Facile preparation of porous carbons derived from orange peel via basic copper carbonate activation for supercapacitors

Journal of Alloys and Compounds 823 (2020) 153747

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Facile preparation of porous carbons derived from orange peel via basic copper carbonate activation for supercapacitors Liu Wan*, Dequan Chen, Jiaxing Liu, Yan Zhang, Jian Chen, Cheng Du, Mingjiang Xie** Hubei Key Lab for Processing and Application of Catalytic Materials, College of Chemical Engineering, Huanggang Normal University, Huanggang, 437000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 September 2019 Received in revised form 13 December 2019 Accepted 7 January 2020 Available online xxx

A facile activation method has been illustrated for the synthesis of porous carbons derived from orange peels using basic copper carbonate as activation agent. The resulting carbon material possesses a high specific surface area of 912.4 m2 g
1, hierarchical pore architecture with interconnected meso-/macropores, and a rich amount of nitrogen, oxygen and sulfur heteroatoms. Benefiting from to its unique pore structure and the co-existence of redox-active nitrogen, oxygen and sulfur functionalities, the obtained porous carbon shows outstanding electrochemical performance when used as electrode material for supercapacitors. The as-prepared porous carbon exhibits a high specific capacitance of 375.7 F g
1 at 1 A g
1 and good rate retention of 50.9% from 1 to 100 A g
1. Additionally, the assembled carbon-based symmetric supercapacitor delivers a high energy density of 31.3 W h kg
1 at a power density of 499.5 W kg
1 in 1.0 M Na2SO4 electrolyte as well as superior long-term cyclic stability (only 7.3% of capacitance loss after 50,000 cycles within a voltage window of 0e2.0 V). This work provides an easy and feasible way for the synthesis of hierarchical porous carbon materials with both high power and energy density. © 2020 Elsevier B.V. All rights reserved.

Keywords: Orange peel Basic copper carbonate Chemical activation Supercapacitor

1. Introduction Recently, supercapacitors (SCs) with high power density and long cyclic lifespan have received extensive attentions [1e4]. It is well-known that there are two types of SCs based on an energy storage mechanism: one is electric double layer capacitors (EDLCs) and the other is pseudocapacitors [5,6]. Porous carbons (PCs) are widely utilized as electrode materials for EDLCs, a the carbon-based devices store energy via physical adsorption without any chemical reactions within the electrode/electrolyte interface [7,8]. Despite traditional microporous carbons are large in specific surface area (>2000 m2 g
1), their pore size distributions are mainly comprised of abundant micropores which restricts the transportation of electrolyte ions [9,10]. As for mesoporous carbons, they have more exposed pore channels for ion diffusion, but because of the low specific surface areas (300e1000 m2 g
1) there is only a limited amount of ion-charge sites [11]. Unlike microporous carbons and

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Wan), [email protected]. edu.cn (M. Xie). https://doi.org/10.1016/j.jallcom.2020.153747 0925-8388/© 2020 Elsevier B.V. All rights reserved.

mesoporous carbons, hierarchical porous carbons (HPCs) have the merits of microporous carbons and mesoporous carbons: large specific surface area, abundant exposed active sites for quick ion transfer and short ion diffusion pathways [12]. Therefore, HPCs are considered advanced electrode materials for high-performance SCs [13,14]. Nevertheless, the HPCs-based SCs are restricted from commercial applications owing to poor specific capacitance and low energy density in comparison with faradaic pseudocapacitors [6,10]. Thus, it is a challenge to develop strategies to overcome these shortcomings. Up to now, the methods for the synthesis of HPCs can be divided into three categories: (i) Template methods based on soft/hard templating [15] or nanocasting [16], (ii) activation methods, and (iii) a combination of activation method and template method. Unfortunately, the template approach usually involves multiple and complicated steps when comes to template synthesis and removal, commonly involving the use of harmful chemicals [17]. As for the activation methods, physical as well as chemical activation are applied [18,19] using activation agents such as KOH, NaOH, Na2CO3, and K2CO3 [20e22]. These corrosive chemicals often etch away a large amount of carbon precursor, leading to poor yield of target material. Moreover, the redox reaction between carbon skeleton

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L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

