Influence of Different Activators on the Structure and Properties of Activated Carbon Based on Bamboo Fiber

In order to explore the influence of different activators on the structure and properties of the prepared activated carbon, bamboo fiber-based activated carbons (BFACs) were prepared by four activators of phosphoric acid, pyrophosphoric acid, zinc chloride, and diammonium biphosphate (BFAC-H3PO4, BFAC-H4P2O7, BFAC-ZnCl2, and BFAC-(NH4)2HPO4) and BFACs adsorption performance and electrochemical properties were investigated. The main conclusions were: the specific surface area of the four BFACs varies greatly, among which BFAC-ZnCl2 was the highest, at 1908.5074 m2/g, and BFAC-(NH4)2HPO4 was the lowest, at 641.5941 m2/g. In terms of the pore structure, BFAC-H3PO4 and BFAC-H4P2O7 are mainly mesopores and BFAC-ZnCl2 and BFAC-(NH4)2HPO4 are mainly micropores. The BFAC-ZnCl2 sample had the largest specific capacitance, with a specific capacitance of 121.2730 F/g at a current density of 0.2 A/g, with a small internal resistance and good electrochemical reversibility and capacitance performance. The adsorption properties were better for BFAC-ZnCl2 and BFAC-H3PO4 and the adsorption amounts were 648.75 and 548.75 mg/g, respectively.


Introduction
In recent years, wastewater treatment is becoming increasingly important as global warming worsens, freshwater resources decrease, and environmental problems become more and more serious. The removal of contaminants from wastewaters is a major challenge in the field of water pollution. Among numerous techniques available for contaminant removal, adsorption using solid materials and named adsorbents is a simple, useful, and effective process [1]. Among the sustainable high-performance electrochemical energy storage technologies, supercapacitors have drawn substantial attention due to their superior power density, ultra-fast charge-discharge rate, high reversibility, long cycle life, and relatively low cost. Nonetheless, the main obstacle presented by supercapacitors is their relatively low energy-density values compared to batteries [2]. Activated carbon, formed by the accumulation of six rings of carbon, is an excellent adsorbent material with a rich internal pore structure and specific surface area. The adsorption capacity of activated carbon depends on its physical and chemical properties such as pore size, pore capacity, specific surface area, and chemical functional groups. In general, the larger the specific surface area, the higher the adsorption capacity of activated carbon [3]. Nowadays, activated carbon has become increasingly popular among scientists because of its simple preparation process and abundant and inexpensive raw materials [4] and it is widely used in industry, agriculture, life, medicine, and health, environmental protection, electronics, and military fields. With the development of carbon material preparation technology, the application of bamboo charcoal and bamboo-based activated carbon will be more promising [5]. The structure and properties of activated carbon are affected by the raw material and activation process, so it has become more and more necessary to explore the preparation of activated carbon from biomass.
In this work, bamboo fiber-based activated carbon was prepared by removing lignin and using the chemical activation method; the effects of different activators on the physicochemical properties of activated carbon are discussed. Our report revealed the inevitable connection between the activators and the activated carbon structure. The effect of activated carbon structure on the electrochemical performance and dye adsorption performance was also analyzed.

Materials and Instruments
The bamboo raw material used in this paper was obtained from natural southern bamboo produced in the Fuyang District, Hangzhou, China. A DF-101S collector type constant temperature heating magnetic stirrer, MFLGKDF406-12 muffle furnace, PHS-2F type PH meter, Nicolet IS Model 50 Fourier Transform Infrared Spectrometer, Mike ASAP2460 fully automatic specific surface and porosity analyzer BET, and UV-5500 UV-Vis spectrophotometer were the instruments used in this paper.

Activated Carbon Preparation
The raw material was first delignified by impregnating the bamboo strips with a solution of sodium chlorite (NaClO 2 ), heating them in a water bath at 80 • C for 6 h. After 6 h, the solution was replaced and the heating was continued, repeating the above steps 8 times until the bamboo strips turned white. Finally, the white bamboo strips were dried in a freeze-drying oven and separated to obtain bamboo fibers. A certain amount of bamboo fiber was mixed with four activators (phosphoric acid, pyrophosphoric acid, zinc chloride, and diammonium biphosphate) at an impregnation ratio of 3:1 and stirred well, dried, and transferred to a tube furnace and activated at 600 • C for 90 min. The impregnation ratio, activation temperature, and activation time were all selected to fully activate the bamboo fiber. Finally, the obtained activated carbon was washed repeatedly until the pH of the aqueous solution was 6-7. The four prepared activated carbon samples were named as BFAC-H 3 PO 4 , BFAC-H 4 P 2 O 7 , BFAC-ZnCl 2 , and BFAC-(NH 4 ) 2 HPO 4 .

