Plane tree bark-derived mesopore-dominant hierarchical carbon for high-voltage supercapacitors

Journal Pre-proofs Full Length Article Plane tree bark-derived mesopore-dominant hierarchical carbon for high-voltage supercapacitors Fang Yu, Zihan Ye, Wanru Chen, Qianya Wang, Hui Wang, Honglei Zhang, Chuang Peng PII: DOI: Reference:

S0169-4332(19)34007-3 https://doi.org/10.1016/j.apsusc.2019.145190 APSUSC 145190

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

28 August 2019 11 December 2019 24 December 2019

Please cite this article as: F. Yu, Z. Ye, W. Chen, Q. Wang, H. Wang, H. Zhang, C. Peng, Plane tree bark-derived mesopore-dominant hierarchical carbon for high-voltage supercapacitors, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145190

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Plane tree bark-derived mesopore-dominant hierarchical carbon for high-voltage supercapacitors Fang Yu a, b #, Zihan Ye a #, Wanru Chen a, b, Qianya Wang a, Hui Wang a, Honglei Zhang a,b

a

*, Chuang Peng a,b *

School of Resource and Environmental Sciences , Wuhan University, 299 Bayi Road,

Wuhan, 430072, China b

Shenzhen Research Institute, Wuhan University, Building 403, Hi-tech Park, Nanshan,

Shenzhen, 518057, China

*Corresponding author. Tel.: +86-027-68778381; fax: +86-027-68778893 E-mail address: [email protected] [email protected] #

These authors contributed equally to this work.

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Abstract: Wide potential range and high specific capacitance of electrode materials are critical to high performance supercapacitors. However, it remains challenging to simultaneously achieve both high capacitance and cell voltage, particularly in organic electrolytes. Herein, three dimensional (3D) mesopore-dominant hierarchical carbons are obtained by one-step pyrolysis-activation of seasonable biowaste, i.e., plane tree bark with nano-ZnO as a mild activator. The optimized biowaste-derived carbon shows an ultrahigh mesopore area and low oxygen content. With these merits, the carbon electrodes show both high capacitance and wide capacitive potential range in both aqueous and organic electrolyte. Particularly, a specific capacitance of 115.6 F g-1 and a high cell voltage of 3 V are achieved with organic electrolyte. This study also reveals a mismatch between the specific capacitance and four commonly used specific surface area values. In a symmetrical supercapacitor with organic electrolyte, the positive electrode is the voltage-determining electrode. These findings may provide new perspectives on materials and device design of high-voltage supercapacitors.

Keywords: Tree bark biomass waste; Mesopore-dominant hierarchical carbon; Supercapacitors; High voltage; Aqueous and organic electrolyte

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1. Introduction With the development and application of portable electronic devices and electric vehicles, supercapacitors based on porous carbons, i.e., electrical double-layer capacitors find growing applications due to their high specific power, rapid charging and discharging ability and superior cycling stability [1]. However, the long endurance demand of emerging applications requires design of supercapacitors with higher specific energy. According to the equation E = 1/2CU2, the specific energy of a supercapacitor can be improved by increasing the specific capacitance (C) or/and expanding the working voltage (V) [2-4]. In general, the specific capacitance is determined by the nature of carbon materials such as their specific surface area, pore volumes, pore size distribution, heteroatom doping and surface functionalities. The working voltage of a supercapacitor depends on the stable electrochemical window of the electrode material and its electrolyte. However, the working voltage of a supercapacitor is usually lower than 1.8 V in aqueous electrolytes to avoid water electrolysis [5]. Despite the higher specific capacitance reported for aqueous electrolytes, commercial supercapacitors commonly use organic electrolytes because of their high working voltages. The maximum working voltages (MWV) of majority of current commercial supercapacitors are limited to 2.7 V. Therefore, it is highly desirable to enhance the specific energy of supercapacitors by further improving their MWV. It is the key to design a matching pair of electrode materials and electrolytes for effectively extending the cell voltage. Pore structure engineering is the most prevailing and effective method for

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obtaining high performance supercapacitor carbons. Currently, chemical activation using KOH, ZnCl2, or H3PO4 is commonly employed to optimize the pore structure of carbon [6-9]. Despite the high specific surface area, the etching effect of these activators mainly generates micropores, which are unfavorable for ion transport. Moreover, these activators are usually corrosive to apparatus and produce acid or alkaline wastewater. Recently, hierarchical porous carbon containing micro-, meso- and macro-pores has become widely recognized as a desirable electrode material for supercapacitors [10,11]. The presence of abundant mesopores is crucial to achieving high capacitance and rate performance [12]. Another issue associated with the-aforementioned activators is their oxidation power which commonly yields carbon products with high oxygen content. The oxygen-containing groups may be detrimental to the MWV of supercapacitors because they tend to decompose or react with electrolyte at high electrode potentials [13,14]. Recently, biomass-derived porous carbon materials have been rigorously studied as supercapacitor electrodes because of their properties of low-cost, sustainable nature and environmental friendliness [15-18]. Nevertheless, biomass-derived carbons with mesopores-dominant hierarchical pore structure are highly desirable, but those concurrently showing both high capacitance and high voltage in organic electrolytes are rarely reported. An effective way of extending high voltage of supercapacitors is to use appropriate electrolyte. Another exploitable strategy is to decrease the oxygen content of electrode carbon materials to avoid irreversible reactions between the electrolyte and electrode

