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Synthesis and characterization of poly(vinyl alcohol)/sulfonated graphene oxide nanocomposite membranes for use in PEMFCs Hossein Beydaghi, Mehran Javanbakht, and Elaheh Kowsari Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502491d • Publication Date (Web): 08 Oct 2014 Downloaded from http://pubs.acs.org on October 13, 2014

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Industrial & Engineering Chemistry Research

Synthesis and characterization of poly(vinyl alcohol)/sulfonated graphene oxide nanocomposite membranes for use in PEMFCs Hossein Beydaghi,† Mehran Javanbakht,*,†, ‡ and Elaheh Kowsari† † ‡

Department of Chemistry, Amirkabir University of Technology, Tehran, Iran

Fuel cell and solar cell Lab, Renewable Energy Research Center, Amirkabir University of Technology, Tehran, Iran

*

Corresponding author at: Department of Chemistry, Amirkabir University of Technology, Tehran, Iran. Tel.: +98 21

64542764; Fax: +98 21 64542762. E-mail address: [email protected] (M. Javanbakht).

ACS Paragon Plus Environment

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ABSTRACT In this study, novel cross-linked nanocomposite membranes have been prepared from poly(vinyl alcohol) (PVA) and aryl sulfonated graphene oxide (SGO) and a way of crosslinking to improve the chemical, thermal and mechanical stabilities of the nanocomposite was adopted. The surface of the graphene oxide nanoparticles was modified by aryl diazonium salt of sulfanilic acid. It was revealed that addition of SGO (5 wt %) into the PVA matrix improves the thermal stability (melting temperature, 223 ºC), mechanical stability (tensile strength, 67.8 MPa) and proton conductivity (0.050 S cm-1) of the nanocomposite proton exchange membranes. A proton exchange membrane fuel cell (PEMFC) fabricated with the PVA/SGO membrane showed a maximum power density of 16.15 mW cm-2 at 30 ºC. As a result, the investigated PVA/SGO nanocomposite membranes have good potential for further studies and applications in PEMFCs.

Keywords: Nanocomposite membrane; Poly(vinyl alcohol); Sulfonated graphene oxide; Proton conductivity; Proton exchange membrane; PEM Fuel cells


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1. INTRODUCTION Fuel cells have received wide recognition as an alternative renewable method of energy generation. Proton exchange membrane fuel cell is potentially suitable for the power supply to all kinds of dynamoelectric vehicles, because it exhibits high efficiency, low emission and simple structure and also is friendly to environment.1 Fuel cells are electrochemical devices that directly convert the chemical energy of fuel into electrical energy. The most prominent proton exchange membranes (PEMs) are perfluorosulphonic acid (PFSA) polymers such as Nafion. This polymer membrane has high proton conductivity, mechanical strength, excellent chemical stability and good flexibility. However, high cost, high methanol permeability in DMFC and low conductivity at low humidity or high temperature (above 80 ºC) have limited its further application.2,3 Organic-inorganic composites consist of polymer backbone and inorganic materials have attracted much interest as polymer electrolyte membranes (PEMs) for fuel cells, since inorganic particles in polymer matrix might improve mechanical strength, thermal stability, proton conductivity, fuel barrier properties and membrane durability.4 The alternative materials such as sulfonated poly (aryl ether ketone) (SPAEK),5 sulfonated poly (ether ether ketone) (SPEEK),6 polybenzimidazole (PBI),1 sulfonated poly (arylene ether sulfone) (SPAES)7 as organic backbone and silicon oxide,8 montmorillonite,9 heteropolyacids,10 zeolite,11 zirconium phosphates12 and metal oxide13 as inorganic filler have been rapidly explored for PEM usages. Zarrin et al.14 prepared the functionalized graphene oxide/Nafion nanocomposites membrane. The GO nanosheets were produced from Hummer’s method and then functionalized by using 3-mercaptopropyl trimethoxysilane (MPTMS) as the sulfonic acid


