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Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
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Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell S. Elakkiya a, G. Arthanareeswaran a,*, K. Venkatesh a, Jihyang Kweon b a
Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirapalli 620015, Tamil Nadu, India b Department of Environmental Engineering, Konkuk University, Seoul, Republic of Korea
article info
abstract
Article history:
The proton exchange membrane (PEM) was synthesized using polyethersulfone (PES),
Received 29 December 2017
sulfonated poly (ether ether ketone) (SPEEK) and nanoparticles. The metal oxide nano-
Received in revised form
particles such as Fe3O4, TiO2 and MoO3 were added individually to the polymer blend (PES
10 April 2018
and SPEEK). The polymer composite membranes exhibit excellent features regarding water
Accepted 12 April 2018
uptake, ion exchange capacity and proton conductivity than the pristine PES membrane.
Available online xxx
Since the presence of sulfonic acid groups provides by added SPEEK and the unique properties of inorganic nanoparticles (Fe3O4, TiO2 and MoO3) helps to interconnect the ionic
Keywords:
domain by the absorption of more water molecules thereby enhance the conductivity
Polyethersulfone
value. The proton conductivity of PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/
Sulfonated poly (ether ether ketone)
SPEEK/MoO3 membranes were 0.22 104 S/cm, 5.18 104 S/cm, 3.57 104 S/cm,
Metal oxide nanoparticle
4.57 104 S/cm and 2.67 104 S/cm respectively. Even though the blending of PES with
Proton exchange membrane
SPEEK has reduced the conductivity value to a lesser extent, hydrophobic PES has vital role
Proton conductivity
in reducing the solvent uptake, swelling ratio and improves hydrolytic stability. Glass transition temperature (Tg) of the membranes were determined from DSC thermogram and it satisfies the operating condition of fuel cell system which guarantees the thermal stability of the membrane for fuel cell application. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction To eradicate the biggest energy crisis challenge, alternative source of energy is needed [1e3]. Polymer electrolyte membrane fuel cell (PEMFC) is an eco-friendly technology to generate electricity from an electrochemical process with less waste generation for the automotive and portable application
[4e8]. The most commonly used membrane for fuel cell application is Nafion which exhibits excellent chemical, mechanical property and proton conductivity. However, high expenditure, fuel loss and oxygen crossover are the drawbacks of Nafion [9]. Development of alternative material instead of Nafion is needed to overcome the drawbacks in fuel cell applications [10]. Nowadays, many polymers such as
* Corresponding author. E-mail address: [email protected] (G. Arthanareeswaran). https://doi.org/10.1016/j.ijhydene.2018.04.094 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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sulfonated poly (ether ether ketone) (SPEEK), sulfonated polyethersulfone (SPES), poly (arylene ethers) and polyimide (PI) are used as PEM. The high thermal, mechanical properties and oxidative resistance have attracted polyethersulfone (PES) as a PEM material for fuel cell system [11,12]. Poly (ether ether ketone) (PEEK) has been used after sulfonation, owing to good chemical, mechanical properties [13,14] and thermal stability in presence of an aromatic and non-fluorinated backbone [15]. SPEEK membrane shows adequate proton conductivity with excellent membrane properties [16e19]. Nowadays, a combination of polymer blends with thermoplastic noneconductive polymer and highly conductive sulfonated polymer has become popular [12]. Since the polymer blends have the advantage over the homo-polymer membranes by compensating the weakness of the individual polymers. The synergistic effect of hybrid organic/inorganic material suggests excellent ways to synthesize a novel membrane with good characteristics [20,21]. Inorganic nanoparticles such as Fe3O4, SiO2, TiO2, ZrO2, Al2O3, etc. have substantial effect on proton conductivity [22,23]. Ferric oxide nanoparticle (Fe3O4) nanoparticles have unique features such as conductive, magnetic, catalytic, eco-friendly and easy way of synthesis [24]. The incorporation of Fe3O4 into Nafion, PEEK, SPEEK and SPES has enhanced the proton conductivity by facilitating specific water hopping mechanism. Hygroscopic property and hydrophilic nature of titanium dioxide (TiO2) facilitates the path for transport of protons and improves the conductivity of SPEEK membrane [25]. The conductivity property of molybdenum oxide (MoO3) was found to be a good material in various applications and also exhibits excellent physicochemical properties for their use in energyerelated fields [26]. Many studies have been conducted on polymer composite membranes with the combination of Nafion/TiO2 (fluorinated polymer/inorganic material), PES/Fe3O4 (nonefluorinated thermoplastic polymer/inorganic material) and PES/SPEEK (nonefluorinated thermoplastic polymer/nonefluorinated sulfonated polymer). The combination of nonefluorinated thermoplastic polymer/nonefluorinated sulfonated polymer/ inorganic material (PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3) has not yet been studied as PEM for fuel cell application. Many researchers have found exact loading rate of inorganic nanoparticle in the polymer phase for fuel application by varying the nanoparticle loading rate from low to high. PES membrane with 5% of Fe3O4 has shown significant fuel cell features [27] and hence constant loading rate of 5% inorganic nanoparticle was selected for the study. The main purpose of this work is to compare the fuel cell performance of three different metal oxide nanoparticles (Fe3O4, TiO2 and MoO3) with the polymer blends (PES and SPEEK). The nanoparticle (Fe3O4, TiO2 and MoO3) were synthesized and is incorporated individually at a constant loading rate of 5% with the organic polymer blends (PES and SPEEK).
Victrex US Inc., USA. Ferrous chloride and ferrous ammonium sulfate were purchased from Loba Chemie Pvt. Ltd., India. Ethanol (99.9%) was obtained from the Hayman Specialty Products Limited, UK. Sodium hydroxide, sodium carbonate anhydrous, phenolphthalein (1%), hydrogen peroxide (30%) and 1emethyle2-pyrrolidone (NMP) were purchased from Merck Specialties Pvt. Ltd., India. Potassium chloride was obtained from Finar Limited, India. Sulfuric acid (98%) and nitric acid were procured from Thermo Fisher Scientific India Pvt. Ltd., India. Sigma Aldrich supplied TiO2 nanoparticle and ammonium molybdate. All these chemicals were used as received without any further purification.
Synthesis of ferric oxide nanoparticle The Iron salts (FeCl2 and FeCl3) and sodium hydroxide solution were prepared individually using deionized water. The prepared iron solutions were added to the sodium hydroxide solution and mixed properly. The ferric oxide nanoparticles were obtained by centrifugation method. The synthesized ferric oxide nanoparticles were rinsed with ethanol for three times and dried at 80 C.
Synthesis of molybdenum trioxide nanoparticle In the synthesis of molybdenum trioxide (MoO3) nanoparticle, ammonium molybdate solution was prepared by dissolving 0.2 M ammonium molybdate in 10 ml of distilled water. After 15 min of continuous stirring, 5 ml of concentrated HNO3 was added and maintained at 90 C. Centrifugation separated the MoO3 nanoparticle. Before drying in an oven at 70 C for 12 h, the MoO3 nanoparticles were rinsed with ethanol and distilled water.
