Electrochemical hydrogen storage of expanded graphite decorated with TiO2 nanoparticles

The electrochemical hydrogen storage of expanded graphite (EG) decorated with TiO₂ nanoparticles (NPs) calcined at different temperatures has been investigated with the galvanostatic charge and discharge method. The TiO₂ NPs are deposited on and between the graphene-like nanosheets of EG by a sol-gel method. The morphology, structure, composition, and specific surface area of the samples were characterized. The electrochemical measurement reveals that the EG decorated with TiO₂ NPs calcined at 500 °C has a discharge capacity of 373.5 mAh/g which is 20 times higher than that of pure EG and quite appealing for the battery applications. The mechanism of enhancement of the electrochemical activity for the TiO₂-decorated EG could be attributed to the preferable redox ability and photocatalytic property of TiO₂ NPs.     

Synergistic effects of multiwalled carbon nanotubes and Al on the electrochemical hydrogen storage properties of Mg2Ni-type alloy prepared by mechanical alloying

Mg_2−xAl_xNi (x = 0, 0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by XRD, SEM and TEM. XRD analysis results indicate that Al substitution results in the formation of AlNi-type solid solution that can interstitially dissolve hydrogen atoms. In contrast, the addition of MWCNTs hardly affects the XRD patterns. SEM observations show that after co-milling with 5 wt. % MWCNTs, the particle sizes of both Mg₂Ni and Mg_1.75Al_0.25Ni milled alloys are decreased explicitly. The TEM images reveal that ball milling is a good method to cut long MWCNTs into short ones. These MWCNTs aggregate along the boundaries and surfaces of milled alloy particles and play a role of lubricant to weaken the adhesion of alloy particles. The majority of MWCNTs retain their tubular structure after ball milling except a few MWCNTs whose tubular structure is destroyed. Electrochemical measurements indicate that all milled alloys have excellent activation properties. The Mg_1.75Al_0.25Ni-MWCNTs composite shows the highest discharge capacity due to the synergistic effects of MWCNTs and Al on the electrochemical hydrogen storage properties of Mg₂Ni-type alloy. However, the improvement on the electrode cycle stability by adding MWCNTs is unsatisfactory.     

Facile synthesis and electrochemical hydrogen storage of bentonite/TiO2/Au nanocomposite

In this study, the electrochemical hydrogen storage of bentonite composites containing TiO₂ and Au nanoparticles (NPs) has been investigated by cyclic voltammetry (CV) analysis. TiO₂ NPs were first deposited on the bentonite substrate by reflux technique. Au NPs were then prepared by laser ablation in liquid (LAL) method under different laser irradiation times (6, 12, and 18 min), and utilized in the decoration of bentonite/TiO₂ nanocomposite by physical mixing. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and elemental mapping were carried out in the characterization of the prepared bentonite/TiO₂/Au nanocomposite. The surface and chemical properties of the acquired nanocomposite were analyzed by Brunauer-Emmett-Teller and Fourier transform infrared spectroscopy, respectively. Electrochemical measurement was performed on stainless steel mesh prefabricated electrodes in 1 M KOH electrolyte solution. The B-T/Au nanocomposite prepared under 12 min laser irradiation displayed the highest hydrogen storage capacity (15 Cg^-1).     

Photocatalytic hydrogen production over CuO and TiO2 nanoparticles mixture

Cheap and efficient photocatalysts were fabricated by simply mixing TiO₂ nanoparticles (NPs) and CuO NPs. The two NPs combined with each other to form TiO₂/CuO mixture in an aqueous solution due to the opposite surface charge. The TiO₂/CuO mixture exhibited photocatalytic hydrogen production rate of up to 8.23 mmol h⁻¹ g⁻¹ under Xe lamp irradiation when the weight ratio of P25 to CuO was optimized to 10. Although the conduction band edge position of CuO NPs is more positive than normal hydrogen electrode, the TiO₂/CuO mixture exhibited good photocatalytic hydrogen production performance because of the inter-particle charge transfer between the two NPs. The detailed mechanism of the photocatalytic hydrogen production is discussed. This mixing method does not require a complicated chemical process and allows mass production of the photocatalysts.     

