Custom-made sulfonated poly (ether sulfone) nanocomposite proton exchange membranes using exfoliated molybdenum disulfide nanosheets for DMFC applications

Accepted Manuscript Custom-made sulfonated poly (ether sulfone) nanocomposite proton exchange membranes using exfoliated molybdenum disulfide nanosheets for DMFC applications Kumar Divya, Meenakshi Sundaram, Sri Abirami Saraswathi, Dipak Rana, Subbiah Alwarappan, Alagumalai Nagendran PII:

S0032-3861(18)30449-X

DOI:

10.1016/j.polymer.2018.05.054

Reference:

JPOL 20615

To appear in:

Polymer

Received Date: 21 February 2018 Revised Date:

7 May 2018

Accepted Date: 18 May 2018

Please cite this article as: Divya K, Sundaram M, Saraswathi SA, Rana D, Alwarappan S, Nagendran A, Custom-made sulfonated poly (ether sulfone) nanocomposite proton exchange membranes using exfoliated molybdenum disulfide nanosheets for DMFC applications, Polymer (2018), doi: 10.1016/ j.polymer.2018.05.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Custom-made sulfonated poly (ether sulfone) nanocomposite proton

exchange membranes using exfoliated molybdenum disulfide nanosheets for DMFC applications

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Kumar Divyaa, Meenakshi Sundaram, Sri Abirami Saraswathia, Dipak Ranab, Subbiah Alwarappanc, Alagumalai Nagendrana,*

Polymeric Materials Research Lab, PG & Research Department of Chemistry, Alagappa

Government Arts College, Karaikudi - 630 003, India

Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur

St., Ottawa, ON, K1N 6N5, Canada

CSIR-Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi- 630003, India

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*Corresponding author: E-mail: [email protected] (A. Nagendran); Tel.: 91-4565-224283; Fax: 91-456527497

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ACCEPTED MANUSCRIPT Abstract Exfoliated molybdenum disulfide (E-MoS2) incorporated sulfonated poly ether sulfone (SPES) nanocomposite proton exchange membranes (PEMs) were made by solution casting method. E-MoS2 was synthesized from bulk MoS2 powder by ultrasonication and

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SPES was prepared by sulfonation of PES using sulfuric and chlorosulfonic acid. Surface characterization of these membranes have been performed using several techniques such as Fourier transformed infra-red spectroscopy, x-ray diffraction, atomic force microscopy, scanning electron microscopy and contact angle measurements. In order to assess the

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physicochemical performance of the membranes, water uptake, swelling ratio and ion exchange capacity (IEC) of the membranes were measured. Thermal and mechanical stability

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of the nanocomposite PEMs were probed by Thermogravimetric, differential scanning calorimetric analysis and tensile strength measurement respectively. Physicochemical characteristics such as water uptake, IEC, swelling ratio, thermal and mechanical stability of SPES/MoS2 nanocomposite PEMs were increased upon comparison with the bare SPES

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PEM. Tensile strength of SPES-1 nanocomposite PEM (117 MPa) is doubled upon comparison with pure SPES PEM (60 MPa). AFM images of PEMs nanocomposite revealed that surface roughness and nodular size were increased upon the addition of E-MoS2.

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Electrochemical performance of nanocomposite membranes such as proton conductivity, selectivity and methanol permeability were investigated and results confirmed that SPES/E-

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MoS2 nanocomposite membranes exhibited better performance than bare SPES PEM. More specifically, SPES membrane incorporated with 1 wt% E-MoS2 (SPES-1) exhibited higher proton conductivity (3.17×10-3 Scm-1), selectivity (8.43×104 Scm-3s) and lower methanol permeability (0.376×10-7 cm2s-1). From the results, it is evident that SPES-1 PEM nanocomposites are better candidate for applications in DMFCs. Keywords: SPES; PEM; Exfoliated MoS2; Nanocomposite; DMFC

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ACCEPTED MANUSCRIPT Introduction Rapid urbanization, ever growing population and advancements in the industrial development have led to the global energy crisis which is considered as a daunting global challenge in the current scenario [1]. Further, the global energy crisis is expected to increase

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in the coming decades. Therefore, various attempts have been made to produce electricity in a cost effective and environmental friendly manner. Amongst various alternate techniques available for the power generation, direct methanol fuel cells (DMFCs) is considered to be a

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fair choice as it has the potential to perform as a renewable energy source with high efficiency and energy density [2,3].

