Stability of Ni-Infiltrated Solid Oxide Fuel Cell Anodes for Intermediate Temperature Operation
With the push for the United States to become energy independent and increasing consciousness of where our energy comes from, sources of clean, renewable energy are needed urgently. Solid oxide fuel cells (SOFCs) are one technology that could satisfy this demand. SOFCs are high-temperature solid-state electrochemical devices that directly convert chemical energy into electrical energy. SOFCs that yield good performance at 500-650 C, well below the typical 750-800 C operating range, are of interest to reduce balance of plant costs, prevent interconnect/seal material issues, and decrease startup times. In order to maintain good performance at reduced temperature, highly active alternatives to Ni-yttria stabilized zirconia anodes are needed. Cells with Sr0.8La0.2TiO3 (SLT) anode supports, La1-xSrxGa0.8Mg0.2O3-δ (LSGM) anode functional layers (AFLs) and thin LSGM electrolytes have yielded low cell resistances and high power densities, e.g. 1.2 W cm^-2 at 600 C. Infiltration is used in these cells to form a nanoscale Ni network in the AFL, which yields increased electrochemically active triple phase boundary length and avoids Ni/LSGM interactions that occur during high-temperature cofiring.
Here the infiltration process and the stability of the Ni in the anode are examined. The effects of infiltration calcination conditions on nanoparticle morphology are discussed using targeted studies of the evolution of infiltrated nickel nitrate to nickel oxide. To demonstrate the stability of nanoscale Ni, short-term (less than 100 h) preliminary tests and long-term (greater than 500 h) accelerated degradation tests were performed by aging cells at 650 C and characterizing cell performance at a nominal 550 C operating temperature. Over the course of the test, the cells exhibited high power densities. Impedance spectroscopy revealed some anode degradation and significant cathode degradation but little impact on the operating voltage with time. The Ni morphology in tested cells changed dramatically over the duration of the test, even though performance was not significantly compromised.