Advances in EC Capacitor Materials and Cell Design
To enhance the value proposition of EC Capacitors by boosting cell performance, improvements in electrode capacity, power performance, electrolyte properties, electrode technology, and cell design are desired. ECCAP Session 1 reviewed advances in materials, including the development of advanced materials and processes to meet the pricing threshold of important markets, and in capacitor design, including the development of advanced asymmetric ECs.
Session Chairman:
Katsuhiko Naoi, Professor of Chemistry, Institute of Symbiotic Science & Technology, Tokyo University of Agriculture & Technology
Dr. Naoi is a professor of chemistry at the Institute of Symbiotic Science & Technology at Tokyo University of Agriculture & Technology (TAT). He also serves as a CTO of K&W Inc., a TAT venture company dealing with "Energy Solutions for the Future." With a Ph.D. from Waseda University, Tokyo, his research interests are advanced supercapacitors, future nanobatteries, and energy, environmental, and materials science.
SESSION AGENDA
Future Perspective of Electrochemical Supercapacitors and Hybrid EES Prof. Katsuhiko Naoi, Professor of Chemistry, Tokyo University of Agriculture & Technology
Abstract
Electrochemical capacitors utilize activated carbons for both positive and negative electrodes that show non-faradaic, double-layer charging-discharging mechanism in symmetric configuration. Thus the electrochemical capacitors are efficient energy storage devices that exhibit long lifespans and extremely rapid charge-discharge characteristics compared with batteries. Today, the capacitor technology is regarded as a promising means and has an additional advantage with increasing effectiveness when combined with solar and wind regenerative energy sources. In recent years, nano-composite battery materials have been vigorously researched in hopes to improve their energy density. Hybridizing battery and capacitor materials overcomes the energy density limitation of existing generation-I capacitors without much sacrificing the cycling performances. Normal battery-capacitor hybrids employ high-energy & sluggish redox electrode and low-energy & fast double-layer electrodes, possibly producing a larger working voltage and higher over-all capacitance. In order to smoothly operate such asymmetric systems, however, the rates of the two different electrodes must be highly balanced. Especially, the redox rates of the battery electrodes must be substantially increased to the levels of double-layer process. In this perspective we summarized various hybrid systems and show representative aqueous and non-aqueous asymmetric configurations for their energy-power enhancement. We attempt to identify the essential issues for the realizable hybrids and suggest ways to overcome the rate enhancement by exemplifying ultrafast performance of the Li4Ti5O12 nanocrystal prepared via a unique in-situ material processing technology under ultra-centrifuging.
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Nanostructured Materials for High-Energy Electrochemical Capacitors Prof. Patrice Simon, Professor, Université Paul Sabatier
Abstract
This presentation will present different strategies for improving the energy density of supercapacitors; it will be focused on the design and synthesis of nanostructured material and materials /electrolyte couples for Electrical Double Layer Capcitors (EDLCs) and pseudocapacitors. We will present results about:
Modelling of ion adsorption in nanoporous carbons, for understanding capacitance increase in small pores
Nano-structured carbons in ionic liquid mixture electrolyte for high voltage and large temperature range EDLCs
Nano-structured metal oxide with high-rate charge-discharge capability, for pseudocapacitive and hybrid systems.
The presentation concludes with a summary and discussion of next steps toward the design of optimized nano-structured materials/electrolytes for high energy density supercapacitors.
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IES: a BMBF Project Devoted to the Development of Greener and Safer Supercapacitors Prof. Stefano Passerini, Professor, University of Muenster
Abstract
IES project has the scope to develop safe and high performance electrochemical double layer capacitors (SCs) for application in hybrid and electric vehicles (HV, EV). Coordinated by the University of Muenster, this BMBF (Bundesministerium für Bildung un Forschung) funded project involves the industrial partners IOLITEC and WIMA. Considering the limitation of the state of the art supercapacitors, especially in terms of safety, the following fields of investigations are contemplated:
Non-flammable (or high flash-point) electrolyte solutions based on mixtures of ionic liquids and organic carbonates (IL-O mixtures) having high performance (ionic conductivity, thermal stability) in the range of temperature between -20°C and 80°C;
Development of activated carbon for double layer electrical storage with improved energy storage;
Synthesis of graphenes by electrochemical exfoliation in ionic liquid media and their application as ultra-fast, high-capacity electrodes for supercapacitor application;
Composite electrodes and separators based on the use of natural cellulose.
The investigations are presently being pursued with the goal to improve the knowledge and the technology on safer and greener materials and processes for supercapacitors. The effectiveness of the investigation will be verified via the realization and performance and safety evaluation of prototypes containing IL-OE mixtures as electrolytes. The status of IES project activities will be presented and discussed.
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Novel Concept of Supercapacitor Based on Electrolyte Activity Prof. Elżbieta Frąckowiak, Professor, Poznan University
Abstract
The performance of electrochemical capacitors is typically based on the electrostatic attraction of ions at the electrode/electrolyte interface, where the value of capacitance depends on the electrochemically available surface area. However, capacitance can be enhanced if additional faradaic reactions will take place; such phenomenon is named pseudocapacitance. Pseudocapacitive properties can be originated from electrode materials (transition metal oxides, conducting polymers, nitrogen/oxygen rich carbons, carbons with electrosorbed hydrogen).