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Nafion, a perfluorinated polymer which shows excellent chemical as well as thermal stability is considered to be the most preferred proton exchange membrane (PEM) for DMFCs. However, at high temperatures, their conducting property is lost and thereby it results in high methanol crossover. Moreover, Nafion is expensive and thereby it has limited

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applications [4,5]. As a result, development of inexpensive sulfonated hydrocarbon polymer based PEMs which is can perform room temperature operation and exhibits low methanol permeability as well as good thermal and chemical stability [6,7] have emerged. Amongst,

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several known polymers poly (ether sulfone) (PES) is considered as a promising polymer as it fulfils the basic requirements for a good PEM such as good oxidative resistance, high thermal

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stability and high group dipole moments. Further the functionalization of PES by sulfonation results in the enhanced membrane properties such as good wettability, improved proton conductivity with low methanol permeability [8, 9]. Nevertheless, in sulfonated polymers based on hydrocarbon backbone, the degree of sulfonation mainly affects the membrane activity. Polymers with high degree of sulfonation show high wettability, which minimizes the mechanical property and its catalytic activities [10]. Several attempts have been made to overcome this trade-off behaviour between proton conductivity and mechanical stability.

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ACCEPTED MANUSCRIPT Organic-inorganic hybrid polymer nanocomposites prepared by incorporating nanomaterial into the PEM matrix have attracted much attention in recent days, since the resulting hybrid nanocomposites bear the unique properties from both polymer and nano-additives. Combination of sulfonated polymer back bone with nanomaterials such as GO [11], SiO2

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[12], ZrO2 [13,14], TiO2 [15], MoS2 [16] etc., are currently used. Of all these, MoS2 was widely used in energy conversion field such us hydro-desulfurization and hydrogen evaluation reaction. Moreover, graphene like 2D MoS2 material possess a hexagonal layered

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structure which is sandwiched between Mo and S atoms. MoS2 may possibly provide active surface area and more hydrophilic channels which can facilitate the proton transport. In

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addition, the high water uptake capacity of the polymer nanocomposite membrane might provide high conductivity together with high mechanical strength. Herein, exfoliated molybdenum disulfide (E-MoS2) was synthesized and incorporated at different concentrations into the SPES matrix using solution casting and evaporation

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method. The proton conductivity, methanol permeability, membrane selectivity, ion exchange

reported in detail. 2. Experimental 2.1 Materials

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capacity, thermal and mechanical stability of the nanocomposite membranes were studied and

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Poly (ether sulfone) (Gafone 3200P, Mw 123 kDa) and N-methyl-2-pyrrolidone

(NMP) were received from Gharda Chemicals Ltd., Mumbai and Merck Millipore, India, respectively. Sodium chloride and chlorosulfonic acid was received from Loba Chemie. Pvt. Ltd. (Mumbai, India), Bulk MoS2 powder and N,N-dimethylformamide (DMF) were procured from Alfa Aesar and Sigma Aldrich, respectively. Methanol solvent was obtained from SRL, India.

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ACCEPTED MANUSCRIPT 2.2 Sulfonation of polyethersulfone Sulfonated poly (ether sulfone) (SPES) was synthesized according to a recent report [8]. Initially, pure PES pellets were dried at 80˚C in an oven after which a predetermined quantity of PES was completely dissolved in 100 mL concentrated sulfuric acid under

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vigorous stirring at room temperature. Further the addition of appropriate amount of chlorosulfonic acid at 45°C for 6 h with constant stirring to attain the essential sulfonation. At the completion of sulfonation, the foretold mixture was added in drops to ice cold water and

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the resultant product was filtered, washed several times with de-ionized water and dried at 50°C in a hot air oven to completely remove the moisture content.

