Determination of Surface Exchange Coefficients of LSM, LSCF, YSZ, GDC Constituent Materials in Composite SOFC Cathodes

Abstract

A novel approach to processing and modeling isothermal isotope exchange (IIE) data was developed to extract kinetic rate coefficients for the oxygen reduction reaction (ORR) on cathode materials used for solid oxide fuel cells (SOFC). IIE is capable of testing powders with particle sizes on the nano-scale, where the effective sample thickness (particle size) was shown to be below the characteristic thickness (Lc) for ionically conducting materials. This allows for accurate kinetic measurements in surface exchange controlled regimes, in contrast to secondary ion mass spectrometry depth profiling and electrical conductivity relaxation techniques where sample thicknesses are typically in the diffusion limited or mixed regimes. Surface exchange coefficients (k*) were extracted and Lc values calculated from cathode (La0.6Sr0.4)(Co0.2Fe0.8)O3–δ (LSCF) and (La0.8Sr0.2)MnO3 (LSM), and electrolyte (Ce0.9Gd0.1)O2–δ (GDC) and (Zr0.8Y0.2)O2 (YSZ) materials. Additionally, diffusion coefficients (D*) were extracted for LSM. In a surface exchange controlled regime LSCF exhibits a low activation energy for k*, while for LSM k* was observed to increase with decreasing temperature consistent with a precursor-mediated mechanism in which there is a negative apparent activation energy for the dissociative chemisorption of O2. GDC was shown to exhibit a low activation energy for k*, lower than YSZ, which is attributed to higher concentration of electrons in GDC than YSZ.