Carbon Nanostructured for Energy Storage

Bingqing (B. Q.) Wei

Department of Mechanical Engineering

University of Delaware

Newark, DE 19716, USA

Email: weib@udel.edu

http://www.me.udel.edu/Faculty/wei_group/index.html
NIST Workshop 2011BQWei 1
[1] S.Iijima, Nature, 1991, 354, 56.

[2] S.Iijima et al., Nature, 1993, 363, 603.

[3] D.S.Bethune et al., Nature, 1993, 363, 605.

[4] A.Thess et al., Science, 1996, 273, 483.

[5] C.Journet et al., Nature, 1997, 388, 756.

[6] H.Zhu et al., Science, 2002, 296, 884. 

C60

MWNT [1] SWNT [2,3]

1985

1991

1993

1996

1997

Arc [5] Laser [4]

[7] Z. Wu et al., Science, 2004, 305, 1273.

[8] K. Hata et al., Science, 2004, 306, 1362.

[9] M. Zhang et al. Science, 2004, 306, 1358. 

[10] M. Zhang et al., Science, 2005, 309, 1217.

2002

2004

2004

1-D Strands [6]

2-D Film [7] 3-D Arrays [8]

Yarns [9]

2005Macro-sheet[10]

2004

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Assembly of nanoscale building blocks (CNTs) 

into controlled 2-D multifunctional macroscopic 

structures for various applications 

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Properties:

• Low resistivity

Ballistic Electron Conduction

• Small Dimensions

• Mechanical Robustness

• Chemical Stability

• Structural Stability

• High Thermal Conductivity

Low Dissipation

• High Current Densities (~109 A/cm2)

• High Surface Areas

3

Why CNT Films for Supercapacitors?
Films are synthesized by utilizing

a liquid free precursor system in

which ferrocene/sulfur powders

(atomic ratio Fe: S = 10: 1) are

mixed uniformly and evaporated in

a CVD reactor under argon

atmosphere

Synthesis

• Si 

•Copper

• Stainless steel

• Aluminum

• Nickel mesh

• Polymer

• ...

Zhu and Wei

Chem. Comm., 2007

Synthesis of Flexible CNT Macro-Films 

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• Characteristic of SWNT film: demonstrates structural combination of 

mesoporous and macroporous materials. 

• Compared with nanoporous carbon materials, counterions are 

expected to easily and quickly reside at the SWNT electrode/electrolyte 

interface without having to enter the pores, leading to an exohedral 

supercapacitor with a higher power density feature.

2 4 6 8 10 12 14

0.00

0.08

0.16

0.24

0.32 Pore Volume (cm3/g)

Pore Width (Ao)

Surface area of SWNT film: 700 m2/g

Structural Features of SWNT Films

5
SWNT 

bundles

Flexible, Machinable, Freestanding SWNT films

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α=148o α=85o α=121o

H2O

H2O

H2O

H2O

C2H5OH

C2H5OH

C2H5OH

DWNT-1 DWNT-2 DWNT-3

α=70o

Hydrophilicity 

Tuning

Hydrophobic

Hydrophilic

Wei, et al.,

Adv. Mater., 

18, 1695 (2006).

Heat treatment in air at 450 0C for

half hour to remove amorphous

carbon

Treatment in 100% hydrochloric

acid at room temperature for half

hour to remove catalyst particles

Purification

Hydrophilicity

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Schematic Representation of Electrical 

Double Layer Capacitor

Separator

Current collector

(+) (-)

Porous electrode

(SWNT film)

Electrolyte 

(TEABF4/PC)

Positive charge

Negative charge

Anions

Cations

Electrical double 

layer

Electrolyte: Tetraethylammonium tetrafluoroborate/Polypropylene carbonate

Charge-Discharge cycles: Diffusion and charge accumulation near the electrodes

8

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Comparison of Li Ion Battery and 

Supercapacitor

Rechargeable Li battery

• Involves chemical reactions

• Long duration charge discharge 

cycles (several hours)

• High energy density low power 

density device

• Low self discharge

• Life time: 1000 - 2000 cycles

• Typical energy density: 10 – 300 

Wh/kg

• Typical power density: 100 – 1000 

W/kg

Supercapacitor

• No major chemical reactions 

involved 

• Short duration charge discharge 

cycles (few seconds to minutes)

