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Multi‐heteroatom self‐doped porous carbon derived from swim
bladders for large capacitance supercapacitors
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Lintong Hu,a Junxian Hou,b Ying Ma, a Huiqiao Li,a,* and Tianyou Zhaia
Multi‐heteroatom self‐doped porous carbon was synthesized via carbonization and activation of the amino‐acids‐riched
swim bladders and used for the high performance supercapacitors. The effects of different activation temperatures on the
pore structure and composition of the carbon were investigated by TEM, Raman, XPS and nitrogen sorption analysis. With
the increasing temperature, the pore size broadens and the doped amount of heteroatom decreases. The obtained
2
‐1
materials have a high surface area up to 3068 m g with nitrogen, oxygen and sulfur multi‐heteroatom doping. Carbon
o
‐1
activated at 550 C shows the highest capacitance of 410 F g and excellent cyclic stability upon 10000 cycles due to the
high surface area and multi‐heteroatom doping. The outstanding performance make this materials promissing for
supercapacitors.
oxygen, sulfur, phosphorous, boron and fluorine, which can
10,
effectively boost the performance of carbon supercapacitors.
Introduction
Carbon‐based materials are widely used in the energy storage
and conversion devices, especially in supercapacitors, for their
high conductivity, stability in different solutions and low cost.
1‐3
Based on the electrostatic attraction mechanism on the
interface of electrode/electrolyte, the capacitance of carbon
based materials has dependence on the surface area that is
4, 5
Thus, to gain high
accessible by the electrolyte ions.
capacitance, carbon materials with high surface area and fine
pore size distribution are extensively explored. However, even
2 ‐1
with a high surface area up to 3000 m g , the capacitance is
‐1
still low, ranging from 100~300 F g . It is still not sufficient for
6, 7
independent devices. To further enhance the capacitance of
the electrical double layer capacitors, a typical strategy is to
trigger the pseudocapacitance of carbon materials in addition
8
of high surface area. Heteroatom introduction has been
proved effective to introduce redox surface‐reaction in carbon
9
materials. Functional groups with diversified methodologies
can be covalently bonded into the carbon backbone and on
the carbon surface. These groups not only can enhance the
electronic conductivity, but also enhance the surface
10, 11
More important, they
wettability of hydrophobic carbon.
8
can produce surface pseudocapacitance. Various carbon
materials doped by different electron‐donating or electron‐
withdrawing elements have been reported, such as nitrogen,
a
State Key Laboratory of Material Processing and Die & Mould Technology, School
of Materials Science and Engineering, Huazhong University of Science and
Technology (HUST), Wuhan 430074, Hubei, P. R. China.
b
Department of composite Materials and Engineering, College of Materials Science
and Engineering, Hebei University of Engineering, Handan, 056038, PR China.
* Corresponding authors: E‐mail: hqli@hust.edu.cn; huiqiaoli@gmail.com
12‐14
Up to now, two methods have been reported to introduce
the heteroatoms into carbon framework. One is post‐
treatment of carbon materials that are subjected to
heteroatom compounds at high temperature. Another is self‐
doping at inert atmosphere through carbonizing precursors
that are rich in the heteroatoms. The former approach is to
mix the carbon materials with gas or solid powder, such as NH3,
urea and melamine for N‐doping, H2S for S doping, and boric
13, 15‐19
This method operates complicatedly
acid for B‐doping.
and is time‐consuming. Besides, the toxic compounds (NH3 and
H2S) are used and the high temperature post‐reaction may
cause the collapse of pore structure. Especially, this method
usually gives rise to a low doping amount, and the doping
mainly happens on the surface rather than the bulk of carbons.
The generated surface functional groups may unstable in the
cycling and as a result, the capacitance would decay. By
contrast, the latter approach is more convenient and easier.
Dislike the former, a homogeneous bulk doping carbon with
20
As the doping
surface functionality can be obtained.
elements are inherited from the precursors, the structure of
final materials and generated functional groups are much
more stable than the post‐treatment one, which show more
excellent cycling stability of supercapacitors.
