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 Energy – the capacity to do work Power – how fast the energy is delivered 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. Regone plot of energy storage systems* *Halper, M.S., & Ellenbogen, J.C., MITRE Nanosystems Group, March 2006 Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007

Capacitive Storage Systems Electrochemical Capacitors EC Double Layer Capacitor Pseudocapacitors Pseudocapacitance Charge transfer through surface Faradaic, redox reactions Non-Faradaic (no transfer of charge) + - Electrode Electrolyte + Electrode Electrolyte M+n M+n+1 - Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007 3

Electrochemical Double Layer Capacitors (EDLC) EDLCs store charge electrostatically at electrode/electrolyte interface as charge separation. Cdl: 10-50 mF/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 + - Electrolyte Electrode Charged E *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 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 RuO2 (700+ F/g)* Electrode materials: Metal oxides Conducting polymers + - e- A Electrolyte H+ or Li+ *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.)? www.answers.com/topic/capacitor Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007

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? A. G. Pandolfo, A. F. Hollenkemp, J. Power Sources 157 (2006) 11-27 Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007

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? Conway, B. E., Birss, V. & Wojtowicz, J. Journal of Power Sources 66, 1-14 (1997) H. Kim, B. Popov, J. Electrochemical Soc. 150 (2003) 1153 Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007

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? lem.ch.unito.it/didattica/infochimica/Liquidi%20Ionici/Composition.html J. Chmiola et al., Science 313 (2006) 1760 Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007

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? electrochem.cwru.edu/ed/encycl/art-c03-elchem-cap.htm www-math.mit.edu/~bazant/research/ Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007

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 * Sub-panel Lead *Core writer Basic Energy Sciences Workshop on “Capacitive Storage Science” April 2-5, 2007