The history of Electrochemical Impedance Spectroscopy (EIS) is briefly reviewed, starting with the foundations laid by Heaviside in the late nineteenth century in the form of Linear Systems Theory (LST). Warburg apparently was the first to extend the concept of impedance to electrochemical systems at the turn of the nineteenth century, when he derived the impedance function for a diffusion process that still bears his name. Impedance spectroscopy was next employed extensively using reactive bridges to measure the capacitance of ideally polarizable electrodes (mostly mercury), leading to the development of models for the electrified interface. However, it was the invention of the potentiostat in the 1940s and the development of frequency response analyzers in the 1970s that led to the use of EIS in exploring electrochemical and corrosion mechanisms, primarily because of their ability to probe electrochemical systems at very low frequencies. These inventions have led to an explosion in the use of EIS for exploring a wide range of systems and processes, ranging from conduction in the solid and liquid states, ionic and electronic conduction in polymers, heterogeneous reaction mechanisms, and the important phenomenon of passivity. It is evident that the use of EIS in identifying reaction mechanisms makes use of pattern recognition, currently through inspection. It is argued that, in the future development of EIS, reaction mechanism analysis (RMA) would be most efficiently done by using artificial neural networks operating in the pattern recognition mode. This strategy would require the creation of libraries of reaction mechanisms for which the theoretical impedance functions are known.
Electrochemical Impedance Spectroscopy (EIS) is now well established as a powerful tool for investigating the mechanisms of electrochemical reactions, for measuring the dielectric and transport properties of materials, for exploring the properties of porous electrodes, and for investigating passive surfaces1-9. The power of the technique arises from: (i) it is a linear technique and hence the results are readily interpreted in terms of Linear Systems Theory; (ii) if measured over an infinite frequency range, the impedance (or admittance) contains all of the information that can be gleaned from the system by linear electrical perturbation/response techniques; (iii) the experimental efficiency (amount of information transferred to the observer compared to the amount produced by the experiment) is extraordinarily high; (iv) the validity of the data is readily determined using integral transform techniques (the Kramers-Kronig transforms) that are independent of the physical processes involved.
In the author?s opinion, the full potential of EIS has yet to be realized, partly because it has been regarded as a specialty technique requiring expensive equipment, but more importantly because the effective interpretation of EIS data requires a level of mathematical skills that is not commonly held by electrochemists and corrosion scientists. When the necessary mathematical skills are present, the power of EIS is extraordinary, in that it is capable of differentiating between closely related mechanisms. Because these skills often are not present, EIS data are often interpreted in terms of electrical equivalent circuits (EECs). However, EECs are analogs not models, and hence the information that they can deliver on the physico-electrochemical processes involved is very limited. In the author?s opinion, EEC analysis, while providing a termination point for a paper, is not the end-of-the-road; the amount of information that is to be gleaned from a full mechanistic analysis far outweighs that which can be
