Journal of Membrane Science 428 (2013) 123–130

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Journal of Membrane Science

0376-73

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n Corr

E-m

journal homepage: www.elsevier.com/locate/memsci

Impact of sulphur contamination on the oxygen transport mechanismthrough Ba0.5Sr0.5Co0.8Fe0.2O3�d: Relevant issues in the developmentof capillary and hollow fibre membrane geometry.

A.A. Yaremchenko a, C. Buysse b, V. Middelkoop b, F. Snijkers b, A. Buekenhoudt b,J.R. Frade a, A.V. Kovalevsky a,n

a Department of Materials and Ceramics Engineering, CICECO, University of Aveiro, Aveiro 3810-193, Portugalb Flemish Institute for Technological Research, VITO NV, Boeretang 200, B-2400 Mol, Belgium

a r t i c l e i n f o

Article history:

Received 6 July 2012

Received in revised form

27 September 2012

Accepted 18 October 2012Available online 26 October 2012

Keywords:

BSCF

Hollow fibre

Oxygen membrane

Surface exchange

Surface activation

88/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.memsci.2012.10.033

esponding author. Tel.: þ351 234 370263; fa

ail address: akavaleuski@ua.pt (A.V. Kovalevs

a b s t r a c t

Fabrication of dense perovskite membranes in the form of capillaries or hollow fibres is considered

attractive for large-scale oxygen separation applications. For the preparation of such membranes by

phase-inversion process polysulphone or polyethersulphone are commonly used as a binder. The

decomposition of the sulphur-containing binder during the calcination leads to the formation of

sulphates, which negatively affect the oxygen permeation through the membrane. The present work

focuses on the comparative analysis of the oxygen transport mechanism through sulphur-free and -

containing Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) membranes. The analysis of the thickness dependence of the

oxygen permeation fluxes indicated that sulphates decrease the permeation rate mostly due to the

partial blocking of the surface oxygen exchange, whilst the bulk ambipolar conductivity remains

essentially unchanged. SEM/EDS studies revealed segregation of BaSO4 at the grain boundaries, which

might be responsible for the fast oxygen exchange in phase-pure BSCF. The negative impact of sulphur

contamination on oxygen permeation was more pronounced at temperatures below 1123 K. It has been

demonstrated, that, by surface activation, the oxygen flux through sulphur-containing BSCF membranes

can be increased to the level of that of sulphur-free membranes.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, it is generally believed that a major cause for globalclimate change is the anthropogenic emission of greenhousegases, in particular, from the combustion of fossil fuels, resultingin the formation of carbon dioxide. An alternative route forcapturing carbon from fuel gas or flue gas may includea modification of the combustion process, aimed at increasingthe concentration of CO2 in the flue gas, thus making it moresuitable for sequestration. A promising technology to accomplishthis is oxy-fuel combustion, in which the fuel is burned withnearly pure oxygen (495%), mixed with recycled flue gas [1,2], toavoid excessively high temperatures. Mixed ionic- and electronic-conducting (MIEC) dense ceramic membranes provide an attrac-tive way for pure oxygen production [3–6], due to infinite oxygenselectivity at elevated temperatures. MIEC membrane technologyis envisaged to replace state-of-the-art cryogenics and reduceoxygen production costs by 35%, which can significantly cut

ll rights reserved.

x: þ351 234 370204.

ky).

energy penalty for CO2 capture by 50%, when integrated in oxy-fuel power plants [6].

Moving forward towards competitive membrane technologyimplies the development of oxide materials, possessing chemicaland mechanical stability under operation conditions, along withsufficiently high oxygen permeation fluxes to be economicallyfeasible. According to the current state of knowledge in the field,Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) perovskites show one of the high-est oxygen fluxes of all known MIEC materials [7–10]. Besidesmaterial science related aspects, research challenges are alsofocused on the optimisation of membrane architecture. For powerplant applications, oxygen mass flows between 2500 and 8000 tper day are common, requiring a maximal ratio of membranesurface area to the volume of the module. Therefore, in suchlarge-scale gas separation applications capillary or hollow fibremembranes are considered to be more feasible, than flat sheet ortubular geometry. However, this requires non-trivial techniquesto obtain gas-tight ceramics.

