Production and characterization of co-sintered NIO/8YSZ - 8YSZ laminates for SOFC applications

Solid Oxide Fuel Cells (SOFCs) are promising electrochemical devices which directly produce electrical energy by electrochemical combination of a fuel gas with an oxidant. The most attractive features of these converters are their high conversion efficiency and the possibility to process a wide variety of fuel gases due to the high operating temperatures. Ni based zirconia cermet and 8YSZ (8 mol% Yttria Stabilized Zirconia) are, respectively, the most studied anode and electrolyte materials due to their well-proven long term stability and to the similarity of the thermal expansion coefficient with the low-cost ferritic stainless steel commonly used for interconnectors in intermediate temperature applications. In the present work NiO/8YSZ and 8YSZ water-based ceramic slurries were prepared using commercial oxides powders and cast by a laboratory scale tape casting apparatus. Graphite flakes and polymethylmetacrylate powders were used as pore-formers in order to increase the porosity in the anode layers. Anodes and electrolytes green layers were stacked together, warm pressed and co-sintered. In this way, half cells with electrolyte thickness in the range of 5-10 ?m were successfully prepared. A dense anode layer was also inserted at the anode/electrolyte interface in order to facilitate electrolyte sintering, to provide a large distribution of three phase boundaries (TPBs) and to improve the mechanical properties of the laminate. In spite of the thermal expansion coefficient mismatch between anode and electrolyte materials, microstructural analysis showed a perfect adhesion between co-sintered elements and no evidence of any delamination phenomena was observed. The aim of the current experimental work was to study the influence of porosity, layers thickness and sintering conditions on the asymmetric laminates mechanical properties and curvature. For each condition electrolyte gas-tightness of the sintered electrolyte was verified. The effect of pore-former content and morphology on the sintered anode porosity was investigated by scanning electron microscopy and porosimetric measurements and the relationship between micro-structure and mechanical behaviour of both reduced and unreduced half cells was discussed.