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.     

Fabrication of nickel boride-coated carbon nanotube films by electrophoresis and electroless deposition for electrochemical hydrogen storage

Network-like carbon nanotube (CNT) films free of polymer binder were deposited directly onto a stainless steel substrate by electrophoresis. Results of experiments indicated that a CNT film with duplex surface treatment (nitric acid etching and nickel boride coating) provides satisfactory electrochemical hydrogen storage. Nitric acid treatment increased the hydrogen storage capacity of the CNT film due to the opening of CNTs, the formation of micropores in the CNT surface, and the improvement of surface wettability. Coating the CNT film with nickel boride (Ni₂B) greatly improved the electrochemical activity of the film and consequently increased the hydrogen storage capacity. The optimal amount of nickel boride coating on the CNT film was found to be about 45 wt%. Smaller amounts of nickel boride cannot provide enough electrochemical activity. Excessive nickel boride, on the other hand, leads to a decrease in the number of active sites on CNTs that are available for hydrogen storage.     

Electroless deposition of Ni nanoparticles on carbon nanotubes with the aid of supercritical CO2 fluid and a synergistic hydrogen storage property of the composite

A hybrid synthesis protocol that combines electroless plating and the supercritical CO₂ (scCO₂) technique is developed for the first time to decorate multi-walled carbon nanotubes (CNTs) with Ni nanoparticles. The scCO₂ fluid, which is immiscible with aqueous plating solution, renders a heterogeneous Ni deposition reaction and suppresses the lateral growth of Ni, which leads to the formation of nanoparticles. A uniform dispersion of tightly anchored particles, a few nanometers in diameter, on CNTs can be achieved. Since the electroless deposition process can be easily manipulated, large-scale production should be realizable. The constructed CNT/Ni nano-composite exhibits a synergistic property in hydrogen storage performance, which is evaluated using a high-pressure microbalance. The deposited nanoparticles enhance the hydrogen spillover reaction on CNTs, tripling the hydrogen storage amount at room temperature as compared to pristine CNTs.     

Enhancement of hydrogen storage by electrophoresis deposition of CNTs into nanoscale pores of silver foams

Hydrogen storage capacity of Ag-CNTs foamed electrodes was studied by chronopotentiometry method. The CNTs (carbon nanotubes) were deposited inside pores of the nanoscale silver foam by electrophoresis deposition (EPD) method and it was believed that the interconnections between the CNTs and the Ag frames were increased and were more stable; therefore, charge transfer process through the electrode became facilitated. XRD, TGA and SEM techniques were employed to examine the purity of the CNTs and the quality of the Ag foam surface.     

Effect of different reductants for palladium loading on hydrogen storage capacity of double-walled carbon nanotubes

The effects of different reductants for palladium loading on the hydrogen sorption characteristics of double-walled carbon nanotubes (DWCNTs) have been investigated. Pd nanoparticles were loaded on DWCNT surfaces for dissociation of H₂ into atomic hydrogen, which spills over to the defect sites on the DWCNTs. When we use different reductants, the reduction capabilities and other effects of the different reductants are different, which affects the hydrogen storage capacity of the DWCNTs. In this work, the amount of hydrogen storage capacity was determined (by AMC Gas Reactor Controller) to be 1.7, 2.0, 2.55, and 3.0 wt% for pristine DWCNTS and for 2.0%Pd/DWCNTs using H₂, l-ascorbic acid, and NaBH₄ as reductants, respectively. We found that the hydrogen storage capacity can be enhanced by loading with 2% Pd nanoparticles and selecting a suitable reductant. Furthermore, the sorption can be attributed to the chemical reaction between atomic hydrogen and the dangling bonds of the DWCNTs.     

