polymer nanocomposite membranes for dmfc application

Journal of Membrane Science 254 (2005) 139–146

Polymer nanocomposite membranes for DMFC application

C.S. Karthikeyana, S.P. Nunesa, ∗, L.A.S.A. Pradob, M.L. Poncea,H. Silvaa, B. Ruffmanna, K. Schulteb

a GKSS Research Centre, Institute of Chemistry, Max-Planck Strasse 1, D-21502 Geesthacht, Germanyb Polymer Composites, Denickestrasse 15, TU Hamburg-Harburg, D-21703 Hamburg, Germany

Received 26 March 2004; received in revised form 17 September 2004; accepted 13 December 2004Available online 7 April 2005

Abstract

Polymer nanocomposite membranes based on sulphonated poly(ether ether ketone) (SPEEK) containing different weight percentages ofsynthetic non-spherical nanofillers such as laponite and MCM-41 were prepared and characterised for direct methanol fuel cells (DMFC).Prior to the preparation of the composite membranes, they were modified using organo silanes. The results showed that there was a decreasein methanol and water permeability with the increasing content of modified laponite and MCM-41. While the membranes containing higher( owed betterv tropic silican , membranesc ct of formo l and waterp containingl©

K

1

ilAedpmDago

t has. Thessed

eentillh ontive.osite

em-e for, and

siteusingsth

0d

>10) weight percentages of silicates displayed lower proton conductivity values than plain polymer, the lower percentages even shalues than the plain. The results are compared with the membranes containing spherical nanofillers, namely Aerosil and an isoetwork system in order to see the effect of shape of nanofillers on the properties of the composite membranes. Among all shapesontaining silica network had the lowest permeability but they also had poor conductivity values. Much more evident than the effer aspect was the influence of the filler surface modification. In all the cases, organic modification drastically decreased methanoermeabilities. A good agreement between the experiment and theory was found for the permeability reduction for membranes

ower weight percentages of layered silicates assuming aspect ratio of 125 for laponite.2005 Elsevier B.V. All rights reserved.

eywords:Membranes; Fuel cells; Nanocomposites; Ionomers; Silicates

. Introduction

Direct methanol fuel cells (DMFC) have potential usagen portable and automobile applications, as they are smaller,ighter, simpler and cleaner than the conventional batteries.lthough the use of methanol as a direct fuel is convenient byliminating the problems of hydrogen storage, still DMFCso not display the current high performance of hydrogenolymer electrolyte fuel cells. New polymer electrolyteembranes are required to improve the performance ofMFC. The main requirements of a desired membranere low methanol crossover, high proton conductivity, andood mechanical and chemical stabilities during DMFCperation. Nafion®, still dominating the market for fuel cells,

∗ Corresponding author. Tel.: +49 4152 872424; fax: +49 4152 872444.E-mail address:[email protected] (S.P. Nunes).

satisfies most of the above-mentioned requirements, buthe disadvantage of high methanol and water crossoverdetrimental effect of methanol crossover has been discuby different groups[1,2] and many membranes have bdeveloped[3–7]. However, the membranes developednow do not satisfy all the criteria and hence the researcthe development of new membranes for DMFC is very acOf the several approaches, organic–inorganic compmembranes have been developed by many groups[8–10].The research on the development of organic–inorganic mbranes at GKSS has been carried out since a long timgas separation, pervapouration and other applicationshas been intensified for DMFC in the last few years[11–14].The previous works deal with the development of compomembranes based on sulphonated polyether ketoneinorganic networks by sol–gel method[11,12], phosphate[13] and heteropolyacids[14]. The present work deals wi

376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2004.12.048

140 C.S. Karthikeyan et al. / Journal of Membrane Science 254 (2005) 139–146

the development of polymer nanocomposite membranesbased on layered silicates, Aerosils®, and silica networkfor DMFC application. The silica network here is a silicanetwork with organic groups, with repeating unit RSiO3/2,generated by the hydrolysis of organo silanes in the polymersolution.

