Performance regeneration in lanthanum strontium manganite cathode during exposure to H₂O and CO₂ containing ambient air atmospheres

Abstract

The regenerability and stability of lanthanum strontium manganite (LSM)/yttria doped zirconia (YSZ)/LSM symmetrical cells have been examined after the cells were exposed to real-world air environment containing H 2 O and CO 2 . An alternate exposure experiment in 20% H 2 O-air and dry air has been conducted to test the reversibility of the cell degradation in H 2 O/air. Long-term experiment in 3% H 2 O-0.5% CO 2 -air, dry air, 0.5% CO 2 -air, and 10% CO 2 -air has shown the stability and regenerability of the cell performance. Additional experiments show that periodic water content fluctuation causes the cell performance fluctuation. Electrochemical performance measurements and post-test microstructure analysis indicate that the segregation of SrO on the LSM surface and formation of La 2 Zr 2 O 7 at the LSM/YSZ interface degraded the electrochemical performance by increasing the polarization resistance. The cell performance degradation can be partially recovered by exposure to dry air. Thermogravimetric analysis (TGA) has been conducted for the LSM as well as its constituent oxides to elucidate degradation mechanisms. Mechanisms related to the performance degradation and regenerability have been proposed.

Thermodynamic Investigation of La0.8Sr0.2MnO3±δ Cathode, including the Prediction of Defect Chemistry, Electrical Conductivity and Thermo-Mechanical Properties

“…# , and composition have been conducted based on the CALPHAD approach and compared with the experimental observations. It is predicted that MnO x is the oxide forms on the LSM surface in both biased/unbiased conditions in accordance with current experimental results and in agreement with previous observations [70,71,74,76]. Additionally, in agreement with the previous experimental data in literature [70,74], formation of SrCO 3 on the surface of LSM-20 in 10 vol.% CO 2air is also confirmed and it is concluded that SrCO 3 is more favorable to form at the lower temperatures, the higher concentration of Sr, and the lower ! "…”

“…However, various phases were identified ex-situ in different works. Overall, the identified secondary phases have been claimed as either SrO [69,70,74,75] or Mn oxides [71,76]. The current…”

“…In condition C#3, SrO is stable at high temperatures, which will react with CO 2 to form SrCO 3 at low temperatures (T<1323 K). Therefore, secondary phase observed in the La-Sr-Mn-O-C system cannot be SrO [69,70,74] .…”

“…Reactions between cell component constituents (cationic species) and atmospheric gases, such as CO 2 or H 2 O [64, 72, 73], can be considered as one of the degradation acceleration factors. In addition to the impurity species in the air, any gas leakage from the anode may increase the amount of CO 2 or H 2 O in cathode side by more than 30% [8] and leads to the formation of secondary phases such as SrO/SrCO 3 [69,70,74,75], MnO x oxides [71,76], La 2 Zr 2 O 7 (LZO) and SrZrO 3 (SZO) [69,71,74], La 2 O 3 and La(OH) 3 [68,71]. It is also suggested that secondary phases impede permeation of oxygen ions (O 2-) into the cell and could block active oxygen reduction reaction (ORR) sites.…”

“…According to the sets of condition nucleation of various secondary phases was reported [8,[67][68][69][70][71]80]. Formation of secondary phases, which were predicted to be SrO by firstprinciples calculations, was predicted for LSM-20/8YSZ/LSM-20 system [70,74,75,81].…”