Porous Layered Carbon with Interconnected Pore Structure Derived

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Porous layered carbon with interconnected pore structure derived from reed membranes for supercapacitors Chao-Lei Ban, Zongying Xu, Dawei Wang, Zhenqi Liu, and Huaihao Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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ACS Sustainable Chemistry & Engineering

Porous layered carbon with interconnected pore structure derived from reed membranes for supercapacitors

Chao-Lei Ban,*,† Zongying Xu,‡ Dawei Wang,‡ Zhenqi Liu,† Huaihao Zhang*,‡

†

School of Materials Science and Technology, Liaocheng University, Liaocheng, 252059, PR

China. ‡

School of Chemistry and Chemical Engineering, Yangzhou University, 180 Si-wang-ting Road,

Yangzhou 225002, PR China.

*Corresponding Author. Email Address: [email protected] (Cao-Lei Ban); [email protected] (Huaihao Zhang)

ACS Paragon Plus Environment

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ABSTRACT In this work, unique porous layer-stacking active carbon (RHC), using natural reed membranes as precursor, was prepared by a green and facile hydrothermal treatment and following carbonization process. The graphene-like RHC possesses abundant and interconnected pore structures, which can provide large active surfaces and wide ion transport paths to ensure rapid electrolyte ions storage and diffusion. In addition, abundant heteroatoms of raw materials have been retained well due to less alkali content and lower carbonization temperature, contributing to additional pseudocapacitance. As a result, the RHC electrode is endowed with desirable electrochemical behaviors. In 6 M KOH electrolyte, the as-prepared electrodes exhibit excellent specific capacitance (353.6 F·g-1 at a current density of 0.5 A·g-1), long cycle stability (3.9% specific capacitance loss after 10000 cycles) and good rate capability (76.9% specific capacitance retention from 0.5 to 10 A·g-1). Meanwhile, the assembled symmetrical supercapacitor (RHC//RHC) holds a superior energy density (11.6 Wh·kg-1 at a power density of 210 W·kg-1) and good cycling stability (99.0% specific capacitance and 80% energy efficiency retention after 5000 cycles). The exciting performance suggests that a simple and common design of porous carbon from layered biomass for supercapacitor is feasible and promising. KEYWORD: Porous carbon; Biomass; Layered structure; Supercapacitor INTRODUCTION High-efficiency energy conversion and storage technologies are the major challenges and important opportunity for human beings at present. Among advanced energy storage techniques, electrochemical energy storage equipment, including rechargeable batteries and supercapacitors, are the most promising storage devices.1-4 Researchers pay great attention to supercapacitors in recent years, not only benefiting from their fast charge-discharge rate, long cycling lifetime and


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good reversibility and stability, but also owing to theirs prospective application for portable electronic devices, energy management systems and hybrid electric vehicles.5-9 According to charge storage theory, supercapacitors can be divided into electric double-layer capacitors (EDLCs) and pseudocapacitors. As we all know that the capacitance of EDLCs roots in the electrostatic charge accumulation at the electrode/electrolyte surfaces, while pseudocapacitors are ascribed to rapid and reversible redox reactions on electrode material’s surface.10-12 In actual application, EDLCs play a significant role in potential energy conversion and storage equipment.3 Porous carbon as active electrode for EDLCs is due to its large active surfaces, excellent stability and good electrical conductivity. However, the low density, leading to inferior volumetric property, would restrain their applications in many fields.13-16 In recent years, layered materials have drawn much more attention because of their large active surfaces and open transport channels to accelerate charge accumulation and promote electron transfer. Besides, the layered structures can maintain the stability and integrity of electrode materials, so as to improve its charge-discharge properties remarkably.17-19 However, stack and agglomeration effect of layered materials results in small active surface areas and long ion diffusion path, which are not beneficial for its capacitive performance.2,

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Thus, much effort has been devoted to addressing the challenge, taking full

advantage of porous architectures and layered structure to maximize the electrode performance. Natural layered biomass as carbon precursors can be converted into active carbon with layered texture and porous framework features, increasing the density of carbon materials and relieving stack/agglomeration of porous layered materials, which would be excellent electrode materials for EDLCs, such as lotus receptacle,13 fungus (Auricularia),16and balsa wood.20 Biomass, as a green sustainable resource, are of distinctive inherent texture and abundant heteroatom composition (such as O, N, S), conducive to prepare carbon with unique porous structure and


