Cathode ORR is the most complicated process in solid oxide fuel cells (SOFC), since it involves oxygen molecule adsorption and dissociation, electron transfer and oxygen ion transport. Most of the cathodes used in SOFC contain mixed ionic electronic conducting (MIEC) oxides [1]. Either they are used as single phase, if the material exhibits sufficient ionic conductivity in addition to electronic conductivity, or as a composite with an oxygen ion conductor such as yttria-stabilized zirconia (YSZ) or rare earth oxide doped ceria. It is well known that microstructure plays a crucial role in determining the polarization resistance [2,3]. Typically, the finer the cathode microstructure, the lower is the polarization resistance [2,3]. We have recently reported on the deposition of cathodes from an aqueous solution directly on electrolyte surfaces by spray-pyrolysis followed by thermal treatment at 700^oC [4]. In this work, we have investigated cathode polarization resistance of many cathodes prepared by the spray pyrolysis process. This would then allow one to determine similarities and differences across many cathodes prepared by a single process.

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 400^oC and 700^oC. Figure 2(a) shows the Arrhenius plots of the polarization resistance over a temperature range from 400^oC to 700^oC 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 [2] (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 [3]

(3)

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.

References:

[1] S. B. Adler, Chem. Rev., 104 (10) 4791-4843 (2004).

[2] C. W. Tanner, K-Z. Fung and A. V. Virkar, J. Electrochem. Soc., 144 (1) 21-30 (1997).

[3] S. B. Adler, J. A. Lane and B. C. H. Steele, J. Electrochem. Soc., 143 (11) 3554-3564 (1996).

[4] L. Zhang, L. Zhu and A. V. Virkar, J. Electrochem. Soc., 163 (13) F1358-F1365 (2016).

Figure 1: A bar graph comparing the crystallite size in single phase and composite cathodes.