Reduced polarization resistance of CoFe2O4 cathodes via in-situ cathode activation for solid oxide fuel cells

CoFe 2 O 4 (CFO) was investigated as a spinel cathode for solid oxide fuel cells (SOFCs), with particular focus on its electrochemical response to cathodic activation. While cathode activation has been applied to enhance the cathode performance in various electrode materials, the electrochemical response and related changes in CFO cathodes after activation have rarely been investigated. Cathodic polarization was applied by chronopotentiometry at −100 mA cm -2 in sequential steps of 5, 10, 15, 30, and 120 min, and electrochemical impedance spectroscopy (EIS) was conducted between each step. The polarization resistance decreased from 24.465 to 5.885 Ω cm 2 after a total activation time of 180 min, corresponding to a reduction of 75.9% relative to the untreated electrode. Distribution of relaxation times (DRT) analysis showed that the largest change occurred in the medium-frequency region. The normalized integrated DRT area decreased to 23.30% after 15 min, and further to 3.07% after 180 min of activation. FE-SEM observations also showed a finer surface microstructure after activation, with the apparent average surface grain size decreasing from 1.957 to 1.052 μm. GI-XRD showed a clear low-angle shift of the CFO (311) reflection after activation, although the shifts were not identical for the other major reflections. The apparent lattice parameter calculated from the (311) reflection changed from 8.379 to 8.402 Å. XPS analysis showed changes in the relative contributions of Co 2+ and Fe 3+ at the tetrahedral and octahedral sites after activation. The relative area of the surface oxygen component in the O 1s spectra increased from 13.29% to 36.87%, indicating a change in the surface oxygen environment of the CFO electrode. Overall, cathodic polarization produced a substantial decrease in the polarization resistance of the CFO cathode together with measurable changes in its surface microstructure and near-surface crystal structure. These findings show that electrochemical activation can modify both the electrochemical and physicochemical properties of spinel-type CFO cathodes.