New sulfonated polybenzimidazole (SPBI) copolymer-based proton-exchange membranes for fuel cells

New sulfonated polybenzimidazole (SPBI)copolymer-based proton-exchange membranes for fuel cellsHe Bai a ,b ,W.S.Winston Ho a ,b ,*a Department of Chemical and Biomolecular Engineering,The Ohio State University,140West 19th Avenue,Columbus,OH 43210-1180,USA bDepartment of Materials Science and Engineering,The Ohio State University,2041College Road,Columbus,OH 43210-1178,USA1.IntroductionIn recent years,great progress has been made on the development of proton-exchange membrane fuel cells (PEMFCs)for both mobile and stationary applications,particularly for fuel cell vehicles.Dupont’s Nafion 1and other perfluorinated sulfonic acid membranes are currently popular to use for low temperature PEMFCs due to their high proton conductivity,as well as desirable mechanical strength and chemical stability.However,some disadvantages,such as high cost,relatively low conductivity at high temperatures (above 1008C)and low humidities,and high dependence of proton conduction on the water content seriously limit the industrial application of these membranes.High temperature operations can increase the anode’s tolerable level of CO in the fuel and accelerate the reaction rates at the anode and cathode.Low humidity operations can facilitate the water management of the fuel cell system.Therefore,it is desirable for a PEMFC to operate at high temperatures (above 1008C)and lowrelative humidities (below 50%RH).As a result,the development of competitive and less expensive proton-exchange membranes (PEMs)that have effective performance at high temperatures is crucial for fuel cell applications.Many efforts have been initiated to synthesize cost-effective and thermally stable alternative membranes,including sulfo-nated poly(arylene ether sulfone)copolymers (Ghassemi et al.,2006;Harrison et al.,2005;Wang et al.,2002),sulfonated poly(aryl ether ketone)copolymers (Gil et al.,2004;Xing et al.,2004),sulfonated polyimide copolymers (Bai and Ho,2008a,b;Ye et al.,2006),and radiation-grafted membranes (Gubler et al.,2005;Shen et al.,2005).Several membranes have continued to attract interest.Polybenzimidazoles (PBIs)have currently been proposed as alternative candidate polymers for high temperature PEMFCs because of their outstanding thermal,oxidative,chemical,and hydrolytic stability under fuel cell operating conditions,particu-larly at high temperatures.However,PBI must be modified since it is not a proton-conductive material by itself.Several different approaches have been carried out to achieve this goal.One method was the introduction of sulfonic acid groups into PBI polymers by chemical modification (Ariza et al.,2002;Asensio et al.,2002;Bae et al.,2002;Glipa et al.,1997;Qing et al.,2005,2006).However,the conductivities of these SPBI membranes wereJournal of the Taiwan Institute of Chemical Engineers 40(2009)260–267A R T I C L E I N F O Article history:Received 1August 2008Received in revised form 5December 2008Accepted 15December 2008Keywords:Sulfonated polybenzimidazole (SPBI)CopolymerProton-exchange membrane (PEM)High temperature Low humidityProton conductivity Fuel cellA B S T R A C TThe synthesis and characterization of sulfonated polybenzimidazole (SPBI)copolymers as proton-exchange membranes (PEMs)for fuel cells are described in this paper.A one-step high temperature polymerization method was used to synthesize the SPBI copolymers from 3,30-diaminobenzidine (DABD),4,40-oxybis(benzoic acid)(OBBA),and two sulfonated dicarboxylic acids,i.e.,5-sulfoisophthalic acid (SIPA)and 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (DSNDA),using polyphosphoric acid (PPA)as the solvent.The pure SPBI copolymer membrane was prepared by the traditional solution-casting technique (technique 1),and the SPBI/H 3PO 4blend membrane was prepared through a new direct hot-casting and in situ phase inversion technique (technique 2).The membranes prepared from both techniques possessed desirable mechanical,chemical,thermal,and hydrolytic stability.However,the pure SPBI copolymer membranes from technique 1exhibited very poor proton conductivities since most of the sulfonated groups in SPBIs were neutralized by the basic imidazole groups and became deprotonated sulfonated groups.On the other hand,the SPBI/H 3PO 4blend membrane from technique 2showed extremely high conductivities (>0.1S/cm)at high temperatures (>1208C)and low relative humidities (RHs)(even at anhydrous conditions),which has great potential to be used as high temperature and low humidity PEMs for fuel cells.