The synthesis of Ni-activated carbon nanocomposites via electroless deposition without a surface pretreatment as potential hydrogen storage materials

This work presents the deposition of Ni nanoparticles on a potassium hydroxide (KOH) activated carbon (AC) support by an electroless deposition (ED) technique without using sensitization and activation surface pretreatments. The hydrogen storage properties of Ni-activated carbon nanocomposites (Ni/AC) were investigated at room temperature and under moderate pressure. The chemical composition, morphology and textural parameters are characterized using an inductively coupled plasma spectrometer (ICP), scanning and transmission electron microscopy (SEM and TEM) and N₂ adsorption isotherms. Fine and well-dispersed Ni nanoparticles were obtained by ED that had spherical shape with an average size of 5 nm. The hydrogen storage capacity of the AC can be improved through Ni loading; which results in a hydrogen storage enhancement factor of two compared with the Ni-free AC. This enhancement factor is due to the greater interactions between the Ni and the AC, which facilitate the hydrogen spillover mechanism.     

Aerogel sheet of carbon nanotubes decorated with palladium nanoparticles for hydrogen gas sensing

In the development of hydrogen sensors, it is required to meet the demands of both high sensor performance as well as the ease of fabrication for mass production. For this purpose we proposed a chemiresistive hydrogen sensors based on an aerogel sheet of carbon nanotubes decorated with palladium nanoparticles (CNT/Pd sheet). The fabrication process is straightforward that a dry-spun CNT aerogel sheet is suspended between concentric electrodes followed by depositing Pd nanoparticles on CNT sheets by thermal evaporation. The present CNT/Pd sheet sensors can detect hydrogen at concentrations as low as 2 ppm at room temperature with a detection range from 2 to 1000 ppm. The aerogel nature of CNT/Pd sheet contributes to low detection limit and broad detection range of the CNT/Pd sensor. Relations between hydrogen concentration and sensor response and response time, and the effects of temperature on sensor performance were investigated.     

Effect of thermal fluorination on the hydrogen storage capacity of multi-walled carbon nanotubes

Thermal fluorination of multi-walled carbon nanotubes (MWCNTs) was performed to improve their hydrogen storage capacity, based on three considerations. First, the surface of the MWCNTs was altered by thermal fluorination to create a pathway for the storage of hydrogen molecules inside the MWCNTs. These surface treatments increased the number of MWCNT defects through attack by fluorine radicals. The defects were identified using Raman peaks and TEM images. Second, thermal fluorination changed the pore structure by enlarging the specific surface area and the pore volume, which increased the number of hydrogen adsorption sites. Last, the induced fluorine groups enhanced the hydrogen storage capacity through attraction effects on the electron in the hydrogen molecules due to the high electronegativity of fluorine. In conclusion, thermal fluorination increased the hydrogen storage capacity of MWCNTs four-fold to 1.69 wt%.     

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.%).     

Chemical processing of double-walled carbon nanotubes for enhanced hydrogen storage

Double-walled carbon nanotubes (DWCNTs) were modified for enhanced hydrogen storage by employing a combination of two techniques: KOH activation for the formation of defects on DWCNT surfaces and loading of the DWCNTs with nanocrystalline Pd. The physical properties of the pristine DWCNTs and chemically modified DWCNTs were systematically characterised by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and Brunauer–Emmett–Teller (BET) surface area measurements. The amounts of hydrogen storage capacity were measured at ambient temperature and found to be 1.7, 2.0, 3.7, and 2.8 wt% for pristine DWCNTS, 2 wt% Pd DWCNTs, activated DWCNTs, and 2 wt% Pd activated DWCNTs, respectively. Hydrogen molecules could be adsorbed on defect sites created by chemical activation in DWCNTs through van der Waals forces. For Pd nanoparticle loaded DWCNTs, H₂ molecules could be dissociated into atomic hydrogen and adsorbed on defect sites. We found that the hydrogen storage capacity of DWCNTs can be significantly enhanced by chemical activation or loading with Pd nanoparticles.     