Preparation of SPEEK from PEEK 20 g of dried PEEK was dissolved in a 400 ml of concentrated sulfuric acid. The reaction mixture was stirred for 5 h at 50 C. The procedure followed according to Arthanareeswaran et al. [14]. The degree of Sulfonation is determined using back titration method and was found as 35%.
Preparation of proton exchange membranes The composition of the casting solutions are mentioned in Table 1. Constant loading of 5% Fe3O4, TiO2 and MoO3 nanoparticles were dissolved in NMP solvent. Before addition of polymers (PES and SPEEK), the casting solutions are sonicated
Table 1 e Composition of the casting solutions. Membrane
Materials and methods Materials Polyethersulfone (PES Grade 3000P) was purchased from Solvay process India Ltd, India and PEEK was purchased from
PES SPEEK PES/SPEEK/Fe3O4 PES/SPEEK/TiO2 PES/SPEEK/MoO3
PES (g) 4.375 4.375 1.6626 1.6626 1.6626
SPEEK Nanoparticle NMP (g) (g) solvent (ml) e e 2.4937 2.4937 2.4937
e e 0.2187 (Fe3O4) 0.2187 (TiO2) 0.2187 (MoO3)
21.7 21.7 21.7 21.7 21.7
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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using ultrasonicator. The pristine PES and SPEEK membranes were prepared by dissolving PES, SPEEK polymer in NMP respectively at 70 C for 24 h. The membrane casting procedure was followed as reported in Mohtar et al. [17]. The thickness of the membrane was measured using a digital micrometer (Insize 0e25 mm digital outside micrometer 3109e25S) and the thickness is mentioned in Table 3.
Nanoparticles and characterization of the membranes The morphologies and characteristics of the synthesized nanoparticles were analyzed using Transmission electron microscopy (TEM) (Model: JEM 2100, Make: Joel, USA). FTIR analysis was recorded for the membranes from 4000 to 400 cm1 at room temperature in an ATR mode using Perkin Elmer/ Spectrum 2, USA. The influence of the polymer and nanoparticle on the membrane surface were analyzed using Field Emission Scanning electron microscope (FESEM) (Model: Sigma, Make: Carl Zeiss, USA). Before the analysis, the dried membrane samples were coated with thin layer of gold by sputter coating. To analysis the crystalline nature of the material, XRD patterns were recorded between 5 and 80 at 2q angle using Perkin Elmer 1621, USA.
Thermal property Differential scanning calorimetry (DSC) analyses for the membranes were performed using DSC 6000-Perkin-Elmer instrument, USA. The thermogram was recorded at a heating rate of 10 C/min with temperature range from 20 to 350 C under nitrogen atmosphere.
Water uptake and swelling ratio Water uptake was noted by measuring the wet weight and dry weight of the membrane. The water uptake content was calculated using Eq (1),
Water Uptake ð%Þ ¼
ðWet weight Dry weightÞ 100 Dry weight
(1)
The swelling characteristic of the membranes were determined by measuring the changes in the membrane geometrical area under hydrated condition [15]. The swelling ratio of the membranes were determined using Eq (2),
Swelling ratioð%Þ ¼
Ion Exchange Capacity (IEC) The dry weight of the membrane was weighed and then soaked in 0.5 M H2SO4 for 24 h to extract the protons from the membrane. The membranes were rinsed with water to remove excess acid and soaked in potassium chloride solution for 24 h to swap the protons to Kþ ions. The solution containing extracted protons were titrated with the 0.01 N concentration of sodium carbonate solution using phenolphthalein as the indicator. The ion exchange capacity was determined by using Eq (3), meq Volume Normality ¼ Ion exchange capacity g Dry weight of membrane ðgÞ (3)
Oxidative stability Pre-weighed membrane was immersed in Fenton's reagent (3% H2O2 and 4 ppm of ferrous ammonium sulfate) at 68 C. The oxidative/chemical stability of the membranes is determined using the procedure reported in Zhang et al. [42].
Hydrolytic stability The hydrolytic stability was determined by immersing membrane in deionized water at 100 C for 2 days. To find the stability under the test condition, the changes in the native appearance of the membrane was recorded.
Proton conductivity analysis The proton conductivity of the membranes was analyzed using potentiostat (Model: VMP3, Make: Bio-logic, France) by two probe impendence method. The membranes were treated with 0.5 M of sulfuric acid for overnight and rinsed with deionized water. Before the conductivity analysis, the membrane was hydrated with deionized water for 24 h. The membrane was placed in between the stainless steel electrodes of 1 cm diameter. The impendence spectrum was recorded between 200 KHz and 100 mHz maintaining the amplitude at 10 mV. The impendence spectrum was connected with EC lab software, and the resistance was recorded from the Nyquist plot. The proton conductivity of the membrane was determined using Eq (4), Proton Conductivity of the membraneðS=cmÞ ¼
ðArea of Wet membrane Area of dry membraneÞ 100 Area of dry membrane
Solvent uptake The methanol uptake of the membrane was determined by soaking in methanol at 60 C for 2 h. The difference in the percentage weight gain and the original weight of the membrane was considered as solvent/methanol uptake [28].
L RA
(4)
(2)
where L is the thickness of the wet membrane in cm. R is the resistance obtained from the Nyquist plot in Ohm and A is the area of the membrane in cm2.
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Results and discussion
Table 2 e Lattice space (d-spacing) and lattice length of the nanoparticles.
Transmission electron microscope (TEM)
Nanoparticle
Fig. 1 shows TEM images and SAED pattern of the nanoparticles. The region created by Fe3O4 appears as the dark image. Fig. 1(a) represents the hydrophilic groups as the darker region, and the hydrophobic groups as the lighter region [27,29]. The Fe3O4 particles have black spot forms, and their sizes are different from 10 to 30 nm. TiO2 particles are all well distinguished with a spherical shape, and the size ranges from 10 to 40 nm. MoO3 nanoparticles show hexagonal rod shape with lattice fringes of crystallographic planes. The diameter of MoO3 varies from 100 to 300 nm, and the length is of few micrometers [30]. Table 2 shows the lattice space (d-spacing) and lattice length of the nanoparticles determined from SAED pattern.
