Tailoring the Stability of Ti-Doped Sr2Fe1.4TixMo0.6−xO6−δ Electrode Materials for Solid Oxide Fuel Cells

In this work, the stability of Sr2(FeMo)O6−δ-type perovskites was tailored by the substitution of Mo with Ti. Redox stable Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) perovskites were successfully obtained and evaluated as potential electrode materials for SOFCs. The crystal structure as a function of temperature, microstructure, redox stability, and thermal expansion properties in reducing and oxidizing atmospheres, oxygen content change, and transport properties in air and reducing conditions, as well as chemical stability and compatibility towards typical electrolytes have been systematically studied. All Sr2Fe1.4TixMo0.6−xO6−δ compounds exhibit a regular crystal structure with Pm-3m space group, showing excellent stability in oxidizing and reducing conditions. The increase of Ti-doping content in materials increases the thermal expansion coefficient (TEC), oxygen content change, and electrical conductivity in air, while it decreases the conductivity in reducing condition. All three materials are stable and compatible with studied electrolytes. Interestingly, redox stable Sr2Fe1.4Ti0.1Mo0.5O6−δ, possessing 1 μm grain size, low TEC (15.3 × 10−6 K−1), large oxygen content change of 0.72 mol·mol−1 between 30 and 900 °C, satisfactory conductivity of 4.1–7.3 S·cm−1 in 5% H2 at 600–800 °C, and good transport coefficients D and k, could be considered as a potential anode material for SOFCs, and are thus of great interest for further studies.


Introduction
Solid oxide fuel cells (SOFCs) with the advantage of high efficiency and fuel flexibility are among the most promising devices for the generation of electrical energy and heat from renewable and traditional energy sources, considerably reducing the emission of CO 2 and other harmful gases (NO x , SO x , CO) [1][2][3][4][5] However, the high operational temperature (above 800 • C) of SOFCs leads to a high operational cost, limiting the choice of materials and delaying commercial applications of SOFCs. For practical use, SOFCs need to operate at much lower temperatures than the current range (≤800 • C) and still be able to generate a high-power production. Therefore, new anode [6][7][8] and cathode materials [9,10] with high stability and electrocatalytic activity are required to maintain a reasonable power output at temperature below 800 • C [11][12][13][14].
One group of the most interesting cathode and anode material candidates for SOFCs is the Sr 2 (FeMo)O 6 -type perovskite with Fe-and Mo-cations at B-site [15][16][17][18][19][20][21]. The B-site rock perovskites were evaluated in terms of their application as new anode material candidates for [47]. Sr 2 ScTi 1−x Mo x O 6 (x = 0.1 and 0.5) double perovskites were studied as both cathode and anode materials for symmetrical solid oxide fuel cells, exhibiting a good power output of 218 mW cm −2 at 800 • C in humidified CH 4 [48]. Ti-containing Sr 2 TiNi 0.5 Mo 0.5 O 6−δ perovskite was also proposed and systematically evaluated as a new anode material for SOFCs, generating a power density of 335 mW cm −2 at 800 • C in humidified H 2 [49].
It has been reported that the electrocatalytic activity In the series of Sr 2 Fe 2−x Mo x O 6−δ double perovskites enhances with the increase of Mo-doping content, which contributes to the improved performance of SOFCs cells in different fuels (H 2 and methanol) [32], and SOFC with Sr 2 Fe 1.4 Mo 0.6 O 6−δ anode material shows much better electrochemical performance than the SOFC cell with redox stable Sr 2 Fe 1.5 Mo 0.5 O 6−δ electrode [32]. Sr 2 Fe 1.4 Mo 0.6 O 6−δ double perovskite possess very high electrical conductivity with 330 S cm −1 at 500 • C, nonetheless unfortunately it decomposes in air above 500 • C [50]. Interestingly, the Tidoping at Fe-site in Sr 2 Fe 1.4−x Ti x Mo 0.6 O 6−δ double perovskites significantly improves the structural stability of materials, which were systematically investigated as new anode material candidates for SOFCs [50]. Sr 2 Fe 1.3 Ti 0.1 Mo 0.6 O 6−δ anode-based SOFC cell delivers a promising power output with >0.64 W cm 2 in wet hydrogen at 900 • C [50]. However, such a material is still not stable in air at a high temperature range. Therefore, in this work, the stability of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) perovskites was tailored by the substitution of Mo with Ti at B-site. Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ materials with excellent stability in both reducing and oxidizing atmospheres have been obtained. The crystal structure as a function of temperature, microstructure, redox stability, and thermal expansion properties in reducing and oxidizing atmospheres, oxygen content change and transport properties in air and reducing condition, as well as the chemical stability and compatibility towards typical solid electrolytes have been evaluated for the studied materials in terms of their application as electrode material candidates for SOFCs.

