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Popcorn-Derived Porous Carbon for Energy Storage and CO Capture Ting Liang, Chunlin Chen, Xing Li, and Jian Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01953 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 26, 2016
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Popcorn-Derived Porous Carbon for Energy Storage
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and CO2 Capture Ting Liang,†,‡ Chunlin Chen,*,† Xing Li,‡ Jian Zhang*,†
3 †
4 5 6 7
Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China.
‡
Faculty of Materials Science and Chemical Engineering, Ningbo University, 818 Fenghua Road, Ningbo 315211, China.
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ABSTRACT: Porous carbon materials have drawn tremendous attentions due to its applications
10
in energy storage, gas/water purification, catalyst support and other important fields. However,
11
producing high-performance carbons via a facile and efficient route is still a big challenge. Here
12
we report the synthesis of microporous carbon materials by employing a steam-explosion method
13
with subsequent potassium activation and carbonization of the obtained popcorn. The obtained
14
carbon features a large specific surface area, high porosity and doped nitrogen atoms. Using as
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an electrode material in supercapacitor, it displays a high specific capacitance of 245 F g-1 at 0.5
16
A g-1 and a remarkable stability of 97.8% retention after 5000 cycles at 5 A g-1. The product also
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exhibits a high CO2 adsorption capacity of 4.60 mmol g-1 under 1066 mbar and 25 oC. Both areal
18
specific capacitance and specific CO2 uptake are directly proportional to the surface nitrogen
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content. This approach could thus enlighten the batch production of porous nitrogen-doped
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carbons for a wide range of energy and environmental applications.
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1. INTRODUCTION
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Energy storage and environmental protection are two determining factors to the sustainable
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development of living and industries. The exceptional properties of porous carbon materials,
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such as high surface area, controllable pore structure, excellent thermal and chemical stability,
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wide availability of raw materials, superior environmental friendliness, high amenability to
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surface functionalization and heterogeneous doping, stimulate fundamental researches towards
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the applications in lithium batteries, supercapacitors, CO2 capture/sequestration, catalysis, and so
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forth. Both high porosity and surface functionality are necessary to achieve a high performance
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of carbons as supercapacitor electrode materials and gas adsorbent. Large porosity can provide
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more active sites for charge accommodation and accessibility of electrolyte ions,1-3 and CO2
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capture is closely dependent on the volume of pores that are smaller than a certain diameter.4-5
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Surface functionality may generate the extra pseudo-capacitance derived from redox reactions of
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surface groups6 and improve the adsorption capacity due to the enhanced interaction force with
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CO2 molecules.
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Numerous researches are being conducted to use abundant and clean resources to directly
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synthesize functional carbon materials. Raw biomass naturally contains nitrogen and other
19
heteroatoms that will be anchored inside of or around the graphitic structure to endow carbon
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materials with a variety of functionalities. With the aid of template chemicals, nitrogen-doped
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carbons with a large specific surface area can be obtained from various biomass like rice,7 wood,8
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gelatin,9 human hair,10 silk proteins,11 soybeans,12 pomelo peel,13 disposable cashmere,3 willow
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catkins,14 yogurt,15 et al. A new synthesis strategy is required to swell cellulose, lignocellulose,
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polysaccharide or other precursors into a porous or open structure before the carbonization
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process, allowing a high fraction of exposed carbon atoms to the pore-making or
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functionalization reagents. One successful example is the microwave popping of corn followed
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by thermal carbonization,16 albeit with the difficulties to be industrialized due to the limited
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processibility of commercial microwave apparatus.
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Herein, we reported an efficient and sustainable method to swell corn by employing steam-
7
explosion method and successfully produce high-surface-area and nitrogen-incorporated carbons
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by subsequent KOH activation. The excellent textural properties with a large SSA up to 1489 m2
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g-1 and high microporosity up to 90% as well as controllable nitrogen doping level (0.88~1.62
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at%), endowed the activated porous carbons as a very promising materials in both energy storage
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and CO2 capture applications. According to the quantitative description of structure-effect
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relationship of the areal specific capacitance or volumetric specific CO2 uptake capacity versus
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surface nitrogen content, our finding demonstrated that the synergistic effect of textural
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parameters and surface chemistry was crucial to both charge storage and CO2 uptake. The fitted
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empirical equations provide guidelines for materials design and structural optimization for high-
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performance carbon materials. These results suggest the widespread and efficient utilization of
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biomass as a treasure resource for sustainable development.
