Synthesis of porous carbons from coal tar pitch for high-performance supercapacitors

NEW CARBON MATERIALS Volume 34, Issue 2, Apr 2019 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2019, 34(2):132-139

RESEARCH PAPER

Synthesis of porous carbons from coal tar pitch for high-performance supercapacitors Feng Wei, Han-fang Zhang, Xiao-jun He, Hao Ma, Shi-an Dong, Xiao-yu Xie School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan, 243002, China

Abstract:

Porous carbons (PCs) for supercapacitors were synthesized by a combined Mg(OH)2 templating and in-situ KOH activation method

using coal tar pitch as the carbon precursor, and were characterized by TEM, Raman spectroscopy, XPS and N2 adsorption. Their electrochemical properties were investigated by galvanostatic charge-discharge, electrochemical impedance spectroscopy and cyclic voltammetry. Results show that the specific surface area of the PCs increases with the KOH dosage and exhibits a maximum with an activation temperature at 800 °C. The optimum PC has a high surface area up to 3 145 m2 g1 with abundant micropores, and exhibits a high specific capacitance of 272 F·g1 at 0.05 A·g1, a rate capability of 217 F·g1 at 20 A·g1 and a good cycle stability with a 96.69% capacitance retention after 10000 cycles in a 6 mol/L KOH electrolyte. This work provides a simple method for the large-scale production of PCs from pitch-based carbon sources for high-performance supercapacitors. Key Words:

Coal tar pitch; Porous carbon; Supercapacitor

1 Introduction With the fast consumption of fossil fuels, numerous efforts have been devoted to design novel energy-storage devices to resolve the energy shortage issue [1–4]. Among them, supercapacitors have attracted great attention owing to their fast charge speed and superior cycle stability to batteries [5–9]. The properties of supercapacitors mainly rely on the electrode materials, e.g. carbon material, metal oxide and conductive polymer. Of which, carbon-based electrode materials including carbon fibers [10,11], carbon nanotubes [12], porous carbons (PCs) [13–15] and graphene nanocapsules [16, 17], have attached much attention owing to their good conductivity, high specific surface area, etc [18–22]. At present, PCs are the key electrode materials of commercial supercapacitors. Cai et al[23] synthesized PCs from moringa oleifera stems, which exhibited a specific capacitance of 248 F·g1 at 1.0 A·g1. Xu et al prepared PCs from methane with porous Mg3(PO4)2 as the template, which had a high surface area up to 1 627.8 m2·g1 and a high power conversion efficiency of 7.08 % as a catalyst of a dye-sensitized solar cell [24]. Template method is one of the efficient methods to prepare PCs because the experiment process is simple, the PCs obtained feature a narrow size distribution and possess a good conductivity [25]. At present, nano-metal oxides including ZnO, MgO, NiO, have been used as templates for synthesizing PCs since these templates are easy to be removed and recycled [26–30] . PCs with hierarchical pores were synthesized using

nano-ZnO as both as a template and an internal activation agent, which exhibited a good cycle stability and excellent rate capability [31, 32]. PCs prepared from polyvinyl alcohol and tetrahydrofuran using MgO as a template, presented a high specific capacitance [33]. 3D hierarchical PCs prepared by an in-situ porous CuO template method, exhibited supercapacitance performance [34]. Yet, the cost of raw materials for the synthesis of aforementioned PCs is relatively high. Hence, it is necessary to synthesize PC-based electrode materials from low-cost raw materials for commercial applications. Coal tar pitch (CTP) is abundant and easy available. It is one of the main ingredients of coal tar, which is a residue fraction of the coal coking process. There are abundant polycyclic aromatic hydrocarbons in CTP, which act as building blocks to synthesize PCs. In present work, PCs for high-performance supercapacitors were prepared from CTP by using Mg(OH)2 as the template coupled with an in-situ KOH activation technique. As-prepared PCs feature a high specific surface area, abundant micropores and mesopores. As the electrode materials for supercapacitors, PCs show a high capacitance, perfect rate capability and cycle stability. As far as we know, there are no reports on the fabrication of PCs from CTP by a combined Mg(OH)2 templating with in-situ KOH activation method.

