Analysis of electrochemical impedance spectra of anodised pure aluminium foil with various etch tunnel length distributions used for electrolytic capacitors

Abstract

The present work analyses the electrochemical impedance spectra of anodized pure Al foil with various etch tunnel length distributions, used for electrolytic capacitors. The lengths of the etch tunnels formed on the Al foil specimen were more widely distributed with increasing etching time. The etch tunnel length distributions were quantitatively measured by scanning electron microscopy. All the measured impedance spectra hardly exhibited the ideal capacitive behaviour in the high-frequency region and they deviated more significantly from the ideal capacitive behaviour with increasing etching time. For the numerical calculation of the total impedance of the electrode, the transmission line model was modified to reflect the etch tunnel length distribution, which represents the respective contributions of etch tunnels with different lengths to the total impedance. The total impedance was calculated by the integration of the impedance of one etch tunnel with respect to the etch tunnel length by taking the value of the etch tunnel length distribution determined experimentally. From the coincidence of the impedance spectra calculated numerically with those spectra measured experimentally, it is concluded that the deviation of the impedance spectra from the ideal capacitive behaviour in the high-frequency region originates from the characteristic frequency dispersion that depends upon the etch tunnel length distribution.