Poo Reum Choi¹, Sang-Gil Kim², Ji Chul Jung^1,♠ and Myung-Soo Kim^1,♠

¹Department of Chemical Engineering, Myongji University, Yongin 17058, Korea

²R&D Center, Vitzrocell Co. Ltd., Yesan 32417, Korea

2.2. Preparation of AC

In order to change the degree of crystallinity of the precursors before the KOH activation process, the PPs were ground and pre-carbonized for 1 h in a temperature range of 500°–1000°C under N2 flow in a mini tube furnace (mini Tube Furnace 锚文字 https://www.tmaxcn.com/tube-furnace_c80 链接) (Xiamen Tmaxcn Inc.). The sample pre-carbonized at 500°C was denoted as P5, and so on. Various ACs were prepared by KOH activation of the pre-carbonized PPs (P5–P10). The pre-carbonized PPs were mixed with KOH powders at a PP/KOH mass ratio of 1:4 and the mixtures were activated at 900°C for 3 h. The obtained ACs were thoroughly washed with distilled water three times and dried at 110°C in an oven for 1 day. The ACs activated from P5–P10 were denoted as P5AC–P10AC, respectively. For comparison, ACs were also prepared from a CTP precursor having a softening point of 292°C, provided by OCI Co. (Korea), using the same carbonization and activation procedures.

2.3. Characterization and electrochemical

properties of AC Pore structures of the obtained ACs were characterized by N₂ adsorption/desorption measurements using an ASAP 2010 (Micromeritics, USA) instrument. SSAs of ACs were calculated by the Brunauer-Emmett-Teller (BET) method. Nitrogen absorption/desorption isotherm, micro-pore analysis method, Barrett-Joyner-Halenda, and non-local density functional theory (NLDFT) analyses were used to obtain the pore volume, pore

diameter, and pore size distribution (PSD) of the ACs. Crystallinity of pristine PP, carbonized PPs,and activated PPs was determined by X-ray diffraction (XRD) measurement (XRD-7000;  Shimadzu, Japan) using Cu-Kα radiation (λ=1.54056 Å) operated at 40 kV and 30 mA. The interlayer spacings (d002 and d10ℓ) were calculated using the Bragg equation (1).

λ=2 d sin(φ)  (1)

where φ is the Bragg angle for the reflection concerned and λ is the wavelength of the radiation. The in-plane crystallite size (La) and the stacking height of the crystallite (Lc) were determined  from the (10ℓ) peak at ~43° and the (002) peak at ~26°, respectively, using the following Scherrer equations [16-19].

La=1.84λ/Ba cos(φa) (2)

Lc=0.89λ/Bc cos(φc) (3)

where λ is the wavelength of the radiation used, Ba and Bc are the width of the (10ℓ) and (002) peaks, respectively, at 50% height, and φa and φc are the corresponding scattering angles. Electrodes were prepared by mixing AC, Super-P, and PVDF at a mass ratio of 8:1:1. PVDF and Super-P were used as a binder and a conductive additive, respectively. The mixture was homogenized in a sufficient quantity of NMP solvent to make a homogeneous slurry. On etched aluminum foil used as a current collector, the mixed slurry was coated by doctor blade apparatus and dried in an oven at 70°C for 24 h. The dried electrode was then pressed using a electrode calender machine (electrode calender machine 锚文字  https://www.tmaxcn.com/Calender_c0_ss 链接) (Xiamen Tmaxcn Inc.) at 80°C and dried in a vacuum oven at 70°C for 24 h.

Coin-type EDLC cells of CR2032 size were fabricated using two symmetrical carbon electrodes, a separator, and a spacer. The electrodes were punched into small disks with 18 mm diameter; these were placed in a glass vial containing 1 M TEABF4/AN electrolyte after measuring the mass and thickness. The bulk density of the AC electrodes was measured by dividing the mass of the dried carbon electrodes without the current collector by the volume, which was obtained by multiplying the area and the thickness of carbon electrodes. The immersion was maintained for 24 h so that the electrolyte could sufficiently permeate the electrode materials. The 19-mm diameter separator, soaked with the electrolyte solution, was placed between the electrodes. Finally, the coin cell was sealed using a coin cell crimper (coin cell crimper 锚文字 https://www.tmaxcn.com/coin-cell-crimper_sp 链接) (Xiamen Tmaxcn Inc.). All processes for assembling the coin-type EDLC cell were carried out in a vacuum glove box (vacuum glove box锚文字 https://www.tmaxcn.com/vacuum-glove-box_c75  链接) (Xiamen Tmaxcn Inc.) filled with N₂ gas.

The electrochemical performance of the assembled coin-type EDLC cells was measured by cyclic voltammetry (CV; Potentiostat/Galvanostat Model 273 A, EG&G, USA) and galvanostatic charge/discharge (C/D; WBCS-3000, WonA Tech Co., Ltd., USA). The CV measurement was conducted at potential scan rates of 10 and 100 mV/s in the voltage range of 0–2.7 V. Constant current loads (1, 3, and, 5 A/g) were applied to the C/D measurement in the voltage range of 0–2.7 V.Gravimetric capacitance (F/g) was calculated by CV using the following equation; volumetric capacitance (F/cc) was calculated by multiplying the electrode density (g/cc) by the gravimetric capacitance.

where I is current, n is the scan rate, m is the mass of the electrode materials, and ∆V is the measured voltage range (0–2.7 V).Capacitance (F/g) was also calculated by charge/discharge test using the following equation.

where I is the current density (A/g), ∆t is the discharge time, and ∆V is the measured voltage range (0–2.7 V).