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Influences of crosslink density on the performance of PVA-diphenylamine-4-sulfonic acid sodium salt composite membranes
Solid State Ionics 338 (2019) 12–19
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Influences of crosslink density on the performance of PVA-diphenylamine-4sulfonic acid sodium salt composite membranes Sajede Shabanpanah, Abdollah Omrani
T
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Faculty of Chemistry, University of Mazandaran, P. O. BOX 453, Babolsar, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: PVA proton exchange membrane Diphenylamine-4-sulfonic acid sodium salt Crosslink density Mechanical properties Morphology
In the present paper, diphenylamine-4-sulfonic acid sodium salt and silica nanoparticles are introduced into poly (vinyl alcohol) (PVA) matrix to improve the thermal and mechanical properties through crosslinking with glutaraldehyde (GA) under acidic conditions. Proton conductivity, ion exchange capacity, and swelling ratio are examined as a function of the crosslinking degree. The membranes showed a degree of crosslinking between 6.1 and 33%. XRD experiments revealed that the amorphous character of the newly developed PVA membranes is improved by increasing the amount of GA. FE-SEM observations confirmed a major change of the surface morphology owing to the variation of GA content. It is also demonstrated that the proton conductivity decreased by increasing the crosslinker content and its maximum value, 6 × 10−2 (S/cm), is obtained for the membrane having 2 wt% of glutaraldehyde. Results showed that the methanol permeability decreased by increasing the concentration of GA. The prepared membrane consisting 5 wt% of silica nanoparticles showed the best thermal and mechanical properties.
1. Introduction Recently, interest in fuel cell technology as a replacement for fossil fuels is increased because of their limited reserves and eco-friendly aspects [1,2]. Among several types of fuel cells, polymer electrolyte membrane fuel cells (PEMFC) have been received as promising candidates for electrical vehicles. The vital component in PEMFC is proton conducting membrane. In this regard, the Nafion-based membranes are extensively used but, in spite of their high conductivity, they have weak resistance to methanol crossover [3,4]. This problem finally reduces the performance of commercially available fuel cells. Many polymer-based membranes have been suggested to overcome this problem and indeed to improve the fuel cell efficiency. Among polymers used in direct methanol fuel cells (DMFCs), PVA-based membranes are of great interest because of their suitable cost, safety, reliable performance in water/alcohol separations, and facile approach of film-forming. Compared with Nafion, PVA-based polymer electrolytes showed poor proton conductivity. Accordingly, attempts have been paid to improve proton conductivity of PVA membranes with simultaneous improvement of the other relevant properties like thermal and mechanical stabilities. As cited in the literatures, different types of possible candidates comprising hydroxyl, amine, carboxylate, sulfonate, and quaternary ammonium groups have been successfully incorporated into the
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membranes to improve their hydrophilicity and charged properties [5–7]. Boroglu et al. developed a PVA-based membrane modified with 4,4-diaminodiphenylether-2,2-disulfonic acid salt [8]. It is demonstrated that the proton conductivity increased with the sulfonic acid group content. In other study conducted by Qiao et al., a novel poly (vinyl alcohol)–2-acrylamido-2-methyl-1-propanesulfonic acid (PVA–PAMPS) blend membranes proposed as suitable alternative materials to perfluorinated ionomers for application in polymer electrolyte fuel cells [9]. High proton conductivity of 0.09 to 0.12 S/cm is obtained for a blend composition of PVA–PAMPS with 1:2 ratio in mass. In 2005, Kim and co-workers prepared a series of new highly proton conductive poly (vinyl alcohol)/poly (acrylic acid)/sulfosuccinic acid/silica hybrid membranes according to blending and chemical crosslinking procedures [10]. Reports indicated that the proton conductivity is enhanced due to the presence of sulfonic acid groups. The hybrid membranes containing PVA/PVP (Poly vinyl pyrrolidone)/MnTiO3 nanocomposite are synthesized by Attaran et al. [11]. Owing to hydrophilic nature of MnTiO3 nanoparticles, the proton conductivity and water uptake of the membranes are increased. A novel approach presented by Liew and coworkers on ionic liquid-based poly (vinyl alcohol) proton conductive polymer electrolytes for fuel cell applications [12]. Detailed studies on proton conductivity of PVA/ammonium acetate/1-butyl-3-methylimidazolium chloride (BmImCl) system are reported. They observed that
Corresponding author. E-mail address: [email protected] (A. Omrani).
