Aligned Nanocomposite Membranes Containing Sulfonated Graphene

Subscriber access provided by University of Otago Library

Article

Aligned Nanocomposite Membranes Containing Sulfonated Graphene Oxide with Superior Ionic Conductivity for Direct Methanol Fuel Cell Application Hossein Beydaghi, and Mehran Javanbakht Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01450 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 8, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Aligned Nanocomposite Membranes Containing Sulfonated Graphene Oxide with Superior Ionic Conductivity for Direct Methanol Fuel Cell Application

Hossein Beydaghi,† ,‡ Mehran Javanbakht,*,†,‡ †

Department of Chemistry, Amirkabir University of Technology, Tehran, 1599637111, Iran ‡

Fuel Cell and Solar Cell Laboratory, Renewable Energy Research Center, Amirkabir University of Technology, Tehran, 1599637111, Iran

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 40

ABSTRACT In this work, iron oxide (Fe3O4) nanoparticles are deposited onto sulfonated graphene oxide (SGO) nanosheets using a solvothermal method. By applying a magnetic field on the solution during casting, the SGO/Fe3O4 nanosheets are drawn to through–plane direction of the membrane. The structures of the nanosheets and membranes are characterized. The aligned

poly(vinyl

alcohol) (PVA)/SGO/Fe3O4 membrane shows

higher proton

conductivity, water uptake, thermal stability, methanol permeability and selectivity compared to non–aligned membrane. By orientation of nanosheets, 5.7 % improvement in the tensile stress of the membranes is observed. The aligned PVA/SGO/Fe3O4 nanocomposite membrane generates the highest power density of 25.57 mW cm−1 at 30 °C. As a result, the aligned PVA/SGO/Fe3O4 nanocomposite membrane appears to be a good candidate for direct methanol fuel cell application.


2

Page 3 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Development of environment–friendly power sources such as fuel cell, solar energy, and wind energy has emerged as a new and promising emission free technology in alternative energy field.1 A special case of fuel cells is that of direct methanol fuel cells (DMFC), which converts methanol solution directly into electrical energy, without use of a reformer.2 The DMFC is one of the most attractive energy sources with a range of applications from portable electronic devices to power generation on a medium scale.3 One of the key components in the DMFC is the proton exchange membrane (PEM) which transports protons and blocks electrons and exhibits high proton conductivity with electrical isolation, low methanol crossover, and good chemical, thermal, and mechanical stability.4 The most utilized PEMs are perfluorosulphonic acid (PFSA) polymers such as Nafion, which has high proton conductivity, good mechanical, thermal and chemical stability, and good flexibility; unfortunately, it has several known disadvantages: operation at temperatures below 80 °C, high cost, and high methanol permeability in DMFCs. One of the effective methods to overcome these limitations is the preparation of the organic–inorganic composite membranes by incorporations of inorganic fillers to Nafion or different organic polymers. An extensive range of the fillers such as TiO2,5 SiO2,6 heteropolyacids,7 and montmorillonite8 are used for preparation of the alternative composite PEMs. In previous studies, we introduced new proton conducting hybrid membranes based on poly(vinyl alcohol) and nanoporous silica containing phenyl sulfonic acid9 or propyl sulfonic acid10, poly(sulfonic acid)–grafted silica nanoparticles,11,12 BaZrO3 nanoparticles,13 La2Ce2O7 nanoparticles14, and Nafion/Fe2TiO5 nanocomposite membranes15 for polymer electrolyte membrane fuel cells (PEMFCs). Recently, the preparation and characterization


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 40

of PVA/aryl sulfonated graphene oxide (SGO) nanocomposite membranes for PEMFCs were investigated.16 Hu et al.17 investigated proton transport mechanisms through one−atom−thick crystals such as graphene. They found that proton transport can be further enhanced by decorating the graphene based membranes with catalytic metal nanoparticles. However, the high electrical conductivity of graphene nanosheets causes more attention to graphene oxide (GO) nanosheets. Although PEMs based on GO nanosheets are reported by several research groups,18−20 but the realization of a complete fuel barrier, mechanical, thermal, and electrochemical properties have been rarely investigated. As examples, Gahlot et al.21,22 investigated the effect of GO and SGO nanosheets content on the sulfonated polyethersulfone (SPES) membrane and its application for DMFC and water desalination. The results showed that electrochemical properties enhanced with addition of nanosheets. Sharma et al.23 synthesized the cross−linked anion exchange membrane based on chemically covalently modified GO and polyethyleneimine (PEI) for fuel cell applications. By incorporation of silica particles within the GO flakes, the ionic conductivity of fGO−PEI (2 wt % fGO) membrane increased to 7.2 × 10−2 S cm−1. GO with 2 D structure containing carbonyl (C=O), hydroxyl (−OH), and carboxyl (−COOH), is one of the best inorganic fillers for nanocomposite membranes because of large surface area, fuel barrier property, high conductivity, unique graphitized plane structure, electronic insulation property, and intrinsic thermal and chemical stability.16,24 In addition to its remarkable physical properties, GO exhibits low density and high aspect ratio which predestines it as a filler for PEM applications. sulfonation of GO can improve proton conductivity and power density of the nanocomposite membranes.16


4

Page 5 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To obtain good mechanical, thermal, and conductivity properties, it is essential to achieve uniform dispersion of SGO nanosheets in the polymer matrix. Poly (vinyl alcohol), a semi– crystalline polymer, is one of the most widely investigated polymers in the composite membranes for DMFCs because of its low cost and good performance in water/alcohol separations.25 PVA is methanol resistant and a water–soluble polymer with a high hydrophilicity, excellent film–forming ability, good chemical stability, and non– toxicity.26,27 Since PVA is a water soluble polymer, PVA needs to be cross–linked with cross–linking agent (for example glutaraldehyde) to use in PEMs.28 Under acidic conditions, the hydroxyl groups of PVA react with –CHO groups of aldehydes to form acetal or hemiacetal linkages. Cross–linking of the –OH groups in PVA can effectively control the swelling of the membranes. Then, the swelling decreases and mechanical properties increase in resultant membrane. The presence of functional –OH groups in the PVA chains allows the reaction with oxygen–containing groups of GO based nanosheets to improve thermal and mechanical properties and decrease water uptake of the nanocomposite membranes. The aim of this study is orientation of SGO nanosheets in matrix of polymer for improvement in water uptake, proton conductivity, mechanical stability and performance of the nanocomposite membranes. Gahlot et al.29 aligned functionalized carbon nanotube (fCNT) in sulfonated poly ether ether ketone (SPEEK) matrix by applying electric field. The graphene oxide based nanosheets are electronic insulation materials. For orientation of SGO nanosheets in polymer matrix, SGO nanosheets were decorated with Fe3O4 nanoparticles. The magnetic characteristics of Fe3O4 nanoparticles help orientation of SGO nanosheets in PVA matrix using the external magnetic field. By applying a magnetic field


