Hierarchical Biocarbons with Controlled Micropores and Mesopores

Mar 28, 2019 - The AC4 biocarbon exhibits a maximum specific capacitance of 332.3 F/g at a current density of 1 A/g, and it still remains a specific c...
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Article Cite This: ACS Omega 2019, 4, 5991−5999

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Hierarchical Biocarbons with Controlled Micropores and Mesopores Derived from Kapok Fruit Peels for High-Performance Supercapacitor Electrodes Shu-Xia Liang,† Fang-Fang Duan,† Qiu-Feng Lü,*,† and Haijun Yang‡ †

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Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), College of Materials Science and Engineering, Fuzhou University, 2 Xueyuan Road, Fuzhou 350116, China ‡ CAS Key Laboratory of Interfacial Physics and Technology & Interfacial Water Division, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China S Supporting Information *

ABSTRACT: Hierarchical microporous and mesoporous biocarbons derived from kapok fruit peels were prepared by a two-step method. First, a carbon precursor was prepared from the kapok fruit peels by precarbonization at lower temperature, and then it was activated at a higher temperature with KOH as an activation agent. Micropores and mesopores of the biocarbons were controlled by adjusting the mass ratio of KOH to the carbon precursor. Especially, the biocarbon AC4 that was prepared with a KOH/carbon precursor mass ratio of 4:1 possesses a sheet structure with hierarchical pores, a high specific surface area of 1257.6 m2/g, and a pore volume of 1.05 cm3/g. The AC4 biocarbon exhibits a maximum specific capacitance of 332.3 F/g at a current density of 1 A/g, and it still remains a specific capacitance as high as 180 F/g with increasing the current density up to 50 A/g. The AC4 electrode material also possesses an excellent life span after 10 000 cycles. On the basis of these obtained results, the relationship between the electrochemical performance and the structure of the biocarbon was discussed. used in hybrid electric vehicles with low CO2 emission13 and fuel cell vehicles.14 Porous carbons (PCs) have been widely used as supercapacitor electrode materials owing to their high specific surface areas, hierarchical pore structures, excellent conductivity for fast transport of the electrolyte ions, and great cycle stability.10,12,15−17 However, most PCs have a single kind of pore structures, presenting only micropores or mesopores, which limit their electrical performances. On the basis of the charge storage mechanism of EDLCs as mentioned before, the specific capacitance of EDLCs is not only related to the specific surface areas but also affected by the pore structures of the electrode materials. Micropores make contributions to increase the specific surface area and provide abundant adsorbing sites for the electrolyte ions.18 At lower current densities, microporous carbon electrodes show high specific capacitances, but the specific capacitances sharply drop with the increase of current densities. It might be caused by the limited electrolyte ion transport resulting from the small pore size of micropores.17 Mesopores in PCs facilitate ion transport by providing smaller resistance and shorter diffusion pathways,19 and hence

1. INTRODUCTION The exponential growth of fossil fuel consumption results in many problems, such as air pollution, greenhouse effect, and acid rain. Until now, many countries have been developing clean, cheap, and sustainable energies, for instance, solar, wind, and geothermal energy.1−3 However, these energy sources are limited by natural conditions because there are many difficulties in energy storage and conversion.4 Electrical capacitors, sometimes called supercapacitors, ultracapacitors, or hybrid capacitors,5 have attracted widespread attention because they exhibit superb energy storage,6 long cycle life, simple principle, and high dynamic charge propagation.7 Supercapacitors show higher energy densities compared with traditional capacitors and larger power densities than batteries. According to their charge storage mechanism, supercapacitors can be divided into pseudocapacitors and double-layer capacitors.8 The pseudocapacitors store charges by redox reactions which occur on the surface of the electrodes.9 In the electrical double-layer capacitors (EDLCs), the charges are stored by the surface adsorption of ions from the electrolyte as a result of electrostatic attractions, thus forming two charged layers (double layer).3 At present, supercapacitors are mainly used in electric vehicles, electric hybrid vehicles, digital communication devices, digital cameras, and mobile phones.10−12 Moreover, their most promising applications are © 2019 American Chemical Society

