Proton transfer in La2-xCaxZr2O7-δ pyrochlores: Reasons for limited water uptake and high grain boundary conductivity

Abstract

Proton-conducting electrolytes based on zirconates with a pyrochlore-like structure are known for their excellent thermal and chemical stability, low grain boundaries (GB) resistance, and low contribution of electronic conductivity. However, the values of proton conductivity seem to be undermined by insufficient incorporation of protons into the structure. The current study aims to identify the causes of these barriers from standpoints of crystal and defect structure, water uptake, and proton transfer. The thermogravimetric analysis (TGA) results show limited water uptake, which is ∼60% of the theoretical value. We show that several oxygen vacancies are not involved in the hydration, due to a significant structural nonequivalence of oxygen sites in the pyrochlore. Additionally, the proton positions themselves are energetically nonequivalent due to the different electrostatic repulsion energy of protons from the nearest cations, which ultimately leads to a weak proton transfer in the bulk of the material. At the same time, we observe the segregation of Ca on the grain boundaries without any impurity formation. This leads to the high apparent GB conductivity according to electrochemical impedance spectroscopy. These findings suggest that the proton conductivity of pyrochlore-like zirconates could be improved by tailoring their GBs instead of perfecting bulk properties.