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.     

Hydrogen adsorption in carbon nanostructures

Hydrogen adsorption, (BET) specific surface area and X-ray diffraction (XRD) measurements have been performed on carbon nanofibers, intercalated and exfoliated carbon materials. Excess adsorption capacity was evaluated at equilibrium pressures and temperatures ranging from 0.1 to 10.5MPa and 77 to 295K, respectively. We find that at room temperature, carbon nanofibers can adsorb up to 0.7wt% at 10.5MPa. We observed that the presence of different nickel–copper ratios in the catalyst particles leads to change in crystalline structure and specific surface area. Furthermore, we noted that the latter can be increased by the addition of hydrogen in the organic gas during the synthesis of the nanofibers. Finally, we will discuss the hydrogen coverage per unit surface area which is substantially larger on nanostructures than on activated carbon.     

Hydrogen storage by carbon materials

In order for fuel cells to become a practical means of supplying power for road vehicles it will be necessary for a viable method of on-board hydrogen storage to be identified and implemented. Target values that must be achieved for the critical parameters in such a system have been drawn up by the US Department of Energy. It is quite clear that these targets cannot be met by systems that store hydrogen at high pressure or as a liquid at cryogenic temperatures. Attention has thus been focused on methods of storing hydrogen in, or on, a solid phase. In such schemes it is obviously desirable to make use of elements from the early part of the periodic table and there has been much interest in the possible use of carbon as a hydrogen host. This paper provides an overview of experimental work on such systems together with an outline of theoretical studies that have been undertaken to estimate the practical limits to the amount of hydrogen that could be stored per unit weight.     

Hydrogen adsorption by microporous materials based on alumina-pillared clays

Hydrogen adsorption capacities of various alumina-pillared clays have been studied. The starting material for the preparation of the samples was a natural montmorillonite, which was intercalated with solutions of hydrolysed aluminium, at various Al concentration/clay weight ratios, and then calcined at various temperatures. The results of the adsorption at 77 K indicate that there is a relation between the hydrogen uptake and the microporous volume of the pillared clays. Equilibrium adsorption data were analysed using the Freundlich, Langmuir and Toth isotherm models, in order to investigate the heterogeneity of the materials.     

Hydrogen adsorption on single walled carbon nanotubes-tungsten trioxide composite

In this work, hydrogen storage property of single walled carbon nanotubes-tungsten trioxide (SWCNTs-WO₃) composite is investigated. The composite is prepared and hydrogenated by electron beam (e-beam) evaporation technique. Hydrogenation is carried out during the preparation of the composite itself. The amount of hydrogen uptake by the composite is 2.7 wt%, which is due to the collective adsorption of hydrogen by CNTs and WO₃ nanostructured materials, the method of preparation and hydrogenation involved. The incorporated hydrogen is completely (100%) released in the temperature range of 175–305 °C which in turn infers that the hydrogenated composite is stable at room temperature. The stored hydrogen has the average binding energy of 0.4 eV and the nature of binding is found to be weak chemisorption. Spillover mechanism is attributed for the hydrogen uptake of the composite.     

Oxygen-promoted hydrogen adsorption on activated and hybrid carbon materials

The effect of heteroatoms on hydrogen adsorption properties of activated and hybrid carbon materials is critically described. For that purpose, olive stones were activated chemically with KOH, and subsequently washed or not, and oxidised with ozone or not. Olive stones were also activated physically with CO₂. A series of activated carbons prepared by chemical activation of sucrose was also investigated for comparison. As a result, many activated carbons with different pore-size distributions, surface areas, average micropore widths, oxygen contents and amounts of mineral matter could be compared. All were thoroughly characterised by adsorption of N₂, CO₂ and H₂O, elemental analysis, XPS, thermogravimetry, and adsorption of H₂ at different pressures. Many correlations between textural parameters, composition and adsorption properties could be evidenced, and were critically discussed. We show that the hydrogen uptake at 77 K is controlled by the following parameters, listed by decreasing order of importance: specific surface area, average micropore size, surface chemistry and shape of the pore size distribution. At room temperature (i.e., at 298 K), the adsorbed hydrogen uptake was in the range of 0.19–0.42 wt %; the presence of large amounts of alkali metals can further improve the hydrogen adsorption properties, but surface chemistry still has a major influence, especially through the acidic surface functions.     

Hydrogen adsorption over Zeolite-like MOF materials modified by ion exchange

Novel porous Zeolite-like metal-organic framework (ZMOF) materials with Rho and Sod topologies are promising adsorbents for hydrogen storage due to their high surface area and, more importantly, to their capacity of being ion-exchanged, potentially changing their affinity for hydrogen. In this work, we have successfully synthesized both Rho and SodZMOF materials, optimizing experimental conditions for scaling-up the procedure already published to produce grams of material. The resultant materials were alkaline-cation-exchanged, widely characterized and finally tested as hydrogen adsorbents.     

Hydrogen evolution reaction at Pd-modified carbon fibre and nickel-coated carbon fibre materials

The present paper reports on A.C. impedance study of hydrogen evolution reaction (HER), carried-out on Pd-modified carbon fibre (CF) and nickel-coated carbon fibre (NiCCF) materials. The HER was examined in 0.5 M H₂SO₄ solution for electrochemically deposited Pd on Hexcel 12K AS4C CF and Toho-Tenax 12K50 NiCCF tow materials. Kinetics of the hydrogen evolution reaction was studied at room temperature, over the cathodic overpotential range: −100 to −1200 mV vs. RHE. Corresponding values of charge-transfer resistance, exchange current–density for the HER and other electrochemical parameters for the examined catalyst materials were derived. Thus, Pd modification of CF and NiCCF materials (at ca. 1.5 wt.% Pd) dramatically increased the exchange current–density parameter by about 5300× and 445× for carbon fibre and nickel-coated carbon fibre tows, correspondingly.     

Hydrogen adsorption on porous silica

A number of MCM-41 samples were produced with different ageing times and zinc dopant levels. Hydrogen adsorption experiments performed at 77 K on pure MCM-41 samples showed a maximum excess wt% uptake of 2.01%. Samples were doped with Zn in the Si:Zn ratio of 50:1, 25:1 and 10:1. These samples showed an increase in d$d$-spacing in their XRD patterns, suggestive of Zn incorporation into the structure but resulting in a decrease in long range pore ordering and surface area with increasing Zn content. Two of the three doped samples showed an increase in _H2${\mathrm{H}}_2$ adsorbed per unit surface area suggestive of increased interaction strength. However, definitive confirmation is difficult due to degradation of the pore structure and surface area. Adsorption at room temperature was minimal, being 0.1 wt% or below.     

Hydrogen adsorption with micro-structure deformation in nanoporous carbon under ultra-high pressure

Hydrogen adsorption with micro-structure deformation under ultra-high pressure in nanoporous carbon (NPC) has been studied. This study proposed a new ultra-high pressurization (UHP) method. It produces a gas atmosphere of over 100 MPa utilizing the cold isostatic pressing (CIP) device. NPC materials were pressurized under a hydrogen atmosphere at 100–400 MPa. NPC fabricated from rice husk via KOH activation possesses a high surface area achieving 3500 cm²/g and a micropore volume of over 2.0 cm³/g. The maximum hydrogen uptake reached 3.2 wt% (77 K, 0.1 MPa). Then, NPC materials were treated with 100–400 MPa pressurization in the hydrogen atmosphere. NPC showed a preferred deformation behavior of 1.1–1.2 nm after pressurization, which is the optimum size for hydrogen adsorption. Additionally, the maximum micropore volume increased to 2.51 cm³/g. However, the hydrogen uptake shows a slight decrease to 3.0 wt%. The isosteric heat of adsorption maintained at 8.0–10.3 kJ/mol.     

Hydrogen adsorption on Li decorated graphyne-like carbon nanosheet: A density functional theory study

Motivated by novel graphyne-like carbon nanostructure C₆₈-GY, spin-polarized DFT calculations with dispersion-correction were performed to investigate the hydrogen adsorption capacity of Li decorated C₆₈-GY nanosheet. The binding energy between Li and C₆₈-GY was larger than the cohesive energy of bulk metal, indicating Li atoms would prefer to separately attached on C₆₈-GY. The ab initio molecular dynamics simulation has been performed to confirm the stability of Li/C complex. When five Li atoms decorated on C₆₈-GY, 14H₂ molecules were captured. The maximum hydrogen storage density was 8.04 wt% with an average hydrogen adsorption energy of −0.227 eV per H₂. The positively charged Li atoms aroused electrostatic field and induced the polarization of H₂. It was notable to observe strong hybridization between the main peak of H-1s orbitals with Li below Fermi level, which was responsible for the enhancement of hydrogen binding energy, indicating its potential application on hydrogen storage.     

Hydrogen storage system based on novel carbon materials and heat pipe heat exchanger

Adsorbed hydrogen is being considered as a potential energy carrier for vehicular applications to replace compressed gas due to its high energy density capability. A new design of hydrogen storage vessel using novel carbon sorbents and heat pipes thermal control is the subject of research program oriented on 5–10 kg of hydrogen be stored on-board. Porous structure and hydrogen-sorption capacities of activated carbon materials are considered. Numerical analysis based on 2D nonequilibrium model of heat and mass transfer in the sorbent cylinder is carried out with the aim to investigate and optimize the sorption storage system. Obtained numerical and experimental data testify the possibilities of a new sorbent bed to reduce the operating pressure and increase a gas storage capacity.     