ß2009Taiwan Institute of Chemical Engineers.Published by Elsevier B.V.All rights reserved.*Corresponding author at:Department of Chemical and Biomolecular Engineer-ing,The Ohio State University,140West 19th Avenue,Columbus,OH 43210-1180,USA.Tel.:+16142929970;fax:+16142923769.E-mail address:ho@ (W.S.Winston Ho).Contents lists available at ScienceDirectJournal of the Taiwan Institute of Chemical Engineersj o u r n a l ho m e p a g e :w w w.e l s e v i er.c om /l o ca t e /j t i c e1876-1070/$–see front matter ß2009Taiwan Institute of Chemical Engineers.Published by Elsevier B.V.All rights reserved.doi:10.1016/j.jtice.2008.12.014relatively low since most of the sulfonic acid groups were neutralized by the basic imidazole groups and could not contribute to proton conductivity any more.Another method was the physical blending of PBI with other sulfonic acid-containing proton-conductive polymers(Hasiotis et al.,2001;Kosmala and Schauer,2002).Similarly,the PBI basic imidazole groups could inactivate some of the sulfonic acid groups in the blending process,thus the resulting membranes exhibited lower conductivities than the pure sulfonated polymers.However, other physical properties of the membranes were much improved by the incorporation of PBI compared to the pure sulfonated polymers.Generally,this blending method is very useful if the pure sulfonated polymer has extremely high proton conductivities but undesirable physical properties.Currently,the acid-doping method is the most successful approach for the application of PBI membranes as high tempera-ture and low humidity PEMs(Asensio et al.,2003;Chuang and Hsu, 2006;He et al.,2003;Li et al.,2001,2004;Lobato et al.,2007; Savadogo and Xing,2000;Xiao et al.,2005).The potential-current characteristics of PEMs using H2SO4or H3PO4-doped PBI were studied in a H2/O2fuel cell in Savadogo’s group(Savadogo and Xing,2000).The results showed that the H2SO4-doped PBI membranes exhibited desirable fuel cell performance at low temperatures(e.g.,508C)and the non-humidified condition.The H3PO4-doped PBI membranes exhibited improved fuel cell performance with a temperature increase to1858C without outside humidification.Furthermore,at1858C,the presence of even3%CO did not have significant effects on polarization curves. This work opened the way to a new approach for the modification of PBI membranes as high temperature and low humidity PEMs.Bjerrum and his co-workers studied the H3PO4-doped PBI system more systematically(He et al.,2003;Li et al.,2001,2004). They reported that with a H3PO4-doping level of5.6(mole number of H3PO4per repeat unit of PBI),the membrane showed conductivity as high as6.8Â10À2S/cm at2008C and5%RH.In addition,the membrane conductivity had higher H3PO4-doping level and temperature dependence than humidity dependence. With the increase of H3PO4-doping level and temperature,the membrane conductivity significantly increased.However,humid-ity did not have a significant effect on membrane conductivity,and the fuel humidification was not necessary in fuel cell operations due to the proposed unique proton conduction mechanism by self-ionization and self-dehydration(He et al.,2003):5H3PO4@2H4PO4þþH3OþþH2PO4ÀþH2P2O72À(1) On the other hand,the increase of humidity could also increase the membrane conductivity due to the dissociation of acid and, therefore,the increased numbers of charge carriers(He et al., 2003):H3PO4þH2O@H3OþþH2PO4À(2)Further