Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study
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,...
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MDPI AG
2023-07-01
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| author | John F. Cassidy Rafaela C. de Carvalho Anthony J. Betts |
| author_facet | John F. Cassidy Rafaela C. de Carvalho Anthony J. Betts |
| author_sort | John F. Cassidy |
| collection | DOAJ |
| description | 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. |
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| language | English |
| last_indexed | 2024-03-10T22:51:23Z |
| publishDate | 2023-07-01 |
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| series | Electrochem |
| spelling | doaj.art-a06efdeb2e08426da4b2363949ff6df22023-11-19T10:20:32ZengMDPI AGElectrochem2673-32932023-07-014331334910.3390/electrochem4030022Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case StudyJohn F. Cassidy0Rafaela C. de Carvalho1Anthony J. Betts2Applied Electrochemistry Group, FOCAS Research Institute, Technological University Dublin, Central Quad, City Campus, Grangegorman Lower, DO7 ADY7 Dublin, IrelandApplied Electrochemistry Group, FOCAS Research Institute, Technological University Dublin, Central Quad, City Campus, Grangegorman Lower, DO7 ADY7 Dublin, IrelandApplied Electrochemistry Group, FOCAS Research Institute, Technological University Dublin, Central Quad, City Campus, Grangegorman Lower, DO7 ADY7 Dublin, IrelandSalts 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.https://www.mdpi.com/2673-3293/4/3/22hexacyanoferrate II/IIIferrocyanide/ferricyanideinner/outer spheremulti-sphere electron transfercyclic voltammetryNicholson analysis |
| spellingShingle | John F. Cassidy Rafaela C. de Carvalho Anthony J. Betts Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study Electrochem hexacyanoferrate II/III ferrocyanide/ferricyanide inner/outer sphere multi-sphere electron transfer cyclic voltammetry Nicholson analysis |
| title | Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study |
| title_full | Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study |
| title_fullStr | Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study |
| title_full_unstemmed | Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study |
| title_short | Use of Inner/Outer Sphere Terminology in Electrochemistry—A Hexacyanoferrate II/III Case Study |
| title_sort | use of inner outer sphere terminology in electrochemistry a hexacyanoferrate ii iii case study |
| topic | hexacyanoferrate II/III ferrocyanide/ferricyanide inner/outer sphere multi-sphere electron transfer cyclic voltammetry Nicholson analysis |
| url | https://www.mdpi.com/2673-3293/4/3/22 |
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