Fe3O4 Fe3O4 TiO2 TiO2 MoO3
2R (1/nm)
R (1/nm)
lattice space d ¼ 1/R (nm)
Lattice length (cm)
3.52 6.20 4.89 7.30 5.58
1.76 3.1 2.445 3.65 2.79
0.56 0.323 0.408 0.27 0.36
1.41 2.45 1.94 2.93 2.19
Fourier transform infrared spectroscopy (FTIR) Fig. 2 shows the FTIR spectra of pristine and polymer composite membranes. For PES membrane, the peak around 1090 cm1 corresponds to symmetric and asymmetric stretching of the O]S]O. The peak at 705 cm1 corresponds
Fig. 1 e TEM and SAED pattern of nanoparticles (a) TEM of Fe3O4, (b) TEM of TiO2, and (c) TEM of MoO3,(d) SAED of Fe3O4, (e) SAED of TiO2, and (f) SAED of MoO3. Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Table 3 e Thickness, ion exchange capacity, chemical degradation and proton conductivity of membranes. Membrane PES SPEEK PES/SPEEK/Fe3O4 PES/SPEEK/TiO2 PES/SPEEK/MoO3
Thickness (mm)
Ion exchange capacity (meq/g)
Chemical degradation (%)
310 70 100 150 110
0.23 0.27 0.33 0.29 0.31
6.87 18.64 19.02 15.94 16.58
to the SeO group. The characteristic peaks of SPEEK appeared at 3410, 1225, 1080, 1023 and 761 cm1 represents the OeH group stretching, asymmetric stretching of O]S]O, the symmetric stretching vibration of O¼ S]O, stretching of S]O, and stretching of SeO of the sulfonic acid group respectively [31]. The polymer composite membranes show the peak of PES, SPEEK and nanoparticle. The characteristic peaks of PES/ SPEEK/Fe3O4 membrane at 1476, 1351, 1158 and 761 cm1 confirm the presence of Fe3O4 nanoparticle. The PES/SPEEK/ TiO2 membrane show characteristic peak at 549 cm1 corresponds to TieO stretching. The band at 1023 and 1080 cm1 corresponds to the association of TieO with the sulfonic acid group of SPEEK which reveals the strong interaction between the polymer and TiO2 nanoparticle [32]. The PES/SPEEK/MoO3 membrane shows the peak at 932 cm1 corresponds to Mo]O stretching of hexagonal phase which represents the characteristics bond between metal and oxygen [30]. The recognized broad peak of SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2, PES/SPEEK/MoO3 membranes at 3410, 3431, 3393 and 3398 cm1 corresponds to OeH bonding. The FTIR spectra of PES membrane exhibit a sharp peak at 3403 cm1 corresponds to the absence of OeH bonding. The FTIR spectra of polymer composite membranes confirmed the presence of sulfonic acid groups.
Field Emission Scanning Electron Microscopy (FESEM) Fig. 3 shows the FESEM images of pristine PES, pristine SPEEK and polymer composite membranes. The membranes were smooth and denser. It was observed that the nanoparticles
Proton Conductivity (S/cm) 0.22 5.18 3.57 4.57 2.67
104 104 104 104 104
were distributed evenly in the polymer composite membrane without any agglomeration. FESEM images of pristine SPEEK and polymer composite membranes (PES/SPEEK/TiO2 and PES/ SPEEK/MoO3) were appeared to be denser (nonporous) with uniform morphologies. The denser membrane enhances the movement of protons and retards substrate loss. In 10.0 KX magnification, the pristine PES and PES/SPEEK/Fe3O4 membrane exhibits porous and void structure. In the polymer composite membrane, interconnected microstructure was formed by the thermoplastic polymer (PES) and the conductive polymer (SPEEK). The interactions among PES, sulfonic acid groups of SPEEK and crystalline nature of nanoparticles change the morphology of the membrane [29].
XRD pattern Fig. 4 shows the XRD pattern of pristine and polymer composite membranes. The XRD pattern of pristine PES membrane showing the peak at 2q ¼ 18 [33]. For SPEEK membrane, the broad signal represents the amorphous nature [34]. The XRD pattern of PES/SPEEK/Fe3O4 membrane showing peaks at 2q ¼ 29 , 35 , 57 and 63 confirmed the presence of Fe3O4 nanoparticle within the membrane [35]. The XRD pattern of PES/SPEEK/TiO2 membrane show the characteristic peaks at 2q ¼ 19 , 25 , 28 , 32 , 47 and 54 represents the crystalline nature of TiO2. The XRD pattern of PES/SPEEK/MoO3 membrane confirmed the presence of MoO3 within the membrane. It is well known that the crystalline peak appears at lower 2q values. In the polymer composite membrane, the crystalline nature is reduced by SPEEK and there is no sharp peak at low 2q which indicates the interactions between polymers and nanoparticle. On addition of nanoparticles to the polymer blend (PES and SPEEK) reduces the amorphous nature and the crystalline peaks are obtained at higher 2q values. The inorganic nanoparticle within the polymer matrix reduces the amorphous nature of the polymer [36].
Glass transition temperature (Tg) and swelling ratio
Fig. 2 e FTIR spectra of PES, SPEEK, PES/SPEEK/Fe3O4, PES/ SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
Fig. 5 illustrates the DSC thermogram of membranes. A single Tg value is obtained for the polymer composite membrane which indicates the miscibility of the polymer blend systems (PES and SPEEK). Glass transition temperature (Tg) from the DSC thermograms for PES, SPEEK, PES/ SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membranes are 205, 194, 210, 217 and 209 C respectively. The blending of sulfonated polymer (SPEEK) with the thermoplastic polymer (PES) forms the interconnected structure by intermolecular interactions. The specific interactions between sulfonic acid groups in the polymeric chain and
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Fig. 3 e FESEM images of synthesized membrane (a) PES, (b) SPEEK, (c) PES/SPEEK/Fe3O4, (d) PES/SPEEK/TiO2, and (e) PES/ SPEEK/MoO3.
nanoparticles decrease the internal rotation and have a substantial impact on Tg. Here, the results revealed that the polymer composite membranes are satisfied the working temperature of the fuel cell system and also give assurance for thermal stability of membranes. In common, to avoid thermal degradation of membranes highest Tg is required for fuel cell operation [37]. The values of swelling ratio are 5.56, 16.28, 11.11, 11.25 and 11.28 for PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membrane respectively. DSC inference confirmed the miscibility of the polymer blend systems (PES and SPEEK) and there is an intermolecular interaction
between PES and SPEEK. Thus, the interactions between the hydrophobic PES and hydrophilic SPEEK reduces swelling ratio of the polymer composite membranes [38].
Water uptake content Fig. 6 shows the water uptake of pristine and polymer composite membranes. The ability of the membrane to absorb water is an important parameter to evaluate its performance in the fuel cell system. The PES/SPEEK/Fe3O4 membrane shows highest water uptake value than all the membranes. The addition of SPEEK to the PES has modified the water uptake
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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low when compared with the pristine SPEEK membrane. Since the miscibility of hydrophobic PES with hydrophilic SPEEK has a substantial impact on reducing the methanol uptake [28]. SPEEK membrane is soluble in pure methanol due to the presence of sulfonic acid groups thus it shows signs of changes in the original appearance with high solvent uptake.
Ion exchange capacity
Fig. 4 e XRD pattern of PES, SPEEK, PES/SPEEK/Fe3O4, PES/ SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
properties. The presence of sulfonic acid groups in the membrane provide by added SPEEK creates a center of attention for the movement of the water molecule and accommodates more water molecules within the polymeric membrane structure [39,40]. Pristine PES membrane exhibit lowest water uptake represents the absence of sulfonic acid group.
