Hybird Membrane with High Proton Conductivity and Selectivity Based on SPEEK for Direct Methanol Fue

CHEM. RES. CHINESE UNIVERSITTES 2010, 26(6), 1031-1034Hybird Membrane with High Proton Conductivity and SelectivityBased on SPEEK for Direct Methanol Fuel CellsXU Dan', WANG Yang, ZHANG Yang, ZHANG Gang',SHAO Ke', LI Shi-wei' and NA Huil*1. Alan G. MacDiarmid Institute, College of Chemistry, Jin University, Changchun 130012, P. R China;2. Jilin Province Product Quality Supervision Test Institute, Changchun 130022, P. R ChinaAbstract Composite membranes based on sulfonated silica/sulfonated poly(ether ether ketone)(SPEEK) were pre-pared by means of sol-gel method so as to gain a high conductivity and reasonable methanol permeability. The sulfo-nated silica is generated in situ via the hydrolysis of sulfonated 3-anminopropyl triethoxysilane(KH550) synthesizednewly from 3-aminopropyl triethoxysilane and 1,4-butane sultone. The membrane with a silica mass fraction of 5%exhibits a conductivity of 0.187 S/cm at 80。C and a methanol coefficient with 9.72x10-7 cm2/s. The compositemembranes show improved condutive ability and better selectivity that can be promisingly used in direct methanolfuel cell.Keywords Fuel cell; Proton exchange membrane; Sulfonated poly(ether ether ketonc)(SPEEK); Sulfonated silicaArticle ID 1005-9040(2010)-06-1031-041 Introductionwith H2O2, however, oxidation reagent(H2O2) alwaysDirect methanol fuel cells(DMFCs) are a envi-corrodes the polymer backbones during the post-ronmentally friendly power source with high enfi-oxidation process, which resulted in a poor mechani-cal and chemical stability. So the method of introdu-cency. The key department of DMFCs is proton ex-cing sulfonated silica needs to be improved.change membrane which should exhibit high protonIn this study, first sulfonated silicon dioxideconductivity, low methanol leakage, good chemicalprecursor was synthesized newly by the reactionstability and mechanical properities. Sulfonated polyof 3-anminopropyl triethoxysilane(KH550) and 1,4-(ether ether ketone)(SPEEK) own excellent methanol-butane sultone, then SPEEK/SiO2-SO3H compositebarrier than commercial Nafion with compared protonmembranes were prepared directly by a sol-gelconductivity and low cost. However, SPEEK is em-brassed by some problems, such as high swelling ratioprocess. This technique avoided the corrosion to theat high sulfonated degre(DS) and deficient protonpolymer matrix caused by the post-oxidation methodand improved the proton conductivity and selectivityconductivity at low DS. Thus modified SPEEK wassignificantly.researched extensivelyl- 41.Silicon dioxide(SiO2) is widely used for compo-2 Experimentalsite membranes due to its good mechanical propertiesand water retention'- -9J. Ruffmann et al.8.9) reported2.1 Materialssulfonated SPEEK-based silica nanocomposite mem-3-AnminopropyI triethoxysilane(99%)branes for DMFCs. Theses membranes own goodwas purchased from Nanjing Lipai Co. Ltd,methanol blocking properties but not high enough1,4-butane sultone from Tianjin Yiheng Co. Ltd,proton conductivity. So much work on modified SiO2HCl(2 mol/L) and N,N-dimethylacetamide (DMAC,such as sulfonated silica(SiO2- SO;H) was researched.99%) from Beijing Chemical Co. Ltd.. SPEEK wasRen et al."o0 and Rhee et al!"n investigated a series ofsynthesized by direct aromatic nucleophilic substitu-Nafion/SiO2-SO3H hybrid membranes. However, intion polymerization from 4,4'- difluorobenzophenone,these researches SiOz-SO3H was primarily fabricatedsodi中国煤化工benzene sufonat)via sol-gel method followed by post oxidation of thiolandenol"!. The valueTYHCNMHG*Corresponding author. E mail: huina@jlu.edu.cnReceived December 8, 