Characterization
The pore structures of the activated carbon samples were obtained using nitrogen adsorption and desorption tests. The total surface area of the activated carbon was calculated using the BET method. The specific surface area and pore volume of the micropores and mesopores were obtained using the t-curve method and the Barrett-Joyner-Halenda method, respectively. The pore size distribution was calculated using the Nonlocal Density Functional Theory method. The functional groups of activated carbon were analyzed using infrared spectroscopy. The high purity potassium bromide and dried activated carbon samples were thoroughly ground in a 100:1 ratio in a natural agate mortar and then pressed into transparent tablets using a tablet press and tested in an infrared spectrometer with a scanning range of 500-4000 cm −1 . In this paper, the crystal composition of the prepared activated carbon samples was analyzed using X-ray diffraction (XRD) with a Rigaku D/MAX 2500 V model (Rigaku, Japan) with a scanning range from 5 to 65 • and a scanning speed of 10 • /min. Finally, the surface elements and valence states of the activated carbon samples were characterized using X-ray photoelectron spectroscopy (XPS).

Electrochemical Performance Test
A certain amount of PTFE, acetylene black, and activated carbon samples (ratio 1:1:8) were weighed in a natural agate body mortar, thoroughly polished and stirred well before being placed on a nickel sheet 2 cm long and 1 cm wide, then wrapped with a film of electrode material and squeezed with a pneumatic press to obtain an electrode sheet. Subsequently, the electrode sheet was placed in an uncovered glass container and transferred to a vacuum drying chamber for evacuation, with the temperature set at 80 • C for 12 h. The electrode sheet was placed in an uncovered glass container and impregnated  , ensuring that the solution does not pass through  the electrode sheets and then transferred to the vacuum drying chamber with the heating  procedure switched off and vacuum evacuation only for 8 h to allow for the complete  penetration of the electrode sheets. The electrode sheet was then removed, assembled with the three electrodes, and tested by adding 6 M potassium hydroxide and 1 M potassium hydroxide solution to the electrolytic cell and reference electrode. Finally, the Faraday EM shield was connected to an electrochemical workstation and linear cyclic voltammetry, constant current charge/discharge, and impedance sweep tests were carried out using CS Studio5 software.

Adsorption Performance Test
Next, 3.6 g of potassium dihydrogen phosphate and 14.3 g of disodium hydrogen phosphate were dissolved in 1000 mL of water and then the buffer solution was heated in a water bath at 60 • C, at which the pH of the buffer solution was about 7. The actual content of the 1.5 g of methylene blue sample was dissolved in the water bath heated buffer solution, stirred well until fully dissolved, cooled to room temperature, and filtered into a 1000 mL volumetric flask; the concentration of the buffer solution was 1.5 g/L. Moreover, 100 mL of the above solution was weighed into a 500 mL volumetric flask and diluted with a buffer solution to configure 300 mg/L of methylene blue solution. Five solutions of methylene blue at 2.4 mg/L, 1.2 mg/L, 0.60 mg/L, 0.48 mg/L, and 0.12 mg/L were prepared. The absorbances of the above five concentrations of methylene blue solution were determined using an ultraviolet spectrophotometer and the standard curve was plotted as Figure 1, with the test wavelength set at 665 nm. Additionally, 20 mg of activated carbon samples were weighed and dissolved in 100 mL of methylene blue solution at a concentration of 300 mg/L and then transferred to conical flasks for two parallel experiments for each activated carbon sample. The conical flask was placed in a water bath thermostatic shaker for 20 min at 25 • C. The samples were filtered through a medium-speed qualitative filter paper and then diluted with buffer solution to a concentration range of 0.12-2.4 mg/L and loaded into the flask for measurement. electrode material and squeezed with a pneumatic press to obtain an electrode sheet. Subsequently, the electrode sheet was placed in an uncovered glass container and transferred to a vacuum drying chamber for evacuation, with the temperature set at 80 °C for 12 h. The electrode sheet was placed in an uncovered glass container and impregnated with a 6M potassium hydroxide solution, ensuring that the solution does not pass through the electrode sheets and then transferred to the vacuum drying chamber with the heating procedure switched off and vacuum evacuation only for 8 h to allow for the complete penetration of the electrode sheets. The electrode sheet was then removed, assembled with the three electrodes, and tested by adding 6 M potassium hydroxide and 1 M potassium hydroxide solution to the electrolytic cell and reference electrode. Finally, the Faraday EM shield was connected to an electrochemical workstation and linear cyclic voltammetry, constant current charge/discharge, and impedance sweep tests were carried out using CS Studio5 software.

