Novel cobalt-free tantalum-doped perovskite BaFe1-y TayO3-δwith high oxygen permeation☆

Qing Liao,Yanjie Wang,Yan Chen,Haihui Wang

School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510640,China

1.Introduction

Since Teraoka first reported that the La1-xSrxCo1-yFeyO3-δmembrane has the stable oxygen permeation at elevated temperatures with in finite selectivity[1],mixed ionic-electronic conductor membranes(MIECs)have attracted much attention because of their wide and potential applications in catalytic membrane reactors[2-12],solid oxide fuel cells(SOFCs)[13-15],and gas sensors[16,17].To acquire the high oxygen permeability for industrial demand,most of the membrane materials always contain cobalt,which is beneficial to activate oxygen molecules and deliver high oxygen permeation flux.However,cobalt can be easily reduced and evaporated,which leads to the poor stability of cobalt-containing membranes at high temperatures or the reducing atmosphere.Meanwhile,it is undesirable for practical application due to the high cost of Co.Therefore,developing new membrane materials with high oxygen permeability,good stability and low cost are required.

At present,BaFeO3-δmembranes have received increasing attentions due to its low cost and high concentration of oxygen vacancy.However,the ionic radius of Ba2+is too large to stabilize the perovskite structure.Most researchers have found that replacing A-site or B-site partly through doping can not just stabilize the perovskite structure[18-27],but also improve the oxygen permeability.Kidaet al.[18]investigated that the oxygen permeation varies with the partially substitution of A-site by Na,Rb,Ca,Y and La.They found that the partial substitution of Ba by La,Y,Ca can successfully stabilize the cubic peroviskite structure.Specially,Ba0.95La0.05FeO3-δshowed the highest oxygen permeability,which can reach 2.95 ml·min-1·cm-2at 930 °C[24].Zhuet al.[20,21]partially substitute Fe with Ce on the B-site and found that BaCe0.15Fe0.85O3-δmembrane exhibits the highest oxygen permeation,which can reach 0.52 ml·min-1·cm-2at 950 °C.Watanabeet al.[19]also reported that the BaFe0.975Zr0.025O3-δmembrane has 1.3 ml·min-1·cm-2oxygen permeation flux at 930 °C under an air/He gradient by doping Zr on the B-site.

Recently,Ta is reported to improve the structure stability of MIECs membrane by some groups[28-33].Our group firstly developed a Ta doping perovskite membrane material,namely BaCo0.7Fe0.2Ta0.1O3-δ,which presents outstanding phase structural stability and oxygen permeability[31-33].After then,Chenet al.[28]found that the phase structure stability of SrCo0.8Fe0.2O3-δmembrane could be improved by doping Ta.Lohne[29]and Liuet al.[30]indicate the Ta can improve the stability of membrane in the reducing atmosphere at high temperature.In this work,we will systematically explore the influence of the partial substitution of Fe by Ta based on BaFeO3-δ.On basis of the definition of Goldschmidt tolerance factor(t)[34],the ideal cubic perovskite structure can be obtained onlytclose to 1.For BaFeO3-δ,thetis calculated to be 1.07(Ba2+:0.161 nm,Fe3+:0.05975 nm,O2-:0.140 nm)[35,36],which is slightly larger than 1.0(the optimum value).It is necessary to introduce a cation with a radius that is larger than that of Fe3+or Fe4+(B site),and simultaneously smaller than Ba2+to stabilize the structure.The ion radius of Ta5+is 0.064 nm,which is larger than Fe3+/Fe4+yet smaller than Ba2+.Therefore,Ta is possibly the good candidate to dope BaFeO3and enhance its stability.In this paper,the BaFe1-yTayO3-δ(0≤y≤0.2)samples were synthesized by a solid state reaction method.The oxygen permeability,phase structure,the rate-determining step for the oxygen transport and the operation stability will be studied in detail.

2.Experimental

Solid state reaction method was used to synthesize BaFe1-yTayO3-δ(0≤y≤0.2)samples.According to the stoichiometry,BaCO3,Ta2O5,and Fe2O3(A.R.purity)were weighted,and hand-milled for 3 h in an agate mortar,and then ball-milled for 24 h in ethanol.After being calcined at 950°C for 10 h,the powder were uniaxially pressed at 20 MPa in a stainless steel module to obtain the green disk membranes.The obtained disk membranes were calcined between 1175°C and 1300°C for 10 h.Only the sintered disk membranes with a relative density over 95%,which were tested by the Archimedes method in ethanol,could be used for oxygen permeation testing.

The crystal structures of BaFe1-yTayO3-δsamples were analyzed by X-ray diffraction(XRD,Bruker-D8 ADVANCE).Scanning electron microscopy(SEM,Quanta 400)was used for analyzing the microstructure of membrane.O2-TPD(Oxygen temperature-programmed desorption)was operated on a Micromeritics AutoChem 2920TM in our previous work[31].

