Numerical Study on the Electron-Blocking Mechanism of Ceria-Related Composite Electrolytes Considering Mixed Conductivities of Free Electron, Oxygen Ion, and Proton

Composite electrolytes have been widely applied in the ceria-related SOFCs free from the internal short circuit; however, a mathematical model has not been proposed to describe the charge transport mechanism in the composite electrolytes to date. In this work, the random distribution model and core–shell model are, respectively, developed to calculate mixed conductivities of the free electron, oxygen ion, and proton in the composite electrolytes with Sm0.2Ce0.8O2−δ (SDC) and BaCe0.8Sm0.2O3−δ (BCS) as an example. The connected probability of the electron transport path consisting of SDC particles is evaluated in the random distribution model, while the electron transport is intercepted by the BCS-based core–shell structure around the inner SDC particles. The simulation results by the core–shell model instead of the random distribution model are very consistent with the experimental data, indicating that the core–shell model can describe in a superior way the charge transport in the composite electrolytes. Meanwhile, the effects of the contact angle of adjacent SDC particles, BCS volume fraction, and temperature on the electrochemical performance are investigated considering the open circuit voltage, I–V curve, leakage current density, proton transport number, and cell efficiency. The results show that the open circuit voltage and proton current density increase while electron and oxygen ion current densities decrease with the increasing BCS volume fraction, leading to the increase of cell efficiency; the electron and oxygen ion conductivities increase while proton conductivity decreases with the increasing contact angle related to the sintering condition.