Codoping Strategy To Improve Stability and Permeability of Ba0.6Sr0

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A co-doping strategy to improve stability and permeability of Ba0.6Sr0.4FeO3-# based perovskite membranes Guanghu He, Zhengwen Cao, Wenyuan Liang, Yan Zhang, Xin Liu, Jürgen Caro, and Heqing Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02134 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016

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Industrial & Engineering Chemistry Research

Contribution to Industrial & Engineering Chemistry Research

A co-doping strategy to improve stability and permeability of Ba0.6Sr0.4FeO3-δδ based perovskite membranes

Guanghu He,a,† Zhengwen Cao,b,†,‡ Wenyuan Liang,a Yan Zhang,a Xin Liu,c Jürgen Caro,b,* and Heqing Jianga,*

a

Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess

Technology, Chinese Academy of Sciences, Qingdao 266101, China b

Institute of Physical Chemistry and Electrochemistry, Leibniz University of Hannover,

Callinstrasse 3-3A, Hannover 30167, Germany c

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China

* Corresponding authors: [email protected] and [email protected] † These authors contributed equally to this work ‡ Present address: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany 1 ACS Paragon Plus Environment


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Abstract To improve the stability and oxygen permeability of Ba0.6Sr0.4FeO3-δ (BSF) based perovskite membranes, an Mg and Zr co-doping strategy is proposed. The characterization by X-ray diffraction, Mössbauer spectroscopy and oxygen permeation measurements revealed that single-element Mg doping could improve the oxygen permeability of BSF-based membranes. However, in-situ XRD measurements indicated that the single-element Mg doping exhibits a poor thermal stability at low oxygen partial pressure. Single-element Zr doping could improve the structure stability of BSF-based perovskites but lead to a serious decrease of oxygen permeability. Compared with the BSF-based perovskites doped by either Mg or Zr alone, Mg and Zr co-doped perovskite Ba0.6Sr0.4Fe0.8Mg0.15Zr0.05O3-δ showed a better stability than single-element Mg-doping and exhibited a higher oxygen permeability than single-element Zr-doping. For the Mg and Zr co-doped BSF, the oxygen permeation flux reached 0.78 ml min-1cm-2 at 950 ˚C under an air/He oxygen partial pressure gradient.

Keywords: oxygen permeable membrane, perovskite oxide, Mg and Zr co-doping, permeability, stability

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Introduction In the past few decades, considerable research efforts have been undertaken for developing mixed ionic-electronic conducting perovskite (ABO3) membranes1 because of their potential applications such as separation of oxygen from air,2-4 partial oxidation of methane (POM) to syngas,5-7 selective oxidation of light hydrocarbons,8-9 and hydrogen production by water splitting.10-11 Among the perovskite membranes, Ba1-xSrxFe1-yCoyO3-δ (BSFC) is considered as one of the most promising materials because of its high concentration of mobile oxygen vacancies over a wide temperature range, with the δ-values of 2.19 ˂ (3-δ) ˂ 2.34 in the range 873 ≤ T/K ≤ 1173 and 10-3 ≤ pO2/bar ≤ 1.0.12 However, in addition to the large thermal expansion coefficient of cobaltites,12-13 cobalt-containing perovskite membranes suffer from spin-state transition as a function of temperature, easy evaporation and reduction of cobalt.14 These intrinsic properties usually lead to a structure transition and cause severe decrease in oxygen ion transport through the membranes.15-16 Therefore, the development of cobalt-free perovskite membranes with high oxygen permeability and structure stability is highly desirable. Owing to the less flexible reduction-oxidation behaviour of iron, Fe-based perovskite oxides have attracted extensive attention as possible alternatives to cobaltites. Considering the oxygen permeability, structure and stability of membranes, a number of Fe-based membranes have been developed by (i) the partial substitution at the A-site in perovskite structures with smaller cations, such as La, Ca, Y, Na and Pr,2, 17-19

and (ii) the partial substitution at the B-site in perovskite structures with larger



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cations, such as Ce, Zr, Ti, Mo, In and Nb.20-25 Recent works have been focused upon lower-valence cation doping on the B-site of Ba1-xSrxFeO3-δ (BSF) based perovskites, with the aim to generate more oxygen vacancies and to improve the oxygen permeability of BSF-based membranes. For example, Wang et al.26 reported a Ba0.5Sr0.5Fe0.8Zn0.2O3-δ material, which exhibited a relative high oxygen permeation flux of 0.35 ml min-1 cm-2 under an air/helium gradient at 950 ˚C. Later, Efimov et al.27 designed a perovskite Ba0.5Sr0.5Fe0.8Cu0.2O3-δ, which showed an increase of oxygen permeation flux after doping Cu2+ on the Fe-site. However, such single-element doping of the Fe site in BSF perovskites suffers from easy evaporation of Zn and the poor stability of Cu under low oxygen partial pressures, which often result in structure changes of the perovskite.28-29 Therefore, it is highly desirable to develop effective doping strategies for a practical application of BSF perovskites with high oxygen permeability and stability. Similar to Zn2+, Mg2+ shows a constant and low valence state of +2. Mg2+ doping of the B-site in BSF perovskites is expected to induce oxygen vacancies and to give a more stable redox behaviour of the material.30 In addition, the possible poor structure stability resulting from excessive oxygen vacancies of Mg2+-doped BSF at high temperature, like Pb0.4Sr0.6Ti0.9Mg0.1O3-δ31 and La0.7Sr0.3Mn0.6Mg0.4O3-δ,32 could be suppressed by introducing cations with higher oxidation states (e.g. Zr4+, Nb5+, Ti4+).21, 25, 33

Inspired by these reports, Mg2+and Zr4+ co-doping on the B-site of Ba1-xSrxFeO3-δ

oxide is probably an effective strategy to adjust oxygen vacancies and tolerance factor,


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respectively, and ultimately develop an oxygen permeable membrane with high oxygen permeability and excellent stability. In this work, we report the development of a new type of cobalt-free Ba0.6Sr0.4Fe1-x-yMgxZryO3-δ (BSF-MZ) perovskite oxide, where Mg2+ and Zr4+ are both on the Fe site. The effect of Mg2+ and Zr4+ co-doping on the phase structure and on the oxygen permeability of BSF was investigated in comparison with the single-element Mg2+ or Zr4+ doped BSF. It is found experimentally that the composition BSF-M15Z5 with 15% Mg2+ and 5% Zr4+ doping shows both high oxygen permeation flux and excellent structure stability at low oxygen partial pressures, and can be steadily operated for over 100 h at 950 ˚C.

