Cristelle Portet

Relation between the Ion Size and Pore Size for an Electric Double-Layer Capacitor

The research on electrochemical double layer capacitors (EDLC), also known as supercapacitors or ultracapacitors, is quickly expanding because their power delivery performance fills the gap between dielectric capacitors and traditional batteries. However, many fundamental questions, such as the relations between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance have not yet been fully answered. We show that the pore size leading to the maximum double-layer capacitance of a TiC-derived carbon electrode in a solvent-free ethyl-methylimmidazolium-bis(trifluoro-methane-sulfonyl)imide (EMI-TFSI) ionic liquid is roughly equal to the ion size (approximately 0.7 nm). The capacitance values of TiC-CDC produced at 500 degrees C are more than 160 F/g and 85 F/cm(3) at 60 degrees C, while standard activated carbons with larger pores and a broader pore size distribution present capacitance values lower than 100 F/g and 50 F/cm(3) in ionic liquids. A significant drop in capacitance has been observed in pores that were larger or smaller than the ion size by just an angstrom, suggesting that the pore size must be tuned with sub-angstrom accuracy when selecting a carbon/ion couple. This work suggests a general approach to EDLC design leading to the maximum energy density, which has been now proved for both solvated organic salts and solvent-free liquid electrolytes.

Effect of Carbon Particle Size on Electrochemical Performance of EDLC

The effect of particle size on the electrochemical performance of electrical double-layer capacitors (EDLCs) has been studied using carbon derived from silicon carbide powders with 20 nm to 20 μm grains at temperatures from 800 to 1200°C. For the same synthesis temperature, similar pore texture and microstructure of carbide-derived carbons produced from different powders have been observed. Nanoparticles exhibited a slight porosity modification with a larger pore volume (1.8 cc/g) and surface area (1300 m 2 /g) as compared to micrometer particles (0.4 cc/g and 1100 m 2 /g). Capacitances as high as 135 F/g associated with a small resistance and time constant have been reached for nano- and sub-micrometer particles synthesized at low temperatures and tested in a tetraethylammonium tetrafluoroborate in acetonitrile solution. This result suggests that small particles facilitate the migration of the ions inside the porous electrodes, allowing them to reach the whole pore volume due to a shorter transport distance within the particle.

Electrical Double-Layer Capacitance of Zeolite-Templated Carbon in Organic Electrolyte

Downlo Electrical Double-Layer Capacitance of Zeolite-Templated Carbon in Organic Electrolyte C. Portet,* Z. Yang, Y. Korenblit, Y. Gogotsi,** R. Mokaya, and G. Yushin A. J. Drexel Nanotechnology Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

Curvature effects in carbon nanomaterials: Exohedral versus endohedral supercapacitors

Capacitive energy storage mechanisms in nanoporous carbon supercapacitors hinge on endohedral interactions in carbon materials with macro-, meso-, and micropores that have negative surface curvature. In this article, we show that because of the positive curvature found in zero-dimensional carbon onions or one-dimensional carbon nanotube arrays, exohedral interactions cause the normalized capacitance to increase with decreasing particle size or tube diameter, in sharp contrast to the behavior of nanoporous carbon materials. This finding is in good agreement with the trend of recent experimental data. Our analysis suggests that electrical energy storage can be improved by exploiting the highly curved surfaces of carbon nanotube arrays with diameters on the order of 1 nm.

Nitrogen modified carbide-derived carbons as adsorbents of hydrogen sulfide

Carbide-derived carbons produced from titanium carbide at temperatures from 600 degrees C to 1000 degrees C and exhibiting different porosities were treated with urea in order to introduce nitrogen containing species to their surface. Adsorption of hydrogen sulfide in the dynamic conditions in the presence of moisture was studied on initial and modified samples. The samples, before and after exposure to hydrogen sulfide, were characterized using adsorption of nitrogen, potentiometric titration, elemental analysis, and thermal analysis. The results showed that the introduction of nitrogen significantly enhances the performance of carbons in the process of hydrogen sulfide removal. The amount adsorbed and the degree of oxidation depended on the porosity. On the samples with very small pores, the adsorption was limited, probably owing to the sterical hindrances. With an increase in the size and volume of micropores, in which water and hydrogen sulfide can be accommodated, the efficiency of H(2)S removal by CDC increased.

High Temperature Functionalization and Surface Modification of Nanodiamond Powders

High temperature annealing in vacuum, air, hydrogen, chlorine, and ammonia are described as a means to change surface chemistry and phase composition of nanodiamond powders of three different grades, which have different sp 2 /sp 3 carbon ratios. The changes in surface chemistry and phase composition of the powders are analyzed using Raman spectroscopy and Fourier Transform Infra Red (FTIR) spectroscopy. Advantages and limitation of high-temperature treatment techniques as well as potential applications of the gas-treated nanodiamond powders are discussed.