Mesopore-rich carbon flakes derived from lotus leaves and it’s ultrahigh performance for supercapacitors

Journal Pre-proof Mesopore-rich carbon flakes derived from lotus leaves and it's ultrahigh performance for supercapacitors Qingjie Lu, Shiqiang Zhou, Bo Li, Haitang Wei, Dongming Zhang, Jicu Hu, Longzhou Zhang, Jin Zhang, Qingju Liu PII:

S0013-4686(19)32353-9

DOI:

https://doi.org/10.1016/j.electacta.2019.135481

Reference:

EA 135481

To appear in:

Electrochimica Acta

Received Date: 21 August 2019 Revised Date:

20 November 2019

Accepted Date: 9 December 2019

Please cite this article as: Q. Lu, S. Zhou, B. Li, H. Wei, D. Zhang, J. Hu, L. Zhang, J. Zhang, Q. Liu, Mesopore-rich carbon flakes derived from lotus leaves and it's ultrahigh performance for supercapacitors, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135481. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Mesopore-rich carbon flakes derived from lotus leaves and it’s ultrahigh performance for supercapacitors Qingjie Lu, Shiqiang Zhou, Bo Li, Haitang Wei, Dongming Zhang, Jicu Hu, Longzhou Zhang, Jin Zhang, Qingju Liu* Department: School of Materials Science and Engineering, Yunnan Key Laboratory for Micro/nano Materials & Technology, Yunnan University, Kunming 650091, China E-mail:

[email protected]

Abstract: Much attractions have been aroused about the biomass-derived carbon used for energy storage due to their high specific surface areas (SSA) and well-developed porosity. Here, we report a novel biomass carbon derived from lotus leaves which has excellent electrochemical performance. A two-step activation method is used for the preparation of the final sample: double activated lotus carbon (DALC). Briefly, HNO3 is used for the first-step activation and the KOH activating process continues. The DALC possesses an ultra-high SSA of 2351 m2 g-1 and abundant micropores and mesopores structures. And importantly, some defects are fabricated during the preparation process. These specific features make the final sample DALC exhibits an ideal specific capacity of 478 F g-1 in a three-electrode system at a current density of 1 A g-1, which is the highest level for all the previously reported biomass-derived carbon materials (without the presence of any metal oxides, which possess excellent specific capacity originally). Besides, the assembled symmetric supercapacitor in a two-electrode system also exhibits a superb specific capacitance of 358 F g-1 at 1 A g-1. All the results indicate that the DALC possesses a great potential for electrode materials of supercapacitors.

Key Words: biomass; carbon nanosheets; supercapacitors 1 Introduction 1

The development and utilization of efficient and environmentally friendly energy storage devices are extremely important, due to the ever-growing of the energy consumptions in recent decades. Supercapacitor has been a hot research spot with the advantages of higher power density, rapid charge-discharge characteristics, and long cycle life[1-3]. Electrode materials, as the essential component of supercapacitors, determine the performance of supercapacitors decisively. Ideal electrode materials possess well-developed structures, simple preparation process, abundant resources, low cost and environmental friendliness, originally[4]. Carbon materials are widely used as the electrode materials of supercapacitors due to their high SSA, strong chemical stability, high electronic conductivity, adjustable microstructure and low cost[5]. Especially, porous carbon is one kind of the ideal candidate for the electrode materials since the rich pore structure can not only provide enough active sites for electrochemical reactions, but also supply channels for the transmission of electrolyte ions to shorten the transmission distance and improve the transmission efficiency[6]. However, most of the carbon materials are currently prepared by fossil fuels which is limitedly stored, unsustainable and eco-unfriendly[7]. Biomass-derived carbon materials have attracted wide attentions due to their sustainability, low pollution and structural diversity[8,9]. To date, many biomass sources have been used as the carbon precursors for energy storage, such as nut shells[10,11], seeds[12,13], shellfish[14,15], fungi[16,17], fructus[18,19], leaves[20,21], abandoned food[22] and animal tissue[23,24]. Naturally, biomass-derived carbon materials generally present various morphologies, such as spheres, fibers, sheets, tubes, and rods[25]. For supercapacitors, the biomass-derived carbon usually possesses lots of merits, including high SSA, ideal pore size, manipulated surface chemistry and excellent electric

2

conductivity.

