Current-overpotential relationships from electrochemical impedance spectroscopy (EIS) under DC bias – A tutorial exercise applied to a Pt||YSZ electrode

Abstract

Electrochemical impedance spectroscopy (EIS) of electrode polarisation offers the possibility to delineate overpotentials into ohmic, charge transfer kinetic, and transport and other mass transfer contributions, commonly applied and interpreted under open circuit conditions. It is sometimes also applied under DC bias, as it in principle then can provide information about net anodic or cathodic processes. However, the impedances so obtained are seldomly converted to overpotentials and therefore remain as qualitative indicators only. Here, we tutorially derive formalism of converting resistances from EIS under DC bias properly into overpotential contributions to the total overpotential, allowing identification of their origins from their dependencies on the current. We illustrate the methodology by generated model current-voltage curves and then apply it to an experimental data set for a Pt electrode on an yttria-stabilised zirconia (YSZ) oxide ion conducting electrolyte. The result reveals that a dominating electrode polarisation easily taken to reflect mass transfer in fact behaves like a second charge transfer step following Butler-Volmer kinetics, allowing us to hypothesise a new model for the O₂,Pt||YSZ electrode. Our tutorial exercise is applicable to both liquid- and solid-state electrochemistry and should apply equally also to EIS under DC bias of any types of junctions.