Study of LiFeO2 coated NiO as cathodes for MCFC by electrochemical impedance spectroscopy

LiFeO2 was coated on porous NiO cathode using a simple combustion process. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the cathode characterizations. The electrochemical behaviors of LiFeO2 coated NiO cathode (LFO-NiO) were also evaluated in a molten 62 mol% Li2CO3 + 38 mol% K2CO3 eutectic at 650 degreesC under the standard cathode gas condition by electrochemical impedance spectroscopy (EIS). The impedance response of the NiO and LFO-NiO at different immersion time is characterized by the presence of depressed semicircles in the high frequency range changing over into the lines with the angle of which observed with the real axis differing 45degrees or 90degrees in the low frequency range. The experimental Nyquist plots can be well analyzed theoretically with a modified model based on the well known Randles-Ershler equivalent circuit model. In the new model, the double layer capacity (C-d) is replaced by the parallel combination of C-d and b/omega to take into consideration the non-uniform of electric field at the electrode/electrolyte interface owing to the roughness of electrode surface. The LFO-NiO showed a lower dissolution and a good catalytic efficiency close to the state-of-the-art NiO value. In the unit cell test, the performance of the cell composed of LiFeO2 coated NiO cathode maintained more stable values than that of the cell composed of NiO cathode. Thus the cathode prepared with coating method to coat LiFeO2 on the surface of NiO cathode is able to reduce the solubility of NiO to lengthen the lifetime of MCFC while maintaining the advantages of NiO cathode. (C) 2004 Elsevier B.V. All rights reserved.