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materials. However, the impact of oxygen content on cell voltage is rarely reported and a high-voltage supercapacitor based on this perception has not been developed. Etching activators are usually introduced during the synthesis of porous carbon in pursuit of high surface area. Meanwhile, the oxygen content also increased, posing a detrimental effect on the maximum achievable voltage of supercapacitors. Therefore, the use of a mild activator that mainly functions as hard template for preparing porous carbon with low oxygen content may be highly desirable in this regard. Recent research shows that nano-ZnO shows site-occupying effects similar to silica hard template which mainly yield mesopores [19]. It also functions as a mild activator to produce hierarchical carbons with micropores [19]. In the current manuscript, low-cost and non-toxic nano-ZnO is proposed as a template and activator to produce biomass-derived carbon with both desired pore structure and low oxygen content for high voltage supercapacitors. Platanus, or plane trees, are a family of rapid-growing ornamental and shade trees, widely planted by roadsides or in city areas. In summer, the outer bark of plane trees exfoliates and flakes off, generating large amounts of seasonal biowaste. Unlike most tree bark that is high in polysaccharide and cork, the outer bark of plane trees contains higher amount of lignin, making it a suitable precursor for producing porous carbon [20]. In this work, we report a facile and effective carbonization-activation approach to prepare bark-derived carbon showing 3D hierarchical porous structure with high

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mesopore volume. This unique feature coupled with its low oxygen content endows the carbon electrode with high capacitance, rate performance and high voltage in both aqueous and organic electrolytes. Furthermore, this work allows us to reexamine two important aspects of the charge storage mechanisms in supercapacitors, i.e., correlation of specific capacitance with pore features and the identification of voltage-determining electrode. 2. Experimental section 2.1. Synthesis of 3D hierarchical porous carbon The bark of Platanus was collected from landscape plane tree in Wuhan University. After thorough washing with deionized water, the bark was crushed into small pieces and dried at 60 Ԩ overnight. Typically, 8 g of dried bark was mixed with 8g of nano-ZnO (Macklin, 30 ± 10nm) by ball milling (Retsch PM 100, Germany) for 2h at

350 rpm. The mixed powder was transferred into tubular furnace for carbonization at 700-900 Ԩ for 2 h under an N2 atmosphere with a heating rate of 10 Ԩ min-1. The

product was thoroughly washed with 1.0 M HCl and deionized water, and then dried at 60 °C for 12 h to obtain the porous carbon. The final carbon samples are denoted as BZnC-T, where T stands for the carbonization temperature. For control experiment, the bark was directly calcined at 800 Ԩ under N2 for 2 h without nano-ZnO, the resulting carbon is labelled as BC-8.

2.2. Materials characterization Sample morphologies were characterized by scanning electron microscopy (SEM,

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LMH, MIRA 3) with energy dispersive X-ray spectroscopy (EDS, X-Max 20, Aztec Energy) and transmission electron microscopy (TEM, JEOL, JEM-2100). The elemental content of the samples was recorded by elemental analysis (Elementar, Vario Macro Cube). The specific surface area of the samples was determined by the Brunauer-Emmett-Teller (BET) method from the N2 adsorption data. The pore sizes were measured by density functional theory method based on the nitrogen adsorption-desorption data (Micromeritics, ASAP 2020 Plus). Micropore surface area and micropore volume were estimated by the t-plot method. The phase composition analysis was carried out using X-ray diffraction (XRD, XPert Pro), and Raman spectra were recorded by Raman microscope at wavelength of 532 nm (Renishaw, RM1000). The chemical bonding states of elements were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALAB250Xi). The binding energy of the C 1s (284.6 eV) peak was considered as an internal standard. 2.3. Electrochemical measurements The working electrode was prepared by mixing the active material, carbon black and polytetrafluoroethylene (PTFE) with a mass ratio of 8:1:1. The mixture was rolled into sheets and pressed onto titanium mesh current collector (1×1 cm2), followed by vacuum drying at 80 Ԩ for 12 h to remove the solvent. The active material loading on each working electrode was approximately 5 mg cm-2. In the electrolyte of 6.0 M KOH

aqueous solution, a platinum foil electrode and Hg/HgO electrode were used as the counter and reference electrode, respectively. The Ag/AgNO3 reference electrode was

