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ADVANCED ELECTRODE MATERIALS FOR ELECTROCHEMICAL SUPERCAPACITORS

DEEPAK KUMARAPPA

SUPERVISOR : DR. ZHITOMIRSKY

MALTS #701

29, April 2011

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OUTLINE

IntroductionLiterature review Problem formulationApproach and methodologyResults and discussionConclusions

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Applications

LED driver

PhoneHybride electric vehicle

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Advantages of supercapacitors (when compared to batteries)

• High power density• Possibility of fast recharging• Large cycling capability

(up to 106 cycles)• Excellent reversibility• Longer lifetime

/dQ dV

C idV dt

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2E CV

2

4

VP

ESR

Capacitance:

Energy:

Power:

V: voltageESR: equivalent series resistance

Two basic charge-storage mechanisms

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++++

++

++

e-

e-

e-

e-

+++++

---

--

+-

+

--

5–10 Å

+

+

+

+

+

+

---

--

+-

+

-- +

+

+++

e-

e-

e-

e- X-

X-

X-

X-

X-

X-

X-

X-

Current Current

Double-layer capacitance Pseudocapacitance

Capacitance arises from charge separation at an electrode/electrolyte interface

Utilize the charge-transfer pseudocapacitance arising from reversible Faradaic reactions occurring at the electrode surface

(Activated carbon) (Metal oxides & Conducting Polymers)

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• RuO2 720 F/g

• MnO2.H2O 700 F/g

• Conductive Polymers 400 – 500 F/g

• SnO2 285 F/g

• NiO 280 F/g

• In2O3 190 F/g

• Co3O4 164 F/g

Materials science aspectsExamples of materials and capacitance

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Polypyrrole• High conductivity • Excellent chemical stability• High Specific Capacitance - 400 F/g (noble current collectors!) • Large voltage window• Corrosion protection of current collectors• Flexibility• Light weight

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Electropolymerization of PolypyrroleDiaz’s mechanism

1. Oxidation of monomer

Different resonance forms

2. Coupling between radicals

3. Stabilization

Forms cation radical on application of anodic potential

Greater unpaired electron density in α-position

Forms dihydromer dication

Forms aromatic dimer on losing two protons

Said Sadki et al., Chem. Soc. Rev., 2000, 29,283-293

Electropolymerization of Polypyrrole

4. Oxidation of dimer

5. Formation of trimer

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Dimer Dimer cation radical

Trimer

Electropolymerization of Polypyrrole

6. Final polymer product

On continues propagation of above sequence, final polymer product is obtained

Electropolymerization doesn’t give neutral non-conducting polypyrrole but its oxidized conducting form (doped) Final polymer chain has a positive charge which is counter balanced by anion Films obtained consists of 65% polymer and 35% anion by weight

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Problem related on anodic electropolymerization on SS

W. Su et al., J. Elec. Acta 46 (2000) 1-8

Anodic dissolution of SS substrate at Epa prevents film formation

Proposed solution

Oxalate additive

Passivation of SS substrate is established

Disadvantages for Supercapacitors

Formation of resistive Iron(II) oxalate layer Poor adhesion

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Mussel-Inspired Surface Chemistry

Dopamine forms strong bonds with metals and oxidesStrong adhesion in water and aqueous solutions of metal salts

Literature related to proposed approach

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J.H. Waite, Nat, Matter. 7 (2008) 8

Literature related to proposed approach

1-hydroxybenzoic acid

benzoic acid

3,5-dihydroxybenzoic acidgallic acid

dopamine

K. Wu, Y. Wang, I. Zhitomirsky, J. Colloid and Interface Science 352 (2010) 371-378Y. Wang, I. Zhitomirsky, Colloids and Surfaces A 369 (2010) 211-217

chromotropic acid (CHR)

• Presence of adjacent OH group bonded to aromatic ring in dopamine and gallic acid enhances the adsorption of molecules on the oxide particles

• Strong adsorption of CHR on oxide particles was observed 13

Fundamental studies of absorption

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Conjugate bond provides high conductance & electron transfer mediation

K. Wu, Y. Wang, I. Zhitomirsky, J. Colloid and Interface Science 352 (2010) 371-378Y. Wang, I. Zhitomirsky, Colloids and Surfaces A 369 (2010) 211-217