In recent years, different materials have been explored as
21‐23
organic salts,17, 24
the carbon precursors, such as polymers,
25, 26
27‐30
and animal precursors,
in which the
plants precursors
biomasses attracted much attention due to their low cost,
wide abundance and good environmental compatibility. The
reported capacitance for these biomass derived carbon ranges
‐1
from 150~350 F g , higher than that of conventional activated
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DOI: 10.1039/C6TA06337C
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ARTIICLE
carrbon. Differe
ent from th
he precursorrs above, aanimal
pre
ecursors have
e amount of carbohydrate and amino acids,
wh
hich are rich in
n N, O, and S elements,
e
mak
king it more feeasible
to obtain a mu
ulti‐heteroatom
m self‐doped carbon. Heree, we
ose swim blad
dders as a ne
ew precursor to develop c arbon
cho
ma
aterials with b
both multi‐he
eteroatom self‐doping and large
surrface area forr supercapacittors by combining carbonizzation
and
d activation prrocesses, as sh
hown in the Fig. 1. Swim blaadders,
the
e by‐product o
of fish industrry, contain rich amino acidss such
as glycine, prolin
ne and hydroxxyproline31, 32 which can co
onvert
atom self‐dop
ped carbon materials. By fu
urther
to multi‐heteroa
H as the activvation agent, high surfacee area
introducing KOH
carrbon with different porosityy can be obtained. The obttained
carrbon materialss are proved to
o be O, N, S multi‐doped
m
an
nd can
2 ‐1
rea
ach an ultrahiggh surface of 3068 m g . The
T doping am
mount
can
n be adjuste
ed by chang
ging the activation tempeerate.
Ele
ectrochemical performance
e tests show
w that the swim
bladders derive
ed carbon materials
m
can
n deliver sup
perior
ele
ectrochemical performancess both in acid and base aqu
ueous
ele
ectrolytes inclu
uding a large capacitance
c
off 410 F g‐1 and
d good
cyccle stability up
pon 10000 cyclles.
Experimentall
Ma
aterial synth
hesis
The
e swim bladde
ers were first carbonized in tube furnacee for 2
h at
a 650 oC un
nder a nitrog
gen atmosphe
ere. Then thee pre‐
carrbonized swim
m bladders we
ere mixed witth KOH in an agate
mo
ortar at a weigght ratio of 1:3.
1 Activation was perform
med at
450
0 oC, 550 oC, 6
650 oC, 750 oC,
C and 850 oC for 2 h in nittrogen
flow
w. The obtain
ned carbon po
owder was wa
ashed with 6 M HCl
and
d rinsed with ultrapure wa
ater until neu
utral. The matterials
were denoted a
as AC‐T, wherre T indicatess the carbonizzation
tem
mperature.
btained
AC were
w
The morphology and structurre of the ob
View Article Online
DOI:
10.1039/C6TA06337C
acterized by scanning
s
elect
ctron microsco
ope (Quanta 650
chara
FEG, FEI) and tran
nsmission eleectron microsccope (Sirion 200,
2
ed with an ASAP
A
FEI). Nitrogen sorption analysiss was obtaine
0, Micrometrittics. The poree size distribution plots were
w
2020
evalu
uated based on the nonloocal density functional
f
the
eory
(NLDFT). Raman was
w done on a LabRAM HR
R800 with a la
aser
elength of 532
2 nm. Combusstion elementtal characterisstics
wave
were
e performed by Vario Microcube (Elementar) for
deterrmination of the C, O, N and S am
mount. Elemental
chara
acterization was
w
perform
med by X‐ray photoelecttron
specttrum (AXIS‐ULTRA DLD‐600W
W, Kratos).
Electtrochemicall tests
To prrepare the ele
ectrode, a slurrry of 80% AC, 15% super P and
5% poly (tetrafluorroethylene) (PPTFE) was mixe
ed and rolled into
m, then dried overnight. A piece of carbon film was
a film
presssed onto a nickel foam (basse electrolyte) or stainless steel
(acid electrolyte) current collectoor at 30 MPa. The load masss of
w
electrode is about 4.00 mg cm‐2 with a thickness of
the work
appro
oximately 100
0 μm. In a threee‐electrode ce
ell, the AC‐loaded
electrode and a Pt electrode wass used as the work
w
and coun
nter
electrode, respecttively. Hg/Hg22SO4 (in saturrated K2SO4) and
HgO (in 1 M KOH) were useed as the refe
erence electro
odes
Hg/H
for the acid an
nd base eleectrolyte, resspectively. Cyyclic
voltammetry (CV) and galvanoostatic charge‐discharge (G
GCD)
arried out onn a CHI 760E
E electrochem
mical
meassures were ca
work
kstation
(Sh
hanghai
Chhenhua
Insttruments
C
Co.).
