Hydrogen adsorption on modified activated carbon

The aim of this work is to investigate hydrogen adsorption on prepared super activated carbon (AC). Litchi trunk was activated by potassium hydroxide under N₂ or CO₂ atmosphere. Nanoparticles of palladium were impregnated in the prepared-AC. Hydrogen adsorption was accurately measured by a volumetric adsorption apparatus at 77, 87, 90 and 303 K, up to 5 MPa. Experimental results revealed that specific surface area of the prepared-AC increased according to KOH/char ratio. The maximum specific surface area reached up to 3400 m²/g and total pore volume of 1.79 cm³/g. The maximum hydrogen adsorption capacity of 2.89 wt.% at 77 K and under 0.1 MPa, was obtained on these materials. The hydrogen adsorption capacity of the 10 wt.% Pd-AC was determined as 0.53 wt.% at 303 K and under 6 MPa. This amount is higher than that on the pristine AC (0.41 wt.%) under the same conditions.     

Studies on the mechanism and capacity of hydrogen uptake by physisorption-based materials

Nanostructure carbons are the most important physisorption-based hydrogen carriers. The mechanism of hydrogen uptake was studied based on the adsorption isotherms collected for a wide range of temperature and pressure. It was concluded that the hydrogen adsorbed is arranged on carbon surface monolayerly; therefore, the storage capacity depends only on the specific surface area of the carbon. This rule applies also for other organic/inorganic materials if the interaction between hydrogen and the solid surface remains the Van der Waals force. Carbon nanotubes cannot be good carriers of hydrogen due to the small surface area, but superactivated carbon is of great potential and a storage capacity over 10 wt% was proven for the condition of 77 K and 6 MPa.     

H2 storage on single- and multi-walled carbon nanotubes

The adsorption of hydrogen on single-walled and multi-walled carbon nanotubes (CNTs) was investigated at 77 and 298 K, in the pressure range of 0–1000 Torr. The adsorption isotherms indicate that adsorption follows the Langmuir model. Hydrogen uptakes were found to depend strongly on the nature of the CNTs. Single-walled CNTs adsorb significantly higher quantities of hydrogen per unit mass of the solid, while the opposite is true on a per unit surface area basis. This observation implies that adsorption takes place selectively on specific sites on the surface. The hydrogen uptake capacity of CNTs was also found to be affected by the purity of the materials, increasing with increasing purity. Temperature programmed desorption indicated that relatively strong adsorption bonds develop between adsorbent and adsorbate and that a single type of adsorption site exists on the solid surface.     

Preparation of activated rectangular polyaniline-based carbon tubes and their application in hydrogen adsorption

Porous materials, especially porous carbon materials, have the most potential as hydrogen adsorbents. In this research, a series of novel rectangular polyaniline tubes (RPTs) are synthesized using hollow carbon nanosphere (HCNS) templates. By changing mass ratios of ammonium persulfate to HCNSs, the sizes of RPTs can be controlled. Chemical activation with KOH gives rise to a large specific surface area (SSA), ranging from 1680 to 2415 m2 g⁻¹, and big pore volumes that range from 1.274 to 1.550 cm³g⁻¹. These observations demonstrate that activated rectangular polyaniline-based carbon tubes ARP-CTs are promising hydrogen adsorbents. Hydrogen uptake measurements show that the highest hydrogen adsorption reaches 5.2 wt% at 5 MPa/77 K and 0.62 wt% at 7.5 MPa/293 K respectively. Notably, the large pore volume can contribute 2.8 wt% to the total hydrogen storage which has approached 8.0 wt% at 5 MPa/77 K.     