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functional group precursor. It was found that proton conductivity increases in SGO/Nafion membranes (4 times) over recast Nafion at 120 ºC with 25% humidity. Chien et al.15 synthesized the sulfonated graphene oxide/Nafion nanocomposite membrane and demonstrated it for DMFCs. They found that the composite membranes show lower methanol and water uptakes, improved proton conductivity in low relative humidity, and extremely high methanol selectivity. Heo et al.16 prepared novel SPEEK composite membrane that was prepared using sulfonated graphene oxide and sulfonated PEEK. The GO was produced from Hummer’s method and then functionalized by propane sultone and HCl/water solution. It was found that the proton conductivity and mechanical strength of the SGO/SPEEK membranes are significantly enhanced because the graphene oxide based materials have better mechanical properties, and the interfacial interaction between SGO and SPEEK was increased by the hydrogen bonding. In previous studies, we introduced new proton conducting hybrid membranes for PEM fuel cells based on poly(vinyl alcohol) / nanoporous silica containing phenyl sulfonic acid17 and poly(vinyl alcohol) / poly(sulfonic acid)-grafted silica nanoparticles.18,19 Recently, the preparation and characterization of Nafion/Fe2TiO5 or PVA/La2Ce2O7 nanocomposite membranes for proton exchange membrane fuel cells (PEMFCs) were investigated.20,21 In this work, a novel strategy for producing nanocomposite proton exchange membrane using PVA and sulfonated graphene oxide (SGO) was reported. The sulfonated graphene oxide dispersed in PVA solution and the mixtures were fabricated into thin film membranes by solution casting method. This work shows a way of cross-linking to improve the chemical, thermal and mechanical stabilities of the nanocomposite, and getting a higher proton conductivity of 0.05 S cm-1 with an addition of 5 wt% SGO, compared to peers’


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results for PVA/sulfonated GO membranes. The structure, morphology, thermal stability, water uptake and proton conductivity of new nanocomposite membranes were investigated. At the end, I-V performance of single cell PEMFC based on new nanocomposite membranes was also investigated. PVA is one of the most important commodity polymers due to its good mechanical and thermal properties. This polymer has good chemical resistance, low cost, film forming ability, chemical cross-linking ability and availability of sites for the formation of a stable membrane with good mechanical properties.22,23 It also has high tensile strength, good water uptake and flexibility although these properties are dependent on humidity. The hydrophilic nature of poly(vinyl alchole) results from the presence of -OH groups, which confers the potential for chemical cross-linking in acidic conditions with aldehydes to form acetal or hemiacetal linkages.4 The content of added cross-linker must be controlled because if too much cross-linker is added, the membrane construction becomes compact and proton conductivity decreases and if cross-linker is added less than needed, the membrane won’t have enough mechanical stability to use in fuel cell. The content of added cross-linker is obtained with trial and error method. The properties of the PVA membranes have to be improved, which could be accomplished by blending them with inorganic additive. Today, remarkable properties of graphene based materials have attracted tremendous attention for researchers.24 Their high surface area, high aspect ratio, high mechanical strength and electronic insulation property make GOs particularly attractive as inorganic fillers in PEM fuel cells.25,26 Hydrophilic graphene oxide can disperse readily in most polar solvents such as water. Upon incorporation in PVA, the unique structure and large surface


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area of the GO may provide more proton transport channels, which could be beneficial for the improvement of the proton conductivity and mechanical properties of proton exchange membranes.27 In addition, for increasing the proton conductivity of membrane, GO is functionalized with sulfonic acid groups on the surface that are expected to act as ion transport sites, respectively. For the first time our group used the PVA polymer for combination with aryl sulfonated graphene oxide (SGO) to produce the nanocomposite membrane used in PEM feul cells. Also in this work for the first time aryl sulfonated graphene oxide was used to produce the nanocomposite membrane used in PEM feul cells. The surface of the graphene oxide nanoparticles is functionalized by aryl diazonium salt of sulfanilic acid to increase the proton conductivity. In addition, the attached sulfonated groups may enhance the interfacial interactions between the graphene oxide and the polymer matrix and enhance thermal and mechanical properties of membrane. The anchored –SO3H group in the sulfonated graphene oxide (SGO) is a stronger H-bonding group compared to –COOH/–OH groups present in graphene oxide and it may strongly supramolecularly interact with PVA through H-bond formation with the –OH group of PVA. The strong and directional nature of H-bonding interaction may yield new supramolecular structure of the membrane.28

2. EXPERIMENTAL 2.1. Material Poly (vinyl alcohol), PVA, (molecular weight of 130000 g mol-1 and degree of hydrolysis min. 99%) and glutaraldehyde, GLA, (25 wt % solution in water) were used as supplied by