Solvent uptake The solvent uptake was determined after soaking the membranes in pure methanol at particular testing condition. The solvent uptake value for SPEEK membrane is 48.42% and the polymer composite membranes are 20%. The results revealed that the pristine PES membrane and polymer composite membranes exhibit any significant changes in the original appearance under the testing condition of pure methanol at 60 C for 2 h. The solvent uptakes of these membranes are also
The number of exchangeable ions present in the membranes was analyzed by titration method, and the results are mentioned in Table 3. The highest IEC value of PES/SPEEK/ Fe3O4 membrane is 0.33 meq$g1. Due to the high water uptake capacity and the presence of sulfonic acid groups provide by added SPEEK. The water uptake has positive effects on ion exchange capacity by improving the ability of the membrane to exchange ions. The IEC value increases with the addition of nanoparticle in the polymer blend (PES and SPEEK) indicate that the nanoparticles do not block the polymer matrix for exchanging ions. Polymer composite membrane exhibit higher IEC than pristine SPEEK membrane because the protons inside the polymeric matrix are also involved in IEC through water hopping mechanism. The lowest IEC value is 0.23 meq$g1 with PES membrane, and it is due to the absence of sulfonic acid group in the polymeric structure. The other factor responsible for IEC is the surface morphology of the membrane. FESEM images represent the interaction between polymer blends and nanoparticle [41]. Hence, the polymer composite membrane exhibit high IEC than the pristine PES.
Oxidative stability The chemical degradation of the polymer membrane was analyzed using Fenton test, and the results are shown in Table 3. All the membranes were starting to degrade after 24 h in the Fenton's reagent. The decomposition of H2O2 catalyzed by iron generates hydroxy (HO) and hydroperoxy (HOO) free radicals. These free radicals are potent to cause the degradation of the membrane [42]. The performances of SPEEK and polymer composite membranes in Fenton test were found to be unsatisfactory after 24 h. Since the membrane is exposed to impractical conditions, the result does not reveal that the membrane is not suitable for PEMFC. The oxidative stability determined by Fenton test method has been widely used to evaluate the performance of the membrane.
Hydrolytic stability
Fig. 5 e DSC thermogram of PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
The polymer composite membranes exhibited no significant appearance changes and withstood the testing condition. This signifies that the surface of the membrane holds the nanoparticles even after exposing at 100 C for 48 h. The rigid structure of the polymer composite membrane provide by added PES with SPEEK has improved the hydrolytic stability by specific interaction which was confirmed by DSC. The membranes exhibited excellent hydrolytic stability without any change in the natural appearance and is shown in Supplementary Fig. S1.
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Fig. 6 e Water uptake content of PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
Proton conductivity of the membranes The proton conductivity of the polymer membranes was analyzed, and the results are mentioned in Table 3. Fig. 7 shows the image of proton conductivity analysis setup. The low conductivity value is 0.22 104 S/cm with pristine PES membrane reveals it has minimum proton conduction [43]. The absence of sulfonic acid groups in PES reduces the ability of the membrane to hold more water molecules. The highest conductivity value is 5.18 104 S/cm with SPEEK membrane. PES/SPEEK/TiO2 membrane showed good conductivity nearly to SPEEK membrane, and its value is 4.57 104 S/cm. The hygroscopic nature of TiO2, the presence of sulfonic acid
groups provide by added SPEEK and the formation of hydrogen bonding between water molecules and hydroxyl groups of TiO2 upon hydration have increased the proton conductivity of PES/SPEEK/TiO2 membrane. The movement of water molecules interconnected the sulfonic ionic domain clusters in the membrane, and it enhances the transport of protons through the membrane by water hopping mechanism [44]. PES/SPEEK/Fe3O4 and PES/SPEEK/MoO3 membranes showed virtually of better conductivity than the pristine PES membrane. In the polymer composite membrane, the specific intermolecular interactions between the PES and SPEEK have diminished the effect of sulfonation pair thereby decreasing the ability of the membrane to absorb more water molecules
Fig. 7 e Proton conductivity analysis setup. Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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to enhance proton transport [12]. The membrane thickness also plays a vital role in the measurement of proton conductivity. The thickness of the polymer composite membranes was found to be less and exhibit excellent proton conductivity than the pristine PES membrane.
[3]
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Conclusion The Polymer composite membrane with different nanoparticles were synthesized using phase inversion technique. PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2, PES/SPEEK/ MoO3 membranes were characterized by FTIR, XRD and FESEM. The characterization studies of the polymer composite membrane have confirmed the presence of nanoparticle within the polymer matrix. A single Tg was obtained for the polymer composite membranes which establish the miscibility of PES and SPEEK. Interactions between the hydrophobic rigid PES and hydrophilic sulfonated SPEEK have a vital role in the reduction of solvent uptake, swelling ratio and enhancement of hydrolytic stability. The water uptake and IEC value of the polymer composite membranes were satisfactory. The proton conductivity value of PES/SPEEK/TiO2 membrane is 4.57 104 S/cm which is higher than the pristine PES membrane. Since the presence of sulfonic acid groups provide by SPEEK and the hydrophilic nature of TiO2 to oxidize water molecule helps to imbibe more water molecules. Thereby interconnecting the ionic domain clusters present within the polymeric membrane and promote the proton conductivity value. The PES/SPEEK/Fe2O3, PES/SPEEK/MoO3 membranes also shown significant proton conductivity value of 3.57 104 and 2.67 104 S/cm respectively. Thus features of the polymer composite membranes were excellent than the pristine PES membrane and suitable to be used as PEM for fuel cell application.
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Acknowledgment Dr. G. Arthanareeswaran thank Konkuk University, South Korea and also acknowledges the Korean Federation of Science and Technology Society (KOFST) for possible support under Brainpool program (Reference Number 161S-5-3-1561).
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Appendix A. Supplementary data Supplementary data related to this article can be found at doi:10.1016/j.ijhydene.2018.04.094.
