Boronium Ionic Liquids for High-Voltage Supercapacitors

Boronium ionic liquids (BILs) are an emergent class of electrolytes with high electrochemical stability afforded by charge delocalization across the cation. BILs are of particular interest for electrochemical energy storage (EES) devices because of their large voltage window. Here, a series of BILs were systematically evaluated as electrolytes in symmetric double-layer capacitors equipped with carbon nanofoam paper (CNFP) architected electrodes. First, the operational voltage window and capacitive properties of supercapacitor cells composed of BILs and CNFP electrodes were evaluated in a two-electrode configuration by using cyclic voltammetry (CV). Then, galvanostatic charge-discharge (GCD) cycling was used to assess the capacitance, energy density, power density, and long-term stability of cells assembled with the BIL electrolyte. Our results show excellent capacitive behavior of the cells assembled with a series of ammonium-, imidazolium-, and pyrrolidinium-based BILs, with nearly rectangular CV curves across a range of scan rates. Specifically, the methylpyrrolidinium-substituted BIL electrolyte ([(1-m-pyrr)-N₁₁₁BH₂]-TFSI, TFSI: bis-(trifluoromethane)-sulfonimide) presents higher ionic conductivity (1.82 mS cm^-1 at 25 °C) compared to other BIL analogues and a wide operating voltage window of ∼3.7 V. These properties of [(1-m-pyrr)-N₁₁₁BH₂]-TFSI deliver an appreciable energy density of 16.3 Wh kg^-1 (at a power density of 36.4 W kg^-1), whereas [(1-a-pyrr)-N₁₁₁BH₂]-TFSI achieves a maximum power density of 13.9 kW kg^-1. Overall, these BILs display excellent power density and sufficient energy density with the advantage of steadily delivering the energy at high power density. High cycling durability is also possible with the BILs supercapacitor cells, which maintain a capacitance retention above 90% after undergoing 1000 charge-discharge cycles at a current density of 0.5 A g^-1. Finally, the specific capacitance, energy density, and power density of ammonium- and pyrrolidinium-based BILs exhibit a delicate dependence on temperature intended to facilitate the diffusion kinetics of BILs, confirming thermal resilience with no additional performance advantage.