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2.2.1 Degree of sulfonation and FT-IR of SPES

The degree of sulfonation (DS) of the as-prepared SPES was determined by titration method and it was found to be 30% [17]. Briefly, SPES was immersed in 3 M NaCl for 24 h at room temperature and titrated against 0.01 M NaOH using phenolphthalein as indicator

. [  × ]

× 100 1

.[  × ]

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 =

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and the DS was calculated using the equation (1):



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Where, M (NaOH) is the concentration of NaOH, V (NaOH) is the volume of NaOH used for neutralization, ‘W’ is the weight of the sample in ‘g’, 81 is the molecular weight of – SO3H group and 244 is the molecular weight of PES repeating unit. The FTIR characterization of SPES was probed with transmission mode by

employing TENSOR 27, Bruker Optik GmbH, Germany with in the wavelength between 400 and 4000 cm-1. 2.3 Synthesis of exfoliated MoS2 (E-MoS2)

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ACCEPTED MANUSCRIPT E-MoS2 nanosheets synthesis was performed by liquid exfoliation method as reported by us [18] using bulk MoS2 powder and DMF as a base material and exfoliation medium respectively. In brief, MoS2 powder was dispersed in DMF and the mixture was sonicated for 3h at room temperature. The resulting product was centrifuged at 3000 rpm, filtered and dried

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in an oven to get nano E-MoS2. 2.3.1 XRD and FT-IR characterization of E-MoS2

The as-synthesized E-MoS2was characterized by PANalyticalX’Pert PRO powder X-

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ray diffractometer at a scan rate of 1º min-1 with the 2θ range from 10º to 80º, using Cu Kα radiation. An FTIR spectrum of E-MoS2 was probed by using a TENSOR 27,Bruker OPTIK

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GmBH, FT-IR spectrophotometer.

2.4 Fabrication of SPES/E-MoS2 composite membrane

SPES/MoS2 nanocomposite membranes were made by the simple solution casting method [19] and the composition of polymer doped solution is given in Table 1. Initially, an

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appropriate amount of E-MoS2 was dispersed in NMP and ultra-sonicated for 30 min. To this mixture, a fixed quantity of dried SPES was added and stirred vigorously to attain a homogeneous dope and left as such for 12 h, to get rid of air bubbles. Finally, the dope was

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cast on a glass plate to a 0.02 mm thick film and dried in a vacuum oven for 24 h at a temperature of 110°C to remove the excess solvent. Composition of neat and nanocomposite SPES membranes

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Table 1

Membrane code

Polymer (wt%) (SPES)

Additive (wt%) (MoS2)

Solvent (wt%) NMP

Neat SPES

10

0

90

SPES-0.5

9.9

0.1

90

SPES-1

9.5

0.5

90

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ACCEPTED MANUSCRIPT 2.5 Physicochemical characterization of SPES/E-MoS2 composite membranes 2.5.1 XRD The XRD spectrum was employed to confirm the presence of different functional groups in the SPES/E-MoS2 nanocomposite membranes using PANalyticalX’Pert PRO

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powder X-ray diffractometer. 2.5.2 AFM, FESEM and contact angle

The surface roughness and morphology of the bare and nanocomposite SPES

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membranes were investigated using Atomic force microscope (AFM, Multimode 8, Bruker Corporation, USA) in tapping mode at room temperature. Field Emission Scanning Electron

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Microscope (FESEM) (Carl Zeiss, Germany) was employed for the surface morphological analysis of SPES/MoS2 nanocomposite membranes. The water contact angle measurements for the membrane was performed by employing a dynamic Surface Tensiometer (VCA Optima surface analyzing system, AST Products Inc., Billerica, MA) at 25°C. In order to

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eliminate the experimental error, the water droplets were placed at different locations and the angles were measured and averaged.

2.5.3 Water uptake, swelling ratio and IEC

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Water uptake of the composite membrane was evaluated by drying the membrane samples in an oven at 80°C (The dry weights are measure before the start of the experiments).

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Subsequently, the membranes were soaked in de-ionized water at room temperature for 24 h and weighed again. The obtained dry and wet membrane weights were substituted in equation (2) to determine the percent water uptake [19].

