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Article

Optimized Solid-State Synthesis of Sr2Fe1.5Mo0.5O6−δ Perovskite: Implications for Efficient Synthesis of Mo-Containing SOFC Electrodes

by
Hui Dong
1,
Meiyu Wang
1,
Yuke Liu
1 and
Zongying Han
2,*
1
College of Chemical and Biological Engineering, Shandong University of Science and Technology, Qingdao 266590, China
2
College of Energy Storage Technology, Shandong University of Science and Technology, Qingdao 266590, China
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(11), 1533; https://doi.org/10.3390/cryst12111533
Submission received: 6 October 2022 / Revised: 22 October 2022 / Accepted: 25 October 2022 / Published: 27 October 2022
(This article belongs to the Special Issue Advances in Solid Oxide Fuel Cells 2022)
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Abstract

:
Sr2Fe1.5Mo0.5O6−δ (SFMO) perovskite has been considered as a promising anode candidate for solid oxide fuel cells. However, the significant inconsistency in the conductivity properties of SFMO perovskite has been reported in the literature through various synthesis procedures, highlighting the necessity of a standard and unified synthesis process. In this work, we propose an optimized solid-state synthesis process of SFMO perovskite based on the thermal properties of the precursors. Our TG analysis indicates that the evaporation of MoO3 during sintering over 752 °C may affect the synthesis of the expected SFMO perovskite. The presence of Fe2O3 has a trap effect on MoO3, based on the TG analysis of the binary mixture. A cubically structured SFMO perovskite without a secondary phase is obtained from the as-proposed stepwise sintering program while an impurity phase of SrMoO4 is observed when adopting a direct sintering program. The as-synthesized SFMO perovskite exhibits high stability in a reducing atmosphere, which is attributed to the self-adjustment of the overall valence states of molybdenum ions and iron ions. Many pure cubically structured perovskites have been successfully synthesized using the as-proposed solid-state synthesis process, suggesting its universality for the synthesis of other Mo-containing SOFC perovskite electrodes.

1. Introduction

Solid oxide fuel cells (SOFCs) have been considered one of the most efficient energy conversion technologies, attracting a remarkable amount of attention in the last several decades [1,2,3]. Benefiting from their high catalytic activity toward fuel conversion [4,5], Ni-based composite anodes have been widely adopted worldwide. However, these state-of-the-art Ni-based anodes regularly suffer from severe coking problems when directly running in carbon-containing fuels [6]. One of the most effective strategies to improve the anti-coking behavior of SOFCs is the adoption of single-phase mixed ionic-electronic conductor (MIEC) perovskites. As one of the most promising MIEC-type anode candidates, Sr2Fe1.5Mo0.5O6−δ (SFMO) has been proven to be sufficiently electrical conductive, redox stable, sulfur tolerant, and coking-resistant [7,8,9,10,11,12], and it has been widely used in symmetrical SOFCs [13] and SOECs [14]. However, a previous study demonstrated that the electrical properties of SFMO exhibit significant inconsistencies between different studies, with the electrical conductivity values varying from 9 to 310 S cm−1 in hydrogen and from 10 to 550 S cm−1 in air [15]. One possible reason for this phenomenon is that the purity or crystallinity of the synthesized SFMO perovskite is distinct from the various fabrication processes.
Briefly, SFMO perovskite can be synthesized by the combustion method [16,17,18,19,20], the freeze-drying of an aqueous cation solution [21], and the solid-state synthetic method [22,23]. Although the combustion method is the most-widely used in the literature, its application to the large-scale production of SFMO perovskite has limited potential. As is well known, specific organic additives (such as poly(vinyl alcohol) (PVA) citric acid [7], glycine [20], ethylenediaminetetraacetic acid (EDTA) [24], etc.) are required to prompt the combustion process and multiple steps are usually necessitated, which leads to the high complexity of the combustion method. Another crucial reason is that the combustion method generally employs nitrates as precursors, which poses a potential risk since these nitrates are usually corrosive, flammable, and easily explosive, especially in large quantities. This drawback is also applicable to the method entailing the freeze-drying of an aqueous cation solution. The solid-state synthetic method is recognized as the simplest method for the synthesis of SFMO perovskite, which is more suitable to the large-scale production of industrial applications. Regardless of which method one takes, a high-temperature sintering process is the final step. However, the sintering programs adopted in previous studies are not in complete accordance, even when using the same method. For instance, in the studies of Chen’s group where the combustion method was adopted [8,16,17,18], a direct sintering process was implemented without setting any intermediate heat preservation steps before reaching the peak temperature of 1000 °C. In another study by Xia et al. [19] using the combustion method, a heat preservation step (600 °C for 2 h) was employed prior to reaching the peak temperature of 1000 °C. This minor difference in the preparation process led to a huge discrepancy in the total conductivity of SFMO perovskite: the former is higher than 310 S cm−1 in hydrogen [7], while the latter is less than 35 S cm−1 in hydrogen [19]. A significant difference was also observed with respect to the oxygen ionic conductivity [18,19]. To the best of our knowledge, these distinct results have not attracted much attention and the underlying reason is still unclear. However, this cannot be ignored since confusing results may be obtained when comparing different SFMO-related studies. Therefore, to avoid potential mistakes, a standard and unified synthesis process is required for the production of SFMO perovskite.
In this study, we propose an optimized solid-state synthesis process for SFMO perovskite based on the thermal properties of the precursors. Moreover, the phase structure, phase stability, and elemental chemical state are systematically investigated. Furthermore, the universality of the as-proposed solid-state synthesis process is verified by applying it to the synthesis of other perovskites. It is expected that the findings in this study can provide references for the efficient synthesis of Mo-containing SOFC perovskite electrodes.

