Microscopic Mechanism Study of 4f Electrons' Positive Effect on the Enhanced Proton Conduction in a Pr-Doped BaCeO3 Electrolyte

Protonic ceramic fuel cells (PCFCs) composed of porous electrodes and dense electrolytes are considered to be a promising alternative to solid oxide fuel cells (SOFCs) at low and intermediate temperatures (<= 600 degrees C) due to their comparative low activity barrier. It is well known that rare earth elements, such as Y, Yb, and Pr, have been applied to dope prototypical perovskite-type oxides BaCeO3 and BaZrO3 to improve both the conductivity and the stability. In this article, we particularly select Pr with a radius similar to that of Ce as a dopant to dope BaCeO3, aiming to exclusively extract the 4f5d electrons' effect on the protonic conductivity as well as the influence on the stability, based on both a static density functional theory (DFT) calculation and an ab initio molecular dynamics (AIMD) simulation. The calculated results demonstrated that the oxygen vacancy formations were lowered owing to the O 2p band center shifting toward the Fermi level and the protonic conductivity was positively reduced due to the energy springboard provided by the Pr 4f orbital, and the p-type semiconductor effect originated from the different covalent states of Pr3+ with Ce4+. AIMD simulations confirmed the proton transition process as previously proposed and thermodynamic stability at a temperature as high as 750 degrees C. In contrast, CO2 and SO2 adsorption calculated results indicate that Pr doping has little effect on the chemical stability. This is consistent with the experimental strategy in which multiple rare earth elements are utilized to secure the high performance of the electrolytes in these acceptor-doped perovskites.