MSE PhD Final Defense: John-In Lee

MSE PhD Final Defense:

TITLE: Degradation Behavior of Non-Infiltrated and GDC-Infiltrated Solid Oxide Cells Under Long-Term Operation

ADVISOR: Uday Pal MSE

COMMITTEE: Chair: Sean Lubner,MSE ; Soumendra Basu MSE; Srikanth Gopalan MSE; Jörg Werner MSE

ABSTRACT: Reversible solid oxide cells (RSOCs) are regarded as promising energy conversion systems that operate efficiently in both modes, fuel cell (FC) and electrolysis cell (EC). Despite their many attractive features, RSOCs are not widely used because of durability issues, resulting in long-term performance degradation. Among these, air electrode barrier layer detachment and microstructural degradation at the fuel electrode are known to be major factors. In this research, solid oxide cells with stable and robust air electrode barrier layers were manufactured, allowing fuel electrode-focused degradation research to be performed. This research focused on identifying the fuel-electrode-related factors leading to long-term degradation and finding ways to modify the fuel electrode structure to improve the performance and durability of the cell. Solid Oxide Cells (SOCs) were operated under two different conditions, EC only and RSOC, alternating between FC and EC. Additionally, 10 mole % gadolinium-doped ceria (GDC) was infiltrated into the fuel electrode to enhance its durability and performance. Non-infiltrated and GDC-infiltrated cells were operated under harsh conditions, galvanostatic electrolysis (EC) for 506 hours, and under less harsh conditions, potentiostatic RSOC conditions for 1008 hours which has EC conditions for 300 hours and FC condition for the remainder time. The cells were electrochemically characterized by measuring I-V curves and performing electrochemical impedance spectroscopy (EIS) and DRT analyses every 24 hours. Under both operating conditions, the barrier layer of the cells remained intact, while the fuel electrode degradation was confirmed. Additionally, the effect of GDC infiltration was demonstrated to improve the performance and durability of cells. GDC increased the performance by mitigating the diffusion-related degradation at the fuel electrode while restricting changes in the nickel microstructure. The working potential of RSOCs can be separated into the open circuit voltage and overpotentials which include, ohmic, activation and concentration. Among the overpotentials, the activation overpotential can be calculated using the Butler-Volmer equation, which shows the relationship between activation overpotential and the exchange current density of the electrode. For the fuel electrode exchange current density is proportional to the triple phase boundary (TPB) length, meaning that degradation at the fuel electrode can be revealed through changes in the exchange current density. To evaluate the overpotentials, I-V curves and EIS data were obtained under various gas conditions and fitted to extract each component of the overpotential. Through this approach, the degradation of the fuel electrode could be tracked during the EC operation for 506 hours.