Journal of Membrane Science 444 (2013) 276–284

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Journal of Membrane Science

0376-73http://d

n CorrE-m

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

Hydrogen separation in La5.5WO11.25−δ membranes

Sonia Escolástico a, Cecilia Solís a, Tobias Scherb b, Gerhard Schumacher b, Jose M. Serra a,n

a Instituto de Tecnología Química, Universidad Politécnica de Valencia -Consejo Superior de Investigaciones Científicas,Avenida de los Naranjos s/n. 46022 Valencia, Spainb Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Hahn-Meitner-Platz 1, 14109 Berlin, Germany

a r t i c l e i n f o

Article history:Received 10 February 2013Received in revised form22 April 2013Accepted 7 May 2013Available online 23 May 2013

Keywords:Hydrogen membraneProton conductorLanthanum tungstateCO2 stableMixed conductor

88/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.memsci.2013.05.005

esponding author. Tel. +34963879448; fax: +ail address: jmserra@itq.upv.es (J.M. Serra).

a b s t r a c t

Tungstates Ln6WO12 are proton conducting crystalline materials which show significant mixed protonicand electronic conductivity and stability in moist CO2 environments. These materials are promisingcandidates for hydrogen separation membranes at temperatures above 700 1C, especially whenintegrated in intensified processes as catalytic membrane reactors. From this family of compounds, La6−xWOy (0.4oxo0.7) presents the highest protonic conductivity and hydrogen permeability. Thiscontribution presents the preparation, hydrogen permeation and stability study of La5.5WO11.25−δ

membranes. Hydrogen separation properties of La5.5WO11.25−δ were systematically analyzed, i.e. theinfluence of the H2 concentration in feed stream, humidification degree and operating temperature wereinvestigated. The integrity of the La5.5WO11.25−δ after the hydrogen permeation experiments wasconfirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy.The stability of La5.5WO11.25−δ permeation when CO2 containing atmosphere was used as sweep gas wasevaluated. Furthermore, the stability of this compound in CO2-rich and sulfur-containing environmentswas confirmed by XRD.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Separation of high purity hydrogen at high temperatures andlow cost is a key step in several industrial processes. One of theoptions under research to accomplish hydrogen separation ineconomically-viable manner is the use of highly selective anddurable membranes. High-temperature membranes would findapplication in the hydrogen separation in gas streams produced inthe gasification of fossil fuels (coal, natural gas) or biomass, as forIGCC (integrated gasification combined cycle) power plants [1,2]implementing CCS (CO2 capture and storage) schemes. In addition,this kind of membranes could be integrated in catalytic membranereactors for in situ hydrogen removal from reactions such as non-oxidative olefin and aromatics production from natural gas, syngasproduction and hydrocarbon reforming. These applications entailthe membrane operation with gas streams typically at tempera-tures above 500 1C and the presence of high concentrations ofsteam and CO2, non-negligible amounts of sulfur and otherpotential poisoning compounds. Mixed protonic and electronicmembranes appear as possible candidates for these operatingconditions albeit the current state-of-the-art materials suffer fromlow hydrogen flux and low thermo-mechanical and chemicalstability in harsh environments. For this reason, there is a need

ll rights reserved.

34963877809.

to develop thermo-mechanically and chemically stable and effi-cient membranes to overcome these pivotal problems.

Dense mixed protonic and electronic membranes allow produ-cing high-purity hydrogen thanks to the high selectivity associatedwith the proton-transport mechanism. Classic perovskite com-pounds, such as SrCeO3 and BaCeO3 based compositions, are themost studied protonic ceramics. They present very high protonicconductivity but are unstable in presence of CO2 and H2O, [3]decomposing in carbonates upon exposure to those atmospheres.On the other hand, zirconates show high protonic conductivity butrequire very high sintering temperatures. It is difficult to sinterthem below 1700 1C, which gives rise to Ba evaporation and theconsequent decrease of the proton conductivity of the resultingceramic.[4–6] Furthermore, they exhibit high grain boundaryresistance and, as a result, the polycrystalline materials presentmuch lower ionic conductivity than the predicted values byMünch et al. [7] In literature, different approaches have beenproposed to circumvent these two problems (high sinteringtemperature and grain boundary resistance) [8–12]. Furthermore,mixed solid solutions of BaCeO3 and BaZrO3 have been developedin different studies [13,14] in order to achieve a compromisebetween the proton conductivity and the chemical stability.

The mixed protonic and electronic conductivity valuesobserved in tungstates (Ln6WO12) [15,16] together with theirstability under atmospheres containing CO2, H2O and H2S,[17,18] make these materials the most promising candidates asprotonic membranes. This kind of materials presents defect

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284 277

fluorite structure. Specifically, the structure of La5.5WO11.25−δ canbe described as an oxygen deficient fluorite lattice with a doublingin lattice parameter due to cation ordering. W builds up a facecentered cubic (fcc) lattice and nearest W neighbors are linked by ahalf occupied split La2 site, which has an average oxygen coordi-nation of seven. About one W has to be substituted on La2 site tostabilize the La5.5WO11.25−δ phase. The oxygen sublattice for theaverage crystal structure is highly disordered especially around theW-sites. The 24 possible oxygen neighbors are grouped around thecorner of a cube and have an occupancy of 25% which leads tooctahedral oxygen coordination for W. In Fig. 1 the average and thelocal structure for La5.5WO11.25−δ are shown according to Scherbet al. [19].

