Manipulating the Activity and Thermal Compatibility of NdBaCoFeO5+δ Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells via Fluorine Doping

The use of LnBaCo 2 O 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 LnBaCo 2 O 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- x F x (NBCFF x, x = 0.05 and 0.10). The maximum power density of the single cell with a configuration of NiO–Ce 0.8 Sm 0.2 O 2 /Ce 0.8 Sm 0.2 O 2 /La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3−δ /NBCFF0.05 attains 504 mW cm –2 at 700 °C in dry H 2 . 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.