JME

ELECTROCHEMICAL CAPACITORS (ECs):

Technology, Applications, and Needs

John R. Miller; JME, Inc.

216-751-9537<jmecapacitor@att.net>

Basic Research Needs for Electrical Energy Storage Workshop—April 2-5, 2007

JME JME, Inc.17210 Parkland Drive

Shaker Heights, OH 44120216-751-9537

<jmecapacitor@att.net>

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

JME CAPACITOR BASICS

Area, A

+ Q

+

_Separation, d

Charge capacitor to voltage V, Then charge Q is on plate

Q = C V

C = oA/d is dielectric constant, o is

constant

vo

Energy in voltage window Vi to Vf

E = ½ C(Vi2 – Vf

2)

Stored energy

E = ½ C V2

Vf Vi

E

Voltage

Energy

Energy in voltage window Vi to Vf

E = ½ C(Vi2 – Vf

2)

Stored energy

E = ½ C V2

Vf Vi

E

Voltage

Energy

Vf Vi

E

Voltage

Energy

Vf Vi

E

Voltage

Energy

Energy density

E/Ad = ½ o (V/d)2

JME

Organicelectrolyte

Most popular today Potential for bulk storage

Primary

ENERGY STORAGE COMPONENTS

Capacitor

Secondary(rechargeable)

Battery

Leadacid

NiCd NMH

electrostatic

electrolytic

electrochemical

asymmetricsymmetricLi ion

Aqueouselectrolyte

Organicelectrolyte

Aqueouselectrolyte

Active research

JMECAPACITOR TYPES

Electrostatic• Air• Mica• Film• Ceramic

Electrolytic• Aluminum• Tantalum

Electrochemical• Carbon-carbon• Metal oxide symmetric• carbon-asymmetric

+

+---

-

+

+

+-

-

-

C

C1

C2

C2 >>C1

C1

JMECAPACITOR TECHNOLOGY COMPARISON

1.0 MJ (277 Wh) Energy Delivery System

Capacitor TypeMass

(kg)

Volume

(m3)

Cost

(k$)

Response

time (s)

Electrostatic 200,000 140 700 10-9

Electrolytic 10,000 2.2 300 10-4

Electrochemical 30- 100 .02- 0.1 2 - 20 ~1

JMEELECTROCHEMICAL 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

JMEDOUBLE LAYER CAPACITOR CONCEPT

• Discovered by Helmholtz• C ~ 10 F/cm

2 on electrode

• Charge stored electrostatically (not chemically)

• Voltage limited by decomposition potential of electrolyte

• Extremely large capacitances from high-surface-area carbon electrodes

V-

V+ Qm

-Qm

+

electrodeelectrode

electrolyte +++++

-----

-----

+++++

C+ C-Rel

R+rx

R-rx

1 1 1

C C Ct

CQ

Vm

CQ

Vm

EC CAPACITOR EQUIVALENT CIRCUIT

JMEElectric Double Layer Model

d~1 nmC ≈ A/d ≈ 5 to 50 F/cm2

Area, A

+ Q

+

_Separation, d

Stored energy

E = ½ C V2

Capacitor

Use of high-surface-area electrodesproduce very high F/cm3

I-13

JMETypical EC Cell Cross-section

•Activated carbon electrode

•Current collectors (positive and negative)

•Micro-porous separator

•Spiral-wound or prismatic

•Aqueous or non-aqueous electrolytes

I-15

Capacitance ~ el. thicknessResistance ~ el. thicknessThus response time =RC~ (el. thickness)2

With electrolyte

With electrolyte

JMECAPACITOR 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• Density• Binders additives

• Separator• thickness• open area• tortuosity• Wettability

• Electrolyte Conductivity

Ion ConcentrationTemperature stability rangeIon size

Operating voltage window Volatility, flammability, flash point

Purity• Design

Both electrodes sameSame material different massesDifferent materials same capacitancesDifferent materials and capacitances

• Construction Bipolar Single cell, spiral wound Single cell prismatic Current collectors and tabbing

JMEEC 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

JMEPorous Electrode--Transmission Line Response

Complex Impedance

)1(1coth

2

)1(3

jr

C

Crn

fZp dl

dl

Where j=(-1)1/2

n= number of pores in the electroder = radius of a cylindrical pore = electrolyte conductivity = angular frequencyCdl = double layer capacitance per unit areal = length of a cylindrical pore

De LevieElectrochim Acta. 8, 751 (1963)

JME

High frequency limit

Low frequency limit

dlCrn

jZp

32

)1(

C

j

Cnl

jZp

dl

2

R equivalent series resistance

= l2/2V = l2 /rS ionic resistance within the porous structure

Porous Electrode Electrical ResponseComplex-Plane Plot

Where l = pore lengthk = electrolyte conductivityV = pore volumer = pore radiusS = 2rlnC = SCdl

n = number of pores

R R+

I I

JME

R

C

Series RC Circuit

CjRZ

1

222 1

CRZ

)1

(tan 1

RC

R Re Z

-Im

Z increasing

|Z|

Model Surfaces

JMEElectrode Porosity

Due to PackingComplex Plane Plot

at Five Temperatures

I

JME

JMETIME CHARACTERISTICS OF A LOAD DICTATE

THE APPROPRIATE EQUIVALENT CIRCUIT MODEL

-

Long times:

C i=a*exp(b*V)