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heteroatom-enriched surface functional groups.21-24 Special pore structures can provide plentiful active sites for rapid ion storage and diffusion to enable outstanding specific capacitance and rate capability. Meanwhile, surface redox reaction can contribute to additional pseudocapacitance from heteroatom functional groups.25, 26 In this work, we develop a novel strategy for green preparation of carbon material with interconnected porous network via hydrothermal treatment under diluted alkaline condition and following carbonization process. Compared with most of reported biomass-derived carbon materials, our work demonstrates advantages as following: 1) Hydrothermal treatment can hydrolyze part of reed membrane to form a lot of pores and promote KOH to enter into precursor deeply to achieve activation from inside to outside, resulting in wide pore size distribution of porous carbon, not only making efficient use of KOH to realize green activation process, but also relieving the stack and agglomeration of layer structure during carbonization to obtain large active surface areas. 2) Small amount of alkali and low carbonization temperature ensure many functional groups retain in RHC to improve the wettability and pseudocapacitance. 3) RHC with uniform and interconnected pore structure endows plentiful active sites and shortens ion diffusion length remarkably, facilitating ion transport and storage in electrode materials. In general, this work aims to convert layered biomass into unique materials with porous and layered framework. The asobtained RHC displays outstanding electrochemical performance in electrochemical tests. RHC possesses a high specific capacitance of 353.6 F·g-1 at 0.5 A·g-1, excellent rate capability and good cyclic stability (96.1% of capacitance retention after 10000 cycles). Furthermore, the symmetric supercapacitor (noted as RHC//RHC) has a broad operating potential 1.4V and delivered a high energy density of 11.6 Wh·kg-1 at 210 W·kg-1 with 80% energy efficiency. EXPERIMENTAL SECTION


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Preparation of porous layer-stacking carbon

Figure. 1 Schematic of preparation process for RHC. Briefly, kibbling reed membranes (1 g) were added into 60 mL 0.2 M KOH solution (Figure. S1B) under stirring until immersion thoroughly, then transferred into a 100 mL Teflon-lined stainless autoclave. Subsequently, the mixture was put in an oven heating at 130℃ for 5 h and then cooled down spontaneously. The solid samples were filtered followed by freeze-drying for 48 h (Figure. S1D). The obtained product was annealed under N2 at 650℃ for 2 h with a heating rate of 2 ℃·min-1. The puffed black materials (Figure. S2C1) were washed until the filtrate was neutral, and then dried at 80℃ overnight. The as-collected products were denoted as RHC. The whole preparation process of RHC is schematically illustrated in Figure. 1. For comparison, carbonized reed membranes (1 g) were added into 60 mL 0.2 M KOH solution, then the other steps are the same as above process. The obtained samples were denoted as RCC. Moreover, traditional KOH activation method was employed to prepare porous carbon. Hydrothermal samples (without KOH) were mixed with KOH completely in a 1:2 mass ratio, followed by activation at 650℃ for 2 h under N2 flow. Finally, the products were repeatedly washed with 1M HCl and deionized water, then dried at 80℃ overnight, denoted as RAC.