2. Materials and Methods

The SFMO was synthesized by a simple solid-state reaction method. (NH4)6Mo7O24·4H2O, SrCO3 and Fe2O3 with analytical purity were used as metal precursors. Stoichiometrically corresponding precursors were mixed and ball-milled thoroughly for 4 h and then sintered at an elevated temperature in air. Based on the thermal analysis of the precursors, we proposed a stepwise sintering program, as shown in Figure 1: firstly, set the heat at 750 °C for 2 h; then, increase the temperature to 1000 °C for 2 h, and finally increase the temperature to 1100 °C for 5 h before natural cooling in air. For comparison, the direct sintering program was also used. To check the phase stability under reducing atmosphere, the as-synthesized SFMO perovskite was treated in H2 with flow rate of 50 mL/min at 800 °C for 2 h.
The thermal properties of the metal precursors were determined with NETZSCH STA 449F3 thermo-gravimetric (TG) analyzer using air as carrier gas. To evaluate the phase structure, the XRD patterns of SFMO powders were determined at room temperature using a Rigaku Miniflex 500 diffractometer with Cu-Ka radiation at a scanning step of 0.01°. The surface morphology of the as-synthesized and reduced SFMO powders was observed by an FEI Apreo scanning electron microscope (SEM). The chemical state analyses of SFMO powders were performed using a Thermo Fisher ESCALAB XI+ X-ray photoelectron spectroscopy (XPS), which has been widely used to analyze chemical states [25,26,27].

3. Results and Discussion

3.1. Thermal Analysis of Precursors

The thermal analytical results of the precursors for SFMO at 20–1200 °C are shown in Figure 2. As can be seen in the TG of the as-received ammonium molybdate (Figure 2a), there are three obvious mass-loss stages: (I) 20–345 °C with a mass loss of 18%, (II) 345–752 °C without an obvious mass loss, and (III) 752–1200 °C with a mass loss of 80%. In stage I, there are three mass-loss peaks at 129 °C, 231 °C, and 321 °C, which are due to the stepwise decomposition of ammonium molybdate in the sequence (NH4)6Mo7O24·4H2O → (NH4)8Mo10O34 → (NH4)2Mo4O13 → MoO3 [28]. Over the temperature range of 345–752 °C, there are no detectable changes in the TG curve, indicating that a thermally stable MoO3 phase has been formed. When the temperature is higher than 752 °C, the remaining mass is greatly reduced from 82% to 2%, which is attributed to the melting and evaporating behavior of MoO3 [29]. This indicates that it is of great importance to prevent the evaporation of MoO3 during the synthesis of SFMO.
A previous study reported that Fe2O3 can react with MoO3 at 710 °C to form Fe2Mo3O12 [30], suggesting that Fe2O3 can stabilize the MoO3 during high-temperature sintering. To verify our conjecture, the thermal analyses of the Fe2O3-MoO3 mixtures with different ratios were conducted as shown in Figure 2c,d. It can be seen that the final remaining mass of the Fe2O3-MoO3 mixture with Fe:Mo = 2:3 is determined to be 26.9%, which is higher than the theoretical value (23.2%) assuming the full evaporation of MoO3. Similarly, the final remaining mass of the Fe2O3-MoO3 mixture with Fe:Mo = 1:1 is determined to be 43.0%, which is also higher than the theoretical value (35.1%) assuming the full evaporation of MoO3. This suggests that the presence of Fe2O3 indeed has a trap effect on MoO3, which can also be seen in the temperature range of 752–1200 °C of the TG curves. However, the direct sintering process does not leave sufficient time for the complete fixation of MoO3. Therefore, we set the heat preservation at 750 °C for 2 h in our optimized sintering procedure. The TG curve in Figure 2b shows that SrCO3 starts to decompose at the temperature of 803 °C and reaches the maximum rate at a temperature of 1063 °C. For the sake of full decomposition before the final formation of SFMO, we also set the heat preservation at 1000 °C for 2 h in our optimized sintering procedure.

