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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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