Large amounts of spent coffee grounds (SCGs) are generated all over the world. Since SCGs contain a lot of carbon and inherently has a porous structure, SCGs are considered to be a valuable industrial resource by carbonization. According to our previous research, it was apparent that the specific surface area of SCGs-derived carbon was greatly improved by potassium hydroxide (KOH) activation. In this research, we prepared an electric double layer capacitor (EDLC) using SCGs-derived carbon activated with KOH, and compared them to phenol resin-derived activated carbon (MSP-20, Kansai Coke and Chemicals Co., Ltd.), which is commonly used as a reference material in EDLC research. Electrodes were prepared by mixing 80 wt% activated carbon, 10 wt% carbon black and 10 wt% polytetrafluoroethylene (PTFE) and the electrolyte used was 1 M triethylmethylammonium-tetrafluoroborate (TEMA-BF 4 )/propylene carbonate (PC). The capacitive performance was evaluated by a constant current chargedischarge cycle measured with various electrical current loads from 5 to 150 mA/cm 2 . Although the capacitance of SCGs-derived carbon activated with KOH was inferior to that of MSP-20 at low current load density, it had high capacitance in high rate charge-discharge cycles. This suggests that the EDLC consisting of SCGs-derived activated carbon electrodes is superior to MSP-20 in capacitance retention rate when used at a high electric current density.
We prepared the spent coffee grounds (SCG) and controlled the steam activation process for them in order to develop SCG-derived activated carbon with uniform micropores that were similar to those of KOH-activated phenolic resin carbon (MSP-20). The SCG-derived activated carbon that we obtained had a high yield, a high specific surface area, and uniform-sized micropores, and it exhibited characteristics different from those of conventional structures containing both micropores and mesopores. The laminated multi-layer EDLC cells achieved a capacitance of higher than 75 F/g. They were produced with only small variations. The Nyquist plot generated from the impedance measurements showed a fairly vertical rise that can be attributed to the blocking electrode in the low frequency range. This confirms that the pores in the electrodes were used effectively. It was demonstrated that the electric double layer characteristics and impedance characteristics of the SCG capacitors can be changed by controlling the raw material preparation and its activation conditions.
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