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Characterization The morphological and microstructural features of samples were analyzed by Hitachi S-4800 II scanning electron microscopy (SEM) and Tecnai G2F30 S-TWIN high resolution transmission electron microscopy (HRTEM). The crystalline structure of material was tested on Bruker D8 super speed X-ray diffraction (XRD). The main element content was determined via Vario EL cube elemental analyzer (EA). Raman drawings were obtained using Renishaw inVia spectrometer. Surface analysis of samples was studied through Thermo Science ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS). The pore volume and specific surface area were collected by Micromeritics ASAP 2460 N2 adsorption analyzer. Electrochemical measurements The electrochemical behaviors of single electrode was measured via three-electrode mode, with Ag/AgCl electrode and platinum sheet as reference electrode and counter electrode, correspondingly (in 6M KOH electrolyte) . The working electrodes were obtained by mixing the obtained active materials (80 wt %), conductive graphite (15 wt %) and polytetrafluoroethylene (PTFE, 5 wt %) with a suitable amount of ethanol. Afterwards, the as-obtained slurry was pressed on nickel foam (1×1 cm2) at 10 MPa, dried at 80℃ for 8 h. The active mass of single electrode is around 2.0 mg. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests were carried out under voltage window -1.0 to 0 V. Electrochemical impedance spectrum (EIS) was collected under open circuit potential between 0.01Hz and 100 kHz. The symmetric supercapacitor (RHC//RHC) was assembled through two same active material electrodes, separated by a piece of polyester fiber paper. The RHC//RHC was measured in voltage range of 0 to 1.4 V. The value of capacitance (Cm) was estimated from GCD discharge curves depending on the following equation:


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Cm  I  t /（V  m）

（1）

Here, specific capacitance is represented by Cm (F·g-1), discharge current is denoted as I (mA), Δt (s) means discharge time, m (mg) is the weight of active materials and ΔV (V) signifies voltage range. Meanwhile, the energy density and power density of RHC//RHC were computed by the equation:

E = Cm  V 2 / 7.2

（2）

P = 3600  E / t

（3）

Where E (Wh·kg-1) is energy density and P (W·kg-1) means power density. Cm (F·g-1) means specific capacitance, ΔV (V) is discharge voltage and Δt (s) is discharge time. RESULTS AND DISCUSSION


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Figure. 2 SEM images of samples: (A) natural reed membrane, (B) carbonized reed membrane and (C and D) RHC. TEM micrographs of samples: (E) RCC, (F) RAC and (G and H) RHC. The morphologies and microstructures of materials were analyzed by SEM and TEM. As depicted in Figure. 2A, natural reed membrane exhibits a layer-stacking morphology, which has coarse surfaces without apparent pores and holes. Carbonized reed membranes (Figure. 2B) display sheet structure with relative flat surface. It has been found that KOH is capable of etching carbon framework to form porous structure. The mainly mechanism of this process has been proposed that KOH reacts with precursor to generate H2O, CO, CO2, K2O, K2CO3 and K at the


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beginning. Then the porosity of carbon is further created through gasification process, which is attributed to the formation of H2O and CO2. Furthermore, K intercalates into the carbon lattice to form intercalated K or other K compounds, which brings about amorphous and porous structure after these compounds removed by washing.27,28 In Figure. 2C, the RHC has a particular framework, layer-stacking textures coupled with porous architectures. The enlarged SEM image (Figure. 2D) reveals that plenty of interconnected pores distribute evenly on both inside and outside surfaces of RHC. The layered structure of reed membranes cannot be retained well, ascribing to the hydrolysis and activation during hydrothermal process. From Figure. S2A, RCC has no obvious charge compared with carbonized reed membranes (Figure. 2B), indicating that carbonization process destroys the layered structure of precursor, hindering KOH enter into samples to create pores and channels. The morphology of RAC (Figure. S2B) and RHC (Figure. 2C) are similar, being of plentiful porous channels. However, RAC consists of irregular lumps without apparent layered structure, suggesting that traditional KOH activation has intensely destroyed the layer texture of reed membranes. According to the analyses of SEM, a probable mechanism is put forward. During hydrothermal process, KOH diffuses into the interlayers of precursor with uniform distribution. Subsequently, KOH acts as not only template for preventing materials from stack and agglomeration, but also activating agent for creating abundant porous network during carbonization process.29 TEM images of samples are illustrated in Figure. 2E-H. RCC (Figure. 2E) shows a dense layer-stacking structure, consistent with the SEM results. Moreover, the RHC in Figure. 2(G, H) further confirm the layered structure and porous network, so as to provide large active surfaces and open ion diffusion channels. The pore features of materials were further revealed by N2 absorption-desorption isotherm. As displayed in Figure. 3A and 3B, all samples show typical Ⅰ isotherms, implying plenty of