Investigation on platinum loaded multi-walled carbon nanotubes for hydrogen storage applications

Molecular hydrogen uptake of modified carbon nanotubes is a prospect for efficient hydrogen storage in fuel cell vehicles. In this study, a simple and efficient method to decorate the surface of multi-walled carbon nanotubes (MWNT) with platinum nanoparticles is presented. To load the Pt nanoparticles, hexachloroplatinic acid (H₂PtCl₆·6H₂O) is used as a precursor. Surface morphology of these Pt loaded MWNT is observed using Scanning and Transmission Electron Microscopy. Both samples are also characterized by X-Ray Diffraction. Thermal Gravimetric Analysis results indicate that both as purchased MWNT and Pt loaded MWNT have decomposition temperature higher than 500 °C in air. N₂ adsorption experiments yields a BET area of the sample close to 500 m²/g. This MWNT/Pt sample was reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 h before hydrogen adsorption measurements. Hydrogen uptake of MWNT/Pt was measured at 2.5 MPa and 77 K. This hydrogen uptake isotherm is also compared with measurements at ambient temperature.     

Experimental investigation of hydrogen storage in single walled carbon nanotubes functionalized with borane

Single walled carbon nanotubes (SWCNTs) dispersed in 2-propanol are deposited on the alumina substrate using drop cast method. The deposited SWCNTs are characterized using the techniques SEM, EDS and FTIR. Then the SWCNTs are functionalized with BH₃ using LiBH₄ as the precursor. FTIR, XPS and CHNS techniques are used to confirm the functionalization. The functional groups are identified from FTIR studies. The various elements present in the functionalized SWCNTs are identified from XPS and CHNS studies. The functionalized samples are hydrogenated and the hydrogen storage capacity of these samples is estimated using CHNS studies.     

Study on hydrogen storage properties of Mg nanoparticles confined in carbon aerogels

Magnesium nanoparticles confined in carbon aerogels were successfully synthesized through hydrogenation of infiltrated dibutyl-magnesium followed by hydrogen desorption at 623 K. The average crystallite size of Mg nanoparticle is calculated to be 19.3 nm based on X-ray diffraction analyses. TEM observations showed that the size of MgH₂ particle is mainly distributed in the range from 5.0 to 20.0 nm, with a majority portion smaller than 10.0 nm. The hydrogenation and dehydrogenation enthalpies of the confined Mg are determined to be −65.1 ± 1.56 kJ/mol H₂ and 68.8 ± 1.03 kJ/mol H₂ by Pressure–composition–temperature tests, respectively, slightly lower than the corresponding enthalpies for pure Mg. In addition, the apparent activation energy for hydrogen absorption is determined to be 29.4 kJ/mol H₂, much lower than that of the micro-size Mg particles. These results indicate that the thermodynamic and absorption kinetic properties of confined Mg nanoparticles can be significantly improved due to the ‘nanosize effect’.     

Chemisorption,physisorption and hysteresis during hydrogen storage in carbon nanotubes

The question of chemisorption versus physisorption during hydrogen storage in carbon nanotubes (CNTs) is addressed experimentally. We utilize a powerful measurement technique based on a magnetic suspension balance coupled with a residual gas analyzer, and report new data for hydrogen sorption at pressures of up to 100 bar at 25 °C. The measured sorption capacity is less than 0.2 wt.%, and there is hysteresis in the sorption isotherms when multi-walled CNTs are exposed to hydrogen after pretreatment at elevated temperatures. The cause of the hysteresis is then studied, and is shown to be due to a combination of weak sorption – physisorption – and strong sorption – chemisorption – in the CNTs. Analysis of the experimental data enables us to calculate separately the individual hydrogen physisorption and chemisorption isotherms in CNTs that, to our knowledge, are reported for the first time here. The maximum measured hydrogen physisorption and chemisorption are 0.13 wt.% and 0.058 wt.%, respectively.     

