Recent advances in the functionalisation of polybenzimidazole and polyetherketone for fuel cell applications

Journal of Membrane Science 185 (2001) 41–58

Recent advances in the functionalisation of polybenzimidazole andpolyetherketone for fuel cell applications

Deborah J. Jones, Jacques Rozière∗Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques, UMR CNRS 5072, Université Montpellier II,

34095 Montpellier Cedex 5, France

Received 29 February 2000; received in revised form 10 April 2000; accepted 10 April 2000

Abstract

This article reviews progress made over the past years in the functionalisation of polybenzimidazole and polyetherketoneswith a view to increasing their proton conduction properties without detriment to their thermohydrolytic and chemicalstability such that corresponding membranes may be employed in hydrogen oxygen (air) or direct methanol fuel cells.The approaches include complexation of polybenzimidazole with acids, grafting of groups containing sulfonic acid moietieson to polybenzimidazole by N-substitution, and direct sulfonation of polyetherketones. A further approach concerns theincorporation of inorganic proton conducting particles in the polymer matrix, and this is developed in detail for the case ofhybrid sulfonated polyetheretherketone–metal(IV) phosphate membranes. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Polybenzimidazole; Polyetherketones; Functionalisation

1. Introduction

Electricity is the fastest growing form of energy.Although it is being used more efficiently, and despiteprogress having been made in switching to fuels otherthan oil, the electricity industry still faces a number ofmajor challenges, one of the most important of whichis public concern about the environmental impactof electricity generation and use. The other princi-pal contributor to urban pollution is road transport.Greater diversification of energy sources is required,with reduced associated emissions. Fuel cells willcontribute to reducing the demands for fossil fuel andnuclear-derived energy, both in the power generationsector and the road transport sector. Medium tempera-

∗ Corresponding author. Tel.:+33-467-14-33-30;fax: +33-467-14-33-04.E-mail address:[email protected] (J. Roziere).

ture (80–180◦C) operation of PEM fuel cells is attrac-tive from the point of view of eventual cogeneration ofheat, and increasingly high tolerance of electrodes toCO. However, current commercially available mem-branes are not suitable for use in this range; indeed,even for low temperature operation (<80◦C), the cur-rent high cost of fuel cells reflects in part the highcost of commercial perfluorosulfonated membranes.

Recent research has approached the developmentof novel protonic conducting membranes in a varietyof strategies [1]. Acid complexation of basic polymers[2–6] has received much attention in the past 5–6years, and will be described in further detail below.The addition of a protogenic group (generally a sul-fonic acid group) to a polymer can be achieved eitherby direct sulfonation of the polymer with sulfuric acidor chlorosulfonic acid [7–14], via sulfination [15],by chemically grafting a group containing a sul-fonic acid function on to a polymer [16], by graft

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0376-7388(00)00633-5

42 D.J. Jones, J. Roziere / Journal of Membrane Science 185 (2001) 41–58

copolymerisation using high-energy radiation follo-wed by sulfonation of the aromatic component [17,18]or, in an alternative route, polymer synthesis frommonomers bearing sulfonic acid groups [19]. Thelevel of sulfonation is a key parameter, since it isaccompanied at too high levels by an unacceptable de-gree of swelling and unsatisfactory mechanical prop-erties. The conductivity of the membrane is howeverdirectly related to the extent of sulfonation (in addi-tion to the degree of relative humidity, temperature,etc.). The presence of an inorganic component [20]can compensate for the restricted level of sulfonationrequired for satisfactory mechanical behaviour. Otherattendant advantages that can be observed includereduced dependence of the conductivity on relativehumidity, reduced reactant crossover, for example, ofmethanol or of degradative oxygen radical diffusion,and increased mechanical strength.

Recent advances centre around thermostable poly-mers including polyethersulfone, polyimides andpolyetherketones, and this review is restricted to twoaspects of the search for innovative membranes formedium temperature fuel cells. The first part con-cerns the functionalisation of polybenzimidazole bycomplexation or chemical grafting, and the secondthe electrochemical characterisation of sulfonatedpolyaryletherketone membranes and the use of thispolymer as a matrix for the incorporation of inorganicproton conductors or inorganic oxides.

2. Polybenzimidazole-based systems

Polybenzimidazoles are synthesised from aromaticbis-o-diamines and dicarboxylates (acids, esters,amides), either in the molten state or in solution. Therepeat unit, benzimidazole, has rather remarkablethermal properties compared with its carbon congenerindene, as illustrated by melting and boiling pointdata [21]. Thus, indene melts at−2◦C and boils at183◦C, whereas the corresponding values for ben-zimidazole are 170 and >360◦C, respectively. Thethermal properties of polybenzimidazoles, which de-pend on the nature of the component tetraamine anddicarboxylic acids, have been largely reported in theearly literature [22]. Aromatic polybenzimidazoles arehighly thermostable, with melting points >600◦C. Thecommercially available polybenzimidazole is poly-

Scheme 1. Poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], PBI.

[2,2′-(m-phenylene)-5,5′-bibenzimidazole], Scheme 1,which is synthesised from diphenyl-iso-phthalate andtetraaminobiphenyl, and this will be referred to here-after simply as “PBI”. It is characterised by excellentthermal and mechanical stability. Under conditionsrelevant to fuel cell use viz. oxidising and reduc-ing environments at elevated temperature and in thepresence of water, Linkous [23] observed no weightgain/loss for PBI in either H2/H2O or O2/H2O at200◦C, although its stability at 300◦C in oxidisingconditions was not satisfactory. It was concluded thatat 300◦C, the imidazole ring is susceptible to hydrol-ysis, and that the products of this hydrolysis are morereadily oxidisable.