BaFe1-yTayO3-δmembranes were measured in homemade hightemperature apparatus,as reported in our previous work[31-33].The BaFe1-yTayO3-δmembrane polished using SiC paper was sealed by a ceramic sealant(Huitian,China).Air was supplied as the feed gas,and helium was supplied as the sweep gas,respectively.The effluents were analyzed by gas chromatography(GC,Agilent Technologies,7890A).The soap bubble flow meter was used to measure the flow rates of the effluents.Due to the imperfect sealing,a little nitrogen can be detected,which was subtracted when calculating the membrane permeability.The particular calculation of the oxygen permeation flux was calculated as follows:

whereCO2andCN2are the concentration of O2and N2,respectively,which is calculated from GC calibration.Sis the effective membrane area,andFis the total flow rate of the effluents.

3.Results and Discussion

Room-temperature XRD patterns of BaFe1-yTayO3-δmembranes after sintering are shown in Fig.1.As can be seen,BaFeO3-δmembrane exhibits a hexagonal phase structure.After doping Ta,BaFe1-yTayO3-δmembranes begin to change into the cubic perovskite phase.Wheny≥0.1,BaFe1-yTayO3-δmembranes show pure cubic perovskite phase.According to the tolerance factor t defined by:

whererA(A-site ionic radii);rB(B-site ionic radii);rO(oxygen ionic radii).In the case of 0.75≤t≤1.0,the cubic structure can be stabilized.For BaFeO3-δ,thetis estimated to be 1.07.(Ba2+:0.161 nm;Fe3+:0.05975 nm;O2-:0.140 nm)[35,36].So as to obtain the cubic perovskite structure,Ta5+(0.064 nm)is a good choice,since its ionic radius is between Fe3+or Fe4+(B site)and that of Ba2+(A site).Fig.2 shows the calculatedtof different Ta doping.With an increasing of doping Ta5+,the tolerance factor is a slight decrease and close to 1.0,which has an obvious influence on the phase structure of membrane,as shown in Fig.1.Therefore,for obtaining the cubic perovskite phase,the concentration of doping Ta should be more than 0.1.Fig.3 shows the XRD patterns of BaFe0.9Ta0.1O3-δ,BaFe0.85Ta0.15O3-δand BaFe0.8Ta0.2O3-δsamples after being exposed to 8%H2-Ar for 5 h at 900°C.It can be seen that all samples still keep cubic perovskite phase after being disposed.The result indicates that doping moderate Ta can stabilize the cubic structure,even under lower oxygen partial pressure.

Fig.1.The room-temperature XRDpatterns of BaFe1-y TayO3-δmembranes after sintering.(P-cubic perovskite phase;Δ-hexagonal structure phase).

Fig.2.Calculated tolerance factor(t)of different Ta dopings.

Fig.3.XRD patterns of BaFe1-y Ta y O3-δ(y≥0.1)samples after exposure to 8%H2-Ar for 5 h at 900°C.

Fig.4 shows the oxygen permeability of BaFe1-yTayO3-δ(y≥0.1)membranes.It can be found that the content of Ta-doping has an obvious influence on oxygen permeability.With increasing the concentration of Ta(y≥0.1),the oxygen permeation flux of BaFe1-yTayO3-δmembranes reduces gradually.For example,when the Ta proportion is 0.1,the oxygen permeation flux can reach 1.26 ml·min-1·cm-2at 950 °C.Nevertheless,it is only 0.47 ml·min-1·cm-2when the proportion of Ta is 0.2.The reason is that the concentration of oxygen vacancy is reduced after the excess Ta-doping,thus resulting in the low oxygen permeability.Similar results could be also found by other researchers[19,37,38].Since it shows both pure cubic perovskite structure and high oxygen permeation,the BFT0.1 membrane was selected for research in detail.

Fig.4.Temperature dependence of oxygen permeation fluxes through different membranes.Conditions:F air=180 ml·min-1,F He=60 ml·min-1,thickness=0.8 mm.

Fig.5 shows the multi-run O2-TPD profiles of BFT0.1 sample.As shown in Fig.5,there is only one peak between 250 and 700°C,which is attributed to the reduction of Fe4+to Fe3+.When the temperature rising,oxygen releases from the lattice,associating with high valence state metal ions change to lower valence state.From Fig.5,it also can be found that the multi-run O2-TPD profiles are the same as the first-run one,which shows outstanding reversibility of phase structure.After multi-run O2-TPD,the BFT0.1 powder still keeps pure cubic perovskite structure,as shown in Fig.6.These results indicate that BFT0.1 has an excellent structure stability.

Fig.5.Multi-run O2-TPD profiles of the BFT0.1 powder.

Fig.6.XRD patterns for the fresh BFT0.1 and the sample after multi-run O2-TPD.

Fig.7 presents the oxygen permeability of BFT0.1 membranes with different thicknesses at different temperatures.With the temperature increasing,the oxygen permeability distinctly increases.For example,the oxygen permeation flux of 0.8 mm BFT0.1 membrane is 0.80 ml·min-1·cm-2at 850 °C.However,the oxygen permeation flux of BFT0.1 membrane increases to 1.26 ml·min-1·cm-2at 950°C,which is due to increasing the rate of the surface exchange kinetics and the oxygen ion diffusion.Meanwhile,the oxygen permeability increases with decreasing the membrane thickness.When the thickness of membrane is 1.1 mm,the oxygen permeation flux is 0.91 ml·min-1·cm-2at 950 °C.Then,the oxygen permeation flux increases to 1.40 ml·min-1·cm-2when the membrane thickness is 0.5 mm at the same conditions.

