Preparation of hierarchical porous carbon through one-step KOH activation of coconut shell biomass for high-performance supercapacitor

Here we report an effective and facile method for preparing porous carbons (CSCK-T-x) with highly developed hierarchical porosity for high-performance supercapacitor by one-step KOH activation of coconut shell carbon. The effects of carbonization temperature (T, °C) and KOH/C ratio (x) on the structure and electrochemical properties were studied systematically. As the KOH/C ratio and activation temperature increase, the SBET rapidly increases and then decreases and reaches a maximum (2143.6m2 g−1) at 800 °C with a KOH/C ratio of 2. Furthermore, CSCK-800-2 displayed abundant micropores and two concentrated mesopores at 4 nm and 14 nm. In a three-electrode test system, CSCK-800-2 exhibits a high specific capacitance of 317 F g−1 at a current density of 0.5 A g−1 and considerable rate retention of 68% at 20 A g−1. The symmetrical supercapacitor that was based on CSCK-800-2 showed a maximum energy density of 13.5 Wh kg−1 at 0.5 A g−1 with a superior cycling stability (99.7% of the capacitance retention after 10,000 cycles at 5 A g−1) in 6 M KOH electrolyte. The large specific surface area and unique hierarchical porosity of CSCK-800-2 enable it to have high ion-accessible surface and low-ion transport resistance. This one-step activation method provides an approach to convert biological waste into high-value hierarchical porous carbon material for electric double-layer capacitors.


Introduction
With the increasing demand for energy from transportation electrification and smart grids, as well as the gradual depletion of fossil fuels, it is significant to develop efficient, low-cost, eco-friendly and highperformance energy storage equipments [1,2]. Supercapacitors (SCs) have received extensive attention for their excellent cycling stability and high power density [3]. However, its low energy density has become a key factor restricting its wider development. Therefore, it is necessary to improve the energy density of SCs to satisfy urgent demand for uninterruptible power supplies. The charge storage mechanisms of SCs can be divided into the electric double-layer capacitors (EDLC), which rely on electrostatic attractions between ions and charged electrode surface, such as carbon materials [4,5], and pseudocapacitance based on the highly reversible redox reactions on the surface or in the bulk phase of the electrode materials, such as transition metal oxides [6], transition metal hydroxides [7] and conductive polymers [8].
Many factors contribute to the performance of SCs, such as the electrochemical performance of electrode materials, electrolyte selection and potential window of the electrode. The electrode material needs to have large specific surface area and reasonable pore structure to maximize the charge storage density of SCs. Some advanced materials including metal oxides, conductive polymers and carbon-based materials are reported [9][10][11][12][13]. Among these materials, carbon-based materials have attracted widespread attention for their large specific surface area, low cost, wide sources, non-toxic and high electrical conductivity [14,15]. However, their theoretical capacity and energy density have not reached a satisfactory level. An effective way to solve the above problem is to construct hierarchical porous structure, which is beneficial to high-density charge storage [16][17][18]. Micropores (\ 2 nm) can enhance surface area to ensure high energy storage capacity, small mesopores (between 4 and 6 nm) increase ion-accessible surface area with a lower ion-transport resistance, large mesopores (between 10 and 50 nm) and macropores ([ 50 nm) act as ion buffering reservoir to shorten ion-transport pathway and reduce the resistance of electron transport [19][20][21]. However, the size of some micropores does not match the size of electrolyte ion or too many large mesopores, usually lead to poor capacity and relatively low specific surface area [22,23].
A variety of strategies have been tried to obtain the ideal porous structure, including high-temperature pyrolysis method [24], template method [25], molten salt activation method [26], chemical vapor deposition [27], physical activation (e.g. CO 2 and H 2 O) [28] and chemical activation (e.g. KOH, K 2 CO 3 and ZnCl 2 ) et al. [29]. Chemical activation creates pores by etching or reacting with carbon and is more efficiently than physical activation [30]. KOH activation is considered to be the most effective chemical activation method. In addition, carbon source is also a critical factor restricting industrial application. Activated carbon (AC) is one of the most widely used electrode materials for EDLCs [31]. The raw materials used for the preparation of AC are very wide, which can be divided into wood raw materials and coal raw materials. The former mainly includes crop straw, fruit husk, pulp waste liquid and other industrial and agricultural wastes. The latter mainly includes petroleum bitumen, bituminous coal, petroleum coke, anthracite and so on [32]. The nanoporous carbon electrodes obtained from natural biomass are more eco-friendly, cost-effective and renewable. For example, Ma et al. [33] fabricated N/O co-doped activated carbon from glucose as raw materials without using any toxic reagents. Its energy density was 17.1 Wh kg -1 , and the cyclic stability reached 88.1% after 10,000 cycles. Wang et al. [34] prepared rod-like activated carbon by KOH activation using aniline-modified lignin. The materials showed high specific surface area with connected cavities, which lead to the specific capacitance of 336 F g -1 and good electrochemical performance. Notably, coconut shell is a low-cost, renewable agricultural waste that is widely distributed on earth and, therefore, is a promising candidate for the preparation of porous carbon precursors [35]. For example, Yin et al. [36] synthesized porous carbon electrode from coconut shell by using carbon dioxide and potassium hydroxide as physical activator and chemical activator. The materials exhibited a high specific capacitance of 266F g -1 at 0.1 A g -1 with a specific surface area of 2898 m 2 g -1 . In short, compared to other agricultural waste materials, coconut shell would be an excellent candidate for the preparation of supercapacitor electrode. Furthermore, KOH activation is one of the simple routes to obtain electrode materials with large surface area, hierarchical porosity, as well as high conductivity [37][38][39][40].
Herein, a simple and effective method for preparing porous carbon with high hierarchical porosity by one-step activation of coconut shell carbon with KOH is proposed. The activated carbon exhibits a high specific surface area up to 2143.6 m 2 g -1 with a unique hierarchical porous structure, a remarkable electrochemical performance with extraordinary cycling stability, showing that our work provides a sustainable and facile strategy for preparing porous carbon materials with promising applications in SCs.

Materials
Coconut shell carbon was purchased from Wenxian Boyuan activated carbon factory. Potassium hydroxide (KOH), muriatic acid (HCl) and N-methyl-2pyrrolidinone (NMP) were purchased from Tianjin Damao Chemical Reagent Factory. Poly (vinylidene fluoride) (PVDF) was purchased from Aladdin Reagent Co., Ltd. Carbon black was acquired by Cabot Investment Co., Ltd. The carbon cloth (WOS 1009, CC) was purchased from the Taiwan carbon energy corporation.

Material synthesis
The coconut shell carbon was ultrasonically cleaned with distilled water for 10 min, filtered and dried at 80°C for 24 h to obtain the pre-treated coconut shell carbon (CSC). Afterward, the obtained black powder was added into KOH solution at a KOH/C ratio of x (x = 0.5, 1, 2, or 3; w/w). Then, the black paste was dried at 80°C for 12 h to remove redundant water and further ground to ensure KOH was mixed evenly. Subsequently, the resultant mixture was placed in a nickel crucible and carbonized at different temperatures (700, 800 and 900°C) for 2 h under N 2 atmosphere and cooled down to room temperature. After that, the samples were cleaned by 1 M HCl and deionized water several times until the filtrate was neutral. Lastly, the synthesized coconut shell carbon was dried at 80°C for over 12 h (Scheme 1). The dried materials were labeled as CSCK-T-x (T = final activation temperature, x = weight ratio of KOH and C).