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 40

to the membrane solution during casting, the SGO/Fe3O4 nanosheets are drawn to through– plane direction of the membrane. With orientation of SGO nanosheets in the direction perpendicular to the membrane, proton transfer in membrane becomes easier and performance of the membrane increases. Also, recently due to special features of Fe3O4 nanoparticles (conductive, magnetic, catalytic, eco–friendly and ease of synthesis), intensive attention has been paid to them.30–32 Magnetite possesses specific water mediated proton hopping mechanism which facilitates the proton conductivity through the ‘‘H’’ diffusion at temperatures as low as room temperature.33 In addition, the hydrogen bonding between surfaces –OH groups of Fe3O4 nanoparticles and –OH groups of PVA enhances thermal and mechanical stability of the membrane.

2. EXPERIMENTAL SECTION 2.1. Materials Poly (vinyl alcohol), PVA, (Sigma−Aldrich), glutaraldehyde, GLA, (Sigma–Aldrich), other materials and solvents (Merck) were used as received without further purification. The molecular weight and degree of saponification of PVA were 130000 g mol−1 and 99 %, respectively. Iron acetyl acetonate, tetra–n–butyl titanate and stearic acid were all of analytical grade reagents. 2.2. Syntheses of Phenyl Sulfonated Graphene oxide/Fe3O4 Nanosheets The SGO nanosheets used in this study were prepared according to the literature procedure as reported in refrence.16 To synthesize the Fe3O4 nanoparticles, FeCl3·6H2O (0.8 g) and FeCl2·4H2O (0.3 g) were dissolved in deionized water. After that, 15 mL of NH4OH aqueous solution was added quickly. The mixture was stirred at room temperature for 2 h.


6

Page 7 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Finally, the product was separated from the solvent and washed with deionized water several times, and then dried in an oven at 60 °C. The SGO/Fe3O4 nanosheets (1:2 in weight) were prepared via solvothermal method. In SGO/Fe3O4 nanosheets with lower Fe3O4 content, the magnetic properties of SGO/Fe3O4 nanosheet is not enough to orient them in matrix of polymer. First, 100 mg of SGO nanosheets was sonicated in 100 mL of deionized water to form a well–distributed suspension. Separately, appropriate weight of Fe3O4 nanoparticles were dispersed in deionized water and subsequently added into the SGO suspension and sonicated for 30 min. Then, resulted solution was loaded into Teflon lined stainless steel autoclave for hydrothermal reaction at 180 °C for 24 h. The final product was washed with deionized water several times, and dried in oven at 60 °C for 24 h. 2.3. Preparation of the Nanocomposite Membranes The membranes were prepared via solution–casting a mixture of PVA and SGO/Fe3O4 powder.

Four

types

of

membranes

PVA/GLA,

PVA/GLA/SGO,

and

PVA/GLA/SGO/Fe3O4 (casted in and out of magnetic field) were prepared and named MP, MPSx, MPSFix and MPSFox (Table S1, Supporting Information) respectively, where x presents the weight percentage of nanosheets in the nanocomposite membranes. The weight ratios for nanosheets varied from 0 to7 %. At first, appropriate weight of PVA were dissolved in 40 mL deionized water to get a 5 wt % solution. Separately, appropriate weight of SGO/Fe3O4 nanosheets was dispersed in deionized water and subsequently added into the PVA suspension and stirred for 1 h at 80 °C to obtain a homogeneous low viscous liquid. Then, glutaraldehyde was added into the solution to carry out the cross–linking reaction. The solution was stirred until the temperature of solution slowly decreased to room


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 40

temperature and then 0.5 mL of 2 M H2SO4 was added to the solution as catalyst for the cross–linking reaction. The resulting solution was poured out into a glass plate. The solution underwent freeze–thaw cycles in order to gain high mechanical stability and elasticity.34 The casting process was applied in magnetic field (0.25 T) to the SGO /Fe3O4 nanosheets drawn to through–plane direction of the PVA membrane. Baseline nanocomposite membranes were also prepared using the same procedure, except that the magnetic field was not applied in this case. A schematic of the membrane casting procedure is shown in Scheme 1. The thickness of the dry nanocomposite membrane is about 150 µm. 2.4. Characterization of Membranes Ion-exchange capacity (IEC) indicates the number of milliequivalents of ions in 1 g of membrane. The IEC of each membrane was determined by the titration method. The weighted dry membranes were soaked in 50 mL of a 1 M NaCl solution for 24 h to exchange H+ with Na+ ions. The amount of H+ liberated was estimated by acid–base titration with 0.01 M NaOH solution with phenolphthalein as the indicator and IEC was calculated using the following equation: IEC ( %) =

V NaOH × C W dry

NaOH

× 100

(1)

were, 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). The water uptake of nanocomposite membranes was determined by measuring the difference between the dry weight (mdry) and wet weight (mwet) of the membranes after the tested sample was immersed in deionized water at room temperature for 12 h. The water uptake of the membranes was calculated using the following equation:


8

Page 9 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


WU (%) =

m wet − m dry m dry

× 100

(2)

where, mwet and mdry are the weight of the wet membranes and dry membranes, respectively. The membrane swelling (SW) was calculated using the wet and the dry length data of the nanocomposite membranes, designated as Lwet and Ldry, respectively. The SW was obtained using a following equation:

SW (%) =

L wet − L dry L dry

× 100

(3)

To investigate the proton conducting performance of the nanocomposite membranes, through–plane proton conductivity of the membranes was tested using an AC impedance spectroscopy. Before test, the membrane was soaked in deionized water for 12 h until got sufficiently wet and saturated. The proton conductivity was determined using the equation:

σ=

(4)