The present work focuses on the preparation and hydrogenpermeation study of a La5.5WO11.25−δ (LaWO) membrane due totheir interesting properties, i.e., high protonic conductivity andhigh hydration degree [16]. Hydrogen permeation was thoroughlycharacterized under different operating conditions (temperature,hydrogen concentration and humidification degree), as thesematerials exhibit the simultaneous conduction of protons, oxygenvacancies and electronic species [20] depending on the pO2 andpH2O gradients and temperature. Previous concise studies of thepO2 and the hydration degree effect on the different multi-speciesconduction were performed in Nd5.5WO11.25−δ, (Nd5/6La1/6)5.5WO11.25−δ and (La5/6Nd1/6)5.5WO11.25−δ membranes [18,21]. In addi-tion, the use of La5.5WO11.25−δ as electrolyte in proton conductingfuel cell has been recently reported when subjected to a large pO2

gradient below 800 1C [22,23].After hydrogen permeation measurements, the sample integ-

rity was checked by X-ray diffraction (XRD), scanning electron

Fig. 1. (a) Average crystal structure for La5.5WO11.25−δ with split oxygen and splitlanthanum sites in space group Fm3m and (b) local crystal structure in space groupPa3 with regular W octahedra.

microscopy (SEM) and Raman spectroscopy. The stability in work-ing conditions, i.e. during hydrogen permeation using CO2 contain-ing atmospheres as sweep gas, was evaluated and finally, thestability of La5.5WO11.25−δ under CO2 and H2S containing atmo-spheres was checked by X-ray diffraction.

2. Experimental

2.1. Preparation and characterization of La5.5WO11.25−δ

Solid state reaction method was used for the manufacture ofLa5.5WO11.25−δ powder. Stoichiometric proportions of La2O3 andWO3 were weighted, thoroughly mixed and milled in a mortar for10 min. La/W ratio of 5.5 was employed to achieve proper phasepurity [17,18].

Transport properties were studied by standard four-point DCconductivity measurements as a function of temperature for4 different environments: dry H2, H2+H2O (2.5% vol.), dry D2 andD2 +D2O (2.5% vol.), where hydrogen and deuterium are diluted inHe (95%). A constant current was supplied by a programmablecurrent source (Keithley 2601) while the voltage drop through thesample was detected by a multimeter (Keithley 3706). Thesemeasurements were performed using rectangular bar samplessintered in air at 1550 1C for 4 h. Preliminary studies showed thatthe four-point conductivity results were similar to those obtainedby AC methods at a fixed frequency of 12 kHz using an Alpha-AHPFRA Novocontrol Tech.

Membrane preparation was performed using La5.5WO11.25−δ

powder sintered at 900 1C for 10 h. This material was uniaxiallypressed at 30 kN. The diameter of the green sample was 23 mm(applied pressure 72 MPa). Finally the material was sintered at1550 1C for 10 h.

X-ray diffraction (XRD) was used to characterize the phasestructure of the La5.5WO11.25−δ samples (bars and membranes).XRD measurements were recorded in the 2θ range from 20 to 901by a PANalytical CubiX FAST equipment using CuKα1,2 radiation andan X'Celerator detector in Bragg–Brentano geometry. Patterns wereanalyzed using X'Pert Highscore Plus software (PANalytical). Ther-mal expansion coefficient analysis was permormed by high-temperature XRD (HT-XRD), in an Anton Paar XRK-900 reactionchamber attached to the diffractometer, in different dry atmo-spheres (25% O2 in He, 100% He and 20% H2 in He) up to 750 1C.

2.2. Hydrogen permeation

The La5.5WO11.25−δ membrane used for the hydrogen permea-tion measurements was a gastight 900 μm thick disc with adiameter of 15 mm sintered at 1550 1C. Helium leakage test wasused to confirm the gas tightness of the membrane. Both disk sideswere coated by a 20 μm porous screen printed layer of a Pt(Mateck, Germany) in order to improve the catalytic activity ofthe sample.

Permeation measurements were performed on a double cham-ber quartz reactor as it is schematized in Fig. 2. Hydrogen wasseparated from a mixture of H2–He (dry or saturated in water atroom temperature) using argon as sweep gas. The influence ofthree different parameters on the hydrogen separation wasstudied:

a)

Temperature b) Hydration degree: four different configurations were selected as

represented in Fig. 2: (1) dry atmosphere in both sides (feed andpermeate side), (2) feed side humidified (pH2O¼0.025 atm),(3) both membrane sides humidified (pH2O¼0.025 atm)and (4) sweep side humidified (pH2O¼0.025 atm).

He Ar+H2

ArHe+H2

e-

H+

Sweep

Feed

He Ar+H2

ArHe+H2

H2O

H+

e-

H2O

He Ar+H2

ArHe+H2

H2O

H+

e-

O2-

H2O

He Ar+H2

ArHe+H2 H+

e-

O2-

1

2

3

4

Permeation set-up

Fig. 2. On the left hand, permeation set-up scheme. On the right hand, hydrationdegree configurations of permeation measurements: (1) dry atmosphere in bothsides (feed and permeate side), (2) feed side humidified (pH2O¼0.025 atm),(3) both membrane sides humidified and (4) sweep side humidified (pH2O¼0.025 atm). pO2 values at both membranes sides are listed in Table 1.

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284278

c)

pH2: different hydrogen concentrations were selected in thefeed stream.

The gas flow rates used were 100 mL min−1 for feed and150 mL min−1 for sweep in all the conditions and they werecontrolled by using mass flow controllers (MFCs). Feed and sweephumidification was accomplished by saturation of Milli-Q water atroom temperature (25 1C). The total hydrogen permeation rate wascalculated from the hydrogen content measured in the argon side(permeate side) and the argon flow rate, assuming ideal gas law.The permeation fluxes (mL min−1 cm−2) were calculated by divid-ing the permeation rates by the effective surface area of themembranes. The hydrogen content in the permeate side wasanalyzed using micro-GC Varian CP-4900 equipped with Molsie-ve5A, PoraPlot-Q glass capillary, and CP-Sil modules. Qualitativeanalysis of water concentration is done in the PoraPlot-Q channel.Sealing was done using a gold ring and an acceptable sealing wasachieved as the helium concentration was lower than 5% of thepermeated H2. Data reported were acquired at steady state afterthirty minutes of stabilization. Each GC analysis was repeatedthree times to minimize errors, obtaining an experimental stan-dard deviation of 10−4.