3.2. Phase Structure

Figure 3 shows the XRD patterns of the SFMO powders obtained by sintering at 1100 °C for 5 h using the direct sintering and stepwise sintering programs. All XRD patterns were fitted using the Rietveld refinement method with Rwp less than 10%. It can be seen from Figure 3a that an impurity phase with the strongest characteristic peak at 27.6° is observed for the XRD pattern of the SFMO powder from the direct sintering program, which is identified as SrMoO4. A similar finding was also reported in a previous study using the direct sintering process [31]. On the contrary, as can be seen in Figure 3b, a cubic perovskite structure without a secondary phase is obtained for the SFMO powder from the stepwise sintering program. This difference could be associated with the potential evaporation of Mo during the direct sintering process. This indicates that our proposed solid-state synthesis process with a stepwise sintering program is more effective than that of the direct sintering program. Although the cubically structured nature of SFMO perovskite has been widely recognized, the attribution of its crystal groups is still controversial [23]. Both refinements using cubic space groups Fm 3 ¯ m (225) and Pm 3 ¯ m (221) can adequately fit most of the characteristic peaks with a high intensity. However, the refinement using Pm 3 ¯ m did not accurately simulate some weak diffraction peaks, especially at 22.6°. Therefore, we suggest that SFMO belongs to the Fm 3 ¯ m space group. The calculated lattice parameter is 7.842 Å, which is very close to the experimental (7.831 Å–7.857 Å [7,8,32]) and theoretical (7.834 Å–7.939 Å [12,23,33,34]) values reported in the literature.

3.3. Stability in Reducing Atmosphere

One of the main requirements of an anode material for SOFC is that it should have high stability in both reducing and oxidizing atmospheres at elevated temperatures [35]. Thus, the phase transformation or partial decomposition of the crystal structure is considered to be detrimental for practical applications due to the possible volume change and decrease in electrical conductivity [36]. The stability of SFMO was studied by XRD after a treatment of the powders in pure H2 at 800 °C for 2 h. It can be seen in Figure 4a that the SFMO perovskite maintains a cubic structure and no detectable additional phases appear after the reduction at 800 °C, suggesting that the as-synthesized SFMO perovskite is stable in H2. A careful comparison manifests that the peaks in the XRD pattern shift to lower diffraction angles after the reduction at 800 °C, indicating the increase in the lattice parameter. The calculated lattice parameter from the Rietveld refinement is 7.856 Å, which is slightly larger than that of SFMO before reduction. A similar finding has also been reported in a previous study [7,37]. This is mainly attributed to the fact that the partial lattice oxygen in SFMO is reversibly released under the reduction. Nevertheless, the crystal structure remains stable without nanoparticle exsolution due to the valence readjustment of Fe/Mo in the lattice [38]. After re-oxidation, the peaks of SFMO shift back to the higher diffraction angles, as shown in Figure 4b, suggesting that the released lattice oxygen has been restored. To confirm the stability of SFMO perovskite under a hydrogen atmosphere, the surface microstructures of the SFMO perovskite powders were evaluated through SEM imaging. In Figure 5, it is evident that no detectable secondary phase can be observed in the particle surface, indicating that no partial decomposition occurred during the reduction. The grain size of the SFMO perovskite is about 1–3 μm, which is slightly larger than the grain sizes reported by Lv et al. (0.5–1 μm) [39] and He et al. (1–2 μm) [19].

