Influence of Sr and Co deficiency on the transport properties and oxygen reduction reaction of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ

The surface oxygen exchange coefficient (k*) and the oxide ion diffusivity (D*) in the La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) related perovskite materials with various strontium (Sr) and cobalt (Co) compositions were determined by a combination of isotope exchange (IE) technique and secondary ion mass spectrometry (SIMS). Results show that the derived values of k* and D* decrease with decreasing Sr and Co content in those LSCF samples. To corroborate the results, electrochemical impedance spectroscopy (EIS) was performed on porous LSCF samples with the same Sr and Co deficiency; obtained impedance spectra were analyzed by the transmission line model (TLM) to derive k* and D*. It showed that those k* and D* values obtained from the isotope exchange experiments are in good agreement with the electrochemically determined values. For further analysis, thermogravimetry was performed to evaluate the oxygen vacancy concentration (δ) for respective samples. It has been found that changes in log D* can be well correlated with the log (δ/(3-δ)) as expected from the vacancy mechanism for the oxide ion diffusion. On the other hand, changes in log k* are not correlated only with the ionic properties like log (δ/(3-δ)) but also with the electron properties; this can be seen in the h plane coordinated with log D* and log k*.