Hydrogen uptake of cobalt and copper oxide-multiwalled carbon nanotube composites

Hydrogen storage capacity of a pristine multi-walled carbon nanotubes is increased 10-fold at 298 K and an equilibrium hydrogen pressure of ～23 atm, upon addition of a hydrogen spillover catalyst cobalt- and copper oxide, from 0.09 to 0.9 wt.%. In situ reduction method is utilized to synthesize Co-oxide/MWCNTs and Cu-oxide/MWCNTs composite. Blocking of channels and pores of MWCNTs by oxide nanoparticles during preparation method is responsible for low BET specific surface area of composites compared to pristine sample. X-ray diffraction, scanning, and transmission electron microscopy demonstrates nanostructural characterization of MWCNTs and composites. Thermogravimetric analysis of two oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage of both composites is attributed to the spillover mechanism due to decoration of Co and Cu-oxide nanoparticles on the outer surface of MWCNTs.     

Hydrogen uptake of Ti-decorated multiwalled carbon nanotube composites

In this study, the hydrogen storage capacity of purified multiwalled carbon nanotubes (MWCNTs) was enhanced from 13- to 15-fold at a temperature of 298 K and pressure of 2.0 MPa, upon incorporation of 1.57–1.88 wt% of ultrafine Ti nanoparticles. The effect of a hydrogen spillover Ti catalyst on MWCNTs prepared using the sputtering method was investigated. A comparison between the hydrogen uptake by MWCNTs sputtered with Ti for 3000 s and that for 6000 s was also performed using the Sievert's volumetric apparatus. The significant enhancement in hydrogen uptake was attributed to the interfacial diffusion of hydrogen from Ti to the MWCNTs. The re-adsorption of hydrogen on the pristine MWCNTs and Ti-decorated MWCNTs dehydrogenated at 200 °C indicated that the samples did not compromise their reversible hydrogen uptake during the hydrogenation–dehydrogenation cycles. It was also found that longer exposure of Ti to the MWCNTs during sputtering improved the hydrogen storage capacity. This improvement could be attributed to the presence of a higher amount of Ti deposited on the MWCNTs, as indicated by thermogravimetric analysis study.     

Preparation,characterization and hydrogen storage studies of carbon nanotubes and their composites: A review

Current challenge for researchers worldwide is to construct a reliable, efficient, and affordable medium that can store hydrogen reversibly at ambient temperature and pressure for on-board applications. Carbon nanotubes (CNTs) and their composites are considered as leading source of solid-state reversible hydrogen storage medium owing to its unique characteristics including high surface area, nanoporous structure, tuneable properties, low mass density, cage like structure, chemical stability, dissociation of hydrogen molecule, and easy synthesis method. Nanocrystalline metal or metal oxide or hydride is doped/embedded into pristine CNTs via in-situ reduction, wetness impregnation, high-energy ball milling and sputtering method. Characterization techniques of pristine and composites are utilized to study morphological, thermal, qualitative, quantative, and elemental analysis. Nanocomposite hydrogen uptake capacity is frequently measured by volumetric and gravimetric methods. Multifold enhancement of hydrogen storage of composites compared to pristine CNTs is attributed to activation, acidification, purification, ball milling and spillover of physisorbed hydrogen by metal catalyst onto CNTs via spillover mechanism. Hydrogen uptake of CNTs and composites follow monotonous dependence on hydrogen pressure. Composites not only present high hydrogen uptake as compared to pristine CNTs but also shows significant cyclic stability upon successive adsorption–desorption cycles.     

