Controllable Growth of Carbon Nano-Onions for Developing High-Performance
Supercapacitors
Y. Gao, Y. S. Zhou, J. Hudgins, and Y. F. Lu*
Department of Electrical Engineering, University of Nebraska, Lincoln
E-mail: ylu2@unl.edu
website: http://lane.unl.edu
What is carbon nano-onion
Motivations
 CNOs are promising electrode material
in fabricating high-performance
supercapacitors
Carbon nano-onions (CNOs) are concentric multilayer giant
fullerenes, which consist of multiple concentric graphitic
shells to form encapsulated structures
Emergency doors on jet planes
Hybrid auto
Battleship ignition
 High specific surface area
 High electrochemical stability
 High electronic conductivity
Carbon Nano-Onion: non-edible
Fresh Onion: edible
Ragone plot: www.imechanica.org
Backup power supply
Wind energy storage
Experiments and results
Capacitive properties of CNOs
A simple method was used to activate the primitive CNOs by using KOH solution to achieve the increased specific surface areas of CNOs.
In brief, (1) CNOs was firstly impregnated in KOH solution for 24 h; (2) The solution was filtered to get the impregnated CNOs. (3) The
obtained CNOs was dried in an oven for 12 h at ~90 ℃; (4) At last, the CNOs were annealed at ~800 ℃ in N2 atmosphere for 1 h.
Capacitive properties of CNOs before KOH activation
(c)
400 W: 16 F/g
10
0
-10
40
800 W: 25 F/g
30
Capacitance (F/g)
Capacitance (F/g)
20
15
0
-15
-30
-0.4
-0.2
0.0
Potential (V)
0.2
0.4
1000 W: 30 F/g
-45
-0.4
-0.2
0.0
Potential (V)
0.2
0.4
Photographs of ethylene-oxygen flames under laser excitation
(The images below show molecular vibration under the excitation conditions).
0
-20
-60
-0.4
-0.2
0.0
Potential (V)
0.2
0.4
CNOs deposited onto
Ni foam
 The SSAs increase with the increase in laser power. Consequently, the capacitance of CNOs increases.
 However, CNOs with much larger SSAs are needed to achieve improved capacitances.
10.333 m
(c)
D1
(d)
10.532 µm-600 W
D3
2D
2000
-1
3000
1200
Raman shift (cm )
1500
-1
Raman shift (cm )
Summary of G-band FWHM and R3 for CNOs grown without laser excitation and with
excitation at wavelengths of 10.333 and 10.532 µm at 1000 W.
CNOs
0
2
4
6
8
KOH concentration for activation (mol/L)
1
10
Pore size (nm)
(c) Before activation
(d)
(c) 150
FWHM of G-band
(cm-1)
R3=ID3/( ID3 +ID2 +IG)
Without laser
71.4
0.24
10.333 m
64.6
0.23
10.532 m
59.5
0.19
6 M activation: 108 F/g
100
50
Before activation: 25 F/g
0
-50
-100
-150
TEM images of
CNOs (c) before
activation (d) after
6M activation.
3000
5nm
1500
2000
2500
-1
3000
Raman shift (cm )
TEM images of CNOs grown (a) without laser excitation
and with different laser powers of (b) 400, (c) 600, and (d)
1000 W at 10.532 µm. (e) Raman spectra of CNOs grown
with different laser powers.
-0.2
0.0
Potential (V)
5nm
(a)
 25 welding torches with 3 mm orifice tips will be used to generate
the flames.
 A wavelength-tunable CO2 laser at a wavelength of 10.532 µm
will be used to resonantly couple laser energy to the flame.
 Another laser at 10.591 µm will be used to control the size of the
CNOs.
 A fume collector will be used to collect CNOs generated from the
flames.
 It is estimated that a production rate of 500 g/h will be achieved.
200
Then it followed by decomposition of K2CO3 and/or reaction of
K/K2CO3/CO2, with carbon.
(c)
Before activation
0
10 mv/s
50 mv/s
100 mv/s
500 mv/s
1000 mv/s
-0.3
110
0.3
6 h deposition
3 h deposition
800
1600
-1
Raman shift (cm )
90
80
0.0
Potential (V)
12 h deposition
Without deposition
100
0
200
400
600
800
Scan rate (mv/s)
1000
(a) Cyclic voltammograms and (b) the capacitances of CNOs after
6 M KOH activation at different scan rates.
3
6
9
Deposition time (h)
12
12 h deposition: 313 F/g
Before activation: 25 F/g
0
-200
Without deposition
400
800
-1
Raman shift (cm )
SEM images of CNOs (a) without MnO2 deposition and with (b) 3
h, (c) 6 h, (d) 12 h deposition. (e) Raman spectra of CNOs without
MnO2 deposition and with (b) 3 h, (c) 6 h, (d) 12 h deposition. (f)
Raman spectra of the samples in the spectra range from 200 to 1000
cm-1 .
-0.4
-0.2
0.0
0.2
0.4
Potential (V)
0.6
(a)
200
300
Time (sec)
400
0.8
10 mv
100 mv
500 mv
1000 mv
5000 mv
600
6 h deposition
3 h deposition
100
(a) Capacitance of CNOs after
MnO2 deposition. (b) Galvanostatic
charge/discharge
curves of CNOs before and after 12
h MnO2 deposition. (c) Cyclic
voltammograms of CNOs before and
after 12 h MnO2 deposition. .
