How Microstructures, Oxide Layers, and Charge Transfer Reactions Influence Double Layer Capacitances. Part 2: Equivalent Circuit Models

In the first part of this study, double layer (DL) capacitances of plane and porous electrodes were related to electrochemical active surface areas based on electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements. Here, these measured data are described with equivalent circuit models (ECMs), aiming to critically assess the ambiguity, reliability, and pitfalls of the parametrization of physicochemical mechanisms. For microstructures and porous electrodes, the resistive–capacitive contributions of DL in combination with resistively damped currents in pores are discussed to require the complexity of convoluted transmission line ECMs. With these ECMs, the frequency-dependencies of the capacitances of porous electrodes are elucidated. Detailed EIS or CV data-based reconstructions of complex microstructures are discussed as impossible due to the blending of individual structural features and the related loss of information. Microstructures in combination with charge transfer reactions and weakly conducting parts require parameter-rich ECMs for an accurate physicochemical description of all physicochemical mechanisms contributing to the response. Nevertheless, the data of such a complex electrode in the form of an oxidized titanium electrode are fitted by an oversimplistic ECM, showing how easily unphysical parameterizations can be obtained with ECM-based impedance analysis. In summary, trends in how microstructures, charge transfer resistances and oxide layers can influence EIS and CV data are shown, while awareness for the overinterpretation of ECM-analysis is raised.