Hydrogen adsorption on Pd-modified carbon nanofibres: Influence of CNF surface chemistry and impregnation procedure

Different carbon nanofibre (CNF) based materials (parent, oxidized, and impregnated with a palladium loading of 1 wt.% using different procedures) have been tested for hydrogen storage at ambient pressure. Parent CNF are completely free of oxygen surface groups, whereas treatment in nitric acid increases mainly the amount of surface anhydrides groups. Add to the surface functionalization, the solvent employed in the palladium impregnation was also varied, using both aqueous and organic precursor solutions. Thermogravimetric analyses of the hydrogen adsorption–desorption cycles suggest that the presence of theses functional groups hinders the adsorption. Concerning the presence of palladium, its influence strongly depends on the previous activation of the surface and on the solvent used for the palladium addition. The use of aqueous precursors and functionalized CNFs leads to increases in the adsorption capacity close to 100% compared to the parent CNF (12.6 vs. 6.7 cm³/g).     

Hydrogen adsorption on graphene sheets and nonporous graphitized thermal carbon black at low surface coverage

Behavior of hydrogen adsorption on nonporous carbon based materials was comparatively studied for selection of an efficient carrier for catalytic metals. Graphene sheets (GS) and graphitized thermal carbon black, which respectively has a specific surface area about 220 m²/g and 36 m²/g, were selected for adsorption equilibrium testes within temperature–pressure range from 77 K–87 K and 0–1 kPa. Henry law constants were employed to calculate the second virial constants and the limit isosteric heat of adsorption. The Weeks, Chandler and Andersen (WCA) perturbing scheme and the fundamental measure theory (FMT) were used to determine the interaction energy between solid atoms and hydrogen molecules. Adsorption potential well was determined by linear interpolation based on the Boltzmann distribution approximation. It shows that the potential well between hydrogen molecules and the GS, BP280 is respectively about 33.55 K and 31.97 K, suggesting that the bonding energy between the GS and hydrogen molecules is larger than that on carbon black.     

Hydrogen adsorption and storage in boron‐substituted and nitrogen‐substituted nano‐carbon materials decorated with alkaline earth metals

Based on ab initio calculations, we have investigated the H₂ adsorption and storage capacity on boron‐substituted and nitrogen‐substituted nano‐carbon materials doped with alkaline earth metal ions (Be²⁺, Mg²⁺, and Ca²⁺) systematically. The calculation results show that the Be²⁺‐decorated, Mg²⁺‐decorated, and Ca²⁺‐decorated carbon‐based materials with B‐substitution and N‐substitution improve the hydrogen storage capacity. H₂ molecules are bound stronger with lighter cations. The adsorption energy of H₂ molecule on the M²⁺‐nano‐carbon complex (M²⁺ = Be²⁺, Mg²⁺, and Ca²⁺) is disproportional to ionic radii of the M²⁺ cations. The interaction between H₂ and M²⁺@nano‐carbon complex is elucidated by Mulliken charge analysis. It is determined that the highest gravimetric density is predicted to be 13.38 and 19.89 wt.% for the B‐substituted and N‐substituted materials, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrogen adsorption around lithium atoms anchored on graphene vacancies

Density functional theory and molecular dynamics were used to study the interaction of a lithium atom with a vacancy inside a graphene layer. It was found that the lithium atom is adsorbed on this vacancy, with a binding energy much larger than the lithium cohesive energy. Then, the adsorption of hydrogen molecules around lithium atoms was studied. We found that at 300 K and atmospheric pressure, this system could store up to 6.2 wt.% hydrogen, with average adsorption energy of 0.19 eV per molecule. Thus, this material satisfies the gravimetric capacity requirements for technological applications. A complete desorption of hydrogen occurs at 750 K. However, a multilayer of this system would be required for practical reasons. Under atmospheric pressure and at 300 K, we found that a system made of multiple layers of this material is stable. The storage capacity remained at 6.2 wt.%, but all adsorbed molecules were dissociated. The average adsorption energy becomes 0.875 eV/H.     

Hydrogen adsorption on graphane: An estimate using ab-initio interaction

The interaction between hydrogen molecule and graphane, material synthesized when a graphene plane is fully functionalized by hydrogen atoms, is assessable by quantum mechanical ab-initio calculations. Therefore for hydrogen, it is possible to estimate the adsorption properties of a porous material similar to activated carbons, the adsorbent surface of which is made of graphane planes instead of graphene or basal graphitic planes. The calculation realized by Monte-Carlo simulations in the grand canonical ensemble shows that the hydrogen adsorption of graphane stays qualitatively similar to that of graphene.     

Characterisation of porous carbon electrode materials used in proton exchange membrane fuel cells via gas adsorption

Porous carbon materials are typically used in both the substrate (typically carbon paper) and the electrocatalyst supports (often platinised carbon) within proton exchange membrane fuel cells. Gravimetric nitrogen adsorption has been studied at a carbon paper substrate, two different Pt-loaded carbon paper electrodes and three particulate carbon blacks. N₂ BET surface areas and surface fractal dimensions were determined using the fractal BET and Frenkel–Halsey–Hill models for all but one of the materials studied. The fractal dimensions of the carbon blacks obtained from gas adsorption were compared with those obtained independently by small angle X-ray scattering and showed good agreement. Density functional theory was used to characterise one of the carbon blacks, as the standard BET model was not applicable.