In order to check the stability of the membrane in reducingatmospheres, microstructural and phase changes in the membranewere analyzed by measuring the membrane before and after thepermeation measurements using XRD, scanning electron micro-scopy (SEM) and Raman spectroscopy techniques on both surfaces(sweep and feed side) and in the bulk of the membrane. SEManalysis was performed using a JEOL microscope (modelJSM6300). Raman spectra were measured with a Renishaw inViaRaman spectrometer equipped with a Leica DMLMmicroscope anda 514-nm Ar+ ion laser as an excitation source. A 50� objective of8-mm optical length was used to focus the depolarized laser beam

on a spot of 3 μm in diameter. A CCD array detector was used forthe Raman scattering collection.

2.3. Stability against CO2 and other acid atmospheres

In order to evaluate the stability in operation of La5.5WO11.25−δ

membrane under CO2 containing atmospheres, permeation mea-surements were also performed using 15% CO2 in Ar as sweep gas.Measurements were made at 800 1C during 3 days, feeding 50% H2

in helium and humidifying both sides of the membrane(configuration 3).

Additionally, two different chemical stability tests were carriedout in CO2 and acid gases containing atmospheres:

(1)

A treatment under continuous gas flow composed of 115 ppmH2S, 4.43% CO2, 2.12% CO and 92.09% H2 at 500 1C, at 30 barsand for 40 h using powders sintered at 1550 1C.

(2)

A treatment under continuous gas flow composed of1000 ppm COS, 100 ppm HCN, 4% CO2, 46% CO, 46% H2 and4% H2O, at 35 bars and following next thermal treatment: 24 hat 160 1C, 24 h at 200 1C, 24 h at 160 1C and finally, 24 h at400 1C using a piece of a pellet sintered at 1550 1C.

3. Results

3.1. Structure and transport properties

Total conductivity measurements are depicted in Fig. 3a as afunction of inverse temperature in H2, D2, H2+H2O and D2+D2O(where H2 and D2 are diluted in helium and pH2O and pD2O are0.025 atm) atmospheres. Proton transport prevails up to 800 1C ascan be deduced from the hydration effect ðsH2þH2O4sH2 andsD2þD2O4sD2 Þ and the isotopic effect ðsH2þH2O4sD2þD2OÞ. Never-theless, both n-type and oxygen-ion conduction become predo-minant with respect to proton conduction at higher temperatures,where isotopic effect becomes almost negligible and an increase ofthe activation energy is observed. This fact is ascribed to the oxidedehydration at high temperatures under the current operatingconditions as it was previously reported [24–26].

Furthermore, a gradual change in the conductivity activationenergy (EA) is observed at around 650–750 1C in wet atmosphereswhile in dry atmospheres there is a constant apparent activationenergy. This change at high temperatures under wet conditionscan be ascribed to: (i) the aforementioned decrease of the protonconcentration in the oxide and the consequent drop in protonconductivity; and (ii) the more thermally-activated nature of theoxygen-ion and n-type electronic conductivities, which becomepredominant at high temperatures. This is also supported by thefact that EA in wet conditions increases from 0.62 and 0.64 eV atlow temperatures for wet H2 and D2 atmospheres, respectively, to0.73 and 0.76 eV at high temperatures. EA values at high tempera-tures are closer to those obtained in dry atmospheres (0.88 and0.96 eV for dry H2 and D2, respectively), which are principallyrelated to mixed electronic and oxygen-ionic transport.

With the aim to evaluate the stability of La5.5WO11.25−δ inreducing conditions, the conductivity was measured during 15days at 800 1C under H2+H2O (2.5% vol.) Fig. 3b presents theevolution of the total conductivity as function of time. Conductiv-ity remains stable during, at least, 400 h (15 days). Some oscilla-tions in the conductivity values can be detected and they can beattributed to the day/night temperature fluctuations in the labora-tory and their direct effect on the stream water concentration(saturated at room temperature). XRD pattern of La5.5WO11.25−δ barshaped sample can be observed in Fig. 3c and it confirms the“cubic” fluorite structure reported by Scherb et al. [19]. The relative

0.75 1.00 1.25 1.501E-6

1E-5

1E-4

1E-3

0.01

1000/T (K-1)

5% H25% H2+ H2O5% D25% D2+D2O

Con

duct

ivity

(S·c

m-1

)

0 100 200 300 4000.020

0.025

0.030

0.035

0.040

0.045

0.050

Con

duct

ivity

(S·c

m-1

)

t (h)

20 30 40 50 60 70 80 900

5000

10000

15000

20000

25000

30000

Inte

nsity

(a.u

.)

2θ (º)

300 400 500 600 700 800 900 1000 1100

0.000

0.002

0.004

0.006

0.008

(a-a

0)/a

0

HeO2+He

H2+He

T (K)

Fig. 3. Total conductivity as a function of inverse temperature measured under four different reducing atmospheres: dry H2, H2+H2O (2.5% vol.), dry D2 and D2+D2O (2.5%vol.) (a). Total conductivity as function of time under H2+H2O (2.5% vol.) atmosphere at 800 1C during 400 h (b). Room temperature XRD patterns of bars used in theconductivity measurements (c). Relative expansion of the cell parameter calculated from HT-XRD as a function of the temperature in different dry atmospheres: He,O2+He and H2+He (d).

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284 279

cell expansion as a function of the temperature in three differentatmospheres (25% O2 in He, 100% He and 20% H2 in He) obtainedfrom HT-XRD is plotted in Fig. 3d. The slopes correspond to theaverage thermal expansion coefficients and the correspondingvalues are 11.1�10−6 K−1 regardless of the atmosphere.