L RA

For through–plane test, σ is the proton conductivity of the membrane (S cm−1), L is the thickness of the membrane (cm), R is the measured resistance of the membrane (Ω), and A is the overlap area of the two electrodes (cm2). The methanol permeability (P) was determined by using a two compartment cell in recirculation mode. Compartment A was filled with the 20 wt % methanol solution and compartment B was filled with the deionized water. Prior to the test, the nanocomposite membranes were hydrated for 12 h at least. Both compartments were continually stirred


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 40

during the experiment and concentration of methanol in compartment B was examined with time using density meter. The methanol permeability was determined using the equation: C B (t ) =

A P L VB

C A (t − t0 )

(5)

where, CA and CB are the concentrations of methanol in Compartment A and B; VB is the volume of deionized water in compartment B; L is the thickness of the membrane and A is the diffusion area. The IEC, water uptake, swelling, methanol permeability and proton conductivity of membranes were tested between 3 to 5 times. Also, selectivity of the nanocomposite membranes can be obtained using the following equation: S =

(6)

σ P

where, S is the selectivity (S s cm−3), P is the methanol permeability (cm2 s−1) and σ is the membrane conductivity (S cm−1). Membrane–electrode assembly (MEA) was prepared by using the conventional painting method. The catalyst ink was prepared via mixing Pt/Ru/C catalyst with Nafion binder solution (Aldrich), suitable amount of isopropyl alcohol (IPA) and deionized water for the anode ink. The catalyst ink was painted onto carbon cloth (E–tek, HT 2500–W) to give a metal loading 2 mg cm–2. The catalyst for the cathode was Pt/C on carbon cloth with a catalyst loading of 1 mg (Pt) cm–2. The MEA was fabricated by hot–pressing of PVA based membranes with electrodes at 60 °C and pressure of 100 kg cm–2 for 5 min.


10

Page 11 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5. Apparatus The Fourier transform infrared (FTIR) spectra (resolution, 4 cm−1) were recorded with Bruker Equinox 55 with ultradry compressed air. X–ray diffraction (XRD) patterns of nanosheets were recorded using Philips X pert pro Diffractometer with Cu Kα radiation. The morphology of nanosheets and membranes was investigated using an AIS2100 Seron Technology scanning electron microscopy (SEM) system and a model Zeiss EM900 transmission electron microscopy (TEM) system. Mechanical properties of the nanocomposite membranes were investigated using a model Z030 Zwick/Roell testing machine with an operating rate of 1 mm min−1. Magnetisation measurement of the sample was carried out using vibrating sample magnetometer (VSM; BHV-55, Riken, Japan) at room temperature. The Thermogravimetric analysis (TGA) of the sample was carried out using a Mettler TGA/SDTA851 thermal analyser under a nitrogen flow of 60 mL min−1. The heating rate was 10 °C min−1. All electrochemical measurements were performed on an Autolab potentiostat/galvanostat. The data of proton conductivity were collected by AC voltage (50 mv) in the frequency range from 0.1 Hz to 100 kHz. The DMFC was operated at room temperature and 5 % relative humidity (RH) with 2 M aqueous methanol solution with a flow rate of 1 mL min−1 at the anode and O2 with a flow rate of 300 mL min−1 at the cathode. The area of testing fixture was 3.5 cm × 3.5 cm.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization XRD patterns of SGO, Fe3O4, and SGO/Fe3O4 have been shown in Figure 1. The natural graphite and GO show a sharp diffraction angle (2θ) at 26.6 ° and 11.9 °, whereas interlayer


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 40

d–spacing is 0.33 nm and 0.75 nm, respectively.16 It can be seen from the Figure 1 that SGO nanosheets have the 2θ and interlayer d–spacing values as 26.3 ° and 0.41 nm, respectively. The interlayer d–spacing is higher than that of graphite, which indicates partial restacking through π−π interaction upon sulfonation.35 For the Fe3O4 nanoparticles, the peaks with 2θ values of 30.0 °, 35.2 °, 43.1 °, 53.1 °, 56.9 °, and 62.3 ° correspond to the crystal planes (220), (311), (400), (422), (511), and (440) of crystalline Fe3O4, respectively.36 In the SGO/Fe3O4 pattern, the low peak at 25 ° confirms the existence of SGO in the structure of SGO/Fe3O4 nanosheets. The FTIR techniques were employed to confirm the formation of SGO/Fe3O4 nanosheets and MPSFi5 nanocomposite membranes. Figure 2 shows the FTIR spectra of the SGO nanosheets, Fe3O4 nanoparticles, SGO/Fe3O4 nanosheets, and gained MPSFi5 nanocomposite membrane. The characteristic peaks at 3425 (O–H stretching vibration), 1724 (C=O stretching vibration), 1623 (C=C skeletal vibration in the unoxidized graphitic domain), 1160, 1115, and 1030 (two νS–O and one νs–phenyl) and 830 cm−1 (out of plane hydrogen wagging) appear in the spectrum of SGO nanosheets.16,37 In SGO/Fe3O4 spectra, the peak at 589 cm−1 assigned to the stretching vibration of Fe–O groups in Fe3O4 nanoparticles, which is consistent with the bare Fe3O4 nanoparticles. The peaks at 1630 and 3448 cm−1 in Fe3O4 38

spectra can be assigned to the H–O–H stretching modes and bending vibration of adsorbed water, respectively. As it can be observed, MPSFi5 membranes show characteristic 39

absorption peaks at 3302 cm−1, indicated existence of strong hydrogen bonding whereas the peak at 1096 cm−1 indicates the C–O stretch of the secondary alcoholic groups. In addition, the peak at 2927 cm−1 is because of the stretch vibration of C–H bands related to the aldehydes groups. The characteristic band at 1727 cm−1 can be seen because of existence of


12

Page 13 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ester groups formed between the –COOH groups of SGO nanosheets and the –OH groups of PVA. The magnetic properties of Fe3O4 nanoparticles, SGO/Fe3O4 nanosheets, and MPSFi5 nanocomposite membrane were investigated using a VSM. Figure 3a presents the magnetization hysteresis loop of Fe3O4 nanoparticles and SGO/Fe3O4 nanosheets obtained at

room

temperature.

