A physically-based modelling to predict the cyclic voltammetry response of LSCF-type electrodes: Impact of the Ohmic Losses and Microstructure

Abstract

Two numerical models have been developed to simulate the Cyclic Voltammetry (CV) response of Lanthanum Strontium Cobalt Ferrite (LSCF) porous electrode at high temperature. The first one (model-I) takes into account the solid-state diffusion in LSCF coupled with a global reaction of oxygen exchange while the second one (model-II) is based on a detailed elementary description of the reaction mechanism. The relevance of model-II to predict the voltammograms has been checked with experimental data obtained at different operating temperatures and imposed scan rates. The CV response of porous and quasi-dense electrodes has been studied with the two models and has been discussed in the frame of the zone diagram method. It has been shown that the peaks of the voltammograms are due to the transient change of the oxygen stoichiometry in LSCF. As this evolution is mainly governed by the oxygen diffusion in the material coupled to the reaction of oxygen exchange, it has been established that model-I can provide a satisfactory approximation of the CV curves if the exchange rate constant kchem is determined far from equilibrium. It has been also shown that the voltammograms are strongly distorted by the Ohmic losses making their interpretation impossible in practice, under the classical operating conditions of the solid oxide cells. To overcome this limitation, a methodology based on the modelling approach has been proposed to remove the Ohmic losses from the voltammograms and hence to reveal the voltammetry peaks. Finally, the impact of the LSCF decomposition on the CV response has been estimated with model-II. It has been established that the surface passivation and the decrease of the chemical diffusivity can substantially affect the shape of the voltammograms.