By cobalt-doping of the mixed conducting phase PSFC, a good combination of high CO2 stability and high oxygen permeability is obtained for the 60 wt% Ce0.9Pr0.1O2-delta d -40 wt% Pr0.6Sr0.4Fe0.5Co0.5O3-delta (CP-PSFC) dual phase membrane, which suggests that CP-PSFC is a promising membrane for industrial applications in the oxyfuel process for CO2 capture

Cao, Zhengwen

Caro, Jürgen

Liang, Fangyi

Liu, Yi

Luo, Huixia

Partovi, Kaveh

Ravkina, Olga

Institutionelles Repositorium der Leibniz Universität Hannover

This journal is©The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 2451--2454 | 2451Cite this:Chem. Commun., 2014,50, 2451A novel CO2-stable dual phase membrane withhigh oxygen permeability†Fangyi Liang,*a Huixia Luo,b Kaveh Partovi,a Olga Ravkina,a Zhengwen Cao,a Yi Liuaand Ju¨rgen CaroaBy cobalt-doping of the mixed conducting phase PSFC, a good combi-nation of high CO2 stability and high oxygen permeability is obtainedfor the 60 wt% Ce0.9Pr0.1O2d–40 wt% Pr0.6Sr0.4Fe0.5Co0.5O3d(CP–PSFC) dual phase membrane, which suggests that CP–PSFCis a promising membrane for industrial applications in the oxyfuelprocess for CO2 capture.CO2 capture and storage technologies in power plants have gainedattention worldwide1 since the combustion of fossil fuel is con-sidered to be the main contribution to CO2 emissions. Oxygentransporting membranes (OTMs)2 based on mixed electronic andionic conductors, can supply oxygen of 100% purity to powerstations for CO2 capture according to the oxyfuel for CO2 captureand storage.3 The oxyfuel concept involves the combustion of fossilfuels with an oxygen/exhaust gas mixture. The pure oxygen used inthe oxyfuel process can be produced using the Linde cryogenictechnique. However, by using OTMs, oxygen can be separated fromair by using a part of the exhaust gas CO2 as a sweep gas. Oxygenproduction by using OTMs can reduce the costs by 35% and save60% energy compared to the conventional cryogenic process.4 A partof the exhaust gas CO2 is sequestrated, and another part is recycledand used as sweep gas for the oxygen separation through the OTMs.Thus, the OTMs for CO2 capture in the oxyfuel process should havenot only good stability in a CO2 atmosphere but also high oxygenpermeation performance at elevated temperatures. Moreover, OTMscan be used in many promising potential applications, such as inhigh-purity oxygen production,5 in catalytic membrane reactors,6and as cathodes in solid oxide fuel cells (SOFCs).7Among OTMs, many cobalt-based single phase perovskite-typemembranes such as Ba1xSrxCo1yFeyO3d exhibit high oxygenpermeability8 since cobalt can provide a high concentration ofmobile oxygen vacancies in the perovskite lattice over a widetemperature range.9 However, under a CO2 atmosphere thesemembranes immediately lose their oxygen permeation flux, sincethe alkaline-earth metals on the A site of the perovskite frameworkform carbonates with CO2.10 Recently, dual phase membranes withgood structural stability and CO2 resistance, made of a micro-scalemixture of well separated grains of the two phases of an oxygenionic conductor and an electronic conductor, have attracted muchattention for the application of the O2 production in the oxyfuelconcept. However, the low oxygen permeability of dual phasemembranes needs to be improved to meet the industrial applica-tion requirements.11 Zhu et al. improved the oxygen permeability oftheir CO2-stable dual phase membranes, such as Ce0.8Sm0.2O1.9–SmMn0.5Co0.5O312 and Ce0.85Sm0.15O1.925–Sm0.6Sr0.4FeO3,13 by coat-ing Co-containing porous layers on the membrane surface toenlarge the oxygen exchange surface. In our group, we havedeveloped some novel Co-free CO2-stable dual phase membranes,such as NiFe2O4–Ce0.9Gd0.1O2d (NF–CG),14 Fe2O3–Ce0.9Gd0.1O2d(F–CG)15 and Ce0.9Pr0.1O2d–Pr0.6Sr0.4FeO3d (CP–PSF).16 The CP–PSF membrane shows a good chemical stability