Activated Carbon Fiber / Poly(3,4-ethylenedioxythiophene) based SupercapacitorsWashington University in St. Louis, Department of Chemistry, Institute of Materials Science & Engineering

Yutong Liu

IntroductionGlobal industry and research centers are coping to address the world-wide energy demand by developing renewable energy sources and complementary energy storage technologies. Capacitors and batteries are two types of devices that can store energy. Capacitors possess excellent power density for fast bursts of electricity while batteries afford high energy density for prolonged utilization times. Supercapacitors can bal-ance the gap between batteries and capacitors, which has prototypes been applied in energy storage sys-term in race and daliy used cars, aircrafts and wind energy system.Among the materials for supercapacitors fabrication, poly(3,4-ethylenedioxythiophene) (PEDOT), being a conjugated heterocyclic conducting polymer, offers the electrical properties of metals with excellent organic processability, and is attractive candidate for narrowing the energy density and power density gap. Another candidate of supercapacitor fabrication is activated carbon fiber (ACF) because it bears ultrahigh surface area up to 4000 m2/g, 1 which provides a suitable load carrier for conducting polymer.

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TOYOTA ConceptYrais Hybrid-R

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Supercapacitor application examples: (A) Electric Plane from Volta Volare GT4 , (B) Pitch control in wind turbine from Loxuc Inc andaccelerator in Toyota (C) Yaris Hybrid-R and (D) Racing TS030 Hybrid.

(A)(D)(C)

(B)

Experimental SectionVapor phase polymerization (VPP) is ideal for fabricating state-of-the-art supercapacitors because it leads to high quality polymers that possess a long π-π conjugated length resulting in high conductivity. PEDOT coated ACF results in electrodes comprised of vertical-brushes of nanofibers that decrease inter-electrode distance, shorten diffusion length, and increase capacitance.Typical characterization methods include SEM, EDS, and Raman Spectroscopy for morphology and bond-ing characterzation along with BET for surface area quantification. Supercapacitors based on integrated ACF/PEDOT nanocomposites and pure ACF will be tested through a variety of electrochemical techniques such as cyclic voltammetry and galvanostatic charge/discharge curves for determining gravimetric capaci-tance and stability.

S

-e-

Oxidant

H H

O O

SH H

O O

S

C

H H

O O

2

SH H

O O

SH H

O O

- 2H

SH H

O O

SH H

O O

Ox.

- 2H

S* *

O O

n

I/V Characterization Scheme

Polymerization Mechanism

ACF

Carbon Fiber Paper

Kapton TapeSeparator

Au Electrode on Si Wafer

ACF

GlassIn Plane Test Through Plane Test

Synthesis Protocol

VPP ReactorFeCl3OxidantDroplet

ACF

PEDOTNanofiber

EDOT Monomer

Supercapacitor Geometry

Results & Discussion: Materials Characterization

10 μm 100 μm

Morphology & Elemental Analysis

C O

Fe

ClS

Fe

ClC K

S KS E1O K

Fe KCl K

3 μm 50 μm10 μm 500 μm

SEM of Pure ACF EDS of ACF/PEDOT demonstrates ACF is well coated by PEDOT

SEM of ACF/PEDOT illustrates ACF is well coated by PEDOT

Raman Shift (/cm) Corresponding Bond1425 Symmetric Cα=Cβ(-O) stretching1539 Asymmetric Cα=Cβ stretching1365 Cβ=Cβ stretching1267 Inter-ring Cα=Cα’ stretching990 Oxyethylene ring deformation

Chemical Structure Analysis

ACF PEDOT

Raman Spectroscopy demonstrates both ACF and PEDOT correspond to reported literature. 2,3