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2. EXPERIMENTAL SECTION
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2.1 Preparation of Popcorn Derived Porous Carbon. Briefly, popcorn was obtained by a
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traditional method using an orient artifact as depicted in Figure 1. It’s essentially a pressure
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vessel manufactured in Shengdi Machinery Plant, Wenling City, Zhejiang province, China. Corn
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with 15%~25% moisture and ~70% starch named Bainong 5 was purchased from Hubei
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Kangnong Seed Industy Limited Company. Typically, 800 g corn was sealed in the vessel and
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then heated on a flame for 6~8 min to reach the pressure of 1.0 MPa. At this point the water
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inside the hot corn should be vaporized and the starch liquefied. The corn popped to form
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popcorn when the pressure relieved suddenly by opening the lip of vessel. After that, popcorn
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was crushed into powders. The white powders were pre-carbonized under N2 atmosphere at 400
5
o
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grounded and mixed with different volume of 1 M KOH aqueous solution. And then the mixture
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was treated with sonic for ten minutes. Afterwards, the black mixture was dried in oven at 80 oC
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to remove the excess water. The solid mixture was further grounded into powders and
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subsequently transferred to tube furnace for activation at 800 oC for 1h with the protection of
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nitrogen. After that the as-prepared activated material was washed with suitable amounts of HCl
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(1 M) to remove any inorganic impurities, then washed with deionized water until the filtrate was
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neutral and dried at 110 oC for whole night. To investigate the effect of KOH dosage on the
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microstructure of the as-obtained samples, a sequence of experiments were carried out under the
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same conditions. The obtained products were marked as PC-xK (x=0.5, 1, 2), when x is the mass
15
ratio of KOH and pre-carbonization product. For comparison, the sample direct carbonized at
16
800 oC without activation was noted as PC.
C for 1 h with the heating rate of 5 oC per minute. Hereafter, the obtained black powders were
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2.2 Characterization and CO2 adsorption. Scanning electron microscopy (SEM, Hitachi S-
18
4800) and transmission electron microscopy (TEM, Tecnai F20) were employed to investigate
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the morphology and microstructure of the as prepared materials. The X-ray photoelectron
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spectroscopy (XPS) tests were carried out on an AXIS ULTRADLD Multifunctional X-ray
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photoelectron spectroscope with an Al Kα radiation source at room temperature and under a
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vacuum of 10–7 Pa. Raman spectra were recorded using a Renishaw inVia Reflex (532 nm laser)
23
system. The textural analysis was carried out at 77 K using a Micromeritics ASAP 2020
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instrument after degassing at 300 °C for 7 h prior to analysis. The CO2 adsorption isotherms of
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the obtained materials were measured on Micromeritics ASAP 2020 instrument at 0 and 25 °C.
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2.3 Electrochemical Measurements. The working electrode was prepared by mixing the
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active materials, carbon black, and binder poly(tetrafluoroethylene) with the mass ratio of 8:1:1
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in ethanol, followed by ultrasonic treatment. Then the mixture was coated onto the nickel foam
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current collector with coating area of 1 cm2, which was further dried at 100 oC over night.
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Electrochemical measurement was performed in a standard three-electrode system in 6 M KOH
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aqueous solution with a saturated calomel electrode (SCE) as reference electrode and a 2×2 cm
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titanium foil plated with iridium-tantalum alloy counter electrodes. Electrochemical
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performances were evaluated by cyclic voltammetry (CV), galvanostatic charge/discharge
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(GCD) and electrochemical impedance spectroscopy (EIS) measurements. All of CV and GCD
12
measurements were evaluated using CHI 760E instrument, while the EIS was measured by
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Zahner Zenium electrochemical system. In three-electrode system, the specific gravimetric
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capacitance (Cs, F g-1) can be calculated from GCD curves according to the formula (1): ூ∆௧
ܥୱ = ∆
15
(1)
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where I(A), ∆t(s), m(g), ∆V(V) are GCD current , discharge time, loading mass of active material
17
on the electrode and voltage change excluding the IR drop during the discharge process,
18
respectively.
