Origin of the Adsorption-Controlled Redox Behavior of Quinone-Based Molecules: Importance of the Micropore Width†

Abstract

Redox-active organic materials have emerged as promising alternatives to inorganic electrode materials in electrochemical devices owing to advantages such as low cost and flexible design. However, the kinetics of their electrochemical reactions are typically slow due to the slow diffusion of organic materials dissolved in the electrolyte. Generally, peak separation of the redox reaction is observed (mass-transfer-controlled system), while no peak separation is obtained when the active molecules, such as high surface carbon material, are adsorbed onto the electrode material (adsorption-controlled system). Aromatic compounds confined in activated carbon (AC) micropores exhibit an adsorption-controlled reaction, improving the reaction kinetics. To elucidate this behavior, a well-defined and accurate understanding of the pore geometry is required. Although various synthetic techniques have been used to tune the micropore size, these afford different surface properties. This study reports an approach to achieve an adsorption-controlled redox reaction of quinone-based molecules and a tool to analyze their reaction environment. AC micropores sized <1 nm were filled with n-nonane without any change occurring in the AC surface properties. It was thus concluded that AC micropores in the sub-nanometer scale are necessary for an adsorption-controlled redox reaction to occur. This study reveals new insights on the micropore confinement effect in electrochemistry.(Picture Presented)