Materials and Methods
The synthesis of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) materials was conducted using the high temperature method (solid state reaction), with the calculated amounts of SrCO 3 , Fe 2 O 3 , TiO 2 , and MoO 3 (all compounds with ≥ 99.9% purity) chemicals. All required chemicals after milling in the high efficiency planetary ball mill (Spex Sample-Prep 8000 M, Spex Sampleprep, Metuchen, UK) were pressed into pellets and fired in air for 10 h at 1200 • C. The crystal structure properties of all obtained materials were investigated by the XRD measurements within 10-110 deg. 2 Theta range using the Panalytical Empyrean diffractometer (CuKα radiation, Malvern, UK).
The chemical stability of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ compounds was studied by the reduction of oxides in 5 vol.% H 2 in argon for 10 h at 1200 • C. The XRD data refinement was applied applying the Rietveld method using GSAS/EXPGUI software [51,52]. Scanning electron microscopy (SEM) studies of reduced powders were conducted using FEI Nova NanoSEM 200 apparatus. The high temperature XRD studies were performed for the in-situ oxidation in air of reduced Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) samples using Panalytical Empyrean apparatus with Anton Paar HTK 1200N oven-chamber. The in-situ oxidation of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ sinters was also investigated with the thermal expansion measurements in air from room temperature to 900 • C using the Linseis L75 Platinum Series dilatometer (Selb, Germany).
Thermogravimetric (TG) studies were carried out on the TA Instruments Q5000IR apparatus (New Castle, DE, USA) and STA PT1600 TG with differential scanning calorimetry (DSC) studies from 30 to 900 • C in different conditions (in air and in 5 vol.% H 2 /Argon) with the rate of 2 • ·min −1 . The buoyancy effect was also taken into account. The electrical conductivity (σ) of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ samples was recorded to 900 • C in air and 5 vol.% H 2 in argon by a four-probe DC technique, on the dense cuboid shape sinters. The porosity effect of the studied sinters was also considered [53]. The oxygen diffusion coefficient D and surface exchange constant k of Sr 2 Fe 1.4 Ti 0.1 Mo 0.5 O 6−δ compound were studied using the mass relaxation technique in TA Instruments Q5000 IR on a very thin-sheet shape sample [54,55]. The mass relaxation measurements were performed with a very fast oxygen partial pressure change between 0.21 atm and 0.01 atm. The determination of coefficients D and k was conducted in custom-made Matlab code, based on Crank's mathematical solutions [56]. The chemical stability and compatibility studies of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) materials towards typical electrolytes, such as CGO20-Ce 0.8 Gd 0.2 O 1.9 and LSGM-La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3−d solid electrolytes, were investigated by examining the XRD data gathered for the respective oxide and solid electrolyte mixtures (with a 50:50 wt.%), fired in air for 10 h at 1200 • C.

