0. Technology, Applications, and Needs
ELECTROCHEMICAL CAPACITORS (ECs):
Technology, Applications, and Needs
John R. Miller;
JME, Inc.
Basic Research Needs for Electrical Energy Storage Workshop—April 2-5, 2007

1. <jmecapacitor@att.net>
JME, Inc.
17210 Parkland Drive
Shaker Heights, OH 44120
Established in 1989 to support electrochemical capacitor
material, product, technology, and industry development
Material evaluations
Prototype fabrication
Performance evaluations
Product reliability testing
Performance modeling
Product optimization
System engineering
Competitive market information
Staff:
Dr. John R. Miller
Dr. Susannah M. Butler
Dr. Arkadiy D. Klementov
Todd Zeigler
Specialization:
Facility
2500 ft2 laboratory
Total EC capacitor focus

2. CAPACITOR BASICS C = eoA/d Charge capacitor to voltage V,
Area, A
+ Q + _
Separation, d
Charge capacitor to voltage V,
Then charge Q is on plate
Q = C V
C = eoA/d
e is dielectric constant, o is constant vo
Energy density
E/Ad = ½ o (V/d)2

3. Capacitor Battery ENERGY STORAGE COMPONENTS Secondary (rechargeable)
Lead
acid
NiCd
NMH
electrostatic
electrolytic
electrochemical
asymmetric
symmetric
Li ion
Aqueous
electrolyte
Organic
Active research
Primary
Organic
electrolyte
Most popular today
Potential for bulk storage

4. CAPACITOR TYPES - + Electrostatic Electrolytic Electrochemical Air
Film
Ceramic
Electrolytic
Aluminum
Tantalum
Electrochemical
Carbon-carbon
Metal oxide symmetric
carbon-asymmetric + - C C1 C2
C2 >>C1
page 1

5. CAPACITOR TECHNOLOGY COMPARISON 1.0 MJ (277 Wh) Energy Delivery System
Capacitor Type
Mass (kg)
Volume (m3)
Cost (k$)
Response time (s)
Electrostatic
200,000
140
700
10-9
Electrolytic
10,000
2.2
300
10-4
Electrochemical
2 - 20 ~1

6. ELECTROCHEMICAL CAPACITORS (ECs)
Often called supercapacitor or ultracapacitor
Invented by Standard Oil of Ohio in the 1960’s
Product line introduced by NEC in 1978 (SOHIO license)
Originally used for computer memory backup
Appreciation of other attractive features in 1990s
Extraordinary power performance
Very high cycle-life
Long maintenance-free operational life
Safe, generally environmentally friendly technologies
I-10

7. DOUBLE LAYER CAPACITOR CONCEPT EC CAPACITOR EQUIVALENT CIRCUIT
Discovered by Helmholtz
C ~ 10 mF/cm2 on electrode
Charge stored electrostatically (not chemically)
Voltage limited by decomposition potential of electrolyte
Extremely large capacitances from high-surface-area carbon electrodes
EC CAPACITOR EQUIVALENT CIRCUIT V- V+
Qm-
Qm+
electrode
electrolyte + - C+ C-
Rel
R+rx
R-rx

8. Electric Double Layer Model
d~1 nm
Capacitor
C ≈ A/d ≈ 5 to 50 mF/cm2
Area, A
+ Q + _
Separation, d
Stored energy
E = ½ C V2
Use of high-surface-area electrodes
produce very high F/cm3
I-13

9. Typical EC Cell Cross-section
With electrolyte
With electrolyte
Activated carbon electrode
Current collectors (positive and negative)
Micro-porous separator
Spiral-wound or prismatic
Aqueous or non-aqueous electrolytes
Capacitance ~ el. thickness
Resistance ~ el. thickness
Thus response time
=RC~ (el. thickness)2
I-15

10. CAPACITOR PERFORMANCE
Electrode
Material
Conductivity
Surface area
Pore size distribution
Density
Pore volume
Wettability
Purity
Crystallinity
Particle size and shape
Surface functional groups
Charge carrier type/conc.
Geometry
Thickness
Binders additives
Separator
thickness
open area
tortuosity
Electrolyte
Conductivity
Ion Concentration
Temperature stability range
Ion size
Operating voltage window
Volatility, flammability, flash point
Purity
Design
Both electrodes same
Same material different masses
Different materials same capacitances
Different materials and capacitances
Construction
Bipolar
Single cell, spiral wound
Single cell prismatic
Current collectors and tabbing