Received: January 16, 2019 Accepted: March 18, 2019 Published: March 28, 2019 5991

DOI: 10.1021/acsomega.9b00148 ACS Omega 2019, 4, 5991−5999

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C800 were also prepared by using directed carbonization of the kapok fruit peels at 650, 700, 750, and 800 °C, respectively. Compared with their specific capacitances (Figure S1) and morphologies (Figure S2), 700 °C was the optimal activation temperature. 2.2. Morphologies. The morphologies of C650, C700, C750, C800 (Figure S2), and the AC biocarbons (Figure 2) were characterized by scanning electron microscopy (SEM). It can be noted that there are many different sized cylindrical holes in the C650, C700, C750, C800, and AC biocarbons, which were like honeycombs as a whole. The SEM images in Figure 2 show two kinds of macrospores, one is about 20 μm cylindrical holes and the other is on the wall of the hole with diameter 500 nm, forming ion-buffering reservoirs. Compared with the AC biocarbons, C700 has smoother and flat surfaces (Figure 2a). In the lower-magnification SEM images, the AC biocarbons with KOH activation broke into pieces. It can be found that on the wall of AC, the biocarbons become thinner and exhibit more crease and pores on their surfaces resulting from KOH etching. For the overactivated AC4 and AC5 biocarbons, their sheets are too thin to support the honeycomb structures, resulting in collapse of the holes. In brief, the morphologies of AC3 and AC3.5 are block structures (Figure 2b,c), whereas AC4 and AC5 (Figure 2d,e) presents uniform and small nanosheets which can increase their specific surface areas. Transmission electron microscopy (TEM) images of AC4 in Figure 2f further prove that there are many micropores and mesopores in the AC4 biocarbon nanosheets. Therefore, the KOH/carbon precursor mass ratio is a key factor controlling the morphologies of biocarbons, and the hierarchical AC4 biocarbon with controlled micropores and mesopores was successfully prepared derived from kapok fruit peels. The reaction between KOH and the carbon precursor during the activation process is carried out by the following steps.31−34 First, KOH (melting point 360 °C) is melted and sufficiently comes in contact with the carbon precursor. At around 400 °C, K2CO3 is formed by a redox reaction of carbon precursor and KOH (eq 1). Then, all KOH molecules transform into K2CO3 at about 600 °C. When the pyrolysis temperature is higher than 700 °C, K2CO3 is decomposed to K2O and CO2 (eq 2) and completely disappeared at 800 °C. In addition, the reduction reactions of CO2, K2O, and K2CO3 by a carbon precursor that produce CO and potassium metal occur at higher temperatures (eqs 3−5, respectively). Therefore, the process of KOH activation develops pore structures. The redox reactions of the carbon precursor and potassium compounds (K2O and K2CO3) lead to the collapse of the carbon precursor and the as-formed potassium metal intercalates into the carbon lattices of the biocarbon that leads to volumetric expansion of the carbon framework. After removing these intercalated potassium compounds (K, K2O, and K2CO3) by washing, a large specific surface area with high porosity is created.

improving the electrical performances of the electrodes. Therefore, PC electrodes possessing both micropores and mesopores are benefitted for charge and storage of supercapacitors. Until now, more common raw materials, such as coffee beans,20 corn stover,1 bamboo,21 and chestnut shells,22 have been used as precursors to produce renewability,23 low-cost,24 eco-friendly,25 and abundance26 porous biocarbon electrodes for EDLCs. Kapok fruit peels, a kind of agricultural waste derived from the fruits of silk cotton trees that are widely distributed in South China, are mainly composed of cellulose, hemicellulose, and lignin. Especially, the kapok fruit peels exhibit honeycomb-like structures that can develop interconnected pore networks. Recently, kapok fibers have received increasing attention as supercapacitor electrode materials with excellent electrical performances.27,28 However, kapok fibers are not agricultural wastes and have been widely applied in textile industry, whereas the kapok fruit peels are discarded directly. Therefore, the kapok fruit peels are more ideal raw precursor materials to prepare porous biocarbons as supercapacitor electrodes. In this study, porous biocarbons were prepared from kapok fruit peels by using a simplified method. The raw materials were first precarbonized at a lower temperature to acquire carbon precursors, and then the carbon precursors were activated by using KOH as an activating agent at higher temperature to develop porous biocarbons. The pore structures of the biocarbons can be controlled by adjusting the mass ratio of KOH to the carbon precursors. Furthermore, electrochemical performances of the biocarbons were systematically evaluated.

