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Available online at www.sciencedirect.com

Journal of Membrane Science 307 (2008) 21–27

Synthesis and characterization of P2O5–ZrO2–SiO2 membranes dopedwith tungstophosphoric acid (PWA) for applications in PEMFC

J. Mosa a, G. Larramona b, A. Duran a, M. Aparicio a,∗a Instituto de Ceramica y Vidrio (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain

b IMRA EUROPE, 220 rue Albert Caquot, BP 213, 06904 Sophia-Antipolis Cedex, France

Received 6 February 2007; received in revised form 15 June 2007; accepted 16 June 2007Available online 22 June 2007

bstract

Homogeneous, transparent and crack-free P2O5–ZrO2 and P2O5–ZrO2–SiO2 membranes have been synthesized by the sol–gel process. A firsttep has been oriented to the optimization of the synthesis and characterization of different compositions by TGA, FE-SEM, FTIR and EIS tohoose the best inorganic composition in terms of chemical and mechanical stability, and proton conductivity. The addition of SiO2 improveshe mechanical and chemical stability. On the other hand, compositions with higher content in P2O5 have demonstrated lower mechanical andhemical stability against water, but higher proton conductivity. The water retention and high porosity of inorganic membranes leads to high protononductivity, 10−2 S/cm, at 140 ◦C and 100% relative humidity. The second step has been focused in the study of doped inorganic membranes ofolar composition 99.65(40P2O5–20ZrO2–40SiO2)–0.35PWA. The high homogeneity, transparency and SEM-EDX analysis of these membranes

ndicates no phase separation suggesting that PWA is well dispersed in the inorganic structure. The incorporation of PWA in sol–gel oxides providesn increase of the proton conductivity at low relative humidity due to the adequate distribution of PWA in the inorganic network. Conductivityncreases in two orders of magnitude at low humidity (10−4 S/cm at 50 ◦C and 62% RH) compared with undoped sol–gel oxide membranes.

2007 Elsevier B.V. All rights reserved.

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eywords: PEMFC; Sol–gel; Proton conductivity; PWA; Inorganic membrane

. Introduction

Proton exchange membrane fuel cells (PEMFC) are exten-ively studied as one of the new energy conversion devices1,2]. The currently developed PEMFC technology is basedpon perfluorosulfonic acid membranes as electrolytes, such asafion®, which exhibits many attractive properties (high proton

onductivity, good mechanical and chemical stability). Nafion®

hows some shortcomings involving high cost, high methanolermeability and low conductivity under elevated temperaturend low humidity conditions due to the loss of water [3–6]. Tomprove energy conversion efficiency and to avoid CO poison-ng of catalysts, PEMFCs operating in the medium temperature

ange (100–200 ◦C) are strongly desired. Many efforts haveeen done to produce adequate proton-conducting membranes.n particular, inorganic membranes have been developed, such

∗ Corresponding author. Tel.: +34 91 735 58 40; fax: +34 91 735 58 43.E-mail address: maparicio@icv.csic.es (M. Aparicio).

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s gels derived of P2O5–ZrO2 and SiO2–P2O5–ZrO2 systems7–15].

The sol–gel method is an excellent process to produce proton-onducting materials because the gels contain a large number oficropores filled with liquid which can be used for fast proton

ransport. The sol–gel chemistry of phosphosilicates has beenxtensively studied, showing that the choice of the phosphorusolecular precursors plays a fundamental role in the character-

stics of the final product, such as the extent of copolymerizationetween silicate and phosphate units. Phosphate esters reactore slowly with water at ambient conditions in comparisonith silicon alkoxides [16], whereas zirconium alkoxides reacts

oo fast and it is necessary their complexation to decrease hydrol-sis rate [17].

