Giselle Sandi

Production of micro- and mesoporous activated carbon from paper mill sludge I. Effect of zinc chloride activation

Abstract A series of micro- and mesoporous activated carbons were produced from paper mill sludge using a modified carbonization methodology. N2-adsorption isotherm data and mathematical models such as the D–R equation, αs-plot, and MP and BJH methods were used to characterize the surface properties of the produced carbons. Results of the surface analysis showed that paper mill sludge can be economically and successfully converted to micro- and mesoporous activated carbons with surface areas higher than 1000 m2/g. Activated carbons with a prescribed micro- or mesoporous structure were produced by controlling the amount of zinc chloride (ZnCl2) used during chemical activation. Pore evolvement was shown to be most affected by the incremental addition of ZnCl2. Increasing the ZnCl2 to sludge ratio from 0.75 to 2.5 resulted in a 600% increase in the mesopore volume. ZnCl2 to sludge ratios less than 1 and greater than 1.5 resulted in the production of micro- and mesoporous carbons, respectively. At a ZnCl2 to sludge ratio of 3.5, an activated carbon with a predominantly (80%) mesoporous structure was produced. The calculated D–R micropore volumes for activated carbons with the suggested microporous structure were in good agreement with those obtained from the αs method, while estimated micropore volumes from the αs method deviated markedly from those obtained from the D–R equation for carbons with a predominantly mesoporous structure.

CHARACTERIZATION OF MONTMORILLONITE SURFACES AFTER MODIFICATION BY ORGANOSILANE

X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA), surface area measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to examine the surface properties of organosilane-modified smectite-type aluminosilicate clays. Organic modified clays derived from the reactions of montmorillonite (containing 93–95% montmorillonite from a bentonite, <1% quartz, and 4–6% opal CT) with octadecyltrichlorosilane (C18H37SiCl3) and octadecyltrimethoxysilane [C18H37Si(OMe)3] are highly hydrophobic. Surface loadings of the modified clays depend on the organosilane and the solvent, and they range from 10 to 25 wt. %. The organic species are probably adsorbed to the outer surfaces and bound to the edges of the clay via condensation with edge-OH groups. Encapsulation of montmorillonite with C18H37SiCl3 and C18H37Si(OMe)3 resulted in a hydrophobic coating that acts like a “cage” around the clay particles to limit diffusion. Basal spacings of the organic modified clays remain at ∼15 Å upon heating to 400°C in N2, whereas those of unmodified clays collapse to ∼10 Å. A considerable reduction in surface area (by 75–90%) for organic modified clays is observed, which is consistent with the existence of a surface coating. The solvent used can affect the amount of organic silane coated on the clay particles, whereas the difference between the products prepared using C18H37SiCl3 and C18H37Si(OMe)3 in the same solvent is relatively small. The carbon and oxygen K-edge NEXAFS spectroscopy of the modified montmorillonite surfaces showed that surface coatings on the outside of the clay particles exist. The encapsulating system may allow for economical remediation and storage of hazardous materials.

7Li NMR study of intercalated lithium in curved carbon lattices

Abstract A device was invented that permits nuclear magnetic resonance (NMR) analysis of the internal elements of a coin cell battery. The Coin Cell Battery Imager was used to record wideline 7 Li NMR spectra of the lithium ions that were electrochemically intercalated into three different types of carbon-based materials. The samples included graphite, corannulene, and carbon derived from sepiolite clay. All samples were excised from 2032-size coin cells that were cycled multiple times and left in a discharged state (i.e., fully lithiated). A comparison of the 7 Li NMR spectra recorded for the three carbons revealed that the curved carbon lattice derived from sepiolite affected the lithium resonances in a manner similar to that observed for the curved molecule corannulene, while both differed from the flat lattice of graphite. In addition, it was possible to observe lithium dendrites on the surface of a hard carbon electrode even in the presence of a large lithium counter electrode using NMR imaging techniques.

New carbon electrodes for secondary lithium batteries

The authors have synthesized and electrochemically tested carbon samples that are suitable as anodes for lithium secondary batteries. The unique synthesis is based on the use of the inorganic templates and organic precursors. Pyrolysis of these carbons was performed at 500 and 700 C. High reversible capacity (up to 825 mAh/g) was found on samples heated to 700 C. The cells were tested continuously for more than 20 cycles with a drop in capacity of only 10%. X-ray powder diffraction showed that the samples heated to 700 C have a highly disordered structures.