Effect of in-situ boron doping on hydrogen adsorption properties of carbon nanotubes

Floating catalyst chemical vapor deposition method was used for the synthesis of boron doped carbon nanotubes (BCNTs) using ethanol, triethyl borate and ferrocene as carbon source, boron source and catalyst precursor, respectively. The synthesized BCNTs were characterized by transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy (XPS). The hydrogen adsorption activity was studied for BCNTs along with undoped single walled and multi walled carbon nanotubes. Significant enhancement in the hydrogen storage value was found in doped CNTs as compared to the other undoped CNTs. Hydrogen storage for BCNTs was found to be 2.5 wt% at 10 bar and 77 K. In-situ doped BCNTs gives higher hydrogen adsorption as compared to ex-situ doped BCNTs. The Langmuir adsorption isotherm was found to be suitable for describing the adsorption isotherm as compared with Freundlich isotherm. Maximum adsorption capacity was about 9.8 wt% at 77 K. Pseudo second order kinetics was followed by BCNTs for hydrogen adsorption.     

High pressure cryo-storage of hydrogen by adsorption at 77 K and up to 50 MPa

In this paper, we focused on hydrogen adsorption on large surface area solids, combining optimal extreme conditions i.e. very high pressure and low temperature for gas storage process purpose. Therefore, a new volumetric device is elaborated to obtain excess adsorption isotherms at 77 K up to 500 bar. Two activated carbons with different micro-porosities are analysed in the view of hydrogen storage investigation. Also, the results are compared to zeolite adsorption properties. Based on these results, the total mass and volumetric storage capacity are calculated using the bulk density relationship. Thereby, we obtained high storage in situ capacities equal to 5.2 wt% and 54.5 kgH₂/m³. Further, we also considered practical application aspects related to hydrogen storage process in highly porous packed materials.     

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.     

Hydrogen storage in Co-and Zn-based metal-organic frameworks at ambient temperature

Hydrogen adsorption properties of some Co-and Zn-based Metal-Organic Framework (MOF) materials were studied at near ambient temperatures. Maximal hydrogen storage capacity of 0.75 wt% was found for a Zn-based material at 175 Bar hydrogen pressure and T = −4 °C. Hydrogen adsorption correlated linearly with BET surface area and strongly depends on temperature. Relatively low structural stability of some MOF's results in framework collapse during degassing and hydrogen adsorption measurements.     

Carbon materials for H2 storage

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

Copper-doped activated carbon from amorphous cellulose for hydrogen,methane and carbon dioxide storage

The transition away from fossil fuel and ultimately to a carbon-neutral energy sector requires new storage materials for hydrogen and methane as well as new solutions for carbon capture and storage. Among the investigated adsorbents, activated carbons are considered especially promising because they have a high specific surface area, are lightweight, thermally and chemically stable, and easy to produce. Moreover, their porosity can be tuned and they can be produced from inexpensive and environmentally friendly raw materials. This study reports on the development and characterization of activated carbons synthesized starting from amorphous cellulose with and without the inclusion of copper nanoparticles. The aim was to investigate how the presence of different concentrations of metal nanoparticles affects porosity and gas storage properties. Therefore, the research work focused on synthesis and characterization of physical and chemical properties of pristine and metal-doped activated carbons materials and on further investigation to analyze their hydrogen, methane and carbon dioxide adsorption capacity. For an optimized Cu content the microporosity is improved, resulting in a specific surface area increase of 25%, which leads to a H₂ uptake (at 77 K) higher than the theoretical value predicted by the Chahine Rule. For CH₄, the storage capacity is improved by the addition of Cu but less importantly because the size of the molecule hampers easy access of the smaller pores. For CO₂ a 26% increase in adsorption capacity compared to pure activated carbon was achieved, which translated with an absolute value of over 48 wt% at 298 K and 15 bar of pressure.     