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Sigma-Aldrich. All of the other materials and solvents (Merck) were used without more purification. 2.2. Syntheses of GO and SGO Scheme 1 briefly illustrates the procedure used to prepare sulfonated graphene oxide. The graphene oxide nanoparticles were prepared from natural graphite by the modified Hummer’s method.29 First, graphite powders (0.5 g) and sodium nitrate (0.5 g) were placed in cold (0-5 ºC) concentrated H2SO4 (23 ml) and then 2 g of KMnO4 was slowly added. After 15 min, the solution was transferred to a 35 ± 2 ºC water bath and stirred for 30 min and then 46 ml deionized water was added and the solution was stirred for 30 min until the temperature raised to 90 ± 2 ºC. Finally, deionized water (140 ml) and 30% H2O2 solution (10 ml) were added to solution. The product was washed several times with 5% hydrochloric acid (HCl) and deionized water and centrifuged until the excess HCl was removed and then filtered and dried in an oven at 70 ºC overnight. The synthesis graphene oxide modified with aryl diazonium salt of sulfanilic acid according to the reported method with minor modifications.30 First, 75 mg dried GO was dispersed in 75 ml deionized water via sonication for 30 min. Then, the pH value of solution was adjusted to 9 –10 with addition of 5 wt % sodium carbonate solution. Then, 600 mg sodium borohydride in 15 ml deionized water was added into the dispersion of GO. The solution was heated at 80 ºC and kept for 1 h under constant stirring to remove the majority of the oxygen functionality. The reduced product was washed with water until its pH became 7 and it is dispersed in 75 ml deionized water for diazonium coupling. The mechanism of sulfonation of graphene oxide by diazonium salt of sulfanilic acid involves the homolytic fission of dinitrogen from the diazonium salt leading to the generation of aryl radical which


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binds to the graphene surface via carbon–carbon covalent bond.31 Then, 18 mg sodium nitrite was dissolved into 5 ml deionized water and the resulting solution was added into 5 ml water containing 46 mg sulfanilic acid and 1.2 ml concentrated HCl in the ice bath. The prepared aryl diazonium salt of sulfanilic acid was added into the RGO dispersion at 0 ºC and is kept for 2 h with stirring. It is centrifuged and washed with deionized water. Then, the product is dispersed in 100 ml deionized water for final reduction with 2 ml hydrazine hydrate solution under refluxed condition for 24 h at 100 ºC to remove the remaining oxygen functionality. Finally, the product was washed with deionized water and dried in oven at 60 ºC. 2.3. Preparation of membranes Three kinds of membranes (PVA/GLA, PVA/GLA/GO and PVA/GLA/SGO) were synthesized by solution casting method and named MPG, MPGGx and MPGSx, respectively, where x present the weight percent of GO or SGO in the nanocomposite membranes. The weight percent of nanoparticles varied from 0% to 7%. Scheme 2 illustrates the preparation of MPGS5 nanocomposite membranes. At first, appropriate amounts of GO based nanoparticles were dispersed in deionized water and then PVA added and stirred for 1 h at 80 ºC to obtain a 10 wt.% solution. Then 1 ml of a cross-linking agent, GLA, was gradually added. The solution was stirred till the temperature of solution slowly decreased to 25 ºC. Then, approximately 0.5 ml of 2 M H2SO4 was added to the solution as a catalyst for the cross-linking reaction. Membrane was cast from the solution using Petri dishes. The solution underwent freeze–thaw cycles in order to enhance mechanical property and elasticity.32,33 The thickness of the dry nanocomposite membrane was about 150 µm. 2.4. Apparatus


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The IR spectra (resolution 4 cm-1) were recorded with a Bruker Equinox 55 with ultra-dry compressed air. The differential scanning calorimetric (DSC) measurements were performed on the dried samples using a Mettler DSC 823. The scanning rate was 10 ºC min1

with nitrogen flow. Powder X-ray diffraction patterns of nanoparticles were recorded

using X pert pro Philips Diffractometer with Cu Kα radiation. Mechanical properties of the membranes were investigated by Z030 Zwick/Roell testing machine with an operating rate of 1 mm/min. The membrane morphologies were investigated by AIS2100 Seron Technology SEM and Zeiss EM900 TEM. The samples were sputtered with gold. Transmission electron microscopy (TEM) was undertaken of ultra-microtome membranes using a TEM instrument. The proton conductivity of nanocomposite membrane was measured by AC impedance spectroscopy with an Autolab potentiostat/galvanostat. The data were collected by applying an AC voltage (50 mV) in the frequency range of 0.1 Hz – 100 KHz. The performances of single cells were tested at 30 ºC using FCT 1505 fuel cell testing system (CHINO Inc., Japan) under dry condition and 5 % RH. The area of testing fixture was 2.3 cm × 2.3 cm. The anode H2 and cathode O2 input flow rates were 300 and 500 ml/min, respectively. 2.5. Water uptake and membrane swelling measurements The membranes were dried at 60 ºC, weighed, soaked in deionized water for 12 h at room temperature and reweighed. Then, water uptake of membrane was calculated from this formula: WU

=

m wet − m dry m dry

× 100 %


(1)