Characterization
A scanning electron microscope (SEM) with energydisperse spectroscopy (EDS) mapping was performed by SIGMA scanning electron microscope from Carl Zeiss AG, Germany, and the sample was magnified to 2000-100,000 times. The X-ray diffraction (XRD) analysis was recorded on a D/max2200PC-X-ray diffractometer using Cuka radiation. The specific surface area of the samples was determined by N 2 adsorption-desorption method (Tristar II 3020 surface area analyzer). All the samples were degassed at 180°C for 10 h prior to the analysis. The specific surface area was measured by Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated by classic Barrett-Joyner-Halenda (BJH) model. The micropore volume was carried out using the-plot model for surface analysis, and the total pore volume was obtained at a relative pressure P/P 0 of 0.99. The characteristic peaks of carbon samples were identified by XploRA Plus (Jobionyvon Technology) for Raman spectra. X-ray photoelectron spectroscopy (XPS) was used for chemical element analysis and obtained by ESCA-LAB MKII spectrometer equipped with a hemispherical analyzer under vacuum conditions using the Mg radiation (E = 1253.6 eV). Transmission electron microscopy (TEM) was used to observe the microstructure of the samples using the JEM-1010 instrument provided by JEOL Company.

Electrochemical measurements
The electrochemical test was carried out at the electrochemical workstation (Chenhua CHI660E, Shanghai) with 6M KOH as electrolyte. In the threeelectrode system, Pt sheet was used as the counter electrode and Hg/Hg 2 Cl 2 as the reference electrode. The working electrode was fabricated as follows. Mix the active materials, polyvinylidene fluoride (PVDF), and carbon black in the mass ratio of 8:1:1, and added an appropriate amount of N-methylpyrrolidone (NMP) to form a uniform slurry. Then, the sticky slurry was coated onto 1-cm 2 carbon cloth and dried at 80°C in oven for 12 h. Cyclic voltammetry (CV) test was carried out in the potential range of -1-0 V at a series of scan rates from 5 to 200 mVÁs -1 . The galvanostatic charge-discharge (GCD) measurement was operated at different current densities from 0.5 to 20 A g -1 with a potential window of -1-0 V.
Electrochemical impedance spectroscopy (EIS) was conducted in a frequency range from 10 -1 to 10 5 Hz with an amplitude of 5 mV. In addition, the electrochemical measurement of symmetrical SCs was carried out by using a two-electrode system. The gravimetric-specific capacitance (C, FÁg -1 ) of the electrode composites based on the three-electrode system and symmetric supercapacitors were calculated from galvanostatic charge-discharge (GCD) curve profiles according to the following equation: where I (A), Dt (s), DV (V) and m (g) refer to the discharge current, discharge time, potential change within Dt and mass of the active material loaded in the working electrode, respectively. Gravimetric energy density (E) in two-electrode system was evaluated through the equation below.
Gravimetric power density (P) in two-electrode system is evaluated through the equation below: where C represents the specific capacitance, DV refers to working potential and t is the discharge time of symmetric supercapacitors.

Results and discussion
3.1 Materials characterization 3.1.1 SEM analysis Figure 1 shows the microstructural properties of the carbon samples. It can be observed from Fig. 1a and b that CSC is composed of numerous irregularly shaped objects with rough surface and no obvious pores. However, the obtained CSCK-800-2 presents a 3D structure with abundant porous structure and the surface is very loose ( Fig. 1c and d). This is because during the preparation of CSCK-2-800, the KOH can enter the inside of the activated carbon, enlarge the pores, and etch the material through oxidizing part of the carbon into carbonate and oxycarbide at high temperatures. It is beneficial to obtain better pore size distribution, more reasonable rational pore structure and high surface area, which is conducive to providing more ion adsorption sites and reducing diffusion resistance [41]. In addition, the EDS spectrum ( Fig. 1e) and element mapping showed that the C (Fig. 1f) and O (Fig. 1g) atoms are uniformly distributed.

