One-step process of hydrogen storage in single walled carbon nanotubes-tin oxide nano composite

Hydrogen intake study on single walled carbon nanotubes (SWCNTs)-tin oxide (SnO₂) nano composite films have been performed. The composite is prepared on glass substrates in hydrogen atmosphere by electron beam evaporation (e-beam) technique. The process of hydrogenation has been done during the preparation of hydrogen storage medium itself, as one-step process. The amount of hydrogen incorporated in the composite is found to be 2.4 wt.%. The entire (100%) amount of stored hydrogen is released in the temperature range of 200–350 °C. The stored hydrogen has weak chemical binding in the SWCNTs-SnO₂ nano composite.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Direct synthesis of nanocrystalline titanium dioxide/carbon composite and its catalytic effect on NaAlH4 for hydrogen storage

Nanocrystalline titanium dioxide/carbon composite (TiO₂/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)₄) and resol as precursors. The prepared TiO₂/C composite was mainly in the anatase structure with an average particle size under 20 nm, which was then introduced in NaAlH₄ as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO₂/C and TiO₂ can obviously improve the kinetics of NaAlH₄, while NaAlH₄ with 3 mol% TiO₂/C exhibits better cycling stability than NaAlH₄ with 3 mol%TiO₂. The hydrogen storage capacity of NaAlH₄ with TiO₂/C remains stable after 5th cycle, and about 94% of initial hydrogen is released, while the capacity of NaAlH₄ with TiO₂ decreases continuously during cycling, and only 88% of initial hydrogen is released after 10th cycle. Furthermore, NaAlH₄ with 3 mol%TiO₂/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload 4.16 and 1.63wt% hydrogen at 50 and 30 bar hydrogen pressures, respectively.     

Improved hydrogen desorption in lithium alanate by addition of SWCNT-metallic catalyst composite

A LiAlH₄/single walled carbon nanotube (SWCNT) composite system was prepared by mechanical milling and its hydrogen storage properties investigated. The SWCNT - metallic particle addition resulted in both a decreased decomposition temperature and enhanced desorption kinetics compared to pure LiAlH₄. The decomposition temperature of the 5 wt.% SWCNT-added LiAlH₄ sample was reduced to 80 °C and 130 °C for the first and second stage, respectively, compared with 150 °C and 180 °C for as-received LiAlH₄. In terms of the desorption kinetics, the 5 wt.% SWCNT-added LiAlH₄ sample released about 4.0 wt.% hydrogen at 90 °C after 40 min dehydrogenation, while the as-milled LiAlH₄ sample released less than 0.3 wt.% hydrogen for the same temperature and time. Differential scanning calorimetry measurements indicate that enthalpies of decomposition in LiAlH₄ decrease with added SWCNTs. The apparent activation energy for hydrogen desorption was decreased from 116 kJ/mol for as-received LiAlH₄ to 61 kJ/mol by the addition of 5 wt.% SWCNTs. It is believed that the significant improvement in dehydrogenation behaviour of SWCNT-added LiAlH₄ is due to the combined influence of the SWCNT structure itself and the catalytic role of the metallic particles contained in the SWCNTs. In addition, the different effects of the SWCNTs and the metallic catalysts contained in the SWCNTs were also investigated, and the possible mechanism is discussed.     

Improvement of the hydrogen storage kinetics of NaAlH4 with nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) by melt infiltration

Nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) with 10wt% TiO₂ were synthesized through an easy one-step method using phenolic resols, titania nanoparticles and Pluronic F127 as organic carbon sources, inorganic precursors and surfactant, respectively. The results show that the as-prepared Ti-CSs composite is spherical shape with a diameter ranging from 0.3 to 2 μm, and rutile TiO₂ nanoparticles are distributed on the surface of the carbon spheres. Then the kinetics of NaAlH₄ was improved through depositing it on the surface of as-prepared Ti-CSs by melt infiltration. The results show that NaAlH₄ with Ti-CSs exhibits better hydrogen desorption kinetics than TiF₃ or nanocrystalline TiO₂ catalysted-NaAlH₄, and it starts to release hydrogen at about 40 °C and releases about 25% of the hydrogen content during heating to 60 °C. The results from SEM and XPS show that hydrogen storage properties of NaAlH₄ were considerably improved due to the formation of special structure during melt infiltration and the nanocrystalline TiO₂ and/or amorphous phase Ti–Al clusters near the subsurface sites, which succeed in combining catalyst addition (TiO₂ nanoparticles) and nanoconfinement to improve the kinetics of NaAlH₄.     