This presentation will show that the electrolytic solution can also play the role of pseudocapacitance source. A few examples of electrolytes with electrochemically active redox couples will be given.
Iodides in contact with well adapted carbon material supply an enormous capacitance exceeding 1,000 F/g. The main redox pair is I-1/I2 ; iodine forms further I3-1 and I5-1 species. The redox reactions with iodides are fully reversible and the capacitor system operates with a long durability over 10,000 cycles. This redox pair operates in a narrow range of potential.
Vanadium species supply a rich variety of redox reactions . They can be used as a source of pseudocapacitive phenomena. They have been tested together with the iodide/iodine redox pair.
Di-hydroxybenzenes dissolved in aqueous solution are another interesting redox couple affecting greatly the capacitor performance. The grafting of quinone/hydroquinone groups should be considered in this case. The process is strongly depending on pH of the electrolytic solution.
In summary, the profits and eventual disadvantages of redox active electrolytes will be discussed.
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Optimization of High-Voltage EDLCs in Salt Aqueous Electrolytes Prof. François Béguin, Professor, Poznan University of Technology
Abstract
Most of the research effort on supercapacitors is oriented to energy density enhancement. As given by formula E = ½CU2, energy is highly depending on voltage U, the latter being controlled by the stability window of the electrolyte. Therefore, the commercially available systems are usually based on organic electrolyte, e.g., NEt4BF4 in acetonitrile, allowing 2.7–2.8 V to be reached. Due to the relatively high cost of these devices and environmental unfriendly character of the electrolyte, alternative solutions such as the use of protic electrolytes must be investigated.
Acid (H2SO4) and base (KOH) solutions are the most often considered aqueous electrolytes for supercapacitors, giving voltages of 0.7–0.8 V. Recently, applying alkali sulfate solutions with pH ≈ 6.5, we have demonstrated good galvanostatic cycling performance of AC/AC capacitors up to ca. 2 V. Such high voltage value is possible owing to the high over-potential for di-hydrogen evolution consequently to the pH increase in the porosity of the carbon negative electrode.
This presentation intends to detail some of the technological/scientific aspects investigated to develop a prototype based on the AC/AC concept in salt aqueous solutions. It will include:
Two- and three-electrode cell investigations in presence of various sulfate salt solutions and stainless steel current collectors
The design of optimized porous carbons
The implementation of electrolyte additive to reduce corrosion of stainless steel under high voltage
Accelerated aging by floating at various temperatures.
The presentation concludes with the construction and testing of prototype pouch cells implementing the optimal components defined from the previous study.
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Composite Separators: a Novel Strategy for Improving EDLC Performance Dr. Charles Gibson, Chief Technical Officer, Shamrock Energy Corporation
Abstract
Separators commonly used in EDLCs include wet-laid cellulose (i.e., paper), microporous polypropylene films, and fibrous mats. This presentation looks at inorganic/polymer composites as alternatives to conventional separators. The focus of this talk will be on comparison of the properties/performance of composite separators versus conventional separators. Main topics include:
Physical characteristics of the separators (e.g., thickness, porosity, tortuosity, resistance).
Basic performance characteristics of EDLCs containing the composite separator versus EDLCs with conventional separators (e.g., capacitance, ESR, aging).
Evidence for improved performance in EDLCs with composite separators versus conventional separators (e.g., higher capacitance, better high-temperature performance.)
This presentation concludes with a summary and discussion of next steps towards commercialization of the composite separators.
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How Electrochemical Science can Improve the EDLC Performance Dr. Yurii Maletin, Chief Scientist, Yunasko
Abstract
In spite of EDLC’s are normally presented as physical devices with no chemical or electrochemical processes (like the charge or mass transfer across the electrode-electrolyte interface) being involved in their operation, these are electrochemical methods that can help to better understand “intimate details” of the EDLC behavior. In our electrochemical studies we have found how a combination of various electrochemical methods can show a way to significantly improve all the key performance characteristics of EDLC devices: energy and power density, efficiency and cycle life. Some examples to illustrate this statement are:
cyclic voltammetry (CV) measurements with a special reference electrode developed by YUNASKO for organic electrolytes have shown a way to select the best materials for positive and negative electrodes in order to increase the working voltage and/or cycle life;
electrochemical impedance spectroscopy (EIS) clearly demonstrates whether the “ideal” capacitor behavior takes place in a selected system or some “parasitic” processes interfere;
special measurements with the use of rotary disc electrode (RDE) enable to best match the carbon electrode and organic electrolyte so as to provide the highest electrolyte mobility in the nanoporous electrode matrix and thus to significantly reduce the EDLC internal resistance;
a combination of electrochemical methods enables to optimize the chemistry and the ratio of Li-ion and EDLC components in the electrodes of hybrid devices thus providing a nine-fold increase in energy density as compared with conventional carbon-carbon EDLC devices.
