Optimization of Extrinsic Impact Factors for Improving Performance and Mitigating Performance Degradation of LSCF-SDC/YSZ/Ni-YSZ Cells Under Solid Oxide Electrolysis Operation

Abstract

Long-term solid oxide electrolysis operation (SOEC) testing was performed to study the performance and performance degradation of LSCF-SDC/YSZ/Ni-YSZ cells under temperatures of 750 oC, 800 oC, and 850 oC, current density of 0.5A/cm2 and 1.0A/cm2, and H2/H2O ratio of 1:1, 1:3, and 2:1. Operational temperature, current density, and H2/H2O ratio played important roles in the cells’ performance and performance degradation. These factors were correlated and should be balanced and/or optimized to maximize overall lifetime performance. LSCF-SDC cells showed better performance and less performance degradation under higher temperature. Electrochemical impedance spectroscopy (EIS) data showed polarization resistance was significantly lower for the cell operated at higher temperature. Therefore, SOEC operation may need relatively higher temperature to be activated. Long-term SOEC testing under different current density showed better performance and less performance degradation at 0.5A/cm2, 850 oC, and 1:1 of H2/H2O ratio. H2/H2O ratio in the functional layer of the cell also played an important role. The cell showed best performance and less performance degradation under 1:1 of H2/H2O ratio. EIS data showed the polarization resistance was significantly lower under 1:1 of H2/H2O ratio. H2/H2O ratio needs to be optimized for different operational temperature and current density. Overdosed steam may react with Ni causing Ni movement in the H2 electrode, adding to cell degradation. Transmission electron microscopy (TEM) studies identified the formation of the (CoFe)Ox along the SDC grain boundaries, which could impact both the SDC conductivity and the LSCF catalytic activity due to the loss of the transition metals in the LSCF. TEM analysis of the H2 electrode identified a large number of NiO clusters relocated into the original pore region, seemingly formed during Ni migration. This NiO formation in the H2 electrode during SOEC operation is another cause of long-term performance degradation.