Nanoconfinement Effects on Enhanced Reversibility of Redox Reactions Coupled with an Irreversible Chemical Process by Electrolysis Acceleration in Nanoporous Carbon Electrodes for a Redox-Enhanced Electrochemical Capacitor

Redox-enhanced electrochemical capacitors (Redox-ECs) in which electrons are stored and released by redox reactions of organic molecules either dissolved in an electrolyte or adsorbed on an electrode surface represent a promising energy storage system with electrochemical characteristics of both rechargeable batteries and electrical double-layer capacitors. However, the choices for redox-active molecules in Redox-ECs are often limited due to an irreversible nature induced by chemical processes, such as hydrolysis, coupled with e^–-transfer reactions. Here, we describe the effects of nanoconfinement on enhanced reversibility in the redox reaction of an electroactive organic molecule undergoing irreversible hydrolysis after e^–-transfer in a nanoporous carbon electrode. The redox reaction between hydrated rhodizonic acid (RDZ·2H₂O) and hexahydroxybenzene (HHB) via tetrahydroxy-1,4-benzoquinone served as a model in which RDZ is irreversibly hydrolyzed to RDZ·2H₂O. This phenomenon results from electrolysis acceleration within confined nanoregimes in a porous carbon matrix, which is analyzed by finite-element analysis. We built asymmetric ECs composed of nanoporous carbon electrodes, one of which was coated with RDZ·2H₂O. Due to the enhanced reversibility of the RDZ·2H₂O/HHB redox reaction in a nanoporous carbon electrode, Coulombic efficiency of the cell remained near 90% despite the irreversible nature of RDZ via hydrolysis. This research provides fundamental insights into the use of organic molecules in energy storage using redox electrolytes such as Redox-ECs and organic redox flow batteries.