Photocatalytic generation of hydrogen coupled with in-situ hydrogen storage

To date, hydrogen generation and storage are two separated processes. We report on a new concept where photocatalytically generated hydrogen is simultaneously stored in-situ within the material photo-generating hydrogen. To this aim, we successfully synthesised a “forest” of vertically aligned TiO₂ nanotubes decorated with Pd nanoparticles acting as the hydrogen store. Upon illumination of TiO₂, hydrogen was effectively generated and full storage of hydrogen within the Pd nanostructures was achieved within 100 min. This result demonstrates new avenues on the possibility of designing hybrid nanostructures for the effective use of hydrogen as an energy vector.     

Field ionization effect on hydrogen adsorption over TiO2-coated activated carbon

Behaviors of hydrogen adsorption over TiO₂-coated activated carbon under various electric potentials were studied. TiO₂ particles were introduced onto carbon via the hydrolysis of TiCl₄ in acid solution. The results showed that the hydrogen adsorption first increased and then decreased with the increase of electric field. The improved storage was due to a stronger interaction between charged carbon surface and polarized hydrogen molecule caused by field induced polarization of TiO₂ coating. When the electric field was sufficient to cause considerable ionization of hydrogen, the decrease of hydrogen adsorption occurred. The electricity detected at 3000 V was a sign of ionization of hydrogen. The DFT calculations showed a much stronger binding between TiO₂-doped coronene and hydrogen molecule under an electric field, which is consistent with our experimental observations.     

Enhanced desorption properties of LiBH4 incorporated into mesoporous TiO2

LiBH₄ nano-particles are incorporated into mesoporous TiO₂ scaffolds via a chemical impregnation method. And the enhanced desorption properties of the composite have been investigated. The LiBH₄/TiO₂ sample starts to release hydrogen at 220 °C and the maximal desorption peak occurs at about 330 °C, much lower compared to the bulk LiBH₄. Furthermore, the composite exhibits excellent dehydrogenation kinetics, with 11 wt% of hydrogen liberated from LiBH₄ at 300 °C within 3 h. X-ray diffraction and Fourier transform infrared spectroscopy are used to confirm the nanostructure of LiBH₄ in the TiO₂ scaffold. This work demonstrates that confinement within active porous scaffold host is a promising approach for enhancing hydrogen decomposition properties of light-metal complex hydrides.     

Enhancing the activity of a SiC–TiO2 composite catalyst for photo-stimulated catalytic water splitting

In this work, effort has been made to design an efficient catalyst for the photo-stimulated water splitting reaction, starting with the modification of TiO₂ (P25) to enhance its activity. A SiC(1 wt%)–TiO₂ composite material shows an activity as high as twice of that of TiO₂. NiO_x, an electron collector, promotes the activity of TiO₂, while IrO₂, a hole capturer, enhances the hydrogen evolution rate of SiC. A SiC(1 wt%)–NiO_x/TiO₂ three-component and an IrO₂/SiC(1 wt%)–NiO_x/TiO₂ four-component composite materials produce 30% and 100% more H₂ than the NiO_x/TiO₂ catalyst during the first 5 h, respectively, with ethanol used as the sacrificial reagent. Furthermore, the SiC(1 wt%)–NiO_x/TiO₂ catalyst is active under visible light, while the NiO_x/TiO₂ catalyst shows no activity under the same irradiation condition. 3C-SiC has a narrow band gap and its band edge well compensates that of the TiO₂. The enhancing effect of dopants on the SiC(1 wt%)–TiO₂ composite material is sensitive to the location of the modifiers, which further proves that an efficient separation of the charge carriers is crucial to the overall activity of the composite catalyst.     

Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite

Mesoporous TiO₂/AC, Pt/TiO₂ and Pt/TiO₂/AC (AC = activated carbon) nanocomposites were synthesized by functionalizing the activated carbon using acid treatment and sol–gel method. Photochemical deposition method was used for Pt loading. The nano-photocatalysts were characterized using XRD, SEM, DRS, BET, FTIR, XPS, CHN and ICP methods. The hydrogen production, under UV light irradiation in an aqueous suspension containing methanol has been studied. The effect of Pt, methanol and activated carbon were investigated. The results show that the activated carbon and Pt together improve the hydrogen production via water splitting. Also methanol acts as a good hole scavenger. Mesoporous Pt/TiO₂/AC nanocomposite is the most efficient photocatalyst for hydrogen production compared to TiO₂/AC, Pt/TiO₂ and the commercial photocatalyst P25 under the same photoreaction conditions. Using Pt/TiO₂/AC, the rate of hydrogen production is 7490 μmol (h g catal.)⁻¹ that is about 75 times higher than that of the P25 photocatalyst.     

Enhanced photocatalytic water splitting activity of carbon-modified TiO2 composite materials synthesized by a green synthetic approach

We report a green and facile approach for the preparation of carbon-modified (C-modified) TiO₂ composite materials by hydrothermal synthesis followed by pyrolytic treatment. The resultant materials were characterized by powder X-ray diffraction (XRD), nitrogen physisorption studies, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). The photocatalytic performances of these materials were evaluated by calculating the amount of hydrogen evolved from the decomposition of water under solar simulated irradiation conditions. An improvement was achieved from no H₂ evolution at all with the bare TiO₂, to an evolution of 0.21 mL g⁻¹ h⁻¹ from a composite material modified with an optimum carbon loading of 3.62%. These results suggested that the interaction of carbon with predominantly rutile form of TiO₂ can promote shallow trapping of photogenerated electrons in the oxygen vacancies. This phenomenon consequently enhances the photocatalytic activity by minimizing charge carrier recombination, a characteristic demonstrated by fluorescence quenching of the TiO₂ emission.     

Hydrogen storage in TiO2 functionalized (10, 10) single walled carbon nanotube (SWCNT) – First principles study

Hydrogen storage in titanium dioxide (TiO₂) functionalized (10, 10) armchair single walled carbon nanotube (SWCNT) is investigated through first principle calculations using density functional theory (DFT). This first principles study uses Vienna Ab-initio Simulation Package (VASP) with ultrasoft pseudopotentials and local density approximation (LDA). The necessary benchmark and other systematic calculations were carried out to project the hydrogen storage capability of the designed system. Interestingly, the TiO₂ molecules functionalized on the outer surface of SWCNT do not undergo any dimerization/clustering thus giving excellent stability and usable gravimetric hydrogen storage capacity of 5.7 wt.% and the value nearly fulfills the US DOE target (i.e. 6 wt.%). The band structure and density of states (DOS) plots suggest that the functionalization can lead a way to transform the nature (metallic → semiconducting) of the pristine SWCNT. The nominal values of H₂ storage capacity and binding energies give much hope for using CNT functionalized with TiO₂ as a practical and reversible hydrogen storage medium (HSM).     

Investigation of single-walled carbon nanotubes-titanium metal composite as a possible hydrogen storage medium

The present experimental work deals with the investigation of hydrogen uptake study of single-walled carbon nanotubes (SWCNTs-Ti)-titanium metal composite. The mixture containing SWCNTs and Ti powder is made into tablet by cold pressing. The composite has been prepared and hydrogenated by evaporating the tablet in hydrogen ambient on glass substrates using electron beam (EB) evaporation technique. Efficient hydrogen uptake of 4.74 wt.% is achieved with the composite and the adsorbed hydrogen posses the average hydrogen binding energy of 0.4 eV. The obtained hydrogen uptake is due to the cumulative adsorption of hydrogen by CNTs and Ti nanostructured materials. The physical properties are characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction study (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Raman analysis. Hydrogenation and dehydrogenation behavior of the composite are studied using CHN-elemental analysis and thermo gravimetric/thermal desorption spectroscopy (TG/TDS) studies, respectively. The stored hydrogen is found to be 100% reversible in the temperature range of 160–310 °C.     

