Combined effect of nitrogen and oxygen heteroatoms and micropores of porous carbon frameworks from Schiff-base networks on their high supercapacitance

Article abstract of DOI:10.1039/c7ta08366a

Nitrogen and oxygen heteroatom doped porous carbon frameworks (HPCFs) have been constructed through the structural evolution of 2D microporous Schiff-base frameworks from the rigid polyquinoneimine and trigonal-symmetrical triformylphloroglucinol. Owing to its predictable and controllable nitrogen and oxygen doping and pore structures, the derived HPCF material exhibits good electrochemical properties as a supercapacitor electrode with remarkable specific capacitance (479.5 F g^-1 at 0.1 A g^-1 and 125 F g^-1 at 10 A g^-1) and excellent long-term cycling stability over 10000 cycles. The combined effect of heteroatom doping and micropores on its electrochemical performance as a supercapacitor electrode is thoroughly investigated. According to the analysis of the electrochemical behavior, pores larger than 5 ? in HPCFs are effective for electrical double-layer capacitance, which is associated with the size of hydrated ions. With regard to the heteroatoms, while the quaternary nitrogen functionalities can improve electron transfer, pyrrolic and pyridinic nitrogens as well as quinone oxygen are the most conducive functional groups for pseudocapacitive performance.

Full text of DOI:10.1039/c7ta08366a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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J oP ul er na as el o df oM na ot et rai ad lj su Cs th me ma ri sg ti rn ys A
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DOI: 10.1039/C7TA08366A
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ARTICLE
Combined Effect of Nitrogen and Oxygen Heteroatoms and
Micropores of Porous Carbon Frameworks from Schiff-Base
Networks on Its High Supercapacitance
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a,c
a
a,b
DOI: 10.1039/x0xx00000x
Man Zhou, Xiaoyan Li, Hong Zhao, Jun Wang, Yaping Zhao, Fengyan Ge, Zaisheng Cai*
www.rsc.org/
The nitrogen and oxygen heteroatoms doped porous carbon frameworks (HPCFs) have been constructed through the
structural evolution of 2D microporous Schiff-base frameworks from the rigid polyquinoneimine and trigonal-symmetrical
triformylphloroglucinol. Owing to its predictable and controllable nitrogen and oxygen doping and pore structures, the
derived HPCFs material exhibits high electrochemical properties as supercapacitors electrode with remarkable specific
-1
-1
-1
-1
capacitance (479.5 F g at 0.1 A g and 125 F g at 10 A g ) and excellent long cycling stability over 10000 cycles. The
combined effect of heteroatoms doping and micropores on its electrochemical performance as supercapacitor electrode is
deeply investigated. According to the analysis of the electrochemical behavior, pores larger than 5 Å in HPCFs are effective
for electrical double-layer capacitance, which associates with the size of hydrated ions. As regards to the heteroatoms,
while the quaternary nitrogen functionalities can improve electron transfer, pyrrolic and pyridinic nitrogens as well as
quinone oxygen are the most conducive functional groups conducing to pseudocapacitive performance.
Introduction
Carbon-based supercapacitors, also called electrical double- electrolyte stability, C of carbon electrode materials is
layer capacitors (EDLCS), are a type of outstanding storing generally controlled by the carbon/electrolyte interface. Thus,
energy devices under vigorous study in view of their low proceeding from the parameter C with carbon electrode,
weight, very fast charging-discharging kinetics, and bipolar considerable research efforts have been put into rational
1
operational flexibility. Supercapacitors characterized by designing new carbonaceous materials and optimizing their
5
, 6
combining the best of the general capacitor and storage structure and functionality, such as increasing the effective
7
battery have been regarded as a solution to mismatch surface area, the adjustment of pore size distribution and
-9
2
between power density and energy density. However, the the heteroatoms introduction into the carbon skeletons,
10, 11
1
2, 13
14, 15
16 17
phosphorus, sulfur and
carbon-based supercapacitve energy storage operates on including nitrogen,
1
oxygen,
3, 18
electric double layers by the reversible physical adsorption of boron.