The phase-inversion spinning technique is commonly used forthe production of hollow fibre or capillary polymer membranes.The process is based on the exchange of a solvent and nonsolventfor the polymer, leading to ‘liquid–liquid’ demixing of the solution

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130124

and subsequent precipitation of the polymer. The technique wasadapted for the fabrication of ceramic membranes, by adding acertain fraction of oxide powder into the polymer solution,in order to obtain gastight hollow fibre membranes after thesintering step. For the preparation of BSCF and other perovskite-based capillaries and hollow fibres by the phase-inversion pro-cess, polysulphone or polyethersulphone are usually used as abinder [11–15], since they are well studied for the production ofmicrofiltration, ultrafiltration, reverse osmosis and gas separationpolymer membranes. In particular, well adjusted experimentalroutes allow the production of BSCF hollow fibres with anadvanced design, e.g., U-shaped configuration [13]. However,decomposition of the sulphur-containing binder during the calci-nation leads to a partial perovskite phase decomposition andformation of sulphates, which negatively affect the oxygenpermeation through the membrane [11,12,15–18]. A possibleexplanation, proposed in [18], considers the presence of non-ionic domains, which hinder oxygen ionic diffusion. However, itwas demonstrated that surface activation of sulphur-containing

Fig. 1. Typical example of the oxygen permeation data obtained for one S

membrane (A), and reproducibility of the fluxes through different nS membranes

with similar thickness.

BSCF capillary membranes with praseodymium oxide leads to animprovement of the oxygen permeation flux at 1023–1223 K by afactor 2.0–4.6, depending on the temperature and sweep gas flowrate [16]. The latter, in particular, suggests that sulphur maynegatively affect the surface exchange kinetics even to a higherextent than bulk diffusion. Although a possible solution involvesthe adaptation of a sulphur-free polymeric binder for the phase-inversion process (Refs. [17–20] and references therein), anotherstrategy to suppress the effect of sulphur contamination mayinclude the tuning of the rate-determining step for the oxygenpermeation, based on the detailed knowledge of the oxygentransport mechanism in perovskite membranes from precursorscontaining sulphur. The present work was focused on the com-parative analysis of the oxygen transport mechanisms throughsulphur-free and -containing flat BSCF membranes, addressingthe relevant issues in the development of capillary and hollowfibre membrane geometry.

2. Experimental

BSCF powder with specific surface area 4.4 m2/g, d50¼1.7 mmwas prepared by Fraunhofer Institute for Ceramic Technologiesand Systems (IKTS) using the standard solid state route. Fordetailed analysis of the oxygen transport mechanism throughsulphur-free and -containing BSCF, a flat disk-shaped membraneconcept was selected in order to avoid complexities associatedwith the preparation and sintering of hollow fibres of variousthicknesses and uniform density, as well as with the oxygenpermeation studies. In particular, the latter offered easier andmore precise control of the oxygen partial pressure at thepermeate side of the membrane, compared to hollow fibregeometry. The phase-inversion spinning technique, describedelsewhere [15,20], was employed for the preparation of greenhollow fibres. Sulphur-containing fibres were fabricated usingpolysulphone and N-methyl-2-pyrrolidone as a binder and

Fig. 2. XRD patterns of the powders, obtained by grinding of sintered nS and S

BSCF membranes.

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130 125

solvent [15]. For the preparation of sulphur-free hollow fibrescellulose acetate, dimethylsulfoxide and de-ionised water wereused as phase-inversion polymer, solvent and additive to thepolymer solution, respectively [20]. Green dried fibres werecrushed into powder, and disk-shaped BSCF membranes(d¼0.38–1.03 mm) were fabricated by uniaxial compactionfollowed by sintering at 1373 K for 5 h. Corresponding sets ofsulphur-containing and -free membranes are further denoted as‘‘S’’ and ‘‘nS’’, respectively. To collect additional information onthe role of surface oxygen exchange in the overall oxygentransport, S-membranes (d¼0.71 mm) were activated bysulphur-free powder from both feed and permeate sides (furtherdenoted as nSact-S membrane). The activation was accomplishedby applying the slurry, containing sulphur-free BSCF powder, onthe membrane surfaces and annealing at 1373 K for 1 hour. Thesurface density of the activation layer after annealing amountedto �6.9 mg/cm2.

