Effect of porosity and crystallinity of activated carbons for electrochemical capacitors

Activated carbons (ACs) are usually utilized for the electrochemical capacitor due to their inherent properties, such as large surface area, high chemical stability, and good electrical conductivity. In this study, the commercial and chemically activated ACs with various surface areas were prepared and evaluated for their capacitance. The capacitances were generally increased as increasing the surface areas, but the highest surface area always did not lead to the highest capacitance. The Brunauer–Emmett–Teller specific surface area of chemically activated coconut-ACs (2,209 m2/g) was lower than chemically activated wood-ACs (2,903 m2/g), but the capacitance was higher. It was found that the major factor for the capacitance was not only the surface area, but also the electrical conductivity. The electrical conductivity is usually associated with crystallinity, which is represented by the nanographitic domain size and crystalline thickness along the c-axis (Lc). The crystalline structure enhances the electron mobility, increasing the capacitance. The conductivities of prepared ACs were measured at different pressures, and their crystalline structure was confirmed by Raman spectroscopy and x-ray diffraction.


Introduction
Supercapacitors or electrochemical capacitors have lots of interests for energy storages due to their properties, such as high-power density, long-life cycle, and superior charge-discharge efficiency [1,2].Not only supercapacitors are a prospective and competent candidate for energy store and conversion, but also it combines the advantages of both conventional dielectric capacitors and rechargeable batteries [3,4].The capacitors can be classified into two principal types based on their storage mechanisms: electrical double-layer capacitor (EDLC) and pseudo capacitors [5][6][7][8].Among them, EDLC s have been considerable attentions for the energy storage device which can replace batteries, due to their similarity of design and manufacturing.In EDLC system, the electrical double layer of electrolyte ions is formed at the electrode surface for energy storage.For the ion collection on the electrodes, the activated carbons have been usually used to electrode materials in EDLC because it has high surface area, relatively high electrical conductivity, and cost-efficiency [1,9].
The commercial activated carbons (denoted as ACs) are usually produced using heat treatment methods from various carbonaceous materials, such as wood, coal, coconut shell, etc [9].
However, the surface area of commercial ACs is relatively low, so the methods to enhance the surface area are newly investigated.The physical and chemical activation methods are widely used for increasing the surface area.Among the activation methods, the physical activations are obtained by the heat treatment at the high temperatures in the oxidizing agents (e.g.steam, CO 2 , air, etc) [10][11][12][13].In contrast, the chemical activations refer to heat treatment at relatively lower temperature with chemical agents, such as H 2 PO 3 , KOH, NaOH, and ZnCl 2 [14].
Those materials can help to develop the inner pores, especially micropores, and the generated pores increase the specific surface areas.The large surface areas can adsorb large number of electrons, due to the abundance of active sites on the carbon surface [15].In the previous literatures have elucidated the correlation between specific surface area and their electrocapacitive properties [16,17].The ACs characterized by a large surface area which in turn contributes to its elevated capacitance.In addition, D. Qu and M. Endo at.al. also reported that the capacitance of activated carbon [6,18] AC was contingent upon the pore size distributions as well as the electrolyte type employed.This is attributed to the optimal size of pores that facilitate efficient ionic mobility within the electrolyte.The capacitance in the previous studies usually was considered by the surface areas and pore distributions of materials [19,20].
In this study, the ACs with various surface areas and pore distributions were produced using the chemical activation method, and their capacitances were tested and compared.Furthermore, the capacitances were considered by using the surface area and electrical conductivity as well.Especially, the conductivities were rarely considered for the capacitances in the former studies.The conductivities of produced ACs were highly focused, and the relationship between the conductivities and specific capacitances was discussed.