Adsorption Performance Test
Next, 3.6 g of potassium dihydrogen phosphate and 14.3 g of disodium hydrogen phosphate were dissolved in 1000 mL of water and then the buffer solution was heated in a water bath at 60 °C, at which the pH of the buffer solution was about 7. The actual content of the 1.5 g of methylene blue sample was dissolved in the water bath heated buffer solution, stirred well until fully dissolved, cooled to room temperature, and filtered into a 1000 mL volumetric flask; the concentration of the buffer solution was 1.5 g/L. Moreover, 100 mL of the above solution was weighed into a 500 mL volumetric flask and diluted with a buffer solution to configure 300 mg/L of methylene blue solution. Five solutions of methylene blue at 2.4 mg/L, 1.2 mg/L, 0.60 mg/L, 0.48 mg/L, and 0.12 mg/L were prepared. The absorbances of the above five concentrations of methylene blue solution were determined using an ultraviolet spectrophotometer and the standard curve was plotted as

Specific Surface Area and Pore Size Distribution Analysis
The specific surface area and pore size distribution of BFAC are one of the effective indicators to characterize its adsorption performance [17]. From Figure 2, the initial segment of the isotherm of the four samples showed a significantly large and steep rise when the relative pressure was less than 0.01. After the relative pressure was greater than 0.01, the adsorption of BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 gradually increased with the relative pressure, which was mainly multilayer adsorption. Additionally, the curves showed an upward curve at a higher relative pressure, which belonged to capillary coalescence phenomenon, and the adsorption isotherms belonged to combined type II and type IV according to IUPAC classification and there was an obvious H2-type hysteresis loop at the end of the curve, indicating that these two samples belonged to ink bottle-shaped mesoporous materials [18]; the pore structure of BFAC-H 4 P 2 O 7 was closer to the ink bottleshaped. The nitrogen adsorption and desorption curves of BFAC-(NH 4 ) 2 HPO 4 belonged to the typical type I. The entire adsorption platform was in a horizontal straight line after the relative pressure is greater than 0.1, indicating that the activated carbon was a microporous material; the data in Table 1 confirmed this conclusion [19]. The curve of BFAC-ZnCl 2 showed a knee-like bending change near the relative pressure of 0.1, mainly because more and more nitrogen molecules were introduced into the system, which was also related to the specific surface area and pore size distribution of the micropores of BFAC-ZnCl 2 (as shown in Table 1 and Figure 3, indicating that the sample had a relatively wide pore size distribution [20]. The specific surface area and pore size distribution of BFAC are one of the effective indicators to characterize its adsorption performance [17]. From Figure 2, the initial segment of the isotherm of the four samples showed a significantly large and steep rise when the relative pressure was less than 0.01. After the relative pressure was greater than 0.01, the adsorption of BFAC-H3PO4 and BFAC-H4P2O7 gradually increased with the relative pressure, which was mainly multilayer adsorption. Additionally, the curves showed an upward curve at a higher relative pressure, which belonged to capillary coalescence phenomenon, and the adsorption isotherms belonged to combined type II and type IV according to IUPAC classification and there was an obvious H2-type hysteresis loop at the end of the curve, indicating that these two samples belonged to ink bottle-shaped mesoporous materials [18]; the pore structure of BFAC-H4P2O7 was closer to the ink bottle-shaped. The nitrogen adsorption and desorption curves of BFAC-(NH4)2HPO4 belonged to the typical type I. The entire adsorption platform was in a horizontal straight line after the relative pressure is greater than 0.1, indicating that the activated carbon was a microporous material; the data in Table 1 confirmed this conclusion [19]. The curve of BFAC-ZnCl2 showed a knee-like bending change near the relative pressure of 0.1, mainly because more and more nitrogen molecules were introduced into the system, which was also related to the specific surface area and pore size distribution of the micropores of BFAC-ZnCl2 (as shown in Table 1 and Figure 3, indicating that the sample had a relatively wide pore size distribution [20].  