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micropores in the samples,30 derived from the removal of N and O elements from carbon framework as well as KOH corrosion function.31 The pore texture values of various samples are listed in Table 1, which obviously confirm that RHC has the largest SBET and pore volume. The curves of pore size distribution are plotted in Figure. 3B based on density functional theory (DFT) model. It is obvious that the distribution of pore diameter for RAC is mainly within 2 nm, while that of RHC is about 0.5~4 nm. For RAC, classical KOH activation method, which has an activation process from outside to inside, leads to formation of many micropores with narrow pore size distribution. For RHC, hydrothermal treatment can hydrolyze part of reed membrane to form a lot of pores and promote KOH to enter into precursor deeply to achieve activation from inside to outside, resulting in wide pore size distribution. Generally, micropores can offer a high active surface area for charge accumulation. Mesopores are able to facilitate rapid ion transport by supplying ion-buffering reservoirs and ion-transport channels. The synergistic effect of micropores and mesopores could shorten ion diffusion path and accelerate the charge accumulation rate, resulting in a large specific capacitance and high energy density.32-34 Comparatively, the large SBET and optimal pore size distribution of RHC can provide abundant electrical charges adsorption sites on the surface of electrodes, and also facilitate ions transfer and infiltration in electrode materials.35


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B 0.14

600 450 300

RAC RCC RHC

150 0.0

C

0.2 0.4 0.6 0.8 Relative Pressure(P/P0)

0.10 0.00016

0.08 0.06 0.04

1.0

0.00

RCC 0.00012

0.00008

0.00004

0.00000

0

3

0

5

10 15 Relative Pressure(P/P0)

6 9 Pore size(nm)

20

12

15

D D

(100)

RHC

RAC RCC 20

0.12

0.02

(002)

10

RAC RCC RHC Quantity Adsorbed(cm3·g-1 STP)

dV/dD(cm3·g-1·nm-1)

750

Raman Intensity (a.u.)

Quantity Adsorbed(cm3·g-1 STP)

A

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

40 50 2θ (Degree)

60

70

G 1590cm-1

-1

1345cm

RCC

RAC

RHC 80

900

1200

1500 1800 2100 Raman Shift (cm-1)

2400

Figure. 3 (A) N2 adsorption/desorption isotherms, (B) pore size distribution curves, (C) XRD pattern and (D) Raman spectra of samples. Table 1 Summary of BET characteristics of activated carbons.

a

Sample

aS

BET (m2·g-1)

bS

micro (m2·g-1)

cS

meso (m2·g-1)

dV t (cm3·g-1)

bV micro (cm3·g-1)

cV meso (cm3·g-1)

eD

(nm)

RAC

1067.4

822.5

231.1

0.64

0.59

0.05

1.08

RCC

297.6

198.0

17.2

0.12

0.07

0.05

6.23

RHC

1288.7

789.6

479.3

1.02

0.82

0.20

1.41

m

SBET: BET surface area. b Smeso: micropore surface area and Vmicro: micropore volume. c Smeso: Mesopore

surface area and Vmeso: Mesopore volume. d Vt: total pore volume.e Dm: average pore size.

Three samples display similar XRD patterns in Figure. 3C. The broad peak located at approximately 23.5° is related to the graphite (002) plane and a weak peak at around 43.3° represent the disorder (100) faces, suggesting the primarily amorphous or disordered structures of all sample.36 Raman spectra give further information about the disordered structural feature of carbon materials. As shown in Figure. 3D, three materials exhibit two obvious peaks at ~1346 (D


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band) and ~1591 cm-1 (G band), related to disordered structures and graphitic texture, respectively. The intensity ratios (ID/IG) represent the amorphous and defect degree of samples. The ID/IG ratios of RAC, RCC and RHC are estimated to be 0.76, 0.86 and 0.78, respectively, indicating that three samples possess high defect degree, due to KOH activation introducing lots of nanopores. The different ID/IG values of samples are likely due to different activation process. RAC delivers a slightly higher degree of graphitization than RHC, attributing to formation of more disordered structures of RHC during activation treatment,37, 38 which is consistent with the XRD analysis. Electrode materials with high conductivity is conducive to improve their capacitance performance. According to the four-point probe method, the electrical conductivity of RAC, RCC and RHC is 3.21, 2.87 and 3.53 S·cm-1, respectively. RCC with a poor conductivity is attributed to high defect structure,38 while RHC with interconnected porous network is beneficial for electron transport, leading to relative good electrical conductivity.39 Table 2 Elemental analysis parameters of samples Samples