Raman Shift (/cm) Corresponding Mode

1340 D BandDisorder-induced D lineDefect induced mode

1580 G BandRaman allowed R line

Graphite i2g mode

Table 1 | Raman Shift of ACF Table 2 | Raman Shift of PEDOT

ACF Thickness Resistance STD

ThroughPlane

T = 1.0 mm 16.00 Ω 1.20T = 0.5 mm 8.10 Ω 0.65T < 0.5 mm 5.63 Ω 0.32

In PlaneT = 1.0 mm 146.09 Ω 10.33T = 0.5 mm 87.47 Ω 3.88T < 0.5 mm 58.11 Ω 4.10

PEDOT Pressure Resistance STD

Through Plane

Little 4.15 Ω 0.32Medium 2.89 Ω 0.34Large 2.62 Ω 0.26

In PlaneLittle 6.70 Ω 0.21

Medium 6.49 Ω 0.53Large 6.32 Ω 0.59

A+P Thickness Resistance STD

Through Plane

T = 1.0 mm 4.15 Ω 0.35T = 0.5 mm 2.89 Ω 0.19T < 0.5 mm 2.62 Ω 0.20

In PlaneT = 1.0 mm 6.70 Ω 1.42T = 0.5 mm 6.49 Ω 1.15T < 0.5 mm 6.32 Ω 1.23

Electrical Resistance Characterization

Table 3 | ACF Resistance Table 4 | PEDOT Resistance Table 5 | ACF/PEDOT Resistance

PEDOT In PlaneACF In Plane ACF/PEDOT In Plane

PEDOT Through PlaneACF Through Plane ACF/PEDOT Through Plane

I/V Curve shows that ACF’s resistance values vary with applied pressure and PEDOT’s resistance values are nearly constant.

Results & Discussion:Cyclic Voltammetry Characterization

Stability Analysis

1 mV/s & 5 mV/s

10 mV/s & 25 mV/s

Reproducibility Anaylsis

5 Smaples @ 1 mV/s

1, 5, 10, 25 & 50 mV/s

Performance Analysis

Our supercapacitor has a stable gravimentric capacitance and CV shape during stability tests.

Multiple identical samples tests demonstrate the reproducibility of our supercapacitors.

The gravimetric of our supercapacitors based on ACF can reach as high as 104 F/g.

Scan Rate (mV/s) Cycle Capacitance

(F)Gravimetric

Capacitance (F/g) STD

11 3.222 104.37

0.190110 3.210 103.99

51 2.991 96.78

0.050010 2.982 96.68

Scan Rate (mV/s) Cycle Capacitance

(F)Gravimetric

Capacitance (F/g) STD

10

1 2.851 92.36

0.025010 2.853 92.42

20 2.852 92.40

25

1 2.582 83.65

0.000210 2.581 83.62

20 2.582 83.65

Sample Cycle Capacitance (F)

GravimetricCapacitance (F/g) STD

1 Last 3.210 103.98

0.0500

2 Last 2.922 96.34

3 Last 2.826 96.91

4 Last 2.426 106.03

5 Last 2.383 108.12

Scan Rate (mV/s) Cycle Capacitance

(F)Gravimetric

Capacitance (F/g) STD

1 Last 2.815 96.36

0.0500

5 Last 2.606 89.16

10 Last 2.483 85.05

25 Last 2.252 77.16

50 Last 1.978 67.56

Table 9 | Gravimetric Capacitance Calculation@ 1, 5, 10, 25 & 50 mV/s

Table 8 | Gravimetric Capacitance Calculationfor multiple samples @ 1 mV/s

Table 7 | Gravimetric Capacitance Calculationfor different cycles @ 10 & 25 mV/s

Table 6 | Gravimetric Capacitance Calculationfor different cycles @ 1 & 5 mV/s

s

ConclusionsSEM, EDS and Raman Spectroscopy illustrate that our ACF and PEDOT materials follow literature.

I/V curves demonstrate that ACF’s resistance decreases when more pressure is applied, while PEDOT’s resistance is independent from applied pressure.

Supercapacitors based on ACF are stable and reproducible. Their gravimetric capacitance reaches 100 F/g. Vapor phase polymerization yields high purity PEDOT, which will facilitate increasing the gravimetric capacitance of supercapacitors based on ACF/PEDOT.

References Acknowledgments1. B. Xu et al., Electrochemistry Communications, 2008, 10, 795.

2. S. Jiang et al., Journal of Power Source, 2014, 272, 16.

3. B.Cho et al., Nano Lett., 2014, 14, 3321.