Study of REBa2Fe3O8+δ (RE = Pr, Nd, Sm) layered perovskites as cobalt-free electrodes for symmetrical solid oxide fuel cells

Abstract

The REBa₂Fe₃O_8+δ (RE = Nd, Sm, Pr) perovskites are investigated as potential cobalt-free electrodes in symmetrical solid oxide fuel cells (SOFCs). After the preparation of samples by a soft chemistry route, we first characterized the intrinsic properties and then determined the electrochemical performance after the deposition of porous electrodes to obtain symmetrical cells. Analytical techniques such as X-ray diffraction (XRD) at room and high temperatures, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), dilatometry (TEC), and 4-probe conductivity measurements were employed to characterize exhaustively structural, thermal and electrical properties of the samples. The electrochemical characterization was further investigated through electrochemical impedance spectroscopy (EIS) as well as fuel cell testing conducted on electrolyte-supported symmetrical cells. XRD showed that all samples have a cubic structure with the $\mathrm{Pm}\overline{3}m$ space group. However, during TEM experiments, it was observed that SmBa₂Fe₃O_8+δ presents a quintuple nano-ordering perovskite structure. Pr-based sample shows the highest electrical conductivity (68 S cm⁻¹ at 500 °C), while NdBa₂Fe₃O_8+δ presents the lowest area specific resistance in air (0.47 Ω cm² at 600 °C) revealing that the disordered perovskite structure is more efficient than the quintuple nano-ordered phase of SmBa₂Fe₃O_8+δ in the oxygen reduction reaction (ORR). The use of NdBa₂Fe₃O_8+δ as electrodes in symmetrical cells has been demonstrated between 500 °C and 600 °C.