Table 1, the BET-specific surface area of activated carbon prepared by ZnCl2 activation was the largest among all the samples (1908.51 m 2 /g), illustrating that ZnCl2 had the potential to prepare activated carbon with a high specific surface area. The specific microporous surface area of BFAC-(NH4)2HPO4 was 613.34 m 2 /g, about 95.6% of the total specific surface area, showing that (NH4)2HPO4 was suitable for the preparation of microporous activated carbon. The mesoporous specific surface areas of BFAC-H3PO4 and BFAC-H4P2O7 were 900.50 and 876.58 m 2 /g, accounting for 68.3% and 69.3% of the total specific surface area, respectively, which had a richer mesoporous structure compared with BFAC-ZnCl2 and BFAC-(NH4)2HPO4. It was explained that the preparation of activated carbon by the phosphoric acid method was more suitable for use as a mesoporous adsorbent, whose adsorbent mechanism was capillary condensation [21]. According to Figure 3a, the samples of pyrophosphate and phosphoric acid pyrophosphate had a From Table 1, the BET-specific surface area of activated carbon prepared by ZnCl 2 activation was the largest among all the samples (1908.51 m 2 /g), illustrating that ZnCl 2 had the potential to prepare activated carbon with a high specific surface area. The specific microporous surface area of BFAC-(NH 4 ) 2 HPO 4 was 613.34 m 2 /g, about 95.6% of the total specific surface area, showing that (NH 4 ) 2 HPO 4 was suitable for the preparation of microporous activated carbon. The mesoporous specific surface areas of BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 were 900.50 and 876.58 m 2 /g, accounting for 68.3% and 69.3% of the total specific surface area, respectively, which had a richer mesoporous structure compared with BFAC-ZnCl 2 and BFAC-(NH 4 ) 2 HPO 4 . It was explained that the preparation of activated carbon by the phosphoric acid method was more suitable for use as a mesoporous adsorbent, whose adsorbent mechanism was capillary condensation [21]. According to Figure 3a, the samples of pyrophosphate and phosphoric acid pyrophosphate had a broader and similar pore size distribution. In contrast, BFAC-ZnCl 2 and BFAC-(NH 4 ) 2 HPO 4 were mainly microporous. Figure 3b shows more clearly the pore size distribution of micropores and micromesopores and BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 were basically the same. BFAC-ZnCl 2 had well-developed micropores and was mainly concentrated below 4 nm. BFAC-(NH 4 ) 2 HPO 4 had the narrowest pore size distribution and the smallest pore size (<2 nm) and pore volume (<0.04 cm 3 /g). Consistent with Table 1, the micropore structure of BFAC-ZnCl 2 has a maximum pore size of 1.3 nm, followed by 1.1 nm and 1.9 nm and there are also some micropores with pore sizes in the range of 1.5 nm. It indicates that the pore size distribution of BFAC-(NH 4 ) 2 HPO 4 was more uniform.
bution of micropores and micromesopores and BFAC-H3PO4 and BFAC-H4P2O7 were basically the same. BFAC-ZnCl2 had well-developed micropores and was mainly concentrated below 4 nm. BFAC-(NH4)2HPO4 had the narrowest pore size distribution and the smallest pore size (<2 nm) and pore volume (<0.04 cm 3 /g). Consistent with Table 1, the micropore structure of BFAC-ZnCl2 has a maximum pore size of 1.3 nm, followed by 1.1 nm and 1.9 nm and there are also some micropores with pore sizes in the range of 1.5 nm. It indicates that the pore size distribution of BFAC-(NH4)2HPO4 was more uniform.