11. EC FREQUENCY RESPONSE Much different from other capacitor types
Due to use of porous electrode materials (multiple time constant)
Self-resonant frequency typically <100 Hz for large systems
Leakage current has exponential dependence on voltage
High dissipation precludes 120 Hz power filtering applications
II-21

12. Porous Electrode--Transmission Line Response
Complex Impedance
Where j=(-1)1/2
n= number of pores in the electrode
r = radius of a cylindrical pore
k = electrolyte conductivity
w = angular frequency
Cdl = double layer capacitance per unit area
l = length of a cylindrical pore
De Levie
Electrochim Acta. 8, 751 (1963)

13. Porous Electrode Electrical Response
Complex-Plane Plot
High frequency limit
Low frequency limit I I
Where l = pore length
k = electrolyte conductivity
V = pore volume
r = pore radius
S = 2prln
C = SCdl
n = number of pores
R R+
R equivalent series resistance
 = l2/2V = l2 /rS ionic resistance within the porous structure

14. Model Surfaces
Series RC Circuit R C R
Re Z
-Im Z
increasing w
|Z|

15. Electrode Porosity Due to Packing
Complex Plane Plot
at Five Temperatures I

17. TIME CHARACTERISTICS OF A LOAD DICTATE
THE APPROPRIATE EQUIVALENT CIRCUIT MODEL
Long times:
Intermediate times: - R C C
i=a*exp(b*V) -
Short times: - C1 R1 R5 R2 R4 R3 C2 C3 C4 C5

18. Typical DLC Design
I-19

19. LARGE EC PRODUCTS ECOND ELIT NESS Nippon Maxwell Chemi-Con ESMA
Power Systems
(Okamura)
ESMA
LS Cable

20. Series Resistance (mW) Specific Energy (Wh/kg)
State of the Art Large EC Cells/Modules
Manufacturer
Electro-lyte
Rated Voltage
(V)
Capacitance
(F)
Series Resistance (mW)
Mass (kg)
Specific Energy (Wh/kg)
response time* (s)
ECOND (module) Aq
270
2.33
300
48.0
0.5
0.7
ELIT (module)
14.5
423
1.0
15.7
0.8
0.4
ESMA (module)
1.5
10,000
0.28
1.1
2.7
3.0
LS Cable (cell) PC
2.8
3,000
0.50
0.63
5.2
Maxwell (cell) AN
0.37
0.55
5.5
0.9
NessCap (cell)
3,600
0.67
5.3
1.8
Nippon Chemi-Con (cell)
2.5
2,400
4.2
1.7
AN: acetonitrile, PC: propylene carbonate, Aq: KOH in water
*response time calculated as of the series resistance--capacitance product

21. BATTERY -- EC COMPARISON
PROPERTY
BATTERY EC
Storage mechanism
Chemical
Physical
Power limitation
Reaction kinetics,
mass transport
Separator ionic conductivity
Energy limitation
Electrode mass
Electrode surface area
Output voltage
Constant value
Sloping value (SOC known precisely)
Charge rate
Very high, same as discharge rate
Cycle life limitations
Physical stability, chem. reversibility
Side reactions
Life limitation
Thermodynamic stability
I-38

22. SUMMARY OF EC CHARACTERISTICS
Extraordinarily high specific capacitance ~100 F/g typical
Very low $/J compared with conventional capacitors
Low unit-cell voltage, ~1 to 3 V
Non-ideal behavior--response time ~1 s
Expensive, on an energy basis, compared with batteries
Very powerful when compared with batteries
Operational life and cycle life can be engineered to exceed application requirements
I-43

23. Capacitor Powered Pure Electric Bus
CAPACITOR ONLY ENERGY STORAGE
Capacitor Powered Pure Electric Bus
50 Passenger, 25 km/hr, 15 km range, 15 min. charge time, 190 V
30 MJ CAPACITOR STORAGE SYSTEM

