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 k(chem) 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.

The classical 1D continuum model of oxygen exchange in a porous mixed conductor (bulk path) is presented in terms of Linear Sweep Voltammetry (LSV) and Cyclic Voltammetry (CV). We show that, from a formal point of view, it corresponds to an insertion, diffusion and chemical reaction mechanism. Some general rules for LSV/CV curves are established. Using a dimensionless description, we evidence eight different limiting behaviors of the electrochemical model, which depend on four dimensionless parameters. For an almost full insertion level at equilibrium, these parameters can be reduced to only two, i.e. the dimensionless electrode thickness L and the dimensionless chemical rate constant λ. We show that the resulting zone diagram, plotted using the representation log L versus log λ, is useful to predict the possible sequences of model behaviors as a function of the potential sweep rate and the electrode thickness. This theoretical analysis is applied to a porous LSCF oxygen electrode under air at high temperature.

La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) electrode deposited on Gadolinium Doped Ceria (GDC) has been characterized by cyclic voltammetry and impedance spectroscopy in the temperature range of 300-700 degrees C. We demonstrate that the LSCF microstructure has a strong influence on the shape of the voltammograms and on the variation of the series resistance, R-s, measured on the impedance diagrams as a function of the dc bias. LSCF was deposited with two different microstructures, either porous layer (similar to 10 mu m thick) by screen-printing (SP) or almost dense layer (similar to 2 mu m thick) by Electrostatic Spray Deposition (ESD). For the denser film, one cathodic peak and one reverse anodic peak were evidenced under air at 300 and 500 degrees C. The R-s value was also found to increase with the dc cathodic bias. On the contrary, no peak was observed with the porous film under the same conditions and R-s was independent on the applied potential from 300 to 700 degrees C. Decreasing the oxygen partial pressure allowed the peaks to be evidenced and R-s to vary. The results are discussed in terms of oxygen exchange rate at the LSCF/gas interface, which depends on the LSCF specific surface area and the oxygen partial pressure. (C) 2019 Published by Elsevier Ltd.

In order to validate the reaction mechanism of porous LSCF oxygen electrodes, a set of experiments has been conducted on two types of symmetrical cells exhibiting different microstructures. In both cases, the polarization curves exhibit a dissymmetry with a transition at low anodic overpotential associated to a modification in the shape of the electrochemical impedance spectra. To interpret the experimental results, a micro-scale electrode model including two reaction pathways has been used. The model considers an oxidation/reduction at TPBs (surface path) in parallel to an oxygen transfer at the gas/LSCF interface (bulk path). Thanks to a 3D electrode reconstruction, the simulations have been performed with a reduced number of unknown parameters. It has been found that the simulated data are in good agreement with the experimental polarization curves and impedance spectra at OCP as well as under anodic polarization. Once validated, the model has been used to unravel the complex electrode operating mechanisms in electrolysis mode. The simulations have shown that the transition detected at low anodic polarization is due to a change in the dominant reaction mechanism passing from the bulk to the surface path. Moreover, the relative contribution of the two pathways has been investigated as a function of temperature.