TEM analysis
Transmission electron microscopy images of CSCK-800-2 at different magnifications were taken to further observe its microstructure, as shown in Fig. 2. A large number of bright pores can be observed, indicating the distribution of a large number of micropores and mesopores. This porous structure provides a larger specific surface area, facilitates ion transport and penetration and contributes to store more charge, which is very beneficial for supercapacitors. This further indicates that KOH plays a crucial role in the formation of pore structure. Figure 3 shows the XRD patterns of all samples. Two evident diffraction peaks at around 21°-23°and 43°b elong to the (002) and (100) planes of the graphitic structure, respectively [42]. The broader peak around 21-23°indicates a certain degree of graphitization structure [43][44][45]. The high intensity peak at 23°of CSCK-800-2 indicates that it has a better graphitized degree. When the KOH ratio increases to 3, the peak intensity becomes weaker, indicating that excessive KOH dosage would increase the amorphous degree of carbonaceous materials [46]. With the increase of Scheme 1 Schematic diagram for the synthesis of CSCK-T-x temperature, the peak for CSCK-T-2 becomes wider and weaker, indicating that high temperature can promote the destruction of carbon microstructure by the activator [47,48]. The weak peak at 2h = 43°s uggests that the existence of graphitic-type carbon frameworks in as-prepared CSCK-T-x. The peak intensity at 43°is gradually increased with the increase of activation temperature from 700 to 900°C, demonstrating that the graphitization of CSCK-T-2 can be effectively improved by high activation temperature [49]. Figure 4 displays the Raman spectra of all samples. Two obvious peaks in the spectra were found at 1360 cm -1 and 1610 cm -1 , respectively, corresponding to the D band and G band [50]. The D band reflected the degree of disorder with substantial defects, and the G band revealed the E 2 g vibration mode of graphitization carbon [51,52]. The intensity ratio of D and G bands (I D /I G ) reflects the graphitization degree of carbon materials, and the corresponding values are shown in Table 1. It appears that the I D /I G value of CSC was 0.91, which is lower than all the KOH-activated samples. The lower I D /I G value of CSC is attributed to its underdeveloped porous structure. As we know, the lower the I D /I G value, the higher the graphitization degree, and the less disordered carbon [53,54]. With the increase of KOH/C ratio, the I D /I G value of CSCK-800-x samples increased, demonstrating the more disordered carbon. Among CSCK-T-2, the CSCK-800-2 possessed the highest I D /I G value of 1.07, indicating that CSCK-800-2 would possess highest defect degree and more active sites which contributes to high energy storage [50]. Figure 5 shows the N 2 adsorption-desorption isotherms and pore size distributions of all samples. The samples after KOH activation are shown as type I and IV with the H 4 hoop, which demonstrates the presence of hierarchical micro-and mesopore architecture ( Fig. 5a and c) [55,56]. Such hierarchical pore structure provides large surface area of the electrode/electrolyte interface for ions adsorption and transport [57,58]. However, CSC only shows a more moderate type IV adsorption isotherm with rapid adsorption under high relative pressure (P/P 0 [ 0.8), indicating that CSC mainly has macroporous structure and a limited number of mesopores [59]. The pore size distribution curves of carbon samples shown in Fig. 5b and d are consistent with the N 2 adsorption/desorption analysis. It can be seen that in all CSCK-T-x samples, micropores are still the major part coexisting with a few of small mesopores (4 nm). However, with increasing the KOH/C ratio to [ 2 large mesopores with size of 14 nm are gradually created, indicating that KOH constructs generous mesoporous structure. This is because as the KOH/C ratio increases, the reaction between KOH and CSC becomes more intense. At the same time, metallic K is effectively intercalated into the carbon lattice of the carbon matrix during the activation, resulting in the expansion of the carbon lattice, which widens the spacing of crystal faces and increases the pore size [60]. Importantly, the small mesopores ensure a high surface area and enough active sites to establish countless double electric layers, thus improving the charge storage capacity of supercapacitors [23]. The mesopores with diameters between 10 and 50 nm provide shorter ion-transport channels with a minimized inner-pore resistance [21]. The presence of double mesopores could result in high retention capability, since they could serve as both electrolyte reservoirs and highways for efficient electrolyte transport, which contributes to improved rate performance.