These specific characters promise

it to

be the ideal candidate for

supercapacitors[26,27]. Although biomass-derived carbon as the electrode material of supercapacitors has achieved many progresses, there are still some bottlenecks and challenges limiting their extensive deployments and further applications. For instance, the surface chemistry of biomass-derived carbon such as SSA and pore structure is difficult to control. In particularly, high SSA is greatly beneficial for the electrochemical performance of biomass-derived carbon materials, however, the high SSA usually derived from chemical activation methods while vast micropores are generated, which are not conducive to the transmission and diffusion of electrolyte ions[28]. Furthermore, The introduction of the heteroatoms can modify the surface properties and internal structure of the biomass-carbon materials to increase the active sites and interlayer spacing, which can be beneficial for electrochemical reactions[29]. However, heteroatoms may decrease the electrical conductivity of biomass-carbon materials[30]. As is known to all, the biomass-derived carbon used for the electrode materials of supercapacitors usually exhibits lower capacitance comparing with the carbon derived from fossil fuels[7,12,20]. In addition, it is necessary to expand the production scale of the biomass-derived carbon materials from the laboratory stage to the real industrial applications, and for this, those all above problems and challenges should be improved for the design of high electrochemical performance electrode materials of supercapacitors[12]. In this work, a biomass nanocarbon derived from lotus leaves by a two-step activation method is presented, which exhibits an excellent electrochemical performance for supercapacitors. Innovatively, a two-step activation method was applied combining the HNO3 activation and the KOH activation. In general, we firstly proceeded the HNO3 activation process after a hydrothermal

3

carbonization treatment (lotus carbon derived by hydrothermal carbonization denoted as LC), creating small amounts of pores and improving the SSA, which is denoted as activated lotus carbon (ALC). But the electrochemical performance of ALC is still unsatisfied. Then the KOH activation process is proceeded and large number of micropores, mesopores and some defects were generated. As a result, the as-obtained sample DALC possesses an ultra-high SSA of 2351 m2 g-1 and abundant micropores and mesopores. When DALC was used as electrode materials, it exhibits a superb specific capacitance of 478 F g-1 at a current density of 1 A g-1 in a three-electrode system. Additionally, the symmetric supercapacitor assembled by two same DALC electrodes also displays an excellent specific capacitance of 358 F g-1 at a current density of 1 A g-1, and a long cycling life with 87.4% specific capacitance retention after 5000 cycles in a two-electrode system.

2 Experimental section 2.1 Preparation of DALC Lotus leaves were used for the preparation of DALC. The leaves picked from the pond were repeatedly rinsed with deionized water, and then dried in an oven at 80

overnight to obtain dried

leaves. Subsequently, the dried leaves was treated by a hydrothermal carbonization method at 160

for 12 h in a 150 mL autoclave with the ratio of 1 g : 20 mL deionized water, then the

sample was dried overnight to obtain the LC. LC and 98% HNO3 solution was mixed with a ratio of 1 g : 20 mL and then the mixture was stirred at room temperature for 24 h. The obtained sample was washed repeatedly by deionized water and then treated in a tubular furnace at 700

for 2 h.

Finally, the sample was washed by deionized water and then centrifugated with a speed of 7000

4

rad s-1 for 10 min, then dried to obtain the ALC, which is the first activation. Subsequently, the second activation was proceeded. ALC was mixed with solid KOH with the mass ratio of 1: 3 and the mixture was fully grinded, then treated in a tubular furnace at 700