Hydrogenation behavior of high-energy ball milled amorphous Mg2Ni catalyzed by multi-walled carbon nanotubes

Amorphous Mg₂Ni alloy was successfully synthesized by means of mechanical alloying. Then, the multi-walled carbon nanotubes (MWCNTs) were added by high-energy ball milling to catalyze the amorphous alloy. The X-ray diffraction (XRD) spectroscopy reveal that the as-cast Mg₂Ni alloy has presented a completely amorphous state under specific conditions of high-energy ball milling process. Different process parameters of ball-to-powder ratio (10:1, 20:1, 40:1) and milling time have been attempted for the preparation of amorphous Mg₂Ni alloy. The results show that the milling time and ball-to-powder weight ratio have significantly influence on the amorphization process of crystalline Mg₂Ni alloy. Before and after the milling, phase compositions and microstructures of the prepared materials were characterized by XRD, scanning electron microscope (SEM), electron energy dispersion spectrum (EDS) and transition electron microscope (TEM) approaches. The morphology of composite Mg₂Ni/MWCNTs was investigated, the TEM images show that the MWCNTs imbed on the surface of the particles after milling for 1 h, and the MWCNTs with and without tubular structure have been observed. The hydrogen storage properties of amorphous Mg₂Ni alloys were improved by the catalytic effect of MWCNTs. The catalytic effect and mechanism of MWCNTs on the hydrogen storage properties of amorphous Mg₂Ni alloy are discussed and investigated.     

Photo-induced reforming of alcohols with improved hydrogen apparent quantum yield on TiO2 nanotubes loaded with ultra-small Pt nanoparticles

Photo-induced reforming of methanol, ethanol, glycerol and phenol at room temperature for hydrogen production was investigated with the use of ultra-small Pt nanoparticles (NPs) loaded on TiO₂ nanotubes (NTs). The Pt NPs with diameters between 1.1 and 1.3 nm were deposited on TiO₂ NTs by DC-magnetron sputtering (DC-MS) technique. The photocatalytic hydrogen rate achieved an optimum value for a loading of about 1 wt% of Pt. Apparent quantum yield for hydrogen generation was measured for methanol and ethanol water solutions reaching a maximum of 16% under irradiation with a wavelength of 313 nm in methanol/water solution (1/8 v/v). Pt NPs loaded on TiO₂ NTs represented also a true water splitting catalyst under UV irradiation and pure distilled water. DC-MS method appears to be a technologically simple, ecologically benign and potentially low-cost process for production of an efficient photocatalyst loaded with ultra-small NPs with precise size control.     

Functionalization of TiO2 nanotubes with palladium nanoparticles for hydrogen sorption and storage

The use of hydrogen as an energy carrier is an attractive solution toward addressing global energy issues and reducing the effects of climate change. Design of new materials with high hydrogen sorption capacity and high stability is critical for hydrogen purification and storage. In this study, titanium dioxide nanotubes (TiO₂NTs) were modified with palladium nanoparticles (PdNPs) utilizing a facile photo-assisted chemical deposition approach. Electrochemical anodization was employed for the direct growth of TiO₂NTs. The PdNP functionalized TiO₂NTs (TiO₂NT/Pd) were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The hydrogen sorption behaviours and stability of the TiO₂NT/Pd nanocomposites were investigated and compared with nanoporous Pd networks that were deposited on a bulk titanium substrate (Ti/Pd) using cyclic voltammetry (CV) and chronoamperometry (CA). Our studies show that the TiO₂NT/Pd nanocomposites possess a much higher hydrogen storage capacity, faster kinetics for hydrogen sorption and desorption, and higher stability than the nanoporous Pd.     