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used in 1 M tetraethylammonium tetrafluoroborate/acetonitrile (TEABF4/AN). The symmetric supercapacitors were assembled by two identical electrodes and Whatman separator with TEABF4/AN as electrolyte, polyvinylidenedifluoride (PVDF) as the binder, and aluminum foil as the current collectors. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were conducted using electrochemical workstation (PARSTAT MC 1000). The EIS measurement was performed at open circuit potential, with amplitude of 5 mV in the frequency range from 0.01 Hz to 100 kHz. The specific capacitance, specific energy and specific power were calculated by the equations in the Supporting Information. 3. Results and discussion 3.1. Characterization of the samples The BC-8 prepared without nano-ZnO consists of large-size lumps with relatively smooth surface and macropores of biologic origin (Fig. 1a). In contrast, carbons prepared with nano-ZnO (BZnC-7, 8 and 9) generally show a loose structure with abundant interconnected pores (Fig. 1b-d). The TEM images (Fig. 1e) of BZnC-8 further reveal its well-developed mesopores and small-sized macropores, while high-resolution TEM image (Fig. 1f) also shows the presence of micropores. The EDS analysis of BZnC-8 indicates that the ash content or other impurities are hardly detectable (Fig. S1). The nitrogen adsorption-desorption isotherms of BZnC-T samples all belong to type IV isotherms (IUPAC classification, Fig. 2a), while BC-8 exhibits typical type I isotherm. The significant hysteresis loops as observed in isotherms of all

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the three BZnC-T samples reveal a combination of microporous/mesoporous structure, which is in good agreement with the TEM analysis. Moreover, the pores of BZnC-T are mainly mesopores distributed between 10 and 50 nm, which is close to the size of the nano-ZnO particles (Fig. 2b). The generation of macropores could be ascribed to the partial collapse of mesopores and micropores as well as aggregation of the nano-ZnO templates during ball milling. As shown in Table 1, the specific surface area, total pore volume and micropore volume increased notably as the activation temperature increased from 700 Ԩ to 800 Ԩ. As the temperature further increased from 800 Ԩ to 900 Ԩ, these

values showed minor increase (Table 1). Furthermore, BZnC-8 presents the highest

mesopore volume, reaching up to 86% of its total pore volume. The high mesopore content can be attributed to the dual role of nano-ZnO during the activation process. As a mild activating agent, nano-ZnO reacts with carbon to produce Zn and carbon monoxide [19], generating pores during the activation process. The nano-ZnO particles also served as a hard template, which is responsible for the ultrahigh mesopore volume in the product carbon. In contrast, alkali such as NaOH and KOH, is an effective activating agent capable of creating even higher specific surface area because of the high micropore volume [21 22, 23]. The interconnected hierarchical pore structure of the BZnC-T samples is considered a favorable feature for both storage and transport of electrolyte ions [24]. The XRD patterns of all samples commonly showed two broad diffraction peaks centered at 24° and 44° (Fig. 2c). These peaks are ascribed to the (002) and (100) planes

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of graphite, indicating the dominant amorphous character of all carbon samples [25]. The Raman spectra (Fig. 2d) of all samples revealed two peaks at ~1350 and 1580 cm−1, corresponding to the D and G band of the carbon materials. As listed in Fig. 2d, the intensity ratio of the D and G band (ID/IG) suggests the samples with nano-ZnO activation possess lower degree of graphitization and more defects. The ID/IG value of BZnC-8 is the highest of all samples. This counterbalance effect of carbonization temperature on ID/IG value is probably because higher temperature promotes activation and results in more defect, while even higher temperature, e.g. 900 Ԩ, increases the degree of graphitization.

The elemental composition of these samples was measured by elemental analysis and XPS (Table S1). Compared with control sample BC-8, the activated products BZnC-T have low oxygen content (<10 wt%, Table S1). XPS result indicates that BZnC-8 is mainly composed of C (92.59 at.%), and O (5.71 at.%) with a small amount of N (1.21 at%) and S (0.49 at.%) (Fig. S2). The high-resolution C 1s spectrum (Fig. 2e) is resolved into four components, corresponding to C-C (~ 284.6 eV), C-N (~ 285.3 eV), C-O (~ 287 eV), and COOR (~ 289.8 eV) respectively [26]. Similarly, the deconvoluted O 1s peak (Fig. 2f) revealed the presence of three oxygen-based components, i.e., C=O (~531.8 eV), C-O (~ 532.9 eV), and COOR (533.6eV) [27]. As discussed, the surface oxygen-functional groups of electrodes could constrain the MWV of a supercapacitor [13], so a low oxygen content is a prerequisite for fabricating high voltage supercapacitors. Unlike the chemical etching effect in acidic and alkali activators (e.g.,