It

is

important

that

the

SGO/Fe3O4

nanosheets

have

superparamagnetic properties for alignment in PVA matrix, when the magnetic field applied. The superparamagnetic properties of Fe3O4 nanoparticles help orientation of SGO nanosheets in PVA membranes. The results of Figure 3a show that both Fe3O4 nanoparticles and SGO/Fe3O4 nanosheets exhibit superparamagnetic properties. The saturation magnetizations (Ms) of Fe3O4 nanoparticles and SGO/Fe3O4 nanosheets are 77.42 and 50.31 emu g–1, respectively. The decrease in Ms for SGO/Fe3O4 nanosheets compared to Fe3O4 nanoparticles can be attributed to lack of magnetic properties of SGO nanosheets.40 However, the Ms of the SGO/Fe3O4 nanosheet is still enough for the alignment of nanosheets by magnetic field. As shown in Figure 3b, Ms of the MPSFi5 nanocomposite membrane is 1.72 emu g–1, which is much lower than SGO/Fe3O4 nanosheets. The low content of SGO/Fe3O4 nanosheets in MPSFi5 membrane is the reason of this result. With increase in content of SGO/Fe3O4 nanosheets, the Ms value is expected to increase. To investigate the morphology of the SGO and SGO/Fe3O4 nanosheets, SEM and TEM images were collected and shown in Figure 4. The SGO nanosheets show wavy and exfoliated layered structure resulted from thermal exfoliation, which GO based nanosheets intrinsically possesses. As shown in Figure 4c and d, when SGO nanosheets were well


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

decorated

with

Fe3O4

nanoparticles,

Fe3O4

nanoparticles

Page 14 of 40

had

been

dispersed

homogeneously on the surface of SGO nanosheets and no aggregation of Fe3O4 nanoparticles was observed. The TEM images confirm that the Fe3O4 nanoparticles were firmly attached to the SGO nanosheets. The SEM pictures of the cross–sectional morphology of the MPSFo5 and MPSFi5 nanocomposite membranes are depicted in Figure 4e and f, respectively. With comparison of Figure 4e and f alignment of SGO/Fe3O4 nanosheets in matrix of polymer was confirmed. From the VSM results, the SGO/Fe3O4 nanosheets have enough magnetization properties to be aligned by magnetic field. Figure 4f shows that with applied magnetic field, the SGO/Fe3O4 nanosheets move following the magnetic force. The alignment of SGO/Fe3O4 nanosheets might also influence the PVA membrane orientation and changed some of properties in membranes. A uniform distribution of SGO/Fe3O4 nanosheets was observed with the ends of the broken nanosheets on the cross–sectional surface of MPSFi5 nanocomposite membrane. The observation that most nanosheets are broken rather than pulled out from the PVA matrix indicates the strong hydrogen bonding between the functionalized groups of SGO/Fe3O4 nanosheets and PVA matrix. 3.2. Ion Exchange Capacity, Water Uptake, Membrane Swelling, and Proton Conductivity IEC is usually considered to correspond to the proton transfer properties in PEMs. IECs of all prepared membranes are shown in (Table S2, Supporting Information). The results of Table S2 in the Supporting Information show that IEC values increased from 0.74 to 0.88 meq g–1 and 0.74 to 0.89 meq g–1 for MPSFo5 and MPSFi5 membranes respectively by increasing filler content, which is due to the presence of supplementary ion-exchangeable


14

Page 15 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


groups (–SO3H groups) from SGO nanosheets. The higher amount IEC decreases the distance between –SO3H groups in membrane and leading to faster proton conduction. The MPS5 membrane shows higher IEC in comparison with MPSFi5 and MPSFo5 membranes due to decrease in -SO3H portion in SGO/Fe3O4 nanosheets with addition of Fe3O4 nanoparticles. It is clear that water content in PEMs is important because it effects on the transport behavior and overall system performance. The water uptake, membrane swelling, and proton conductivity of different nanocomposite membranes are shown in (Table S2, Supporting Information). The water uptake and swelling of MPS5 nanocomposite membrane are lower than that of MP membrane because the functionalized groups of SGO nanosheets attach to the hydroxyl groups of PVA, so free space for water transferring decreased, water sorption in membrane decreases, and dimensional stability of membrane increases.16 Decrease of water uptake in membranes, decreases proton conductivity of these membranes. As it can be seen in (Table S2, Supporting Information), the water uptakes of the MPSFix and MPSFox nanocomposite membranes were higher than that of the MP and MPSx membrane.16 The water uptake of the MPSFix and MPSFox nanocomposite membranes were rather increased with increase of the SGO/Fe3O4 content. The increase in water uptake of these membranes can be attributed to the water retention of the incorporated SGO/Fe3O4 nanosheets. The formation of hydrogen bonds between hydroxyl groups of the Fe3O4 nanoparticles and free water, help to increase water uptake of the MPSFix and MPSFox nanocomposite membranes. Then, improve in water uptake of the SGO/Fe3O4 based membranes can increase proton conductivity of these membranes. The results of Table S2 in the Supporting Information show that swelling of the MPSFix and MPSFox membranes


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

increased compared to MPSx membrane and decreased compared to MP membrane. The results show, increase in SGO/Fe3O4 density decreases the penetration of the water molecules through the membrane. Therefore, a higher amount of SGO/Fe3O4 nanosheets compact structure of membrane by hydrogen bonding between surfaces –OH groups of Fe3O4 nanoparticles and –OH groups of PVA, and consequently a reduction in the spaces that accommodate water molecules in the membrane and decrease membrane swelling and increase dimensional stability of membrane. The results of Table S2 in the Supporting Information show water uptake and swelling of the MPSFix membranes is higher than MPSFox membranes. The orientation of SGO/Fe3O4 nanosheets in PVA matrix confirms with SEM images. With orientation of the SGO/Fe3O4 nanosheets, the water transferring channels in the membrane become wide and the empty spaces that accommodate water molecules in the membranes increase, and water uptake and swelling of membranes increase. Then, orientation of SGO/Fe3O4 nanosheets can increase proton transfer in membrane with increase of water molecules and Vehicle mechanism. Two types of bound water and free water exist in nanocomposite membranes. Some of the water molecules are bound to –SO3H and –OH groups of filler and are called bound water. The increase in the amount of the absorbed water in MPSFix and MPSFox membranes compared to MPSx membrane upon addition of SGO/Fe3O4 nanosheets is because of the increase in the amount of the bound water. The other type is free water that occupies the free space from the influence of the ionic sites. Generally, the proton transfers through the membrane by two different mechanisms. One is near the channel wall via the bound water. Proton is transported via the Grotthuss mechanism and hopping from one ionic site


