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JME, Inc. 17210 Parkland Drive Shaker Heights, OH 44120

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Presentation on theme: "JME, Inc. 17210 Parkland Drive Shaker Heights, OH 44120 "— Presentation transcript:

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

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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

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

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

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


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

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

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

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




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

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

. . 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?)

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