Proton-conducting barium stannates: Doping strategies and transport properties

Abstract

High temperature proton conductors (HTPCs) find their applications in steam electrolysis, gas sensing, and most importantly, fuel cells. In this work, proton conductivities and transport properties of doped BaSnO3 are investigated. Samples of BaSn0.9D0.1O2.95 (D = In, Lu, Er, Y, and Gd) were prepared by solid-state reaction and relative densities >93% were achieved after sintering at 1600 °C or lower. Although Y doping is commonly known to yield the highest conductivities for BaCeO3 and BaZrO3, In-doped BaSnO3 exhibits the highest conductivity and conductivities decrease in the order In > Lu > Er > Y > Gd. Ionic radius and electronegativity matching between dopants and host Sn4+ is shown to be an important doping strategy for enhancing conductivities of BaSnO3. Measurements of H/D isotope effect and electromotive force (EMF) were performed on BaSnO3 to give direct evidence for proton conduction and to examine transport properties. The ratio between conductivities in H- and D-atmosphere (σH/σD) is 2.62 in reducing conditions, indicating protons transfer via the Grötthus mechanism. Proton transport numbers reached above 0.75 at 450 °C, and n- and p-type electronic conduction is identified to be secondary in reducing and oxidizing atmospheres, respectively. Electronic contribution to conductivity is found to increase with temperatures. Conductivities of BaSnO3 are seen to be comparable to Y-doped BaZrO3 (e.g., grain conductivities for both Y-doped BaSnO3 and BaZrO3 are ∼1.5 × 10−4 S cm−1 at 350 °C), and are much higher than other common HTPCs, such as LaPO4 and LaNbO4. The high conductivity and good sinterability make BaSnO3 a promising HTPC.