https://doi.org/10.1016/j.ssi.2019.05.003 Received 29 August 2018; Received in revised form 1 May 2019; Accepted 5 May 2019 0167-2738/ © 2019 Published by Elsevier B.V.
Solid State Ionics 338 (2019) 12–19
S. Shabanpanah and A. Omrani
the ionic conductivity could be increased with the ionic liquid content. The highest value of ionic conductivity (5.74 mS/cm) is obtained upon addition of 50 wt% of the ionic liquid. Moreover, the novel organic/ inorganic hybrid membranes based on PVA and sulfonated polyhedral oligosilsesquioxane (sPOSS) crosslinked by ethylenediaminetetraacetic dianhydride are fabricated and characterized by Chang et al. [13]. It is evidenced that the proton conductivity could be enhanced directly by the content of sPOSS in the hybrid membranes. On the other hand, stability (thermal and hydrolysis stability) is a critical parameter for the membranes having soluble components. Therefore, to enhance the stability of PVA-based membranes they should be crosslinked with an appropriate crosslinking agent. Pure PVA has high hydrophilicity and crosslinking reaction can affect the hydrophilicity and mechanical strength of the final products. Crosslinking of PVA occurs through its hydroxyl groups and sometimes resulting in the formation of water-insoluble hydrophilic networks which could be used for a wide range of applications [14–17]. The purpose of the present study is to correlate the conductivity, proton exchange capacity, morphology and structure, thermal stability, and mechanical properties with the crosslinking degree of PVA-based membranes composed of diphenylamine-4-sulfonic acid sodium salt and silica nanoparticles. SiO2 nanoparticles are selected for its better water retention properties. For that aim, PVA membranes synthesized with 2, 3, 4 and 5 wt% of glutaraldehyde and abbreviated as PVA1, PVA2, PVA3 and PVA4 samples, respectively. It is seen that high GA concentrations enhanced the overall stability of PVA membranes. Combining a potential strategy based on silica nanoparticles allowed further improvement of the membrane stability.
(flow rate of 50 ml/min). About 15 mg of the sample is used for the TGA tests. The surface morphology, elemental analysis, and X-ray mapping of the prepared membranes are studied through Field Emission Scanning Electron Microscopy (FE-SEM, Model Mira 3-XMU) equipped well with the required accessories. The samples are dried firstly and then coated with a thin layer of Au before imaging at an accelerating voltage of 15 kV. The X-ray diffraction patterns are obtained from XRD instrument (Model Philips PW1730) with Cu Kα radiation (λ = 1.5418 Å) at ambient temperature. X Ray Diffraction recorded in the 2θ range from 10 to 80° at scanning step of 0.05°. The mechanical strength of the composite membranes is evaluated by a Santam tensile tester machine (model STM-20, Iran) with a crosshead speed of 2 mm/ min. Atomic force microscopy (AFM) is used to examine the surface morphology and roughness of the prepared membranes. The morphological measurements are carried out on the atomic force microscope Ara (Ara. AFM, model N0.0101/A, Iran). Small squares of the prepared membranes (approximately 1 cm2) were cut and glued on glass substrate.