3.4. Chemical State Analysis

An XPS analysis was carried out to reveal the valence evolution of the Fe/Mo elements in the SFMO perovskite before and after reduction. Both the Fe 2p and Mo 3d spectra exhibit two contributions, which result from the spin–orbit splitting [40]. Figure 6a shows the Fe 2p spectra of SFMO perovskite before and after reduction, in which three valence states of Fe are simultaneously present [41,42,43]. In the Fe 2p3/2 characteristic peak of the unreduced SFMO perovskite, the resolved peaks situated at binding energies of 709.6 eV, 710.6 eV, and 712.8 eV correspond to Fe2+, Fe3+, and Fe4+, respectively [44]. The corresponding proportion of Fe2+, Fe3+, and Fe4+ are determined to be 36.75%, 40.77%, and 22.48%, with an average valence state of 2.86. For the reduced SFMO perovskite, the proportion of iron ions in the various valence states has slightly changed due to the increase in the oxygen vacancy. It was found that the proportion of Fe2+ increases to 44.43%, while Fe3+ and Fe4+ decrease to 32.79% and 22.78%, leading to the decrease in the average valence state to 2.78. Figure 6b displays the Mo 3d spectra of SFMO perovskite before and after reduction, and the Mo5+ and Mo6+ valence states are used for fitting the Mo 3d peaks [20,39]. The binding energies of the Mo5+ and Mo6+ valence states in the Mo 3d5/2 characteristic peak are 231.6 eV and 232.0 eV, respectively [37]. The peak-fitting results show that the relative contents of Mo5+ and Mo6+ in unreduced SFMO perovskite account for 25.76% and 74.24%, respectively. After the reduction, the proportion of Mo5+ increases to 31.05% while that of Mo6+ decreases to 68.95%. As a consequence, the average valence state of the molybdenum ions changes from the initial 5.74 to 5.69 after reduction. The self-adjustment of the overall valence states of the molybdenum ions and iron ions should account for the stability of the SFMO perovskite under a reduction atmosphere and the slight cell expansion. Figure 6c shows the O1s spectra of SFMO perovskite before and after reduction, where two main peaks are clearly observed [44]. The peaks of the binding energies at about 528.6 eV correspond to the lattice oxygen (Olat), and the peaks at 530.7 eV are attributed to the surface oxygen species (Oads) [45]. For the as-synthesized SFMO perovskite, the relative contents of lattice oxygen and surface oxygen species are determined to be 39.95% and 60.05%, respectively. After reduction, the relative content of lattice oxygen in the SFMO perovskite declined to 30.72%, suggesting an increase in oxygen vacancy.

3.5. Application to other Mo-Containing Perovskites

To confirm the practicability of the as-proposed solid-state synthesis process, we applied it to the synthesis of other Mo-containing perovskites, including Sr2Fe1.3Ni0.2Mo0.5O6−δ, Sr2Fe1.3Ni0.1Co0.1Mo0.5O6−δ, Sr2Fe1.3Ni0.1Co0.05Cu0.05Mo0.5O6−δ, Sr2Fe1.3Ni0.05Co0.05Cu0.05Mn0.05Mo0.5O6−δ, and Sr2Fe1.3Ni0.05Co0.05Cu0.05Ag0.05Mo0.5O6−δ. Firstly, we estimated the tolerance factor of these perovskites according to the equation proposed by Goldschmidt [46,47], and all the corresponding values fall into the range of 0.978–0.988, indicating that all of these perovskites possess a stable cubic structure. This can be verified in the XRD patterns of the perovskites prepared by the as-proposed solid-state synthesis process, as shown in Figure 7. Clearly, all perovskites exhibit cubic structures, and no detectable additional phases appear, suggesting that the as-proposed solid-state synthesis process is universal for the synthesis of other Mo-containing SOFC perovskite electrodes.

4. Conclusions

In this study, we have proposed an optimized solid-state synthesis process of SFMO perovskite based on the thermal properties of the precursors. The TG analysis indicates that the evaporation of MoO3 during sintering over 752 °C may affect the synthesis of the expected SFMO perovskite. The presence of Fe2O3 has a trap effect on MoO3 based on the TG analysis of the binary mixture. A cubically structured SFMO perovskite without a secondary phase is obtained from the as-proposed stepwise sintering program, while an impurity phase of SrMoO4 is observed when adopting the direct sintering program. The as-synthesized SFMO perovskite exhibits high stability in a reducing atmosphere, which is attributed to the self-adjustment of the overall valence states of molybdenum ions and iron ions. Many pure, cubically structured perovskites have been successfully synthesized, suggesting that the as-proposed solid-state synthesis process is universal for the synthesis of other Mo-containing SOFC perovskite electrodes.