3.2. Hydrogen permeation

The hydrogen permeation tests presented in this section wereperformed on a unique La5.5WO11.25−δ disk membrane (thickness0.9 mm) and both disk sides were coated by a 20 μm-thick porouscatalytic layer made of Pt in order to improve hydrogen surfaceexchange.

Fig. 4. Hydrogen flux through a 0.9 mm-thick La5.5WO11.25−δ membrane in the fourdifferent hydration degree configurations: (1) dry atmosphere in both sides (feedand permeate side), (2) feed side humidified (pH2O¼0.025 atm), (3) both mem-brane sides humidified and (4) sweep side humidified (pH2O¼0.025 atm). Lines areeye guides. The activation energy for each configuration is: (1) 0.54 eV; (2) 0.58 eV;(3) 0.87 eV; and (4) 0.98 eV.

3.2.1. Influence of environment humidification degreeHydrogen permeation was measured as a function of the

humidification degree in order to study the multispecies transport,i.e. protons, oxygen ions, electrons and holes, depending on themeasurement configuration and on the oxidizing and reducingconditions of the membrane as it was explained elsewhere [27].In this study, four different conditions were selected (see Fig. 2):(1) dry atmosphere in both sides (feed and permeate side), (2) feedside humidified (pH2O¼0.025 atm), (3) both membrane sideshumidified (pH2O¼0.025 atm) and (4) sweep side humidified(pH2O¼0.025 atm). The corresponding hydrogen permeationfluxes as a function of temperature are showed in Fig. 4 for thesefour configurations when feeding with 50% H2 in He.

When hydrogen permeation is measured in the configuration 1,i.e. dry atmosphere in both sides, the maximum value of hydrogenflux is below 0.005 mL min−1 cm−2 due to the low concentration ofproton charge carriers. However, when hydrogen feed side is

750 800 850 900 950 10000.00

0.02

0.04

0.06

0.08

0.10

0.12

J H2 (m

L·m

in-1

·cm

-2)

T (ºC)

JH2 experimental

JH2 total calculated

JH2 permeting calculated

JH2 water splitting calculated

Fig. 5. Experimental hydrogen flux through a 0.9 mm-thick La5.5WO11.25−δ mem-brane (■) as compared with the calculated total hydrogen flux (□), the calculatedhydrogen permeating flux (blue Δ) and the calculated hydrogen flux produced bywater splitting in the sweep side (orange Δ) as a function of the temperature. Linesare eye guides (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.).

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284280

humidified (configuration 2) the hydrogen flux increases up to0.012 mL min−1 cm−2. This effect can be directly ascribed to thehydration of the membrane that allows the formation of twoprotonated oxygen atoms (lattice or interstitial oxygen) [26],which are the essential charge carriers involved in hydrogentransport.

When both sides are humidified (configuration 3) the hydrogenflux increases notably, approximately eight times at 1000 1C withrespect to the preceding conditions (configuration 2). Due to thepO2 and pH2O conditions in this configuration (see Table 1),hydrogen production in the sweep side can occur by two differentprocesses: (1) transport of protons through the membrane fromthe higher pH2 (hydrogen feed) to the lower pH2 side (Ar sweep)(same protonic transport as in configuration 1 and 2 but increasedby the higher hydration of the membrane) and (2) hydrogenproduction by water splitting in the sweep side. Water splittingreaction (hydrogen splits fromwater in the sweep side) occurs dueto the oxygen diffusion from the higher pO2 (Ar sweep-pII

O2) to the

lower pO2 side (hydrogen feed-pIO2) and it is favored by the

predominant oxygen-ion transport at high temperatures (above850 1C).

In configuration 3, the hydrogen permeating flux can becalculated by the Eq. (1), assuming that the hydrogen transportoccurs in accordance with the ambipolar diffusion mechanism [28]

JH2¼ RT

4F2L

ðsHþ þ sO2� Þ⋅ðse� þ shÞsT

lnpH2

ðf eedÞpH2

ðpermÞ ð1Þ

where JH2is the hydrogen flux, R is the ideal gases constant, F is

the Faraday's constant, L is the membrane thickness, si is thepartial conductivity of protons (H+), oxygens (O2), electrons (e−)and holes (h) and sT total conductivity, and pH2

the hydrogenpartial pressure in the feed and permeate (perm) side.

Furthermore, if Eq. (1) is rewritten, it can be observed that thefirst term of Eq. (2) corresponds to the hydrogen permeation andthe second to the hydrogen produced by the water splitting in thepermeate side due to the oxygen permeation from the sweep sideto the feed side

JH2¼ RT

4F2L

ðsHþ ⋅se� ÞsT

lnpH2

ðf eedÞpH2

ðpermÞ þRT

4F2L

ðsO2� ⋅se� ÞsT

lnpH2

ðf eedÞpH2

ðpermÞ ð2Þ

Theoretical fluxes were calculated applying Eq. (2) and makinguse of two datasets: (1) the partial conductivities of La5.5WO11.25−δ

determined by Haugsrud et al. [13,26] in wet reducing atmo-spheres employing the EMF (electromotive force) method [29–31]and (2) the experimental results from this work, i.e., partialpressures of hydrogen, thickness and temperature. Fig. 5 showsthe temperature dependency of the measured hydrogen flux (filledblack squares) as compared with the theoretical total hydrogenflux produced (open black squares), theoretical hydrogen per-meating flux from feed side to sweep side (open blue triangles),and theoretical hydrogen produced by water splitting in the sweepside (open orange triangles).

