Novel Nanoporous Carbon Derived from Coal Tar Pitch/polyethylene Glycol Diacid Blends as Electrodes for Ultracapacitors
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Novel Nanoporous Carbon Derived from Coal Tar Pitch/Polyethylene Glycol Diacid Blends as Electrodes for Ultracapacitors Ramakrishnan Rajagopalan1, Keith Perez2, and Henry Foley2 1 Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802 2 The Pennsylvania State University, University Park, PA, 16802
ABSTRACT Nanoporous carbon materials with high surface area (1500 – 2000 m2/g) and narrow pore size distribution ranging from 1 – 3 nm were synthesized using polyfurfuryl alcohol/polyethylene glycol diacid and coal tar pitch/polymer blends. Electrical double layer capacitance of synthesized carbon was measured using cyclic voltammetry. There is a strong correlation between the surface area of the carbon and the specific capacitance. Carbon that had surface area smaller than 1000 m2/g had specific capacitance less than 50 F/g while the carbons having surface area from 1000 – 1500 m2/g showed specific capacitances in the order of 200 -250 F/g. It was shown that the mesoporosity and macroporosity in the parent carbon are critical for both activation and as well as the specific capacitance of the material. The use of these carbons in EDLCs was also demonstrated by fabricating a two-electrode ultracapacitor. INTRODUCTION Porous carbon research has also fueled interest in some recent applications like electrochemical capacitors and batteries. Microporous carbon materials possess huge surface area in the order of 600 – 3000 m2/g. This property makes them excellent candidates for electrode materials for double layer electrochemical capacitors. The specific capacitance of carbon is about 20 µF/cm2 and thus it is possible to achieve double layer capacitance of 175 – 200 F/g with these materials depending upon the surface area of the carbon. Another important feature that determines the capacitance is the accessibility of the pores by the electrolyte ions. This is determined by the pore size of the carbon. Typically, pores less than 1 – 2 nm are not accessible by the electrolyte ions, on the other hand, pores greater than 5 nm leads to loss of surface area and density [1]. In addition to this, capacitance of carbon-based capacitor are also affected by conductivity of carbon-binder composite, blockage of carbon pores by the binder, thickness of the composite, electrolyte characteristics and their properties. As a result, charge accumulated in a practical capacitor varies widely from one activated carbon to another. The proper selection of carbonaceous precursor is thus very important for further electric double layer capacitors (EDLC) development. Several different forms of carbon such as carbon black, carbon aerogels, glassy carbon and carbon fibers have been investigated as potential electrodes for ultracapacitors [2]. Specific capacitances as high as 100 – 250 F/g can be achieved when these carbons are activated thermally using carbon dioxide or chemically using sodium or potassium hydroxide. However, the volumetric capacitances of most of these carbons are low. In terms of high v
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