24. V-36

25. V-37

28. Bridge Power Example (Four systems deployed in Japan)
Time (s)
Voltage (V)
Current (A)
V-34

29. V-75

33. ENERGY STORAGE TECHNOLOGY
COMBINATIONS
Hypothetical energy-power behavior
Combination
Technology 1
Specific Energy
Technology 2
Specific Power
The technologies must be decoupled to effectively exploit the combination
Decoupling approaches
active system (dc-dc converter)
resistor, often the ESR of the less powerful technology
switches and diodes
Examples
Electrochemical capacitor + film capacitor
Electrochemical capacitor + battery
Electrochemical capacitor + fuel cell
409

35. ECs Provide Immediate Cost Savings in System

36. Important EC Metrics Energy density and specific energy
Response time (63.2% charge for series-RC model)
Cycle efficiency
Cycle life and operational life property fade
Life distribution (reliability issues)
Performance under specific functional tests
Ragone plots—poor for technology comparison
Obtained at constant power using full discharge
Says nothing about charging performance, cycle efficiency, life, cycle life, safety
Power density and specific power—poor for technology comparison
Generally same for charge and discharge
Strongly depends on voltage
Usually adequate for an application—capacitor sized by energy needs

37. EC Discharge/Charge Cycle for Energy-Efficiency Model Calculations (Use Series-RC Circuit Model)
current T
voltage Vo
Vo/2
Time 2T
T ~2T ~3T ~4T ~5T
Efficiency depends on the applied power profile
Series-RC circuit analytical solution: scales as the ratio of charge time T to EC time-constant: n = T/RC
Energy efficiency = (n+4/3)/(n+8/3)
Eout / Ewindow = n(n+4/3)/(n+2)2
T = charge time

38. Series-RC Circuit Model Results
Energy Cycle Efficiency
T = charge time
Discharge Energy Out
CC Charge/discharge: Vo /2 -Vo -Vo /2
n = T/RC

39. ELECTROCHEMICAL CAPACITOR DESIGNS
Double layer + - _
electrolyte
Symmetric
Asymmetric Q
Lower Limit
Upper Limit V + -
Double layer
Faradaic and other processes +
electrolyte _ Q
Lower Limit
Upper Limit V + -
Battery electrode

40. Doubling capacitance of carbon electrode over symmetric device
Advantages of the Aqueous Electrolyte Asymmetric Electrochemical Capacitor Design
Doubling capacitance of carbon electrode over symmetric device
Higher operating voltage than symmetric device
Capacitance boost at high charge states
Tolerant to over-voltage conditions
Voltage self-balance in series strings
Cycle life dependent on capacity asymmetry of the two electrodes
Very high specific energy and energy density demonstrated
Response times of 2 to 100 seconds typical
Lower packaging and manufacturing costs since carbon drying and hermetic packaging unnecessary

41. Anomalous Capacitance of
Some Carbon at Low Potentials
Discharge energy proportional to area under curve
Substantial increase in stored energy with charge voltage

42. Anomalous Capacitance of Carbon
Asymmetric Carbon // H2SO4 // PbO2 Capacitor
Discharge energy after constant current charge to: 1.9, 2.05, 2.25 V
Stored energy proportional to (voltage)7.9, not (voltage)2
Specific capacitance of carbon increases many times

43. Cyclic Voltammogram of Carbon Electrode
Acidic Electrolyte, Scans From +0.9 to –1.1 V vs SHE
that becomes available at very low
Note all of the area (capacitance)
potentials (<0 V SHE).
Double Layer Capacitor Seminar, Deerfield Beach, FL, Dec. 6-8, 2004

44. CAPACITOR POWERED PURE ELECTRIC TRUCK. .
V-93

45. EC Technology Needs Lower cost cells Longer life cells
Increase cell operating voltage to >4.0 V with RC<1 s, high cycle life electrode/electrolyte system
Use lower cost design—exploit anomalous capacitance observed in asymmetric aqueous electrolyte ECs
Use electrolyte additive to reduce drying costs and control other impurities
Longer life cells
Well-sealed cells always fail with package rupture (except valved caps)
Use electrolyte additive to prevent or control gas generation
Devise more effective ways for removing impurities
Carbon composite electrode may obviate current collector in asymmetrics
Higher cycle efficiency cells
Higher conductivity electrolyte
Thinner, more open separator
Resistances need to be reduced everywhere
Lower embedded energy costs, particularly if technology “explodes”
Increased capacitive operating frequency (electrode/device structure)
Dynamic cell voltage balancing (electrolyte additives?)