under the harshconditions of the POM reaction and in a CO2 atmosphere at hightemperatures.16 Thus, cobalt-doping of the PSF phase in the dualphase CP–PSF membrane can enhance the oxygen vacancy concen-tration in the membrane lattice, which should result in a combi-nation of good CO2 stability and high oxygen permeability.In this paper, we present the novel dual phase membrane,60 wt% Ce0.9Pr0.1O2d–40 wt% Pr0.6Sr0.4Fe0.5Co0.5O3d (abbreviatedas CP–PSFC) prepared via a one-pot sol–gel synthesis method.17 Inthis dual phase system, the CP phase is mainly for ionic transport,and the PSFC phase for both electronic and ionic transport. TheXRD pattern in Fig. 1 shows that after sintering at 1200 1C for 5 h inair, the dual phase CP–PSFC membrane consists of only the cubicflourite CP phase (space group 225: Fm3m) and the orthorhombicdistorted perovskite PSFC phase (space group 74: Imma). The phasestability of the dual phase CP–PSFC material against CO2 wasevaluated by in situ XRD measurements (Fig. S2, ESI†) between30 and 1000 1C in an atmosphere of 50 vol% CO2/50 vol% air.No carbonate formation was observed in the in situ XRD patternsa Institute of Physical Chemistry and Electrochemistry, Leibniz University ofHannover, Callinstr. 3a, D-30167 Hannover, Germany.E-mail: fangyi.liang@pci.uni-hannover.de; Fax: +49-511-762-19121;Tel: +49-511-762-2942b Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA† Electronic supplementary information (ESI) available: Experimental details. SeeDOI: 10.1039/c3cc47962eReceived 16th October 2013,Accepted 9th January 2014DOI: 10.1039/c3cc47962ewww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 10 January 2014. Downloaded by TIB und Universitaetsbibliothek Hannover on 19/11/2015 13:06:04. View Article OnlineView Journal  | View Issue2452 | Chem. Commun., 2014, 50, 2451--2454 This journal is©The Royal Society of Chemistry 2014from 30 to 1000 1C, which is in good agreement with the in situ XRDfinding in previous reports of the CP–PSF16 and La0.6Sr0.4Co0.8-Fe0.2O3d18 under similar experimental conditions. In this tempera-ture range, the CP phase retains the cubic structure. Moreover, twophase transitions of the PSFC phase from the orthorhombic to therhombohedral symmetry at ca. 600 1C and from the rhombohedralto the cubic symmetry at ca. 800 1C could be observed. However, thePSF phase without Co in the dual phase CP–PSF powder retains theorthorhombic symmetry under the same conditions.16 The phasetransition from rhombohedral to cubic symmetry at high tempera-tures is in agreement with the findings for the comparable materialsLa0.6Sr0.4Co0.8Fe0.2O3d18 and La0.6Ca0.4Co0.8Fe0.2O3d.19 The forma-tion of the cubic structure of the PSFC phase at high temperatures isbeneficial for oxygen permeability.17,20 Thus, the dual phaseCP–PSFC material exhibits a stable co-existence of both the CPand the PSFC phases and a good tolerance against CO2.Fig. 2 shows the scanning electron microscopy (SEM), back-scattered SEM (BSEM), and energy-dispersive X-ray spectroscopy(EDXS) images of the dual phase CP–PSFCmembrane after sinteringat 1200 1C for 5 h in air. SEM (Fig. 2a) indicates that themembrane isdense. In the BSEM (Fig. 2b and c) at different magnifications, thegrains of the two phases are well distributed forming a 3-dimensionalpercolation network with clear grain boundaries, which is beneficialto the oxygen ion and electron transport through the membrane.The light grains in the BSEM are CP and the dark grains are PSFCsince the contribution of the backscattered electrons to the SEMsignal intensity is proportional to the atomic number. The grainsize of CP in these composite membranes is larger than that ofPSFC. The same information about the phase distribution isprovided by the EDXS (Fig. 2d–f) of the membrane. Fig. 2d showsthe color version EDXS of themembrane where the turquoise color(light in BSEM) is