2. RESULTS AND DISCUSSION 2.1. Thermogravimetric Analysis. Carbonization temperature is an important factor in determining the pore structures of carbon materials; thus, thermogravimetric analysis (TGA) was conducted in order to choose the right pyrolysis temperature. TGA and derivative thermogravimetry (DTG) curves of raw kapok fruit peel powder are displayed in Figure 1a, from which it can be seen that weight loss mainly occurs at

Figure 1. (a) TGA and DTG curves and (b) FT-IR spectrum of raw kapok fruit peel powder.

200−500 °C because of decomposition of organics.29 Two large peaks are observed at around 310 and 440 °C from the DTG curve, the peak at 300 °C on account of the primary pyrolysis of hemicellulose and the peak at 440 °C as a result of the primary pyrolysis of cellulose. Meanwhile, the weight loss below 100 °C with a tiny peak is caused by water evaporation.30 According to the TGA curve, the kapok fruit peels were precarbonized at 500 °C. The biocarbons C650, C700, C750, and 5992

6KOH + 2C → 2K + 2K 2CO3 + 3H 2

(1)

K 2CO3 → K 2O + CO2

(2)

CO2 + C → 2CO

(3)

K 2CO3 + 2C → 2K + 3CO

(4)

C + K 2O → 2K + CO

(5) DOI: 10.1021/acsomega.9b00148 ACS Omega 2019, 4, 5991−5999

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Figure 3. (a) N2 adsorption−desorption isotherms, (b) H−K absorption PSDs, (c) BJH desorption PSDs, and (d) specific surface areas and pore volumes of C700, AC3, AC3.5, AC4, and AC5.

number of mesopores and micropores. It can be found that there are obvious uptakes at low relative pressure, which are characteristics of microporous materials. The isotherms with a wide knee, remarkable hysteresis slope at intermediate and high relative pressure do not show clear platforms, proving the existence of mesopores. In addition, it can be observed that the isotherms with hysteresis loops of the AC biocarbons become wider, compared with the isotherm of C700, indicating that there are more mesopores in the AC biocarbons. Pore size distributions (PSDs) of the AC biocarbons and C700 computed using Horwath−Kawazoe (HK) and Barrett− Joyner−Halenda (BJH) are illustrated in Figure 3b,c. The HK model based on the continuous pore filling method is applicable for micropores only.36 The BJH theory is applicable only for condensation process on mesopores. It can be seen from Figure 3b,c that the porosity of all samples is composed of micropores and mesopores as mentioned before and that the pore sizes are mainly concentrated in 3−4 nm and 0.3−0.4 nm. Although the pore size regions of the AC biocarbons have no significant change compared to that of C700, the amount of micropores and mesopores improved greatly according to the BJH and HK curves. Furthermore, the intensities of BJH and HK curves of the AC4 biocarbon are highest, confirming that the AC4 biocarbon presents the largest number of micropores and mesopores. The physical properties of the AC biocarbons and C700 are summarized in Figure 3d and Table 1. The Brunauer− Emmett−Teller (BET)-specific surface area (SBET) of C700 without activation is 766.3 m2/g. After KOH activation, the specific surface areas of the AC biocarbons improved obviously from 1003.1 m2/g of AC3 to 1257.6 m2/g of AC4, whereas by increasing the mass ratio of KOH to the carbon precursor from 4:1 to 5:1, the SBET of AC5 decreased to 1083.0 m2/g. In Table 1, the specific surface area of micropores (Smic) of AC5 enhanced slightly, whereas the specific surface area of mesopores (Smes) decreased obviously. It can be deduced from the variation of Smic and Smes values that overactivation of the carbon precursor at high KOH usage would cause collapse of the mesopores and lead to the decrease in specific surface area.37 Therefore, the AC4 biocarbon possesses the highest specific surface area among all the four AC biocarbons.

