Ranjan Pati1, Osifo Akhuemonkhan1, Michael B. Pomfret2, Hillary Sadoff1, Robert A. Walker2, and Sheryl Ehrman3. (1) Chemical and Biomolecular Engineering, University of Maryland, 2113 Building 90, College Park, MD 20742, (2) Department of Chemistry and Biochemistry, University of Maryland, 0107 Chemistry Building, Building 091, College Park, MD 20742, (3) Department of Chemical and Biomolecular Engineering, University of Maryland, Bldg. # 090, College Park, MD 20742
Nickel oxide/yttria stabilized zirconia (NiO/YSZ) cermets are the most commonly used anode materials for high temperature solid oxide fuel cells (SOFCs). The electrochemical performance of the NiO/YSZ cermets strongly depends on the microstructure and distribution of the Ni and YSZ phases in the cermets. Addition of CeO2 in the NiO/YSZ cermets not only improves the electrochemical performance but also protects the anode from carbon deposition during fuel reformation. With this motivation, homogeneous mixtures of nanocrystalline NiO/YSZ, NiO/CeO2/YSZ cermet powders were prepared by flame synthesis method from a mixture of nickel acetate, cerium nitrate yttrium nitrate and zirconyl oxynitrate precursors. In this process, aqueous solutions of precursors were atomized using a nebulizer. The atomized solution was then passed through the flame reactor. In the reactor, the precursors react in the solid phase, forming the final product. The particles were collected by thermophoresis onto a water-cooled surface. The as prepared materials were characterized by using x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and BET surface area measurement. X-ray diffraction study shows the formation of cubic and tetragonal phases of the as prepared powders along with the presence of NiO and CeO2. The particles are crystalline with size in the range of 20-30 nm.
To test the anode performance, a SOFC electrolyte was prepared by pressing 8 mole% YSZ at room temperature. The resulting disk was heated at 1500°C for 3 hrs. Anodes were prepared by tape casting the as prepared nanoparticles over an electrolyte disk. The assembly was then sintered at 1000°C for 12 hrs to ensure adherence to the electrolyte. Electrochemical performance showed that the maximum power density increases as the amount of CeO2 in the nanoparticle sample decreases. This result is expected as Ni is a better fuel decomposition catalyst and therefore more active in SOFCs. The maximum power density obtained was 8.3 mW/cm2 for NiO/YSZ anode material with NiO:YSZ mole ratio of 50:50. However, CeO2 is an excellent oxygen storage material, and a maximum power density of 40.2 mW/cm2 was obtained for NiO/CeO2/YSZ having a NiO:CeO2:YSZ mole ratio of 20:30:50, emphasizing the role of CeO2 in the electrochemical performance of the cermet materials.