They can be summarized into the micropore effect
electrolyte ions on porous electrodes without electrochemical and the heteroatom effect. For the micropore effect, Chmiola
reactions, which contributes to EDLCS low energy density, and co-workers have revealed that certain kinds of
3
, 4
although with high power density and long cycle life.
micropores, especially those with the width close to the size of
electrolyte ions in the electrode can contribute significantly to
19
2
1
E =
CV
(1) the electrical double-layer formation. This was supported by
2
20
other researchers, although it is somehow conflictive to the
Based on the equation (1), the energy density (E) of a traditional opinion that micropores are barely conducive to the
supercapacitor depends on the specific capacitance (C) and capacitance since the electrolyte ions cannot enter these
2
1
operational voltage (V). While V is mainly relevant to the pores. Regarding to the heteroatom effect, the doping of the
heteroatoms can affect the electron distribution in carbon
materials and introduce the acid-base properties by surface
2
2
groups, and further produce the wettability.
And
heteroatom-doped carbons are promising to improve the
capacitance via faradaic reactions without sacrificing the long
cycle life. Therefore, it is the great concern to understand the
combined effect of various heteroatoms and the pore
structure on supercapacitance to develop high performance
carbon electrodes.
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Porous polymer frameworks are built by the polymerization both the powder samples and the thin films COFs are still
of rigid organic molecules, called tectons and symmetric limited. The main factor is relatively low conductivity of porous
2
3-25
functional groups pointing.
introduced into the networks of the materials from intrinsic examples of investigating porous polymer frameworks related
Heteroatoms can be selectively polymer frameworks. Therefore, the successful pioneering
2
6, 27
heteroatom-containing building blocks.
Meantime, the materials as supercapacitors electrode remain scarce. Xiong’s
defined pore sizes can be obtained by selecting molecules with group used pyrrolidine as a catalyst to obtain melamine-based
2
4, 25
different sizes.
have abundant micropores with high specific surface area and and then SNW was calcined with ZnCl
intrinsic heteroatoms doping, which should be quite conducive porous carbon with adaptable conductivity and high
Consequently, porous polymer frameworks Schiff-base network (SNW) materials by solvothermal method
2
to prepare activated
3
1
to the supercapacitor performance when applied as electrode capacitance. But the works need a complicated two-step
materials. Jose L. Segura and co-workers summarized different preparation. Recently, the molten-salt method emerges as an
synthetic strategies successfully used for the production of a effective strategy to prepare graphene-like carbon materials.
́
3
2-
3
4
kind of porous polymer frameworks, Covalent Organic
Zhi’s research group constructed a conjugated microporous
Frameworks (COFs), based on Schiff-base chemistry and covalent triazine-based framework (CTF) through the
2
8
studied their properties and potential used in different areas.
trimerization reactions with only one step under ionothermal
However, typical COFs lack intra-sheet π-conjugation and conditions and showed that the conductivity problem can be
chemical stability, which significantly limit their practical utility. solved by increasing the polymerization temperature without
3
5, 36
In 2013, Dichtel et al. reported for the first time the use of a sacrificing their intrinsic advantages.
COF based on Schiff-base chemistry for pseudocapacitive Herein, we choose a nitrogen-containing monomer 2,6-
energy storage. The redox-active 2,6-diaminoanthraquinone diamino-anthraquinone (DAAQ) as a linker to connect 3,4,9,10-
DAAQ) moieties was incorporated into a 2D-COF linked by β- perylenetetracarboxylic dianhydride (PTDA) into the
ketoenamines with 1,3,5-triformylphloroglycinol (TFP) under polyquinoneimine (PQI) with rigid structure. Then the PQI and
(
2
9
solvothermal conditions. And in 2015 they described the first trigonal-symmetrical triformylphloroglucinol (TFP) are
oriented thin films of β-ketoenamine-linked 2D-COFs, which incorporated as the building blocks into a porous polymer
were formed through the slow introduction of TFP into a framework which can be used to construct a conjugated
3
0
solution of DAAQ in the presence of Au substrates. With this microporous network through the Schiff-base reactions based
strategy, DAAQ-TFP COF thin films provide areal capacitances on the molten-salt method. This conjugated Schiff-base porous
-2
-2
of 3.0 mF cm , as compared to 0.40 mF cm for powder network with well-defined nitrogen and oxygen incorporation
electrodes mentioned above. However, the capacitances of
Figure 1. Illustration of the preparation of HPCFs, including the polymerization of PQI and TFP and high-temperature treatment
to form a nitrogen and oxygen heteroatom-doped porous carbon frameworks HPCFs.