Effect of electroless nickel coating on the electrochemical hydrogen storage characteristics of Al and Zr including Mg-based alloys

Mg₂Ni, Mg_1.5Al_0.5Ni, Mg_1.5Zr_0.5Ni and Mg_1.5Al_0.2Zr_0.3Ni alloys were synthesized by the mechanical alloying and the effect of electroless Ni coatings on the electrochemical hydrogen storage characteristics of these alloys were investigated. X-ray diffraction studies showed that the Ni particles formed on the alloy surface during the electroless coating resulted in the formation of two new broad Ni peaks. The application of the Ni coating improved the capacity retention rates of all the alloys. This improvement was attributed to the hydrogen diffusion pathways provided by the external Ni particles. The capacity retention of the coated Mg₂Ni alloy was relatively lower than those of the coated Al and Zr including alloys. This observation was assumed to arise from the contribution of the underlying electrocatalytically active Ni sites formed as a result of the dissolution of the disseminated Al and Zr oxides throughout the Mg(OH)₂ layer to the alloy capacity retention rate. The electrochemical impedance spectroscopy experiments indicated that the Ni crystals nucleated during the electroless coating make the alloy surface more catalytic for the electrochemical charge transfer reactions.     

Hydrogen storage in Li-doped charged single-walled carbon nanotubes

Density functional calculations have been carried out to investigate the interaction of hydrogen molecules with Li-doped charged single-walled carbon nanotubes (SWNTs). The results show that binding of H₂ on positively charged Li-doped SWNTs is enhanced compared with that on uncharged systems. As the charge of the Li-doped SWNTs increases, the binding energy of H₂ increases with the largest binding energy of 0.26 eV. The reason for the increase in H₂ binding energy is that the positively charged tube improves the charge transfer from Li atom to the tube and make the Li atom more ionized, which strengthens the polarization interaction between H₂ and Li atom.     

Hydrogen storage in coiled carbon nanotubes

We report a density functional calculation of the adsorption of molecular hydrogen on the external surface of coiled carbon nanotube (CCNT). Binding energies of single molecule have been studied as a function of three different orientations and at three different sites like hexagon, pentagon and heptagon. The binding energy values are larger than linear (5,5) armchair nanotube, which has approximately same diameter as that of coiled carbon nanotube. The curvature and topology of CCNT are responsible for this considerable enhancement. The system with full coverage is also studied. When the nanotube surface is fully covered with one molecule per graphitic hexagon, pentagon and heptagon gives the 6.8 wt% storage capacity. The binding energy per molecule decreases due to repulsive interactions between neighbor molecules. It gives good storage medium for hydrogen. Almost it meets the DOE target.     

Synthesis and hydrogen storage capacity of exfoliated turbostratic carbon nanofibers

Turbostratic carbon nanofibers (CNFs) with a rough surface, open pore walls, and a defect structure were continuously produced by the thermal decomposition of alcohol in the presence of an iron catalyst and a sulfur promoter at 1100 °C under a nitrogen atmosphere in a vertical chemical vapor deposition reactor. A graphite exfoliation technique using intercalation and thermal shock was employed to expand the graphene layers of the as-produced turbostratic CNFs. The hydrogen storage capacity of the turbostratic CNF samples was measured using the volumetric method with a pressure of up to 1 MPa at 77 K. The hydrogen storage capacities of the as-produced and exfoliated turbostratic CNFs were 1.5 and 5 wt%, respectively. The defects on the surface and expandable graphitic structure are considered important keys to increasing the hydrogen uptake in turbostratic CNFs.     

Fabrication and electrochemical activity of Ni-attached carbon nanotube electrodes for hydrogen storage in alkali electrolyte

The electrochemical activity of an electrode of carbon nanotubes (CNTs) attached with Ni nanoparticles was investigated. A surface modification technique enabled different Ni particle densities to coat onto the CNT surface, which was chemically oxidized by nitric acid. It was found that each nickel nanoparticle has an average size of 30–50  nm, and the Ni-attached CNTs still possessed a similar pore size distribution. Cyclic voltammetry measurements in 6  M KOH showed that the electrochemical adsorption and desorption amount of hydrogen is a linearly increasing function of the Ni loading. This enhancement of electrochemical activity was ascribed to a fact that Ni particle acts as a redox site for hydrogen storage, thus leading to a greater specific peak current. According to our calculation, the electrochemical capacitance of nickel nanocatalyst in KOH electrolyte was estimated to be the value of 217  F/g. Charge/discharge cycling demonstrated that the Ni-attached CNT electrode maintains fairly good cycleability during 50 cycles.