Insights into Barriers to Increasing Proton Conductivity of Lanthanum Scandate Ceramics via High Sr-Doping

Abstract

Maximizing proton conductivity is a necessity in the design of efficient oxide proton-conductive membranes for electrochemical applications. Acceptor doping makes it possible to achieve a high concentration of charge carriers but encounters problems of phase stability or defect interaction. Sr is the optimal dopant for perovskite-like lanthanum scandate. La1–x Sr x ScO3−δ (0.05 ≤ x ≤ 0.20) solid solutions demonstrate phase and structural stability and high levels of hydration. Compositions with x = 0.1 and 0.15 reach proton conductivity of 5 mS/cm at 600 °C and air humidity 2.8 kPa; however, for compositions x ≥ 0.15, an increase in temperature above 500 °C causes a drastic loss of protons. It is probably associated with the clustering of oxygen vacancies, which is confirmed by an anomalous increase in the thermodynamic parameters of hydration, chemical expansion values, and activation energies of proton and oxide-ion transfer. Thus, both high proton transfer activation energies and chemical expansion values for compositions x ≥ 0.15 can become a challenge in the production and operation of electrolytic membranes, whereas compositions x ∼ 0.1 are most preferred.