Experimental Preparation of Powders and Membranes A series of Ba0.6Sr0.4Fe1-x-yMgxZryO3-δ (BSF-MZ) powders was prepared by a combined citric acid and ethylene-diamine-tetraacetic acid (EDTA) method.34 Stoichiometric amounts of nitrates were dissolved in de-ionized water, followed by the addition of citric acid and EDTA with molar ratio of citric acid : EDTA : total metal cations at 1.5 : 1 : 1. After, NH3·H2O was added to adjust the pH value of the solution to around 9. With stirring using a magnetic stirrer, the solution was heated to obtain a dark purple gel. Afterwards, the gel was burnt in a heating mantle to obtain a black mixed oxide powder, which was subsequently calcined for 10 h at 950 ˚C in air to form BSF-MZ precursor powders with final composition. The BSF-MZ powders were compressed by a uniaxial single-acting press at 10 MPa into disc-shaped bodies and 5 ACS Paragon Plus Environment


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sintered at 1300 ˚C for 10 h to form membrane disks with a thickness of 1.0 mm. The relative densities of the sintered pellets measured by the Archimedes method were around 94%, which is acceptable for oxygen permeation measurement.35-36 In this paper, the Ba0.6Sr0.4Fe1-x-yMgxZryO3-δ composition is referred to as BSF-MZ with x and y denoting the content of Mg2+ and Zr4+ in mol %. Characterization The as-synthesized BSF-MZ membranes were structurally analysed by X-ray diffraction (PHILIPS-PW1710) using Cu Kα radiation from 20˚ - 60˚ (2θ) with intervals of 0.02˚. In-situ XRD for BSF-M10, BSF-Z10 and BSF-M10Z10 powders were performed using a high temperature cell (Buehler HDK 2.4 with REP 2000), with steps of 100 ˚C at 6 ˚C/min heating rate under vacuum (pO2 = 10-6 bar) in the temperature range 30-1000 ˚C. The data acquisition time for each spectrum was 30 min. The microstructures of the BSF-MZ membrane were studied by scanning electron microscopy (SEM) (JSM-6700F, JEOL).

57

Fe Mössbauer spectra of BSF-M20 and

BSF-M10Z10 perovskites were recorded at room temperature on a conventional constant acceleration spectrometer which used a

57

Co/Rh source and was calibrated

using a standard α-Fe foil. The velocity range for these spectra measurement was -3 to 3 mm/s. All spectra were evaluated by the MössWinn fitting package. Oxygen Permeation Measurement Oxygen permeabilities of BSF-MZ membranes were measured using a homemade apparatus from 850 ˚C to 950 ˚C, which is described in a previous paper.37 The flow rate of synthetic air (20% O2 and 80% N2) on the feed side was 100 ml min-1,


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while the sweep side was introduced by a gaseous mixture of He (49 ml min-1) and Ne (1 ml min-1). It is noted that Ne mixed with He was used as an internal standard gas for determining the absolute flow rate of the effluent on the permeate side. The oxygen permeation fluxes were calculated from the composition analysis of effluent stream by an Agilent 6890 Gas Chromatograph (GC). The leakage of oxygen from feed side was less than 5 % of the total oxygen flux. Assuming this leakage is controlled by Knudsen diffusion mechanism. The oxygen permeation rate was then rectified as follows: jO = ൬xO2 2

21 F xN2 ൰ 79 A

where jO2 is the oxygen permeation flux, x is the molar fraction of nitrogen or oxygen, F is the volumetric flow rate of the effluent stream, and A is the effective area of membrane disk.

Results and discussion The effect of single-element Mg2+ doping The stoichiometric Ba0.6Sr0.4Fe1-xMgxO3-δ (x=0, 0.05, 0.1, 0.15, 0.2) perovskite oxides were synthesized via a sol-gel method, which delivers high purity and precise stoichiometry control. Figure 1 displays room-temperature XRD patterns of BSF-M membranes sintered at 1300 ˚C. The samples BSF, BSF-M5 and BSF-M10 exhibit pure cubic phase with the lattice constant 3.9531 Å, 3.9825 Å and 3.9838 Å, implying that the Mg2+ cation was incorporated successfully into the BSF perovskite structure. Although the main diffraction peaks were perfectly indexed to the cubic perovskite phase of BSF-based oxide, additional peaks with weak intensities are observed at



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approximately 30˚, 37˚ and 43˚ (2θ) for x=0.15 and 0.2. These additional peaks could be assigned to MgO (JPCD 87-0653), Fe3O4 (JPCD 89-0950) and Ba2Fe2O5 (JPCD 79-1486). Figure 2 and Table 1 show the 57Fe Mössbauer spectra of the BSF-M20 and the extracted parameters including isomer shift (IS), quadrupole splitting (QS) and relative peak area (A%) to ascertain the oxidation state(s) of the iron species. The spectra are made up of three doublets, with isomer shift falling in the range of -0.13 to 0.42 mm/s, indicating that iron species assume both tetravalent (Fe4+) and trivalent (Fe3+) states in BSF-M20 oxide.38-40 In particular, the hyperfine parameter difference between the two quadrupole doublets (IS=0.16 mm/s, QS=0.73 mm/s and IS=0.42 mm/s, QS=0.75 mm/s) could be attributed to two states of Fe3+ in BSF-M20. As reported for CaFe1.2Al10.8O1941 and LaFeAl11O19,42 the smaller isomer shift of 0.16 mm/s is much more related to Fe3+ ions in tetrahedral (Th) sites (IS=0.18-0.19 mm/s) indicating that approximately 50% of Fe3+ occupied tetrahedral sites in the sample BSF-M20 (Table 1). According to the Mössbauer analysis of a previous study,39 the tetrahedral Fe3+ ions in BSF-M20 sample may come from both the perovskite phase (i.e. BSF-M) and the additional phases (i.e. Fe3O4 and Ba2Fe2O5). Such a high percentage of Fe3+ in tetrahedral sites may cause a great coordination difference of Fe-O polyhedrons and lead to distortions of the BSF-M20 structure, and the formation of additional phases as observed in XRD pattern. Figure 3a shows the effect of Mg2+ doping on oxygen permeation flux of Ba0.6Sr0.4Fe1-xMgxO3-δ (x=0.05, 0.1, 0.15, 0.2) membranes between 850 to 950 ˚C. As expected, the oxygen permeation fluxes of all BSF-M membranes increase with 8 ACS Paragon Plus Environment

