Boronium Ionic Liquids for High-Voltage Supercapacitors

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• Boronium ionic liquids (BILs) are a novel class of electrolytes known for their high electrochemical stability due to cation charge delocalization.
• BILs possess a large voltage window, making them highly attractive for electrochemical energy storage (EES) devices.
• Carbon nanofoam paper (CNFP) is utilized as a high-performance electrode material in supercapacitors.

Purpose of the Study:

• To systematically evaluate a series of BILs as electrolytes in symmetric double-layer capacitors with CNFP electrodes.
• To assess the electrochemical performance, including voltage window, capacitance, energy density, and power density, of BIL-based supercapacitors.
• To investigate the long-term stability and temperature dependence of BIL electrolytes in supercapacitor applications.

Main Methods:

• Supercapacitor cells were assembled using CNFP electrodes and various BIL electrolytes.
• Cyclic voltammetry (CV) was employed to determine the operational voltage window and capacitive properties.
• Galvanostatic charge-discharge (GCD) cycling was used to evaluate capacitance, energy density, power density, and cycling stability.

Main Results:

• All tested BILs, including ammonium-, imidazolium-, and pyrrolidinium-based types, exhibited excellent capacitive behavior with nearly rectangular CV curves.
• The methylpyrrolidinium-substituted BIL ([ (1-m-pyrr)-N111BH2 ]-TFSI) showed superior ionic conductivity (1.82 mS cm−1 at 25 °C) and a wide operating voltage window of approximately 3.7 V.
• Supercapacitors achieved significant energy density (16.3 Wh kg−1 at 36.4 W kg−1) and high power density (up to 13.9 kW kg−1), with over 90% capacitance retention after 1000 cycles.

Conclusions:

• Boronium ionic liquids demonstrate significant potential as electrolytes for high-performance supercapacitors.
• Pyrrolidinium-based BILs, particularly the methylpyrrolidinium derivative, offer a promising combination of ionic conductivity, voltage stability, and energy/power density.
• The supercapacitors exhibit excellent cycling durability and thermal resilience, confirming their suitability for demanding electrochemical energy storage applications.