| Summary: | Pure zirconia nanofibers were fabricated by electrospinning zirconia-polymer precursor and subsequent annealing. Fiber properties such as polymer decomposition, crystallization formation, phase transformation, surface morphologies, etc., were investigated by various techniques, including thermogravimetric analysis (TGA) and differential thermal analysis (DTA), high temperature differential scanning calorimeter (HTDSC), powder X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), etc. It was found the crystallization of as-spun fibers started at 450 °C and the initial crystallized zirconia phase was tetragonal (t), which began transforming to monoclinic (m) phase at 650 °C as evidenced by XRD; HTDSC showed at different thermal circles, the m-to-t transformation temperatures remained virtually unchanged while the reverse t-to-m temperatures systematically shifted from 924.9 to 978.6 °C as the progress of thermal circles; FESEM examinations revealed that fibers calcined to 1000 °C went through thermal grooving due to surface diffusion during heat treatment; fibers heated to 1370 °C formed the so-called bamboo wires, where volume diffusion was the dominant driving force.
A novel route to fabricate nanofiber-based anodes for solid oxide fuel cells (SOFCs) was also presented. Uniform YSZ nanofibers were first synthesized by electrospinning of 8YSZ dispersion. The fiber surfaces were then electrolessly plated with a layer of Ni after sintering. The Ni-YSZ nanofibers were slurry-coated on a commercial half cell as the anode and the cell performance was tested; the Ni content was quantified by XPS. A second cell with the same Ni content in the anode as the first one, prepared by conventional ball-milling of powders, was also fabricated and tested. We found the peak power density for the cell with the fiber-based anode is twice of that with the powder-based anode; the FESEM images of the two cells showed that the fiber-coated anode mainly consisted of nanofibers, which formed an interconnected network within the anode; on the other hand, the particles in the powder-coated anode formed sphere-like granules that are unorganized and are not well-connected, which will not be advantageous for anode functionality. In the end we came up with two anode models that are based on FESEM observations and they explained the superiority of the fiber-based anode.
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