for 2h. After the

temperature dropped to room temperature, 1M HCl was used to wash the sample for 4 h, and followed the deionized water was used to wash the sample until the pH was neutral, then centrifugated with a speed of 7000 rad/s for 10 min and dried to obtain the final carbon DALC (the reason for the order of the activators is explained: KOH activation is beneficial for the electrochemical performance by improving the SSA and optimizing the pore structure of carbon materials. The activation mechanism considered with two aspects. Firstly, the carbon materials react with KOH, which causes part of carbon is etched and the sites are etched would arise pores by washing and removing the generated salts and redundant KOH. Another aspect considered as the introduction of KOH can promote the removal of non-carbon atoms. HNO3 activation is beneficial for the electrochemical performance by generating some reactions with the carbon materials, such as hydrolysis、dehydration、cyclization and pore-forming. These reactions can improve the pore content、oxygen-containing functional groups and SSA, suggesting the abundant electrochemical active sites. But the activation degree is relative lower which limited by the reaction degree between carbon materials and activator in one kind of activation mechanism. It suggests that the second activation step has a low effect in the KOH-KOH and HNO3-HNO3 activation methods. In addition, From the activation mechanism of KOH, It can be predicted that the more reasonable pore structure and larger SSA will achieve as the KOH activation can promote the removal of N atoms which are introduced by HNO3 activation. The removal process can generate some defects and more pores. So the HNO3-KOH was chose as the activation order.

5

The subsequent tests confirmed the right thoughts of ours. According to Fig. S1, ESI† and Table S1, ESI†, high SSA and more reasonable pore structure of the samples which activated by HNO3-KOH activation will be observed intuitively and then the performance exhibits a higher value as we expected Fig. S2, ESI†).The whole process is concisely illustrated in Fig. 1.

Fig. 1 Scheme diagram for the fabrication of the DALC from lotus leaf.

2.2 Characterization Fourier-transform infrared spectroscopy (FT-IR) was conducted with a FT-IR-2000 spectrometer (Ican 9) in the wave number range of 400-4400 cm-1. Thermogravimetry (TG) and differential scanning Calorimetry (DSC) analyses were performed using a synchronous thermal analyzer (STA ZCT-B) in the range of 0-1000

at a heating rate of 10

min-1 under air

atmosphere. Raman measurements were conducted with a Renishaw laser Raman microscope

6

(inVia) with a 633 nm argon ion laser. The chemical compositions were detected by X-ray photoelectron spectroscopy (XPS, K-Alpha spectrometer, Thermo Fisher Scientific Co. Ltd) with Al Ka excitation (1486.6 eV). Field emission scanning electron microscopy (FESEM, Nova NanoSEM 450, Thermo Fisher Scientific Co. Ltd) and high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL Co. Ltd., Japan) were used to observe the microscopic morphology

of

the

samples.

The

specific

surface

area

was

measured

by

the

Brunauer-Emmett-Teller (BET) method and the pore size distribution was evaluated by the Barrett-Joyner-Halenda (BJH) method, respectively.

2.3 Electrochemical measurements For the electrochemical performance measurements of the samples, a typical three-electrode system was used as the test system in which a platinum electrode was used as the counter electrode and an Ag/AgCl electrode as the reference electrode. Preparation of the working electrodes: The sample LC, ALC and DALC were respectively mixed with carbon black and Polytetrafluoroethylene (PTFE) emulsion in a mortar with a mass ratio of 8: 1: 1, and an appropriate amount of absolute ethyl alcohol was added, the mixtures were thoroughly ground to form the slurry, and the slurry was evenly spread on the nickel foam (Before use, nickel foam would be put into acetone for ultrasonic washing for 30min, and then put into absolute ethyl alcohol、deionized water、absolute ethyl alcohol for ultrasonic washing for 30min one by one). Then dried in a vacuum oven at 120 ℃ for 24 h, and tableted to obtain the working electrodes.