General characterisation of the membrane samples includedX-ray diffraction (XRD) analysis, scanning electron microscopycoupled with energy-dispersive spectroscopy (SEM/EDS) and gas-tightness control; description of the experimental procedures andequipment can be found elsewhere (Refs. [15–17,20,21] andreferences therein). For all membranes studied in this work, gastightness was confirmed under a total pressure gradient of 200–400 kPa. Oxygen permeation measurements were performedunder air/(O2þAr) gradients at 973–1223 K, supplying the sweep

Fig. 3. Typical SEM micrographs and results of EDS

gas (argon) onto the membrane permeate side. The experimentalsetup, comprising a dense membrane disk hermetically sealedonto YSZ tube and two YSZ sensors at the inlet and outlet of thesystem, was described in Refs. [21,22]. Fig. 1A shows a typicaldataset for S membrane sample, including results of isothermalmeasurements upon cooling from 1223 K to 973 K and afterheating up back to 1223 K. At 1123–1223 K a good reproducibilityof the oxygen permeation fluxes was observed, the variations in j

values did not exceed 3–5% for all measured membrane samples.Poorer reproducibility and noticeable influence of the thermalpre-history on the oxygen permeation at lower temperatures aredetermined by slow phase transitions in BSCF; the impact of thelatter is discussed below. The variations in oxygen permeationfluxes through the different membranes of the same compositionand similar thicknesses were found to be less than 5% (Fig. 1B).

3. Results and discussion

The XRD patterns of the powders, prepared by grindingsintered S and nS BSCF disc membranes, are shown in Fig. 2.As expected, the formation of single-phase cubic perovskitestructure was observed for sulphur-free samples. In the case ofS membranes, a noticeable amount of BSCF was converted intoBaSO4 (5–7 wt.%) and Co3O4–Fe3O4, due to the reaction with theproducts of decomposition of the polysulphone binder from the

mapping of the surface of S BSCF membrane.

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130126

spinning suspension. SEM studies revealed similar grain sizes atthe surface of both S and nS BSCF membranes; typical SEMmicrographs of nS and S BSCF membranes surface are presentedin Fig. 3. However, EDS studies of the surface of sulphur-containing samples (Fig. 3) show sulphur-rich impurities, mostlysegregated at the grain boundaries, which are also enriched withbarium; this is consistent with XRD detection of BaSO4 (Fig. 2).Although SrSO4 formation was reported in a similar case [18], noenrichment in strontium content along the grain boundaries wasdetected by SEM/EDS in the present work.

Results on the dependence of oxygen permeation fluxes(j) through S and nS BSCF membranes, having various thickness(d), on the oxygen partial pressure drop at 1123 and 1223 K arepresented in Fig. 4A and C. As expected, the presence of sulphurdecreases the permeation rate, in accordance with the datapreviously obtained for BSCF capillary membranes [15,17,20].The fluxes increase with decreasing membrane thickness, indicat-ing that, for both sets of membranes, the overall rate of oxygentransport is at least partially affected by bulk ionic diffusion.However, the corresponding values of specific oxygen perme-ability J(O2) still show dependence on thickness; this is given by[23]:

J O2ð Þ ¼ j� d� lnp2

p1

� ��1

ð1Þ

where p2 and p1 are the oxygen partial pressures at the membranefeed and permeate sides, respectively, and should be independentof membrane thickness if the surface limitations are negligible.According to Fig. 4B and D, the oxygen permeability of both S andnS BSCF membranes depends on the thickness in the range 0.38–1.03 mm, decreasing with a reduction in d. In combination withthe thickness dependence of the oxygen permeation fluxes

Fig. 4. Dependence of oxygen permeation flux (A and C) and specific oxygen permeabil

partial pressure gradient at 1223 K (A and B) and 1123 K (C and D).