BET analysis
Based on the above analysis, the process of the porous carbon formation was proposed as follows. The carbon was activated with KOH at high temperature, and a series of redox reactions took place to etch the carbon frame and generate H 2 , CO 2 and CO, which were occupied by the resultant potassium compounds and then washed with distilled water and diluted HCl and boost number of micropores and small mesopores effectively. The activation mechanism is based on the following equations [61].
The specific surface area (S BET ) and total pore volume (V total ) parameters of all samples are summarized in Table 1. The CSC exhibits a low surface area (S BET = 340.4 m 2 Á g -1 ), whereas the CSCK-T-x samples exhibit much higher surface areas (865.3 * 2143.6 m 2 Á g -1 ), which confirms that KOH makes a vital contribution to the formation of the pore structure. The higher surface area could increase the electrode/electrolyte interface area during the physical process of electrolyte ion transfer/  adsorption and provide more active sites for charge storage, which is conducive to improving specific capacitance and energy density [30]. As the KOH/C ratio and activation temperature increase, the S BET rapidly increases and then decreases, reaches a maximum (2143.6m 2 g -1 ) at 800°C with a KOH/C ratio of 2. It is worth noting that the V total of all CSCK-T-x samples increased from 0.53 to 1.44 g -1 cm 3 .
These results indicate that KOH as pore-causing agent plays a decisive role in increasing the S BET and V total of carbon skeleton during pyrolysis. In addition, the change of micropores area (S micro ) of CSCK-Tx showed peak-like shape, increasing rapidly from 669.1 m 2 g -1 at 700°C to 1299.2 m 2 g -1 at 800°C and then decreasing to 1067.4 m 2 g -1 at 900°C. As we know, micropores play a key role in the formation of a double layer of activated carbon. The decrease in micropores area at 900°C indicated that some of micropores were transformed into mesopores and macropores at higher activation temperature [62].

XPS analysis
The surface elements of CSC, CSCK-700-2, CSCK-800-2, CSCK-800-3 are measured by XPS; the test results are shown in Fig. 6 and summarized in Table 2. As can be seen from the XPS full scan spectrum (Fig. 6a), there are two peaks at around 284 and 532 eV that correspond to the C 1 s peak of sp 2 carbon and the O 1 s spectrum. As shown in Fig. 6b, d, f and h, detailed bonding configurations of carbon atoms can be obtained from the high-resolution C1s spectra. The C1s shows three peaks of C-C (285 eV), C-O (285.9 eV), O=C-O (288.4 eV) [63], which indicates the presence of functional carbon in the sample [64], while the O1s (Fig. 6c, e, g and i) consists of two peaks, which can be attributed to C=O (531.2 eV) and    C-O (532.7 eV) [63]. As can be seen from Table 2, with the increase of temperature, O content decreases and C content increases, which is due to the decomposition of O from the carbon lattice and the pyrolysis of C-O bond at high temperatures, resulting in the stabilization and/or elimination of the sp 3 -saturated oxygen-containing groups and the structural recovery of the sp 2 -conjugated grapheme basal plane [65]. However, with the increase of KOH/C ratio, the oxygen content increases gradually. After activation, the content of C-O increases with the content of C=O decrease. These chemical bonds can improve the wettability of electrode materials surface, reduce the charge transfer resistance and may afford stable pseudo-capacitance via faradic reactions [63,66].

