing to the existence of periodic channels in phosphorene, this 2D material can be a good candidate for room temperature reversible hydrogen storage. The density functional theory calculations (DFT), including van der Waals interactions (vdW-DF2) coupled with the cooper exchange functional (C09), has been applied to study the potential of phosphorene as a new 2D material for hydrogen storage. Our results show that the adsorption energy (−292 to −277 meV) of H₂ on phosphorene is appropriate for storage. The analysis of diffusion pathways between different physisorbed states on phosphorene shows that a single hydrogen molecule diffuses very easily along the open channel (less than 1 meV along the zigzag direction), as compared to 14 meV for diffusion across the channels (along the armchair direction). The potential energy surfaces for the dissociative chemisorption of H₂ was computed on highly symmetric sites of phosphorene and the highest activation barrier was found to be 2.77 eV. The very large dissociation energy coupled with a strong physisorption of H₂ on phosphorene and facile diffusion, makes this 2D material a promising candidate for H₂ storage at room temperature.     

Chemical strategies in molybdenum based chalcogenides nanostructures for photocatalysis

Since the last two decades, plenty of environmental issues have risen up due to the damage which humans have caused to the planet for the sake of development. The continual ignorance of global climate change and the stalemate approach of major oil producing industries led to the catastrophic melting of glaciers in Arctic and Antarctica and very recently the highest mountain peak of Sweden have become 24 m shorter which is the evident outcome of climatic disturbance. The chaotic unbalancing in the reservoirs of natural resources is leading us to the several crisis which has a potential to affect the livelihood. Among the various techniques used for the development of sustainable energy, photocatalysis is regarded as one of the simplest technique which can yield enormous amount of energy by the utilization of solar energy for meeting the world's demand of an energy requirement and which can be exploited in the degradation of toxic pollutants i.e. organic as well as inorganic pollutants for environment remediation. Transition metal chalcogenides (TMCs) have a potential to get adsorb easily and be utilized for solving the energy-related problems. Large number of photocatalysts has been fabricated, among them Molybdenum (Mo) chalcogenides nanostructures, which also belong to the class of TMCs exhibit exceptional properties such as non-toxicity, low cost and structural flexibility which give them edge over the other materials. Furthermore, the tunable band gap of Mo chalcogenides nanostructures makes them the promising candidates for efficient hydrogen evolution via photocatalytic water splitting in the visible light illumination. This review deals with the photocatalytic applications of Mo based chalcogenides nanostructures in efficient hydrogen production via water splitting and degradation of dyes. It also discusses the recent developments in fabricating Molybdenum chalcogenides nanostructures, their role in the photocatalytic water splitting and discusses the efforts which have been made to improve their photocatalytic activity for extending their applications to the scalable point.     

Carbon materials for H2 storage

In this work a series of carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The materials used were different types of commercial carbons: carbon fibers, carbon cloths, nanotubes, superactivated carbons, and synthetic carbons (carbon nanospheres and carbon xerogels). Their textural properties (i.e., surface area, pore size distribution, etc.) were related to their hydrogen adsorption capacities. These H₂ storage capacities were evaluated by various methods (i.e., volumetric and gravimetric) at different temperatures and pressures. The differences between both methods at various operating conditions were evaluated and related to the textural properties of the carbon-based adsorbents. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. Furthermore, hydrogen storage capacity seems to be proportional to surface area, especially at 77 K. The micropore size distribution and the presence of narrow micropores also notably influence the H₂ storage capacity of carbons. In contrast, morphological or structural characteristics have no influence on gravimetric storage capacity. If synthetic materials are used, the textural properties of carbon materials can be tailored for hydrogen storage. However, a larger pore volume would be needed in order to increase storage capacity. It seems very difficult approach to attain the DOE and EU targets only by physical adsorption on carbon materials. Chemical modification of carbons would seem to be a promising alternative approach in order to increase the capacities.     

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.     