 !% =

#$%&'()* +()*

7

× 100

(2)

ACCEPTED MANUSCRIPT The swelling ratio of the nanocomposite membranes was determined by measuring the thickness of the wet ,+-.  and dry (,/01 ) membrane samples.

/#$% /()*

 % =

(3)

× 100

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/()*

Ion exchange capacity (IEC) of the nanocomposite membranes was determined using simple acid base titration [20]. The membranes were soaked in a freshly prepared 0.1 M NaCl

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for 24 h in order to initiate the exchange of H+ and Na+ ions to occur. Following this, it was titrated against 0.01 M NaOH by using phenolphthalein as an indicator. Finally, the IEC

234 =

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values were calculated from the following equation (4).

5678 95678 ()*

(4)

of membrane sample.

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Where, :  and 4  is the volume and molarity of the titrant, and /01 is the dry weight

2.5.4 Mechanical and thermal stability

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Mechanical strength of the nanocomposite membranes was performed by a Universal Testing Machine (Tinius Olsen, H10) at room temperature. The thermal stability of the

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prepared nanocomposite membranes were investigated by the thermogravimetric analyzer (SDTQ600, TA instruments, USA) from 25 to 800°C at 10°C min-1 scan rate under nitrogen atmosphere.

2.6 Electrochemical characterization of SPES/MoS2 composite membranes 2.6.1 Proton conductivity Proton conductivity of the nanocomposite membranes was tested under different conditions by AC impedance spectroscopy with an Auto Lab potentiostat/galvanostat electrochemical system. Initially, the membrane samples are completely hydrated by 8

ACCEPTED MANUSCRIPT immersing them in deionized water for 24 h at room temperature. Later, the sample was placed between the two stainless steel electrode assemblies and operated in the frequency range between 1 Hz and100 KHz with as oscillating voltage of 10 mV. Proton conductivity of

;=

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the nanocomposite membrane was calculated using the relation (5) [21].

<

(5)

=>

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Where, ‘L’ is the thickness of the membrane sample (cm), ‘A’ is the active surface of the electrode (cm2), and ‘R’ is the resistivity (Ω) of the membrane.

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2.6.2 Methanol permeability

The methanol permeation properties of the nanocomposite SPES membranes were determined, before which the sample membranes was immersed in water for 24 h for the purpose of membrane hydration. A cell with two compartment separated by the test

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membrane. The one side of the cell compartment contains methanol while the other carries de-ionized water and both the compartments were kept under constant stirring throughout the experiment. At the end, the methanol permeability of the test membrane was determined by

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measuring the concentration of methanol on the water side using an ABBE NAR-3T and

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calculated using the equation (6):

? - = @

A >

×

< 9>

(6)

2.6.3 Electrochemical selectivity The electrochemical selectivity of a PEM can be defined as the ratio between the proton conductivity and methanol permeability and represented as the overall PEM performance when used in DMFCs.

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; E

3. Results and discussion 3.1 Characterization of SPES and E-MoS2 The FTIR spectrum of SPES is depicted in Fig.1. The peaks at 1575 cm-1 and 1292

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cm-1 are attributed to the vibration of aromatic skeleton and aryl oxide respectively. The peaks at 3427 and 1011 cm-1 are assigned to the stretching vibrations of hydroxyl group present in the -SO3H and the symmetric stretching of SO3- in pure SPES, respectively [21].

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Thus the presence of SO3H group confirms the sulfonation of PES.

FT-IR spectrum of SPES.

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Figure 1

FTIR spectra of E-MoS2 powder as shown in Fig.2a shows peaks at 3460 cm-1, 1715

cm-1, 1380 cm-1 and 975 cm-1. The peaks are assigned to the stretching vibrations of O-H, C=O, -CH3 and Mo-O, respectively [22]. The XRD of E-MoS2 powder is shown in Fig. 2b. The peaks at the 2θ angle of 15, 29.4, 45.2 and 60.3º are associated to the MoS2 and assigned as (002), (004), (104) and (110) planes, respectively [23].

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FT-IR spectrum (a) and XRD (b) of SPES.