Electrochemical Im
mpedance Specctra (EIS) were
e performed on
o a
Solarrtron 1260 + 1287
1
Interfacee with amplitu
ude of 5 mV frrom
10 mHz
m to 100 kHz. The capacittance was callculated from the
GCD tests using the
e following eqquation of C = IΔt/mΔV, whe
ere I
(A) iss the current density,
d
Δt (s)) is the discharge time, m (g
g) is
the weight
w
of active materials onn the working electrode and
d ΔV
(V) iss the voltage window.
w
Ma
aterial chara
acterization
Fig. 1 Schematic illustration off the multi‐hetteroatom self‐‐doped porouss carbon derived from swim bladders.
Thiss journal is © Th e Royal Society of
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J. Name ., 2013, 00, 1‐3
3|2
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Re
esults and d
discussion
Fig. 2 (a) SEM im
mage of AC‐550
0. TEM imagess of (b) AC‐4500, (c) AC‐550, (d) AC‐650, (e) AC‐750 and (ff) AC‐850.
a
-1
0.1
16
micropore
meso
opore
3
1200
1000
800
600
400
200
0
0.1
12
0.0
08
AC-550
AC-650
AC-750
0.0
04
0.0
00
0.0
c
b
AC-550
AC-650
AC-750
1400
Pore Volume (cm g )
3
-1
Volume Adsorption (cm g , STP)
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The
e typical morp
phology of the
e swim bladders derived carb
bon is
sho
own in Fig. 2a
a. It consists of
o irregular mo
onoliths with sharp
corrners. In spite of the differe
ent activation temperaturees, the
SEM
M of AC‐450, AC‐550, AC‐6
650, AC‐750 and
a
AC‐850 sh
how a
sim
milar morphology. The mon
nolithic morphology is diffferent
from the hierarcchical structure of other carrbon materialss. The
es of these ACs
A
were further observeed by
dettailed texture
HRTEM. Fig. 2b‐‐f give the HR
RTEM images of AC‐450, AC
C‐550,
AC‐‐650, AC‐750 and AC‐850, respectively. All the speciimens
ain micro‐ and
d mesoporous that are surro
ounded
by currved
conta
View Article Online
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and multilayer carrbon. The exissting mesopores allow for the
d diffusion of electrolyte ioons. And it can
n be clearly seen
s
rapid
that the pore size broadens withh the increasing temperatu
ures.
he same amplification, the ddensity of the
e ACs is obserrved
At th
decre
easing and the average porosity in
ncreases as the
temp
peratures incrrease due too the higher degree of KOH
K
etching.
The
T
porosity was
w further ccharacterized by the nitro
ogen
adsorption‐desorption isotherm al analysis, as seen in the Fiig. 3.
0.2
0.4
0.6
0.8
Relative Pre
essure (P/P0)
1.0
1
10
Pore Size (nm
m)
SBEET (m2 g-1)
Vtotal (ccm3 g-1)
Daveragee (nm)
AC-5
550
1591
0
0.7
1.9
9
AC-6
650
3047
1
1.6
2.3
3
AC-7
750
3068
2
2.0
2.8
Fig. 3 (a) Nitroge
en adsorption isotherms at 77 K, (b) NLD
DFT pore‐size distribution
d
an
nd (c) pore chaaracteristics of
o AC‐550, AC‐‐650
and
d AC‐750.
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AC-850
AC-750
AC-650
AC-550
AC-450
1000
1500 2000 250
00
-1
1
Raman
n Shift (cm )
3000
Fig. 4 Raman sp
pectra of ACs obtained at different
d
activvation
tem
mperatures.
Table
e 1 Chemical composition
c
off ACs at different temperatu
ures
View Article Online
DOI:
10.1039/C6TA06337C
deterrmined by com
mbustion analyysis.