Figure 2. FE-SEM images of C700 (a1,a2), AC3 (b1,b2), AC3.5 (c1,c2), AC4 (d1,d2), and AC5 (e1,e2) and TEM images of AC4 (f1,f2).

2.3. Surface Area and Porosity. Nitrogen adsorption− desorption isotherms at 77 K of the AC biocarbons are exhibited in Figure 3. All the isotherms were categorized as a mixed type in the IUPAC classification (Figure 3a), type I at low relative pressures (P/P0) and type IV at intermediate and high relative pressures,35 implying the presence of a large 5993

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Table 1. Porosity Parameters of C700, AC3, AC3.5, AC4, and AC5 sample C700 AC3 AC3.5 AC4 AC5

a

SBET (m2 g−1) 766.3 1003.1 1048.6 1257.6 1083.0

c

Smic (m2·g−1) 522.3 574.6 714.2 651.5 721.2

d

Smes(m2·g−1) 244.0 428.6 334.4 606.1 361.7

b

Vtot (cm3·g−1) 0.45 0.65 0.63 1.05 0.83

c

Vmic (cm3·g−1) 0.27 0.30 0.37 0.35 0.38

d

Vmes (cm3·g−1) 0.18 0.35 0.26 0.70 0.45

a

The surface area (SBET) was calculated by the multipoint BET method at the relative pressure range of P/P0 = 0.05−0.20. bThe total pore volume (Vtotal) was estimated at P/P0 = 0.993. cMicropore surface area (Smic) and micropore volume (Vmic) were calculated using the t-plot method. d Mesopore surface area (Smes) and mesopore volume (Vmes) were calculated by using Smes = SBET − Smicr and Vmes = Vtotal − Vmicr, respectively.

and the stretching peaks of −C−O−C at 1047.4 cm−1 can be observed; the peaks at 2908.6 and 1733.4 cm−1 correspond to the stretching vibration of −CH2 and CO, respectively. Compared with the raw kapok fruit peels, the spectra of C700 and the AC biocarbons are simpler and smoother, indicating that some functional groups degraded during the pyrolysis process. The residue groups on biocarbons are −OH, CC, and −C−O−C, and these two peaks of −CH2 and CO disappear in the spectra of the AC biocarbons (Figure 4a). The stretching vibration peaks of −OH groups are at 3453 cm−1 in Figure 4a, and the intensities of these peaks for the AC biocarbons are stronger and sharper than that of 700, indicating higher −OH group contents of the AC biocarbons. The stretching vibration peaks of CC at 1556 cm−1 in aromatic compounds and the stretching peaks of −C−O−C at 1064 cm−1 can still be observed in the spectra of the AC biocarbons, which are similar to the raw materials. The FT-IR spectra declare that the AC biocarbons contain oxygencontaining functional groups. Raman analysis was employed to further confirm the structures. The Raman spectra of C700, AC3, AC3.5, AC4, and AC5 in (Figure 4b) displayed two characteristic peaks, a Dband at 1335 cm−1 and a G-band at 1595 cm−1. The D and G bands are assigned to the crystal defects or imperfections and a hexagonal carbon plane, respectively.27 Hence, the intensity ratio of D band to G band (r = ID/IG) is used to measure disorder of samples.40 In Figure 4c, the ID/IG of C700, AC3, AC3.5, AC4, and AC5 are 1.13, 0.997, 0.954, 0.940, and 0.971, respectively. The results demonstrate that the graphitization degrees of the samples are lower even if the carbon precursors are activated at a very high temperature. 2.5. XRD Analysis. XRD tests were used to determine the crystalline structures of C700 and the AC biocarbons. As we can see in Figure 4d, there are two typical peaks at approximately 2θ of 22.58° and 42°, which are assigned to the (002) and (100) crystal planes of biocarbons, respectively. These peaks indicate the existence of graphite crystallites in the AC biocarbons, and they should greatly improve the electrical conductivity41 of the AC biocarbons. The shapes of the two peaks are wide and short, indicating that the AC biocarbons are amorphous layered structures. 2.6. XPS Analysis. X-ray photoelectron spectroscopy (XPS) was employed to determine whether the AC4 biocarbon contained oxygen and nitrogen groups. From Figure 5a, AC4 contains two distinct peaks, confirming only two chemical elements. The significant peak at 284 eV is C 1s, and the weak peak concentrate at 532 eV is O 1s. The C 1s region was deconvoluted to three peaks that imply the presence of CC (284.6 eV), C−O−C (285.6 eV), and C−OH (288.2 eV) bonds42 (Figure 5b). The high-resolution O 1s region in Figure 5c shows the coexistence of C−OH (531.3 eV) and C−