2 2.1

Experimental Synthesis of PCs

Received date: 03 Feb 2019; Revised date: 31 Mar 2019 *Corresponding author. E-mail: [email protected] Copyright©2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(19)60006-5

Feng Wei

et al. / New Carbon Materials, 2019, 34(2): 132-139

High temperature CTP (softening point of 110 oC, C/H=19.78) was supplied by Maanshan Iron & Steel Co. Ltd., Mg(OH)2 was purchased from Aladdin Co. Ltd. and other reagents were purchased from Sinopharm Group Chemical Reagent Co. Ltd. In a typical run, 3 g CTP, 6 g KOH and 18 g Mg(OH)2 were added to a mortar and mixed. The resultant mixture was loaded into a corundum crucible that was placed into a tubular furnace, first heated to 300 oC at 5 oC·min1 for 30 min under Ar gas flow (60 mL·min1), then to 800 oC within 1 h. The product was purified by diluted HCl and distilled water, and dried at 110 oC for 12 h before use. The resultant PC is denoted as PC6-800, the subscript 6 is the dosage of KOH (g) and 800 represents the heat treatment temperature. The PC made from 3 g CTP, 18 g Mg(OH)2, 12 g KOH and 3 g CTP, 18 g Mg(OH)2, 18 g KOH at 800 oC is named as PC12-800 and PC18-800, respectively. For comparison, PC prepared from 3 g CTP, 18 g Mg(OH)2, 18 g KOH at 900 oC is named as PC18-900. 2.2

Characterization

The morphology and microstructure of the PCs were investigated by transmission electron microscopy (TEM, JEM-2100, Japan) and Raman spectra were collected using an RamHR800 Raman spectrometer with an excitation length of 532 nm. The X-ray photoelectron spectroscopy (XPS) measurement was carried out on a ThermoESCALAB250 apparatus (USA). The pore structure of the PCs was examined using N2 adsorption at -196 oC (Micrometrics, ASAP2010), and the specific surface area of the PCs was calculated by the BET (Brunauer-Emmett-Teller) method in a relative pressure range from 0.05 to 0.3 [16]. 2.3

Fabrication and test of PC electrodes

Polytetrafluoroethylene (10 wt%) and PC (90 wt%) were blended in deionized water to form a slurry, then the slurry was rolled into a thin film, which was machined into several round films (12 mm in diameter) and dried at 110 oC under vacuum for 2 h. Subsequently, as-prepared films were pressed onto nickel foam and immersed in a 6 mol/L KOH electrolyte under vacuum for 1 h. The symmetric supercapacitor was fabricated using the immersed electrodes. The cycle voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD) were performed on an electrochemical workstation (Chenhua Instrument, CHI760C, Shanghai, China), a Solartron impedance analyzer (Solartron Analytical, SI 1260, UK) and a supercapacitor test system (SCTs, Arbin Instruments, USA), respectively. The specific capacitance (F·g1) of the PCs was obtained according to Eq. (1).

C

4I m ΔV Δt

(1)

Where I (A) presents the discharge current, Δt (V·s1) stands for the slope of the discharge curve, and m (g) signifies the mass of the PCs in two electrodes.

The energy density (E, Wh·kg1) and average power density (P, W·kg1) of the PC electrodes were calculated based on Eq. (2) and Eq. (3).

E

1 CV 2 2   

P E t

(2)

(3)

Where V (V) represents the usable voltage after IR drop, t(h) stands for the discharge time.

