Structural and Mechanical Stability of Reduced Nickel Oxide/Yttria-stabilized Zirconia Anode/Electrolyte Structures for
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1098-HH03-26
Structural and Mechanical Stability of Reduced Nickel Oxide/Yttria-stabilized Zirconia Anode/Electrolyte Structures for Solid Oxide Fuel Cell Applications S. Biswas, N. Thangamani, N. T. Saraswathi, J. Zhang, and S. Bandopadhyay College of Engineering and Mines, Institute of Northern Engineering, University of Alaska Fairbanks (UAF), Fairbanks, AK, 99775 ABSTRACT Highly porous Ni-8YSZ anodes supported by a thin and dense electrolyte layer of 8YSZ have been developed for solid oxide fuel cell applications by reducing a NiO-8YSZ anode/electrolye precursor structure in a gas mixture of 5% H2-95% Ar at 800°C for selected time periods up to 8 h. It appears that 2 h of exposure to the reducing conditions is enough to reduce ~ 80% of NiO. XRD and SEM analyses in the reduced samples disclose the formation of the Ni-8YSZ cermet structure with desired porosity and microstructure. The porosity in the anode samples, which increases with the increase in the fraction of reduced NiO, severely affects the hardness and elastic moduli of the anode samples. Vickers indentation tests show that a hardness value of 5.5 GPa in the unreduced anode samples (12% porosity) reduces to less than 1 GPa in the 8 h reduced samples (36.68 % porosity). Similarly, a decrease of ∼ 44% in the Young’s modulus and ∼ 40% in shear modulus is observed in the 8 h reduced samples through impulse excitation techniques, in comparison to the unreduced anode precursor. Since the elastic properties of fully dense Ni, NiO and YSZ are comparable to each other, the decrease in the magnitude in elastic moduli and hardness is attributed to the colossal increase in porosity as a result of the reduction of NiO in H2 atmosphere. INTRODUCTION Solid oxide fuel cells (SOFCs), based on oxide-ion conducting electrolytes have emerged as an alluring alternative to traditional energy conversion systems due to several potential advantages including high efficiency, fuel flexibility, modularity, reliability and low levels of environmental impacts [1-5]. Because of their high operating temperature and reactive conditions, the performance and reliability of SOFC systems are highly dependent on the structural stability and electro-catalytic activity of the anode material. It is well established that the key to better anode performance is the optimization of the anode cermet structure. Here, we report a systematic analysis of the phase formation and development of cermet microstructure in the anode samples derived by reducing a NiO-8YSZ precursor structure. The mechanical properties of the reduced samples were also studied at room temperature. EXPERIMENT The half-cell precursor samples used in this investigation have a dense electrolyte layer (∼8 µm thickness) of 8YSZ supported by a thick NiO-8YSZ (70:30 vol%) anode precursor layer (∼500 µm thickness). The as-received samples were cut into suitable sizes and reduced in a gas mixture of 5% H2 – 95% Ar at 800°C in an autoclave set-up for selected time periods of 10 min, 30 min, 2 h and 8 h.
The porosity in the as-received
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