荞麦芯衍生的富氮和富氧可控多孔碳用于高性能超级电容器

1 Introduction

With the improvement of the requirements of the living environment and the rapid development of economy, people have problems in the storage and utilization of energy. With the large-scale development and utilization of new energy, the existing energy storage equipment has been unable to meet the demand for energy source storage. Supercapacitor (SC), a potential energy storage device, has attracted much attention on account of its advantages (such as fast charge propagation dynamics, high power density and long cycle life) which may replace commercial batteries [1-2]. Generally speaking, supercapacitors include electric double layer capacitors (EDLCs) and pseudocapacitors (PCs), considering the different energy storage mechanisms [3-4]. The former mainly generates stored energy through the adsorption of pure electrostatic charges on the electrode surface. PC stores energy through rapid surface redox reactions [5]. The structure and characteristics of electrode materials determine the charge storage mechanism of supercapacitors, which determines the electrochemical performance of supercapacitors [6-8].

In the last few years, numerous studies have been carried out by researchers on the development of electrode materials, which commonly include carbon materials, conducting polymers and metal oxides [9-13]. Although metal oxides have high specific capacitance and redox activity, they are not practical because of their high cost (such as RuO2, MnO2 and Co3O4) [14-16]. Conductive polymers, such as PANI and PPY, generally store and release charge through reversible doping and de-doping processes on the material surface, and the expansion and contraction produced in this process may damage the electrode structure [17-18]. Up to now, carbon has been the only electrode material employed in commercial SCs owning to its large surface area, good processability and physicochemical stability [19], mainly including CNTs, activated carbon and graphene [20-22]. Electric-double layer capacitance (EDLC) energy storage of carbon materials could accumulate and separate electrostatic charges at the electrode/electrolyte interface, yielding the high specific capacitance [23]. Recently, a lot of attention has been paid to porous carbon materials for EDLC behavior [24-26]. The pore structures play different roles in improving capacitance performances. Parameters such as specific surface area (SSA) and pore structure determine the electrochemical performance of carbon electrodes, and a large SSA is not necessarily favorable for their capacitive performance. Therefore, it is necessary to find an extensible approach to prepare high available surface area carbon materials with reasonable pore structure.

Porous carbon, attributed to its large SSA and interconnected cell network, is considered to be an electrode material with high performance SCs [27-29]. Waste biomass is regarded as one of the promising precursors for the fabrication of activated carbon from the perspective of raw material availability and low cost. Currently, the reported biomass carbon materials used as electrode materials for SCs include buleberry peel [30], puffball spores [31], tofu [32], pine nut shells [33] and shiitake mushroom [34], miscanthus waste [35],methyl cellulose [36], etc. Buckwheat is an abundant resource with world buckwheat production exceeding one million tons. Compared with the other biomass, buckwheat has been widely employed as a health care product such as buckwheat tea, and seldom applied in other fields especially for energy. Its main proteins and carbohydrates are globulin (70%) and starch (10%), respectively, and contains trace elements and dietary fiber [37-38]. The abundant starch is beneficial in the high temperature carbonization process to obtain a rich carbon structure, and a degree of protein content is beneficial in the establishment of a N-doped structure in the carbon matrix [39-40]. We demonstrate a low-cost, green, simple and easily scalable approach to fabricate porous carbons using buckwheat core as raw material.

Hence, we prepared porous carbon (BCPC-3) with high available surface area and reasonable pore structure using buckwheat core powder as precursor by activation of KOH at 800 ℃. We investigated the capacitance of the acquired samples, likewise the effects of the KOH activation ratio and the nitrogen/oxygen content. The prepared carbon materials also displayed tunable shape and pore structure due to the different KOH ratios. This 3D hierarchically porous structure consisted of various diameters of pores, which may enhance the electrochemical performance of the active material. In particular, the presence of a great number of micropores strengthens the electric-double-layer capacitance. Additionally, N-5, N-6 and phenol-O could enhance the wettability. Overall, BCPC-3 showed a high capacitance (330 F/g at 0.5 A/g) and excellent rate performance (140 F/g at 100 A/g) in a three-electrode system with 6 mol/L KOH as electrolyte. Besides, the symmetrial supercapacitor based on the BCPC-3 sample delivers a large energy density of 6.1 W·h/kg at a power density of 250 W/kg in 6 mol/L KOH electrolyte.