As to the nature of materials that are used in the presentwork, nanocomposites are particle-filled polymers for whichat least one of the dimensions of the particles is in the rangeof nanometre. These materials are classified into three typesdepending on how many dimensions of the fillers are in thenanometre scale. When all three dimensions are in the order ofnanometres then they are known as isodimensional nanoparti-cles, such as spherical silica particles[15,16]. Nanotubes andwhiskers[17,18]come under second type having two dimen-sions in the nanometres scale and the third being larger form-ing an elongated structure. The third type of nanocompositesis featured by only one dimension in the nanometre range; fewexamples being layered silicates, wherein the filler is presentin the form of sheets of few nanometres thickness and hun-dreds to thousand nanometres long. Polymer nanocompositesbased on layered silicates have gained considerable attention[19–21] for which few reviews have been reported[22,23]on their preparation and properties. These layered silicateshave high aspect ratio and are low priced. They improve me-chanical and thermal properties and decrease gas/vapour per-m rc ller.I ei n toe ereds

atedp n inp , goodm en iti EEKaH be-c h. Ast s aren nce,i tiono rs ino werm ericaln sil-i d fort om-p icatesf ca-t t andt ud-i nceo vity,t men-

sional systems such as spherical silica and in situ generatedsilica networks.

2. Experimental

2.1. Materials

Sulphonated poly(ether ether ketone) with 67% degreeof sulphonation was prepared in our laboratory, as describedelsewhere[13], by functionalising poly(ether ether ketone)supplied by Victrex. Synthetic layered silicates were usedfor the present investigation. Aerosil® 380, a hydrophilicfumed silica was supplied by Degussa AG, Germany.Glycidoxypropyl triethoxysilane silane (GPTES), 3-2-imidazolin-1-yl-propyltrimethoxysilane and aminopropyltrimethoxy silane (APTMS) purchased from Aldrich wereused as inorganic precursors to produce the silica networkinside the polymer and to modify the layered silicates.Imidazole was supplied by Merck.

2.2. Laponite

Laponite, a synthetic layered silicate, was supplied bySolvay Soda Deutschland GmbH, Rheinberg. Laponite, ane h ond stals[ ys ick-n

2

ure[ ro-c fs iumb dedt w-e sr tew cipi-t for 7d ithh

2

du S)a ),r here[ ndS di-fi

eability. It is claimed[24,25] that permeability of polymeomposites is largely affected by the aspect ratio of the fit is also believed that flakes[26] are much more effectivn decreasing the permeability. This was the motivatioxplore inorganic filler with high aspect ratio such as layilicates for preparing composites.

The base polymer chosen for this work was sulphonoly(ether ether ketone) (SPEEK). PEEK is quite knowolymer research, as it possesses good thermal stabilityechanical properties and a reasonable conductivity wh

s sulphonated. However, the mechanical properties of Pre found to degrade with the degree of sulphonation[27].ighly sulphonated PEEK swells strongly in water andomes soluble if the sulphonation degree is high enougo the low degree of sulphonation, the conductivity valueot that much appreciable for fuel cell applications. He

t was decided to use PEEK with a degree of sulphonaf ca. 67% and subsequently incorporate inorganic fillerder to make up for the mechanical properties and loethanol and water crossover. Of the several non-sphanofillers, ranging from natural to synthetic, synthetic

cates such as laponite and MCM-41 have been utilisehis work. The preparation and characterisation of the cosite membranes based on SPEEK and synthetic sil

or DMFC application are reported here. A new modifiion of silicates using organo silanes has been carried ouheir effect on permeability and proton conductivity is sted in this investigation. In order to understand the influef shape of the fillers on the permeability and conducti

he results are compared with the ones containing isodi

ntirely synthetic silicate, has a layered structure, whicispersion in a solvent is in the form of disc-shaped cry

28]. Unlike most other clays[28], laponite crystal is vermall in size with a very low aspect ratio (diameter to thess ratio) of 25–100[28].

