Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study

Abstract

Salts of hexacyanoferrate II/III anions have been widely used as redox couple probe molecules to determine the characteristics of electrode surfaces. Examples include the assessment of electrocatalysts for energy applications and electrocatalysts for the detection of biological or chemical species, as well as the determination of electrochemically active surface areas. An examination of the electrochemical literature, based largely on cyclic voltammetric investigations, reveals a wide range of peak separation and/or heterogeneous electron transfer rate constants, classified sometimes as inner or outer sphere electron transfer processes. Originally developed for the mechanistic interpretation of inorganic transition metal compounds in solution, this terminology has since been extended to account for heterogeneous electron transfer occurring at electrodes. In the case of the hexacyanoferrate II/III anions, there can be a number of reasons why it sometimes behaves as an outer sphere probe and at other times displays inner sphere electron transfer characteristics. After examining some of the structural and chemical properties of the hexacyanoferrate II/III species, the methods used to determine such classifications are described. The most common method involves measuring peak-to-peak separation in a cyclic voltammogram to ascertain a heterogeneous rate constant, but it has inherent flaws. This paper reviews the reasons for the classification disparity, including the effects of various oxygen surface species, the influence of organic surface films, the nature of the cation counter-ion, surface adsorption and surface hydrophilicity/hydrophobicity. Other surface interactions may also take place, such as those occurring with Au corrosion or pH effects. These can impact the electrical double layer and thus may affect the electron transfer process. Consequently, it is recommended that hexacyanoferrate II/III should be considered a multi-sphere or alternatively a surface-sensitive electron transfer species.

Self-assembled monolayers for electrostatic electrocatalysis and enhanced electrode stability in thermogalvanic cells

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Simultaneous Electrochemical Detection of Dopamine and Tryptophan Using 3D Goethite–Spongin Composites

“…The ferro- and ferricyanide redox reactions on the CPE and 3DGS/CPE electrode surface displayed a quasi-reversible behavior with peak-to-peak separation of 250 and 200 mV, respectively, which are much higher than the ideal value of 57 mV, predicted for reversible single electron transfer reactions. However, the higher peak current value and the smaller peak-to-peak distance of hexacyanoferrate II/III anions at 3DGS/CPE compared to the CPE are attributed to the increase in the electrochemically accessible surface area (EASA) of the electrode and the facilitation of electron transfer [ 59 ].…”

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