Capacitive Storage Science

1. Capacitive Storage Science Chairs: Bruce Dunn and Yury Gogotsi Panelists: Michel Armand (France) Martin Bazant Ralph Brodd Andrew Burke Ranjan Dash John Ferraris Wesley Henderson Sam Jenekhe Katsumi Kaneko (Japan) Prashant Kumta Keryn Lian (Canada) Jeff Long John Miller Katsuhiko Naoi (Japan) Joel Schindall Bruno Scrosati (Italy) Patrice Simon (France) Henry White

2. Capacitive Storage Science 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 provide versatile solutions to a variety of emerging energy applications including harvesting and regenerating energy in transportation, industrial machinery, and storage of wind, light and vibrational energy. This is enabled by their sub-second response time. *Halper, M.S., & Ellenbogen, J.C., MITRE Nanosystems Group, March 2006

3. Capacitive Storage Science:technology challenges • Capacitor Systems and Devices - Increased energy density - Longer life cells - Self-balancing - Cost • Electrolytes for Capacitor Storage Design electrolytes for EC operation: high ionic conductivity; wide electrochemical window, chemical and thermal stability; non toxic, biodegradable and/or renewable • EDLC and Pseudocapacitive Charge Storage Materials New strategies are needed to improve power and energy density of charge storage materials

4. Capacitive Storage Science:current status • Capacitor Systems and Devices High specific capacitance (100 F/g) and fast response time (~ 1 sec), but energy storage (2-10 wh/kg) not sufficient for many apps Long shelf (10 yr) and cycle (>1M) life • Electrolytes for Capacitor Storage Traditional Electrolytes: - aqueous (KOH, H2SO4) - corrosive, low voltage - organic (AN or PC and [Et4N][BF4] or [Et3MeN][BF4]) - low capacitance, toxicity and safety concerns Ionic Liquid Electrolytes - safer, but viscosity too high, conductivity too low for capacitor applications; improvements in properties from mixing with organic solvents • Theory and Modeling Variety of approaches available – continuum, atomistic, ab initio; all have advantages and limitations

5. Capacitive Storage Science: current status EDLC Charge Storage Materials: Majority of present day EDLC devices are based on activated carbon Multifunctional Materials for Pseudocapacitors: Pseudocapacitive materials generally exhibit higher specific capacitance and energy density relative to high-surface-area carbon

6. Capacitive Storage Science:basic-science challenges, opportunities, and needs • EDLC Charge Storage Materials - Materials utilizing only double layer storage require understanding of pore structure and ion size influences on charge storage - Identify new strategies in which EDLC materials exploit both multiple charge storage mechanisms; combine double layer charging and pseudocapacitance to enhance energy and power densities • Multifunctional Materials for Pseudocapacitors - The underlying charge-storage mechanisms for pseudocapacitive materials are not well understood. - Opportunities for new directions in pseudocapacitor materials; single phase and multi-phase; nanostructure design of novel 3-D electrode architectures with tailored ion and electronic transport

7. Capacitive Storage Science:basic-science challenges, opportunities, and needs • Electrolytes for Capacitor Storage - Create new electrolyte formulations enabling high voltage devices and revolutionary electrode combinations for capacitive storage; - New salts, new solvents, immobilizing matrices designed for capacitor storage • Theory and Modeling - Structure and dynamics of solvent and ions in non-polar nanopores. - Electronic characteristics of carbon and MOx electrodes. - Validation against simple model experiments.

8. Capacitive Storage Science:basic-science challenges, opportunities, and needs • Capacitor Systems and Devices Higher volumetric and gravimetric energy density with less than one second response time: Increased voltage, increased specific capacitance Improved device safety: Non-toxic, non-flammable electrolyte Regenerative Energy Capture using Capacitors: 40% of energy is recovered

9. Ralph Brodd Patrice Simon Ranjan Dash John Ferraris* * Subpanel leader Capacitive Storage Science:Materials for Electrical Double Layer Capacitors Subpanel members

