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Wang et al. J Mater Inf 2023;3:3 https://dx.doi.org/10.20517/jmi.2022.45 Page 5 of 15

Similar to the treatment in our previous works [15,17] , the Gibbs free energy of the missing sulfur-containing
[26]
[28]
[27]
secondary phases La O S , La O SO and La (SO ) were added together with the SSUB5 database,
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4 3
2
2
2
2
2
which did not consider these phases in the database. With this, the final obtained LSCF + S database is able
to perform thermodynamic simulations of the sulfur poisoning phenomena in LSCF cathode materials.
More importantly, to make thermodynamic predictions, a fixed number of thermodynamic conditions are
needed to obtain reliable results, which is based on the following concerns. Firstly, the simulation
conditions should reflect the current experimental operating conditions as shown above, which include a
-5
temperature range from 800 up to 1,000 °C, a P(O ) range from Argon (10 atm) up to ambient atmosphere
2
(0.21 atm) and a fixed 10 ppm P(SO ) based on the concentration of the actual gas we received from Airgas.
2
Only in this way can the simulation results be comparable to our experimental observations. In addition,
simulations should also mimic the general sintering and operation conditions under the actual or
accelerating testing circumstances. Here, P(SO ), ranging from ppb level (~100 ppb in the atmosphere) to
2
ppm level (accelerated testing condition), with P(O ), from ambient air on the surface down to the reducing
2
conditions on TPBs due to the polarization effect is considered as the atmospheric conditions in the
[29]
simulations. And the temperature is from 600 to 800 °C as the general operating temperature for IT-SOFCs
(Intermediate-Solid Oxide Fuel Cells). Finally, the influence of the Sr content is considered in the current
simulation to investigate the relationship between the Sr content and sulfur poisoning phenomenon, of
which LSCF-8228, LSCF-7328 and LSCF-6428 are chosen. Finally, the sulfur poisoning results of the current
[17]
LSCF cathode were cross-compared with the most widely-used LSM20 (La Sr MnO ) cathode in our
0.2
0.8
3
previous literature to better understand the sulfur-tolerant cathode in the future.
RESULTS AND DISCUSSIONS
The CALPHAD simulation approach introduced above was applied to the LSCF-6428 cathode in 10 ppm
SO conditions, shown in Figure 1 as a function of temperature, where Figure 1A is for dry air and Figure 1
2
is for Argon. It should be noted that the simulation is based on 0.1 mole molecule of the LSCF-6428
cathode. More importantly, the dry air condition is for the surface of the LSCF cathode (the cathode-gas
interface), while the argon condition is for the TPBs (the cathode-electrolyte-gas interface) due to the
polarization effect . It can be seen that the sulfur-containing phase, SrSO , will be thermodynamically
[14]
4
stable over the temperature range (600-1,000 °C) in both dry air and argon atmospheric conditions.
However, at higher temperatures, the amount of SrSO formed drops more severely in the argon condition
4
than in dry air, indicating that the SrSO secondary phase is more thermodynamically favorable in higher
4
P(O ), which agrees well with the literature [5,12] and suggests that the formation of the sulfates necessitates
2
the oxidation of SO . Furthermore, the stability of the sulfate phase decreases with increasing temperature in
2
both dry air and Argon, suggesting that sulfate formation favors lower temperature conditions. In addition,
a halite phase, instead of the spinel phase, shows up as the stable Co-Fe oxide phase at high temperatures in
Argon [Figure 1B], but not in dry air, which is also intuitive due to the lower valance of the cations in
reducing conditions. It is also worth mentioning that in both conditions, the structure of the spinel phase
will change with increasing temperature from a Co-rich spinel (spinel #2) to a Fe-rich spinel (spinel #1) due
to the miscibility gap of the spinel phase. The stability of two spinel structures is predicted
thermodynamically but may be different from the actual experimental observations.

Results from the experimental characterization of LSCF-6428 samples heat-treated in both dry air and
Argon were then cross-compared with the simulation results for empirical validation. The XRD spectra of
the pre-sintered LSCF-6428 and the heat-treated samples are shown in Figure 2. It is apparent from the
figure that some secondary phases do form during sulfur poisoning heat treatment experiments, which
mainly correspond to the SrSO phase. Meanwhile, a few isolated peaks cannot be identified due to their
4
limited signals, which the simulations suggest could be related to the formation of other oxide phases. More