Estimation of Chemical and Transport Processes in Porous, Stoichiometric LSM Cathodes Using Steady-State Polarization and Impedance Modeling

Abstract

Electrochemical data collected for the porous La1−xSrxMnO3±δ (LSM) cathode–yttria-stabilized zirconia electrolyte system were analyzed using a mathematical model capable of simulating both the steady-state and impedance responses. The model considered the distributed nature of the porous electrode and a parallel pathway for oxygen transport including gas transport and surface diffusion. Data were collected in low oxygen partial pressures ranging between 10−4 and 10−3atm , where LSM was considered to be stoichiometric with respect to oxygen. Thermodynamically consistent kinetic and transport parameters were regressed from the steady-state polarization data. Three different parameter sets fit the general trend of the experimental data. The surface diffusion and adsorption/desorption parameters were cross-correlated, which underlines the difficulty in finding a unique set of fitting parameters. The parameter set with four adjustable parameters resulted in a high surface diffusion coefficient but slower adsorption/desorption kinetics compared to the other two parameter sets for which a lower surface diffusion coefficient was fixed. The predicted overall impedance at zero-dc bias conditions for three parameter sets had varying degrees of agreement with the data. Low and high frequency arcs attributed to adsorption and an oxygen transport process were observed in the faradaic impedance response.

Understanding the electrochemical behaviour of LSM-based SOFC cathodes. Part II - Mechanistic modelling and physically-based interpretation

“…Recently modelling studies [12][13][14][15][16][17][18] have emerged to provide a mechanistic interpretation and quantitatively corroborate these experimental evidences, providing deeper insights than those obtained from the application of phenomenological equivalent circuits [19][20][21]. Nielsen and Hjelm [15] applied the porous electrode theory to interpret the impedance spectra of composite LSM/YSZ cathodes over a wide range of temperatures.…”

“…Oxygen adatoms diffuse on the surface of the cathode and reach the TPB at the electrolyte interface, wherein they undergo a charge transfer reaction with the electrons carried by the electronic conductor, as follows [17]:…”

“…The system in frequency domain Eqs. (15)(16)(17)(18)(19) depends on the steady-state solution  (see Eqs. (15a), (16a) and (16b)), or, in other words, the impedance Z depends on the applied dc bias bias.…”

“…(9-11), then Eqs. (15)(16)(17)(18)(19) are solved for each frequency to compute the impedance of the cathode. The commercial finite-element software COMSOL Multiphysics [36] is used in this study to perform the numerical simulations.…”

“…The second part of this series of papers focuses on model development, mechanistic interpretation and identification of the transition overpotential. The model, which is an extension of that proposed by Fu et al [16] and is similar to that developed by Kenney and Karan [17], which in turn is based upon van Heuveln et al [24,25], considers the surface path only and it is kept as simple as possible to reduce the number of unknown parameters.…”