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10 mv/s 50 mv/s 100 mv/s 500 mv/s 1000 mv/s

Potential (V)

5 nm5 nm

10.333 m(b)

800 1600

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nsity

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What is carbon nano-onion

Experiment set-up

Controllable Growth of Carbon Nano-Onions for Developing High-Performance Controllable Growth of Carbon Nano-Onions for Developing High-Performance SupercapacitorsSupercapacitors

Y. Gao, Y. S. Zhou, J. Hudgins,Y. Gao, Y. S. Zhou, J. Hudgins, and Y. F. Lu*and Y. F. Lu*Department of Electrical Engineering, University of Nebraska, LincolnDepartment of Electrical Engineering, University of Nebraska, Lincoln

E-mail: E-mail: ylu2@unl.edu website: http://lane.unl.edu website: http://lane.unl.edu

Fresh Onion: edibleCarbon Nano-Onion: non-edible

Carbon nano-onions (CNOs) are concentric multilayer giant fullerenes, which consist of multiple concentric graphitic shells to form encapsulated structures

Motivations

High specific surface area

High electrochemical stability

High electronic conductivity

Emergency doors on jet planesEmergency doors on jet planes Hybrid autoHybrid auto

Backup power supplyBackup power supply Wind energy storageWind energy storage

Battleship ignitionBattleship ignition

Experiments and results

Synthesis of CNOs Capacitive properties of CNOs Capacitive properties of MnO2/CNO hybrid structure

Illustration of the experimental setup for CNO growth with resonant excitation by a wavelength-tunable CO2 laser.

10.532 m10.333 mWithout laser

Photographs of ethylene-oxygen flames under laser excitation (The images below show molecular vibration under the excitation conditions).

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.

Without laser

10.333 m

10.532 m2D

CNOs 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

DG

10.532 m

5 nm5 nm

Experiment results

1500 2000 2500 3000

2400 2700 3000

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5 nm5 nm

5 nm5 nm

10.532 µm-400 W

10.532 µm-600 W

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.

5 nm5 nm

Without laser(a) (c)

1500 2000 2500 3000

In

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Raman shift (cm-1)

(d)

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.

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.

Without laser(a) (b)

(c) (d)

(e)

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 N℃ ℃ 2 atmosphere for 1 h.

Capacitive properties of CNOs before KOH activation

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-0.4 -0.2 0.0 0.2 0.4-45

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(a) Specific surface areas (SSAs) of CNOs grown at different laser powers. (b - d) Cyclic voltammograms of CNO electrodes.

(a) (b) (c) (d)400 W: 16 F/g 800 W: 25 F/g 1000 W: 30 F/g

Capacitive properties of CNOs after KOH activation

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.

1 10 100

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Without activation 4M KOH activation 5M KOH activation 6M KOH activation 7M KOH activation

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(a) SSAs and (b) Pore size distributions of CNOs activated at different KOH concentrations.

The SSAs increase with the increase of KOH concentration. After activation, pores (<= 5 nm) contribute significantly to the total pore volume of CNOs.

It is suggested that the activation of carbon with KOH proceeds as

6KOH + C ↔ 2K +3H2 + 2K2CO3 Then it followed by decomposition of K2CO3 and/or reaction of K/K2CO3/CO2, with carbon.

5nm5nm 5nm5nm

(b)(a)

TEM images of CNOs (c) before activation (d) after 6M activation.

Before activation After activation(c) (d)

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6 M activation: 108 F/g

Before activation: 25 F/g

6 M activation

Before activation

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ap

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(a) Capacitance of CNOs after KOH activation. (b) Galvanostatic charge/dischargecurves of CNOs before and after 6 M KOH activation. (c) Cyclic voltammograms of CNOs before and after 6 M KOH activation.

(a) Cyclic voltammograms and (b) the capacitances of CNOs after 6 M KOH activation at different scan rates.

Deposition of MnO2 on CNOs

Experiment steps:

• 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.℃

Capacitive properties of MnO2/CNO hybrid structure

Electrophoretic deposition of CNOs on Ni foamsElectrophoretic deposition of CNOs on Ni foams

• 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.

Deposition of MnODeposition of MnO22 on CNO/Ni foam on CNO/Ni foam

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-600

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6 h MnO2 deposition 12 h MnO2 deposition(c) (d)

12 h deposition

6 h deposition

3 h deposition

Without deposition

12 h deposition

6 h deposition

3 h deposition

Without deposition

MnO2

(e) (f)

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 .

(a) Capacitance of CNOs after MnO2 deposition. (b) Galvanostatic charge/dischargecurves of CNOs before and after 12 h MnO2 deposition. (c) Cyclic voltammograms of CNOs before and after 12 h MnO2 deposition. .

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(a) Cyclic voltammograms and (b) the capacitances of CNOs after 12 h MnO2 deposition.

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(b)(a)

(c)

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(b)(a)

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CNOs deposited onto Ni foam

MnO2 deposited onto CNO/Ni foam

Future directions Acknowledgements

Scalable production of CNOs

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.

High-performance supercapacitors using hierarchical three-dimensional 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.

(e)

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12 h deposition: 313 F/g

Before activation: 25 F/g

12 h deposition

Before activation500 nm500 nm

500 nm500 nm 500 nm500 nm

500 nm500 nm

CNOs are promising electrode material in fabricating high-performance supercapacitors

Ragone plot: www.imechanica.org

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.