Development of a high frequency supercapacitor based on highly concentrated aqueous electrolytes

“Could ion transportation be very fast even with high-concentration electrolytes with high viscosity?”
A team led by Professor Kim Woong implemented a high-frequency supercapacitor using the fast ion transport properties of highly concentrated salt aqueous electrolytes. 

The research results were published online in ACS Energy Letters.

▲ Professor Park Jin-woo (first author), Lee Jun-young (MS Student), Professor Kim Woong (corresponding author)

Professor Kim Woong (KU College of Engineering’s Department of Materials Science and Engineering) and his team proved that through the implementation of an ultrafast supercapacitor, ion transport in highly concentrated salt aqueous electrolytes can occur as effectively and quickly as in conventional dilute aqueous electrolytes, despite their high viscosity.
*Salt: A compound in which positive and negative ions are combined by electrostatic attraction
*Electrolyte: A solution in which salt is dissociated into cations and anions in a solvent such as water that is a major component of an energy storage device that allows current to flow through the movement of ions
*Supercapacitor: An energy storage device that stores and discharges energy by physically adsorbing and desorbing electrolyte ions on the electrode surface by electrostatic attraction

Aqueous electrolytes based on water have many advantages over non-aqueous organic electrolytes, such as high conductivity, low volatility, low flammability, high environmental friendliness, and low cost. However, since water decomposes when a voltage higher than 1.23 V is applied, the fields of application were limited due to the inability to use it at higher voltages. Currently, highly concentrated salt aqueous electrolytes are attracting the attention of many researchers as they have been found to have a wide operating voltage range (2~3 V) because electrochemical water decomposition is inhibited by the strong interactions between a small number of free water molecules and ion water molecules. However, as the salt concentration increases, the viscosity of the electrolyte increases as well, so it was expected that the transport of ions in highly concentrated salt aqueous electrolytes would be generally much more difficult than in ordinary dilute aqueous electrolytes.

Through various electrochemical characterizations and demonstrations of high-frequency supercapacitor operation, the Korea University research team proved that, contrary to these general expectations, ion transport can be very fast in highly concentrated salt aqueous electrolytes. The biggest factor that enables ultrafast ion transport, even in highly concentrated salt aqueous electrolytes with high viscosity, is the special electrolyte structure of highly concentrated salt aqueous electrolytes. Such electrolytes are composed of a salt network structure formed by the agglomeration of large amounts of salts and a water network structure formed by a small amount of water connected to fill the space. As ions move along this water network, they can move as quickly as in a dilute electrolyte.

The highly concentrated salt aqueous electrolytes-based supercapacitor developed through this research also served without any problem as a capacitor in an AC line filtering circuit that converts AC voltage that rapidly changes at 60Hz to DC voltage. Currently, the capacitor mainly used in AC line filtering circuits is a bulky aluminum electrolytic capacitor. A supercapacitor based on highly concentrated salt aqueous electrolytes maintains the fast ion transport properties of aqueous electrolytes and operates at a voltage (~2.3 V) much higher than the water decomposition voltage. In addition, since its size is small, it is expected to contribute to the miniaturization of various electronic devices by replacing the existing aluminum electrolytic capacitors.

The research results were published online on February 1 (based on local time in the U.S.) in ACS Energy Letters (IF: 19.003), a renowned academic journal in the field of energy. The research was supported by the National Research Foundation of Korea’s Mid-career Researcher Support Program and Korea University’s Research Faculty Support Program.
- Author Information: Research Professor Park Jin-woo (first author, Korea University), Lee Jun-young (MS Student, Korea University Department of Materials Science and Engineering), Professor Kim Woong (corresponding author, Korea University Department of Materials Science and Engineering)
- Paper Title: Water-in-Salt Electrolyte Enables Ultrafast Supercapacitors for AC Line Filtering
- Journal Title: ACS Energy Letters. 2021, 6, 769-777

[ Figure Description ]

▲Figure 1. Solvation shell structures of low-concentration (upper left, 1 m NaClO4/H2O) and high-concentration (upper right, 17 m NaClO4/H2O) salt aqueous electrolytes (white: H; red: O; green: cl; purple: Na). A comparison of the electrochemical characterization of high-concentration (17 m NaClO4/H2O) and low-concentration (1 m NaClO4/H2O) salt-based supercapacitors: Phase angle change according to frequency change (bottom left). Cyclic voltage-current curve (bottom right, scan rate = 10 V s-1).

▲Figure 2. The AC line filtering performance of the highly concentrated salt aqueous electrolytes-based supercapacitor.
AC input (Vpeak = ±2.49 V, 60 Hz, black line), rectified pulsating DC (V = 0–2.28 V, 120 Hz, blue line), constant DC output filtered by the supercapacitor (~2.08 V, red line).