Early reports of the proton conductivity of PBIare conflicting. Thus, whereas values in the range2 × 10−4–8× 10−4 S cm−1 at relative humidities be-tween 0 and 100% were published [24], other authors[16,21,25] observed proton conductivity of magni-tude some two to three orders of magnitude lower.These latter values are those generally accepted fornon-modified PBI, and are clearly too low for anyuse of PBI membranes in fuel cell applications. Twoprincipal routes have been developed to improve theproton conduction properties, and these repose uponthe particular reactivity of PBI, which is twofold,and arises from the –N= and –NH-groups of theimidazole ring. Due to its basic character (pK valueof ∼5.5) PBI complexes with inorganic and organicacids [6,26]. In addition however, the –NH-group isreactive; hydrogen can be abstracted, and functionalgroups then grafted on to the anionic PBI polymerbackbone [27,28]. It should be mentioned also thatunlike for other polyaromatic polymers, the directsulfonation of PBI using sulfuric or sulfonic acid isnot appropriate for the preparation of proton conduct-ing polymers for fuel cell membranes, since it tendsto lead to a polymer of low degree of sulfonation andincreased brittleness [29].

D.J. Jones, J. Roziere / Journal of Membrane Science 185 (2001) 41–58 43

2.1. Complexation with acids

“Stabilised” and “plastified” PBI refer to PBI treatedwith sulfuric and phosphoric acid, respectively [21].In the former, a subsequent heat treatment confers par-ticular properties of reduced flame shrinkage in drawnfibres, as well as other intrinsic properties which leadto its textile and non-textile applications. Early workon films and treatment of PBI films also generated con-ductivity data on plasticised PBI, in which the poly-mer was allowed to absorb phosphoric acid from anaqueous solution. The conductivity of water swollenacidified PBI was reported to reach 9× 10−3 S cm−1,and its resistance to decomposition in air, as demon-strated by the percent weight loss after heating at500◦C for 3 h, was higher than that of unmodifiedPBI [21,30]. Systematic study of the complexation ofPBI with phosphoric acid with the particular objectiveof formation of thermo-oxidatively stable proton con-ducting membranes for fuel cell applications beganin 1994 [6], using various types of fuel including hy-drogen [31], methanol [26,32,33], trimethoxymethane[34] and formic acid [35]. This polymer electrolytemembrane has also been used for other electrochemi-cal applications, including a hydrogen sensor [36,37].

Acid “doped” is the term generally used to describethe homogeneous polymer electrolyte system formedby dissolution of phosphoric [6,26,36–39] (sulfuric[36–39], hydrochloric [39], hydrobromic [36,37],nitric [39], perchloric [39]) acid in PBI. The basiccharacter of the PBI polymer allows doping levels ofup to ca. 50 wt.%. Two routes to the complexation ofH3PO4 by PBI have been reported, the first in whichPBI films are immersed in an acid solution of molarityM for time t, [6,30–33,36–39] and the second wherefilms are cast directly from a solution of the polymerand phosphoric acid in a suitable solvent [26]. Thissecond manner produces doped films directly, andso reduces the preparation time. Depending on thequantity of acid in the complex PBI/H3PO4, suchsystems have a conductivity between 5× 10−3 and

Table 1Conductivity at room temperature after immersion of PBI film for 10 days in acids of indicated concentration [39]

PBI/HCl(11.8 mol dm−3)

PBI/HClO4

(11.6 mol dm−3)PBI/HNO3

(15.8 mol dm−3)PBI/H3PO4

(14.4 mol dm−3)PBI/H2SO4

(16 mol dm−3)

Conductivity (S cm−1) 1.4 × 10−3 1.6 × 10−3 1.8 × 10−3 1.9 × 10−2 6 × 10−2

2 × 10−2 S cm−1 at room temperature [26,38,39] andeven 3.5 × 10−2 S cm−1 at 190◦C [6,32]. The natureof the acid influences the conductivity of doped PBI,and after contact with acid of high concentration(≥11 mol dm−3) the conductivity follows the orderH2SO4 > H3PO4 > HNO3 > HClO4 > HCl [39],Table 1. Importantly, for the functioning of a fuelcell at temperatures above 100◦C, the electro-osmoticdrag number of PBI/H3PO4 is almost zero [40]. A lowlevel of gas hydration can therefore be used withoutdrying out of the membrane, which may also assist inreducing reactant crossover, a point of particular im-portance in the context of the search for membranessuitable for direct methanol fuel cells (DMFC).

Recently, the dependence of the room temperatureconductivity of PBI/H3PO4 and PBI/H2SO4 films hasbeen investigated as a function of the concentration ofacid solution and their immersion time in acid solu-tion [38]. As shown in Fig. 1, two types of behaviourare observed. The first type of membrane, preparedat shorter doping times, displays a conductivity inthe range 10−5–10−4 S cm−1, whilst the second, ofconductivity >10−3 S cm−1, is formed after moreprolonged immersion. There is a “switch-over” fromone state to the other which occurs after 10–11 hin H3PO4 and after 2–3 h in H2SO4. Most notably,the cross-over to more highly conducting system isindependent of the concentration of the acid solution.

Based on this observation, a representative exampleof the two types of state, viz. PBI/H3PO4 membranesobtained after 1 and 16 h immersion in acid concentra-tion of 1–10 mol dm−3, were analysed for their H3PO4(elemental analysis for phosphorus) and water content(thermogravimetric analysis), and the concentration ofthe acid in the membrane was derived. Similar anal-yses were made for PBI/H2SO4 membranes after 5 himmersion, and the results are shown in Fig. 2. Aciduptake by the membrane depends both on immersiontime and concentration of the acid bath and, for allimmersion times studied, it increases with concentra-tion. Uptake of H3PO4 after 1 h is low, 0.02–0.7 mol


Fig. 1. Conductivity at 25◦C of PBI as a function of the: (a)H3PO4; (b) H2SO4 concentration and for immersion times of upto 24 h.