and corrosive agent not only leads to broad pore-size distribution (PSD), but also causes loss of heteroatoms and damage of equipment. With low heteroatom content and inappropriate pore structure, the fabricated HPCs are poor in electrochemical performance [23,24]. It is hence necessary to develop green routes for the synthesis of efficient HPCs. Beside the creation of a hierarchical porous structure, the incorporation of heteroatoms (such as N, S, P, B and so on) into a carbon framework is another way to enhance capacity and energy density of carbon-based SCs [25,26]. Heteroatom doping not only promotes the wettability and electronic conductivity, but also functions as redox-active sites and induces extra pseudocapacitance [27]. For instance, Han et al. employed polydopamine as precursor for the construction of nitrogen-containing porous carbons, and reported specific surface area of 3965 m2 g
1 and high specific capacitance of 342 F g
1 at 1 A g
1 [28]. Li. et al. synthesized N/S co-doped porous carbon nanosheets with high porosity and rich N, S species, which exhibits a remarkable energy density of 24.5 W h kg
1 [29]. Yang et al. obtained P-doping HPCs by direct pyrolysis of a mixture of P-containing metal salt and carbon precursor, and achieved a specific capacitance of 367 F g
1 at 0.3 A g
1 and good cyclic stability [30]. Global orange production was over 50 million tons per year according to the Food and Agriculture Organization (FAO) of the United Nations [31]. A large amount of orange residue (peel) can be collected after most oranges being processed as orange juice. Unfortunately, most of orange peel are discarded, which exhibits negligible value. It is noted that orange peel is composed of cellulose, hemicellulose, and lignin, which contains numerous carbon, oxygen, nitrogen and sulfur species. Therefore, it is meaningful to develop a simple and feasible method to use orange peel as carbon precursor for the synthesis of N, O, S-containing porous carbon materials. Herein, we demonstrate for the first time a facile method for the preparation of N,O,S-doped HPCs (denoted herein as OPC) from orange peel using basic copper carbonate as activating agent. It was found that the temperature adopted for activation has an influence not only on heteroatom content, but also on pore architecture. Basic copper carbonate functions as an oxidizing agent, which can etch carbon precursor via redox reactions and result in the formation of numerous nanopores in the carbon skeleton. As a result, OPC synthesized under optimized conditions is not only high in specific surface area and heteroatom content, but also endowed with interconnected meso-macro pores. When used as electrode material for SCs, the obtained optimal OPC delivers high specific capacitance and good rate capability. Furthermore, the assembled symmetric SCs show a high energy density and excellent long-term cyclic stability. 2. Experimental section 2.1. Materials Basic copper carbonate (Cu2(OH)2CO3), sulphuric acid (H2SO4), and hydrogen peroxide (H2O2) were purchased from Sinopharm Chemical Reagent, China. All of the reagents were analytical grade and directly utilized without any purification. The orange peel was collected from local fruit markets in Huanggang city, Hubei Province, China. The orange peel were thoroughly washed with deionized water for at least three times, and dried at 80  C for 12 h.

designated pyrolysis temperature (700, 800 and 900  C) at a ramping rate of 5  C min
1 and kept at this temperature for 2 h in an Ar atmosphere. After cooling to room temperature, the resultant product was immersed into a 50 mL solution of 1 M H2SO4 and 2 M H2O2 (volume ratio ¼ 1:1), and was subject to vigorous stirring for 12 h. Then, the solid substance was filtered out, washed with plenty of deionized water, and dried at 100  C for 12 h. The final product is herein named as OPC-x, where x denotes the pyrolysis temperature (x ¼ 700, 800 and 900). For comparison purpose, a carbonized sample (herein named as OPC-0) was obtained following the same procedure but without basic copper carbonate at a pyrolysis temperature of 800  C. The schematic of the preparation of porous carbons derived from orange peel is depicted in Scheme 1. 2.3. Material characterization The morphology of the samples were investigated by scanning electron microscopy (SEM, JEOL JSM-7400F) equipped with an energy dispersive spectrometer equipment (EDS) and transmission electron microscopy (TEM, JEM-2010, Japan). The crystalline structure of samples was studied by powder X-ray diffractometry (Philips, PW-1710, Netherlands). Raman spectroscopy was performed on a HR800 Raman spectrometer (HORIBA Jobin Yvon). Xray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250XI spectrometer with Al Ka source. Thermogravimetric analysis (TGA) was carried on a SDT Q600 TA instrument from room temperature to 900  C under nitrogen atmosphere. Nitrogen isotherms at 77 K were obtained using a Micromeritics ASAP 2020 porosimeter. 2.4. Electrochemical measurement With the adding of ethanol (2.0 mL), a mixture made up of the prepared carbon material (80 wt%), polytetrafluoroethylene (PTFE, 10 wt%) and acetylene black (10 wt%) was turned to slurry. To prepare the working electrode, the slurry was coated onto one side nickel foam (active area of 1  1 cm2) and vacuum dried at 80  C for 12 h. Subsequently, the electrodes with mass loading of 1.5e2.0 mg cm
2 was subject to pressing of 5 MPa. In a threeelectrode cell, platinum foil and Hg/HgO electrode were used as counter and reference electrode, respectively. The electrochemical measurements were performed on a CHI660E electrochemical workstation (Shanghai Chenhua Co., China). Electrochemical impedance spectroscopy (EIS) was conducted at open circuit potential in a frequency range from 0.01 Hz to 100 kHz. The cycling stability of the OPC electrodes was evaluated using a LANHECT2001A Battery Testing system. Gravimetric specific capacitance (C, F g
1) was calculated from the galvanostatic charge-discharge (GCD) profiles according to Equation (1):

C¼

I Dt mDV

The energy density (E, Wh kg
1) and power density (P, kW kg
1) of OPC-based symmetric SCs were calculated using Equations (2) and (3):

2.2. Synthesis of porous carbons derived from orange peel In a typical synthesis process, dried orange peel (1 g) and basic copper carbonate (12.0 g) was ground in a mortar for 60 min. Then, the obtained powder was placed in a tube furnace and heated to a

(1)

Scheme 1. Schematic of the synthesis of porous carbons from orange peel.