.3. MCM-41

MCM-41, an acid silicate with mesoporous struct29,30]was prepared in the laboratory by the following pedure. The preparation[29] involves the polymerisation oodium silicate in the presence of cetyltrimethyl ammonromide (CTAB). Anhydrous sodium metasilicate was ad

o the 33 wt.% CTAB solution in water. The pH was then lored to 10.8 by the addition of 85% HNO3. The reaction waefluxed at 80◦C for 4 h to form a precipitate. The precipitaas filtered and washed profusely with water. The pre

ate was then extracted with ethanol/heptane mixtureays at pH 1 to obtain MCM-41, known for its structure wexagonal channels[30].

.4. Modification of the silicates

Laponite, MCM-41 and Aerosil® were further modifiesing imidazole glycidoxypropyl triethoxysilane (IGPTEnd 3-2-imidazolin-1-yl-propyltrimethoxysilane (IPTMSespectively, by a procedure, which is described elsew31]. Scheme 1shows the preparation of IGPTES acheme 2depicts the structure of IPTMS. Organic mocation was done with the following objectives:

C.S. Karthikeyan et al. / Journal of Membrane Science 254 (2005) 139–146 141

Scheme 1. Structure of imidazolin-1-yl-glycidylpropyltriethoxysilane (IGPTES).

(i) to have a better compatibility with the polymer, i.e., tohave a good interface bonding between polymer andsilicates/Aerosils;

(ii) to decrease methanol permeability;(iii) to aid the proton conductivity of the whole composite

membrane system.

2.5. Preparation of the membranes

Membranes were prepared by a normal casting pro-cess. The preparation of composite membranes involved twostages. A good dispersion of the silicates or Aerosil® in N-methyl pyrollidone (NMP) solvent was obtained first. Then,the polymer was added to the silicate dispersion and allowedto stir till a homogeneous system was obtained. Finally, theentire system was cast on a silylated glass plate. The finalmembrane was obtained by solvent evaporation at 70◦C for24 h followed by vacuum drying at 100◦C for another 24 h.Thus, membranes based on SPEEK with 2, 5, 10, 20 and30 wt.% of modified laponites and MCM-41 were prepared.Preparation of membrane with 30 wt.% of modified MCM-41 could not be accomplished due to the non-dispersion ofthe silicates in the polymer solution. Similar procedure wasfollowed for the preparation of 20 and 10 wt.% of unmodifiedlaponite and MCM-41, respectively. Membranes containing9 ® -p

pylt xys l pro-c werefi pre-

S ES).

pared, and allowed to stir for 3–4 h followed by the additionof small quantity of water in order to induce hydrolysis. Tocomplete the hydrolysis of the alkoxide, the polymer solutioncontaining silane was again stirred for 24 h at 60◦C. The finalmembranes were obtained after casting followed by dryingas mentioned before.

Imidazole glycidyl propyl triethoxy silane was preparedin the laboratory by mixing Imidazole and glycidyl propyltritethoxy silane in the molar ratio 1:1 and allowing to stirfor 10–12 h. Prior to the addition of GPTES, imidazole wasdissolved in dimethyl formamide.

2.6. Characterisation

2.6.1. Methanol and water permeabilityThe water and methanol permeabilities across the mem-

branes were measured by pervaporation at 25◦C, usinga Millipore cell [13,14] with 47 mm membrane diameter.A 20 wt.% methanol solution was circulated on the feedside while the permeate side was evacuated. The perme-ate was then collected in a trap immersed in liquid nitro-gen, in the time intervals of 1 h. The amount of permeatewas weighed and the composition was determined by mea-suring the refractive index or by gas chromatography. Theindividual mass of each component (water and methanol)w y di-v nentm abil-iw nceo ess,l

2sing

i ahnerI imi-l tswe elec-

and 17% of unmodified and modified Aerosilswere preared.

