In recent years, a great emphasis has been placed on replacing fossil fuels with clean, renewable energy for use in vehicles. One potential solution is the use of hydrogen gas as a fuel source to power a fuel cell. For vehicular use, the US Department of Energy (DOE) has identified major challenges to implementing a hydrogen-powered which include design hydrogen storage systems capable of delivering a driving range of hundreds of kilometers. Mechanism of hydrogen adsorption in carbon porous structures is a fundamental problem for these applications. It can be shown that it is not possible to increase hydrogen storage capacity only by modification of slit geometry without simultaneous increase of the specific surface. So, we have introduced structures with higher surfaces and analyzed their adsorption properties. These new models of hypothetical structures represent ordered carbon structure with low density architecture required for effective application of porous carbons for mobile storage. We call them Open Carbon Frameworks (OCF). Theoretically they may have the specific surfaces exceeding 6000 m2/g.

Properties of a new class of hypothetical high-surface-area porous carbons (open carbon frameworks) have been discussed. The limits of hydrogen adsorption in these carbon porous structures have been analyzed in terms of competition between increasing surface accessible for adsorption and the lowering energy of adsorption. From an analysis of an analytical model and simulations of adsorption the physical limits of hydrogen adsorption have been defined: (i) higher storage capacities in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets into nano-metric elements which creates surface areas in excess of 2600 m2/g, the surface area for infinite graphene sheets; (ii) the positive influence of increasing surface area is compensated by the decreasing energy of adsorption in the carbon scaffolds of nano-metric sizes; (iii) for open carbon frameworks (OCF) built from coronene and benzene molecules with surface areas 6500 m2 g-1, we find an impressive excess adsorption of 75-110 g H2/kg C at 77 K, and high storage capacity of 110-150 g H2/kg C at 77 K and 100 bar; (iv) the new OCF, if synthesized and optimized, could lead to required hydrogen storage capacity for mobile applications.

A class of high-surface-area carbon hypothetical structures has been investigated that goes beyond the traditional model of parallel graphene sheets hosting layers of physisorbed hydrogen in slit-shaped pores of variable width. The investigation focuses on structures with locally planar units (unbounded or bounded fragments of graphene sheets), and variable ratios of in-plane to edge atoms. Adsorption of molecular hydrogen on these structures was studied by performing grand canonical Monte Carlo simulations with appropriately chosen adsorbent-adsorbate interaction potentials. The interaction models were tested by comparing simulated adsorption isotherms with experimental isotherms on a high-performance activated carbon with well-defined pore structure (approximately bimodal pore-size distribution), and remarkable agreement between computed and experimental isotherms was obtained, both for gravimetric excess adsorption and for gravimetric storage capacity. From this analysis and the simulations performed on the new structures, a rich spectrum of relationships between structural characteristics of carbons and ensuing hydrogen adsorption (structure-function relationships) emerges: (i) Storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets, which creates surface areas exceeding of 2600 m2/g, the maximum surface area for infinite graphene sheets, carried mainly by edge sites; we call the resulting structures open carbon frameworks (OCF). (ii) For OCFs with a ratio of in-plane to edge sites ≈1 and surface areas 3800-6500 m2/g, we found record maximum excess adsorption of 75-85 g of H2/kg of C at 77 K and record storage capacity of 100-260 g of H2/kg of C at 77 K and 100 bar. (iii) The adsorption in structures having large specific surface area built from small polycyclic aromatic hydrocarbons cannot be further increased because their energy of adsorption is low. (iv) Additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. We conclude that OCF structures, if synthesized, will give hydrogen uptake at the level required for mobile applications. The conclusions define the physical limits of hydrogen adsorption in carbon-based porous structures

In recent years, a great emphasis has been placed on replacing fossil fuels with clean, re-newable energy for use in vehicles. One potential solution is the use of hydrogen gas as a fuel source to power a fuel cell. For vehicular use, the US Department of Energy (DOE) has identified three major challenges to implementing a hydrogen-powered solution: (i) hydrogen production costs must be substantially lowered, (ii) fuel-cell costs should be reduced and (iii) hydrogen storage systems capable of delivering a driving range of hundreds of kilometers, without major detrimental effects to vehicle cost, safety, or cargo capacity must be developed. Hydrogen adsorption in carbon porous structures is a fundamental problem for these applications. Although many porous carbons exist in the nature, or have been prepared during recent decades, most of them show hydrogen storage capacity that is too low for practical applications.It has been shown that it is not possible to increase hydrogen storage capacity only by modification of pore geometry without simultaneous increase of the material specific surface. So, we have conceived graphene-based structures with high surfaces and analyzed their adsorption properties. These new models of hypothetical activated carbons have low density architecture required for effective storage media in mobile applications. We call them Open Carbon Frameworks (OCF). Theoretically they may have the specific surfaces exceeding 6000 m2/g. The question arises: Is it possible to prepare a porous carbon structure that will have properties required for hydrogen adsorption? Our discussion is focused on hypothetical model materials that have not been yet practically synthesized. It shows a vision and one possible way of searching for new porous carbon-based structures with properties required for storage applications. From such perspective it is interesting to discuss also the recent exceptional predictions of enhanced storage capacity of Covalent Organic Frameworks (COF’s) materials and Porous Aromatic Frameworks (PAF’s).

Open Carbon Frameworks (OCF) - new hypothetical ordered carbons

New Open Carbon Frameworks(nano-porous carbons with high surface area): Theoretical vision.

Numerical estimation of hydrogen storage in activated carbon