due to overlapping of the Ce and Pr signals,whereas the orange color (dark in BSEM) stems from an average ofthe Pr, Sr, Fe and Co signals. The EDXS elemental distributions ofCe in the CP grains (Fig. 2e) and of Sr in the PSFC grains (Fig. 2f)indicate the phase separation in the membrane and proof that nointermixing of Ce and Sr between the two phases exists.The oxygen permeation fluxes through the dual phase CP–PSFCmembrane as a function of temperature with pure He and CO2 assweep gases are shown in Fig. 3. For both the sweep gases He andCO2, the oxygen permeation flux increases with increasing tempera-tures. By increasing the temperatures from 800 to 1000 1C, the oxygenpermeation fluxes increase from 0.24 to 1.08 cm3 (STP) min1 cm2for using He and from 0.11 to 1.01 cm3 (STP) min1 cm2 for usingCO2 as the sweep gas. If pure CO2 has been used as the sweep gas, theoxygen permeation flux is only slightly lower than in the case whenpure He was used as the sweep gas, which is in good agreement withthe observations in previous reports.13,14,21 The reason for thisexperimental finding can be attributed to the weaker adsorptiveinteraction of He in comparison with CO2 with the membranesurface, as the influence of He on the oxygen exchange reaction isless than that of CO2.12,14,22,23 Furthermore, it is also found that thedifference in the oxygen permeation flux between He and CO2 assweep gases is larger at lower temperatures and becomes smaller athigher temperatures, which can be ascribed to the decreasingadsorptive interaction with increasing temperatures. In comparisonwith the Co-free dual phase CP–PSFmembrane at 950 and 1000 1C,16the oxygen permeation fluxes of the Co-containing dual phaseCP–PSFC membrane are increased by approximately a factor of 3,which is a clear indication of an increased concentration of mobileoxygen vacancies by Co-doping, which significantly enhances theoxygen permeability of the PSFC phase.Fig. 1 XRD patterns of the dual phase CP–PSFC membrane, and of thesingle phases CP and PSFC, all sintered at 1200 1C in air for 5 h.Fig. 2 SEM (a), BSEM (b and c), and EDXS (d–f) images of the dual phaseCP–PSFCmembrane after sintered at 1200 1C for 5 h in air. In BSEM (b and c)for different magnifications, the dark grains represent PSFC, the lightones CP. Superimpositions of the Pr, Sr, Fe and Co (orange), and Pr andCe (turquoise) signals have been used in Fig. 2(d). Element distribution ofCe in the CP grains (Fig. 2e) and Sr in the PSFC (Fig. 2f).Fig. 3 Oxygen permeation fluxes through the dual phase CP–PSFC andCP–PSF25membranes as a function of temperature with pure He and CO2 assweep gases. Conditions for CP–PSFC: 150 cm3 (STP) min1 air as the feedgas, 50 cm3 (STP) min1 He or CO2 as the sweep gas; membrane thickness:0.6 mm. Conditions for CP–PSF:16 150 cm3 (STP) min1 air as the feed gas,30 cm3 (STP) min1 CO2 as the sweep gas; membrane thickness: 0.5 mm.Communication ChemCommPublished on 10 January 2014. Downloaded by TIB und Universitaetsbibliothek Hannover on 19/11/2015 13:06:04. View Article OnlineThis journal is©The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 2451--2454 | 2453The oxygen permeability of an OTM strongly depends on itscomposition, the membrane thickness, the coating of themembrane surface (catalytic coating or coating to enlarge thesurface, thus enhancing the exchange reaction), and the operationconditions (type of sweep gas, temperature, or the oxygen partialpressure). Table S1 (ESI†) contains the oxygen permeation fluxesof several OTMs evaluated with CO2 as the sweep gas at ca. 950 1C.As shown in Table S1 (ESI†), our dual phase CP–PSFC membraneexhibits the highest oxygen permeation flux compared to theother dual phase membranes. It should be noted that the fluxesmeasured on our dual phase CP–PSFC membrane were obtainedfor a relatively thick membrane (0.6 mm) without any coating