Electronic and Morphological Characterization of Nanostructured Ni-Doped (Ce,Gd)O2-δ Anodes for IT-SOFCs

It is well known that CeO₂-based materials present excellent catalytic properties for the oxidation of H₂ and CH₄ fuels [1] and with the incorporation of metal oxides (Gd₂O₃, Sm₂O₃, Y₂O₃) in the CeO₂ lattice the oxygen storage capacity, the ionic conductivity and the specific surface area can be significantly improved. Furthermore, the addition of metals such as Ni or Cu enhances the electronic conductivity of the material, thus enabling them as efficient IT-SOFC anodes [2, 3]. 

A previous study on the microstructure influence in the electrochemical properties of commercial Ce_0.9Gd_0.1O_1.95 (GDC) with several grain sizes, impregnated with NiO [3] to form 40:60 (%wt) composites showed that better performances were achieved with nanocrystalline samples supporting the fact that the optimization of microstructure and morphology are crucial for the development of efficient anodes. 

A particular approach to optimize the anode microstructure is to tailor its porosity. In this way, the addition of activated carbon in an intermediate step followed by calcination at 1450ºC resulted in an increase of more than 30% in the anode efficiency [4]. Other composite morphologies can be obtained modifying the way that Ni is incorporated into the material. A common procedure is the impregnation method: the GDC material is submerged in a Ni(NO₃)₂*6H₂O solution, dried in an oven at 90ºC and calcinated at 350ºC to generate NiO [4]. 

In this work, we present a new modified sol-gel method to incorporate Ni into the precursor solution. This route produces a powder with a very homogeneous Ni distribution. Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) analysis indicated that this modification resulted in smaller particle sizes with a narrower size-distribution than the observed in commercial Ni-GDC cermets. 

The study of the oxidation state and coordination of Ce and Ni in these cermets, simulating in-operando conditions, was performed using synchrotron DXAS technique.

Electrochemical impedance spectroscopy (EIS) analyses allowed to establish a correlation between the electrochemical properties with the sample´s characteristics (microstructure, Ni distribution, etc.).

[1] M.G. Zimicz, S.A. Larrondo, R.J. Prado and D.G. Lamas, Int. Journal of Hydrogen Energy 37 (2012) 14881–14886. 

[2] W.C. Chueh, Y. Hao, W. Jung and S.M. Haile, Nature Materials 11 (2012) 155-161.

[3] M. G. Zimicz, P. Núñez, J. C. Ruiz-Morales, D. G. Lamas, S. A. Larrondo, Journal of Power Sources, vol. 238 (2013) 87-94

[4] A. Fernandez Zuvich, A. Caneiro, C. Cotaro and A, Serquis. Procedia Materials Science 1 (2012) 628-635.