[16]
[17]
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[20] Li S, Zhou Z, Liu M, Li W, Ukai J, Hase K, et al. Synthesis and properties of imidazole-grafted hybrid inorganic-organic polymer membranes. Electrochim Acta 2006;51:1351e8. [21] Guhan S, Sangeetha D. Evaluation of sulfonated poly(ether ether ketone) silicotungstic acid composite membranes for fuel cell applications. Int J Polym Mater 2009;58:87e98. [22] Lakshminarayana G, Nogami M, Kityk IV. Synthesis and characterization of anhydrous proton conducting inorganicorganic composite membranes for medium temperature proton exchange membrane fuel cells (PEMFCs). Energy 2010;35:5260e8. [23] Zhang G, Zhou Z. Organic/inorganic composite membranes for application in DMFC. J Membr Sci 2005;261:107e13. [24] Chen J, Wang F, Huang K, Liu Y, Liu S. Preparation of Fe3O4 nanoparticles with adjustable morphology. J Alloys Compd 2009;475:898e902. [25] Jalani NH, Dunn K, Datta R. Synthesis and characterization of Nafion®-MO2 (M ¼ Zr, Si, Ti) nanocomposite membranes for higher temperature PEM fuel cells. Electrochim Acta 2005;51:553e60. [26] Manivel A, Lee GJ, Chen CY, Chen JH, Ma SH, Horng TL, et al. Synthesis of MoO3 nanoparticles for azo dye degradation by catalytic ozonation. Mater Res Bull 2015;62:184e91. [27] Prabhu NV, 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;243:564e71. [28] Wu HL, Ma CCM, Liu FY, Chen CY, Lee SJ, Chiang CL. Preparation and characterization of poly(ether sulfone)/ sulfonated poly(ether ether ketone) blend membranes. Eur Polym J 2006;42:1688e95. [29] Lim SS, Daud WRW, Jahim JM, Ghasemi M, Chong PS, Ismail M. Sulfonated poly (ether ether ketone)/poly(ether sulfone) composite membranes as an alternative proton exchange membrane in microbial fuel cells. Int J Hydrogen Energy 2012;37:11409e24. [30] Chithambararaj A, Bose AC. Hydrothermal synthesis of hexagonal and orthorhombic MoO3 nanoparticles. J Alloys Compd 2011;509:8105e10. [31] Leong JX, Daud WRW, Ghasemi M, Ahmad A, Ismail M, BenLiew K. Composite membrane containing graphene oxide in sulfonated polyether ether ketone in microbial fuel cell applications. Int J Hydrogen Energy 2015;40:11604e14. [32] Zhengbang W, Tang H, Mu P. Self-assembly of durable Nafion/TiO2 nanowire electrolyte membranes for elevatedtemperature PEM fuel cells. J Membr Sci 2011;369:250e7. [33] Gohari JR, Halakoo E, Lau WJ, Kassim MA, Matsuura T, Ismail AF. Novel polyethersulfone (PES)/hydrous manganese
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Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell S. Elakkiya a, G. Arthanareeswaran a,*, K. Venkatesh a, Jihyang Kweon b a
Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirapalli 620015, Tamil Nadu, India b Department of Environmental Engineering, Konkuk University, Seoul, Republic of Korea
article info
abstract
Article history:
The proton exchange membrane (PEM) was synthesized using polyethersulfone (PES),
Received 29 December 2017
sulfonated poly (ether ether ketone) (SPEEK) and nanoparticles. The metal oxide nano-
Received in revised form
particles such as Fe3O4, TiO2 and MoO3 were added individually to the polymer blend (PES
10 April 2018
and SPEEK). The polymer composite membranes exhibit excellent features regarding water
Accepted 12 April 2018
uptake, ion exchange capacity and proton conductivity than the pristine PES membrane.
Available online xxx
Since the presence of sulfonic acid groups provides by added SPEEK and the unique properties of inorganic nanoparticles (Fe3O4, TiO2 and MoO3) helps to interconnect the ionic
Keywords:
domain by the absorption of more water molecules thereby enhance the conductivity
Polyethersulfone
value. The proton conductivity of PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/
Sulfonated poly (ether ether ketone)
SPEEK/MoO3 membranes were 0.22 104 S/cm, 5.18 104 S/cm, 3.57 104 S/cm,
Metal oxide nanoparticle
4.57 104 S/cm and 2.67 104 S/cm respectively. Even though the blending of PES with
Proton exchange membrane
SPEEK has reduced the conductivity value to a lesser extent, hydrophobic PES has vital role
Proton conductivity
in reducing the solvent uptake, swelling ratio and improves hydrolytic stability. Glass transition temperature (Tg) of the membranes were determined from DSC thermogram and it satisfies the operating condition of fuel cell system which guarantees the thermal stability of the membrane for fuel cell application. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction To eradicate the biggest energy crisis challenge, alternative source of energy is needed [1e3]. Polymer electrolyte membrane fuel cell (PEMFC) is an eco-friendly technology to generate electricity from an electrochemical process with less waste generation for the automotive and portable application
[4e8]. The most commonly used membrane for fuel cell application is Nafion which exhibits excellent chemical, mechanical property and proton conductivity. However, high expenditure, fuel loss and oxygen crossover are the drawbacks of Nafion [9]. Development of alternative material instead of Nafion is needed to overcome the drawbacks in fuel cell applications [10]. Nowadays, many polymers such as
* Corresponding author. E-mail address: [email protected] (G. Arthanareeswaran). https://doi.org/10.1016/j.ijhydene.2018.04.094 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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sulfonated poly (ether ether ketone) (SPEEK), sulfonated polyethersulfone (SPES), poly (arylene ethers) and polyimide (PI) are used as PEM. The high thermal, mechanical properties and oxidative resistance have attracted polyethersulfone (PES) as a PEM material for fuel cell system [11,12]. Poly (ether ether ketone) (PEEK) has been used after sulfonation, owing to good chemical, mechanical properties [13,14] and thermal stability in presence of an aromatic and non-fluorinated backbone [15]. SPEEK membrane shows adequate proton conductivity with excellent membrane properties [16e19]. Nowadays, a combination of polymer blends with thermoplastic noneconductive polymer and highly conductive sulfonated polymer has become popular [12]. Since the polymer blends have the advantage over the homo-polymer membranes by compensating the weakness of the individual polymers. The synergistic effect of hybrid organic/inorganic material suggests excellent ways to synthesize a novel membrane with good characteristics [20,21]. Inorganic nanoparticles such as Fe3O4, SiO2, TiO2, ZrO2, Al2O3, etc. have substantial effect on proton conductivity [22,23]. Ferric oxide nanoparticle (Fe3O4) nanoparticles have unique features such as conductive, magnetic, catalytic, eco-friendly and easy way of synthesis [24]. The incorporation of Fe3O4 into Nafion, PEEK, SPEEK and SPES has enhanced the proton conductivity by facilitating specific water hopping mechanism. Hygroscopic property and hydrophilic nature of titanium dioxide (TiO2) facilitates the path for transport of protons and improves the conductivity of SPEEK membrane [25]. The conductivity property of molybdenum oxide (MoO3) was found to be a good material in various applications and also exhibits excellent physicochemical properties for their use in energyerelated fields [26]. Many studies have been conducted on polymer composite membranes with the combination of Nafion/TiO2 (fluorinated polymer/inorganic material), PES/Fe3O4 (nonefluorinated thermoplastic polymer/inorganic material) and PES/SPEEK (nonefluorinated thermoplastic polymer/nonefluorinated sulfonated polymer). The combination of nonefluorinated thermoplastic polymer/nonefluorinated sulfonated polymer/ inorganic material (PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3) has not yet been studied as PEM for fuel cell application. Many researchers have found exact loading rate of inorganic nanoparticle in the polymer phase for fuel application by varying the nanoparticle loading rate from low to high. PES membrane with 5% of Fe3O4 has shown significant fuel cell features [27] and hence constant loading rate of 5% inorganic nanoparticle was selected for the study. The main purpose of this work is to compare the fuel cell performance of three different metal oxide nanoparticles (Fe3O4, TiO2 and MoO3) with the polymer blends (PES and SPEEK). The nanoparticle (Fe3O4, TiO2 and MoO3) were synthesized and is incorporated individually at a constant loading rate of 5% with the organic polymer blends (PES and SPEEK).
Victrex US Inc., USA. Ferrous chloride and ferrous ammonium sulfate were purchased from Loba Chemie Pvt. Ltd., India. Ethanol (99.9%) was obtained from the Hayman Specialty Products Limited, UK. Sodium hydroxide, sodium carbonate anhydrous, phenolphthalein (1%), hydrogen peroxide (30%) and 1emethyle2-pyrrolidone (NMP) were purchased from Merck Specialties Pvt. Ltd., India. Potassium chloride was obtained from Finar Limited, India. Sulfuric acid (98%) and nitric acid were procured from Thermo Fisher Scientific India Pvt. Ltd., India. Sigma Aldrich supplied TiO2 nanoparticle and ammonium molybdate. All these chemicals were used as received without any further purification.
