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Aliphatic-aromatic sulphonated polyimide and acid functionalized polysilsesquioxane composite membranes for fuel cell applications

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Aliphatic-aromatic sulphonated polyimide and acid functionalized polysilsesquioxane composite membranes for fuel cell applications
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  Aliphatic-aromatic sulphonated polyimide and acidfunctionalized polysilsesquioxane compositemembranes for fuel cell applications † Ravi P. Pandey ab and Vinod K. Shahi* ab Forthedesignofhighlystableand protonconductivepolymerelectrolytemembranes (PEM), wesynthesizednucleophillic attack resistant sulphonated polyimide (SPI), in which electron-withdrawing sulphonic acidgroups were not directly attached through amino-phenyl rings, using diamines with high basicity anddianhydride with low electron a ffi nity. SPI- sulphonated silica precursor (SSP) composite PEM was assessedfor its high water activity (water retention capacity) and direct methanol fuel cell (DMFC) applications.SPI/SSP-40 (composite membrane with 40 wt% SSP content) showed 10.25    10  8 cm 2 s  1 water di ff usioncoe ffi cient; 1.86 mequiv. per g ion-exchange capacity (IEC); and 6.34    10  2 S cm  1 proton conductivity.The prepared PEM was classi 󿬁 ed as  water enriched   because of the presence of a high percentage ofbound water content. A relatively high proton mobility (5.52    10  4 cm 2 s  1 V  1 ) across the PEM wasattributed to a percolated network of ionic clusters (ions and water). Frictional data con 󿬁 rmed thereduction in frictional coe ffi cient between the proton and membrane matrix at a high SSP content. Thereported PEM, especially SPI/SSP-40, were assessed for their suitability for DMFC applications. Introduction PEMs may be classi  ed as promising polymeric materials andhave found a wide range of applications in fuel cells (a cleansource of electricity), ion-exchangers, ionomers for batteriesand sensors. 1 – 4 Per  urosulfonicionomers (Na  on R  ) are state-of-the-art PEM, because of their polytetra  uroethylene (PTFE)backbone and highly acidic per  urosulphonic groups, respon-sible for high proton conductivity and stability. 1,2,4 – 7 However,serious e ff  orts have been made to develop alternative PEMs,especially sulphonated aromatic polymers, because of thedeterioration of Na  on's properties above its glass transitionpoints, high fuel permeability, cost and environmentalinadoptability. 2,7 – 9 For developing PEMs, sulphonated aromatic polymers, suchas poly-(ether ether ketone)s, 10,11 poly(arylene ether sulfone)s, 12 – 14 polyimides, 2,6,15,16 polyphosphazenes, 17 polybenzimidazoles, 18,19 and polyphenylenes, 20 have been extensively studied due to theirexcellent stability and easy functionalization. Among thesepolymers, SPIs based on naphthalenic moieties were consideredas promising alternative materials because of their high stability (thermal, mechanical and chemical), good   lm forming ability and signi  cantly low fuel cross-over. 2,6,21 Since, imide groups aresusceptible to nucleophillic attack by water, the hydrolyticstability of SPI thus a ff  ects its fuel cell suitability. 2,21 In SPI, theimide linkage is derived from diamine with high basicity anddianhydride, in which the carbonyl carbon should bear a highelectron density. Their structure showed great in  uence on thehydrolytic stability of SPI. 2,16 Introduction of aliphatic segmentsin both the main and side chains, was also considered to reducethe chances of nucleophillic attack on the imide linkage. 6,16 SPIsbased on 3,3 0 -bis(sulphopropoxy)-4,4 0 -diaminobiphenyl (BSPA),1,6-hexamethylenediamine (HMDA), (diamines) have beenreportedintheliterature. 