(Invited) Enhancing High Temperature Electrode Performance with Secondary Phases

Abstract

This work summarizes an exhaustive effort intended to identify the mechanisms responsible for improved electrode performance that results from adding small amounts of aluminum titanate (Al2TiO5, ALT) to Ni-YSZ cermet anodes used in solid oxide fuel cells (SOFCs). Specifically, high temperature Raman spectroscopy and X-ray diffraction combined with electrochemical impedance measurements and ex situchemical analysis show that during electrode processing, ALT reacts to form secondary phases including NiAl2O4and Zr5Ti7O24. These secondary phases confer unexpected and cooperative benefits to the electrode’s mechanical and electrocatalytic properties as well as its operational resilience. Electrodes in high-temperature energy conversion devices must fulfill several critical tasks in order for these devices to realize their full potential of efficient operation, fuel flexibility and scalable architecture. Specifically, electrodes must remain stable at temperatures as high as 900˚C and be tolerant of incident contaminants. Traditional SOFC anodes consist of ceramic-metallic (or cermet) composite materials that are comprised of an oxide ion conducting yttria-stabilized zirconia (YSZ) and nickel oxide that is reduced to elemental Ni prior to SOFC operation. Ni is an outstanding and inexpensive electrocatalyst but is susceptible to particle coarsening during extended operation and poisoning by contaminants including sulfur and chlorine. Furthermore, when operating with carbon containing fuels, Ni-YSZ anodes can suffer from carbon accumulation (or coking) that blocks electrochemically active sites and will eventually disintegrate the electrode through metal dusting. A final liability of Ni-based SOFC electrodes is their inability to withstand reduction-oxidation cycling. Repeated Ni oxidation followed by NiO reduction creates large mechanical stresses in the anode microstructure that lead to electrode delamination from the electrolyte and electrode cracking. Recent studies have reported that small amounts (3-5% by mass) of ALT added to the Ni-YSZ cermet during anode fabrication endow electrodes with improved performance, enhanced strength, carbon tolerance and resilience to redox cycling.1-4 Specifically, adding 5% ALT (by mass) to a Ni-YSZ cermet coupon improves its mechanical strength measured with three point bending by 40% and functional ALT-enhanced Ni-YSZ anodes show two to three fold improved resilience to both electrochemical and environmental redox cycling. Additional studies presented elsewhere at this meeting demonstrate that adding ALT to Ni-YSZ anodes also improves electrode carbon tolerance, suppressing carbon accumulation by up to 75% in operating SOFCs. While the benefits of enhancing Ni-YSZ cermet anodes with ALT are clear, the mechanisms responsible for observed enhancements are not. Results presented in this work suggest that alumina (Al2O3) formed when NiAl2O4is reduced serves to anchor Ni particles and preserve microstructure. Al2O3also appears to play a reforming role that limits carbon accumulation on ALT-enhanced anodes. Electrochemical measurements demonstrate that Zr5Ti7O24has both oxide and electron conducting capabilities, allowing this MIEC material to expand the anode’s three phase boundary. Prior studies have shown that Zr5Ti7O24’s electronic conduction results from electron hopping from Ti3+and Ti4+sites and suggest that this n-type conduction can occur when titanium ions occupy as few as 4% of the YSZ cation sites.5 References. M. Welander, et al.“Operando Studies of Redox Resilience in ALT Enhanced NiO-YSZ SOFC Anodes” J. Electrochem. Soc. 165(2018) F152-F157.Hunt, et al.“Degradation Rate Quantification of Solid Oxide Fuel Cell Performance with and without Al2TiO5Addition” Int. J. Hydrogen Energy 43(2018) 15531-15536.R. Driscoll, et al. “Enhancement of High Temperature Metallic Catalysts: Aluminum Titanate in the Nickel-Zirconia System” Applied Catalysis A - General 527(2016) 36-44.McCleary and R. A. Amendola “Effect of Aluminum Titanate (Al2TiO5) on the Mechanical Performance of Solid Oxide Fuel Cell Ni-YSZ Anodes” Fuel Cells 17(2017) 862-868.K. E. Swider and W. L. Worrell “Electronic Conduction Mechanism in Yttria Stabilized Zirconia-Titania under Reducing Atmospheres” J. Electrochem. Soc. 143(1996) 3706-3711.