Synthesis of ferric oxide nanoparticle The Iron salts (FeCl2 and FeCl3) and sodium hydroxide solution were prepared individually using deionized water. The prepared iron solutions were added to the sodium hydroxide solution and mixed properly. The ferric oxide nanoparticles were obtained by centrifugation method. The synthesized ferric oxide nanoparticles were rinsed with ethanol for three times and dried at 80 C.
Synthesis of molybdenum trioxide nanoparticle In the synthesis of molybdenum trioxide (MoO3) nanoparticle, ammonium molybdate solution was prepared by dissolving 0.2 M ammonium molybdate in 10 ml of distilled water. After 15 min of continuous stirring, 5 ml of concentrated HNO3 was added and maintained at 90 C. Centrifugation separated the MoO3 nanoparticle. Before drying in an oven at 70 C for 12 h, the MoO3 nanoparticles were rinsed with ethanol and distilled water.
Preparation of SPEEK from PEEK 20 g of dried PEEK was dissolved in a 400 ml of concentrated sulfuric acid. The reaction mixture was stirred for 5 h at 50 C. The procedure followed according to Arthanareeswaran et al. [14]. The degree of Sulfonation is determined using back titration method and was found as 35%.
Preparation of proton exchange membranes The composition of the casting solutions are mentioned in Table 1. Constant loading of 5% Fe3O4, TiO2 and MoO3 nanoparticles were dissolved in NMP solvent. Before addition of polymers (PES and SPEEK), the casting solutions are sonicated
Table 1 e Composition of the casting solutions. Membrane
Materials and methods Materials Polyethersulfone (PES Grade 3000P) was purchased from Solvay process India Ltd, India and PEEK was purchased from
PES SPEEK PES/SPEEK/Fe3O4 PES/SPEEK/TiO2 PES/SPEEK/MoO3
PES (g) 4.375 4.375 1.6626 1.6626 1.6626
SPEEK Nanoparticle NMP (g) (g) solvent (ml) e e 2.4937 2.4937 2.4937
e e 0.2187 (Fe3O4) 0.2187 (TiO2) 0.2187 (MoO3)
21.7 21.7 21.7 21.7 21.7
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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using ultrasonicator. The pristine PES and SPEEK membranes were prepared by dissolving PES, SPEEK polymer in NMP respectively at 70 C for 24 h. The membrane casting procedure was followed as reported in Mohtar et al. [17]. The thickness of the membrane was measured using a digital micrometer (Insize 0e25 mm digital outside micrometer 3109e25S) and the thickness is mentioned in Table 3.
Nanoparticles and characterization of the membranes The morphologies and characteristics of the synthesized nanoparticles were analyzed using Transmission electron microscopy (TEM) (Model: JEM 2100, Make: Joel, USA). FTIR analysis was recorded for the membranes from 4000 to 400 cm1 at room temperature in an ATR mode using Perkin Elmer/ Spectrum 2, USA. The influence of the polymer and nanoparticle on the membrane surface were analyzed using Field Emission Scanning electron microscope (FESEM) (Model: Sigma, Make: Carl Zeiss, USA). Before the analysis, the dried membrane samples were coated with thin layer of gold by sputter coating. To analysis the crystalline nature of the material, XRD patterns were recorded between 5 and 80 at 2q angle using Perkin Elmer 1621, USA.
Thermal property Differential scanning calorimetry (DSC) analyses for the membranes were performed using DSC 6000-Perkin-Elmer instrument, USA. The thermogram was recorded at a heating rate of 10 C/min with temperature range from 20 to 350 C under nitrogen atmosphere.
Water uptake and swelling ratio Water uptake was noted by measuring the wet weight and dry weight of the membrane. The water uptake content was calculated using Eq (1),
Water Uptake ð%Þ ¼
ðWet weight Dry weightÞ 100 Dry weight
(1)
The swelling characteristic of the membranes were determined by measuring the changes in the membrane geometrical area under hydrated condition [15]. The swelling ratio of the membranes were determined using Eq (2),
Swelling ratioð%Þ ¼
Ion Exchange Capacity (IEC) The dry weight of the membrane was weighed and then soaked in 0.5 M H2SO4 for 24 h to extract the protons from the membrane. The membranes were rinsed with water to remove excess acid and soaked in potassium chloride solution for 24 h to swap the protons to Kþ ions. The solution containing extracted protons were titrated with the 0.01 N concentration of sodium carbonate solution using phenolphthalein as the indicator. The ion exchange capacity was determined by using Eq (3), meq Volume Normality ¼ Ion exchange capacity g Dry weight of membrane ðgÞ (3)
Oxidative stability Pre-weighed membrane was immersed in Fenton's reagent (3% H2O2 and 4 ppm of ferrous ammonium sulfate) at 68 C. The oxidative/chemical stability of the membranes is determined using the procedure reported in Zhang et al. [42].
Hydrolytic stability The hydrolytic stability was determined by immersing membrane in deionized water at 100 C for 2 days. To find the stability under the test condition, the changes in the native appearance of the membrane was recorded.
Proton conductivity analysis The proton conductivity of the membranes was analyzed using potentiostat (Model: VMP3, Make: Bio-logic, France) by two probe impendence method. The membranes were treated with 0.5 M of sulfuric acid for overnight and rinsed with deionized water. Before the conductivity analysis, the membrane was hydrated with deionized water for 24 h. The membrane was placed in between the stainless steel electrodes of 1 cm diameter. The impendence spectrum was recorded between 200 KHz and 100 mHz maintaining the amplitude at 10 mV. The impendence spectrum was connected with EC lab software, and the resistance was recorded from the Nyquist plot. The proton conductivity of the membrane was determined using Eq (4), Proton Conductivity of the membraneðS=cmÞ ¼
ðArea of Wet membrane Area of dry membraneÞ 100 Area of dry membrane
Solvent uptake The methanol uptake of the membrane was determined by soaking in methanol at 60 C for 2 h. The difference in the percentage weight gain and the original weight of the membrane was considered as solvent/methanol uptake [28].
L RA
(4)
(2)
where L is the thickness of the wet membrane in cm. R is the resistance obtained from the Nyquist plot in Ohm and A is the area of the membrane in cm2.
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Results and discussion
Table 2 e Lattice space (d-spacing) and lattice length of the nanoparticles.
Transmission electron microscope (TEM)
Nanoparticle
Fig. 1 shows TEM images and SAED pattern of the nanoparticles. The region created by Fe3O4 appears as the dark image. Fig. 1(a) represents the hydrophilic groups as the darker region, and the hydrophobic groups as the lighter region [27,29]. The Fe3O4 particles have black spot forms, and their sizes are different from 10 to 30 nm. TiO2 particles are all well distinguished with a spherical shape, and the size ranges from 10 to 40 nm. MoO3 nanoparticles show hexagonal rod shape with lattice fringes of crystallographic planes. The diameter of MoO3 varies from 100 to 300 nm, and the length is of few micrometers [30]. Table 2 shows the lattice space (d-spacing) and lattice length of the nanoparticles determined from SAED pattern.
