Unlocking the nature of the co-doping effect on the ionic conductivity of CeO2-based electrolyte

Doped CeO2 is a very promising electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). To further improve the performance of the CeO2-based electrolyte, co-doping two different elements into CeO2 is a feasible method, however the co-doping effect on the ionic conductivity is not well understood and whether it is synergistic or average is even controversial. In order to gain a fundamental understanding of the co-doping effect, the microscopic properties of co-doped CeO2 are calculated using the DFT + U method. Density of states, band structures, oxygen vacancy formation energies, defect association energies, and oxygen vacancy migration energies are systematically calculated for In3+, Sm3+ single-doped and co-doped CeO2. Based on our calculations, we find that the coexistence of the two doped ions in the local structures of the doped CeO2 can suppress the reduction of Ce4+ to Ce3+, which is beneficial for the decrease of the internal short circuit current of the CeO2-based electrolyte. For In3+ and Sm3+ co-doped CeO2, when the distance between the two doped ions is the first nearest neighbor, the co-doping effect is average. However, when the distance between the two doped ions extends to the second nearest neighbor, the availability of the free oxygen vacancies is synergistically enhanced. Therefore whether the co-doping effect on the ionic conductivity is average or synergistic is highly dependent on the local structures of the co-doped CeO2 which are difficult to control in experiments, offering a reasonable explanation for controversial experimental results. Our work provides a new atomistic level insight into the co doping effect in CeO2 which would be helpful for high performance electrolyte development.