Silicophosphate gels have shown to be fast proton-conductingolids. The mobility of protons increases when these are strongly

ydrogen-bonded. Compared with Si–OH, phosphate glassesre more efficient for high proton conduction because theydrogen ions are more strongly bounded to the non-bridgingxygen. Moreover, the hydrogen in the P–OH group is more

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ydrogen-bonded with molecules, resulting in an increase inhe temperature necessary to remove the water from P–OH. Athe same time, the silicate network improves the mechanicaltrength and chemical durability while a high content in phos-horous decreases the chemical stability. The introduction ofr4+ into silicophosphate gels also results in improved chemicaltability [12]. Phosphotungstic acid, H3(P(W3O10)4) (PWA) isn interesting compound to be incorporated as dopant in sol–gelembranes to provide high proton conductivity [18–25]. These

eteropoliacids can be used as inorganic modifiers because theyave demonstrated to be highly conductive at low relative humid-ty and are thermally stable. The basic structural unit of PWAs the Keggin anion (PW12O40)3− which consists of a centralO4 tetrahedron surrounded by four W3O13 sets linked together

hrough oxygen atoms. They form channels that contain up to 29ater molecules in different hydrate phases. This variety leads

o different stable proton species (up to 180 ◦C) and hydrogenonds of different strength; the result is high proton conduc-ivity at room temperature. Hydrated PWA is known to presentalues at around 0.18 S/cm [19]. In spite of the promising ini-ial results obtained using PWA in PEMFC membranes, the celleakes some dissolved acid and performance decay. To overcomehis problem, it would be interesting to block the PWA in the

embrane by chemical bonding. The polyanion PWA clustersan be stabilized within the membrane through a strong coulom-ic interaction with an inorganic framework. If the assembledWA is nanosegregated, the strongly acidic phase may originaten ionic conduction path with continuous channels [18,19].

In this work, we have prepared membranes with differentompositions in the systems P2O5–ZrO2 and P2O5–ZrO2–SiO2sing a sol–gel technique. SiO2 was incorporated to the binaryystem looking for an improvement of the chemical stabilityf the materials. After a screening study, one composition waselected taking into account both, chemical stability and pro-on conductivity to be doped with PWA. The objective was toncrease the proton conductivity of membranes, especially atigh temperature and lower relative humidity, compared withndoped sol–gel oxides.

. Experimental

Seven compositions have been studied in two different sys-ems: P2O5–ZrO2–SiO2 and P2O5–ZrO2. Table 1 shows the

olar ratio of each composition. Membranes were producedy sol–gel using trimethyl phosphate [PO(OCH3)3, TMP] from

able 1olar ratios of compositions in systems P2O5–ZrO2 and P2O5–ZrO2–SiO2

omposition %P2O2 %ZrO2 %SiO2

50 50 –60 40 –80 20 –25 25 5035 35 3040 20 4060 30 10

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e Science 307 (2008) 21–27

erck and zirconium tetrapropoxide [Zr(OC3H7)4, TPZr] fromluka for binary compositions, and also tetraethylorthosilicateSi(OC2H5)4, TEOS] from ABCR in the case of ternary com-ositions. Sols were prepared by mixing two solutions. Therst one was prepared at room temperature by mixing TMP,EOS (for ternary compositions), half the volume of propanol

solvent) (molar ratio of propanol/TEOS + TMP + TPZr = 2) andater as HCl 0.1N (molar ratio of water/TMP = 0.75 andater/TEOS = 1), stirring for 1 h. The second solution wasrepared by mixing TPZr, the other half of propanol and acety-acetone (molar ratio of acetylacetone/TPZr = 1) stirring at roomemperature for 1 h. Both solutions were subsequently mixedogether for 10 min. The remaining amount of water as HCl.1N (total molar ratio of water/TEOS + TMP + TPZr = 3) wasdded drop by drop. The resulting homogenous and transpar-nt solutions were stirred for 30 min. In the case of membranesoped with PWA from Fluka (99.65 oxides–0.35 PWA in molaratio), the heteropolyacid was dissolved in the sol in the last stepf the synthesis, after adding acidulated water.

Sols were cast in polytetrafluoroethylen (PTFE) and glassoulds, and left at room temperature and 50 ◦C in order to obtain

omogeneous gels. The membranes were additionally treated at50 ◦C in air for 2 h with a heating rate of 0.1 ◦C/min.