Table 1Formal oxygen partial pressures for the four hydration degree configurations in hydroge(sweep) and supposing that pH2 (feed)¼0.5 atm, pH2O (wet)¼0.025 atm and pH2O (dr

T (1C) Configuration 1 Configuration 2

pIO2

(atm) pIIO2

(atm) pIO2

(atm) pIIO2

(atm)

1000 1.21E−24 1.31E15 1.89E−18 2.08E−16950 1.96E−25 3.97E−16 3.06E−19 4.72E−17900 2.60E−26 7.05E−17 4.06E−20 8.63E−18850 2.89E−27 7.80E−18 4.52E−21 1.50E−18800 2.62E−28 8.53E−19 4.10E−22 2.42E−19750 1.88E−29 5.76E−20 2.94E−23 3.63E−20

It is worth noting the good agreement between theoretical andthe experimental data (Fig. 5). It can also be pointed out that themain contribution to the total hydrogen production at hightemperatures (from 850 1C up to 1000 1C) is mainly due to thewater splitting in the sweep side. This fact agrees with the highoxygen-ion conductivity in this range of temperatures, and it isrelated to the low proton concentration in the oxide at hightemperatures and/or under relatively low steam partial pressures.

In addition, the transient process produced by a step-changefrom wet to dry sweep gas (change from hydration condition 3 tohydration condition 2) was measured at 1000 1C and by feedingwith 50% H2 (Fig. 6a). In this process, hydrogen flux decreaseswhen the permeate side changes from humidified to dry environ-ment and two phenomena can be distinguished [18]: oxidedehydration and ending of water splitting reaction. Firstly, theflux decreases slowly for 30 min until reaching a value of0.077 mL min−1 cm−2. This first step can be attributed to thedehydration of the membrane, releasing additional water in thesweep side, and the subsequent reduction in the protonic con-ductivity through the membrane. Secondly, it can be observed adrop until reaching stable flux values of 0.0125 mL min−1 cm−2 in30 min. This second step is attributed to the cessation of watersplitting since no more water is available for the reaction.

When conditions were changed from configuration 3 to con-figuration 4, hydrogen flux increases 1.4 times at 1000 1C (Fig. 6b).This hydrogen increase can be related to the increase in thehydrogen production by water splitting in the sweep side due tothe higher pO2 gradient between permeate side (Ar sweep-pII

O2)

n permeation measurements. Calculated taking into account the experimental pH2

y)¼2�10−5 atm.

Configuration 3 Configuration 4

pIO2

(atm) pIIO2

(atm) pIO2

(atm) pIIO2

(atm)

1.89E−18 4.95E−12 1.21E−24 2.53E−123.06E−19 1.48E−12 1.96E−25 1.05E−124.05E−20 3.89E−13 2.60E−26 2.65E−134.52E−21 9.17E−14 2.89E−27 7.33E−144.09E−22 1.95E−14 2.62E−28 1.52E−142.94E−23 3.52E−15 1.88E−29 2.49E−15

0 50 100 150 200 2500.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16Transient from configuration 3 to configuration 2Feed concentration: 50% H21000ºC

J H2(m

L·m

in-1

·cm

-2) J

H2 (m

L·min

-1·cm-2)

t (min)0 200 300

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

t (min)

Transient from configuration 3 to configuration 4Feed concentration: 50% H21000ºC

Fig. 6. Hydrogen flux variation produced by the step-change from configuration 3 to configuration 2, i.e., from wet to dry permeate side (a); and from configuration 3 toconfiguration 4 (b) at 1000 1C using 50% H2 in the feed stream.

0.00

0.05

0.10

0.00

0.05

0.10

0.02

% H25

10204050

Configuration 4

% H25

20

% H25

2050

Configuration 3

J H2 (m

L·m

in-1

·cm

-2)

Configuration 2

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284 281

and feed side (hydrogen feed-pIO2) as it is indicated in Table 1. On

the other hand, the hydrogen flux due to the proton transportshould decrease due to the lower hydration of the membranewhich diminishes the incorporation of protons in the lattice (samecase than in configuration 2). Consequently, the main contributionto the hydrogen permeation at 1000 1C in this configuration is dueto the water splitting in the sweep side due to the diffusion ofoxygen from the higher pO2 (Ar sweep-pII

O2) side to the low pO2

side (hydrogen feed-pIO2). Furthermore, the quick increase of the

hydrogen flux when conditions are changed from configuration3 to configuration 4 (Fig. 6b) provides an evidence of theimprovement of water splitting reaction due to the higher pO2

gradient across the membrane.In general, the values of the apparent activation (EA) derived

from the permeation measurements for each configuration are inagreement with the transport mechanisms suggested above. Forconfigurations 1 and 2 (0.54 and 0.58, respectively), the EA areclose to the values expected when both proton and n-typeconduction are governing the permeation process. However, theEA values related to configuration 3 and 4 (0.87 and 0.98 eV,respectively) are more in line with the values associated tooxygen-ion conduction.

750 800 850 900 950 10000.00

50

T (ºC)

Fig. 7. Hydrogen flux through a 0.9 mm-thick La5.5WO11.25−δ membrane as functionof temperature measured in configurations 2, 3 and 4, feeding different concentra-tions of hydrogen. Lines are eye guides.

3.2.2. Influence of pH2

After the study of the hydration degree effect on the hydrogenflux, the influence of the feed stream concentration was studied inthe different configurations. In the case of configurations 2 and 3,three different feed compositions were selected (pH2¼0.05, 0.2and 0.5 atm) whilst five hydrogen partial pressures (pH2¼0.05, 0.1,0.2, 0.4 and 0.5 atm) were considered in configuration 4. Fig. 7illustrates the temperature dependence of hydrogen permeation atdifferent pH2 for the three configurations. Hydrogen flux increaseswith increasing temperature and pH2 in the feed stream, asexpected from Wagner equation.