6,16 Sulphonatedaromaticdiaminesmay beclassi  edintotwotypes.Inthe  rsttype,electronwithdrawing sulphonic acid groups are directly attached to aminophenylrings (such as 2,2 0 -benzidinedisulphonic acid, and 4,4 0 -dia-minodiphenyl ether-2,2 0 -disulphonic acid). In the second type,sulphonic acid groups are attached with an aromatic ring other than aminophenyl ring (such as 4,4 0 -bis(4-aminophenoxy)biphenyl-3,3 0 -disulphonic acid (BAPBDS)). Because of the strong electronwithdrawinge ff  ect ofsulphonic acid groups, the basicity of the second type of diamine should be comparatively higherthan the   rst type. 21 – 23 The hydrolysis of imide groups occursbecause of the nucleophillic attack of a water molecule on thecarbonyl carbon atoms. Thus dianhydrides with a high electrondensity in the carbonyl carbon atoms should produce hydrolyti-callystablepolyimides.Theelectrona ffi nity(  E  a )isthemeasureof electron density of the carbonyl carbon atoms, and the lower theabsolute value of   E  a  is, the higher the electron density is in thecarbonyl carbon atoms. 2 Further, di ff  erent dianhydrides, such as a  Academy of Scienti    c and Innovative Research, India. E-mail: vkshahi@csmcri.org;vinodshahi1@yahoo.com; Fax: +91-278-2567562/2566970; Tel: +91-278-2569445 b  Electro-Membrane Processes Division, CSIR-Central Salt & Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364002, Gujarat, India †  Electronic supplementary information (ESI) available. See DOI:10.1039/c3ta12755a Cite this: DOI: 10.1039/c3ta12755a Received 16th July 2013Accepted 21st September 2013DOI: 10.1039/c3ta12755a www.rsc.org/MaterialsA This journal is  ª  The Royal Society of Chemistry 2013  J. Mater. Chem. A  Journal of Materials Chemistry A  PAPER     P  u   b   l   i  s   h  e   d  o  n   2   6   S  e  p   t  e  m   b  e  r   2   0   1   3 .   D  o  w  n   l  o  a   d  e   d   b  y   C  e  n   t  r  a   l   S  a   l   t   &   M  a  r   i  n  e   C   h  e  m   i  c  a   l  s   R  e  s  e  a  r  c   h   I  n  s   t   i   t  u   t  e   (   C   S   M   C   R   I   )  o  n   2   3   /   1   0   /   2   0   1   3   1   7  :   4   7  :   1   6 . View Article Online View Journal  1,4,5,8-naphthalenetetracarboxylic dianhydride (TCND), 4,4 0 -binaphthyl-1,1 0 ,8,8 0 -tetracarboxylic dianhydride (BNTDA), werealso explored. These dianhydrides show relatively high electrona ffi nity (  E  a : 4.01 eV and 3.79 eV, respectively), in which BNTDA isrelatively more hydrolytically stable in comparison with TCND,due to the high electron density in carbonyl carbon atoms. 2,16 Forthe architecture of the imide linkage, we propose a diamine(BAPBDS) and benzophenone-3,3 0 ,4,4 0 -tetracarboxylic dianhy-dride (BTDA) (  E  a : 1.55 eV). Using this approach, we synthesizedSPI, in which the electron-withdrawing sulphonic acid groups were not directly attached through amino-phenyl rings.For designing the PEM, the dependency of its protonconductivity on water activity (water retention capacity) shouldalso be taken into account along with the membrane stabilities.Generally, the proton conductivity drops several orders of magnitude with a decrease in membrane humidity. 1 Theincorporation of inorganic materials in the membrane matrix isexpected to enhance the membrane stability and water reten-tionproperties. 5 Compositematerials arepromisingsystems forPEM applications due to their extraordinary combination of properties derived by di ff  erent building blocks. 9 The incorpo-ration of inorganic compounds, such as silica, 7,24 – 26 clay, 27,28 titania, 29,30 zirconia, 31,32 alumina, 33 etc.  are reported to improvethe water retention properties of membranes for high levels of proton conductivity at elevated temperatures. Although, thesehybrid PEMs showed better mechanical, thermal properties andlow methanol permeability, in some cases a distinctive negativeimpact on proton conductivity was observed. 1 To avoid theproblem, attention was paid to developing organic – inorganiccomposite PEMs, in which both blocks (organic & inorganic) were functionalized. 