C (%
%)
N (%)
H (%)
S (%)
Others (%)
AC‐450
62.3
32
9.01
3.10
0.35
25.22
AC‐550
70.0
02
5.27
2.95
0.26
21.50
AC‐650
85.5
57
2.28
1.13
0.33
10.69
AC‐750
90.0
03
1.67
0.77
0.35
7.18
AC‐850
91.16
0.81
0.58
0.32
7.13
Fig. 5 EDX spectrum of AC‐550.
‐1
‐1
garded as the
e D
nearlly 1320 cm and 1590 ccm are reg
(disorder and defe
ects) and G ((graphitic) ban
nds, respectivvely.
w evaluated
d by
The degree of ordering in thee materials was
nsity ratio betw
ween D and G band. The in
ntensity of G peak
p
inten
is a litter higher tha
an that of D peeak, indicating
g that the ACs are
ed. The ID/IG ratio is calculated to be 0.93
0
partially graphitize
450), 0.93 (AC
C‐550), 0.94 (A
AC‐650), 0.98 (AC‐750) and 1.0
(AC‐4
(AC‐8
850). It is wortth noting thatt the ID/IG ratio
o changes slightly
with the increasing
g temperaturees. It is known
n that during KOH
K
activation, pores are created byy etching of the carbon and the
35
ore,
latticces are expanded by inteercalation off K. Therefo
activation leads to
o the increase of disorder and defects in the
struccture. While on the othher hand, high temperatture
anne
ealing would favour
f
to prom
mote the stru
uctural alignm
ment
and orders.
o
The fin
nal graphitic ddegree is the trade‐off betw
ween
KOH activation and
d high‐temperrature carbonization.
Combustion
C
elemental aanalysis wa s employed to
invesstigate the composition
c
oof the ACs. The results are
deteccted by pyroly
ysis the samplees and analysiis produced ga
ases.
Beyo
ond the elemen
nts listed in thhe Table 1, the
e amount of otther
elements, such as O, are presennted in the forrm of others. It is
clearly seen that the N‐amountt decreases wiith the increassing
temp
peratures.28, 344 The AC‐450 has the highest N‐amount of
Thiss journal is © Th e Royal Society of
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J. Name ., 2013, 00, 1‐3
3|4
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Journal of Materials Chemistry A Accepted Manuscript
Porre size distribu
ution simulate
ed by nonlocal density funcctional
the
eory (NLDFT) a
and the resultt is shown in the Figure 3b‐cc. It is
obvvious that the
e temperature
e makes a big
g difference o
on the
surrface area and pore size distribution.
d
All
A the ACs sh
how a
typ
pe‐I sorption issotherm, whicch exhibit the characterizat ion of
mo
onolayer‐multiilayer adsorption with a hyssteresis loop a t high
pre
essure.33 A stteep slope below
b
P/P0 = 0.5 indicatee the
existence of abu
undant micro
opores. For AC
C‐650 and AC
C‐750,
the
ey both show
w a broad kn
nee at low pressure, wh ich is
reg
garded to th
he existing of plentiful mesoporous.. The
em
merging hyste
eresis loop results fro
om the cap
pillary
con
ndensation in
oporous. As the temperrature
n the meso
increases form 5
550 oC to 650 oC, the surface
e area and thee pore
AC‐550
volume increase nearly two times. The surfface area of A
2 ‐1
d AC‐650 wass calculated to
o be 1591 and
d 3047 m g , with
and
the
e pore volume
e of 0.7 and 1.6 cm3 g‐1, resp
pectively. How
wever,
further increase the temperature, the surfface area and
d pore
volume increase slightly. The surface
s
area of
o AC‐750 wass 3068
2 ‐1
pore volume off 2.0 cm3 g‐1. It indicates thaat 650
m g with the p
o
C is enough for the activation. AC‐550 possesses
p
a naarrow
mber of micro
opores
porre size distribution in which a large num
are
e less than 2 nm. While AC‐6
650 and AC‐75
50 have a widee pore
size
e distribution and contain both
b
microporres and mesop
pores.