Crystal Structure and Microstructure
The obtained XRD results with the Rietveld refinement in Figure 1 and Table 1 show that as-synthesized Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) samples exhibit simple perovskite crystal structure, belonging to the cubic Pm-3m space-group. However, the Sr 2 Fe 1.4 Ti 0.1 Mo 0.5 O 6−δ material synthesized in air possesses about 3.6% secondary phase (SrMoO 4 ), which can be successfully removed by annealing the compound in reducing condition (see Figure 2a). As evidenced, the substitution of molybdenum by titanium in Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ perovskites does not change the crystal symmetry but leads to a decrease of the unit cell parameter a. The doping of Ti 4+ (with smaller oxidation state) at Mo 6+ site contributes to the increase of Fe 4+ (reducing the amount of Fe 3+ ) and the content of oxygen vacancies in materials. The larger difference in ionic radius between Fe 3+ (r Fe3+ = 0.645 Å) and Fe 4+ (r Fe4+ = 0.585 Å) causes the decrease of the unit cell parameter, despite Ti 4+ (r Ti4+ = 0.605 Å) presenting a slightly bigger ionic radius than Mo 6+ (r Mo6+ = 0.59 Å). The geometric tolerance factor t g of all studied materials was calculated using the following  [39] oxides, also show a Pm-3m simple perovskite structure. Meanwhile, the Ti-doping at Fe-site in Sr 2 Fe 1.4−x Ti x Mo 0.6 O 6−δ (x= 0 and 0.1) leads to a double perovskite structure with Fm-3m space group [50]. geometric tolerance factor was calculated to be tg = 1.00 for all measured Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) samples, which indicates the presence of a regular crystal structure in the studied materials. Similar compositions, such as Sr2Fe1.5Mo0.5O6−δ and SrFe0.5Mn0.25Mo0.25O3−δ [30], Sr2TiFe0.5Mo0.5O6−δ [57], and SrFe0.45Co0.45Mo0.1O3−δ [39] oxides, also show a Pm-3m simple perovskite structure. Meanwhile, the Ti-doping at Fe-site in Sr2Fe1.4−xTixMo0.6O6−δ (x= 0 and 0.1) leads to a double perovskite structure with Fm-3m space group [50].  Table 1). Interestingly, among all investigated Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ materials, Sr 2 Fe 1.4 Ti 0.1 Mo 0.5 O 6−δ oxide presents the smallest relative volume change ∆V = 0.52% between the reduced and oxidized samples. This indicates the possible application of such a compound in oxidizing and reducing conditions [54]. For comparison, the relative volume changes between the oxidized and reduced Sr 2 Fe 1.5 Mo 0.5 O 6−δ and Sr 2 Fe 0.9 Mg 0.4 Mo 0.7 O 6−δ oxides are larger and reach 1.18% [30] and 0.55% [34], respectively. The increase of Ti-doping in Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.2 and 0.3) leads to a significantly large volume change ∆V, which can contribute to larger thermal expansion coefficients (TEC) in reducing and oxidizing atmospheres.   Figure 3. No substantial differences were observed in the microstructure for all studied materials, and the grain size is approximately 1 µm. Interestingly, the Ti-doping at Fe-site in Sr 2 Fe 1.4−x Ti x Mo 0.6 O 6−δ materials (x = 0-0.2) introduces a totally different microstructure with well-sintered aggregates (primary grains/crystallites are not visible) reported in the work [50]. SEM microphotographs of reduced Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) powders are shown in Figure 3. No substantial differences were observed in the microstructure for all studied materials, and the grain size is approximately 1 μm. Interestingly, the Tidoping at Fe-site in Sr2Fe1.4−xTixMo0.6O6−δ materials (x = 0-0.2) introduces a totally different microstructure with well-sintered aggregates (primary grains/crystallites are not visible) reported in the work [50].