Two different inorganic precursors, namely aminoprorimethoxy silane and imidazole glycidyl propyl triethoilane, were used to generate a silica network by sol–geess in the polymer matrix. In both cases, the silanesrst added to the polymer solution, which was previously

cheme 2. Structure of 3-2-imidazolin-1-yl-propyltriethoxysilane (IPT

as then converted to an approximate volumetric flux biding each component mass flux by the pure compoass density. Finally, the individual component perme

ties were calculated asPi = ([volumetric flux of i]/[�Pi /l])here�Pi is the trans membrane partial pressure differef componenti acting across the membrane of thickn

.

.6.2. Proton conductivityProton conductivity measurements were conducted u

mpedance spectroscopy. The set-up consisted of a ZM6 electrochemical workstation connected to a cell sar to the one described by Alberti et al.[32]. Measuremenere performed in the frequency range of 1–106 Hz. In thesexperiments the membrane was pressed between Etek


Fig. 1. Schematic representations of the shape of the nanofillers used in thepresent work.

trodes and the impedance at temperatures ranging from 50 to90◦C at 100% RH was determined.

3. Results and discussion

The results illustrate quite well the effect of form, aspectratio and surface modification and content of the nanofillerson the methanol and water permeability as well as protonconductivity of the membranes.Fig. 1 is the schematic rep-resentation of the form and aspect ratio.

3.1. Laponites

Low methanol and water permeabilities are important re-quirements for a membrane in direct methanol fuel cells.As mentioned before methanol permeability was determinedusing pervapouration method. The results of methanol andwater permeability of the composite membranes based onSPEEK and modified laponites are given inTable 1. The per-meability values were obtained using the flux of methanol andwater normalized for thickness and pressure difference. The

Table 1Methanol and water permeability of membranes prepared from SPEEK(67% of sulphonation degree) and laponite modified with imidazoleglyci-d

results are compared with those for the plain polymer. It isevident from the table that the methanol and water permeabil-ity decreases on the incorporation of layered silicates in thepolymeric system. It is also found that except for 10 wt.% thevalues continuously decrease with the increase in content ofthe silicates. Nanocomposites are usually prepared with smallamounts of nanofillers. At rather low concentrations (up to5%), strong interactions between polymer matrix and the lay-ered silicates usually lead to drastic changes in macroscopicproperties such as tensile strength and modulus. Increasingthe content to higher levels lead to smooth property changes.It seems that also here the effect of concentration of laponiteor MCM-41 (as reported in Section3.2) on permeability ismore accentuated up to about 5 wt.% filler. The permeabil-ity decreases further with addition of 20 or 30 wt.% laponite,but the effect is less accentuated. From 5 to 10 wt.% evenan increase of permeability was detected. Specially, in thecase of silicates modified with the basic imidazole groups, astrong interaction with the acid polymeric matrix is expected.The effect of organic modification is evident, when compar-ing membranes with the same amount of laponite with andwithout modification. SPEEK membranes with 20% modi-fied laponite had methanol and water permeabilities of 7 and74, respectively, while the analogous membranes preparedwith unmodified laponite had 11 methanol and 79 water per-meability values. The proton conductivity values of the com-p es ofl -i out2 sw uese posi-t mayb etainw so esf g de-g lues,i EKh andi w.

F with(

oxypropyl triethoxysilane

Thickness (�m) Permeability (×10−17 m2 s−1 Pa−1)

Methanol Water

Nafion 117a 175 21 87

SPEEK membranes with wt.% of laponite0 90 18 1292 132 12 1005 102 9 66

10 112 13 9120 111 7 7430 179 3 54

a From Ref.[13].

osite membranes containing different weight percentagaponite are given inFig. 2. Nafion 117 chracterized in simlar conditions had a conductivity value varying from ab5 mS/cm at 50◦C to about 40 mS/cm at 100◦C. Membraneith 2, 5 and 10% modified laponite had conductivity valven higher than the plain membrane and rest of the comions. The higher conductivity in membranes up to 10%e due to an improved capacity of the membranes to rater. It was reported in the literature[33] that small amountf silica give a similar effect. The lower conductivity valu

or higher contents of laponites may be due to increasinree of obstacles for the proton mobility. From these va

t is clear that the incorporation of laponites in to SPEas a twin effect of lowering the methanol permeability

ncreasing proton conductivity if the silicate content is lo

ig. 2. Proton conductivities of SPEEK/modified laponite membranes�) 0, (�) 2, (�) 5, (×) 10, ( ) 20 and (�) 30 wt.% laponite.