2
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Figure 2. (a) FT-IR spectrum of PQI-TFP (green), PQI (cayenne), TFP (black). (b) XPS N 1s deconvolution of PQI-TFP. (c) XPS O 1s
deconvolution of PQI-TFP. (d) AFM images of 2D PQI-TFP. Thin layers with ~4nm thicknesses can be observed.
4
0
at higher temperatures leads to a nitrogen and oxygen 2c). In addition, atomic force microscopy (AFM, Figure 2d)
heteroatoms-containing porous carbon frameworks (HPCFs) by analysis demonstrates the layered structure of PQI-TFP.
structural evolution (Figure 1). From the experimental design,
we choose conjugated structures and increase the HPCFs
polymerization temperature by molten-salt method to stretching/deformation vibration signals (
Compared with PQI-TFP, the detailed structure signals of
have
disappeared
and
only
show
the
∼
1580 and
∼1220
−
1
significantly improve the conductivity. As a result, high specific cm ) of the frameworks resulting from the rigid structure with
-
1
-1
capacitances in 1 M H SO (e.g., 479.5 F g at 0.1 A g and 125 the increase of the polymerization temperature (Figure 3a).
2
4
-1
-1
-1
F g at 10 A g ) and excellent cycling stabilities can be The signal intensity at ~1220 cm decreasing with the increase
obtained by tuning the synthesis temperature of HPCFs. of the temperature suggests that further cross-linking
What’s more, we study the combined effect of nitrogen and reactions have occurred. XPS analysis demonstrates the
oxygen heteroatoms doping and porous structure on nitrogen configurations of N 1s evolve from two kinds in PQI-
electrochemical performance.
TFP to three kinds in HPCFs, including still the pyridinic
nitrogen (N-6) and pyrrolic/pyridone nitrogen (N-5) but also
the quaternary nitrogen (N-Q) (typical N 1s deconvolution of
HPCF@500 in Figure 3b and others in Figure S6-9), suggesting
the formation of graphitized fragments. The chemical formulas
of three kinds of nitrogen configurations are schematically
shown in Figure 3c. In addition, quantitative analysis of XPS
suggests that with the increase of the reaction temperatures,
the ratios of N-Q in HPCFs gradually increase (Figure 3d)
corresponding to the increasing graphitized fragments, which
also confirms further cross-linking rearrangement under higher
temperatures. And the ratios of N-5 and N-6 changing
Results and Discussion
To verify the basic structure of HPCFs resembles the Schiff-
base porous network, the polymerization was first carried out
at relatively low temperature of 300 °C (see details in
Experimental Section) and the as-prepared material was
marked as PQI-TFP. The Fourier transform infrared (FT-IR)
spectrum and X-ray photoelectron spectroscopy (XPS) of PQI-
TFP were first performed to characterize the nitrogen- and
oxygen-contained chemical structure of PQI-TFP. The Fourier
transform infrared (FT-IR) spectrum (Figure 2a) indicates that it
is linked by Schiff-base reaction, evidenced by the appearance
o
o
dramatically between 400 C and 450 C can indicate the
rearrangement takes place drastically at this moment. The
oxygen configurations of O 1s are shown in Figure 3e and
Figure S6-9. As seen from Table S1, C=O groups (O-I) still
predominates in oxygen-containing surface functionalities
after high temperature. While XPS inspects the composition on
the surface, the bulk composition is revealed by elemental
analysis. In consequence, the relative contents of surface
carbon, nitrogen and oxygen obtained from elemental analysis
differ from the results of XPS. The results of elemental analysis
-
1
of a characteristic C=N stretch at 1220 cm and the
3
7
disappearance of the N-H stretches. The N 1s deconvolution
peaks (Figure 2b) at 397.87~398.27 eV and 399.47 400.15 eV
∼
indicate that only the pyridinic nitrogen (N-6) and pyrrolic/
pyridonic nitrogen (N-5) in PQI-TFP are present, while there is
3
no quaternary nitrogen (N-Q) at 401.09~401.86 eV.
