Fundamentally Addressing Bromine Storage through Reversible Solid-State Confinement in Porous Carbon Electrodes: Design of a High-Performance Dual-Redox Electrochemical Capacitor

Abstract

Research in electric double-layer capacitors (EDLCs) and rechargeable batteries is converging to target systems that have battery-level energy density and capacitor-level cycling stability and power density. This research direction has been facilitated by the use of redox-active electrolytes that add faradaic charge storage to increase energy density of the EDLCs. Aqueous redox-enhanced electrochemical capacitors (redox ECs) have, however, performed poorly due to cross-diffusion of soluble redox couples, reduced cycle life, and low operating voltages. In this manuscript, we propose that these challenges can be simultaneously met by mechanistically designing a liquid-to-solid phase transition of oxidized catholyte (or reduced anolyte) with confinement in the pores of electrodes. Here we demonstrate the realization of this approach with the use of bromide catholyte and tetrabutylammonium cation that induces reversible solid-state complexation of Br/Br. This mechanism solves the inherent cross-diffusion issue of redox ECs and has the added benefit of greatly stabilizing the reactive bromine generated during charging. Based on this new mechanistic insight on the utilization of solid-state bromine storage in redox ECs, we developed a dual-redox EC consisting of a bromide catholyte and an ethyl viologen anolyte with the addition of tetrabutylammonium bromide. In comparison to aqueous and organic electric double-layer capacitors, this system enhances energy by factors of ca. 11 and 3.5, respectively, with a specific energy of ∼64 W·h/kg at 1 A/g, a maximum power density >3 kW/kg, and cycling stability over 7000 cycles.

Redox‐Mediator‐Enhanced Electrochemical Capacitors: Recent Advances and Future Perspectives

“…Many efforts have been devoted to the redox‐mediator‐enhanced supercapacitors. As a result, a variety of redox mediators and electrolyte systems have been developed …”

“…Therefore, the CV curve of the positive electrode now shows a pair of redox peaks and the negative one a rectangular shape (Figure g) and the GCD curve of the positive electrode exhibits the sloping potential plateaus whereas the potential of the negative electrode varies linearly with time (Figure h). After the first successful attempt of enhancing the capacitance by introducing Ag + as mediator, a large number of redox mediators were developed for supercapacitors . Based on the used redox mediators, the single‐redox‐mediator‐enhanced supercapacitors can be divided into three categories: metal‐mediator‐based supercapacitors, inorganic‐mediator‐based supercapacitors, and organic‐mediator‐based supercapacitors.…”

“…Metal ions are typical mediators that can be deposited on the electrode during charging and reversibly dissolved into the electrolyte during discharging. In the case of soluble mediators for which redox processes do not involve solid species, adding a foreign stabilizer has proved to be an effective method . As mentioned above, the self‐discharge of bromine was suppressed by adding TBABr, which can form complexes with Br 3 − in the pores of carbons .…”

“…In the case of soluble mediators for which redox processes do not involve solid species, adding a foreign stabilizer has proved to be an effective method . As mentioned above, the self‐discharge of bromine was suppressed by adding TBABr, which can form complexes with Br 3 − in the pores of carbons . Moreover, increasing the ion size also can inhibit the diffusion of redox mediators.…”

Electrochemical Double‐Layer Capacitor Energized by Adding an Ambipolar Organic Redox Radical into the Electrolyte

“…[30] However,t he reported redox mediators are usually oxidized/reduced at specific potentials,w hich can only provide capacitance for one electrode (positive or negative) in EDLCs.The enhanced capacitance on single electrode aggravates the asymmetric behavior of two electrodes,g enerally resulting in al imited voltage window and corresponding energy density. [35][36][37][38][39][40][41] However,s uch dual redox mediators would make the system quite complex. [35][36][37][38][39][40][41] However,s uch dual redox mediators would make the system quite complex.…”