research showed that the PBI membrane with a H3PO4-doping level of5.6exhibited very attractive fuel cell performance at high temperatures and low RHs.A maximum power output of over1.0W/cm2could be achieved at2008C and3atm without gas humidification,and a CO tolerance of up to several percent could be obtained.Moreover,the lifetime of continuous operation for over5000h at1508C and the shutdown–restart thermal cycle testing for47times were achieved(Li et al.,2004).Benicewicz and co-workersfirst proposed a new method for the preparation of PBI/H3PO4membranes(Xiao et al.,2005).The polymerization solution of PBI was cast directly at high tempera-tures,and the membrane could be obtained in situ after the solvent PPA hydrolyzed into non-solvent phosphoric acid(PA).This method could reserve the H3PO4material used in post-acid-doping,save time for the post-acid-doping process(normally around1week),and increase the H3PO4-doping level to improve the membrane performance.The resulting membrane showed better conductivities than the membranes prepared from the traditional post-acid-doping method at high temperatures and low RHs.Moreover,the membrane showed very attractive fuel cell performance at1608C without gas humidification,and the long-term stability of more than1200h was obtained at this condition.In this work,we report the synthesis of the new SPBI copolymers.Both the pure SPBI copolymer membranes and the SPBI/H3PO4blend membrane were successfully prepared.The physical properties and proton conductivities of the resulting membranes were investigated,and their potential application for high temperature PEMFCs was explored.2.Experimental2.1.MaterialsPolyphosphoric acid(PPA,115wt.%H3PO4equivalent),potas-sium hydroxide(KOH,85+%),fuming sulfuric acid(27–30wt.% SO3),phosphoric acid(PA,85wt.%aqueous solution),and dimethyl sulfoxide(DMSO,99.5+%),all from Aldrich(Milwaukee,WI),were used as received without further purification.3,30-Diaminobenzi-dine(DABD,99%,Aldrich),4,40-oxybis(benzoic acid)(OBBA,99%, Aldrich),and5-sulfoisophthalic acid monosodium salt(SIPA-Na, 98%,Acros Organics,Morris Plaines,NJ)were dried in a vacuum oven at1208C overnight before the reaction.2,6-Naphthalenedi-carboxylic acid(NDA,98+%)was purchased from TCI America (Portland,OR).A sample(from National Chemical Laboratory, Pune,India)of PBI polymer(structure:poly(2,20-m-phenylene)-5,50-bibenzimidazole(Li et al.,2004))was kindly provided by Unitel Technologies,Inc.(Mt.Prospect,IL)for use in a comparison study.4,8-Disulfonyl-2,6-naphthalenedicarboxylic acid(DSNDA)was self-synthesized by direct sulfonation of NDA as described elsewhere(Qing et al.,2005).NDA in the amount of 5.0g (22.7mmol,98+%)was charged into a four-neckflask equipped with a mechanical stirring device and a nitrogen inlet.Then, fuming sulfuric acid(27–30wt.%SO3)in the amount of22.5mL was slowly added with vigorous stirring and nitrogen purge.The sulfonation reaction proceeded at1508C for 6.5h.After the reaction,the mixture was poured into400mL ice-water to obtain a clear brown solution.Next,100g of NaCl was added into the solution to salt out the product(white powder).The raw product precipitate wasfiltered and was re-dissolved in400mL of5wt.% Na2CO3solution.The product was completely dissolved in the aqueous solution again,and a brown solution was obtained. Finally,the solution was acidified with37wt.%HCl aqueous solution,and the white pure product DSNDA was precipitated.The product was collected and dried in vacuum at1108C overnight.A total of7.36g of white powder DSNDA was obtained,yield=84.6%.2.2.Synthesis of new SPBI copolymersA typical procedure for the preparation of sulfonated poly-benzimidazole copolymers is described below using the SPBI-1 copolymer with the composition of DABD/SIPA(70mol%)/OBBA (30mol%)as an example(Fig.1).In the preparation,PPA in the amount of57g was charged into a250-mL,completely dried four-neckflask equipped with a mechanical stirring device and a nitrogen inlet.Theflask was heated to808C under stirring with a nitrogenflow for around half an hour,and PPA became less viscous after heating.Then,2.164g(10.0mmol,99%)of DABD,1.916g (7.0mmol,98%)of SIPA-Na,and0.782g(3.0mmol,99%)of OBBA were added successively into the PPA solvent to obtain the solutionH.Bai,W.S.W.Ho/Journal of the Taiwan Institute of Chemical Engineers40(2009)260–267261with a solid concentration of 7.87wt.