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f GO based polymer electrolyte membranes for electrochemical applications
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Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications Vikrant Yadav a,b, Prem P. Sharma a,b, Vaibhav Kulshrestha a,b,* a
CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Gijubhai Badheka Marg Bhavnagar, 364002 Gujarat, India b Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Gijubhai Badheka Marg Bhavnagar, 364002 Gujarat, India
article info
abstract
Article history:
We report the synthesis of anion exchange membrane based on brominated poly (2, 6-
Received 28 March 2017
dimethyl-1, 4-phenylene oxide) (PPO) and polyethyleneimine (PEI) modified graphene
Received in revised form
oxide (GO). The prepared membrane are characterized electrochemically in terms of ion
23 July 2017
exchange capacity (IEC), water uptake (Wt), dimensional stability (Sd), and membrane ionic
Accepted 3 August 2017
conductivity (ɸ). The surface morphology and structural analysis of membrane were per-
Available online xxx
formed by using AFM, TEM, FT-IR, XRD and 1HNMR. Thermal and mechanical analysis was done by using TGA, DMA and UTM. QP-5 membrane show good thermal and mechanical
Keywords:
stability among all prepared membranes. The ionic conductivity and IEC value was found
PPO
maximum in QP-5 and the value is 7.1 102 S cm1 and 3.59 meq g1. Mechanical study
Anion conducting membranes
was determined by universal testing machine (UTM) analysis shows QP-5 membrane best
Ionic conductivity
among the membranes with modulus 6.5 MPa. The methanol crossover resistance for QP-5
Methanol crossover resistance
membrane is calculated to be 2.27 107 cm2/s with the selectivity of 3.4 104. The power consumption during salt removal is also found to be lower compared to other synthesized membranes. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In present era, research is proceeding towards the synthesis of anion exchange membranes as a separator that might be suitable towards alkaline membrane fuel cells (AMFC) as well as for other electro-membrane based energy transformation system such as redox flow batteries (RFB), reverse
electrodialysis (RED) and many kind of storage systems [1e4]. The reason of this interest is due to the apparent advantage over acidic proton exchange membranes [5e7]. On the other hand the synthetic procedure involves greener route that should be environmental friendly and cost effective [8]. The energy based technology such as fuel cells provide a better straight way for energy production conventionally. Anion conducting membranes show better performance in Fuel cell
* Corresponding author. CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Gijubhai Badheka Marg Bhavnagar, 364002 Gujarat, India. E-mail addresses: [email protected], [email protected] (V. Kulshrestha). http://dx.doi.org/10.1016/j.ijhydene.2017.08.029 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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application because the direction of transport for methanol is opposed by hydroxide ion and hence therefore reduces methanol crossover [9e12]. Fuel cells are the best devices used to convert energy of fuel such as H2, CH4 and CH3OH in electricity by using polymer electrolyte membrane. Polymer electrolyte membrane fuel cells (PEMFCs) are the operable electrochemical converter with a number of advantages although with some disadvantages such as short electrolyte life time, metal catalyst poisoning and limited performance under harsh conditions. Inorganic chemical compound (SiO2, titania, zirconia, etc.) are used to raise working temperature as well as stability during long term operations [13]. Graphene significantly improve the performance of fuel at the same time by increasing operational life time as well as durability of fuel cell. Graphene and its composite are the key component for the commercialization of fuel cell technology [14]. Graphene is used as an anode or cathode supporting material, as a nano filler and additive to polymer electrolyte in polymer electrolyte membrane fuel cells. Graphene oxide shows a positive effect to the catalyst such as increase in electrochemical surface area, decrease in average pore width, facilitate anodic reaction and significantly increase the ionic conductivity [15]. Although Nafion is a benchmark commercially available proton exchange membrane bearing very good physiochemical as well as electrochemical properties. However its use is little bit hindered because of its poor performance in harsh anhydrous condition. GO being an organic filler also used as nano-additive when we make nanocomposite membranes. GO contain several functional groups on its surface which help to provide sites for bonding, hence increase durability. It also restrict methanol crossover by making barrier throughout the membrane and hence increase the tunability of membrane for its application towards Fuel cells [16]. Unlike sulfonic acid based per fluorinated cation exchange membrane such as Nafion, which has high ionic conductivity as well as high thermal and chemical stability, anion exchange membranes are not showing good hydroxide conductivity over a wide range of temperature and pH, hence research activities are performed regularly in account to improve these factors for anion exchange membranes [17]. Briefly good anion exchange membranes should have a moderate amount of swelling when they are placed in water for the hydroxide movement, but sometimes this swelling may be harmful if it occur at higher degree. This higher water uptake value cause diminishing in the mechanical stability. Sometimes it can be improved by filling the polymer matrix with organic filler such as GO [18,19]. If such type of membranes are incorporated with fillers like GO they will present high performance towards mechanical and thermal surrounding. Because GO exhibit unique properties having multifunctional groups over it for better permeation also provide better mechanical stability by making bond with the polymer matrix. Many of the anion exchange membranes are used for applications. Among these membranes which are derived from partially fluorinated polymer show significant results when they are subjected to use in halide form such as Cl and F instead of OH anion because of their good thermal and chemical stability [20,21]. The anion exchange membranes that use OH ion and are aimed for fuel cell application
are limited, hence in last decade maximum research is carried out just to synthetize alkaline exchange membrane for fuel cell application that has good oxidative stability in methanol and having low coast as compared to fluorinated membrane [22,23]. Among all the engineering plastics PPO is widely used thermoplastic polymer now a days due to its good solubility, high thermal and mechanical stability in preparation of ion exchange membrane [24]. PPO is a commonly used forbear for preparation of functionalized polymer by chemical modifications. When compared with other aromatic polymers, PPO can easily undergo a number of polymer analogous reaction at both arylic and benzylic positions such as electrophilic substitution, nucleophilic substitution and radical substitution [25]. Br-PPO is susceptible to nucleophilic attack. Br-PPO contains abundant CH2Br group which easily react with amines and cross linker without penetrating base membrane [26]. GO have excellent mechanical and thermal stability, a large no oxygen containing functional groups (epoxy, carboxyl and hydroxyl) present on GO surface enables to react with other material and polymer [27]. The composites of GO possess improved tensile strength, elastic modulus and good dispersibility in polymer matrix [28]. Herein, a green synthetic route have been proposed for synthesis of Br-PPO based anion exchange membrane for electro membrane applications. Br-PPO was modified with different weight content of polyethyleneimine modified graphene oxide and termed as QP-n. The prepared membranes were characterized in term of FTIR, 1H NMR, XRD, AFM, thermal stability, mechanical stability, ion exchange capacity, water uptake and dimensional stability. All the composite membranes were employed to pervaporation separation and electrodialysis experiment and their performance toward methanol crossover resistance and salt removal from aqueous solution was coined in terms of permeability, selectivity and power consumption.
