The inhibition of self-discharge in a redox-enhanced electrochemical capacitor (Redox-EC) is crucial for excellent energy retention. Heptyl viologen dibromide (HVBr) 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 HV is electrochemically reduced to HV and form a solid complex, [HV·Br], on an anode while Br is electro-oxidized to Br and renders [HV·2Br] 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 [HV·2Br] and [HV·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 NaSO) and acidic (1 M HSO) 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.
