proton conducting polymer blends and hybrid organic inorganic materials

Ž .Solid State Ionics 145 2001 37–45www.elsevier.comrlocaterssi

Proton conducting polymer blends and hybrid organicinorganic materials

J.C. Lassegues), J. Grondin, M. Hernandez, B. Maree` ´( )Laboratoire de Physico-Chimie Moleculaire UMR 5803 , UniÕersite Bordeaux I, 351 Cours de la Liberation,´ ´ ´

33405 Talence Cedex, France

Received 25 September 2000; received in revised form 12 January 2001; accepted 8 February 2001

Abstract

Literature results concerning proton conducting polymer electrolytes are reviewed with emphasis on acid doped polymerblends and on derived systems obtained by adding an inorganic filler or a plasticizer or both. In addition, conductivity resultsare provided on anhydrous polyamiderH PO blends in a wide acid concentration range and on new membranes involving a3 4

fluorinated polymer, silica and H PO or H SO aqueous solutions. The room temperature conductivity of 10y1 S cmy13 4 2 4

reached by these membranes is discussed using in situ Raman spectroscopy measurements of the acid concentration. Generaltrends and prospects are tentatively drawn from the available results.q2001 Elsevier Science B.V. All rights reserved.

Ž .Keywords: Proton conductivity; Polyamide; Poly vinylidenedifluoride ; H PO , H SO ; Raman spectroscopy3 4 2 4

1. Introduction

It is well established that proton conducting poly-mer electrolytes can be obtained by doping polymersbearing ether, alcohol, imine, amide or imide groups

w xwith strong acid such as H PO or H SO 1–13 .3 4 2 4

This very simple concept is now extended to morecomplex systems where either an inorganic fillerw x w x w x14,15 or a plasticizer 16–18 or both 19,20 areadded to the binary polymer blend in order to im-prove the mechanical andror conducting properties.When three or four components are involved, thenumber of possible combinations becomes very largeand they are far from being all explored. In particu-

) Corresponding author. Fax:q33-5-56-84-84-02.E-mail address: [email protected]

Ž .J.C. Lassegues .`

lar, hybrid organicrinorganic materials are likely toprovide a wide range of new properties.

The aim of the present contribution is to describesome selected systems in order to identify character-istic trends that might be useful for the investigationof improved proton conducting materials. Polymersbearing sulfonic or carboxylic groups constitute an-other very important class of starting materials thatcan be combined with plasticizers and inorganicfillers. They are excluded from the present review asthey are treated in other contributions of this confer-ence.

2. Experimental section

Elvamides are amorphous copolymers of nylon 6,6-10 and 6-6 produced by Du Pont. We have usedElvamide 8061 with a melting temperatureT sm

0167-2738r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0167-2738 01 00909-2

( )J.C. Lassegues et al.rSolid State Ionics 145 2001 37–45`38

Ž .147–1628C and mean repeat unit—NH CH CO2 5.6

—as determined by elemental analysis. AnhydrousElvamiderH PO blends have been prepared by3 4

adding the required quantity of H PO to a solution3 4

of the polymer in methanol. As self-supported filmsare difficult to obtain, samples for conductivity mea-surements are prepared in two different ways in-

w xside an argon-flushed glove box. Forxs H PO r3 4w xrepeat unit-5, the ElvamiderH POrmethanol3 4

solution was poured into one half of the conductivityw xcell described in Ref. 2 . Evaporation of the solvent

and dehydration are performed in situ by heating atabout 608C under vacuum for 12 h. The cell is thenclosed and the thickness of electrolyte is measuredfrom the final length of the cell. Forx)5, theviscosity of the blends is so low that a Tacusselconductivity cell for liquids had to be used. It ismade of two platinum electrodes of constant surfaceand spacing giving a cell constant of 0.85 cm. Thecell is inserted into a hermetically closed glass cylin-der equipped with two lateral taps. After introductionof the ElvamiderH POrmethanol solution, solvent3 4

evaporation and dehydration are again performed insitu under vacuum. For both experiments, the cellwas placed in a hermetic metallic container filledwith argon. The container internal temperature isthen equilibrated at the required value either in a

w xcooling bath or in an oven 21 .In parallel to the above preparations, infrared

measurements have been performed on thin filmssubmitted to the same thermal and vacuum treat-ments in order to check that methanol and water

w xhave been eliminated 21,22 .Composite membranes involving a fluorinated

polymer, an inorganic filler and an aqueous acidicsolution have been prepared in a similar way as

w xdescribed by Peled et al. 20 . Among the varietyof copolymers of vinylidene difluoride and hexa-

