Impact of Leakage Current on Performance of Solid Oxide Fuel Cell Stacks with Gadolinia-Doped Ceria Electrolytes

Gadolinia-doped ceria (GDC) has been studied as an electrolyte material for solid oxide fuel cells (SOFCs) at intermediate temperatures (500–650°C) due to its high oxide-ion conductivity and better compatibility with high-activity lanthanum strontium cobalt ferrite (LSCF) cathodes. Recent studies have shown high surface activity for fuel oxidation and suppression of carbon deposition in GDC-based SOFC anodes due to mixed ionic and electronic conductivity (MIEC) at the reducing environments^{1, 2}. On the other hand, the MIEC behavior results in deleterious leakage (electronic) current, which reduces the open circuit voltage (OCV) and SOFC stack efficiency during operation³. The electronic current depends strongly on electrolyte thickness and external current load. In this study, a 3-D model has been developed in COMSOL which calculates the leakage current through GDC-based electrolytes and evaluates its impact on the cell OCV and overall performance of an GDC-based SOFC stack operating from 550 to 650°C.

The model incorporates both ionic and electronic current in the GDC electrolyte driving by the electrochemical gradient. The model employs irreversible electrodes and allows for a nonlinear potential distribution across the electrolyte to capture the distribution of oxygen defect concentrations following the work of Duncan et al³. In the current study, the model is used to analyze the relationship between leakage current and electrolyte thickness for a full stack cell wherein the conditions and effective oxygen partial pressure P_O2 can vary significantly particularly in the anode channels. Figure 1 plots average ionic, electronic, and total current density as a function of cell voltage for a cell with a 20 µm GDC electrolyte operating at 600°C. OCV deviation from theoretical value due to electrode/electrolyte interfaces overpotentials and Ohmic losses contributed by the ionic and electronic current. The effect of the leakage current on stack performance and efficiency provides a basis for understanding how fuel oxidation due to the leakage current impacts the relationship between stack current and fuel utilization. Results provide guidance for designing high-performance intermediate-temperature SOFCs with GDC electrolytes.

References

1. S. Baron, N. Brandon, A. Atkinson, B. Steele, and R. Rudkin, Journal of Power Sources, 126(1-2), pp. 58-66 (2004).

2. T. Nakamura, T. Kobayashi, K. Yashiro, A. Kaimai, T. Otake, K. Sato, J. Mizusaki, and T. Kawada, Journal of the Electrochemical Society, 155(6), pp. B563-B569 (2008).

3. K.L. Duncan, and E.D. Wachsman, Journal of the Electrochemical Society, 156(9), pp. B1030-B1038 (2009).