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Fabrication and characterisation of nickel/yttria stabilised zirconia cermet anodes for solid oxide fuel cells

The primary objective of this thesis was to study the Ni/YSZ system from powder preparation to electrochemical and morphological characterisation. Nickel/yttria stabilised zirconia (YSZ) cermet systems are employed as anode materials in high temperature solid oxide fuel cell (SOFC) applications. The ionically conducting YSZ powder was modified, by sintering and milling to vary the particle distribution. This allowed the fabrication of cermet powders with varying particle sizes and therefore microstructures. Bimodal particle distributions were present for the milled YSZ, which indicated two particle sizes. The finest particles were individual YSZ particles of approximately 0.4 μm, and represented the grinding limit, which was independent of calcining temperature. The agglomerate size decreased and the amount of fine YSZ increased as milling times were extended. A ‘paint’ consisting of the cermet powder (NiO/YSZ), a solvent, and a binder was developed in order to use a wet powder spraying (WPS®) technique, to apply the electrode layer to the electrolyte substrate. By this method, reproducible anodes with good morphological properties were produced. The slurry was prepared with ethanol as the solvent and a PVP binder, to form a stable suspension. Anode sintering temperature was studied with results indicating that a temperature of 1300 °C provided the optimum sintering effect. Anodes sintered at 1300 °C showed no agglomeration of NiO. There was no discernable increase of Ni particle size after reducing in hydrogen. A number of Ni/YSZ cermet anodes consisting of 40 vol% nickel and 60 vol% YSZ were fabricated with varying thickness of the active layer. The anode materials were milled to the appropriate particle sizes, air-sprayed onto YSZ substrates and subsequently sintered to produce the anode component. The thicknesses ranged from 3.5 μm to 54 μm in order to find the minimum thickness of active layer for a high performance anode using the least material. The anodes were studied in an atmosphere of hydrogen, water and nitrogen in a 3-electrode setup. Impedance spectroscopy was carried out at a temperature of 1000 °C. It was found that approximately 12 μm was the limiting thickness for a low polarisation resistance. Below a thickness of 12 μm, the polarisation resistance increased in an erratic manner as the reaction zone was reduced. Ni/YSZ cermet anodes consisting of 90 vol% Ni and 10 vol% YSZ were fabricated in order to create an anode with significantly higher polarisation resistance. Hereby a structure-related impedance arc at high frequencies could be examined in detail. The anode was studied in mixtures of hydrogen, water and nitrogen in a 3-electrode setup. Impedance measurements were carried out at temperatures ranging between 700°C and 1000°C. The present work confirmed that a high frequency arc from impedance spectroscopy data was indeed related to the structure of the cermet. Four types of Ni/YSZ anodes were characterised and compared to illustrate similarities and differences from state-of-the-art cermet anodes to coarse structure cermets to two-dimensional Ni anodes with significantly higher polarization resistance. The anode structure-dependent high frequency section of the impedance spectrum was studied. The thesis concludes with recommendations for further investigations.
Type of thesis
The University of Waikato
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