Study of the electrostatic repulsion of the carboxylic surface in the diffusion layer by scanning electrochemical microscopy

The behavior of electrostatic repulsion in the diffusion layer on the electrode is significant for understanding electrochemical processes with a diffusion control step. In this work, the electrostatic repulsion at the carboxylic acid-containing surface of a 3-mercaptopropionic acid self-assembled monolayer (MPA SAM) in the diffusion layer on the tip electrode was revealed by scanning electrochemical microscopy (SECM). A direct surface repulsion system was designed between [Fe(CN)₆]⁴⁻ and the carboxylate groups in the diffusion layer. The pH-modulated electrostatic repulsion of the carboxylic surface led to the repulsion of [Fe(CN)₆]⁴⁻ during diffusion, interrupting the regeneration of [Fe(CN)₆]⁴⁻ to [Fe(CN)₆]³⁻ on MPA SAM. Therefore, the approaching curve underwent a characteristic inversion from positive feedback to negative feedback dramatically. Electrostatic repulsion of the carboxylic surface was confirmed by linear sweep voltammetry and substrate generation/tip collection. Furthermore, the electrostatic repulsion of the carboxylic surface was visualized by SECM imaging technology. This work has contributed to broadening the recognition of electrostatic repulsion in the diffusion layer.