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Here, mwet is the weight of the wet membranes and mdry is the weight of the dry membranes. Membrane swelling (SW) was obtained by measuring lengths of the dry and wet membranes. The membranes were cut into 2 cm × 3 cm and dried overnight at 60 ºC before measuring the length. Then, the dried membranes were immersed for 12 h in deionized water at room temperature. The wet membranes were wiped dry with tissue paper and the length was measured again. SW was calculated by following formula:

SW

=

L wet − L dry L dry

× 100 %

(2)

where, Lwet is the length of the wet membranes and Ldry is the length of the dry membranes. 2.6. Ion-exchange capacity (IEC) The IEC of the nanocomposite membrane was measured by acid-base titration method. At first, the dried membrane was soaked in 50 ml of a 1 M NaCl solution for 24 h to liberate the H+ ions to the solution by ion exchange reaction with Na+ ions. Titration was then accomplished with 0.01 M NaOH solution with phenolphthalein as the indicator and IEC was calculated by following formula: IEC =

V NaOH × C W dry

NaOH

× 100 %

(3)

Where IEC is the ion exchange capacity (meq g-1); VNaOH is the added titrant volume (ml); CNaOH is the molar concentration of the titrant and Wdry is the dry mass of the sample (g). 2.7. Conductivity measurement


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Proton conductivities of membranes were measured by AC impedance method. The proton conductivity was calculated from the measured current resistance (R) using following equation: σ =

L RA

(4)

Where, σ is the proton conductivity (S cm-1), L is the membrane thickness (cm), A is the surface area of electrodes (cm2) and R is the membrane resistance (Ω). Before test, the membrane was soaked in deionized water for 24 h until got sufficiently wet and saturated. 2.8. Fuel cell test Fuel cell performance was tested by measuring the polarization curves of membrane electrode assembly (MEA) prepared from catalyst-coated on electrode (CCE) method. The catalyst solution was prepared by using Pt/C catalyst (10 wt % of Pt), 15 wt % Nafion binder solution (Aldrich), isopropyl alcohol (IPA) and suitable amount of deionized water. The catalyst ink was painted onto carbon cloth (E-tek, HT 2500-W) and dried at 190 ºC in an oven to calculate catalyst loading and then dried in oven at 110 ºC. The nanocomposite membranes were sandwiched between two electrodes (E-tek, 0.2 mg cm-2 Pt/C for the anode and 0.2 mg cm-2 Pt for the cathode) and then hot-pressed under a pressure of about 100 kg cm-2 at 60 ºC for 5 min to obtain a MEA. The I–V measurement of the single cell was recorded after activation.

3. RESULTS AND DISCUSSION 3.1. Structural characterization of nanoparticles and nanocomposite membranes.


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The XRD analyses of prepared GO, SGO and nanocomposite membranes are shown in Figure 1. Natural graphite shows a sharp diffraction peak at 2θ = 26.6◦ corresponding to the (002) plane, indicating a highly organized crystal structure with a layer-to-layer distance of 0.33 nm.34 As for GO, the peak (001) is observed at 2θ = 11.9◦, which indicates that the interlayer spacing is about 0.75 nm calculated using Bragg’s law (λ = 2dsin θ). This value is much larger than that of graphite due to the generation of oxygen containing functional groups between layers.35 The corresponding XRD patterns given in Figure 1A show a peak shift from 2θ = 11.9° for GO to 2θ = 26.3° (d = 0.41 nm) (002) for SGO implying that the largely exfoliated graphene oxide sheets partially restacked through π−π interaction upon sulfonation. On the other hand, for the MPG membrane a broad peak was observes at 2θ = 19.2° indicating the semi-crystalline nature of the membrane. The intensity of the diffraction peak decreases and becomes wider with addition of GO based nanoparticles, indicating the decrease in crystallinity and generation of hydrogen bonding between -SO3H and -COOH groups of nanoparticles and the -OH groups of PVA.36 In the XRD patterns of the nanocomposite membranes, the GO and SGO peaks could be hardly detected; this is reflected in a complete exfoliation and uniform dispersion of the GO based nanoparticles in the PVA matrix.37 The structure of graphene based nanoparticles was characterized by FT-IR spectroscopy. The infrared spectrum of GO and SGO nanoparticles are shown in Figure 2A. In the FT-IR spectrum of GO, we observe a strong and broad absorption at 3425 cm-1 due to O-H stretching vibration. The peaks at 1372 cm-1 and 1224 cm-1 correspond to the skeletal vibrations of C-OH and C-O-C in unoxidized graphitic domains. The bands at 1724 cm-1 and 1051 cm-1are associated with C=O and C-O stretching vibration bands of COOH


