Electrochemical impedance of microelectrodes

The influence of the radius (10 < a < 1500 mu m) of a Pt disk electrode on the impedance frequency spectrum of the [Fe(CN)(6)](3-/4-) reversible system is studied in the frequency range 5 x 10(-3) to 10(3) Hz. The impedance is calculated by applying the Fourier transform to potential and current pulses of various lengths obtained when imposing a step potential. For electrodes with a < 100 mu m, the spectrum in the complex plane has the form characteristic of microelectrodes, while for electrodes of higher radii, it corresponds to a finite Warburg impedance. The main impedance parameters, such as the charge transfer resistance, the diffusion resistance, and the frequency in the maximum of the imaginary constituent are determined. The latter decreases with an increase in a by the law f* similar to 1/a(2) at low a and is independent of a on electrodes with a > 100 mu m, which agrees with the impedance theory for microelectrodes.