Characterization of activated carbons
The commercial-activated carbons from wood and coconut were activated by the chemical activation method.Figure 1 exhibits the N 2 adsorption-desorption isotherms at 77 K for commercial and activated samples.As shown in figure 1(a), the adsorption of both CAC and C-CAC was sharply increased at low-pressure region, and then being gradual.It indicates microporous structure and also follows type I isotherms curve, which is classified by International Union of Pure and Applied Chemistry (IUPAC) cases [16,21].As comparing the curves, the adsorption capacity of C-CAC is higher than that of CAC due to the large specific surface area.Whereas the curves from the wood-based ACs (WAC and C-WAC) belong to type IV, which represents the mono-and multilayer adsorption behavior.In addition, the symbol with empty squares, which indicate desorption curves, have a different pattern with adsorption.The shape of isotherm and hysteresis loop represents mixed micro-and mesoporous structure [20].The adsorption capacity of C-WAC with a large surface area is also higher than the non-chemically activated AC (WAC).
The pore size distributions of the activated carbons were measured using the desorption branch of the adsorption-desorption isotherms using the Barrett-Joyner-Halenda (BJH) method [11], as shown in figure 2. It shows the pore distributions of coconut-based ACs with and without the chemical activation.The pore width of CAC was between 1 to 2 nm in size, but the pore of C-CAC was increased between 1 to 3 nm.Whereas the pore distributions of wood-based ACs have a different pattern from coconut-based ACs.The micropores (< 2 nm) of WAC was expanded to the larger pore (> 2 nm), and the mesopores (> 10 nm) was disappeared and shifted to smaller pores after the chemical activation, generating the pores with around 2 nm in size.Table 1 shows the summary of specific surface areas and pore volumes.The surface area of CAC was 1,169 m 2 /g, but it was increased to 2,209 m 2 /g through the activation.The surface area of C-WAC was increased to 2,903 m 2 /g, which was almost double of commercial WAC (1,398 m 2 /g).The surface areas of both chemically activated ACs were increased two times comparing with starting ACs.However, the micropore volumes were decreased after activations, because the micropores were expanded by the activation.It is because the potassium (K) from KOH were penetrated into the micro-and meso-pores, and then reacted inside carbons [11].
The SEM images indicate that the surface roughness and pores, as shown in figure S1.After chemical activation, the surface roughness is increasing as developed pores regardless of raw material characteristics.Thes SEM results are consistent with the results showing an increase in specific surface area and porosity [22,23].

Electrochemical properties of prepared ACs
The electrochemical properties of ACs were examined through cyclic voltammetry (CV).The CV tests were performed on commercial and activated AC samples using a 1 M KOH solution as the electrolyte, at different scan rates (10 ∼ 200 mV s −1 ). Figure 3 exhibits the CV curves of CAC, C-CAC, WAC, and C-WAC.The CV curve of CAC shows the typical rectangular shape at various scan rates, with an increase in current density corresponding to higher scan rates.However, the CV curves of the other ACs exhibit a different shape increasing the scan rate.Notably, these ACs demonstrate higher current densities than CAC at all tested scan rates, a phenomenon attributed to their larger surface areas [9,24,25].
The specific capacitances of all the samples were calculated from CV curves using the equation (1).
Where Cs is the specific capacitance, ∫IdV represents the integrated area of the CV curve, m is the mass of the prepared ACs on the electrode, ΔV is the potential window, and S is the potential scan rate [26,27].Table 2 and figure 4 present the calculated specific capacitances both commercial and activated ACs at varying scan rates.As    4, the specific capacitance is generally correlated with their surface areas.The CAC with the lowest surface area exhibited lower specific capacitance than the others.This phenomenon is consistent with the principle that larger surface areas on ACs enable more extensive electron collection, thereby leading to increased capacitance [15].According to the data in table 1, the specific surface areas of ACs increase in sequence: CAC < WAC < C-CAC < C-WAC.Interestingly, the C-WAC with the highest surface area did not have the highest capacitance at all scan rates, even lower than C-CAC at 20 and 100 mV s −1 in the scan rates.
The specific capacitance values for the ACs are comprehensively summarized in table 2. Generally, ACs with lower specific surface area tend to exhibit lower capacitance.However, an exception is observed with C-CAC, which demonstrates the highest capacitance in the scan rates from 20 to 100 mV s −1 .In this study, the difference of surface areas between C-WAC and C-CAC was 694 m 2 /g, which was initially expected to result in increased capacitance for C-WAC.However, the capacitance of C-WAC was lower than that of C-CAC at all scan rates, except for 10 mV s −1 .This indicates that the specific surface area is not only the factor to determine the capacitance.