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groups.38 The peak at 1623 cm-1 may be from skeletal vibrations of unoxidized graphitic domains and the adsorbed water molecules. After sulfonation of GO with aryl diazonium salt of sulfanilic acid, Three peaks at 1163 cm-1, 1113 cm-1, and 1029 cm-1 (two νS-O and one νs-phenyl) confirm the presence of a sulfonic acid groups in sample. The peak at 830 cm-1 (out of plane hydrogen wagging) indicates characteristic vibrations of a p-disubstituted phenyl group.39 After reduction by the present method, the absorption peak of C=O in COOH groups at about 1724 cm-1 apparently diminishes, demonstrating the successful reduction of GO to reduced graphene oxide. In the FTIR spectra of the membranes (Figure 2B), the O-H stretching absorption peak between 3000 and 3500 cm-1 indicates the existence of strong intermolecular and intramolecular hydrogen bonding.4 This absorption peak is shifted to a lower wavenumber with addition of SGO nanoparticle. This phenomenon indicates the existence of hydrogen bonding between the hydroxyl group in PVA and the remaining oxygen functional groups in graphene based nanoparticle. The absorption peak between 1710 and 1720 cm-1, corresponding to the stretching vibration of C=O ester groups, appears with higher intensity in the MPGG5 spectrum, indicating hydrogen bonding between carboxylic acid groups of GO and SGO nanoparticles and the alcohol groups of PVA has been established.40 The membranes show a stretching vibration band at 2800-3000 cm-1 (CH and CH2 groups), at 1300-1500 cm-1 (CH/CH2 deformation vibrations) and at 1700-1720 cm-1 for stretching vibration of C=O ester groups. The elemental analysis of GO and SGO nanoparticles was performed by CHNS test and an average of wt % data is presented in Table 2. The content of sulfur in SGO was estimated to be 3.83 wt %. The results indicate that the graphene oxide nanoparticles are successfully modified with aryl sulfonic acid groups without damaging to the graphene oxide structure.


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The CHNS analysis indicates that the aryl sulfonic group is introduced on the surface of graphene oxide and part of oxygen functional groups in SGO decrease compared to GO nanoparticles after sulfonation. The morphology of synthesized GO and SGO is characterized by scanning electron microscope (SEM) and Transmission electron microscopy (TEM). The exfoliated layered structure of GO and SGO is shown in Figure 3. SEM images showed that the samples have nanoporous structures resulted from thermal exfoliation. The TEM images of the nanoparticles indicated the wrinkled sheet-like two-dimensional structure of graphene based nanoparticles. Also, TEM images showed that SGO nanoparticles look much darker in color as compared to that of GO nanoparticles.41 The surface and cross-sectional morphology of the prepared membranes are shown in Figure 4. From the surface SEM images, MPGS5 membrane exhibits rougher surface and porous compared to MPG membrane which is related to SGO nanoparticles. The cross-sectional images display a homogeneously distributed of GO based nanoparticles and layered structure for the nanocomposite membranes comparing with the smooth surface for MPG membrane, which is for the strong interfacial adhesion and good compatibility between the GO based nanoparticles and PVA matrix. 3.2. Ion exchange capacity (IEC), water uptake, membrane swelling and proton conductivity The ion exchange capacity of the nanocomposite membranes was tested and the results are shown in Table 1. The IEC value provides an indication of the number of proton exchangeable groups present in the nanocomposite membranes. The IEC values of MPGGx membranes show little fluctuation with increasing GO content, which is for presence of


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COOH groups. The IEC of MPGGx membranes is much lower than MPGSx membranes because the GO nanoparticles do not have the –SO3H group. As shown in Table 1, IEC of MPGSx membranes increases with increasing SGO content in the nanocomposite membranes. The IEC values of MPGSx membranes are ranged from 0.81 to 0.96 meq g-1. With the increase in the content of SGO nanoparticles, the membrane becomes more hydrophilic and enhances the suitable sulfonating sites which correspond to the increase in the proton carrier of the nanocomposite membranes. This trend coincides with the increase in proton conductivity with SGO content as shown in Table 1. These results indicate that SGO contributes to proton conduction due to the increase in the sulfonic acid groups as a proton donor and carrier in the membrane. Water has important effects in proton exchange membranes because the adsorbed water can help the transport of protons in membrane. PVA based membranes have high water uptake, which is for hydrophilic property of PVA. Cross-linker has important role in water uptake and mechanical properties of PVA based membranes. The effect of cross-linking agent in water uptake of PVA membrane was investigated before.17 The water uptake of PVA membranes decreases gradually with addition of the cross-linking agent. However, addition of cross-linking agent to PVA is obligatory for enhancing in thermal and mechanical property of membrane. The nanocomposite membranes (PVA/GO and PVA/SGO) show decreased water uptake compare to PVA membrane (Table 1) because –SO3H and COOH groups of nanoparticles attach to the -OH groups of PVA with hydrogen bonds and increase the interfacial adhesion of membrane, so the membranes’ structures become compact, and the water transferring channels become narrow and water uptake of membrane decreases. The formation of hydrogen bonding between graphene based nanosheets and PVA was