The AC biocarbons can produce micropores and mesopores during the KOH etching. The data in Figure 3d and Table 1 show that the pore volumes of all the AC biocarbons increase disorderly. The pore volume is not much different between AC3 and AC3.5. AC4 presents a maximum pore volume, whereas the pore volume of AC5 is lower. According to the data of Smic, Smes, Vmic, and Vmes in Table 1 and Figure 3d, it can be found that AC3.5 mainly develops micropores by KOH activation leading to lower pore volume. As for AC4, it is speculated that the collapse of its micropores led to the formation of small mesopores.38 Thus, the collapse of the mesopores in AC5 caused smaller pore volume. As we know, the mesopores could serve as a reservoir of electrolyte that can accelerate the diffusion of ions, while the micropores can provide a large contact area between the electrode and the electrolyte, resulting in the significant enhancement of electrical double-layer area and exposed area of active sites.39 Because the AC4 biocarbon possesses the highest SBET and pore volume as well as balanced micropores and mesopores, it is expected to be a promising electrode material for supercapacitors.37 2.4. FT-IR and Raman Spectra Analyses. Surface oxygen-containing functional groups of the AC biocarbons were confirmed by the Fourier transform infrared (FT-IR) spectra. The spectra of the raw kapok fruit peels and the AC biocarbons were illustrated in Figures 1b and 4a, respectively. In Figure 1b, the peaks at 3428.6 cm−1 result from the stretching vibration of the −OH groups and the stretching vibration peaks of CC at 1641 cm−1 in aromatic compounds

Figure 4. (a) FT-IR, (b) Raman spectra, (c) ID/IG, and (d) XRD curves of C700, AC3, AC3.5, AC4, and AC5. 5994

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Figure 5. (a) XPS survey spectrum, (b) C 1s and (c) O 1s spectra of AC4, and (d) elemental content of the four AC biocarbons and C700.

O−C (532.4 eV). The contents of carbon and oxygen elements displayed in Table 2 and Figure 5d indicate that Table 2. Elemental Contents (at. %) of AC4 Obtained from XPS Analysis element

C700

AC3

AC3.5

AC4

AC5

C O

95.89 4.11

92.59 7.41

91.57 8.43

90.00 10.00

91.23 8.77

Figure 6. (a) CV curves of C700, AC3, AC3.5, AC4, and AC5 at a scan rate of 100 mV/s; (b) GCD curves of C700, AC3, AC3.5, AC4, and AC5 at a current density of 1 A/g; (c) CV curves of AC4 at various scan rates; (d) GCD curves of AC4 at various current densities; and (e) specific capacitances of AC3, AC3.5, AC4, and AC5 at different current densities.

