Direct Measurement of Effective Electrical Capacitance in Systems with a Constant-Phase Element Behavior Using the Example of Barrier Coatings

For various reasons, many dielectric and electrochemical systems exhibit behavior described by a constant-phase element (CPE) instead of a pure capacitor. However, in many applications, it is desirable to determine the electrical capacitance of such systems as a physically meaningful quantity. For this purpose, models are developed that allow for the conversion of the parameters of the studied system into the so-called effective electrical capacitance of the system. This study aims to replace model-based effective capacitance estimation with an approach based on direct measurements using electrochemical impedance spectroscopy (EIS). This approach utilizes the phenomenon of electrical resonance in the studied system. Using the described approach, the effective electrical capacitance of the coating system on steel was determined during over 300 h of exposure in Harrison’s solution. The CPE parameters were also determined during exposure. Using the Brasher–Kingbury equation, the kinetics of water absorption by the coating were compared using the obtained effective capacitance and the CPE parameter (pseudocapacitance). It was observed that using the effective capacitance yields significantly lower values of water content in the coating. The proposed method eliminates the uncertainty associated with speculative modeling and enables reliable tracking of the dielectric and electrochemical properties of systems, which is particularly important in batteries, supercapacitors, sensors, and protective coatings, where capacitance is a key indicator of performance and degradation.