Electrochemical Impedance Studies on SOC

Abstract

In the institute for fundamental electrochemistry IEK-9 at the Forschungszentrum Jülich GmbH the electrochemical properties of solid oxide cells are electrochemically characterized at a variety of sizes, configurations and operating conditions. In various projects different aspects of the complete cell and the reversible oxygen electrode (ROE) are studies. Both electrode supported cells of 5 x 5 cm² and 20 mm diameter are characterized by galvanostatic DC methods and electrochemical impedance spectroscopy (EIS). Also electrolyte supported symmetric cells are studied using a configuration with reference electrodes to investigate the reversible air electrode in detail. The main focus lies on the use of EIS to try and understand the different fundamental (electrochemical) reactions within the electrode process. Measurements are conducted on 5 x 5 cm² electrode supported cells comprising a 0.6 mm thick Ni/8YSZ fuel electrode, a 10 µm thick 8YSZ electrolyte, a 10 µm thick GDC diffusion barrier layer and a 50 µm thick LSFC air electrode. The impedance of the cells shows a continuous increase when progressing from fuel cell to electrolysis conditions (see Figure 1A). At high currents densities under both positive and negative current densities an inductive loop appears in the low frequency range. Due to the high currents involved and the use of a power booster during the measurements the EIS data are limited to ~10 kHz in the high frequency range. Therefore the EIS data do not directly show a high frequency intercept with the real axis and the Ohmic resistance has to be determined using a fitting procedure. Similar measurements were also performed on 20 mm round or button cells of the same composition and microstructure. These cells show similar behavior (see Figure 1B). The overall shape of the impedance spectra is comparable for both cell types although the contributions of the underlying processes seem to vary with the cell size. Here the high frequency intercept can be determined but the measured data have to be corrected for an inductive contribution originating in the measurement setup. To separate the contribution of the ROE from the total cell resistance, measurements on electrolyte supported cells with symmetrical CGO/LSCF electrodes and a platinum reference electrode are performed. Also here a continuous transformation of the EIS spectra can be observed when going from oxygen reduction in the fuel cell mode to oxygen evolution in the electrolysis mode. By combining the data of all three different cell geometries an attempt is made to separate contributions from both electrodes and the influence of cell size on the total cell resistance. Figure 1