Experimental Materials and methods Graphite powder, polyethyleneimine branched (PEI) with average molecular Weight~25000, poly (2,6-dimethyl-1,4phenylene oxide) (PPO), N-bromosuccinimide (NBS) and Nmethylmorpholine (NMM) are purchased from Sigma Aldrich. N-methyl-2-pyrollidone (NMP) supplied by S D fine chemicals limited. All other chemicals were obtained commercially and used without further purification. Graphene oxide was prepared by using modified Hummer's method [28,29]. Briefly, in a round bottom flask 46 ml H2SO4, 1 g graphite powder, 1 g NaNO3 are taken under vigorous stirring in an ice bath at 0e5 C for 2 h 8 g of KMnO4 is added to the reaction mixture at room temperature and stirred for 4 h, 30 ml water is added dropwise with stirring for 3 h as reaction is highly exothermic, produce enormous amount of heat. Finally, 200 ml, 5% H2O2 is added and washed repeatedly and centrifuged to recover graphene oxide (GO). Functionalization of GO with PEI was done by “one spot” method with a modification reported by Liu et al. [30]. In a typical synthesis, 500 mg
Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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of GO were dispersed in 30 ml of DI water by ultra sonication for 30 min and 2 g PEI (AMW~25000) was mixed and resulting solution is stirred for 2 h at 80 C. PEI modified GO was obtained by repeat centrifugation with DI water [31]. The schematic representation of product obtained f GO i.e. PEI-GO was represented in Scheme 1. PEI-GO was dispersed in NMP at an appropriate concentration for further use. In a typical synthesis of brominated PPO, 12 g PPO in 100 ml Chlorobenzene on complete dissolution, 8.9 g N-bromosuccinimide (NBS) and 0.5 g crystalized 2, 2-azobisisobutyronitrile were added and the reaction carried out at 135 C under reflux conditions for 3 h. After cooling, the reaction mixture poured to excess of ethanol to precipitate the product. The polymer was filtered and again redissolved into chloroform followed by precipitate into excess of ethanol. Precipitated polymer was filtered and collected as light yellow powder and dried under vacuum to get final Br-PPO [32].
Membrane preparation The membranes were prepared via simple solution casting method. Casting solution contain an appropriate amount of Br-PPO in NMP, a desired amount of N-methylmorpholine (NMM) was added and reaction was stirred vigorously for 15 h at 40 C as per Scheme 1 [33,34]. Different concentrations of f GO (PEI-GO) was dispersed in NMP and added to above solution to get different f GO concentration followed by ultrasonication. Finally, the solution was casted on glass plate at 60 C to complete removal of solvent The obtained membranes were designated as QP, QP-1, QP-2 and QP-5 for QPPO, 1 wt% f GO, 2 wt% f GO and 5 wt% f GO with respect to polymer, respectively.
Chemical, structural, and thermal characterization of materials and membranes The study of structural and chemical characterization of the f GO and composite membrane were done by using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). GO, PEI-GO and composite membranes are characterized by X-ray diffraction (XRD) at room temperature on Rigaku miniflex-II X-ray diffractometer. Transmission electron microscope (TEM) images of GO are obtained by JEOL, JEM 2100 microscope with an accelerating voltage of 200 KeV. Surface analysis of composite films was done by Atomic Force Microscopy (AFM) by using NTEGRA AURA (NTMDT) for surface roughness of the samples. Thermomechanical analysis of membrane sample was done by using thermogravimetric analysis (TGA) of NETZSCH TG 209F1 Libra and Dynamic mechanical analysis (DMA) of NETZSCH DMA 242. Stress/strain curve for composite membranes are measured by using Zwick Roell, Z2.5 universal testing machine (UTM).
Physiochemical characterization Water Uptake behavior of membranes is determined by recording the weight gain after equilibrating in water for 24 h. Ion exchange capacity (IEC) of composite membranes was estimated by the acid base titration. Ionic conductivity of the
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membranes was measured on potentiostat/galvanostat (CH608). Details of the experiments are given in ESI.
Methanol permeability measurement Methanol permeability was carried out with two stainless steel half cells setup with a capacity of 100 ml and effective area of 10.2 cm2. A Varian rotary vacuum pump (DS-302) was used to maintain the vacuum on the downstream side of membranes. All the prepared membranes are equilibrated in feed solution for 12 h before performing the experiment. An aqueous solution (20%) of methanol was recirculated in a stainless steel half cell setup. Before starting the vacuum pump feed solution was circulated through the membrane for 10 min. Pervaporation process was started after it and operated for 2 h at room temperature. Permeate is collected in cold trap immersed in liquid nitrogen while the retantate was circulated back to feed solution in a cross flow mode. The compositions of feed solution and permeate was determined with a digital refractometer (Mettler Toledo RE-40D) having accuracy up to four digits. Total permeate flux J (kg m2 h1) was calculated by following equation: J¼
W A:t
where w, A and t are the total weight of permeate (kg), effective area of membrane (m2) and operating time (h). For the suitability of membrane for DMFC, the selectivity of membrane was calculate by following equation: Sp ¼
s PM
where PM is membrane permeability (cm2/s) and s is ionic conductivity of membrane (S/cm).
Salt removal efficiency by electrodialysis The performance of prepared composite membranes were checked in a locally fabricated PVC based ED unit. The details of the experiments are given in ESI section.
Results and discussion Structural and chemical characterization of GO, PEI-GO and different nanocomposite membranes The chemical characterization of GO and PEI-GO was performed by FTIR analysis as shown in Fig. 1. FTIR spectra of prepared GO shows OeH stretching at 3391 cm1, C]O stretching at 1719 cm1, 1620 cm1, CeO stretching at 1051 cm1 and a broad peak at 1211 cm1 is present because of epoxy group in prepared GO [28,35]. Spectra of f GO shows not only peaks at 2945 cm1, 2828 cm1, because of methylene group of PEI but two new peaks at 1650 cm1, 1568 cm1 due to NHeC]O stretching. It shows that the carboxyl group of GO react with amine group of PEI result in the formation of amide [31]. Fig. 1 and S-1 shows FTIR spectra of the different composite membranes, shows a sharp peak in the region 1114 cm-1 confirming the reaction of brominated PPO with the
Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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Scheme 1 e Schematic representation of quaternized PPO based f GO nanocomposite membrane.
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A
Fig. 1 e FTIR spectra of GO, PEI-GO and QP-5 nanocomposite membrane.
N-methylmorpholine. Another peak in the region 1600 cm-1 is assigned for stretching vibration of eC]Ce in the phenyl ring. A dominant peak in the region 1185 cm1 indicating the stretching of eCeOeCe bond in between the phenyl ring [33,38]. The absorption peaks at 1470 cm1 is arises due to symmetric and asymmetric stretching vibration of phenyl group [33]. 1H NMR spectra of base polymer, bromomethylated polymer and composite membranes was presented in Fig. 2 and S-2, shows the characteristics value of chemical shift at 6.4 ppm is attributed to the aromatic proton, while a sharp peak at d ¼ 4.35 ppm corresponds to the methylene protons of bromomethyl functional group. Another peak at 2.1 ppm is indicating the existence of methyl groups at PPO [39,40]. X-ray diffraction pattern of GO and f GO is shown in Fig. 3(A), sharp and intense peak at 10.8 corresponds to the formation of GO from graphite with an interlayer spacing of
Fig. 2 e 1H NMR spectra of PPO, Br-PPO and QP-5 nanocomposite membrane.