L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

E¼

C DV 2 7:2

(2)

P¼

3600E Dt

(3)

Where I (A) is current, Dt (s) is discharging time, DV (V) is potential change, m (g) is mass of OPC in a single electrode. 3. Results and discussion 3.1. Morphology of OPC Fig. 1 shows the morphology of the as-prepared OPC samples. As shown in Fig. 1a and b, OPC-0 has irregular shape, which is composed of micrometer-sized particles. No cavities or holes can be seen on its smooth surface. As for the OPC-x samples prepared with basic copper carbonate activation, there is the detection of cavities and holes on the particle surface. Over OPC-700, numerous cavities with a diameter from 300 to 700 nm can be found on the surface (Fig. 1c and d). It is apparent that the generation of cavities is a result of interaction between biomass and basic copper carbonate during the activation process, followed by a subsequent removal of Cu species in the presence of H2SO4 and H2O2. When the activation temperature increases from 700 to 800  C, OPC-800 shows more developed cavities than that of OPC-700 (Fig. 1e and f). The

3

formation of such 3D interconnected macropores on the surface of OPC-800 can be attributed to the strong etching effect of basic copper carbonate on orange peel at higher activation temperature. When the activation temperature reached to 900  C, apparent cracks as well as large amount of macropores were homogeneously occupied on the surface of OPC-900 (Fig. 1g). The appearance of crevices and highly developed porous structure for OPC-900 might result from the severe reactions between carbon and Cu species at 900  C (Fig. 1h). The elemental mapping of OPC-800 reveals uniform distribution of carbon, nitrogen, oxygen, and sulfur elements on the surface (Fig. 1i, j, k, l and m). The pore structure of the OPC materials can be further elucidated through TEM. The TEM image of OPC-0 confirms the absence of meso- and macropores (Fig. S1 a,b). Noticeably, OPC-700 has well-developed meso- and macropores in the carbon framework (Fig. 2a and b), which may result from the effective basic copper carbonate activation. Furthermore, few partially ordered graphitic layers can be clearly found in the high-resolution TEM of OPC-700 (Fig. 2b), indicating the presence of short-range order graphitic domain. OPC-800 possesses a highly porous (Fig. 2c), hierarchical structure comprising interconnected mesopores and macropores (Figs. 2d and S1c). As for OPC-900, more interconnected mesopores a macropores can be detected in the carbon network (Fig. 2e and f). With the promotion of activation temperature, more ordered carbon layers can be observed in the microstructures of OPC-700, OPC800 and OPC-900 (Fig. 2b, d, and f). It can be ascribed to the graphitization and rearrangement of carbon under the assistance of

Fig. 1. SEM images of (a,b) OPC-0, (c,d) OPC-700, (e,f) OPC-800, (g,h) OPC-900 and (i,j,k,l,m) elemental mappings of OPC-800.

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L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

Fig. 2. TEM images of (a, b) OPC-700, (c, d) OPC-800 and (e, f) OPC-900.

copper species with the increased temperature. It should be noted that such hierarchical pore structure of OPC materials are beneficial for fast electrolyte ion diffusion. The meso- and macropores not only shorten transport distance but also provide space for the formation of ion-buffering reservoir.

3.2. Structure characterization of OPC Fig. 3a shows the XRD patterns of the OPC samples. Two broad peaks at 2q around 23 and 43 can be obviously seen for OPC-0, OPC-700, OPC-800, and OPC-900, which belong to (002) and (100) planes of graphitic carbon materials, respectively [10,26]. The

Fig. 3. (a) XRD patterns, (b) Raman spectra, (c) Nitrogen adsorption/desorption isotherms and (d) the NLDFT pore size distributions of OPC-0, OPC-700, OPC-800 and OPC-900.

L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

XRD results suggest that all OPC samples are amorphous. Furthermore, there is no detection of signals ascribable to other species, implying complete removal of Cu species. Noticeably, when the activation temperature increases from 700 to 900  C, the intensities of these two diffraction peaks increase evidently. It implies that activation at high temperature contributes to the improvement of graphitization degree and the formation of more ordered structure [32]. The Raman spectra of all OPC samples are presented in Fig. 3b. Two obvious peaks at 1348 and 1595 m
1 can be seen for all OPC, which can be attributed to D band and G band, respectively [5,12]. In details, the D band is related to sp3-hybridized bonds of carbonaceous materials and the existence of disordered structure, vacancies or defects [33]. The G band is associated with the ordered graphitic sp2 carbon in 2D hexagonal lattice structure [34]. The intensity ratio (ID/IG) reflects the degree of ordering of carbonbased materials [35]. A higher ID/IG value reflects the higher disorder in the carbon skeleton as well as a lower degree of graphitization. The intensity ratio ID/IG of OPC-0, OPC-700, OPC0-800 and OPC-900 is 1.07, 1.05, 1.01 and 0.98, respectively. Notably, the value of ID/IG decreases with the increase of activation temperature, suggesting the presence of fewer structural defects or disorders in the carbon framework [36]. This is well in accordance with XRD result. Moreover, the ID/IG value of OPC-800 is lower than that of OPC-0 obtained at the same temperature, indicating that OPC-800 possesses less disordered structure. It can be ascribed to the loss of N, O, S species (Table S1) and the promotion of graphitic carbon during activation process, which can be confirmed by TEM results (Fig. 2d and Fig. S1b). Fig. 3c shows the nitrogen adsorption/desorption isotherms of the OPC samples. According to the IUPAC classification, the isotherm of OPC-0 corresponds to type I isotherm without hysteresis loop [37]. It indicates that the pore structure of OPC-0 is mainly dominated by micropores, which can be confirmed by its PSD curve as shown in Fig. 3d. Instead, the other three isotherms of OPC-700, OPC-800 and OPC-900 belong to a typical type IV with obvious H4 hysteresis loop, suggesting the presence of mesopores in the carbon framework [38]. Moreover, a sharp increase of nitrogen uptake at a relative pressure P/P0 smaller than 0.1 implies the existence of abundant micropores. Fig. 3d shows that OPC-700, OPC-800 and OPC-900 exhibit similar PSDs: their micropore size distributions are mainly centered at around 0.77 and 1.19 nm, mesopore size distributions range from 2.31 to 4.88 nm, and macropore size distributions in the range of 5.11e13.50 nm. However, when the activation temperature increases from 700 to 800  C, more micropores and mesopores are generated via strong etching effect between carbon and basic copper carbonate, while decrease evidently for OPC-900 probably due to the collapse of pore structure at high temperature [39]. Table 1 presents the pore parameters of all OPC samples. Without chemical activation, the SSA value of the OPC-0 is merely 198.2 m2 g
1, exhibiting a low porosity. When the activation temperature increased from 700 to 900  C, the SSA value increases from 621.3 to 912.4 m2 g
1 but decreases to 754.4 m2 g
1. When compared to OPC-0, the greatly enhanced specific surface area and pore volume for OPC-700, OPC-800, and OPC-900 proves that basic copper carbonate is an effective activator for the synthesis of hierarchical porous carbons with high porosity.

