Square-wave thin-film voltammetry: influence of uncompensated resistance and charge transfer kinetics

An electrode reaction occurring in a thin-film having a low electrical conductivity was considered under conditions of square-wave voltammetry (SWV). The ohmic polarization of the system owing to the low conductivity of the film is represented by the complex resistance parameter defined as ρ=RΩ(n2F2/RT)ScOx*Df, where RΩ is the resistance of the film, S is the electrode surface area, f is the SW frequency and the other symbols have their usual meanings. The influence of the thickness of the film is given by the thickness parameter Λ=Lf/D, where L is the thickness of the layer, and D is the diffusion coefficient. For a thin film (Λ⩽0.949) the dimensionless net-peak current of a reversible electrode reaction depends parabolically on the resistance parameter, exhibiting a well-developed maximum. A detailed study of this phenomenon revealed that it resembles the charge transfer kinetics effect as well as the property known as a “quasireversible maximum” that is typical for an adsorption complicated electrode reaction. The theoretically predicted properties of the SW response are illustrated by experiments with iodine, decamethylferrocene, and azobenzene at the three-phase electrode system. This methodology [Electrochem. Commun. 2 (2000) 112] consists of a single droplet of an organic water immiscible solvent containing an electroactive probe that is attached to the surface of a graphite electrode and immersed in an aqueous electrolyte solution. A good agreement between theory and experiment indicates that the SW voltammetric response of the three-phase electrode system exhibits properties of an electrode reaction occurring under limiting diffusion conditions.