Electrochemical performance in a threeelectrode test
The electrochemical performances of CSC and CSCK-T-x samples were characterized by CV, GCD and EIS in 6 M KOH aqueous electrolyte using a three-electrode system. Figure 7a and b exhibits the CV curves of all samples at the scan rate of 50 mV s -1 . All of CV curves reveal the typical rectangular shape, which can be attributed to the EDLC behavior [67]. The area enclosed by the CV curves represents capacitance [68]. The area of CSCK-800-2 is the largest, indicating the best specific capacitance. Figure 7e shows the CV curves of CSCK-800-2 at various scan rates in detail. It can be observed that the CV curves remained a rectangle-like shape within the scanning rate (5-200 mVÁs -1 ) without distinct deformation even at 200 mV s -1 , suggesting the rapid transfer of charge with good rate capability [69]. Figure 7c and d illustrates the GCD curves of all samples at the current density of 0.5 A g -1 . CSCK-800-2 shows the longest discharge time among all samples, demonstrating that CSCK-800-2 can adsorb/desorb more electrolyte ions in the charging and discharging process with outstanding capacitance performance [70]. This is due to the etching of KOH and many micropores and mesopores are increased on the surface of the pristine CSC, which can not only improve the specific surface area of the material, but also conducive to the diffusion and penetration of electrolyte ions and charge transmission, which plays a positive role in improving the electrochemical performance of SCs. It can be seen from Fig. 7c that the specific capacitance increases with the increase of KOH/C ratio. When the ratio of KOH/C is 2:1, the capacitance reaches the maximum value. Further increasing the amount of KOH will not increase the capacitance. The trend of specific capacitance is consistent with that of specific surface area. It is noteworthy that of all the CSCK-T-2 samples, CSCK-700-2 shows the lowest specific surface area, but its specific capacitance was higher than CSCK-900-2, which can be interpreted that the capacitance performance depends not only on the specific surface area but also on the reasonable pore size distribution. The capacitance performance of the CSCK-800-2 at the different current density was further evaluated by GCD test, and the corresponding curves are shown in Fig. 7f. In particular, the GCD curves exhibit a linear shape with small internal resistance (IR) drop (0.0037 and 0.11 V at 0.5 and 20A g -1 , respectively), demonstrating the lower equivalent series resistance, which will be confirmed by EIS [71]. Figure 7g shows the effect of the current density on gravimetric-specific capacitance of the CSC and CSCK-800-2 samples calculated from the GCD curves. In detail, the specific capacitance of CSC maintains 150.3 and 75.6 F g -1 at 0.5 A g -1 and 20 A g -1 with capacitance retention of 50.3%. In contrast, good rate capability is observed in CSCK-800-2. It displays gravimetric specific capacitance of 317.0 and 216.0 F g -1 at 0.5 and 20 A g -1 with capacitance retention of 68.1%, respectively. The specific capacitance is 2.1 times higher than that of pristine coconut shell-based commercial activated carbon at 0.5A g -1 , which attributed to the presence of double mesopores (4 and 14 nm), as they could serve both as electrolyte reservoirs and as highways for efficient electrolyte transport, contributing to improved rate performance. The supercapacitor performance of CSCK-800-2 is superior to that of biomass-derived materials reported in the literature (Table 3). Therefore, coconut shell-derived hierarchical porous carbon is an ideal electrode material with potential application prospect in SCs. Figure 7h and i exhibits the Nyquist plots of all samples. EIS curves are composed of a semicircle and a straight line. In the low-frequency region, the slope of the straight line corresponds to Warburg resistance [72]. All the samples showed nearly vertical lines, demonstrating ideal capacitive behavior. The equivalent series resistance (Rs) is determined by the curve's intercept on the Z' axis at the high-frequency region, which influenced by the contact resistance between the active material and current collector and the overall electrolyte resistance in solution [73,74].
The charge transfer resistance (R ct ) is calculated from the diameter of the semicircle in the medium-highfrequency region. It can be seen from Table 1 Fig. 7 Electrochemical performance of all samples in a three-electrode system: a-b CV curves at 50 mVÁs -1 , c-d GCD curves at a current density of 0.5 AÁg -1 , e CV curves of CSCK-800-2 at different scan rates, f GCD curves of CSCK-800-2 at different current density, g variation of specific capacitance with current density of CSC and CSCK-800-2, h-i Nyquist plots, j Bode plot of CSCK-800-2 Rs of the CSC was about 1.2 X, which is 1.5 times that of CSCK-800-2 (0.8 X). The localized graphitized structure of CSCK-800-2 contributes to the lower Rs. The charge-transfer resistance (R ct ) mainly comes from the faradaic reaction between the interfaces of electrode and electrolyte [34,75]. As can be observed from Table 1, all the CSCK-T-x samples show low R ct value (0.11 * 0.19X) than that of CSC (0.38X), demonstrating small internal resistance, low ions transfer resistance and excellent electrical conductivity [76,77]. High specific surface area and welldeveloped mesoporous and microporous structure shorten the ion transport path and reduce the contact resistance of the CSCK-T-x materials, leading to low Rct values. Figure 7(j) shows the Bode plot of CSCK-800-2 with a phase angle of -80°at low frequencies, which has proved ideal supercapacitor behavior [78].
The characteristic frequency f 0 is *0.68 Hz at -45°p hase angle, and the corresponding time constant s 0 (= 1/f 0 ) is *1.47 s. The fast frequency response demonstrates the high ion/charge transport rate [79].

