L. Firlej

Nanospace engineering of KOH activated carbon

This paper demonstrates that nanospace engineering of KOH activated carbon is possible by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. High specific surface areas, porosities, sub-nanometer (<1 nm) and supra-nanometer (1-5 nm) pore volumes are quantitatively controlled by a combination of KOH concentration and activation temperature. The process typically leads to a bimodal pore size distribution, with a large, approximately constant number of sub-nanometer pores and a variable number of supra-nanometer pores. We show how to control the number of supra-nanometer pores in a manner not achieved previously by chemical activation. The chemical mechanism underlying this control is studied by following the evolution of elemental composition, specific surface area, porosity, and pore size distribution during KOH activation and preceding H(3)PO(4) activation. The oxygen, nitrogen, and hydrogen contents decrease during successive activation steps, creating a nanoporous carbon network with a porosity and surface area controllable for various applications, including gas storage. The formation of tunable sub-nanometer and supra-nanometer pores is validated by sub-critical nitrogen adsorption. Surface functional groups of KOH activated carbon are studied by microscopic infrared spectroscopy.

Hypothetical High-Surface-Area Carbons with Exceptional Hydrogen Storage Capacities: Open Carbon Frameworks

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 m(2)/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 m(2)/g, we found record maximum excess adsorption of 75-85 g of H(2)/kg of C at 77 K and record storage capacity of 100-260 g of H(2)/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.

Enhanced hydrogen adsorption in boron substituted carbon nanospaces

Activated carbons are one of promising groups of materials for reversible storage of hydrogen by physisorption. However, the heat of hydrogen adsorption in such materials is relatively low, in the range of about 4-8 kJ/mol, which limits the total amount of hydrogen adsorbed at P=100 bar to approximately 2 wt % at room temperature and approximately 8 wt % at 77 K. To improve the sorption characteristics the adsorbing surfaces must be modified either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. In this letter we present ab initio calculations and Monte Carlo simulations showing that substitution of 5%-10% of atoms in a nanoporous carbon by boron atoms results in significant increases in the adsorption energy (up to 10-13.5 kJ/mol) and storage capacity ( approximately 5 wt % at 298 K, 100 bar) with a 97% delivery rate.

Spectroscopic studies of poly(4,4′-dialkyl-2,2′-bithiophenes) — the ‘head-to-head’ analogues of poly(3-alkylthiophenes)

Abstract Poly(4,4′-dialkyl-2,2′-bithiophenes) prepared by chemical polymerizatiok of 4,4′-dialkyl-2,2′-bithiophenes show highly regular structure. Due to different coupling patterns (equivalent to ‘head-to-head’ and ‘tail-to-tail’ coupled rings), their spectroscopic features are different to corresponding, mainly ‘head-to-tail’ coupled poly(3-alkylthiophenes). In particular, their π−π ∗ transition occurs at much shorter wavelengths due to lower conjugation caused by close vicinity of alkyl chains in adjacent thiophene rings. On the basis of the comparative investigation of poly(4,4′-dialkyl-2,2′-bithiophenes) and poly(3-alkylthiophenes) we discuss 13C solid state NMR and FT-IR spectra.

Understanding universal adsorption limits for hydrogen storage in nano porous systems.

Despite of more than 15 years of research, no materials possess the adsorbing properties required for mobile storage. At this time of state-of-the-art technology, the essential question should be asked: why is it so difficult to prepare a material with the desired properties? Here, we discuss the sources of physical limitations of existing materials and indicate the directions for further material research.

Monte Carlo simulations of krypton adsorption in nanopores: Influence of pore-wall heterogeneity on the adsorption mechanism

We present molecular simulation results of the adsorption of krypton in a model of the mesoporous material MCM-41. The adsorption isotherm and adsorption enthalpies are studied at 77 K. Comparison of the experimental and simulation data allows us to analyze how the available interaction models (Kr–Kr and Kr–walls) are able to reproduce the experimental situation. The role of the heterogeneous interactions versus the homogenous model is studied and compared with the previous simulation results of nitrogen adsorption in MCM-41. The results show that a model of ideal cylindrical pores gives qualitatively and quantitatively different results. A distribution of the adsorption sites must exist to explain the loading at low pressure (below capillary condensation). Such a distribution in MCM-41 is a consequence of inhomogenous walls that contain a wide variety of attractive sites ranging from weakly attractive silica-type to highly attractive regions. In our simulations the MCM-41 structure is modeled as an amorp...

Microscopic mechanism of adsorption in cylindrical nanopores with heterogenous wall structure.

We study the microscopic mechanism of adsorption in nanometric cylindrical pores with strongly heterogeneous walls using grand canonical Monte Carlo simulations. The pore surface structure is modeled by a new lattice-site approach. Each site is characterized by two amplitudes--structural and energetic--that locally modify the structural and energetic properties of the surface. The amplitudes are randomly distributed over the pore wall. We have shown that different structural and energetic distribution functions lead to different mechanism of adsorption. The energetic site distribution plays the most crucial role in the submonolayer region. The structural site distribution modifies the multilayer adsorption. A method to analyze the stability of the adsorbed system using static susceptibility is proposed. Potential applications in multiscale modeling are discussed.

Properties of conductive polycarbonate films reticulate doped with MPht(TCNQ)2 and PrPht(TCNQ)2 salts: A highly-conductive form of PrPht-TCNQ by crystallization in a polymer matrix

Abstract Conductive polymer films obtained by reticulate doping with N -methyl phthalazinum (MPht) and N -propyl phthalazinum (PrPht) TCNQ complex salts are investigated. It is found that the spectral, electrical and magnetic properties of the MPht(TCNQ) 2 in a polymer matrix are not significantly different from the properties of single crystals, while in the case of PrPht a new highly-conductive form is obtained by rapid crystallization during film casting. It is concluded that the dimerization of the TCNQ stacks in the new form is not as strong but the disorder is higher as compared with single crystals of PrPht(TCNQ) 2 . It is possible that the stoichiometry is also different.

Melting of Hexane Monolayers Adsorbed on Graphite: The Role of Domains and Defect Formation

We present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduce all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) a fully atomistic representation of hexane; (ii) an explicit site-by-site interaction with carbon atoms in graphite; (iii) the CHARMM force field with carefully chosen adjustable parameters of nonbonded interaction, and (iv) numerous >or=100 ns runs, requiring a total computation time of ca. 10 CPU years. The exhaustive studies have allowed us to determine the mechanism of the transition: proliferation of small domains through molecular reorientation within lamellae and without perturbation of the overall adsorbed film structure. At temperatures greater than that of melting, the system exhibits dynamically reorienting domains whose orientations reflect the graphite substrates symmetry and whose size decrease with increasing temperature.

Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite: an explicit hydrogen molecular dynamics study.

We discuss molecular dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes into a rectangularly centered structure that is not commensurate with underlying graphite. Above T=200 K, as the molecules start to lose their translational and orientational order via gauche defect formation a weak smectic mesophase (observed experimentally but never reproduced in united atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as for a UA force field), then the melting temperature increases by approximately 70 K with respect to the experimental value. When the nonbonded 1-4 interaction is not scaled, the melting temperature decreases by approximately 90 K. If the scaling factor is set to 0.5, then melting occurs at T=350 K, in very good agreement with experimental data.