%.The reaction was continued at 808C for 1–2h and 2008C for 8–10h with the nitrogen purge.Finally,a very viscous brown solution was observed.After the reaction,part of the highly viscous copolymer solution was poured into 400ml water with vigorous stirring,and the brown fiber-like precipitate was filtered and washed thoroughly with 200mL of 10wt.%KOH aqueous solution.The resulting brown SPBI-1-K copolymer powder was collected after drying at 1208C in vacuum overnight.A yield of more than 98%could be obtained.The SPBI-1-K copolymer was used for membrane preparation (pure SPBI-1copolymer membrane)by the traditional solution-casting techni-que (technique 1).In addition,another part of the polymerization solution was used directly for membrane preparation (SPBI-1/H 3PO 4blend membrane)by an in situ phase inversion technique (technique 2)(Xiao et al.,2005).A similar procedure was followed for the preparation of another copolymer SPBI-2.Copolymer SPBI-2had the composition of DABD/DSNDA (70mol%)/OBBA (30mol%)(Fig.1).2.3.Membrane preparationThe pure SPBI copolymer membranes were prepared by the traditional solution-casting technique (technique 1).The SPBI copolymer (in potassium salt form)was dissolved in DMSO to obtain a homogeneous solution with the concentration of 5wt.%.The membranes with a controlled thickness were prepared by casting the solutions onto clean glass plates,using a GARDCO adjustable micrometer film applicator (Paul N.Gardner Company,Pompano Beach,FL).The as-cast films were dried in a hood overnight,followed by drying in an oven at 808C for 6h and 1208C for 12h.Then,the membranes were carefully removed from the glass substrates,and were soaked for the purpose of proton-exchange treatment in 1.0M hydrochloric acid at room tempera-ture for more than 24h.The resulting free-standing membranes (in acid form)were thoroughly washed with de-ionized water and dried in vacuum at 908C overnight (Fig.2(a)).A Mitutoyo Electronic Indicator (Model:543-252B,Mitutoyo America Cor-poration,Aurora,IL)was used to measure the membrane thicknesswith an accuracy of Æ0.5m m.The final membranes had a thickness of approximately 30m m.The SPBI-1/H 3PO 4blend membrane was prepared through a new direct hot-casting and in situ phase inversion technique (technique 2)(Xiao et al.,2005).After polymerization,the hot viscous solution with a concentration of 7.87wt.%was cast directly on a glass plate in an oven at 1808C with a controlled thickness.Then,the wet membrane was cooled from the polymerization temperature to room temperature in a hood (258C and 45%RH)for 24h.In this period,in situ phase inversion (transition from the solution state to the gel state)of the membrane was observed during the hydrolysis of the solvent PPA (a good solvent for SPBI-1)to PA (a poor solvent for SPBI-1)since both SPBI-1and PPA are extremely hygroscopic and the moisture could be easily absorbed to hydrolyze PPA into PA.Finally,a very uniform and flexible SPBI-1/H 3PO 4blend membrane was obtained (Fig.2(b)).The membrane was wiped with a tissue paper to remove all remaining H 3PO 4on the membrane surface and was dried in an oven at 1208C for 10h.The final membrane had a thickness of about 70m m.2.4.Membrane characterizationThe pure SPBI copolymer membranes (from technique 1)and the SPBI-1/H 3PO 4blend membrane (from technique 2)were characterized with ion-exchange capacity (IEC),Fourier transform infrared (FTIR)spectrometry,thermogravimetric analysis (TGA),H 3PO 4-doping level,and proton conductivity measurements.2.4.1.Ion-exchange capacityThe IEC was measured by means of a classical titration method.A membrane sample