16

Page 17 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(H3O+SO3–) to the other. The other is via the free water via the vehicle mechanism, which means proton is carried by the water molecules moving through the interconnected central channel space.9 Table S2 in the Supporting Information shows the proton conductivity of the different synthesis membranes at room temperature. The proton conductivity for the MP and MPS5 membranes are 0.0006 and 0.05 S cm−1, respectively.16 Addition of the SGO nanosheets can enhance proton conductivity with increasing the bound water–SO3H contents in the membranes via the Grotthuss mechanism. Also, the hydroxyl groups in SGO nanosheets can attach to free water molecules and increase the proton conductivity of membranes with Vehicle mechanism.16 The proton conductivity of the MPSFix and MPSFox nanocomposite membranes increases from 0.023 to 0.064 S cm−1 and 0.019 to 0.058 S cm−1 with increases in SGO/Fe3O4 loading values from 1 to 5 wt % (Table S2, Supporting Information), respectively. This increase may have been resulted from the interaction between –SO3H groups of the SGO nanosheets and free water molecules and increase proton conductivity via the Grotthus mechanism and formation of hydrogen bonding between –OH groups of Fe3O4 nanoparticles and free water and increase proton conductivity via the Vehicle mechanism.15 The results of Table S2 in the Supporting Information show that proton conductivity of the MPSFix and MPSFox membranes starts to decrease when the amount of added SGO/Fe3O4 nanosheets is over 5 wt %, because addition of too many planar SGO sheets obstructe the polymer chain movement in the proton cluster (blocking effect) .20 The MPSFix membrane shows higher proton conductivity compared to MPSFox membrane. With orientation of SGO/Fe3O4 nanosheets in PVA matrix, the proton transferring channels in membrane become wide and proton transfer in membrane occurs faster.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

The proton conductivity of the MPSFi5 and MPSFo5 membranes are measured at a temperature range between 25 and 100 °C and shown in Figure 5. The activation energy (Ea) in nanocomposite membranes is the minimum energy required for proton transport from one functional group to other group and calculated from Figure 5 according to the 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–1), R is universal gas constant (J mol–1 K–1) and T is the temperature in Kelvin. The results of Figure 5 show nanocomposite membranes have greater proton conductivity by increasing temperature. This is because increase in temperature favors proton transport mechanisms, thus the mobility of water will be enhanced favoring proton transport.41 The bound water does not evaporate at elevated temperatures and improves the proton conduction in the PEMs.42 At each temperature, the orientation of SGO nanosheets in PVA matrix with magnetic field enhances the proton conductivity of membranes. The proton can transfer in MPSFi5 membrane easier compared to MPSFo5 membrane, because of lower Ea in MPSFi5 membrane. 3.3. TGA TGA was performed to investigate the effect of orientation of the SGO nanosheets in PVA matrix on the thermal stability of the nanocomposite membranes. TGA curves of MPSFo5 and MPSFi5 membranes are shown in Figure 6. The TGA curve of the MPSFi5 membrane shifted toward a higher temperature compared to MPSFo5 membrane, because of orientation of the SGO nanosheets in PVA matrix. For MPSFo5 and MPSFi5 membranes, there are three major loss mass steps upon the increase of temperature. The initial mass loss at around 80– 100 °C is attributed to the expulsion of water molecules, the mass loss at around 320–340 °C is ascribed to the loss of sulfonic acid groups and a breakage of some portion of polymer


18

Page 19 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chains as well as breakage of the ester bonds and that at around 430–450 °C is because of the cleavage of the backbone of PVA membrane.43 The degradation temperatures were observed at about 84, 323, and 440 °C for MPSFo5 membrane and 92, 331, and 442 °C for MPSFi5 membrane, thus increased by 8, 8, and 2 °C, respectively. These results showed the orientation of the SGO nanosheets can improve thermal stability of the nanocomposite membranes. 3.4. Mechanical Property The mechanical properties of the different prepared membranes were determined in order to investigate the effect of orientation of the SGO nanosheets in PVA matrix on the mechanical properties of the membranes. The mechanical properties of the synthesis membranes were tested for 3 times and results were summarized in Table 1. The tensile strength (TS) of the MPS5 nanocomposite membrane is much higher than MP membrane, because of high mechanical strength of SGO nanosheets, and formation of hydrogen bonding between –SO3H groups of the SGO nanosheets and –OH groups of the PVA.16 The results of Table 1 show TS of the MPSFo5 and MPSFi5 nanocomposite membranes was 6.2 % and 12.3 % higher than that of MPS5 nanocomposite membrane, respectively. The formation of hydrogen bonds between –OH groups of Fe3O4 nanoparticles and PVA increases TS in MPSFo5 and MPSFi5 nanocomposite membranes. The results show that orientation of SGO nanosheets in magnetic field increases the stiffness of the nanocomposite membrane and it can improve 5.7 % TS in nanocomposite membranes. As shown in the Table 1, similar to the TS results, the tensile modulus of membranes improves with the addition of Fe3O4 nanoparticles. The improvement in the rigidity of nanocomposite membranes implied that the SGO/Fe3O4 nanosheets played an important role in the structure stability of the


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

nanocomposite membranes. The results of Table 1 show elongation at break (Eb) of the nanocomposite membranes decreases with addition of SGO/Fe3O4 nanosheets, because of decrease in mobility of PVA chain with inclusion of SGO/Fe3O4 nanosheets.44 3.5. Methanol Permeability and Selectivity The methanol crossover through a PEM has been considered as one of the major impediments hindering the commercialization of DMFCs, since the permeated methanol could not only lead to a decrease in power density and fuel utilization in DMFCs, but also to a reduction of the electrocatalytic activities of the cathode catalysts because of the poisoning of the catalysts by the crossed–over methanol.45 The methanol permeability and selectivity of the different prepared membranes are shown in Table 2. The results of Table 2 show the methanol permeability of the MPS5 nanocomposite membrane (P = 4.42 × 10−6 cm2 s−1) was 48 % lower than the values measured for the MP membrane (P = 8.43 × 10−6 cm2 s−1), suggesting that the incorporation of 5 wt % SGO nanosheets was effective in blocking the passage of methanol and reducing the methanol crossover. The higher swelling of membranes increases the ionic transport channels of membranes and increase methanol permeability of the membranes. The formation of hydrogen bonding between SGO nanosheets and PVA could compacts structure of membranes and restricts the formation of the methanol passage channels in membranes and decreased methanol permeability in nanocomposite membranes. The methanol permeability of the MPSFo5 and MPSFi5 nanocomposite membranes are 4.31 × 10−6 and 4.56 × 10−6 cm2 s−1, respectively. With orientation of the SGO nanosheets in matrix of PVA, the methanol transport channels in membrane increase and methanol permeability in membranes increases.