FT-IR Analysis
The FT-IR spectra of the four BFACs and BF were shown in Figure 4. The spectra showed a strong absorption peak at 3400 cm −1 associated with the O-H stretching vibration, which increased in intensity after activation, indicating an increase in the -OH content. The absorption peak in the region (4000-3750 cm −1 ) was caused by the absorption of water molecules by the activated carbon sample [22]. The absorption peak observed at 2920 cm −1 and 2850 cm −1 was associated with the stretching vibration of the -CH 2 -group. BF had a cellulose -CH 2 -functional group, so it had a strong peak here. After activation by (NH 4 ) 2 HPO 4 , some of the -CH 2 -groups remained. In contrast, the weak peak of the other sample indicated that a small number of saturated hydrocarbons are present in the other sample and that most of the -OCH 3 groups attached to the aromatic ring have been Polymers 2022, 14, 5500 7 of 14 dissociated during activation [23]. The band at 1620 cm −1 can be ascribed to C-C aromatic ring stretching vibration [24]; this band of BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 was much higher than BFAC-ZnCl 2 and BFAC-(NH 4 ) 2 HPO 4 , indicating that phosphoric acid and pyrophosphate have a certain aromatization effect. The strong peak at 1080 cm −1 indicates the presence of alcohol groups, which indicates the presence of polysaccharides [25].
water molecules by the activated carbon sample [22]. The absorption peak observed at 2920 cm −1 and 2850 cm −1 was associated with the stretching vibration of the -CH2-group. BF had a cellulose -CH2functional group, so it had a strong peak here. After activation by (NH4)2HPO4, some of the -CH2-groups remained. In contrast, the weak peak of the other sample indicated that a small number of saturated hydrocarbons are present in the other sample and that most of the -OCH3 groups attached to the aromatic ring have been dissociated during activation [23]. The band at 1620 cm −1 can be ascribed to C-C aromatic ring stretching vibration [24]; this band of BFAC-H3PO4 and BFAC-H4P2O7 was much higher than BFAC-ZnCl2 and BFAC-(NH4)2HPO4, indicating that phosphoric acid and pyrophosphate have a certain aromatization effect. The strong peak at 1080 cm −1 indicates the presence of alcohol groups, which indicates the presence of polysaccharides [25].

X-ray Diffraction Analysis
The prominent diffraction peaks of BF were around 15.8°, 22.76°, and 34.7°, which were consistent with the crystallographic planes of (110), (200), and (004), respectively [26]. The XRD pattern of BFACs in Figure 5 revealed characteristic diffraction peaks (2θ = 23° and 43°) that correspond to the (002) and (100) signatures, respectively [27]. Additionally, the existence of broad XRD peaks in the spectrum indicated the amorphous nature of the BFACs' structure. This characteristic is beneficial for the adsorption behavior material, because gas or liquid generally penetrated the cellulose matrix through the amorphous phase [28]. BFAC-(NH4)2HPO4 showed a higher intensity (002) crystallographic surface diffraction characteristic peak compared with the other three groups. It can be seen from Table 2 that the 2θ002 of the four samples were 23.0, 22.5, 23.0, and 24.3 and the crystal plane spacing d002 was 0.386, 0.395, 0.372, and 0.366, respectively. BFAC-(NH4)2HPO4 had the smallest d002 value, which indicates that this activated carbon sample has a higher degree of graphitization and a more ordered internal structure, indicating that the activation of diammonium impregnation is more favorable to the formation of graphitic microcrystalline structure in activated carbon. Therefore, the adsorption capacity of the BFAC-(NH4)2HPO4 might be poor.