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KOH or HNO3) [28, 29], nano-ZnO predominantly serve as a hard template during carbonization, leading to the high carbon and low oxygen content. The low oxygen content reduces electrolyte decomposition, and hence increasing the cell voltage without harming the cycle performance [14, 30]. 3.2. Electrochemical performance in aqueous electrolyte The capacitive potential range of all carbon samples was assessed using cyclic voltammetry at a slow scan rate of 5 mV s-1. The resulting CVs (Fig. 3a) show the onset potentials for hydrogen and oxygen evolution as evidenced by the negative and positive faradaic currents. No obvious faradaic current was observed between -1.2 and 0 V (vs Hg/HgO) so this is chosen as the capacitive potential range for all the carbon electrodes. This potential range is among the highest of aqueous supercapacitors with biomass-derived carbon (Table S2). Within this potential range, the CV curves (Fig. 3b) of BZnC-7, BZnC-8 and BZnC-9 all exhibit rectangular shape, i.e., constant current over the whole potential range and fast current switching at negative and positive vertex potentials. In contrast BC-8 shows a distorted rectangular shape and notably higher CV current at positive potentials. This is probably due to its high ratio of micropore/total surface area (Table 1), i.e., the micropores can accommodate the cations but are unfavourable for anion adsorption [31.32]. BZnC-8 displays the highest CV current and hence the highest specific capacitance among all samples. The galvanostatic charge-discharge (GCD) plots (Fig. 3c) of all carbon samples displayed isosceles triangle shape, suggesting high reversibility and high coulombic efficiency during their

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capacitive charge-discharge process. BC-8 shows much higher IR drop than the other carbons, indicating nano-ZnO activation enhances the charge-discharge kinetics by improving the specific surface area and pore structure. The rate capability of BZnC-8 was studied by running CV and GCD tests at different scan rates and current densities respectively. The resulting curves (Fig. S3a-b) show that BZnC-8 still exhibit excellent capacitive behavior at scan rate of 200 mV s-1 and current density up to 20 A g-1. Fig 3d provides a comparison of the specific capacitance values of the four samples at different current densities as derived from their GCD curves. BZnC-7, 8 and 9 all showed high capacitance and excellent rate performance due to their mesopore-dominant structure and high specific surface area. In accordance with the CV curves, BZnC-8 exhibits the highest specific capacitance of 286 F g−1 at 0.5 A g−1. Even at a high current density of 20 A g−1, it still shows a high specific capacitance of 216 F g-1. The Nyquist plots (Fig. 3e) show that BZnC-8 has a steeper line in low frequency region that is almost vertical, which revealed its good capacitive behavior [25]. The intercept with the Z' axis suggests the equivalent series resistances (Rs) in the high frequency region. Meanwhile, Z' intercept of BZnC-8 is considerably smaller than those of other samples, implying its better conductivity [33]. In the high-frequency region, the diameter of the semicircle presents the charge transfer resistance (Rct). Moreover, the Rct values of BZnC-T are distinctly lower than that of the without activated BC-8, which can be ascribed to the 3D hierarchical porous structure with high mesopore volume. The capacitance retention curve (Fig. 3f) derived from repeated GCD test shows that BZnC-8 retain 88.9% of the

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initial capacitance after 5000 charge/discharge cycles conducted at 5 Ag-1 within the potential range of -1.2 to 0 V. The above electrochemical tests suggest that BZnC-8 is a high-performance supercapacitor electrode in terms of its specific capacitance, rate performance and cycle stability. Both the high capacitance and rate performance can be attributed to its 3D hierarchical porous structures and high mesopore volume, while its cycle performance and wide capacitive potential range may be due to the low oxygen content. The desired electrochemical performance of BZnC-8 renders its promising application

in

supercapacitors,

verifying

the

feasibility

of

synthesizing

high-performance carbon electrodes using nano-ZnO and biowaste materials. Although BZnC-9 shows the highest specific surface area, the specific capacitance values follow the sequence of BZnC-8 > BZnC-9 > BZnC-7 > BC-8, as shown in Fig. 3b-d. The sum of mesopore surface area and external surface area (Smes+Sext, calculated by deduction of micropore area from the total BET specific surface area, S BET-Smic) shows the same trend. However Smes+Sext is not considered an appropriate capacitance indicator because large-sized micropores also contribute to the specific capacitance. A more plausible model [34] considering the size of the pores and counter ions is adopted to calculate the effective specific surface area (E-SSA, detailed calculation provided in Table S3 of the Supporting Information). BZnC-9 showed slightly higher E-SSA value than BZnC-8 despite that the latter exhibited higher specific capacitance. This discrepancy is probably because the solvation effect and diffusion kinetics of electrolyte ions needs to be further considered in the current model. Fig. S4 shows the trend of SBET,

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SBJH, Smes+Sext, and E-SSA values together with the specific capacitance of all the four carbon samples. It is found that none of the four specific-surface-area values exhibits linear correlation with the specific capacitance. Therefore, a universal capacitance indicator based on specific surface area is yet to be developed. 3.3. Electrochemical performance in organic electrolyte Organic electrolytes deliver higher voltage and hence higher specific energy than aqueous electrolytes in supercapacitors. But carbon-based electrodes often show lower specific capacitance in organic electrolytes because of the larger-sized counterions. The wide-potential-range cyclic voltammograms (Fig. S5a) show that BZnC-8 possesses much higher specific capacitance than BC-8, particularly at positive potentials in TEABF4/AN electrolyte. This is because the mesopore-dominant structure of BZnC-8 can better accommodate the large-sized TEA+ and BF4- in organic electrolyte. The CV (Fig. S5a) also displays a stable capacitive potential range of -2.0 to -2.3 V (vs. Ag/AgNO3) in TEABF4/AN. The capacitive potential range of BZnC-8 is further verified by extending its positive potential limit from 1.0 to 1.4 V (Fig. S5b), as well as extending its negative potential limit from -2.0 to -2.5V (Fig. S5c). The CVs still do not show significant faradaic current, confirming that -2.3 V to +1.4 V (vs. Ag/AgNO3) is a stable capacitive potential range for BZnC-8 in the organic electrolyte (Fig. S5a). This fact suggests that the maximum achievable voltage of a symmetric supercapacitor with BZnC-8 electrodes may reach up to 3.7 V in TEABF4/AN. But in the real case the cell voltage of a supercapacitor is determined by one electrode (denoted as the

