Molecular engineering of redox electrolytes for size-matched interfacial coupling with microporous carbon in redox-enhanced electrochemical capacitors

Abstract

Redox-enhanced electrochemical capacitors (redox ECs), leveraging soluble organic redox-active species, exceed traditional electric double-layer capacitors (EDLCs) in achieving higher energy density and delivering stable power output. While prior studies have predominantly focused on improving redox EC performance by enhancing the solubility of redox-active species, solubility alone is insufficient to fully unlock their potential. A critical yet underexplored factor is the interfacial compatibility between organic redox species and electrode surfaces. This limitation contrasts sharply with the well-established strategies in EDLCs, where capacitance optimization has been rigorously demonstrated through the precise matching of electrolyte ion size with electrode pore size. Here, we systematically investigate interfacial interactions by tailoring the molecular size of viologen derivatives through substituent modifications, enabling precise adjustments at the angstrom scale to achieve optimal compatibility with porous carbon electrodes. Our findings reveal that redox EC performance-spanning capacity, rate capability, and self-discharge characteristics-is strongly governed by the size matching between viologen derivatives and carbon pore structures. Notably, butyl viologen (BV), when paired with microporedominant activated carbon (MSC30; pore size <0.8 nm), achieves optimal cell performance with exceptional cycle stability over 10,000 cycles.