Ž .fluoropropylene VF –HFP distributed by Elf Ato-2

chem or Solef, we have chosen the Solef 21508 co-wŽ . Žpolymer of general formula –CH CF – –CF2 2 0.85 2

Ž . . xCF CF – and molecular mass Mws1.15=3 0.15

105. The crystallinity of this copolymer is 27.5%, itsglass transition temperatureT sy298C and itsg

melting temperatureT s1308C. One gram ofm

copolymer is dissolved in a mixture of 5 cm3 2-3 Ž .butanone and 1.4 cm propylene carbonate PC . The

Žrequired quantity of fumed silica Aldrich, 99.8%,

2 y1 .7-nm particle size, 380 m g specific surface areais then slowly added under stirring. The resulting

Žviscous mixture is spread onto a poly tetrafluoro-.ethylene substrate and left at 308C under a stream

of dry air for 1 day. 2-Butanone evaporates and aflexible plastified film is obtained. It is dipped intodistilled water for 5 h to exchange PC by water, andfinally for 2 h in the required acidic aqueous solutionto introduce the acid within the film. For conductiv-ity measurements, films of 300-mm typical thickness

w xare used in the previously described cell 2 . Thesurface of the stainless steel electrodes are cleanedand polished before each experiment. Indeed, if theelectrodes are used several times without polishing,corrosion effects may influence the conductivity re-sults. Impedance spectra for all samples are recordedwith a Solartron 1260 frequency analyser.

The composite films have also been analyzed byŽ .Raman spectroscopy. We used a Labram IB Dilor

Žspectrometer in its standard configuration confocal.microscope, HerNe laser, CCD detector . The con-

focal arrangement minimizes fluorescence effects andallows good quality spectra to be recorded in about20 s from a sample area of about 1mm2. Details onthe vibrational assignments of Elvamide and El-

w xvamide complexes can be found in Refs. 21,22 .

3. Results and discussion

3.1. Binary polymer blends

To illustrate some main features of the acid dopedpolymers, we have compared the conductivities ofH POrH O solutions and of H POrElvamide3 4 2 3 4

blends in a very wide range of acid concentration atŽ . w x308C Fig. 1 21 . The supercooled melted acid has

a conductivity of 0.053 S cmy1 at 308C, and thishigh value is known to come from the extensive

w xself-ionization of H PO 23,24 . Addition of water3 4

increases the dissociation, decreases the viscosityand brings the conductivity to a maximum of 0.27 S

y1 w xcm for 45% acid weight 25 . Further dilutiondecreases the number of charge carriers more rapidlythan the viscosity and the conductivity drops finallyto the value of pure water. When Elvamide is addedto H PO , the conductivity of the blend decreases3 4

continuously towards the very small value of the

( )J.C. Lassegues et al.rSolid State Ionics 145 2001 37–45` 39

ŽFig. 1. Conductivity at 308C of H PO aqueous solutions black3 4. Ž .circles and of H POrElvamide blends open circles as a3 4

function of the acid content. The conductivity of a dry polyamideis about 10y15 S cmy1 and increases up to about 10y11 S cmy1

after water uptake. The conductivity of pure water is 4=10y8 Scmy1 at 188C.

Ž .pure polymer. If H PO or H SO are replaced by3 4 2 4Ž .monoacids HX XsCl, Br, ClO , . . . , the polymer4

blends become less conductive by several orders ofmagnitude.

These observations apply to many other solvatingpolymers and lead to the following general com-ments.

3.1.1. SolÕationThe repeat unit of the polymer must involve

groups such as ethers, alcohols, imines, amides,imides, etc. to be able to establish hydrogen bondswith the acid. In other words, the polymer has to besufficiently basic to dissolve and complex the acid. Itacts as a solvent in which the acid still undergoessome dissociation. Infrared spectroscopy is a use-ful tool to investigate the polymerracid interactions.We have already shown that the amide group is

` ` ` ` yinvolved in a P O H. . . O5C NHl P O` ` q. . . H O C5N H hydrogen bonded network with

w xH PO 4 . At a given concentration, a decrease of3 4

the temperature favours the protonated amidonium

q` yŽ .form C OH5N H interacting with H PO . On2 4

the contrary, the self-ionization of pure phosphoricacid decreases when the temperature is loweredw x23,24 . However, the state of protonation of El-vamide and of dissociation of H PO does not seem3 4

to induce peculiar effects on the conductivity. Thelatter is shown in Fig. 2 to obey a classical VTFŽ .Vogel, Tamman and Fulcher law.

