Evaluation and origins of the difference between double-layer capacitance behaviour at Au-metal and oxidized Au surfaces

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 ( C dl) 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 C dl 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. C dl is derived from constant-phase element (CPE) values and the CPE parameter, ϕ, which is near unity for most conditions. At oxidized Au surfaces C dl 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, C dl 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 C dl for the oxidized and unoxidized Au surfaces indicate that double-layer charging corrections cannot be made simply by extrapolation of C dl data for unoxidized Au metal surfaces into the potential region for oxide formation.