Highly porous activated glassy carbon film sandwich structure for electrochemical energy storage in ultracapacitor appli
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Soenke Seifert and Jan Ilavsky Argonne National Laboratory, Advanced Photon Source, Argonne, Illinois 60439 (Received 15 December 2009; accepted 14 April 2010)
Glassy carbon plates were thermochemically gas phase oxidized to obtain monolithic sandwichlike electrode assemblies with high surface area porous films for electrochemical energy storage applications. Film thicknesses were varied by variation of oxidation parameters time, temperature, and oxygen concentration and measured with electron microscopy. The mass density of the porous carbon film material was estimated by fitting a geometrical model to experimental gravimetric data. Optical Raman spectroscopy line scans suggest that the porosity has a gradient between the surface and the film/bulk interface, which is supported by pore-size distribution data obtained from small-angle x-ray scattering (SAXS) on slightly oxidized and fully oxidized samples. Detailed inspection of the power law behavior of SAXS data suggests that the internal surface area of well-oxidized glassy carbon (GC) is compact and extends over the entire probed volume and thus has optimal pore connectivity. This effect goes along with pore enlargement and a relative decrease of internal surface area per volume. Slightly oxidized carbon has no pore space with a compact, high connectivity internal surface area. The corresponding SAXS power law and the x-ray density suggest that this high volumetric surface area must be interpreted as a result of surface roughness, rather than true geometric or volumetric surface area. In consequence, this surface area is of limited use for electrochemical energy storage.
I. INTRODUCTION
High surface area carbons are important electrode materials in electrochemical energy storage, including battery, capacitor, and fuel-cell applications. Glassy carbon (GC) is a solid and monolithic form of a hard carbon with a high number density of nanosized voids, which are not connected.1,2 GC is a good diffusion barrier even to helium. Generation of high surface area in carbon is called activation. One unique property of GC is that it comes in monolithic form. Thin sheets make it suitable for microdevice applications. Activation of glassy carbon plates and sheets by gas phase or electrochemical oxidative treatment can be carried out in a controlled way, with theoretically well-understood kinetics and experimentally well-mastered accuracy.3,4 Oxidative treatment5 can open and connect these voids and create an active a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0197
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http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 8, Aug 2010 Downloaded: 28 Jan 2015
carbon film with a spongelike structure and open porosity on the surface of the monolith, with internal surface areas exceeding 1000 m2/g.6–10 Under heat in the furnace, oxygen attacks the GC surface, preferentially burns away carbon atoms which are in a fourfold atomic coordination, rather than in the threefold coordination of graphene sheets, leaving b
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