(Fig. 4A and C) these observations indicate that the oxygenpermeation through S and nS membranes, fabricated in thepresent work, is limited by both oxygen bulk diffusion and surfacereactions in the studied thickness range.

Estimated activation energies (Ea) for both S and nS mem-branes, calculated from the data presented in Fig. 5, vary for bothS and nS membranes in the range 45–59 kJ/mol at 1123–1223 K atfixed oxygen partial pressure drop, being close to the activationenergy of bulk diffusion in BSCF (30–50 kJ/mol at 673–1173 K)[7,24–27]. At temperatures below 1123 K the Ea values increaseup to 75–79 kJ/mol for nS membranes, whilst for sulphur-containing BSCF the increase is more pronounced (80–100 kJ/mol).Typically, for perovskite-type mixed conducting membranes and,in particular, for BSCF, Ea for bulk diffusion is lower than that foroxygen exchange [25,26,28]. The observed differences indicatehigher surface kinetics limitations at temperatures below 1123 K.However, the literature data for Ea of the surface exchange vary ina wide range of 64–160 kJ/mol at 650–1173 K [26,29–31],depending mainly on the preparation route and measurementmethod. The shift in the oxygen permeation mechanism isillustrated by a change in the slope of the Arrhenius plots foroxygen fluxes under fixed oxygen partial pressure gradient(Fig. 5). Similar results for sulphur-free BSCF were obtained in[7,24,31]. Thus, higher activation energies, observed for S BSCFmembranes, in particular, indicate prevailing negative impact ofsulphur presence on the surface exchange kinetics at least atTo1123 K, if compared to nS samples.

One should note that behaviour of membranes below 1123 Kand change in slope of Arrhenius plots can be partially related totransformation from cubic to hexagonal phase characteristic forBSCF [32,33]. Indeed, experimental results showed gradualdegradation of permeation fluxes with time at 973–1073 K, as

ity (B and D) of dense BSCF ceramic membranes of different thicknesses on oxygen

Fig. 5. Temperature dependence of oxygen permeation fluxes through nS and S

BSCF membranes under a fixed oxygen partial pressure gradient.

Fig. 6. Examples of time dependence of the oxygen permeation flux through nS

and nSact-S BSCF membrane under fixed temperatures and sweep gas flow rates.

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130 127

illustrated in Fig. 6A for one nS membrane. This degradation isslow, but still may affect the apparent values of Ea at lowertemperatures. Thus, only initial values of permeation fluxes ateach temperature (within 10–15 h after temperature stabiliza-tion) were used for comparative analysis below 1123 K. At thesame time, at higher temperatures (1123–1223 K), the permea-tion fluxes were stable (Fig. 6) and reproducible after temperaturecycling (Fig. 1A) and for different samples with the same thick-ness (Fig. 1B).

The contributions from the bulk ambipolar transport andsurface exchange to the oxygen transport mechanism may befurther estimated by decomposing the overall oxygen partialpressure driving force, expressed as ln(p2/p1), into three resistivecontributions associated with membrane bulk and surfaces.A detailed description of the corresponding model can be foundelsewhere [34]:

lnp2

p1

� �=j¼

16F2

RTsamb

" #dþ kf

ex

h i�1þ kp

ex

� ��1ð2Þ

where samb is the ambipolar conductivity, kex is the exchangecoefficient, and the superscripts f and p denote the feed- andpermeate sides, respectively. Fig. 7 presents the dependences ofambipolar conductivity and average oxygen surface exchangerate, estimated from the variations in permeation fluxes throughS and nS membranes with thickness, on the oxygen partialpressure gradient across the membrane. The bulk ambipolarconductivity, which, in the case of BSCF, is similar to the ionicconductivity, at 1173–1223 K is almost unaffected by sulphurpresence, and monotonically increases with the oxygen partialpressure drop due to an increase in average concentration ofoxygen vacancies. A possible explanation for unexpectedly negli-gible influence of sulphates on the ambipolar conductivity of BSCFmay rely on the microstructural features of S membranes.Possible pathways for the oxygen ions during the permeationprocess include diffusion through the grain bulk and grainboundary transport. Although no influence of grain size onoxygen permeation through BSCF membranes was found in Ref.[27], several studies have pointed out higher oxygen fluxes for themembranes with larger grains [35,36]. The latter, in particular,