2 Experimental

2.1 Sample fabrication

BCPC-x (x is the mass ratio of KOH to buckwheat core) was obtained by chemical activation of the buckwheat core for preparation. The mixture of Buckwheat core and KOH (mBuckwheat core/mKOH=1:1/1:3/1:5) was allowed to stand for 24 h at room temperature and then completely dried in a blast dryer and moved to a high-temperature tube furnace raised to 800 ℃ (5 ℃/min) and keep in Ar for 1 h. The black powder was obtained in order to remove potassium compounds and other impurities soaked in 2 mol/L HNO3 for 24 h and washed several times with distilled water, followed by a pH test paper to measure the pH value of the filtrate around 7. Finally, the black paste was dried at 70 °C and named as BCPC-x. The final yields of the carbon materials were 1.35%, 1.18% and 0.94% respectively. As a comparison, the samples were calcined at 700°C and named as BCPC700 °C-x. The preparation of the BCPC-x is shown in Scheme 1.

Scheme 1  Schematic of the synthesis steps for the BCPC-x

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2.2 Characterization of materials

The porous carbon prepared was tested by X-ray diffraction (XRD) (Bruker D8 Advance). Raman and infrared spectrum were operated by Lab RAM HR800 (France) and NICOLET5700 FTIR spectrometer, respectively. The N2 adsorption and desorption tests were conducted by Autosorb IQ (USA). Element valance was studied by the X-ray photoelectron spectroscopy (XPS) (Kratos XSAM 800). The morphological structures of the samples at different nanometer sizes were studied by scanning electron microscopy (SEM) (Hitachi S4800) and transmission electron microscopy (TEM) (Tecnai G2 20). Thermogravimetry (TG) was operated by TGA Q500 in argon atmosphere.

2.3 Electrochemical tests

2.3.1 Three-electrode measurement

To estimate the electrochemical capabilities of the porous carbon obtained from buckwheat, four main methods are used for electrochemical testing: constant current charge/discharge, cyclic voltammetry, AC impedance, and charge/discharge cyclic stability of 5000 cycles. Black paste was obtained by mixing active substance BCPC-x, acetylene black, and PVDF in the conventional ratio of 80 :10 :10, adding appropriate nitromethyl pyrrolidone as the solvent, and then evenly coated on carbon cloth (area: 1 cm×1 cm; thickness: 0.33 mm). It was maintained at 60 ℃ under vacuum and the working electrode was obtained. Here, the working electrode mass was about 2 mg. The devices were tested in 6 mol/L KOH aqueous electrolytes, the Hg/HgO electrode acted as reference electrode and the graphite sheet was used as counter electrode.

Depending to the discharge time, the specific capacities (C, F/g) were calculated by:

C=IΔtΔVma

(1)

In the above equation, I(A), C(F/g), Δt(s), ma(g), ΔV(V) represent the discharge current, specific capacitance, discharge time, active mass and potential window, respectively.

2.3.2 Two-electrode measurement

A coin-shaped symmetric supercapacitor was assembled using the electrodes prepared above, where the separator is a glass fiber (Whatman) and the electrolyte is 6 mol/L KOH. The specific capacitance of a unitary electrode of carbon material is computed as follows:

C=IΔtΔVmT 

(2)

where mT(g) represents the total mass of active materials in both electrodes. The energy density and power density of SC were obtained by Eqs. (3) and (4), respectively:

 E=0.5C(ΔV)23.6 

(3)

 P=3600EΔt

(4)

In the above formulas, E(W·h/kg), C(F/g), ΔV(V), P(W/kg) and Δt(s) represent the energy density, specific capacitance, potential window, power density and discharge time, respectively.

3 Results and discussion

3.1 Physicochemical characterization

The gathered samples were characterized by XRD and the patterns are shown in Figure 1(a). The three sets of samples exhibited two typical wide peaks at nearly 22° and 43°, representing (002) and (100) planes of the hexagonal graphite, respectively [41].