Thermal behavior of the membranes crushed to powder andreviously dried at room temperature, 50 and 150 ◦C was stud-ed by thermogravimetric analysis (TGA) in air up to 900 ◦Cith a heating rate of 10 ◦C/min using a Netzsch STA 409.he fracture surfaces of the membranes were analyzed with aITACHI S-4700 field emission scanning electron microscope

FE-SEM), and analysis by EDX (NORAN system six) was usedo study the distribution of elements across the film thickness.ourier transform infrared spectroscopy (FTIR Spectrum 100,erkin-Elmer) was used to study the influence of incorpora-

ion of PWA in the oxide structure. FTIR spectra were recordedetween 400 and 4000 cm−1 with a resolution of 2 cm−1. Theurface area was measured by the single-point N2 adsorptionethod using a Monosorb Surface Area Analyser MS-13 (Quan-

achrome Co., USA). Proton conductivity was measured as aunction of both relative humidity (RH) and temperature by elec-rochemical impedance spectroscopy (EIS) with a Gamry FAS2rom 1 Hz to 1 MHz. Two parallel silver electrodes were paintedn both sides of the transparent films. The measurements wereade at lab conditions (25 ◦C, 35–40% RH), and allowing sam-

les to equilibrate at different RH for 24 h inside a sealed cellontaining saturated solutions of Ca(NO3)2 (24–100%), NaNO362–100%) and KNO3 (90–100%). Temperature and relativeumidity have been measured using a Rotronic HYGROCLIPK 25.

. Results and discussion

Transparent and yellowish glass-like gels were obtained in allhe cases after drying at 50 ◦C and further treatment at 150 ◦C.

or ternary system, gelling time is much shorter that in binaryystems because the incorporation of SiO2 (glass former) makeshe network development easier. Gelling time changed from 5o 1 day for silica contents of 10 and 50%, respectively.

J. Mosa et al. / Journal of Membrane Science 307 (2008) 21–27 23

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ig. 1. Photograph of 40P2O5–20ZrO2–40SiO2 samples prepared with PFTEoulds and treated at 150 ◦C.

Quite big non-cracked samples with thickness around00 �m were obtained after treating at 50 and 150 ◦C. Theddition of a small amount of silica to the binary system isighly effective for obtaining crack-free transparent gels of big-er size in shorter time. The additional treatment at 150 ◦Cnly produces a change in color from yellow to orange, due toxidation of organic compounds, without loss of transparency.ig. 1 shows an example of a membrane with the composition0P2O5–20ZrO2–40SiO2 treated at 150 ◦C.

The thermal behavior of the different binary and ternary

ompositions is similar. Fig. 2 shows TGA curves (weighthange and derivate curves) of samples with composition 535P2O5–35ZrO2–30SiO2) treated previously at different tem-eratures. The curves can be divided into four regions. The first,

ig. 2. Thermal gravimetric analysis (weight change and derivate) of samplesith composition 35P2O5–35ZrO2–30SiO2 treated at different temperatures.

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ig. 3. Nyquist plots of the membrane 40P2O5–20ZrO2–40SiO2 at differentemperatures and a relative humidity between 35 and 40%.

etween room temperature and 200 ◦C, with two maximum ratesf weight loss (derivative curve) mainly attributed to desorptionf physically adsorbed water and residual solvents from the solreparation. Other region, between 200 and 350 ◦C, is assignedo the elimination of water and alcohol from condensation reac-ions. A third region, between 350 and 500 ◦C, is associatedith decomposition of organic residues from unhydrolyzed lig-

nds bound to metals and combustion of acetylacetone. Finally,bove 500 ◦C no changes in weight are observed. The treatmentt 150 ◦C before TGA test produces a decrease of weight losst low temperature due to the previous elimination of water andlcohol. However, the sample treated at 150 ◦C has demonstratedhe capacity to absorb water very quickly reflected in the 7% ofeight loss up to 150 ◦C.Fig. 3 presents the Nyquist plots of the membrane

0P2O5–20ZrO2–40SiO2 at different temperatures and a rela-ive humidity between 35 and 40%. Diagrams show semicirclescapacitance of the membrane in parallel with its pore resis-ance) at high frequencies corresponding to the bulk electricalroperties, and a line at around 45◦ in the low frequencies regionue to the diffusion process. The increase of temperature orig-nates a slight reduction of relative humidity, and consequentlybigger semicircle associated with a lower conductivity. Pro-

on conductivity can be estimated on around 10−7 S/cm in allamples in these conditions. Hydroxyl groups and water reten-ion capacity of sol–gel oxides are not enough to create adequateaths for proton conduction when the water content is so low. Allamples measured at higher RH (above 60%) present analogousyquist diagrams Z′–Z′′. As an example, Fig. 4 presents Nyquistlots of the membrane 50P2O5–50ZrO2 at different tempera-ures and relative humidity. The increase of temperature andelative humidity leads to smaller semicircles (lower resistance)ndicating that proton conductivity in these membranes follows ahermal and water humidity stimulated process. A higher relativeumidity originates an increase of the amount of water adsorbed

n hydroxyl groups attached to inorganic network, enhancing theransport of protons through the material. On the other hand, therrhenius equation predicts an increase of proton conductivity

