Moving electrode electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a powerful tool for analysing material properties and studying interfacial electrochemical processes. During measurement, the sample is contacted via electrodes, and impedance as a function of frequency is determined by applying an AC voltage and measuring the resulting current. In conventional EIS, where electrodes are located at a fixed position, sample homogeneity is a crucial requirement for determining intrinsic electrical properties such as conductivity or permittivity. However, many substances, such as suspensions, multiphasic liquids, or crystallising media, are heterogeneous in nature or become so over time, strongly limiting the use of EIS.

In this thesis, a new measurement concept called moving electrode electrochemical impedance spectroscopy (MEEIS) is presented. Unlike EIS with immovable electrodes, MEEIS employs a flexible cell design to precisely adjust the inter-electrode distance, enabling local probing of sample properties. By determining properties not from the absolute value of impedance, but from the impedance increment as a function of electrode distance, (quasi-)static contributions such as the resistance of wires, the influence of electrode geometry, electrode passivation, and corrosion effects are eliminated.

In combination with a robust cell design, this opens new perspectives in the electrochemical characterisation of heterogeneous media. As experimentally demonstrated, it can, for example, be used to analyse particle sedimentation in suspensions, perform accurate conductivity measurements of chemically aggressive substances, and monitor phase transformation processes such as crystal formation. The latter was demonstrated in the case of zeolite synthesis in highly alkaline media. Here, conductivity was identified as a key parameter that is directly linked to the fraction of crystals formed over time. This enables in-situ monitoring of the crystallisation process, provided that the small conductivity changes can be tracked accurately over a long time.