Investigations on hydrogenation behaviour of CNT admixed Mg2Ni

The aim of the present paper is to report results on hydrogenation behaviour of the new composite material Mg₂Ni: CNT. Admixing of carbon nanotubes (CNT) in storage material Mg₂Ni leads to noticeable enhancement in desorption kinetics as well as storage capacity. We have found that the composite material Mg₂Ni–2 mole% CNT is the optimum material. The Mg₂Ni–CNT composite exhibits hydrogen desorption rate of 5.7 cc/g/min as against 3.0 cc/g/min for Mg₂Ni alone (enhancement of ∼ 90%) and storage capacity of ∼ 4.20 wt% in contrast to ∼3.20 wt% for Mg₂Ni alone (increase of ∼ 31%). Feasible mechanisms for the enhancement of hydrogen desorption kinetics and storage capacity have been put forward.     

High photocatalytic hydrogen production over the band gap-tuned urchin-like Bi2S3-loaded TiO2 composites system

A new system using Bi₂S₃-loaded TiO₂ photocatalysts (Bi₂S₃/TiO₂) was developed to enhance the production of hydrogen. The Bi₂S₃ (5, 10, 15 wt%) particles in an urchin-like morphology with a length of about 2∼3 μm and a diameter of 15–20 nm, which can absorb all wavelengths in UV–visible radiation, were prepared by solvothermal method and loaded onto nano-sized TiO₂ (10∼15 nm) for photocatalysis on hydrogen production. The evolution of H₂ from methanol/water (1:1) photo splitting over the Bi₂S₃/TiO₂ composite in the liquid system was enhanced, compared with that over pure TiO₂ and Bi₂S₃. In particular, 14.2 ml of H₂ gas was produced after 12 h when 0.5 g of a 10 wt% Bi₂S₃/TiO₂ composite was used. On the basis of cyclic voltammetry (CV) results, the high photoactivity was attributed to the increase of band gap in the Bi₂S₃/TiO₂ composite, due to the decreased recombination between the excited electrons and holes.     

Synthesis of reduced graphene oxide–TiO2 nanoparticle composite systems and its application in hydrogen production

The utilization of solar energy for the conversion of water to hydrogen and oxygen has been considered to be an efficient strategy to solve crisis of energy and environment. Here, we report the synthesis of reduced graphene oxide–TiO₂ nanoparticle composite system through the photocatalytic reduction of graphite oxide using TiO₂ nanoparticles. Photoelectrochemical characterizations and hydrogen evolution measurements of these nanocomposites reveal that the presence of graphene enhances the photocurrent density and hydrogen generation rate. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm⁻² and 127.5 μmole cm⁻²h⁻¹ in 0.5 M Na₂SO₄ electrolyte solution under 1.5AM solar irradiance of white light with illumination intensity of 100 mW cm⁻². In graphene–TiO₂ nanocomposite, photogenerated electrons in TiO₂ are scavenged by graphene sheets and percolate to counter electrode to reduce H⁺ to molecular hydrogen thus increasing the performance of water-splitting reaction.     

MoS2-supported on free-standing TiO2-nanotubes for efficient hydrogen evolution reaction

The electrochemical production of hydrogen is a promising and flexible approach towards the conversion of intermittent renewable energy sources into clean chemical fuel. However, low-cost, efficient, and durable electrocatalysts are yet to be developed to attain economies of scale in hydrogen generation. In this study, we fabricated highly ordered free-standing TiO₂ nanotube arrays (TiO₂-NTs), by simply anodizing Ti foils. The tube length, diameter, wall thickness, and surface structure of the TiO₂-NTs can be controlled by adjusting the anodization conditions. Subsequently, we synthesized and supported MoS₂ layers on free-standing TiO₂-NTs as an active material for the hydrogen evolution reaction (HER), using a slow evaporation method. The layers of MoS₂ uniformly disperse on the entire surface of the TiO₂-NTs composite. The electrochemical test shows that the MoS₂-supported free-standing (MoS₂/TiO₂-NTs) system exhibits excellent HER performance in acidic media with a low overpotential at 10 mA cm⁻² (170 mV), as well as small Tafel slope (70 mV decade⁻¹). Also, the MoS₂/TiO₂-NTs displays superior durability for HER after 5000 continuous potential cycles between −0.4 and + 0.2 (V vs. RHE). Our results demonstrate the potential application of MoS₂/TiO₂-NTs composite for cost-effective electrochemical production of hydrogen.     

