Transcript Slide 1 - Course Notes

ADVANCED ELECTRODE MATERIALS FOR
ELECTROCHEMICAL SUPERCAPACITORS
DEEPAK KUMARAPPA
SUPERVISOR : DR. ZHITOMIRSKY
MALTS #701
29, April 2011
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OUTLINE
Introduction
Literature review
Problem formulation
Approach and methodology
Results and discussion
Conclusions
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Applications
Hybride electric vehicle
Phone
LED driver
3
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
Capacitance:
dQ
dV
C
i/
dV
dt
Energy:
1
2
E  CV
2
Power:
V2
P
4  ESR
V:
voltage
ESR: equivalent series resistance
4
Two basic charge-storage mechanisms
Double-layer capacitance
(Activated carbon)
(Metal oxides & Conducting Polymers)
Capacitance arises from charge
separation at an
electrode/electrolyte interface
Current
5–10 Å
e-+
e-+
+
e
+
e+
- ++ -++
+- - -++ - +
- - +- - ++
-- - + +
+ +
-
 Pseudocapacitance
Utilize the charge-transfer
pseudocapacitance arising
from reversible Faradaic
reactions occurring at the
electrode surface
Current
+
+
ee- +
e- +
e-
+
+
+
+
-
X
-
X
X
-
X
-
X
-
-
-
X
X
-
X
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Materials science aspects
Examples of materials and capacitance
•
•
•
•
•
•
•
RuO2
MnO2.H2O
Conductive Polymers
SnO2
NiO
In2O3
Co3O4
720 F/g
700 F/g
400 – 500 F/g
285 F/g
280 F/g
190 F/g
164 F/g
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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 Polypyrrole
Diaz’s mechanism
Said Sadki et al., Chem. Soc. Rev., 2000, 29,283-293
1. Oxidation of monomer
Forms cation radical on application
of anodic potential
Different resonance forms
Greater unpaired electron density in α-position
2. Coupling between radicals
Forms dihydromer dication
3. Stabilization
Forms aromatic dimer on losing two protons
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Electropolymerization of Polypyrrole
4. Oxidation of dimer
Dimer
Dimer cation radical
5. Formation of trimer
Trimer
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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
Anodic dissolution of SS substrate at Epa prevents
film formation
Proposed solution
W. Su et al., J. Elec. Acta 46 (2000) 1-8
Oxalate additive
Passivation of SS substrate is established
Disadvantages for Supercapacitors
 Formation of resistive Iron(II) oxalate layer
 Poor adhesion
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Literature related to proposed approach
Mussel-Inspired Surface
Chemistry
J.H. Waite, Nat, Matter. 7 (2008) 8
Dopamine forms strong bonds with metals and oxides
Strong adhesion in water and aqueous solutions of metal salts
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Literature related to proposed approach
benzoic acid
gallic acid
1-hydroxybenzoic acid
3,5-dihydroxybenzoic acid
dopamine
chromotropic acid (CHR)
K. Wu, Y. Wang, I. Zhitomirsky, J. Colloid and Interface Science 352 (2010) 371-378
Y. Wang, I. Zhitomirsky, Colloids and Surfaces A 369 (2010) 211-217
• 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
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Fundamental studies of absorption
K. Wu, Y. Wang, I. Zhitomirsky, J. Colloid and Interface Science 352 (2010) 371-378
Y. Wang, I. Zhitomirsky, Colloids and Surfaces A 369 (2010) 211-217
Conjugate bond provides high conductance & electron transfer mediation
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Objectives
• Development of electropolymerization method for the
fabrication of PPY films on SS using new anionic additives
Chromotropic acid (CHR)
Gallic acid
• 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
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Approach and methodology
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
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Approach and methodology
Selected additive
Suggested Complexation mechanism
SS
Chromotropic acid (CHR)
SS
Gallic acid
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Methodology
Fabrication of Ppy film on Stainless steel
Pyrrole + Additive
SSt
+
Pt
-
Galvanostatic
Water
H2O
Electropolymerization
Characterization
Ppy film on
Substrate
• SEM
• Electrochemical testing
– Cyclic Voltammetry (CV)
– Electrochemical Impedance
Spectroscopy (EIS)
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Results and Discussion
Galvanostatic behavior
Oxalic acid and Pyrrole
5mM CHR and 50 mM Pyrrole
• No Induction time is required
• Good adhesive film is formed
W. Su et al., J. Elec. Acta 46 (2000) 1-8
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Results and Discussion
Deposition mass Vs Time
5mM CHR and 50mM Pyrrole
Pyrrole without additive
Pyrrole + CHR
Current density is 0.7 mA cm-2
Mass of the film can be controlled by deposition time
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Cyclic Voltammetry
5mM CHR & 50mM Pyrrole
15mM CHR and 150mM Pyrrole
Electrolyte:0.5M Na2SO4
Charging
[Ppy]f + [A-]s
Discharging
[Ppy·+/A- ]f + e-
A-: Anions of electrolyte
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Electrochemical Impedance
Spectroscopy (EIS)
5mM CHR and 50mM Pyrrole
104 μg
104 μg
227 μg
227 μg

Limited depth of ion
penetration
 Pore size
 Mobility of ions
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Optimization of CHR and Pyrrole concentrations
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
a – 5mM CHR & 50mM Pyrrole
b – 5mM CHR & 100mM Pyrrole
e – 15mM CHR & 150mM Pyrrole
f – 50mM CHR & 150mM Pyrrole
Film mass is approximately 100 μg
Scan rate is 2 mV/s
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SEM Analysis
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
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Results and Discussion
Cyclic Stability
50mM CHR and 150mM Pyrrole
Cyclic Voltammetry
1
1000
Mass - 147 μg
scan rate - 50 mV s-1
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Results and Discussion
Gallic Acid
50mM Gallic acid and 250mM Pyrrole
Deposition mass Vs Time
Cyclic Voltammetry
Mass - 116 μg
Electrolyte:0.5M Na2SO4
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Results and Discussion
50mM Gallic acid and 250mM Pyrrole
Scan Rate Vs Specific Capacitance
EIS
83 μg
188 μg
116 μg
a - 83 μg
b - 116 μg
c - 188 μg
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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
 My Supervisor, Dr. Zhitomirsky
 Steve Koprich, Canadian Centre for
Electron Microscopy, McMaster
University
All my group members
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Thank You
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Electropolymerization of Polypyrrole
6. Electro-oxidation of trimer
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