The effect of various acids treatment on the purification and electrochemical hydrogen storage of multi-walled carbon nanotubes

The effects of HCl, HNO₃, H₂SO₄ and HF acids on the purification and the electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) were studied. The MWCNTs were synthesized on Fe–Ni catalyst by thermal chemical vapor deposition method. The X-ray diffraction and thermal gravimetric analysis results indicated that the MWCNTs purified by HF acid had the highest impurities as compared with the other acids. The N₂ adsorption results at 77 K indicated that all the samples were mainly mesoporous and the purified MWCNTs by HF acid had the highest surface area as compared with the other acids. The hydrogen storage capacities of the purified MWCNTs by the following acids were in ascending order as: H₂SO₄, HCl, HNO₃ and HF. It was found that the 1–2 nm micropores in the MWCNTs are very important for hydrogen storage. Further, the presences of catalyst and defective sites in MWCNTs influence the hydrogen storage capacity.     

The effects of bromine treatment on the hydrogen storage properties of multi-walled carbon nanotubes

The effects of bromine treatment on the properties of multi-walled carbon nanotubes (MWCNTs) such as surface porosity, sp² hybridization, functional groups and hydrogen storage capacity were studied and compared with treated MWCNTs by HCl, HNO₃ and H₂SO₄ acids. The treatments affect the graphitization properties (sp² hybridization) and porous structures of MWCNTs by enlarging the specific surface area and the micro-pore volume. In addition, the hydrogen storage capacity of the treated MWCNTs was also investigated by volumetric technique. It is found that the destroying of sp² hybridization of bromine treated MWCNTs increases hydrogen adsorption sites and decreases hydrogen desorption temperature.     

Formic acid oxidation reaction on a PdxNiy bimetallic nanoparticle catalyst prepared by a thermal decomposition process using ionic liquids as the solvent

The nano-catalysts of Pd_xNi_y bimetallic nanoparticles (NPs, the nominal atomic ratios of Pd to Ni are 2:1, 3:2 and 1:1) supported on multi-walled carbon nanotubes (MWCNTs) (denoted as Pd_xNi_y/MWCNTs) have been synthesized by a thermal decomposition process using room temperature ionic liquids (RTILs) of N-butylpyridinium tetrafluoroborate (BPyBF₄) as the solvent. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were employed to characterize the morphology of the samples, revealing that the prepared Pd_xNi_y NPs were quite uniformly dispersed on the surface of MWCNTs with an average particle size of ∼8.0 nm. Formic acid oxidation reaction (FAOR) was investigated on the as-prepared catalysts by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), demonstrating that the peak current on the Pd₃Ni₂/MWCNTs catalyst was about three times higher than that on the Pd/MWCNTs. The lower electrode potential and easier hydrogen evolution, based on the results obtained from chronopotentiometry and CV, respectively, were thought as the main reasons for the excellent electrocatalysis of the Pd₃Ni₂/MWCNTs toward formic acid oxidation reaction (FAOR) when compared to other samples.     

Single-step preparation and hydrogenation of single walled carbon nanotubes-titanium dioxide composite

The composite material made up of carbon nanotubes (CNTs) and metal oxide nanostructures have been investigated for hydrogen storage application. The present experimental work deals with the investigation of hydrogen storage in single walled carbon nanotubes-titanium dioxide (SWCNTs-TiO₂) composite. The SWCNTs-TiO₂ composite has been made by depositing the pellet containing the mixture of SWCNTs and TiO₂ using electron beam (EB) evaporation technique in hydrogen ambient. The preparation and hydrogenation of the SWCNTs-TiO₂ composite have been done in a single-step. The characterization results expose that the deposition of SWCNTs with TiO₂ material is possible using EB evaporation technique without any significant structural decomposition of SWCNTs. The amount of hydrogen incorporated is found to be 3.2 wt.%, and it is attributed to both the synergetic action of SWCNTs-TiO₂ nanostructures and the method of preparation. The stored hydrogen is found to be released completely in the temperature range of 120–215 °C.     

