High-Performance Protonic Ceramic Fuel Cell with Ytterbium-Doped Barium Zirconate: Reducing Cathode Polarization by Improving Electrolyte Surface Condition

Protonic ceramic fuel cells (PCFCs) have great potential to realize ultrahigh energy-conversion efficiency, but higher power density is required for future commercialization. The present work reports the effect of the electrolyte surface condition related to chemical composition such as stoichiometry and element distribution on cathode performance as a key for high-performance PCFCs. In our PCFCs, Ce-free Yb-doped BaZrO₃ (BZYb20) electrolytes are prepared using two BZYb20 raw powder materials with different A/B ratios, i.e., Ba_0.97Zr_0.8Yb_0.2O_3−δ (Cell-97) and Ba_0.99Zr_0.8Yb_0.2O_3−δ (Cell-99). These PCFCs exhibit different element distributions on their BZYb20 electrolyte surfaces, namely, a heterogeneous element distribution with segregation of Yb₂O₃ for Cell-97 and a homogeneous distribution for Cell-99. The maximum power density of Cell-99 is higher than that of Cell-97 and reaches exceptionally high values, e.g., ∼1.3 W cm^–2 at 600 °C and ∼0.7 W cm^–2 at 500 °C, which are the highest attained for PCFCs with Ce-free BaZrO₃-based electrolytes. The distribution of relaxation times and fitting analysis reveals that the cathode polarization resistance of Cell-99 is much lower than that of Cell-97 even when using the same cathode material. In conclusion, an optimal electrolyte surface condition significantly reduces cathode polarization resistance, leading to the achievement of high-performance PCFCs.