Improved hydrogen adsorption of ZnO doped multi-walled carbon nanotubes

Hydrogen storage is still one of the most important problems to improve hydrogen energy usage widespread. New materials capable of storing hydrogen with high efficiency must be introduced to overcome this problem. In recent years, addition of metals or inorganic compounds to multiwalled carbon nanotubes (MWCNTs) has been generally used for hydrogen uptake studies to enhance adsorption property of the nanotubes. In this study, Zinc oxide (ZnO) nanoparticles doped MWCNTs (ZnO-MWCNTs) have been produced as new reversible hydrogen storage materials, and we have investigated characterization of ZnO-MWCNTs by XRD, SEM, TGA, TEM and BET analyses. The functionalized MWCNTs and ZnO doped MWCNTs were subjected to hydrogenation step by dynamic gas sorption analyser under pressure of 5–50 bar. The hydrogen uptake capacities of the materials under different pressures were measured gravimetrically. It was indicated that by controlling the pressures for hydrogenation of ZnO-MWCNTs induces the spillover of ZnO nanoparticles in the layer of MWCNTs which in return with high hydrogen adsorption capacity. Consequently, the hydrogen adsorption of the functionalized MWCNTs (f-MWCNTs) and the ZnO-MWCNTs were achieved to be 1.05 wt% and 2.7091 wt% under pressure of 50 bar as maximum.     

Optimizing the conditions of multi-walled carbon nanotubes surface activation and loading metal nanoparticles for enhanced hydrogen storage

In this study, the effect of surface activation of multi-walled carbon nanotubes (MWCNTs) by KOH along with loading of cobalt and lithium nanoparticles on the surface of MWCNTs are investigated. In the first step, surface activation parameters, i.e. MWCNT/KOH weight ratio, activation temperature, and activation time are optimized to give the highest hydrogen uptake. According to obtained results, the optimum synthesis conditions are MWCNT/KOH weight ratio of 1:5, 800 °C, and 1 h of activation duration. Afterward, cobalt and lithium metal nanoparticles are doped discretely on the surface of activated nanotubes. It is demonstrated that amounts of loaded cobalt and lithium metals are 5.5 and 1.9% wt, respectively. In addition, it is revealed that the amount of hydrogen storage capacity for cobalt-loaded and lithium-loaded MWCNTs are 1.06% wt. and 1.33% wt., respectively (at 278 K) which are higher than the capacity of pristine and activated MWCNT samples.     

Improved hydrogen storage performance of defected carbon nanotubes with Pd spillover catalysts dispersed using supercritical CO2 fluid

Hydrogen storage properties of carbon nanotubes (CNTs) modified by oxidative etching and decoration of Pd spillover catalysts are investigated. A mixed H₂SO₄/H₂O₂ solution containing ferrous ions (Fe²⁺) is useful to open the caps, to shorten the length, and to generate defects on CNTs. The Pd catalysts are deposited on the CNTs with the aid of supercritical carbon dioxide (scCO₂); as a result, a highly dispersed Pd nanoparticles and an intimate connection between Pd and carbon surface can be obtained. Combination of the two approaches can optimize a hydrogen spillover reaction on CNTs, resulting in a superior hydrogen storage capacity of 1.54 wt% (at 25 °C and 6.89 MPa), which corresponds to an enhancement factor of ∼4.5 as compared to that of pristine CNTs.     

Spillover enhanced hydrogen uptake of Pt/Pd doped corncob-derived activated carbon with ultra-high surface area at high pressure

Corncob-derived activated carbon (CAC) was prepared by potassium hydroxide activation. The Pt/Pd-doped CAC samples were prepared by two-step reduction method (ethylene glycol reduction plus hydrogen reduction). The as-obtained samples were characterized by N₂-sorption, TEM and XRD. The results show the texture of CAC is varied after doping Pt/Pd. The Pd particles are easier to grow up than Pt particles on the surface of activated carbon. For containing Pt samples, the pore size distributions are different from original sample and Pd loaded sample. The hydrogen uptake results show excess hydrogen uptake capacity on the Pt/Pd-doped CAC samples are higher than pure CAC at 298 K, which should be attributed to hydrogen spillover effects. The 2.5%Pt and 2.5%Pd hybrid doped CAC sample shows the highest hydrogen uptake capacity (1.65 wt%) at 298 K and 180 bar, The particle size and distribution of Pt/Pd catalysts could play a crucial role on hydrogen uptake by spillover. The total hydrogen storage capacity analysis show that total H₂ storage capacities for all samples are similar, and spillover enhanced H₂ uptakes of metal-doped samples could not well support total H₂ storage capacity. The total pore volume of porous materials also is a key factor to affect total hydrogen storage capacity.     