C (%)

N (%)

O (%)

H (%)

Precursor Carbonized sample

41.98 66.22

1.82 0.22

42.72 28.62

6.41 2.97

RAC

73.82

0.46

22.84

2.56

RCC RHC

71.90 76.18

0.43 0.74

24.75 20.14

2.54 2.54

With regard to elemental analysis (EA), the major element content of samples is listed in Table 2. The reed membranes mainly contain C, O and N. After treatments, the amount of C increases and the content of O and N decreased obviously.


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B

A O 1s

C 1s C-C 284.4eV

Intensity (a.u.)

Intensity (a.u.)

C 1s

N 1s

0

C

200

400 600 800 Binding Energy (eV)

1000

C-O 286.5eV

C-N

C=O

285.5eV

288.0eV

290

D

N 1s

288 286 284 Binding Energy (eV)

282

O 1s

N-5

Intensity (a.u.)

399.8eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N-Q 400.4eV

N-X

N-6

404.0eV

398.0eV

C-O 532.2eV

C=O 531.1eV

O-C=O 535.3eV

406

404

402 400 398 396 Binding Energy (eV)

394

538

536

534 532 530 528 Binding Energy (eV)

526

Figure. 4 XPS spectra of RHC: (A) survey spectra, (B) high-resolution C 1s, (C) N1s and (D) O1s. To further study the exterior feature and chemical state of samples, the XPS was employed. According to the fitting spectra in Figure. 4A, RHC contains mainly carbon (62.34 at%), oxygen (26.84 at%) and small amounts of nitrogen (4.44 at%) without other heteroatoms. The total content of heteroatoms is higher than those determined by EA, suggesting abundant surface functional groups. The surface chemical states of RHC samples are research by high-resolution XPS spectra (C 1s, O 1s and N1s). The C 1s spectra fit well with four peaks in Figure. 4B, as follows: C–C (284.4 eV), C–N (285.5 eV), C–O (286.5 eV) and C=O (288.0 eV).40 In Figure. 4C, the N 1s spectra can be included to four independent peaks, centered at 398.0 eV, 399.8 eV, 400.4 eV and 404.0 eV, matching with pyridine-N (N-6), pyrrolic-N (N-5), quaternary-N (N-Q) and pyridineN-oxides (N-X).41 The relative amount of surface functional groups was computed and listed in Table 3. The N-5 (61.64%) and N-6 (24.66%) possess major N composition, which can enhance


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electrode activity and improve electrochemical performance, as reported previously.39 Substitutional introduction of more electron-rich N into the C network could bring more electrons to the delocalized π-system of carbon materials, which is beneficial for improving electrical conductivity. N-containing carbons are incorporated with more heterogeneous species and usually contribute to its wettability. Moreover, carbon materials with nitrogen functional group provide additional pseudocapacitors by redox reactions.42 The O 1s spectra (Figure. 4D) can be decomposed into three states, C=O (531.1 eV), C–OH/C–O–C (532.2 eV) and O-C=O (535.3 eV).43 Generally, these results reveal that heteroatoms are essential to improve wettability and pseudocapacitance of carbon materials. Table 3 Percentage of total O1s and N 1s analyzed by XPS. Functionality

% of total O 1s

% of total N 1s

C=O

C-O

O-C=O

N-5

N-6

N-Q

N-X

B.E.(eV)