2. Experimental
The IEC values are determined by a titration approach. A small fraction of each sample is immersed in 1 M H2SO4 solution for 24 h. The samples are washed with distilled water to remove excess amount of H2SO4, then equilibrated with 50 mL of 1 M NaCl solution, and finally heated to 40 °C for 24 h allowing the exchange between protons and sodium ions to be completed. The resulting solution is titrated with 0.01 N NaOH solution in the presence of phenolphthalein. The IEC value is estimated using the following equation:
2.4. Water uptake and swelling ratios Water uptake of the membranes is monitored by the weight change of the samples due to hydration. In this way, a small piece of the membrane is immersed in deionized water for 24 h at 25 °C and then the wet sample weight obtained. Meanwhile, this test was performed three times for each sample. 2.5. Ion exchange capacity (IEC)
2.1. Materials Poly (vinyl alcohol) (PVA, Mw = 27,000 g/mol) and diphenylamine-4-sulfonic acid sodium salt are supplied from Fluka. Glutaraldehyde (25 wt% solution in water) and SiO2 nanoparticles, average particle size 10–20 nm, are purchased from Sigma-Aldrich. Sodium hydroxide, sodium chloride and sulfuric acid (H2SO4) are also purchased from Merck. De-ionized (DI) water is used for the all experiments.
IEC = (VNaOH × 0.01)/Wdry
(1)
where, VNaOH is the volume of NaOH used in the titration, and Wdry is the dry weight of the membrane in g. The ion exchange experiment was repeated three times for each sample.
2.2. Membrane preparation PVA aqueous solutions are prepared in double distilled water under stirring at 50 °C for 4 h by solution casting method. Then, 80 wt% of diphenylamine-4-sulfonic acid sodium salt is mixed with the solution under stirring for 4 h. The desired amount of GA (0.2, 0.3, 0.4 and 0.5 mL) is added to the PVA solutions under acidic conditions (pH = 2.5 to 3.1) and the resultant viscous solutions transferred to petri-glass dishes. The films are dried at ambient temperature and subsequently cured in a vacuum oven at 50 °C for 24 h. In order to highlight the role of silica nanoparticles on the membrane properties, 5 wt% of SiO2 nanoparticles is added to the PVA solution having the sulfonic acid salt and ultrasound samples taken for 2 h to obtain well homogeneous mixtures. Finally, the optimum value of the crosslinker (0.5 ml) is added to the solution and the membrane fabricated according to the same procedure as described earlier.
2.6. Solvent extraction The solvent extraction method is employed to determine the degree of crosslinking of the membranes. A small portion of the samples is covered in filter paper and kept in distilled water at room temperature. The solvent replaced in every 15 h until no further solubility is observed in the polymer. The remaining part of the samples is then dried and weighed. 2.7. Proton conductivity measurement Proton conductivity of the fabricated membranes is obtained using impedance spectroscopy. A frequency response detector (EG&G model 1025) with an electrochemical setup that works under M 398 software is used for the measurements over a frequency range of 0.005 Hz to 100 kHz and at a sinusoidal potential of 10 mV. Prior to the impedance tests, each membrane sample is cut into pieces with dimensions of 1 cm × 1 cm and then kept immersed in 1 M H2SO4 solution for 24 h. The membranes are sandwiched between two brass plates acting as anode and cathode electrodes, and the proton conductivity obtained using the following equation:
2.3. Membranes characterization The FT-IR spectra of the fabricated PVA membranes are recorded over the frequency range 500–4000 cm−1 by a spectrometer (Thermo Nicolet, model Avatar 370). Thermal stability of the fabricated composite membranes is evaluated by a thermogravimetry analyzer (BÄHR Thermoanalyse GmbH STA 450, Germany). The programmed heating rate was 10 °C/min from 25 to 600 °C under a dry nitrogen atmosphere
σ = L/RA 13
(2)
Solid State Ionics 338 (2019) 12–19
S. Shabanpanah and A. Omrani
where, σ is the proton conductivity (S/cm), L is the membrane thickness (cm), R is the resistance from the impedance data (Ω), and A is the cross-sectional area of the membrane (cm2).
considerably with an increase in the content of glutaraldehyde confirming major part of the hydroxyl groups converted to the acetal rings and ether linkages.
