Dispersion phenomena in EIS and DIS spectra of porous materials and their representation as transmission line bases ‘diffusion’ elements—part I: a case study of lead-acid systems

A number techniques exist to assess the porosity of materials, however a large number of them cannot be used to monitor the behaviour of such in ‘live’ systems. This problem can be overcome by the usage of Electrochemical Impedance Spectroscopy (EIS). However, porous systems and their qualities, can not be easily described using regular equivalent circuit and basic elements. An approximation of such has to be made using transmission lines, which can, in turn, be equated to specific diffusion elements. The parameters of these elements can be related to porous material qualities. And in turn, the changes of these parameters can then be related to various processes—such as ageing or degeneration. In this part of the material a case study was performed on a number of lead-acid systems—a VRLA battery, a maintenance-free one and system consisting of a lead electrode and a platinum mesh counter electrode. This was done in order to test the validity of using the Warburg impedance element in equivalent circuits. During the course of the tests it was found that not only a Gerischer element is a better choice, but the changes in its parameters can be related to different ageing-related processes.