Abstract
The sulfur poisoning and performance recovery of the state-of-the-art SOFC cathodes (La0.80Sr0.20)0.95MnO3±δ (LSM) and (La0.60Sr0.40)0.95Co0.20Fe0.80O3–δ (LSCF), have been studied. Electrochemical impedance spectroscopy measurements of LSCF|GDC and LSM|YSZ half-cells are carried out in alternating atmospheres of air and SO2–air at 700°C for hundreds of hours. In the presence of SO2, the electrochemical performance of both the cells decays with ohmic and non-ohmic losses, owing to the absorption and chemical interaction of SO2 with the electrodes. In LSCF, the SrO segregated on the surface tends to absorb and react with SO2, forming SrSO4 followed by the exsolution of Co-Fe. As for LSM, SO2 is absorbed onto the Sr-rich areas of LSM, including the active reaction sites near the TPBs, leading to Sr exsolution and SrSO4 formation, leaving a Sr-deficient LSM. During the subsequent exposure to air, the performance of the sulfur-contaminated LSM is almost restored. The LSM particles, exposed to alternating atmospheres of air and SO2-air during the electrochemical tests, show a relatively clean surface with sparsely distributed SrSO4 particles, indicating a high stability against sulfur poisoning. It is suggested that the loosely adsorbed SO2 at the TPBs is readily swept away by the SO2-free air flow, recovering its ORR activity, whereas the Sr-deficient LSM due to Sr-exsolution stays modified, contributing to the incomplete performance restoration. Unlike the case of LSM, the performance of the sulfur-poisoned LSCF partially recovers during the subsequent exposure to air. Correspondingly, the LSCF particles have a modified morphology covered with numerous nanoparticles, mostly SrSO4, showing the irreversible aspect of the sulfur poisoning. The morphology modification is not concentrated near the electrode/electrolyte interface but over the entire cathode, indicating that the degree of recovery from sulfur poisoning is closely related to the presence of SrO and chemical activity of Sr in the electrodes at the solid-gas interface. These results also show the potential application of LSM for a sulfur sensor available in high-temperature harsh conditions.
Highlights
MATERIALS AND METHODSThe need for enhancement in the efficiency and durability of solid oxide fuel cells (SOFC) has stimulated extensive research
distribution of relaxation times (DRT) analysis has been frequently used, especially in SOFCs, for electrochemical impedance spectroscopy (EIS) data interpretation as it allows for deconvolution in a simple way (Wan et al, 2015), compared with the conventional non-linear least square fitting that requires an accurate equivalent circuit model
The rise of the low-frequency signal at 10−2–100 Hz (P4) is remarkable. This indicates the occurrence of physi- and chemisorption of SO2 on the LSCF surface, inhibiting the catalytic activity for oxygen reduction reaction (ORR), given that the low-frequency response (
Summary
MATERIALS AND METHODSThe need for enhancement in the efficiency and durability of solid oxide fuel cells (SOFC) has stimulated extensive research. The main causes of the cathode degradation include microstructure coarsening, phase separation, chemical interaction with electrolyte, delamination at electrode/electrolyte interfaces, and poisoning by airborne contaminants (Chen et al, 2016). The concentration of SO2 is very low in air (75 ppb for hourly primary standard) (National Ambient Air Quality Standards by US Environmental Protection Agency, 2010), the long-term exposure of the cathode to air flow can lead to contaminant deposition and reaction, degrading the cathodic activity for oxygen reduction reaction (ORR) (Singh and Birks, 1978; Mori et al, 2015). The airborne SO2 contaminates LSM electrodes, the effect may not be as severe as on LSCF (Liu et al, 2011; Daio et al, 2016). Higher concentrations of SO2 (0.1, 1, 10, and 100 ppm) showed an accelerated degradation in the SOFC performance (Wang et al, 2020)
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