Changes in hydrogen storage properties of carbon nano-horns submitted to thermal oxidation

The effect of thermal oxidation on the hydrogen storage properties of carbon nano-horns was investigated by gravimetric and electrochemical methods. The pristine nano-horn sample was oxidised at 673 K in air for different periods (15, 30 and 60 min) and the resulting materials were characterised. The N₂ adsorption experiments reveal a marked increase in the surface area, from 267 m² g⁻¹, for the pristine sample, up to 1360 m² g⁻¹ for the sample oxidised for the 60 min period, and a reduction in the average pore diameter. The gravimetric investigation, conducted at low temperature (77 K) showed an increase in the hydrogen storage, from 0.75 wt% for the pristine sample up to 2.60 wt% for the oxidised material. Reproducible and stable hydrogen storage was found for all the samples examined apart from the sample oxidised for 60 min. For the latter, a decrease in the amount of hydrogen stored between the first and second cycles was found. Electrochemical loading of hydrogen in the samples was performed at room temperature (298 K) in alkaline solution by the galvanostatic charge/discharge technique. The results obtained here however show a much lower hydrogen storage level by the samples as compared to the gas storage method, with a maximum value of 0.124 wt% H₂ and with very little dependence on the thermal oxidation treatment.     

A study on the hydrogen storage capacity of Ni-plated porous carbon nanofibers

In this study, we prepared highly porous carbon-nanofiber-supported nickel nanoparticles as a promising material for hydrogen storage. The porous carbons were activated at 1050 °C, and the nickel nanoparticles were loaded by an electroless metal-plating method. The textural properties of the porous carbon nanofibers were analyzed using N₂/77 K adsorption isotherms. The hydrogen storage capacity of the carbons was evaluated at 298 K and 100 bar. It was found that the amount of hydrogen stored was enhanced by increasing nickel content, showing 2.2 wt.% in the PCNF-Ni-40 sample (5.1 wt.% and 6.4% of nickel content and dispersion rate, respectively) owing to the effects of the spill-over of hydrogen molecules onto the metal–carbon interfaces. This result clearly indicates that the presence of highly dispersed nickel particles can enhance high-capacity hydrogen storage.     

Hydrogen sorption in transition metal modified ETS-10

Hydrogen adsorption studies in nickel, rhodium and palladium exchanged and in situ loaded titanosilicate ETS-10 were performed at 77.4 K using a static volumetric adsorption system up to 1 bar, and 303 K in a gravimetric adsorption system up to 5 bar. The hydrogen adsorption isotherms at 77.4 K were reversible with pressure but chemisorption of hydrogen was noticed at 303 K. Rhodium exchanged ETS-10 showed the highest hydrogen adsorption capacity of 82.6 cc/g at 77.4 K. The hydrogen adsorption isotherm analysis at 303 K was repeated up to three adsorption runs to check the repeatability of hydrogen uptake. At 303 K palladium loaded ETS-10 showed the highest hydrogen uptake capacity of 33.1 cc/g. The DRIFT spectra analysis of ETS-10 samples before and after hydrogen adsorption was conducted, which confirmed that the hydrogen adsorbed in transition metal modified ETS-10 at 303 K was due to the chemical interactions in the form of transition metal hydrides inside ETS-10. The absorbed hydrogen at 303 K can be desorbed by heating the ETS-10 sample up to 413 K.     

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.     

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.     

Synthesis of boron and nitrogen co-doped carbon nanotubes and their application in hydrogen storage

Boron and nitrogen codoped carbon nanotubes (B,N-CNTs) were synthesized by floating catalyst chemical vapor deposition (FCCVD) using ethanol, ferrocene, boric acid and imidazole as carbon source, catalyst, boron and nitrogen precursors, respectively. The samples were analyzed using transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoemission spectroscopy. 1.5 at% B and 1.34 at% N could be doped in the resultant structure, which has higher length (few μm) with higher thermal stability (621 °C). At pressure 16 bar, hydrogen adsorption for B,N-CNTs was found to be 1.96 and 0.35 wt% at 77 K and 303 K, respectively. Hydrogen storage as function of time was also reported for both the cases. The adsorption process follow pseudo second order kinetics. The present study reveals that the codoping of CNTs aid in tuning properties of CNTs for hydrogen storage application.     

