Why can organic molecules slow an electrochemical reaction by blocking an electrode surface?

Organic molecules inhibit electrochemical reactions primarily by adsorbing onto the electrode surface, which physically blocks the sites necessary for the reaction to occur.1

When these organic species—such as certain ions or dipoles—adsorb onto the electrode, they effectively cover the surface area that would otherwise be available for the electrochemical process. This blocking effect can be potential-dependent; for some molecules, the inhibition begins abruptly at a specific, well-defined potential. When this threshold is reached, the typical current density-potential relationship is interrupted by a sudden, abnormal decrease in current, signaling that the surface has been occupied by the adsorbed organic layer.1

By occupying these active sites, the organic molecules prevent reactants from reaching the electrode surface, thereby slowing or stopping the reaction rate.1