Electrical conductivity measurement
To further investigate the unexpected capacitance results mentioned above, the electrical conductivity of the ACs was measured at various pressures as shown in table 3.As increasing the pressure, the resistance is slightly decreased, while the conductivity is increased.Notably, the coconut-based ACs consistently exhibited the higher conductivities than wood-based ACs in the various pressure ranges.Unusually conductivities of WAC could not be measured in all the pressures.This lower conductivity in wood-based ACs can be attributed to their amorphous and complex pore structures [28].The ACs with well development pores have an advantage for the increase of surface area, but their multiple and complex pore structures obtain the poor carbon networks formation, subsequently leading to reduced electrical conductivities.In contrast, coconut-based ACs, despite their amorphous structure, tend to possess larger graphitic domains and narrower pore sizes compared to woodbased ACs.This structural difference suggests their relatively higher conductivities.David et al reported the correlation between the nanographite domain and micropores.The nanographite domains bounded each other, and the pores existed between the domains [29].They observed the larger graphitic sheets and narrow pores are known to facilitate increased electron movement and hopping, respectively [29,30].The efficiency of electron  transport via hopping is affected by the distance between the nanographite domains.As depicted in figure 2, the commercial wood-based ACs (WAC) possess larger pores compared to CAC, which in turn reduces electron hopping and leads to significantly lower conductivity, to the extent that it was not measurable in this study.On the other hand, CAC initially had narrower pores, leading to low resistance and high conductivity.The carbon structure and the degree of crystallinity of these ACs were further analyzed and confirmed through Raman spectroscopy and x-ray diffraction (XRD) in the below.

Raman analysis of ACs
Raman spectroscopy is known to be an effective analysis method for confirming the graphitic structure of materials and identifying defects in samples.Figure 5 presents the Raman spectra of CAC, C-CAC, WAC, and C-WAC.The two prominent broad bands were observed in the range of 1000 to 2000 cm −1 , which indicates D and G bands as shown in figures 5(a) and (c) at 1350 and 1575 cm −1 , respectively [31].The D band indicates the disordered graphite structures, while the G band typically represents crystalline graphite, such as few layers of graphene [32].The intensity ratio of the D and G band (I D /I G ) usually presents the amount of disorder structure [33,34].In addition, the broaden bands also indicate dis-order increases, because of merging the disordered bands.Especially the broaden G band was caused by the overlapped D' band at 1620 cm −1 as shown in   Furthermore, the bands identified in the range of 2200 to 3400 cm −1 are indicative of more ordered structures, as delineated in reference [35].Typically, the 2D band at approximately 2700 cm −1 represents the graphitic structure and surface crystallization, which are known to facilitate high electron mobility [36,37].In the spectra of all samples, the D+D', 2D, D+D', and 2D' bands were detected at all the samples in the spectrum, but a distinct and sharp 2D peak was uniquely observed at the C-CAC.It means that the C-CAC exhibits partial graphitization on its surface, potentially contributing to enhanced electrical conductivity.The chemical activation process using KOH at high temperatures partially converts amorphous activated carbons to a graphitic structure, particularly noticeable in coconut-based ACs.However, the surface structure of wood-based ACs showed rarely change post activation, whereas the surface of coconut-based materials was converted to the nanogarphitic structure.

X-ray diffraction pattern of ACs
Figure 6 presents the x-ray diffraction (XRD) patterns of CAC, C-CAC, WAC, and C-WAC.The diffraction patterns of all the ACs were normalized to compare with the band width and intensity.The XRD patterns were measured at 2θ in the range 0°to 40°, and three distinct peaks were detected in the spectra.The diffraction peak at 2θ = 12°and 22°can be indexed to (001) and (002) planes, respectively [38].The assigned (001) peak in figure 6 exhibits the layered graphene oxides, and the (002) peak indicates the crystallization of carbon.Those peaks of the samples are summarized in table 4. The full-width at half-maximum (FWHM) of (001) planes was increased, while that of (002) planes was decreased after the chemical activation.The interlayer stacking distance (L c ) was calculated by the Scherrer equation (2) [39].
Where β 00l is the FWHM of (00l) planes, λ is the radiation wavelength, and θ 00l is the reflection angle.After the chemical activation process, the calculated L c of (001) planes was decreased, while L c of (002) was increased.It is indicative of a partial reduction of the oxidized surface layers, culminating in their more crystallized structures.In addition, the chemical activation using KOH helps to develop the crystalline structure of ACs because the chemical agent simply removes the oxygen functional group on the surface [40].
Both Raman in figure 5 and XRD in figure 6 show the development of nanographitic domain and crystalline structure of ACs after the chemical activations.In addition, the thermogravimetric analysis (TGA) in figure S2 also shows the maximal rate of weight loss, and it indicates the crystallinity of the samples [41].For instance, the C-CAC exhibited the most elevated oxidation temperature, which represented superior crystallinity.This trend in oxidation temperature is consistent and corroborative with the results in figures 5 and 6.The development of nanographitic and crystalline structure is attributed to the increase in electrical conductivity.Figure 7 represents the relationship between the specific surface area, electrical conductivity, and capacitance.It is observed that there is a progressive increase in specific capacitance concomitant with the augmentation of surface areas, following the sequence of CAC < WAC < C-CAC < C-WAC.
However, the AC with the largest surface area does not always exhibit the highest capacitance.For instance, the specific surface area of C-CAC (2,209 m 2 /g) was smaller than C-WAC (2,903 m 2 /g), but the capacitance was even higher, as shown in table 5.It was found that the capacitance was dependent on not only specific surface area but also electrical conductivity.In the former studies, the capacitance was mostly considered by their surface areas [16,17].As shown in figure 7, the major parameter for capacitance is the specific surface areas, but the electrical conductivity is also another parameter to considered capacitance.