Determination of fractal dimensions of solid carbons from gas and liquid phase adsorption isotherms

The total surface area, micropore volume, and fractal dimensions of five different carbons (Sorbonorite 4, GAC 1240, and three amorphous carbons) were evaluated from analysis of gas (N2) and liquid (phenanthrene) adsorption isotherm data. The modified BET and fractal Frenkel–Halsey–Hill (FHH) models were used to estimate surface fractal dimensions. Micropore volumes were estimated from Dubinin–Radushkevich (DR) plots and were compared to those calculated from standard N2 adsorption isotherm data using de Boer’s t-method. The estimated surface fractal dimensions using the modified BET and FHH models (DS=3+3h, and P/P0 from 0.0 to 0.4) were (2.7, 2.6, 2.1, 2.4, and 2.1) and (2.5, 2.6, 1.9, 2.4, and 1.9), respectively. The FHH fractal analysis suggested that van der Waals forces are the dominant interaction forces between nitrogen and carbon surfaces. Depending on the method of analysis, the fractal dimensions of the carbons with suggested micropore structure, Sorbonorite 4 and GAC 1240, were 2.5–2.9 and 2.6–2.9, respectively. Analysis of the adsorption–desorption data suggested that amorphous carbons with fractal dimensions of 2.1 (from the modified BET model) have smooth surfaces, with respect to their micropore structure. Further analysis of the adsorption data showed that the slopes of the linear segment of the plots of adsorption potential versus relative amount adsorbed are dependent on the pore size range and surface structure (fractal dimension) of the carbons.

Computational Modeling of Ionic Transport in Continuous and Batch Electrodialysis

ABSTRACT We describe the transport of ions and dissociation of a single salt and a solvent solution in an electrodialysis (ED) stack. An ED stack basic unit is made of a two-compartment cell: dilute and concentrate. We use the fundamental principles of electrochemistry, transport phenomena, and thermodynamics to describe mechanisms and to predict the performance of the ED process. We propose and analyze three model formulations for a single salt (KC1). The first and the second models are for a one- and two-dimensional continuous ED, and the third examines batch ED. The models include the effect of the superficial velocity in the boundary layer near the ion-exchange membranes. We examine the diffusion and electro- migration of ions in the polarization region and consider electromigration and convection in the bulk region. We show that the ionic surface concentration of both membranes in the dilute compartment is affected by two parameters: flow rate and current density. We also show that in the dilute compartment, concentration changes along the x-axis are greater than along the y-axis because the ionic flux along the x-axis is greater and is in the direction of the current. In simulations where the KC1 dilute concentration ranges from 200 to 500 mol m-3 with a constant concentrate concentration, the dilute voltage drop accounts for more than 36% of the total voltage drop. This value is reduced to 7% as the dilute concentration increases and contributions of both ion-exchange membrane drops account for more than 50% of the total cell voltage drop. All the three models were validated experimentally.

Carbons for lithium battery applications prepared using sepiolite as an inorganic template

Carbon anodes for Li-ion cells were prepared by the in situ polymerization of olefins such as propylene and ethylene in the channels of a sepiolite clay mineral. Upon dissolution of the inorganic framework, a disordered carbon was obtained. The carbon was tested as an anode in coin cells, yielding an average reversible capacity of 633 mAh/g discounting the first cycle, which is 1.70 times higher than the capacity delivered by graphitic carbon assuming 100% efficiency. The coulombic efficiency was higher than 90%. Morphologies of the clay, carbon/clay composite, and final carbon were examined by TEM. The structure of the carbon and its electrochemical performance were monitored in situ by small angle x-ray scattering.

