Session 1:
Advances in EC Capacitor Materials and Cell Design
In this session, we will explore the latest 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
Prof. Katsuhiko NAOI is a professor of chemistry at Tokyo University of Agriculture & Technology (TAT). He got his BS, MS and Ph.D. from Waseda University, Tokyo and post-doc at The University of Minnesota. His major is energy chemistry, environmental, and materials science. His research interests are advanced supercapacitors, high-power Li-ion batteries, and fuel cells for automotive/stationary applications. He serves as the chair of The Capacitor Technology Committee of The Electrochemical Society of Japan.
SESSION AGENDA
The Next Generation Supercapacitors in Synergy with Li-Ion Technology
Katsuhiko Naoi, Professor of Chemistry, Institute of Symbiotic Science & Technology, Tokyo University of Agriculture & Technology
Dynamic energy storage by supercapacitors is an important energy and environmental technology that is highly influential in advancing our future automotive and wireless society. Unlike batteries, supercapacitors are efficient energy storage devices that exhibit long lifespans and rapid charging and discharging. Figure shows the comparative studies of Li-ion battery and supercapacitor with respect to many different characteristics. Thus, the capacitor technology is recently regarded as a reliable, durable and safe technology with increasing effectiveness when utilized in smart mobility and wireless applications. However, since electrochemical capacitors generally have low energy densities (below 10 Wh L-1), their uses are limited and cannot fully meet performance demands required by existing electrical equipment and electronic devices. In this connection, supercapacitors have been vigorously researched in hopes to improve their energy density. Hybrid approaches combining lithium-ion battery (LIB) and electrochemical capacitor (EDLC) electrodes can overcome the energy density limitation and can triple and more that of the conventional electrochemical capacitors.
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Development of a High-Voltage Capacitor Prototype in Environmental Friendly Salt Aqueous Electrolyte
François Béguin, Professor, Poznan Technical University
Most of the commercially available electrochemical capacitors are based on activated carbon (AC) electrodes in an organic electrolyte, e.g., tetraethylammonium tetrafluoroborate in acetronitrile or propylene carbonate. The reason of this choice is linked to the high voltage (2.7-2.8 V), and consequently high energy density, which is reached with these electrolytes as compared to conventional aqueous electrolytes, such as KOH or H2SO4 solutions, where the maximum voltage is lower than 0.8 V. Recently, we have shown that voltage values up to 1.6-1.8 V can be reached with AC/AC systems in salt aqueous electrolytes, such as alkali sulfates or nitrates, giving promising perspectives for low cost, safe and environment friendly devices.
This presentation looks at the development of an optimized prototype based on salt aqueous electrolytes, using realistic materials. The selected current collectors are made from stainless steel which has been surface treated in order to improve the contact resistance. Since the voltage reached in the salt media is lower than in organic electrolyte, we have paid attention to the enhancement of capacitance by adding pseudo-capacitive contributions to the EDL capacitance. In particular, the addition of corrosion inhibitors to the electrolyte allows simultaneously to enhance the maximum potential of the positive electrode and to provide a pseudo-faradic redox contribution. The EDL capacitance has been improved by adjusting the porosity of the carbon electrodes to the size of electrolyte ions; carbons obtained by self-activation of biomass were found to present pores fitting the size of ions, with much narrower pore size distribution than the traditionally prepared activated carbons. Overall, by combining appropriate materials and aqueous electrolytes, we were able to design an AC/AC capacitor reaching the same energy density as in organic medium.
The presentation concludes by showing the performance of pouch-cell prototypes in various conditions of temperature and charge/discharge current. Parameters such as capacitance, series resistance, leakage current and self-discharge were measured during ageing of the capacitors in floating conditions.
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Redox Active Electrolyte for Supercapacitors
Elżbieta Frąckowiak, Professor, Poznan University of Technology
The capacitance of electrochemical capacitors can significantly be increased by faradaic reactions (pseudocapacitance) originated from the electrode material or electrolytic solution. The texture of the carbon material plays an important role because of the character of faradaic reactions which involve transport (diffusion) phenomena at the electrochemically available surface area. This presentation will be dedicated to electrolytic solutions with inorganic and organic redox species rich in iodides, bromides, vanadium, cerium, di-hydroxybenzenes and their derivatives. Generally, the main elements of these species point out a rich variety of oxidation states. The values of capacitance can easily exceed 1000 F/g per one electrode in some cases.