B
Fig. 3 e (A): XRD of GO and PEI modified GO. (B): XRD of different composite membranes.
8.03 A [36,37] with high degree of exfoliation and a disordered structure. In case of f GO long chains of PEI interact with GO sheets by covalent bonding results in broadening of peak and slight shifting at 2q ¼ 6.4 with an increased interlayer spacing of 10.7 A [31] this is due to the disturbance of PEI in GO. XRD analysis of QP, QP-1, QP-2 and QP-5 membranes is shown in Fig. 3(B), revels that the crystalline nature appear in composite membrane. The surface morphology of all prepared membranes are performed by AFM analysis through NT-MDT, Ntegra Aura software (Fig. 4). It describes a trend of uniform distribution of f GO in polymer matrix and increasing roughness of the membrane due to increase of f GO content. The calculated roughness of QP membrane is found to be 3.264 nm. On the other hand the roughness of the composite membranes increases and found to be 4.18, 5.79 and 12.06 nm for QP-1, QP-2 and QP-5 respectively. Roughness of composite membrane with a highest concentration of f GO is three times more than that of pristine membrane. Fig. 5 shows the TEM images of GO (A, A0 ) and f GO (B, B0 ) at low and high magnification. The
Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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Fig. 4 e AFM images of QP (A, Aˈ), QP-1 (B, Bˈ), QP-2 (C, Cˈ) and QP-5 (D, Dˈ) prepared nanocomposite membranes.
Fig. 5 e TEM images of GO (A,A′) and PEI-GO (B,B′) at different magnification.
overlapping and folding feature at some particular area expose the property of well dispersion in water. This fact leads us to a conclusion that grapheme oxide sheets try to agglomerate with each other by some interaction. The wrinkles in the nano
sheets are suggesting fully exfoliation when these are oxidized. The average size of the GO sheets is about 0.6 mm. The uniform dispersion of f GO into the membrane has been confirmed by TEM analysis and presented in Fig. S-3.
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Thermal and mechanical analysis Thermal and mechanical stability of prepared membranes were investigated with the help of TGA, DMA and UTM analysis. Thermal gravimetric analysis (TGA) of composite anion exchange membrane was performed from RT to 600 C at a heating rate of 10 C per minute under nitrogen environment. It is clear from Fig. 6, that there is four step weight loss in the membranes corresponding to desorption of water, thermal degradation of functional groups and thermal oxidation of polymer backbone. In first step all membranes shows a weight loss before 100 C attributed to evaporation of bound water present in samples. Second step weight loss occur at 230e280 C due to release of decomposed product of quaternary ammonium group. Third step weight loss at 300e330 C is likely ascribed to the release of residual quaternary ammonium group. Fourth and last weight loss observed around 400e500 C attributed to the degradation of polymer chains [41,42]. Composite membranes shows lower weight loss compared to QP membrane. This study shows that polymer is thermally very stable and with the addition of f GO thermal stability of polymer significantly increases. DMA analysis (Fig. 7) of different prepared membranes shows that as we increase f GO content in polymer matrix value of storage modulus as well as temperature of tan d increases significantly [28,43]. Value of storage modulus and T tan d for QP-5 membrane were 2787 MPa and 215 C which is highest among all the prepared membranes and least for QP membrane which is 1150 MPa and 199 C respectively. A uniform and good dispersion of f GO in polymer matrix lead to a good accession and stiffness of membranes. Mechanical analysis of prepared membranes was done by stress-strain curve of each membrane and presented in Fig. 8 and Table 1. It can be observed that, QP membrane with a minimum elastic modulus of 2.75 MPa and maximum elongation of 6.37. It show the flexibility of membrane as well as amorphous nature. But QP-5 membrane with a maximum elastic modulus of 6.50 MPa which is almost double from pristine QP membrane and minimum elongation of 3.58. It reveals that as well as the PEIGO content in the polymer matrix increases, there is a gradual change in amorphous to crystalline nature of membranes. It
Fig. 7 e DMA analysis of membranes with storage modulus and T tan d
Fig. 8 e Stress vs strain curves of different nanocomposite membranes.
Table 1 e Mechanical strength for different hybrid membranes. Membrane QP QP-1 QP-2 QP-5
Modulus (MPa)
Stress (MPa)
Strain (%)
2.75 4.00 5.51 6.50
10.31 18.47 18.92 23.80
6.37 5.54 4.74 3.58
was observed that f GO content reduce the flexibility and increase the crystallinity of fabricated membrane [43,44].
Electro-chemical properties
Fig. 6 e TGA profile for different prepared membranes.
Water uptake, ion exchange capacity, hydration number and dimensional stability are the important parameters of ion exchange membrane. Table 2, shows the values of water uptake, ion exchange capacity, hydration number and dimensional change values of different composite membranes. Water uptake and bound water gradually decreases as
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Table 2 e Ion exchange capacity (IEC), water content/ hydration number (l), water uptake (Wt), bound water (Wb) and free water (Wf), dimensional stability (Sd), ionic conductivity (F) for different hybrid membranes.
QP QP-1 QP-2 QP-5
IEC (meq/gm)
l
2.15 2.39 3.14 3.59
4.02 4.08 4.67 5.06
Wt Wb (%) (%) 27.7 23.1 20.1 19.6
0.58 0.27 0.25 0.18
Wf (%) 27.07 22.79 19.83 15.36
Sd F X 102 (%) (S/cm) 31.6 28.7 18.4 14.2
1.3 1.5 2.6 7.1
concentration of f GO increases because f GO act as a nano filler to the polymer matrix. f GO and PPO matrix leads to a network in which f GO serve as a joint and cross linkage by covalent bonding interactions. Covalent bonding interaction results in crosslinking of polymer chains so that the water retention capacity of composite membrane decreases, hence bound water within membrane decreases. Dimensional stability of polymer membranes increases i.e. 44.9% for QP-5, as f GO content increases this due that swelling of membrane decreases because of crosslinking of polymer chains and membrane form more stability. Least dimensional changes observed in QP-5 membrane with highest f GO content. The IEC value of composite membrane increases as the content of f GO increases in PPO matrix due to covalent bonding interaction in between f GO and Br-PPO which results in decrease in bound water in polymer matrix and approaches to 3.59 meq g1 for QP-5 membrane.
Ionic conductivity Ionic conductivity is also an important parameter of membrane for their application in different electro-membrane processes. Ionic conductivity of AEMs is highly dependent on ionic mobility and fixed charge concentration in the membrane matrix. Ion conducting channels inside the membrane provide a path for transportation of ion. Incorporation of fGO with the polymer matrix enhance conduction of ion via hydrophilic channels. Being hydrophilic, fGO also induces water trapping capacity for the membrane, which in results provide a route for ionic transportation. Water uptake also play an important role for obtain desirable results towards electro-membrane applications. Water uptake should not be as higher because it may create low stability. Thus incorporation of fGO also maintains stability of the membrane as well as also enhance ion transportation property. Increase in ionic conductivity, lowers the value of energy barrier for ion transport through polymer. Ionic conductivity of the composite membranes increases with increasing the f GO content into PPO matrix and reaches up to 7.1 102 S/cm for QP-5 membrane. The value of ionic conductivity for QP-5 membrane is around 5.5 times higher than the QP membrane (1.3 102 S/cm) the increment in the ionic conductivity may be due to the addition of more functional group in the form of f GO content as also confirmed by IEC values (Table 2).
