Preparation and characterization of SMA/S-POSS hybrid membranes for direct methanol fuel cell applic

RARE METALS .Vol. 25 ,Spec. Issue , Oct 2006 ,p. 224Preparation and characterization of SMA/ 'S- POSS hybridmembranes for direct methanol fuel cell applicationsWANG Erdong') , SHI Pengfeil) , and CHANG Young-twook21 ) Department of Applied Chemistry , Harbin Institute of Technology , Harbin 150001 , China2 ) Department of Chemical Engineering , Hanyang University , AnSan 425-791 , South Korea( Received 2006-06-30 )Abstract : A novel sulfonated polyhedral oligomeric silsesquioxane( S-POSS ) monomer was synthesized successfully in thisarticle. S-POSS acted as a donor of sulfonic acid group and reacted with poly( styrene-co-maleic anhydride )( SMA ). FT-IR spectroscopy confirmed the modification through introduction of peaks characteristic of ester linkages and carboxylicgroups. The SMA/S-POSS hybrid membranes were fabricated from different S -POSS contents. The proton conductivity andmethanol permeability of the hybrid membranes were studied with changing S-POSS content from 5wt. % to 30wt. %. Itwas found that the proton conductivity and the methanol permeability were dependent on the S- POSS content. Both of pro-ton conductivity and methanol permeability properties improve with increasing S-POSS content. The proton conductivitiesof the hybrid membranes are in the range of 10-3- 10-2 S. cm -1 , and the range of methanol permeabilities was between10-8 and 10~7 cm2. s~' 。 The membranes show good thermal properties characterized by thermogravimetric analysis( TGA ).Key words : SMA ;sulfonated polyhedral oligomeric silsesquioxane ; proton conductivity imethanol permeabilitydisperse in polymer matrix well , so homogenous mem-1. Introductionbrane can be obtained. Proton conductivity and meth-anol permeability were measured in this article. In ad-The development of direct methanol fuel cells hasdition , thermal analysis was performed to gain infor-beenreceiving considerable attention because of itsmation on the thermal stability of the membranes.high power efficiency , low cost of fuel and simple cellstructure[ 1-3 ] Membrane plays an important role in2. ExperimentalDMFC , not only conducting proton , but also preven-2.1. Materialsting methanol crossover. Because of the disadvantage-ments of Nafion , such as high methanol permeability ,The poly( styrene- co-maleic anhydride )( SMA )low working temperature and high cost , many resear-vas purchased from Aldrich Chemical ( Milwaukeeches have been done in order to search suitable substi-WI ,USA ). The Polyhedral Oligomeric Silsesquioxanetutes[ 4-5 ]( POSS ) derivative was purchased from the HybridIn this article , a novel method to prepare SMAPlastics Co. , USA. The tetrafuran ( THF ) , methanolbased nano composite membranes used for DMFCs wasand chloroform were purchased for Junsei Chemical. 4-reported. Sulfonated Polyhedral Oligomeric Silsesqui-hyd中国煤化THBSA )( 65wt. % solu-oxane( S-POSS ) succesfully was synthesized. Hyti:YHCNMH Gom Aldrich Chemical.droxyl group will be formed after sulfonation reaction.2. 2. Membrane preparationDue to this hydroxyI group , maleic anhydride group inSMA can be linked by chemical reaction. POSS canSame mole numbers of POSS and HBSA reacted in，Corresponding a! SHI PengfeiE-mail : phshi@ hit. edu. cnWang E. D. et al. , Preparation and characterization of SMA/ S-POSS hybrid membranes225THF ,and were stirred for 3 h at 50 C. Then the solu-S-POSS could react with the maleic anhydride groupstion was dried in air for 24 h , and then dried in vacuumin SMA , so sulfonic acid groups located in S-POSSovenat 30 C for 48 h.molecules can contribute to proton conductivity of theSMA and S-POSS were dissolved in chloroformmembrane , at the same time , due to the special struc-separately. After the solutions were homogeneous ,ture of POSS , we also expected there would be betterboth solutions were mixed and stirred vigorously at 60thermal stability , chemical stability and lower metha-C for 6 h，then poured solution into Teflon dish ,nol permeability for the SMA/S-POSS membrane.dried in air for 24 h , afterwards , dried in vacuum ov-enat30Cfor24h.Table 1. Composition of SMA/S-POSS samplesThe weight ratios of S-POSS in SMA/S-POSSSamples .SMA SMAS SMA10 SMA20 SMA30system are listed in Table 1.SMA( w.% )1009S90807(Fig.1 illustrates the structure of SMA/S-POSSS-POSS( wt.% )-510203(membrane , here we expected that hydroxyl groups inH2H口p=RsO,HR=phenylH,OHR、sOyHFig. 1. Structure of SMA/S-POSS after graft reaction. .2.3. Membrane characterization2.4. Water uptake and IECThe FT-IR spectra of SMA , S-POSS and SMA30The samples were dried in vacuum at 40 C for 48were measured by a Nicolet IR 860 spectrometerh ,and weighed at once. Afterwards , samples were im-( Thermo Nicolet , Madison , WI , USA ) in the rangemersed in deionized water for at least 24 h , then tippedof 4000 -500 cm - '.off the water left on the surface , weighed again. WaterThe degration process and the thermal stability ofupta中国煤化Ilowing equatior[ 6]:the membranes were performed using thermogravimeticWateMYHCNMHGyx 100% .(1 )analyzer( C-R4A , Shimadzu ). The measurements ofW..TGA were carried out under nitrogen condition using aThe ion exchange capacity( IEC ) was measuredheating rate of 10 C. min-' from 25 to 800 C.using the classical titration method[ 7 ] The IEC val-ue( in meq./g ) is defined as milliequivalents of sul-226RARE METALS , Vol. 25 ,Spec. Issue , Oct 2006fonic acid groups per gram of dried sample , and calcu-where Cg and C、are the methanol concentration oflated using the following equation :permeated and feed sides , respectively ,A ,L and VgMp+are effective area of membrane , membrane thicknessIEC=Way(2)and volume of permeated compartment ,and D ,K andto are the methanol diffusivity , the solubility and thewhere Mμu+ is milliequivalents of Ht existing in NaCltime lag , respectively. All experiments were carriedsolution ,and Wary is the weight of dried membrane.out at 25 C , and the uncertainty of the obtained val-The reported values were the mean of at least threeues was less than 2% .measurements.3. Results and discussion2.5. Proton conductivity3.1. FT-IR spectroscopyThe proton conductivity of membranes was meas-Fig.2 shows the FT-IR spectra of SMA , SMA30ured using the four-point probe technique. The imped-and S-POSS. S-POSS sample shows a characteristicance of the membranes was determined using an HP4192A LF Impedance Analyzer. Each sample was cutband at 1220 and 1060 cm ( characteristic of theinto sections 4 cm x 1 cm prior to being mounted onasymmetric Si-O- Si stretch ) ,at 1596 cm ~'( char-the cell. The proton conductivity( σ ) was obtained u-acteristic of S- CH = CH2 stretch ) and at 3373 cmsing the following equation[ 8 ]:arose from - OH group formed by sulfonation reac-ltion. The pure SMA sample shows a characteristicσ=(3)RSband at 1779 and 1857 cm~ ! which arose from the an-where σ is the proton conductivity( S. cm-' ) ,and lhydride groups ,and both of the peaks were reduced inis the distance between the electrodes used to measureFig. I( b) , the peak at 1043 cm-' represents the S=the potential( 1.7 cm ). R is the impedance of mem-0 stretch in - SO;H. After reaction , Ester groups arebrane( Q). S is the cross-sectional area of the mem-brane( 1 x thickness of sample in cm2 ). The impen-formed which can be seen that at 1726 cm-'. So ,itdance of each sample was measured five times to en-can be seen SMA and S-POSS reacted succssfully.sure good data reproducibility.3.2. Water uptake and IEC2. 