Oxidation-Induced Degradation of SOFC Ni Anodes at High Fuel Utilizations

In the downstream of SOFC systems, higher oxygen partial pressure may cause oxidation-induced Ni anode degradation, associated with the formation of NiO and/or Ni(OH)₂. Under experimental conditions for Ni anodes exposing to high oxygen partial pressures, conduction pathways could be destroyed resulting in cell performance degradation. This study is therefore focusing on the changes in microstructure and cell performance at high fuel utilizations. The objective of this study is to derive guidelines for high fuel utilization operation towards higher power generation efficiency.

The cells used consist of Ni-based anode with scandia-stabilized zirconia (ScSZ), ScSZ electrolyte, and lanthanum-strontium-manganite(LSM)-ScSZ cathode. The electrochemical properties of the cells were measured at 800^oC by feeding humidified fuel, H₂ (20cc/min)-H₂O (80cc/min), to the anode, and air (150cc/min) to the cathode. The condition for Ni oxidation was estimated by thermochemical equilibrium calculation software, HSC Chemistry. Performance stability was examined at a constant cell voltage for a constant partial oxygen pressure at the anode for 100h. I-V characteristics before and after the 100h test were measured and focused-ion-beam-scanning electron microscopy (FIB-SEM), and SEM-energy dispersive X-ray spectroscopy (SEM-EDX) analysis were conducted for microstructural characterizations.

Anode voltage threshold was derived from the oxygen partial pressure at the boundary where both Ni and NiO coexist in the phase stability diagram. The threshold voltage was 0.701 V at 800^oC. The performance stability was examined for 100h at the constant cell voltage of 0.6 V, showing stable performance as shown in Image1. The resistances did not change before and after the test, and no microstructural change of the anode was observed. The performance stability was also examined at the constant cell voltage of 0.5 V in order to set the experimental condition near the threshold. As shown in Image 2, anode voltage started to vibrate during the test. The anode-side ohmic resistance increased during the vibration, suggesting Ni oxidation. But the electrochemical measurements revealed no change in anode resistances, indicating that the oxidation of Ni is limited to their surfaces. While slight agglomeration of Ni particles was observed, the number of isolated Ni particles was still small judging from FIB-SEM reconstruction images, maintaining their conduction pathways.