of about 0.200g was soaked in 50mL of 1.0M NaCl solution for 2days.The released proton concentration was titrated using a 0.01-M NaOH solution.2.4.2.Fourier transform infrared spectraFTIR spectra were recorded on a Nexus 4701FTIR Spectrometer (Thermo Nicolet,Madison,WI),using Smart MIRacle TMSingleFig.1.Schematic synthesis of the new sulfonated polybenzimidazole copolymers.H.Bai,W.S.W.Ho /Journal of the Taiwan Institute of Chemical Engineers 40(2009)260–267262Reflection Horizontal ATR(attenuated total reflectance).Each of the membrane samples was put in appropriate physical contact with the sampling plate of the spectrometer accessory,yielding high quality and reproducible spectra.2.4.3.Thermogravimetric analysisTGA was performed to estimate the thermal stability of the membranes with a Pyris1TGA thermogravimetric analyzer (PerkinElmer,Shelton,CT)at a heating rate of208C/min in N2 in the temperature range of30–8008C.2.4.4.H3PO4-doping levelThe H3PO4-doping level of the SPBI-1/H3PO4blend membrane was calculated as the numbers of H3PO4per repeat unit of SPBI-1 (two basic imidazole groups).Firstly,the SPBI-1/H3PO4blend membrane was dried in vacuum and weighted as W b.Then,the membrane was soaked in de-ionized water at room temperature for several days to remove all the combined H3PO4,and the resulting pure SPBI-1membrane was dried in vacuum and weighted as W p.Thus,the H3PO4-doping level could be calculated from the following equation:H3PO4-doping level¼ðW bÀW pÞ=98:0W p=396:4(3)where98.0is the molecular weight of H3PO4,and396.4is the molecular weight of the SPBI-1copolymer repeat unit(70%of the SIPA-based segment and30%of the OBBA-based segment).Three membrane samples were measured using the above method,and the average value was obtained.2.4.5.Proton conductivity measurementThe method for proton conductivity measurements was based on a four-point-probe electrochemical impedance spectroscopy (EIS)technique(Guo et al.,2002;Yin et al.,2003).The existing fuel cell(25cm2effective membrane area)hardware(ElectroChem, Inc.,Woburn,MA),gas paths,and test station(Model890C, Scribner Associates,Inc.,Southern Pines,NC)were used for proton conductivity measurements.As shown in Fig.3(a),a conductivity cell(BekkTech LLC,Loveland,CO)was installed between the anode and cathode plates of the fuel cell hardware,and the membrane conductivity was measured through the connected four probes. Fig.3(b)shows the inside structure of the conductivity cell,which consisted of a Teflon block,a membrane clamp,and four platinum wires.The two outside platinum wires were used as working and counter electrodes to apply a current to the sample membrane (2.5cmÂ0.5cm)through the two connected platinum gauzes, and the two inside platinum wires at0.425cm apart were used as reference electrodes.The membrane sample was equilibrated by the incoming hydrogen gas with a specific humidity level through the existing gas path.The relative humidity level of the hydrogen gas was controlled by the set temperature of the self-designed humidifier. The temperature of the membrane sample was maintained by the fuel–cell test station,and the back pressure regulator was used to adjust the system pressure.The AC impedance measurements at various cell temperatures and relative humidity levels were carried out over the amplitude of300mV and the frequency range from10kHz to1Hz using a Solartron1260frequency response analyzer and a Solartron1287potentiostat(Solartron Analytical, Houston,TX).The resistance value associated with themembrane Fig.2.Appearance of(a)the pure SPBI-1copolymer membrane from technique1and(b)the SPBI-1/H3PO4blend membrane from technique2.Fig.3.The pictures of the apparatus for proton conductivity measurements:(a)outside appearance of the assembled conductivity cell and(b)inside structure of the conductivity cell.H.Bai,W.S.W.Ho/Journal of the Taiwan Institute of Chemical Engineers40(2009)260–267263conductance was determined from the intercept of the impedance with the real axis using the