Capacitance spectra extracted from EIS by a model-free generalized phase element analysis

Abstract

A new graphical analysis of impedance spectra of electrochemical interfaces is proposed that takes into account the possible existence of distributed elements and filters out portions of spectra that are dominated by faradaic contributions. The proposed method is based on the concept of a generalized phase element, αgpe. It is already known that, in contrast to the so called constant phase element, αgpe is a function of frequency and is very sensitive to changes in impedance spectra caused by underlying electrochemical processes. The novelty in this work is to further extend the concept of generalized functions, which enables a direct translation of impedance spectra into effective capacitance spectra, without the intervening use of predefined electric equivalent circuit models. As a practical example, we investigate the capacitive behavior of a polycrystalline Au electrode, immersed in an aqueous 0.1 M H2SO4 electrolyte. Despite the presence of a 2D distribution of time constants, the capacitance spectra display non-dispersive values of capacitance for the electric double layer. Furthermore, the gpe analysis is able to track capacitive processes occurring at potentials close to the onset of hydrogen evolution, thus revealing the presence of a non-faradaic adsorption capacitance that could be related to a surface activated deprotonation of bisulfate ions.