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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. Established in 1989 to support electrochemical capacitor

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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


Established in 1989 to support electrochemical capacitor

material, product, technology, and industry development

Staff:

Dr. John R. Miller

Dr. Susannah M. Butler

Dr. Arkadiy D. Klementov

Todd Zeigler

Specialization:

  • Material evaluations

  • Prototype fabrication

  • Performance evaluations

  • Product reliability testing

  • Performance modeling

  • Product optimization

  • System engineering

  • Competitive market information

JME, Inc.

17210 Parkland Drive

Shaker Heights, OH 44120

216-751-9537

<jmecapacitor@att.net>

Facility

2500 ft2 laboratory

Total EC capacitor focus


  • e is dielectric constant, o is constant

  • Charge capacitor to voltage V,

    Then charge Q is on plate

    Q = C V

Separation, d

_

+ Q

+

Area, A

vo

  • Energy density

  • E/Ad = ½ o (V/d)2

CAPACITOR BASICS


ENERGY STORAGE COMPONENTS

Battery

Capacitor

Secondary

(rechargeable)

electrostatic

electrolytic

electrochemical

Lead

acid

Li ion

NiCd

NMH

symmetric

asymmetric

Organic

electrolyte

Aqueous

electrolyte

Aqueous

electrolyte

Organic

electrolyte

Most popular today

Potential for bulk storage

Active research

Primary


Electrostatic

  • Air

  • Mica

  • Film

  • Ceramic

+

-

C

  • Electrolytic

    • Aluminum

    • Tantalum

C1

-

-

+

-

+

C2 >>C1

  • Electrochemical

  • Carbon-carbon

  • Metal oxide symmetric

  • carbon-asymmetric

+

-

-

+

C2

C1

CAPACITOR TYPES


CAPACITOR TECHNOLOGY COMPARISON

1.0 MJ (277 Wh) Energy Delivery System


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

ELECTROCHEMICAL CAPACITORS (ECs)

I-10


electrolyte

-

-

-

-

-

+

+

+

+

+

-

-

-

-

-

+

+

+

+

+

electrode

electrode

V-

Qm+

Qm-

V+

C-

C+

Rel

R-rx

R+rx

DOUBLE LAYER CAPACITOR CONCEPT

  • 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


Separation, d

_

+ Q

+

Area, A

C ≈ A/d ≈ 5 to 50 mF/cm2

d~1 nm

Stored energy

E = ½ C V2

Electric Double Layer Model

Capacitor

Use of high-surface-area electrodes

produce very high F/cm3

I-13


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


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

      • Density

      • Binders additives

  • Separator

    • thickness

    • open area

    • tortuosity

    • Wettability

  • 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


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


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)

Porous Electrode--Transmission Line Response


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Sionic resistance within the porous structure


R

C

increasing w

-Im Z

|Z|

R

Re Z

Model Surfaces

Series RC Circuit


Electrode Porosity

Due to Packing

Complex Plane Plot

at Five Temperatures

I


R

C

-

-

R5

R1

R2

R3

R4

C2

C5

C1

C3

C4

-

TIME CHARACTERISTICS OF A LOAD DICTATE

THE APPROPRIATE EQUIVALENT CIRCUIT MODEL

Long times:

Intermediate times:

C

i=a*exp(b*V)

Short times:



Power Systems

(Okamura)

LARGE EC PRODUCTS

ECOND

ELIT

NESS

Nippon

Chemi-Con

Maxwell

ESMA

LS Cable


State of the Art Large EC Cells/Modules

AN: acetonitrile, PC: propylene carbonate, Aq: KOH in water

*response time calculated as of the series resistance--capacitance product



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

SUMMARY OFEC CHARACTERISTICS

I-43


30 MJ CAPACITOR STORAGE SYSTEM

CAPACITOR ONLY ENERGY STORAGE

Capacitor Powered Pure Electric Bus

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


V-36


V-37


Bridge power example four systems deployed in japan

Current (A)

Voltage (V)

Time (s)

Bridge Power Example(Four systems deployed in Japan)

V-34


V-75


ENERGY STORAGE TECHNOLOGY

COMBINATIONS

  • Hypothetical energy-power behavior

Combination

Technology 1

Technology 2

Specific Energy

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



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

Important EC Metrics


Ec discharge charge cycle for energy efficiency model calculations use series rc circuit model

io

2T

current

T

-io

Vo

voltage

Vo/2

Time

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

EC 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

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

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

T = charge time


Series rc circuit model results
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


Double layer

_

+

+

+

+

+

+

-

-

-

-

-

electrolyte

Upper Limit

V

+

-

Lower Limit

_

+

Upper Limit

+

+

+

+

Q

+

V

electrolyte

-

Lower Limit

Q

ELECTROCHEMICAL CAPACITOR DESIGNS

  • Symmetric

  • Asymmetric

Double layer

Faradaic and other processes

Battery electrode


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

Advantages of the Aqueous ElectrolyteAsymmetric Electrochemical Capacitor Design


Anomalous Capacitance of device

Some Carbon at Low Potentials

  • Discharge energy proportional to area under curve

  • Substantial increase in stored energy with charge voltage


Anomalous Capacitance of Carbon device

Asymmetric Carbon // H2SO4 // PbO2Capacitor

  • 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


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

Cyclic Voltammogram of Carbon Electrode

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

Note all of the area (capacitance)

that becomes available at very low

potentials (<0 V SHE).



Ec technology needs

Lower cost cells 6-8, 2004

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?)

EC Technology Needs


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