Inccreasing the te
emperature, from
f
550 oC to
t 750 oC, nott only
increases the su
urface area, but also wid
dens the poree size
o
C, the
distribution. Further increase the temperature to 850 C
2
00 m /g, but th
he carbon yie ld will
surrface can keptt at about 300
be lower than A
AC‐650 and AC‐750
A
due to
o the more seeverer
34
ults are in goo
od agreementt with
etcching of KOH. These resu
the
e observation of TEM. It turrns out that KOH
K
activation
n is an
effective metho
od to create the micropores and activvation
tem
mperature playys an important role on the pore size.
The graphitizzation degree of these samples was estim
mated
by the Raman sp
pectra. As see
en in Fig. 4, tw
wo peaks locatted at
Raman Intensity (a. u.)
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a
b
N-6
N-Q
406
d
404 402 400 398
Banding Energy (eV)
S 2p
O 1s
C=O
536
534
532
530
Banding Energy (eV)
396
528
C-S
S-O
172
S-S
170 168 166 164 162
Banding Energy (eV)
160
Fig. 6 XPS spectra of AC‐550. (a) High‐resolution C 1s, (b) N 1s, (c) O 1s and (d) S 2p.
9.01%. When the temperature was increased to 550 oC, the N‐
amount decreased to 5.27%. Higher temperatures lead to
lower N‐amount, so that AC‐650, AC‐750 and AC‐850 show the
N‐amount of 2.28%, 1.67% and 0.81%, respectively. These
specimens also contain some amount of S, which do not
change with the temperatures. Otherwise, some other
elements are found in all these samples and their amount
decreases with the temperature increasing. The AC‐450 shows
the largest amount of others with an amount of 25.22%, which
in fact contains a large number of oxygen due to the
incomplete carbonization of the precursor in such a low
temperature. EDX spectrum of AC‐550 is given in the Fig. 5.
Only three peaks can be detected that are identified as C, N
and O, indicating the others in the Table 1 mainly refers to the
oxygen amount.
To get more detailed information of the surface functional
group, XPS measurements was conducted on AC‐550. The
Table 2 Surface content of nitrogen species by fitting the N1s
AC‐450
AC‐550
AC‐650
AC‐750
AC‐850
N‐5
77.34
75.8
72.49
69.80
68.68
N‐6
15.91
17.59
18.97
23.7
24.13
N‐Q
6.76
6.61
8.53
6.45
7.18
high‐resolution C 1s peak is shown in the Fig. 6a, containing C‐
C peak at 284.4 eV, C‐N peak at 285.1 eV, C‐O peak at 286.1 eV
36
and C=O peak at 288.6 eV. The N 1s (Fig. 6b) can be
deconvoluted into three peaks, pyrrolic nitrogen (N‐5 centered
at 400.2 eV), pyridinic N (N‐6 centered at 398.3 eV) and
37, 38
The relative
quaternary N (N‐Q centered at 403.0 eV).
percentages of N functionalities are given in the Table 2. The
amount of N‐5 decreases with the temperature while the
amount of N‐6 increases. It attribute to the different stability
of N‐5 and N‐6. It has been report that the N‐6 is more stable
than N‐5 at high temperature.39 The amount of N‐Q changes
slightly in spite of the different activation temperatures. It is
well known that N‐5 and N‐6 are the main configuration to
contribute the pseudocapacitance and N‐Q can enhance the
40, 41
In the O 1s spectra (Fig. 6c), three
conductivity of carbon.
peaks located at 531.4 eV, 532.9 eV and 534.2 eV represent
the carbonyl groups (C=O), phenol‐type groups (C‐
OH)/ethertype groups (C‐O‐C) and chemisorbed oxygen and/or
adsorbed water (Chemisorbed O) which can enhance the
37, 38, 42
The S
wettability and contribute to pseudocapacitance.
2p spectrum is shown in the Fig. 6d. The peak of S 2p can be
splitted into two components, 2p3/2 and 2p1/2, with the 2:1
intensity and 1.2 eV energy difference. The 2p3/2/2p1/2 peaks at
162.7 eV/163.9 eV, 163.7 eV/164.9 eV, and 167.8 eV/169.0 eV
12, 43
.
are assigned to S‐S, C‐S and S‐O species, respectively.
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry A Accepted Manuscript
Intensity (a. u.)
282
Chemisorbed O
538
N-5
Intensity (a. u.)
Intensity (a. u.)
