Biredox ionic liquids: new opportunities toward high performance supercapacitors

Abstract

Nowadays commercial supercapacitors are based on purely capacitive storage at the porous carbons that are used for the electrodes. However, the limits that capacitive storage imposes on energy density calls to investigate new materials to improve the capacitance of the device. This new type of electrodes (e.g., RuO, MnO…) involves pseudo-capacitive faradaic redox processes with the solid material. Ion exchange with solid materials is, however, much slower than the adsorption process in capacitive storage and inevitably leads to significant loss of power. Faradaic process in the liquid state, in contrast can be similarly fast as capacitive processes due to the fast ion transport. Designing new devices with liquid like dynamics and improved specific capacitance is challenging. We present a new approach to increase the specific capacitance using biredox ionic liquids, where redox moieties are tethered to the electrolyte ions, allowing high redox concentrations and significant pseudo-capacitive storage in the liquid state. Anions and cations are functionalized with anthraquinone (AQ) and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) moieties, respectively. Glassy carbon, carbon-onion, and commercial activated carbon electrodes that exhibit different double layer structures and thus different diffusion dynamics were used to simultaneously study the electrochemical response of biredox ionic liquids at the positive and negative electrode.

Ionic Liquids as Environmentally Benign Electrolytes for High‐Performance Supercapacitors

“…While maintaining cyclic and power stability, the research team documented a doubled increase in specific energy compared to the same cell without electroactive electrolyte. Likewise, Bodin et al functionalized the cations and anions of bi redox‐IL with AQ and TEMPO groups . The redox center of BIL underwent fast faradic reaction at positive and negative electrodes compared to other devices utilizing redox species immobilized to the surface.…”

“…Likewise, Bodin et al functionalized the cations and anions of bi redox-IL with AQ and TEMPO groups. [192] The redox center of BIL underwent fast faradic reaction at positive and negative electrodes compared to other devices utilizing redox species immobilized to the surface. Mourad et al investigated bioredox-ILs named IP1 and IP2 comprising of TEMPO tethered to an N-methylimidazolium cation and AQ tethered to a tetrafluoroethansulfonate anion (Figure 7).…”

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“…These limitations substantially influence the performance of the supercapacitors, particularly their operating temperature range and operating potential range, hence, specific energy. To overcome these restrictions and to improve supercapacitor performance, a different approach has been adopted to use redox additive added ionic liquid (IL) electrolytes . Sun et al first introduced this approach by adding CuCl 2 in an IL 1‐ethyl‐3‐methylmidazolium tetrabuoroborate (EMImBF 4 ) to use as electrolyte in EDLC.…”

“…About 83% improvement in specific energy and substantial improvement in self‐discharge behavior have been noted. Fontaine and coworkers have recently introduced a novel concept of biredox IL as redox active electrolyte in EDLCs. In these reports, anion and cation of IL are functionalized by anthraquinone and 2,2,6,6‐tetramethylpiperidinyl‐1‐oxyl (TEMPO) moieties, respectively.…”

“…To overcome these restrictions and to improve supercapacitor performance, a different approach has been adopted to use redox additive added ionic liquid (IL) electrolytes. [40][41][42][43][44][45][46] Sun et al [40] first introduced this approach by adding CuCl 2 in an IL 1-ethyl-3-methylmidazolium tetrabuoroborate (EMImBF 4 ) to use as electrolyte in EDLC. Enhanced specific capacitance and energy (%225 F g À1 and 45 Wh kg À1 , respectively) are reported, which is attributed to the faradic behavior at the interfaces due to the reversible redox reaction of Cu 2þ /Cu 0 ion pairs.…”