24 J. Mosa et al. / Journal of Membran

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4Tw1apresidual solvents. The second region, between 100 and 220 ◦C,presents a slight weight loss (around 4% to PWA and 6% to

ig. 4. Nyquist plots of the membrane 80P2O5–20ZrO2 at different temperaturesnd relative humidity.

ith temperature, without taking in account others parameterss dehydration of the membrane. Fig. 5 illustrates conductivityehavior of membranes with different compositions as func-ion of temperature and relative humidity. Proton conductivityncreases with both temperature and relative humidity up to0−2 S/cm at 140 ◦C and 100% RH. These results are very inter-sting taking into account the absence of groups as sulfonate inafion® membranes. In this case, proton conductivity is onlyased on hydroxyl groups and a microporous structure withigh water retention capacity. At RH below 90% proton con-uctivity seems to be dependent on P2O5 content, although SiO2ontent also plays an important role compared with ZrO2. Theigher the P2O5 content, the higher is the proton conductiv-ty. Compared with Si–OH and Zr–OH, phosphate glasses areore efficient for high proton conduction because the hydro-

en in the P–OH group is more strongly hydrogen-bonded withater molecules, resulting in an increase in the temperatureecessary to remove the water from P–OH. The explanations based on the electronegativity of the elements involved thatecreases in the order P > Si > Zr. At RH above 90%, the amount

f available water molecules is very high and the influence ofomposition is weaker, showing similar values of conductivity.ompositions in ternary systems demonstrated higher stabil-

ig. 5. Conductivity values as a function of temperature in different rangef relative humidity of different membranes. Square points show values of0P2O5–50ZrO2, triangle points for 80P2O5–20ZrO2 and circle points for0P2O5–20ZrO2–40SiO2.

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e Science 307 (2008) 21–27

ty, since the addition of SiO2 improves the mechanical andhemical properties, related to higher density of bonds and atiffer network. On the other hand, compositions with higherontent of P2O5 showed lower mechanical and chemical stabilitygainst water, but higher proton conductivity. The best compo-ition corresponds to 40P2O5–20ZrO2–40SiO2 that presents aood combination of water uptake and water retention proper-ies leading to high proton conductivity and enough mechanicalnd chemical stability. However, the conductivity is highlyependent on relative humidity, as is shown in Fig. 5. Thus,possible way to improve the proton conductivity at low rela-

ive humidity is to dope the membrane with PWA, that provideew points for proton conduction via proton transfer reactions inhe same way as water, but in absence of significant quantities ofater.Inorganic membranes of composition 40P2O5–20ZrO2–

0SiO2 doped with PWA, 99.65(40P2O5–20ZrO2–40SiO2)–.35PWA, are homogeneous, transparent, crack-free and yellow-rown colored after treatment at 150 ◦C. The high homogeneitynd transparency of these membranes indicates no phase separa-ion suggesting PWA is well dispersed in the inorganic structure.ig. 6 shows a FE-SEM photograph of a fracture surface of a9.65(40P2O5–20ZrO2–40SiO2)–0.35PWA membrane treatedt 150 ◦C. The figure shows a homogeneous material formedy nanosized agglomerates (∼50 nm) without phase separation,ith low size porosity (∼10 nm), very important to retention ofater at high temperature. EDX analyses present a homogeneousistribution of the elements across the membrane.

TGA curves of pure PWA and 99.65(40P2O5–20ZrO2–0SiO2)–0.35PWA inorganic membrane are shown in Fig. 7.hermogravimetric curves present four regions with differenteight loss. A first region, between room temperature and00 ◦C, shows a weight loss (around 8% for pure PWA [18]nd ∼3% for inorganic membranes) attributed to desorption ofhysically absorbed water from PWA and sol–gel oxides, and

norganic membrane) assigned to the release of more structural

ig. 6. FE-SEM photograph of a fracture surface of 99.65(40P2O5–20ZrO2–40iO2)–0.35PWA membrane.