Hydrogen storage in hybrid nanostructured carbon/palladium materials: Influence of particle size and surface chemistry

Hydrogen adsorption on porous materials is one of the possible methods proposed for hydrogen storage for transport applications. One way for increasing adsorption at room temperature is the inclusion of metal nanoparticles to increase hydrogen–surface interactions. In this study, ordered mesoporous carbon materials were synthesized by replication of nanostructured mesoporous SBA-15 silica. The combination of different carbon precursors allowed to tailor the textural, structural and chemical properties of the materials. These carbons were used for the synthesis of hybrid nanostructured carbon/palladium materials with different sizes of metal nanoparticles. The hydrogen sorption isotherms were measured at 77 K and 298 K between 0.1 and 8 MPa. Hydrogen storage capacities strongly correlate with the textural properties of the carbon at 77 K. At room temperature, Pd nanoparticles enhance hydrogen storage capacity by reversible formation of hydride PdH_x and through the spillover mechanism. The hydrogen uptake depends on the combined influences of metal particle size and of carbon chemical properties. Carbons obtained from sucrose precursors lead to the hybrid materials with the highest storage capacities since they exhibits a large microporous volume and a high density of oxygenated surface groups.     

Decorating carbon nanotubes with Ni particles using an electroless deposition technique for hydrogen storage applications

A facile and low-cost electroless deposition technique is utilized to decorate multi-walled carbon nanotubes (CNTs) with Ni. The obtained composites are attempted to use as hydrogen storage materials, whose performance is evaluated with a high-pressure microbalance. Effects of the concentration of plating solution, deposition time, and reaction temperature on the loading amount, particle size, morphology, and distribution density of Ni are studied using a transmission electron microscope. With proper deposition parameters, highly dispersed Ni nanoparticles with a uniform diameter can be fabricated on CNTs, causing a notable hydrogen spillover reaction on the composite. The optimum hydrogen storage capacity of the prepared Ni-decorated CNTs with a average diameter of 5 nm, measured at 6.89 MPa and 25 °C, is 1.02 wt%, which is almost three times higher than that (0.35 wt%) of pristine CNTs.     

Hydrogen storage performance in palladium-doped graphene/carbon composites

In this study a two-dimensional graphene sheet (GS) doped with palladium (Pd) nanoparticles was physically mixed with a superactivated carbon (AC) receptor and used as a hydrogen adsorbent. The hydrogen adsorption/desorption isotherm of the Pd-doped GS catalyst/AC composite (Pd-GS/AC) is determined using a static volumetric measurement at room temperature (RT) and pressure up to 8 MPa. The experiments show that the H₂ uptake capacity of 0.82wt.% for Pd-GS/AC is obviously enhanced, measuring 49% more than the 0.55wt.% for Pd-free GS/AC at RT and 8 MPa. Highly reversible behavior of Pd-GS/AC is also observed. Moreover, the isosteric heat of adsorption for Pd-GS/AC (−14 to −10 kJ/mol) is higher than that for pristine AC (−8 kJ/mol). An increase in H₂ uptake in the Pd-GS/AC suggests the occurrence of a relatively strong interaction between the spilt-over H and the receptor sites due to the spillover effect.     