290 288 286 284
Banding Energy (eV)
C-O
Intensity (a. u.)
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C-N
C=O C-O
c
N 1s
C 1s
C-C
292
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-1
5 mV s , 1M KOH
-1
AC-450
AC-550
AC-650
AC-750
AC-850
-3
-4
-1.0
-0.8
-0.6
-0.4
-0.2
Potential (V, vs. Hg/HgO)
0
0.0
)
-1
-0.6
d
1
-2
-0.4
0.4
2
0
-0.2
100 200 300 400 500 600 700
Time (s)
AC-450
AC-550
AC-650
AC-750
AC-850
-0.2
-0.4
-0.6
-0.8
0
100 200 300 400 500 600
Time (s)
AC-550
AC-650
AC-750
AC-850
400
300
200
100
0
0
f
5
10 15 20 25
-1
Current density (A g )
30
AC-450
AC-550
AC-650
AC-750
AC-850
400
300
200
100
0
0
5
10 15 20 25
-1
Current density (A g )
30
Fig. 7 Electrochemical performances of swim bladders derived carbons in a three‐electrode setup. CVs in (a) 1 M H2SO4 and (b) 1 M KOH;
‐1
GCD curves at 1 A g in (c) 1 M H2SO4 and (d) 1 M KOH; Capacitances at different current densities in (e) 1 M H2SO4 and (f) 1 M KOH.
These signals indicate the N, O, and S heteroatoms are
bounded into the carbon framework or attached at the edge
With such high surface area and multiple heteroatom doping,
the ACs derived from swim bladders are expected as promising
electrode materials for supercapacitors.
To evaluate the electrochemical performances of these
synthesized carbon materials, CV was first carried out in both
acid (1 M H2SO4) and base (1 M KOH) electrolytes using a
three‐electrode system. The results are shown in the Fig. 7a‐b.
Unlike the rectangular shapes of typical electrical double layer
capacitors, these curves show distinct humps in acid and base
solutions. The CVs of low temperature activated carbon (AC‐
450, AC‐550 and AC‐650) show prominent redox peaks, which
located at ‐0.2 to 0 V (vs. Hg/Hg2SO4) in H2SO4 and ‐0.6 to ‐0.4
V (vs. Hg/HgO) in KOH. These redox peaks indicate the
contribution of pseudocapacitance by the surface reaction of N,
O and S functional groups. Among them, the curve of AC‐450
shows the biggest hump that overlays the contribution of
electrical double layer capacitance. This can be attributed to
its high N amount (9.01%), high oxygen amount and low
conductivity. Due to the low heteroatom amount and high
conductivity, the CVs of high temperature activated carbon
(AC‐750 and AC‐850) show quasi‐rectangular shape.
Interestingly, the peak of AC‐850, located in ‐0.2 V, in H2SO4
Table 3 The reported capacitance for activated carbon obtained from different biomasses.
Precursor
SBET (m2 g‐1)
C (F g‐1)
Electrolyte
Reference
Pomelo peel
Chicken eggshell membranes
Chicken eggshell membranes
Willow catkin
Shrimp shells
Enteromorpha prolifera
Yogurt
Gelatin
Cotton
Egg white
Swim bladders
Swim bladders
2725
221.2
221.2
1589
1946
1240
1300
416
1716
805.7
3068
3068
342
297
284
298
322
296
225
358
175
390
419
350
6 M KOH
1 M KOH
1 M H2SO4
6 M KOH
6 M KOH
30% KOH
1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M KOH
25
30
30
44
45
46
47
13
48
29
This article
This article
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 6
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Journal of Materials Chemistry A Accepted Manuscript
-0.6 -0.4 -0.2
0.0
0.2
Potential (V, vs. Hg/Hg2SO4)
0.0
Capacitance (A g
-3
0.2
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AC-450
DOI: 10.1039/C6TA06337C
-1
)
-1
AC-450
AC-550
AC-650
AC-750
AC-850
-2
AC-450
AC-550
AC-650
AC-750
AC-850
Capacitance (F g )
-1
b
Current density (A g
-1
5 mV s , 1M H2SO4
0
Potential (V, vs. Hg/Hg2SO4)