8, 39
On the
other hand, according to the O 1s deconvolution peaks, PQI-
TFP has C=O groups (O-I) at 531.25~531.80 eV and C-OH
groups and/or C-O-C groups (O-II) at 533.00~533.20 eV (Figure
(Table S2) shows that the nitrogen content reached more than
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6% and oxygen content reached more than 20%. And less than HPCF@400 as seen from the Table 1, which might be
elemental mapping with field-emission scanning electron explained by rearrangement and graphitization in accordance
microscopy (FESEM) demonstrates homogeneously distributed with the dramatical ratio tendency change of N-5 and N-6
o
o
carbon, nitrogen and oxygen in the skeleton of HPCF@500 between 400 C and 450 C. With the temperature increasing
(Figure 3f).
to 500 and 550°C, these ultramicropores volume regain
Although the surface chemistry can affect the material because of the molten salt porogen possibly, as two
4
performance, only if the chemistry is accessible for the species publications attribute a template role to the molten salt.
3-45
taking part in the process, which is dependent on the porous
structure and surface area of the material. Remarkably, further the electrochemical performances in 1 M H
cross-linking and rearrangement reactions induced by high voltammetry (CV) curves for all HPCFs at 2 mV s , 10 mV s
A conventional three-electrode system was used to evaluate
2
SO solution. Cyclic
4
-1
-1
-1
temperature also cause evolution of the pore structures in and 50 mV s are shown in Figure 5a and Figure S11. We can
-1
HPCFs. The X-ray diffraction (XRD) patterns of HPCFs indicate see the CV curves display quasi-rectangular shape at 2 mV s ,
the dominant features of amorphous carbon by displaying two suggesting the excellent capacitive performance of all samples,
-1
broad diffraction peaks at 2θ values of 26.2° and 43.8° (Figure while at 10 and 50 mV s , only CV curves of HPCFs prepared at
a), which are relevant to the (002) and (101) planes of more than 450 °C (HPCF@500, HPCF@550 and HPCF@600)
4
graphite. The TEM images (Figure 4b,c and Figure S10) show can maintain the quasi-rectangular shape, and CV curves of
that extremely abundant pores have emerged. According to HPCF@400 and HPCF@450 are gradually changed to fusiform
the nitrogen adsorption/desorption measurements (Figure 4d), shape, suggesting that HPCFs prepared at higher temperatures
HPCF@600 exhibits typical non-porous materials isotherm, have better rate capabilities. And broad redox peaks
while the HPCF@400, HPCF@450, HPCF@500 and HPCF@550 superposed on CV curves maybe linked to the heteroatoms
samples display combined type I and type IV sorption present in the skeleton of HPCFs, as the XPS and elemental
isotherms with the microporous sorption at low pressure and analyses implied. Moreover, in Figure 5a the CV curves of the
the type H4 hysteresis loop resulting from the mesopores. HPCFs electrodes from various temperatures samples
Evaluation of the pore size distributions based on the density demonstrate moderate distortion, which can be a result of
functional theory (DFT) further verifies this finding (Figure 4e). prominent contribution of pseudocapacitance, mainly relating
The pore size distributions show that pores near 1~2 nm to the nitrogen and oxygen surface functional groups on the
predominate similarly in all HPCFs except for HPCF@600, in surface of the carbon networks. The CV curve for HPCF@500
accordance with the dominant proportion of micropores in has a larger integrated area within the potential window,
Figure 4f. The porosity parameters are shown in Table 1 in which implies that the specific capacitance of HPCF@500 is
detail. It’s found that the structural character is seriously always higher than that of other samples under the same
affected by the reaction temperature. With the temperature measurement conditions. Then, galvanostatic charge-
increase (400-500 °C), both the surface area and pore volume discharge (GCD) experiments at different current densities
-1
are gradually increased through the high temperature from 0.1 to 10 A g (Figure 5b,c and Figure S17) were carried
4
1, 42
-
activation.
Whereas, after the higher temperature (550- out to assess their electrical performances further. At 1 A g
1
6
00 °C), the surface area and pore volume are reduced
(Figure 5c), the GCD curve of HPCF@600 shows a nearly
remarkably, especially HPCF@600 exhibits almost no porosity, triangular shape, similar to that of theoretical electrochemical
4
1
because of the collapse of pores.
double layer capacitors. This is mainly because the
As we all know, precise information about the volume of conductivity of the material becomes better with the increase
pores smaller than 5 Å cannot be provided by nitrogen of the reaction temperature, which can be confirmed by
adsorption owing to the diffusion limitation, but the electrochemical impedance spectroscopy (EIS) testing results
3
8
magnitude at very low pressure can be an indication of the in Figure 5e. The overall performances of HPCFs electrodes
pore volume change. It can be noted that the volume of are summarized in Figure 5d from GCD. In consistent with the
ultramicropores smaller than 5 Å size of HPCF@450 is much
Table 1. Structural parameters of HPCFs calculated from nitrogen adsorption isotherms.