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temperature according to the Wagner equation43-44 describing oxygen transport in mixed ionic and electronic conductors. Extracted from Figure 3a, the oxygen permeation fluxes of BSF membranes with different Mg content at 950 ˚C are shown in Figure 3b. When the Mg2+-doping is less than 0.1, the oxygen flux increases with increasing Mg2+ content and reaches a maximum of 1.15 ml (STP) min-1 cm-2 for the BSF-M10 membrane. Doping Mg2+ into the BSF structure could produce oxygen vacancies for maintaining the electroneutrality of ABO3 perovskite and, therefore, result in a higher oxygen ionic conductivity. However, further increasing the Mg2+ content (x>0.1) may cause the perovskite structure distortion and form a new phase (Figure 1). As a result, with increasing Mg doping content in BSF the oxygen permeation flux for BSF-M membranes decreases from 1.15 ml (STP) min-1 cm-2 (x=0.10) to 0.78 ml (STP) min-1 cm-2 (x=0.15) and 0.47 ml (STP) min-1 cm-2 (x=0.20). These results suggest that Mg doping at or below 0.1 (x ≤ 0.1) in BSF oxide resulted in significant increase in oxygen permeation flux, which probably is attributed to the increase in oxygen vacancies. Whereas further raising Mg-doping amount leads to secondary phase formation in BSF oxides, delivering low oxygen permeation fluxes. The thermal stability of BSF-M10 was investigated by in-situ XRD measurements over the temperature range of 30-1000 ˚C with steps of 100 ˚C under vacuum (pO2 =10-6 bar), as displayed in Figure 4. In the heating mode (Figure 4a), the XRD pattern of BSF-M10 at 30 ˚C shows a highly crystalline perovskite phase. With increasing temperature, the diffraction peaks assigned to the perovskite phase were maintained up to 600 ˚C. However, few additional peaks at around 31˚ occurred at 700



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˚C, and became more visible with further increasing temperature, these weak peaks around 31˚ (2θ) should be attributed to Fe3O4 (JPCD 89-0951), suggesting that thermal decomposition may occur for BSF-M10 at high temperatures under low oxygen partial pressures. Moreover, these impurity peaks were maintained during the cooling procedure (Figure 4b), indicating the phase transition was not reversible. Obviously, single element Mg2+ doping of the perovskite BSF exhibits poor thermal stability at low oxygen partial pressure. Hence, single element Mg2+ doping in Ba0.6Sr0.4FeO3-δ significantly affects the perovskite

structure

and

oxygen

permeability.

The

pure

perovskite

phase

Ba0.6Sr0.4Fe1-xMgxO3-δ could be obtained, and the oxygen permeation flux increased with increasing Mg2+ content till x=0.1. However, single element Mg doped perovskite BSF-M10 exhibited poor thermal stability at low oxygen partial pressure.

The effect of single element Zr4+ doping To improve the stability of BSF perovskites, the doping effect of Zr4+ with the constant and high valence of +4 on the oxygen permeation flux and stability of BSF membrane was investigated. Figure 5 shows the XRD patterns of BSF-Z membranes with different Zr4+ content. It can be seen that all samples show the cubic perovskite structure. In particular, the peak at 2θ=31.5° shifts gradually towards smaller angles with increasing Zr4+ content from x=0 to 0.2, corresponding to the enlargement of cell parameters of cubic phase ( a= 3.9531 Å at x=0, 3.9697 Å at x=0.05, 3.9863 Å at x=0.10 and 4.0255 Å at x=0.20) since the ionic radius of Zr4+ (0.72 Å) is larger than


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that of Fe4+ (0.585 Å) and Fe3+ (0.645 Å).45-46 The analysis of the spectrum for the BSF-Z20 sample reveals the presence of two non-equivalent sites for both Fe4+ and Fe3+ cations (see Figure S1 and Table S1). The oxygen permeation fluxes of BSF-Z membranes with different Zr4+ content are presented in Figure 6. With raising Zr4+ doping, the oxygen flux slightly decreases to 0.58 ml min-1 cm-2 (x=0.05), 0.44 ml min-1 cm-2 (x=0.10), and 0.12 ml min-1 cm-2 (x=0.20) for the BSF-Z membranes. This trend of decline suggests that Zr4+ doping of BSF is not in favour of oxygen permeation, which is in agreement with previous studies by other researchers.47-48 This finding can be ascribed to a lower oxygen vacancy concentration in BSF-Z by Zr4+ doping because of the higher and constant +4 valence state of Zr compared with Fe. Another possible reason is that the much higher bonding energy of Zr-O (776.4 kJ/mol) in comparison with Fe-O (390.4 kJ/mol)49 increases the oxygen ion migration energy and leads to the diminishment of the oxygen permeation flux through Zr-doped BSF membranes. Figure S2 shows the oxygen permeation fluxes through BSF-Z membranes at different temperatures (850 – 950 ˚C). The temperature-related activation energy for oxygen permeation flux was over 113.7 kJ mol-1 for BSF-Z5, 116.1 kJ mol-1 for BSF-Z10 and 120.1 kJ mol-1 for BSF-Z20, compared with 70 kJ mol-1 for Ba0.5Sr0.5Fe0.2Co0.8O3-δ membrane.50 In-situ high temperature XRD characterization of

Ba0.6Sr0.4Fe0.9Zr0.1O3-δ

(BSF-Z10) oxide was performed to investigate its thermal stability. Figure 7 shows the XRD patterns of BSF-Z10 membrane under vacuum (pO2 = 10-6 bar) from 30 ˚C to 1000 ˚C. It is clearly seen from XRD patterns that the cubic perovskite was stable in



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the entire temperature range. With increasing temperature, the diffraction peaks shifted progressively to lower 2θ angle, a sign of lattice expansion with increasing temperature. As compared with Mg2+ doping, partial substitution of Fe in the BSF oxides by Zr4+ with a larger ionic radius and higher constant valence improved the structure stability of BSF-based perovskites under a low oxygen partial pressure in a wide temperature range. However, the oxygen permeation flux of BSF-Z membrane was observed to decline significantly with increasing Zr4+ doping to a value of only 0.20 ml min-1 cm-2 at 950 ˚C for BSF-Z20. Therefore, single element Zr4+ doping of BSF gives also not the optimal membrane material.

