Co-Deposition and Poisoning of Chromium and Sulfur Contaminants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δCathodes of Solid Oxide Fuel Cells

Abstract

The presence of both chromium and sulfur (Cr/S) contaminants on the microstructure and electrocatalytic activity properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) electrodes of solid oxide fuel cells (SOFCs) is studied, using Confocal laser Raman spectroscopy, XRD, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and electrical conductivity relaxation (ECR) methods. LSCF dense bar samples were heat treated in the presence of Cr 2 O 3 and 20 ppm SO 2 and in the temperature range of 600-900 • C. The deposition and reaction products between LSCF and Cr/S depend on the temperature: SrCrO 4 only forms on LSCF samples at 900 • C and 800 • C, while formation of SrSO 4 phase occurs at all temperatures studied. The results indicate that sulfur shows a higher activity with LSCF, as compared to gaseous Cr species. Segregated SrO is more likely to react with gaseous Cr species at higher temperatures, however, reaction with SO 2 is more pronounced at lower temperatures, forming SrSO 4 . ECR results indicate that co-deposition of Cr and sulfur significantly deteriorates the surface exchange and diffusion processes for the O 2 reduction reaction on LSCF electrodes. The durability of a solid oxide fuel cell (SOFC) is critically related to the degradation behavior of its cathodes such as La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) in the presence of impurity species such as chromium from the chromia-forming interconnect, silica, boron and volatile alkaline elements from the glass seals sealant and sulfur from air.1-8 Among them, gaseous chromium species vaporized from the chromium oxide scale of chromia-forming metallic interconnect are probably the most investigated contaminants affecting the performance of SOFCs' cathodes. The mechanism and process of the deposition and poisoning of chromium species at the cathodes of SOFCs have been extensively investigated, including LSM, 9-11 LSCF 12 and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF). 13 In the case of LSCF electrodes, deposition of Cr species preferentially take place on the surface of the LSCF electrode, resulted from the interaction between the segregated SrO and gaseous Cr species.14,15 Both the humidity in the air stream and operation temperature have a significant effect on the Cr deposition. 16 Cr deposition decreases significantly with the decrease of temperature, most likely due to the reduced Sr segregation as well as the decrease of the partial pressure of gaseous Cr species at reduced temperatures. 17The sulfur in the form of SO 2 or H 2 S in the air stream is another important contaminant affecting the performance stability of SOFC cathodes. SO 2 content in air can be in the range of 10-340 μg m −3 (3.5 × 10 −3 -0.12 ppm) in cities. 18 Despite the low concentration of SO 2 , sulfur in the air stream can accumulate at the cathodes of SOFC cells, degrading the performance.5 Wang et al. 19 studied the polarization performance behavior of LSCF in the presence of 0.1 ppm SO 2 and observed two-stages of performance degrad...

SrCo1−xTixO3−δ perovskites as excellent catalysts for fast degradation of water contaminants in neutral and alkaline solutions

“…The peaks at 786.4 eV and 802.9 eV were the additional satellite peaks for Co 2p 1/2 and Co 2p 3/2 , respectively. The peak located at BE of 781.2 eV (Co 2p 3/2 ) represents Co 3+ while the peak located at BE of 780.1 eV (Co 2p3/2) represents Co 2+ , consistent with the literature274344. The peaks corresponding to Co 3+ and Co 2+ can be deconvoluted to determine their peak areas.…”

“…Accordingly, Sr 3d 5/2 peak at 131.8 eV and Sr 3d 3/2 peak at 133.0 eV correspond to Sr in the perovskite phase42. Sr 3d peak at approximately 133.8 eV, on the other hand, is likely related to the strontium carbonate on the surface344344.…”

Integrated Cr and S poisoning of a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode for solid oxide fuel cells

“…18 The increase in R E illustrated that the presence of Cr species had a signicant effect on the oxygen exchange, and the diffusion process generally associated with the electrode low frequencies. 19 Fig . 1 shows the polarization and the impedance curves for ORR on the LSCF electrode as a function of polarization time at 200 mA cm À2 in the presence of 1 ppm SO 2 at 800 C for 20 h. At 800 C, E cathode was stable, except for a small decrease from 250 to 215 mV initially (Fig.…”

Factors influencing the measured surface reaction kinetics parameters

“…The surface micro‐structure and composition could be poisoned by some minor elements such as chromium, sulfur, boron and silicon, which are possibly introduced during the preparation or measuring process. Wang et al researched the poisoning process of LSCF under the existence of gaseous chromium, sulfur and chromium/sulfur. The experimental results reveal that deposition of Cr and S significantly deteriorates the surface exchange and diffusion processes.…”