Structural, electrical, and electrochemical properties of cobalt-doped NiFe_2O_4 as a potential cathode material for solid oxide fuel cells

摘要

In this work, Co-doped NiFe_2O_4 spinels (NFCO-x) are successfully fabricated and characterized as possible cathode materials for the intermediate-temperature solid oxide fuel cells (SOFC). Results of the binding energy calculations using the density functional theory suggest that the reverse spinel structure is stable when Co~(3+) occupies the octahedral interstitial sites. Total and ionic-only conductivities indicate that NFCO-x are a kind of mixed electronic-ionic conductors. Ionic transferring numbers are approximately 0.049 and 0.006 for NFCO-0.1 and NFCO-0.5, respectively, measured at 700 ℃ in air. Co dopant in the NFCO-x improves the electronic conductivity at the expense of the ionic conductivity. For NFCO-0.5, electronic and ionic conductivities are approximately 0.24 and 9.6 × 10~(-4) S cm~1 respectively, measured also at 700 ℃ in air. Unlike behaviour of the conductivities, the polarization resistance of symmetric cells with NFCO-x electrodes decreases when increasing the Co content (x) to a certain level, and then increases. The cell containing the NFCO-0.5 electrode exhibits the lowest polarization resistance (R_p), which is approximately 1.51 Ω cm~2 at 650 ℃. For single cells, the maximum power density is 320 mW cm~(-2) measured at 650 ℃ using a 38-μm-thick SDC electrolyte and an NFCO-0.5 cathode.