The effect of Mg2+ and Zr4+ co-doping Based on the above results and taking into account the trade-off between oxygen permeability and stability of BSFM and BSFZ, Mg and Zr co-doped BSF perovskites with compositions Ba0.6Sr0.4Fe0.8Mg0.2-xZrxO3-δ (BSF-MZ, x= 0.05, 0.1) were synthesised by EDTA-citric acid method. The X-ray diffraction patterns for BSF-M10Z10 and BSF-M15Z5 membranes can be indexed as a cubic perovskite without any impurities (Figure 8), with a lattice constant of 4.0127 Å and 4.0053 Å, respectively. From the SEM micrograph of BSF-M10Z10 membrane, no cracks were found and a dense structure was visible on the BSF-M10Z10 membrane surface. The mixed Fe3+/Fe4+ valence states of iron in BSF-M10Z10 was quantified by Mössbauer spectroscopy. Figure 9 shows the Mössbauer spectra obtained at room temperature, and the extracting parameters after spectra analysis are listed in Table 2. 12 ACS Paragon Plus Environment

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It can be seen that the isomer shift (IS) values (-0.13- 0.40 mm/s) of BSF-M10Z10 are close to BSF-M20 (Table 1), indicating that the iron ions in sample BSF-M10Z10 are in the +3 and +4 oxidation states.38-40 Nevertheless, the relative area of tetrahedral Fe3+ in the sample BSF-M10Z10 decreases significantly to 25% in comparison with 49% in sample BSF-M20, and more octahedral Fe3+ and Fe4+ cations were found, indicating less structure distortion in the Mg2+ and Zr4+ co-doped BSF sample. Accordingly, different from the only Mg2+ doped perovskite BSF-M20, no additional phase could be found in sample BSF-M10Z10 as it follows from the XRD results (Figure 8). In order to further confirm the stability of Mg and Zr co-doped BSF, in-situ high temperature XRD characterization of the BSF-M10Z10 was performed. Figure 10 shows the related patterns at variable temperature from 30 to 1000 ˚C. At low and high temperatures, the BSF-M10Z10 sample demonstrated the same diffraction pattern, which adopts a cubic perovskite structure and indicates the high thermal stability of BSF after Mg and Zr co-doping. Figure

11

displays

the

oxygen

permeation

fluxes

through

Ba0.6Sr0.4Fe0.8Zr0.1Mg0.1O3-δ (BSF-M10Z10) at different temperatures (850 – 950 ˚C), in comparison with a Ba0.6Sr0.4Fe0.8Mg0.15Zr0.05O3-δ (BSF-M15Z5) membrane, a Ba0.6Sr0.4Fe0.8Mg0.2O3-δ

(BSF-M20)

membrane,

and

a

Ba0.6Sr0.4Fe0.8Zr0.2O3-δ

(BSF-Z20) membrane. As expected, the oxygen permeation fluxes of all samples increase with increasing temperatures. Among these membranes, the single-element Mg doped sample BSF-M20 with poor stability exhibits an oxygen permeation flux of 0.36 ml min-1 cm-2 at 850 ˚C, and BSF-Z20 membrane shows the lowest oxygen flux



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at 850 ˚C. After doping Mg and Zr into BSF, the co-doped BSF-MZ membranes demonstrate higher oxygen permeation fluxes than BSF-M20 sample at all temperatures. For examples, the oxygen permeation fluxes of the BSF-M10Z10 and BSF-M15Z5 membranes are 0.61 and 0.78 ml min-1 cm-2 at 950 ˚C, respectively, which are obviously higher than the flux of the BSF-M20 membrane (0.47 ml min-1 cm-2). Moreover, the activation energies (Ea) of oxygen permeation through BSF-M10Z10 and BSF-M15Z5 membranes were 110.1 kJ mol-1 and 108.5 kJ mol-1 respectively, which were lower than that of the BSF-Z20 membrane (120.1 kJ mol-1). For a wide application from industrial perspective, the perovskite-based membranes are required to exhibit high oxygen permeability and also possess good stability at high temperature. Thus, oxygen permeation experiments as a function of time were carried out at 950 ˚C for BSF-Z20 and BSF-M15Z5 membranes, as depicted in Figure 12. During the first 65 h, a slow decrease in the permeation flux of BSF-M15Z5 membrane from an initial value of 0.75 ml min-1 cm-2 to about 0.70 ml min-1 cm-2 was detected, but then the oxygen flux remained stable until the permeation study was stopped at 100 h. On the other hand, the membrane made of Ba0.6Sr0.4Fe0.8Zr0.2O3-δ (BSF-Z20) had the same excellent stability but much lower oxygen permeation flux (less than 0.20 ml min-1 cm-2). Obviously, the Mg and Zr co-doped BSF perovskite membranes shown a better stability than single-element Mg-doping BSF perovskite and exhibited a higher oxygen permeability than single-element Zr-doping BSF perovskite. Furthermore, the CO2 and H2 tolerance of BSF-M10Z10 sample were also examined at 900 ˚C for 1 h (Figure S3). After CO2


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treatment, carbonate was observed in all samples. This observation may be attributed that the nature of Ba cation at the A site is inclined to react with the acidic CO2 in the atmosphere. After H2 treatment, both the phase composition and cubic structure of these three samples remained unchanged, demonstrating their better reduction tolerance in comparison with cobalt based perovskite.51

