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Sulphonated Polyimide/Acid Functionalized Graphene Oxide Composite Polymer Electrolyte Membranes with Improved Proton Conductivity and Water Retention Properties

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Sulphonated polyimide (SPI)/sulphonated propylsilane graphene oxide (SPSGO) was assessed to be a promising candidate for polymer electrolyte membrane (PEM). Incorporation of multi-functionalized (–SO3H and -COOH) SPSGO in SPI matrix improved proton
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  1  Sulfonated Polyimide/Acid-Functionalized Graphene Oxide 2  Composite Polymer Electrolyte Membranes with Improved Proton 3  Conductivity and Water-Retention Properties 4  Ravi P. Pandey, †  , ‡  Amit K. Thakur, † and Vinod K. Shahi *  , †  , ‡ 5  † Electro-Membrane Processes Division and  ‡  Academy of Scienti 󿬁 c and Innovative Research, Central Salt and Marine Chemicals 6  Research Institute, Council of Scienti 󿬁 c & Industrial Research, Gijubhai Badheka Marg, Bhavnagar 364 002, Gujarat, India 7  * S  Supporting Information 8  ABSTRACT:  Sulfonated polyimide (SPI)/sulfonated propyl- 9  silane graphene oxide (SPSGO) was assessed to be a 10  promising candidate for polymer electrolyte membranes 11  (PEMs). Incorporation of multifunctionalized (-SO 3 H and 12  -COOH) SPSGO in SPI matrix improved proton conductivity  13  and thermal, mechanical, and chemical stabilities along with 14  bound water content responsible for slow dehydration of the 15  membrane matrix. The reported SPSGO/SPI composite PEM 16  was designed to promote internal self-humidi 󿬁 cation, respon- 17  sible for water-retention properties, and to promote proton 18  conduction, due to the presence of di ff  erent acidic functional 19  groups. Strong hydrogen bonding between multifunctional groups thus led to the presence of interconnected hydrophobic 20  graphene sheets and organic polymer chains, which provides hydrophobic − hydrophilic phase separation and suitable architecture 21  of proton-conducting channels. In single-cell direct methanol fuel cell tests, SPI/SPSGO-8 exhibited 75.06 mW  · cm − 2 maximum 22  power density (in comparison with commercial Na 󿬁 on 117 membrane, 62.40 mW  · cm − 2 ) under 2 M methanol fuel at 70  ° C. 23  KEYWORDS:  sulfonated graphene oxide, sulfonated polyimide, polymer electrolyte membranes, improved water-retention properties, 24  direct methanol fuel cell 25 ■  INTRODUCTION 26  Advanced nanostructured materials for energy applications, 27  such as water splitting, hydrogen pumps, fuel cells, batteries, 28  and other electrochemical devices, require polymer electrolyte 29  membranes (PEMs) with high proton conduction, chemical 30  stability, and electron insulation. 1,2 Direct methanol fuel cells 31  (DMFCs) provide an attractive alternative to rechargeable 32  batteries in electronic devices because of high e ffi ciency, long 33  lifetimes, ability to ref uel, operation at moderate temperatures, 34  and low emission. 3 − 7 For DMFC applications, PEMs should 35  exhibit several demanding properties, such as good mechanical, 36  thermal, and chemical stabilities, high proton conductivity, and 37  imperviousness toward methanol. 7 − 9 Generally, per 󿬂 uorosulfo- 38  nated polymers, such as DuPont ’ s Na 󿬁 on membrane , are 39  considered as a benchmark due to their desired properties. 10 − 15 40  Several practical drawbacks of per 󿬂 uorosulfonated membranes, 41  such as high methanol permeability, deterioration in proton 42  conductivity above 100  ° C, cost, and environmental inadapt- 43  ability, led to serious e ff  orts to develop alternative PEMs, 44  especially sulfonated aromatic polymers. 8,9 ,16  Among the 45  sulfonated aromatic polymers, sulfonated polyimides (SPIs) 46  were considered as promising alternative materials because of  47  their high stabilities (thermal and mechanical), good  󿬁 lm- 48  forming ability, low cost, and low fuel crossover. 