J. Mosa et al. / Journal of Membrane Science 307 (2008) 21–27 25

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Fig. 9. Conductivity values as a function of temperature of99.65(40P2O5–20ZrO2–40SiO2)–0.35PWA membranes in different range ofrelative humidity. Square points represent the values for Ca(NO3)2 salt indifferent range of relative humidity: 24–76% (�); 76–99% (�); 100% ( ).Triangle points show values for NaNO salt in different range of relativehp8

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ig. 7. TGA curves of pure PWA (solid line) and a 99.65(40P2O5–20ZrO2–40iO2)–0.35PWA membrane (dash line).

ater from PWA, mainly lower PWA hydrates containing sixater molecules [18,19,23], and water generated during sol–gel

eactions. In the third region, in the range 220–400 ◦C, the TGAurve of the inorganic membrane shows an important weightoss (∼14%) attributed to an oxidative decomposition of organicesidues from unhydrolyzed ligands bound to metals, and com-ustion of acetylacetone, and a loss of phosphorous due to thetructure collapse of PWA. In the last region, above 400 ◦C,he weight keeps up stable up to 900 ◦C. The small weight lossbserved at low temperatures indicates that the thermal stabil-ty of these inorganic membranes is quite sufficient and will betable enough within temperature range of PEMFC application.

maximum temperature of 140 ◦C can be used with this mem-rane for fuel cell application in order to maintain enough waterolecules for proton conduction and avoid the degradation ofWA clusters [18].

Fig. 8 shows the FTIR spectra of pure PWA,0P2O5–20ZrO2–40SiO2 and 99.65(40P2O5–20ZrO2–40SiO2)0.35PWA membranes. The main bands of the oxide structurere observed in the 1200–900 cm−1 region, where some of theibrations modes of hydrolyzed TEOS molecules (1168 and082 cm−1) overlap those from the siloxane network (1250

−1

nd 1080 cm ) [26]. The spectra of both membranes alsohow the adsorption bands associated to Zr–O–Si bonds at34 cm−1, stretching modes of the (PO4)3− (1160–980 cm−1),–O–P and Si–O–Si symmetric stretching (840 cm−1) and

ig. 8. FTIR spectra of (1) pure PWA, (2) 40P2O5–20ZrO2–40SiO2 membrane,nd (3) 99.65(40P2O5–20ZrO2–40SiO2)–0.35PWA membrane.

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umidity: 62–70% ( ); 70–81% ( ); 81–99% (�) and 100% (�). Rhombusoints represent the values for KNO3 salt in different range of relative humidity:9–99% (♦); 100% (�).

i–OH stretching (960 cm−1) [27]. The comparison of FTIRpectra of pure PWA and doped inorganic membrane indicateshe preservation of Keggin ions geometry inside the doped

embrane. It can be noticed that there is not change in theosition of W = O terminal oxygen (972 cm−1) band. However,he bands assigned to the edge shared octahedral of Kegginnit (W–Ob–W) and to the corner-shared octahedral of Kegginnit (W–Oc–W) at 883 and 754 cm−1 in pure PWA, appeart higher wavenumbers in the doped membrane, 895 and10 cm−1, respectively. These frequency shifts are attributed tohe interaction of oxygens of PWA with hydroxyl groups fromxide structure, indicating an improved stability of PWA insidehe material [20,22,28,29].

The surface area measurements of 99.65(40P2O5–20ZrO2–0SiO2)–0.35PWA membranes present an average value of7 m2/g. Very different values of surface area for phosphosili-ate materials have been reported in literature [24,26,30]. Thesealues ranged from 4 to 600 m2/g, indicating the existence ofifferent structures. In theory, the higher the surface area theigher the proton conductivity is. However, other variables havelso an important influence in the proton-conducting mechanisms the pore size, pore connectivity and pore orientation along theembrane.Fig. 9 presents the behavior of proton conductivity as a

unction of temperature and relative humidity of the inorganicembrane 99.65(40P2O5–20ZrO2–40SiO2)–0.35PWA testedith different saturated salts solutions. The membrane was kept

tabilizing for 24 h in these salts. The membrane showed anncrease of conductivity with temperature and relative humidityn all RH. The most relevant behavior of this membrane con-isted on the increase in almost two orders of magnitude withncreasing relative humidity from 24 to 84%, at low tempera-ures. The amount of absorbed water in the gels increases withn increase in relative humidity. The gel materials prepared by

ol–gel consist of an interconnected porous network, so that con-inuous paths suitable for fast proton conduction are formed inhe gels due to water adsorption. The acidic solid clusters ofWA have three protons in the unit structure, which dissoci-