Conclusions A series of Ba0.6Sr0.4Fe0.8MgxZr0.2-xO3-δ (BSF-MZ, x=0, 0.05, 0.1, 0.2) perovskites through single-element and co-doping of the Ba0.6Sr0.4FeO3-δ parent oxide with Mg2+ and Zr4+ were synthesized and evaluated as oxygen permeable membranes. For single-element doping, the oxygen permeation fluxes of BSF-M membranes increased with the Mg2+ content, but some foreign phases as impurity were observed for Mg2+ contents ≥ 0.1. The structure stability of all the prepared Zr4+ doped BSF membranes was excellent, but their oxygen permeation fluxes were relatively low. The Mg2+ and Zr4+ co-doped materials with cubic perovskite structure combined the positive aspects of the single-element doping and exhibited good phase stability at high temperatures and good oxygen permeation performance. The co-doped BSF-M15Z5 membrane had an oxygen permeation flux of about 0.78 ml min-1 cm-2 at 950 ˚C which was about two times larger than that of BSF-M20 membrane, whilst possessing a good stability like the BSF-Z20 membrane. This co-doping strategy of B-site cations in BSF provides a general route to develop perovskite-type membranes



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with high oxygen permeability and phase stability, which holds promise for industrial applications such as air separation and catalytic membrane reactors.

Acknowledgements This work was financially supported by the Shandong Provincial Natural Science Foundation, China (BS2015NJ002, BS2014NJ003), the National Natural Science Foundation of China (21506237, 21501186), the project of Science and Technology Development Program in Shandong Province (2014GSF117031), Applied Basic Research Programs for Young Scientists of Qingdao (14-2-4-82-jch), Qingdao Institute of Bioenergy and Bioprocess Technology Director Innovation Foundation for Young Scientists (QIBEBT-DIFYS-201509). Dr. Konstantin Efimov is acknowledged for the in-situ XRD measurements.

Supporting Information. Fe Mössbauer spectra of BSF-Z20 powder at room temperature. The sub-spectra are fitted in color and are vertically shifted for clarify, (magenta ■) Octahedral Fe4+; (green □) Octahedral Fe3+; (blue ○) Tetrahedral Fe3+. Hyperfine parametersa and iron site assignment in BSF-Z20 sample at room temperature. Oxygen permeation fluxes (JO2) through Ba0.6Sr0.4Fe1-xZrxO3-δ (x= 0.05, 0.1, 0.2) membranes denoted as BSF-Z5, 10 and 20, respectively, as a function of temperature. X-ray diffraction patterns of BSF-M10 (a), BSF-Z10 (b) and BSF-M10Z10 (c) oxides in air, 5% CO2 and 10% H2 at 900 ˚C for 1 h. ★: BaCO3 (JPCD: 41-0373) 16 ACS Paragon Plus Environment

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References (1) Sunarso, J.; Baumann, S.; Serra, J. M.; Meulenberg, W. A.; Liu, S.; Lin, Y. S.; Diniz da Costa, J. C. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation. J. Membr. Sci. 2008, 320, 13. (2) Watenabe, K.; Yuasa, M.; Kida, T.; Teraoka, Y.; Yamazoe, N.; Shimanoe, K. High-performance oxygen-permeable membranes with an asymmetric structure using Ba0.95La0.05FeO3−δ perovskite-type oxide. Adv. Mater. 2010, 22, 2367. (3) Sunarso, J.; Liu, S.; Lin, Y. S.; Diniz Da Costa, J. C. High performance BaBiScCo hollow fibre membranes for oxygen transport. Energy Environ. Sci. 2011, 4, 2516. (4) Liao, Q.; Zheng, Q.; Xue, J.; Wei, Y. Y.; Wang, H. H. U-Shaped BaCo0.7Fe0.2Ta0.1O3−δ Hollow-fiber membranes with high permeation for oxygen separation. Ind. Eng. Chem. Res. 2012, 51, 15217. (5) Ritchie, J. T.; Richardson, J. T.; Luss, D. Ceramic membrane reactor for synthesis gas production. AIChE J. 2001, 47, 2092. (6) Chen, C. S.; Feng, S. J.; Ran, S.; Zhu, D. C.; Liu, W.; Bouwmeester, H. J. M. Conversion of methane to syngas by a membrane-based oxidation-reforming process. Angew. Chem., Int. Ed. 2003, 42, 5196. (7) Tan, X. Y.; Li, K. Oxidative coupling of methane in a perovskite hollow-fiber membrane reactor. Ind. Eng. Chem. Res. 2006, 45, 142. (8) Jiang, H. Q.; Cao, Z. W.; Schirrmeister, S.; Schiestel, T.; Caro, J. A coupling strategy to produce hydrogen and ethylene in a membrane reactor. Angew. Chem., Int. Ed. 2010, 49, 5656.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Dong, X. L.; Jin, W. Q.; Xu, N. P.; Li, K. Dense ceramic catalytic membranes and membrane reactors for energy and environmental applications. Chem. Commun. 2011, 47, 10886. (10) Jiang, H. Q.; Liang, F. Y.; Czuprat, O.; Efimov, K.; Feldhoff, A.; Schirrmeister, S.; Schiestel, T.; Wang, H. H.; Caro, J. Hydrogen production by water dissociation in surface-modified BaCoxFeyZr1-x-yO3-δ hollow-fiber membrane reactor with improved oxygen permeation. Chem. Eur. J. 2010, 16, 7898. (11) Franca, R. V.; Thursfield, A.; Metcalfe, I. S. La0.6Sr0.4Co0.2Fe0.8O3-δ microtubular membranes for hydrogen production from water splitting. J. Membr. Sci. 2012, 389, 173. (12) McIntosh, S.; Vente, J. F.; Haije, W. G.; Blank, D. H. A.; Bouwmeester, H. J. M. Oxygen stoichiometry and chemical expansion of Ba0.5Sr0.5Co0.8Fe0.2O3-δ- measured by in situ neutron diffraction. Chem. Mater. 2006, 18, 2187. (13) Vente, J. F.; Haije, W. G.; Rak, Z. S. Performance of functional perovskite membranes for oxygen production. J. Membr. Sci. 2006, 276, 178. (14) Bouwmeester, H. J. M. Dense ceramic membranes for methane conversion. Catal. Today. 2003, 82, 141. (15) Švarcová, S.; Wiik, K.; Tolchard, J.; Bouwmeester, H. J. M.; Grande, T. Structural instability of cubic perovskite BaxSr1-xCo1-yFeyO3-δ. Solid State Ionics 2008, 178, 1787. (16) Arnold, M.; Gesing, T. M.; Martynczuk, J.; Feldhoff, A. Correlation of the formation and the decomposition process of the BSCF perovskite at intermediate temperatures. Chem. Mater. 2008, 20, 5851.