7

In order to detect the electrochemical performance of LC, ALC and DALC, electrochemical workstation CHI760 was used for cyclic voltammetry (CV) tests, galvanostatic charge-discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS) tests. Meanwhile, 6M KOH aqueous solution was used as electrolyte. For the three-electrode system, CV tests was carried out at various scan rates from 5 to 200 mV s-1 while the potential range is between -0.7 to 0.3 V vs Ag/AgCl electrode. The GCD tests were performed with potential range between -0.7 V to 0.3 V in various current densities from 1 to 20 A g-1. The EIS measurements were performed at open circuit potential from 100 KHZ to 0.01 Hz with an amplitude of 5 mV. The symmetric supercapacitor was assembled using the same two DALC electrodes and tested in a two-electrode system and the cycling tests were evaluated by GCD measurements at a current density of 5 A g-1 for 5000 cycles, 6M KOH solution was used as electrolyte. For quantitative considerations, the specific capacitance C is calculated from the CV and GCD values tested on three-electrode system according to the formula:

C=

∫ idV m∆VS

(1)

where i (A) is the response current, S (V s-1) is scan rate, m (g) is mass of active material on the electrode, and ∆V (V) is potential window in cyclic voltammetry.

C=

i • ∆t m • ∆V

(2)

In two-electrode system, the specific capacitance Cs of the working electrodes is calculated from the GCD values using the following formula:

Cs =

2i • ∆t m • ∆V 8

(3)

In formulas (2) and (3), C is the specific capacitance of the working electrodes calculated from the discharge curves based on the three-electrode system, Cs is the specific capacitance of the working electrode calculated from the discharge curves based on the two-electrode system, i (A) is the charge-discharge current, ∆t (s) denotes discharging time, m (g) is the mass of the carbon materials on the Ni foam, ∆V (V) is the voltage window. The energy density and power density of symmetrical supercapacitor systems is calculated by using the following formulas:

E=

Cs • ∆V 8× 3.6

(4)

P=

E t

(5)

Where E (Wh kg-1 ) is the specific energy density, P (W kg-1 ) is the specific power density,

∆V (V) is the voltage window for charging and discharging process, and t (h) is the discharge time.

3 Results and discussion XRD was used to confirm the crystalline structures of all samples. As shown in Fig. S3(ESI†), all XRD patterns of these samples present disordered structure, which is a typical property of carbon materials with an amorphous structure. Specifically, the broad diffraction peak at ~23

corresponds to the partially graphitic nature of the carbon materials. From the FT-IR

spectra (Fig. S4), there is a poignant peak with medium absorption intensity at ~1611.2cm-1 on the curve of ALC, which can be vested in double-bond between carbon and nitrogen of pyridine. From the TG and DSC curves (Fig. S5a, ESI† and S5b, ESI†), LC show the first weight loss at

9

0-305

and endothermic phenomenon, simultaneously, which could be regarded as the

elimination of the moisture. Moreover, the elimination of the volatile matters of the LC proceeds at the range of 305-435

. After 435

, LC shows a steady weight with an exothermic

phenomenon, suggesting the happening of chemical reactions between LC and oxygen gas. For ALC, weight loss and heat release happened at temperature range of 0-70 attributed to dehydration. At 70-400

, which can be

, ALC has no weight loss but a strong endothermic peak,

suggesting the happening of phase transition. After 700

, ALC has no weight loss but

exothermic phenomenon, which may be due to gas adsorption at the surface of sample. By comparison, DALC does not show apparent weight loss while its DSC curve displays a wide endothermic peak at the temperature range of 0-500

, which suggests that phase change might

have taken place without gasification, water loss and chemical reactions. After 500

, DALC

shows a fast weight loss and rapid exothermic, suggesting the reaction between DALC and oxygen gas. FESEM and transmission electron microscope (TEM) were used to observe the microscopic morphology of the samples. As shown in Fig. 2(a), the LC derived from lotus leaves by hydrothermal carbonization display a relatively dense structure, which is not beneficial for its electrochemical performance. A more porous structure of ALC and DALC are shown in Fig. 2(b) and 2(c), respectively. It can be seen that DALC exhibits a multilayered structure assembled by lots of carbon flakes which possess a thickness of 30 nm (Fig. 2d). From Figure 2(e), it can be more doubtless that the carbon flakes of DALC are ultra-thin. As shown in Fig. 2(f) and 2(g), the existence of plenty of mesopores and micropores is demonstrated. Such a porous carbon flake-like