3.1.2. Conduction mechanismThe conductivity of the polymer blends remains

always lower than fused H PO and a fortiori of3 4

H PO aqueous solutions. No polymer can compete3 4

with water as a solvent or with other small polarmolecules such as imidazole or pyrazole that havebeen shown to increase the conductivity of pure

w xH SO or H PO 26 . As far as we know, no2 4 3 4

evidence has been found of a specific polymer thatcould provide enhanced conducting properties byparticipating efficiently in proton transport. In partic-ular, the state of protonation of the amide groups

Fig. 2. Arrhenius plot of the conductivities of H POrElvamide3 4Ž . Ž .blends open symbols , of melted H PO black circles and of a3 4

7.3 M H PO aqueous solution. A fit of these data with the VTF3 4Ž y1r2. w Ž .xequation s s AT expy E rk T y T from 100%a B 0

Ž .H PO to 38% H PO in Elvamidexs0.75 indicates thatE3 4 3 4 a

varies from 0.057 to 0.14 eV,A from 125 to 10 S cmy1 K 1r2

w xand T is roughly constant at 173"10 K 21 .0


does not seem to affect the conductivity. It seemsthat the high viscosity of the interacting polymerracid system prevents any protonic ‘superconducti-vity’ being reached. Conductivity values of about10y3 S cmy1 at room temperature seem to be amaximum for binary polymer blends that can beprocessed as thin self-supported films. This conduc-tivity occurs essentially through the partially dissoci-ated and hydrogen bonded acid network. Althoughthe transference numbers are not known for thissystem, one can infer that the proton is the majorcharge carrier as in fused phosphoric acid. One ofthe more relevant types of experiment in this fieldremains the measurement of the1H and31P self-dif-fusion coefficients by PFG-NMR. Recent resultsshow that the phosphate moieties are considerablymore immobilized in the binary polymer-H PO3 4

w xblends as compared to pure H PO 12 .3 4

3.1.3. Mechanical propertiesWith amorphous polymers such as Elvamide or

Ž . Ž .branched poly ethyleneimine BPEI , it is not possi-w xble to obtain mechanically stable films 3,5,6 .

Polyamide homopolymers of the Nylon family keepsome crystallinity and films can be processed up to

w xxs3 4 . However, much better quality films can bew xobtained either by reticulation of BPEI 5,6 or with

Ž . Ž .high Tg polymers such as poly benzimidazole PBIw x Ž . w x7–12 or poly oxadiazole 13 although this is gen-erally achieved at the expense of conductivity.

3.1.4. Chemical stabilityIt is satisfactory for the ElvamiderH PO blends3 4

up to about 908C but degradation is faster withH SO . This is a general rule coming from the more2 4

acidic and oxidizing properties of H SO . Much2 4

better chemical stabilities are achieved with highlythermostable polymers including polyimides,polyetherketones or polyethersulfones.

3.1.5. Water contentThe hygroscopicity of the polymer blend in-

creases with its conductivity. It is virtually impossi-ble to obtain a fully anhydrous polymer blendbecause H SO and H PO undergo some auto-2 4 3 4

dehydration process in addition to the self-dissocia-w xtion one 23,24 . The conflicting conductivity results

sometimes obtained by different groups on a given

system come generally from differences in the dry-ing process.

3.2. Ternary polymer blends

To overcome the above limitations of binary sys-tems, several new strategies have been proposed.They lead to ternary systems where an inorganiccharge or a plasticizer is added to the polymerracidblend.

3.2.1. Polymerracidr inorganic fillerThe BPEIrxH PO system is a typical case where3 4

addition of acid first produces a solid polycation oflow conductivity at xs0.35 and then a paste forx)1 because of the plastifying effect of the excessacid. The room temperature conductivity is about

y4 y1 w x10 S cm atxs1.5 2,5 . Unfortunately, at thisx value, the material is too soft to be processed as

w xthin films. Senadeera et al. 14 have added silicaparticles of 200 m2 gy1 BET surface area and 12-nmdiameter to improve the mechanical properties.Although rheological studies are not available,the authors indicate that silica addition makes theelectrolytes stiffer, and they succeeded in preparingpellets under pressure. However, the more interest-ing result is an increase of the conductivity by one

Ž y3 y1order of magnitude up to 10 S cm at 278C for.xs1.5 when the silica content reaches 10%. This

enhancement is tentatively interpreted by a contribu-tion of the silica surface conductivity that wouldprovide an alternative path for the proton conduction.Above 10% silica content, the conductivity de-creases. This can be simply due to the adverse effectof the addition of an inert filler.