33,34 The objective of the present work is to prepare nucleophillicattack resistant SPI using diamines (BAPBDS and 1,4-dia-minobutane (DAB)) with high basicity and dianhydride (BTDA) with low electron a ffi nity, and to develop organic – inorganiccomposite PEM (sulphonic acid gra   ed blocks) with highstability and water retention properties for DMFC applications. Experimental section Materials Benzophenone-3,3 0 ,4,4 0 -tetracarboxylic dianhydride(BTDA), (3-gly-cidoxypropyl)trimethoxysilane (GPTMS), and 1,4-diaminobutane(DAB) (99%), were obtained from Aldrich, while 4,4 0 -bis(4-amino-phenoxy)biphenyl (BAPB) was obtained from TCI (>97%), andused as received. Dimethyl sulphoxide (DMSO), dimethyl acet-amide (DMAc), triethylamine (TEA), benzoic acid,  m -cresol,acetone, H 2 SO 4 , HCl, NaOH, methanol, NaCl  etc.  of AR grade wereobtained from SD   ne chemicals India, and used with properpuri  cation. All the other chemicals used were of analytical grade.Milli-Q water (deionized) was used for all of the experiments. Synthesis of sulphonated polyimide (SPI) The method adopted for the synthesis of 4,4 0 -bis(4-amino-phenoxy)biphenyl-3,3 0 -disulphonic acid (BAPBDS) from 4,4 0 -bis(4-aminophenoxy)biphenyl (BAPB) has been reported in theliterature (the detailed procedure and  1 H NMR spectra areincluded in Section S1(ESI) and Fig. S1(ESI † ), respectively). 4 SPI was synthesized by a copolymerization reaction reportedearlier. 16 In a typical reaction, a 100 ml three-neck round-bottomed   ask was charged with BAPBDS (2 mmol), DAB(2 mmol), and TEA (8.6 mmol). The copolymerization reaction was carried out in  m -cresol (14 ml) under stirring and nitrogenconditions at 70   C for 30 min. To the obtained clear solution,BTDA (4 mmol) benzoic acid (8 mmol), and  m -cresol (12 ml) were added. The reaction mixture was cooled to room temper-ature and stirred for 24 h under a nitrogen stream. The mixture was then heated at 175   C for 15 h followed by heating at 195   Cfor 3 h, for completion of the reaction. Then the mixture wasprecipitated in an excess of acetone, and the yellow powderedprecipitate was  ltered, washed with acetone, and dried at 60   Cfor 12 h to obtain SPI. Synthesis of sulphonated silica precursor (SSP) andmembrane preparation SSP was synthesized by an epoxide ring opening reaction. 35 In atypical synthetic procedure, to a 10 ml conical   ask, equipped with a mechanical stirring device, BAPBDS (1 mmol), TEA (2 mmol) and GPTMS (2 mmol) were added. The reaction pro-ceeded in DMSO (4 ml) under constant stirring at 80   C for 6 hin a nitrogen atmosphere. The obtained dark yellow coloredsolution (SSP) was assessed by   1 H, and  29 Si NMR. The SPI/SSPcomposite organic – inorganic membrane of the desiredcomposition was prepared by the sol – gel method in DMAc inacidic medium. 1 The obtained gel was transformed as a thin  lm of the desired thickness onto a cleaned glass plate anddried at 80   C in a vacuum oven for 24 h. During the sol – gelprocess SSP was converted into sulphonated polysilsesquioxane(SPS). The thickness of the membranes was measured by adigital micrometer (Mitutoyo Digimatic) with a standard devi-ation 1.0  m m. The obtained 140  m m thick membranes wereconditioned by successive equilibration in HCl and designatedas SPI/SSP-  X  , where  X   is the weight percentage of SSP to the SPI(where  X   is 20, 30, and 40). Instrumental characterization of the membranes Detailed instrumental characterization for the  1 H,  13 C and  29 SiNMR spectra, FTIR spectra, TGA, DSC, DMA, SEM, TEM and AFM analysis are included in Section S2 (ESI † ). For the calcu-lation of bound water content, wet membrane samples wereheated in a TGA at a rate of 10   C min  1 under nitrogenatmosphere and the weight loss percentage of the membrane(100 – 150   C) was calculated. 4  Water uptake and water retention studies  Water uptake was analyzed for the wet membrane sample(equilibrated in distilled water for 24 h). The water retentionability of the developed membranes was evaluated by measuring the water mobility during the dynamic deswelling test. 4,36,37 The detailed procedures for the estimation of wateruptake and water retention are included in Section S3 (ESI † ).  J. Mater. Chem. A  This journal is  ª  The Royal Society of Chemistry 2013 Journal of Materials Chemistry A Paper     P  u   b   l   i  s   h  e   d  o  n   2   6   S  e  p   t  e  m   b  e  r   2   0   1   3 .   