Methanol permeability and selectivity for membranes High methanol permeation resistance is the basic requirement of ion exchange membrane for fuel cell. Methanol
Fig. 9 e Methanol permeability and selectivity of different nanocomposite membranes. permeability and selectivity of different composite membranes are shown in Fig. 9. The methanol permeation resistance for different composite membranes increases with f GO content in membrane matrix. The interaction between f GO and QPPO leads to low methanol permeability across the membrane. f GO prevent the movement of methanol across the composite membrane and acts as a barrier for connected channels. Total permeate flux for different composite membranes QP-1, QP-2 and QP-5 was found to be 7.18 109, 4.78 109 and 1.78 109 kg cm2 h1 respectively. The lowest value is found for QP-5 membrane among all composite membranes due to higher f GO content. Methanol permeability for different composite membranes is found to be 8.7 107, 6.64 107 and 2.27 107 cm2sec1 for QP-1, QP-2 and QP-5 membranes respectively. The successive reduction in methanol permeability of membranes is due to increment in strong interfacial adhesion between f GO and QPPO matrix. For the suitability for fuel cell selectivity of membranes is calculated which is directly proportional to the conductivity and inversely proportional to the permeability of membranes. Selectivity of composite membranes is found to
2
Current density (mA/cm2)
Membrane
QP QP-1 QP-2 QP-5 1
0
0
1
2
3
4
Potential (V/Cell pair)
5
6
Fig. 10 e Current density-potential curve for different membranes.
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Table 3 e DV, Di and Ilim values, power consumption (P) during salt removal for different hybrid membranes. Membrane QP QP-1 QP-2 QP-5
DV (V)
Di (mA cm2)
Ilim (mA cm2)
P/kWh kg1
2.84 3.85 3.54 5.07
0.98 0.66 1.02 1.47
1.00 0.64 0.83 1.05
1.10 0.98 0.93 0.79
be highest for QP-5 membrane which is 3.4 104 and lowest for QP-1 which is 1.8 104. The methanol permeability of QP-5 membrane are found to be one fourth to QP-1 with 1.9 times higher selectivity, low methanol permeability and high selectivity towards methanol crossover make QP-5 membrane suitable for fuel cell application.
Salt removal efficiency of composite membranes Current and potential characteristics of composite membranes was plotted by equilibrating the membrane in 0.1 M NaCl solution. I-V plot (Fig. 10) of composite membranes show
A
B
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typically three characteristic regions viz, ohmic, plateau and non-ohmic which reveals about the ion transport and concentration polarization across the membranes [28,45]. In ohmic region current density increases with an applied potential due to presence of a large number of ions, in plateau region current density remain almost constant and in nonohmic region ions get depleted causing water splitting with a higher current density. The value of DV, Di and Ilim calculated from Fig. 10 and presented in Table 3 [45]. It is clear from the table that the region from 2 V to 4 V is of our interest for electro-dialysis (ED) experiment. ED experiment was done at a constant applied potential of 2.0 V per cell pair using 0.1 M NaCl feed solution and 0.05 M Na2SO4 solution was recirculating as electrode wash to avoid electrode reaction and rinse the electrodes. In CC the change in conductivity of NaCl with time is represented in Fig. S-4. It is clear that conductivity linearly decrease with time. QP-5 membrane decreases concentration more efficiently than other prepared membranes, membrane highly facilitate the migration of ion from CC to DC. For QP-5 membranes concentration goes to 813 ppm but it is 1442 ppm for QP membrane which is higher in same time interval. The change in concentration of solutions with time presented in Fig. 11(A). Concentration of CC decreases with time and it increases for DC. Variation in current density with time presented in Fig. 11(B) initially current with time increases faster because concentration gradient in CC is very high and movement of ions from CC to DC is very high after passage of time concentration of both compartments almost same and there is no significant change in current takes place. Further when concentration of ions in DC increases hence due to it the movement of the ion start from DC to CC, but with a slower rate than previous and hence value of current goes down. The relative concentration of CC and DC with time shown in Fig. S-5 concentration of CC decrease with time simultaneously a significant increase in concentration of DC is observed. The accessibility of membranes was estimated in terms of power consumption during salt removal and depicted in Table 3. The power consumption during salt removal for QP-5 membrane found to be 0.79 kW h kg1 while for QP membrane it is 1.1 kW h kg1, about 30% lower power consumption is noted for QP-5 membrane.
Conclusion
Fig. 11 e (A) Concentration vs. time plot for different membranes during salt removal. (B) Current density vs. time plot for different membranes during salt removal.
Br-PPO based anion exchange membrane was synthesized successfully with greener method by incorporating PEI modified GO in membrane matrix. FTIR characterization of PEI-GO and prepared membrane confirms the successful synthesis of f GO and quarternized PPO. Prepared membranes show high thermal as well as mechanical stability. QP-5 membrane with a highest elastic modulus of 6.50 MPa found to be highly stable among all prepared membranes. QP-5 membrane shows the best performance among the membranes in terms of physico-chemical, electro-chemical and mechanical stability. The ion exchange capacity and ionic conductivity of QP-5 is found to be 3.59 meq g1 and 7.1 102 S/cm respectively which is 55% (IEC) and 5.5 times higher than that of QP membrane. The methanol permeation resistance and selectivity of all the composite membranes are
Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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calculated and QP-5 shows both higher permeation resistance and selectivity for methanol that is 2.27 107 cm2/s and 3.436 104 S cm3 s1. The membranes are also checked for their salt removal efficiency in terms of power consumption, which is about 0.79 kW h kg1 for QP-5 membrane. These result suggest that composite membrane is suitable for fuel cell and other electro-membrane applications.
Acknowledgments Author V. Kulshrestha is thankful to SERB, Department of Science and Technology, New Delhi, for providing financial support. Authors are also thankful to Analytical Discipline and Centralized Instrument facility, CSMCRI, Bhavnagar for instrumental support.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.08.029.
references
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lower cytotoxicity and long term antibacterial activity. Carbon 2012;50:3407e15. [38] Wu Y, Wu C, Gong M, Xu TW. New anion exchanger organic inorganic hybrid materials and membrane from a copolymer of glycidylmethacrylate and g-metacryloxypropyl trimethoxy silane. J Appl Polym Sci 2006;102:3580e9. [39] Ran J, Wu L, Vercoe JR, Ong AL, Poynton SD, Xu TW. Development of imidazolium type alkaline anion exchange membrane for fuel cell applications. J Membr Sci 2012;415e416:242e9. [40] Yang Y, Knauss DM. Poly(2,6-dimethyl-1,4-phenylene oxide)b-poly(vinylbenzyltrimethylammonium) diblock copolymer for highly conductive anion exchange membrane. Macromolecules 2015;48(13):4471e80. [41] Thakur AK, Gahlot S, Kulshrestha V, shahi VK. Highly stable acid-base complex membrane for ethanol dehydration by pervaporation separation. RSC Adv 2013;3:22014e22. [42] Li Y, Xu TW. Fundamental studies of a new series of anion exchange membranes: membrane prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) and 4-vinylpyridine. J Appl Polym Sci 2009;114:3016e25. [43] Gahlot S, Kulshrestha V. Dramatic improvement in water retention and proton conductivity in electrically aligned functionalized CNT/SPPEK nano hybrid PEM. ACS Appl Mater Interfaces 2015;7:264e72. [44] Luo Z, Gong Y, Liao X, Pan Y, Zhang H. Nanocomposite membrane modified by graphene-based material for anion exchange membrane for fuel cell. RSC Adv 2016;6:13618e25. 45 Sharma PP, Gahlot S, Gupta H, Kulshrestha V. Hybrid anion conducting membranes (ACM) for industrial applications: Excellent salt removal and efficiency electro-chemical properties. Separation and Purification Technology 2017. http://dx.doi.org/10.1016/j.seppur.2017.08.040.
