Effect of Transition Element Dissolution on Ytterbium-Doped Barium-Zirconate-Based Protonic Ceramic Fuel Cells

Protonic ceramic fuel cells (PCFCs) use a proton-conducting oxide as the electrolyte, leading to a high power density and energy conversion efficiency at intermediate temperatures. Ytterbium-doped barium zirconate has a high proton conductivity and chemical stability against carbon dioxide, making it an attractive electrolyte material for PCFCs. The durability of fuel cells is a critical issue for realizing practical applications of PCFCs, and dissolution of the transition metals from the electrodes into the electrolyte is expected to decrease the durability. In this study, we investigated the effect of dissolution of transition metals (Ni, Co, Fe, and Mn) into BaZr0.8Yb0.2O3−δ on the proton transport properties and on the cell properties. The proton conductivity and proton transport number decreased with an increasing concentration of transition-metal elements, and the Co was found to have a particularly large effect on the material properties of BaZr0.8Yb0.2O3−δ. As for cell properties, anode-supported PCFCs with different cathode firing temperatures were evaluated. The OCV and the maximum power density were low in the cell with high concentrations of Co and Fe in the BaZr0.8Yb0.2O3−δ electrolyte, suggesting the decrease of proton transport number due to dissolution of Co and Fe. Furthermore, based on the measured proton transport properties, the effects of transition-metal dissolution on the power density and energy efficiency of PCFCs were estimated, and the trends were consistent with the results of the cell test. These results indicate that the dissolution of transition metals, especially Co, has a significant effect on the performance and durability of PCFCs using the BaZr0.8Yb0.2O3−δ electrolyte.