ELECTROCHEMICAL CAPACITORS (ECs): Technology, Applications, and Needs

1. 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

2. 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

3. C = eoA/d • 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

4. 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

5. Electrostatic • Air • Mica • Film • Ceramic + - C • Electrolytic • Aluminum • Tantalum C1 - - + - + C2 >>C1 • Electrochemical • Carbon-carbon • Metal oxide symmetric • carbon-asymmetric + - - + C2 C1 CAPACITOR TYPES

6. CAPACITOR TECHNOLOGY COMPARISON 1.0 MJ (277 Wh) Energy Delivery System

7. 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

8. 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

9. 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

10. 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

11. 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. 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

13. 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

14. 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

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

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

18. 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:

19. Typical DLC Design I-19

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

21. 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

22. BATTERY -- EC COMPARISON I-38

23. 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

24. 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

25. V-36

26. V-37

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

30. V-75

34. 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

36. ECs Provide Immediate Cost Savings in System V-85

37. 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

38. 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

39. 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

40. 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

41. 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

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

43. Anomalous Capacitance of Carbon 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

44. 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).

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

46. 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?) EC Technology Needs