Preparation of Furfural Resin-Based Active Carbon with Acid Treated Pore Surface Electric Double Layer Capacitor

Abstract

Demand for high capacity and high power energy saving devices is recently increasing for environmental conservation and energy saving. Electric double layer capacitor (EDLC) is a capacitor utilizing an electric double layer generated at the interface between an activated carbon electrode and an electrolyte. While high power and high durability are attractive, low energy density remained as a crucial issue. We reported that EDLC capacity increased as the amount of acidic surface functional groups increases. In this study, we employed mixed-acid treatment and KMnO4 treatment to introduce acidic surface functional groups on active carbon and investigated the relationship between physicochemical properties and electrical properties.Furfural resin-based particles (1 µm in diameter) were carburized in a nitrogen atmosphere for 3 h at a temperature ramping rate of 1°C/min to 400°C. The carbonized particles were mixed with KOH at a KOH : carbonized particles ratio of 4:1; the mixture was then activated at 800°C under flowing nitrogen gas at a temperature ramping rate of 10°C/min. After heat treatment had been performed, the active carbon was impregnated with mixed acid and KMnO4. Specific surface area/pore size distribution was evaluated by N2 adsorption method. Surface functional groups were evaluated by Boehm method. Active carbon, carbon black and polytetrafluoroethylene were mixed at a mass ratio of 8: 1: 1 (=0.08 g:0.01 g:0.01 g), then pellets (14 mm in diameter and 2 mm in thickness) were prepared and dried at 115ºC for 24 hours to obtain activated carbon electrodes. Constant current charge/discharge test (potential width: 0-1.0 V, current density: 20-500 mA/g) and was performed at room temperature by two-electrode cell using KOH aqueous solution (6 M) as an electrolytic solution.Table 1 shows the pore structure and specific capacity per weight at 20 mA/g of active carbon treated mixed-acid treatment and KMnO4 treatment. Due to acid treatment, the surface structure was destroyed and the specific surface area decreased in the following order: Control (1291 m2/g) > Mixed-acid (883 m2/g) > KMnO4 (793 m2/g) KMnO4 & Mixed-acid (358 m2/g). Although acid treatment result in a smaller specific surface area (883, 793, 358 m2/g) than that of control (1291 m2/g), they had higher specific capacity per weight (160, 219, 149 F/g) than that of no acid treatment (111 F/g). The improvement in wettability or pseudo-capacity due to redox reaction was a possible reason why the high the specific capacity per weight and high rate characteristics were obtained by the acid treatment. Figure 1