Influence of iron content on water uptake and charge transport in BaCe0.6Zr0.2Y0.2−xFexO3−δ triple-conducting oxides

Abstract

In this work, we studied the BaCe_0.6Zr_0.2Y_0.2−xFe_xO_3−δ system which belongs to the triple-conducting oxides (TCOs) group. The electrochemical properties of BaCe_0.6Zr_0.2Y_0.2−xFe_xO_3−δ were investigated using electrochemical impedance spectroscopy (EIS) and the water uptake was analyzed using thermogravimetry (TG). All investigated materials exhibited water uptake, with proton concentration increasing with decreasing iron content. BaCe_0.6Zr_0.2Y_0.18Fe_0.02O_3−δ and BaCe_0.6Zr_0.2Y_0.15Fe_0.05O_3−δ showed a significant electrical conductivity increase in wet air at low temperatures, indicating a hydration reaction occurring in these materials. The oxygen nonstoichiometry was determined using iodometric titration, revealing a linear decrease in oxygen vacancy concentration with increasing iron content in the materials. The partial oxygen ionic and electronic conductivity was determined at 600 and 800 °C. All compositions exhibited p-type electronic conductivity in air, and both types of partial conductivity decreased with increasing iron content. Water uptake kinetics was analyzed using electrical conductivity relaxation (ECR). Water uptake followed single-fold monotonic relaxation for BaCe_0.6Zr_0.2Y_0.15Fe_0.05O_3−δ and BaCe_0.6Zr_0.2Y_0.1Fe_0.1O_3−δ across the entire temperature range and, in the case of BaCe_0.6Zr_0.2Y_0.18Fe_0.02O_3−δ, below 550 °C. At 550 °C and above, a two-fold relaxation kinetics in BaCe_0.6Zr_0.2Y_0.18Fe_0.02O_3−δ was observed. The surface exchange coefficient and the chemical diffusion coefficient of water for different compositions were determined. The influence of water vapor on the oxidation and reduction kinetics in BaCe_0.6Zr_0.2Y_0.18Fe_0.02O_3−δ and BaCe_0.6Zr_0.2Y_0.1Fe_0.1O_3−δ was investigated. In both materials, a significant increase in the oxygen surface exchange and diffusion coefficients was observed in wet conditions in the case of oxidation.