20

Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Low methanol permeability and high proton conductivity are the basic requirements for the PEM’s for DMFCs. The Selectivity which defined as the ratio of proton conductivity to methanol permeability is an important factor for evaluating membrane performance. The higher selectivity value implies better applicability in DMFC application. The selectivity of MP, MPS5, MPSFo5 and MPSFi5 membranes are 7.12 × 101, 1.13 × 104, 1.34 × 104, and 1.40 × 104 S s cm−3, respectively. It can be observed from the results that the MPSFi5 nanocomposite membrane shows the best results for use in DMFCs. The obtained selectivity of MPSFi5 membrane has good agreement with the value obtained in the other PVA based literature.43,46,47 3.6. DMFC Test The current density–potential (I–V) and the current density–power density curves of DMFCs measured at the 30 °C and 5 % RH are given in Figure 7. By comparing the performance of the MPSFo5 and MPSFi5 membranes, we may evaluate the effect of orientation of SGO nanosheets in PVA matrix on the DMFC performance. As it can be seen, the MPSFi5 membrane has higher open circuit voltage (OCV) (0.67 V) than MPSFo5 membrane (0.65 V). In spite of a low catalyst loading (2 mg cm–2 Pt/Ru/C on the anode and 1 mg cm–2 Pt/C on the cathode), the MEA showed a good current density and power density (PD). The maximum current density of the MPSFi5 membrane prepared in the presence of a magnetic field was 107.11 mA cm–2, which is higher than the MPSFo5 membrane (96.95 mA cm–2). The maximum power density of the MPSFi5 membrane (25.57 mW cm–2) is better than MPSFo5 membrane (20.70 mW cm–2) because of the higher water uptake and proton conductivity of the MPSFi5 membrane. By increasing the temperature and RH of the DMFCs, a significant increase in the performance occurred.48


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

4. CONCLUSIONS The SGO/Fe3O4 nanosheets have been synthesized successfully using a solvothermal method and oriented in PVA matrix with casting in external magnetic field. The characterization results of XRD, FTIR, SEM, and TEM showed that the Fe3O4 nanoparticles were deposited onto SGO nanosheets, nicely. The magnetic behavior was documented by VSM test and it was found that the SGO/Fe3O4 nanosheets have good superparamagnetic properties. The enhancement of thermal and mechanical stability of MPSFi5 membranes compared to MPSFo5 membranes were attributed to the orientation of SGO nanosheets in PVA matrix. A literature survey on water uptake, proton conductivity, methanol permeability, and power density of PVA–based membranes and Nafion 117 is presented in (Table S3, Supporting Information). It is evident that the water uptake, proton conductivity, methanol permeability, and power density of MPSFo5 and MPSFi5 nanocomposite membranes were comparable to that of PVA–based membranes, while these properties are better than Nafion 117.

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

Supporting Information Detail of the composition of membranes (Table S1), analytical measurements (ion exchange capacity, water uptake, swelling, and proton conductivity) (Table S2) and


22

Page 23 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Comparison between properties of synthesis membranes, PVA–based membranes and Nafion 117 (Table S3) are provided. This material is available free of charge via the Internet at http://pubs.acs.org.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

References (1) Del rio, C.; Morales, E.; Escribano, P. G. Nafion/sPOSS hybrid membranes for PEMFC. Single cell performance and electrochemical characterization at different humidity conditions. Int. J. Hydrogen Energy 2014, 39, 5326. (2) Knights, Sh. D.; Colbow, K. M.; St–pierre, J.; Wilkinson, D. P. Aging mechanisms and lifetime of PEFC and DMFC. J. Power Sources 2004, 127, 127. (3) Goswami, G. K.; Nandan, R.; Barman, B. K.; Nanda, K. K. Excellent performance of Pt–free cathode in alkaline direct methanol fuel cell at room temperature. J. Mater. Chem. A. 2013, 1, 3133.

(4) Devrim, Y.; Erkan, S.; Bac, N.; Eroglu, I. Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique. Int. J. Hydrogen Energy 2012, 37, 16748. (5) Chen, S. Y.; Han. C. C.; Tsai, CH.; Huang. J.; Chen. Y. W. Effect of morphological properties of ionic liquid–templated mesoporous anatase TiO2 on performance of PEMFC with Nafion/TiO2 composite membrane at elevated temperature and low relative humidity. J. Power Sources 2007, 171, 363. (6) Dinoto. V.; Gliubizzi. R.; Negro. E.; Pace. G. Effect of SiO2 on relaxation phenomena and mechanism of ion conductivity of [Nafion/(SiO2)x] composite membranes. J. Phys. Chem. B 2006, 49, 24972.

(7) Staiti, P.; Arico, A. S.; Baglio, V.; Lufrano, F.; Passalacqua, E.; Antonucci,V. Hybrid Nafion–silica membranes diped with HPA for applications in DMFC. Solid State Ionics 2001, 145, 101. (8) Sxengu, E.; Erdener, H. L.; Akay, R. G.; Yucel, H.; Bac, N.; Eroglu, I. Effects of sulfonated polyether–etherketone (SPEEK) and composite membranes on the proton exchangemembrane fuel cell (PEMFC) performance. Int. J. Hydrogen Energy 2009, 34, 4645. (9) Beydaghi, H.; Javanbakht, M.; Badiei, A. Cross–linked poly(vinyl alcohol)/sulfonated nanoporous silica hybrid membranes for proton exchange membrane fuel cell. J. Nanostru. Chem. 2014, 97 (DOI 10.1007/s40097–014–0097–y).


24

Page 25 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Beydaghi, H.; Javanbakht, M.; Salar Amoli, H.; Badiei, A.; Khaniani, Y.; Ganjali, M. R.; Norouzi, P.; Abdouss, M. Synthesis and characterization of new proton conducting hybrid membranes for PEM fuel cells based on poly(vinyl alcohol) and nanoporous silica containing phenyl sulfonic acid. Int. J. Hydrogen Energy 2011, 36, 13310. (11) Salarizadeh, P.; Javanbakht, M.; Abdollahi, M.; Naji, L. Preparation, characterization and properties of proton exchange nanocomposite membranes based on poly(vinyl alcohol) and poly(sulfonic acid)–grafted silica nanoparticles. Int. J. Hydrogen Energy 2012, 38, 5473. (12) Salarizadeh, P.; Abdollahi, M.; Javanbakht, M. Modification of silica nanoparticles with hydrophilic sulfonated polymers by using surface–initiated redox polymerization Iranian Polym. J. 2012, 21, 661.