Analysis of suitability of TPP ash-slag waste as materials for hydrogen fuel storage

The current trends in energy were described, the main of which is the use of alternative energy sources, especially hydrogen. The most common methods of hydrogen accumulation were proposed: accumulation of compressed gaseous hydrogen in high-pressure tanks; accumulation of liquid hydrogen in cryogenic tanks; storing hydrogen in a chemically bound state; accumulation of gaseous hydrogen in carriers with a high specific surface area. Based on the combination of advantages and disadvantages, the most promising methods of accumulation were selected: storage of liquid hydrogen and storage of hydrogen in carriers with a high specific surface area. The main requirement for materials for hydrogen storage by these methods was revealed – a high specific surface area. Prospects for the development of waste-free low-emission technologies due to the recycling of secondary raw materials and the development of low-temperature technologies for the synthesis of functional and structural materials were substantiated. The applicability of large-scale ash and slag waste from coal-fired thermal power plants as a raw material for obtaining materials by low-temperature technologies was shown. The traditional ways of using ash and slag waste as a raw material, active additive and filler in the production of cements were described. Modern technologies for the production of innovative materials with a unique set of properties were presented, namely carbon nanotubes, silica aerogel and geopolymer materials. The prospect of using geopolymer matrices as a precursor for the synthesis of a number of materials was described; the most promising type of materials was selected – geopolymer foams, which are mainly used as sorbents for purifying liquids and gases or accumulating target products, as well as heat-insulating materials. The possibility of obtaining products of any shape and size on the basis of geopolymer matrices without high-temperature processing was shown. The special efficiency of the development of the technology of porous granules and powders obtained from a geopolymer precursor using various methods was substantiated. The obtained granules can be used in the following hydrogen storage technologies: direct accumulation of hydrogen in porous granules; creation of insulating layers for liquid hydrogen storage units.     

Review on the interface engineering in the carbonaceous titania for the improved photocatalytic hydrogen production

Hydrogen is the prime source of energy with enormous attention in the current research development process as it is safe, clean, eco-friendly, and can be produced from renewable resources through simple catalytic reactions. Scalable production of hydrogen through photocatalysis has been achieved using carbon-modified semiconductors since 2009. In this direction, this review delivers comprehensive understandings into the interface and structural interactions between TiO₂ and carbonaceous materials such as carbon, carbon nanotubes, graphene, activated carbon, graphitic carbon nitride, carbon quantum dots, etc., and their influences toward improving the hydrogen generation activity of these systems. Besides, recently developed carbonaceous materials such as 3-D graphene, carbon nanohorns, and carbon nanocones have also been discussed on their character in the photocatalytic water splitting procedure. In general, the observed improvements in this carbon-modified TiO₂ attributed to the synergetic effects, which offer the active migration of charge carriers and reduced recombination rates in the photocatalyst. Finally, highlighting the future perspectives of the carbonaceous materials in photocatalytic applications are concluded.     

Boosted charge transfer from single crystalline titanium nanotubes for a dual-purpose PEC system

Photoelectrocatalytic (PEC) technology has promising applications. It is capable of effectively generating hydrogen and removing organic pollutant in water at the same time. Though TiO₂ materials have been proven an excellent candidate for fabricating PEC electrodes for its low toxicity, high oxidation potential and photocatalytic activity, the catalytic performance of TiO₂ electrodes is limited by their low electrons transporting efficiency. In this study, using a facile electrochemical method, we produced single crystalline TiO₂ nanotubes (s-TNTs) exposing {001} facets with enhanced charge transfer efficiency than that of common TiO₂ nanotubes electrodes (m-TNTs). In this system, the electrochromic s-TNTs electrodes exhibit better performance in generating hydrogen and higher degradation rate of atrazine on s-TNTs than that on common TiO₂ nanotubes electrodes (m-TNTs) in varying conditions: electrochemical (EC), photocatalytic (PC), and PEC. Using s-TNTs and s-TNTs@Pd nanocomposites as both photoanode and cathode, the photon-to-current conversion efficiency was significantly enhanced, and the promoted hydrogen production rose obviously. The enhanced charge transfer efficiency of s-TNTs might induce a highly enhancement in the whole PEC system, which could be primarily attributed to the single-crystalline structure and exposed {001} facet. This study could provide new possibility of utilizing single crystalline TiO₂ nanotubes for efficient dual-purpose PEC system for its high activity, great stability, low cost and no toxicity.     

