Dopants in BaFeO3-based triple-conducting perovskites - bigger and more is not always better for proton uptake

The proton uptake of triple-conducting perovskites based on BaFeO 3-δ is sensitive to redox-inactive dopants on the Fe-site. Depending on charge, size mismatch, and preferred coordination number the dopant solubilities are found to range from up to 0.2 (for Ce 4+ , Y 3+ , Zn 2+ ) to (almost) complete solid solutions for Sn 4+ , Zr 4+ , Sc 3+ , In 3+ . The observation of very different and complex trends of proton uptake with dopant concentration indicates strong defect interactions. Effects of local lattice distortions and modified oxide ion basicity are discussed. The largest proton uptake is found for moderately oversized dopants (Zn 2+ , Sc 3+ , In 3+ , also combined (Zn,Zr) doping) and reaches 0.1–0.12 protons per formula unit at 250 °C ( p H 2 O = 17 mbar, samples quenched from 700 °C in N 2 ). These trends are expected to be transferable also to the optimization of other triple-conducting perovskites. For the solid solutions, a significant further increase of proton uptake is found only at dopant contents exceeding 0.6. However, the electronic conductivity of such materials will be insufficient for an application as air electrode in protonic ceramic electrochemical cells. Standard enthalpy and entropy of the hydration reaction show a linear correlation, but with different slope and axis intercept than for electrolyte perovskites.