Processing of high performance composite cathodes for protonic ceramic fuel cells by exsolution

Abstract

La_0.5Ba_0.5CoO_3−δ–BaZrO₃ (LB–BZ)-based composite materials were prepared by a modified Pechini sol–gel method combined with exsolution. Two different LB–BZ composites were prepared through two alternative thermal treatments of the precursor gel. A metastable single phase with a perovskite crystal structure was first obtained upon annealing the precursor in an inert atmosphere, and it was further transformed into a two-phase composite by in situ exsolution in air. Comparatively, direct calcination of the LB–BZ gel in air resulted in a two-phase composite with different microstructures and compositions of the two phases. The composite cathode formed by exsolution consisted of a matrix of BZ-phase with ∼45 nm grain size embedding ∼20 nm grains of LB-phase, while the composite cathode obtained by direct calcination consisted of a mixture of both phases with 50–60 nm grain size. Electrodes of symmetric half-cells were spray-coated on the BaZr_0.9Y_0.1O_2.95 electrolyte to examine the electrochemical performance by impedance spectroscopy. The lowest area specific resistance (ASR) was obtained for the composite cathode produced by exsolution with an excellent ASR of 1.54 Ω cm² at 600 °C and 18 Ω cm² at 400 °C and an activation energy (E_a) of 0.60 eV in 3% moist synthetic air. This work demonstrates the potential of fabricating high performance nanocomposite cathodes with tailored chemical composition by a novel exsolution method.