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Objectives• Development of electropolymerization method for the

fabrication of PPY films on SS using new anionic additives

• Investigation of kinetics of deposition and deposition mechanism

• Optimization of bath composition and deposition parameters• Investigation of electrochemical properties of PPY films for

application in electrochemical supercapacitors

Chromotropic acid (CHR) Gallic acid

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Suggested role of anionic additives

Anionic doping of conducting polymer during electropolymerization

Improves adhesion and reduces anodic dissolution of stainless steel due to complexation with metal ions

Act as electron transfer mediator

Approach and methodology

Approach and methodology

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Selected additive

Chromotropic acid (CHR)

Suggested Complexation mechanism

SS

Gallic acid

SS

MethodologyFabrication of Ppy film on Stainless steel

H2OWater

Pyrrole + Additive

Ppy film on Substrate

Electropolymerization

Galvanostatic

-+SSt Pt

Characterization

• SEM • Electrochemical testing

– Cyclic Voltammetry (CV)

– Electrochemical Impedance Spectroscopy (EIS)

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Results and Discussion

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Galvanostatic behavior

W. Su et al., J. Elec. Acta 46 (2000) 1-8

5mM CHR and 50 mM Pyrrole

• No Induction time is required• Good adhesive film is formed

Oxalic acid and Pyrrole

Results and Discussion

5mM CHR and 50mM Pyrrole

Mass of the film can be controlled by deposition time

Current density is 0.7 mA cm-2

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Pyrrole without additive Pyrrole + CHR

Deposition mass Vs Time

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Cyclic Voltammetry

Electrolyte:0.5M Na2SO4

[Ppy]f + [A-]s [Ppy·+/A- ]f + e-Charging

Discharging

5mM CHR & 50mM Pyrrole 15mM CHR and 150mM Pyrrole

A-: Anions of electrolyte

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Electrochemical Impedance Spectroscopy (EIS)

Limited depth of ion penetration Pore size Mobility of ions

104 μg

227 μg

5mM CHR and 50mM Pyrrole

104 μg

227 μg

Optimization of CHR and Pyrrole concentrations

Film mass is approximately 100 μgScan rate is 2 mV/s

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a – 5mM CHR & 50mM Pyrrole b – 5mM CHR & 100mM Pyrrole e – 15mM CHR & 150mM Pyrrole f – 50mM CHR & 150mM Pyrrole

Conc. of CHR (mM)

Conc. of Pyrrole (mM)

Specific Capacitance

(F/g)

5 50 206

5 100 228

5 150 250

15 150 302

50 150 341

SEM Analysis

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5mM CHR & 50mM Pyrrole 15mM CHR &100mM Pyrrole 50mM CHR & 150mM Pyrrole

• PPY particles are uniformly distributed• Porosity of the film increases with increase in CHR and Pyrrole concentration• Porous structure improves the ions accessibility into the film pores

Results and Discussion

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Mass - 147 μg

scan rate - 50 mV s-1

50mM CHR and 150mM Pyrrole

Cyclic Stability Cyclic Voltammetry

11000

Results and Discussion

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Gallic Acid50mM Gallic acid and 250mM Pyrrole

Mass - 116 μg

Electrolyte:0.5M Na2SO4

Cyclic VoltammetryDeposition mass Vs Time

Results and Discussion

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83 μg

116 μg

188 μga - 83 μg b - 116 μgc - 188 μg

EIS Scan Rate Vs Specific Capacitance

50mM Gallic acid and 250mM Pyrrole

Conclusions• Electropolymerization method has been developed

for the fabrication of PPY coatings on stainless steel• The electropolymerization mechanism in the

presence of CHR and Gallic acid has been investigated.

• Films prepared from CHR showed better capacitive behavior than the one prepared from Gallic acid.

• The highest specific capacitance was 341 F/g when CHR is used as additive at optimized deposition conditions.

• The films prepared by the electropolymerization method are promising materials for application in electrochemical supercapacitors using low cost stainless steel current collectors.

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Acknowledgements

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My Supervisor, Dr. Zhitomirsky

Steve Koprich, Canadian Centre for Electron Microscopy, McMaster University

All my group members

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Thank You

Electropolymerization of Polypyrrole6. Electro-oxidation of trimer

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