Adsorption and Diffusion of Hydrogen in Carbon Honeycomb

International Journal of Hydrogen Energy, 2007

This work aims at resolving the discrepancy between theoretical predictions on the physical adsorption of molecular hydrogen on carbonaceous solids, by exploiting molecular dynamics simulations of the adsorption process. In continuance of our previous work, three models were constructed for the depiction of the microporous carbonaceous structure. The first one (SSM) consisted of only two parallel sheets, being the lightest one used. The second (IHM) and third (HWM) models comprised structural imperfections in the form of pits and holes into their structure. Structural imperfections seemed to have a slight augmentative effect on the adsorption process. It was concluded that the addition of extra sheets to the walls did not result to any enhancement of the adsorption efficiency of the solid model. On the contrary, the lightest model exhibited superb results for the % weight-by-weight adsorption of hydrogen, approaching the highest value reported. Finally, a couple of suggestions on the development of a material for the storage of hydrogen were derived, based on the above conclusions.

International Journal of Hydrogen Energy, 2007

This work attempts to shed light on molecular hydrogen adsorption in carbonaceous microporous materials by exploiting molecular dynamics simulations combined with geometry optimization calculations of the solid structures. Carbon structures were considered here because of evidence suggesting that they may be efficient media for hydrogen storage. The inclusion of oxygen functional groups in these solid structures was also examined since they could affect hydrogen adsorption. Insertion of oxygen functional groups causes a decrease in hydrogen adsorption and this effect is more evident in narrow pores. Hydrogen molecules adsorb in the pores as structured layers, depending on pore slit width. The amount of hydrogen adsorbed reached 4.41% w/w for the purely carbonaceous materials, whereas for the oxygenated materials adsorption was limited to 3.30% w/w. The estimated adsorption density inside the pores exceeded the liquid hydrogen density for both solid structures investigated.

Chemical Physics Letters, 2010

The interaction between atomic hydrogen and microporous carbon is investigated by density functional theory (DFT) calculations. To reveal how the nanographene structures affect the atomic hydrogen uptake, which is caused by hydrogen spillover, chemisorption energies of a hydrogen atom on four graphene-like fragments are compared: a condensed hexagonal plane, two buckybowls, and a heptagon-containing curved structure. It is shown that hydrogen atoms adsorb strongly at the edge sites and on convex surfaces. Two hydrogen diffusion paths on the carbon surface are examined: hydrogen migration along the C-C bond and hydrogen desorption. The results suggest that a probable path depends on the nanographene structure.

International Journal of Hydrogen Energy, 2014

Keywords: Hydrogen storage Super diamond CNT network Grand canonical monte carlo simulations Porous carbon based material a b s t r a c t A multi-technique theoretical approach was used to investigate hydrogen storage in a three-dimensional diamond-like architecture composed by interconnected carbon nanotubes (CNT). This is achieved with nodes formed by four nanotubes joined together by the inclusion of heptagonal rings placed appropriately. This novel nanoporous material, named Super Diamond has, by design, tunable pore size and exhibit large free volume and surface area, which can reach the values of 95% and 2535 g/m 2 respectively. The interaction and the adsorption properties of this material with hydrogen were studied thoroughly via ab-initio and Grand Canonical Monte Carlo simulations. Our results show that a large pore Super Diamond can surpass the gravimetric capacity of 20% at 77 K and can reach the high value of 8% at room temperature.

The Journal of Physical Chemistry B, 2004

Hydrogen adsorption isotherms at 77, 87, and 298 K have been measured on three samples of single-wall carbon nanotubes. The highest adsorption capacity (1.58 wt % at 77 K, 1.15 wt % at 87 K, and 0.02 wt % at 298 K) at atmospheric pressure has been observed in a chemically activated sample (activated with KOH), which has hybrid porosity between a carbon nanotube material and a microporous activated carbon. According to CO 2 adsorption at 273 K and density functional theory pore size distributions from N 2 adsorption, it is deduced that pores up to approximately 0.5-0.7 nm can adsorb hydrogen at ambient conditions. Isosteric heat of hydrogen adsorption has been calculated for the three samples, having initial values around 7-7.5 kJ mol -1 . It is concluded that the hydrogen adsorption capacity of carbon nanotubes depends both on the extent of their surface area and on the adsorption energy of the surface sites.

Journal of Non-Crystalline Solids, 2006

Porous carbon is considered a promising material to store hydrogen. It can be visualized as a defective relaxed sample and therefore some of the methods we have developed to deal with porous silicon are presently applied to this material. Porous atomic structures with 50% porosity that, due to the size of the supercells fall within the regime of nanoporous carbon, are generated using our procedure. Two pure nanoporous samples of densities 1.75 g/cm 3 and 1.31 g/cm 3 were hydrogenated, relaxed and their total energy obtained. The hydrogenated samples were first stripped of the hydrogen atoms and their total energy obtained. Then the original samples were stripped of the carbon atoms and the total energy calculated. From these values the average energy per hydrogen atom was then deduced. We compare our results to CH bond energies; conclusions are drawn.