Author Contributions

Validation, H.D., M.W. and Y.L.; formal analysis, H.D. and M.W.; writing—original draft preparation, Z.H. and H.D.; writing—review and editing, H.D.; funding acquisition, Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Shandong Provincial Natural Science Foundation (Grant Nos. ZR2022MB060 and ZR2018BB069), China Postdoctoral Science Foundation (Grant No. 2018M632692) and Project of Shandong Province Higher Educational Young Innovative Talent Introduction and Cultivation Team (Hydrogen Energy Chemistry Innovation Team).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Two different sintering programs for solid-state synthesis of Sr2Fe1.5Mo0.5O6−δ perovskite.
Figure 1. Two different sintering programs for solid-state synthesis of Sr2Fe1.5Mo0.5O6−δ perovskite.
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Figure 2. TG−DTG curves of the precursors for SFMO at 20−1200 °C. (a) As−received (NH4)6Mo7O24·4H2O; (b) As-received SrCO3; (c) Fe2O3-MoO3 mixture with Fe:Mo = 2:3; (d) Fe2O3-MoO3 mixture with Fe:Mo = 1:1.
Figure 2. TG−DTG curves of the precursors for SFMO at 20−1200 °C. (a) As−received (NH4)6Mo7O24·4H2O; (b) As-received SrCO3; (c) Fe2O3-MoO3 mixture with Fe:Mo = 2:3; (d) Fe2O3-MoO3 mixture with Fe:Mo = 1:1.
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Figure 3. Rietveld refinements of room temperature XRD patterns of SFMO powders prepared from direct sintering program (a) and stepwise sintering program (b).
Figure 3. Rietveld refinements of room temperature XRD patterns of SFMO powders prepared from direct sintering program (a) and stepwise sintering program (b).
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Figure 4. Rietveld refinements of room-temperature XRD patterns of SFMO powders after reduction in pure H2 at 800 °C for 2 h (a) and re-oxidation in air at 800 °C for 2 h (b).
Figure 4. Rietveld refinements of room-temperature XRD patterns of SFMO powders after reduction in pure H2 at 800 °C for 2 h (a) and re-oxidation in air at 800 °C for 2 h (b).
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Figure 5. SEM images of surface microstructure of SFMO perovskite powders before (a) and after (b) reduction in pure H2 at 800 °C for 2 h.
Figure 5. SEM images of surface microstructure of SFMO perovskite powders before (a) and after (b) reduction in pure H2 at 800 °C for 2 h.
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Figure 6. Experimental and deconvoluted Fe 2p (a), Mo 3d (b) and O 1s (c) XPS spectra for SFMO before and after reduction in pure H2 at 800 °C for 2 h.
Figure 6. Experimental and deconvoluted Fe 2p (a), Mo 3d (b) and O 1s (c) XPS spectra for SFMO before and after reduction in pure H2 at 800 °C for 2 h.
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Figure 7. XRD patterns of several other Mo-containing perovskites synthesized by the as-proposed solid-state synthesis process.
Figure 7. XRD patterns of several other Mo-containing perovskites synthesized by the as-proposed solid-state synthesis process.
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Dong, H.; Wang, M.; Liu, Y.; Han, Z. Optimized Solid-State Synthesis of Sr2Fe1.5Mo0.5O6−δ Perovskite: Implications for Efficient Synthesis of Mo-Containing SOFC Electrodes. Crystals 2022, 12, 1533. https://doi.org/10.3390/cryst12111533

AMA Style

Dong H, Wang M, Liu Y, Han Z. Optimized Solid-State Synthesis of Sr2Fe1.5Mo0.5O6−δ Perovskite: Implications for Efficient Synthesis of Mo-Containing SOFC Electrodes. Crystals. 2022; 12(11):1533. https://doi.org/10.3390/cryst12111533

Chicago/Turabian Style

Dong, Hui, Meiyu Wang, Yuke Liu, and Zongying Han. 2022. "Optimized Solid-State Synthesis of Sr2Fe1.5Mo0.5O6−δ Perovskite: Implications for Efficient Synthesis of Mo-Containing SOFC Electrodes" Crystals 12, no. 11: 1533. https://doi.org/10.3390/cryst12111533

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