2.8. Methanol permeability measurement
3.2. Morphology of the prepared membranes
Resistances to methanol crossover of the membranes are evaluated using gas chromatography equipped with a thermal conductivity detector (GC-7890A with a DB-5 column, Agilent, USA). For this purpose, a glass diffusion cell is employed to measure methanol permeability. The membranes are clamped between two compartments A and B (40 ml, respectively). Compartments of (A) and (B) are filled by 2 M methanol solution and double distilled water, respectively. Magnetic stirrers are used in each compartment to ensure uniform concentration in each compartment during the experiment. The methanol concentration in the receiving compartment as a function of time is given by the following equation:
SEM photographs of the crosslinked PVA membranes are presented in Fig. 2. The PVA membrane showed a flat and smooth surface morphology as evidenced in Fig. 2a. Crosslinking by GA results in a considerable change of the surface morphology (Fig. 2b and c). A distinct and uniform layered morphology is the characteristic of crosslinked PVA which could be seen in Fig. 2b. When the concentration of GA increased from 2 wt% (case of PVA1 membrane) to 5 wt% (case of PVA4/SiO2 membrane) the surface morphology changed significantly. The layered structure has been completely destroyed in the case of PVA1 membrane and a spongy structure with some hollow pores (Fig. 2c) was obtained for the PVA4/SiO2 membrane. The decrease in the available free volume of the membranes with addition of the crosslinker content has some contributions to the formation of acetal covalent linkages after crosslinking reaction. Energy-dispersive X-ray (EDXA) spectrum of the crosslinked PVA membrane consisting SiO2 nanoparticles is shown in Fig. 2d. The presence of Si, Na, and S elements in the spectrum is a directly evidence for the existence of the sulfonic acid salt and silica nanoparticles within the composite membrane.
P = CB (t) VB L/A CA (t − t 0)
(3)
where, CB and CA are the methanol concentrations in the two compartments. A, L, and VB are the membrane effective area, membrane thickness, and volume of permeated compartment, respectively. 3. Results and discussion 3.1. FT-IR ATR spectroscopy
3.3. XRD results Attenuated total reflection Fourier transform infrared spectroscopy (FT-IR ATR) is used to characterize the existence of specific chemical groups in the membranes. Fig. 1 represents the ATR FT-IR spectra of the pure PVA, PVA1, PVA2, PVA3 and PVA4 samples. The large bands are observed at around 3000 and 3400 cm−1 (region (I)) attributed to –OH group stretching of the PVA chain, which indicates the presence of hydroxyl groups [18]. The band occurring at around 2924 cm−1 (region (II)) is corresponding to the stretching of –CH groups [19]. Obviously, after cross-linking with GA, intensity of the -OH peak is abridged and shifted towards higher frequency. The formation of ether bonds (C-O-C) as the result of reaction between the hydroxyl groups of PVA and –CHO groups of GA is previously demonstrated in the literatures [20]. The characteristic bands at 1250–1270 cm−1 are accordingly attributed to the formation of C-O-C linkage. Moreover, the carbonyl group stretching at 1720 cm−1 (region (III)) is observed in the crosslinked membranes. Besides, the absorption peaks at 1030–1145 cm−1 and 830 cm−1 are assigned to S]O and SeO stretching vibrations of SO3− groups [21]. The observed bands at around 800 and 1050 cm−1 are characteristic of Si-O-Si symmetric and asymmetric vibrations, respectively, which confirms the presence of silica in PVA4/SiO2 membrane. However, the intensity of the OeH vibration peak decreased