The obtained fluxes for the La5.5WO11.25−δ membrane (valuesrange from 0.005 mL min−1 cm−2 up to 0.136 mL min−1 cm−2, at1000 1C depending on the feed composition) are higher than thosepreviously reported for cerates [32,33], zirconates [34] and solidsolutions of cerates and zirconates [35]. For example, a hydrogenflow of 0.03 ml min−1 cm−2 was obtained with a BaZr0.8Y0.15

Mn0.05O3−δ membrane at 1000 1C humidifying both sides of themembrane [34]. Furthermore, tungstates based membranes pre-sent the advantage of lower sintering temperatures and high

stability in CO2 and acid gases atmospheres as it is discussedbelow. On the other hand, the hydrogen fluxes obtained in thiswork are orders of magnitude below those reported for mem-branes based on Pd and other hydrogen permeable metals such asV, Nb, Ta and Ni [36,37]. However, ceramic mixed proton conduct-ing can become competitive in certain application niches, wherePd membranes are not stable, i.e. at temperatures above 500 1Cand in harsh gas environments. Specifically, Pd membranes exhibitphase transformation at high temperatures and are prone to bepoisoned by impurity gases such as H2S and CO [36], which are

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284282

important disadvantages for key industrial applications in theenergy and petrochemistry sector.

3.2.3. Post-mortem membrane analysisAfter hydrogen permeation measurements, the integrity of the

membrane was analyzed by XRD, SEM and Raman spectroscopy.As can be seen in the XRD patterns of the membrane before andafter the permeation measurements (Fig. 8) the membrane doesnot suffer any phase change or the apparition of any secondaryphases, however there are minor changes in the peak shapes,which can be attributed to different effects: slight deviation in themembrane positioning on the sample holder during XRD analysis,variations in the grain size and/or preferred orientation.

In addition, the possible differences between the surfaceexposed to more reducing conditions (feed side), the other surfaceexposed to less reducing conditions (sweep side) and the bulkmaterial were studied by SEM analysis. Fig. 9 shows the SEMmicrographs of both surfaces and the bulk (fracture cross-section

20 30 40 50 60 70 80 900

10000

20000

30000

40000

50000

60000

La5.5WO11.25-δ fresh

Inte

nsity

(a.u

.)

2θ (º)

La5.5WO11.25-δ membrane post treatment

Fig. 8. XRD patterns of LWO membrane before and after the permeationmeasurements.

20 μm

20 μm

20 μm

Fresh sample Post permeationmeasurement sam

20 μm

20 μm

20 μm

Fig. 9. SEM micrographs of fracture cross-section corresponding to a fresh

analysis) of a fresh and a tested La5.5WO11.25−δ membrane. It can beascertained that no difference or apparent degradation wasobserved after the measurements regardless the analysis locationalong the membrane thickness.

The absence of structural changes in the membrane is further-more evidenced by Raman spectroscopy. Fig. 10 shows the Ramanspectra of La5.5WO11.25−δ membrane as sintered in air and afterhydrogen permeation measurements in the bulk material (fracturecross-section analysis) and on both surfaces (feed and sweepsides) of the membrane recorded at room temperature. From thisexperiment, no formation of secondary phases during the hydro-gen permeation can be perceived, as exclusively the originalstructural peaks appear in the Raman spectra. Specifically, theRaman peak at 800 cm−1 corresponds to the ν(O–W–O) stretchingmode and the peak between 350 and 420 cm−1 is due to the mostintense band of La–O [38]. Moreover, no peak shift or broadeningcan be detected, which evidences the stability of the crystallinestructure despite the thermal treatment or gas environment.

3.3. Stability against CO2 and acid gases containing atmospheres

In order to evaluate the stability of La5.5WO11.25−δ membraneunder CO2 atmospheres, hydrogen permeation measurementswere performed using 15% CO2 in Ar as sweep gas. The measure-ments were carried out at 800 1C during 3 days using 50% H2 inhelium as feed gas and both sides of the membrane werehumidified (configuration 3). Fig. 11 shows the hydrogen flux asa function of time under these conditions. The hydrogen permea-tion is stable in CO2 atmospheres as inferred from the constantvalue of the flux (note that the observed variations are due tosmall temperature fluctuations in the reactor). However, the valueof the hydrogen flux is lower than that obtained when using pureAr as sweep gas (Fig. 4), which can be attributed to the CO2/O2

competitive adsorption on the membrane surface. Moreover, theoriginal H2 flux obtained without CO2 in the sweep gas was fullyrecovered when the membrane side was swept again usingpure Ar.

Sweep side

Feed side

ple

Bulk

He

Ar+H2Ar

He+H2

sample and a spent sample in the bulk, the feed and the sweep sides.

200 400 600 800 1000 1200 1400

Inside the sample

Sweep side

After treatment

I (a.

u.)

Raman Shift (cm-1)

Before treatment

Feed side

Fig. 10. Raman spectra of the La5.5WO11.25−δ membrane as sintered and afterhydrogen permeation measurements.

0 500 1000 1500 2000 2500 3000 3500 40000.00

0.01

0.02

0.03

0.04

0.05

J H2 (m

L·m

in-1

·cm

-2)

72 hours48 hours

t (min)

800 ºCSweep gas = 15% CO2-85% Ar+ H2OFeed stream = 50% H2+ H2O

24 hours

Fig. 11. Hydrogen flux as a function of time using 15% CO2–85% Ar as sweep gas and50% He–50% H2 as feed gas at 800 1C. Both sides of the membrane were humidified.

20 30 40 50 60 70 80 902θ (º)

Fresh sample

Treatment 1

Inte

nsity

(a.u

.)

Treatment 2

Fig. 12. XRD patterns of La5.5WO11.25−δ samples before and after the stabilitytreatments 1 and 2.

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284 283

Additionally, two different stability treatments were performedon La5.5WO11.25−δ: treatment 1 using powders sintered at 1550 1Cand treatment 2 employing a pellet sintered at 1550 1C (moredetails are provided in the experimental section).