Figure 1  (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption/desorption isotherms, and (d) pore size distribution of the BCPC-1, BCPC-3 and BCPC-5 samples

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The interlayer distance (002) of graphitic layers of carbon is estimated 0.39-0.42 nm following the Bragg equation,

2dsin θ=λ

(5)

La and Lc are the average transverse length and thickness of the graphite segment, respectively. Following the the Scherrer Eq. (6),

La/c=Kλβcos θ 

(6)

Among them, β and K denote the diffraction peak half-height width and Scherrer constant, respectively [42]. Raman spectra (Figure 1(b)) delivered two peaks locating at 1345 cm-1 (D band) and 1590 cm-1 (G band), related to disorder feature and graphitic degree [30]. We generally consider that the intensity ratio of G-band to D-band (IG/ID) reflected the crystallinity of carbon [27]. As a result, the value of BCPC-3 (1.04) was higher than that of BCPC-1 (0.95) and BCPC-5 (1.01), which implied that the graphitization of BCPC-3 was higher than the other two groups. The textural properties of the three types of BCPC-x were further analyzed using the N2 adsorption/desorption analysis. This clearly demonstrates the characteristics of the micropores seen in Figure 1(c), where all the isotherms can be classified to type IV absorption isotherms [43]. Figure 1(d) shows the pore size distributions of all samples. Numerous micropores (mainly range from 0.2 to 2 nm) can be observed in BCPC-x after KOH activation (inset in Figure 1(d)). In Table 1, the SSA of the carbons increases with the KOH ratio. Notably, BCPC-3 exhibited the largest mesoporous and total pore volume, which are considered as beneficial for the diffusion of the ions. Microporosity of the three samples was 85%, 41% and 71%, respectively. Previous reports indicate that excessive ultra-micropores are not beneficial for charge storage [44]. This is because in aqueous KOH solutions, the K+ and OH- ions involved in the conductivity have a diameter of about 0.3 nm, and it is difficult for K+ and OH- to quickly enter some ultramicroporous pores (<0.55 nm) due to the presence of ionic solvent layers and the irregularity of the pores [45]. Consequently, BCPC-3, with the high SSA and reasonable pore size gradient, was desirable to exhibit outstanding electrochemical performance in terms of rate and capacitance aspects.

Table 1  SBET, Vt, Vmic and IG/ID for the BCPC-1, BCPC-3, and BCPC-5 samples

Sample D002/nm Lc/nm La/nm SBET/(m2·g-1) Vt/(cm3·g-1) Vmic/(cm3·g-1) IG/ID

BCPC-1 0.41 2.33 4.82 799.89 0.41 0.35 0.95

BCPC-3 0.39 1.57 3.25 805.91 0.60 0.25 1.04

BCPC-5 0.42 2.21 4.57 923.94 0.53 0.38 1.01

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To investigate the microscopic morphology of the materials, scanning electron microscopy (SEM) characterization was conducted. Firstly, when the ratio of buckwheat core powder to KOH is 1 : 1, the material contains a significant number of mesopores and macropores (see Figure 2(a)). As the ratio increases to 1:3, the BCPC-3 formed a more rational porous foam-like interconnected carbon skeleton (see Figure 2(b)). Subsequent further etching of BCPC-5 resulted the crash of the micro-structure and generated a bulky amount of material debris (Figure 2(c)), which indicated the disruption of the interconnected structure. Energy elemental mapping (Figures 2(d)-(g)) obviously displayed that the elements of C, O and N were distributed homogeneously in the BCPC-3. The elements effectively increase the capacitance and rate capability of the active material in the charging/discharging procedures [46]. The detailed structural characterization of BCPC-3 was further revealed by typical transmission electron microscopy (TEM). Simultaneously, in Figure 2(h), we found myriad shiny and shadowy patches uniformly dispersed in BCPC-3, which are microporous structures in the carbon material from the pyrolysis of organic components and the activation of KOH. Besides, the lattice fringe spacing measured in Figure 2(j) is also consistent with the calculation of XRD patterns. This result may be ascribed to the combination of the low carbonation temperature and KOH activation and further leads to an increased graphitization of the material, but not a fully formed graphitic structure. The prepared carbon materials also displayed tunable shape and pore structure due to the different KOH ratios. This 3D hierarchically porous structure consisted of various diameters of pores, which may enhance the electrochemical performance of the active material. In particular, the presence of a great number of micropores strengthens the electric-double-layer capacitance [28]. Further, the thermal decomposition process of buckwheat core powder in argon atmosphere was analyzed by TGA. It is mainly composed of starch and protein, and the carbonation process is mainly attributed to the pyrolysis of these compounds. The TGA curves in Figure S1 show a significant mass loss in the temperature interval 250 ℃-500 ℃. The decomposition of these polymers during carbonization leads to the gas volatilization and the interconnected porous structure (micropores, mesopores and macropores).

