NdBaCo₂O_5+δ layered perovskites are promising mixed ionic-electronic conductive cathodes for Intermediate Temperature SOFCs, thanks to their high electronic conductivity, high oxygen vacancy concentration, good ion transport capability and high oxygen surface exchange activity [1]. These compounds are arranged in double perovskite structure (NdCoO_3-δ–BaCoO_3-δ), wherein Nd and Ba occupy alternated single perovskite layers. The partial substitution of Fe for Co in the structure allows for lowering the thermal expansion. However, the introduction of Fe also modifies the oxygen reduction activity and lowers the electronic conductivity: optimization of the Fe substitution is required. In this work, NdBa_1-xCo_2-yFe_yO_5+δcathodes with different levels of Ba deficiency and Fe substitution (x = 0, 0.1 and y = 0, 0.1, 0.2, 0.3, 0.4) were prepared and characterized with several techniques. The Oxygen Reduction Reaction (ORR) was investigated by EIS and the results were numerically analyzed with a multistep, physically-sound model to derive the main kinetic dependences and the consequences of Fe doping on the electrochemical activity.

Experimental and numerical methods

NdBa_1-xCo_2-yFe_yO_5+δ (NBCF) samples were prepared via the molten citrate technique, to achieve improved powder microstructure. XRD was applied to verify the phase purity. Lattice parameters were refined using the Rietveld method. Thermogravimetric analyses (TG-DTA, air, 25-850°C) and Temperature Programmed Oxidation tests (TPO, 2% O₂ in He, 20 Ncc/min, 25-850°C) allowed to investigate the oxygen exchange activity. The oxidation state of the samples was determined by cerimetric redox titration and by O₂ pulse chemisorption. The electrical conductivity was measured via a four-electrode DC method on sintered bars between 25 and 850°C. EIS tests were performed at OCV using a symmetric cell configuration with GDC electrolyte. The EIS tests were performed with O₂/N₂ mixtures at varying O₂concentration (100%, 21%, 10% and 5% v/v) between 550 and 750°C. SEM allowed to access the morphologic characteristics of both powders and electrodes. A one-dimensional, heterogeneous, dynamic, physically-based model was applied to kinetically analyze the EIS spectra collected on the NBCF samples. The model simulates the spectra in the time domain by accounting for the gas diffusion inside the electrode pores, and for the solid state diffusion of oxygen vacancies in the bulk of the material. A detailed kinetic scheme is applied to describe the ORR, including steps for adsorption and desorption, first and second electronation at the gas/electrode interface, and ion transfer at the electrode/electrolyte interface [2].

Results

The XRD patterns show that the samples have an orthorhombic structure (space group P/mmm) at 10% and 20% Fe substitution, and an A-site ordered tetragonal crystal structure (P4/mmm) at 30% and 40% Fe, with expansion of the cell. A transition from the orthorhombic to the tetragonal structure is observed also at increasing the Ba deficiency, with shrinkage of the cell. The cerimetric titration reveals that the oxidation state of Co increases at decreasing Ba, with a decrease of the oxygen content: instead, increasing the Fe amount increases the average oxidation state and reduces the vacancy concentration. The TG analyses coupled with the titration results (Fig. 1A) show that the oxygen content decreases at increasing temperature: the average Co-Fe oxidation state decreases, reaching a value of 2.9 at 850°C. The samples possess a high electrical conductivity (500 S/cm at 700°C, at y = 0), which decreases with the Fe content, although maintaining over 200 S/cm. The EIS tests (Fig. 1B) reveal that the lowest polarization is achieved on the Fe-free sample: increasing the Fe amount hinders the ORR activity, coherently with the lower reducibility of the Fe³⁺/Fe⁴⁺ redox couple compared to the Co³⁺/Co⁴⁺ couple. At increasing the Fe content the ASR grows but the apparent activation energy maintains nearly constant (1.2 eV). The numerical investigation based on the kinetic model is performed for each sample at all the O₂levels (Fig. 1C). The main reaction steps of the ORR mechanism result the bulk vacancy transfer and the first electronation of the O adatom, this latter being Rate Determining: increasing the Fe substitution mainly affects this electronation step.

Conclusions

The operating temperatures of IT-SOFCs (500-700°C) are especially detrimental for ORR kinetics. Double-layered perovskites based on Nd and Ba cobaltites substituted with Fe are promising cathodes: the Ba-site deficiency generates negative defects in the structure, balanced by oxygen vacancy formation. Fe substitution requires careful optimization: amounts larger than 10% may impair the electrocatalytic performance.

References

[1] J.H. Kim, A. Manthiram, Journal of the Electrochemical Society, 155 (2008), B385.

[2] A. Donazzi, M. Maestri, G. Groppi, Electrochimica Acta, doi:10.1016/j.electacta.2016.11.072, (2016).