(13) Attaran, A.M.; Javanbakht, M.; Hooshyari, Kh.; Enhessari, M. New proton conducting nanocomposite membranes based on poly vinyl alcohol/poly vinyl pyrrolidone/BaZrO3 for proton exchange membrane fuel cell, Solid State Ionics, in press, doi: 10.106/j.ssi.2014.11.003. (14) Javanbakht, M.; Hooshyari, Kh.; Enhessari, M.; Beydaghi, H. Novel PVA/La2Ce2O7 hybrid nanocomposite membranes for application in proton exchange membrane fuel cells. Iranian J. Hydrogen & Fuel Cell 2014, 1, 105. (15) Hooshyari, Kh.; Javanbakht, M.; Naji, L.; Enhessari, M. Nanocomposite proton exchange membranes based on Nafion containing Fe2TiO5 nanoparticles in water and alcohol environments for PEMFC. J. Membr. Sci. 2014, 454, 74–81. (16) Beydaghi, H.; Javanbakht M.; Kowsari, E. Synthesis and characterization of poly(vinyl alcohol)/sulfonated graphene oxide nanocomposite membranes for use in PEMFCs. Ind. Eng. Chem. Res. 2014, 53, 16621.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

(17) Hu, S.; Hidalgo, M. L.; Wang, F. C.; Mishchenko, A.; Schedin, F.; Nair, R. R.; Hill, E. W.; Boukhvalov, D. W.; Katsnelson, M. I.; Dryfe, R. A. W.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K. Proton transport through one-atom-thick crystals. Nature 2014, 516, 227. (18) Chien, H. Ch.; Tsai, L. D.; Huang, Ch. P.; Kang, Ch. Y.; Lin, J. N.; Chang, F. Ch. Sulfonated graphene oxide/Nafion composite membranes for high–performance direct methanol fuel cells. Int. J. Hydrogen Energy 2013, 38, 13792. (19) Zarrin, H.; Higgins, D.; Jun, Y.; Chen, Zh.; Fowler M. Functionalized Graphene Oxide Nanocomposite Membrane for Low Humidity and High Temperature Proton Exchange Membrane Fuel Cells. J. Phys. Chem. C 2011, 115, 20774. (20) Heo, Y.; Im, H.; Kim, J. The Effect of Sulfonated Graphene Oxide on Sulfonated Poly (Ether Ether Ketone) Membrane for Direct Methanol Fuel Cells. J. Membr. Sci. 2013, 425, 11.

(21) Gahlot, S.; Sharma, P. P.; Gupta, H.; Kulshrestha, V.; Jha, P. K. Preparation of graphene oxide nano–composite ion–exchange membranes for desalination application. RSC Adv. 2014, 4, 24662.

(22) Gahlot, S.; Sharma, P. P.; Kulshrestha, V. Dramatic improvement in ionic conductivity and water desalination efficiency of SGO composite membranes. Sep. Sci. Technol. 2015, 50, 446. (23) Sharma, P. P.; Gahlot, S.; Bhil, B. M.; Gupta, H.; Kulshrestha, V. An environmentally friendly process for the synthesis of an fGO modified anion exchange membrane for electro−membrane applications. RSC Adv. 2015, 5, 38712.


26

Page 27 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Lee, D. C.; Yang, H. N.; Park, S. H.; Kim, W. J. Nafion/graphene oxide composite membranes for low humidifying polymer electrolyte membrane fuel cell, J. Membr. Sci. 2014, 452, 20. (25) Chang, Y. W.; Wang, E.; Shin, G.; Han, J. E.; Mather, P. T. Poly(vinyl alcohol) (PVA)/sulfonated polyhedral oligosilsesquioxane (sPOSS) hybrid membranes for direct methanol fuel cell applications. Polym. Adv. Technol. 2007, 18, 535. (26) Moon, Y. E.; Yun, J.; Kim, H. I.; Lee, Y. S. Effect of graphite oxide on photodegradation behavior of poly(vinyl alcohol)/graphite oxide composite hydrogels. Carbon Lett. 2011, 12, 138.

(27) Lin, C. W.; Huang, Y.F.; Kannan, A. M. Semi–interpenetrating network based on cross–linked poly(vinyl alcohol) and poly(styrene sulfonic acid–co–maleic anhydride) as proton exchange fuel cell membranes. J. Power Sources 2007, 164, 449. (28) Kim, D. S.; Park, H. B.; Lee, Ch. H.; Lee, Y. M. Preparation of Ion Exchange Membranes for Fuel Cell Based on crosslinked Poly(vinyl alcohol) with Poly(acrylic acid–co–maleic acid). Macromol. Res. 2005, 13, 314. (29) Gahlot, S.; Kulshrestha, V. Dramatic improvement in water retention and proton conductivity in electrically aligned functionalized CNT/SPEEK nanohybrid PEM. ACS Appl. Mater. Interfaces 2015, 7, 264.

(30) Chen, J.; Wang, F.; Huang, K.; Liu, Y.; Liu, S. Preparation of Fe3O4 nanoparticles with adjustable morphology. J. Alloys Compd. 2009, 475, 898. (31) Mulakaluri, N.; Pentcheva, R. Hydrogen Adsorption and site–selective reduction of the Fe3O4 (001) surface: insights from first principles. J. Phys. Chem. C 2012, 116, 16447. (32) Iida, H.; Takayanagi, K.; Nakanishi, T.; Osaka, T. Synthesis of Fe3O4 nanoparticles with various sizes and magnetic properties by controlled hydrolysis, J. Colloid Interface Sci. 2007, 314, 274.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

(33) Prabhu, N. V.; Sangeetha, D. Characterization and performance study of sulfonated poly ether ether ketone/Fe3O4 nano composite membrane as electrolyte for microbial fuel cell. Chem. Eng. J. 2014, 243, 564. (34) 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. (35) Gahlot, S.; Sharma, P. P.; Kulshrestha, V.; Jha, P. K. SGO/SPES–Based Highly Conducting Polymer Electrolyte Membranes for Fuel Cell Application. ACS Appl. Mater. Interfaces 2014, 6, 5595.