531.3

532.2

536.3

399.8

398.0

400.4

404.0

RHC

41.78

51.88

6.34

61.64

24.66

11.62

2.08

RAC

35.95

57.98

6.07

57.83

19.80

15.62

6.75

To assess the capacitance properties of as- fabricated electrodes, CV and GCD were analyzed in 6M KOH electrolyte with a three-electrode setup. The typical CV curves at scan rate of 5 mV·s-1 are plotted in Figure. 5A. All CV curves of samples display a quasi-rectangular shape with an obvious redox peaks at potential ranging from -1.0 to -0.2 V, owing to a stable charge/discharge process, implying the coexistence of EDLC and pseudocapacitance. The redox peaks are ascribed to abundant surface functional groups, such as C-N, C=N, N-O, C-O and C=O, which can provide additional pseudocapacitance by fast reversible redox reaction.44 Figure. S4 (A1 ， B1 and C1) exhibit CV profiles of samples at different scan rates. When the scan rate reaches to 100 mV·s-1, the rectangular shape could be well-maintained without obvious distortion, suggesting its small


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equivalent series resistance, fast ion diffusion capability and good capacitive response.45 Especially, the integral area of RHC is bigger compared to those of RAC and RCC, suggesting its best capacitance behavior. B

2 1 5 mV s-1

-1

RAC RCC RHC

-2 -3

-1.0

-0.8 -0.6 -0.4 -0.2 Potential (V vs. Ag/AgCl)

C 400

RAC RCC RHC

-0.2 -0.4 -0.6

0.5 A·g-1

-0.8 -1.0

0.0 RAC RCC RHC

0

D

-Z''(Ohm)

300

0.0

200

300

1500

2.5

80

2.0

RAC RCC RHC

1.5 1.0 0.5 0.0 0.0

0

1200

RAC RCC RHC

120

40

100

600 900 Time (s)

-Z''(Ohm)

0

Potential (V vs.Ag/AgCl)

Current Density (A·g-1)

A

Specific Capacitance (F·g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0

2

4 6 8 Current Density (A·g-1)

10

0

40

0.5

1.0 1.5 Z'(Ohm)

80 Z'(Ohm)

2.0

2.5

120

Figure.5 Electrochemical performance of samples: (A) CV curves at 5 mV·s-1, (B) GCD curves at 0.5 A·g-1, (C) specific capacitance at various current densities and (D) Nyquist plots and partial magnified Nyquist plots (insect). Figure. 5B depicts the GCD profiles of samples at current density of 0.5 A·g-1. Obviously, the nearly symmetric triangular and linear GCD feature evidences the excellent electrochemical reversibility.46 The curves with a slight curvature could be due to surface functional groups of redox reaction, consistent with the resulting from Figure. 5A. As depicted in Figure. S4, the GCD profiles at different current densities retain good symmetry without apparent IR drop on discharge curves, demonstrating their superior reversibility and good conductivity.47 The capacitance of as-


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prepared RAC, RCC and RHC electrodes were estimated by discharge curves. Notably, RHC has the maximum Cm of 353.6 F·g-1 (233.4 F·cm-3) which is better than other biomass-based porous carbon (Table S2). The Cm of RAC and RCC was 269.2 (234.2 F·cm-3) and 100.2 F·g-1 (161.3 F·cm-3) at 0.5 A·g-1, respectively. From Figure. 5C, while current density increase from 0.5 A·g-1 to 10 A·g-1, the capacitance retention values of RAC, RCC and RHC were still 201, 61 and 271 F·g-1 corresponding to 74.7%, 60.9% and 76.9% retention rates, demonstrating their remarkable rate capability. The Cm of all electrode materials decreases owing to deficient surface reaction and limited ion transport with current density increasing.48 The Cm of porous layered RHC possesses almost 3 times higher than non-porous RCC, indicating that porous structure is helpful to improve capacitance performance. Synergetic feature combining interconnected porous and layered structure contributes to abundant active sites and open diffusion channels to improve its electrochemical properties. The EIS results are illustrated in Figure. 5D. The slope of RHC curve in low-frequency part is largest, suggesting its lowest ion transfer resistance derived from unique porous structure.49 From the magnified Nyquist plots (inset of Figure. 5D), the Rs of RAC, RCC and RHC are 0.44, 0.45 and 0.41 Ω, respectively, proving the best conductivity of RHC. Based on ZSimpWin software, impedance parameters simulated by equivalent circuit are clearly displayed in Table S1. The Rct of RAC, RCC and RHC are estimated to be 0.21, 0.46 and 0.12 Ω, respectively, meaning the fastest charge transfer rate of RHC. The involved equivalent circuit for Nyquist plot simulation is shown in Figure. S5A. Interestingly, the experimental results and simulation data of Nyquist plots in Figure. S5 exhibit a high degree of similarity in shape, proving outstanding capacitive behaviors.


