**3. Results**

#### *3.1. X-ray Di*ff*raction Analysis*

It is well known that XRD is a powerful technique for revealing detailed and precise microstructure, such as the interlayer spacing (d002) of AC, which is composed of thin layers with the same atomic positions as graphite within the layers. Figure 1 shows the X-ray diffraction profiles of the APHS prepared under different activation conditions. The APHS exhibits very broad diffraction peaks and the absence of a sharp peak reveals a predominant HC structure. The XRD pattern of the AC was very similar to that of the HC. In this work, the interlayer spacing of the APHS is about 3.62 to 3.76 Å, and that of the HC is 3.68 Å, as seen in Figure 2.

**Figure 1.** X-ray diffraction patterns of activated polymer-based hard carbon under various steam activation conditions.

**Figure 2.** Structural characteristics of the activated polymer-based hard carbon as a function of various steam activation conditions: (**a**) structural parameters; (**b**) interplanar distance.

The oxidation behaviors of HC tend to operate in the amorphous regions and graphite edges [11]. It is widely known that amorphous carbon can be more easily oxidized than crystalline carbon can during the activation reactions [11–13]. If carbon atoms in the amorphous region of HC were removed first, the overall crystallinity of the HC could be increased due to decrease in a portion of the amorphous region, resulting in the increase of Lc or La in the final APHS.

As shown in Figure 2, Lc does not exhibit a large change at an activation temperature of 900 ◦C, but does gradually increase with the activation time. However, La increases steadily with increasing activation time. In particular, the largest increase is found for the sample that was activated for 10 min, followed by 30 to 40 min. It is believed that much oxidation of amorphous parts or small crystallites occurred in this section. This was confirmed in the yield of Table 1. Both Lc and La increase significantly at the activation temperature of 1000 ◦C. These results sugges<sup>t</sup> that the activation reaction at high temperature is strong and rapid.


**Table 1.** Textural properties of polymer-based hard carbon activated under various steam activation conditions.

1APHS: polymer-based hard carbon activated using steam activation. 2 Yield: *Weight o f activated sample Weightofhardcarboninputted*× 100. 3

SBET: Specific surface area by the Brunauer-Emmett-Teller (BET) method. 4 VTotal: Total pore volume; BET method. 5 VMeso: Mesopore volume by the Barrett-Joyner-Halenda (BJH) method. 6 VMicro: Micropore volume; VTotal − VMeso. 7 Cg: Specific capacitance.

The interlayer spacing exhibited a large change at d002 and a small change at d10*<sup>l</sup>*. The interlayer spacing decreased up to APHS-9-1, and then increased. The increase in the interplanar distance d002 was clearly observed with increase in the activation time. More specifically, the distance decreased from 3.68 Å for the HC to 3.62 Å for APHS-9-1. The amorphous oxidation within HC is presumed to cause this decrease in the interplanar distance of d002. In addition, a similar trend is observed in APHS-10-1. After amorphous oxidation, the small crystallite is oxidized and increases the interplanar distance of d002. According to the activation progression, the specific surface area of APHS was increased by oxidation of small crystallites. Based on these observations, it can be postulated that long activation time and high temperature burn off the less ordered components and small crystallites, resulting in the increased specific surface area of the APHS.

#### *3.2. Adsorption Isotherm and Textural Properties*

The adsorption isotherms of N2 at 77 K contain a large amount of information related to pore-structure. Because changes in the isotherms imply an alteration of pore structures; their measurements are the basis upon which to estimate the pore structure parameters. The specific surface area and pore structures of the APHS before and after activation were determined using N2 adsorption/desorption isotherms (Figure 3). There are six kinds of adsorption isotherm patterns according to the International Union of Pure and Applied Chemistry (IUPAC) classification [32], and each is indicated by a distinct pore structure. According to the IUPAC standards, the curves in Figure 3 all have Type I shape, largely consisting of micropores; longer activation time was associated with increased specific surface area and quantity of mesopores.