5

Additionally, compared with OPC-800, the decreased SSA and pore volume for OPC-900 can be related to the severe corrosion effect between carbon skeleton and basic copper carbonate at higher temperature (Table 1), resulting in the collapse of microspores and formation of more mesopores as shown in Fig. 3d. The activation mechanism of the Cu2(OH)2CO3 activator can be further studied by TGA technique (Fig. 4a). For raw orange peel, the weight loss of about 5.5% below 200  C can be attributed to water desorption. The weight loss of 43.6% between 200 and 360  C can be associated with the pyrogenic decomposition of carbohydrates and plant proteins [14]. The gradual weight loss from 360 to 900  C can be related to the pyrolysis of orange peel and the formation of condensation cross-linking ring in the carbon network [18]. For Cu2(OH)2CO3 alone, there is fast weight loss in the temperature range of 270e330  C. As for the mixture of orange peel and

Fig. 4. (a) TGA curves of raw orange peel, Cu2(OH)2CO3 and a mixture of orange peel and Cu2(OH)2CO3 with weight ratio of 1:8 (orange peel/Cu2(OH)2CO3), (b) XRD patterns of a mixture of orange peel and Cu2(OH)2CO3 with weight ratio of 1:8 (orange peel/Cu2(OH)2CO3) at various temperatures (200e900  C) under a Ar flow without acid washing.

Table 1 Pore parameters, yields and intensity ratios of the D band and G band of OPC samples. Samples

Yield (%)

SBET (m2 g
1)

Smicro (m2 g
1)

Vtotal (cm3 g
1)

Vmicro (cm3 g
1)

ID/IG

OPC-0 OPC-700 OPC-800 OPC-900

45.2 40.4 38.7 33.6

198.2 621.3 912.4 754.4

162.5 394.8 489.9 419.7

0.101 0.350 0.576 0.470

0.064 0.159 0.201 0.172

1.07 1.05 1.01 0.98

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L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

Cu2(OH)2CO3 with the weight ratio of 1:8, the weigh loss is different from that of raw orange peel. The fast weight loss between 270 and 330  C is similar to that of Cu2(OH)2CO3 but in larger amount, indicating the decomposition of Cu2(OH)2CO3 as well as the carbonization of orange peel. The obvious weight loss after 580  C is due to the loss of heteroatoms (such as N, O and S) as well as the deduction of Cu2(OH)2CO3, which is much faster than that of raw orange peel and pure Cu2(OH)2CO3. It indicates that the strong interactions between orange peel and Cu2(OH)2CO3 between 580 and 900  C occurred, which might favor the generation of highly porous structure in the carbon skeleton. The crystallinity of a mixture of orange peel and Cu2(OH)2CO3 with a weight ratio of 1:8 collected at 200e900  C was characterized by XRD. As displayed in Fig. 4b, the solid product obtained at 200  C shows typical 2q peaks at 17.4 , 24.2 , 31.8 and 35.3 , which are assigned to the (011), (
102), (021), (022) planes of Cu2(OH)2CO3 (JPCDS card no. 11e0682). When the activation temperature increases to 300 and 400  C, the characteristic peaks of Cu2(OH)2CO3 can not be detected, while those of CuO occurs. The phenomenon indicates that Cu2(OH)2CO3 have been completely decomposed into CuO, CO2 and H2O above 200  C. Notably, one new peak at about 37.0 appeared in the intermediates activated at 500  C, which belongs to the (111) plane of Cu2O (JPCDS card no. 34e1354). At 600  C, the peak at 37.0 intensified, indicating the generation of large number of Cu2O. Meanwhile, two new peaks at around 43.3 and 50.4 can be seen, which are well consistent with the standard diffraction data of Cu (JPCDS card no. 04e0836). It suggests that Cu2O is gradually reduced to metallic copper at 600  C. Above 600  C, the typical diffraction peaks of Cu2O disappear and those of metallic Cu remain. Based on the above XRD and TGA results, it can be inferred that Cu2(OH)2CO3 as an activating agent participate the following reactions (Equations (4)e(7)) between Cu species and carbon at different temperatures. Cu2(OH)2CO3 / 2CuO þ CO2 þ H2O (200 < T < 300  C