Static and dynamic hydrogen adsorption on Pt/AC and MOF-5

Hydrogen adsorption has been studied by static and dynamic methods on activated carbon (AC), platinum/activated carbon (Pt/AC), metal organic frameworks (MOF-5), and Pt/AC_MOF-5.The static method showed that all of adsorbents used in this study exhibited a Langmuir (type I) adsorption isotherm at 77 K and a linear function of hydrogen partial pressure at 298 K. The dynamic method produced breakthrough curves, indicating (i) slow rate of hydrogen diffusion in the densely packed activated carbon and Pt/AC beds and (ii) high rate of hydrogen diffusion in the loosely packed bed with large MOF-5 crystallites. Temperature variable adsorption resulted in the higher hydrogen uptake on Pt/AC than other adsorbents. The results suggested that temperature variable adsorption enhanced the hydrogen storage process by (i) initiating hydrogen dissociation at high temperature and (ii) facilitating spillover at low temperature on Pt/AC.     

Hydrogen storage behavior of Mg/Ni layered nanostructured composite materials produced by accumulative fold-forging

An advanced and newly developed severe plastic deformation (SPD) method called accumulative fold-forging (AFF) was applied to produce layered nanostructured MgNi alloys exhibiting superior hydrogen storage capacity. Microstructural developments and storage properties were characterized in depth to correlate the structure and performance of this advanced material. The enhanced hydrogen storage performance of the magnesium-based layered composite material was investigated in comparison to the pristine state by conducting hydrogenation and dehydrogenation testing. It was also shown that the hydrogen uptake and release characteristics can be controlled by adjusting the layered structure or the Mg: Ni stoichiometry ratio. Refining the grain structure of the magnesium alloy down to the nano-scale range (～400–900 nm) by applying high cycles AFF consolidation to promote creation of multi-million nanometric interfaces led to superior storage performance with a remarkable hydrogen absorption capacity of up to ～1.425 wt%. X-ray diffraction (XRD) analysis of the hydrogenated products revealed the formation of MgH₂ that indicates the dominance of the magnesium matrix for controlling the hydrogen storage behavior of the layered Mg/Ni composite material. Finally, the relationship between the directional hydrogen storage behavior and the induced structural features upon AFF treatment were also established using quantitative characterization and analytical tools.     

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.     

Hydrogen storage in microwave-treated multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) treated by microwave and heat treatment were used for hydrogen storage. Their storage capacity was measured using a quadruple quartz crystal microbalance in a moisture-free chamber at room temperature and at relatively low pressure (0.5 MPa). Deuterium was also used to monitor the presence of moisture. The hydrogen storage capacity of the microwave-treated MWCNTs was increased to nearly 0.35 wt% over 0.1 wt% for the pristine sample and increased further to 0.4 wt%, with improved stability after subsequent heat-treatment. The increase in the storage capacity by the microwave treatment was mostly attributed to the introduction of micropore surfaces, while the stability improvement after the subsequent heat treatment was related to the removal of functional groups. We also propose a measurement method that eliminates the moisture effect by measuring the storage capacity with hydrogen and deuterium gas.     

Inorganic nanotube composites based on polyaniline: Potential room-temperature hydrogen storage materials

Inorganic nanotubes as a support material for polyaniline were used for hydrogen storage. To this end, a solid-state preparation method has been developed for controlling the hydrogen storage capacity of these nanocomposites. The hydrogen storage capacities have measured at room temperature and at a low pressure of 0.5 MPa using the precise quadrupole quartz crystal microbalance technique in a chamber free of moisture. The optimum nanocomposite shows an enhanced hydrogen storage capacity of 0.78 wt.% with excellent reversibility when compared to less than 0.019 wt.% capacity of the pristine nanotubes and 0.05 wt.% of polyaniline. This large increase in the hydrogen capacity can be attributed to the chemisorption hydrogen uptake, which was enhanced by the sorption sites created through the milling process of polyaniline with the nanotubes. This is in addition to the hydrogen adsorption contribution by a controlled lumen size that is suitable for a maximum hydrogen adsorption through inserting polyaniline chains into the nanotubes.     

