Nanocomposite Cathode and Anode Materials for Intermediate Temperature Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFC) are the most efficient and environment friendly power generators. To ensure their high and stable performance in the intermediate temperature (IT) range, design of new stable to carbonation nanocomposite cathodes and stable to coking in the internal reforming (IR) mode of operation anodes are required. Results of research aimed at solving these problems are reviewed here. Main approach was based upon enhancing oxygen mobility and reactivity of these materials by combining in cathode nanocomposites perovskite-like oxides (Pr2NiO4, PrNi1-xCoxO3, (La,Sr)(Fe,Ni,Co)O3, etc) with doped ceria or zirconia electrolytes, while Ni-YSZ anode composite was promoted by fluorite LnCeZrO (Ln = Pr, La, Sm) or perovskite LaPrMnCrO oxides and Pt/Ru. Nanocrystalline oxides were synthesized by Pechini method. Nanocomposites were prepared via ultrasonic dispersion of powders in isopropanol with addition of polyvinyl butyral followed by sintering in air using conventional sintering as well as sintering by microwave heating or radiation-thermal treatment by e-beam. Samples real structure was characterized by XRD and HRTEM/SEM with EDX. Oxygen mobility and reactivity were characterized by O2 TPD, oxygen isotope exchange (including 18O2 and C18O2 SSITKA), weight loss, unit cell and conductivity relaxation studies, H2 (CH4, C2H5OH) TPR. Electrochemical characteristics of thin film SOFC with cathode nanocomposites supported on anode substrates (YDC buffer layer/thin YSZ electrolyte/NiO/YSZ) were estimated using wet H2 as fuel, while those for anode nanocomposites supported on Ni-Al foam plate and attached to the Ni/YSZ anode side were characterized in the CH4 internal reforming mode. Synthesis and sintering procedures were optimized to ensure developed interfaces in composites. P and F domains in nanocomposite cathodes are nanosized and disordered even in dense materials due to elements redistribution between phases. This microstructure provides fast oxygen diffusion in such domains and along their interfaces. For PrNi1-xCoxO3-YDC nanocomposites pronounced transfer of Pr cations into domains of YDC creates a broad (up to 70% of all oxygen) path of fast diffusion Do up to 10 7 cm2/s at 700°C much exceeding that for standard LSCF perovskite. Microwave radiation provides increased sample density, improved phase purity and enhanced oxygen mobility.
Optimized nanocomposite anodes show a high efficiency of natural gas and ethanol transformation into syngas and coking stability at a low (~1-2) steam/carbon ratio. Nanocomposite cathodes and anodes ensure a stable SOFC performance in the IT range and a high power density (up to 800 mW/cm2 at 700 oC) using H2 or CH4 + H2O feed in the IR mode.
Support by Russian Scientific Foundation (16-13-00112 Project) is gratefully acknowledged