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

Abstract

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 environments1, 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.