6. Methanol permeabilityThe results of water uptake and IEC wereThe methanol permeability of the membranes wasillustrated as a function of S - POSS contentdetermined using a diffusion cell described ir[ 7 ] Pri-or to testing , the membranes were equilibrated in dis-tilled water for at least 24 h. Initially , one reservoirw (@)SMApi\wllcontains 2 mol. L~ methanol aqueous solution andthe other reservoir contains distilled water , and each(b) SMA30Vwcell was 50 ml liquid contained. Increases of the con-centration of methanol in the water reservoir with timewere measured by gas chromatogram. During the ex-中国煤化工periment，solutions in each cell were stirred. TheMHCNMHG0000 15000 1000methanol permeability was calculated using the follow-Wave number/cm-'ing equatior[ 7 ]:Fig.2. FT-IR spectra of( a)SMA ,( b) SMA30 and( c)A DKCi(t)=2 D-C(1-to)(4 )S-POSS.Wang E. D. et al. , Preparation and characterization of SMA/ S-POSS hybrid membranes227Both of them are increased with the increasing of S-the degradation temperatures were increased with thePOSS content. But because of the hydrophobic probili-increasing of S-POSS content , the thermal property ofty of SMA itself and low content of S- POSS relatively ,the membrane has been improved.water uptake and IEC value are somewhat low com-pared to nafion and some other types of membranes.3.4. Proton conductivity and methanol permea-120.5bility十Water uptake甘IEC+0.4Fig. 5 shows the proton conductivity and methanol10}+0.3permeability results of SMA/S-POSS membranes. It8|40.20can be seen that proton conductivity is increased with7the increasing of S-POSS content in the range of 1.5 X40.1610-3 -1 x10~ S. cm -1 , by contraries , the methanol5o5202530→0.0permeability value is decreased between4.7 x10~ 7 -S-POSS content/wt%7.8 x 10-8. Compared to the water uptake and IECFig. 3.Water uptake and IEC value of SMA/S-POSSresults , we know that proton conductivity depends onmembranes.water uptake , more hydrophilic - SO,H groups areneeded to increase water content , and proper channels3.3. TGAfor proton transport can be formed to increase the pro-ton conductivity. Methanol permeability is decreasedThe TGA curves measured are shown in Fig. 4.which is because POSS molecules are cage type Si- 0The TGA curves of the membranes were ftted by threestructure , maybe they can prevent methanol moleculesmain degradation stages arising from the processes ofpassing through the membrane.thermal salvation( above 100 C ) , thermal desulfona-1E-11E-6tion( 250 -400 C ) , and thermooxidation of the poly-七Proton conductitymer matrix( T>400 C ). The first stage is associated“Methano permeabilitywith the loss of absorbed water molecules. For the sec-1E-2:}1E-7ond stage , the weight loss corresponds to the loss ofsulfonic acid groups from S-POSS , and for the thirdstage , SMA main chains were decomposed. BecauseE-355 10520253035，J1E-8of the presence of S-POSS ,S-POSS content/wt.%120- Pure SMAFig. 5. Proton conductivity and methanol permeability re-100. SMAS. SMA10sults of SMA/S-POSS membranes.-SMA20-SMA304.Conclusions. Hvbrid membranes based on SMA/S-POSS wereprep中国煤化工,proton conductivity in--20creMHC NM H Cebilily decreased with200400600800the increasing of S-POSS content. It was proved thatTemperature/Cdue to the presence of S-POSS , the hybrid membraneFig.4. TGA curves of SMA/S-POSS membranes with dif-was a more effective methanol barrier. Because of theferent PosS高敝据228RARE METALS , Vol. 25 ,Spec. Issue , Oct 2006nano size of POSS molecules and sulfonic acid groupsJ. Membrane Science , 2005 ,259 : 10.located in S-POSS molecules , it can be contributed to[5] ChenJ. 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