2009; acepted December 29, 2009.Supported by the National High-Tech Research and Development Program of China(No.2007AA03Z218).1032CHEM. RES. CHINESE UNIVERSITESVol.26of IEC used in this study was 1.85 mmol/g. SPEEKinto 0.5 mol/L HCl at 80 °C and refluxing for 12 h.polymers in acid form were obtained by immersingThe structure is shown in Scheme 1.H,C,,CHsHCHsSO;NaCHCHs SO,NaScheme 1 Chemical structure of SPEEK used in composite membranes2.2 Preparation of Composite Membranesthe hydrolysis of S-KH550. The mixed solution wasSulfonated KH550(S-KH550) was synthesizedpoured on a glass plate after stiring for 6 h and thenheated at 50 °C for 24 h to remove the residualfrom 3- aminopropyl triethoxysilane and 1,4-butanesultone with a molar ratio of l:l in a beaker via stir-DMAC. Each membrane was peeled off from the platering for 24 h at room temperature(shown in Scheme 2).and the thickness was about 60- 120 um. Compositemembranes with sulfonated silica mass fraction of 5%,Composite membrane was prepared by solution10% or 20% were signed respectively as SPEEK/casting. First, 2 g of SPEEK in acid form wasS-SiO2 5%, SPEEK/S-SiO2-10% or SPEEK/S-SiO2-dissolved in 20 mL of DMAC, to which a certain20% below.amount of S-KH550 was then added followed byaddition of 1 mL of HCI aqucous solution to induceOCsHs0C;HsDMSOC:H,O-OC:H3 NHEStirred at room temperature0C:Hs NH3-Aminopropyl tiethoxyilane 1,4-Butane sultoneKH5503-({3-(Toichoxsll)prplamino}butane-sulfonic acid(S-KH550)Scheme 2 Synthesis of sulfonated precursor S-KH5502.33 Instrumentsreaction between the animo group attached on KH550FTIR analysis was carried out on a Bruker Vectorand 1,4-butane sultone. The successful synthesis wasconfirmed by the Fourier transform infrared(FTIR)22 spectrometer at a resolution of 4 cm "/min fromspectra(Fig. 1), where - -NH strong characteristic peak4000 to 400 cm+. Scanning electron microscopyappears at 830 cm-' instead of that of - -NH2 at 850(SEM) measurement was performed on a Shimadzucm~*. Meanwhile the new peaks at 607, 1033 and 1165SSX-550 microscope. Thermal analysis was per-:m~' can be assigned to asymmetric and symmetricformed on a Pyris TGA(Perkin-Elmer) thermal0= =S= =0 stretching vibrations of - -SO3H group'5].analyzer system under nitrogen gas4. Proton con-The results prove that sulfonated precursor has beenductivity was measured by a four-electrode ACsynthesized as expected and its IEC value is 2.66impedance method at 10-107 Hz, 50- -500 mV withmmol/g measured by titration.an AC impedance spectroscopy, on Salton 1260+ Sal-1033ton 128713,14. Methanol permeability cefficient,Dx(cm2/s), was determined via a cell consisted of twoCMhalf cells separated by the membrane. The concentra-tion of the methanol was measured on a Shimadzu-NH2,GC-8A chromatograph.3 Results and Discussion中国煤化工H3.1 Characterization of S-KH550YHCNMHG.00S-KH550 was obtained by the ring openingFig.1 FTIR spectra of KH550(a) and S-KH550(6)No.6XU Dan et al.10333.2 Morphology of Membranes3.3 IEC, Water Uptake and Swelling RatioFig.2 shows the micrograph of the cross sectionAs listed in Table 1, the values of IEC of compo-of membrane with loaded 20%(mass fraction) sulfo-site membranes all became higher except that of thenated silica(SPEEK/S-SiO2 20%). It indicates that noSPEEK/S-SiO2-20%. Water uptake increases with theparticle aggregatation is visible and nano-silica is dis-increased content of sulfonated silica. The swellingpersed well in the membrane prepared by the sol-gelratio changes first with a decrease then a sequentialprocess.increase. Cross-linking sulfonated silica dioxide affectthe performance above from two aspects. SO;H andSi-OH on the sulfonated silica absorb more water mo-lecules