Table 2Methanol and water permeability of membranes prepared from SPEEK(67% of sulphonation degree) and MCM-41 modified with imidazole glyci-doxypropyl triethoxysilane


Methanol Water

Nafion 117a 175 21 87

Membranes wt.% of MCM-410 90 18 1292 106 12 875 108 4 19

10 205 8 9220 132 3 18

a From Ref.[13].

3.2. MCM-41

The results of SPEEK membranes containing differentamounts of MCM-41 are given inTable 2. Here again the val-ues are compared with the plain polymer. The values show adecreasing trend similar to that observed for laponite. Hence,it emphasises that the incorporation of organically modifiedMCM-41 into the SPEEK has decreased the methanol flux,which is again an encouraging result. The 10 wt.% shows anexception in this case as well. The effect of organic modifi-cation is visible in this case as well, where the membraneswith 10 wt.% of unmodified MCM-41 have a permeability of17 for methanol and 116 for water and similar membraneswith modified MCM-41 had 8 and 92 for methanol and water,respectively.

The proton conductivity values are shown inFig. 3. Theplot shows that membrane containing 5 or 10% has higherconductivity than the plain. This can be explained again by theability to hold water. Another feature is that when the MCM-41 concentration increases to 20% the conductivity valuesmerge with those of plain polymer at all temperatures. In thiscase, it can be said that the membrane containing 20 wt.% ofMCM-41 has decreased the permeability, at the same timeretaining the conductivity of the polymer thus making the in-corporation of silicates in the SPEEK polymer advantageous.

Interesting aspect worth to be mentioned at this juncture ist uc-t than1 2, 5

F with(

Table 3Methanol and water permeability of SPEEK/Aerosil® membranes


Methanol Water

Nafion 117a 117 21 87

Membranes with Aerosil0 90 18 1299 88 68 332

17 99 82 3409b 57 4 48

17b 56 3 29a Aerosil modified by 3-2-imidazolin-1-yl-propyltrmethoxysilane.b From Ref.[13].

and 10% have higher values than 20 and 30 wt.% of laponites.Same situation occurs again in the case of MCM-41 whereinthe 20 wt.% shows lower conductivity values (Fig. 3).

3.3. Aerosils

Aerosils are spherical nanoparticles with average particlesize of 7 nm and high surface area. The influence of theirincorporation into SPEEK on the properties is reported anddiscussed in this section.Table 3gives the methanol and waterpermeability values of the membranes with 9 and 17 wt.% ofAerosil. Here the effect of filler surface modification is morepronounced than in the case of layered silicates. The addi-tion of unmodified Aerosil® has increased the permeabilityof methanol and water compared to the plain polymer. Mem-branes with modified Aerosils have shown much lower valuesand they decrease with the increase in content of filler fol-lowing the usual trend. Following the conventional equationbased on Maxwell[34] model, which was used by differ-ent groups[26,35]for calculating permeability reduction forspherical fillers,

P0

P= 1 + φ/2

1 − φ(1)

whereP is the permeability of the composite system,P0 thep es les( ldl ever,t ad-h ifiedA in-t d as ac ugh,s sils od-i ilityd onlytA thes e in-

hat in both laponites and MCM-41 there is a drop in condivity values if the compositions increase to values higher0 wt.%. FromFig. 2, it can be seen that membranes with

ig. 3. Proton conductivities of SPEEK/modified MCM-41 membranes�) 0, (�) 2, ( ) 5, (�) 10 and (�) 20%.

