Layered perovskites based on cobalt are a class of compounds suitable for the application as cathodic materials in IT-SOFCs, thanks to their excellent properties of mixed ionic and electronic conduction and high catalytic efficiency towards the Oxygen Reduction Reaction (ORR) [1]. Recently, the improvement of the electrochemical activity through the introduction of Ba deficiency was demonstrated for layered perovskite structures [2, 3]. To reduce thermal stresses and increase long-term stability, the substitution of Co with Fe is a well-known solution. However, the introduction of large amounts of Fe drastically reduces the cathodic efficiency, while a small doping can even improve the performance [4]. In this work, the effect of Fe doping together with Ba deficiency are investigated through the production and characterization of the series of compounds NdBa_1-xCo_2-yFe_yO_5+δ (NBCF), with x = 0, 0.1 and y = 0, 0.1, 0.2, 0.3, 0.4. The aim of this project is to evaluate the introduction of both negative defects (Ba deficiency) and Fe doping into Co sites on the performance and on the kinetics of the electrochemical processes.

The materials were synthesized via the molten citrate technique, to achieve an excellent powder microstructure typical of wet synthesis procedures. The physico-chemical properties were characterized with several techniques: X-ray diffraction (XRD), thermogravimetric analyses (TG-DTA), scanning electron microscopy (SEM), temperature programmed oxidation (TPO) and cerimetric titration. The electrical conductivity was measured via a four-electrode DC method on sintered bars between 25 and 850°C. Electrochemical Impedance Spectroscopy (EIS) measurements were performed at OCV using a symmetric cell configuration on GDC electrolyte pellets as support. The EIS tests were performed with O₂/N₂ mixtures at varying O₂ concentration (100%, 21%, 10% and 5% v/v) between 550 and 750°C. The results were simulated with Equivalent Circuit Model (ECM) technique and numerically analyzed with a multistep, physically-sound model to derive the main kinetic dependences and the consequences of Fe doping on the electrochemical activity [5].

The compounds show an A-site ordered perovskite structure. The iron-free compound presents an orthorhombic symmetry (space group Pmmm), while the iron containing compounds were refined using a tetragonal cell (space group P4/mmm). Their cell volume increases with the Fe content. The expansion or contraction of the crystal lattice volume is strictly connected to the oxygen content (5+δ) which is decreased by the deficiency and increased by Fe doping (Table 1). The introduction of negative defects is partially compensated by the formation of oxygen vacancies, which reduce the oxygen content and the mean oxidation state of Fe³⁺/Fe⁴⁺ and Co³⁺/Co⁴⁺ redox couples. The effect of Ba deficiency on the oxygen content decrease is more effective on samples with Fe doping, with a percentage reduction of δ equal to 6% for Fe = 0, to 17% for Fe = 0.1 and 21% for Fe = 0.4. However, the generation of large amount of oxygen vacancies seem to hamper the conduction path inside the material, reducing the performance of the material. The TGA reveals that the increase of the temperature above 300°C reduces the oxygen content, introducing additional vacancies in the lattice for all the compounds. This trend is consistent with the conductivity measurements that show a maximum at about 300°C and a metal-type behavior as the temperature further increases. In addition, increasing Fe doping reduces the conductivity but even NBCF4 shows values higher than the minimum threshold of 100 S/cm. On the contrary, Ba deficiency increases the conductivity of the correspondent stoichiometric compound.

The EIS results show that Fe doping improves the electrochemical activity and the sample with the lowest polarization is NBCF4, achieving a value of 0.17 Ω·cm². On the contrary, Ba deficiency enhances the performance for Fe-free sample, but impairs the activity of Fe doped compounds (Figure 1A). The numerical investigation based on ECM analysis and the physically-sound model is performed for each sample at all the O₂ levels (Figure 1B). From ECM analysis, the surface electronation at the gas/electrode interface (middle frequencies arc) and the subsequent oxygen ion transfer (high frequencies) are the main ORR contributions. This result is confirmed by the kinetic model, which indicates the first electronation of the O adatom as the Rate Determining Step. In addition, the increase of Fe doping mainly affects this electronation step.

[1] R. Pelosato, et al., J. Power Sources, 298 (2015) 46-67.

[2] A. Donazzi, et al., Electrochim. Acta, 182 (2015) 573-587.

[3] S. Pang, et al., Int. J. Hydrogen Energy, 37 (2012) 3998-4001.

[4] J. Zou, et al., Solid State Ionics, 206 (2012) 112-119.

[5] A. Donazzi, et al., Electrochim. Acta, 222 (2016) 1029-1044.