(36) Baker, A. M.; Wang, L.; Advani, S. G.; Prasad, A. K. Nafion membranes reinforced with magnetically controlled Fe3O4–MWCNTs for PEMFCs. J. Mater. Chem. 2012, 22, 14008. (37) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy−3rd−Edition; Academic Press: London, 1990. (38) Yadav, M.; Rhee, K. Y.; Park. S. J.; Hui, D. Mechanical properties of Fe3O4/GO/chitosan composites. Composites 2014, 66, 89. (39) Ahangaran, F.; Hassanzadeh A.; Nori, S. Surface modification of Fe3O4@SiO2 microsphere by silane coupling agent. Int. Nano Lett. 2013, 3, 23. (40) Farghali, M. A.; Salah El-Din,T. A.; Al-Enizi, A. M.; El Bahnasawy, R. M. Graphene/ Magnetite Nanocomposite for Potential Environmental Application. Int. J. Electrochem. Sci 2015, 10, 529.

(41) Tseng, Ch. Y.; Ye, Y. Sh.; Kao, K. Y.; Joseph, J.; Shen, W. Ch.; Rick J.; Hwang, B. Interpenetrating network–forming sulfonated poly(vinyl alcohol) proton exchange membranes for direct methanol fuel cell applications. Int. J. Hydrogen Energy 2011, 36, 11936. (42) Lue, S. J.; Shieh, S. J. Water states in perfluorosulfonic acid membranes using differential scanning calorimetry. J. Macromol. Sci. Phys. 2009, 48, 114.


28

Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Maiti, J.; Kakati, N.; Lee, S. H.; Jee, S. H.; Yoon, Y. S. PVA nano composite membrane for DMFC application. Solid State Ionics, 2011, 201, 21. (44) Zhao, X.; Zhang, Q. H.; Chen, D. J.; Lu, P. Enhanced Mechanical Properties of Graphene–Based Poly(vinyl alcohol) Composites. Macromolecules 2010, 43, 2357. (45) Jiang, Zh.; Zhao X.; Manthiram, A. Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel. Int. J. Hydrogen Energy 2013, 38, 5875. (46) Yang, Ch. Ch.; Lee, Y. J.; Yang, J. M. Direct methanol fuel cell (DMFC) based on PVA/MMT composite polymer membranes. J. Power Sources 2009, 188, 30. (47) Yang, Ch. Ch.; Chien, W. Ch.; Li, Y. J. Direct methanol fuel cell based on poly(vinyl alcohol)/titanium oxide nanotubes/poly(styrene sulfonic acid) (PVA/nt-TiO2/PSSA) composite polymer membrane. J. Power Sources 2010, 195, 3407. (48) Qiao, J.; Okada, T.; Ono, H. High molecular weight PVA–modified PVA/PAMPS proton–conducting membranes with increased stability and their application in DMFCs. Solid State Ionics 2009, 180, 1318.

(49) Yang, Ch. Ch. Fabrication and characterization of poly(vinyl alcohol)/montmorillonite /poly(styrene sulfonic acid) proton–conducting composite membranes for direct methanol fuel cells. Int. J. Hydrogen Energy 2011, 36, 4419. (50) Pandey, J.; Mir, F. Q.; Shukla, A. Synthesis of silica immobilized phosphotungstic acid (Si–PWA)–poly(vinyl alcohol) (PVA) composite ion–exchange membrane for direct methanol fuel cell. Int. J. Hydrogen Energy 2014, 39, 9473.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

Figure legends Scheme 1. Schematic of synthesis procedure of the nanocomposite membranes casted in and out of magnetic field. Figure 1. XRD patterns of the Fe3O4 nanoparticles, SGO, and SGO/Fe3O4 nanosheets. Figure 2. FTIR spectra of the SGO nanosheets, Fe3O4 nanoparticles, SGO/Fe3O4 nanosheets, and MPSFi5 membrane. Figure 3. Hysteresis loops of (A) Fe3O4 nanopatticles and SGO/Fe3O4 nanosheets, and (B) MPSFi5 nanocomposite membrane. Figure 4. SEM and TEM images of (a, b) SGO nanosheets, and (c, d) SGO/Fe3O4 nanosheets, and cross–sectional SEM images of (e) MPSFo5 and (f) MPSFi5 membranes. Figure 5. Arrhenius plot of the proton conductivity and activation energy (Ea) of membranes. Figure 6. TGA curves of the MPSFo5 and MPSFi5 membranes. Figure 7. Current density–potential (I–V) and power density curves of the MPSFo5 and MPSFi5 membranes


30

Page 31 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Mechanical Properties of Membranes at Room Temperature. membranes

tensile strength, TS (MPa)

elongation at break, Eb (%)

modulus (GPa)

MP

48.46 ± 0.6

174.75 ± 1

0.23 ± 0.03

MPS5

67.83 ± 0.6

97.80 ± 1

0.43 ± 0.03

MPSFo5

72.09 ± 0.6

33.45 ± 1

0.49 ± 0.03

MPSFi5

76.20 ± 0.6

51.45 ± 1

0.53 ± 0.03


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

Table 2. Comparison of Proton Conductivity, Methanol Permeability, and Selectivity of the Membranes

membranes

proton conductivity, σ (S cm−1)

methanol permeability, P (× 10−6 ) (cm2 s−1)

selectivity, S (S s−1 cm−3)

MP

0.0006 ± 0.0001

8.43 ± 0.1

7.12 ± 1 × 101

MPS5

0.050 ± 0.001

4.42 ± 0.1

1.13 ± 0.05 × 104

MPSFo5

0.058 ± 0.001

4.31 ± 0.1

1.34 ± 0.05 × 104

MPSFi5

0.064 ± 0.001

4.56 ± 0.1

1.40 ± 0.05 × 104


32

Page 33 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


282x352mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

282x352mm (300 x 300 DPI)


Page 34 of 40

Page 35 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


282x352mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

282x352mm (300 x 300 DPI)


Page 36 of 40

Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


656x928mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

282x352mm (300 x 300 DPI)


Page 38 of 40

Page 39 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


282x352mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

282x352mm (300 x 300 DPI)


Page 40 of 40

Aligned Nanocomposite Membranes Containing Sulfonated Graphene

Recommend Documents