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The electrochemical measurement of the symmetric supercapacitor (SC) was carried out in a
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2032 stainless steel coin cell. Two pieces (diameter of 1.5 cm) of nickel foam were used as the
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current collectors, using a glass fiber membrane as separator and 6 M KOH aqueous solutions as
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the electrolyte. The working electrodes were prepared by the same procedure in the above three-
23
electrode system with nearly identical loading mass of active materials. The electrochemical
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performance of SC was evaluated by the CV and GCD. The specific capacitance of the SC cell
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(Ccell, F g-1) was calculated from GCD curves based on the following formula: ܥୡୣ୪୪ =
3
ூ∆௧ ∆
(2)
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Where I(A) is the GCD current, m(g) is the total mass of active materials on the anode and
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cathode, ∆t(s) is the discharge time and ∆V(V) is the voltage change excluding the IR drop
6
during the discharge process .
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The power and energy density were calculated based on the total mass of active materials
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loading on both electrodes. The energy density (E, Wh kg-1) was calculated from the following
9
formula:
10 11 12 13
ଵ
= ܧଶ ܥୡୣ୪୪ (߂ܸ)ଶ
(3)
The average power density (P, W kg-1) during the discharge time of the symmetrical supercapacitor systems were calculated according to: ா
ܲ = ∆௧
(4)
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3. Results and Discussions
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The history of popcorn can be traced back to 5000 years ago and Chenda Fan a Chinese poet
16
also described the home-made process that was popular in 1150's.17 As illustrated in Figure 1,
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corn, millet or rice is heated in a sealed container and then the water is vaporized to keep a
18
balance of pressure across the outer shell. As the overall pressure inside the container reached 5-
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10 atm, the lid is suddenly opened to cool down the whole system. Rapid temperature drop will
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cause a large pressure difference to lead the fast explosion of the grain. The obtained popcorn
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displayed an open feature with a honeycomb-like structure and the productivity of popcorn is
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estimated to around 50 kg h-1 by using an automatic commercial machine. This demonstrates a
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great potential for large-scale non-pollution production of porous carbon precursors. Then we
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carbonized the popcorn powder with the assistance of KOH as activation agent to improve the
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porosity of the carbon materials. The products exhibited excellent textural properties with a
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specific surface area (SSA) up to 1489 m2 g-1, high microporosity of ~90% and a controllable
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nitrogen doping level (0.88~1.62 at%).
5 6
Figure 1. Schematic diagram for the preparation of PC-xK.
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The final carbon materials were marked as PC-xK (x= 0.5, 1, 2), where x is the mass ratio of
8
KOH to popcorn before carbonization. For comparison, the sample directly carbonized at 800 oC
9
without activation was noted as PC. The textural properties of all samples were investigated in
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detailed by nitrogen physisorption. Tiny N2 absorbance and no distribution in H-K region
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revealed that the PC sample only contained macropores with the size of a few microns, as shown
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in Figures. 2a&b. After the KOH activation, the PC-xK samples exhibited characteristic I type
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isotherms and H4 type hysteresis curves, being typical for micropores dominated texture with
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little amount of mesopores. Such a hierarchical pore feature was reflected in the narrow
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distribution of pore size (Figure 2b) by the Barrett-Joy-Halenda (BJH) and Horvath-Kawazoe
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(H-K) methods. The slight rise in isotherms at the relatively high pressure (0.95~1 p/p0) implied
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the retention of some slit-sharped macropores, which can be assigned to the remained
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honeycomb-like walls and stacking carbon particles. The BET surface area increased from 39 to
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1489 m2 g-1 and the total pore volume reached 0.706 cm3 g-1, benefiting from an open and thin-
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walled structure of the popcorn precursor. The pore diameter of the most probable aperture
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increased from 0.57 to 0.59 nm with the dosage of KOH. This phenomenon is related with the
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chemical etching mechanism by potassium ions at elevated temperatures,18 in accordance with
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the raised weight loss from 37.3% to 49.4% with the increasing amount of KOH.
b)
450
1000
400
-1
dV/dW (cm g nm )
350 300 250
-1
PC-2K PC-1K PC-0.5K PC
50
PC-2K
0 20 10
PC-1K
0 60 30
PC-0.5K
0 1.0 0.5
PC
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
c) 20
0.6
0.7
0.8
10
100
Pore diameter (nm)
ID/IG d)
C
0.70
10 0.80 0 0.90 -10
N1s
Intensity (a.u.)
0.0
PC-2K PC-1K PC-0.5K PC
Intensity (a.u.)
