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 micro/mesoporous 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.

Using the concepts learned from this 1^st generation configuration, we further developed high-performance aqueous-based redox ECs by (1) synthesizing differently functionalized/substituted viologen molecules and (2) optimizing the interfacial interactions between the viologens and hierarchically porous-structured carbon electrodes. In the second part of this presentation, we show systematic approaches to explore optimal parameters to improve the energy storage capacity of the device.