Green synthesis of well dispersed TiO2/Pt nanoparticles photocatalysts and enhanced photocatalytic activity towards hydrogen production

Deposition of Pt NPs with preferred dispersion and morphologies on TiO₂ have been the focus of studies in photocatalytic and photoelectrochemical hydrogen production. Green synthesis of TiO₂/Pt NPs nanocomposites with narrow size distribution of Pt NPs still remain a challenge. Herein, we report that sucrose is highly efficient for the preparation of well-dispersed TiO₂/Pt NPs photocatalysts. Moreover, the sucrose could act as an electron donor, showing higher hydrogen production activity under simulated sunlight than pure water. The as-synthesized photocatalysts have been characterized by techniques of transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), and diffuse reflectance spectroscopy (DRS). Compared with TiO₂/Pt NPs photocatalysts prepared through conventional photodeposition, the photocatalysts as prepared showed higher photocatalytic efficiency. Moreover, the catalyst could be reused easily without apparent degradation of their original photocatalytic activities. This approach presents a promising and low-cost strategy to improve the photocatalytic performance of TiO₂ from biomass.     

Enhanced electrochemical hydrogen storage performance of titanate-based nanostructures synthesized by facile auto-combustion: Li2TiO3 nanostructures versus LaTiO3 nanoperovskites

Green energies are vital for near-future energy needs. Hydrogen is a promising secondary energy career that counted as a clean-burning fuel. However, hydrogen suffers from a low volumetric energy density at ambient temperature and pressure. This deficiency has been overcome by “solid-state hydrogen storage” technologies, where the hydrogen is adsorbed/absorbed - depending on the type of materials - on a solid surface. Mixed metal oxides (MMOs) particularly transition-based metal oxides have been recently developed for hydrogen adsorption with a superior affinity for hydrogen. Here, we demonstrated two nanosized-MMOs based on (mono-) perovskite structure, Li₂TiO₃, and LaTiO₃. These two MMOs are successfully synthesized via the auto-combustion method in the presence of starch fuel. After confirmation of their structures and morphologies, the samples are used for electrochemical hydrogen storage in an alkaline medium. The average particle diameters of Li₂TiO₃ and LaTiO₃ are calculated to be around 16.74 and 24.46 nm, respectively. The results indicate a higher discharge capacity of LaTiO₃ nanoperovskites (1140 mAh/g) as compared to Li₂TiO₃ nanoparticles (680 mAh/g); as confirmed primarily by cyclic voltammetry (CV), with the theoretical hydrogen capacities of 4.1% and 2.4%, respectively. We believe that novel MMOs can be potentially fulfilled the requirements of future energy targets, arranged and reported by US-department of energy (DOE).     

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.     

Improving the direct methanol fuel cell performance with poly(vinyl alcohol)/titanium dioxide nanocomposites as a novel electrolyte additive

The present investigation deals with the fabrication of new poly(vinyl alcohol)/titanium dioxide (PVA/TiO₂) nanocomposites (NCs) with different titanium dioxide (TiO₂) loading by using ultrasound irradiation. For the improvement of nanoparticles (NPs) dispersion and increasing possible interactions between NPs and PVA, the surface of TiO₂ NPs was modified by γ-aminopropyltriethoxy silane. The as-prepared NCs were characterized by spectroscopic, thermogravimetric analysis and electron microscopy methods. The results demonstrate that NPs dispersed homogeneously within the PVA matrix on nanoscale, which could be assigned to the hydrogen and covalent bonds formed between PVA and NPs. The results indicated that heat stability of NCs was improved in the presence of modified TiO₂ NPs. The mechanisms of surface modification and a possible mechanism of ultrasonic induced interaction between polymer and NPs have been analyzed.     