onthe membrane surface. It is believed that the oxygen permeabilityof our CP–PSFC membrane can be further improved by reducingthe membrane thickness like a hollow fiber membrane23,24 or anasymmetric membrane,25 or by coating for an enlargement of themembrane surface.12,13Fig. 4 presents the long-term oxygen permeation operation oftwo dual phase CP–PSFCmembranesM1 andM2. For the CP–PSFCmembrane M1, when He is used as the sweep gas at 950 1C, theoxygen permeation flux increases slowly during the activationtime in the first 10 h. After this activation step, the oxygenpermeation flux reaches steady-state with a constant value of0.84 cm3 (STP) min1 cm2 (inset in Fig. 4). When pure CO2 wasused instead of He as the sweep gas, the oxygen permeation flux onlyslightly decreased to the lower value of 0.70 cm3 (STP) min1 cm2.This oxygen permeation flux was found to be constant during thewhole oxygen permeation operation by using pure CO2 as the sweepgas at 950 1C for 400 h. When the operation temperature waselevated to 1000 1C, the oxygen permeation flux increased signifi-cantly to the higher value of 1.01 cm3 (STP) min1 cm2 and wasconstant for at least 100 h. Whereas the CP–PSFC membrane M1shows a phase transition from the rhombohedral to the cubic phase(Fig. S2, ESI†) and a fast activation step over 10 h only at the highertemperature of 950 1C, when cooling the CP–PSFC membrane M2from 950 to 800 1C, a phase transition from the cubic to the ortho-rhombic phase takes place (Fig. S2, ESI†). After this phase transition,a slow activation step takes place over ca. 100 h at the lowertemperature of 800 1C. As a result of (i) the phase transforma-tion, and (ii) the lower permeation temperature, for theCP–PSFC membrane M2 with pure CO2 as the sweep gas at800 1C, the oxygen permeation flux decreases slowly from 0.19 to0.13 cm3 (STP) min1 cm2 in the first 100 h. After this phasetransformation and the activation step, the oxygen permeation fluxreaches a steady-state for 200 h. For the membranes M1 and M2after the long-term oxygen permeation operation, no carbonateformation was observed in the XRD patterns (Fig. S3 and S5, ESI†).No carbon species enrichment was found at the strontium positionsin the EDXS images on the surfaces of the membranes M1 and M2(Fig. S6 and S7, ESI†) and their cross-sections (Fig. S4 and S8, ESI†),which excludes SrCO3 formation. This long-term operations suggestthat our dual phase CP–PSFC membrane exhibits an excellentstability in CO2 and shows a high oxygen permeation performance.In summary, we have prepared a novel CO2-stable dual phase60 wt% Ce0.9Pr0.1O2d–40 wt% Pr0.6Sr0.4Fe0.5Co0.5O3d (CP–PSFC)membrane with Co-doping of the PSF phase for high oxygenpermeability. Cobalt-doping of the PSF phase can enhance theoxygen vacancy concentration in themembrane lattice and stabilizethe cubic structure above 800 1C which is beneficial for oxygenpermeability. The in situ XRD and the long-term operationsdemonstrate that the CP–PSFC dual phase membrane shows avery good tolerance against CO2. Moreover, the very high oxygenpermeation flux of 0.70 and 1.01 cm3 (STP)min1 cm2 were foundto be constant at 950 and 1000 1C during the long-term operationfor over at least 400 h. The dual phase CP–PSFC membrane withexcellent CO2 stability and high oxygen permeation performance isa promising membrane material for industrial applications in theoxyfuel process for CO2 capture.The EU is thanked for financing the 7th Framework Program ofthe IP Innovative Catalytic Technologies & Materials for the Next Gasto Liquid Processes (NEXT-GTL). The authors are thankful for thefinancial support by DFG 147/18-1 and to the Sino-German Center forPromoting Research (GZ 676). 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A novel CO2-stable dual phase membrane with high oxygen permeability

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