Fe3O4 Fe3O4 TiO2 TiO2 MoO3
2R (1/nm)
R (1/nm)
lattice space d ¼ 1/R (nm)
Lattice length (cm)
3.52 6.20 4.89 7.30 5.58
1.76 3.1 2.445 3.65 2.79
0.56 0.323 0.408 0.27 0.36
1.41 2.45 1.94 2.93 2.19
Fourier transform infrared spectroscopy (FTIR) Fig. 2 shows the FTIR spectra of pristine and polymer composite membranes. For PES membrane, the peak around 1090 cm1 corresponds to symmetric and asymmetric stretching of the O]S]O. The peak at 705 cm1 corresponds
Fig. 1 e TEM and SAED pattern of nanoparticles (a) TEM of Fe3O4, (b) TEM of TiO2, and (c) TEM of MoO3,(d) SAED of Fe3O4, (e) SAED of TiO2, and (f) SAED of MoO3. Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Table 3 e Thickness, ion exchange capacity, chemical degradation and proton conductivity of membranes. Membrane PES SPEEK PES/SPEEK/Fe3O4 PES/SPEEK/TiO2 PES/SPEEK/MoO3
Thickness (mm)
Ion exchange capacity (meq/g)
Chemical degradation (%)
310 70 100 150 110
0.23 0.27 0.33 0.29 0.31
6.87 18.64 19.02 15.94 16.58
to the SeO group. The characteristic peaks of SPEEK appeared at 3410, 1225, 1080, 1023 and 761 cm1 represents the OeH group stretching, asymmetric stretching of O]S]O, the symmetric stretching vibration of O¼ S]O, stretching of S]O, and stretching of SeO of the sulfonic acid group respectively [31]. The polymer composite membranes show the peak of PES, SPEEK and nanoparticle. The characteristic peaks of PES/ SPEEK/Fe3O4 membrane at 1476, 1351, 1158 and 761 cm1 confirm the presence of Fe3O4 nanoparticle. The PES/SPEEK/ TiO2 membrane show characteristic peak at 549 cm1 corresponds to TieO stretching. The band at 1023 and 1080 cm1 corresponds to the association of TieO with the sulfonic acid group of SPEEK which reveals the strong interaction between the polymer and TiO2 nanoparticle [32]. The PES/SPEEK/MoO3 membrane shows the peak at 932 cm1 corresponds to Mo]O stretching of hexagonal phase which represents the characteristics bond between metal and oxygen [30]. The recognized broad peak of SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2, PES/SPEEK/MoO3 membranes at 3410, 3431, 3393 and 3398 cm1 corresponds to OeH bonding. The FTIR spectra of PES membrane exhibit a sharp peak at 3403 cm1 corresponds to the absence of OeH bonding. The FTIR spectra of polymer composite membranes confirmed the presence of sulfonic acid groups.
Field Emission Scanning Electron Microscopy (FESEM) Fig. 3 shows the FESEM images of pristine PES, pristine SPEEK and polymer composite membranes. The membranes were smooth and denser. It was observed that the nanoparticles
Proton Conductivity (S/cm) 0.22 5.18 3.57 4.57 2.67
104 104 104 104 104
were distributed evenly in the polymer composite membrane without any agglomeration. FESEM images of pristine SPEEK and polymer composite membranes (PES/SPEEK/TiO2 and PES/ SPEEK/MoO3) were appeared to be denser (nonporous) with uniform morphologies. The denser membrane enhances the movement of protons and retards substrate loss. In 10.0 KX magnification, the pristine PES and PES/SPEEK/Fe3O4 membrane exhibits porous and void structure. In the polymer composite membrane, interconnected microstructure was formed by the thermoplastic polymer (PES) and the conductive polymer (SPEEK). The interactions among PES, sulfonic acid groups of SPEEK and crystalline nature of nanoparticles change the morphology of the membrane [29].
XRD pattern Fig. 4 shows the XRD pattern of pristine and polymer composite membranes. The XRD pattern of pristine PES membrane showing the peak at 2q ¼ 18 [33]. For SPEEK membrane, the broad signal represents the amorphous nature [34]. The XRD pattern of PES/SPEEK/Fe3O4 membrane showing peaks at 2q ¼ 29 , 35 , 57 and 63 confirmed the presence of Fe3O4 nanoparticle within the membrane [35]. The XRD pattern of PES/SPEEK/TiO2 membrane show the characteristic peaks at 2q ¼ 19 , 25 , 28 , 32 , 47 and 54 represents the crystalline nature of TiO2. The XRD pattern of PES/SPEEK/MoO3 membrane confirmed the presence of MoO3 within the membrane. It is well known that the crystalline peak appears at lower 2q values. In the polymer composite membrane, the crystalline nature is reduced by SPEEK and there is no sharp peak at low 2q which indicates the interactions between polymers and nanoparticle. On addition of nanoparticles to the polymer blend (PES and SPEEK) reduces the amorphous nature and the crystalline peaks are obtained at higher 2q values. The inorganic nanoparticle within the polymer matrix reduces the amorphous nature of the polymer [36].
Glass transition temperature (Tg) and swelling ratio
Fig. 2 e FTIR spectra of PES, SPEEK, PES/SPEEK/Fe3O4, PES/ SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
Fig. 5 illustrates the DSC thermogram of membranes. A single Tg value is obtained for the polymer composite membrane which indicates the miscibility of the polymer blend systems (PES and SPEEK). Glass transition temperature (Tg) from the DSC thermograms for PES, SPEEK, PES/ SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membranes are 205, 194, 210, 217 and 209 C respectively. The blending of sulfonated polymer (SPEEK) with the thermoplastic polymer (PES) forms the interconnected structure by intermolecular interactions. The specific interactions between sulfonic acid groups in the polymeric chain and
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Fig. 3 e FESEM images of synthesized membrane (a) PES, (b) SPEEK, (c) PES/SPEEK/Fe3O4, (d) PES/SPEEK/TiO2, and (e) PES/ SPEEK/MoO3.
nanoparticles decrease the internal rotation and have a substantial impact on Tg. Here, the results revealed that the polymer composite membranes are satisfied the working temperature of the fuel cell system and also give assurance for thermal stability of membranes. In common, to avoid thermal degradation of membranes highest Tg is required for fuel cell operation [37]. The values of swelling ratio are 5.56, 16.28, 11.11, 11.25 and 11.28 for PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membrane respectively. DSC inference confirmed the miscibility of the polymer blend systems (PES and SPEEK) and there is an intermolecular interaction
between PES and SPEEK. Thus, the interactions between the hydrophobic PES and hydrophilic SPEEK reduces swelling ratio of the polymer composite membranes [38].
