Effective Suppression of LSCF Air Electrode Degradation by Air Cleaning

Abstract

The performance degradation of air electrodes in solid oxide cells is strongly affected by impurities such as sulfur dioxide. Air filters are applied in most fuel cell systems to avoid poisoning and subsequent degradation of fuel cell stacks [1, 2]. Filtering solutions designed for the high air purity demands of proton-exchange membrane fuel cells (PEMFC) are commercially available.This study evaluates the applicability of such a filter for the LSCF air electrode in an electrolyte-supported SOC. Three different air qualities were applied in single-cell tests to distinguish the degradation mechanism caused by the ambient air impurities from the intrinsic aging phenomena of the cell. The utilized gases are (i) compressed air, (ii) filtered compressed air, and (iii) synthetic air. The compressed air (i) consists of ambient air supplied by a conventional compressor with a downstream particle filtering and oil removal system. In (ii), an additional filter layer was applied between the mass flow controller and the cell housing to filter the compressed air. The filter consisted of specially modified activated carbons designed to remove harmful gases and was operated at ambient temperature and pressure. The active area of the filter was 0.95 cm², corresponding to an air velocity of 0.088 m s-1 at an airflow rate of 500 sccm. The synthetic air is a mixture of 21% oxygen (99.95% O2, the main impurities are H2O and Ar) and 79% nitrogen (99.999% N2) provided by the Air Liquide company. Symmetrical cells based on a 3YSZ substrate coated with GDC buffer layers and LSCF electrode applied on both sides with an active surface area of 1 cm2 were applied in this study.Figure 1 shows the polarization resistance of the LSCF electrode operated in different types of air. Similar to previous reports [3-5], a significant degradation rate is recorded within the first 300 hours of operation. Filtering the air reduces the degradation rate significantly. The performance degradation of the cell with filtered air is comparable to that of the cell with synthetic air, demonstrating the capability of the filter to adsorb the gas impurities. Fig. 1 polarization resistance of the LSCF-cathode of 3 symmetrical cells operated at 750 °C with unfiltered and filtered compressed air and synthetic air, respectively.To better understand the degradation mechanism in the LSCF air electrode, we analyzed the impedance spectra measured during aging tests using the distribution of relaxation times (DRT) and then a complex nonlinear least squares (CNLS) fit. This contribution discusses differences in the temporal development of air electrode impedance and degradation mechanisms related to surface poisoning and subsequent changes in LSCF bulk.Keywords: LSCF air electrode, Impedance spectra, Degradation mechanisms, Air filtration.[1] M. Harenbrock, A. Korn, A. Weber, E. Hallbauer, in, SAE Technical Paper, 2020.[2] M. Harenbrock, E. Hallbauer, T. Heilmann, M. Hanselmann, ATZheavy duty, 14 (2021) 34-37.[3] C. Endler, A. Leonide, A. Weber, F. Tietz, E. Ivers-Tiffée, Journal of the Electrochemical Society, 157 (2010) B292-B298.[4] C. Endler-Schuck, A. Leonide, A. Weber, S. Uhlenbruck, F. Tietz, E. Ivers-Tiffée, Journal of Power Sources, 196 (2011) 7257-7262.[5] C. Endler-Schuck, J. Joos, C. Niedrig, A. Weber, E. Ivers-Tiffée, Solid State Ionics, 269 (2015) 67-79. Figure 1