10

Fig. 2 (a-b) SEM images of LC and ALC, respectively. (c-d) SEM images of DALC. (e-g) TEM images of DALC. structure is very conducive to the progress of the electrochemical reactions, the storage and transport of ions[7,29]. This further predicts the ideal electrochemical performance of DALC. The N2 adsorption/desorption system was used to determine the SSA, pore volume and pore size distribution of the samples. As shown in Fig. S6 (ESI†), DALC possess a much higher SSA of 2351 m2 g-1 than LC (292.8 m2 g-1) and ALC (759.7 m2 g-1). The pore size distribution of DALC is more reasonable, which means the DALC possesses more micropores and mesopores than that of LC and ALC (Fig. 3a). Intuitively, the SSA and pore structure properties of the 11

Fig. 3 (a) the pore size distribution of LC, ALC, DALC. (b) XPS patterns of LC, ALC, DALC. (c) High resolution of N1s spectra of LC, ALC, DALC. (d) Raman patterns of LC, ALC, DALC.

samples are summarized in table S2 (ESI†). When used as electrode materials, a high SSA and large number of micropores and mesopores can provide abundant active sites and electrolyte ion transport channels for electrochemical reactions, which can accelerate the electrochemical reactions and improve electrode efficiency[31,32]. So the more ideal electrochemical performance of DALC is predictable when used as electrode materials for supercapacitors. XPS was used to analyze the elements in the samples. As shown in Fig. 3(b), the N1s peak was detected with 2.24 at% nitrogen in the LC, while the content of nitrogen exhibits as 4.88 at% in the ALC and almost zero in the DALC. According to the high resolution analysis of the N1s

12

spectra (Fig. 3c) after peak deconvolution, it can be revealed that two main peaks can be fitted at 398.3 eV and 401.0 eV, which can be ascribed to pyridinic and graphitic nitrogen, respectively. Naturally, the XPS patterns of LC, ALC, DALC show that a certain amount of nitrogen may be introduced in the structure of LC after the HNO3 activation process, and all of the nitrogen in ALC were removed after the KOH activation process. According to some reports, it can be known that the introduction and removal of nitrogen atoms would produce some defects in the structures[33]. Raman spectrum was used to observe the different intensity of the D and G bands between the LC, ALC, DALC samples (Fig. 3d). For LC, the intensity of D band is lower than that of G band with an ID/IG ratio of 0.84 (Fig. 3d inset), indicating that the high regularity of the carbon structure. For ALC, the nitrogen atoms are introduced so the D band intensity got increased with an ID/IG ratio of 0.90 (Fig. 3d inset), suggesting that the defective degree increased after the HNO3 activation process. As is known to all, heteroatom doping is beneficial for the electrochemical performance of the biomass-derived carbon. On the one hand, more active sites for electrochemical reactions would be generated and thus the SSA gets increased; on the other hand, heteroatom doping can increase the porosity and layers spacing to accelerate the transmission of the electrolyte ions and finally improve the specific capacitance[29,34-35]. For DALC, the intensity of D band increases likewise and the removal of the all nitrogen atoms results in a further increase of defects as the ID/IG ratio has reached 1.02 (Fig. 3d inset), which might be much beneficial for the electrochemical performance for the biomass-derived carbon when it is used for the electrode materials of supercapacitors[36-37]. LC, ALC and DALC were prepared for working electrodes and formed three-electrode system with a platinum electrode and an Ag/AgCl electrode for electrochemical measurements in 6M KOH electrolyte. As shown in Fig. 4(a), the CV curves of LC, ALC, DALC were compared at