H3PO4/PBI repeat unit in aqueous acid of concentra-tion 1–10 mol dm−3. For longer contact times, acid up-take increases rapidly, and reaches 4.2 mol H3PO4/PBIafter 16 h immersion in an acid bath of concentration10 mol dm−3. These data complete and confirm thoseof other authors, who report uptake of 3.4 [6] and5 mol [31] H3PO4/PBI repeat unit after 16 h contactwith phosphoric acid of 7 and 11 mol dm−3, respec-tively. The difference in concentration of acid in PBImembranes after 1 and 16 h of soaking is particularlystriking. Whereas for the former, the acid concentra-tions of the acid bath and within the membrane arealmost equal, after 16 h there is a strong concentrationeffect, and above a concentration in solution of ca.3 mol dm−3, that the membrane is close to that of con-centrated phosphoric acid (14.7 mol dm−3). In H2SO4,uptake of acid reaches 3 mol H2SO4/PBI repeat unitafter 5 h immersion. Highly doped (highly conduct-ing) and slightly doped PBI membranes also differ in

Fig. 2. Number of moles of acid (above) and water (middle) takenup by PBI in: (a) H3PO4 after 1 h immersion; (b) H3PO4 after 16 himmersion; (c) H2SO4 after 5 h immersion, and the concentrationof H3PO4 so derived in phosphoric acid-doped PBI (bottom).

their textural properties. Qualitatively, in the higherconductivity regime, membranes are more swollenand flexible than unmodified PBI. Quantitatively, dy-namic mechanical testing of a PBI film doped with320 mol% H3PO4 shows that in the range of interestfor a medium temperature fuel cell (100–200◦C), themodulus is relatively constant and is extremely high(ca. 109 Pa) [6]. For comparison, the storage modulusof NafionTM is roughly 1000 times lower (ca. 106 Paat 150◦C) than the modulus of the PBI film.


The ionic interactions between the acid and PBIhave been characterised using infrared [38] and nu-clear magnetic resonance [41] spectroscopies.13CNMR of unmodified PBI and phosphoric acid dopedPBI are different, providing evidence for an inter-action between polymer and acid. The13C NMRspectrum does not change further after heating thedoped membrane to 250◦C for 1 h in air, however the31P NMR spectrum shows a shift from a single signalat 0 ppm to three at 0,−11 and−22 ppm, which indi-cates condensation to have occurred, giving, amongstother possible species, pyrophosphoric acid [41]. TheIR spectrum of PBI is significantly modified aftercomplexation with phosphoric acid, in particular, bythe presence of a very broad and intense absorptionin the range 2000–3500 cm−1 characteristic of stronghydrogen bonding. In addition however, there arechanges, more easily seen in weakly doped mem-branes, in the mid-frequency region 1300–1650 cm−1,and in the region of phosphorus–oxygen stretching vi-brations 900–1200 cm−1, which are compatible withthe protonation of the imino nitrogen, i.e. formation ofthe benzimidazonium cation, Scheme 2, and with thepresence of H2PO4

−, respectively. In strongly dopedmembranes, e.g. PBI·4H3PO4, a spectrum very similarto that given by pure phosphoric acid is obtained, withcharacteristic maxima at 890 and 1004 cm−1 [38].

In general, polybenzimidazoles are known for theirvapour barrier properties. The permeability of PBIcomplexed by H3PO4 is considerably lower than thatof NafionTM, in particular, with respect to methanol,reflecting the dense, non-porous character of PBIfilms [6]. Table 2 reproduces data from [6] on the per-meability of phosphoric acid doped PBI to methanol,hydrogen and oxygen and compares these values withthose for NafionTM-117. For hydrogen and oxygen,the comparisons are for temperatures correspond-ing to normal or intended fuel cell operation. Basedon this permeation data, a direct methanol fuel cellusing such a phosphoric acid doped membrane and

Scheme 2. Benzimidazonium cation present in acid doped PBI.

Table 2Permeability of H3PO4-doped PBI and NafionTM-117 to methanol,hydrogen and oxygen [6]

Electrolyte Gas Temperature(◦C)

Permeability(barrer)a

PBI/H3PO4 Methanol 80 270Nafion-117 Methanol 80 80000PBI/H3PO4 Hydrogen 150 180Nafion-117 Hydrogen 80 180b

PBI/H3PO4 Oxygen 150 10Nafion-117 Oxygen 80 90b

a 1 barrer= 10−10 cm3 (STP) cm/cm2 s cmHg.b From [42].

operating on a 50/50 methanol/water feed to the anodeat 150◦C would have a methanol crossover equivalentto less than 10 mA cm−2, [6] which can be comparedwith crossover rates in excess of 100 mA cm−2 whenNafionTM is employed.

Comparison of the performance at 200◦C of adirect methanol fuel cell using acid complexed PBIprepared by doping of pre-formed membranes and bycasting from a solution containing PBI and phospho-ric acid has been made [26]. Membranes of the formertype contain 5H3PO4/PBI repeat unit and, of the lat-ter type, 6H3PO4/PBI. With 4 mg cm−2 Pt–Ru alloyelectrode as anode and 4 mg cm−2 Pt black electrodeas cathode, this DMFC produced power densities of0.21 and 0.16 W cm−2, respectively, at 500 mA cm−2

(atmospheric pressure feed of methanol/water mix-ture in 2/1 mole ratio, and oxygen) [26]. Increasedperformance in the temperature range 150–200◦Cis attributed to lower methanol crossover due to thelower solubility of methanol in the membrane athigher temperatures, and to higher electrolyte con-ductivity. On increasing the water content of the feed,the increase in cathode performance (a result of lowermethanol crossover) is greater than the loss in anodeperformance (as a result of lower methanol concentra-tion). The very low water-drag number (almost zero)[40] is important for the use of acid-doped PBI in ahydrogen–oxygen (air) fuel cell, since it allows thecell to be operated at very low humidification level.On air, with gas humidification at room temperature,a power density of 0.25 W cm−2 has been obtained, at700 mA cm−2 [31]. Impregnation of the membrane–electrode assembly with polymer electrolyte improvesthe electrode performance, and it is expected thatfurther improvements in the performance of fuel cells


using PBI-based electrolytes will accompany optimi-sation of the electrode structures [33,43].

