Current-distribution effects on the impedance of porous electrodes and electrodes covered with films

• Models for the impedance of ion-insertion and porous electrodes were established.
• The models include the effects of current distribution on the impedance.
• Both disk and square electrodes were modeled.
• Below a critical frequency the impedance data are unaffected by the current distribution.
• Above the critical frequency the impedance for the two electrode geometries is different.

An analytical formula for the impedance of a disk electrode for general local interfacial impedances has been derived. The formula indicates that the current and potential distributions will affect the impedance data when the measurement frequency is close to or higher than certain critical value. For capacitive or Warburg-like local interfacial impedances, both relevant for porous and intercalation electrode films, the criterion is the same as that previously obtained by Orazem, Tribollet and co-workers through numerical simulation (Huang et al., 2007). Numerical simulation performed here for square and rectangular electrodes show that the criterion will be similar to that for the disk but with the shorter edge length of the electrode playing the role of the disk radius. The local impedances calculated for the square and rectangular electrodes are qualitatively similar to those of the disk. However, for square or rectangular electrodes the current-distribution effects lead to a different appearance in the various plots of the global impedance (impedance-plane plots, Bode plots, etc.) than those for the disk. This is suggested to be due to the “edge-like” and “center-like” local admittances receiving different weight when summed up to the global impedance of the electrode.

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