Zero-Overpotential Redox Reactions of Quinone-Based Molecules Confined in Carbon Micropores

Quinone-based aromatic compounds have been studied as electrode materials for various energy-storage devices. However, the relatively large activation barrier of the charge-transfer process of these redox-active molecules causes sluggish reactions and a decrease in energy efficiency. To lower the activation barrier, aromatic compounds must be strongly adsorbed on the electrode surface, preferably via π-π stacking interactions. Molecules in slit-shaped micropores strongly adsorb on the graphitic walls, thus experiencing unique micropore-confinement properties. In this study, the micropore-confinement effect is extended to the adsorption of quinone-based redox-active molecules in 0.8 nm slit-shaped micropores of activated carbon, which produces a drastic reduction in the activation barrier of the charge-transfer process and creates a zero-overpotential redox reaction. The property originates from the short distance (approximately 0.3 nm) between the quinone molecules and the graphitic wall due to the strong adsorption of the aromatic compound. Our results provide the first demonstration that the micropore-confinement effect can reduce and nearly eliminate the activation barrier of an electrochemical reaction. We also demonstrate the applicability of this approach via the charge/discharge performance of a two-electrode cell. Cells comprising the aromatic compound/activated carbon material as positive and negative electrodes exhibit a greater retention capacity than those without activated carbon. The technique described herein can guide the development of high-performance, rapid charging/discharging electrodes for energy-storage devices such as batteries, supercapacitors, and hybrid devices using organic materials.