Manipulating the Activity and Thermal Compatibility of NdBaCoFeO_5+δ Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells via Fluorine Doping

The use of LnBaCo₂O_5+δ (Ln = rare earth) cathodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) is limited by their high thermal expansion coefficients (TECs). Unfortunately, the electrocatalytic activity of LnBaCo₂O_5+δ cathodes is also decreased significantly while reducing the TECs. Here, we report the simultaneous enhancement of the oxygen-reduction activity and thermal expansion compatibility of the NdBaCoFeO_5+δ (NBCF) double-perovskite cathode for IT-SOFCs via fluorine doping at the oxygen sites, that is, NdBaCoFeO_5+δ‑xF_x (NBCFFx, x = 0.05 and 0.10). The maximum power density of the single cell with a configuration of NiO–Ce_0.8Sm_0.2O₂/Ce_0.8Sm_0.2O₂/La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ/NBCFF0.05 attains 504 mW cm^–2 at 700 °C in dry H₂. Compared with the pristine NBCF, the polarization resistances of NBCFF0.05 are down by 18.9, 17.0, 13.9, and 10.5% from 650 to 800 °C with an interval of 50 °C, respectively, while the average TEC is decreased by 3.6%. Based on the analysis of distribution of relaxation time from impedance data, the oxygen surface exchange and bulk diffusion are the rate-limiting steps in the oxygen-reduction reaction, which can be improved by fluorine doping. Our results suggest that the fluorine doping at oxygen sites shows a better ability to promote the performance of the cathode material in IT-SOFCs.