Unreliability of simultaneously determining kchem and Dchem via conductivity relaxation for surface-modified La0.6Sr0.4Co0.2Fe0.8O3 − δ

Electrical conductivity relaxation (ECR) was utilized to determine the oxygen surface exchange coefficient, kchem, and bulk diffusion coefficient, Dchem, for La0.6Sr0.4Co0.2Fe0.8O3 − δ (LSCF 6428) solid oxide fuel cell (SOFC) cathodes. Measurements were performed at 1073 K in the 100% to 3.3% pO2 range. Both kchem and Dchem were nearly one order of magnitude larger than previously reported values. This discrepancy is attributed to the low reliability, yet commonly utilized, two-parameter fit procedure that seeks to simultaneously determine kchem and Dchem from a single measurement. The value of kchem is shown to vary by almost one order of magnitude depending on the number of terms chosen in the data fit, while the fit quality, SR, remained constant throughout. Polished LSCF 6428 samples were also individually surface-doped with La, Sr, Co, and Fe, as well as with LSCF 6428 nanoparticles (25–50 nm) to investigate trends in kchem and Dchem. Fe surface doping caused a decrease in kchem, while La, Sr, Co and nanoparticle doping did not affect the oxygen surface exchange or bulk diffusion coefficients.