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Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications Vikrant Yadav a,b, Prem P. Sharma a,b, Vaibhav Kulshrestha a,b,* a
CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Gijubhai Badheka Marg Bhavnagar, 364002 Gujarat, India b Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Gijubhai Badheka Marg Bhavnagar, 364002 Gujarat, India
article info
abstract
Article history:
We report the synthesis of anion exchange membrane based on brominated poly (2, 6-
Received 28 March 2017
dimethyl-1, 4-phenylene oxide) (PPO) and polyethyleneimine (PEI) modified graphene
Received in revised form
oxide (GO). The prepared membrane are characterized electrochemically in terms of ion
23 July 2017
exchange capacity (IEC), water uptake (Wt), dimensional stability (Sd), and membrane ionic
Accepted 3 August 2017
conductivity (ɸ). The surface morphology and structural analysis of membrane were per-
Available online xxx
formed by using AFM, TEM, FT-IR, XRD and 1HNMR. Thermal and mechanical analysis was done by using TGA, DMA and UTM. QP-5 membrane show good thermal and mechanical
Keywords:
stability among all prepared membranes. The ionic conductivity and IEC value was found
PPO
maximum in QP-5 and the value is 7.1 102 S cm1 and 3.59 meq g1. Mechanical study
Anion conducting membranes
was determined by universal testing machine (UTM) analysis shows QP-5 membrane best
Ionic conductivity
among the membranes with modulus 6.5 MPa. The methanol crossover resistance for QP-5
Methanol crossover resistance
membrane is calculated to be 2.27 107 cm2/s with the selectivity of 3.4 104. The power consumption during salt removal is also found to be lower compared to other synthesized membranes. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In present era, research is proceeding towards the synthesis of anion exchange membranes as a separator that might be suitable towards alkaline membrane fuel cells (AMFC) as well as for other electro-membrane based energy transformation system such as redox flow batteries (RFB), reverse
electrodialysis (RED) and many kind of storage systems [1e4]. The reason of this interest is due to the apparent advantage over acidic proton exchange membranes [5e7]. On the other hand the synthetic procedure involves greener route that should be environmental friendly and cost effective [8]. The energy based technology such as fuel cells provide a better straight way for energy production conventionally. Anion conducting membranes show better performance in Fuel cell
* Corresponding author. CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Gijubhai Badheka Marg Bhavnagar, 364002 Gujarat, India. E-mail addresses: [email protected], [email protected] (V. Kulshrestha). http://dx.doi.org/10.1016/j.ijhydene.2017.08.029 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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application because the direction of transport for methanol is opposed by hydroxide ion and hence therefore reduces methanol crossover [9e12]. Fuel cells are the best devices used to convert energy of fuel such as H2, CH4 and CH3OH in electricity by using polymer electrolyte membrane. Polymer electrolyte membrane fuel cells (PEMFCs) are the operable electrochemical converter with a number of advantages although with some disadvantages such as short electrolyte life time, metal catalyst poisoning and limited performance under harsh conditions. Inorganic chemical compound (SiO2, titania, zirconia, etc.) are used to raise working temperature as well as stability during long term operations [13]. Graphene significantly improve the performance of fuel at the same time by increasing operational life time as well as durability of fuel cell. Graphene and its composite are the key component for the commercialization of fuel cell technology [14]. Graphene is used as an anode or cathode supporting material, as a nano filler and additive to polymer electrolyte in polymer electrolyte membrane fuel cells. Graphene oxide shows a positive effect to the catalyst such as increase in electrochemical surface area, decrease in average pore width, facilitate anodic reaction and significantly increase the ionic conductivity [15]. Although Nafion is a benchmark commercially available proton exchange membrane bearing very good physiochemical as well as electrochemical properties. However its use is little bit hindered because of its poor performance in harsh anhydrous condition. GO being an organic filler also used as nano-additive when we make nanocomposite membranes. GO contain several functional groups on its surface which help to provide sites for bonding, hence increase durability. It also restrict methanol crossover by making barrier throughout the membrane and hence increase the tunability of membrane for its application towards Fuel cells [16]. Unlike sulfonic acid based per fluorinated cation exchange membrane such as Nafion, which has high ionic conductivity as well as high thermal and chemical stability, anion exchange membranes are not showing good hydroxide conductivity over a wide range of temperature and pH, hence research activities are performed regularly in account to improve these factors for anion exchange membranes [17]. Briefly good anion exchange membranes should have a moderate amount of swelling when they are placed in water for the hydroxide movement, but sometimes this swelling may be harmful if it occur at higher degree. This higher water uptake value cause diminishing in the mechanical stability. Sometimes it can be improved by filling the polymer matrix with organic filler such as GO [18,19]. If such type of membranes are incorporated with fillers like GO they will present high performance towards mechanical and thermal surrounding. Because GO exhibit unique properties having multifunctional groups over it for better permeation also provide better mechanical stability by making bond with the polymer matrix. Many of the anion exchange membranes are used for applications. Among these membranes which are derived from partially fluorinated polymer show significant results when they are subjected to use in halide form such as Cl and F instead of OH anion because of their good thermal and chemical stability [20,21]. The anion exchange membranes that use OH ion and are aimed for fuel cell application
are limited, hence in last decade maximum research is carried out just to synthetize alkaline exchange membrane for fuel cell application that has good oxidative stability in methanol and having low coast as compared to fluorinated membrane [22,23]. Among all the engineering plastics PPO is widely used thermoplastic polymer now a days due to its good solubility, high thermal and mechanical stability in preparation of ion exchange membrane [24]. PPO is a commonly used forbear for preparation of functionalized polymer by chemical modifications. When compared with other aromatic polymers, PPO can easily undergo a number of polymer analogous reaction at both arylic and benzylic positions such as electrophilic substitution, nucleophilic substitution and radical substitution [25]. Br-PPO is susceptible to nucleophilic attack. Br-PPO contains abundant CH2Br group which easily react with amines and cross linker without penetrating base membrane [26]. GO have excellent mechanical and thermal stability, a large no oxygen containing functional groups (epoxy, carboxyl and hydroxyl) present on GO surface enables to react with other material and polymer [27]. The composites of GO possess improved tensile strength, elastic modulus and good dispersibility in polymer matrix [28]. Herein, a green synthetic route have been proposed for synthesis of Br-PPO based anion exchange membrane for electro membrane applications. Br-PPO was modified with different weight content of polyethyleneimine modified graphene oxide and termed as QP-n. The prepared membranes were characterized in term of FTIR, 1H NMR, XRD, AFM, thermal stability, mechanical stability, ion exchange capacity, water uptake and dimensional stability. All the composite membranes were employed to pervaporation separation and electrodialysis experiment and their performance toward methanol crossover resistance and salt removal from aqueous solution was coined in terms of permeability, selectivity and power consumption.