The structural changes created by the presence of GA are evaluated using X-ray diffraction. Fig. 3 shows the diffraction patterns of the PVA membranes crosslinked by 2 to 5 wt% of GA. According to Fig. 3, the peak occurring at 2θ = 20° is attributed to the pure PVA membrane. Clearly, this characteristic peak is somewhat broadened and its intensity reduced by an increase in the crosslink density. The decreased concentration of the hydroxyl groups of PVA owing to crosslinking reaction is responsible for the reduced crystallinity [22]. This indicates that the crosslinked PVA membrane becomes more amorphous and the amount of amorphous portion increases by the crosslinking reaction. Also, the amorphous character is developed in the presence of SiO2 nanoparticles. The decreased crystallinity could be described based on a reduction in the hydroxyl groups concentration of the PVA side-chain because of the reaction with hydroxyl groups of silica nanoparticles and GA. 3.4. Water uptake and swelling ratio analysis Fig. 4 shows the changes in water uptake and swelling ratio of the composite membranes in terms of GA content. What is clear is that water absorption capacity of the membranes is decreased with increasing the crosslinker concentration. Apparently, the crosslinking reaction decreased the PVA's hydrophilicity based on a reduction in the number of water molecules adsorbed on the membrane surface. In other words, the reduced free space in the membranes will make its structure harder, hence, the water absorption capacity decreased. The similar trend is observed for the changes in swelling ratio by GA content as can be seen in Fig. 4. Again, an increase in the crosslinker concentration results in a decrease of the swelling ratio. This can be described based on a reduction in the polymer chains mobility causing the membrane structure be more compact. Such compact structures have the limited available free space for swelling. 3.5. IEC evaluation Changes in the value of IEC with the crosslinker concentration in the composite membranes are shown in Fig. 5. The highest IEC value is obtained for the PVA1 sample where the content of GA was the
Fig. 1. FT-IR spectra of: (a) PVA, (b) PVA1, (c) PVA2, (d) PVA3, (e) PVA4 and (f) PVA4/SiO2 membranes. 14
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Fig. 2. FE-SEM images of the pure PVA (a), PVA1 (b) and PVA4/SiO2 (c) membranes, and EDXA profile of the PVA4/SiO2 membrane (d).
Fig. 3. X-ray diffraction patterns of: (a) pure PVA, (b) PVA1, (c) PVA2, (d) PVA3, (e) PVA4 and (f) PVA4/SiO2 membranes.
Fig. 4. Water uptake and swelling ratios of the PVA membranes.
3.6. Crosslink density assessment
minimum value of 2 wt%. This can be described according to the membrane structure. The membrane structure becomes more compact as the content of GA increased. Therefore, the available free sites in its structure, where the protons could be exchanged, are decreased.
The variation of crosslink density with GA concentration is shown in Fig. 6. Clearly, the crosslink density improved as the content of GA increased. The PVA4 sample showed the highest amount of crosslink density, i.e. 33%. 15
Solid State Ionics 338 (2019) 12–19
S. Shabanpanah and A. Omrani
Fig. 8. The Arrhenius plot of a) PVA1 b) PVA2, c) PVA3, d) PVA4.and e) PVA4/ SiO2 membranes.
Fig. 5. Ion exchange capacity of the PVA membranes.
3.7. Proton conductivity measurement Fig. 7 represents the effect of GA concentration on the membrane's AC impedance spectra and proton conductivity at 25 °C. When the crosslinker content in the membrane composition is increased from 2 to 5 wt% a significant decrease in the conductivity observed (about 463.6%). The observed decreasing trend of proton conductivity suggests that an increase in GA content makes compression of the membrane structure via filling its pores preventing the proton transfer process. This phenomenon ultimately reduces the proton conductivity of the membranes having higher amount of GA [17,24,25]. Temperature dependence of the conductivity for the fabricated membranes are determined. Generally, the change in the proton conductivity with temperature is complied with an Arrhenius type equation which can be used to obtain the activation energy (Ea) of proton transfer. The Ea values are obtained from the linear regression of lnσ vs 1000/T plots. The results are shown in Fig. 8 and Table 1. Obviously, the ionic conductivity is decreased by increasing the degree of crosslinking. As it is expected, the proton transfer facilitates at higher temperatures. Due to development of a compact structure, the activation energy is increased by increasing the content of GA. As evidenced earlier, the water uptake is diminished and hence, the Ea values increased.