3

Results and discussion

The TEM images of Mg(OH)2 and the PCs are shown in Fig. 1. The sizes of some Mg(OH)2 particles in Fig. 1a are from 200 to 400 nm, which are expected to serve as templates. The capsule-like carbons with thin walls are observed in PC6-800 (Fig. 1b), which are formed after the removal of the MgO particles derived from the Mg(OH)2 template by acid washing. Many pores can also be seen in PC12-800 (Fig. 1c), whose the sizes are from 200 to 400 nm, suggesting that the pores are derived from the Mg(OH)2 particles. The PC18-800 in Fig. 1d exhibits the large thin sheet-like morphology within holes. Fig. 2a shows the N2 adsorption-desorption isotherms of the PCs. All the isotherms present a strong N2 adsorption at a low relative pressure (p/p0 < 0.01), indicating the existence of plentiful micropores due to the in-situ activation by KOH. The small hysteresis loop located at 0.4 < p/p0 < 0.95 indicates the existence of some mesopores in the PCs. Fig. 2b exhibits the pore size distributions of PCs. It can be observed that the pore size of the samples is mostly concentrated at ca. 0.57, 1.27, 2.61, 3.55 and 4.92 nm due to different etching levels of carbon by KOH. The pore structure parameters of the PCs are listed in Table 1. The SBET of PCs increases from 2 438 to 3 145 m2·g1 when the KOH dosage rises from 6 to 18 g at 800 o C. With a further increase of temperature to 900 oC, the SBET of PC18-900 decreases to 3 079 m2·g1 due to the collapse of some small pores at an elevated temperature. The SBET of PC18-800 is the biggest among the carbon materials in literatures (300-2 600 m2·g1) [35–39]. The Vt of PCs is in the order of PC6-800
Feng Wei

Fig. 1

Fig. 2

et al. / New Carbon Materials, 2019, 34(2): 132-139

TEM images of (a) Mg(OH)2; (b) PC6-800; (c) PC12-800 and (d) PC18-800.

(a) N2 adsorption/desorption isotherms and (b) pore size distribution curves of PCs. Table 1

The pore structure parameters of PCs.

Samples

Dap (nm)

SBET (m2· g1)

Smic (m2·g1)

Vt (cm3· g1)

Vmic (cm3· g1)

Vmic/Vmes

PC6-800

2.45

2438

1830

1.49

0.80

1.18

PC12-800

2.54

2619

1955

1.65

0.91

1.25

PC18-800

2.30

3145

2807

1.68

1.13

2.08

PC18-900

2.90

3079

2197

2.23

1.25

1.31

Dap: average pore diameter by the formula of Dap=4Vt/SBET; SBET: specific surface area by BET method; Vmic: micropore volume from the t-plot method; Vt: total pore volume at p/p0=0.99.

Feng Wei

et al. / New Carbon Materials, 2019, 34(2): 132-139

the N content in PCs decreases from 1.68% to 1.44% as the dosage of KOH rises from 6 to 18 g at 800 oC, and then drops to 1.35% when the activation temperature rises to 900 oC. The O1s spectrum of the PC18-800 is deconvoluted into three different oxygen-containing functional groups, e.g. OH

(530.99 eV), C=O (532.18 eV) and CO (533.30 eV), as shown in Fig. 3b. The hydrophilic functional groups in the PCs are expected to increase the affinity and wettability of PC electrode materials in electrolytes [40].

Fig. 3 (a) XPS spectra of PCs and (b) O1s spectra of PC18-800. Table 2

The contents of C, O, N elements and surface oxygen-containing functional groups in PCs.

Samples

C1s (at.%)

O1s (at.%)

N1s (at.%)

PC6-800

93.03

5.29

PC12-800

87.50

PC18-800 PC18-900

CO (%)

O—H (%)

1.68

2.15

2.11

1.03

10.88

1.62

3.19

4.20

3.49

89.21

9.35

1.44

3.87

5.18

0.30

86.60

12.05

1.35

2.47

4.61

4.97

The crystal structures of PCs were further researched by Raman spectroscopy and the corresponding results are shown in Fig. 4. It can be clearly observed that there are two characteristic peaks in the samples. One peak at ca. 1 325 cm1 (D band), corresponds to structural defects. Another peak at ca. 1 585 cm1 (G band), is associated with stretching bond of sp2 hybridized carbon [41,42]. The peak intensity ratio (ID/IG) of the PC18-900 (0.96) is the smallest among the PC6-800 (1.05), PC12-800 (1.03), PC18-800 (0.98), indicating that the PC18-900 sample possesses the highest graphitization degree among the four samples.

