Abstract
In this work, we investigate electrode overpotential of positrodes under anodic and cathodic operation. We study how the impedance of several state-of-the-art positrode materials is affected by applied bias and propose a model for decoupling the kinetic and mass transfer contributions to the total electrode polarisation. Furthermore, we investigate how partial electronic conductivity – affected by applied bias – disguise the true electrode impedance. The model is tested for two different electrolytes – BaZr0.7Ce0.2Y0.1O3 (BZCY721) and BaZr0.4Ce0.4Y0.2O3 (BZCY442) – displaying different ionic- to electronic transference numbers and revealing the true influence of p-type conductivity on measured impedance under anodic and cathodic bias. The results enable decoupling of three main charge carriers – electron holes, protons and oxygen vacancies – and show how the partial currents are affected by temperature, pO2 and pH2O. Moreover, the kinetic part of the overpotential is revealed as more influential than previously interpreted from the small low-capacitance charge transfer reaction at the electrolyte-electrode interface as seen by EIS at OCV. The results are obtained by applying voltage over BZCY button-cells with three-electrode configuration in symmetrical atmosphere, and stepwise modelling of electrolyte and electrode impedances for three charge carriers. We propose a correction for the effect of the counter electrode overpotential on p-type conductivity, enabling full modelling of electrode impedance under bias