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voltage-determining electrode) while the potential range of the other electrode is often underexploited [35, 36]. The symmetric supercapacitor with two identical BZnC-8 electrodes exhibits ideal supercapacitor behavior with no irreversible faradaic current (Fig. 4a) or voltage delay (Fig. 4b) as the cell voltage increase from 2.5 to 3.0V. To identify the voltage-determining electrode, the cell voltage and the potential of the two electrodes were simultaneously measured and plotted in Fig 4c. The plots show that the potential of the positive electrode attains the positive limit of 1.4 V but the negative electrode only reaches -1.6 V at cell voltage of 3 V. These observations indicate that the positive electrode is the voltage-determining electrode while the potential range of the negative electrode stays underexploited. The promising strategy for higher voltage of supercapacitor can be achieved by tuning and raising the potential of the positive electrode. Since both the positive and negative electrode potentials are in the capacitive potential range of BZnC-8, it further verifies that 3 V is a safe and stable cell voltage for the symmetric supercapacitor. The highly symmetric GCD curves (Fig. 4d) indicate high coulombic efficiency and reversibility of the symmetric cell at various current densities. As listed in Table S4, a cell voltage of 3 V is higher than most of the previously reported symmetric supercapacitors using biomass-derived carbons with organic electrolytes, and is even comparable to those with ionic liquid electrolyte. The high voltage in organic electrolyte could be due to the 3D hierarchical porous structure that provides easily accessible high surface area for storage of counterions [37, 38]. Besides, the low oxygen content (Table S4) of the carbon ensures a stable

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electrode-electrolyte interface at high cell voltage [13, 39, 40]. The oxygen contents of the listed biomass-derived samples are generally higher than ours and their working voltage are mostly lower than 3.0 V. This comparison validates our claims on the effect of oxygen content on cell voltage, i.e., a low oxygen content is a prerequisite for fabricating high voltage supercapacitor. The symmetrical cell also showed high rate capability, i.e., a high capacitance retention of 85.2% was observed as the current density increased from 1 to 20A g−1) (Fig. 5a). The specific capacitance of individual electrode is 115.6 F g-1 at 0.5 A g−1, which is superior to most of the reported biomass-derived carbon using the same electrolyte. The specific energy and specific power were calculated from Eq. S(3) and S(4) (Supporting Information) and plotted in Fig5. b. BZnC-8 based symmetric supercapacitor delivers specific energy of 34.6 Wh kg−1 at specific power of 366.8 W kg−1. At high specific power of 4350.5 W kg−1, it shows specific energy of 11.2 Wh kg−1. Additionally, a coin-type supercapacitor was assembled which displays 70.8% capacitance retention after 5000 cycles at a high cell voltage of 3.0 V (Fig. 5c). 4. Conclusion In summary, 3D mesopore-dominant hierarchical carbons were fabricated by the nanosized zinc oxide-assistant carbonization-activation method using plane tree bark as precursor. The optimized BZnC-8 sample shows the sum of mesopore surface area and external surface area of 717 cm2 g-1, and total specific surface area of 1512 cm2g-1. The corresponding carbon electrode exhibited high capacitance (286 F g-1 at 0.5 A g-1) and

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excellent rate capability in aqueous electrolyte from -1.2 V to 0 V (vs Hg/HgO). In the symmetric supercapacitor with organic electrolyte, the carbon showed a specific capacitance of 115.6 F g-1, and excellent cycle performance at a high cell voltage of 3.0 V. The superior capacitive performance can be ascribed to its mesopore-dominant high specific surface area, easily accessible 3D hierarchical porous structure and low oxygen content. This study also shows that the use of specific surface area as a capacitance indicator may not be a prudent approach. Furthermore, it is found that in a symmetrical supercapacitor, the positive electrode is the voltage-determining electrode while the potential range of the negative electrode is not fully exploited. This work not only proposed facile preparation of high-performance supercapacitor carbon using biowaste, but also prompted materials and device considerations for future design of high-voltage supercapacitors.

