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 μm and are randomly distributed at a surface density of ∼180 sites/cm2. Localized current densities as large as 0.14 A/cm2 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 ∼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 ∼10 μm; a precipitous decrease in the electron-transfer rate is observed at electroactive sites of slightly larger dimensions.