1
-4
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e
c
2
Potential (V, vs. Hg/HgO)
-1
Current density (A g )
a
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a 140
b
80
14
12
20
550
650
750
850
4
0
0
0
20
40
80
14
60
12
10
6
2
0
100
40
8
2
4
6
8
-1
Z' ohm
10
12
60 80 100 120 140
-1
Z' ohm
AC-550
AC-650
AC-750
AC-850
-80
-60
-40
-20
0.1
1
10
Frenquency (Hz)
100
6
550
650
750
850
2
0
0
0
14
8
4
20
0
20
40
2
4
6
8
-1
Z' ohm
10
12
14
60 80 100 120 140
-1
Z' ohm
d-100
Phase angle (degree)
40
0
0.01
-1
-1
10
AC-450
AC-550
AC-650
AC-750
AC-850
120
Z'' ohm
60
Z'' ohm
Z'' ohm
-1
100
140
Z'' ohm
AC-450
AC-550
AC-650
AC-750
AC-850
120
c-100
capacitance decrease as the result of inadequate
surface
View Article Online
DOI:inner
10.1039/C6TA06337C
reaction and difficult ion diffusion into the
pores. At the
current of 30 A g‐1, the AC‐550 remains capacitance of 203 F g‐1
‐1
in 1 M H2SO4 and 296 F g in 1 M KOH. AC‐750 and AC‐850
have more excellent rate performance, which can be relate to
the amounts of mesoporous and high conductivity. The
capacitance of these swim bladders derived ACs is better than
most of the reported biomass‐derived carbon, as shown in
Table 3. The colossal capacitance of swim bladders derived
carbon comes from the multiple heteroatom doping effect and
the high surface area.
Fig. 8 gives the Nyquist plot of the ACs. The typical curves
show a semicircle at high frequency and an approximate
vertical line at low frequency. The intercept on the real axis
represents the internal resistances, which consists of three
parts: electrolyte resistance, electrode resistance and interface
resistance between active materials and current collector. As
shown in the Fig. 8a‐b, there is no obvious difference of the
equivalent series resistance. The diameter of the semi‐circles
decreases with the increasing temperature, which indicates
the decreased charge transfer resistance due to the enhanced
47
conductivity. The vertical line along the Z’’ axis is the
characteristics of ideal capacitive behavior of electrical double
layer capacitors. The deviation of AC‐450 at the low frequency
indicates that it is no longer a capacitive behavior, also can be
-1
shifts to negative potential, which may attribute to the
different amount of N‐5 and/or N‐6.41 The areas of AC‐550 are
the biggest while the curves of AC‐450 are highly distorted.
The capacitance of materials consists of two parts: electrical
double layer capacitance stored on the interface of
electrolyte/materials and pseudocapacitance from the quick
redox reaction of functional groups on the surface.
The pseudocapacitance also causes the distortion of the
GCD curves, as shown in the Fig.7c‐d. As same as the CV results,
the AC‐550 shows the largest capacitance. Calculated by the
‐1
discharge time at 1 A g , the capacitances of AC‐450, AC‐550,
‐
AC‐650, AC‐750 and AC‐850 are 306, 385, 338, 300 and 230 F g
1
‐1
in 1 M H2SO4 and 326, 346, 328, 246 and 232 F g in 1 M KOH,
respectively. The highest capacitance of AC‐550 is attributed to
its high heteroatom doping and high surface area. Though AC‐
650, AC‐750 and AC‐850 possess higher surface area than AC‐
550, their low heteroatom amount would contribute a low
pseudocapacitance. As for AC‐450, the low activation
temperature results in a low surface area and low conductivity,
thus it delivers poor capacitive performance.
Fig. 7e‐f give the capacitance at different current densities.
