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

Abstract

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 center dot 2H(2)O) and hexahydroxybenzene (HHB) via tetrahydroxy-1,4-benzoquinone served as a model in which RDZ is irreversibly hydrolyzed to RDZ center dot 2H(2)O. 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 center dot 2H(2)O. Due to the enhanced reversibility of the RDZ center dot 2H(2)O/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.