Stability of Nickel-Infiltrated Anodes in Intermediate Temperature SOFCs

Solid oxide fuel cells (SOFCs) with a thin La_1-xSr_xGa_0.8Mg_0.2O_3-δ (LSGM) electrolyte plus Ni-infiltrated LSGM anode functional layer and Sr_0.8La_0.2TiO₃ (SLT) anode support have shown high power density (> 1 W cm^-2) at temperatures ≤ 650°C and good stability in short-term testing (< 100 h).  However, the long term stability of these cells is still largely unstudied. In this work, ex situ studies on the evolution of aqueous infiltrated Ni(NO₃)₂ to NiO and finally to Ni metal nanoparticles on LSGM pellets determined that the size and number of Ni particles can be controlled by dehydrating the samples before calcination, which could be used to advantageously control the microstructure of infiltrated anodes. SOFCs with Ni-infiltrated anodes were tested in humidified hydrogen and air at open circuit voltage (OCV) to determine stability for durations up to 600 h. Scanning electron microscopy indicated that Ni nanoparticles coarsened over time, which negatively affected performance. The polarization resistance of the anode doubled in magnitude over the course of the test, and ohmic polarization was found to increase with time, which could result from coarsening Ni closing off active areas in the anode functional layer. OCV remained within ~0.05 V of the theoretical OCV by the end of the test. In all cases, the majority of the polarization resistance resulted from degradation of the La_0.6Sr_0.4Fe_0.8Co_0.2O_3-δ-Ce_0.8Gd_0.2O_1.95 (LSCF-GDC) cathode.  To better understand the effect of Ni coarsening on cell performance, a model was developed to predict the changes in triple phase boundary (TPB) length during coarsening and its effect on cell performance. Additional testing is required to determine how these preliminary results will extend to operating lifetimes and how to use the ex situ test results to control microstructures in processed cells.