the AC biocarbons have higher oxygen contents. Compared with C700, the four AC biocarbons contain more oxygen and lower carbon contents, and the AC4 biocarbon has the highest oxygen content of 10 at. %. These results are consistent with the analysis of its FT-IR spectrum. According to the current reports, the oxygen-containing functional group is an important factor in enhancing the hydrophilicity and wettability of the material surface and facilitating the accessibility of the electrolyte ions.25 Therefore, oxygencontaining functional groups can be good for the increase of specific capacitance of the AC biocarbons. 2.7. Electrochemical Performances. Figure 6a shows the CV curves of C700 and the AC biocarbons with a threeelectrode system. The CV curves present approximately rectangular shapes, suggesting that the AC biocarbons present electric double-layer structures. However, the CV curves are not perfect rectangles because the AC biocarbons and C700 have little pseudocapacitive contribution because of to their oxygen-containing functional groups.25 Clearly, the AC4 biocarbon obtains the largest CV curve area among the five biocarbons, signifying that AC4 has the largest specific capacitance among the supercapacitor electrodes. As shown in Figure 6b, the specific capacitances of C700, AC3, AC3.5, AC4, and AC5 at 1 A/g current density are 197.2, 228.8, 241.9, 332.3, and 252.1 F/g, respectively. It can be observed that the specific capacitances of the AC biocarbon electrodes have been significantly enhanced than that of C700. Moreover, AC4 has the longest discharge time, implying that it possesses superior electrochemical performance than other AC biocarbon electrodes. As was mentioned above, high specific surface area, more oxygen-containing functional groups, and

low internal resistance43 of AC4 contribute to its highest specific capacitance. The CV curves of the AC4 biocarbon at scan rates within 10−200 mV/s are illustrated in Figure 6c. It can be seen that when the scan rate increased to 100 mV/s, the CV curves could keep the nearly rectangular shape. This result reveals the superb rate capability of AC4 biocarbon in the process of a fast charge−discharge, which attributes to the surface oxygencontaining functional groups and interworking of hierarchical porous structures. The galvanostatic charge−discharge (GCD) curves of the AC4 electrodes at various current densities from 1 to 50 A/g are depicted in Figure 6d. Clearly, no obvious potential drop can be discovered at the start of the discharge process for the AC4 electrode, and all lines are regularly in triangular shape. Moreover, Figure 6e provides the specific capacitances of the four AC biocarbon electrodes at various current densities over 1−50 A/g. The AC4 biocarbon electrode has higher specific capacitance than those of the other three biocarbon electrodes. The largest specific capacitance of the AC4 electrode is 332.3 F/g at a current density of 1 A/g, and it still keeps 180 F/g even at a current density of 50 A/g. At higher current densities, the decreases in specific capacitances are due to the sterical limitations of materials over which ions can only partially penetrate into the micropores.25 Furthermore, the specific capacitances of previously published activated biocarbons from biomass feedstocks are summarized in Table 3. These results show that the AC4 biocarbon electrode exhibits an outstanding electrochemical 5995

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C700 exhibit lowest Rct and highest Rct, respectively, which prove that the appropriate KOH/carbon precursor mass ratio of 4:1 can improve the conductivity of the AC4 biocarbon. Furthermore, the Nyquist plot of the AC4 biocarbon is closer to the imaginary axis in the low-frequency region than the C700 Nyquist plot, indicating diffusion of ions in the AC4 biocarbon with less limitation. Cycle stability is a very significant parameter for measuring the supercapacitor performance. Thence, repeated CV tests were used to evaluate the stability of the AC biocarbons at a scan rate of 100 mV/s for 10 000 cycles. The cycling stability curves of C700 and the AC biocarbons in Figure 7c reveal that all the biocarbons have excellent cycle lives. Especially, after 10 000 cycles, the AC4 biocarbon still maintained ultrahigh specific capacitance, and the capacitance retention ratio is 96.8%. This benefits from the unique microporous and mesoporous ratios, stable porous biocarbon structures, high oxygen content, lower impedance, and the specific surface area of the AC4 biocarbon itself.22 In a two-electrode supercapacitor, the specific capacitance of AC4 is 80 F/g (Figure 7c,d), and this assembled two-electrode supercapacitor shows a maximum energy density of 12.36 W h/kg with a power density of 500 W/kg in 6 mol/L KOH electrolyte at 1 A/g. The inset image in Figure 7d further shows that a red light-emitting diode with a working voltage of 2 V can be lightened by the symmetric supercapacitor connected in series with the AC4 electrodes, proving that the AC4 biocarbon has a great commercial application as a highperformance supercapacitor.