Redox Stability and Thermal Expansion Properties
The in-situ oxidation of reduced Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) samples was examined by the high temperature XRD (HT-XRD) measurements in air. As can be observed in Figure 4, the oxidation of the reduced materials starts around 250 °C and finishes below 400 °C, accompanied by the significant change of unit cell parameter a. This is associated with the oxidation of reduced B-site cations (Fe, Ti, and Mo). The introduction of a higher content of titanium in Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.2 and 0.3) leads to a more considerable unit cell parameter change. The initial linear increase of unit cell parameter recoded below 250 °C is attributed to the thermal expansion of reduced samples. Above 400 °C, the oxidized materials present a linear increase of unit cell parameter (indicating a linear thermal expansion behavior) to 850 °C in air, and the HT-XRD data allow to calculate the thermal expansion coefficient (TEC). The increase of Ti content in Sr2Fe1.4TixMo0.6−xO6−δ leads to an increase of TEC values. Sr2Fe1.4Ti0.1Mo0.5O6−δ sample shows the lowest TEC among all studied materials, with TEC = 17.4 × 10 −6 K −1 . While Sr2Fe1.4Ti0.3Mo0.3O6-δ presents a rather high TEC value of 22.1 × 10 −6 K −1 , which can be a shortcoming for applications as electrode materials for SOFCs. No phase transition was observed in the HT-XRD studies for all samples, and all Sr2Fe1.4TixMo0.6−xO6−δ materials are stable to 850 °C in air ( Figure 5), possessing the same simple Pm-3m perovskite structure.

Redox Stability and Thermal Expansion Properties
The in-situ oxidation of reduced Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) samples was examined by the high temperature XRD (HT-XRD) measurements in air. As can be observed in Figure 4, the oxidation of the reduced materials starts around 250 • C and finishes below 400 • C, accompanied by the significant change of unit cell parameter a. This is associated with the oxidation of reduced B-site cations (Fe, Ti, and Mo). The introduction of a higher content of titanium in Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.2 and 0.3) leads to a more considerable unit cell parameter change. The initial linear increase of unit cell parameter recoded below 250 • C is attributed to the thermal expansion of reduced samples. Above 400 • C, the oxidized materials present a linear increase of unit cell parameter (indicating a linear thermal expansion behavior) to 850 • C in air, and the HT-XRD data allow to calculate the thermal expansion coefficient (TEC). The increase of Ti content in sample shows the lowest TEC among all studied materials, with TEC = 17.4 × 10 −6 K −1 . While Sr2Fe1.4Ti0.3Mo0.3O6-δ presents a rather high TEC value of 22.1 × 10 −6 K −1 , which can be a shortcoming for applications as electrode materials for SOFCs. No phase transition was observed in the HT-XRD studies for all samples, and all Sr2Fe1.4TixMo0.6−xO6−δ materials are stable to 850 °C in air ( Figure 5), possessing the same simple Pm-3m perovskite structure.   In addition, the thermal expansion behavior of oxidized Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) was studied in air ( Figure 6b). For all three samples, two slopes with an obvious bending at 400 °C can be observed. The slope in the range of 400-900 °C, corresponding to a higher TEC, is related with the chemical expansion, caused by the reduction of B-site cations and loss of lattice oxygen, which was observed in the TG studies ( Figure 7a). Sr2Fe1.4Ti0.3Mo0.3O6−δ possesses the largest TEC value (22.1 × 10 −6 K −1 ), while the TEC of Sr2Fe1.4Ti0.1Mo0.5O6−δ is the smallest (15.3 × 10 −6 K −1 ), which is close to the TEC values of typical solid electrolytes for SOFCs, such as: La0.9Sr0.1Ga0.8Mg0.2O3−δ (TEC = 12.17 × 10 −6 K −1 ), Zr0.85Y0.15O2−δ (TEC = 10.8 × 10 −6 K −1 ), and Ce0.8Gd0.2O2−δ (TEC = 12.5 × 10 −6 K −1 ) [26]. Moreover, the in-situ reduction of oxidized Sr2Fe1.4Ti0.1Mo0.5O6−δ sample was conducted in 5 vol. % H2/argon (Figure 6c), and a considerable nonlinear change due to the reduction of B-site cations occurs above 400 °C, which corresponds well with the significant mass reduction of Sr2Fe1.4Ti0.1Mo0.5O6−δ recorded in the TG measurement in Figure 7b. Meanwhile, a linear thermal expansion of reduced material was observed above 575 °C and it presents a relatively low TEC of 15.0 × 10 −6 K −1 . For the reduced Sr2Fe1.4Ti0.1Mo0.5O6−δ sample in 5 vol. % H2/argon, it exhibits a linear thermal expansion, with a small TEC value of 13.7 × 10 −6 K −1 , which favours the application of such a material in reducing conditions. Therefore, in the case of the application of redox stable Sr2Fe1.4Ti0.1Mo0.5O6−δ perovskite as electrode material for SOFCs, the TEC mismatch (causing delamination) problem is alleviated, possibly providing stable cell performance. As can be observed in Table 2, the substitution of Mo by Ti at B-site in Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) materials leads to an increase of TEC values. Hence, the Ti-doping in Sr2Fe1.4TixMo0.6-xO6-δ does not benefit the thermal expansion properties and the titanium content in those materials should be restricted to a rather small level (such as x = 0.1).