Hydrogen Storage Based on Physisorption

Physisorption of molecular hydrogen based on neutral and negatively charged aromatic molecular systems has been evaluated using ab initio calculations to estimate the binding energy, DeltaH, and DeltaG at 298 ( approximately 77 bar) and 77 K (45 bar) in order to compare calculated results with experimental measurements of hydrogen adsorption. The molecular systems used in this study were corannulene (C(20)H(10)), dicyclopenta[def,jkl]triphenylene (C(20)H(10)), 5,8-dioxo-5,8-dihydroindeno[2,1-c]fluorene (C(20)H(10)O(2)), 6-hexyl-5,8-dioxo-5,8-dihydroindeno[2,1-c]fluorene (C(26)H(22)O(2)), coronene (C(24)H(12)), dilithium phthalocyanine (Li(2)Pc, C(32)H(16)Li(2)N(8)), tetrabutylammonium lithium phthalocyanine (TBA-LiPc, C(48)H(52)LiN(9)), and tetramethylammonium lithium phthalocyanine (TMA-LiPc, C(36)H(28)LiN(9)). It was found (a) that the calculated term that corrects 0 K electronic energies to give Gibbs energies (thermal correction to Gibbs energy, TCGE) serves as a good approximation of the adsorbent binding energy required in order for a physisorption process to be thermodynamically allowed and (b) that the binding energy for neutral aromatic molecules varies as a function of curvature (e.g., corannulene versus coronene) or if electron-withdrawing or -donating groups are part of the adsorbent. A negatively charged aromatic ring, the lithium phthalocyanine complex anion, [LiPc](-), introduces charge-induced dipole interactions into the adsorption process, resulting in a doubling of the binding energy of Li(2)Pc relative to corannulene. Experimental hydrogen adsorption results for Li(2)Pc, which are consistent with MD simulation results using chi-Li(2)Pc to simulate the adsorbent, suggest that only one side of the phthalocyanine ring is used in the adsorption process. The introduction of a tetrabutylammonium cation as a replacement for one lithium ion in Li(2)Pc has the effect of increasing the number of hydrogen molecules adsorbed from 10 (3.80 wt %) for Li(2)Pc to 24 (5.93 wt %) at 77 K and 45 bar, suggesting that both sides of the phthalocyanine ring are available for hydrogen adsorption. MD simulations of layered tetramethylammonium lithium phthalocyanine molecular systems illustrate that doubling the wt % H(2) adsorbed is possible via such a system. Ab initio calculations also suggest that layered or sandwich structures can result in significant reductions in the pressure required for hydrogen adsorption.

Efficient simultaneous reverse Monte Carlo modeling of pair-distribution functions and extended x-ray-absorption fine structure spectra of crystalline disordered materials.

An efficient implementation of simultaneous reverse Monte Carlo (RMC) modeling of pair distribution function (PDF) and EXAFS spectra is reported. This implementation is an extension of the technique established by Krayzman et al. [J. Appl. Cryst. 42, 867 (2009)] in the sense that it enables simultaneous real-space fitting of x-ray PDF with accurate treatment of Q-dependence of the scattering cross-sections and EXAFS with multiple photoelectron scattering included. The extension also allows for atom swaps during EXAFS fits thereby enabling modeling the effects of chemical disorder, such as migrating atoms and vacancies. Significant acceleration of EXAFS computation is achieved via discretization of effective path lengths and subsequent reduction of operation counts. The validity and accuracy of the approach is illustrated on small atomic clusters and on 5500-9000 atom models of bcc-Fe and α-Fe(2)O(3). The accuracy gains of combined simultaneous EXAFS and PDF fits are pointed out against PDF-only and EXAFS-only RMC fits. Our modeling approach may be widely used in PDF and EXAFS based investigations of disordered materials.

Layered carbon lattices and their influence on the nature of lithium bonding in lithium intercalated carbon anodes

Abstract Ab initio molecular orbital calculations have been used to investigate the nature of lithium bonding in stage 1 lithium intercalated carbon anodes. This has been approximated by using layered carbon lattices such as coronene, (C 24 H 12 ), anthracene, and anthracene substituted with boron. With two coronene carbon lattices forming a sandwich structure and intercalated with either two, three, four or six lithiums, it has been found that the predominant mode of bonding for the lithium is at the carbon edge sites as opposed to bonding at interior carbon hexagon sites. With a single planar coronene molecule approximating a graphene sheet, the bonding of four lithiums with this molecule is near the interior carbon hexagon sites. For anthracene and boron substituted anthracene, lithium bonding takes place within the carbon hexagon sites. The separation between lithiums in a sandwich type structure with two anthracenes in the eclipsed conformation is 5.36 A. The effect of boron substitution is to increases lattice flexibility by allowing the lattice to twist and lithium to bond at adjacent hexagon sites.