Electrostatic spray deposited Ca3Co4O9+δ and Ca3Co4O9+δ/Ce0.9Gd0.1O1.95 cathodes for SOFC: A comparative impedance analysis study

Abstract Mixed conducting Ca3Co4O9+δ (CCO) is an interesting cathode material for application in Solid Oxide Fuel Cells (SOFC). In a previous study it has been shown that addition of Ce0.9Gd0.1O1.95 (CGO) significantly enhances the electrode properties, reducing the Area Specific Resistance (ASR) to ∼0.5 Ω cm2 at 700°C for the CCO/CGO 50/50% composition. As the microstructure of a composite electrode has a significant influence on the frequency dispersion, it is of interest to prepare electrodes with quite different microstructures. Electrostatic Spray Deposition (ESD) is a technique that is able to produce a large variety of microstructures by modifying the process parameters. Very different microstructures can help in elucidating the major charge transport and transfer processes in an electrode. In this study pure CCO and a CCO/CGO composite (50/50%), both prepared with ESD, are compared and analyzed with Electrochemical Impedance Spectroscopy (EIS). The analysis of the ESD-CCO/CGO composition showed remarkable similarities with the screen-printed cathodes from the previous study, although a clear change in the magnitudes of the separate contributions (low-frequency redox, mid-frequency Gerischer and high-frequency diffusion) was observed. The ASR was close to the screen-printed one, but showed two apparent activation energies in the Arrhenius graph. A tentative model indicates that the Gerischer process is related to dissociative oxygen adsorption and (surface) diffusion at the CGO phase. The limiting factor is the density of the triple-phase boundaries (TPBs) between CCO, CGO and the ambient, which is for the ESD-CCO/CGO cathode apparently lower than for the microstructure of the screen-printed cathodes. It was noted that for the ESD-CCO/CGO cathodes the Distribution Function of Relaxation Times (DFRT) presented a more consistent image of the temperature dependence than the standard Complex Nonlinear Least Squares (CNLS) analysis. The pure ESD-CCO cathode showed a remarkable dispersion, which could be interpreted with a Finite Length Warburg (FLW) model. Although analysis with a simple Equivalent Circuit (EqC) was not feasible, partial CNLS-analysis of the high- and low-frequency regimes resulted in a parameter set that is consistent with the FLW-model. Considering the coral-like microstructure, the oxygen reaction at the electrode could be interpreted as slow oxygen dissociation and fast diffusion towards a more dense CCO layer at the electrolyte interface, followed by a fast oxygen exchange step at the CCO-layer/platelets + ambient interface. Combining FLW parameters with published chemical diffusion values, an apparent effective layer thickness of 2.2 μm could be estimated.