Oxygen Content and Transport Properties
The oxygen content change of Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) was recorded by TG measurements in air and in 5 vol.% H2 in argon (see Figure 7a,b), respectively. For the TG studies in air, a significant mass loss of all three samples occurs above 300 °C, As can be observed in Table 2

Oxygen Content and Transport Properties
The oxygen content change of Sr 2 Fe 1.4 Ti x Mo 0.6−x O 6−δ (x = 0.1, 0.2 and 0.3) was recorded by TG measurements in air and in 5 vol.% H 2 in argon (see Figure 7a,b), respectively. For the TG studies in air, a significant mass loss of all three samples occurs above 300 • C, indicating the generation of additional oxygen vacancies in the studied perovskites, according to the following reaction: In the supplementary DSC studies (Figure 7c), no endothermic or exothermic effects were observed for the investigated compounds, confirming no phase transition recorded, which was also documented in the in-situ HT-XRD studies (see Figure 4).
For Sr2Fe1.4Ti0.1Mo0.5O6-δ sample, the chemical diffusion coefficient D and surface exchange constant k were evaluated by the mass relaxation study (see Figure 8b

Chemical Stability and Compatibility with Electrolytes
The stability of cathode and anode oxides and their compatibility towards applied electrolytes at high temperature range are very critical for the electrode layer preparation (sintering) and a stable performance of cells. Therefore, the chemical stability of Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) oxides and their compatibility with typical solid electrolyte/buffer layer materials, such as Ce0.8Gd0.2O1.9-CGO20 and La0.8Sr0.2Ga0.8Mg0.2O3−d-LSGM, were evaluated by analyzing XRD data gathered for the respective materials and solid electrolyte powders (50:50 wt.%), which were fired in air for 10 h at 1200 °C. The Rietveld refined XRD patterns of Sr2Fe1.4TixMo0.6-xO6-δ with electrolyte (50:50 wt.%) powder mixtures are shown in Figures 9 and 10. The XRD patterns present no chemical reaction occurring between the studied compounds and solid electrolytes (CGO20 and LSGM), confirming the good stability and compatibility of Sr2Fe1.4TixMo0.6−xO6−δ (x = 0.1, 0.2 and 0.3) materials with used electrolytes.

Summary
In this work, the stability of Sr 2 (FeMo)O 6−δ -type perovskites was successfully tai- Redox stable Sr 2 Fe 1.4 Ti 0.1 Mo 0.5 O 6−δ seems to be the most interesting among studied materials, with large oxygen content change of 0.72 mol·mol −1 between 30 and 900 • C, satisfactory conductivity of 4.1-7.3 S·cm −1 in 5% H 2 at 600-800 • C, and good transport coefficients D and k, which indicate that such a material can be considered as a potential anode material for SOFCs and is of great interest for further studies.
Author Contributions: Conceptualization, methodology, investigation, validation, formal analysis, resources, data curation, writing-original draft preparation, funding acquisition, supervision, K.Z.; visualization, investigation, M.A.; writing-review and editing, M.C., K.Q. and P.C. All authors have read and agreed to the published version of the manuscript.