10. Capacitive Storage Science:PRD: Charge Storage Materials by Design Scientific challenges Summary of research direction Enhance EDLC materials performance by creating designed architectures, surface functionality, tailored porosity, and thin conformal films, matched synergistically with appropriate electrolyte systems. Identify new strategies in which EDLC materials simultaneously exploit multiple charge storage mechanisms. Potential scientific impact Potential impact on EES Establish nanodimensional spatial control of the interface utilizing tethered functionalized molecular wires. Understand ion transport across interfaces EDLC systems will be rationally designed to revolutionize their utilization throughout the energy sector Develop new EDLC materials and architectures to dramatically boost energy and power densities Anticipate impact in decades

11. Capacitive Storage Science:Materials for Electrical Double Layer Capacitorstechnology challenges New strategies are required to improve both power and energy density of EDLC materials • Materials Synthesis • Designed Architectures • Modeling Input/Output

12. Capacitive Storage Science:PRD Charge Storage Materials by Design Systematic guidelines are currently lacking for development of improved charge storage materials Materials utilizing only double layer charge storage • Requires fundamental understanding of pore structure and “effective” ion size • Requiresnew synthesis methodology

13. Capacitive Storage Science:PRD Charge Storage Materials by Design Materials utilizing mixed charge storage • Highly reversible redox-active functionalities on high surface area electrodes • Thin dielectric or conducting coatings on ordered high surface area materials • Surfaces decorated with nanowires having active functionality • Requires new synthesis methodology

14. Capacitive Storage Science:PRD Charge Storage Materials by Design Materials utilizing synthetic ordered architectures Electrode materials with controlled pore size and surface area deposited in ordered geometries with intimate contact to current collectors • Requires new synthesis methodology

15. Capacitive Storage Science:PRD: Charge Storage Materials by Design • Materials Synthesis • Designed Architectures Development of new EDLC materials and architectures will dramatically boost: Power and Energy!

16. Samson Jenekhe, sub-Panel lead Prashant Kumta Jeffrey Long Katsuhiko Naoi John Newman Capacitive Storage Science: Sub-panel on Materials for Pseudocapacitors and Hybrid Devices

17. Capacitive Storage Science: PRD: Multifunctional Materials for Pseudocapacitors and Hybrid Devices Motivation: Pseudocapacitors enable energy densities significantly higher than for double-layer capacitors. Challenge: Simultaneously maximize both energy density and power density, and enhance lifetime. New Research Directions • Investigation of new materialsbeyond metal oxides • Multifunctional architecture. • Rational design of materials and structures. • Understand fundamental charge-storage mechanisms.

18. Capacitive Storage Science: Multifunctional Materials for Pseudocapacitors and Hybrid Devices New Materials + Architectures Vanadium Nitride, VN nanocrystals = New opportunities for fundamental understanding and scientific advances.

19. Keryn Lian* Bruno Scrosati Michel Armand Wesley Henderson Capacitive Storage Science: Electrolyte subpanel members * Subpanel lead

20. Bulk Device Performance Interfacial Capacitive Storage Science:technology challenges Aqueous and non-aqueous electrolytes with the following properties: ■ immobilized matrix ■ produced from sustainable sources ■ high ionic conductivity ■ chemical and thermal stability ■ large electrochemical stability window (>5V) ■ non-toxic, biodegradable and/or recyclable ■ exceptional performance with long device lifetime

21. Capacitive Storage Science:PRD Topic:Molecular Understanding of Electrolyte Interactions in Capacitor Science Fundamental lack of understanding: solvent-salt structure and physical properties. • Bulk Properties • Diverse materials (salt, solvent, immobilizing matrices, …) • Various conditions (temperature, concentration, …) • Experimental measurements (phase diagrams, spectroscopy, …) • Modelling and simulations • Interfacial Effects • Same approaches to explore interfacial and confined pore interactions differ from the bulk • Performance • Create a fundamental understanding of link between device performance and bulk/interfacial molecular interactions.