13

a same scan rate of 200 mV s-1 and the rectangular-like shapes suggest the double layer capacitance of the samples[46]. It can be seen that the CV curve area of DALC is much larger than that of LC and ALC, which indicates that the DALC exhibits better electrochemical performance as predicted. Calculated from the CV curves, the specific capacitance of DALC is 508 F g-1, much higher than ALC (191 F g-1) and LC (53 F g-1), but these capacitance values might be on the high side of the exact value because the rectangular shapes of the CV curves are not so normative. Fig. 4(b) shows the GCD curves of the samples at 1 A g-1, the symmetrical triangular-like shapes suggest the remarkable charge and discharge processes of the samples[46]. Calculated from the GCD curves, the specific capacitance of DALC is 478 F g-1, much higher than ALC (173 F g-1) and LC (40 F g-1). It should be mentioned here that the nickel foam itself showed capacitance. But the capacitance value will not influence the accuracy of the calculation results of the samples (Fig. S7, ESI†). For the ideal specific capacitance, the porous and flake-like structure is very conducive to the storage and transport of electrolyte ions[47-48], the ultra-high SSA and large number of micropores and mesopores can provide abundant active sites and electrolyte ion transport channels for electrochemical reactions, which can accelerate the electrochemical reactions and improve electrode efficiency[49-50]. Besides, the defects are beneficial for the electrochemical performance for the biomass-derived carbon when it is used for the electrode materials of supercapacitors[51-52]. Table 1 is the performance comparison of DALC with the biomass carbon reported latest. It can be seen that DALC exhibit a superb specific capacitance of 478 F g-1. The porous structure of DALC can provide ion transport channels and electrolyte reservoirs which ensure effective accessibility of ions transport at high charge/discharge rates[53]. As shown in Fig. 4(c) and 4(d), DALC shows steady quasi-rectangular shapes of CV curves at different scan rates from 5 to 200 mV s-1 and isosceles-like triangles shapes of GCD curves at different current densities from 1 A g-1 to 20 A g-1. These suggest the remarkable rate performance and superb capacitive behavior[46]. Compared with the CV curves at different scan rates and GCD curves at

14

Fig. 4 Electrochemical measurements in a three-electrode system. (a) CV curves of LC, ALC, DALC at a scan rate of 200 mV s-1. (b) GCD curves of LC, ALC, DALC at a current density of 1 A g-1. (c-d) CV and GCD curves of DALC at different scan rates and current densities, respectively. (e) Nyquist plots of LC, ALC, DALC. (f) Specific capacitance of LC, ALC, DALC at different current densities. (g) Cycling stability at 5 A g-1 up to 5000 cycles. different current densities of LC and ALC (Fig. S8a, ESI†, S8b, ESI†, S8c, ESI† and S8d, ESI†), the superb electrochemical performance of DALC is further confirmed. The Nyquist plots of DALC samples is shown in Fig. 4(e). It can be observed directly that DALC possesses steepest linear curve in the low frequency region, which suggests an ideal specific capacitance performance and a low equivalent series resistance of ion diffusion and migration. And the smallest semicircle diameter of DALC in the high frequency region suggests a low charge transfer resistance. For the lower resistance of DALC, the abundant micropores and mesopores can provide transport channels for electrolyte ions to accelerate electrochemical eactions, which can decrease the resistance[47,54]. As shown in Fig. 4(f), DALC still exhibits a high

15

Table 1. Comparison with the latest reports of biomass carbon. Specific Raw Materials

Current Density (A g-1)

Ref. Capacitance (F g-1)

bamboo char

0.5

222

[38]

Pomelo peel

0.5

240

[39]

yeast

1

255

[40]

perilla frutescens

0.5

270

[41]

cattail wool

1

314

[42]

tofu

0.5

315

[43]

Corn straw

0.3

327

[44]

soybean

0.5

330

[45]

cornstalk

1

407

[46]

Lotus leaves

1

478

This work

specific capacitance of 198 F g-1 at 5 A g-1 and 125 F g-1 at 10 A g-1, which means DALC possesses an ideal rate capacity when it is used for the electrode materials of supercapacitors. As shown in Fig. 4(g), the DALC electrode materials displayed an ideal specific capacitance retention of 87.4% in a three-electrode system. Theoretically, the lower ions transport resistance and short diffusion distance are beneficial for the ideal rate capacity[46-47,54]. Symmetric supercapacitor was further assembled by employing two same DALC electrodes and tested in a two-electrode system in 6M KOH aqueous solution. As shown in Fig. 5(a), the GCD curves of DALC at different current densities from 1 to 20 A g-1 display a triangular-like shapes and the CV curves also exhibits a rectangle-like shapes at different scan rates (Fig. S9, ESI †), which suggests that DALC exhibits a typical electrochemical capacitive behavior when used for the electrode materials of symmetric supercapacitor[47]. Calculated from the CV curve at a

