Why Electrochemical Impedance Spectroscopy is the Ultimate Tool in Mechanistic Analysis

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). Extension of the impedance concept to electrochemical systems was apparently first made by Warburg at the turn of the nineteenth century, when he derived the impedance for a diffusional process. 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 by Hickling in the early 1960s that led to the use of EIS in exploring electrochemical and corrosion mechanisms, primarily because of the ability to probe electrochemical systems at very low frequencies. 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, this 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.