Grand canonical Monte Carlo simulations of hydrogen adsorption in carbon aerogels

Hydrogen storage plays a fundamental role in the future hydrogen energy system, and carbon aerogel is one of the most potential hydrogen storage materials because of its high gravimetric and volumetric density on hydrogen adsorption. In this paper, the amorphous structure of carbon, obtained by a numerical simulation process by using the molecular dynamic and Monte Carlo methods, as well as the primary unit method, was intercepted as a sphere structure for numerical annealing to build a carbon nanosphere, which serves as the basic unit to reconstruct the carbon aerogel's skeleton by the Diffusion Limited Cluster Aggregation (DLCA) method. The hydrogen adsorption in carbon aerogel was simulated by using the self-coding parallel grand canonical Monte Carlo (GCMC) method. The influences of particle diameter, density, temperature, pressure, and specific surface area on the hydrogen adsorbing capacity in carbon aerogel were analyzed in detail. The results showed that the carbon aerogel's hydrogen storage capacity with a specific surface area of 2680 m²/g could reach 4.52 wt % at 77 K and 3.0 MPa.     

Scale-up activation of carbon fibres for hydrogen storage

In a previous study, we investigated, at a laboratory scale, the chemical activation of two different carbon fibres (CF), their porosity characterization, and their optimization for hydrogen storage [1]. In the present work, this study is extended to: (i) a larger range of KOH activated carbon fibres, (ii) a larger range of hydrogen adsorption measurements at different temperatures and pressures (i.e. at room temperature, up to 20 MPa, and at 77 K, up to 4 MPa), and (iii) a scaling-up activation approach in which the obtained activated carbon fibres (ACF) are compared with those from laboratory-scale activation. The prepared samples cover a large range of porosities, which is found to govern their ability for hydrogen adsorption. The hydrogen uptake capacities of all the prepared samples have been analysed both in volumetric and in gravimetric bases. Thus, maximum adsorption capacities of around 5 wt% are obtained at 77 K, and 1.1 wt% at room temperature, respectively. The packing densities of the materials have been measured, turning out to play an important role in order to estimate the total storage capacity of a tank volume. Maximum values of 17.4 g l⁻¹ at 298 K, and 38.6 g l⁻¹ at 77 K were obtained.     

Grand canonical Monte Carlo simulation of hydrogen physisorption in single-walled boron nitride nanotubes

The properties of hydrogen physisorption in single-walled boron nitride nanotubes (SWBNNTs) and single-walled carbon nanotubes (SWCNTs) are investigated in detail by the grand canonical Monte Carlo simulations. A great deal of our computational results show that the hydrogen storage capacity of SWBNNTs is slightly larger than the capacity of SWCNTs at any time when their diameters were equal and in the same conditions, and indicate that the hydrogen storage capacity of SWBNNTs at 293 K and 10 MPa with a diameter of more than 30 nm or at 293 K and 15 MPa with a diameter of more than 25 nm could exceed the 2010 goal of 6 wt%, which is presented by the U.S. Department of Energy. In addition, these results are discussed in theory.     

Preparation of a porous carbon from ferrocene-loaded polyaniline and its use in hydrogen adsorption

A porous carbon made of polyaniline with different ferrocene loadings was prepared through carbonization and thermal chemistry activation with KOH. The ferrocene served as a pore-forming agent and a resource of iron nanoparticles. N₂ adsorption/desorption measurements showed that the specific surface area and pore volume ranged from 2681 to 3246 m² g⁻¹ and from 1.56 to 2.06 cm³ g⁻¹, respectively, with increasing ferrocene loadings. Similarly, hydrogen adsorption also increased from 5.3 to 6.2 wt% at 77 K/5 MPa and 0.6 wt% to 0.85 wt% at 293 K/8 MPa. Scanning electron microscopy, X-ray diffraction and energy dispersive X-ray analysis showed that iron nanoparticles were embedded in the carbon matrix or dispersed on the surface. The large specific surface area and big pore volume improved the original hydrogen adsorption heat up to 7.2 kJ mol⁻¹ for the best sample.