Physical Properties of Furfural Resin-Based Active Carbon for Improved Electric Double Layer Capacitor

Abstract

Specific surface area, pore size distribution and surface functional group of carbonaceous material for electric double layer capacitor (EDLC) are controlling factors to improve energy density. High specific surface area would lead to high specific capacitance, however some micropores (smaller than 2 nm) may be inaccessible to electrolyte ions due to the narrow pore gate effect at the pore entrance, thereby resulting in specific capacity reduction. Surface functional groups also affect the wettability of the activated carbon electrode and pseudo-capacity was yielded due to redox reaction. The relationships between capacity and multiple factors (i.e., specific surface area, pore size distribution, and surface functional groups) are still unclear. In this study, the specific surface area was fixed at 1200 ± 100 m2/g to eliminate the influence of specific surface area on specific capacity. The influences of pore size distribution and surface functional groups on specific capacity were then evaluated.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 700–800°C for the prescribed holding time, ranging from 0 to 0.5 h, under flowing nitrogen gas at a temperature ramping rate of 10°C/min. After heat treatment had been performed, the active carbon was cooled to room temperature, washed with 1 M HCl, rinsed with distilled water, and dried in an oven at 150ºC for 24 h. 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, 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 of 1-μm-diameter active carbon particles treated at 700–800°C for different holding times to set SBET at ca. 1200 m2/g, allowing elucidation of the factor controlling the specific capacity. For a holding time of 0.5 h, the specific surface area (SBET) and total pore volume (Vtot) increased with increasing activation temperature. We found that increasing the holding time led to higher Vmicro and lower Vmeso. The three experimental conditions (700°C-0.5 h, 750°C-0 h, and 800°C-0 h) yielded active carbons with an SBET of 1200 ± 100 m2/g; these are indicated by the orange squares in Table 1. Active carbon by 750°C-0 h or 800°C-0 h had 2 to 3 times larger Vmeso and 1.9 times larger mesopore ratio than the one with 700°C-0.5 h. Specific capacity per weight increases in the order of 750ºC-0 h (148 F/g), 800ºC-0 h (111 F/g), and 700ºC-0.5 h (66 F/g). Figure 1 shows pore size distribution of these active carbons. Although 750ºC-0 h, 800ºC-0 h have similar pore size distributions, specific capacity per weight are very different. This indicates that factors other than pore structure may have a great influence on specific capacity per weight.Figure 2 shows the quantitative results of acid-base surface functional group by acid-base back titration (Boehm method). The total amount of acidic groups increased from 700°C-0.5 h (2.48 mmol/g), 800°C-0 h (2.45 mmol/g) to 750°C-0 h (2.21 mmol/g) and correlates with the large of the capacity. The fractional order of carboxyl groups increased from 750°C-0 h (18.1%), 800°C-0 h (23.0%) to 700°C-0.5 h (31.4%). The increase of carboxyl would be one possible reason for better specific capacity. Carboxyl groups cause pseudo-capacity due to redox reaction and improve wettability, and contribute to high capacity. From these relationships, we assumed that the amount of surface functional groups has a larger influence on the capacity than the pore structure. Figure 1