**Figure 3.** N2/77 K adsorption-desorption isotherm curves of activated polymer-based hard carbon under various steam activation conditions.

When the hysteresis curves were examined, hysteresis curves were rarely observed except for APHS-9-4 and APHS-10-2. This seems to be because the mesopores of these two samples were well-developed, so that many pores underwent interior development so that the shape of such pores changed to the shape of a jar.

The effect of the steam activation conditions on the specific surface area, total pore volume, and micropore volume are given in Table 1. The specific surface area and total pore volume increase with increasing activation temperature and time. As activation time and temperature increase during the steam activation process, the specific surface area increases from 50 to 2410 m<sup>2</sup>/g, and the total pore volume increases from 0.03 to 1.22 cm<sup>3</sup>/g.

When the pore characteristics were examined in detail at the activation temperature of 900 ◦C, APHS activated for 10 min developed only micropores. At up to 40 min of activation, both micropores and mesopores increased, with the former increasing from 0.02 to 0.78 cm<sup>3</sup>/g and the latter increasing from 0.01 to 0.38 cm<sup>3</sup>/g. In the case of APHS activated for 40 min, the volume of micropores increased from 0.74 to 0.78 cm<sup>3</sup>/g, while that of mesopores increased from 0.17 to 0.38 cm<sup>3</sup>/g. Because the activation yield was greatly reduced, mainly mesopores were increased. This is because micropores collapsed and developed into mesopores due to the activation reaction induced by steam oxidation. However, as the volume of micropores continued to increase, it was recognized that new micropores continued to be formed as the activation time increased. Activation at 1000 ◦C was observed to develop pores more quickly than activation at 900 ◦C. APHS-10-2 sample exhibited the highest specific surface area and total pore volume.

Figure 4 exhibits the mesopore size distributions after application of the BJH method. All samples have their highest intensity peak around 2.5 nm pore diameter. APHS-9-1 and APHS-10-1, and APHS-9-4 and APHS-10-2, were produced using different activation processes, but have similar specific surface areas. However, the APHS produced at higher temperatures exhibits greater mesopore volume. That is, the high activation temperature produced APHS that was richer in mesopores.

**Figure 4.** Pore size distribution of polymer-based hard carbon activated under various steam activation conditions using the BJH method.

Figure 5 exhibits the micropore size distributions created by the NLDFT method. As activation time increases, narrow micropores develop. High micropore volume is observed from up to 20 min of activation time in the diameter range from 0.75 to 1.0 nm. When activation time exceeds 20 min and stretches to 30 min, micropore volume decreases in the diameter range from 0.75 to 1.0 nm, while the micropore volume increases in the diameter range from 1.0 nm and more. Moreover, the pore size distribution becomes broader.

**Figure 5.** Pore size distribution of polymer-based hard carbon activated under various steam activation conditions using the non-local density functional theory method.

It is widely known that steam activation is a process that develops pores by oxidizing the carbon atoms of the precursors [13,16]. During the initial increase of activation time, micropores with small pore diameters are formed first as a result of the reaction between steam and carbon. With increasing activation time, the oxidation of crystallites increases, leading to increased specific surface areas. However, with additional activation time, the pores developed in the initial state start to deepen, enlarge, and perhaps merge, resulting in the observed increase in the average pore width. In addition, new micropores develop within the mesopores, and the specific surface area continues to increase.

#### *3.3. Electrochemical Characterization*

The electrochemical properties, including the galvanostatic discharge curves of the APHS electrodes, were studied using an electrolyte of 1M TEABF4/PC. Theoretically, the capacitance of the AC is proportional to its specific surface area [6]. However, only the surface of the pores that the ions can access can contribute to double layer capacitance [5,7]. In particular, the pore size distribution has been considered the most important parameter, because the accessibility of ion molecules in an electrolyte strongly depends on the pore size of the electrodes. The sizes of non-solvated ions and the sizes of solvated ions in 1M TEABF4/PC is 0.34 to 1.40 nm [7]. Therefore, mesopores are more useful than micropores for EDLCs, especially for non-aqueous and ionic liquid (IL) EDLCs with larger ions. Baek et al., reported a close relationship between specific capacitance and the pore size of activated carbon (2–5 nm and >5 nm) in 1M TEABF4/PC [7].