16

Fig. 5 Electrochemical performances of the symmetric supercapacitor in a two-electrode system, 6M KOH aqueous solution used for the electrolyte. (a) GCD curves and (b) Specific capacitance and (c) Energy densities of DALC at different current densities. (d) Cycling stability at 5 A g-1 up to 5000 cycles. scan rate of 200mV s-1, the specific capacitance of DALC is 376 F g-1 (a value on the high side of the exact value based on the previous discussion). Calculated from the GCD curve, the device consisted by the two same DALC electrodes shows a superb specific capacitance of 358 F g-1 at 1 A g-1, suggesting an excellent capacitance performance. As discussed previously, the ultra-high SSA provides enough interfaces for the storage of electrolyte ions, the porous, loose and layered carbon flakes structure can ensure the diffusion and transport of electrolyte ions in a fast and efficient level[29,47-50]. Deservedly, the device exhibits an ideal rate capacity of 142 F g-1 at a current density of 5 A g-1 (Fig. 5b). In addition, as shown in Fig. 5(c), the device exhibits an ideal 17

energy densities of 14.2 Wh k-1g at 1 A g-1 and calculated from this, a power density of 285.6 W kg-1 is obtained. Cycling stability is a vital indicator of a kind of electrode materials. As shown in Fig. 5(d), the DALC electrode materials displayed an ideal specific capacitance retention of 89.1%. Additionally, the small bulb will give off a bright light when connected with

the symmetric

supercapacitor device (Fig. 5d inset). All of the above suggest the great potential of DALC for the electrodes used in the supercapacitors, further in the energy storage applications.

4 Conclusions In summary, a novel porous carbon was fabricated from lotus leaves, which possesses an ultra-high SSA (2351 m2 g-1) and well-developed pore structures. After the electrochemical measurements, the final sample DALC exhibits a high specific capacitance of 478 F g-1 at a current density of 1 A g-1 and ideal rate performance (198 F g-1at 5 A g-1). Furthermore, the assembled symmetric supercapacitor device in a two-electrode system also exhibits a superb specific capacitance of 358 F g-1 and a long cycle life with 89.1% capacitance retention after 5000 cycles at 5 A g-1. This superior performance for energy storage can be attributed to porous structure, high SSA and the defects fabricated by removing the nitrogen atoms. It is worth mentioning that a two-step activation method was used flexibly to modify the properties of the biomass-derived carbon and the thought about that defects are a potential factor for the ideal electrochemical performance. Briefly, a kind of biomass-derived porous carbon with excellent electrochemical performance was fabricated, which means that it possesses the great potential for the energy storage. And a more comprehensive explanation for the mechanism was explored, which could be meaningful for the fabrication and modification of the biomass-derived carbon.

18

Acknowledgements This work was supported by National Natural Science Foundation of China (no.51562038), Key Project of Natural Science Foundation of Yunnan (2018FY001(-011)) and Yunnan basic applied research project (no. 2017FB086). QL acknowledges Mingpeng Chen and Jianhong Zhao for data analysis and useful discussions.

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Author Contributions: Conceptualization, Qingjie Lu; Methodology, Qingjie Lu; Software, Shiqiang Zhou; Validation, Qingjie Lu, Shiqiang Zhou, Bo Li, Dongming Zhang; Formal Analysis, Shiqiang Zhou, Haitang Wei, Jicu Hu; Investigation, Qingjie

Lu and Longzhou Zhang; Resources, Qingju Liu; Data Curation, Qingjie Lu; Writing

Original Draft Preparation, Qingjie Lu; Writing

Review and Editing,

Shiqiang Zhou; Visualization, Jin Zhang; Supervision, Qingju Liu; Project Administration, Qingju Liu; Funding Acquisition, Qingju Liu.

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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: