Functionally Graded IT-MOFC Electrolytes Based on Highly Conductive δ-Bi₂O₃–0.2 wt % B₂O₃ Composite with Molten Grain Boundaries

Recently, the concept of an intermediate-temperature molten oxide fuel cell (IT-MOFC) has been suggested. A MOFC combines the advantages of both the MCFC (highly conductive molten electrolyte) and SOFC (air oxygen is used as an oxidant). Here, we report the performances of IT-MOFCs based on functionally graded double-layer GDC/δ-Bi2O3–0.2 wt % B2O3 and YSZ/δ-Bi2O3–0.2 wt % B2O3 electrolytes in which δ-Bi2O3–0.2 wt % B2O3 composite with molten grain boundaries (GBs) on the cathode side (oxidant) provides the highest oxygen ionic conductivity of 2 S cm–1 at 750 °C. The GDC or YSZ layer on the anode side (reductant) protects δ-Bi2O3–0.2 wt % B2O3 from decomposing in a reducing atmosphere. The optimal thicknesses of GDC and YSZ protective layers are evaluated. By reducing the thickness of the GDC and YSZ layers to 15 and 5 μm, respectively, the power density of these IT-MOFCs can theoretically reach a value of 1 W cm–2. This value is significantly higher than the previously reported power output for IT-MOFCs based on the matrix TeO2–Te4Bi2O11 (11.5 mW cm–2 at 640 °C) and Al2O3–Na2W2O7 (121 mW cm–2 at 750 °C) electrolytes.