Page 18 of 39

Page 19 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Kharton, V. V.; Waerenborgh, J. C.; Viskup, A. P.; Yakovlev, S. O.; Patrakeev, M. V.; Gaczyński, P.; Marozau, I. P.; Yaremchenko, A. A.; Shaula, A. L.; Samakhval, V. V. Mixed conductivity and mössbauer spectra of (La0.5Sr0.5)1-xFe1-yAlyO3-δ (x=0-0.05, y=0-0.30). J. Solid State Chem. 2006, 179, 1273. (18) Kida, T.; Takauchi, D.; Watanabe, K.; Yuasa, M.; Shimanoe, K.; Teraoka, Y.; Yamazoe, N. Oxygen permeation properties of partially A-site substituted BaFeO3-δ perovskites. J. Electrochem. Soc. 2009, 156, E187. (19) Kharton, V. V.; Kovalevsky, A. V.; Patrakeev, M. V.; Tsipis, E. V.; Viskup, A. P.; Kolotygin, V. A.; Yaremchenko, A. A.; Shaula, A. L.; Kiselev, E. A.; Waerenborgh, J. C. Oxygen nonstoichiometry, mixed conductivity, and mössbauer spectra of Ln0.5A0.5FeO3-δ (Ln = La-Sm, A = Sr, Ba): Effects of cation size. Chem. Mater. 2008, 20, 6457. (20) Zhu, X. F.; Wang, H. H.; Yang, W. S. Novel cobalt-free oxygen permeable membrane. Chem. Commun. 2004, 1130. DOI: 10.1039/B400857J (21) Watanabe, K.; Takauchi, D.; Yuasa, M.; Kida, T.; Shimanoe, K.; Teraoka, Y.; Yamazoe, N. Oxygen permeation properties of Co-free perovskite-type oxide membranes based on BaFe1-yZryO3-δ. J. Electrochem. Soc. 2009, 156, E81. (22) Takahashi, Y.; Kawahara, A.; Suzuki, T.; Hirano, M.; Shin, W. Perovskite membrane of La1−xSrxTi1−yFeyO3−δ for partial oxidation of methane to syngas. Solid State Ionics. 2010, 181, 300 (23) Dong, X. L.; Jin, W. Q.; Xu, N. P. Reduction-tolerant oxygen-permeable perovskite-type oxide Sr0.7Ba0.3Fe0.9Mo0.1O3−δ. Chem. Mater. 2010, 22, 3610.



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(24) Lu, Y.; Zhao, H.; Cheng, X.; Jia, Y.; Du, X.; Fang, M.; Du, Z.; Zheng, K.; Swierczek, K. Investigation of In-doped BaFeO3-δ perovskite-type oxygen permeable membranes. J. Mater. Chem. A. 2015, 3, 6202. (25) Yi, J.; Schroeder, M.; Martin, M. CO2-tolerant and cobalt-free SrFe0.8Nb0.2O3-δ perovskite membrane for oxygen separation. Chem. Mater. 2013, 25, 815. (26) Wang, H. H.; Tablet, C.; Feldhoff, A.; Caro, J. A cobalt-free oxygen-permeable membrane based on the perovskite-type oxide Ba0.5Sr0.5Zn0.2Fe0.8O3–δ. Adv. Mater. 2005, 17, 1785. (27) Efimov, K.; Halfer, T.; Kuhn, A.; Heitjans, P.; Caro, J.; Feldhoff, A. Novel cobalt-free oxygen-permeable perovskite-type membrane. Chem. Mater. 2010, 22, 1540. (28) Daza, Y. A.; Maiti, D.; Hare, B. J.; Bhethanabotla, V. R.; Kuhn, J. N. More Cu, more problems: Decreased CO2 conversion ability by Cu-doped La0.75Sr0.25FeO3 perovskite oxides. Surf. Sci. 2016, 648, 92. (29) Nagai, T.; Ito, W.; Sakon, T. Relationship between cation substitution and stability of perovskite structure in SrCoO3-δ-based mixed conductors. Solid State Ionics 2007, 177, 3433. (30) Li, M.; Pietrowski, M. J.; De Souza, R. A.; Zhang, H.; Reaney, I. M.; Cook, S. N.; Kilner, J. A.; Sinclair, D. C. A family of oxide ion conductors based on the ferroelectric perovskite Na0.5Bi0.5TiO3. Nat. Mater. 2014, 13, 31. (31) Li, X. T.; Huo, W. L.; Mak, C. L.; Sui, S.; Weng, W. J.; Han, G. R.; Shen, G.; Du, P. Y. Effect of Mg doping on ferroelectric PST thin films for high tunable devices. Mater. Chem. Phys. 2008, 108, 417.