which can enhance the IEC and water uptake,otherwise inorganic network limits the freedomvolume of polymer matrix that depresses the absorb-ing of water and the swelling of the membrane.'2m3.4 Proton Conductivity, Methanol Permeabitily(Dx) and SelectivityFig.2 SEM image of composite membraneeros-section of SPEEK/S SiO-20%Proton conductivity is the most important forTable 1 Properties of SPEEK composite membranesIECIWater upake(WU(%)Swelling ratio(SW)(%)0/Scm^ )Membrane10' /(cm'sh)(mmol:g" )25 eC80°C25 °C80 oC25 °cSPEEK1.8555.967.311.314.510.950.0370.1211SPEEK/S-SiO-5%1.9258.810.213.69.720.166SPEEK/S- siO-10%1.9761.674.211.515.013.740.0980.187SPEEK/S-SiO-20%19169.484.013.79.319.590.0930.181 .Nafion117.9218.37.91.07.221.600.0760.146Dx: methanol difusion coficient;。proton conductivity.PEM which determines the performance of fuel0.20(A)cell directly. As shown in Fig.3(A), each compositemembrane shows improved proton conductivity thandistinct SPEEK. Among these membranes SPEEK/S-SiO2- 10% exhibits the highest proton conductivity0.10(25 °。C/0.098 Scm-', 80 °C/0.187 S.cm ) and is better0.05han Nafion117(25 。C/0.076 S:cm~, 80。C/0.146S.cm |) obviously. Proton conductivity is affected by2(4(608(Temperature/Cmany factors such as IEC, water uptake, microstruc-2.0F (B)ture and so on. Generally, high IEC or water uptakeleads to a high proton conductivity, however, mem-brane absorbing too much water will result in a dilu-1.2tion effect on conductive groupTherefore,0.8SPEEK/S-SiO2-20% membrane shows lower protonconductivity than SPEEK/S-SiO2- 10%.0.4Methanol diffusion coefficients(Dx) are listed inTable I, the composite membranes exhibit higher me-Samplethanol diffusion coefficient than SPEEK exceptFig_3_ Proton condutivities(A) and selectivities(B) ofSPEEK/S-SiO2-5%. A compact structure formned by中国煤化工fo1 at dif-the sol-gel process blocked the channel of methanoltransferring, which decreased the methanol diffusionTYHCNMHGSEKsSiO-10%,▼SPEEK/S-SiO-20%;。Nafion117.coefficient. The high loading of SOzH made methanol(B) a. SPEEK; b. SPEEK/S-SiO2-5%, c. SPEEK/S -SiO2-CrOSS the membrane more easily. The performance of10%; d. SPEEK/S SiO2-20%, e. Nafionll7.1034CHEM. RES. CHINESE UNIVERSITESVol.26the membranes was affected by both the reasons5820above simultaneously. Membrane loading 5% silica[2] ZhaoC.J.,Lix. F. WangZ, Dou z. Y, Zhong s. L, Na H.JMembr. Sci, 2006, 280, 643was affected mainly by the first reason, so Dk showed[3] ZhaoC.J. LinH. D. ShaoK, Lix. E, NiH. z, J. Power .a slightly decrease. However, composite membranesSources, 2006, 162, 1003still exhibited good methanol barrier properties than[4] WangG. Q.2zhuX. L, Zhang s. H, Liang YF, Jian X G, AcaNafionl 17.Selectivity is defined as the ratio of protonPolym, 2006, 2, 209conductivity to methanol permeability, and it is often[S] Kimball s. R, Thomas H, Thomas s, J. Membr. Sci, 2010, 346,215used to evaluate the potential performance of mem-branes. High value of selectivity corresponds to good[6} Nunecs s. P. Ruffmann B.. Rikowski E, Vetter s, Richau K, J.Membr. Sci, 2002, 203, 215performance. Fig.3(B) shows the selectivity of com-[7] Techicaya-Bouckary L, Jones D. J, Roziere 1, Fuel Cells, 2002, 2,posite membranes, SPEEK and Nafion117. It is seen40that SPEEK/S-SiO2-5% has the best selectivity.[8] Rufimann B.. Siva H, Scule B.. Nunes s. P, Solid Slate Ionics,2003, 162, 2694 Conclusions[9] Silva V. s, Ruffmann B.. Vetter s, Mendes A, Madeidra L. M.,Nunes s. P, Caral. Today, 2005, 104, 205Composite sulfonated silica/SPEEK membranes[10] Ren s, Sun G., LiC. LiangZ x, WuZ. M. Jin w, Qin x, Yangwere prepared via the hydrolysis of sulfonated KH550x. F.J Membr. 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