Figure 6a exhibits a change in the specific capacitance according to the charge/discharge cycle. All APHS exhibit stable initial specific capacitance. The specific capacitance is estimated from the galvanostatic charge/discharge curve (Figure 6b), which corresponds to the calculated specific capacitance of the electrode. The specific capacitance of all the APHS samples increases with increasing activation times and temperature. APHS-9-1 has very low capacity due to small pore size and low pore volume. The mobility of the ions within the pores is greatly influenced by pore size. If the pores are too small to allow easy access to electrolyte ions, they will not contribute to double-layer capacitance. APHS-9-2 has specific capacitance higher than that of APHS-9-1 because micropores and mesopores were significantly increased by activation. APHS-9-3 exhibits micropore and mesopore volume slightly increased over those of APHS-9-2; however, the specific capacitance increases significantly from 73.2 to 115.8 <sup>F</sup>/g. As discussed in Figure 5, the main pore diameter is distinctively enlarged due to expansion of the previously generated micropores and to collapse of micropore walls. Therefore, it is considered that the adsorption capacity of ions would be increased to provide easy access to electrolyte ions by the enlarged pores. In this work, APHS-9-4 exhibits the best mesopore fraction and 136.1 <sup>F</sup>/g of energy storage ability. The galvanostatic discharge curve of APHS-9-4 exhibited a straight line, typical of EDLC, with a non-IR drop. (Because the discharge data was not converted to the value per weight of the electrode material, they cannot be compared with the absolute value.) This is about 148% and 108% higher than the YP50F (95 <sup>F</sup>/g, coconut shell origin, physical activation with steam, Kuraray Chemical Co., LTD., Osaka, Amagasaki, Japan) and MSP20 (125 <sup>F</sup>/g, phenol resin origin, chemical activation with KOH, Kansai Coke and Chemicals, Japan) electrode capacity, respectively [16,33]. The porosity of APHS-9-1 and APHS-10-1 are very similar, but the specific capacitance of APHS-10-1 is higher than that of APHS-9-1. In Figure 5, the pore diameter of APHS-10-1 is larger than that of APHS-9-1, with easy access to electrolyte ions. APHS-10-2 has the highest specific surface area and mesopore volume. However, APHS-10-2 exhibits lower specific capacitance than APHS-9-3 and APHS-9-4. As seen in Figure 5, the pore width of APHS-10-2 is wider than that of APHS-9-3 and APHS-9-4. Therefore, APHS-10-2 has low capacitance despite the fact that APHS-10-2 has the best pore properties.

Figure 7 exhibits the result of plotting the pore volume according to pore diameter in 0.5 nm units using NLDFT, and then plotting the coefficient of determination with specific capacitance. An empirical linear fit was used to evaluate the contributions from each divided pore volume. The *X*-axis is exhibited using the average value of each pore size distribution. The R<sup>2</sup> coefficient of determination (R<sup>2</sup> = 1 − SSres/SStot, SSres is residual sum of squares and SStot is total sum of squares) exhibits a trend of increase and then decrease again after the occurrence of the highest value in the results (1.5–2.5 nm pore diameter).

**Figure 6.** Electrochemical performance of the activated polymer-based hard carbon as a function of a cycle under various steam activation conditions: (**a**) initial specific capacitance; (**b**) discharge curves; (**c**) specific capacitance curves. Because the discharge data was not converted to the value per weight of electrode material, they cannot be compared with absolute value.

