Effect of Covalent Modification on Proton-Coupled Electron Transfer at Quinone-Functionalized Carbon Electrodes

The attachment of molecular species to conductive carbon electrodes is attracting attention as a strategy for developing high-performance functional materials for a large variety of electrochemical energy storage and conversion applications. It is critical to the optimal design of these materials that there is a fundamental understanding of how these functionalized molecular species interact with the electrical double layer at the electrode|electrolyte interface. In this work, we investigate the aqueous electrochemical behavior of glassy carbon electrodes that were functionalized with 1-aminoanthraquinone-2-sulfonic acid (AAQS) via two conjugation techniques: amide coupling and radical-mediated diazonium reduction. The two conjugation routes gave rise to electrodes with distinct redox potential versus pH (i.e., Pourbaix) slopes, suggestive of distinct proton-coupled electron transfer (PCET) energetics. We relate the measured Pourbaix slope in each case to the electrostatic potential drop experienced by the quinone. Additionally, using a simple multilayer dielectric model of the double layer, we show how differing PCET energetics as a function of AAQS conjugation mode and electrolyte concentration can be explained in large part by differing distances between the ketone moiety in each quinone and the electrode, and thus differing driving forces for interfacial electric field-driven protonation of each quinone. Our results highlight the potential for the use of PCET thermochemistry to map out the electric field/electrostatic potential profile at electrode|electrolyte interfaces of relevance to many emerging electrochemical technologies.