S
BET
-1
V
<5 Å
-1
V
5~10 Å
-1
V
mic
3
V
meso
-1
V
t
Sample
2
3
3
-1
3
3
-1
V
mic/Vt
[
m g ] [cm g ] [cm g ] [cm g ] [cm g ] [cm g ]
HPCF@400
HPCF@450
HPCF@500
HPCF@550
HPCF@600
323
566
838
455
82
0.02959 0.03111 0.13418 0.05101 0.19478
0.00270 0.13088 0.23739 0.06440 0.31081
0.02764 0.15755 0.35443 0.04700 0.40451
0.03983 0.07438 0.18563 0.06518 0.25873
0.69
0.76
0.88
0.72
0.25
-
-
0.02321 0.05977 0.09418
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Figure 5. Electrochemical properties of HPCFs electrodes measured in a three-electrode system. (a) Cyclic voltammetry curves of
-1
-1
HPCFs at 2 mV s . (b) Galvanostatic charge-discharge curves of HPCFs at 0.1 A g . (c) Galvanostatic charge-discharge curves of
-1
HPCFs at 1 A g . (d) Specific capacitances of HPCFs at various current densities. (e) Nyquist plots (AC frequency ranges from
00000 to 0.01 Hz) of HPCFs electrodes. The equivalent circuit is shown in the inset. (f) Cycle life tests of HPCF@500 at 5.0 A g .
The first and last fifteen cycles of GCD curves are shown in the inset.
−1
1
CV, the results of GCD reveal that HPCF@500 shows relatively electrode materials relative to other samples, ensuring much
higher capacitive property. Specifically, it can be noted that easier kinetics in accordance to better rate capability. In
-1
the specific capacitance increases from 426 F g (HPCF@400) addition, the cycling life of the HPCFs electrodes is tested.
-1
-
1
to 567 F g (HPCF@450), and then decreases to 480 F g
Typically, after charge and discharge for 10000 cycles at a
-1
-1
HPCF@500), 384 F g (HPCF@550) and 317 F g (HPCF@600) current density of 5 A g , the HPCF@500 electrode shows
-1
(
-1
at a current density of 0.1 A g with the increase of synthesis almost no deterioration in capacitance and the charge-
temperature. Obviously, the specific capacitances of both discharge curves maintain triangular (Figure 5f).
HPCF@400 and HPCF@450 decrease much quicker and HPCFs
The insight of the relationship between the various
synthesized at higher temperatures have better rate capability, performances of HPCFs and the different reaction temperature
which is apparently associated with reaction temperature. should be more interesting. Therefore, the following
From the XPS analysis, the higher temperature induced the paragraphs elaborate the combined effect of the heteroatoms
relatively high amounts of N-Q among HPCFs (Figure 3d and and the micropores on the supercapacitance. As mentioned
Table S1), which is positively charged and can help electron above, Chmiola and co-workers have revealed that certain
transfer and improve the capacitive performance at high kinds of micropores, especially those with the width close to
current densities. Figure 5e shows the Nyquist plots of the the size of electrolyte ions in the electrode can contribute
prepared materials. When the intercept on real impedance (Z’) significantly to the electric double-layer formation. In our
2
-
+
in the high frequencies stands for the ohmic resistances (R
including the electrolyte and the contact resistance, the corresponding solvated ions are SO
semicircle in the medium-high frequencies is the charge sizes are 5.33 Å and 4.2 Å respectively as reported. On the
s
)
research, the electrolyte ions are SO
4
3
and H O . The
2
-
+
4
2 3
·12H O and H O , whose
4
6
2
-
transfer resistance (Rct) and the slope of the line in the low basis of it, the pore size of ~5 Å matches the solvent ions SO
·
frequencies is in relation to the diffusive resistivity of the 12H O, simultaneously, is not too big to accommodate the
2
4
electrolyte ions within the pores. The equivalent circuit is H3O+. According to these, considering the formation of the
illustrated in Figure 5e as an inset. The equivalent series double-layer, the most efficient ultramicroporous structure
resistances (ESR) of all HPCFs electrode are similar around 0.90 among these samples are in HPCF@450, who has the least
Ω. And HPCF@500 clearly shows the shortest semicircle in the pore with the size of less than 5 Å. This conclusion can be
medium-high frequencies range conforming. It is worth confirmed by the fact that the HPCF@450 shows the biggest
-1
pointing out that the steeper low-frequency tails of HPCF@500, capacitance at 0.1 A g current density.