(4)

2CuO þ C / Cu2O þ CO (300 < T < 500  C)

(5)

CuO þ C / Cu þ CO (500 < T < 600  C)

(6)

Cu2O þ C / 2Cu þ CO (T > 600  C)

(7)

The chemical compositions of the OPC samples were examined by XPS, and the C, N, O and S contents are listed in Table S1. As shown in Fig. S2, the XPS spectra displays four kinds of peaks located at 167.9, 284.4, 399.8, 531.4 eV, which belong to S2p, C1s, N1s and O1s, respectively [18,40]. No other elements can be seen in the XPS spectra, further demonstrating the absence of any Cu species. The contents of N, O, S of all OPC samples are 2.15e3.29 at%, 11.71e18.43 at%, and 0.36e0.85 at% respectively (Table S1), which can be only derived from the precursor orange peel. Moreover, N, O and S contents decrease with the increase of activation temperature from 700 to 900  C, suggesting the loss of heteroatoms at high temperature. Fig. 5a shows the high-resolution C1s spectrum of all OPCs, which can be deconvoluted into five peaks located at 284.5, 285.5, 286.3, 287.5 and 288.9 eV, assigning to graphite carbon, CeO/ CeN, CeOeC, C]O and O]CeO, respectively [41]. The N1s spectra exhibits four peaks at 398.7, 400.3, 401.4 and 403.0 eV, which are attributed to pyridinic N (N-6), pyrrolic N (N-5), quaternary N (N-Q) and oxidized N (N-X), respectively (Fig. 5b) [42]. It should be noted that N-6 and N-5 in the carbon network can not only promote the wettability and hydrophilicity, but also function as contactable defects for active sites and provide extra faradaic capacitance [43]. Besides, the existence of N-Q contributes to the enhancement of electrical conductivity [44]. It is noted that the N-Q content

increases with the rise of activation temperature due to the transformation of N-6 and N-5 to N-Q [45]. As a result, OPC-900 exhibits the highest graphitic degree, which is accordance with XRD and Raman results (Fig. 3a and b). The O1s spectra can be deconvoluted into four peaks at 530.6, 532.2, 533.2 and 535.1 eV (Fig. 5c) corresponding to pyridone, eC] O, -C-OH/-C-O-C- and eCOOH, respectively [46]. Among the four OPC samples, OPC-800 possesses the highest quantitative compositions of N-5 (39.41 at%) and eC]O (43.44 at%) (Table S2), which might provide significant faradaic pseudocapacitance. Furthermore, the S2p spectra can be divided into three peaks: two peaks at 163.7 and 164.6 eV is ascribed to the -C-S-C and eC]S, respectively; the other peak at 168.5 eV is assigned to the oxidized sulfur moieties (-C-SOx-C-) (Fig. 5d) [26,47]. The S functionalities in the carbon network can promote the electron density of carbon and participate the reversible redox reactions of sulfones to sulfoxides together with sulfoxides to hydroxylated sulfoxides [48]. Overall, the presence of these N, O, S functional groups in the OPCs is beneficial for the enhancement of electrochemical properties of carbonaceous materials.

3.3. Electrochemical performance of the OPC materials 3.3.1. Three-electrode configuration In order to assess the electrochemical performance of the OPC samples, cyclic voltammetry (CV) and GCD measurements are conducted in a three-electrode cell using 6 M KOH as electrolyte. Fig. 6a shows the CV curves of the OPC electrodes at a scan rate of 10 mV s
1. OPC-0 electrode shows a triangle-like CV profile of small area, suggesting poor charge storage performance. OPC-700, OPC800, and OPC-900 electrodes exhibit nearly rectangular shape with small and broad humps, which is a combined outcome of EDLC behavior and pseudocapacitive reactions [32,49]. When the scan rate increases from 10 to 500 mV s
1, the CV profiles of OPC-700, OPC-800 and OPC-900 electrodes still maintain ideal rectangular shape (Figs. S3a, S4a, c, and e), implying quick dynamics for EDLC formation and excellent rate capability [37,50]. Fig. 6b shows the GCD curves of all OPC electrodes at a current density of 1.0 A g
1. All OPC electrodes present symmetrical triangle-like shape with negligible Ohmic drop, indicating ideal capacitive characteristics [18,23]. The small tails at the end of discharge curves at 1.0 A g
1 can be attributed to pseudocapacitance derived from the redox reactions of N, O and S species. The possible redox reactions participated by redox-active N, O, S functionalities can be described by Equations (8)e(16) [51e55]: >C]O þ e
4 >CeO-

(8)

>CeOH þ OH
4 >C]O þ e
þ H2O

(9)

-COOH þ OH
4 eCOOe þ e
þ H2O

(10)

(11)

(12)

L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

7

Fig. 5. High-resolution XPS survey of (a) C1s, (b) N1s, (c) O1s and (d) S2p spectra of OPC-0, OPC-700, OPC-800 and OPC-900.

(13)

(14)

(15)

(16) When the current density increased from 1.0 to 20 A g
1, the GCD curves still retain triangle shape (Figs. S3b, S4b, d, and f), further demonstrating good electrochemical reversibility. Fig. 6c exhibits the CV profiles of OPC-800 electrode at different scan rates.