2.2. Grafting of functional groups on to PBI

Sulfonation of the PBI backbone occurs on heattreatment of the polymer–hydrogen sulfate complexformed when pristine PBI is cast from sulfuric acid,or when PBI films are immersed in sulfuric acid,Scheme 3 [21]. This type of sulfonated PBI is referredto as “stabilised PBI” due to the remarkable propertiesof drawn fibres to flame resistance, hot air shrinkageand strength retention after immersion in inorganicacids and bases, and organic liquids. Even heated sol-vents which dissolve PBI do not affect the strength ofstabilised PBI [21]. This virtual insolubility rendersdifficult the casting of films and also suggests thatsome crosslinking occurs, which is supported by theobserved high brittleness of cast sulfonated PBI films[29].

In an alternative approach however, which allowscontrol over the degree of sulfonation, PBI can bederivatised by replacing the imidazole hydrogen withalkyl or aryl substituents [16,27,28]. This methodwas developed as a route for further improving thechemical stability of PBI by introduction into the imi-dazole ring of groups less reactive than the imidazolehydrogen, and provides the opportunity of tuning the

Scheme 3. Synthesis route to sulfonated (“stabilised”) PBI.

Scheme 4. Synthesis route to benzylsulfonated substituted PBI[16,44].

properties of the polymer by the choice of substituent.The synthesis [16,44] and electrochemical characteri-sation [16] of benzylsulfonate N-substituted PBI wasrecently described. In this synthesis, a PBI anion isformed by reaction with a soluble base, such as an al-kali metal hydride, followed by reaction with sodium(4-bromomethyl)benzenesulfonate, Scheme 4.

The extent of sulfonation can be controlled at oneor both of the reaction steps, i.e. by limiting thenumber of sites ionised and/or by limiting the ratioof sodium (4-bromomethyl)benzenesulfonate to PBI,and the extent of sulfonation determined by elementalanalysis or1H NMR. In the latter method, the ratio ofthe integrated intensity of the resonance at 5.5–6 ppmgiven by the benzyl protons to those arising fromthe PBI polyanion increases with the amount of ben-zylsulfonate grafted, and so is a direct method fordetermination of the degree of sulfonation. Complete


substitution at both imidazole –NH-groups would rep-resent a degree of sulfonation of 100%. In practice,such extensive functionalisation is undesirable sincethe textural properties are increasingly significantlymodified. The influence of sulfonation on thermal,water uptake and conduction properties in the range40–75% has been studied [16]. Thermogravimetricanalysis in air shows degradation of the sulfonatedpolymer commences at around 360◦C, notably higherthan polymers in which the sulfonic acid group is di-rectly substituted onto the aromatic backbone, whereweight loss is generally observed above 260◦C. Asexpected following addition of a functional group ableto interact strongly by hydrogen bonding, the glasstransition temperature is increased in benzylsulfonategrafted PBI, from 417◦C in pristine PBI to 447◦C in asample of 50% degree of sulfonation (i.e. on average,substitution of one –NH-group per PBI repeat unit,ion exchange capacity 2.1 meq g−1). At higher de-grees of sulfonation, the glass transition temperaturecannot be determined, as it occurs in the same rangeas the combustion of sulfonated PBI. Importantly,under isothermal conditions at 200◦C, no weight lossof sulfonated PBI was detected in air, even after 90 h.

The extent of sulfonation directly affects the wateruptake and the conductivity. The conductivity of asulfonated PBI membrane is closely related to theamount of water it contains, and this water uptakecorresponds to 4, 7, 9 and 11 water molecules/PBIrepeat unit with degrees of sulfonation of 0, 50, 65and 75%, respectively [16]. This leads to a hydrationnumber (i.e. number of water molecules associatedwith each sulfonic acid group) of ca. 7, lower byat least a factor 2 than in NafionTM or sulfonatedpolyetherketone membranes [45]. Fig. 3 shows theconductivity of benzylsulfonate-grafted PBI, pristinePBI and NafionTM-117 derived from membrane resis-tance measurements at 25◦C in aqueous phosphoricacid [46,47]. Membranes were allowed to equilibratefor 8 h prior to measurement and, correspondingly,curve (c) corresponds to the lower curve of Fig. 1(a).The conductivity of the benzylsulfonate grafted PBIranges from 3× 10−3 to 2× 10−2 S cm−1, lower thanthat of NafionTM (1× 10−1 to 3× 10−2 S cm−1 underthese conditions of measurement) and is significantlyhigher than that of PBI (ca. 10−5 S cm−1), which isslightly doped by phosphoric under these conditions.Benzylsulfonate grafted PBI displays a high conduc-

Fig. 3. Conductivity at 25◦C of: (a) NafionTM-117; (b) benzyl-sulfonate-grafted PBI and (c) PBI as a function of the H3PO4

concentration (immersion time 8 h) [46,47].

tivity as long as the corresponding polymer films aremaintained in an environment of high relative humid-ity. To an extent which increases with the initial degreeof sulfonation, they shrink and become brittle if leftto dry, and their initial flexibility cannot be recoveredby simply soaking in water [16,48]. Partial loss ofconductivity (to ca. 10−4 S cm−1) accompanies thistextural change. Infrared spectra of sulfonated PBIfilms show broad and intense absorption between ca.2000 and 3500 cm−1, which is characteristic of stronghydrogen bonding [16]. Such intra- and inter-chainhydrogen bonding crosslinks the sulfonated polymer,and modifies its mechanical and conduction proper-ties. This observation parallels that made by otherauthors on sulfonated (stabilised) PBI prepared bydirect sulfonation of the polymer backbone [29].