Fig. 6. Crosslink density of the synthesized PVA membranes.
As it is demonstrated, the formation of acetal ring networks in the crosslinked membranes reduces their solubility considerably via forming a hydrophobic protective layer [23].
Fig. 7. The Nyquist plots of (1) PVA1, (2) PVA2, (3) PVA3, (4) PVA4 and (5) PVA4/SiO2 membranes (a) and the corresponding proton conductivities (b). 16
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Table 1 Apparent activation energy (Ea) of proton conduction for the different PVA membranes. Membrane
% additive
Ea/kJ mol−1
PVA/GLA/salt PVA/GLA/salt PVA/GLA/salt PVA/GLA/salt PVA/GLA/salt/SiO2
GA GA GA GA GA
9.8 10.3 10.6 10.9 11.7
2 3 4 5 5/SiO2 5
Fig. 10. Changes in the tensile strength, elongation, and modulus of the crosslink PVA membranes as a function of the sample composition.
obtained for the PVA1 membrane which is 95.6% of Nafion 117 conductivity. 3.9. Tensile test results The mechanical properties are one of the key parameters for polymeric membranes. Fig. 10 shows the effect of crosslink density on the membrane's tensile strength, elongation, and Young's modulus at failure point. With further increase in the crosslink density, the tensile strength of the membranes is enhanced with the maximum value of 48.39 MPa for the PVA4 sample. Also, the percent elongation at break is decreased by increasing the crosslink density. The enhancement of mechanical strength is due to the change of the membrane structure after making crosslinked networks which restricted the mobility of the PVA chains [32]. Similarly, the addition of silica nanoparticles to the membrane composition having 5 wt% of the crosslinker improved the mechanical stability as well.
Fig. 9. Methanol permeability of the PVA membranes.
3.8. Evaluation of methanol permeability Resistance to methanol crossover of the membranes are calculated using Eq. (3), and the results shown as a function of the crosslinker concentration in Fig. 9. Methanol permeability of Nafion 117 was measured under the same experimental conditions yielding a value of 1.83 × 10−6 cm2/s which is in good agreement with the value reported in the literature [25]. Interestingly, the methanol permeability of all the crosslinked PVA membranes is lower than that of Nafion 117. The methanol permeability is found to be 4.38 × 10−7 cm2/s for the crosslinked membrane containing 2 wt% of GA (PVA1 membrane) and decreased continuously until reaching a value of 1.37 × 10−7 cm2/s for the crosslinked membrane having 5 wt% of GA (PVA4 sample). Generally, the change in the methanol permeability of the membranes might be attributed to the crosslinking effect of the PVA matrix by glutaraldehyde. Further decrease of the methanol permeability is achieved by incorporation of SiO2 nanoparticles (5 wt%) within the membrane composition. The lowest amount of 1.20 × 10−7 cm2/s is obtained for the PVA4/SiO2 hybrid membrane. However, it may be implied that the simultaneous actions of GA and SiO2 nanoparticles in the PVA membranes result in a blocking effect for methanol transport. Data in Table 2 highlights that the highest proton conductivity is