16

400

RAC RCC RHC

300

200

100

-1 0.0 2 A·g

RHC

-0.2 -0.4 -0.6 -0.8 -1.0

0

C

B

0

2000

8

4000 6000 Cycle number

8000

10000

0

700

1400 2100 Time (s)

2800

3500

D 100

4 -Z''(Ohm)


A


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Potential (V vs.Ag/AgCl)

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20 mV·s-1

0

50

-4

Before Cycle After Cycle

Before Cycle After Cycle

-8 -1.0

-0.8 -0.6 -0.4 -0.2 Potential (V vs.Ag/AgCl)

0.0

0

0

50 Z'(Ohm)

100

Figure.6 (A) cycling performance and (B) the initial 10 GCD cycles of RHC at current density of 2 A·g-1, (C) CV curves and (D) Nyquist plots of RHC measured before and after 10000 cycles. The cycling stability of samples was tested at current density of 2 A·g-1 for 10000 circulations, as depicted in Figure. 6A. Especially, after 10000 consecutive cycles, capacitance retention of RAC, RCC and RHC reach to 90.5%, 94.0% and 96.1%, respectively, suggesting their good electrochemical stability and outstanding reversibility. The superb performance ascribes to synergy feature of porous and layered structure, which can alleviate volume effect of electrode during the continuous cycles.50 In Figure. 6B, the initial 10 cycle of RHC show a symmetric and repetitive CV curves, testifying its good electrochemical reversibility. In addition, the CV profiles of RHC plotting at 20 mV s-1 are similar in shape without deformation before and after 10000 cycles in Figure. 6C, revealing its excellent electrochemical stability. Moreover, Nyquist plots of RHC display in Figure. 6D before and after cycles. The slope of latter straight line in low frequency


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part is smaller, with a little change for Rs and Rct values, further proving its excellent cycling durability.51, 52 The remarkable capacitance performance of RHC may benefit from the followings. 1) Open layer channels provide large active surface areas and open ion diffusion channels, so as to accelerate charge accumulation and enhance ion transfer kinetics during charge/discharge process. 2) Interconnected porous structure shortens ion transport pathway and increases the contact areas between electrode and electrolyte to enhance utilization rate of porous carbon. 3) The heteroatoms functional groups can improve the wettability and conductivity of carbon electrode and endow additional pseudocapacitors via redox reactions. 4) The smaller Rs and Rct of RHC enable the fast electrochemical kinetics, and enhance the capacitive property of electrode materials. To further investigate RHC material for potential application, the symmetric supercapacitor cell (RHC//RHC) was assembled based on RHC electrode and tested in in 6 M KOH solution. Its operable working voltage is determined by CV and GCD curves of RHC//RHC under different voltage windows, as shown in Figure. S6. Notably, the CV curves retain quasi-rectangular shape without evident polarization curves with the potential extending to 1.4 V, and display no obvious distortion at various scan rates (Figure. 7A), demonstrating its practical working potential within the range of 0 to 1.4 V. The stable voltage window is attributed to abundant oxygen-, nitrogencontaining functional groups in the carbon framework, leading to the high oxygen or hydrogen evolution potential.53, 54 In addition, GCD curves exhibit perfect linearity and symmetry (Figure. 7B and Figure. S6B), showing desirable electrochemical reversibility and Columbic efficiency for RHC//RHC symmetric supercapacitor. The Cm of optimal symmetric supercapacitor are computed to be 42.8 and 28.6 F·g-1 at 0.3 and 5 A·g-1, respectively. Moreover, the capacitance retention rate reach to 66.8% from 0.3 to 5.0 A·g-1, verifying its remarkable rate capability. The continuous


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4

1.6

B

2 Voltage (V)

0 -2

100 mV s-1 50 mV s-1 20 mV s-1 10 mV s-1 5 mV s-1

-4 -6

0.0

0.3

0.6 0.9 Voltage (V)