Surprisingly, this interesting method has been lit-tle used in spite of the availability of more and moreoxides of high specific surface area and nanometricsizes. It is certainly possible to play with the surface

Ž .acidity basicity of the various oxides of Si, Al, Ti,etc. as in the field of ion exchangers. The onlydifficulty in preparing these ternary mixtures is toensure a homogoneous dispersion of non-aggregatednanosize oxide particles.

The sol–gel techniques constitute another interest-ing and more elegant way of preparing hybrid or-

w xganicrinorganic electrolytes. Matsuda et al. 15Ž .claim that poly vinyl alcohol -containing silica gels


doped with HClO can reach a conductivity of 5=4

10y2 S cmy1 at room temperature. However, thethermal stability and water content of these gels arenot commented upon. The presence of water wouldrather class these systems into the category of thequaternary polymer blends treated in Section 3.3.

3.2.2. PolymerracidrplasticizerOne of the main advantages of using a plasticizer

is to increase considerably the range of usable poly-mers and acids. Indeed, the role of acid solvation isnow played by the plasticizer, and it is enough thatthe polymer act as a containing matrix with filmo-genic properties. The resulting material is often called

˙ w xa polymer gel electrolyte. Zukowska et al. 16 havestudied the PVdFrDMFrH PO system and mea-3 4

sured conductivities of about 5=10y4 S cmy1 atw x208C. Ericson et al. 17 propose a concept derived

q Žfrom Li -ion conducting gels where poly methyl-. Ž .methacrylate PMMA is gelified by a solution of

Ž .weak organic acid benzoic or salicylic in typicalmixtures of polar solvents used in the battery elec-

Ž .trolytes EC, PC, DMF . Conductivities are in the10y5–10y4 S cmy1 ranges at 258C. The transpar-ent membranes thus obtained can be used in elec-trochromic devices. However, their thermal stabilityis limited by the solvent vapour pressure.

The more ubiquitous polymer plasticizer is water.As already pointed out, it is difficult to obtain a fullyanhydrous polymerracid blend but in some in-stances, water has been intentionally introduced.

w xThus, Wieczorek and Stevens 18 report room tem-perature conductivities greater than 10y2 S cmy1 forpolyacrylamide hydrogels doped with H PO or3 4

H SO with the advantage of increased mechanical,2 4

chemical and thermal stabilities as compared to thew xbinary Paamracid blends 3 .

The PBIracid system has been studied by manygroups owing to its outstanding properties of thermal

w xand chemical stabilities 7–11 . The presence ofvariable water contents seems to be again the causeof differences between various conductivity mea-surements. Thus, with monoacids, the conductivityof a dried membrane is lower by four or five orders

w xof magnitude 8 than a membrane immersed inw xaqueous solutions 10 . This difference is less pro-

nounced with H PO or H SO owing to the intrin-3 4 2 4

sic conductivity properties of these acids.

3.3. Quaternary polymer blends

In a logical continuation of the previous studies,some authors have proposed to combine the benefi-cial effects of an inorganic charge and of a plasti-cizer in polymerracidrceramicrsolvent systems.

w xStaiti et al. 19 have developed a compositemembrane for fuel cell applications based on phos-

Ž .photungstic acid PWA adsorbed on silica and mixedwith PBI. At 100% relative humidity and 1008C, amaximum conductivity of 3=10y3 S cmy1 is ob-tained. According to the authors, these membranesare chemically stable in boiling water and thermallystable up to 4008C but the conductivity decreasesabove 1108C because of a lowering of the watercontent. Silica is essential in these systems for retain-ing water and providing a conducting path for theproton. PBI brings the advantages of a mechanicallyand chemically stable polymer matrix. It is interest-ing to note the progression achieved with thesequaternary PBIrPWArsilicarwater membranes bycomparison with the previous studies performed on

w xeither binary PBIrPWA blends 10 or PWA dopedw xsilica gels 27 .

w xPeled et al. 20 have also presented a novelproton-conducting membrane based on PVdF andnanosize ceramic powder that can reach a roomtemperature conductivity of up to 0.21 S cmy1 afterbeing soaked in concentrated sulphuric acid. It isimportant to understand how such high conductivitylevels can be obtained. We have repeated Peled’sexperiments and extended our study to variations ofthe silica content and acid concentration. In addition,confocal Raman microspectrometry has been used tocharacterize the acid dissociation state inside themembrane.