• High power density-low energy 

density device

• High self discharge

• Life time: hundreds of thousands 

of cycles

• Typical energy density: 0.1 – 5 

Wh/kg

• Typical power density: 1000 –

1000,000 W/kg

9

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Punching Nanotube Electrodes

Purified SWNT films are dispersed

in water and are deposited on a

wire mesh to get a free standing

film

SWNT electrodes are directly

obtained by punching using an arc

punch with required diameter

Flexible, Machinable, Freestanding SWNT films

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Motivation

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Wavy Silicon Structures

Khang, Jiang, Huang, Rogers, Science, 2006

• Flat Si ribbon becomes buckled due to pre-strained PDMS

Fabricate thin ribbon

Si device elements

Si

Bond elements to prestrained

elastomeric substrate PDMS

PDMS

Peel back PDMS;

flip over

pre dL L

pre

PDMS

mother wafer

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Buckled CNT-Macro-films on PDMS

4 m

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Adv. Mater., 2009.

13
Interface Bonding

• Surface treatment on PDMS substrate

Ultraviolet/ozone (UVO) treatment to 

oxide PDMS surface and generate active 

OH- group 

• Functional group in CNT macro-films

contact condensation reaction

PDMS CNT macro-films

interface PDMS

CNT macro-film

hole

FIB/SEM image 

3 m

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4 m

14
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-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-0.0016

-0.0008

0.0000

0.0008

0.0016

0.0024 Stretchable supercapacitor

Stretchable supercapacitor

 subjected to 30% applied strain

Scan rate: 1000 mV/s

Current (A)

Voltage (V) 0 200 400 600 800 1000

0

15

30

45

0

15

30

45

Stretchable supercapacitor

Supercapacitor

Specific capacitance (F/g)

0

15

30

45

Stretchable supercapacitor

subjected to 30% applied strain

I = 1 A/g

I = 1 A/g

Cycle number

I = 1 A/g

100 1000 10000

10

8

6

4

2

Stretchable supercapacitor

Stretchable supercapacitor

 subjected to 30% applied strain

Supercapacitor

Energy density (Wh/Kg)

Power density (W/Kg)

Reversibly Stretchable Supercapacitor 

15
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Wettability and Ion-size Effect of 

Different Aqueous Electrolytes

ACS Nano, 2010

16

Electrochemical Behavior of SWNT Supercapacitors 

under Compressive Stress
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Pressure (psi) Knee frequency (Hz)

LiOH NaOH KOH LiCl NaCl KCl KNO3

0.37 0.16 0.16 0.16 4 4 13 4 

1.12 0.85 0.85 0.85 13 13 34 13

6.58 41 41 41 72 72 72 72 

19.74 126 126 126 385 385 385 385

118.44 672 672 672 672 672 672 672

250.04 1172 1172 1172 1172 1172 1172 1172

Knee frequency (experiment data) vs. Pressure

An Excellent High-power Capability under Pressure

17
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-0.0010

-0.0005

0.0000

0.0005

0.0010

25 0C

50 0C

75 0C

100 0C Current (A)

Voltage (V)

Scan rate: 50 mV/s

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

25 0C

50 0C

75 0C

100 0C Current (A)

Voltage (V)

Scan rate: 1000 mV/s

Capacitance obtained from CV at Different 

Temperatures with Different Scan Rates

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Masarapu, et al., ACS Nano, 2009

18
0 2500 5000 7500 10000

0

5

10

15

20

25

30

25 0C

100 0C Capacitance (F/g) 50 A/g

100 A/g

Cycle Number

20 A/g

0

6

12

18

24

30

25 0C

100 0C

0

7

14

21

28

35

25 0C

100 0C

c

0 2 4 6 8 10 12 14

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Over Shoot Voltage (V)

Time (sec)

Over Shoot

20 A/g

100 oC

a

0 1 2 3 4 5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0 Voltage (V)

Time (sec)

100 oC

100 A/g

b

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Super-high Power Density
Long Lifetime Supercapacitors at a High Temperature

