An Elementary Kinetic Model for the LSCF and LSCF-CGO Electrodes of Solid Oxide Cells: Impact of Operating Conditions and Degradation on the Electrode Response

Abstract

An elementary kinetic model was developed to predict the electrochemical response of porous LSCF and LSCF-CGO electrodes. The model was validated thanks to experiments performed on symmetrical cells using a three-electrode setup. After the model calibration on polarization curves, it has been shown that the model is able to simulate accurately the experimental impedance diagram at OCP and under polarization without additional fitting.Moreover, the evolution of the electrode polarization resistance with the oxygen partial pressure is well reproduced by the model. The electrodes reaction mechanism was thoroughly analyzed and it has been shown that the transition from the bulk path to the surface path depends on the temperature, the polarization and the oxygen partial pressure. The rate-determining steps for the LSCF electrode have been identified at OCP as function of the oxygen partial pressure.Finally, a sensitivity analysis has been performed to study the impact of LSCF demixing on the electrode performances. For a given decomposition, it has been highlighted that the surface passivation would be more impacting than the decrease of the ionic conductivity. Moreover, the impact of the LSCF decomposition would be more detrimental for the electrode performances evaluated in electrolysis mode.

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

“…However, we have recently considered the coexistence of a second parallel pathway, called 'surface' path defined as the direct oxidation/reduction at the Triple Phase Boundaries lengths (TPBls) [21]. The predominance of one path over the other is strongly dependent on the operating conditions (polarization, temperature and oxygen partial pressure) and LSCF composition [21,22,23,24]. In this frame, it has been shown that the surface path is activated under anodic polarization (SOEC mode).…”

“…In this frame, it has been shown that the surface path is activated under anodic polarization (SOEC mode). Moreover, it has been demonstrated that several steps limit simultaneously the reaction rate on the classical porous La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ electrode [24]. Therefore, a complete set of electrochemical characterizations appears to be necessary for the interpretation of the OER mechanism for LSCF.…”

“…Several models have been based on this simplified approach with improvements towards a more sophisticated description of the reaction mechanism [21,36,37]. As an illustration, some authors have implemented full elementary kinetic models for a better comprehension of the electrode surface behavior [24,40,41]. These models take into account the electrostatic surface potential at the MIEC/gas interface related to the presence of adsorbed charged species on the electrode surface [42,43].…”

“…Through the analysis of the polarization curves, the authors observed that the surface potential strongly affects the electrode kinetic by introducing rate-limiting steps. Besides, Effori et al [24] extended a pre-existing model for the LSCF and LSCF-CGO oxygen electrodes, by decomposing the reaction mechanism into six elementary reaction steps. This elementary expression of the kinetic resulted in an improvement of the model capability to predict accurately the electrode dependence on .…”

“…In this work, the detailed full elementary model for the OER on LSCF electrode, described in [24] and validated for steady-state and impedance measurements, was extended to compute the CV curves as well. The model validation was carried out on experimental results recorded by testing a symmetrical LSCF cell, for different operating temperatures (700 °C and 650 °C) and imposed scan rates, at = 0.1 atm.…”