17 − 20 49 In SPIs, hydrolysis of imide groups occurs due to 50 nucleophilic attack on the carbonyl carbon atoms under fuel 51 cell operating conditions. However, dianhydrides with a high 52 electron density in the carbonyl carbon atoms produced 53 hydrol ytically stable and nucleophilic attack-resistant poly- 54 imides. 21 Recently, we reported aliphatic − aromatic sulfonated 55 polyimide and acid-functionalized polysilsesquioxane composite 56 membranes. 21 The possibility of incorporating graphene oxide 57 (GO) (potential proton transport vehicle) in SPI matrix was 58 also explored, as GO contains di ff  erent functional groups (-O-, 59 -OH, -COOH) responsible for hydrogen bonding with SPI and 60 formation of proton conducting channels. 22 − 24 61 Polymer/GO composites are attractive materials for potential 62 applications in supercapacitors, photovoltaic devices, actuators, 63 and biosensors. 25 − 30 GO is exfoliated from graphite oxide 64 (GtO), prepared by oxidation of graphite by modi 󿬁 ed 65 Hummers method. GO provides a more facile environment 66 for proton conduction, by a  “ hopping ”  mechanism, and 67 enhanced water-retention properties of composite PEM 68 (necessary for proton conduction in nonhumid conditions), 69  because of its large surface area and the presence of hydrophilic Received:  July 14, 2014  Accepted:  September 10, 2014 Research Articlewww.acsami.org © XXXX American Chemical Society  A  dx.doi.org/10.1021/am504597a  |  ACS Appl. Mater. Interfaces  XXXX, XXX, XXX − XXX  jwp00  |  ACSJCA   |  JCA10.0.1465/W Unicode  |  research.3f (R3.6.i5 HF03:4230  |  2.0 alpha 39) 2014/07/15 09:23:00  |  PROD-JCAVA   |  rq_3932353  |  9/16/2014 17:39:15  |  10  |  JCA-DEFAULT  70  functional groups. Thus, GO was considered as an attractive 71  organic  󿬁 ller in PEM, because its incorporation enhances 72  proton conductivity and water retention and provides an 73  electron-insulating environment. 22 ,31 But these properties of  74  GO could be further improved after acid functionalization by  75  grafting of sulfonic acid groups. Although free-standing 76  sulfonated GO has been reported with 0.04 S · cm − 1 proton 77  conductivity at 303 K, its mechanical stability is a serious 78  concern. 32 Furthermore, incorporation of sulfonated GO in 79  sulfonated polymer matrix (SPI) is expected to improve the 80  proton conductivity (high concentration of sulfonic acid 81  groups) along w ith water-retention and mechanical properties 82  of the PEM. 33,34 83  Herein, we report a sulfonated propylsilane graphene oxide 84  (SPSGO)/SPI composite membrane with improved proton 85  conductivity, water retention, and mechanical properties for 86  DMFC applications. Strong interactions between SPI and 87  SPSGO were responsible for homogeneous dispersion with 88  hydrophobic − hydrophilic phase separation, and thus formation 89  of proton-conducting channels. Aliphatic − aromatic sulfonated 90  polyimide, previously reported by our laboratory, was chosen 91  because of its h ydrolytically stable and nucleophilic attack- 92  resistant nature. 21 93 ■  EXPERIMENTAL SECTION 94  Materials.  Graphite (Gt) powder (size 100  μ m) was purchased 95  from SD Fine Chemicals India. Benzophenone-3,3 ′  ,4,4 ′ -tetracarboxylic 96  dianhydride (BTCDA), (3-mercaptopropyl)trimethoxysilane 97  (MPTMS), and 1,4-diaminobutane (DAB) (99%) were received 98  from Aldrich. Dimethylacetamide (DMAc), tetrahydrofuran (THF), 99  triethylamine (TEA), benzoic acid,  m -cresol, acetone, NaNO 3  , 100  KMnO 4  , H 2 O 2  , H 2 SO 4  , HCl, NaOH, methanol, and NaCl of analytical 101  reagent (AR) grade were obtained from SD Fine Chemicals India and 102  used with proper puri 󿬁 cation. Other chemicals are of commercial 103  grade and used as received. In all experiments, Milli-Q water was used. 104 4,4 ′ -Bis(4-aminophenoxy)biphenyl-3,3 ′ -disulfonic acid (BAPBDS) 105  was synthesized from 4,4 ′ -bis(4-aminophenoxy)biphen y l (BAPB) 106 (>97%, TCI) according to the procedure reported earlier. 