‐1
At the current density of 0.5 A g , the capacitance of the AC‐
450, AC‐550, AC‐650, AC‐750 and AC‐850 are 320, 419, 352,
‐1
312 and 239 F g in 1 M H2SO4 and 326, 350, 330, 255 and 237
‐1
F g in 1 M KOH. With the increasing current density, the
AC-550
AC-650
AC-750
AC-850
-80
-60
-40
-20
0
0.01
0.1
1
10
Frenquency (Hz)
100
Fig. 8 Electrochemical impedance spectra measured in (a) 1M H2SO4 and (b) 1M KOH. Bode plots measured in (c) 1M H2SO4 and
(d) 1M KOH.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 7
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Journal of Materials Chemistry A Accepted Manuscript
ARTICLE
Phase angle (degree)
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0.2
0
2000
4000 6000
Cycles
8000
10000
e
1.0
0.8
1 M H2SO4
0.6
1 M KOH
0.4
0.2
0.0
0
2000
4000 6000
Cycles
8000
10000
0.6
0.4
0.2
0.0
0
2000
4000 6000
Cycles
8000
10000
f
1.0
0.8
1 M H2SO4
1 M KOH
0.6
0.4
0.2
0.0
0
2000
4000 6000
Cycles
8000
10000
View Article Online
1.0
DOI: 10.1039/C6TA06337C
0.8
1 M H2SO4
0.6
1 M KOH
0.4
0.2
0.0
0
2000
4000 6000
Cycles
8000
10000
350
300
250
200
AC-450
AC-550
AC-650
AC-750
AC-850
150
100
50
0
0
2000
4000 6000
Cycles
8000
10000
Fig. 9 Cycle performance of (a) AC‐450, (b) AC‐550, (c) AC‐650, (d) AC‐750 and (e) AC‐850 measured in 1 M H2SO4 and KOH, (f)
cycle performance of ACs in 1 M H2SO4.
seen in the CV and GCD measurements. As seen in the Fig. 8c‐d,
from AC‐550 to AC‐850, the phase angle gradually close to 90o,
indicating that their behaviours are more and more close to
the ideal electrical double layer capacitor.
The long‐term cycling performance is one of the important
factors of supercapacitors. It was evaluated by the GCD at the
‐1
current density of 5 A g . As shown in the Fig. 9, the long cycle
performance was carried out for all the samples in both H2SO4
and KOH electrolyte. All samples, from AC‐450 to AC‐850,
show excellent stability in the acid electrolyte with the
capacitance maintain at 86% above upon 10000 cycles. The
capacitance lose mainly occurs at the initial 100 cycles and
after that, it can keep stable with slight fluctuation during the
following cycles. The long‐time stability comes from the self‐
doping of heteroatoms, which doped in the carbon framework
by covalent bonds. However, in base electrolyte, the
capacitance maintains lower than that in the acid electrolyte.
The capacitance decrease quickly at the initial 100 cycles and
then gradually decrease during the cycling. The different
cycling performance can be attributed to the different stability
of functional groups in acid and base electrolytes.
Conclusions
In summary, multi‐heteroatom self‐doped carbon derived
from swim bladders was synthesized by carbonization and KOH
activation. Different activation temperatures were conducted
to study the effect on the structure and electrochemical
performance of the materials. As the activation temperature
increase, the density of carbon materials reduce for the
etching of KOH, the pore size broadens and the doped
amounts of heteroatoms decrease rapidly. A largest surface
2 ‐1
area of 3068 m g can be obtained for these swim bladders
derived carbon. Among the samples obtained at different
activation temperature, the as‐synthesized materials activated
o
‐1
at 550 C shows the largest capacitance of 410 F g and good
stability upon 10000 cycles. This large capacity is attributed to
the contribution of pseudocapacitance derived from the high‐
amount doped heteroatoms as well as the double layer
capacitance from high surface area. The high‐amount
heteroatom doping, high surface area, excellent stability of
structure and convenient synthesis route endow this carbon a
promising material for supercapacitors.
Acknowledgements
We acknowledge the support from the National Basic Research
Program of China (973 program, 2015CB932600), the National
Natural Science Foundation of China (21571073, 51302099,
51551205), the Program for New Century Excellent Talents in
University (NCET‐13‐0227), the Program for HUST
Interdisplinary Innovation Team (2015ZDTD038) and the
Fundamental Research Funds for the Central University. The
authors also thank the Analytical and Testing Center of HUST
for the measurements.
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Journal of Materials Chemistry A Accepted Manuscript
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View Article Online
DOI: 10.1039/C6TA06337C
Multi-heteroatom self-doped porous carbon is synthesized via carbonization and
activation of the amino-acids-riched swim bladders, it shows a large capacitance when
applied in supercapacitors.
Journal of Materials Chemistry A Accepted Manuscript
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