Zplot/Zview software(Beattie et al., 2001).From the measured membrane resistance,the proton con-ductivity,s,of the membrane was calculated using the following equation:s¼L(4)where L is the distance between the two reference electrodes,R is the measured membrane resistance value,and D and W are the thickness and width of the sample membrane at the ambient conditions,respectively.The possible effects of membrane swelling on the changes of the dimensions during measurements were not considered.3.Results and discussionPBI has been reported to be a very thermally and hydrolytically stable polymer material(Li et al.,2004).SPBI copolymers were self-synthesized based on the chemistry,as shown in Fig.1.A one-step high temperature polymerization method was utilized in the solvent of PPA.This method has been extensively employed in the literature(Xiao et al.,2005).The monomer SIPA or DSNDA was copolymerized into SPBIs to introduce sulfonated groups,which could provide proton transport ability.A controlled relative ratio (m:n=7:3in Fig.1)of OBBA was copolymerized into SPBIs to increase theflexibility of the resulting membranes(the prepared membranes were very brittle without using OBBA).3.1.Ion-exchange capacity valuesThe measured IEC values of the pure SPBI copolymer membranes are listed in Table1.As shown in table,the measured values were much lower than the calculated ones.The reason is that most of the sulfonic acid groups in SPBIs were neutralized by the basic imidazole groups and became deprotonated sulfonated groups.3.2.Infrared spectrumThe FTIR spectrum of the pure copolymer SPBI-1is shown in Fig.4,which is consistent with the literature(Qing et al.,2006).No residual carbonyl absorption between1780and1650cmÀ1could be observed,suggesting the nearly complete closure of the imidazole rings.The band at3390cmÀ1was ascribed to the stretching vibration of the non-hydrogen bonded N–H groups, the peak located at3180cmÀ1was attributed to the stretching vibration of the hydrogen bonded N–H groups,and the very broad band due to the N+–H stretching mode in the range of2650–2950cmÀ1might be related to the protonation of nitrogen in imidazole rings.The protonation was caused by the fact that the sulfonic acid group in the copolymer SPBI-1reacted with the basic imidazole.The benzimidazole characteristic bands were clearly observed at1630cmÀ1(C Na C sTable1IEC values and conductivities(at1208C and100%RH)of the pure SPBI copolymer membranes.Membranes Calculated IEC(mmol/g)Measured IEC(mmol/g)Conductivity(S/cm)at1208Cand100%RHSPBI-1 1.770.30 2.17Â10À4 SPBI-2 2.900.59 6.10Â10À4Fig.4.FTIR spectrum of the copolymerSPBI-1. Fig.5.TGA curve of the copolymer SPBI-1.H.Bai,W.S.W.Ho/Journal of the Taiwan Institute of Chemical Engineers40(2009)260–267 264thermally stable up to around 4608C.Thus,it is suitable to be used for high temperature PEMFCs.The physical properties of the SPBI-2copolymer were not measured in this research.However,similar physical properties are expected since they are the same class of polymer matrix (SPBI).3.4.H 3PO 4-doping levelThe H 3PO 4-doping level of the SPBI-1/H 3PO 4blend membrane from technique 2was calculated to be 7.7,which is higher than that of the PBI/H 3PO 4membrane prepared from the traditional post-acid-doping method (H 3PO 4-doping level =5.6)(He et al.,2003;Li et al.,2004).The reason is that the solvent for the synthesis of the new SPBI-1copolymer was PPA (115wt.%PA equivalent);thus,during the in situ phase inversion process,PPA hydrolyzed into PA,and the membrane was equilibrated with 115wt.%PA.However,by using the traditional post-acid-doping method,the membrane was equilibrated with 85wt.