Figure 2  SEM images of (a) BCPC-1, (b) BCPC-3 and (c) BCPC-5, (d-g) elemental mapping and (h-j) HRTEM graphics for the BCPC-3

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In order to study the elemental composition in carbon materials, Figure 3 presents the XPS spectra of BCPC-3. The full XPS spectrum of the sample seen in Figure 3(a) around 284, 532 and 399 eV exhibit three peaks corresponding to C (85.5%), O (14%), and N (0.5%) elements. The property indicates that N in the sample is self-doped, which originates from the N-containing compounds in the buckwheat core. In Figure 3(b), it is shown that the C 1s peak consisted of four peaks located at 284.6, 285.9, 287.4 and 289.3 eV, corresponding to C—C, C—O, C=O and COOH groups, respectively. As shown in Figure 3(c), O 1s spectra could be deconvoluted into two single peaks as phenol-O (532.3 eV) and carboxyl-O (533.7 eV). Additionally, as presented in Figure 3(d), N 1s revealed two peaks at 398.8 (pyrrolic-N abbreviated to N-5) and 400.6 eV (pyridinic-N abbreviated to N-6) [28]. Since N-5 has superior electron donation capability and rapid charge flux, and N-6 possesses electron pairing with π-conjugated rings. [47]. It is reported that pyrrolic-N/phenol-O/carboxyl-O species can serve as active pseudocapacitive species to further provide additional pseudocapacitance in alkaline electrolyte [48-49]. Moreover, the resultant sample was quite hydrophilic, which will facilitate ion diffusion and maximize the effective ion-accessible surface area. Importantly, N-5, N-6 and phenol-O could enhance the wettability [50]. In summary, the presence of these C, N and O functional groups plays an essential part in enhancing the electrochemical properties of the material. One of the oxygen-containing functional groups can efficiently increase the defects and disorder in the carbon structure, and this matches with the results displayed by XRD and Raman spectroscopy. Moreover, the components of BCPC-x were firstly estimated by EA (analytic functional testing). Three chemical elements of carbon, nitrogen and hydrogen can be detected from Table S1. In Figure S2, FTIR patterns of the BCPC-1, BCPC-3 and BCPC-5 samples presented three peaks surrounded at 3440, 1630 and 1090 cm-1 belonging to the O—H, N-6 or N-5 groups, and aromatic ether (C—O) stretching vibrations, respectively [51].

Figure 3  Full XPS spectra of BCPC-3 samples (a) and XPS test of C (b), O (c) and N (d)

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3.2 Electrochemical evaluation

The electrochemical behaviors of the samples were further researched in three-electrode system. Figure S3 showed the CV plots of different BCPC-x from -1 to 0 V at 50 mV/s. Among them, BCPC-3 encloses a higher area than BCPC-1 and BCPC-5. It can be observed that there are no obvious redox peaks for the three curves. Excellent symmetry implied good EDLC behavior and favorable stability of the materials. BCPC-1, BCPC-3 and BCPC-5 electrodes were tested from 5 to 100 mV/s (Figures 4(a), (c) and (e)). The CV curves of BCPC-1, BCPC-3 and BCPC-5 still displayed a near-rectangle shape even at scan rates of 100 mV/s, which indicated the desirable rate capability and fast ion transfer ability. The GCD profiles of BCPC-1, BCPC-3 and BCPC-5 at 0.5-100 A/g are presented in Figures 4(b), (d) and (f). The galvanostatic charge dischange (GCD) curves showed no significant IR drop and all exhibited a standard symmetric shape, proving the good reversibility of the three samples at a low equivalent series resistance. Furthermore, as the KOH ratio increased, the specific capacitance at 0.5 A/g was 226, 330 and 196 F/g for the three sets of carbons calculated according to Eq. (1), respectively. Compared with the other two groups of samples, the discharge time of BCPC-3 was longer at equal current densities, implying that BCPC-3 had a higher specific capacitance, which coincided with the CV curves. KOH was used for the activation of precursor and the reaction mechanism is given as follows [52]:

4KOH+C→K2CO3+K2O+2H2

(7)

K2CO3+2C→2K+3CO

(8)

K2O+C→2K+CO

(9)

Figure 4  Electrochemical performance of the samples tested in a three-electrode system using 6 mol/L KOH electrolyte: CV curves of BCPC-1 (a), BCPC-3 (c) and BCPC-5 (e) at the scan rate from 5 to 100 mV/s; GCD curves of BCPC-1 (b), BCPC-3 (d) and BCPC-5 (f) at 0.5-100 A/g

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According to these chemical reactions, the amount of KOH is critical to the process of activation and the resultant well-developed porous structure. During the pore-forming process, KOH can create pores by reacting with carbon, and potassium is generated. When the temperature exceeds the boiling point of potassium (762 ℃), it will exist in the gaseous form and be extremely reactive. Subsequently, potassium will unfold the aromatic layer and create more internal pores and interconnected channels [53]. As the amount of KOH employed in the production of sample BCPC-1 is rather low, the insufficient etching of carbon framework may result in an uneven pore size distribution. The higher KOH ratio of BCPC-5 resulted in the collapse of microstructured pores. In addition, Faraday reaction process related to abundant N, O doping could give rise to pseudocapacitance effect. Among them, the pseudocapacitance contribution of BCPC-3 can be attributed to more available exposed numerous defect sites and N, O functional groups especially the effective N-5 and N-6 species under high specific surface area [54]. As predicted above, the superior capacitance performance of BCPC-3 materials mainly origins from high specific surface area with favorable structure distribution synergized with abundant oxygen doping and N-6 and N-5 dominated N doping.

To investigate the rate property deeply, Figure 5(a) showed the capacitance versus discharge current density for all samples. It is clear that BCPC-3 has a higher capacitance (0.5-100 A/g), which represents a favorable rate capability of BCPC-3. It is worth noting that the BCPC-3 still delivers a capacitance of 140 F/g at a high rate of 100 A/g. Meanwhile, the GCD curves and rate properties of the three groups of materials at 700 ℃ are shown in Figure S4. BCPC700 ℃-1, BCPC700 ℃-3 and BCPC700 ℃-5 have relatively low capacitance and comparable rate performance. The low capacitive behavior is due to the underdeveloped pore structure and incomplete surface chemistry caused by the low temperature. In addition, the area capacitance (Ca) and volumetric capacitance (Cv) are calculated by the following equations respectively [55-56].

Ca=SC 

(10)

Cv=ρpC 

(11)

Figure 5  (a) Gravimetric specific capacitance of the BCPC-1, BCPC-3 and BCPC-5 at various rates (0.5, 1, 5, 10, 20, 50, 100 A/g); (b) EIS curves of the BCPC-1, BCPC-3 and BCPC-5

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where S stands for the mass loading of a single electrode (2 mg/cm2). ρp=1Vp+1/ρcarbon, where Vp is the pore volume of the carbon particles which can be determined by nitrogen physisorption (0.6 cm3/g) and ρcarbon is the true density of the carbon that can be measured by helium pycnometry and is usually around 2 g/cm3. As shown in Figure S5, the area capacitance and volumetric capacitance of BCPC-3 at 0.5 A/g were 660 mF/cm2 and 300 mF/cm3, respectively. We speculate that this may be due to the proper KOH ratio and temperature resulting in a tunable porosity. Meanwhile, this reasonable pore structure can act as a reservoir for aqueous electrolytes to promote transport of ions [57]. BCPC-3 exhibits a higher specific capacitance comparing to the existing biomass-based carbon materials reported in recent years. For instance, porous carbon obtained by one-step carbonizing biomass cashew nut husk (305.2 F/g at 1 A/g) [43], camellia petals (227 F/g at 0.5 A/g) [58], N-doped porous carbon derived by hydrothermal carbonization of macroalgae (228 F/g at 0.1 A/g) [59], a novel zirconia-based carbon nanofiber (referred to as CNF-20ZrO2) fabricated using a simple electrospinning method (140 F/g at 1 A/g) [60], NaOH-activated peanut shells (199 F/g at 0.5 A/g) [61]. This further proves that BCPC-3 exhibits the supreme capacitance.