2.3. Doping with organic and inorganic bases

Immersion of pristine PBI in aqueous inorganicbases has recently been reported to increase its con-ductivity by almost an order of magnitude [49].However, the kinetics of this reaction are slow (im-mersion for 10 days), and the concentration of base(e.g. 8 M NaOH) high compared with the conditionsdescribed in Section 2.1 above required to complexPBI with acids. Uptake of base is very much fasterby benzylsulfonate-grafted PBI [46,47]; it has beenobserved that after a short contact time (15–60 min)with an aqueous solution of an organic or inor-ganic base, shrunken benzylsulfonate grafted PBI


Table 3Conductivity (25◦C/100% RH) of benzylsulfonate grafted (85%sites sulfonated) PBI following dehydration and immersion in basicsolutions (1 M, 15 min, 25◦C) [46,47]

Membrane Conductivity (S cm−1)

PBI-S 4.2× 10−4

PBI-S/NH4OH 1.5 × 10−2

PBI-S/(CH3)4OH 8.2 × 10−3

PBI-S/imidazole 7.9× 10−3

PBI-S/DABCOa 1.2 × 10−2

PBI-S/LiOH 1.2× 10−2

PBI-S/NaOH 1.2× 10−2

PBI-S/KOH 1.7× 10−2

PBI-S/CsOH 1.7× 10−2

a 1,4-Diazabicyclo[2.2.2]octane.

membranes achieve satisfactory textural and protontransport properties. In each case, the conductivity isclose to 10−2 S cm−1 at 25◦C/100% RH. In addition,if such base-treated membranes are exposed to a dryatmosphere, they will dry and wrinkle but recovertheir flexibility after a few seconds’ immersion inwater. Table 3 summarises the conductivity values ofsulfonated PBI after dehydration, and after immersionin basic solution.

It is interesting also to note that early work mademention of the conductivity of “sodium salted sul-fonated PBI”, 1.6 × 10−3 S cm−1 [21]; in view ofthe observations reported here, this high conductivitymay be that of a directly sulfonated PBI subsequentlytreated with NaOH.

In the IR spectra of base treated benzylsulfonategrafted PBI, the strong absorption in the highwavenumber region is much reduced in intensityin each case, and it is concluded that the hydrogenbonding network is disrupted by addition of OH−or an organic base. Rehydration of the membraneis assisted by the local formation of water throughthe reaction between base and protonated polymer.In the mid-frequency region, signals which can beinterpreted in terms of the presence of –SO3

− groups(at 1040 cm−1) and of protonated imidazole ring(νC=NH− at 1497 cm−1) indicate the polymer to be ina zwitterionic form [46,47].

Analysis of the alkali metal content shows themole ratio of SO3−/M+ to be ca. 1, and in orderto eliminate the possibility that alkali metal cationsmight contribute to the charge transport, the EMFof a cell operated with hydrogen at both electrodes

Fig. 4. EMF as a function of the log of the ratios of pressure ofhydrogen gas at the anode and cathode given by NafionTM-117and CsOH treated benzylsulfonate grafted PBI.

was measured. In initial experiments, a membrane ofbenzylsulfonate-grafted PBI/CsOH and a Nafion-117membrane were sandwiched between E-TEK elec-trodes, with 1 atm pressure hydrogen at both sides[50]. The EMF measured under such conditions was0.04 mV, close the value of zero expected if protonsare the only mobile species [51]. Additional proofwas provided by the variation in the EMF with thepartial pressure, Fig. 4, measured with 1 atm hydrogenat the anode side and 0.8–2.6 atm at the cathode side.The EMFs given by membranes of NafionTM-117and by base-treated sulfonated PBI are closely sim-ilar, providing good evidence that the proton is theonly charge carrier. However, preliminary investiga-tions of this membrane in a hydrogen oxygen fuelcell generated water at the anode side, implyingthat hydrogen is not transferred as H+, but rather asOH−. This system merits further study, and a fullerevaluation of the potential of such base-doped ben-zylsulfonate grafted PBI as an electrolyte membranein cells using both hydrogen and methanol fuels isunderway in our laboratory. In parallel, the stabil-ity of benzylsulfonate-grafted PBI to degradation byHO• radicals, such as can be present in a fuel cellenvironment, is being assessed by accelerated ageingtests with encouraging first results.

3. Polyaryletherketone-based systems

The polyetherketones are a family of non-fluorinatedpolymers made up of ether and ketone units, thatcan be either “ether-rich”: PEEK and PEEKK, or


Scheme 5. General formula of polyetherketones.

“ketone-rich”: PEK and PEKEKK, for example,Scheme 5. The oxidation stability is expected to in-crease with increasing content of ketone segments,and with a decrease in ether segments. In the studyof the thermohydrolytic stability of polymers alreadyreferred to above, Linkous [23] examined three mem-bers of the PEK family, and the results are reproducedin Table 4. PEK, PEKEKK and PEKK are all stableunder water-saturated hydrogen even up to 400◦C,and under water-saturated oxygen to 300◦C. PEKKunderwent least weight loss under oxygen/water at400◦C.