Fig.7.Temperature dependence of the oxygen permeation fluxes through BFT0.1 membranes with different thicknesses.Conditions:F air=180 ml·min-1,F He=60 ml·min-1.

For the MIEC membrane,the oxygen permeability is affected by both the surface reaction exchange and the bulk diffusion.If the bulk diffusion is predominant,the oxygen permeability through the MIEC membrane could be described by the following equation theoretically:whereR(gasconstant),T(temperature),σe(electronic conductivity),σi(ionic conductivity),F(Faraday constant),L(membrane thickness),Ph(oxygen partial pressures of the feed side),andPl(oxygen partial pressures of the sweep side).If the bulk diffusion is the rate-determining step,a plot ofJO2should be proportional to 1/L,and goes through the origin of the coordinates.So as to estimate the step of rate control,the oxygen permeation flux of different thickness at various temperatures is measured,as shown in Fig.8.When the thickness of membrane is thicker than 0.8 mm,the oxygen permeation flux is proportionally to 1/L.However,it does not bring about corresponding linear increase of the oxygen permeability when the thickness is thinner than 0.8 mm.These results indicate that the bulk diffusion is the rate-limiting step when the thickness of membrane is thicker than 0.8 mm.Meanwhile,the surface oxygen exchange reaction comes into playing a significant effluence with the thickness being thinner than 0.5 mm.

Fig.8.Relationship between oxygen permeation fluxes and the reciprocal thickness of BFT0.1 membranes at different temperatures.

For practical oxygen separation applications,both good oxygen permeability and excellent structure stability are essential.Fig.9 presents the oxygen permeability varied with time.During 180 h stability testing,the oxygen permeation flux of membrane was kept at 1.26 ml·min-1·cm-2without any decrease.The result shows that the BFT0.1 membrane possesses an excellent operational stability.Fig.10 presents the micro structural features of the fresh and spent BFT0.1 membrane.As shown in Fig.10(a)and(b),the fresh sintered membrane has clear grain boundaries and dense structure.After 180 h testing,the cross sections of the BFT0.1 membrane still keep its density,although the membrane surface has changed slightly,as shown in Fig.10(c),(d),(e)and(f).The spent membrane of BFT0.1,which is exposed to both He and air side,was characterized by XRD,as shown in Fig.11.The membrane exposed to He(sweep)side still maintains the cubic perovskite structure.However,there is small BaSO4peak on the Air(feed)side.After polishing membrane in several micrometers depth,the pure cubic structure was presented,which manifest that the BaSO4only exist in the surface of membrane.

Fig.9.Oxygen permeation flux through the BFT0.1 disk membrane as a function of time at 950 °C.Conditions:F air=180 ml·min-1,F He=60 ml·min-1,thickness=0.8 mm.

4.Conclusions

In conclusion,a novel cobalt-free Ta-doped perovskite membrane was developed successfully by solid state reaction method.The BaFe0.9Ta0.1O3-δmembrane has a high oxygen permeation flux of 1.6 ml·min-1·cm-2at 950 °C.A stable oxygen permeation flux of 1.26 ml·min-1·cm-2was achieved during 180 h operation.For the BaFe0.9Ta0.1O3-δmembrane with a thickness over 0.8 mm,the oxygen permeation is limited by the bulk diffusion.While for the membrane with a thickness in a range of 0.5-0.8 mm,the oxygen permeation is limited by both the bulk diffusion and the surface reaction.The O2-TPD,SEM and XRD results have shown that the BFT0.1 membrane exhibits excellent stability and reversibility,which holds promise for the practical industrial applications.

Nomenclature

CO2the concentration of O2

CN2the concentration of N2

FFaraday constant,96485 °C·mol-1

JO2the permeation flux of oxygen,ml·min-1·cm-2

Lmembrane thickness,mm

Phoxygen partial pressures of the feed side,Pa

Ploxygen partial pressures of the sweep side,Pa

Rgas constant,8.314 J·mol-1·K-1

rAA-site ionic radii,nm

rBB-site ionic radii,nm

rOoxygen ionic radii,nm

seffective membrane area,cm2

Ttemperature,K

tthe tolerance factor

σeelectronic conductivity,S·cm-1

σiionic conductivity,S·cm-1

Fig.10.SEM micrographs of the BFT0.1 disk-membrane:(a,b)membrane surface and cross section of the fresh membrane;(c,d)membrane surface and cross section of the spent membrane exposed to the feed side;(e,f)membrane surface and cross section of the spent membrane exposed to the sweep side.

Fig.11.XRD patterns of the fresh and spent BFT0.1 membrane after 180 h oxygen permeation.(P-cubic perovskite phase).

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