Crystal structure, oxygen nonstoichiometry, and mass and charge transport properties of the Sr-free SOFC/SOEC air electrode material La0.75Ca0.25FeO3-δ

Abstract

The mixed ionic-electronic conducting perovskite La0.75Ca0.25FeO3-δ (LCF7525) was synthesized via a citric acid – ethylenediaminetetraacetate sol-gel route. Crystal structure, phase purity, and lattice constants were determined by powder X-ray diffraction and Rietveld refinement. The oxygen exchange kinetics (chemical surface exchange coefficients and chemical diffusion coefficients of oxygen) and the electronic conductivity were studied by in-situ dc-conductivity (relaxation) measurements at 600–800 °C and 1 × 10−3 ≤ pO2/bar ≤ 0.1. The thermal expansion coefficient was determined by dilatometry at 30–1000 °C and 1 × 10−3 ≤ pO2/bar ≤1. The oxygen nonstoichiometry was measured as a function of temperature and oxygen partial pressure by thermogravimetry and could be described by a point defect model. Experimental data of the chemical diffusion coefficient of oxygen and results from defect chemical modelling were used to estimate self-diffusion coefficients of oxygen and oxygen vacancies, as well as the ionic conductivity. Based on the results obtained for the mass and charge transport properties and the thermal expansion behaviour, it can be concluded that LCF7525 may be an attractive Sr- and Co-free material for air electrodes in intermediate temperature solid oxide fuel cells and solid oxide electrolyser cells.