# Presentation: 186725

## Capacitive Storage Science

- Chairs:	Bruce Dunn (UCLA), Yury Gogotsi (Drexel)
- Panelists:
- 		Michel Armand (Amiens)		Martin Bazant (MIT)
- 		Ralph Brodd (Broddarp)		Andrew Burke (UC Davis)
- 		Ranjan Dash (Maxwell)		John Ferraris (UT Dallas)
- 		Wesley Henderson (USNA)	Sam Jenekhe (U. Washington)		Katsumi Kaneko (Chiba Univ.) 	Prashant Kumta (Carnegie-Mellon)
- 		Keryn Lian (U. Toronto)		Jeff Long (NRL)				John Miller (JME)			Katsuhiko Naoi (Tokyo Univ.)		Joel Schindall (MIT)			Bruno Scrosati (Rome)			Patrice Simon (Toulouse)		Henry White (Univ. Utah)

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

## Energy vs. Power

**Supercapacitors bridge between batteries and conventional capacitors**

- Supercapacitors are able to attain greater energy densities while still maintaining the high power density of  conventional capacitors.
- Supercapacitors are a potentially versatile solution to a variety of emerging energy applications based on their ability to achieve a wide range of energy and power density.

**Energy – the capacity to do work**
**Power – how fast the energy is delivered**

***Halper, M.S., & Ellenbogen, J.C., MITRE Nanosystems Group, March 2006**

- Regone plot of energy storage systems*

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

## Capacitive Storage Systems

**Electrochemical **
**Capacitors**

**EC Double Layer Capacitor**

**Pseudocapacitors**

***Non-Faradaic***
***(no transfer of charge)***

***Pseudocapacitance***
***Charge transfer through surface***
***Faradaic, redox reactions***

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

## Electrochemical Double Layer Capacitors

-  (EDLC)

- EDLCs store charge electrostatically at electrode/electrolyte interface as charge separation.
- 	Cdl: 10-50 F/cm2*
- There is no charge transfer between electrode and electrolyte.
- Intrinsically high power devices (short response time), limited energy storage, very high cycling stability (~106).
- Different forms of high surface area carbon are used as an electrode material:
-   	activated carbon
-   	carbon aerogels
-   	carbon nanotubes

***Conway, B. E., Birss, V. & Wojtowicz, J. **
***Journal of Power Sources***** 66, 1-14 (1997)**** **

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

## Pseudocapacitors

**H****+**** or Li****+**

- Pseudocapacitors store by charge transfer between electrode and electrolyte.
- The charge is transferred at the surface or in the bulk near the surface through adsorption, redox reaction and intercalation of ions.
- Pseudocapacitors can achieve higher specific capacitance and energy density than EDLCs.
- 	**ex. Hydrous RuO****2 ****(700+ F/g)***
- Electrode materials:
- 	**Metal oxides**
**	****Conducting polymers**

***Zheng, J.P., Jow, T.R., *****J. Power Sources***** 62 (1996) 155**

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

## Capacitive Devices and Systems

- How do we improve the specific gravimetric and volumetric capacitance without compromising response time?
- Are asymmetrical configurations (e.g. battery cathode/capacitor anode) a viable trade-off between lifetime decrease and energy density increase?
- Can form factor and packaging changes increase energy and power density?  Can we put multi-cells together in a simple package?
- How can we improve the current collector properties (contact resistance, corrosion resistance, etc.)?

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

**www.answers.com/topic/capacitor**

## Materials for EDLCs

- What is the role of surface functional groups in determining specific capcitance?
- How does the pore-solid architecture and atomic arrangement affect the high current density behavior of carbon? Can we design this?
- What is the relationship between electrolyte ion size and pore size? Will the same dependence be observed for anions and cations?
- Can carbon nanotubes deliver a high specific capacitance and high power?

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

**A. G. Pandolfo, A. F. Hollenkemp, **
***J. Power Sources***** 157 (2006) 11-27**

## Materials for Pseudocapacitors

- Are there materials that exhibit energy and power density better than RuO2?
- Are transition metal non-oxides viable materials for super-capacitors?  How do they interact with the electrolyte?  How can the chemical stability be improved?
- Can pseudocapacitance be added through chemical or electrochemical modification on top of EDLC to enhance capacity?
- How can we exploit organic redox couples to improve energy/power density?

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

**H. Kim, B. Popov, *****J. Electrochemical Soc.***** 150 (2003) 1153**

**Conway, B. E., Birss, V. & Wojtowicz, J. **
***Journal of Power Sources***** 66, 1-14 (1997)**

## Electrolytes

- Are there organic electrolytes with the benefits of acetonitrile, without the safety issues?
- How can we exploit ionic liquids to improve safety, temperature window, electrochemical window, etc.?
- What effect do the IL anions and cations have individually on various properties (viscosity, pore size sieving, performance, etc.)?
- What about IL/organic mixtures?  Polymer electrolytes?  IL/polymer composites?  Other mixtures/composites?

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

**lem.ch.unito.it/didattica/infochimica/Liquidi%20Ionici/Composition.html**

**J. Chmiola et al., *****Science***** 313 (2006) 1760**

## Theory and Modeling

- How do we model electrostatic correlations (i.e. move away from mean-field approximations)?
- What can theory tell us about the double layer at the nano scale?  How does pore size and geometry affect capacitance?
- What do we know about electron-transfer at the nano scale? Can we achieve higher rates?
- Can we use theory to identify new materials and architectures for supercapacitors?
- How do we experimentally test/guide the models?

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

**electrochem.cwru.edu/ed/encycl/art-c03-elchem-cap.htm**
**www-math.mit.edu/~bazant/research/**

## Panel Discussions (Monday PM)

| Sub-Panel Topic | Primary Speaker
 Review state-of-the-art
 Identify research needs | Sub-Panel Members |
| --- | --- | --- |
| Capacitive Devices and Systems  | Andrew Burke 
        (2:15) | John Miller
Joel Schindall |
| Materials for Electrical Double Layer Capacitors |           Patrice Simon
                  (2:35) | Ralph Brodd
Ranjan Dash
John Ferraris
 |
| Pseudocapacitors and Hybrid Devices |           Sam Jenekhe
                   (2:55) | Jeff Long
Prashant Kumta
Katsuhiko Naoi |
| Electrolytes |            Michel Armand
                    (3:30) | Wesley Henderson
Keryn Lian
Bruno Scrosati |
| Theory and Modeling  |          Katsumi Kaneko
                    (3:50) | Martin Bazant
Henry White |

***Basic Energy Sciences          Workshop on “Capacitive Storage Science” April 2-5, 2007***

*** Sub-panel Lead****               *****Core writer**