Fig. 4

O1s C=O (%)

Raman spectra of PCs.

Fig. 5 illustrates the preparation scheme of the PCs from CTP. First, CTP was added into the mixture of Mg(OH)2 and

KOH, then ground and mixed. As the increase of heat treatment temperature, liquefied CTP diffused and coated onto the surface of Mg(OH)2 particles to form a large network after carbonization. Second, Mg(OH)2 particles acted as the template to confine and direct the transformation of CTP into capsule-like carbons. Third, on one hand, many pores are formed on the capsule-like carbons when the dosage of KOH is not high enough to destroy the capsule walls formed in carbonization by KOH activation at a high temperature. On the other hand, the capsule-like carbons are tailored into sheet-like carbons with a certain curvature by KOH, and many pores are formed on the sheet-like carbons when the dosage of KOH is excessive (up to 18 g). Last, the capsule-like and sheet-like PCs are obtained after repeated leaching with HCl solution and distilled water. Fig. 6a is the GCD curves of the PC electrodes at a current density of 1.2 A·g1 in a 6 mol/L KOH electrolyte. The GCD curves are nearly symmetrical, indicating that the PCs have good electrochemical reversibility. Interestingly, the PC18-800 electrode exhibits the longest discharge time, meaning the largest capacitance of the PC18-800 electrode among the four PC electrodes. Fig. 6b shows the specific capacitance of the PC electrodes at different current densities. The specific capacitances of the PC6-800, PC12-800, PC18-800 and PC18-900 are 206, 244, 272 and 250 F·g1 at a current density of 0.05 A·g1, respectively. Even at a high current density of 20 A·g1, the

Feng Wei

et al. / New Carbon Materials, 2019, 34(2): 132-139

specific capacitances of the PC6-800, PC12-800, PC18-800 and PC18-900 remain at 170, 195, 217 and 185 F·g1 with the capacitance retention rates of 82.5%, 79.9%, 79.8% and 74%, respectively, indicating a good rate performance due to the abundant short pores for ion adsorption and transport. The capacitance retention rate of the PC18-800 electrode reaches 96.69% after 10 000 cycles at 5 A·g1 (Fig. 6c), showing an excellent cycle stability. The Ragone plots of the PC

electrodes are shown in Fig. 6d. The energy densities of the PC18-800 electrode reach 9.47 and 3.85 Wh·kg1 at the power densities of 26.1 W·kg1 and 7.54 kW·kg1, respectively. Obviously, the energy density of the PC18-800 electrode (9.47 Wh·kg1) is the highest among the PC6-800 (7.17 Wh·kg1), PC12-800 (8.47 Wh·kg1), and PC18-900 electrodes (8.70 Wh·kg1) at 0.05 A·g1, suggesting that the PC18-800 electrode exhibit the biggest energy density among them.

Fig. 5 The preparation scheme of PCs from coal tar pitch.

Fig. 6 (a) Charge-discharge curves of PC electrodes at 1.2 A g1; (b) Specific capacitance of PC electrodes at different current densities; (c) Cycle stability of PC electrodes at 5 A g1; (d) Ragone plots of PC electrodes.

