Doping effects on the structure and electrical properties of La2Ce2O7 proton conductors

Abstract

Recent studies have reported that the conductivity of La2Ce2O7 proton conductors is improved by aliovalent doping, despite the high concentration of oxygen vacancies in the pristine material. In this study, we provide new insights into the structural and conducting properties of La2-x AxCe2O7±δ (A = Na+, Ca2+, Sr2+, Ba2+, Y3+ and W6+; x ≤ 0.2). The materials are apparently single phase compounds with a disordered fluorite-type structure by conventional X-ray diffraction; however, the local structure, studied by Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases, which depends on dopant type and the sintering temperature. In addition, phase segregations are observed for Ca, Sr and Ba-doped samples by scanning electron microscopy. The different substitutions do not improve the bulk conduction at low temperature, and the conductivity differences between the samples are mainly ascribed to microstructural changes. High sintering temperatures produce a significant reduction of the bulk conductivity, i.e. 1.38 and 1.01 μS cm−1 at 300 °C for samples sintered at 1400 and 1500 °C, respectively. The use of Zn as sintering aid greatly improves the densification of La2Ce2O7 at lower temperature, without affecting negatively the conductivity.