3.3.1. Preparation of the membranesAs reported in Section 2, our preparation protocol

w xdiffers very little from that of Peled et al. 20 : thepolymer and inorganic filler are not exactly the sameand we have explored a larger range of acid andfiller concentrations.

3.3.2. Raman characterizationEach step of the membrane preparation can be

followed by Raman spectroscopy as shown in Fig. 3.Sulphuric acid is a favourable case where character-


Ž . Ž .Fig. 3. Raman spectra of PC a , VF –HFPrsilicarPC b ,2Ž . Ž .VF –HFPrsilicarH O c , 5.9 M H SO d and VF –2 2 2 4 2

Ž . y1HFPrsilicarH SO e . The polymer line at 796 cm can be2 4

used as an internal intensity reference. The solid line spectra allowthe membrane preparation to be followed step by step.

istic lines of the HSOy and SOy anions are well4 4

separated. So, the acid content and its local concen-tration can be evaluated in situ with a surface resolu-tion of the order of 1mm2.

3.3.3. ConductiÕity measurementsAs reported in Fig. 4, we have first used H SO2 4

5.9 M and H PO 7.3 M dipping solutions. They3 4

correspond to conductivity values of 0.64 at 188Cy1 w xand 0.235 S cm at 258C 25 , respectively. The

Žsilica content, defined as the mass ratio SiOr SiO2 2.qpolymer , has been varied from 5% to 40%. If no

silica is added to the polymer, the hydrophobicity ofthe latter prevents any acidic solution from beinginserted but 5% silica already gives a room tempera-ture conductivity in the 10y4 S cmy1 range. Theconductivity further increases with the silica contentup to values of about 10y1 S cmy1 with H SO and2 4

10y2 S cmy1 with H PO for 40% silica. Silica3 4

contents higher than 40% give films of poor mechan-ical properties.

We have then selected a film with 33% silica andvaried the concentration of the initial sulphuric acid

Fig. 4. Variation of the conductivity of VF –HFPracidrsilica2

blends at 258C as a function of the silica content. The dippingŽ .solutions were 5.9 M H SO black circles or 7.3 M H PO2 4 3 4

Ž .open circles . The arrows indicate the conductivity of thesesolutions.

solution. It can be seen in Fig. 5 that the conductivityof the membranes is lower than the initial sulphuricacid solutions by about a factor of 7. However, the

Fig. 5. Variation of the conductivity of VF –HFPrH SO r33%2 2 4

silica blends at 258C as a function of acid concentration in theŽ .dipping solution black circles . The conductivity of the dipping

Ž .solutions at 188C is shown for comparison dotted line .


y y1 Ž .Fig. 6. Integrated intensity of the HSO line at 1050 cm R4 12y y1 Ž .and of the SO line at 981 cm R as a function of silica4 2

content in VF –HFPr5.9 M H SO rsilica blends at 258C. The2 2 4

ratio RsR rR s3"0.2 corresponds to a local acid concentra-1 2

tion of about 7.5 M.

shape of the variation as a function of the concentra-tion is similar within experimental errors. The resultsof Figs. 4 and 5 confirm the high conductivity values

w xannounced by Peled et al. 20 .

3.3.4. State of the acid within the membraneObviously, the hydrophilic character of the silica

particles allows acidic aqueous solutions to be in-serted into an otherwise hydrophobic polymer matrixin a similar way as grafted sulfonic groups allownafion membranes to be acidified. It can be notedthat the two families of electrolytes reach a maxi-mum conductivity level of about 10y1 S cmy1 at258C. From the above results, one can qualitativelyconclude that the silica particles are surrounded byinterconnected liquid domains. The volume of thesedomains is proportional to the amount of silica em-bedded in the polymer matrix as shown by theconductivity data of Fig. 4 and by a roughly linearincrease of the Raman intensity of the anion lines as

Ž .a function of silica content Fig. 6 . However, Fig. 6reveals that the intensity ratioRsHSOrSO2y

4 4

takes a value of 3"0.2 in the membrane, whereas, it

is 2.5 for the 5.9 M H SO dipping solution. We2 4

have varied the initial acid concentrationC and0

established a calibration curve ofR versusC . Then0Žthe R-values found for the various membranes Fig.

