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JME ELECTROCHEMICAL CAPACITORS (ECs): Technology, Applications, and Needs John R. Miller; JME, Inc. 216-751-9537 Basic Research Needs for Electrical Energy.

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Presentation on theme: "JME ELECTROCHEMICAL CAPACITORS (ECs): Technology, Applications, and Needs John R. Miller; JME, Inc. 216-751-9537 Basic Research Needs for Electrical Energy."— Presentation transcript:

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

2 JME JME, Inc Parkland Drive Shaker Heights, OH 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 ft 2 laboratory Total EC capacitor focus

3 JME CAPACITOR BASICS Area, A + Q + _ Separation, d  Charge capacitor to voltage V, Then charge Q is on plate Q = C V  C =  o A/d   is dielectric constant,  o is constant vovo  Energy density E/Ad = ½  o (V/d) 2

4 JME Organic electrolyte Most popular today Potential for bulk storage Primary ENERGY STORAGE COMPONENTS Capacitor Secondary (rechargeable) Battery Lead acid NiCdNMH electrostatic electrolytic electrochemical asymmetricsymmetric Li ion Aqueous electrolyte Organic electrolyte Aqueous electrolyte Active research

5 JME CAPACITOR TYPES Electrostatic Air Mica Film Ceramic Electrolytic Aluminum Tantalum Electrochemical Carbon-carbon Metal oxide symmetric carbon-asymmetric C C1C1 C2C2 C 2 >>C 1 C1C1

6 JME CAPACITOR TECHNOLOGY COMPARISON 1.0 MJ (277 Wh) Energy Delivery System Capacitor Type Mass (kg) Volume (m 3 ) Cost (k$) Response time (s) Electrostatic200, Electrolytic10, Electrochemical ~1

7 JME 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

8 JME DOUBLE 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- V+V+ Qm-Qm- Qm+Qm+ electrode electrolyte C+C+ C-C- R el R + rx R - rx EC CAPACITOR EQUIVALENT CIRCUIT

9 JME Electric Double Layer Model d~1 nm C ≈ A/d ≈ 5 to 50  F/cm 2 Area, A + Q + _ Separation, d Stored energy E = ½ C V 2 Capacitor Use of high-surface-area electrodes produce very high F/cm 3 I-13

10 JME Typical 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. thickness Resistance ~ el. thickness Thus response time  =RC~ (el. thickness) 2 With electrolyte

11 JME 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

12 JME 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-2 1

13 JME 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  = electrolyte conductivity  = angular frequency C dl = double layer capacitance per unit area l = length of a cylindrical pore De Levie Electrochim Acta. 8, 751 (1963)

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

15 JME R C Series RC Circuit R Re Z -I m Z increasing  |Z| Model Surfaces

16 JME Electrode Porosity Due to Packing Complex Plane Plot at Five Temperatures I

17 JME

18 TIME CHARACTERISTICS OF A LOAD DICTATE THE APPROPRIATE EQUIVALENT CIRCUIT MODEL - Long times: Ci=a*exp(b*V) Intermediate times: - R C Short times: - C1C1 R1R1 R5R5 R2R2 R4R4 R3R3 C2C2 C3C3 C4C4 C5C5

19 JME Typical DLC Design I-19

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

21 JME Manufacturer Electro- lyte Rated Voltage (V) Capacitance (F) Series Resistance (m  ) Mass (kg) Specific Energy (Wh/kg) response time* (s) ECOND (module)Aq ELIT (module)Aq ESMA (module)Aq1.510, LS Cable (cell)PC2.83, Maxwell (cell)AN2.73, NessCap (cell)PC2.73, Nippon Chemi-Con (cell)PC2.52, 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

22 JME BATTERY -- EC COMPARISON PROPERTYBATTERYEC Storage mechanismChemicalPhysical Power limitationReaction kinetics, mass transport Separator ionic conductivity Energy limitationElectrode massElectrode surface area Output voltageConstant valueSloping value (SOC known precisely) Charge rateReaction kinetics, mass transport Very high, same as discharge rate Cycle life limitationsPhysical stability, chem. reversibility Side reactions Life limitationThermodynamic stability Side reactions I-38

23 JME 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

24 JME Capacitor 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

25 JME V- 36

26 JME V- 37

27 JME

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

30 JME V- 75

31 JME

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34 ENERGY 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 Examples Electrochemical capacitor + film capacitor Electrochemical capacitor + battery Electrochemical capacitor + fuel cell Specific Energy Specific Power Technology 1 Technology 2 Combination 409

35 JME

36 V- 85 ECs Provide Immediate Cost Savings in System

37 JME 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

38 JME 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 T = charge time ioio -i o current  voltage Vo Vo/2 Time 2T Energy efficiency = (n+4/3)/(n+8/3) E out / E window = n(n+4/3)/(n+2) 2 0 T ~2T ~3T ~4T ~5T

39 JME Series-RC Circuit Model Results Energy Cycle Efficiency T = charge time Discharge Energy Out n = T/RC CC Charge/discharge: V o /2 -V o -V o /2

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

41 JME 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

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

43 JME Anomalous Capacitance of Carbon 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 // H 2 SO 4 // PbO 2 Capacitor

44 JME Cyclic 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 Note all of the area (capacitance) that becomes available at very low potentials (<0 V SHE).

45 JME CAPACITOR POWERED PURE ELECTRIC TRUCK..... V- 93

46 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?)


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