2.3. Preparation of Porous Carbon Fiber (PCF) and Activated Porous Carbon Fiber (APCF)

We considered that the molecular weight of PVA and the ratio between PVA and PAN are important factors with regards to optimizing the carbon fiber porosity. With this in mind, porous carbon fibers (PCFs) were prepared by the carbonization of the as-prepared PPFs in a silicon carbide tube furnace. For the carbonization of the PPFs, the temperature ramped with a constant rate of 5 °C min⁻¹ from 25 °C to 280 °C under airflow and stabilized at 280 °C for 1 h. After that, the temperature was raised from 280 °C to 1200 °C with the temperature ramping rate of 5 °C min⁻¹ under nitrogen gas and remained at the 1200 °C for 1 h and cooled to 25 °C. The prepared carbon fibers were denoted PCF_H 0.5, PCF_H 0.3, PCF_H 0.1 (or PCF_L 0.5, PCF_L 0.3, PCF_L 0.1), and CF depending on the concentration of PAN and PVA_H (or PVA_L) in the dope solutions, with weight ratios of 5:5, 7:3, 9:1 and 1:0 (PAN alone), respectively in Figure 1c.

For the creation of more pores, chemical activation of the PCF was carried out using a KOH solution. A bundle of PCFs (0.5 cm × 5 cm, 0.2 g) was immersed in a KOH solution (8 M, 50 mL) at 60 °C for 4 h and then dried at 100 °C for 24 h. Then, the samples were calcined in a tube furnace under nitrogen flow. The temperature in the furnace was raised at a constant rate of 5 °C min⁻¹ from 25 °C to 900 °C and remained there for 1 h. To remove any residual potassium from the activated PCFs (APCFs), the fibers were immersed in an HCl (0.1 N, 50 mL) solution for 30 min and then washed repeatedly with deionized water until the pH of the samples became neutral. Finally, the APCFs were obtained after the drying of the samples at 100 °C for 12 h in an oven. The as-prepared APCFs were denoted APCF_H 0.5, APCF_H 0.3, APCF_H 0.1 (or APCF_L 0.5, APCF_L 0.3, APCF_L 0.1), ACF depending on the concentration of PAN and PVA_H (or PVA_L) in the dope solution with weight ratios of 5:5, 7:3, 9:1 and 1:0 (PAN alone), respectively (Figure 1d).