Hydrogen sorption properties of Pd-doped carbon molecular sieves

Decoration with transition metal catalysts has been reported to enhance H₂ storage capacity of carbon materials at ambient temperature. Furthermore, it has been proposed that surface oxygen groups may improve the process. In this study, a carbon molecular sieve was subjected to controlled oxidation and consequent doping with Pd nanoparticles. The H₂ sorption performance of the pristine and oxidized, undoped and doped materials was examined at 298 K up to 20 bar. It was found that the non-oxidized carbon-Pd composite did not show any spillover based sorption increase. On the other hand the oxidized samples reveal a slight enhancement that could be attributed to a weak chemisorption process initiated by the so-called ‘‘spillover’’ effect. Overall, the contribution of spillover to the total hydrogen storage capacity of this system (under the conditions studied) was not found to be of great significance.     

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 in Ni-B nanoalloy-doped 2D graphene

Two-dimensional graphene material is doped with Ni-B nanoalloys via a chemical reduction method, and shows that the optimal graphene doped with Ni (0.14 wt.%) and B (0.63 wt.%) has a hydrogen capacity of 2.81 wt.% at 77 K and 106 kPa, which is more than twice of that of the pristine graphene. The measured adsorption isotherms of hydrogen and nitrogen suggest that the Ni-B nanoalloys function as catalytic centers to induce the dissociative adsorption of hydrogen (spillover) on the graphene. The Ni-B nanoalloys without using any noble metal may be a promising catalyst for hydrogen storage application.     

Enhancement of hydrogen storage capacity on co-functionalized GaS monolayer under external electric field

Hydrogen storage properties of co-functionalized 2D GaS monolayer have been systematically investigated by first-principles calculations. The strength of the binding energy of hydrogen (H₂) molecules to the pristine GaS surface shows the physisorption interactions. Co-functionalized GaS sheet by Li, Na, K and Ca atoms enhanced the capacity of binding energies of hydrogen and strength of hydrogen storage considerably. Besides, DFT calculations show that there is no structural deformation during H₂ desorption from co-functionalized GaS surface. The binding energies of per H₂ molecules is found to be 0.077 eV for pristine GaS surface and 0.064 eV–0.37 eV with the co-functionalization of GaS surface. Additionally, in the presence of applied external electric field enhanced the strength of binding energies and it is found to be 0.09 eV/H₂ for pristine GaS case and 0.19 eV/H₂ to 0.38 eV/H₂ for co-functionalized GaS surface. Among the studied GaS monolayer is found to be the superior candidate for hydrogen storage purposes. The theoretical studies suggest that the electronic properties of the 2D GaS monolayer show the electrostatic behavior of hydrogen molecules which confirms by the interactions between adatoms and hydrogen molecules before and after hydrogen adsorption.     

Hydrogen sorption studies of palladium decorated graphene nanoplatelets and carbon samples

With the increasing population of the world, the need for energy resources is increasing rapidly due to the development of the industry. 88% of the world's energy needs are met from fossil fuels. Since there is a decrease in fossil fuel reserves and the fact that these fuels cause environmental pollution, there is an increase in the number of studies aimed to develop alternative energy sources nowadays. Hydrogen is considered to be a very important alternative energy source due to its some specific properties such as being abundant in nature, high calorific value and producing only water as waste when burned. An important problem with the use of hydrogen as an energy source is its safe storage. Therefore, method development is extremely important for efficient and safe storage of hydrogen. Surface area, surface characteristics and pore size distribution are important parameters in determining the adsorption capacity, and it is needed to develop new adsorbents with optimum parameters providing high hydrogen adsorption capacity. Until recently, several porous adsorbents have been investigated extensively for hydrogen storage. In this study, it was aimed to develop and compare novel Pd/carbon, Pd/multiwalled carbon nanotube, and Pd/graphene composites for hydrogen sorption. All the palladium/carbon composites were characterized by t-plot, BJH desorption pore size distributions, N₂ adsorption/desorption isotherms, and SEM techniques. The maximum hydrogen storage of 2.25 wt.% at −196 °C was achieved for Pd/KAC composite sample. It has been observed that the spillover effect of palladium increases the hydrogen sorption capacity.