7 107 Preparation of Graphene Oxide.  Graphite oxide (GtO) was 108 synthesized by the modi 󿬁 ed Hummers method from puri 󿬁 ed natural 109 󿬂 ake graphite (Gt) powder. A 500 mL round-bottom  󿬂 ask was charged 110  with Gt (2.0 g), NaNO 3  (2.0 g), and H 2 SO 4  (96 mL) and kept in an 111 ice bath. KMnO 4  (6.0 g) was added gradually under stirring below 20 112 ° C. Gradually, reaction temperature was increased to 35  ±  3  ° C for 30 113 min and then to 95  ±  3  ° C for 30 min, followed by slow addition of  114 H 2 O (180 mL). The reaction was terminated by addition of a large 115 amount of distilled water. Further 30% H 2 O 2  solution was added to 116 neutralize the excess amount of permanganate. Finally, the mixture was 117 centrifuged and the precipitate was washed with 5% aqueous HCl 118 solution, and water. The  󿬁 nal precipitate was dried overnight under 119  vacuum. GO was exfoliated from GtO by ultrasonication. 30 120 Preparation of Sulfonated Propylsilanegraphene Oxide, 121 Sulfonated Polyimide, and Membrane.  In a typical preparation 122 s1 procedure for SPSGO (Scheme 1), GO (10 mg), MPTMS [100 mg 123 (in 10:1 MPTMS/GO)], and anhydrous THF (100 mL) were added 124 to a three-neck round-bottom  󿬂 ask equipped with a magnetic stir bar 125 and condenser. The resulting mixture was re 󿬂 uxed at 60  ° C for 15 h. 126 Then the reaction mixture was cooled to room temperature and THF 127  was removed by   󿬁 ltration. The obtained  󿬁 ltrate was treated with 30 wt 128 % H 2 O 2  solution at 25  ° C (24 h) for oxidation of mercapto groups. 129 Finally, the oxidized product was  󿬁 ltered, washed with water/ 130 methanol, and dried overnight under vacuum. 31,35 Sulfonated 131 polyimide (SPI) was prepared by our laboratory method described 132 earlier. 21 133 Membranes were prepared by solution casting method. In a typical 134 procedure, a known amount of SPSGO (0 − 8 wt % relative to the SPI) 135  was homogeneously dispersed in DMAc (100 mL) under sonication, a 136 known amount of SPI (20% w/v) was added to the solution, and the 137 resulting mixture was stirred for 24 h at room temperature. The 138 obtained highly viscous solution was cast as a thin  󿬁 lm on a cleaned 139 glass plate and dried in a vacuum oven at 80  ° C for 12 h. A schematic 140 route for preparation of these composite membranes is depicted in Scheme 1. Preparation of Sulfonated Propylsilane Graphene Oxide ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am504597a  |  ACS Appl. Mater. Interfaces  XXXX, XXX, XXX − XXX B  s2 141  Scheme 2. Prepared membranes were designated as SPI/SPSGO-X, 142  where X is the weight percentage of SPSGO (0, 4, 6, or 8 wt %). 143  Instrumental Analysis.  Detailed instrumental analysis such as IR  144  spectra, solid-state  13 C NMR spectra, wide-angle X-ray di ff  raction 145  (XRD), transmission electron microscopy (TEM), scanning electron 146  microscopy (SEM), optical images, thermogravimetric analysis (TGA), 147 and dynamic mechanical analysis (DMA) are included in section S1 of  148 Supporting Information. Bound water content was estimated from 149  weight loss percentage obtained by sample heating in TGA with 10 150 ° C · min − 1 rate under nitrogen atmosphere between 100 and 150  ° C. 36 151 Water Uptake, Ion-Exchange Capacity, and Stability 152 Measurements.  Detailed procedures for determination of water Scheme 2. Schematic Route for Preparation of SPI/SPSGO Composite Membranes Figure 1.  Solid-state  13 C NMR spectrum of SPSGO. ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am504597a  |  ACS Appl. Mater. Interfaces  XXXX, XXX, XXX − XXX C  153  uptake, swelling ratio, and number of water molecules associated per 154  ionic sites (  λ ) are included in section S2 of  Supporting Information. 