ermeability of the plain system,φ the concentration of thpherical filler, the addition of 20 wt.% of spherical particof density about 1.85 g/cm3) to a polymer system shouead to a decrease of permeability of about 17%. Howhis would only be the case if the particles have goodesion with the polymer. The adhesion between unmoderosil particles and the matrix is probably very poor. The

erface between Aerosil and polymer can even be vieweavity, an easy pathway for water or methanol to go thropecially taking into account the hydrophilicity of the Aerourface. Quite different results are obtained with Aerosil mfied with imidazole. For those systems, water permeabecrease to values even lower than those expected if

he effect of concentration was taken into effect (Eq.(1)).strong interaction between the imidazole groups and

ulphonic groups of the polymer characterizes now th


Fig. 4. Proton conductivities of SPEEK/Aerosil membranes with (�) 0, (�)9 and (�) 17% unmodified Aerosil and with (×) 9 and ( ) 17% modifiedAerosil.

terface between the Aerosil and the polymer matrix. Thisinterface contributes to additional decrease of permeabilityto water and methanol. The proton conductivity values aredepicted inFig. 4 where the values are compared with theplain polymer. With 9% of Aerosil, the proton conductivityis practically the same as for the plain membrane. The modi-fication with imidazole increased the conductivity. When thereinforcement is higher, about 17%, the proton conductivitydecreases to values below to those of plain membrane, despitemodification.

3.4. In situ generated silica

As mentioned before two different precursors were usedto generate the silica network by sol–gel process insidethe SPEEK polymer. Nuclear magnetic resonance (NMR)investigation confirmed the formation of polysilsesquioxanenetwork with a peak at−68 ppm (comparable to the earlierstudies [31]) for RSiO3/2. Table 4 details the methanoland water permeability of the membranes with the silicanetwork network. As one can see, the system with aminogroup is more effective in decreasing the permeability thanone containing the imidazole group. Both the membraneswith NH2 and imidazole–siloxane network have lowerpermeability values than the plain polymer. However, thee ountw

F sil-i s.O iOh than

TM s

W ci-d

Fig. 5. Proton conductivities of (�) plain SPEEK and SPEEK/silica networkmembranes with (�) amino or (×) imidazolinyl propyl groups.

the plain polymer at the same time retaining the conductiv-ity of the latter. The basicity of the imidazole is lower thanthat of NH2 . The interaction between NH2 groups withthe sulphonic groups of the matrix is much stronger than thatof the imidazole group decreasing their availability for pro-ton transport. Another explanation for the low permeabilitiesand conductivities can be given by the difference betweenthe chain lengths of the two silanes used in this work, amino-propyl trimethoxysilane being shorter. It is possible that thelonger side chain interferes with the formation of the silicanetwork making it less cross-linked than in the case of shorteramino chains.

4. Effect of shape and modification of the nanofillers

From the results discussed in this work for nanofillers suchas layered silicates, Aerosil and silica network, the effect ofshape and aspect on the membrane performance can be sum-marised in this section (Fig. 6). The in situ generation ofa modified silica network in the membrane is more effec-tive in decreasing the permeability of methanol comparedto the addition of non-spherical nanofillers like laponite andMCM-41 or spherical silica, Aerosil®. However, the modi-fied nanofillers (laponite, MCM-41 and Aerosil®) had betterconductivities. The membrane containing 10% MCM-41 has

F ofd

ffect on proton conductivity must also be taken into acchen choosing a better membrane.The proton conductivity values are displayed inFig. 5.

rom the figure, it is evident that the membranes withca based on NH2 SiO3/2 have lower conductivity value

n the other hand, membranes based on imidazole–S3/2ave better conductivities, i.e., it has lower permeability

able 4ethanol and water permeability of SPEEK/silica network membrane

Membranes Permeability (×10−17 m2 s−1 Pa−1)

Methanol Water

SPEEK 18 129SPEEK/10% NH2 SiO 3 27SPEEK/10% Im SiO 4 58

here NH2, amino propyl trimethoxy silane and Im, imidazoleglyoxypropyl triethoxy silane.

ig. 6. A plot of conductivity vs. methanol permeability for nanofillersifferent shapes.


a better conductivity among all the membranes reported inthis work. It also has a reasonable permeability. Membranecontaining 5% MCM-41 also has lower permeability and rea-sonably good conductivity values. Hence, membranes with 5or 10% MCM-41 are promising for DMFC application.