Notably, even at an ultrahigh scan rate of 1000 mV s
1, OPC-800 electrode sustains a quasi-rectangular shape, reflecting excellent rate performance [56]. The GCD curves of OPC-800 electrode at current densities ranging from 1 to 100 A g
1 show isosceles triangle and good linearity (Fig. 6d), which agrees well with the CV results. Fig. 6e summarizes the specific capacitances versus current densities. The specific capacitances of OPC-0, OPC-700 OPC-800, and OPC-900 electrodes are calculated to be 130.1, 344.3, 375.7, and 232.4 F g
1 at 1 A g
1, respectively. Such high specific capacitance for OPC-800 is comparable or much higher than those of the previously reported biomass-derived carbon materials as shown in Table S3. Remarkably, even at an ultrahigh current density of 100 A g
1, the specific capacitance of OPC-800 electrode is as high as 191.4 F g
1 with a capacitance retention of 50.9%, which is much higher than that of OPC-700 (139.8 F g
1) and OPC-900 (100.2 F g
1). Although OCP-700 has higher N, O, and S contents than OPC-800, the lower specific capacitance for OCP-700 results from its smaller specific surface area, less developed porosity, and low degree of graphitization. Moreover, compared with OPC-700 and OCP-800, OPC-900 electrode exhibits inferior electrochemical performance due to its low heteroatom contents and inappropriate pore structure. EIS measurement was performed to explore the charge transfer resistance as well as reaction kinetics of OPC electrodes. Fig. 6f presents the Nyquist plots of all OPC electrodes. It can be seen that each plot displays a depressed semicircle, a 45 Warburg resistance region, and an almost vertical line [57]. At the high frequency region, semicircle represents the charge transfer resistance (Rct) at

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L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

Fig. 6. Electrochemical performance of the OPC-based carbon electrodes tested in 6 M KOH electrolyte: (a) CV curves at a scan rate of 10 mV s
1, (b) GCD curves at a current density of 1.0 A g
1, (c) CV curves of the OPC-800 electrode at different scan rates, (d) GCD curves of the OPC-800 electrode at different current densities, (e) specific capacitance versus current density, (f) Nyquist plots of the OPC electrodes and the inset is the magnified image of high-frequency region and the equivalent circuit model, (g) the Bode phase diagrams, (h) specific capacitance vs. t
1/2, and (i) the EDLC and pseudocapacitance contributions of OPC samples.

the electrolyte/electrode interface [58]. Notably, OPC-800 electrode has a low Rct value of 0.15 U, which is much lower than that of OPC0 (0.49 U), OPC-700 (0.24 U), and OPC-900 (0.36 U). It can be ascribed to its high content of N-Q functionality and moderate graphitization level. In addition, the intercept on the X axis represents the intrinsic ohmic resistance (Rs) [59]. As shown in Table S4, OPC-800 electrode possesses a lowest value of Rs (0.71 U) among four OPC electrodes, indicating greatly reduced charge transfer resistance and fast diffusion of electrolyte ions in comparison with other three OPC electrodes. At the medium-frequency region, the inclined lines with a slope of
45 stand for the Warburg resistance (Zw) which are associated with the electrolyte ions diffusion within the carbon electrode [60]. Moreover, at the low frequency region, OPC-800 electrode shows a more vertical line than other three OPC electrodes. Overall, such low value of Zw and vertical line for OPC800 electrode further reveals its low ion diffusion resistance (Table S4), owing to its well-developed pore structure with numerous macropores on its surface. Fig. 6g shows the Bode phase diagrams of the OPC samples. The characteristic frequency f0 at phase angle of
45 is 0.97, 2.81, 3.14, and 2.15 Hz for OPC-0, OPC-

700, OPC-800, and OPC-900 electrodes, respectively. The corresponding time constants t0 are 1.03, 0.35, 0.32, and 0.46 s, which are much lower than those of traditional carbon-based SCs (10 s) [61]. The small t0 value of OPC-800 electrode implies its high efficient charge transfer and quick frequency response [62]. To further demonstrate the merits of the hierarchical pore structure and the high N, O, S contents, the electrochemical kinetics of the OPC electrodes were analyzed. Generally, the total capacitance (CT) can be divided into two parts: the rate-independent part k1 (related to the EDLC, CE) and the diffusion limited part k2t1/2 (ascribed to pseudocapacitance, CP) determined by current density, as described in the following Equation (17) [63e65]:

CT ¼ k1 þ k2 t 1=2

(17)

Fig. 6h shows the relations of CT versus square root of discharge time. The CT, CE, and CP values of all OPC electrodes at 1 A g
1 are presented in Fig. 6i and summarized in Table S5. Notably, CE values of OPC0, OPC-700, and OPC-800 electrodes rise with the increase of specific surface area (Table 1), while slightly decreases for OPC-900 owing to its lower specific surface. It can be found that OPC-800