D  o  w  n   l  o  a   d  e   d   b  y   C  e  n   t  r  a   l   S  a   l   t   &   M  a  r   i  n  e   C   h  e  m   i  c  a   l  s   R  e  s  e  a  r  c   h   I  n  s   t   i   t  u   t  e   (   C   S   M   C   R   I   )  o  n   2   3   /   1   0   /   2   0   1   3   1   7  :   4   7  :   1   6 . View Article Online  Oxidative and hydrolytic stabilities The oxidative stability was examined by immersing themembrane samples in Fenton's reagent (3% H 2 O 2  aqueoussolution containing 2 ppm FeSO 4 ) at 80   C for 1 h. For thehydrolytic stability test, a small piece of membrane was boiledin water for 24 h at 140   C in a pressurized closed vial. Thestability was evaluated by the weight loss observed in themembrane a   er stability evaluation and the appearance of the test samples. 16 Ion exchange capacity (IEC) measurements The ion-exchange capacity (IEC) of the membrane wasmeasured by an acid-base titration method and the detailedmethodology is included in Section S4 (ESI). † Proton conductivity and methanol permeability  The proton conductivity of the membranes was measured by apotentiostat/galvanostat frequencyresponseanalyzer(Auto Lab,Model PGSTAT 30). The detailed procedure is given in SectionS5 (ESI † ). The method adopted for measuring the methanolpermeability is described in Section S6 (ESI † ). Results and discussion Structure characterization of SPI and SSP SPI was synthesized by the copolymerization of BAPBDS,DAB, and BTDA in  m -cresol (Scheme 1), and characterized by  1 H and  13 C NMR spectra (Fig. 1 and S2 (ESI † )). Peaks at   d  ¼ 7.07, 7.34, 7.41, 7.63, 8.04, 8.08, and 8.16 ppm in the  1 H NMR spectra are present due to the aromatic protons, while peaks at  d  ¼  1.64, 2.09, and 3.09 ppm are observed due to the aliphaticprotons. The  13 C NMR spectra is also in good agreement  with the given structure of SPI. TEM images of SPI show    neblack dots in the represented hydrophilic (ionic) domains, while bright areas represent the hydrophobic domains Fig. S3(ESI † ). 16 SSP was synthesized by an epoxide ring opening reactionusing GPTMS and BAPBDS in DMSO, and was con  rmed by   1 Hand  29 Si NMR spectra (Fig. S4 and S5(a) (ESI † ) respectively). SSPshowed two peaks at   d  ¼  41.80, and   50.62 ppm, which areassignable to T 0 , and T 1 (T represents tri-functional silicon andthe superscript represents the number of siloxane bridges)(Scheme 2). 1 Characterization of the SPI/SSP composite membrane SPI/SSP composite membranes of varied compositions werepreparedbyasol – gelmethod. Membraneswithmorethan40wt % of SSP content became sti ff  er, while the   exibility andtransparency were retained even under dry conditions. Theformation of polysilsesquioxane was con  rmed by   29 Si NMR spectrum (Fig. S5(b) (ESI † )), in which a broad peak at   d  ¼  67.91ppm (T 3 ) silicon indicated complete dehydration and the poly-condensation reaction. The FT-IR spectrum of the SPI/SSPcomposite membrane (Fig. S6 (ESI † )) con  rmed the presence of sulphonic acid groups (absorption bands at   y  ¼  1019, 1166(sym. SO 2  stretch), and 1239 cm  1 (asym. SO 2  stretch)). Theabsorption band at   y  ¼  1090 cm  1 (asym. Si – O – Si stretch)con  rmed the formation of polysilsesquioxane, while the broadabsorption band at   y  ¼  3436 cm  1 con  rmed the hydrogenbonding interaction between the SPS and SPI matrix. 16,38 The microstructure of the membrane showed that it had agreat in  uence on its macroscopic properties, such as wateruptake, swelling, and proton conductivity. SEM micrographs of the pristine SPI membranes exhibited dense and homogeneousmorphology (Fig. 2(a)). A comparison between the surfacemorphologies of the pristine and SPI/SSP-40 membrane(Fig. 2(a) and (b)) revealed the relatively rough surface of thelatter, which may be due to the presence of sulphonated poly-silsesquioxane (SPS) in the membrane matrix. A cross sectionimage of the SPI/SSP-40 membrane is shown in Fig. 2(c). Also,the elemental mapping data supported the membrane compo-sition (Fig. 2(d) and (e)). Further, the surface morphology of theSPI/SSP-40 membrane showed uniformity and good compati-bility between SPI and SPS due to interfacial interactions orhydrogen bonding. The AFM images (height) of the compositemembrane are presented in Fig. 3. Bright regions were Scheme 1  Schematic reaction steps involved in the preparation of the sulphonated polyimide (SPI). This journal is  ª  The Royal Society of Chemistry 2013  J. Mater. Chem. A Paper Journal of Materials Chemistry A    P  u   b   l   i  s   h  e   d  o  n   2   6   S  e  p   t  e  m   b  e  r   2   0   1   3 .   