ADVANCED ELECTRODE MATERIALS FOR
ELECTROCHEMICAL SUPERCAPACITORS
DEEPAK KUMARAPPA
SUPERVISOR : DR. ZHITOMIRSKY
MALTS #701
29, April 2011
1
OUTLINE
Introduction
Literature review
Problem formulation
Approach and methodology
Results and discussion
Conclusions
2
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
5
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
6
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
7
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
8
Electropolymerization of Polypyrrole
4. Oxidation of dimer
Dimer
Dimer cation radical
5. Formation of trimer
Trimer
9
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
10
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
11
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
12
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
13
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
14
14
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
15
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
16
Approach and methodology
Selected additive
Suggested Complexation mechanism
SS
Chromotropic acid (CHR)
SS
Gallic acid
17
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)
18
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
19
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
20
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
21
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
22
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
23
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
24
Results and Discussion
Cyclic Stability
50mM CHR and 150mM Pyrrole
Cyclic Voltammetry
1
1000
Mass - 147 μg
scan rate - 50 mV s-1
25
Results and Discussion
Gallic Acid
50mM Gallic acid and 250mM Pyrrole
Deposition mass Vs Time
Cyclic Voltammetry
Mass - 116 μg
Electrolyte:0.5M Na2SO4
26
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
27
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.
28
Acknowledgements
 My Supervisor, Dr. Zhitomirsky
 Steve Koprich, Canadian Centre for
Electron Microscopy, McMaster
University
All my group members
29
Thank You
30
Electropolymerization of Polypyrrole
6. Electro-oxidation of trimer
31