Sulfonation of polyetherketones can be carried outdirectly in concentrated sulfuric acid or oleum, theextent of sulfonation being controlled by the reactiontime and temperature [10–12], although it has beenreported that this method is not appropriate for thepreparation of polymers with a low degree of sul-fonation (<30%) since the sulfonation reaction takesplace at the same time as polymer dissolution; theresulting sulfonation is heterogeneous and the poly-mer microstructure difficult to reproduce [10]. The

Table 4Thermohydrolytic stability of polyetherketones, 24 h exposure [23]

Sample Atmosphere Weight change (%)

200◦C 300◦C 400◦C

Kadel E-10 (PEK) H2/H2O −0.2 −0.5 −1.0O2/H2O −0.3 −1.4 −26.9

Ultrapek (PEKEKK) H2/H2O −0.2 −0.0 −1.3O2/H2O −0.9 −0.4 −78.5

Declar (PEKK) H2/H2O −0.3 −0.9 −2.7O2/H2O −1.5 −1.1 −19.9

NafionTM H2/H2O −4.6 – –O2/H2O −6.9 – –

polyetherketone most readily available commerciallyis Victrex PEEK. Fig. 5 is the thermogravimetricanalysis of sulfonated PEEK of equivalent weight of625 g/mol, which corresponds on average to ca. 0.6acid groups per repeat unit. Although degradation ofthe polymer does not occur below 400◦C, its usefulrange is limited by the temperature of the onset ofdegradation of sulfonic acid groups, which occursat around 240◦C, in common with other sulfonatedpolymers: sulfonated polymides for example [19].Thermomechanical analysis showed that sulfonatedPEEK-S membranes undergo reversible elongation of0.6% in the temperature range 140–180◦C [52].

Various studies have been made of the conductivityof sulfonated PEEK [29,45,2–54]. The conductivityincreases as a function of the degree of sulfona-tion, the ambient relative humidity, temperature andthermal history. Fig. 6 compares the conductivity ofPEEK-S for degrees of sulfonation between 20 and65% at 100% RH as a function of temperature. For the

Fig. 5. Thermogravimetric analysis of sulfonated PEEK in air(heating rate 5◦C min−1).


Fig. 6. Comparison of the conductivity of sulfonated PEEK ofdegrees of sulfonation 20–65% as a function of temperature at100% RH (redrawn from ref. [29]).

PEEK-S with 65% sites sulfonated, the conductivitywas higher than that of NafionTM-117 measured underthe same conditions [29] and it reaches 0.04 S cm−1

at 100◦C/100% RH. The conductivity of PEEK-Sand NafionTM-117 has also been measured at 100◦Cas a function of RH between 50 and 100%, Fig. 7[52]. The PEEK-S was not pre-treated prior to mea-surement. Under these conditions, the dependence ofthe conductivity on RH is more marked for PEEK-Sthan for NafionTM; the former increases by an orderof magnitude between 66 and 100% RH to reach

Fig. 7. Dependence of the conductivity of sulfonated PEEK (equiv-alent weight 625 mol g−1; 60% sites sulfonated) on relative humid-ity at 100◦C [52]. No pre-treatment of PEEK prior to mountingin the conductivity cell.

Fig. 8. Temperature variation at 100% RH of the conductivity ofsulfonated PEEK (60% sites sulfonated): (a) no pre-treatment; (b)water-swollen by boiling the membrane in water for 4 h beforemounting in the conductivity cell [52].

0.02 S cm−1, whilst NafionTM increases by a factor ofca. 4. This value determined for PEEK-S is however,lower than that measured at room temperature on afully swollen sample. A second study compared thetemperature dependence up to 150◦C of the conduc-tivity of two PEEK-S membranes, one of which hadbeen pre-treated in boiling water for 4 h prior to mea-surement [52]. As shown in Fig. 8, the conductivityof the pre-swollen membrane has a weak temperaturedependence from 0.03 to 0.07 S cm−1 over the temper-ature range 25–150◦C, whereas that of the non-treatedmembrane increases by a factor 10 over the samerange. Treatment in boiling water therefore causes anincrease in membrane hydration which is maintainedwhen the membrane is cooled to room temperature,and this same degree of hydration is not attained insitu at 100% RH using a non-treated membrane.

In this respect a comparison of the water uptakeof NafionTM and generic PEK (not otherwise identi-fied) as a function of temperature is of interest, Fig. 9[45,55]. Both membranes have a hydration number ofaround 20 up to 80◦C but there is an abrupt increasein hydration number corresponding to an irreversibleswelling at 80◦C for generic PEK, which only occursat 140◦C for NafionTM. The primary hydration sphereis made up of 3 water molecules in NafionTM, and5 in sulfonated PEK-type polymers. Further differ-ence is found in the number of loosely bound water


Fig. 9. Water uptake of: (a) generic PEK (not further defined) and(b) NafionTM in water as a function of temperature (redrawn from[45]).

molecules: 11 in NafionTM but only 5 in PEK-type,and water is therefore present as a second phase be-yond 14 water molecules in the former and 10 inthe latter. As shown in Fig. 10, proton conductivityabove a value of 10−2 S cm−1 relies more heavilyon the amount of water as a second phase for sul-fonated polyetherketone polymers than for NafionTM

[45]. Fuller description is given by Kreuer in thisissue [55].

Fig. 10. Room temperature proton conductivity of: (a) NafionTM

and (b) sulfonated generic PEK as a function of water content(redrawn from ref. [45]). Two phase region (water as a secondphase) betweenn = 13 and 16 for NafionTM and betweenn = 10and 30 for sulfonated PEK.

The fuel cell performance of sulfonated PEEK hasrecently been reported [52]. Fig. 11 demonstratesthe polarisation characteristics at 90◦C of an 18mmPEEK-S membrane on air and on oxygen. Tests wereperformed discontinuously (shutdown overnight) onair without encountering any mechanical problems,and a peak power density of 0.57 W cm−2 was ob-tained with 3.5 bar pressure hydrogen and 4 barpressure air. Use of lower pressure (pH2 = pair =1.5 bar atm), with correspondingly higher volumet-ric flow rates, tended to lead to some drying out ofthe membrane above 80◦C. Using oxygen instead ofair results in approximately 45–90 mV voltage gainwithin the current density range of 0.2–0.8 A cm−2.These results are particularly encouraging since theelectrodes used were standard ELAT (1 mg Pt cm−2),not modified for medium temperature functioning noroptimised for use with sulfonated PEEK. Long-termtests of up to 4300 h in a hydrogen/oxygen fuel cellat 50◦C and atmospheric pressure have been reported[56] using a current density of 0.5 A cm−2 at a cellvoltage of 500 mV.