155  Ion-exchange capacity (IEC) was measured by the back-titration 156  method; detailed procedure is included in section S3 of  Supporting 157  Information. Procedures for studying oxidative and hydrolytic 158  stabilities of the composite membranes are included in section S4 of  159  Supporting Information. 160  Membrane Conductivity and Methanol Permeability.  Mem- 161  brane conductivity was measured in through-plane direction. Detailed 162  procedures for measurement of proton conductivity and electronic 163  conductivity are included in section S5 of  Supporting Information. 164  Methanol permeability of the composite membranes was determined 165  in a diaphragm di ff  usion cell (see section S6 of  Supporting 166  Information). 167  Direct Methanol Fuel Cell Performance.  Single-cell DMFC 168  performance of prepared SPSGO/SPI composite membranes was 169  compared with pristine membrane (SPI) and Na 󿬁 on 117 with the help 170  of a MTS-150 manual fuel cell test station (ElectroChem Inc.). Fuel 171  cell test station was equipped with controlled fuel  󿬂 ow and pressure 172  and temperature regulation attached with electronic load control ECL- 173  150 (ElectroChem Inc.). For studying DMFC single-cell performance, 174  the anode was made by coating a slurry of catalyst (50 wt % Pt + 50 wt 175  % Ru on carbon), 5 wt % Na 󿬁 on ionomer solution, 2-propanol, and 176  Millipore water (catalyst ink) on gas di ff  usion layer at a loading of 5 177  mg · cm − 2 Pt and Ru. The cathode was obtained by coating the same 178 catalyst ink lacking Ru at the same loading. 37 Measurements were 179 performed in the air mode of operation at 10 psi pressure with a 2 M 180 methanol fede at the anode side with pressure 7 psi at 70  ° C for a 181 representative membrane. 3. RESULTS AND DISCUSSION 182 Structural Characterization of GO and SPSGO.  GO was 183 synthesized by modi 󿬁 ed Hummers method, and the presence 184 of oxygenated functional groups (such as hydroxyl, carboxyl, 185 carbonyl, and oxygen epoxide) was con 󿬁 rmed by the Fourier 186 transform infrared (FT-IR) spectrum presented in Figure S1a 187 (Supporting Information). Absorption bands at  υ  = 3403, 1720, 188 1620, and 1053 cm − 1  were observed due to O − H stretching, 189 C  O stretching, adsor bed water, and C − O stretching 190  vibration, respectively. 38 SPSGO was prepared by the 191 condensation reaction of GO and MPTMS followed by  192 oxidation of mercapto groups, and its structure was con 󿬁 rmed 193  by FT-IR and solid-state  13 C NMR spectrum. FT-IR spectrum 194 of SPSGO showed an absorption band at  υ  = 2927 cm − 1  , due to 195 the presence of C − H stretching of methylene groups (Figure 196 S1b, Supporting Information). Further absorption bands at  υ  = 197 1096 and 689 cm − 1  were observed due to Si − O stretching and 198 Si − C stretching, respectively. Absorption bands at  υ  = 1026 Figure 2.  TEM images: (a) GO and (b) SPSGO. Figure 3.  Solid-state  13 C NMR spectrum of SPI/SPSGO-8 composite membrane. ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am504597a  |  ACS Appl. Mater. Interfaces  XXXX, XXX, XXX − XXX D  199  (SO 2  symmetric stretch) and 1256 cm − 1 (SO 2  asymmetric 200  stretch) con 󿬁 rmed the oxidation of mercapto groups. 39 ,40 Solid- f1 201  state  13 C NMR spectrum (Figure 1) showed a peak at 68.22 202  ppm due to the presence of epoxide carbon and peaks at 12.02, 203  25.38, and 41.34 ppm due to the presence of methylene carbon 204  chain in SPSGO. The minor peaks at 129.36 and 168.35 ppm 205  were assigned to aromatic functionalities (C  C) and to 206  carbonyl carbons (C  O), respectively. 