Impact of SO₂on the Oxygen Exchange Kinetics of the Promising SOFC/SOEC Air Electrode Material La_0.8Ca_0.2FeO_3-δ

Abstract

The mass and charge transport properties of La 0.8 Ca 0.2 FeO 3-δ (LCF82) were determined at 600-800 • C and pO 2 = 0.1 bar. The electronic conductivity is in the range of 112 S cm −1 at 700 • C. The chemical surface exchange coefficient (k chem ) and the chemical diffusion coefficient of oxygen (D chem ) were measured. LCF82 shows exceptionally fast oxygen exchange kinetics in pure O 2 -Ar at 700 • C. Long-term measurements at 700 • C in pure O 2 -Ar showed an excellent stability of the kinetic parameters during 1000 h. However, when 2 ppm of sulfur dioxide were added, k chem decreased by a factor of 40 within the first 24 h. After further 1000 h, the total decrease in k chem amounted to two orders of magnitude. Post-test analyses of LCF82 were performed by SEM-EDXS, XPS, and STEM. Ca-enrichment and La-/Fe-depletion of the LCF82 surface occurred during the first 1000 h without SO 2 . At the surface of the sample exposed to 2 ppm SO 2 for additional 1000 h, CaSO 4 crystals with diameters of 1-2 μm and an underlying layer of Fe 2 O 3 (thickness approx. 300-450 nm) were found. Remarkably, LCF82 shows faster oxygen exchange kinetics than La 0.6 Sr 0.4 CoO 3-δ even in the degraded state. Therefore, LCF82 is suggested as a promising material for SOFC and SOEC air electrodes.

In situ techniques reveal the true capabilities of SOFC cathode materials and their sudden degradation due to omnipresent sulfur trace impurities

“…the formation of a charged adsorbate layer) is fundamentally different from sulfur poisoning due to a second phase formation as discussed in Refs. 22,29,[31][32][33][34]36,37,64 , see also above. With regard to the detailed mechanism of how SO 2 reversibly deactivates the electrode surface, it is important to note that the oxygen reduction mechanism itself is not fully understood yet.…”

“…Besides determination of the overall electrode surface composition, special focus was laid on the detection of sulfur traces on the electrode surface, as sulfur contaminations (typically from SO 2 and H 2 S in the gas phase) are well known for the poisoning of SOFC cathodes. 11,22,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] To determine the sulfur concentration on the surface of the LSC electrodes, XPS spectra of the S2p core levels and O1s core levels were recorded.…”

“…As already mentioned above, the detrimental effect of SO 2 on SOFC cathode performance was also observed in previous work. 22,29,61,62 However, in most studies SO 2 was intentionally added to the feed gas in concentration of a few ppm and long term degradation effects were investigated.…”

Hydroxylation and Cation Segregation in (La0.5Sr0.5)FeO3−δ Electrodes

“…23 In addition to water vapor, SO 2 can also react with surface segregants to form secondary surface phases. 24 T h i s c o n t e n t i s Even N 2 has been reported to affect oxygen reduction. 25 To deepen the mechanistic understanding of degradation in perovskite oxides, and ultimately to mitigate them, multiple degradation pathways need to be decoupled and investigated individually (e.g., those involving Si, SO 2 and water vapor).…”

A novel Ag based sealant for solid oxide cells with a fully tunable thermal expansion

“…Solid oxide fuel/electrolysis cells (SOFC/SOEC) are promising electrochemical conversion devices due to the high electrical efficiency, fuel flexibility, quiet operation and ultra-low pollution 2 [1][2][3][4][5]. During operation, the fuel needs to be separated from the oxidant in order to avoid the direct combination of these reactants and thereby loss of efficiency [6,7].…”