26 J. Mosa et al. / Journal of Membran

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ig. 10. Conductivity values as a function of temperature and rela-ive humidity of 40P2O5–20ZrO2–40SiO2 (�) and 99.65(40P2O5–20ZrO2–0SiO2)–0.35PWA (�) membranes.

tes as oxionium ions as H3O+ and H5O2+ increasing density

rotons compared to undoped membranes. Moreover, incorpo-ation of PWA in sol–gel oxides provides an increase of theroton conductivity at low relative humidity by the high disper-ion of PWA in the inorganic network [18]. At high humidity, theater molecule density is comparable with undoped inorganicembrane and in both cases it is enough for maintaining a high

onductivity. From a technological point of view, proton conduc-ive solid materials showing acceptable proton conductivities in

low relative humidity are very important for the applicationn PEMFC due to the simplification of the stack because theumidifier systems would not be necessary. The proton conduc-ivity increases with increasing the number of absorbed water

olecules that depends on the pore volume and relative humidity31].

Fig. 10 presents the conductivity as a function of temper-ture and relative humidity of 40P2O5–20ZrO2–40SiO2 and9.65(40P2O5–20ZrO2–40SiO2)–0.35PWA membranes. Theehavior of both membranes is analogous: an increase of con-uctivity with temperature and relative humidity even above00 ◦C due to the water retention capability of sol–gel oxides andWA. Conductivity is higher in membranes doped with PWA,howing a difference up to two orders of magnitude at low tem-erature and relative humidity. The divergence of conductivityecreases with increasing temperatures and relative humidity,eaching similar values of conductivity, around 10−2 S/cm above00 ◦C and 100%RH. The high proton conductivity of dopedembranes at low temperature and RH is attributed to the gel

etwork containing PWA acid ions as proton donors and stronglyydrogen-bonded hydroxyl groups. The mechanism of conduc-ion in these materials is likely promoted by proton jump calledrotthus mechanism, where protons can move at different ratessing different paths and may jump from one to other positionwater molecules or hydroxyl groups) [32]. The raise of tem-erature improves proton conductivity indicating a thermallyctivated process of proton dissociation from the PWA [33].

. Conclusions

Transparent and yellowish glass-like P2O5–ZrO2 membranesere obtained after drying at 50 ◦C and further treatment at50 ◦C. Samples treated at 150 ◦C have demonstrated the capac-

e Science 307 (2008) 21–27

ty to absorb water. For compositions in P2O5–ZrO2–SiO2ystem, conductivity reaches values of 10−2 S/cm at 140 ◦Cnd 100% relative humidity. In this case, conductivity seemso be dependent on the P2O5 content, although the SiO2 con-ent also plays an important role compared with zirconiumxide. A possible way to improve the proton conductiv-ty at low relative humidity is to dope the membrane withWA that provides new points for proton conduction via pro-

on transfer reactions. Inorganic membranes of composition9.65(40P2O5–20ZrO2–40SiO2)–0.35PWA are homogeneous,ransparent and crack-free at 150 ◦C. The high homogeneity,ransparency and EDX analysis of these membranes indicateso phase separation suggesting that PWA is well dispersed in thenorganic structure. The incorporation of PWA in sol–gel oxidesrovides an increase of the proton conductivity at low relativeumidity. The conductivity of this membrane at low tempera-ure rises in almost two orders of magnitude with an increasef relative humidity from 24 to 84% compared with undopedembranes. The divergence of conductivity values between both

ind of membranes decreases with the increasing of temperaturend relative humidity, reaching similar values of conductivityf around 10−2 S/cm. In conclusion, PWA doped membraneshows adequate values of proton conductivities for applicationn PEMFC working at higher temperature and lower relativeumidity compared with Nafion® based fuel cells.

cknowledgements

The authors thank the Spanish Ministry of Education andcience for the concession of a FPI grant (MAT2003-05902-02-01) and a Ramon y Cajal contract. This work has beennanced by the company IMRA SA and the Spanish projectAT2006-4375, and developed in the frame of CSIC-University

uel Cells and Advanced Batteries Network. The authors thankaura Pelaez, Luis Contreras and Socorro Benito their assistanceith the experimental techniques.

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