**Figure 7.** Correlation between the specific capacitance of activated polymer-based hard carbon with various pore volume. The *X*-axis exhibits the average pore size distribution. It was plotted using the pore volume according to the pore diameter in 0.5 nm units and the average value of each pore size distribution.

The typical cyclic voltammograms (CV) of the capacitor cells are shown in Figure 8. All the CV curves are rectangular without obvious redox current on both positive and negative sweeps over the whole potential range of 0.1 to 2.5 V at 30 mV/s in the organic electrolyte (1M TEABF4/PC). At longer activation time, CV curves gradually change into rectangles. Moreover, APHS-9-3 and APHS-9-4 show a symmetric, quasi-rectangular shape profile typical of ideal EDLCs. This is consistent with the results of pore characteristic analysis and seems to be because the pores are better developed than those of other ACs. Generally, the specific capacitance of an electrode is in proportion to the integrated area of its CV profile under the same scan rate and voltage window (i.e., the larger the integrated area is, the greater the specific capacitance). As activation time increases, the area of the CVs tends to increase. The capacitance of the electrodes calculated from the CV curves decreases in the following order: APHS-9-4 > APHS-9-3 > APHS-10-2 > APHS-10-1 > APHS-9-2 > APHS-9-1. Among the carbon samples, APHS-9-4 shows the highest specific capacitance due to its uniform mesopores and high specific surface area.

**Figure 8.** Cyclic-voltammetry curves of polymer-based hard carbon activated under various steam activation conditions.

Impedance spectroscopy, which distinguishes the resistance and capacitance of devices, was used to perform a comprehensive analysis of the EDLC cells. Figure 9 shows corresponding impedance plots for an organic (1M TEABF4/PC) electrolyte. The frequency was swept from 10 mHz to 300 kHz. Each curve presents a depressed semicircle in the middle (high-frequency region) and a nearly perpendicular line in a low-frequency region. In general, the impedance spectrum of EDLCs consists of three frequency-sensitive regions showing the characteristic shape of a Z = f(Z) curve. The semicircle present at high frequencies is due to (i) electrode porosity and (ii) the charge transfer resistance of possible pseudo-capacity contributing to the total observed capacity. The electrolyte resistance is in series with the latter resistance. The electrolyte resistance influences not only the shape of the plot but also the value of ohmic resistance at which the vertical line cuts off the real resistance axis.The middle-frequency region, represented by the 45◦ line, is rather due to the frequency dependent resistance R(ω) associated with electrolyte penetration of the electrode pores.

All samples had the same sized semicircles at 900 ◦C activation temperature. Additionally, as the activation temperature increased, the size of the semicircle decreased. The higher the activation temperature, the larger were the diameter pores that developed. The charge transfer resistance decreased due to the easy movement of ions.

Pore shapes affect impedance behavior, specifically, the form of the impedance [34]. At 900 ◦C activation temperature, the morphology of the pore changes from a cylindrical shape (Type I) to a jar shape (Type IV) as the activation time increases. Moreover, the pore shape changes from wedge shape (Type V) to jar shape with increasing activation time at 1000 ◦C. At 900 ◦C, activation starts with pores of cylindrical form because of the slow oxidation of the graphite crystals, but at 1000 ◦C activation, because the oxidation of graphite crystals occurs rapidly, they develop from wedge-shaped pores. In the case of physical activation, small crystals inside the activated carbon are oxidized to form pores. Therefore, the pores of activated carbon are narrow at the entrance, and many additional pores develop inside. Finally, the pore shape of the activated carbon becomes a jar shape.

**Figure 9.** Nyquist plots of polymer-based hard carbon activated under various steam activation conditions.

As the activation time increases, the pores develop toward the inside of the activated carbon. Therefore, as the activation time increases, the ions must move deep within the activated carbon for adsorption. As the electrolyte ion movement time increases, resistance is generated, and thereby, the slope of the straight line decreases.