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(32) Gan, L.; Zhong, Q.; Zhao, X. L.; Song, Y.; Bu, Y. F. Structural and electrochemical properties of B-site Mg-doped La0.7Sr0.3MnO3-δ perovskite cathodes for intermediate temperature solid oxide fuel cells. J. Alloys Compd. 2016, 655, 99. (33) Xu, N. S.; Zhao, H. L.; Shen, Y. N.; Chen, T.; Ding, W. Z.; Lu, X. G.; Li, F. S. Structure, electrical conductivity and oxygen permeability of Ba0.6Sr0.4Co1−xTixO3−δ ceramic membranes. Sep. Purif. Technol. 2012, 89, 16. (34) Liang, F. Y.; Partovi, K.; Jiang, H. Q.; Luo, H. X.; Caro, J. B-site La-doped BaFe0.95−xLaxZr0.05O3−δ perovskite-type membranes for oxygen separation. J. Mater. Chem. A. 2013, 1, 746. (35) Shao, Z.; Yang, W.; Cong, Y.; Dong, H.; Tong, J.; Xiong, G. Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3 oxygen membrane. J. Membr. Sci. 2000, 172, 177. (36) Fan, C. G.; Zuo, Y. B.; Li, J. T.; Lu, J. Q.; Chen, C. S.; Bae, D. S. Highly permeable La0.2Ba0.8Co0.8Fe0.2−xZrxO3−δ membranes for oxygen separation. Sep. Purif. Technol. 2007, 55, 35. (37) Luo, H. X.; Jiang, H. Q.; Efimov, K.; Liang, F. Y.; Wang, H. H.; Caro, J. CO2-tolerant oxygen-permeable Fe2O3-Ce0.9Gd0.1O2-δ dual phase membranes. Ind. Eng. Chem. Res. 2011, 50, 13508. (38) El Shinawi, H.; Marco, J. F.; Berry, F. J.; Greaves, C. LaSrCoFeO5, LaSrCoFeO5F and LaSrCoFeO5.5: New La-Sr-Co-Fe perovskites. J. Mater. Chem. 2010, 20, 3253. (39) Feldhoff, A.; Martynczuk, J.; Arnold, M.; Myndyk, M.; Bergmann, I.; Šepelák, V.; Gruner, W.; Vogt, U.; Hähnel, A.; Woltersdorf, J. Spin-state transition of iron in



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(Ba0.5Sr0.5) (Fe0.8Zn0.2)O3-δ perovskite. J. Solid State Chem. 2009, 182, 2961. (40) Berry, F. J.; Eadon, D.; Holloway, J.; Smart, L. E. Iron-doped zirconium silicate. Part 1. - The location of iron. J. Mater. Chem. 1996, 6, 221. (41) Glasser, F. P.; Woodhams, F. W. D.; Meads, R. E.; Parker, W. G. A mössbauer and ESR study of CaO6Al2O3CaO6Fe2O3 solid solutions. J. Solid State Chem. 1972, 5, 255. (42) Tronc, E.; Laville, F.; Gasperin, M.; Lejus, A. M.; Vivien, D. Structural and spectroscopic investigations of LaFeAl11O19 compounds. J. Solid State Chem. 1989, 81, 192. (43) Schroeder, M. Modeling of dense ceramic bilayer membranes. Phy. Chem. Chem. Phys. 2005, 7, 166. (44) Merkle, R.; Maier, J.; Bouwmeester, H. J. M. A linear free energy relationship for gas-solid interactions: Correlation between surface rate constant and diffusion coefficient of oxygen tracer exchange for electron-rich perovskites. Angew. Chem., Int. Ed. 2004, 43, 5069. (45) John, A. D. Properties of atoms, radicals, and bonds. In: Lang’s Chemistry Handbook Version 15th; McGraw-Hill, 1998. (46) Shannon, R. D.; Prewitt, C. T. Effective ionic radii in oxides and fluorides. Acta Cryst. 1969, B25, 925. (47) Chen, W.; Zuo, Y. B.; Chen, C. S.; Winnubst, A. J. A. Effect of Zr4+ doping on the oxygen stoichiometry and phase stability of SrCo0.8Fe0.2O3-δ oxygen separation membrane. Solid State Ionics 2010, 181, 971. (48) Yang, L.; Tan, L.; Gu, X. H.; Qi, H.; Jin, W. Q.; Zhang, L. X.; Xu, N. P. Effect of the


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size and amount of ZrO2 addition on properties of SrCo0.4Fe0.6O3−δ. AIChE J. 2003, 49, 2374. (49) Lide, D. R. CRC Handbook of Chemistry and Physics (internet version 2005), CRC Press: Boca Raton, 2005. (50) Zhang, Z.; Chen, Y.; Tade, M. O.; Hao, Y.; Liu, S.; Shao, Z. Tin-doped perovskite mixed conducting membrane for efficient air separation. J. Mater. Chem. A. 2014, 2, 9666. (51) Liang, W. Y.; Megarajan, S. K.; Liang, F. Y.; Zhang, Y.; He, G. H.; Liu, Z. Z.; Jiang, H. Q. Coupling of N2O decomposition with CO2 reforming of CH4 in novel cobalt-free BaFe0.9Zr0.05Al0.05O3−δ oxygen transport membrane reactor. Chem.Eng. J. 2015, DOI:10.1016/j.cej.2015.10.067



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Figure Captions Figure 1. XRD patterns of pervoskite Ba0.6Sr0.4Fe1-xMgxO3-δ (x= 0, 0.05, 0.1, 0.15, 0.2) denoted as BSF, BSF-M5, 10, 15 and 20, respectively, disc membranes sintered at 1300 ˚C in air. Figure 2. Fe Mössbauer spectra of BSF-M20 powder at room temperature. The sub-spectra are fitted in color and are vertically shifted for clarify, (magenta ■) Octahedral Fe4+; (green □) Octahedral Fe3+; (blue ○) Tetrahedral Fe3+. Figure 3. Oxygen permeation fluxes (JO2) through Ba0.6Sr0.4Fe1-xMgxO3-δ (x= 0.05, 0.1, 0.15, 0.2) membranes denoted as BSF-M5, 10, 15 and 20, respectively, as a function of (a) temperature, and (b) Mg2+ content at 950 ˚C. Figure 4. In situ powder x-ray diffraction patterns for BSF-M10, heated from 30 to 1000 ˚C (a) and cooled again (b) in steps of 100 ˚C with an equilibration time of 30 min at each temperature under vacuum (pO2 = 10-6 bar). Figure 5. XRD patterns of pervoskite Ba0.6Sr0.4Fe1-xZrxO3-δ membranes with different Zr4+ content (x=0, 0.05, 0.1, 0.2) sintered at 1300 ˚C in air. Figure 6. Oxygen permeation fluxes (JO2) through BSF-Z5, BSF-Z10 and BSF-Z20 membranes at 850 ˚C. Figure 7. In situ powder X-ray diffraction patterns for BSF-Z10, heated from 30 to 1000 ˚C in steps of 100 ˚C (heating rate 6 ˚C/min) with an equilibration time of 30 min under vacuum (pO2 = 10-6 bar). Figure 8. Typical XRD patterns of Ba0.6Sr0.4Fe0.8Mg0.2-xZrxO3-δ membranes (x=0.05, 0.1) sintered at 1300 ˚C and SEM micrograph of BSF-M10Z10 membrane.


