Phase composition and proton uptake of acceptor-doped self-generated Ba(Ce,Fe)O_3-δ – Ba(Fe,Ce)O_3-δ composites

Self-generated Ba(Ce,Fe,In)O_3-δ composites were prepared by one-pot sol-gel synthesis. They consist of Ce-rich and Fe-rich phases, and are intended to supply the required protonic and electronic transport for air electrode materials in protonic ceramic fuel and electrolysis cells (PCFC, PCEC). Crystal structure, lattice parameters, and the relative phase amounts of the composites were obtained from X-ray diffraction. The local chemical composition and distribution of cations within the individual phases were characterized by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. Annealing experiments indicate that the miscibility gap of the BaCe_0.8-xFe_xIn_0.2O_3-δ system ranges from [Ce]/([Ce] + [Fe]) ratios of ~ 0.2 to ~ 0.9. The In³⁺ acceptor shows a tendency to accumulate in the Fe-rich phase, with the ratio In(Ce-rich phase)/In(Fe-rich phase) being in the range of 0.3–0.7. The proton uptake capacity of the materials, which was analyzed by thermogravimetry, increases with an increasing amount of In and decreasing amount of Fe in the precursor. Proton concentrations are in the range of 1–4 mol% at 400 °C. Further measurements on BaCe_0.4Fe_0.4Acc_0.2O_3-δ (Acc = Y, Yb, Gd, Sm, Sc) composites show that proton uptake is generally increased compared to the undoped system BaCe_0.5Fe_0.5O_3-δ. However, variations in the acceptor ion can tune the proton uptake only to a limited extent.