.7 have been transformed into concentrationsC inm

order to plot Fig. 8. It can be seen that the acidconcentration in the liquid domains within the mem-brane, C , is higher thanC for C -8 M andm 0 0

becomes lower forC )8 M. We have no clear0

explanation to propose for this evolution. Possibly,there is either no detectable effect on the conductiv-ity variation in Fig. 4 although one could considerthat the accuracy of the conductivity measurementson the membrane is not sufficient to reveal signifi-cant differences with the aqueous solution. It cansimply be concluded that the silica surface modifiesthe dissociation equilibrium of H SO .2 4

3.3.5. Chemical and thermal stabilitiesAs soon as the membranes were heated up to

about 80–1208C, we realized that their chemicalstability was very bad and, as usual, worse withH SO than with H PO . After a few hours at2 4 3 4

1008C, the transparent membranes become brown

y y1 Ž .Fig. 7. Integrated intensity of the HSO line at 1050 cm R4 12y y1 Ž .and of the SO line at 981 cm R in VF –HFPrH4 2 2 2

SO r33% silica blends at 258C as a function of the acid concen-4

tration in the dipping solution.


Fig. 8. Concentration of H SO within the membrane as a func-2 4

tion of acid concentration in the dipping solution for VF –2

HFPrH SO r33% silica blends at 258C. If these concentrations2 4

were equal, the data points would be situated on the dotted line.

with H PO and dark with H SO . Actually, VF –3 4 2 4 2

HFP derivatives are known to be degraded not onlyby strong acids but also by oxides such as SiO ,2

TiO , Al O . The stability is sufficient to perform2 2 3

measurements at room temperature but not to envis-age any application where even moderate heating isneeded.

We believe, however, that the concept of thesequaternary blends remains valid with more stablepolymers. The field is also open to the large varietyof available nanosize oxides, including aerogels; theyoffer very high specific surface area and quite differ-ent chemical surface properties. Some of them mightallow a strong base to be introduced instead of astrong acid in order to get proton conductivity viaOHy ions.

Cheap and easy to prepare membranes reaching aconductivity of 10y1 S cmy1 at 258C are veryinteresting for devices such as supercapacitors wherepower considerations are so important. OHy conduc-tors are attractive for alkaline batteries. Their appli-cation in the field of fuel cells is less obvious as theacid concentration might well vary under workingconditions. Indeed, the acid is certainly less tightly

bound to the silica surface than, for example, to thesulfonic groups of a fluorinated membrane.

4. Conclusion

The proton-conducting polymer blends describedabove are all very easy to prepare from commer-cially available and relatively cheap materials. De-pending on the foreseen application, the conducting,chemical, thermal and mechanical properties, the wa-ter content, etc. can be varied by introducing inor-ganic charges or plasticizers or both into the poly-merracid system.

In binary polymerracid blends, the water contentcan be reduced to very low values and maximumroom temperature conductivities of about 10y3 Scmy1 can be obtained. However, the mechanical andchemical stabilities are often limited. This field isbeing renewed by using polymers of outstanding

Ž .thermostability such as poly benzimidazole orŽ .poly oxadiazole .

Introduction of an inorganic filler either under theform of nanosize oxides or via sol–gel processesimproves the mechanical properties and the conduc-tivity. Introduction of a plasticizer allows a widerrange of polymers to be used, including chemicallyinert polymers having good mechanical properties.Finally, the combination of all these components in

Ž .quaternary blends or more opens many furtherpossibilities. In terms of conductivity, they practi-cally fill the gap between polymer and liquid protonconducting electrolytes, although the presence ofwater inside the polymer has then to be accepted toreach the highest conductivity values.

Another development along the same lines is tobe expected with polymer blends using a strong baseinstead of a strong acid. Proton conduction via OHy

groups is very important for electrochemical devicesŽ .such as alkaline batteries. Poly ethylene oxider

w xKOHrH O blends have already been described 28 .2

Better performances have recently obtained by thesame authors using a copolymer of epichlorohydrin

w xand ethylene oxide instead of PEO 29 . The roomtemperature conductivity reaches 10y3 S cmy1, theanionic transference number is 0.93 and the potentialstability window 0.9 V. Another interesting example


of KOH doped polymer is given in this conferencew xwith PBI 30 . No doubt that this new family will

benefit from the different strategies described abovefor acid doped polymers.

Acknowledgements

ŽThe authors are grateful to Ph. Guiriec Labo-ratoire d’electrochimie moleculaire, Universite Paris´ ´ ´.7 for his help in the study of quaternary blends.

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