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Figure 9. Fe Mössbauer spectra of BSF-M10Z10 powder at room temperature. The sub-spectra are fitted in color and are vertically shifted for clarify, (magenta ■) Octahedral Fe4+; (green □) Octahedral Fe3+; (blue ○) Tetrahedral Fe3+. Figure 10. In-situ powder X-ray diffraction patterns for BSF-M10Z10, heated from 30 to 1000 ˚C in steps of 100 ˚C (heating rate 6 ˚C/min) with an equilibration time of 30 min at each temperature under vacuum (pO2 = 10-6 bar). Figure 11. Influence of different B-site doping of (Ba0.6Sr0.4Fe0.8MgxZr0.2-xO3-δ, x=0, 0.05, 0.1, 0.2) membranes on oxygen permeation flux (JO2) at 850 - 950˚C. Feed side: 100 ml min-1 Air, Sweep side: 49 ml min-1 He and 1 ml min-1 Ne as an internal standard gas. Figure 12. Oxygen permeation flux (JO2) as a function of time for the membranes Ba0.6Sr0.4Fe0.8Zr0.05Mg0.15O3-δ and Ba0.6Sr0.4Fe0.8Zr0.2O3-δ at 950 ˚C, Feed side: 100 ml min-1 Air, Sweep side: 49 ml min-1 He and 1 ml min-1 Ne as an internal standard gas.

Table Captions Table 1. Hyperfine parametersa and iron site assignment in BSF-M20 sample at room temperature. Table 2. Hyperfine parametersa and iron site assignment in BSF-M10Z10 sample at room temperature.



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Figure 1. XRD patterns of pervoskite Ba0.6Sr0.4Fe1-xMgxO3-δ (x= 0, 0.05, 0.1, 0.15, 0.2) denoted as BSF, BSF-M5, 10, 15 and 20, respectively, disc membranes sintered at 1300 ˚C in air.


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Figure 2. Fe Mössbauer spectra of BSF-M20 powder at room temperature. The sub-spectra are fitted in color and are vertically shifted for clarify, (magenta ■) Octahedral Fe4+; (green □) Octahedral Fe3+; (blue ○) Tetrahedral Fe3+.



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Figure 3. Oxygen permeation fluxes (JO2) through Ba0.6Sr0.4Fe1-xMgxO3-δ (x= 0.05, 0.1, 0.15, 0.2) membranes denoted as BSF-M5, 10, 15 and 20, respectively, as a function of (a) temperature, and (b) Mg2+ content at 950 ˚C.


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Figure 4. In situ powder x-ray diffraction patterns for BSF-M10, heated from 30 to 1000 ˚C (a) and cooled again (b) in steps of 100 ˚C with an equilibration time of 30 min at each temperature under vacuum (pO2 = 10-6 bar).



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Figure 5. XRD patterns of pervoskite Ba0.6Sr0.4Fe1-xZrxO3-δ membranes with different Zr4+ content (x=0, 0.05, 0.1, 0.2) sintered at 1300 ˚C in air.


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Figure 6. Oxygen permeation fluxes (JO2) through BSF-Z5, BSF-Z10 and BSF-Z20 membranes at 850 ˚C.



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Figure 7. In situ powder X-ray diffraction patterns for BSF-Z10, heated from 30 to 1000 ˚C in steps of 100 ˚C (heating rate 6 ˚C/min) with an equilibration time of 30 min under vacuum (pO2 = 10-6 bar).


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Figure 8. Typical XRD patterns of Ba0.6Sr0.4Fe0.8Mg0.2-xZrxO3-δ membranes (x=0.05, 0.1) sintered at 1300 ˚C and SEM micrograph of BSF-M10Z10 membrane.



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Figure 9. Fe Mössbauer spectra of BSF-M10Z10 powder at room temperature. The sub-spectra are fitted in color and are vertically shifted for clarify, (magenta ■) Octahedral Fe4+; (green □) Octahedral Fe3+; (blue ○) Tetrahedral Fe3+.


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Figure 10. In-situ powder X-ray diffraction patterns for BSF-M10Z10, heated from 30 to 1000 ˚C in steps of 100 ˚C (heating rate 6 ˚C/min) with an equilibration time of 30 min at each temperature under vacuum (pO2 = 10-6 bar).



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Figure 11. Influence of different B-site doping of (Ba0.6Sr0.4Fe0.8MgxZr0.2-xO3-δ, x=0, 0.05, 0.1, 0.2) membranes on oxygen permeation flux (JO2) at 850 - 950 ˚C. Feed side: 100 ml min-1 Air, Sweep side: 49 ml min-1 He and 1 ml min-1 Ne as an internal standard gas.


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Figure 12. Oxygen permeation flux (JO2) as a function of time for the membranes Ba0.6Sr0.4Fe0.8Zr0.05Mg0.15O3-δ and Ba0.6Sr0.4Fe0.8Zr0.2O3-δ at 950 ˚C, Feed side: 100 ml min-1 Air, Sweep side: 49 ml min-1 He and 1 ml min-1 Ne as an internal standard gas.



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Table 1. Hyperfine parametersa and iron site assignment in BSF-M20 sample at room temperature.

Assignment IS (mm/s) QS (mm/s) A (%) a

Fe4+ Octahedral -0.13 0.74 28

Fe3+ Octahedral 0.42 0.75 23

Fe3+ Tetrahedral 0.16 0.73 49

IS, isomer shift relative to metallic iron; QS, quadrupole splitting; A, relative area of the

Mössbauer spectroscopy. Uncertainty is ± 5% of reported value.

Table 2. Hyperfine parametersa and iron site assignment in BSF-M10Z10 sample at room temperature.

Assignment IS (mm/s) QS (mm/s) A (%) a

Fe4+ Octahedral -0.13 0.74 32

Fe3+ Octahedral 0.40 0.94 43

Fe3+ Tetrahedral 0.14 0.88 25

IS, isomer shift relative to metallic iron; QS, quadrupole splitting; A, relative area of the

Mössbauer spectroscopy. Uncertainty is ± 5% of reported value.


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Codoping Strategy To Improve Stability and Permeability of Ba0.6Sr0

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