Furthermore, in Figure 5(b), the electrochemical impedance spectrum (EIS) of BCPC-1, BCPC-3 and BCPC-5 is performed (0.01 Hz-100 kHz). Nyquist plot contained a semicircle in the high-frequency region and an oblique straight line in the low-frequency region. The intercept of the semicircle and the horizontal axis in the high-frequency region represents the intrinsic resistance (Rs) of the electrode, including the resistance of the material, the resistance of the electrolyte, and the resistance between the electrolyte and the collector. By simulating and calculating the equivalent circuit model (inset illustration), the electrode intrinsic resistance of BCPC-1, BCPC-3 and BCPC-5 is 0.85 Ω, 0.74 Ω and 0.76 Ω, respectively. It demonstrates that a reasonable KOH ratio and the surface chemistry of the nitrogen-oxygen functional group reduces the self-resistance of the material. At the same time, the diameter of the semicircle in the mid-low frequency region represents the charge transfer resistance between the material and the electrolyte, which is related to the Rct in the equivalent circuit model. The Rct values of BCPC-1, BCPC-3 and BCPC-5 is     0.91 Ω, 0.27 Ω and 0.41 Ω, respectively. The lower charge transfers resistance (Rct) of BCPC-3 is supposed to be due to the enhanced wettability of N-5, N-6 and phenol-O [53].

We prepared two-electrode symmetric SC with BCPC-3 in order to research the electrochemical capability deeply. As presented in Figure 6(a), under the same CV testing condition, quasi-rectangular shape can be observed in both two-electrode symmetric SC and three-electrode system. At 100 mV/s, the quasi-rectangular shape did not disappear. This fine retention of the CV curve illustrated the capacitive behavior of the sample. A decent symmetrical triangular shape still maintained at 20 A/g in GCD curve (Figure 6(b)), which agrees well with the above CV results. The character corresponded to the superior rate capability. The Ragone plot of the relating power and energy densities were depicted in Figure 6(d). The energy density of 6.1 W h/kg was estimated through Eq. (4) at a power density of 250 W/kg (3.3 Wh/kg at        10 kW/kg). Cyclic test at a current density of 5 A/g is exhibited in Figure 6(f), and the SC delivers capacitance retention of 90% and coulombic efficiency of 96% after 5000 cycles. Besides, as depicted in Figure 6(e), the charge transfer impedance of BCPC-3 assembled symmetric SC is low due to the presence of a semicircle with diameter and an almost vertical line in the low frequency. In conclusion, symmetric SC assembled from BCPC-3 electrodes have excellent electrochemical properties and provide a promising candidate material for the development of energy storage equipment.

Figure 6  Electrochemical performances of BCPC-3//BCPC-3 SC in 6 mol/L: (a) CV testing; (b) GCD testing; (c) Rate testing; (d) Ragone plot of the SC; (e) Nyquist plots; (f) 5000 laps of cycle stability

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4 Conclusions

In hierarchical porous carbon was prepared by pyrolysis process of buckwheat core with activation of KOH. The structure of the carbon material with 3D interconnected porous is controllable and the characteristic was determined by calcination temperature and the amount of KOH. Moreover, carbons with different pore structure display different capacitance performance. The BCPC-3 exhibits a superior capacitance of 330 F/g at a current density of 0.5 A/g in three-electrode system in 6 mol/L KOH. In carbon materials, a certain amount of nitrogen (0.5 wt%) acts an indispensable character in the development of pseudocapacitance. The BCPC-3 based symmetrical supercapacitor achieves over 90% capacitance retention after 5000 cycles. It shows that buckwheat core-derived porous carbon is promising for practical high-power supercapacitors.