2001: ELECTRIC DOUBLE LAYER CAPACITANCE OF POROUS CARBON DERIVED FROM PTFE WITH Li METAL

Recently, electric double layer capacitor (EDLC) with high capacitance [1] has been proposed as a sub-power source for a pure electric vehicle (PEV) or a hybrid electric vehicle (HEV) because power density of EDLC is high enough to use as the power source at start-up of the electric vehicles. However, since energy density of EDLC is quite lower than that of rechargeable batteries, the double layer capacitance must be increased considerably in order to apply EDLC to PEV or HEV. The high capacitance of EDLC is related to high specific surface area of activated carbon electrode. The high surface area is derived from many micropores. Therefore, the improvement of the capacitance has been conducted with increasing the surface area and modification of the pore size distribution. However, it seems to be difficult to improve the low energy density of EDLC using activated carbon electrode because the double layer capacitance is not always proportional to specific surface area of the activated carbons [2-5]. As one of the reasons for this, the effective surface area, which is an available surface area of micropores to form the electric double layer, is quite low due to low permeability of the electrolyte into micropores. Generally, activated carbons are prepared by activation process such as gasification reaction of carbon. So, new porous carbon materials prepared by the other processes except the activation can be candidates of porous carbon electrodes with high capacitances. On the other hand, Jansta and Dousek reported in 1980 that porous carbon could be prepared by defluorination of polytetrafluoroethylene (PTFE) with Li-amalgam [6]. From this literature, the material was microporous and its BET surface area was higher than 2500 m g [6]. Therefore, adsorption behavior and electrochemistry of the porous carbon derived from PTFE are very interesting. This carbon is expected to be used practically as an EDLC electrode, a lithium ion battery anode, a storage medium for methane or hydrogen, or an adsorbent of toxic substance. However, the possibility has never been clarified yet. Recently, our group also found that highly porous carbons for EDLC can be prepared by defluorination of PTFE with lithium metal or lithium naphthalenide (radical anion) in ether solution. In our previous paper [4], we already reported that the porous carbons prepared from PTFE with Li metal showed high specific surface area and large capacitance in dilute H 2 SO 4 aqueous electrolyte. In this paper, pore structure of the porous carbon derived from PTFE with radical anion (referred to as PCPTFEr) and its double layer capacitance in nonaqueous electrolyte is discussed based on the comparison with those of activated carbon fiber (ACF) prepared from phenol resin with steam activation.