When comparing unmodified and modified Aerosil®

(spherical) and layered silicates (flakes) composites withsimilar concentrations, the flakes were much more effective.But the effect of surface modification is much more evidentthan the effect of form in this work. Only after the chemicalmodification of the silicates and Aerosil®, they became effec-tive in reducing water and methanol permeability. The effectof surface modification using amino silanes of inorganicfillers has been carefully discussed by Koros[35] for mixedmatrix membranes for gas separation. Modification is impor-tant to avoid the formation of pathways between the filler andpolymer matrix due to lack of compatibility. Amino silanesand imidazolinyl silanes have also been used previously byour group to improve compatibility between inorganic phaseand polymer[13]. The reaction of silcates and Aerosil® withsilanes leads only to partial surface modification whereasin situ generated inorganic network has a very high degreeof modification, each silicon atom of the network beingbonded to an imidazole-containing group. This considerablyincreased the interaction between the inorganic phase andthe polymer resulting a more effective reduction of the watera s fors f thec ayb mera t rolei ationo facea ticles

alsod atesw reda Them rousn werp

5

ithi phe-n pheres[ if-f later[ asg umef

likel ion

Table 5Comparison of experiment and calculated values of the permeability reduc-tion for membranes containing silicates for the aspect ratio 125

Silicates (wt.%) Permeability reduction

Predicted Laponite

2 0.66 0.645 0.45 0.51

10 0.25 0.7020 0.08 0.37

Fig. 7. A plot of volume fraction of silicates vs. calculated permeabilityreduction for different aspect ratios.

for a random dispersion of parallel non-overlapping identicallayered sheets.

P

P0= exp

[−

(x

x0

)β]

(2)

wherex= af, a= aspect ratio,f= volume fraction,xo= 3.47andβ = 0.71.

This equation was derived by Gusev and Lusti[39] basedon a numerical calculation using a computer model and itsays that the permeability reduction is governed by the prod-uctx= af. In the present work, membranes containing 2, 5, 10,20 and 30 wt.% of layered silicates were prepared. The cor-responding volume fractions are given inTable 5. Assuminga filler density of 2.45 g/cm3 for laponite, the permeabilityreduction of the composite membranes for a series of aspectratios ranging from 10 to 200 was calculated and shown inFig. 7. From the calculation andFig. 7, it was found that thepermeability decrease with the increase in aspect ratio andincrease in the silicate content.

After comparing the experimental and calculated valuesof permeability reductions for a series of aspect ratios, thebest fit found for modified laponite was for the aspect ratio125 (Table 5).

A

ject“ ell.

nd methanol permeabilities. Both the general equationpherical particles and flakes predict an independency oomposite permeability on particle size. Although this me in principle true for ideal two-phase systems (polynd inorganic), the interphase region plays an importan

n the permeability acting as a third phase. The concentrf the interphase is certainly proportional to the interrea and hence is also indirectly a function of parize.

The silica network decreased the permeability but itecreased the proton conductivity. Although layered silicere not so effective in reducing permeability, they offebetter balance keeping the conductivity values high.embranes with 5 or 10% modified MCM-41, a macropoanofiller, had better proton conductivities with rather loermeabilities for methanol and water.

. Comparison of theory and experiment

Diffusion of a small molecule through a membrane wmpermeable objects is a classic problem in transportomena and has been studied when the objects are s

34] and cylinders[36]. However, investigations on the dusion through flakes/layered materials started much37,38,26]. It was found that decrease in permeability woverned mainly by aspect ratio and concentration (vol

raction) of the flakes used[25,26,39].Since the present work deals with layered silicates

aponite Eq.(2)shown below gives the permeability reduct

cknowledgements

The work was part of the HGF-Strategiefonds proMembranes and Electrodes for Direct Methanol Fuel C


The authors thank Dr. Serge Vetter for sulphonating the poly-mer.

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