# Presentation: 352577

## Technology, Applications, and Needs

**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, Inc.

- 17210 Parkland Drive
- Shaker Heights, OH 44120
- 216-751-9537
- <jmecapacitor@att.net>

- _Facility_
-   2500 ft2 laboratory
-    Total EC capacitor focus

## CAPACITOR BASICS

## Primary

**Primary**

## CAPACITOR TYPES

## 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 | 30- 100 | .02- 0.1 | 2 - 20 | ~1 |

## 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**

## DOUBLE LAYER CAPACITOR CONCEPT

- Discovered by Helmholtz
- C_ ~_ 10 F/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**

## Electric Double Layer Model

**Capacitor**

- Use of high-surface-area electrodes
- produce very high F/cm3

**I-13**

## Typical  EC  Cell Cross-section

**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

- With electrolyte

## 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-2****1**

## Porous Electrode--Transmission Line Response

## Porous Electrode Electrical Response

- Low frequency limit

- R  equivalent series resistance
-  = l2/2V = l2 /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 = SCdl
- 	n = number of pores

- R             R+

- I

- I

## Series RC Circuit

**Series RC Circuit**

**Model Surfaces**

## Slide 16

## Slide 17

## TIME  CHARACTERISTICS  OF  A  LOAD  DICTATE

**THE  APPROPRIATE  EQUIVALENT  CIRCUIT  MODEL**

- Long times:

- C

- i=a*exp(b*V)

- Intermediate times:

- Short times:

## Typical DLC Design

**Typical DLC Design**

**I-19**

##    LARGE EC PRODUCTS

- ESMA

- Nippon
- Chemi-Con

- ECOND

- ELIT

- NESS

**   ****LARGE EC PRODUCTS**

- LS Cable

- Maxwell

## State of the Art Large EC Cells/Modules

| 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

## BATTERY --  EC COMPARISON

| 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 | 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**

## 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**

## Capacitor Powered Pure Electric Bus

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

** **

- CAPACITOR ONLY ENERGY STORAGE

## V-36

## V-37

## Slide 27

## Slide 28

## Bridge Power Example(Four systems deployed in Japan)

**V-****34**

## V-75

**V-****75**

## Slide 31

## Slide 32

## Slide 33

## 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

## ECs Provide Immediate Cost Savings in System

**ECs Provide Immediate Cost Savings in System**

## 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

## 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

**Energy efficiency =  (n+4/3)/(n+8/3)**
**E****out**** / E****window**** =  n(n+4/3)/(n+2)****2**

**  **

## Series-RC Circuit Model Results

- Energy Cycle Efficiency
- 				 T = charge time

- Discharge Energy Out

- n = T/RC

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

** **

## ELECTROCHEMICAL CAPACITOR DESIGNS

- Symmetric
- Asymmetric

- Double layer

- Battery electrode

- Faradaic and other processes

**ELECTROCHEMICAL CAPACITOR DESIGNS**

## 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

## Anomalous Capacitance of 

**Some Carbon at Low Potentials**

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

## 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**

## Cyclic Voltammogram of Carbon Electrode

**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).  **

## CAPACITOR POWERED PURE ELECTRIC TRUCK

- .
- .

- .

**V-****93**

## 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?)