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0 80000 160000 240000

0

8

16

24

0

8

16

24

20 A/g

25 oC Efficiency: 82%

Capacitance (F/g)

Cycle number

20 A/g

100 oC Efficiency: 84% 20

Masarapu, et al., ACS Nano, 2009
SWNT Supercapacitor

0.1 1 10 100 1000

0.1

1

10

100

1000

DOE

target

SWNT capacitor 

from this experiment

Batteries

Carbon capacitors

Metal oxide capacitors Energy density (Wh/kg)

Power density (kW/kg)

Flywheels

Department of Energy 

(DOE) Target

Ragone Chart

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100 1000 10000

0.01

0.1

1

10 Energy density (Wh/Kg)

Power density (W/Kg)

25 oC

 100 oC

21
The best energy retention capability of SCs 

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We have studied and understood that there 

are different mechanisms over selfdischarge of capacitors, indicating 

combination of electrode/electrolyte 

governs the self-discharge mechanism. By 

selecting the right combination of 

electrode/electrolyte, more than 70%

charged energy retained after selfdischarge for 36 hrs

22
Si subtrate MOSFET

Gate CMOS via

CMOS wiring SiO2 Insulation CMOS to CNT Via SWNT film Polymer electrolyte film SWNT film Drain Source

The CNT SCs will be attached to the 

top of CMOS chip with 

interconnection between them, using 

one extra post-CMOS process. 

Therefore, it will provide a new lowcost manufacturing method for 

highly functional and compact 

systems.

Fully on-chip capacitive devices for SoC design

Besides acting as the energy sources, CNT SCs can also be used to meet the 

requirements on large capacitor design for signal processing, sample and hold, 

frequency compensation, noise decoupling, tuned resonator, memory design and so on, 

enabling fully on-chip capacitive devices.

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Heterogeneous Integration of CNT 

supercapacitors on CMOS process
New Energy Storage and Management Structure 

Power Converter 1

Power Converter 2

Power Converter 3

Power Converter 4

Module 1

Module 2

Module 3

Module 4 Main Storage SC0

SC1

SC2

SC3

SC4 Point-of-Load (POL) Sources

Energy Harvester

Energy Storage and Management System

Power Management Unit Energy Storage and Sources: a main storage SC0 and four point-of-load (POL) sources, SC1~SC4.

PMU: hardware-based executing platform for power management, mainly composed of power converters, i.e., DC-DC 

converters. (four modules are shown as an example)

Features: 

Multi-energy-source structure: SC0~SC4 are all energy sources for the MSN, which provides the system with 

flexibility to assign appropriate voltages to different functional modules. .

Global power management capability: a global energy management strategy can be introduced to deliver energy 

among energy storage and sources.

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IEEE TRANSACTIONS ON POWER ELECTRONICS, 2010.
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•Si makes it an excellent candidate for electrode materials 

in Li-ion batteries. However, volumetric change (400%) 

upon insertion and extraction of Li (Li22Si5 alloy), resulting 

in pulverization and early capacity fading.

Si: An Excellent Material for Li-ion Batteries

0 10 20 30 40

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800 Specific Capacity (mAh/g)

Cycle Number

Si anode

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Specific Capacity (mAh/g)

Graphite

Silicon

25
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Si thin film

CNT film

Current collector

Tandem Structure Formation
Electrode Preparation

High-Vacuum 

Electron-Beam 

Evaporation

Tandem Structure

Si-SWNT-Cu

Si-Cu

Substrates

Vapor

Si target

Electron Source

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ACS Nano, 2010

27
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Preliminary Results

Si film thickness easily controlled within 1 

nm, with ultimate thickness up to 1-2 μm 

achievable

Increased Energy Density 

by 3.6 times

Improved Cycle Life

28
Preliminary Results

Si Structure Evolution

Lithiation Lithiation

SWNT

Si

Cu

Delithiation Delithiation

Si

Cu

Si

Cu

Si

Cu

SWNT

29 NIST Workshop 2011BQWei
Conclusions

• Different supercapacitors can be assembled that 

demonstrated different functions for promising 

applications in electronics:

High Flexibility/stretchability

High operation temperature

Long lifetime

Super-high power density

Mechanical robust under compression

High energy retention with small leakage

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CNT Power Sources for: