Diffusion Impedance in a Thin-Layer Cell: Experimental and Theoretical Study on a Large-Disk Electrode

Abstract : Using an accurate thin-layer cell, the electrochemical impedance corresponding to the diffusion of ferro-cyanide ions was measured on a large-disk electrode covered by thin films of electrolyte. Experimental impedance diagrams obtained in a large frequency range (100 kHz−5 mHz) were in good agreement with those expected in the case of a spatially restricted linear diffusion impedance. For electrolyte film thicknesses lower than 100 μm, an increase of the low-frequency limit of the real part of the impedance and a decrease of the high-frequency phase angle were however observed experimentally as the electrolyte film thickness decreases. This specific behavior, in disagreement with the classical linear diffusion theory, was explained by the potential distribution existing in the thin-layer cell and was modeled using an original transmission line. In a very low-frequency range (lower than 1 mHz), an additional time constant was also evidenced. In consistency with previous work of Gabrielli et al.1 on microelectrodes, this feature was ascribed to the radial contribution of diffusion processes existing within a thin-layer cell.