Feng Wei

et al. / New Carbon Materials, 2019, 34(2): 132-139

The CV tests of the PC18-800 and PC18-900 electrodes are performed at sweep rates from 2 to 200 mV·s1, as shown in Fig. 7a and 7b, respectively. The results show that the CV curves of the PC electrodes are symmetric without an obvious redox peak, exhibiting an ideal electric double-layer capacitive behavior. Fig. 7c shows the Nyquist plot of the PC18-800 supercapacitor, the nearly vertical curve in the low-frequency region suggests the ideal capacitive behavior.

Fig. 7

4

Fig. 7d shows the Bode plot of phase angle versus frequency of the PC18-800 electrode. The phase angle is close to -90o at the low-frequency region, suggesting the ideal capacitive behavior of the supercapacitor [43]. In all, the PCs synthesized from the low-cost carbon precursor exhibit a high capacitance, excellent rate capability and good cycle stability, confirming their potential applications as supercapacitor electrode materials.

CV curves of: (a) PC18-800 and (b) PC18-900 at different scan rates; (c) Nyquist plot of PC18-800 and (d) Bode plot of PC18-800.

Conclusions

[2]

Li W Q, Liu S M, Pan N, et al. Post-treatment-free synthesis of highly mesoporous carbon for high performance supercapacitor

We report a facile approach to prepare PCs from CTP by a combined Mg(OH)2 templating and in-situ KOH activation method for high-performance supercapacitors. The best PC possesses a large specific surface area (3 145 m2·g1) with abundant short pores. As the electrode for a supercapacitor, the PC shows a high specific capacitance (272 F·g1 at 0.05 A·g1), excellent rate capability (217 F·g1 at 20 A·g1) and cycle stability (96.69% of capacitance retention after 10 000 cycles). This work provides a facile method for large-scale production of PCs from pitch-based carbon sources for high-performance supercapacitors.

in aqueous electrolytes[J]. J Power Sources, 2017, 357: 138-143. [3]

supercapacitors obtained by co-carbonization of coal-tar pitch and sawdust[J]. J Mater Sci, 2017, 52: 760-769. [4]

Tian W J, Zhang H Y, Sun H Q, et al. Template-free synthesis

batteries[J]. New Carbon Mater, 2017, 32: 517-526. [5]

Carbon, 2017, 118: 98-105.

Boota M, Anosori B, Voigt C, et al. Pseudocapacitive electrodes produced by oxidant-free polymerization of pyrrole between the layers of 2D titanium carbide (mxene) [J]. Adv Mater, 2016, 28: 1517-1522.

[6]

Ding L, Zou B, Liu H, et al. A new route for conversion of corncob to porous carbon by hydrolysis and activation[J].

of N-doped carbon with pillared-layered pores as bifunctional materials for supercapacitor and environmental applications[J].

Zhang H W, Lu J M, Yang L, et al. N, S co-doped porous carbon nanospheres with a high cycling stability for sodium ion

References [1]

Meng X Y, Cao Q, Jin L, et al. Carbon electrode materials for

Chem Eng J, 2013, 225: 300-305. [7]

Wang H, Sun X, Liu Z, et al. Creation of nanopores on graphene

planes

with

MgO

template

for

preparing

Feng Wei

[8]

[9]

et al. / New Carbon Materials, 2019, 34(2): 132-139

high-performance supercapacitor electrodes[J]. Nanoscale,

waxberry-derived hierarchical porous carbon materials[J]. Adv

2014, 6: 6577-6584.

Energy Mater, 2018, 1702695.

Liang C, Bao J P, Li C G, et al. One-dimensional hierarchically

[22] Xia J S, Zhang N, Chong S K, et al. Three-dimensional porous

porous carbon from biomass with high capacitance as

graphene-like

supercapacitor materials[J]. Micropor Mesopor Mater, 2017,

low-temperature graphitization for a supercapacitor[J]. Green

215: 77-82.

Chem, 2018, 20: 694-700.