Preparation of activated carbons
The chemical activation procedure was employed to enhance the specific surface areas of the ACs.The ACs with high surface areas were synthesized using commercial ACs, which are from wood and coconut (Ja Yeon Science  Ind. Co., Seoul, South Korea).The commercial ACs derived from the wood and coconut were denoted as WAC and CAC, respectively.A mass of 1.0 g of the commercial ACs and 3.0 g of potassium hydroxide (95% KOH, Samchun Chem.Co., Seoul, South Korea) were homogeneously mixed with 1:3 weight ratio (AC: KOH).The mixture was placed into the tubular furnace, and the temperature was increased until 850 °C and maintained at this temperature for a duration for 4 h [11,12].After the activation process, the chemically activated ACs were washed by water three times, and then neutralized by sulfuric acid (H 2 SO 4 , Samchun Chem.Co., Seoul, South Korea).An additional water wash was then performed, and the samples were subsequently dried in an oven at controlled conditions overnight.The washing and neutralized processes can help to remove the remained potassium on the carbon surface.The chemically activated WAC and CACs were also referred as C-WAC and C-CAC, respectively.

Characterization of prepared activated carbons
The specific surface area and pore distribution were calculated via Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods based on the N 2 adsorption obtained at 77 K using an adsorption analyzer ASAP2010 (Micromeritics, Norcross, USA).Scanning electron micrographs of the adsorbent were recorded using a Mira 3 scanning electron microscope (Tescan, Brno, Czech Republic).
The electrical conductivity of ACs was measured at various pressures (0.3 to 60 MPa).The samples were loaded in the cylindrical cell having 0.7 cm inner diameter, and then compressed and monitored using Newton NT-501A indicator.The height changes of samples and conductivity were measured by vernier caliper and Keithley DC Current Model 6220 with four-point probe, respectively.Ohmic conductivity was measured using following equation (3).

• ( )
Where σ is electrical conductivity, L is sample distance, R is resistivity, and A is area of the piston surface [45].
Raman spectroscopy analysis was used to confirm the crystallization of commercial and activated ACs using a Raman spectrometer (LabRAM HR Evolution, HORIBA Ltd, Kyoto, Japan) equipped with an integral microscope.514.54 nm radiation from a 16 mW air-cooled Ar Ion laser, was used as an excitation source.X-ray diffraction (XRD) patterns were recorded on a x-ray diffractometer (XRD-6100, Shimadzu, Kyoto, Japan) with Cu Kα radiation source.
Thermogravimetric analysis (TGA) was conducted under an air atmosphere using a Thermogravimetric Analyzer (TA Instruments Q50, TA Instruments, USA) with a heating rate of 5 °C/min up to 280 °C.Once the temperature reached 280 °C, the samples were maintained for 10 h.Approximately 10 mg of sample was utilized for each test.
The morphology and microstructure of the ACs were observed by field-emission scanning electron microscopy (FE-SEM, Mira 3, Tescan Inc., Brno, Czech Republic).The acceleration voltage and magnification were 15.0 kV and 100,000 times, respectively.

Test for electrochemical capacitance
The capacitance of commercial and activated ACs was measured using the cyclic voltammetry (CV) method.The CV of the samples was carried out using the three electrode system consisted of the counter (platinum wire), reference (Ag/AgCl electrode), and working electrodes equipped with an electrochemical workstation (AUT302M, FRA2, Metrohm Autolab, Utrecht, Netherlands).15 mg of activated carbon powders were mixed with 100 μl of 5 wt% Nafion solution (Sigma-Aldrich, St. Louis, USA) and 1 ml of N,N-Dimethylformamide (DMF, Daejung Chem.Co., Seoul, South Korea) with brief sonication.8.0 μl of the prepared slurry was dropped on the glassy carbon electrode (GCE) as working electrode and dried it in the oven at 50 °C for 30 min [46].The tests were carried out in the potassium hydroxide solution (1 M KOH, Sigma-Aldrich, St. Louis, USA) as an electrolyte at different scan rates (10 ∼ 200 mV s −1 ).