Experimental Materials and methods Graphite powder, polyethyleneimine branched (PEI) with average molecular Weight~25000, poly (2,6-dimethyl-1,4phenylene oxide) (PPO), N-bromosuccinimide (NBS) and Nmethylmorpholine (NMM) are purchased from Sigma Aldrich. N-methyl-2-pyrollidone (NMP) supplied by S D fine chemicals limited. All other chemicals were obtained commercially and used without further purification. Graphene oxide was prepared by using modified Hummer's method [28,29]. Briefly, in a round bottom flask 46 ml H2SO4, 1 g graphite powder, 1 g NaNO3 are taken under vigorous stirring in an ice bath at 0e5 C for 2 h 8 g of KMnO4 is added to the reaction mixture at room temperature and stirred for 4 h, 30 ml water is added dropwise with stirring for 3 h as reaction is highly exothermic, produce enormous amount of heat. Finally, 200 ml, 5% H2O2 is added and washed repeatedly and centrifuged to recover graphene oxide (GO). Functionalization of GO with PEI was done by “one spot” method with a modification reported by Liu et al. [30]. In a typical synthesis, 500 mg
Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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of GO were dispersed in 30 ml of DI water by ultra sonication for 30 min and 2 g PEI (AMW~25000) was mixed and resulting solution is stirred for 2 h at 80 C. PEI modified GO was obtained by repeat centrifugation with DI water [31]. The schematic representation of product obtained f GO i.e. PEI-GO was represented in Scheme 1. PEI-GO was dispersed in NMP at an appropriate concentration for further use. In a typical synthesis of brominated PPO, 12 g PPO in 100 ml Chlorobenzene on complete dissolution, 8.9 g N-bromosuccinimide (NBS) and 0.5 g crystalized 2, 2-azobisisobutyronitrile were added and the reaction carried out at 135 C under reflux conditions for 3 h. After cooling, the reaction mixture poured to excess of ethanol to precipitate the product. The polymer was filtered and again redissolved into chloroform followed by precipitate into excess of ethanol. Precipitated polymer was filtered and collected as light yellow powder and dried under vacuum to get final Br-PPO [32].
Membrane preparation The membranes were prepared via simple solution casting method. Casting solution contain an appropriate amount of Br-PPO in NMP, a desired amount of N-methylmorpholine (NMM) was added and reaction was stirred vigorously for 15 h at 40 C as per Scheme 1 [33,34]. Different concentrations of f GO (PEI-GO) was dispersed in NMP and added to above solution to get different f GO concentration followed by ultrasonication. Finally, the solution was casted on glass plate at 60 C to complete removal of solvent The obtained membranes were designated as QP, QP-1, QP-2 and QP-5 for QPPO, 1 wt% f GO, 2 wt% f GO and 5 wt% f GO with respect to polymer, respectively.
Chemical, structural, and thermal characterization of materials and membranes The study of structural and chemical characterization of the f GO and composite membrane were done by using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). GO, PEI-GO and composite membranes are characterized by X-ray diffraction (XRD) at room temperature on Rigaku miniflex-II X-ray diffractometer. Transmission electron microscope (TEM) images of GO are obtained by JEOL, JEM 2100 microscope with an accelerating voltage of 200 KeV. Surface analysis of composite films was done by Atomic Force Microscopy (AFM) by using NTEGRA AURA (NTMDT) for surface roughness of the samples. Thermomechanical analysis of membrane sample was done by using thermogravimetric analysis (TGA) of NETZSCH TG 209F1 Libra and Dynamic mechanical analysis (DMA) of NETZSCH DMA 242. Stress/strain curve for composite membranes are measured by using Zwick Roell, Z2.5 universal testing machine (UTM).
Physiochemical characterization Water Uptake behavior of membranes is determined by recording the weight gain after equilibrating in water for 24 h. Ion exchange capacity (IEC) of composite membranes was estimated by the acid base titration. Ionic conductivity of the
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membranes was measured on potentiostat/galvanostat (CH608). Details of the experiments are given in ESI.
Methanol permeability measurement Methanol permeability was carried out with two stainless steel half cells setup with a capacity of 100 ml and effective area of 10.2 cm2. A Varian rotary vacuum pump (DS-302) was used to maintain the vacuum on the downstream side of membranes. All the prepared membranes are equilibrated in feed solution for 12 h before performing the experiment. An aqueous solution (20%) of methanol was recirculated in a stainless steel half cell setup. Before starting the vacuum pump feed solution was circulated through the membrane for 10 min. Pervaporation process was started after it and operated for 2 h at room temperature. Permeate is collected in cold trap immersed in liquid nitrogen while the retantate was circulated back to feed solution in a cross flow mode. The compositions of feed solution and permeate was determined with a digital refractometer (Mettler Toledo RE-40D) having accuracy up to four digits. Total permeate flux J (kg m2 h1) was calculated by following equation: J¼
W A:t
where w, A and t are the total weight of permeate (kg), effective area of membrane (m2) and operating time (h). For the suitability of membrane for DMFC, the selectivity of membrane was calculate by following equation: Sp ¼
s PM
where PM is membrane permeability (cm2/s) and s is ionic conductivity of membrane (S/cm).
Salt removal efficiency by electrodialysis The performance of prepared composite membranes were checked in a locally fabricated PVC based ED unit. The details of the experiments are given in ESI section.
Results and discussion Structural and chemical characterization of GO, PEI-GO and different nanocomposite membranes The chemical characterization of GO and PEI-GO was performed by FTIR analysis as shown in Fig. 1. FTIR spectra of prepared GO shows OeH stretching at 3391 cm1, C]O stretching at 1719 cm1, 1620 cm1, CeO stretching at 1051 cm1 and a broad peak at 1211 cm1 is present because of epoxy group in prepared GO [28,35]. Spectra of f GO shows not only peaks at 2945 cm1, 2828 cm1, because of methylene group of PEI but two new peaks at 1650 cm1, 1568 cm1 due to NHeC]O stretching. It shows that the carboxyl group of GO react with amine group of PEI result in the formation of amide [31]. Fig. 1 and S-1 shows FTIR spectra of the different composite membranes, shows a sharp peak in the region 1114 cm-1 confirming the reaction of brominated PPO with the
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Scheme 1 e Schematic representation of quaternized PPO based f GO nanocomposite membrane.
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A
Fig. 1 e FTIR spectra of GO, PEI-GO and QP-5 nanocomposite membrane.