L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

possesses the highest CE of 188.8 F g
1 resulting from its highest surface area, open and accessible pore surface, and interconnected pore structure. Furthermore, the contributions of EDLC and pseudocapacitance of the OPC-800 in the total specific capacitance are 50.3 and 49.7%, respectively. Such large contribution of pseudocapacitance can be ascribed to the coexistence of numerous N, O, S functionalities (2.71 at% N, 13.54 at% O, and 0.58 at% S). Although the pseudocapacitance proportion of total capacitance for OPC-800 is lower than that of other three OPC electrodes (Fig. 6i), the large total capacitance of OPC-800 can be attributed to a combined effect of high porosity and moderate contents of N, O, S species. 3.3.2. Two-electrode configuration To further investigate the workability of the OPC-800-based SC, a symmetric supercapacitor device was fabricated and tested in 1 M Na2SO4. Fig. 7a shows the CV curves of the as-prepared device at various potential windows at 20 mV s
1. Obviously, when the

9

working voltage rises from 1.0 to 2.0 V, CV curves keep an almost rectangular shape with no evident distortion. It suggests that large voltage window (0e2.0 V) can be attained in neutral Na2SO4 electrolyte, because the equilibrium of Hþ and OH
in Na2SO4 effectively inhibits the evolution of H2 or O2 at the electrode [66,67]. Fig. 7b shows the CV curves at different scan rates within a window of 0e2.0 V. Even at a high scan rate of 500 mV s
1, the CV curve retains a quasi-rectangular profile, denoting ideal capacitive performance and good rate capability. Fig. 7c exhibits the GCD profiles of the symmetric SC at various current densities. The GCD curves show a typical triangle shape with small IR drop, further indicating outstanding reversibility and quick charge transfer. As shown in Fig. 7d, the specific capacitance of the OPC-800based symmetric SC based on total mass of active materials on two electrodes are calculated to be 56.4, 53.2, 51.4, 46.4, 43.5, and 42.4 F g
1 at 0.5, 1, 2, 5, 8, and 10 A g
1, respectively, which are much higher than those of OPC-0 based symmetric SC. The Ragone

Fig. 7. Electrochemical performances of the OPC-800-based symmetric supercapacitor in 1 M Na2SO4: (a) CV curves at different voltage windows at 20 mV s
1, (b) CV curves at various scan rates, (c) GCD curves at various current densities, (d) specific capacitances at various current densities, (d) the Ragone plots of the OPC0-0//OPC-0 and OPC0-800//OPC800 SC, (f) cyclic stability of the OPC0-800//OPC-800 SC at 5 A g
1 for 50,000 cycles.

10

L. Wan et al. / Journal of Alloys and Compounds 823 (2020) 153747

plot of the OPC-800 based SC is depicted in Fig. 7e. The OPC-800based device possesses high energy densities of 31.3, 29.5, 27.1, 23.3, 20.7 and 19.4 W h kg
1 at power densities of 499.5, 999.0, 1899.3, 4517.2, and 8244.6 W kg
1, respectively. Such high energy density for OPC-800-based symmetric SC is not only much higher than those of OPC-0-based symmetric SC but also higher than those of previously reported carbon-based symmetric SCs (Fig. 7e). Moreover, the cyclic stability of the OPC-800-based symmetric SC was tested at 5 A g
1. For the first 3000 cycles, the specific capacitance boosts evidently due to the carbon electrode activation process during the tests. After 50,000 cycles, a high capacitance retention of 92.7% with the Coulombic efficiency staying at around 100% is achieved after 50,000 GCD cycles, indicating excellent cycling stability. Such excellent cyclic stability is probably related to its well-designed hierarchical pore structure, interconnected pore channels, 3D carbon network with high electrical conductivity, and high degree of graphitization. All in all, the superior electrochemical performance for OPC800-based electrode material can be ascribed to the following factors. First, OPC-800 possesses high specific surface area and unique hierarchical pore structure with interconnected meso-/ macropores. The presence of ample amount of accessible pores shortens the diffusion distance, and facilitates fast electrolyte ions transport and charge accumulation. Second, the 3D numerous macropores on the surface can expose more available pore surface and serve as ion-buffering reservoir, which favors ion penetration and improve the rate capability especially at high current loads. Third, the coexistence of redox-active N, O, S functionalities in the carbon skeleton can not only promote its wettability, but also involves reversible faradaic reactions and supply extra pseudocapacitance. Fourth, a moderate level of graphitization endows a conductive network for quick electron transfer and leads to low internal resistance. 4. Conclusion Nitrogen, oxygen, sulfur-containing hierarchical porous carbons were successfully synthesized from orange peel using basic copper carbonate as activating agent. The activation temperature has a significant effect on the heteroatoms content and pore structure of the carbon materials. The resultant OPC-800 has a high specific surface area (912.4 m2 g
1), ample surface N, O, S functionalities (2.71 at% N, 13.54 at% O, and 0.58 at% S), and moderate degree of graphitization. Benefiting from its unique hierarchical pore structure with interconnected pore channels and high redox-active heteroatom contents, the as-prepared OPC-800 electrode exhibits outstanding electrochemical performance. A high specific capacitance of 375.7 F g
1 is achieved at 1 A g
1 with a good rate capability with 191.4 F g
1 at 100 A g
1. Moreover, the as-fabricated OPC-800-based symmetric supercapacitor delivers an outstanding energy density of 31.3 W h kg
1 and excellent cycling stability with a capacitance retention of 92.7% after 50,000 cycles in 1 M Na2SO4 electrolyte. This work demonstrates a novel activating agent for the preparation of porous carbons derived from biomass to be used as electrode materials in the manufacture of high-performance supercapacitors. Author contribution section Liu Wan: Writing- Reviewing and Editing. Dequan Chen: Original draft preparation. Jiaxing Liu: Visualization, Investigation. Yan Zhang: Conceptualization, Methodology. Jian Chen: Visualization, Investigation. Cheng Du: Supervision.