Intermediate times:

-

R C

Short times:

-

C1

R1 R5R2 R4R3

C2 C3 C4 C5

JME Typical DLC Design

I-19

JME

ESMA

NipponChemi-Con

ECONDELIT

NESS

Power Systems (Okamura)

LARGE EC PRODUCTS

LS Cable

Maxwell

JME

Manufacturer Electro-lyte

Rated Voltage

(V)

Capacitance(F)

Series Resistance

(m)

Mass (kg)

Specific Energy (Wh/kg)

response time* (s)

ECOND (module) Aq 270 2.33 300 48.0 0.5 0.7

ELIT (module) Aq 14.5 423 1.0 15.7 0.8 0.4

ESMA (module) Aq 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 1.5

Maxwell (cell) AN 2.7 3,000 0.37 0.55 5.5 0.9

NessCap (cell) PC 2.7 3,600 0.50 0.67 5.3 1.8

Nippon Chemi-Con (cell) PC 2.5 2,400 0.7 0.5 4.2 1.7

AN: acetonitrile, PC: propylene carbonate, Aq: KOH in water *response time calculated as of the series resistance--capacitance product

State of the Art Large EC Cells/Modules

JMEBATTERY -- EC COMPARISON

PROPERTY BATTERY ECStorage 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 Reaction kinetics,

mass transport

Very high, same as discharge rate

Cycle life limitations Physical stability, chem. reversibility

Side reactions

Life limitation Thermodynamic stability

Side reactions

I-38

JMESUMMARY 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

JMECapacitor Powered Pure Electric Bus

50 Passenger, 25 km/hr, 15 km range, 15 min. charge time, 190 V

CAPACITOR ONLY ENERGY STORAGE

30 MJ CAPACITOR STORAGE SYSTEM

JME

V-36

JME

V-37

JME

JME

JMEBridge Power Example

(Four systems deployed in Japan)

Time (s)

Vo

lta

ge

(V

)

Cu

rre

nt

(A)

V-34

JME

V-75

JME

JME

JME

JMEENERGY STORAGE TECHNOLOGY

COMBINATIONS

• Hypothetical energy-power behavior

• 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

• ExamplesElectrochemical capacitor + film capacitorElectrochemical capacitor + batteryElectrochemical capacitor + fuel cell

Spe

cific

Ene

rgy

Specific Power

Technology 1

Technology 2

Combination

409

JME

JME

V-85

ECs Provide Immediate Cost Savings in System

JMEImportant 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

JMEEC Discharge/Charge Cycle for

Energy-Efficiency Model Calculations (Use Series-RC Circuit Model)

• 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

T = charge time

-1.5

0 6

io

-io

curr

ent

0

2.5

0 1 2 3 4 5 6

volt

age

Vo

Vo/2

Time

2T

Energy efficiency = (n+4/3)/(n+8/3)

Eout / Ewindow = n(n+4/3)/(n+2)2

0 T ~2T ~3T ~4T ~5T

JMESeries-RC Circuit Model Results

3834

n

n

E

E

in

out

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1 10 100 1000

= /RC

Eff

icie

nc

y =

Eo

ut/E

in

0

0.2

0.4

0.6

0.8

1

1.2

En

erg

y O

ut/T

ota

l En

erg

y S

tore

d in

V

olta

ge

Win

do

w

2)2(

)34

(

n

nn

E

E

window

out

Energy Cycle Efficiency T = charge time

Discharge Energy Out

n = T/RC

CC Charge/discharge: Vo /2 -Vo -Vo /2

JME

• Symmetric

• Asymmetric

Double layer

+-

-

-

-

-

+

+

+

+

+

_

elec

trol

yte

Double layer

+

+

+

+

elec

trol

yte

_ +

Battery electrode

Faradaic and other processes

ELECTROCHEMICAL CAPACITOR DESIGNS

Q

Lower Limit

Upper LimitV +

-

QLower Limit

Upper Limit

V +

-

JMEAdvantages 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

JMEAnomalous Capacitance of

Some Carbon at Low Potentials

PbO2 –H2SO4 –C Asymmetric Electrochemical CapacitorConstant current charge to 1.9, 2.05, and 2.25 V; 30 min. open; constant current discharge

PbO2 –H2SO4 –C Asymmetric Electrochemical CapacitorConstant current charge to 1.9, 2.05, and 2.25 V; 30 min. open; constant current discharge

• Discharge energy proportional to area under curve• Substantial increase in stored energy with charge voltage

JMEAnomalous Capacitance of Carbon

y = 0.0285x7.9267

0

5

10

15

20

0 0.5 1 1.5 2 2.5

Charge Voltage (V)

Dis

char

ge

En

erg

y (J

)

• 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

Asymmetric Carbon // H2SO4 // PbO2 Capacitor

JMECyclic Voltammogram of Carbon Electrode

Acidic Electrolyte, Scans From +0.9 to –1.1 V vs SHE

Double Layer Capacitor Seminar, Deerfield Beach, FL, Dec. 6-8, 2004

No

te a

ll o

f th

e a

rea

(c

apac

ita

nce

) t

hat

bec

om

es

ava

ilab

le a

t ve

ry l

ow

po

ten

tial

s (<

0 V

SH

E).

JMECAPACITOR POWERED PURE ELECTRIC TRUCK

. .

.

V-93

JME EC Technology Needs• Lower cost 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?)