Scanning electrochemical microscopy: Measurement of the current density at microscopic redox-active sites on titanium

Absolute values of the current density at microscopic electroactive sites on polycrystalline Ti electrodes have been measured with a scanning electrochemical microscope (SECM). The active sites on the oxide-film-coated Ti electrode have radii ranging from 3.3 to 28.6 mu m and are randomly distributed at a surface density of similar to 180 sites/cm(2). Localized current densities as large as 0.14 A/cm(2) are observed for the oxidation of Br- in aqueous solution, approximately 4 orders of magnitude larger than the average current density based on the geometrical area of the electrode. Quantitative analysis of large-area SECM images demonstrates that at least similar to 65% of the total current during Br- oxidation is associated with sites that occupy a minuscule fraction (0.01-0.1%) of the total exposed electrode area. The SECM data also suggest that the rate of Br- oxidation approaches the diffusion-limited value at sites having radii less than similar to 10 mu m; a precipitous decrease in the electron-transfer rate is observed at electroactive sites of slightly larger dimensions.