Fig. 12 represents the XRD patterns of the La5.5WO11.25−δ

samples before and after these treatments. Up to the limit of theXRD analysis, neither phase changes nor formation of carbonates,sulfides or sulfates could be detected.

Despite the relatively low temperatures used in these stabilitytests against CO2 and acid gases containing atmospheres, from thevery high pressure used (30 and 35 bars) and the fact consideringthat (i) sulfides and carbonates are stable at lower temperaturesand (ii) the treatment time was sufficient for the sulphidation/carbonation kinetics, it is possible to infer that the material ischemically stable and could be applied in catalytic membranereactor in a range of industrial applications.

4. Conclusions

La5.5WO12−δ was prepared by solid state reaction as single phasematerial. The stability of the conductivity was proved at 800 1Cduring 400 h. The influence of the feed stream concentration,

humidification degree and temperature on the hydrogen separationflux was systematically studied. H2 flux values for La5.5WO11.25−δ at1000 1C range from 0.005 mLmin−1 cm−2 up to 0.136 mL min−1 cm−2,depending on the feed composition, i.e., H2% and H2O%. The presentexperimental results are in good agreement with the expected valuescalculated considering the partial conductivities of the differentinvolved charge carriers (O−2, H+ and e−). These calculations, togetherwith the thorough experimental study of the humidification, evi-dence the important contribution of oxygen-ion transport to theoverall hydrogen separation rate at temperatures above 800 1C andfor the relatively low steam partial pressures (pH2O) employed here.Therefore, La5.5WO11.25−δ behaves as a good and very mixed (n-type)electronic - oxygen-ion conductor at temperatures above 850 1C andat low pH2O. Note that nowadays there are no reports on mixedelectronic-oxygen ion conductors, which present high stability inreducing and CO2-rich and H2S-containing atmospheres. Therefore,La5.5WO11.25−δ could additionally find application in industrial pro-cesses where (atomic) oxygen separation is required but the opera-tion conditions imply severe reducing atmospheres, e.g. processescoupling two reactions in each side of the membrane i.e., watersplitting and methane reforming or coupling hydrocarbon hydro-genation/dehydrogenation reactions.

The integrity of the sample after the hydrogen permeationmeasurements, even in CO2-rich environments, was confirmed bySEM and Raman spectroscopy analysis of treated membranes.Hydrogen permeation was also measured using 15% CO2 in Ar assweep gas and the hydrogen flux remained stable. Furthermore,chemical stability against CO2, H2S, CO, HCN, COS and H2 contain-ing atmospheres was evaluated and materials remained stable asconcluded by XRD.

Acknowledgments

Financial support by the Spanish Government (Grants JAE-Pre08-0058, ENE2011-24761, CSD-2009-0050 and SEV-2012-0267)and the Helmholtz Association of German Research Centersthrough the portfolio topic MEM-BRAIN is kindly acknowledged.The authors are indebted to S. Jiménez for sample preparation.

References

[1] K. Jordal, R. Bredesen, H.M. Kvamsdal, O. Bolland, Integration of H2-separatingmembrane technology in gas turbine processes for CO2 capture, Energy 29(2004) 1269.

S. Escolástico et al. / Journal of Membrane Science 444 (2013) 276–284284

[2] M.L. Fontaine, Y. Larring, T. Norby, T. Grande, R. Bredesen, Dense ceramicmembranes based on ion conducting oxides, Ann. Chimie. Sci. Materiaux 32(2007) 197.

[3] K.H. Ryu, S.M. Haile, Chemical stability and proton conductivity of dopedBaCeO3–BaZrO3 solid solutions, Solid State Ionics 125 (1999) 355.

[4] Y. Yamazaki, C. Yang, S.M. Haile, Unraveling the defect chemistry and protonuptake of yttrium-doped barium zirconate, Scr. Mater. 65 (2011) 102.

[5] K.D. Kreuer, W. St. Adams, A. Münch, U. Fuchs, J. Klock, Maier, Protonconducting alkaline earth zirconates and titanates for high drain electroche-mical applications, Solid State Ionics 145 (2001) 295.

[6] S.M. Haile, G. Staneff, K.H. Ryu, Non-stoichiometry, grain boundary transportand chemical stability of proton conducting perovskites, J. Mater. Sci. 36(2001) 1149.

[7] W. Münch, K.D. Kreuer, G. Seifert, J. Maier, Proton diffusion in perovskites:comparison between BaCeO3, BaZrO3, SrTiO3, and CaTiO3 using quantummolecular dynamics, Solid State Ionics 136 (2000) 183.

[8] P. Babilo, S.M. Haile, Enhanced sintering of yttrium-doped barium zirconate byaddition of ZnO, J. Am. Ceram. Soc. 88 (2005) 2362.

[9] J.M. Serra, W.A. Meulenberg, Thin-film proton BaZr0.85Y0.15O3 conductingelectrolytes: toward an intermediate-temperature solid oxide fuel cell alter-native, J. Am. Ceram. Soc. 90 (2007) 2082.

[10] J.M. Serra, O. Büchler, W.A. Meulenberg, H.P. Buchkremer, Thin BaCe0.8Gd0.2O3

−δ protonic electrolytes on porous Ce0.8Gd0.2O1.9–Ni substrates, J. Electrochem.Soc. 154 (2007) B334.

[11] Z. Sun, E. Fabbri, L. Bi, E. Traversa., Lowering grain boundary resistance of BaZr0.8Y 0.2O 3−δ with LiNO 3 sintering-aid improves proton conductivity for fuelcell operation, Phys. Chem. Chem. Phys. 13 (2011) 7692.

[12] W.G. Coors, A. Manerbino, Characterization of composite cermet with 68 wt.%NiO and BaCe0.2Zr0.6Y0.2O3−δ, J. Memb. Sci. 376 (2011) 50.

[13] S. Ricote, G. Caboche, O. Heintz, Synthesis and proton incorporation in BaCe0.9−xZrxY0.1O3−δ, J. Appl. Electrochem. 39 (2009) 553.