Mingjiang Xie: Software, Validation, and Editing. Declaration of competing interest There are no conflicts to declare. Acknowledgements This work is financially supported by National Natural Science Foundation of China (Grant No. 51902122), and Natural Science Foundation of Hubei Province (Grant No. 2019CFB262). The authors thank Prof. C.T. Au for useful suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153747. References [1] Y.G. Wang, Y.Y. Xia, Recent progress in supercapacitors: from materials design to system construction, Adv. Mater. 25 (2013) 5336e5342. [2] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci. 8 (2015) 702e730. [3] Z.J. Fan, Y. Liu, J. Yan, G.Q. Ning, Q. Wang, T. Wei, L.J. Zhi, F. Wei, Template directed synthesis of pillared-porous nanosheet architectures: highperformance electrode materials for supercapacitor, Adv. Energy Mater. 2 (2012) 419e424. [4] H. Peng, G.F. Ma, K.J. Sun, J.J. Mu, Z. Zhang, Z.Q. Lei, Formation of carbon nanosheets via simultaneous activation and catalytic carbonization of macroporous anion exchange resin for supercapacitors application, ACS Appl. Mater. Interfaces 6 (2014) 20795e20803. [5] L.J. Xie, G.H. Sun, F.Y. Su, X.Q. Guo, Q.Q. Kong, Hierarchical porous carbon microtubes derived from willow catkins for supercapacitor applications, J. Mater. Chem. 4 (2016) 1637e1646. [6] M.Y. Liu, J. Niua, Z.P. Zhang, M.L. Dou, F. Wang, Potassium compound-assistant synthesis of multi-heteroatom doped ultrathin porous carbon nanosheets for high performance supercapacitors, Nano Energy 51 (2018) 366e372. [7] L.Z. Sheng, L.L. Jiang, T. Wei, Z. Liu, Z.J. Fan, Spatial charge storage within honeycomb-carbon frameworks for ultrafast supercapacitors with high energy and power densities, Adv. Energy Mater. (2017) 1700668. [8] T. Lin, I. Chen, F. Liu, C. Yang, H. Bi, F. Xu, F. Huang, Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage, Science 350 (2015) 1508e1513. [9] C.J. Cui, W.Z. Qian, Y.T. Yu, C.Y. Kong, B. Yu, L. Xiang, F. Wei, Highly electroconductive mesoporous graphene nanofibers and their capacitance performance at 4 V, J. Am. Chem. Soc. 136 (2014) 2256e2259. [10] Y. Han, Z. Lai, Z. Wang, M. Yu, Y. Tong, X. Lu, Designing carbon based supercapacitors with high energy density: a summary of recent progress, Chem. Eur J. 24 (2018) 7312e7329. [11] W. Zhou, S. Lei, S. Sun, X. Ou, Q. Fu, Y. Xu, Y. Xiao, B. Cheng, From weed to multi-heteroatom-doped honeycomb-like porous carbon for advanced supercapacitors: a gelatinization-controlled one-step carbonization, J. Power Sources 42 (2018) 203e212. [12] J.G. Wang, H. Liu, H. Sun, W. Hua, H. Wang, X. Liu, B. Wei, One-pot synthesis of nitrogen-doped ordered mesoporous carbon spheres for high-rate and longcycle life supercapacitors, Carbon 127 (2018) 85e92. [13] F. Wu, J. Gao, X. Zhai, M. Xie, Y. Sun, H. Kang, Q. Tian, H. Qiu, Hierarchical porous carbon microrods derived from albizia flowers for high performance supercapacitors, Carbon 147 (2019) 242e251. [14] X. Wei, J.S. Wei, Y. Li, H. Zou, Robust hierarchically interconnected porous carbons derived from discarded Rhus typhina fruits for ultrahigh capacitive performance supercapacitors, J. Power Sources 414 (2019) 13e23. [15] L. Borchardt, M. Oschatz, S. Kaskel, Tailoring porosity in carbon materials for supercapacitor applications, Mater. Horiz. 1 (2014) 157e168. [16] M. Oschatz, S. Boukhalfa, W. Nickel, J.T. Lee, S. Klosz, L. Borchardt, A. Eychmueller, G. Yushin, S. Kaskel, Kroll-carbons based on silica and alumina templates as high-rate electrode materials in electrochemical double-layer capacitors, J. Mater. Chem. 2 (2014) 5131e5139. [17] D. Leistenschneider, K. Wegner, C. Eßbach, M. Sander, C. Schneidermann, L. Borchardt, Tailoring the porosity of a mesoporous carbon by a solvent-free mechanochemical approach, Carbon 147 (2019) 43e50. [18] Z. Li, Z. Xu, X. Tan, H. Wang, C.M.B. Holt, T. Stephenson, B.C. Olsen, D. Mitlin, Mesoporous nitrogen-rich carbons derived from protein for ultra-high capacity battery anodes and supercapacitors, Energy Environ. Sci. 6 (2013) 871e878. [19] Y. Zhang, S. Yang, S. Wang, X. Liu, L. Li, Microwave/freeze casting assisted fabrication of carbon frameworks derived from embedded upholder in tremella for superior performance supercapacitors, Energy Storage Mater. 18

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