indicates that diffusion along the grain boundaries in BSCF may beslower than through the grain bulk. Since sulphur impurities in S

membranes are mostly localised at the grain boundaries (Fig. 3),one may expect that fast oxygen diffusion pathways through thegrains bulk will remain essentially unaffected by sulphates, and,thus, will be responsible for the high level of ambipolar con-ductivity. Still, one cannot exclude some negative impact ontransport properties across grain boundaries due to segregationor constrictions exerted by sulphur-rich precipitates (Fig. 3).

In contrast, presence of sulphur has a dramatic influence mainlyon the surface exchange kinetics (inset in Fig. 7). At the same time,SEM/EDS studies revealed a uniform distribution of sulphur along thecross-section of S membranes after oxygen permeation measure-ments (Fig. 8) with no particular enrichment with sulphur near feedand permeate side surfaces. The latter emphasises a particular role ofthe grain boundaries for the surface oxygen exchange process in BSCF.Recent studies have demonstrated clearly that the grain boundariesat the surface provide preferential reaction sites for facilitatedoxygen incorporation in the case of ionic conductors such as GDCand YSZ [37,38]. In particular, the elementary reactions of surfaceexchange, i.e., oxygen adsorption/desorption, charge transfer or

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130128

oxygen dissociation/recombination may be enhanced on the grainboundaries, due to high defect concentration. Obviously, partialblocking of those active sites at the membrane surface by sulphatesis expected to suppress surface oxygen exchange.

More evidence of the profound impact of sulphur contamina-tion on oxygen exchange follows from the results presented inFigs. 5 and 9. If the slower bulk diffusion would be the mostcritical parameter for the overall permeation rate through S BSCFmembranes, no surface activation method is expected to provide

Feed side

Fig. 8. Typical EDS maps of the cross-section of S BSCF membrane near the su

Fig. 7. Ambipolar conductivity and average oxygen surface exchange rate

(kn

ex ¼ kfex � kp

ex=kfexþkp

ex), estimated from the thickness dependence of oxygen

permeation fluxes through nS (d¼0.46–0.70 mm) and S (d¼0.38–1.03 mm) BSCF

membranes, using Eq. 2.

a significant improvement in the oxygen flux. However, forsimilar membrane thickness and experimental conditions, activa-tion of the surface of S membrane with pure BSCF powder leads toan increase in the permeation fluxes up to those characteristic forthe nS sample, e.g., by a factor of �2 at 1173 K. Generally, theactivation effect may be provided by a faster exchange rate of thesurface layer and/or by a simple enlargement of the membranesurface. As an example, an asymmetric phase-pure BSCF mem-brane concept, comprising a thin dense 170 mm thick layer,sandwiched between two layers with high open porosity of�50%, provided only 1.5–1.8 times increase in the permeationrate at 1173 K, compared to far thicker (1.00 mm) dense mem-brane [21]. This moderate improvement in the flux was achievedby an enlargement of the active surface of the membrane with a

Permeate side

rface of feed and permeate sides after oxygen permeation measurement.

Fig. 9. Comparison of the oxygen permeation fluxes through sulphur-free and -

containing non-modified and surface-activated BSCF membranes.

Fig. 10. SEM micrographs of the cross-section (A) and the surface (B) of nSact-S

BSCF membrane.

Fig. 11. Comparison of the literature data on oxygen permeation fluxes through

sulphur-free and -containing BSCF hollow fiber and capillary membranes

[11,12,16,19,20,39].