N-methylmorpholine. Another peak in the region 1600 cm-1 is assigned for stretching vibration of eC]Ce in the phenyl ring. A dominant peak in the region 1185 cm1 indicating the stretching of eCeOeCe bond in between the phenyl ring [33,38]. The absorption peaks at 1470 cm1 is arises due to symmetric and asymmetric stretching vibration of phenyl group [33]. 1H NMR spectra of base polymer, bromomethylated polymer and composite membranes was presented in Fig. 2 and S-2, shows the characteristics value of chemical shift at 6.4 ppm is attributed to the aromatic proton, while a sharp peak at d ¼ 4.35 ppm corresponds to the methylene protons of bromomethyl functional group. Another peak at 2.1 ppm is indicating the existence of methyl groups at PPO [39,40]. X-ray diffraction pattern of GO and f GO is shown in Fig. 3(A), sharp and intense peak at 10.8 corresponds to the formation of GO from graphite with an interlayer spacing of
Fig. 2 e 1H NMR spectra of PPO, Br-PPO and QP-5 nanocomposite membrane.
B
Fig. 3 e (A): XRD of GO and PEI modified GO. (B): XRD of different composite membranes.
8.03 A [36,37] with high degree of exfoliation and a disordered structure. In case of f GO long chains of PEI interact with GO sheets by covalent bonding results in broadening of peak and slight shifting at 2q ¼ 6.4 with an increased interlayer spacing of 10.7 A [31] this is due to the disturbance of PEI in GO. XRD analysis of QP, QP-1, QP-2 and QP-5 membranes is shown in Fig. 3(B), revels that the crystalline nature appear in composite membrane. The surface morphology of all prepared membranes are performed by AFM analysis through NT-MDT, Ntegra Aura software (Fig. 4). It describes a trend of uniform distribution of f GO in polymer matrix and increasing roughness of the membrane due to increase of f GO content. The calculated roughness of QP membrane is found to be 3.264 nm. On the other hand the roughness of the composite membranes increases and found to be 4.18, 5.79 and 12.06 nm for QP-1, QP-2 and QP-5 respectively. Roughness of composite membrane with a highest concentration of f GO is three times more than that of pristine membrane. Fig. 5 shows the TEM images of GO (A, A0 ) and f GO (B, B0 ) at low and high magnification. The
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Fig. 4 e AFM images of QP (A, Aˈ), QP-1 (B, Bˈ), QP-2 (C, Cˈ) and QP-5 (D, Dˈ) prepared nanocomposite membranes.
Fig. 5 e TEM images of GO (A,A′) and PEI-GO (B,B′) at different magnification.
overlapping and folding feature at some particular area expose the property of well dispersion in water. This fact leads us to a conclusion that grapheme oxide sheets try to agglomerate with each other by some interaction. The wrinkles in the nano
sheets are suggesting fully exfoliation when these are oxidized. The average size of the GO sheets is about 0.6 mm. The uniform dispersion of f GO into the membrane has been confirmed by TEM analysis and presented in Fig. S-3.
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Thermal and mechanical analysis Thermal and mechanical stability of prepared membranes were investigated with the help of TGA, DMA and UTM analysis. Thermal gravimetric analysis (TGA) of composite anion exchange membrane was performed from RT to 600 C at a heating rate of 10 C per minute under nitrogen environment. It is clear from Fig. 6, that there is four step weight loss in the membranes corresponding to desorption of water, thermal degradation of functional groups and thermal oxidation of polymer backbone. In first step all membranes shows a weight loss before 100 C attributed to evaporation of bound water present in samples. Second step weight loss occur at 230e280 C due to release of decomposed product of quaternary ammonium group. Third step weight loss at 300e330 C is likely ascribed to the release of residual quaternary ammonium group. Fourth and last weight loss observed around 400e500 C attributed to the degradation of polymer chains [41,42]. Composite membranes shows lower weight loss compared to QP membrane. This study shows that polymer is thermally very stable and with the addition of f GO thermal stability of polymer significantly increases. DMA analysis (Fig. 7) of different prepared membranes shows that as we increase f GO content in polymer matrix value of storage modulus as well as temperature of tan d increases significantly [28,43]. Value of storage modulus and T tan d for QP-5 membrane were 2787 MPa and 215 C which is highest among all the prepared membranes and least for QP membrane which is 1150 MPa and 199 C respectively. A uniform and good dispersion of f GO in polymer matrix lead to a good accession and stiffness of membranes. Mechanical analysis of prepared membranes was done by stress-strain curve of each membrane and presented in Fig. 8 and Table 1. It can be observed that, QP membrane with a minimum elastic modulus of 2.75 MPa and maximum elongation of 6.37. It show the flexibility of membrane as well as amorphous nature. But QP-5 membrane with a maximum elastic modulus of 6.50 MPa which is almost double from pristine QP membrane and minimum elongation of 3.58. It reveals that as well as the PEIGO content in the polymer matrix increases, there is a gradual change in amorphous to crystalline nature of membranes. It
Fig. 7 e DMA analysis of membranes with storage modulus and T tan d
Fig. 8 e Stress vs strain curves of different nanocomposite membranes.
Table 1 e Mechanical strength for different hybrid membranes. Membrane QP QP-1 QP-2 QP-5
Modulus (MPa)
Stress (MPa)
Strain (%)
2.75 4.00 5.51 6.50
10.31 18.47 18.92 23.80
6.37 5.54 4.74 3.58
was observed that f GO content reduce the flexibility and increase the crystallinity of fabricated membrane [43,44].
Electro-chemical properties
Fig. 6 e TGA profile for different prepared membranes.
Water uptake, ion exchange capacity, hydration number and dimensional stability are the important parameters of ion exchange membrane. Table 2, shows the values of water uptake, ion exchange capacity, hydration number and dimensional change values of different composite membranes. Water uptake and bound water gradually decreases as
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Table 2 e Ion exchange capacity (IEC), water content/ hydration number (l), water uptake (Wt), bound water (Wb) and free water (Wf), dimensional stability (Sd), ionic conductivity (F) for different hybrid membranes.
QP QP-1 QP-2 QP-5
IEC (meq/gm)
l
2.15 2.39 3.14 3.59
4.02 4.08 4.67 5.06
Wt Wb (%) (%) 27.7 23.1 20.1 19.6
0.58 0.27 0.25 0.18
Wf (%) 27.07 22.79 19.83 15.36
Sd F X 102 (%) (S/cm) 31.6 28.7 18.4 14.2
1.3 1.5 2.6 7.1
concentration of f GO increases because f GO act as a nano filler to the polymer matrix. f GO and PPO matrix leads to a network in which f GO serve as a joint and cross linkage by covalent bonding interactions. Covalent bonding interaction results in crosslinking of polymer chains so that the water retention capacity of composite membrane decreases, hence bound water within membrane decreases. Dimensional stability of polymer membranes increases i.e. 44.9% for QP-5, as f GO content increases this due that swelling of membrane decreases because of crosslinking of polymer chains and membrane form more stability. Least dimensional changes observed in QP-5 membrane with highest f GO content. The IEC value of composite membrane increases as the content of f GO increases in PPO matrix due to covalent bonding interaction in between f GO and Br-PPO which results in decrease in bound water in polymer matrix and approaches to 3.59 meq g1 for QP-5 membrane.