Acknowledgements This work received financial support from the Science and Technology Bureau of Shenzhen (Grant No. JCYJ20170306171540744) and the Science and Technology Bureau of Ningbo (Grant No. 201501CX-C01006)

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References [1] W. Raza, F. Ali, N. Raza, Y. Luo, K.-H. Kim, J. Yang, S. Kumar, A. Mehmood, E.E. Kwon, Recent advancements in supercapacitor technology, Nano Energy 52 (2018) 441-473. [2] C. Peng, S. Zhang, X. Zhou, G.Z. Chen, Unequalisation of electrode capacitances for enhanced energy capacity in asymmetrical supercapacitors, Energy Environ. Sci. 3 (2010) 1499-1502. [3] J. Li, N. Wang, J. Tian, W. Qian, W. Chu, Cross-Coupled Macro-Mesoporous Carbon Network toward Record High Energy-Power Density Supercapacitor at 4 V, Adv. Funct. Mater. 28 (2018) 1806153. [4] W. Zuo, C. Xie, P. Xu, Y. Li, J. Liu, A Novel Phase-Transformation Activation Process toward Ni-Mn-O Nanoprism Arrays for 2.4 V Ultrahigh-Voltage Aqueous Supercapacitors, Adv. Mater. 29 (2017) 1703463. [5] C. Wang, D. Wu, H. Wang, Z. Gao, F. Xu, K. Jiang, Biomass derived nitrogen-doped hierarchical porous carbon sheets for supercapacitors with high performance, J. Colloid Interf. Sci. 523 (2018) 133-143. [6] X. Liu, C. Ma, J. Li, B. Zielinska, R.J. Kalenczuk, X. Chen, P.K. Chu, T. Tang, E. Mijowska, Biomass-derived robust three-dimensional porous carbon for high volumetric performance supercapacitors, J. Power Sources 412 (2019) 1-9. [7] Y. Cui, Q. Zhang, J. Wu, X. Liang, A.P. Baker, D. Qu, H. Zhang, H. Zhang, X. Zhang, Developing porous carbon with dihydrogen phosphate groups as sulfur host for

18

high performance lithium sulfur batteries, J. Power Sources 378 (2018) 40-47. [8] J. Qu, C. Geng, S. Lv, G. Shao, S. Ma, M. Wu, Nitrogen, oxygen and phosphorus decorated porous carbons derived from shrimp shells for supercapacitors, Electrochim. Acta 176 (2015) 982-988. [9] J.M. Zhang, Q. Hua, J. Li, J. Yuan, T. Peijs, Z. Dai, Y. Zhang, Z. Zheng, L. Zheng, J. Tang, Cellulose-Derived Highly Porous Three-Dimensional Activated Carbons for Supercapacitors, ACS Omega 3 (2018) 14933-14941. [10] M.H. Sun, S.Z. Huang, L.H. Chen, Y. Li, X.Y. Yang, Z.Y. Yuan, B.L. Su, Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine, Chem. Soc. Rev. 45 (2016) 3479-3563. [11] J. Hu, W. He, S. Qiu, W. Xu, Y. Mai, F. Guo, Nitrogen-doped hierarchical porous carbons prepared via freeze-drying assisted carbonization for high-performance supercapacitors, Appl. Surf. Sci. 496 (2019) 143643. [12] S.Z. Yanhong Lu, Jiameng Yin, Congcong Bai, Junhao Zhang, Yingxue Li, Yang Yang , Zhen Ge, Miao Zhang, Lei Wei, Maixia Ma, Yanfeng Ma, Yongsheng Chen, Mesoporous activated carbon materials with ultrahigh mesopore volume and effective specific surface area for high performance supercapacitors, Carbon 124 (2017) 64-71. [13] M. Pumera, H. Iwai, Multicomponent Metallic Impurities and Their Influence upon the Electrochemistry of Carbon Nanotubes, J. Phys. Chem. C 113 (2009) 4401-4405. [14] Z. Yang, J. Tian, Z. Yin, C. Cui, W. Qian, F. Wei, Carbon nanotube- and

19

graphene-based nanomaterials and applications in high-voltage supercapacitor: A review, Carbon 141 (2019) 467-480. [15] Z. Wang, D. Shen, C. Wu, S. Gu, State-of-the-art on the production and application of carbon nanomaterials from biomass, Green Chem. 20 (2018) 5031-5057. [16] F. Yu, S. Li, W. Chen, T. Wu, C. Peng, BiomassǦ Derived Materials for Electrochemical Energy Storage and Conversion: Overview and Perspectives, Energy Environ. Mater. 2 (2019) 55-67. [17] Z. Wang, S. Yun, X. Wang, C. Wang, Y. Si, Y. Zhang, H. Xu, Aloe peel-derived honeycomb-like bio-based carbon with controllable morphology and its superior electrochemical properties for new energy devices, Ceram Int. 45 (2019) 4208-4218. [18] X. Wang, S. Yun, W. Fang, C. Zhang, X. Liang, Z. Lei, Z. Liu, Layer-Stacking Activated Carbon Derived from Sunflower Stalk as Electrode Materials for High-Performance Supercapacitors, ACS Sustainable Chem. Eng. 6 (2018) 11397-11407. [19] H.W. Shukai Yu, Chen Hu, Qizhen Zhu, Ning Qiao Bin Xu, Facile synthesis of nitrogen-doped, hierarchical porous carbons with a high surface area: the activation effect of a nano-ZnO template, J. Mater. Chem. A 4 (2016) 16341-16348. [20] J. Xu, K. Zhou, F. Chen, W. Chen, X. Wei, X.-W. Liu, J. Liu, Natural Integrated Carbon Architecture for Rechargeable Lithium–Sulfur Batteries, ACS Sustainable Chem. Eng. 4 (2016) 666-670. [21] J. Liu, Q. Wu, Q. Zhu, Y. Guan, B. Xu, Hierarchical porous carbon prepared from