Conclusions
Activated carbons (ACs) with extensive surface areas are conventionally favoured in capacitor applications due to their inherent properties.Typically, a larger surface area provides numerous active sites for electron accumulation.However, this study observed a non-linear correlation between the surface area of ACs and their capacitance.For instance, the specific surface area of C-CAC (2,209 m 2 /g) was lower than W-CAC (2,903 m 2 /g), but C-CAC demonstrates a higher capacitance.
Additionally, the capacitance was also related to electrical conductivity due to their electron mobility.The conductivities of CAC, C-CAC, WAC, and C-WAC were evaluated in the cylindrical cell as increasing the pressures.It was observed that the ACs based on the coconut materials have exhibited superior conductivity than wood-based materials.Especially, the electrical conductivity of C-CAC surpassed the others, and the capacitance was also high.The conductivity is highly related to both nanographitic area and crystallization on the AC surface, so it was confirmed by Raman spectroscopy, XPS, and TGA.
In this study, the ACs with different surface areas were produced and examined for their capacitance.It was found that the surface areas were not the only factor for the capacitance.The electrical conductivity also plays an important role to enhance the capacitance.

Figure 1 .
Figure 1.N 2 adsorption-desorption isotherms of commercial and activated ACs, (a) CAC and C-CAC and (b) WAC and C-WAC.Solid (+) and empty (,) rectangular symbols show nitrogen adsorption and desorption, respectively.

Figure 2 .
Figure 2. Pore size distributions of commercial and activated ACs, (a) CAC (solid line) and C-CAC (dotted line) and (b) WAC (solid line) and C-WAC (dotted line).

Figure 4 .
Figure 4. Specific capacitance of CAC, C-CAC, WAC, and C-WAC at different scan rates.

Figure 5 .
Figure 5. Raman spectra of coconut based ACs (a and b) and wood based ACs (c and d).The black and red lines indicate commercial and activated ACs, respectively.

figures 5 (
figures 5(a) and (c)[31].The D' band represents the defects of graphene, which also indicates the poor crystalline structure.In the case of C-CAC in figure5(a), the sharp G band and shoulder known as D' band are detected, while the two bands in the other spectrum (CAC, WAC, and C-WAC) were merged.The inset of figures 5(a) and (c) shows the deconvoluted bands of G and D'.Following the chemical activation of coconut-based ACs (C-CAC), the intensity of the D' band was sharply decreased.This suggests that the reduction in defect sites within the CAC, leading to enhanced surface crystallization.Conversely, the intensity and broadening of the bands for the wood-based ACs remained largely unchanged post chemical activation.This indicates the persistence of numerous defects and a lack of significant increase in crystalline structure.Furthermore, the bands identified in the range of 2200 to 3400 cm −1 are indicative of more ordered structures, as delineated in reference[35].Typically, the 2D band at approximately 2700 cm −1 represents the graphitic structure and surface crystallization, which are known to facilitate high electron mobility[36,37].In the spectra of all samples, the D+D', 2D, D+D', and 2D' bands were detected at all the samples in the spectrum, but a distinct and sharp 2D peak was uniquely observed at the C-CAC.It means that the C-CAC exhibits partial graphitization on its surface, potentially contributing to enhanced electrical conductivity.The chemical activation process using KOH at high temperatures partially converts amorphous activated carbons to a graphitic structure, particularly noticeable in coconut-based ACs.However, the surface structure of wood-based ACs showed rarely change post activation, whereas the surface of coconut-based materials was converted to the nanogarphitic structure.

Figure 7 .
Figure 7. Relationship between specific surface area, electrical conductivity, and capacitance of commercial and activated ACs.

Table 1 .
Textural properties of the commercial (CAC and WAC) and chemically activated ACs (C-CAC and C-WAC).

Table 2 .
Summary of specific capacitance of ACs with different scan rates from 10 to 200 mV s −1 .

Table 3 .
Electrical conductivity of commercial and chemically activated ACs at different pressures.
a N/A means not detected.

Table 4 .
Full-width at half-maximum (FWHM) of XRD data and calculated L c at the various planes of ACs.

Table 5 .
Summary of specific capacitance of ACs from biomass.