D  o  w  n   l  o  a   d  e   d   b  y   C  e  n   t  r  a   l   S  a   l   t   &   M  a  r   i  n  e   C   h  e  m   i  c  a   l  s   R  e  s  e  a  r  c   h   I  n  s   t   i   t  u   t  e   (   C   S   M   C   R   I   )  o  n   2   3   /   1   0   /   2   0   1   3   1   7  :   4   7  :   1   6 . View Article Online  composed of the hydrophobic backbone, endowing themembranes with mechanical strength, while the dark domains were formed by the hydrophilic sulphonic acid groups andabsorbed water, responsible for proton transport. The sche-matic structure of the SPI/SSP composite membrane basedon its structural and morphological elucidation is presentedin Fig. 4. Membrane stabilitiesThermal and mechanical stability.  The thermal stability of the pristine SPI and SPI/SSP composite membranes was inves-tigated by TGA (Fig. S7 (ESI † )), and these membranes showed athree-step weight loss. The   rst step weight loss (<100   C) wasobserved due to absorbed water in the polymer matrix. Thesecond step weight loss (250 – 400   C) was attributed to the de-functionalization (decomposition of sulphonic acid groups), while the third weight loss (above 470   C) occurred due to thedecomposition of polyimide backbones. Finally, the content residues (above 800   C) increased with the SSP content inthe SPI matrix. This con  rmed the improvement in themembrane ’ s thermal properties with the introduction of SSPcontent in the SPI matrix. The DSC thermograms for the pris-tine SPI membrane (Fig. S8 (ESI † )) showed a   rst glass transi-tion ( T  g  ) temperature around 50   C. The  T  g   values of thecomposite membranes increased with the SSP content in themembrane matrix. This may be attributed to the restrictivesegmental motion of the polymer chains as the result of strong ionic interactions between SPI and SPS. Moreover, themechanical stability of the membranes was evaluated  via  thedynamic mechanical analysis (Fig. S9 (ESI † )) and the SPI/SSP-40membrane exhibited a 2628 MPa storage modulus. It was Fig. 1  1 H NMR spectrum of SPI in DMSO-d 6 . Scheme 2  Synthesis route for the sulphonated silica precursor (SSP).  J. Mater. Chem. A  This journal is  ª  The Royal Society of Chemistry 2013 Journal of Materials Chemistry A Paper     P  u   b   l   i  s   h  e   d  o  n   2   6   S  e  p   t  e  m   b  e  r   2   0   1   3 .   D  o  w  n   l  o  a   d  e   d   b  y   C  e  n   t  r  a   l   S  a   l   t   &   M  a  r   i  n  e   C   h  e  m   i  c  a   l  s   R  e  s  e  a  r  c   h   I  n  s   t   i   t  u   t  e   (   C   S   M   C   R   I   )  o  n   2   3   /   1   0   /   2   0   1   3   1   7  :   4   7  :   1   6 . View Article Online  Fig. 2  SEM images of (a) a surface image of the SPI membrane; (b) a surface image of the SPI/SSP-40 membrane; (c) a cross section image of the SPI/SSP- 40membrane; (d) EDX mapping of the SPI/SSP-40 membrane; (e) EDX spectrum of the SPI/SSP-40 membrane. Fig. 3  AFM images of the SPI/SSP-40 membrane: (a) 5 m m 2D image; (b) 3D image (height 120 nm). This journal is  ª  The Royal Society of Chemistry 2013  J. Mater. Chem. A Paper Journal of Materials Chemistry A    P  u   b   l   i  s   h  e   d  o  n   2   6   S  e  p   t  e  m   b  e  r   2   0   1   3 .   D  o  w  n   l  o  a   d  e   d   b  y   C  e  n   t  r  a   l   S  a   l   t   &   M  a  r   i  n  e   C   h  e  m   i  c  a   l  s   R  e  s  e  a  r  c   h   I  n  s   t   i   t  u   t  e   (   C   S   M   C   R   I   )  o  n   2   3   /   1   0   /   2   0   1   3   1   7  :   4   7  :   1   6 . View Article Online
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