%PA.Therefore,the membrane prepared from this new in situ phase inversion technique could absorb and maintain more PA,and the perfor-mance of the resulting blend membrane should be better.3.5.Proton conductivitiesThe conductivities of the pure SPBI copolymer membranes at 1208C and 100%RH (with the corresponding high pressure applied)are also listed in Table 1.The membranes showed very low conductivities since most of the sulfonic acid groups were neutralized by the basic imidazole groups and became deproto-nated sulfonated groups,which could not contribute to proton conductivity any more.This phenomenon has been proven by IEC value measurements.The proton conductivity of the new SPBI-1/H 3PO 4blend membrane was measured at high temperatures (120–1608C)and low RHs (0–50%)at atmospheric pressure.As shown in Fig.6,the blend membrane exhibited very desirable conductivities (>0.1S/cm)even without external humidification,which was due to the H 3PO 4self-ionization and self-dehydration mechanism,as proposed in Eq.(1).Moreover,with the increase of humidity,the membrane conductivity also increased due to the dissociation of acid and,therefore,the increased numbers of charge carriers,as proposed in Eq.(2).However,the humidity effect was not assignificant as Nafion 1since this kind of PBI-based membranes were much less humidity-dependent,as reported in the literature (He et al.,2003;Li et al.,2004).In addition,at the same humidity,with the increase of temperature,the membrane conductivity increased due to the activation energy effect,which was consistent with the literature (He et al.,2003;Li et al.,2004).These results are very promising and nearly non-existing in the literature,which could essentially meet the DOE (Department of Energy)target for PEM materials.Thus,the new SPBI-1/H 3PO 4membrane has great potential for high temperature and low RH fuel cell applications.Fig.7shows the conductivity comparison for different membranes at 1208C and low RHs (0–50%).The PBI/H 3PO 4membrane (b)was prepared in our lab from the traditional post-acid-doping method following the literature (He et al.,2003;Li et al.,2004):firstly,the powder of the PBI polymer sample was dissolved in DMSO with a concentration of 7wt.%for membrane preparation (around 50m m);after drying,the free-standing membrane was soaked in 85wt.%H 3PO 4aqueous solution for 7days;finally,the PBI/H 3PO 4membrane was wiped dry and used for conductivity measurements.The conductivity data of the PBI/H 3PO 4membrane (b)were comparable with the literature (He et al.,2003;Li et al.,2004).From Fig.7,it could be clearly seen that the Nafion 1membrane (c)exhibited much lower con-ductivities than the SPBI-1/H 3PO 4membrane (a)and the PBI/H 3PO 4membrane (b)because the Nafion 1membrane was much more humidity-dependent and thus showed low conductivities at low RHs.In comparison of the SPBI-1/H 3PO 4membrane (a)with the PBI/H 3PO 4membrane (b),it could be seen that the new SPBI-1/H 3PO 4membrane (a)prepared from this new in situ phase inversion method showed much higher conductivities than the PBI/H 3PO 4membrane (b)prepared from the traditional post-acid-doping method.This could be explained by the following:(1)The membrane prepared from this new in situ phase inversion technique had a higher H 3PO 4-doping level than the membrane prepared from the traditional post-acid-doping method (7.7vs.5.6)and thus showed higher conductivities.(2)The new SPBI-1copolymer synthesized in this work contained many sulfonic acid groups with a theoretical IEC value of 1.77mmol/g (Table 1).Without combining any PA,the pure SPBI-1copolymer membrane showed a lower measured IEC value of 0.3mmol/g (Table 1)since most of the sulfonic acid groups were neutralized by the basic imidazole groups and became deprotonated sulfonated groups.However,when the membrane was prepared from this new insituFig.6.Proton conductivity as a function of relative humidity for the new SPBI-1/H 3PO 4blend membrane at high temperatures (120–1608C)and atmosphericpressure.Fig.7.Conductivity comparison for different membranes at 1208C and low RHs (0–50%):(a)SPBI-1/H 3PO 4blend membrane prepared from the in situ phase inversion method,(b)PBI/H 3PO 4membrane prepared from the traditional post-acid-doping method,and (c)Nafion 1115.H.Bai,W.S.W.Ho /Journal of the Taiwan Institute of Chemical Engineers 40(2009)260–267265。