High-Resolution Studies on Nanoscaled Ni/YSZ Anodes

The performance of Ni/YSZ anodes has been shown to substantially benefit by a reverse current treatment of a solid oxide fuel cell. Within seconds, a solid state reaction builds a nanoscaled layer at the anode/electrolyte interface, thereby increasing the triple-phase boundary density considerably. The reaction mechanism is thoroughly studied using a Ni thin film/YSZ single crystal/Ni/YSZ counter electrode as a model system. The microstructure was analyzed by scanning transmission electron microscopy (STEM) and the chemical composition by atom probe tomography (APT). With these insights, supported by thermodynamic calculations, we propose the following reaction model: The reverse current treatment reduces YSZ congruently with Ni, temporarily forming a Ni-Zr-Y alloy, and Y₂O₃ precipitates. Afterward, at open circuit voltage conditions, Ni-Zr-Y is reoxidized instantaneously until thermodynamic equilibrium is reached. High-resolution STEM/APT studies disclose a newly formed nanoscaled layer consisting of (i) interconnected Ni with inclusions of 10 nm (Y_xZr_1-x)₂O₃ precipitates, (ii) a continuous (Y,Zr)-oxide with varying Y:Zr ratio, (iii) unaffected YSZ, and (iv) pores. The results obtained suggest that formation of the nanoscaled layer results from a diffusion-controlled mechanism, with Ni being the fastest species, but also involving Y or Zr, since alterations of the Y:Zr ratio are detected.