Water uptake content Fig. 6 shows the water uptake of pristine and polymer composite membranes. The ability of the membrane to absorb water is an important parameter to evaluate its performance in the fuel cell system. The PES/SPEEK/Fe3O4 membrane shows highest water uptake value than all the membranes. The addition of SPEEK to the PES has modified the water uptake
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
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low when compared with the pristine SPEEK membrane. Since the miscibility of hydrophobic PES with hydrophilic SPEEK has a substantial impact on reducing the methanol uptake [28]. SPEEK membrane is soluble in pure methanol due to the presence of sulfonic acid groups thus it shows signs of changes in the original appearance with high solvent uptake.
Ion exchange capacity
Fig. 4 e XRD pattern of PES, SPEEK, PES/SPEEK/Fe3O4, PES/ SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
properties. The presence of sulfonic acid groups in the membrane provide by added SPEEK creates a center of attention for the movement of the water molecule and accommodates more water molecules within the polymeric membrane structure [39,40]. Pristine PES membrane exhibit lowest water uptake represents the absence of sulfonic acid group.
Solvent uptake The solvent uptake was determined after soaking the membranes in pure methanol at particular testing condition. The solvent uptake value for SPEEK membrane is 48.42% and the polymer composite membranes are 20%. The results revealed that the pristine PES membrane and polymer composite membranes exhibit any significant changes in the original appearance under the testing condition of pure methanol at 60 C for 2 h. The solvent uptakes of these membranes are also
The number of exchangeable ions present in the membranes was analyzed by titration method, and the results are mentioned in Table 3. The highest IEC value of PES/SPEEK/ Fe3O4 membrane is 0.33 meq$g1. Due to the high water uptake capacity and the presence of sulfonic acid groups provide by added SPEEK. The water uptake has positive effects on ion exchange capacity by improving the ability of the membrane to exchange ions. The IEC value increases with the addition of nanoparticle in the polymer blend (PES and SPEEK) indicate that the nanoparticles do not block the polymer matrix for exchanging ions. Polymer composite membrane exhibit higher IEC than pristine SPEEK membrane because the protons inside the polymeric matrix are also involved in IEC through water hopping mechanism. The lowest IEC value is 0.23 meq$g1 with PES membrane, and it is due to the absence of sulfonic acid group in the polymeric structure. The other factor responsible for IEC is the surface morphology of the membrane. FESEM images represent the interaction between polymer blends and nanoparticle [41]. Hence, the polymer composite membrane exhibit high IEC than the pristine PES.
Oxidative stability The chemical degradation of the polymer membrane was analyzed using Fenton test, and the results are shown in Table 3. All the membranes were starting to degrade after 24 h in the Fenton's reagent. The decomposition of H2O2 catalyzed by iron generates hydroxy (HO) and hydroperoxy (HOO) free radicals. These free radicals are potent to cause the degradation of the membrane [42]. The performances of SPEEK and polymer composite membranes in Fenton test were found to be unsatisfactory after 24 h. Since the membrane is exposed to impractical conditions, the result does not reveal that the membrane is not suitable for PEMFC. The oxidative stability determined by Fenton test method has been widely used to evaluate the performance of the membrane.
Hydrolytic stability
Fig. 5 e DSC thermogram of PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
The polymer composite membranes exhibited no significant appearance changes and withstood the testing condition. This signifies that the surface of the membrane holds the nanoparticles even after exposing at 100 C for 48 h. The rigid structure of the polymer composite membrane provide by added PES with SPEEK has improved the hydrolytic stability by specific interaction which was confirmed by DSC. The membranes exhibited excellent hydrolytic stability without any change in the natural appearance and is shown in Supplementary Fig. S1.
Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
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Fig. 6 e Water uptake content of PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2 and PES/SPEEK/MoO3 membranes.
Proton conductivity of the membranes The proton conductivity of the polymer membranes was analyzed, and the results are mentioned in Table 3. Fig. 7 shows the image of proton conductivity analysis setup. The low conductivity value is 0.22 104 S/cm with pristine PES membrane reveals it has minimum proton conduction [43]. The absence of sulfonic acid groups in PES reduces the ability of the membrane to hold more water molecules. The highest conductivity value is 5.18 104 S/cm with SPEEK membrane. PES/SPEEK/TiO2 membrane showed good conductivity nearly to SPEEK membrane, and its value is 4.57 104 S/cm. The hygroscopic nature of TiO2, the presence of sulfonic acid
groups provide by added SPEEK and the formation of hydrogen bonding between water molecules and hydroxyl groups of TiO2 upon hydration have increased the proton conductivity of PES/SPEEK/TiO2 membrane. The movement of water molecules interconnected the sulfonic ionic domain clusters in the membrane, and it enhances the transport of protons through the membrane by water hopping mechanism [44]. PES/SPEEK/Fe3O4 and PES/SPEEK/MoO3 membranes showed virtually of better conductivity than the pristine PES membrane. In the polymer composite membrane, the specific intermolecular interactions between the PES and SPEEK have diminished the effect of sulfonation pair thereby decreasing the ability of the membrane to absorb more water molecules
Fig. 7 e Proton conductivity analysis setup. Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
to enhance proton transport [12]. The membrane thickness also plays a vital role in the measurement of proton conductivity. The thickness of the polymer composite membranes was found to be less and exhibit excellent proton conductivity than the pristine PES membrane.
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Conclusion The Polymer composite membrane with different nanoparticles were synthesized using phase inversion technique. PES, SPEEK, PES/SPEEK/Fe3O4, PES/SPEEK/TiO2, PES/SPEEK/ MoO3 membranes were characterized by FTIR, XRD and FESEM. The characterization studies of the polymer composite membrane have confirmed the presence of nanoparticle within the polymer matrix. A single Tg was obtained for the polymer composite membranes which establish the miscibility of PES and SPEEK. Interactions between the hydrophobic rigid PES and hydrophilic sulfonated SPEEK have a vital role in the reduction of solvent uptake, swelling ratio and enhancement of hydrolytic stability. The water uptake and IEC value of the polymer composite membranes were satisfactory. The proton conductivity value of PES/SPEEK/TiO2 membrane is 4.57 104 S/cm which is higher than the pristine PES membrane. Since the presence of sulfonic acid groups provide by SPEEK and the hydrophilic nature of TiO2 to oxidize water molecule helps to imbibe more water molecules. Thereby interconnecting the ionic domain clusters present within the polymeric membrane and promote the proton conductivity value. The PES/SPEEK/Fe2O3, PES/SPEEK/MoO3 membranes also shown significant proton conductivity value of 3.57 104 and 2.67 104 S/cm respectively. Thus features of the polymer composite membranes were excellent than the pristine PES membrane and suitable to be used as PEM for fuel cell application.
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Acknowledgment Dr. G. Arthanareeswaran thank Konkuk University, South Korea and also acknowledges the Korean Federation of Science and Technology Society (KOFST) for possible support under Brainpool program (Reference Number 161S-5-3-1561).
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Appendix A. Supplementary data Supplementary data related to this article can be found at doi:10.1016/j.ijhydene.2018.04.094.
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Please cite this article in press as: Elakkiya S, et al., Enhancement of fuel cell properties in polyethersulfone and sulfonated poly (ether ether ketone) membranes using metal oxide nanoparticles for proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.04.094