3.10. Evaluation of the membranes thermal stability The thermal stability of the membranes is evaluated with TGA and the results are shown in Fig. 11. According to the previously reports, thermal degradation of the PVA membranes occur in three main steps: loss of water or moisture absorbed from the membrane, degradation of inter-component bonds by breaking of the side chains, and breakdown of the polymer backbone [33–36]. To compare thermal stability of the synthesized membranes, the TGA curves are analyzed further by defining two parameters of Tonset and Wresidual. Tonset is the temperature at which the weight loss of the membrane sample is 5% while Wresidual shows the value of char yield at 600 °C. The values of Tonset are 136.9, 143.6, 157.5, 160.2, and 170.3 °C for the PVA1, PVA2, PVA3, PVA4, and
Table 2 Properties of Nafion 117 and various PVA membranes reported in references at ambient temperature. Membrane
Conductivity (S/cm)
IEC (m equiv g−1)
Water uptake (%)
Methanol permeability (cm2/s × 10−6)
Ref
PVA/SSA PVA/SSA/GA PVA/SiO2/SSA PVA/PAMPS PVA/SPES PVA/SA/GA PVA/CS/GA Nafion 117 PVA/Nafion PVA membrane
0.001–0.01 0.01–0.0526 0.001–0.01 0.099–0.11 0.0021–0.0051 0.01 0.0099 0.064 0.0009–0.02 0.0129–0.0612
0.5–2.24 0.576–2.676 0.2–1.1 1.71–1.91 0.62–1.24 0.79 0.62 0.91 0.09–0.91 0.12–0.52
10–80 – 45–60 136–163 29.3–35 – – 33.6 13–43 90–250
0.1–1 0.14–2.9` 0.01–0.1 – 0.0468–0.0614 0.069 0.094 2 0.31–0.65 0.12–0.438
[25] [23] [26] [9] [27] [28] [28] [29] [30,31] This work
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Fig. 11. TGA profiles of (a) PVA1, (b) PVA2, (c) PVA3, (d) PVA4, and (e) PVA4/SiO2 composite membranes.
the membranes. A possible interpretation is that the reduce of hydrophilicity can lead to lower water content and ion exchange rate, and decrease the proton conductivity.
Table 3 Results of the PVA membranes from AFM experiment. Membranes
Ra (nm)
Rq (nm)
PVA PVA1 PVA2 PVA3 PVA4 PVA4/SiO2
0.9875 1.5885 1.8947 3.9222 4.0171 7.0363
1.423 2.0936 2.6408 4.8107 4.9843 9.1658
4. Conclusions The novel crosslinked PVA/diphenylamine-4-sulfonic acid sodium salt membranes with varied glutaraldehyde concentration are synthesized via solution casting. Increased crosslink density resulted in reductions of water uptake, swelling, IEC, and proton conductivity while thermal and mechanical properties of the membranes are improved. Also, methanol permeability of the PVA membranes is decreased by increasing the crosslinker concentration and in the presence of silica nanoparticles. It is demonstrated that by increasing the crosslink density the membrane sites are filled and hence, its mobility rate decreased. Observations also revealed that the presence of 5 wt% silica nanoparticles increases the thermal and mechanical stability of the membranes, therefore, the membrane is stable. It is also noticed that adding silica nanoparticles did not change the amount of ion exchange, water uptake, and proton conductivity of PVA4 membrane. In this case, the presence of crosslinker seems to have a greater impact on the properties of the system.
PVA4/SiO2 membranes, respectively. These results showed that both the crosslinking reaction with GA and reinforcement by SiO2 nanoparticles enhanced the thermal stability of PVA-based membranes. Correspondingly, the values of Wresidual at 600 °C are 12.1%, 14.7%, 17.2%, 20.7%, and 25.3% for the PVA1, PVA2, PVA3, PVA4, and PVA4/ SiO2 membranes, respectively. The highest value of char yield for the PVA4/SiO2 membrane confirmed that the flame retardancy property of the membrane is improved owing to crosslinking and reinforcement. The strong interaction between PVA and GA reduces the mobility of polymer segments resulting in an improvement of the thermal stability. Moreover, results from this part highlights that the incorporation of SiO2 nanoparticles in the membrane composition leads to an enhancement of the decomposition temperature.
