Indole-based conjugated macromolecules as a redox-mediated electrolyte for an ultrahigh power supercapacitor

Abstract

aBalancing energy density and power density has been a critical challenge since the inception of supercapacitors. Introducing redox-active additives in the supporting electrolyte has been shown to increase the energy density, however the power density and cycling stability are severely hampered in the process. Herein, an extensively conjugated indole-based macromolecule consisting of 5,6-dihydroxyindole/5,6-quinoneindole motifs, prepared by electrochemical polymerization of dopamine under acidic conditions, was employed as a redox-active additive. By utilizing the conjugation effect, the HOMO-LUMO gap (HLG) of the extensively conjugated indole-based macromolecule was reduced to ca. 2.08 eV, which enhanced the electronic transfer kinetics, in turn improving the power density and reversibility of redox reactions. When coupled with a porous honeycomb-like carbon (PHC) electrode, the assembled supercapacitor delivered an excellent rate performance with a high specific capacitance of 205 F g À1 at 1000 A g À1 . This work reports one of the highest power densities recorded at 153 kW kg À1 for redox-mediated electrolyte systems with a respectable energy density of 8.8 W h kg À1.In addition to an excellent cycling stability of 97.1% capacitance retention after 20 000 charge/discharge cycles, the conjugation degree has to be considered when engineering the redox-active electrolyte so as to improve the power density and stability. Broader contextPortable and reliable energy storage devices (ESDs) have become gradually integrated into technological advancements. Supercapacitors have gained significant recognition as important ESDs due to their high power density arising from fast physical ion adsorption based on the electric double layer (EDL) principle. However, the fast EDL principle is dependent on the electrode surface area which ironically is responsible for poor energy density. Thus, such a limitation has resulted in a research interest shift towards pseudocapacitive capacitors as additional pseudocapacitance can be achieved from the Faradaic reaction. One strategy to incorporate pseudocapacitance is via the addition of small redox species into the supporting electrolyte (resulting in 5-7 times energy density enhancement). However, the use of small redox species typically leads to a poor power density and poor cycling stability which renders such energy density enhancement insignificant. In this work, a supercapacitor based on a novel indole-based conjugated macromolecule redox-mediated electrolyte is reported to achieve one of the highest power densities in redox mediated electrolyte supercapacitor research, while excellent cycling stability was simultaneously attained in this system.

o‐Benzenediol‐Functionalized Carbon Nanosheets as Low Self‐Discharge Aqueous Supercapacitors

“…As observed in the FTIR spectra (Figure 1a), for NDC, the absorption bands in the range of ñ = 3283-3273 cm À1 were attributed to NÀHs tretching and OÀHo fs urfacew ater. [30] Twob ands were observed at ñ = 1190 and 1400 cm À1 and were assigned to the CÀOa nd CÀNv ibrations, respectively. [30] Twob ands were observed at ñ = 1190 and 1400 cm À1 and were assigned to the CÀOa nd CÀNv ibrations, respectively.…”

“…[29] Another band was recordeda tñ = 1610 cm À1 ,w hich indicated C=Cv ibrations. [30][31][32] For NDC-400 th ,anew band associated with phenolic hydroxylg roups appeared at ñ = 3201 cm À1 . [30][31][32] For NDC-400 th ,anew band associated with phenolic hydroxylg roups appeared at ñ = 3201 cm À1 .…”

Controlled Air‐Etching Synthesis of Porous‐Carbon Nanotube Aerogels with Ultrafast Charging at 1000 A g−1

“…In comparison, most of other porous carbon–based two‐electrode systems with aqueous or organic electrolytes show lower capacitance retentions tested in a smaller current density range (up to 200 A g −1 ) (Figure c and Table S2 (Supporting Information)) . Those electrodes were generally tested to current densities of 100 or 200 A g −1 , with only a few reaching 500 A g −1 (57% retention of the low‐current value), and one work reaching 1000 A g −1 (37% retention) by adding indole‐based macromolecules into electrolyte . Our PCN electrodes can not only extend to high current density, but also maintain a minimum decrease in the capacitance as the current density increases.…”

“…However, there exist several kinetic issues such as the ion‐transport resistance and the diffusion distance within the hierarchical 3D porous structure which limit the charging rate and capacitance retention particularly at high rates, and it remains unclear how fast the charging/discharging process can reach in optimized porous carbon EDLC electrodes. So far, very high charging rate (1000 A g −1 ) was only tested in a macromolecule redox‐mediated electrolyte system, whereas the pseudocapacitance dropped substantially to 37% …”

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 …”