Performance Optimization of Cermet SOFC Anodes: An Evaluation of Nanostructured NiO

Anode cermets of metallic Ni and stabilized zirconia are among the most common  SOFC materials. The electrochemical performance of the composite anodes is essentially governed by the triple phase boundary formed by electronic conductor, solid electrolyte and gaseous phase. One important approach to optimize the anode morphology relates to decreasing particle size of the initial powders. On the other hand, a drastic reduction of the component particle size may lead to dramatic changes in the material properties. The tendencies to agglomeration, a presence of modified surface layers and fast rates of microstructural alterations under SOFC operating conditions are all expected to considerably affect the electrode preparation procedure and anode behavior.

The present work is centered on the analysis of key relationships between the morphology of initial NiO powders used to form Ni-containing cermets and overall quality of the composite SOFC anodes. The commercial Nickel (II) oxide nanopowder (99.8% purity, Sigma-Aldrich) was employed as a model starting material for these studies.

A large series of NiO powders were prepared by annealing of initial NiO at different temperatures (RT-1100 ^oC). Scanning and transmission microscopy, X-ray diffraction, thermogravimetry, X-ray photoelectron and Raman spectroscopic analyses were employed for morphological, chemical and structural characterization. Then the powders were mixed with highly dispersed solid electrolyte of 10 mol.% Sc₂O₃ and 1% mol.% CeO₂ stabilized zirconia (10Sc1CeSZ) and an organic solvent to obtain screen-printable pastes, used to deposit anode layers onto 10 mol.% Sc₂O₃ and 1% mol.% Y₂O₃ stabilized ZrO₂ membranes for further microstructural and electrochemical studies. The results showed, in particular, that starting NiO nanopowder (Fig. 1) possesses a core-shell structure, with the crystalline core covered by amorphous shell rich of excessive oxygen and adsorbed water. These surface layers, having critical impact on the integrity and performance of resultant anodes, can be eliminated by thermal pre-treatment.

Fig. 1. TEM image of starting NiO powder.