0
BJH region
3
3
H-K region
500
-1
Quantity adsorbed (cm g )
a)
Y Axis (µm)
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O
395
400
405
Binding energy (eV)
N
O KLL
Si
1.00 -20 -20
6
-10
0
10
X Axis (µm)
20
1.10
0
200
400
600
800
1000
Binding energy (eV)
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Figure 2. (a) N2 adsorption-desorption isotherms and (b) Pore size distribution curves of
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obtained materials. (c) Raman ID/IG mapping of PC-2K. (d) XPS survey spectra of all samples.
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(e) TEM, (f) HRTEM, (g) STEM HAADF image and elemental maps of PC-1K. Inset to (d): the
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high-resolution N1s spectra of PC.
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Raman mapping is a useful technique to elucidate the lattice ordering of carbon materials in a
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large scope. Two distinct peaks positioned at around 1337 and 1594 cm-1 (Figure S1) were
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assigned to the characteristic D-(defects and disorder) and G-(graphitic) bands of carbon,
9
respectively. The Raman ID/IG mapping of PC-2K in Figure 2c showed a narrow distribution of
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ID/IG ratio with an average value at 0.93. The slight fluctuation of the entire area revealed that a
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similar degree of graphitization varied to a limited extent in space, while the spatial
12
inhomogeneity may originate from the coexistence of surface functionalities and well-
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graphitized domains. Surface chemical composition was determined by X-ray photoelectron
14
spectroscopy (XPS) and the survey curves in Figure 2d contained three major peaks centering at
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284.6, 401 and 533 eV, being corresponded to C1s, N1s and O1s, respectively. Deconvolution of
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the N1s spectrum of PC (inset to Figure 2d) identified four nitrogen-containing components:
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pyridinic N (398.4 eV), pyrrolic N (400.4 eV), graphitic N (401.5 eV), and oxidized N (402.8
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eV). The surface content of nitrogen dropped from 2.47% to 0.88% with the increasing dosage of
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KOH (Table S1), suggesting that the chemical etching process by potassium ions may start from
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nitrogen-rich debris.19 KOH activation did not crush the interconnected macropores but produced
3
sufficient porosity to construct a hierarchical pore structure (Figure 2e & Figure S2), in
4
accordance with the physisorption results. High-resolution TEM images of the boundary area
5
depicted a long-range disordered structure with short-range curved graphitic lattices (Figure 2f).
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Uniform distribution of both nitrogen and oxygen atoms throughout the carbon framework can
7
be seen in the elemental maps by the STEM in Figure 2g.
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Owning to the highly porous structure with abundant surface functionalities, the activated PC-
9
xK samples were investigated for their potential uses as the electrode in supercapacitor and
10
absorbent for CO2 capture. The electrochemical capacitive properties of the four samples were
11
evaluated by conducting the cyclic voltammetry (CV), galvanostatic charge/discharge (GCD)
12
and electrochemical impedance spectroscopy (EIS) tests in a three-electrode system with 6 M
13
KOH aqueous solution. The CV profiles at scan rate of 5 mV s-1 (Figure 3a) showed that the
14
activated samples were of the better reversibility because their CV curves were much closer to a
15
rectangular shape than that of untreated PC. The activated samples also exhibited a greatly
16
improved capacitance than PC, as revealed by the longer charge/discharge times in the GCD
17
profiles at low current density of 0.5 A g-1 (insert to Figure 3b). The calculation by using the
18
discharge branches demonstrated that the activation of 1 equiv. KOH can efficiently enhanced
19
the specific capacitance from 60 F g-1 to 245 F g-1. Despite the capacitance inferior to some
20
elaborately manufactured carbons,20-22 the activated samples were superior to many other
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biomass-derived carbon materials in literature (detailed comparison in Table S2). Of all activated
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samples, PC-1K displayed the highest value and there is no proportional relationship between the
23
capacitance and KOH dosage. The rate performance at a range of current density from 0.5 A g-1
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to 100 A g-1 reported that the capacitance retention of all activated samples is 57-58% while that
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of untreated PC sample is only 5% (Figure 3b). The significantly improved rate capability can be
3
related with the hierarchical pore structure of PC-xK samples. The mesopores and macropores
4
are able to furnish a convenient transport and diffusion channels for electrolyte ions, while the
5
micropores provide the intrinsic area for charge accommodation.23 Note that the size of different
6
ions in KOH aqueous electrolyte is presented as follows: OH-