Rajagopal R, Lee Y S, Ryu K S. Synthesis and electrochemical analysis

of

Nb2O5-TiO2/H-rGO

sandwich

type

sheets

synthesized

from

biocarbon

via

[23] Cai Y J, Ying L Y, Dong H D, et al. Hierarchically porous

layered

carbon nanosheets derived from Moringa oleifera stems as

architecture electrode for supercapacitor application[J]. Chem

electrode material for high-performance electric double-layer

Eng J, 2017, 325: 611-623.

capacitors[J]. J Power Sources, 2017, 353: 260-269.

[10] Jin Z, Yan X D, Yu Y H, et al. Sustainable activated carbon

[24] Xu X W, Wang Y, Chen B, et al. Phosphorus-doped porous

fibers from liquefied wood with controllable porosity for

graphene nanosheet as metal-free electrocatalyst for triiodide

high-performance supercapacitors[J]. J Mater Chem A, 2014, 2:

reduction reaction in dye-sensitized solar cell[J]. Appl Surf Sci,

11706-11715.

2017, 405: 308-315.

[11] Qin T F, Peng S L, Hao J X, et al Flexible and wearable

[25] Hedayat N, Du Y H, Ilkhani H. Review on fabrication

all-solid-state supercapacitor with ultrahigh energy density

techniques for porous electrodes of solid oxide fuel cells by

based on a carbon fiber fabric electrode[J]. Adv Energy Mater,

sacrificial template methods[J]. Renew Sust Energy Rev, 2017,

2017, 1700409.

77: 1221-1239.

[12] Wang Z Q, Wu Z Q, Benedetto G D, et al. Microwave

[26] Wang G P, Zhang L, Zhang J J. A review of electrode materials

synthesis of highly oxidized and defective carbon nanotubes for

for electrochemical supercapacitors[J]. Chem Soc Rev, 2012,

enhancing the performance of supercapacitors[J]. Carbon, 2015,

41: 797-828.

91: 103-113.

[27] Deka B K, Hazarika A, Kwon O, et al. Multifunctional

[13] He X J, Ma H, Wang J X, et al. Porous carbon nanosheets from

enhancement of woven carbon fiber/ZnO nanotube-based

coal tar for high-performance supercapacitors[J]. J Power

structural

Source, 2017, 357: 41-46.

solid-polymer electrolytes[J]. Chem Eng J, 2017, 325: 672-680.

[14] Huan Y, Sen X, Ya X Y, et al. Advanced porous carbon

[28] He

X

supercapacitor J,

Zhang

N,

and Shao

polyester X

L,

resin-domain et

al.

layered-template-nanospace-confinement

Energy Mater, 2017, 7: 1700530.

production of corrugated graphene nanosheets from petroleum

[15] Su Y Z, Yao Z Q, Zhang F, et al. Sulfur-enriched conjugated

strategy

A

materials for high efficient lithium metal anodes[J]. Adv

for

pitch for supercapacitors. Chem Eng J, 2016, 297: 121-127.

polymer nanosheet derived sulfur and nitrogen co-doped

[29] Zhang Z X, Wang H, Zhang Y X, et al. Carbon

porous carbon nanosheets as electrocatalysts for oxygen

nanotube/hematite core/shell nanowires on carbon cloth for

reduction reaction and zinc-air battery[J]. Adv Funct Mater,

supercapacitor anode with ultrahigh specific capacitance and

2016, 26: 5893-5902.

superb cycling stability[J]. Chem Eng J, 2017, 325: 221-228.

[16] He X J, Li X J, Wang J X, et al. From diverse polycyclic

[30] Wang Y L, Chang B B, Guan D X, et al. Preparation of

aromatic molecules to interconnected graphene nanocapsules

nanospherical porous NiO by a hard template route and its

for supercapacitors[J]. Micropor Mesopor Mater, 2017, 245:

supercapacitor application[J]. Mater Lett, 2014, 135: 172-175.

73-81.