1.2

0

80

60 60

40 40 20

0

1000

2000 3000 Cycle number

4000

0

5000

D

Coulombic Efficiency (%) Energy Efficiency (%)

100

20

0.4

1.5 120

80

0.8

0.0

C 100

0

0.3 A g-1 0.5 A g-1 1 A g-1 2 A g-1 5 A g-1

1.2

Energy Density (Wh·Kg-1)


A


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

200 Time (s)

300

400

15 This work 10

Ref 38 Ref 34 Ref 39 Ref 40

5

Ref 41 Ref 43

Ref 42

100

1000 Power Density (W·Kg-1)

Figure.7 Capacitive performances of RHC//RHC symmetrical supercapacitor: (A) CV curves at various scan rates, (B) GCD curves at different current densities, (C) cycling performance, coulombic efficiency and energy efficiency at current density of 2A·g-1, and (D) Ragone plots. cycling capacity in Figure. 7C proves that the Cm maintains 99.5% of the initial value and becomes stable gradually after 5000 cycles. Then, RHC//RHC delivers high Coulombic efficiency (almost 100%) and energy efficiency (around 80%), confirming its good cycling durability property. The energy efficiency is much lower than Coulombic efficiency, ascribing to non-ideal charge/discharge behavior. From the Ragone plots (Figure. 7D), the RHC//RHC cell delivers an optimal energy density of 11.6 Wh kg-1 matching with a power density of 210 W kg-1, better than other biomass-based carbon symmetric supercapacitor, as reported previously. In Table 4, a series of energy density and power density derived from different biomass are listed. Generally, those satisfying properties make the RHC a prospective candidate as supercapacitors electrode active materials.


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Table 4 Comparison of energy density and power density of various biomass carbon materials

6 M KOH

Energy density (Wh·kg-1) 11.6

Power density （W·kg-1） 210

This work

6 M KOH

8.75

500

47

Dead ginkgo leaves

1M H2SO4

9.2

48

55

Pomelo peel Pueraria Graphite flakes Tobacco rods Shrimp shell

6 M KOH 6 M KOH 6 M KOH 6 M KOH 6 M KOH

9.4 8.46 6.8 5.5 5.2

96 123 240 1500 1162

56 57 58 59 60

Materials

Electrolyte

Reed membranes Osmanthus flower

Reference

CONCLUSION In summary, the porous layer-stacking active carbon, using renewable reed membranes as precursor, was successfully prepared via a effective and facile hydrothermal treatment and following carbonization process. During hydrothermal treatment, KOH diffuses evenly into the interlayers of reed membranes, with efficient KOH utilization to realize less alkali activation process. Moreover, KOH as template alleviate the stack and agglomeration of layer structure, and also as activating agent to create porous network, realizing green activation. The as-obtained RHC with interconnected porous framework can provide plentiful active surfaces, shorten ion transport pathway and relieve electrode volume expansion. In addition, its abundant surface functional groups are essential to increase wettability and pseudocapacitance of porous carbon. In KOH aqueous electrolytes, RHC is endowed with desirable electrochemical behaviors, including large specific capacitance, remarkable rate capability and cycling durability. Thus, this work has vital practical significance to convert layered biomass into superior carbons for energy storage. ASSOCIATED CONTENT Supporting Information.


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Photographic images of reed membranes, SEM and TEM images, Summary of XPS spectra, Detailed data regarding electrochemical performance, Equivalent circuit model and Nyquist plots, Comparison of the specific capacitances of reported carbon electrodes. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Priority Academic Program Development of Jiangsu

Higher

Education

Institutions

(PAPD),

the

Natural

Science

Foundation

of Shandong Province, China (Grant No. ZR2017MEM019) and the Natural Science Foundation of China (NO. 21375116), The related measure and analysis instrument for this work was supported by the Testing Center of Yangzhou University.


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For Table of Contents Use Only Porous layer-stacking active carbon derived from reed membranes was prepared by a facile hydrothermal treatment and subsequent carbonization for supercapacitors.


31

Porous Layered Carbon with Interconnected Pore Structure Derived

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