Understanding the ionic conductivity maximum in doped ceria: trapping and blocking

Abstract

Materials with high oxygen ion conductivity and low electronic conductivity are required for electrolytes in solid oxide fuel cells (SOFC) and high-temperature electrolysis (SOEC). A potential candidate for the electrolytes, which separate oxidation and reduction processes, is rare-earth doped ceria. The prediction of the ionic conductivity of the electrolytes and a better understanding of the underlying atomistic mechanisms provide an important contribution to the future of sustainable and efficient energy conversion and storage. The central aim of this paper is the detailed investigation of the relationship between defect interactions at the microscopic level and the macroscopic oxygen ion conductivity in the bulk of doped ceria. By combining ab initio density functional theory (DFT) with Kinetic Monte Carlo (KMC) simulations, the oxygen ion conductivity is predicted as a function of the doping concentration. Migration barriers are analyzed for energy contributions, which are caused by the interactions of dopants and vacancies with the migrating oxygen vacancy. We clearly distinguish between energy contributions that are either uniform for forward and backward jumps or favor one migration direction over the reverse direction. If the presence of a dopant changes the migration energy identically for forward and backward jumps, the resulting energy contribution is referred to as blocking. If the change in migration energy due to doping is different for forward and backward jumps of a specific ionic configuration, the resulting energy contributions are referred to as trapping. The influence of both effects on the ionic conductivity is analyzed: blocking determines the dopant fraction where the ionic conductivity exhibits the maximum. Trapping limits the maximum ionic conductivity value. In this way, a deeper understanding of the underlying mechanisms determining the influence of dopants on the ionic conductivity is obtained and the ionic conductivity is predicted more accurately. The detailed results and insights obtained here for doped ceria can be generalized and applied to other ion conductors that are important for SOFCs and SOECs as well as solid state batteries.

The ionic conductivity of Sm‐doped ceria

“…The kink in the conductivity is often assumed to correlate with the association energy of a dopant‐oxygen vacancy pair in literature . However, in our earlier work, we demonstrated that the association is actually a two‐step process (catch‐and‐hold) and that the observed activation enthalpy difference is merely an effective association energy …”

“…Figure 4 also shows that the bulk conductivities for the x = 0.07 and x = 0.1 compositions are similar above 200°C. The shift in the ratio between both conductivities indicates a shift of the dopant fraction of the maximum in conductivity as found in experiments and simulations . The higher thermal energy enhances the probability for oxygen vacancies to exit the association radius of the dopant ions (trapping).…”

“…Alternatively, cerium oxide (CeO 2 , ceria) doped with a rare earth oxide (RE 2 O 3 ) exhibits sufficiently high ionic conductivities around 600°C and is extensively investigated in literature, as reviewed in our earlier publication . The high ionic conductivity is caused by both the creation of oxygen vacancies by doping, as shown in Eq.1 in Kröger‐Vink notation, and weak defect‐defect interactions …”

The grain‐boundary resistance of CeO2 ceramics: A combined microscopy‐spectroscopy‐simulation study of a dilute solution

“…(Under very reducing conditions, singly‐charged oxygen vacancies, and even neutral oxygen vacancies, have to be taken into account, but in our study we remain in the oxidizing regime and so both these species can be safely neglected.) For the dilute regime ( < 1%), we also neglect defect–defect interactions . We note that the electrons are localized on Ce 4+ ions as small polarons, forming Ce 3+ ions.…”

“…This is due to such compositions displaying the highest oxide‐ion conductivities, and the driving force behind the research has been optimizing the ionic conductivity of CeO 2 ‐based ceramics, so that they can be used as electrolytes in solid oxide fuel cells (SOFC). Such solid solutions, however, constitute materials systems in which defect–defect interactions are evidently important . The analysis of the SCLs at the GBs of such concentrated solid solutions, on the other hand, has been based on the dilute‐solution approximation (Poisson–Boltzmann ansatz), in which defect–defect interactions are neglected.…”