HPCF@550 and HPCF@600 suggest higher conductivity in the
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On the other hand, it is worth noting that although the as abscissa of HPCF@600 is far larger than others resulting
porosity of HPCF@600 is much less (the BET surface area 82 from the lowest specific surface area, which affects the degree
2
−1
g ) than other HPCFs samples, its specific capacitance of dispersion of the lines. So, the respective insets in Figure 6
m
−
1
-1
−1
-1
values (317 F g at 0.1 A g and 112 F g at 1 A g ) are not are the lines fitted in the same way removing HPCF@600. The
extremely poor. The faradaic redox reaction occurring on the linearly increasing tendency of C is the evidence of N- and O-
S
surface due to the co-doping of nitrogen and oxygen containing functionalities inducing pseudocapacitance. In
heteroatoms is indirectly confirmed by the capacitance values. consideration of the electrochemical activity of individual N
4
7-49
In view of the small BET specific surface area, the contribution and O functionalities as shown in Figure S18,
the
of the pseudocapacitance to the total capacitance values is far relationships between C with the combination of I-N5, I-N6
S
greater than the double-layer capacitance.
and I-OI are exhibited in Figure 6d,e,f. it can be verified that
With respect to the pseudocapacitance, substantial amounts pyrrolic nitrogen, pyridinic nitrogen and quinone oxygen are
of heteroatoms such as nitrogen and oxygen in surface conducive to the pseudocapacitance by increased C with the
S
functionalities of carbon samples must be taken into account. increase of the combination of I-N5, I-N6 and I-OI. In contrast,
In Figure 6, while the ordinate is the specific capacitance per when the I-OII indices are included in Figure 6g,h,I, as
-
surface area C (in F m ) calculated by the capacitance dividing predicted, correlation degree decreases between the C and
2
S
S
the specific surface area SBET, Nitrogen (I-N) and oxygen indices sum of indices in accordance with the researches that the
(
I-O) as abscissa are obtained by multiplying the percentage of oxygen functionalities in phenol and ether groups are
5
0, 51
each N or O species with the total content of N or O from XPS, electrochemically inactive in acidic electrolyte.
4
then divided by surface area. The correlation between C and results from the insets are consistent with the findings
S
And all
0
the sum of N and O indices is shown in Figure 6a,b,c. It can be including HPCF@600 sample.
noted that the nitrogen indices (I-N) and oxygen indices (I-O)
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In order to further demonstrate the practical Å are beneficial to electrical double-layer capacitance in 1 M
electrochemical performance of prepared HPCFs, the H SO , which closely related to the size of hydrated ions. While
2
4
electrodes were assembled into a conventional two-electrode the quaternary nitrogen functionalities have improved
symmetric supercapacitor system (a coin-type cell of 2032 size, electron transfer, the most conducive functionalities for the
Figure 7a), consisting of two symmetric working electrodes (80 pseudocapacitance are pyrrolic and pyridinic nitrogen along
wt% HPCFs, 10 wt% carbon black and 10 wt% PTFE) with the with quinone oxygen.
ionic liquid (EMIMBF4) electrolyte (see details in the
Electrochemical measurements). The cyclic voltammetry
Acknowledgements
curves (CVs) of HPCF@500 supercapacitor assembled in a two-
electrode cell are performed from 0 to 3.5 V at various scan
rates (Figure 7b). They display quasi-rectangular shape and
large integrated area within the potential window,
corresponding to the decent capacitive character.
Galvanostatic charge-discharge (GCD) experiments were also
carried out at different current densities (Figure 7c). Notably,
the HPCF@500 symmetric supercapacitor was employed to
drive a commercial red LED. It could light it for over 1 min after
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 51303022) and the
Fundamental Research Funds for the Central Universities
(
Grant No. 2232015D3-17), Program for Specialized Research
Fund for the Doctoral Program of Higher Education in China
Grant No. 20130075130002) and PhD Foundation for
Innovation of Donghua University (Grant No. 17D310513).
(
-1
being fully charged at 0.1 A g without connection in series, as
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