[14] J.H. Tong, D. Clark, L. Bernau, A. Subramaniyan, R. O’Hayre, Solid State Ionics181 (2010) 1486.

[15] T. Shimura, S. Fujimoto, H. Iwahara, Proton conduction in non-perovskite-typeoxides at elevated temperatures, Solid State Ionics 143 (2001) 117.

[16] R. Haugsrud, Defects and transport properties in Ln6WO12 (Ln¼La, Nd, Gd, Er),Solid State Ionics 178 (2007) 555.

[17] S. Escolástico, V.B. Vert, J.M. Serra, Preparation and characterization ofnanocrystalline mixed proton-electronic conducting materials based on thesystem Ln6WO12, Chem. Mater 21 (2009) 3079.

[18] S. Escolástico, C. Solís, J.M. Serra, Hydrogen separation and stability study ofceramic membranes based on the system Nd5LnWO12, Int. J. Hydrogen Energy36 (2011) 11946.

[19] T. Scherb, S.A.J. Kimber, C. Stephan, G. Schumacher, S. Escolastico, J M. Serra, P.F. Henry, A. H. Hill, J. Seeger, J. Banhart, 2013. Submitted for publication.

[20] M. Sanders, R. O´Hayre, Coupled transport and uphill permeation of steam andoxygen in a dense ceramic membrane, J. Memb. Sci. 376 (2011) 96.

[21] S. Escolástico, C. Solís, J.M. Serra, Study of hydrogen permeation in (La5/6Nd1/

6)5.5WO12−δ membranes, Solid State Ionics 216 (2012) 31.

[22] C. Solís, L. Navarrete, S. Roitsch, J.M. Serra, Electrochemical properties ofcomposite fuel cell cathodes for La 5.5WO 12−δ proton conducting electrolytes,J. Mater Chem. 22 (2012) 16051.

[23] C. Solís, V.B. Vert, M. Balaguer, S. Escolástico, S. Roitsch, J.M. Serra, Mixedproton–electron conducting chromite electrocatalysts as anode materials forLWO-based solid oxide fuel cells, Chem. Sus Chem. 5 (2012) 2155.

[24] C. Solís, S. Escolástico, R. Haugsrud, J.M. Serra, La5.5WO12−δ characterization oftransport properties under oxidizing conditions: A conductivity relaxationstudy, J. Phys. Chem. C 115 (2011) 11124.

[25] R. Hancke, Z. Li, R. Haugsrud, Thermogravimetric relaxation study of theproton conductor lanthanum tungstate, La28−xW4+xO54+δv2−δ, x¼0.85, Int. J.Hydrogen Energy 37 (2012) 8043.

[26] R. Haugsrud, C. Kjølseth, Effects of protons and acceptor substitution on theelectrical conductivity of La6WO12, J. Phys. Chem. Solids 69 (2008) 1758.

[27] M. Sanders, R.P. O´Hayre, Development of a multi-species transport spacetheory and its application to permeation behavior in proton conducting dopedPerovskites, J. Mater. Chem. 20 (2010) 6271.

[28] H. Matsumoto, H.T. Shimura, H. Iwahara, T. Higuchi, K. Yashiro, A. Kaimai,T. Kawada, J. Mizusaki, Hydrogen separation using proton-conducting per-ovskites, J. Alloys Compd. 408 (2006) 456.

[29] T. Norby, EMF method determination of conductivity contributions fromprotons and other foreign ions in oxides, Solid State Ionics 28 (1988) 1586.

[30] D.P. Sutija, T. Norby, P. Björnbon, Transport number determination by theconcentration-cell/open-circuit voltage method for oxides with mixed elec-tronic, ionic and protonic conductivity, Solid State Ionics 77 (1995) 167.

[31] H.K. Bentzer, N. Bonanos, J.W. Phair, EMF measurements on mixed protonic/electronic conductors for hydrogen membrane applications, Solid State Ionics181 (2010) 249.

[32] X. Wei, J. Kniep, Y.S. Lin, Hydrogen permeation through terbium dopedstrontium cerate membranes enabled by presence of reducing gas in thedownstream, J. Membr. Sci. 345 (2009) 201.

[33] S.J. Song, E.D. Wachsman, J. Rhodes, S.E. Dorris, U. Balachandran, Hydrogenpermeability of SrCe1−xMxO3−δ (x¼0.05, M¼Eu, Sm), Solid State Ionics 167(2004) 99.

[34] S. Escolástico, M. Ivanova, C. Solís, S. Roitsch, W.A. Meulenberg, J.M. Serra,Improvement of transport properties and hydrogen permeation of chemicallystable proton-conducting oxides based on the system BaZr1−x−yYxMyO3−δ, RSCAdv. 2 (2012) 4932.

[35] J. Li, H. Yoon, E.D. Wachsman, Hydrogen permeation through thin supportedSrCe0.7Zr0.2Eu0.1O3−δ membranes; dependence of flux on defect equilibria andoperating conditions, J. Membr Sci. 381 (2011) 126.

[36] G.Q. Lu, J.C. Diniz da Costa, M. Duke, S. Giessler, R. Socolow, R.H. Williams,T. Kreutz, Inorganic membranes for hydrogen production and purification: Acritical review and perspective, J. Colloid Interface Sci. 314 (2007) 589.

[37] M.V. Mundschau, X. Xie, C.R. Evenson, A.F. Sammells, Dense inorganicmembranes for production of hydrogen from methane and coal with carbondioxide sequestration, Catal. Today 118 (2006) 12–23.

[38] N. Dilawar, S. Mehrotra, D. Varandani, B.V. Kumaraswamy, S.K. Haldar, ARaman spectroscopic study of C-type rare earth sesquioxides, Mat. Charact. 59(2008) 462.