Remarkable hydrogen properties of MgH2 via combination of an in-situ formed amorphous carbon

Carbon-based materials have been proposed as an ideal medium to reduce the reaction energy barriers and improve the (de)hydrogenation kinetics of magnesium-based hydrogen storage material (MgH₂) in term of their excellent dispersion. However, tedious preparation process and uneven distribution of carbon restrict the application. Therefore, in this paper, we cover MgH₂ by in-situ formed amorphous carbon via a facile approach of co-sintering Mg with fluorene followed by hydriding combustion and ball milling processes, named as MgH₂-carbonization product of fluorene (MgH₂-CPF). As a result, the MgH₂-CPF composite prepared at 823 K initially dehydrogenates at 557 K, 94 K lower than the as-milled MgH₂ (651 K). Meanwhile, the composite can release 5.67 wt% H₂ within 1000 s at 623 K. Even at a lower temperature of 423 K, the MgH₂-CPF composite still reabsorbs 5.62 wt% H₂ within 3600 s, while the as-milled Mg can hardly absorb hydrogen under a same condition. Furthermore, by addition of CPF, the apparent activation energy of the system is decreased from 161.2 kJ/mol to 87.2 kJ/mol. Our finding suggests that the carbon layer can keep the MgH₂ from aggregation, promote hydrogen transport and improve the efficiency of hydrogen absorption and desorption.     

Superior catalytic effect of titania - porous carbon composite for the storage of hydrogen in MgH2 and lithium in a Li ion battery

A novel nanocomposite (0.2TiO₂ + AC) with two promising applications is demonstrated, (i) as an additive for promoting hydrogen storage in magnesium hydride, (ii) as an active electrode material for hosting lithium in Li ion batteries (surface area of activated carbon (AC): 491 m²/g, pore volume: 0.252 cc/g, size of TiO₂ particles: 20–30 nm). Transmission electron microscopy study provides evidence that well dispersed TiO₂ nanoparticles are enclosed by amorphous carbon nets. A thermogravimetry-differential scanning calorimetry (TG-DSC) study proves that the nanocomposite is thermally stable up to ∼400 °C. Volumetric hydrogen storage tests and DSC studies further prove that a 3 wt% of 0.2TiO₂+AC nanocomposite as additive not only lowers the dehydrogenation temperature of MgH₂ over 100 °C but also maintains the performance consistency. Moreover, as a working electrode for Li ion battery, 0.2TiO₂+AC offers a reversible capacity of 400 mAh/g at the charge/discharge rate of 0.1C and consistent stability up to 43 cycles with the capacity retention of 160 mAh/g at 0.4C. Such cost effective-high performance materials with applications in two promising areas of energy storage are highly desired for progressing towards sustainable energy development.     

Electrospun fibrous active bimetallic electrocatalyst for hydrogen evolution

Fabricating earth-abundant bifunctional water splitting electrocatalysts with high efficiencies to replace noble metal-based Pt and IrO₂ catalysts is in great demand for the development of clean energy conversion technologies. Molybdenum disulfide (MoS₂) nanostructures have attracted much attention as promising material for hydrogen evolution reaction (HER). The production of hydrogen gas by help of potential efficient earth abundant metal oxides, and stable electrolysis seems a promising for hydrogen evolution reaction pathway in 1 M potassium hydroxide electrolyte media is a hot research topic in the field for clean energy conversion, renewable energies and storage. Here we propose asystem composed NiO nanostructures and MoS₂ deposited on (MoS₂@NiO). Here, by hydrothermal method NiO prepared and MoS₂@NiO by an electrospinning technique complex, can be used as catalyst to produce a large amount of hydrogen gas bubbles. The NiO nanostructures composite having highest synergistic behavior fully and covered by the MoS₂. For the MoS₂@NiO nano composite catalyst, experiment applied in 1 M KOH for the production of hydrogen evolution reaction which exhibits distinct properties from the bulk material. Overpotential values recorded low 406 mV and current density 10 mA cm⁻² measured. Co-catalysts characterized by using different techniques for deep study as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Owing to their unique structure, as-prepared nanocomposite exhibited enhanced catalytic performance for HER due to high electroactive surface area and swift electron transfer kinetics. Based on the HER polarization curves at low potential electrochemical to examine the effects of intercalants HER catalytic efficiency. Our findings establish low Tafel slope (44 mV/decade) and the catalyst stable for at least 13 h. This simple exploitation of MoS₂@NiO composite catalysts depending on the intended application of their electrochemistry.