Page 24 - Read Online

Page 2 of 15 Wang et al. J Mater Inf 2023;3:3 https://dx.doi.org/10.20517/jmi.2022.45

INTRODUCTION
Mixed Ionic and Electronic Conducting (MIEC) (La, Sr)(Co, Fe)O (LSCF) is one of the most promising
3
Solid Oxide Fuel Cell (SOFC) cathodes due to its comprehensive electrochemical properties. It has been
widely applied at intermediate conditions in the energy conversion from fossil fuels to electricity to tackle
[1,2]
the penalty of CO emission and poor efficiency from traditional oxy-fuel combustion . However, SOFC
2
cathode materials suffer from thermodynamic instability in their long-term durability in the presence of
[3,4]
several impurities, of which SO is known to be a concern . Thus, developing a comprehensive
2
understanding of the sulfur poisoning of LSCF cathodes in the presence of SO impurity is necessary to
2
address the long-term degradation of the SOFC cathode systems at operating conditions.
It is reported that a trace amount of SO in air, as low as in part per million (ppm) level, can still cause a
2
performance drop and shorten the service life of LSCF cathodes due to the formation of the detrimental
[6]
[5]
secondary phases . Liu et al. first reported the sulfur poisoning of an LSCF cathode in the presence of 1
and 20 ppm of SO at 800 °C for 1,000 h. Results suggested that the LSCF has a low tolerance to sulfur
2
poisoning even with 1 ppm SO . Moreover, the degradation rate of LSCF was 8 times higher than that of
2
LSM at 20 ppm SO because of the formation of SrSO in LSCF. In the same year, Wang et al. reported the
[7]
4
2
formation of SrSO , La O SO and CoFe O in 100 ppm SO at 800 °C for 24 h, SrSO and CoFe O in 10 and
2
2
4
2
4
2
4
2
4,
4
1 ppm SO at 800 °C for 24 h, respectively. The cell was free from sulfur poisoning in 0.1 ppm SO for 24 h
2
2
and SO -free air for 100 h at 800 °C, respectively, indicating a thermodynamic threshold or a diffusion-
2
controlled mechanism for the sulfur poisoning process. Wang et al. reported the formation of SrSO in the
[8]
4
presence of 20mg/L SO at 800 °C for 500 h and concluded that LSCF exhibited poorer durability than LSM
2
against SO poisoning. Later, Wang et al. investigated the effect of Sr concentration on the sulfur
[9]
2
poisoning of LSCF cathodes, namely (La Sr )(Co Fe )O (LSCF-6428) and (La Sr )(Co Fe )O (LSCF-
0.2
3
0.6
0.8
0.8
3
0.2
0.4
0.2
0.8
8228), at 800 °C with 1 ppm SO . They concluded that the sulfur poisoning is much more severe for the less
2
active LSCF-8228 cathode, even though the volume of SrSO formed is much larger in LSCF-6428 due to it
4
having more available electrochemically active sites. Moreover, Wang et al. evaluated the SO poisoning
[10]
2
phenomena of LSCF-6428 cathodes with 0.1 ppm SO -air at 800 °C for 100 h by varying the flow rate (25,
2
50, and 90 mL min ). They concluded that SrSO formed on the surface of the LSCF-6428 first due to the
-1
4
fast absorption of the SO and then concentrated in the vicinity of the triple phase boundaries (TPBs) under
2
a polarization condition. Subsequently, Wang and Jiang reviewed the mechanism of SO poisoning on
[11]
2
both LSCF and LSM cathodes. They reported that the sulfur poisoning effect was more pronounced at lower
temperatures for both LSCF and LSM cathodes. More importantly, it was most pronounced on the surface
of the LSCF cathode and more evident on the interface of LSM/YSZ TPBs. Wang et al. investigated the
[12]
interrelation between sulfur poisoning and performance degradation of LSCF in the presence of 1 ppm SO
2
at 700 °C, 750 °C, and 800 °C, respectively. They found that the performance degradation was more severe
at either 700 °C or 750 °C, even though no significant sulfur was detected in either of these two samples.
Up to now, there are two principal methods to investigate the relationship between the stabilities of the
electrodes and the long-term degradation under SOFC applications, CALPHAD [13-17] and DFT (Density-
Functional Theory) [18,19] . Both can provide a potential approach to designing highly efficient and stable
[15]
electrode materials with excellent impurity tolerance for SOFCs. Darvish et al. reported the impacts of
temperatures, SO partial pressure (P(SO )), oxygen partial pressure (P(O )), and Sr content on the sulfur
2
2
2
poisoning phenomena of the LSCF cathode using the CALPHAD approach and electrochemical analysis.
They concluded that the formation of SrSO and spinel phase is thermodynamically in favor of higher P
4
(SO ), higher P(O ), lower temperatures, and higher Sr content. Later, Wang et al. reviewed the sulfur
[5]
2
2
poisoning of SOFCs, especially the LSCF cathode, using a very detailed literature review with a
corresponding chemical reaction sequence taking into account the parameters like P(SO ), temperature,
2