Theoretical Study on the Ionic Transport Numbers of the Mixed Oxygen Ionic-electronic Conducting Oxides Measured with EMF Method

The mixed oxygen ionic-electronic conducting oxides have been widely used in the fields of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). The accurate measurement of ionic transport numbers in the mixed oxygen ionic-electronic conducting oxides is significant for their performance optimization and application. Electromotive force (EMF) method is one of the most widely used approaches to measure the ionic transport numbers. However, the measured result of EMF method is the apparent ionic transport number (t_i^app). CeO₂-based electrolyte was selected as an object, while a model of defect distribution was established. With this model, the distribution of ionic transport numbers across the CeO₂-based electrolyte membrane was analyzed and the effect of test conditions on t_i^app was examined by using the EMF method, thus evaluating the accuracy of the EMF method. It is demonstrated that the ionic transport number is inhomogeneous across the electrolyte membrane. t_i^app is significantly dependent on thickness of the electrolyte membrane when the electrolyte membrane is relatively thin. When there is a large gradient of oxygen chemical potential on both sides of the electrolyte membrane (P_O2^high/P_O2^low>10²⁵), t_i^app tends to be ionic transport number of the electrolyte layer at high P_O2 side and decreases with increasing temperature and the electronic conductivity coefficient of the electrolyte. t_i^app tends to be ionic transport number of the electrolyte layer at the low P_O2 side when the two sides of the electrolyte layer are in reducing atmospheres(P_O2<10^‒20 atm). t_i^app mainly reflects the ionic transport number of the electrolyte layer at the high P_O2 side, when P_O2 on the two sides of the electrolyte layer are large (P_O2>10^‒15 atm).