41,42 207  TEM images of GO exhi bited  󿬂 at, exfoliated structure with f2 208  some wrinkles (Figure 2a), 26  while TEM images of SPSGO 209  showed some black dots on the surface, attributed to the 210  clustering of sulfonic acid groups (Figure 2 b). Presence of silica, 211  sulfur, and oxygen on the surface of SPSGO was further 212  con 󿬁 rmed by scanning transmission electron microscopy  213  (STEM), elemental mapping images, and energy-dispersive 214  spectroscopy (EDS) spectrum (Figure S2, Supporting 215  Information). 216  SPI/SPSGO Composite Membrane.  SPI/SPSGO compo- 217  site membranes of di ff  erent composition were prepared by  218  solution casting in DMAc. The FT-IR spectrum of SPI/SPSGO 219  composite membrane showed absorption bands at  υ  = 670 and 220  1088 cm − 1  , due to the presence of Si − C and Si − O groups, 221  respectively (Figure S1c, Supporting Information). Absorption 222  bands at  υ  = 1021, 1166 (SO 2  symmetric stretch), and 1239 223  cm − 1 (SO 2  asymmetric stretch) con 󿬁 rmed the presence of  224  sulfonic acid groups in the membrane matrix. Peaks at  υ  = 225  1638, 1773, and 1705 cm − 1  were attributed to the C  C bond 226  stretching of phenol ring and symmetric and asymmetric 227  stretching of C  O groups, respectively. The broad absorption 228  band at  υ  = 3431 cm − 1  was observed due to the strong 229  hydrogen-bond interaction between SPI and SPSGO matrix. 230  Solid-state  13 C NMR spectrum of SPI/SPSGO composite 231  membrane showed all the expected peaks, clearly assigned in f3 232  Figure 3. 233 SEM images of pristine SPI and SPI/SPSGO-8 composite 234 f4 membranes (Figure 4a,b) revealed the change in surface 235 morphology after grafting of SPSGO with SPI. Pristine SPI 236 membrane exhibited a smooth surface, while composite 237 membrane showed a relatively rough surface. Cross-sectional 238 image for SPI/SPSGO-8 composite membrane revealed 239 homogeneous grafting of SPSGO in SPI matrix (Figure 4c). 240 Pristine SPI membranes lost their transparency with progressive 241 increase in SPSGO content in the membrane matrix Figure 242 4d − g. 243 The XRD spectrum of graphite (Gt) showed a di ff  raction 244 peak at about 2 θ   = 26.39 °  , corresponding to the interplanar 245 distance between the di ff  erent graphene layers (Figure S3a, 246 Supporting Information). Due to chemical oxidation of  247 graphite, the order of graphene layers was disturbed and 248 interlayer spacing between graphene sheets was increased. 249 Thus, for GO, the value of the di ff  raction peak decreased to 250 around 2 θ   = 11.80 °  , con 󿬁 rming the successful oxidation of  251 graphite. Furthermore, broad di ff  raction peaks around 2 θ   = 11 ° 252 and 20 °  for SPSGO also con 󿬁 rmed the successful modi 󿬁 cation 253 of GO surface by MPTMS. 32 254 The XRD pattern of pristine SPI membrane showed a peak  255 at around 2 θ   = 22.98 °  , associated with the intricacy of an 256 amorphous region and a crystalline region (Figure S3b, 257 Supporting Information). The SPI/SPSGO-8 composite 258 membrane showed a broad peak, due to strong interaction 259  between SPI matrix and SPSGO. Broad peak at 11 ° 260 corresponds to the  󿬁 ller (SPSGO). Thus, exfoliation of  261 SPSGO layers in the SPI polymer matrix has been ruled out. 262 Thermal and Mechanical Properties.  Thermogravimetric 263 analysis curves for GO and SPSGO are compared in Figure S4 264 (Supporting Information). Below 150  ° C, GO exhibited 13 wt 265 % weight loss, while SPSGO showed 20 wt % weight loss, 266  because of evaporation of adsorbed and bound water. Between 267 150 and 250  ° C, GO exhibited 14 wt % weight loss, due to Figure 4.  (a − c) SEM images: (a) SPI (surface), (b) SPI/SPSGO-8 (surface), and (c) SPI/SPSGO-8 (cross-section). (d − g) Optical images: (d) SPI,(e) SPI/SPSGO-4, (f) SPI/SPSGO-6, and (g) SPI/SPSGO-8. ACS Applied Materials & Interfaces  Research Article dx.doi.org/10.1021/am504597a  |  ACS Appl. Mater. Interfaces  XXXX, XXX, XXX − XXX E

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May 18, 2018
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