A Study on Oxidized Glassy Carbon sheets for Bipolar Supercapacitor Electrodes
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ABSTRACT Electrochemical Double Layer Capacitors (EDLC) for high energy and power density applications, based on glassy carbon (GO) electrodes, are being developed in our laboratory. In the context of this project, GC sheets were oxidized and investigated with Small Angle X-ray Scattering (SAXS), Electrochemical Impedance Spectroscopy (EIS) and Nitrogen Gas Adsorption (BET). During oxidation an active film with open pores is built on the surface of the GC. Upon oxidation, the internal volumetric surface area of the active film decreases, whereas the volumetric electrochemical double layer capacitance increases. We show that this effect is correlated with the opening, the growth and the coalescence of the pores.
INTRODUCTION There exists a number of promising electrode materials for EDLC, such as high surface area carbons (fibers, foams, aerogels, composites, glassy carbon), doped conducting polymers (polyaniline) and mixed metal oxides (ruthenium, iridium and tantalum oxides and, recently, perovskites) [11. The most important material properties of EDLC electrodes are the specific capacitance (F/g) and the electronic and ionic resitivity (flcm), which eventually determine the specific energy and the specific power of the capacitor. Electric energy storage and conversion devices are frequently classified in a so-called Ragone diagram, which relates the specific power of devices (such as batteries, capacitors and fuel cells) with their specific energy. Batteries store energy chemically by redox reactions and have a very high energy density. Their power density, however, is rather poor, because the chemical reactions to release the energy are rather slow and represent a particular resistance in the electrochemical system. In contrast to batteries, conventional dielectric capacitors have a very small energy density. However, they can release their stored energy very rapidly, and therefore, they exhibit a very high power density. Fig. 1 displays the specific power and energy of various currently available devices. The EDLC fill in the gap between the capacitors and batteries. We focus on EDLC based on Glassy Carbon (GO) electrodes with a high power density. GC can be prepared by pyrolysis of polymers (phenolic resins or furfuryl alcohol) under inert atmosphere at heat treatment temperatures (HTT) between 6000C and 30000C [2]. When monolithic GC plates are thermochemically oxidized, a sandwichlike structure with a porous film on top and bottom of the non-oxidized GC is obtained. While the porous film on each side can be filled with electrolyte and therefore is ion conducting, the non-oxidized part in between is only electronically conducting and acts as a current collector. Such samples can be utilized as monolithic bipolar
electrodes for EDLC [3]. GC is well suited as an electrode material for EDLC for several reasons: (i) Activated GC has a high internal surface area accessible for liquid electrolytes and therefore a correspondingly high double layer capacitance [1]; (ii) GC has a good electronic conductivity (200 S/
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