Electrochemical performance in a two-electrode test
A symmetrical two-electrodes SC was assembled by CSCK-800-2, and the capacitive performance was measured in 6 M KOH (Fig. 8). As can be seen from Fig. 8a, the nearly ideal rectangular shape of CV curves demonstrates a typical EDLCs behavior. The CV curves retain quasi-rectangular shape without distinct deformation even at a high scan rate of 200 mV s -1 , suggesting its high reversibility and fast transportation of charge [89]. As shown in Fig. 8b, all GCD curves exhibit similar equilateral triangle shape and the deformation can be ignored, also indicating its good electrochemical performance. The specific capacitance was calculated to be 97.8 F g -1 at 0.5 A g -1 . It is observed from Fig. 8c that the energy density was 13.6 Wh kg -1 at a high power density of 250 W kg -1 . When the power density was increased to 10 kW kg -1 , the energy density decreased to 8.6 Wh kg -1 . This performance is significantly better than those reported carbon materials (Fig. 8c) [59,[90][91][92][93][94]. Moreover, 99.7% of the initial capacitance can still be maintained at 5 A g -1 after 10,000 cycles of the CSCK-800-2-based symmetrical SC demonstrates that it has the potential of practical application with outstanding cycling stability (Fig. 8d). All in all, the coconut shell-derived porous carbon is an excellent and potential alternative electrode material with desirable electrochemical performance for SCs.

Conclusion
In summary, a simple one-step KOH activation approach to prepare a cost-effective coconut shellderived hierarchical porous carbon for high-performance supercapacitor was described. The effects of carbonization temperature and KOH/C ratio on the structure and electrochemical properties were studied systematically, with the aim to design a low-cost biomass carbon material for efficient supercapacitor. It is proved that the KOH activation can significantly increase higher surface area and generate mesopores mainly at 4 nm and a small amount near 14 nm. This unique hierarchical structure enables it to have low ion transport resistance and high ion-accessible surface area, which contributes to outstanding electrochemical performance. Furthermore, the synthesized CSCK-800-2 with an ultra-high surface of 2143.6 m 2 g -1 exhibited the good supercapacitor performance. It delivers high specific capacitances up to 317 F g -1 at a current density of 0.5 A g -1 and maintains 216 F g -1 even at a current density of 20 A g -1 . The energy density of 13.5 Wh kg -1 at a power density of 250 W kg -1 can be achieved. Moreover, the capacitance loss is kept below 0.3% after 10000 chargedischarge cycles at 5 A g -1 . It has proved that the simple, cost-effective synthesis technique and the attractive electrochemical performance of the CSCK-800-2 make it have great application potential in energy storage systems.

Author contributions
YZ performed conceptualization, methodology, writing-original draft, formal analysis, validation and project administration. YW did validation and investigation. YL performed formal analysis. HW contributed to funding acquisition. HS done writing

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.