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approved in FTIR measurements. As shown in Figure 5, the membranes with sulfonic acid functionalized GO show higher water uptakes in compare to the membranes with GO which is for hydrophilic property of –SO3H groups. As shown in Table 1, the membrane swelling of the membranes decreases with addition of nanoparticles, this helps to keep good dimensional stability of the nanocomposite membranes. With increasing of nanoparticles in the PVA matrix, the swelling decreases because of formation of hydrogen bonds between nanoparticles and hydroxyl groups of PVA matrix. The pure PVA membrane swelled 30 %, whereas the membrane with 5 wt % GO and SGO swelled 15 and 19 %, respectively. The proton conductivity of different nanocomposite membranes at room temperature is shown in Table 1. Generally, proton transport in membrane follows by two principle mechanisms of Vehicle mechanism and Grotthus mechanism (hopping). The hydroxyl groups in GO based nanoparticles can attach to free water molecules and increase the proton conductivity of membranes with Vehicle mechanism.16 In sulfonated nanocomposite -

membranes, the proton can jump from one (H3O+SO3 ) molecule to the next molecule by Grotthus mechanism (hopping). It can be observed that the incorporation of SGO nanoparticles into the matrix of PVA has significantly increased the proton conductivity. This phenomenon may suggest that there is an interaction between –SO3H and –OH groups of nanoparticles and water molecules in the membrane. The PVA based membranes have high water uptake and this property helps to increase proton conductivity in membrane. The water uptake of PVA/SGO membranes is much better than Nafion 117 (18%),4 so the PVA/SGO membranes can show high proton conductivity. Addition of the aryl solfunated group to graphene oxide can enhance proton conductivity with increasing the bound water–


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SO3H contents in the membranes. SGO nanoparticles have high surface area with an enormous amount of sulfonated functional groups; it holds more water and facilitates the transfer of protons in membrane. It can be noticed that the proton conductivity of nanocomposite membranes increase with an increase in filler content. As shown in Table 1, proton conductivity of nanocomposite membranes decreases with load of 7 wt % filler, which is for blocking effect in membranes. That is, the addition of too many planar nanoparticles obstructs the polymer chain movement in the proton cluster.16 Figure 6 shows the Arrhenius plot of MPGG5 and MPGS5 nanocomposite membranes at different temperatures. Proton conductivity increases with increasing temperature. At temperature higher than 80 °C, water of membrane slowly evaporates and proton conductivity decreases. The high water uptake of nanocomposite membrane helps to slightly decrease of proton conductivity in high temperature. The activation energy (Ea), the minimum energy required for proton transport through a membrane, was obtained from the gradient of the Arrhenius plot using Arrhenius equation: σ = A exp (-(Ea/RT)), where σ is proton conductivity (S cm-1), A is a pre-exponential factor, Ea is the activation energy (kJ/mol), R is universal gas constant (J mol-1 K-1) and T is the temperature in Kelvin. The activation energy of MPGS5 membrane (3.9 kJ mol-1) is lower than MPGG5 membrane (5.9 kJ mol-1). Then proton can transport in MPGS5 membrane easier compared to MPGG5 membrane. 3.3. Thermal analysis The differential scanning calorimetry (DSC) thermograms of the nanocomposite membranes are shown in Figure 7. The endothermic peaks located between 200 and 215 °C correspond to the melting of the crystalline phases of PVA membrane. These peaks show that PVA membranes have several melting points and indicate different crystal structures in