X-ray Diffraction Analysis
The prominent diffraction peaks of BF were around 15.8 • , 22.76 • , and 34.7 • , which were consistent with the crystallographic planes of (110), (200), and (004), respectively [26]. The XRD pattern of BFACs in Figure 5 revealed characteristic diffraction peaks (2θ = 23 • and 43 • ) that correspond to the (002) and (100) signatures, respectively [27]. Additionally, the existence of broad XRD peaks in the spectrum indicated the amorphous nature of the BFACs' structure. This characteristic is beneficial for the adsorption behavior material, because gas or liquid generally penetrated the cellulose matrix through the amorphous phase [28]. BFAC-(NH 4 ) 2 HPO 4 showed a higher intensity (002) crystallographic surface diffraction characteristic peak compared with the other three groups. It can be seen from Table 2 that the 2θ 002 of the four samples were 23.0, 22.5, 23.0, and 24.3 and the crystal plane spacing d 002 was 0.386, 0.395, 0.372, and 0.366, respectively. BFAC-(NH 4 ) 2 HPO 4 had the smallest d002 value, which indicates that this activated carbon sample has a higher degree of graphitization and a more ordered internal structure, indicating that the activation of diammonium impregnation is more favorable to the formation of graphitic microcrystalline structure in activated carbon. Therefore, the adsorption capacity of the BFAC-(NH 4 ) 2 HPO 4 might be poor.     Figure 6 showed the XPS spectra of the BFACs and BF, which indicated that the BFACs contained the elements C (284 eV), O (532 eV), and N (398 eV). The N1s XPS peak was not found in the spectra of BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 , but the (NH 4 ) 2 HPO 4 activation brings nitrogen-containing functional groups to BF, resulting in the appearance of the N active group.   Figure 6 showed the XPS spectra of the BFACs and BF, which indicated that the BFACs contained the elements C (284 eV), O (532 eV), and N (398 eV). The N1s XPS peak was not found in the spectra of BFAC-H3PO4 and BFAC-H4P2O7, but the (NH4)2HPO4 activation brings nitrogen-containing functional groups to BF, resulting in the appearance of the N active group.  The C1s spectra for BFACs samples from Figure 7 and Figure S1 mainly displayed three peaks corresponding to the C=O (oxygen doubly bonded to aromatic carbon), C-O (oxygen singly bonded to aliphatic carbon), and C-C [29]. In addition, the O1s spectra for BFACs samples from Figure 8 and Figure S2 (Figure 7d). This was mainly due to the contribution of C-OH/C-O-C bonds (Figure 8d). The large oxygen-containing functional groups on the sample's surface could improve the electrochemical wettability and dye adsorption properties [30].

Electrochemical Performance Analysis
The effects of bamboo fiber-based activated carbon by different activators on the capacitive characteristics in a 6 M KOH electrolyte were investigated using a three-electrode system. The electrochemical energy storage performance of BFACs was assessed by CV measurements and the linear sweep rates set in this experiment are 10, 20, 50, and 100 mV/s Polymers 2022, 14, 5500 9 of 14 ( Figure 9). With the increase in scan rates from 10 to 100 mV/s, the current densities and area under the forward and reverse scan of the CV curves increased. This trend is because, at slower scan rates, electrolyte ions were entirely diffused into the electrode materials [31]. Four BFACs samples, having a similar shape of a CV curve at a scan rate of 10 mV/s, which exhibits a quasi-rectangular shape, showed good electrochemical properties. However, the four BFACs samples exhibit different electrochemical properties at a scan rate of 100 mV/s. The CV curve of BFAC-H 3 PO 4 showed fusiform, indicating electrolyte diffusion is difficult. The other three samples presented the rectangle at a scan rate of 100 mV/s, illustrating excellent reversibility for electrolyte ions to adsorb/desorb among the surface of the electrode materials and the incredible rate performance [32]. BFAC-ZnCl 2 had the most significant current because it had the largest specific surface area and the wealthiest pore structure, from which it could be inferred that BFAC-ZnCl 2 had the most prominent specific capacitance. The pore structures of BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 were mainly mesoporous, but the shape of the CV curve at a scan rate of 100 mV/s was different; it is speculated that it was related to the pore shape. The electrolyte diffusion rate was related to the shape of the pore. The shape of BFAC-H 4 P 2 O 7 was closer to the rectangle, so the electrolyte was spreading faster at a high scanning rate.
The GCD curves of BFACs over 0.2, 0.5, 1, and 2 A/g current densities were displayed in Figure 10. The charge/discharge graphs of four BFACs samples revealed symmetrical triangular shapes at various current densities, demonstrating its electrical double layer capacitors (EDLCs) behavior and excellent reversibility [33]. The GCD figures were presented in Table 3. Compared with other samples, BFAC-ZnCl 2 exhibited the longest charging (760.7 s) and discharging times (844.93 s), calculating that the BFAC-ZnCl 2 electrode had the maximum specific capacitance of 169.79 F/g at 0.2A/g. As the reasonable pore structure distribution and maximum specific surface area of the BFAC-ZnCl 2 electrode enhanced the acceptance of K+ and OH− hydrated ions, the performance of the BFAC-ZnCl 2 electrode materials was the most excellent compared to the other samples [34]. The C1s spectra for BFACs samples from Figure 7 and Figure S1 mainly displayed three peaks corresponding to the C=O (oxygen doubly bonded to aromatic carbon), C-O (oxygen singly bonded to aliphatic carbon), and C-C [29]. In addition, the O1s spectra for BFACs samples from Figure 8 and Figure S2 displayed mainly three peaks corresponding to the O-C=O, C-OH/C-O-C, and C=O. Figure 7 showed the C1s spectra of BFACs activated by different activators. BFAC-H3PO4 (Figure 7a), BFAC-H4P2O7 (Figure 7b), and BFAC-ZnCl2 (Figure 7c) had a similar C1s range, explaining the carbon and oxygen content was identical. However, the contents of oxygen-containing functional groups were not identical in the three samples. Compared to BFAC-H3PO4 (Figure 8a), BFAC-H4P2O7 (Figure 8b) had a lower C=O content, while BFAC-ZnCl2 (Figure 8c) had a lower C-OH/C-O-C, and a higher O-C=O content. The highest C-O was found in the (NH4)2HPO4 activated samples (Figure 7d). This was mainly due to the contribution of C-OH/C-O-C bonds (Figure 8d). The large oxygen-containing functional groups on the sample's surface could improve the electrochemical wettability and dye adsorption properties [30].