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Figure 2

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3.2. Physicochemical characterization of SPES/E-MoS2 nanocomposite membranes

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3.2.1 XRD

The XRD of bare SPES and SPES-1 membrane were shown in Fig.3. As evident from the figure that the bare SPES membrane exhibited a broad band at a 2θ angle around 19º, which is considered to be the major amorphous peak of SPES [24]. The SPES-1

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nanocomposite membrane exhibited the characteristic peaks of both SPES andE-MoS2 at 15,

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45.2 and 61.3º and are assigned to (002), (104) and (110) planes, respectively [22].

Figure 3.

XRD of bare SPES and SPES-1 membranes. 11

ACCEPTED MANUSCRIPT 3.2.2 AFM, FESEM and contact angle The AFM images of the membranes and their surface roughness values are depicted in Fig.4 and Table 2, respectively. The AFM images reveal that the incorporation of E-MoS2 into the SPES membrane matrix has enhanced the nodule size and surface roughness.

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Compared to the bare membrane, SPES-0.5 and SPES-1 membranes showed a rougher surface. This is due to the formation of hydrogen bond between the sulfonic acid and

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hydroxyl groups of SPES and E-MoS2, respectively.

Figure 4.

Three dimensional AFM images of bare SPES, SPES-0.5, and SPES-1 membranes.

FESEM images of the SPES/MoS2 nanocomposite membranes were shown in Fig. 5. Herein, we noticed the presence of small dots/nanoparticles on the surface of the SPES nanocomposite membranes confirming the uniform distribution of E-MoS2 on the membranes matrix without any cracks and pinholes formation. Also, these results confirmed that the E12

ACCEPTED MANUSCRIPT MoS2 nanosheets is distributedall over the SPES matrix due to their interfacial interaction

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between the SPES with E-MoS2 particles [18].

Table 2.

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Figure 5. FE-SEM images of SPES-0.5 (a) and SPES-1 (b) membranes. Roughness profile of neat SPES and SPES/E-MoS2 nanocomposite membranes. Membrane code

Surface roughness (nm) FH

SPES

7.2

10.5

SPES-0.5

22.8

30.6

SPES-1

42.7

51.5

Table 3.

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FG

Physicochemical

Performances

of

bare

SPES

and

SPES/E-MoS2

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nanocomposite membranes.

SPES

Water uptake (%) 15.7±0.5

SPES-0.5

20.8±0.2

18.3±0.3

1.99

113

70.8

SPES-1

25.7±0.4

23.1±0.5

2.90

117

66.3

Nafion 117

38

12.6

0.90

43

-

Nafion 212

50

9.9

0.92

32

-

Membrane code

1.13

Tensile strength (MPa) 60

Contact angle (°) 82.3

Swelling ratio (%)

IEC (meq. g-1)

14.5±0.2

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ACCEPTED MANUSCRIPT The contact angle values of the nanocomposite membranes is found to decrease in the following order SPES>SPES-0.5> SPES-1 as shown in Table 3. It indicates that the addition of E-MoS2 enhanced the hydrophilicity of SPES nanocomposite membranes. This is due to the exposed sulfur atoms in the E-MoS2 that makes the entire surface of MoS2 hydrophilic

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and that provides high water affinity to the nanocomposite membranes through hydrogen bonding. Moreover, the E-MoS2 has a fish-bone like structure that creates plenty of channels on the membrane matrix through which the water permeation occur [25, 26].

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3.2.3 Water uptake, swelling ratio and ion-exchange capacity

The water uptake and swelling ratio of composite membranes are listed in the Table 3.

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Results confirm that the incorporation of E-MoS2 sheets increase the wettability and hydrophilicity from bare SPES to SPES-1 membranes. The percentage water absorption facilitates the proton transport via vehicle and Grotthuss mechanism due to the existence of hydrogen bonding with membrane [27].

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Ion-exchange capacity (IEC) provides information about the charge density of a PEM, which is closely associated with its proton conductivity and transport property [28]. The bare SPES shows IEC of 1.13 meq.g-1 and an increasing trend with the increasing concentration of

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E-MoS2 as shown in Table 3. This is due to the formation of hydrogen bonding that favors IEC between the sulfonic acid groups and hydroxyl groups present in SPES and E-MoS2

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respectively.

