A NOVEL METHOD FOR GENERATING QUANTITATIVE LOCAL ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

A local electrochemical impedance spectroscopy (LEIS) technique for mapping the ac impedance distribution, as a function of frequency, of an electrode has been developed. In LEIS, as in traditional ac impedance methods, a sinusoidal voltage perturbation between the working and reference electrode is maintained by driving an ac current between the working electrode and a distant counterelectrode with a potentiostat. Local ac impedances are then derived from the ratio of the applied ac voltage and the local ac solution current density. The local ac current density is obtained from potential difference measurements near the electrode surface using a probe consisting of two micro-electrodes. By measuring the ac potential difference between the micro-electrodes, and knowing their separation distance and the solution conductivity, the local ac solution current density is derived. The accuracy of the local ac impedance data generated with this technique was established by investigating two model systems. The first provided a homogeneous electrode which allowed LEIS measurements to be compared to traditional EIS, while the second system provided a heterogeneity of known size and location whose components were easily characterized with traditional techniques. It is shown that area-normalized scanning ac impedance measurements of the homogeneous electrode agreed well with traditional results. In addition, because LEIS maps the impedance properties of an electrode, the defect in the heterogeneous electrode was easily detected, while traditional ac impedance of this electrode gave little indication of its presence.