Electrochemical Performance Analysis
The effects of bamboo fiber-based activated carbon by different activators on the capacitive characteristics in a 6 M KOH electrolyte were investigated using a three-electrode system. The electrochemical energy storage performance of BFACs was assessed by CV measurements and the linear sweep rates set in this experiment are 10, 20, 50, and 100 mV/s (Figure 9). With the increase in scan rates from 10 to 100 mV/s, the current densities and area under the forward and reverse scan of the CV curves increased. This trend is because, at slower scan rates, electrolyte ions were entirely diffused into the electrode materials [31]. Four BFACs samples, having a similar shape of a CV curve at a scan rate of 10 mV/s, which exhibits a quasi-rectangular shape, showed good electrochemical properties. However, the four BFACs samples exhibit different electrochemical properties at a scan rate of 100 mV/s. The CV curve of BFAC-H3PO4 showed fusiform, indicating electrolyte diffusion is difficult. The other three samples presented the rectangle at a scan rate of 100 mV/s, illustrating excellent reversibility for electrolyte ions to adsorb/desorb among the surface of the electrode materials and the incredible rate performance [32]. BFAC-ZnCl2 had the most significant current because it had the largest specific surface area and the wealthiest pore structure, from which it could be inferred that BFAC-ZnCl2 had the most prominent specific capacitance. The pore structures of BFAC-H3PO4 and BFAC-H4P2O7 were mainly mesoporous, but the shape of the CV curve at a scan rate of 100 mV/s was different; it is speculated that it was related to the pore shape. The electrolyte diffusion rate was related to the shape of the pore. The shape of BFAC-H4P2O7 was closer to the rectangle, so the electrolyte was spreading faster at a high scanning rate. The GCD curves of BFACs over 0.2, 0.5, 1, and 2 A/g current densities were displayed in Figure 10. The charge/discharge graphs of four BFACs samples revealed symmetrical triangular shapes at various current densities, demonstrating its electrical double layer capacitors (EDLCs) behavior and excellent reversibility [33]. The GCD figures were presented in Table 3. Compared with other samples, BFAC-ZnCl2 exhibited the longest charging (760.7 s) and discharging times (844.93 s), calculating that the BFAC-ZnCl2 electrode  In order to understand the ion and electron transport of the BFAC electrodes in 6M KOH electrolyte, electrochemical impedance spectroscopy (EIS) measurements were performed and the results were shown in Figure 11. The Nyquist plots of all the samples exhibited a linear trend at a low frequency, indicating an ideal capacitive behavior of the electrodes [35]. Compared to the other samples, the Nyquist plots of BFAC-ZnCl2 was closer to vertical in the low-frequency region, demonstrating better capacitance characteristics and faster ion diffusion rates about BFAC-ZnCl2. At a high frequency, series resistance (Rs) was obtained from the intercept at the real axis (Z') in the Nyquist plot; the Rs of BFAC-ZnCl2 is the lowest [36].  In order to understand the ion and electron transport of the BFAC electrodes in 6M KOH electrolyte, electrochemical impedance spectroscopy (EIS) measurements were performed and the results were shown in Figure 11. The Nyquist plots of all the samples exhibited a linear trend at a low frequency, indicating an ideal capacitive behavior of the electrodes [35]. Compared to the other samples, the Nyquist plots of BFAC-ZnCl 2 was closer to vertical in the low-frequency region, demonstrating better capacitance characteristics and faster ion diffusion rates about BFAC-ZnCl 2 . At a high frequency, series resistance (Rs) was obtained from the intercept at the real axis (Z') in the Nyquist plot; the Rs of BFAC-ZnCl 2 is the lowest [36].