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.     

Nanoconfinement effects on hydrogen storage properties of MgH2 and LiBH4

To find a solution to efficiently exploit renewable energy sources is a key step to achieve complete independence from fossil fuel energy sources. Hydrogen is considered by many as a suitable energy vector for efficiently exploiting intermittent and unevenly distributed renewable energy sources. However, although the production of hydrogen from renewable energy sources is technically feasible, the storage of large quantities of hydrogen is challenging. Comparing to conventional compressed and cryogenic hydrogen storage, the solid-state storage of hydrogen shows many advantages in terms of safety and volumetric energy density. Among the materials available to store hydrogen, metal hydrides and complex metal hydrides have been extensively investigated due to their appealing hydrogen storage properties. Among several potentials candidates, magnesium hydride (MgH₂) and lithium borohydride (LiBH₄) have been widely recognized as promising solid-state hydrogen storage materials. However, before considering these hydrides ready for real-scale applications, the issue of their high thermodynamic stability and of their poor hydrogenation/dehydrogenation kinetics must be solved. An approach to modify the hydrogen storage properties of these hydrides is nanoconfinement. This review summarizes and discusses recent findings on the use of porous scaffolds as nanostructured tools for improving the thermodynamics and kinetics of MgH₂ and LiBH₄.     

Capacity development of Pd doped Si2BN nanotube for hydrogen storage

Using first principles calculations employing density functional theory (DFT) we have investigated the electronic properties of Si₂BN nanotubes having 10 Å and 15 Å (armchair and zigzag) diameters. The obtained electronic band structures reveal all structures to be metallic in nature. The partial density of states (DOS) shows that the contributions of 3p state of Si is most prominent in the conduction band and majority contribution of the valence band is due to the 2p state of N atoms for all nanotubes. H₂ storage in Pd doped Si₂BN nanotubes (Si₂BN@Pd) was investigated up to the maximum limit. Our calculations have uncovered the possibility of storing 2H₂ and 3H_2, respectively, inside, and outside of the Si₂BN@Pd in each unit cell. When diameter increases from 10 Å to 15 Å the static dielectric constant, ε₁(0), upsurges in value from 6.9 to 10.7. The variation in n(ω) for the 10 Å diameter nanotube, with and without H₂ molecules, indicates no significant changes after the sensing of H₂ molecules, while in the 15 Å diameter nanotube, the n(0) value in the near infra-red (NIR) region is found to decrease and shows a further decrease if we store more H₂. We have also calculated the adsorption energies for the Si₂BN@Pd and Si₂BN@PdH₂ structures (for 10 Å and 15 Å diameters). Our theoretical approach has revealed that the Si₂BN nanotube can efficiently store hydrogen and we propose efficient ways for hydrogen storage for future improvements.     

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.     

Metal hydride materials for solid hydrogen storage: A review

Hydrogen is an ideal energy carrier which is considered for future transport, such as automotive applications. In this context storage of hydrogen is one of the key challenges in developing hydrogen economy. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Chemical or physically combined storage of hydrogen in other materials has potential advantages over other storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties. The present review reports recent developments of metal hydrides on properties including hydrogen-storage capacity, kinetics, cyclic behavior, toxicity, pressure and thermal response. A group of Mg-based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for on-board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system and as well as by ball-milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower decomposition temperatures, however, the state-of-the-art materials are still far from meeting the aimed target for their transport applications. Therefore, further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides.