Nonlinear analysis of a classical system: The Faradaic process

Electrochemical impedance spectroscopy (EIS) is a well established technique and adopted in characterizing heterogeneous interface properties in various fields, such as batteries, fuel cells and corrosion. As an extension to traditional linear EIS technique, nonlinear EIS (NLEIS) is a developing technique that benefits from considerably higher resolution, better sensitivity in determining reaction mechanisms with no extra cost for hardware and measurement time. However, the fundamental concepts of harmonic impedance and optimal experimental as well as analysis procedure have yet to be fully developed. This paper explores the potential and some fundamental characteristics of NLEIS by revisiting one of the most stable and well understood electrochemical systems – the ferri–ferrocyanide redox couple. It was found that the inflections on odd harmonic curves follow a predictable trend, shifting towards lower frequency with increasing order. Even harmonics behave differently and contain critical information about the symmetry of the system. In contrast to EIS where the transfer coefficient is obtained by measurements on solutions of different concentrations, it is demonstrated that analysis of the even harmonics can yield the transfer coefficient for the reaction from measurements of a single solution.

► Nonlinear electrochemical impedance spectroscopy was applied on a classic redox couple. ► The behaviour of odd harmonics agreed with theory. ► Even harmonics showed high sensitivity to the reversibility of the reaction. ► Transfer coefficient can be precisely measured by tracking the even harmonics.