Table 3. Specific Capacitances of Biocarbons Derived from Various Biomass Feedstocks biomass feed stock

current density (A/g)

Ca (F/g)

ref

rice stem loofah sponge Osmanthus flower olive pits AC4

20 50 20 10 10 20 50

32 100 57.3 76 243.6 200 180

44 45 46 47 this work this work this work

a

KOH as an activated agent, 6 mol/L KOH as an electrolyte solution.

performance. This is attributed to the controlled porous structure, high specific surface area, more oxygen-containing functional groups, and reasonable micropore/mesopore ratio of the AC4 biocarbon. The electrochemical impedance spectroscopy (EIS) measurement was used to study the conductivity of all the biocarbon samples in a frequency range from 100 kHz down to 10 mHz at open-circuit potential. In Figure 7a, the Nyquist plots of C700 and the AC biocarbons demonstrated relatively semicircles in the high-frequency region, followed by the 45° slope lines, then nearly vertical lines in the low-frequency region, which are typical characteristics of double-layer supercapacitors. The semicircle of the Nyquist plot intercept with real axis represents the combined series resistance of the electrolyte, electrode, current collectors, and electrode/current collector contact resistance (Rs). The diameter of the semicircle denotes the charge-transport resistance (Rct). A straight line with a slope of 45° in the low-frequency range corresponds to the semi-infinite Warburg impedance resulting from the frequency dependence of ion diffusion/transport in the electrolyte. A vertical line at very low frequency is caused by the accumulation of ions at the bottom of the pores of the electrode.48 Comparing the curves of C700, AC3, AC3.5, AC4, and AC5, their similar Rs values are resulted from application of the same electrolyte and electrode preparation technique.41 As shown in the inset image of Figure 7a, the AC4 biocarbon and

3. CONCLUSIONS Microporous and mesoporous biocarbons derived from kapok fruit peels were successfully prepared from kapok fruit peels by KOH activation. The AC biocarbons have reasonable hierarchical porous structures, high specific surface areas and pore volumes, and high oxygen-containing functional groups. The morphologies of the AC biocarbons changed significantly with the increase of KOH/carbon precursor mass ratios. When the mass ratio of KOH/carbon precursor is 4:1, the as-

Figure 7. (a) EIS and (b) cycling stability of C700, AC3, AC3.5, AC4, and AC5 biocarbons during long-term CV tests at 100 mV/s, (c) GCD curve of AC4 at a current density of 1 A/g (the two-electrode system), (d) CV curve of AC4 at scan rate of 100 mV/s (the two-electrode system), and the inset image of the 2.0 V light-emitting diode bulb lit by three sets of two solid-state supercapacitors in series. 5996

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diffraction (XRD, Rigaku, Japan). TGA was conducted on a SDT-Q600 thermogravimetric analyzer (TA, America) in a nitrogen atmosphere at a heating rate of 5 °C/min. The functional groups of biocarbons were performed by FT-IR spectroscopy (Nicolet FT-IR 360). Raman spectra were measured from a Raman spectrometer (532 nm, Invia Reflex, China). The specific surface areas were analyzed by BET method (Micromeritics Instrument Corporation, USA). Elemental analysis was recorded by an X-ray photoelectron spectrum from an X-ray photoelectron spectrometer (ESCALAB 250). 4.4. Electrochemical Measurements. Electrodes were composed of porous biocarbons (85 wt %), polytetrafluoroethylene (5 wt %, as binder), and acetylene carbon black (10 wt %). Then, 2 mg of electrode material mixture was pressed onto a clean stainless steel mesh (1 cm × 1 cm) at 10 MPa and dried at 60 °C for 24 h.49 All works were performed by a CHI660E electrochemical workstation (Chenhua Instrument Co.) at room temperature. A typical three-electrode cell was assembled with Ag/AgCl as a reference electrode, carbon as a working electrode, Pt wire as a counter electrode, and 6 mol/L KOH as an electrolyte solution. A symmetrical two-electrode supercapacitor was assembled by two biocarbon electrodes (containing 85 wt % porous biocarbons, 5 wt % polytetrafluoroethylene binder, and 10 wt % acetylene carbon black) in a beaker where the two electrodes were separated by an air-laid paper, and 6 mol/L KOH served as an electrolyte solution.49 Cyclic voltammetry (CV) was investigated at scan rates of 10, 20, 50, 100, and 200 mV/s. GCD was performed at a current density load of 1, 2, 5, and 10 A/g. EIS was obtained with frequencies from 0.01 to 100 kHz. The cyclic stability of the biocarbons was measured by CV at a scan rate of 100 mV/ s for 10 000 cycles. The specific capacitance of these biocarbons from galvanostatic charge−discharge curves was calculated by the equations in ref 50.