[31] Wang H R, Yu S K, Xu B. Hierarchical porous carbon

[17] He X J, Li X J, Wang J X, et al. ZnO template strategy for the

materials prepared using nano-ZnO as a template and activation

synthesis of 3D interconnected graphene nanocapsules from

agent for ultrahigh power supercapacitors[J]. Chem Commun,

coal tar pitch as supercapacitors electrode materials[J]. J Power

2016, 52: 11512-11515.

Source, 2017, 340: 183-191.

[32] Yu S K, Wang H R, Hu C, et al. Facile synthesis of

[18] Wang H Q, Zhao Z B, Chen M, et al. Nitrogen-doped

nitrogen-doped, hierarchical porous carbon with high surface

mesoporous carbon nanosheets from coal tar as high

area: activation effect of nano-ZnO template[J]. J Mater Chem

performance anode materials for lithium ion batteries[J]. New

A, 2016, 4: 16341-16348.

Carbon Mater, 2014, 29: 280-286. [19] Blackburn

J

L,

Ferguson

[33] Shao J Q, Ma F W, Wu G, et al. In-situ MgO (CaCO3) A

J,

Cho

C,

et

al.

templating coupled with KOH activation strategy for high yield

Carbon-nanotube-based thermoelectric materials and devices[J].

preparation of various porous carbons as supercapacitor

Adv Mater, 2018, 1704386.

electrode materials[J]. Chem Eng J, 2017, 321: 301-313.

[20] Choudhary N, Li C, Moore J L, et al. Asymmetric

[34] Zhao J, Jiang Y F, Fao H, et al. Porous 3D few-layer

supercapacitor electrodes and devices[J]. Adv Mater, 2017, 29:

graphene-like carbon for ultrahigh-power supercapacitors with

1605336.

well-defined structure-performance relationship[J]. Adv Mater,

[21] Dong X M, Jin H L, Wang R Y, et al. High volumetric capacitance,

ultralong

life

supercapacitors

enabled

by

2017, 29: 1604569.

Feng Wei

et al. / New Carbon Materials, 2019, 34(2): 132-139

[35] Zhao J, Lai H W, Jiang Z Y, et al. Hydrophilic hierarchical

[40] Zhang C, Kong R, Wang X, et al. Porous carbons derived from

nitrogen-doped carbon nanocages for ultrahigh supercapacitive

hypercrosslinked porous polymers for gas adsorption and

performance[J]. Adv Mater, 2015, 27: 3541-3545.

energy storage. Carbon, 2017,114: 608-618.

[36] Guo Y, Shi Z, Chen M, et al. Hierarchical porous carbon

[41] Zhang Z Y, Lee C S, Zhang W J. Vertically aligned graphene

derived from sulfonated pitch for electrical double layer

nanosheet arrays: synthesis, properties, and applications in

capacitors[J]. J Power Sources, 2014, 252: 235-243.

electrochemical energy conversion and storage[J]. Adv Energy

[37] Xiao P W, Meng Q H, Zhao L, et al. Biomass-derived flexible porous carbon materials and their applications in supercapacitor and gas adsorption[J]. Mater Des, 2017, 129: 164-172. [38] Zhang W L, Xu C, Ma C Q, et al. Nitrogen-superdoped 3D graphene networks for high-performance supercapacitors[J]. Adv Mater, 2017, 29: 1701677. with

coal

tar

[42] Xu B, Wang H R, Zhu Q Z, et al. Reduced graphene oxide as a multi-functional

conductive

binder

for

supercapacitor

electrodes[J]. Energy Storage Mater, 2018, 12: 128-136. [43] Bu Y F, Sun T, Cai Y J, et al. Compressing carbon nanocages by capillarity for optimizing porous structures toward

[39] Wang L Q, Wang J Z, Jia F, et al. Nanoporous carbon synthesized

Mater, 2017, 29: 1700678.

pitch

and

its

capacitive

performance[J]. J Mater Chem A, 2013, 1: 9498-9507.

ultrahigh-volumetric-performance Mater, 2017, 29: 1700470.

supercapacitors[J].

Adv