On the intrinsic coupling between constant-phase element parameters α and Q in electrochemical impedance spectroscopy

This paper presents experimental evidences of an intrinsic coupling between the α and Q parameters of constant phase elements (CPE) used in equivalent electrical circuits for EIS data fitting. Clear correlations have been found for two different experimental conditions, anodic dissolution and scale deposit, for which CPE behavior appears as the result of a time constant distribution originated from surface inhomogeneity. Results are in agreement with a coupling function that relates CPE parameters with interfacial quantities such as the ohmic and charge transfer resistances, and also with a characteristic interfacial capacitance whose meaning is addressed here. Although this relationship was initially derived from a theoretical model in which perfect CPE behavior was caused by a double-layer capacity distribution along the interface, it has been extensively used in the literature to estimate interface capacitance from CPE parameters in very different complex systems without any definitive evidence of it. However, its exportability to real systems can be explained from the fact that it can be deduced from simple and general arguments. In this paper we argue on these ideas in the basis of experimental evidences.