Adsorption Performance Analysis
In previous studies, f single molecular layers, as a porous adsorption material, in addition to the surface chemical composition of dye adsorption, a pore structure, also had a certain impact on adsorption.
As can be seen from Table 4, the adsorption performance of BFAC-ZnCl2 is the best (648.75 mg/g); this sample contains more micropores with larger pore sizes and smaller pore size mesopores, indicating that the content of larger micropores and mesopores plays a major role in the adsorption of activated carbon fibers to methylene blue. In addition, compared to BFAC-H3PO4 (373.33 mg/g), BFAC-H4P2O7 of similar void distribution, crystal form, and chemical composition had a higher MB adsorption capacity (548.75 mg/g), which proved that the ink bottle-shaped pore structure was advantageous to the adsorption of MB. BFAC-(NH4)2HPO4 had the lowest amount of MB adsorption (218.75 mg/g) due to the lowest specific surface area mostly correlating with adsorption capacity.

Conclusions
In this work, the effects of four activators, namely phosphoric acid, pyrophosphoric acid, zinc chloride, and ammonium dihydrogen phosphate, on the structure and properties of bamboo fiber cell-based activated carbon were investigated. As previously shown, BFAC-(NH4)2HPO4 had the smallest BET specific surface area and the pore structure was dominated by micropores, with the pore size distribution concentrated from 0.5 nm to 0.8 Figure 11. Electrochemical impedance spectroscopy of BFACs.

Adsorption Performance Analysis
In previous studies, f single molecular layers, as a porous adsorption material, in addition to the surface chemical composition of dye adsorption, a pore structure, also had a certain impact on adsorption.
As can be seen from Table 4, the adsorption performance of BFAC-ZnCl 2 is the best (648.75 mg/g); this sample contains more micropores with larger pore sizes and smaller pore size mesopores, indicating that the content of larger micropores and mesopores plays a major role in the adsorption of activated carbon fibers to methylene blue. In addition, compared to BFAC-H 3 PO 4 (373.33 mg/g), BFAC-H 4 P 2 O 7 of similar void distribution, crystal form, and chemical composition had a higher MB adsorption capacity (548.75 mg/g), which proved that the ink bottle-shaped pore structure was advantageous to the adsorption of MB. BFAC-(NH 4 ) 2 HPO 4 had the lowest amount of MB adsorption (218.75 mg/g) due to the lowest specific surface area mostly correlating with adsorption capacity.

Conclusions
In this work, the effects of four activators, namely phosphoric acid, pyrophosphoric acid, zinc chloride, and ammonium dihydrogen phosphate, on the structure and properties of bamboo fiber cell-based activated carbon were investigated. As previously shown, BFAC-(NH 4 ) 2 HPO 4 had the smallest BET specific surface area and the pore structure was dominated by micropores, with the pore size distribution concentrated from 0.5 nm to 0.8 nm and from 1 nm to 1.5 nm. On the contrary, BFAC-ZnCl 2 has the highest specific surface area and the pore structure was mainly micropores and mesopores, with the pore size distribution concentrated from 1 to 2 nm and from 2.1 to 4 nm, respectively. In contrast, BFAC-H 3 PO 4 and BFAC-H 4 P 2 O 7 have moderate specific surface areas, with basically similar pore size distributions and the pore structures are mainly mesoporous.
In terms of electrochemistry, the specific capacitance of BFAC-ZnCl 2 was the largest. At the current density of 0.2 A/g, the specific capacitance was 121.2730 F/g. Additionally, the internal resistance of the sample was small, which reflects the good electrochemical reversibility and capacitance performance. From the perspective of the adsorption performance, the adsorption amount of methylene blue is affected by the BET specific surface area, pore volume, surface functional group content, and other factors. Considering various factors, BFAC-ZnCl 2 has the best adsorption performance. The superior adsorption capacity of activated carbon has the potential for applications in catalysis, drug loading, and heavy metal enrichment.