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membrane. As shown in Figure 8, final Tm for MPG, MPGG5 and MPGS5 membranes are 214, 221 and 223 °C, respectively. It was found that the melting temperature, Tm, of the PVA nanocomposite membrane slightly shifts toward higher temperature when graphene based nanoparticles are added into the PVA polymer matrix. This indicates that the addition of graphene based fillers, decreases the crystallinity of PVA membrane and improves the thermal stability of the nanocomposite membranes. So stiff quality of graphene based material and interactions (hydrogen bonds between the PVA and graphene based nanoparticles) in the hybrid interfacial regions of PVA matrix increase the melting temperature of the nanocomposite membrane. MPGS5 membrane shows higher Tm compared to MPGG5 membrane, which was due to the increase in interactions of membrane with addition of –SO3H groups. 3.4. Mechanical property The effect of graphene based nanoparticles on the tensile properties of nanocomposite membranes was studied and the results are shown in Figure 8. The tensile strength (TS) values vary in the range of 48.46–67.83 MPa and elongation at break (Eb) is in the range of 80.19% –174.75%. It is manifest from the Figure 8 that tensile strength (TS) increases and elongation at break (Eb) decreases with addition of GO and SGO. From the images, the TS and Eb of MPG membrane are 48.5 MPa and 174.75%, respectively. The tensile stress of MPGG5 and MPGS5 membranes are 63 and 67.8 MPa, corresponding to improvements of 30% and 40%, respectively. The high mechanical strength of graphene based nanoparticles can enhance tensile stress of prepared nanocomposite membranes. Graphene based nanoparticles can disperse nicely throughout the polymer matrix and polymer chains can interact with these nanoparticles through hydrogen bonds. The –SO3H and COOH groups


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of GO and SGO nanoparticles attach to the –OH groups of PVA chain with hydrogen bonds and increase the interfacial adhesion in membrane. This indicates an increase in the stiffness of the nanocomposite membrane due to addition of GO and SGO nanoparticles. Also, results of DSC curves show PVA is a semi-crystalline polymer. The degree of crystallinity has an important role in mechanical stability of membranes. The decreased crytallinity can enhance mechanical properties of membranes. The incorporation of graphene based nanoparticles can lower the crystallinity of PVA, which is smilar to previous studies conducted by others.41,42 The elongation of the nanocomposite membrane decreases with addition of graphene based nanoparticles, which means the membrane becomes soft. These results indicate that the nanocomposite membrane is more stiff and brittle than the neat PVA membrane. The MPGS5 membrane shows the better TS and Eb compared to MPGG5 membrane. So the –SO3H and –COOH groups of SGO is influencing the mechanical property of PVA much more effectively than that of –COOH groups of GO.28 3.5. Fuel cell performances Figure 9 presents the potential-current density (I-V) and the power density-current density curves for MEAs comprising MPGG5 and MPGS5 membranes with H2 fuel and O2 oxidant at 30 °C. The open-circuit voltage (OCV) for MPGS5 membrane (0.76 V) was higher compared to MPGG5 membrane (0.7 V). Also, the maximum current density of MPGG5 membrane (56 mA cm-2 at cell voltage of 0.105 V) was lower than that of MPGS5 membrane (108 mA cm-2 at cell voltage of 0.12 V) and this is consistent with higher ohmic resistance of this membrane. On the other hand, the maximum power density of MPGS5 membrane (16.15 mW cm-2) was 2.2 times higher than that of MPGG5 membrane (7.22 mW cm-2), which can


19


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be attributed to the higher proton conductivity of MPGS5 membrane. The results indicate that the PEMFC comprised of the MPGS5 membrane showed good electrochemical performance under ambient conditions. The aim of this work was to investigate a class of protonconducting non-fluorinated nanocomposite membranes for PEMFC operating at low temperatures and humidity. However, by increasing the temperature and humidity of the fuel cell a significant increase in the membrane performance was occured.43

4. CONCLUSIONS A novel procedure for solution casting of PVA/SGO nanocomposite membranes was developed based on the aryl diazonium salt of sulfanilic acid. Enhancement of physicochemical and thermal properties due to addition of glutaraldehyde (GLA) as crosslinking agent in a PVA matrix have been studied. The characterizations of the graphene oxide nanoparticles were examined and verified by XRD, CHNS, SEM, TEM and FT-IR technique. The structure of membrane was confirmed by FT-IR and SEM technique. The synthesis membranes have a uniform structure, with good mechanical and thermal stabilities. Tensile stress (TS) of the nanocomposite membranes increased and elongation at break (Eb) decreased with addition of GO and SGO nanoparticles. Concerning the composite membrane, enhanced thermal stability was observed for the nanocomposites with addition of GO and SGO nanoparticles, comparing to the neat polymer. The SGO nanoparticle showed the better properties compared to GO nanoparticle, so the –SO3H group of SGO is more effectively than -COOH group of GO. The proton conductivity of the nanocomposite membranes are also significantly increased because the –SO3H groups of SGO and –OH groups of graphene oxide enhance proton transfer in membrane by


20

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Grotthus mechanism and Vehicle mechanism, respectively. The different properties of the MPGS5 membrane and Nafion 117 are included in Table 3 for comparison.20,43-46 The result shows water uptake of the nanocomposite membrane at room temperature (58.3%) is much higher than that of Nafion 117 (18%) at the same condition. In addition, thermal stability of this membrane (Tm = 223 ºC) is also higher than that of Nafion 117 (Tm = 137 ºC). The results in Table 4 show that, the tensile s strength in nanocomposite membrane (67.8 MPa) enhances compared to Nafion 117 (43 MPa). At the end, the proton conductivity of the synthesized membrane is higher than that of Nafion 117 in room temperature.20

Acknowledgement The authors are grateful to Amirkabir University of Technology (Tehran, Iran) and Renewable Energy Organization of Iran (SUNA) for the financial support of this work.