22. Capacitive Storage Science:PRD Topic:Molecular Understanding of Electrolyte Interactions in Capacitor Science Scientific challenges Summary of research direction Explore new salts, new solvents, immobilizing matrices designed for capacitor storage Examine bulk properties (solvent-salt interactions), interfacial effects and behavior in confined spaces using measurements and modelling Understand effect of additives and impurities The ideal electrolyte is an immobilized material produced from sustainable sources, which has high ionic conductivity; wide electrochemical, chemical and thermal stability; and is non toxic, biodegradable and/or renewable Potential scientific impact Potential impact on EES Understanding the mechanism of charging and degradation New electrolyte formulations enabling revolutionary novel electrochemical capacitor devices Knowledge will cross-over to battery systems Enable high power technologies for load levelling, improve energy efficiency. Enable novel energy recovery applications, HEVs and PHEVs

23. Martin Bazant (MIT), sub-panel lead Katsumi Kaneko (Chiba University, Japan) Lawrence Pratt (Los Alamos) Henry White (University of Utah) Capacitive Storage Science: Theory & Modeling sub-panel members

24. Capacitive Storage Science:current status of modeling • Equivalent circuit models (transmission-line models) • Pros: Simple formulae, fit to experimental impedance spectra • Cons: No nonlinear dynamics, microstructure, chemistry… • Continuum models (Poisson-Nernst-Planck equations). • Pros: analytical insight, nonlinear, microstucture • Cons: point-like ions, mean-field approximation, no chemistry • Atomistic models (Monte Carlo, molecular dynamics). • Pros: molecular details, correlations, atomic mechanisms. • Cons: <10,000 atoms, < 10ns, limited chemical reactions. • Quantum models (ab initio quantum chemistry and DFT) • Pros: Mechanisms and chemical reactions from first principles. • Cons: <100 atoms, <ps, periodic boundary conditions VERY FEW MODELS HAVE BEEN APPLIED TO SUPERCAPACITORS

25. Capacitive Storage Science:priority research directions for modeling • Mathematical theory (beyond equivalent circuits) • Derivation of nonlinear transmission line models for large voltages • Modified Poisson-Nernst-Planck equations (steric effects, correlations…) • Continuum models coupling charging to mechanics, energy dissipation,… • Physics & chemistry of electrolytes • Develop accurate models for MD and MC simulations • Entrance of ions into nanopores -- desolvation energy and kinetics. • Ion transport, wetting, surface activation, and chemical modification. • Physics & chemistry of electrode materials • Electron and ion transport in capacitor electrodes. • Theory of capacitance of metal oxidesand conducting polymers. • Validation against simple model experiments • Ordered arrays of monodisperse pores, single carbon nanotubes. • Spectroscopic and x-ray analysis of ions and solvent in confined spaces

26. Capacitive Storage Science:Theory and Modeling Scientific challenges Summary of research direction Fundamental understanding and modeling tools for supercapacitors across all length and time scales. Continuum, atomistic, & quantum models Potential scientific impact Potential impact on EES • Discovery of new physical phenomena- nanopore behavior, nonlinear dynamics… • New models at system, microstructure, molecular, and electronic levels • New multi-scale simulation methods • Models for rational design of EES systems • Prediction of new materials • Increased power and energy density • Time scale: decades to centuries

27. Andrew Burke John R. Miller Pat Moseley Joel Schindall Capacitive Storage Science: Sub-panel members: Capacitive Devices and Systems

28. Capacitive Storage Science:Capacitive devices and systems Scientific challenges Summary of research direction Develop and use efficient, low cost and safe capacitive products to efficiently harvest and recover waste energy in applications that include electrical grid storage, renewable solar and wind energy, transportation, industrial stop-go machinery, mining, and microstorage of light, vibration, and motion energy New approaches for higher specific capacitance :electrode materials with improved morophology, uniform micropores, higher cell voltages, non-toxic, high conductivity, electrolytes, and low resistance separator materials Potential scientific impact Potential impact on EES Improved understanding of fundamental capacitive energy storage and optimization of a device as a system Improved material synthesis and processing Efficient, fast, distributed capacitive energy storage for a wide range of applications

29. Capacitive Storage Science:PRDs: Basic science of Capacitive Devices and Systems • Increased energy density • Longer life at high voltages and temperatures • Self-balancing series strings of cells without electronics • Safe failure modes under extreme conditions • Technologies to enable reduced device cost