Studies on preparation and applications of polymeric precursor-based activated hard carbons: I. Activation mechanism and microstructure analyses

This study examined the influence of CO2 activation on pore characteristics and crystalline structures in the preparation of activated hard carbons (ACs). Changes in pore characteristics (specific surface area, total pore volume, mesopore volume) under varying conditions were analyzed using the Brunauer–Emmett-Teller (BET), Horváth-Kawazoe (HK), and Barrett-Joyner–Halenda (BJH) equations based on the adsorption isotherm (N2/77 K). X-ray diffraction (XRD) was employed to calculate the interplanar distance of crystalline structures, crystal size, and crystal diameter. As activation time increased during the CO2 activation process, the specific surface area surged from 50 to 2030 m2/g, and the total pore volume surged from 0.03 to 0.85 cm3/g. In particular, with an activation time of 60 min, the volume of mesopores increased rapidly to 0.10 cm3/g (250% increase compared to volume at 50 min). The interplanar distance of the (002) peak, crystal size, and crystal diameter varied in the range of 3.68∼3.51 Å, 9.40∼8.98 Å, and 29.87∼35.81, respectively. The oxidation of amorphous carbon atoms led to a decrease in interplanar distance and crystal size. As oxidation time increased, small-sized crystals also became oxidized, which resulted in an increase in crystal diameter and mesopore volume.