Impedance Spectroscopy Study of an SDC-based SOFC with High Open Circuit Voltage

• Developed an improved circuit model considering an inter-diffusion layer for a novel ceria-based SOFC.
• Elucidated the effect of inter-diffusion layer on ion diffusion, electron blocking, and catalyst activity.
• Separated polarization loss into three sub-processes for ceria-based SOFC.
• Identified several physicochemical processes via characteristic frequencies.

A relatively high open circuit voltage (OCV) of 1.047 V at 600 °C was reported recently for a cell based on a BaZr0.1Ce0.7Y0.1Yb0.1O3–δ(BZCYYb)-NiO anode-supported thin SDC electrolyte, demonstrating a peak power density of 0.50 W/cm2. In this study, an equivalent circuit model was developed for interpreting the behavior of this SDC-based SOFC. The mechanism behind the high OCV and the corresponding high peak power density were elucidated via separating the polarization processes and the corresponding characteristic frequencies, especially those for oxygen ion diffusion through the interlayer at the anode/electrolyte interface. Theoretical analysis and data fitting based on the presented circuit model indicate that the inter-diffusion layer between Ni-BZCYYb and SDC effectively suppresses electronic conduction while maintaining the catalytic activity and ionic conductivity. More importantly, careful analysis of the characteristic frequencies offers a powerful approach to assigning a specific part of the impedance data (e.g., an impedance arc or loop) to the corresponding physicochemical process. Further, any sharp change in the characteristic frequency for a physicochemical process also reflects a change in the inherent nature of that process under the testing conditions. Once validated by more experimental results under a broader range of testing conditions, the presented equivalent circuit model, in turn, may be used to predict fuel cell performances and optimize the operating conditions.