3.2.4 Mechanical and thermal stability It has been reported that the incorporation of nanoparticles as fillers into the base

membrane matrix provides enhanced mechanical and thermal stability to the resulting nanocomposite PEM [29, 30]. Similarly, SPES/E-MoS2 nanocomposite PEMs exhibited remarkable mechanical property than that of bare SPES membrane as shown in Table 3. This is due to the fact that the fine dispersion of nanoparticle into the polymer matrix effectively

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ACCEPTED MANUSCRIPT interacts with the polymer chain through hydrogen bonding and increases the compactness of

Figure 6.

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membrane structure and imparts enhanced mechanical stability.

TGA curves of bare SPES, SPES-0.5 and SPES-1 membranes.

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TGA profiles of the nanocomposite membranes were shown in Fig.6. It is evident from the figure that, both the bare and nanocomposite SPES membranes exhibits three stages of degradation such as thermal dehydration, thermal degradation and finally thermal

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decomposition of the polymer backbone [31, 32]. Initially, the loss of absorbed water molecules from the membranes occurs between 30°C and 150°C. The second weight loss

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occurs approximately between 280-320°C and is attributed to the loss of the sulfonic acid group present in SPES. This phenomenon is called as the desulfonation of membrane. The third weight loss occurred due to the oxidation of E-MoS2 in the nanocomposite membrane between 450-520°C. Further weight loss starts at 530°C and is attributed to the thermal degradation of SPES backbone [33]. For the SPES/E-MoS2 membranes with different blending ratios, their TGA also show a similar profile in terms of transition temperatures. From the TGA results, it is observed that the nanocomposite membranes withstand higher desulfonation and polymer backbone degradation temperatures upon comparison with bare 15

ACCEPTED MANUSCRIPT SPES membranes and the relatively high stability of nanocomposite membranes can be attributed to the high interaction between SPES and added E-MoS2 that affects the polymer chain mobility [34]. This indicates the enhanced thermal stabilities of SPES/E-MoS2 membranes and is sufficiently high to meet the requirements of the DMFC.

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3.3. Electrochemical characterization of SPES/E-MoS2 nanocomposite membranes 3.3.1 Proton conductivity

Proton conductivity is the most significant parameter in PEM, which determines the

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membrane performance in the fuel cell system. Table 4 represents the proton conductivity of bare SPES and SPES/E-MoS2 nanocomposite membranes at 25°C and 80°C. It indicates that

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the proton conducting ability of SPES increases with increasing concentration of E-MoS2. MoS2 being a dichalcogenide it has effective polarizable anionic framework and due to the large size of S2- compared with O2-, a much more connectivity of anionic framework leads to the facilitation the proton mobility in polymer matrix. Further, the water uptake and swelling

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ratio of the nanocomposite membrane has a great influence on its proton conductivity [28]. This is because, if a membrane possess high percent water uptake, the protons can easily pass through the hydrogen bond network of the water molecules, which will result in increasing

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proton conducting behaviour [35]. This observation is in good agreement with the observed

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percent water uptake results.

3.3.2 Methanol permiability Table 4 represents the methanol permeability of the perpared nanocomposite

mememranes measured at room temperature. As shown in the table, SPES membrane displayed a very low methanol permiability of (0.949×10-7 cm2s-1) comaperd to Nafion 117 and Nafion 212. This observed decrease in methanol permiability can be explained based on their

microstructure.

The

microstructure

of

SPES

membrane

has

smaller

hydrophobicity/hydrophilicity differences (the back bone is less hydrophobic and the sulfonic

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ACCEPTED MANUSCRIPT acid group is less acidic) compared with Nafion. Consequently, the separation into the hydrophilic and hydrophobic region is less pronounced, creating narrow ionic channels, which directs lower methanol permeation than Nafion-117 and Nafion–212 [36-38]. Furtheremore, the methanol permiability of SPES membrane was singnificantly supressed

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from 0.514×10-7cm2s-1 to 0.376×10-7cm2s-1 upon incorporating the E-MoS2 from 0.05 to 0.1wt%. The existence of E-MoS2 along the ionic channels increased tortuosity of the membrane and thereby it suppressed the methanol transport through the membrane and

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simultaneously improved its proton conductivity as a result of numerous connective ionic cluster domains [39]. In addition, the interfacial interaction between E-MoS2 and SPES also

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minimizes the methanol permeability. The proton transport through the membrane also requires relatively broad hydrophilic channels. However, the strong interfacial adhesion between SPES and E-MoS2 restrict the establishment of continuous channels and thereby it effectively obstructs the migration of methanol through the SPES/E-MoS2 nanocomposite

Table 4.

