A total of eight different cathodes containing transition metal ions were selected for this study. Two types of cathodes were synthesized: Single phase MIEC, and MIEC with gadolinia-doped ceria (GDC) composite cathodes. These were: La0.6Sr0.4CoO3-x (LSC), La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF), LaNi0.6Fe0.4O3-x (LNF), PrNi0.6Co0.4O3-x (PNC), Sm0.5Sr0.5CoO3-x (SSC), Ba0.5Sr0.5Co0.8Fe0.2O3-x (BSCF), Pr0.6Sr0.4Co0.8Fe0.2O3-x (PSCF), and PrBa0.5Sr0.5Co1.5Fe0.5O5+y (PBSCF). Water soluble nitrates of the respective elements were used. Thin films of about 100 nm were spray-coated onto 1mm thick 8YSZ discs to fabricate symmetric cells for EIS measurement. Thick films of about 1 um were spray-coated for XRD and EDS measurement.
Cathode particle size was calculated from XRD peak widening using Scherrer formula. The particle sizes of all sixteen cathodes were plotted in figure 1. All composite cathodes have smaller particle size than their corresponding single phase cathodes. This was caused by diffusion blocking during co-sintering of composite cathodes. Cathode composition was characterized by EDS. Stoichiometry of these cathodes were very close to target value.
Electrochemical impedance spectroscopy was measured over a temperature range between 400oC and 700oC. Figure 2(a) shows the Arrhenius plots of the polarization resistance over a temperature range from 400oC to 700oC for all eight single phase cathodes. Figure 2(b) shows the Arrhenius plots of the polarization resistance of all composite cathodes. As seen in the figures, all plots are linear consistent with Arrhenius behavior over the temperature range. Also, all lines are nearly parallel to each other, indicating essentially the same activation energy. The measured activation energy ranged between ~1.14 eV and ~1.35 eV.
The polarization resistance for composite cathodes is given by  (1)
Therefore, the thermal activation part is contributed by charge transfer resistance and ionic resistance. As a result, the activation energy can be expressed by (2)
While, the polarization resistance for single phase cathodes is given by 
Therefore, the thermal activation part is contributed by oxygen diffusion and surface exchange reaction. As a result, the activation energy can be expressed by (4)
Activation energy of the polarization resistance is similar for these materials, which is reasonable as all of them contain one or more transition metals, Co, Fe and Ni. The large difference in polarization resistance is attributed to the pre-exponential factor. One of the factors in the pre-exponential part is the geometrical part. Possible differences in porosity, agglomeration, degree of mixing (uniformity) of the two phases in composite electrodes, may substantially alter the pre-exponential part.
Acknowledgements: This work was supported by the National Science Foundation under grant Number NSF-CBET-1604008 and by the US Department of Energy under grant Number DE-FG02-06ER46086.
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