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mulberry leaves for supercapacitors, Ionics (2019). DOI: 10.1007/s11581-019-03023-3 [22] F. Yu, L. Sun, Y. Zhou, B. Gao, W. Gao, C. Bao, C. Feng, Y. Li, Biosorbents based on agricultural wastes for ionic liquid removal: An approach to agricultural wastes management, Chemosphere 165 (2016) 94-99. [23] H. Jin, S. Wu, T. Li, Y. Bai, X. Wang, H. Zhang, H. Xu, C. Kong, H. Wang, Synthesis of porous carbon nano-onions derived from rice husk for high-performance supercapacitors, Appl. Surf. Sci. 488 (2019) 593-599. [24] S. Chabi, C. Peng, D. Hu, Y. Zhu, Ideal three-dimensional electrode structures for electrochemical energy storage, Adv. Mater. 26 (2014) 2440-2445. [25] L. Li, Y. Zhou, H. Zhou, H. Qu, C. Zhang, M. Guo, X. Liu, Q. Zhang, B. Gao, N/P Codoped Porous Carbon/One-Dimensional Hollow Tubular Carbon Heterojunction from Biomass Inherent Structure for Supercapacitors, ACS Sustainable Chem. Eng. 7 (2019) 1337-1346. [26] G. Zhao, C. Chen, D. Yu, L. Sun, C. Yang, H. Zhang, Y. Sun, F. Besenbacher, M. Yu, One-step production of O-N-S co-doped three-dimensional hierarchical porous carbons for high-performance supercapacitors, Nano Energy 47 (2018) 547-555. [27] M. Liu, J. Niu, Z. Zhang, M. Dou, F. Wang, Potassium compound-assistant synthesis of multi-heteroatom doped ultrathin porous carbon nanosheets for high performance supercapacitors, Nano Energy, 51 (2018) 366-372. [28] K. Nanaji, V. Upadhyayula, T.N. Rao, S. Anandan, Robust, Environmentally Benign Synthesis of Nanoporous Graphene Sheets from Biowaste for Ultrafast

21

Supercapacitor Application, ACS Sustainable Chem. Eng. 7 (2018) 2516-2529. [29] D. Zhang, J. Wang, C. He, Y. Wang, T. Guan, J. Zhao, J. Qiao, K. Li, Rational Surface Tailoring Oxygen Functional Groups on Carbon Spheres for Capacitive Mechanistic Study, ACS Appl. Mater. Interfaces 11 (2019) 13214-13224. [30] Y.W. Chi, C.C. Hu, H.H. Shen, K.P. Huang, New Approach for High-Voltage Electrical Double-Layer Capacitors Using Vertical Graphene Nanowalls with and without Nitrogen Doping, Nano letters 16 (2016) 5719-5727. [31] A.C. Forse, C. Merlet, J.M. Griffin, C.P. Grey, New Perspectives on the Charging Mechanisms of Supercapacitors, J. Am. Chem. Soc. 138 (2016) 5731-5744. [32] C. Prehal, D. Weingarth, E. Perre, R.T. Lechner, H. Amenitsch, O. Paris, V. Presser, Tracking the structural arrangement of ions in carbon supercapacitor nanopores using in situ small-angle X-ray scattering, Energy Environ. Sci. 8 (2015) 1725-1735. [33] D. Zhang, C. He, Y. Wang, J. Zhao, J. Wang, K. Li, Oxygen-rich hierarchically porous carbons derived from pitch-based oxidized spheres for boosting the supercapacitive performance, J. Colloid Interf. Sci. 540 (2019) 439-447. [34] L. Zhang, X. Yang, F. Zhang, G. Long, T. Zhang, K. Leng, Y. Zhang, Y. Huang, Y. Ma, M. Zhang, Y. Chen, Controlling the effective surface area and pore size distribution of sp2 carbon materials and their impact on the capacitance performance of these materials, J. Am. Chem. Soc. 135 (2013) 5921-5929. [35] J. Yang, J. Hu, M. Zhu, Y. Zhao, H. Chen, F. Pan, Ultrahigh surface area meso/microporous carbon formed with self-template for high-voltage aqueous