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membrane.

Proton conductivity, methanol permeability and electrochemical selectivity of SPES and SPES/E-MoS2 nanocomposite membranes.

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Proton conductivity (Scm-1)

Membrane code

Electrochemical Methanol permeability selectivity (cm2s-1) (Scm-3s)

80°C

SPES

1.42×10-3

2.17×10-3

0.949 ×10-7

1.50×104

SPES-0.5

2.95×10-3

3.52×10-3

0.514 ×10-7

5.73×104

SPES-1

3.17×10-3

4.21×10-3

0.376 ×10-7

8.43×104

Nafion - 117

8.1×10-2

1.29×10-1

1.77 ×10-6

4.57×104

Nafion - 212

4×10-2

-

5.5 ×10-6

0.72×104

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25°C

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ACCEPTED MANUSCRIPT 3.3.3 Electrochemical selectivity The relative selectivity of the bare SPES and SPES/E-MoS2 nanocomposite membrane are represented in Table 4. From the results, it is evident that the addition of EMoS2 in to the SPES membrane matrix promotes high relative selectivity to the SPES/E-

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MoS2 nanocomposite membranes. It is due to the low methanol permeability and high proton conductivity of the nanocomposite membranes. This result reveals that E-MoS2 based the nanocomposite membrane exhibits higher selectivity than Nafion-117 and Nafion-212.

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4. Conclusions

SPES based nanocomposite membranes were successfully synthesized by embedding

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E-MoS2 by the solution casting method. The FTIR, XRD and AFM results indicated that, the SPES membrane matrix is effectively modified by the addition of E-MoS2. While analyzing various results, it was observed that by increasing E-MoS2 concentration in polymer dope, a significant enhancement is noticed in the overall performance of the membrane for DMFC

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applications. Water contact angle, percent water uptake and the swelling ratio results indicates that the hydrophilicity of the nanocomposite membranes is increased upon the addition of E-MoS2. The sulphur termination available in the E-MoS2 is possibly the reason

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for the hydrophilicity of MoS2 and there by provides high water affinity to the nanocomposite

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membranes through hydrogen bonding. It is observed that, by enhancing the E-MoS2 from 0.5 to 1wt.%, the proton conductivity (at 25°C and 80ºC) and membrane ion exchange capacity increases, which may be due to the formation of hydrogen bonding between the sulfonic acid groups and hydroxyl groups present in SPES and E-MoS2 respectively. In addition, the dichalcogenide MoS2 has a large polarizable anionic framework due to the large size S2̶- compared than O2̶-. As a result, a much more connectivity of anionic framework facilitates the proton mobility in polymer matrix that results in excellent proton conductivity, selectivity and reduced methanol permeability of SPES/E- MoS2 nanocomposite membranes.

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ACCEPTED MANUSCRIPT Due to all these aforementioned reasons, SPES/E-MoS2 is a potential alternative candidate for DMFC applications. Acknowledgements This work was supported by the Science and Engineering Research Board (SERB),

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Government of India under project number EMR/2016/007670. This support is gratefully acknowledged. Dr A.N dedicated his 50th international publication in memory of his loving father Late Mr. S. Alagumalai.

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Highlights

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High proton conducting and methanol resistant SPES/E-MoS2 nanocomposite PEMs

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were effectively fabricated. SPES-1 PEM exhibited higher proton conductivity (3.17×10-3 Scm-1, at 25°C) and selectivity (8.43×104 Scm-3s) whereas lower methanol permeability (0.376 ×10-7cm2s-

SPES/E-MoS2 nanocomposite PEMs were found appropriate and promising for use in

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DMFCs.

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).

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