Evaluation and origins of the difference between double-layer capacitance behaviour at Au-metal and oxidized Au surfaces |
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Authors: | Patrick S Germain |
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Affiliation: | Department of Chemistry, University of Ottawa, 10 Marie Curie Street, Ottawa, Ont., Canada K1N 6N5 |
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Abstract: | In studies of processes at oxidized compared with unoxidized electrode surfaces by transient methods corrections for double-layer charging are usually required and have often been made by extrapolation of double-layer capacitance (Cdl) data for the metallic surface, e.g. at Au or Pt, into the potential region of oxide-film formation. Voltammetry and impedance spectroscopy provide direct information on Cdl values determined at unoxidized, i.e. metallic, Au surfaces compared with those of anodic oxide films generated potentiostatically to various extents that are stable in time, and characterized by reductive linear-sweep voltammetry. Cdl is derived from constant-phase element (CPE) values and the CPE parameter, ?, which is near unity for most conditions. At oxidized Au surfaces Cdl depends on potential for various extents of oxide formation; it increases from 15 (±1) μF cm−2 at 1.75 V (RHE) to 25 (±1) μF cm−2 at 1.45 V (RHE) and is independent of added Cl− or Br− for concentrations 0-10−3 M of both anions, while, at unoxidized Au electrodes in the absence of halide anions, Cdl has a maximum value of 60 (±2) μF cm−2 at 0.80 V (RHE) and is now dependent on concentration of added Cl− or Br− ion. These major differences of Cdl for the oxidized and unoxidized Au surfaces indicate that double-layer charging corrections cannot be made simply by extrapolation of Cdl data for unoxidized Au metal surfaces into the potential region for oxide formation. |
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Keywords: | Impedance Au electrode Bromide Chloride Au oxide formation Perchloric acid |
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