Viologen-Bromide Dual-Redox Ionic Solid Complexes: Understanding Their Electrochemical Formation and Proton-Accompanied Redox Chemistry

Abstract

The inhibition of self-discharge in a redox-enhanced electrochemical capacitor (Redox-EC) is crucial for excellent energy retention. Heptyl viologen dibromide (HVBr2) was chosen as a strong candidate of a dual-redox species in Redox-EC due to its solid complexations during the charging process, at which HV2+ is electrochemically reduced to HV+center dot and form a solid complex, [HV+center dot center dot Br-], on an anode while Br- is electro-oxidized to Br-3(-) and renders [HV2+center dot 2Br(3)(-)] on a cathode. The solid complexes could not transfer across the separator, resulting in significant diminution of the self-discharge. In this Article, we present detailed electrochemical studies of formation of [HV2+center dot 2Br(3)(-)] and [HV+center dot center dot Br-], their redox features, and galvanic exchange reactions between the two types of dual-redox ionic solids on a Pt ultra-microelectrode (UME) in neutral (0.33 M Na2SO4) and acidic (1 M H2SO4) solutions. Most importantly, through voltammetric and particle-impact electrochemical analyses, we found that the redox and galvanic exchange reactions of the two dual-redox ionic solid complexes involve H+ transfer, which is the key process to limit the overall kinetics of the electrochemical reactions. We also rationalize the proton-accompanied galvanic exchange reaction based on computational simulation.