Ionic conductivity Ionic conductivity is also an important parameter of membrane for their application in different electro-membrane processes. Ionic conductivity of AEMs is highly dependent on ionic mobility and fixed charge concentration in the membrane matrix. Ion conducting channels inside the membrane provide a path for transportation of ion. Incorporation of fGO with the polymer matrix enhance conduction of ion via hydrophilic channels. Being hydrophilic, fGO also induces water trapping capacity for the membrane, which in results provide a route for ionic transportation. Water uptake also play an important role for obtain desirable results towards electro-membrane applications. Water uptake should not be as higher because it may create low stability. Thus incorporation of fGO also maintains stability of the membrane as well as also enhance ion transportation property. Increase in ionic conductivity, lowers the value of energy barrier for ion transport through polymer. Ionic conductivity of the composite membranes increases with increasing the f GO content into PPO matrix and reaches up to 7.1 102 S/cm for QP-5 membrane. The value of ionic conductivity for QP-5 membrane is around 5.5 times higher than the QP membrane (1.3 102 S/cm) the increment in the ionic conductivity may be due to the addition of more functional group in the form of f GO content as also confirmed by IEC values (Table 2).
Methanol permeability and selectivity for membranes High methanol permeation resistance is the basic requirement of ion exchange membrane for fuel cell. Methanol
Fig. 9 e Methanol permeability and selectivity of different nanocomposite membranes. permeability and selectivity of different composite membranes are shown in Fig. 9. The methanol permeation resistance for different composite membranes increases with f GO content in membrane matrix. The interaction between f GO and QPPO leads to low methanol permeability across the membrane. f GO prevent the movement of methanol across the composite membrane and acts as a barrier for connected channels. Total permeate flux for different composite membranes QP-1, QP-2 and QP-5 was found to be 7.18 109, 4.78 109 and 1.78 109 kg cm2 h1 respectively. The lowest value is found for QP-5 membrane among all composite membranes due to higher f GO content. Methanol permeability for different composite membranes is found to be 8.7 107, 6.64 107 and 2.27 107 cm2sec1 for QP-1, QP-2 and QP-5 membranes respectively. The successive reduction in methanol permeability of membranes is due to increment in strong interfacial adhesion between f GO and QPPO matrix. For the suitability for fuel cell selectivity of membranes is calculated which is directly proportional to the conductivity and inversely proportional to the permeability of membranes. Selectivity of composite membranes is found to
2
Current density (mA/cm2)
Membrane
QP QP-1 QP-2 QP-5 1
0
0
1
2
3
4
Potential (V/Cell pair)
5
6
Fig. 10 e Current density-potential curve for different membranes.
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Table 3 e DV, Di and Ilim values, power consumption (P) during salt removal for different hybrid membranes. Membrane QP QP-1 QP-2 QP-5
DV (V)
Di (mA cm2)
Ilim (mA cm2)
P/kWh kg1
2.84 3.85 3.54 5.07
0.98 0.66 1.02 1.47
1.00 0.64 0.83 1.05
1.10 0.98 0.93 0.79
be highest for QP-5 membrane which is 3.4 104 and lowest for QP-1 which is 1.8 104. The methanol permeability of QP-5 membrane are found to be one fourth to QP-1 with 1.9 times higher selectivity, low methanol permeability and high selectivity towards methanol crossover make QP-5 membrane suitable for fuel cell application.
Salt removal efficiency of composite membranes Current and potential characteristics of composite membranes was plotted by equilibrating the membrane in 0.1 M NaCl solution. I-V plot (Fig. 10) of composite membranes show
A
B
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typically three characteristic regions viz, ohmic, plateau and non-ohmic which reveals about the ion transport and concentration polarization across the membranes [28,45]. In ohmic region current density increases with an applied potential due to presence of a large number of ions, in plateau region current density remain almost constant and in nonohmic region ions get depleted causing water splitting with a higher current density. The value of DV, Di and Ilim calculated from Fig. 10 and presented in Table 3 [45]. It is clear from the table that the region from 2 V to 4 V is of our interest for electro-dialysis (ED) experiment. ED experiment was done at a constant applied potential of 2.0 V per cell pair using 0.1 M NaCl feed solution and 0.05 M Na2SO4 solution was recirculating as electrode wash to avoid electrode reaction and rinse the electrodes. In CC the change in conductivity of NaCl with time is represented in Fig. S-4. It is clear that conductivity linearly decrease with time. QP-5 membrane decreases concentration more efficiently than other prepared membranes, membrane highly facilitate the migration of ion from CC to DC. For QP-5 membranes concentration goes to 813 ppm but it is 1442 ppm for QP membrane which is higher in same time interval. The change in concentration of solutions with time presented in Fig. 11(A). Concentration of CC decreases with time and it increases for DC. Variation in current density with time presented in Fig. 11(B) initially current with time increases faster because concentration gradient in CC is very high and movement of ions from CC to DC is very high after passage of time concentration of both compartments almost same and there is no significant change in current takes place. Further when concentration of ions in DC increases hence due to it the movement of the ion start from DC to CC, but with a slower rate than previous and hence value of current goes down. The relative concentration of CC and DC with time shown in Fig. S-5 concentration of CC decrease with time simultaneously a significant increase in concentration of DC is observed. The accessibility of membranes was estimated in terms of power consumption during salt removal and depicted in Table 3. The power consumption during salt removal for QP-5 membrane found to be 0.79 kW h kg1 while for QP membrane it is 1.1 kW h kg1, about 30% lower power consumption is noted for QP-5 membrane.
Conclusion
Fig. 11 e (A) Concentration vs. time plot for different membranes during salt removal. (B) Current density vs. time plot for different membranes during salt removal.
Br-PPO based anion exchange membrane was synthesized successfully with greener method by incorporating PEI modified GO in membrane matrix. FTIR characterization of PEI-GO and prepared membrane confirms the successful synthesis of f GO and quarternized PPO. Prepared membranes show high thermal as well as mechanical stability. QP-5 membrane with a highest elastic modulus of 6.50 MPa found to be highly stable among all prepared membranes. QP-5 membrane shows the best performance among the membranes in terms of physico-chemical, electro-chemical and mechanical stability. The ion exchange capacity and ionic conductivity of QP-5 is found to be 3.59 meq g1 and 7.1 102 S/cm respectively which is 55% (IEC) and 5.5 times higher than that of QP membrane. The methanol permeation resistance and selectivity of all the composite membranes are
Please cite this article in press as: Yadav V, et al., Facile synthesis of Br-PPO/f GO based polymer electrolyte membranes for electrochemical applications, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.029
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calculated and QP-5 shows both higher permeation resistance and selectivity for methanol that is 2.27 107 cm2/s and 3.436 104 S cm3 s1. The membranes are also checked for their salt removal efficiency in terms of power consumption, which is about 0.79 kW h kg1 for QP-5 membrane. These result suggest that composite membrane is suitable for fuel cell and other electro-membrane applications.
Acknowledgments Author V. Kulshrestha is thankful to SERB, Department of Science and Technology, New Delhi, for providing financial support. Authors are also thankful to Analytical Discipline and Centralized Instrument facility, CSMCRI, Bhavnagar for instrumental support.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.08.029.
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