22

supercapacitors, J. Power Sources 365 (2017) 362-371. [36] M. Yu, D. Lin, H. Feng, Y. Zeng, Y. Tong, X. Lu, Boosting the Energy Density of Carbon-Based Aqueous Supercapacitors by Optimizing the Surface Charge, Angew. Chem. 56 (2017) 5454-5459. [37] J.M. Griffin, A.C. Forse, W.Y. Tsai, P.L. Taberna, P. Simon, C.P. Grey, In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors, Nature Mater. 14 (2015) 812-819. [38] C. Zhao, Y. Huang, C. Zhao, X. Shao, Z. Zhu, Rose-derived 3D carbon nanosheets for high cyclability and extended voltage supercapacitors, Electrochim. Acta 291 (2018) 287-296. [39] E. Frackowiak, Q. Abbas, F. Béguin, Carbon/carbon supercapacitors, Journal of Energy Chemistry 22 (2013) 226-240. [40] C. Kong, W. Qian, C. Zheng, Y. Yu, C. Cui, F. Wei, Raising the performance of a 4 V supercapacitor based on an EMIBF4-single walled carbon nanotube nanofluid electrolyte, Chemical communications 49 (2013) 10727-10729.

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Table 1 The SSA, porosity parameters of the as-prepared carbon materials.a SBET Samples (m2 g-1)

Smic Smic/SBET Smes+Sext SBJH 2 -1 2 -1 (m g ) (m g ) (m2 g-1) (%)

E-SSA KOH (m2 g-1)

E-SSA TEABF4 (m2 g-1)

Vtotal (cm3 g-1)

Vmic (cm3 g-1)

BC-8 83.52 53.76 61.21 270.08 100.15 0.15 326.31 272.55 0.11 BZnC-7 487.89 329.14 67.46 158.75 235.56 376.42 205.91 1.06 0.14 BZnC-8 1511.91 794.87 52.57 717.04 626.40 962.74 710.34 2.28 0.32 BZnC-9 1587.62 985.65 62.08 601.97 686.40 973.48 753.16 2.33 0.40 At P/P0 = 0.99. a SBET, total specific surface area was obtained by the BET method based on the adsorption data in the relative pressure (P/P0) range of 0.05 - 0.3.; Smic, micropore surface area; Smes, mesopore surface area; Sext, external surface area; SBJH, cumulative mesopore surface area were determined using Barrett-Joyner-Halenda (BJH) methods; E-SSA , the E-SSA was obtained according to the cumulative DFT SSA, the pore size distribution of the carbon materials and the electrolyte ion size; Vtotal, Vmic, Vmes, pore volume of total pore volume micropore, mesopore.

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Vmes (cm3 g-1) 0.04 0.92 1.96 1.93

Fig. 1. SEM images of BC-8 (a), BZnC-7 (b), BZnC-8 (c), and BZnC-9 (d), and TEM images of BZnC-8 (e-f).

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Fig. 2. Nitrogen adsorption-desorption isotherms (a), pore size distribution (b), XRD patterns (c), Raman spectra (d) of BC-8, BZnC-7, BZnC-8 and BZnC-9, high-resolution XPS spectra of the C1s (e) and O1s (f) for BZnC-8.

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Fig. 3. Electrochemical properties of carbon electrodes in 6.0 M KOH using a three-electrode setup: CV curves at a scan rate of 5 mV s−1 in the potential range from -1.3 to 0.2 V (a), CV curves at 20 mV s-1 (b), GCD curves at a current density of 1 A g−1(c), specific capacitance at different current densities (d), Nyquist plots (e), and 5000-cycle stability test of BZnC-8 at 5 A g-1 (f). 27

Fig. 4. Electrochemical characterization of BZnC-8 in a two-electrode symmetric cell using 1.0 M TEABF4/AN: CV curves measured at different cell voltages (a), GCD curves at current density of 1.0 A g-1 at different voltages (b), Individual electrode potential profile and cell voltage of the symmetric cell with GCD current density of 2.0 A g-1 (c), GCD curves at current densities from 0.5 to 20 A g-1 (d).

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Fig. 5. Electrochemical properties of BZnC-8 in a two-electrode system using 1.0 M TEABF4/AN: the specific capacitance at different current densities with a voltage of 3.0 V (a), the ragone plot (b), and the cycling stability of the symmetric supercapacitor device at 2A g−1 with a voltage of 3.0 V and it lightened LED bulb (c).

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Graphical Abstract

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Highlights l

Biomass-derived 3D hierarchical porous carbons are synthesized by one-step method.

l

The optimized carbon exhibits an ultrahigh mesopore area and low oxygen content.

l

The carbon electrodes showed high capacitance in aqueous and organic electrolytes.

l

The symmetric supercapacitor delivers high cell voltage and energy density.

l

Cell voltage is determined by the upper limit of the capacitive potential range.

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Author contributions: H. Zhang and C. Peng proposed the research and supervised the project. F. Yu and Z. Ye designed and conducted the experiments and analyzed the experimental results. W. Chen, Q. Wang and H. Wang performed the structural characterization of the carbon materials. F. Yu and W. Chen wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 

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