In this presentation, the redox activity of the electrolytic species was investigated in various pH solutions (acidic, neutral, alkaline). Experiments in two- and three-electrode cells could easily separate the charging/discharging processes for both electrodes, estimate the potential range of single electrodes and define the optimal operating voltage of the total capacitor system using typical galvanostatic discharging as well as floating measurements. A special attention will be devoted to the reversibility of the redox reactions as well as the capacitor stability during long term cycling, and eventual corrosion of the current collectors during cycling. The capacitance retention in some cases (iodides) was more than 100% without corrosive phenomena.
Prons and cons of redox active electrolytes will be presented.
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Design of Advanced Electrolytes for High-Voltage Electrochemical Double-Layer Capacitors
Andrea Balducci, Professor, University of Muenster, Institute of Physical-Chemistry--MEET Battery Research Center
In order to develop high-energy electrochemical double layer capacitors the introduction of advanced electrolytes is a key factor.
Advanced electrolytes need to display the following properties:
- High concentration
- High conductivity and low viscosity
- Large electrochemical stability window
- Low flammability
- High safety
In this presentation several types of electrolytes (solvent-free as well containing solvent) will be considered and critically investigated. Particular attention will be dedicated to two aspects:
- Influence of Aluminum corrosion of the performance of high voltage electrochemical double layer capacitors
- Influence of the salt concentration on the practical and theoretical energy of high voltage electrochemical double layer capacitors
The presentation concludes with a summary and discussion of next steps necessary for the further optimization of electrochemical double layer capacitors containing these advanced electrolytes.
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Novel Experimental Probes of Supercapacitor Performance
David J. Wesolowski, Director, Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center, Oak Ridge National Laboratory
The goal of the FIRST Energy Frontier Research Center is to develop experimentally-guided and validated computational models of the atomic-nanoscale interfacial processes that control the macroscopic behavior of electrode/electrolyte interfaces encountered in electrical energy storage and electrocatalysis. Novel neutron and synchrotron X-ray scattering methodologies and in situ scanning probe microscopies are used to validate the predicted structure and transport dynamics of polar solvent and room temperature ionic liquid electrolytes at nanotextured electrode surfaces as a function of surface morphology and hierarchical pore size distributions. This integrated computational and experimental approach has led to simple rules for the synthesis of nanotextured interfaces that maximize supercapacitor performance.
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Synthesis of Vertically Oriented Graphene Nanosheet Electrical Double-Layer Capacitors
Ronald Outlaw, Research Professor, Department of Applied Science, College of William and Mary
The growth of vertically oriented graphene nanosheet (VOGN) thin films by radio frequency plasma enhanced chemical vapor deposition (RF PECVD) for EDL capacitors is presented. The nanosheets are grown using an RF power of 1000 W on Ni foil substrates at temperatures between 550 and 850ºC using CH4/H2 or C2H2/H2 feedstock and proceeds by planar Volmer-Weber island impingement followed by vertical nanosheet growth. In this temperature range, significant C dissolution into the Ni substrate was observed and resulted in an ESR of 0.05 to 0.07 Ohms. The growth rate observed was found to be 70 nm/min using CH4 and 190 nm/min using C2H2 and is approximately linear with time. The specific capacitance of symmetric parallel plate EDLC devices fabricated, with films ~3 mm tall, was ~ 120 mF/cm2 (1 kHz) using CH4 and ~300 mF/cm2 (1kHz) using C2H2. The height of the films increased linearly with growth time with a slope of 0.5. Morphology deterioration with growth temperature, as indicated by SEM and Raman D band, shows increased specific capacitance up to 850º, but a corresponding reduction in frequency response. However, good frequency response (phase angle of -85º) at 120 Hz was observed for all growths tested, suggesting that EDL capacitors fabricated by this technique are excellent for filtering applications. The presentation includes:
- RF-PECVD synthesized VOGN on Ni growth mechanism using CH2 and C2H2 feedstock
- Ni substrate characterization
- Composition and surface analysis of VOGN films
- VOGN morphology vs growth temperature
- VOGN growth rate
- Ni substrate bias effects
- Specific capacitance and frequency response vs morphology
- Argon ion and plasma bombardment effects on capacitance
- Summary
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