A multiscale model for electrochemical reactions in LSCF based solid oxide cells

Lanthanum Strontium Cobalt Ferrite (La_1−xSr_xCo_1−yFe_yO_3−δ or LSCF), a mixed ionic-electronic conductor is widely used as an electrode in solid oxide cells (SOCs). The reactions and transport at this oxygen electrode are complex and distributed across several length and time scales. However, a comprehensive computational model that could provide accurate and reliable linkages between different scales to predict the electrochemical characteristics of these electrodes is missing. Here, we develop a multiscale model combining density functional theory calculations, transition state theory and continuum modeling for the oxygen-based reactions at the cathode/anode in LSCF based solid oxide fuel cells (SOFC)/electrolysis cells (SOEC), to elucidate the critical reaction steps and predict the performance of the device. Density functional theory calculations were used to obtain the free energy barriers for different reaction steps while transition state theory was used to predict the reaction rate constants/diffusivities for each step based on the free energy barriers. The continuum theory utilized these reaction rate constants and diffusivities to obtain the overpotential-current density relations. The proposed multiscale model yields a quantitative agreement with the overpotential-current density data from experiments. The results indicate that oxygen exchange at the LSCF surface, rather than the triple-phase boundary, is the main contributing reaction pathway for a single phase LSCF electrode. For the entire cathodic and anodic reaction pathway, the reaction step involving the splitting of the surface peroxo units into oxo units under SOFC mode or the combination of surface oxo units into peroxo units under SOEC mode is the rate-limiting reaction step, and the diffusion of oxide ions in bulk LSCF is the rate-limiting diffusion step. We also predict that the SrO terminated LSCF surface is not an efficient surface structure for oxide ion diffusion.