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130 129

simultaneous reduction in thickness, whilst the main permeation-limiting factor was mostly associated with molecular diffusion inpores. For nSact-S membrane, SEM inspection revealed a signifi-cant densification of the activation layer (Fig. 10), suggesting thatthe main difference between permeation through modified andnon-modified S membranes originates from remarkably fasteroxygen exchange of sulphur-free BSCF, thus confirming theinformation derived from the thickness dependence of the oxygenflux (Eq. 2). It is also noteworthy that nSact-S membrane showedsubstantially enhanced initial permeation flux at lower tempera-tures, 973–1023 K, followed, however, with stronger degradation(Fig. 6B). As an example, the decrease of permeation flux throughnSact-S membrane after first 144 h at 973 K was �50% largercompared to nS sample (Fig. 6). The latter indicates that cubic tohexagonal transformation might affect in first place surfaceexchange kinetics and, thus, may result in certain ambiguities inthe comparative analysis of the oxygen permeation fluxesthrough various types of BSCF membranes at temperatures below1123 K.

Finally, Fig. 11 compiles selected literature data on the oxygenpermeation through sulphur-containing and sulphur-free BSCF mem-branes, having hollow fibre or capillary geometry [11,12,16,19,20,39].Expectedly, in similar conditions sulphur-free membranes showhigher performance than sulphur-containing ones; for the latter acertain discrepancy in the literature data can most probably beascribed to the different microstructures, possibly with emphasis on

the surface area, available for the oxygen exchange. Although absenceof porosity in hollow fibres is beneficial in terms of their mechanicalstrength, macrovoid-free membranes show much lower oxygenpermeation fluxes [16], than one may predict from the difference inthickness of membranes (see [11,12,39]) even when only consideringthe bulk limitations of the oxygen transport. The crucial role of thesurface oxygen exchange in sulphur-containing membranes is welldepicted by the favourable and well-pronounced effect of themacrovoids and surface activation on the oxygen permeation fluxes.One may notice that the surface modification of sulphur-containingmacrovoid-free BSCF capillary with praseodymium oxide brings upthe permeation fluxes even almost to the level of sulphur-freemembranes with comparable dimensions [16]. A similar effect wasobserved in present work for nSact-S membrane, emphasising thepotential for the successful tuning of the oxygen transport mechan-ism in sulphur-containing BSCF membranes, aimed at attaining thehighest permeation fluxes. One promising approach may thus com-bine the fabrication of asymmetric hollow fibre membranes withoriented macrovoids at the internal and external surfaces, modifiedwith a suitable electrocatalyst.

4. Conclusions

Sulphur-free and -containing BSCF flat disk-shaped mem-branes of various thicknesses (0.38–1.03 mm) were preparedusing a route, similar to those established for the fabrication ofhollow fibres by the phase-inversion spinning technique. Pre-sence of sulphur resulted in a decreasing oxygen permeation rate,in accordance with literature data. Relative roles of the bulkambipolar diffusion and surface exchange in overall oxygentransport process were estimated from the thickness dependenceof the oxygen permeation fluxes through sulphur-free and -containing membranes. The bulk ambipolar conductivity at1173–1223 K was found to be essentially unaffected by sulphurpresence. In contrast, the partial blocking of the grain boundariesby BaSO4 at the surface of sulphur-containing BSCF membranes,confirmed by SEM/EDS, had a dramatic influence on the surfaceexchange kinetics, leading to lower permeation rates. The surfacemodification of sulphur-containing membranes with sulphur-freeBSCF powder led to an increase of the permeation fluxes to a levelcharacteristic for sulphur-free membranes, e.g., by a factor of �2at 1173 K. The results suggest that high oxygen permeation fluxesthrough sulphur-containing BSCF hollow fibres may still beattained by implementation of asymmetric concept, including athin dense layer in the middle and well-structured microvoids at

A.A. Yaremchenko et al. / Journal of Membrane Science 428 (2013) 123–130130

the internal and external surfaces, modified with a suitableelectrocatalyst.

Acknowledgements

This work was supported by the by the FCT, Portugal (projectPEst-C/CTM/LA0011/2011 and Ciencia-2008 program) and theGerman Helmholtz Alliance Project ‘‘MEM-BRAIN’’. Experimentalassistance of R. Kemps, M. Schoeters and M. Mertens (VITO) isgratefully acknowledged.

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