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(29) Hummers, W.; Offeman, R. Preparation of graphitic oxide. J. Am. Chem. Soc 1958, 80, 1339-1339. (30) Si, Y.; Samulski, E. T. Synthesis of water soluble graphene. Nano Lett. 2008, 8, 16791682. (31) Schmidt, G.; Gallon, S.; Esnouf, S.; Bourgoin, J. P.; Chenevier, P. Mechanism of the coupling of diazonium to single-walled carbon nanotubes and its consequences. Chem. Eur. J. 2009, 15, 2101-2110. (32) Hassan, C. M. Peppas, N. A. Advances in polymer science. Berlin Heidenberg, Springer-Verlag, 2000; Vol. 153. (33) Peppas, N. A.; Stauffer, S. R. Reinforced uncrosslinked poly (vinyl alcohol) gels produced by cyclic freezing-thawing processes: a short review. J. Controlled Release 1991, 16, 305-310. (34) Guo, J.; Ren, L., Wang, R.; Zhang, Ch., Yang, Y. ; Liu, T. Water dispersible graphene noncovalently

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(45) Zhao, Ch.; Lin, H. Na, H. Novel cross-linked sulfonated poly(arylene ether ketone) membranes for direct methanol fuel cell. Int. J. Hydrogen Energy 2010, 35, 21762182. (46) Kim, S. O.; Kim, J. S. Preparation of hybrid proton conductor by sol-gel process from Nafion solution. Macromol. Res. 2002, 10, 174-177.


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Figure legend Scheme 1. Schematic presentation of aryl sulfonated graphene oxide preparation from graphite Scheme 2. Schematic presentation of reaction between PVA and SGO nanoparticles Figure 1. XRD patterns of (A) gaphene based nanoparticles and (B) MPG, MPGG5 and MPGS5 membranes. Figure 2. FTIR spectra of (A) GO and SGO nanoparticles (B) MPG, MPGG5 and MPGS5 membranes. Figure 3. SEM images of the (A) GO (B) SGO and TEM image of (C) GO (D) SGO. Figure 4. Surface SEM micrographs of (A) MPG and (B) MPGS5 and cross-sectional SEM images of (C) MPG, (D) MPGG5 and (E) MPGS5 membrane. Figure 5. Water uptake of nanocomposite membranes. Figure 6. Arrhenius plot of proton conductivity of nanocomposite membranes. Figure 7. DSC thermogram comparison between PVA and nanocomposite membranes. Figure 8. Stress–strain curves of the PVA and nanocomposite membranes. Figure 9. I-V and power density curves of nanocomposite membranes. The single cell operated at temperature of 30 ºC and ambient pressure.


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Table 1. Comparison of ion exchange capacity (IEC), water uptake (WU), swelling (SW) and proton conductivity (σ) for prepared membranes.

IEC Membrane

Composition

σ WU (%)

SW (%)

(meq g-1) MPG MPGG1 MPGG3 MPGG5 MPGG7 MPGS1 MPGS3 MPGS5 MPGS7

(S cm-1)

PVA/GLA

-

65

30

0.0006

PVA/GLA/ GO (1 wt %)

0.17

57.2

18

0.0023


0.18

56.8

17

0.0026


0.21

56.2

15

0.003


0.21

55.6

14

0.0024

PVA/ GLA/ SGO (1 wt %)

0.81

61.4

23

0.017


0.86

59.6

21

0.036


0.92

58.3

19

0.050


0.96

56.7

18

0.041


30

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Table 2. Weight percentages of GO and SGO determined from CHNS test. Nanoparticle

C

O

S

GO

56.36

43.18

0

SGO

64.77

30.12

3.83


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Table 3. Comparison of the Tm, water uptake, tensile strength and proton conductivity of the MPGS5 membrane and Nafion 117 at room temperature.

a

Membranes

Tm ( ºC)

WU (%)

TS (MPa)

σ (S cm-1)

MPGS5

223

58.3

67.83

0.050

Nafion 117

137

18

43a

0.021

50% RH, 23 °C


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