Carbon, 2011

H 2 adsorption in carbon nanohorns and nanocones has been simulated at 77 K using the grand canonical Monte Carlo method. The models used for the solid adsorbents were nanosized curved graphene sheets of conical shape with five different apex angles, corresponding to the introduction of 1-5 pentagons to the hexagonal carbon network; nanohorns are a subclass of the carbon cone family (5 pentagons). Hydrogen molecules have been treated as Lennard-Jones spherical particles with quantum behavior. Details of the adsorption process have been revealed by studying carefully the cone-hydrogen interactions as well as the adsorption capacities and energies, in cones of different dimensions. Additionally, for comparison purposes similar studies have been carried out for carbon nanotube model structures and the effect of pore shape/size as well as elements of the role of confinement on sorption have been highlighted.

Carbon, 2021

Our investigations into molecular hydrogen (H 2) confined in microporous carbons with different pore geometries at 77 K have provided detailed information on effects of pore shape on densification of confined H 2 at pressures up to 15 MPa. We selected three materials: a disordered, phenolic resin-based activated carbon, a graphitic carbon with slit-shaped pores (titanium carbide-derived carbon), and single-walled carbon nanotubes, all with comparable pore sizes of <1 nm. We show via a combination of in situ inelastic neutron scattering studies, high-pressure H 2 adsorption measurements, and molecular modelling that both slit-shaped and cylindrical pores with a diameter of~0.7 nm lead to significant H 2 densification compared to bulk hydrogen under the same conditions, with only subtle differences in hydrogen packing (and hence density) due to geometric constraints. While pore geometry may play some part in influencing the diffusion kinetics and packing arrangement of hydrogen molecules in pores, pore size remains the critical factor determining hydrogen storage capacities. This confirmation of the effects of pore geometry and pore size on the confinement of molecules is essential in understanding and guiding the development and scale-up of porous adsorbents that are tailored for maximising H 2 storage capacities, in particular for sustainable energy applications.

2011

Hydrogen physisorption CO 2 adsorption a b s t r a c t Hydrogen adsorption capabilities of different nanoporous carbon, i.e. amorphous carbons obtained by chemical activation (with KOH) of a sucrose-derived char previously ground by ball milling and carbon replicas of NH 4 -Y and mesocellular silica foam (MSU-F) inorganic templates, were measured and correlated to their porous properties. The porous texture of the prepared carbon materials was studied by means of N 2 and CO 2 adsorption isotherms measured at À196 C and 0 C, respectively. Comparison with nanoporous carbons obtained without pregrinding the sucrose-derived char shows that the ball milling procedure favours the formation of highly microporous carbon materials even at low KOH loadings, having a beneficial effect of the interaction between the char particles and the activating agent. Hydrogen adsorption isotherms at À196 C were measured in the 0.0e1.1 MPa pressure range, and a maximum hydrogen adsorption capacity of 3.4 wt.% was obtained for the amorphous carbon prepared by activation at 900 C with a KOH/char weight ratio of 2. Finally, a linear dependence was found between the maximum hydrogen uptake at 1.1 MPa and the samples microporous volume, confirming previous results obtained at À196 C and sub-atmospheric pressure . (B. Bonelli).

Nanotechnology, 2009

It is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities (∼80 g H 2 /kg carbon, ∼50 g H 2 /liter carbon, at 50 bar and 77 K). Nanopores generate high storage capacities (a) by having high surface area to volume ratios, and (b) by hosting deep potential wells through overlapping substrate potentials from opposite pore walls, giving rise to a binding energy nearly twice the binding energy in wide pores. Experimental case studies are presented with surface areas as high as 3100 m 2 g −1 , in which 40% of all surface sites reside in pores of width ∼0.7 nm and binding energy ∼9 kJ mol −1 , and 60% of sites in pores of width >1.0 nm and binding energy ∼5 kJ mol −1 . The findings, including the prevalence of just two distinct binding energies, are in excellent agreement with results from molecular dynamics simulations. It is also shown, from statistical mechanical models, that one can experimentally distinguish between the situation in which molecules do (mobile adsorption) and do not (localized adsorption) move parallel to the surface, how such lateral dynamics affects the hydrogen storage capacity, and how the two situations are controlled by the vibrational frequencies of adsorbed hydrogen molecules parallel and perpendicular to the surface: in the samples presented, adsorption is mobile at 293 K, and localized at 77 K. These findings make a strong case for it being possible to significantly increase hydrogen storage capacities in nanoporous carbons by suitable engineering of the nanopore space.