Understanding Temperature Stability of Surface Cation Dopants on LSCF Electrodes

Abstract

The widespread application of solid oxide fuel cells (SOFCs) is primarily inhibited by degradation issues impacting the stability and oxygen exchange kinetics of the oxygen electrode. State-of-the-art SOFC materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) boast fast oxygen ion and electron conduction as well as high oxygen reduction activity. Furthermore, materials such as LSCF provide a potential pathway to reduce the operating temperature of SOFCs from 800-100 ° C to 400-800 ° C. But degradation issues continue to impede LSCF, primarily through strontium segregation to the electrode surface and subsequent passivating poisoning reactions due to chromium- and sulfur-containing species.In this work, we investigate a solution infiltration technique to introduce cation dopants to the surface of LSCF porous electrodes on symmetrical cells to improve the stability and performance through suppression of Sr segregation. We have previously demonstrated that introducing more oxidizable transition metal cations on the LSC electrode surface improves both the stability and oxygen exchange kinetics. Others have demonstrated that lanthanides can electrochemically stabilize the LSCF surface as well (Yu Chen, et al. Energy Environ. Sci., 2017, 10, 964-971). However, the temperature dependence of these cation dopants on electrode surface composition and electrochemical stability is not well understood. Thus, we are interested in exploring more oxidizable transition metal and lanthanide cations as infiltrated dopants on the porous LSCF electrode as a function of temperature.Preliminary experiments demonstrate that both transition metal and lanthanide cation dopants diffuse into the LSCF lattice at SOFC operating temperatures (above 700 ° C) and Sr segregates to the surface with increasing temperature. We do also observe some fundamental differences between the transition metal and lanthanide cations, such as the temperature dependence of A:B site ratio. Our initial electrochemical measurements indicate that these cation dopants are found to improve the stability of LSCF electrodes when tested at SOFC operating temperatures (800 ° C in ambient air) despite having largely diffused into the lattice at these temperatures.These preliminary results indicate a potential electrode surface reconstruction as a function of cation type and temperature. We aim to elucidate 1) the temperature stability of these cations at the surface, 2) the impact of these cations on the electrode surface composition and cell electrochemical stability as a function of temperature, and 3) the mechanism for electrochemical influence of these sub-surface cations.We utilize electrochemical impedance spectroscopy and chronoamperometric stability studies to characterize the impact of infiltrated cations on electrode activity and stability. We investigate the stability of cation dopants and reasons for observed electrochemical changes through x-ray photoelectron spectroscopy and x-ray diffraction studies. This work aims to elucidate the impact of transition metal and lanthanide cation dopants on electrode stability and performance as well as the stability of the dopants at the surface at different operating conditions, with a focus on developing a mechanistic understanding of sub-surface dopant influence on electrochemical performance.