prepared AC4 biocarbon presented sheetlike structures. The AC4 biocarbon exhibits an excellent electrochemical performance, possessing a highest specific capacitance of 332.3 F/g at the current density of 1 A/g, and a specific capacitance of 180 F/g even at the current density of 50 A/g, and it also possessed a high rate capability and wonderful cycle life span. Therefore, the AC4 biocarbon could be used as a promising electrode material for supercapacitors.

4. EXPERIMENTAL SECTION 4.1. Materials. Kapok fruit peels were obtained from Fuzhou University, washed by deionized, and dried at 60 °C. Potassium hydroxide (KOH) was acquired from Zhiyuan Reagent Co., Ltd. (Tianjin, China). Hydrochloric acid (HCl) was received from Lanxi Xuri Chemical Engineering Co., Ltd. (Zhejiang, China). 4.2. Preparation of Biocarbons. A preparation process of porous biocarbon from kapok fruit peels is displayed in Scheme 1. First, the kapok fruit peels were rinsed with Scheme 1. Illustration for the Preparation Process of AC Biocarbons from Kapok Fruit Peels

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deionized water to remove dust, dried at 60 °C, and then ground in a mixer for 3 min to make a powder. The dried kapok fruit peel powder was precarbonized at 500 °C under an air atmosphere for 2 h in a tube furnace with a heating rate of 5 °C/min, thus a carbon precursor was obtained. The carbon precursor was washed with 1 mol/L HCl aqueous solution and distilled water, respectively, until the pH value of the filtrate reached 7, and then dried at 60 °C for 24 h. Next, a mixture of water (20 mL), the precarbonized precursor product, and KOH was prepared and dried at 60 °C for 24 h. Subsequently, this mixture was heated to 700 °C under an air atmosphere for 2 h in a tube furnace at a rate of 5 °C/min to prepare porous biocarbon. The porous biocarbon was then transferred to 1 mol/L HCl aqueous solution and rinsed to a neutral pH value with distilled water. Finally, the biocarbon product was dried at 60 °C for 24 h and referred as AC. The AC porous biocarbons prepared with a mass ratio of KOH to the carbon precursor of 3:1, 3.5:1, 4:1, and 5:1 were named AC3, AC3.5, AC4, and AC5, respectively. By contrast, biocarbon C700 was prepared by using directed carbonization of the kapok fruit peels at 700 °C under an air atmosphere without adding KOH. 4.3. Characterization. Microstructures and morphologies of the biocarbons were performed by using field emission scanning electron microscopy (FE-SEM, Carl Zeiss ULTRA55, Germany), TEM (Fei TECNAIG2F2O), and powder X-ray

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00148.

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GCD curves and FE-SEM images (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: +86 591 22 866 540. Fax: +86 591 22 866 539. ORCID

Qiu-Feng Lü: 0000-0003-0361-8781 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support extended by the Open Research Fund of CAS Key Laboratory of Interfacial Physics and Technology, Chinese Academy of Sciences (grant no. CASKL-IPT1704), the Key Program of the Youth Natural Science Foundation of the Fujian Province University, China (grant no. JZ160413), and the Natural 5997

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Science Foundation, Fujian Province, China (grant no. 2016J01729).

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DOI: 10.1021/acsomega.9b00148 ACS Omega 2019, 4, 5991−5999