-Spontaneous reduction of diazoniums: TEMPO in its intermediate redox state (-N-O•) can spontaneously reduce diazonium salts in the diffusion layer, forming reactive aryl radicals. These radicals, generated too far from the electrode surface, are likely to deactivate by reacting with other species in the medium. This process decreases the concentration of diazonium salts near the electrode interface, limiting their availability for immobilization on the surface.