Abstract
Research in electrochemical energy storage is converging to target systems with battery-level energy density, and capacitor-level cycling stability and power density. One approach is to utilize redox-active electrolytes that add faradaic charge storage to increase energy density of supercapacitors. Aqueous redox-active electrolytes are simple to prepare and to up-scale; and, can be synergistically optimized to fully utilize the dynamic charge/discharge and storage properties of activated-carbon based electrode systems. However, aqueous redox-enhanced electrochemical capacitors (redox ECs) have performed relatively poorly, primarily due to the cross-diffusion of soluble redox couples, reduced cycle life, and low operating voltages. In this presentation, we show that these challenges can be met by the use of liquid-to-solid phase transitions of redox electrolyte molecules, and their reversible confinement in the pores ( >2 nm) of high-surface-area electrodes. This approach is demonstrated by the use of bromide catholyte and modified hydrophobic cations (e.g., viologens and tetrabutylammonium) that together induce reversible solid-state complexation of Br2/Br3–. This mechanism solves the cross-diffusion issue of redox ECs without using costly ion-selective membranes, and has the added benefit of stabilizing the reactive bromine generated during charging. Furthermore, our device is simple to fabricate and uses large >10 mg carbon electrodes (BET specific surface area of 2470 m2/g, mass loading of 12.7 mg/cm2) to ensure that our performance metrics are practical.
Using the concepts learned from this 1st generation configuration, we created a hybrid electrochemical storage system by designing and implementing stackable bipolar pouch cells with corrosion-resistant, non-metallic current collectors. The device using ZnBr2 battery chemistry and tetrabutylammonium cations enables high-power aqueous electrochemical energy storage. The specific energy of this system can be made competitive with that of a Pb-acid battery and has a specific power that is well within the range of commercial capacitors.
Biography
Seung Joon Yoo is an Assistant Professor of Materials Science and Engineering at GIST (Gwangju Institute of Science and Engineering) in Korea. He received his Ph.D. in Chemistry under the supervision of Professor R. Daniel Little from the University of California, Santa Barbara (UCSB) and was a postdoctoral research associate in Professor Galen D. Stucky’s research group at UCSB. He has published 13 papers in peer-reviewed journals and has received two NSF sponsored Fellowships. His research focuses on design and synthesis of redox-active electrolytes and device engineering for electrochemical energy storage. He is particularly interested in a fundamental understanding of the structure-function-performance relationships in organic redox species and applying that knowledge to make commercially viable electrochemical energy storage devices using aqueous redox-active electrolytes.