PEEK-S membranes have also been cast incorpo-rating woven polymer supports and non-woven glassfibre mats, and the fuel cell performance has beenreported [52]. Whilst the mechanical robustness ofreinforced membranes is superior to that of their nonreinforced congeners, the polarisation characteristicsare not significantly affected. A major improvementin mechanical strength of thin perfluorosulfonatedmembranes was brought about by the development ofmicro-reinforcements. GoreTM-type membranes areof higher conductance (due to reduced thickness) al-though lower ionic conductivity [57]. Reinforcementsmust of course show the same thermal stability andresistance to oxidative attack as the polymer elec-trolyte and, insofar as these conditions are respectedit may be concluded that this represents a useful wayforward for further improvement.

Sulfonated PEEK has also been used as a matrixfor the inclusion of inorganic oxides and inorganicproton conductors [58]. The association of organicpolymers and inorganic proton conductors has beeninvestigated since the beginning of the 1990s, firstlyusing non-modified polymers such as PBI, polyether-sulfone (PES), etc. [20]. Fig. 12 illustrates the con-duction properties of an inorganic proton conductorof bulk conductivity 3× 10−2 S cm−1 after shaping


Fig. 11. Polarisation characteristics of PEEK-S membrane, thickness 18mm, 16 cm2 active area single cell, Nafion impregnated ELATelectrodespH2 = 3.5 and (above)pair = 4 bar abs, cell temperature 100◦C, humidifiers: 105◦C; cell temperature 90◦C, humidifiers: 90◦C;cell temperature 110◦C, humidifiers: 120◦C; (below)pO2 = 4 bar abs, cell temperature 90◦C, humidifiers: 100◦C [52].

in an insulating polymer matrix, in this case PES.These membranes were prepared by dispersing asieved powder of protonic conductor in a polymersolution. The conduction properties of the inorganiccomponent are “diluted” and, for example, when amembrane is cast from a mixture containing 40 wt.%proton conductor and 60 wt.% PES, the conductivityis lowered by a factor 102. It is not until the propor-tions are increased to 70–75% electrolyte and 30–25%PES that a percolation effect is observed and that theconductivity of the membrane approaches that of thebulk inorganic conductor. However, at such relativeproportions the membranes are mechanically unsatis-

factory, being friable and fragile. An important resultof this early work however, was the demonstrationthat hybrid inorganic–organic membranes have lowermethanol crossover than commercial perfluorinatedmembranes [59].

The methanol crossover of NafionTM can also bereduced using this approach. When it is recast fromaqueous alcohol solution containing dispersed silica,and heated above 150◦C, the composite electrolytemembrane has been demonstrated to function as anefficient separator in a DMFC operating on oxygen at145◦C [60], without dehydration, phase transition ormechanical instability. The high open circuit voltage


Fig. 12. Conductivity of composite membranes with polyethersul-fone and polybenzimidazole as a function of the weight percentof proton conductor embedded in the polymer. Conductivity ofbulk inorganic proton conductor: 0.03 S cm−1. All measurementsat 70◦C and 100% RH [20].

of 0.95 V indicates low methanol crossover. In fuelcells operating with hydrogen and air or oxygen,the dispersion of fine silica particles inside perflu-orosulfonic membrane favours their water retentiveproperties, permitting operation of the fuel cells witha reduced water content in the reactant gases [60].

The method of inclusion of inorganic proton con-ductor or inorganic particle has also evolved fromone whereby a bulk powder is dispersed in a polymersolution to methods leading specifically to particlesof highly dispersed inorganic proton conductors ofparticle size in the sub-micronic range. These meth-ods make use of mild chemical techniques, includ-ing intercalation/exfoliation, sol–gel chemistry, andion-exchange [58]. Such approaches generally avoidany sedimentation of the inorganic component. Inti-macy of contact between the inorganic and organiccomponents at the molecular level assures the great-est possible interface and, at such small particle size,the mechanical properties can be improved comparedwith those of a polymer-only membrane. In addi-tion, since in many proton conductors of conductivitysuitable for electrochemical applications the protontransfer process takes place on the surface of the par-ticles, increase in surface area (small particle size)will increase the conductivity displayed.

The layered metal(IV) phosphates (zirconiumphosphate (ZrP), tin phosphate (SnP)) are well-known

ion-exchangers and proton conductors that areprepared in bulk form by precipitation from a solu-tion containing M(IV) ions with phosphoric acid [61].Fig. 13 shows a transmission electron micrographof a sulfonated PEEK membrane incorporating ZrPparticles, and that of a non-modified PEEK mem-brane for comparison. Both membranes are flexibleand transparent, the presence of 20 wt.% ZrP in theformer not affecting the light scattering properties. Inthe transmission electron micrograph of sulfonatedPEEK, only dark and light regions can be seen, whilein the hybrid membrane a homogeneous distributionof inorganic particles which are of approximatelyrectangular form may be discerned. These particlesare small, of dimensions ca. 15–30 and 5–10 nm.Fig. 13 also shows a micrograph of a sulfonatedPEEK membrane incorporating tin phosphate. Here,the particles are smaller in size and more isotropic inform. This observation is confirmed by the particle(domain) size that can be calculated from the widthof X-ray diffraction lines. The diffraction patterns ofsulfonated PEEK–ZrP and sulfonated PEEK–SnP areshown in Fig. 14, with those of non-modified PEEKand of the bulk proton conductors. The diffractionpatterns of the hybrid membranes are characterisedby relatively intense reflections from theh k0 planes,and broad and weak reflection from the 0 0 2 plane,compared with the relative intensities observed forthe bulk metal phosphates. The domain size esti-mated from the width of the 0 0 2 diffraction line isca. 11 nm, which would correspond for ZrP, with aninterlayer distance of 7.56 Å, to about 13–15 stackedlayers. The tin phosphate in sulfonated PEEK is oflow degree of crystallinity, and the first diffractionline is to lower angle than that of bulk SnP, indicat-ing the presence of a slightly expanded, and morehydrated form of tin phosphate.