200
-600
MnO2
0
(c) 600
(f)
12 h deposition
(b)
0
-400
(e)
200
-400
500 nm
500 nm
0.0
-0.4
400
(a) Capacitance of CNOs after
KOH activation. (b) Galvanostatic
charge/discharge
curves of CNOs before and after 6
M KOH activation. (c) Cyclic
voltammograms of CNOs before
and after 6 M KOH activation.
120
-200
(d) 12 h MnO2 deposition
6 h MnO2 deposition
0.4
100
100
0.4
400
It is suggested that the activation of carbon with KOH proceeds as
6KOH + C ↔ 2K +3H2 + 2K2CO3
0.2
50
Time (Sec)
0.8
12 h deposition
500 nm
500 nm
Future directions
Scalable production of CNOs
(b)
300
Capacitance (F/g)
Potential (V)
100
After activation
2D
2700
(a)
0
-0.4
2400
6 M activation
0
0
2
4
6
8
KOH concentration for activation (mol/L)
1800
TEM images of CNOs grown (a) without laser excitation and with laser excitations at
(b) 10.333 and (c) 10.532 µm. (d) Raman spectra of CNOs grown without laser
excitation and with laser excitations at 10.333 and 10.532 µm . (e) Typical curve fitting
of a first-order Raman spectrum.
-0.2
-0.4
(e)
2500
0.0
40
0.00
D4
5 nm
60
20
The SSAs increase with the increase of KOH concentration. After activation,
pores (<= 5 nm) contribute significantly to the total pore volume of CNOs.
D2
80
0.02
G
Intensity (a.u.)
1500
0.04
(a) SSAs and (b) Pore size distributions of CNOs activated at different KOH
concentrations.
5 nm
10.532 m
0.06
3 h MnO2 deposition
0.2
Intensity (a. u)
Without laser
500
0.08
100
Capacitance (F/g)
Intensity (a.u.)
D
5 nm
600
400
(e)
G
Intensity (a.u.)
(d)
5 nm
700
0.10
(b)
(a) Before MnO2 deposition
Before activation
Intensity (a. u)
5 nm
800
Without activation
4M KOH activation
5M KOH activation
6M KOH activation
7M KOH activation
(b)0.4
Capacitance (F/g)
10.532 µm-400 W
MnO2 deposited onto
CNO/Ni foam
Capacitive properties of MnO2/CNO hybrid structure
(a)120
Capacitance (F/g)
(b)
Without laser
2
(a)
3
10.532 m
(c)
Pore volume (cm /g/nm)
10.333 m
(b)
Without laser
(b)0.12
(a)
Specific surface area (m /g)
(a)
Capacitive properties of CNOs after KOH activation
Capacitance (F/g)
Experiment results
• After drying, the CNO coated Ni foams were immersed into
the precursor solution ( mixture of 0.1 M Na2SO4 and 0.1 M
KMnO4) for MnO2 coating.
• After immersing, the samples were rinsed using deionized
water and then heated at 120 ℃ for 12 h in air.
• CNOs were dispersed in ethanol. Al(NO3)3 was added into
the solution to stabilize the CNO particles in the solution;
• The suspension was ultrasonicated for 30 min;
• Then a layer of CNOs was deposited onto Ni foams by
electrophoretic deposition.
(a) Specific surface areas (SSAs) of CNOs grown at different laser powers. (b - d) Cyclic voltammograms of CNO electrodes.
Illustration of the experimental setup for CNO growth with resonant
excitation by a wavelength-tunable CO2 laser.
Deposition of MnO2 on CNO/Ni foam
Electrophoretic deposition of CNOs on Ni foams
20
-40
-30
-20
Experiment steps:
(d)
Voltage (V)
10.532 m
30
(b)
Capacitance (F/g)
10.333 m
Without laser
(a)
Deposition of MnO2 on CNOs
Capacitance (F/g)
Experiment set-up
Carbon materials have high SSAs, long cycle life, and high conductivity but low capacitance. Metal oxides have high theoretical
capacitance but suffer from low SSAs, short cycle life, and low conductivity. Metal oxide/CNO composite is a potential approach to
improve both capacitance and conductivity. In this study, capacitive properties of MnO2/CNO hybrid structure were investigated.
Capacitance (F/g)
A laser-assisted combustion process for growing CNO in open air was developed by using laser irradiations to achieve resonant excitation
of precursor molecules. The laser energy was much more effectively coupled into the flame through the resonant excitation of ethylene
molecules at 10.532 µm than other non-resonant wavelength.
Capacitive properties of MnO2/CNO hybrid structure
300
0
-300
(b)
Capacitance (F/g)
Synthesis of CNOs
300
200
100
-600
-0.2
0.0
0.2
0.4
Potential (V)
0.6
0.8
0
1000
2000 3000
Potential (V)
4000
5000
(a) Cyclic voltammograms and (b) the capacitances of CNOs after
12 h MnO2 deposition.
Acknowledgements
High-performance supercapacitors using hierarchical threedimensional micro/nanoelectrodes
 To achieve increased SSAs and reduced internal resistance at the same time;
 The Ni foams will serve as conductive scaffolds to house CNTs and CNOs to
reduce the matrix resistivity;
 The CNTs will be grown within the Ni foams to reduce the matrix resistivity
and increase the SSA significantly;
 CNOs will be used to fill the remaining spacing among the Ni forms and to
further increase the total SSA;
 Since all CNTs and CNOs will be filled within the Ni foams, no binder is
required.
The authors gratefully thank Nebraska Center for Energy
Sciences Research (NCESR) and National Science
Foundation (NSF) for financial support.