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).     

The synthesized nickel-doped multi-walled carbon nanotubes for hydrogen storage under moderate pressures

Hydrogen adsorption capacity of Multiwalled carbon nanotubes (MWCNTs) decorated with Nickel (Ni) nanoparticles has been presented at room temperature and under moderate pressures of 4–20 bar. The functionalization of carbon nanotubes was carried by H₂SO₄-HNO₃ reducing agents and the Ni supported MWCNTs (Ni-MWCNTs) were prepared by wet chemical method. The structure and morphology characterization of samples were performed by XRD, TEM, EDX and SEM analyses. These nanotubes then subjected to hydrogenation step by using Sievert's-like apparatus. The hydrogenation of the Ni-MWCNTs was performed at 298 K and moderate hydrogen pressures of 4–20 bar. The obtained results show that there is a correlation between hydrogen storage capacity and hydrogen pressure that; as the pressure was increased, hydrogen uptake capacity enhanced due to physisorption. In addition, maximum hydrogen storage capacity of Ni-MWCNTs was found to be 0.298 wt % at room temperature and under pressure of 20 bar.     

Effect of electrolyte on electrochemical characteristics of MmNi3.55Co0.72Al0.3Mn0.43 alloy electrode for hydrogen storage

A series of experiments have been performed to investigate the effects of three electrolytes of different compositions (EO, EA and EM) on the electrochemical characteristics of MmNi_3.55Co_0.72Al_0.3Mn_0.43 hydrogen storage alloy electrode. Electrolytes EA and EM were obtained by adding appropriate amounts of Al₂(SO₄)₃ and MnSO₄ to the original electrolyte EO (6 M KOH + 1 wt% LiOH), respectively. Electrode activation, maximum capacity, cycle life, self-discharge and high-rate discharge characteristics have been studied. It was found that a maximum capacity of about 260 mA h/g has been obtained for the alloy electrodes in all these electrolytes after 5–7 cycles of charging/discharging. The alloy electrodes have a good durability in electrolytes EA and EM, especially after 175 cycles. Using the capacity retention as an indication of self-discharge resistance, almost identical degree of capacity retention (82% after 4 days and 45% after 16 days) has been observed at 298 K, regardless of the electrolytes used. When tested at higher temperature, however, a higher capacity retention (46% after 3 days) at 333 K has been observed for electrodes in electrolyte EA, and about 32% for electrodes in both electrolytes EO and EM. As to high-rate discharge behavior of the results of high-rate discharge tests indicated that about 50% of discharge efficiencies were obtained in the three electrolytes at 333 K by continuous-model high-rate discharge method, at a discharge rate of 7C, and 22% in 298 K. The alloy electrode in electrolyte EM has the best durability, in which about 50% of discharge efficiency at DC = 9C was obtained by step-model high-rate discharge method at 333 K, which was even higher than that at 298 K.     

Enhanced hydrogen storage capacity of Pt-loaded CNT@MOF-5 hybrid composites

We report on an easy synthesis method for the preparation of a hybrid composite of Pt-loaded MWCNTs@MOF-5 [Zn₄O(benzene-1,4-dicarboxylate)₃] that greatly enhanced hydrogen storage capacity at room temperature. To prepare the composite, we first prepared Pt-loaded MWCNTs, which were then incorporated in-situ into the MOF-5 crystals. The obtained composite was characterized by various techniques such as powder X-ray diffractometry, optical microscopy, porosimetry by nitrogen adsorption, and hydrogen adsorption. The analyses confirmed that the product has a highly crystalline structure with a Langmuir specific surface area of over 2000 m²/g. The hybrid composite was shown to have a hydrogen storage capacity of 1.25 wt% at room temperature and 100 bar, and 1.89 wt% at cryogenic temperature and 1 bar. These H₂ storage capacities represent significant increases over those of virgin MOF-5s and Pt-loaded 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.