The conductivity of sulfonated PEEK membranesincorporating zirconium phosphate particles [58] ishigher than either that of the polymer-only membranerecorded under the same conditions, and having un-dergone the same treatment prior to measurement, orthat of bulk zirconium phosphate. The conductivityof ZrP is known to strongly depend on degree ofcrystallinity, since its electrical properties result fromsurface conduction. This implies that the sulfonatedmembrane stabilises particles of ZrP smaller (and ofmore developed surface area) than those that can be


Fig. 13. Transmission electron micrographs of: (a) sulfonated PEEK; (b and c) hybrid membrane of sulfonated PEEK incorporating zirconiumphosphate particles; (d) hybrid membrane of sulfonated PEEK incorporating tin phosphate particles. For (a) and (b) magnification× 25,000;for (c) and (d) magnification× 50,000.


Fig. 14. Powder X-ray diffraction patterns of (above) sulfonatedPEEK, bulk a-zirconium phosphate and a hybrid membrane ofsulfonated PEEK incorporating 25 wt.% zirconium phosphate and(below) bulk tin phosphate and a hybrid membrane of sulfonatedPEEK incorporating 20 wt.% tin phosphate

Fig. 16. Polarisation characterisations of hybrid membrane of thickness 50mm of sulfonated PEEK–zirconium phosphate (25 wt.% inorganiccomponent). E-TEK electrodes 30% Pt/C; 1 mg Pt cm−2. 10 cm2 active area. Electrodes not hot-pressed.pH2 = pO2 = 2.6 bar abs [58].

Fig. 15. Variation of the conductivity of hybrid membranes ofsulfonated PEEK–zirconium phosphate at 100◦C and 100% RHwith wt.% ZrP. All membranes (PEEK-S and PEEK-S–ZrP) werepre-swollen in 85% H3PO4 for 5 d at 80◦C followed by treatmentin water at 100◦C for 4 h [58].

prepared in a bulk synthesis, even by rapid precipi-tation. This interpretation is supported by the data ofFig. 15, which show the conductivity of the hybridsystem to increase with the wt.% of inorganic com-ponent, varied in the range up to 30 wt.% [58]. Theseobservations provide a further advantage of a veryhighly dispersed inorganic component, in addition tothose of water management or improved mechanicalstability already referred to above.

The polarisation characteristics in a hydrogen–oxygen fuel cell of a sulfonated PEEK–ZrP mem-brane of thickness 70mm are shown in Fig. 16,


where improvement in performance is observed as thetemperature is increased from 85 to 100◦C.

The association of two kinds of polymer able tointeract either by hydrogen bonding or followingpartial proton transfer from, e.g. the sulfonic acidgroup of one polymer to, e.g. the amino group ofanother is a means of creating ionic crosslinks, theadvantages of which include improved mechanicalstrength due to reduced tendency to swelling. On theother hand the cation exchange capacity is lowered,which could affect conduction properties. SulfonatedPEEK has been blended with basic polymers suchas polyethyleneimine (PEI), PBI, aminated polysul-fone (aminated PSU), and poly(4-vinylpyridine), andthe corresponding electrolyte membranes used inboth hydrogen–oxygen and direct methanol fuel cells[62,63]. Fuller results are described in the article byKerres in this issue [64].

4. Conclusion

The functionalisation, by acid doping, chemicalgrafting of protogenic groups or direct sulfonation byelectrophilic substitution on the polymer backbone,of commercially available and hydrothermally stablepolymers, such as those of the polyetherketone fam-ily or polybenzimidazole taken as examples in thiscontribution, leads to a significant improvement intheir proton conduction properties. This is partly dueto the increase in density of mobile protons but alsoto the increased water uptake that is authorised bythe presence of the protogenic group. The degree offunctionalisation must, however, be controlled, sincethis enhanced hydrophilicity can lead to increasedsoftness of the polymer, irreversible swelling, andwater solubility in extreme cases. Even with this lim-itation, the proton conductivity of phosphoric acidcomplexed PBI, base-doped benzylsulfonate-graftedPBI, and sulfonated PEEK exceeds 10−2 S cm−1 atthe respective potential operating temperatures, ad-equate for fuel cell use. Sulfonated PEEK showsexcellent polarisation characteristics in H2–O2 (air)fuel cells, rivalling those of commercial perfluori-nated membranes, however other questions remainto be resolved, in particular, the long-term stabilityof membranes based on these polymers in fuel cells.This long term stability depends on factors such as

reactant crossover, susceptibility to radical degrada-tion, need for a pressurised system to maintain a sat-isfactory degree of hydration, etc. Optimisation of theelectrode both for higher temperature functioning andfor use with polymer electrolytes other than NafionTM

is also essential. Despite the efforts that still need tobe made, the advances made in the past decade repre-sent considerable progress, and viable non-fluorinatedpolymer alternatives to perfluorinated fuel cell mem-branes, and membranes capable of extending thetemperature range of PEM fuel cells into the 150◦Crange may be foreseen within the near future.

Acknowledgements

We thank Giulio Alberti and Mario Casciola, of theUniversity of Perugia, Enrico Ramunni of De Nora,and Bernd Bauer, of FuMA-Tech GmbH, for many dis-cussions and stimulating exchanges in the frameworkof joint European projects. We gratefully acknowledgefinancial support by the European Commission underBRPR-CT97-0408.

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Documents

Recent advances in the functionalisation of polybenzimidazole and polyetherketone for fuel cell applications