REDOX STABLE SOFC · Resistance of Ni-Cermet anode supports against Reduction-Oxidation cycling: A step towards better durability of Solid Oxide Fuel Cells

Final Activity Report Summary - REDOX STABLE SOFC (Resistance of Ni-Cermet anode supports against reduction-oxidation cycling: a step towards better durability of solid oxide fuel cells)

Solid oxide fuel cells (SOFC) are a promising energy technology suitable for clean and efficient direct conversion of hydrogen carrying fuels into electricity and heat. The main potential market areas of SOFC systems are within distributed energy generation (combined heat and power) from small residential applications up to MW class, and Auxiliary Power Units (APU) for both land and marine transport. The SOFC technology is currently under intensive research and development with an expected market entry of products within 1-3 years. Research and development are directed towards reducing cell and system cost, and improving the performance and durability of the cells under operation. The current project tackles the durability issue with the focus on a practical problem related to the operation of most current SOFC cells or systems, the so-called RedOx stability (= stability after a cycle of re-oxidation and reduction of an operating cell). The state-of-the-art SOFC anode is made of a composite of nickel and yttria stabilised zirconia. The anode supported cell design is operating at temperatures of 700-800 degrees Celsius with reducing atmosphere on the anode side. Due to cost issues, many small scale SOFC systems are not expected to have an inert sweep gas supply. In the case of fuel interruption, an oxidation of the nickel at these temperatures either severely degrades or completely destroys most current cells. The destruction is induced by the macroscopic expansion of the anode support, which results in destruction of the support itself and by cracking of the supported electrolyte layer. The large volumetric expansion is mainly caused by the phase change from Ni to NiO, which is the so called redox instability. The aim of the project has been to better understand the mechanisms and processes related to redox instability and through modifications to the half cell to improve the RedOx stability of Ni-based SOFC. We have demonstrated a redox stable Ni-YSZ composite and are in the process of implementing a modified structure into technologically relevant half cells. We have followed a pre-defined track of development work aiming to find microstructures and/or compositions that enable more stable structures under a variety of operation conditions. In simple terms the redox instability is a thermo mechanical problem: excessive stresses in the layered ceramic cell structure may cause cracking and loss of tightness of the cell, with the electrolyte being the critical component. In the anode supported SOFC, the origin of the stress exerted on the electrolyte is the dimensional expansion of the anode support on re-oxidation. Therefore, the main goal of the development work was to design and manufacture anode substrates with improved dimensional stability during redox cycling. The target level, based on literature, was set to a linear expansion of 0.1 % on re-oxidation. The current anode substrate could not reach this target even at the mildest conditions. As a result of the project, we have gained an improved understanding on the processes of importance during redox cycling. We designed and manufactured several modified anode support substrates and the best of these reach the stability target of 0.1 % maximum expansion after 3 redox cycles at 850 oC under certain conditions. These results were based on modifications in the composite microstructure and composition by ceramic processing, by changing the sizes and distributions of particles and pores in the composite. Dopants were added to the nickel phase to improve the creep resistance and suppress nickel grain growth during operation or redox cycling. Within the project it was difficult to fully prove the effectiveness of these dopants compared to simultaneously introduced changes in microstructure. The implementation of the best modified anode support structures in technological half cells was started within this project. While knowledge about an improved anode support structure were obtained, further work in ceramic processing, mainly related to understanding sintering and co-sintering, is still needed to implement these structures in technologically relevant cells.

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