Acknowledgement The financial support from University of Mazandaran, Iran, (grant ID: 93172322102) is gratefully acknowledged.
3.11. Atomic force microscopy (AFM) observations In order to have deep insight on the surface morphology of the synthesized membranes, the atomic force microscopy (AFM) test is carried out. Table 3 shows the results of average roughness (Ra) and root mean square (RMS or Rq) roughness obtained from the AFM images. By comparing the results shown in Table 3, it can be noticed that the surface of pure PVA membrane is quite smooth. However, by increasing the crosslinker concentration the surface of membranes become further rough. Images recorded for PVA4/SiO2 membrane confirm that SiO2 nanoparticles were successfully coated onto the membrane and produced rougher surfaces. The average surface roughness for this membrane is 7.0363 nm which is considerably high when compared to the rest of the membranes demonstrating that incorporation of SiO2 nanoparticles can significantly improve surface roughness. AFM analysis reveals a possible correlation between the average roughness and hydrophilicity of
References [1] J. Pandey, A. Shukla, PVDF supported silica immobilized phosphotungstic acid membrane for DMFC application, Solid State Ionics 262 (2014) 811–814. [2] S.L. Chavan, D.B. Talange, Modeling and performance evaluation of PEM fuel cell by controlling its input parameters, Energy. 138 (2017) 437–445. [3] Y. Devrim, S. Erkan, N. Baç, I. Eroglu, Nafion/titanium silicon oxide nanocomposite membranes for PEM fuel cells, Int. J. Energy Res. 37 (2012) 435–442. [4] F.A. Zakil, S.K. Kamarudin, S. Basri, Modified Nafion membranes for direct alcohol fuel cells: an overview, Renew. Sust. Energ. Rev. 65 (2016) 841–852. [5] D.S. Kim, M.D. Guiver, M.Y. Seo, H.I. Cho, D.H. Kim, J.W. Rhim, G.Y. Moon, S.Y. Nam, Influence of silica content in crosslinked PVA/PSSA-MA/silica hybrid membrane for direct methanol fuel cell (DMFC), Macromol. Res. 15 (2007) 412–417. [6] H.B. Park, S.Y. Nam, J.W. Rhim, J.M. Lee, S.E. Kim, J.R. Kim, Y.M. Lee, Gas transport properties through cation-exchanged sulfonated polysulfone membranes, J. Appl. Polym. Sci. 86 (2002) 2611–2617.
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[22] C. Shao, H.Y. Kim, J. Gong, B. Ding, D.R. Lee, S.J. Park, Fiber mats of poly (vinyl alcohol)/silica composite via electrospinning, Mater. Lett. 57 (2003) 1579–1584. [23] C.E. Tsai, C.W. Lin, B.J. Hwang, A novel crosslinking strategy for preparing poly (vinyl alcohol)-based proton-conducting membranes with high sulfonation, J. Power Sources 195 (2010) 2166–2173. [24] V. Kumar, P. Kumar, A. Nandy, P.P. Kundu, Crosslinked inter penetrating network of sulfonated styrene and sulfonated PVdF-co-HFP as electrolytic membrane in a single chamber microbial fuel cell, RSC Adv. 5 (2015) 30758–30767. [25] J.W. Rhim, H.B. Park, C.S. Lee, J.H. Jun, D.S. Kim, Y.M. Lee, Crosslinked poly (vinyl alcohol) membranes containing sulfonic acid group: proton and methanol transport through membranes, J. Membr. Sci. 238 (2004) 143–151. [26] D.S. Kim, H.B. Park, J.W. Rhim, Y.M. Lee, Preparation and characterization of crosslinked PVA/SiO2 hybrid membranes containing sulfonic acid groups for direct methanol fuel cell applications, J. Membr. Sci. 240 (2004) 37–48. [27] D. Kumar, S.S.A.S. Meenakshi, A. Subbiah, N. Alagumalai, R. Dipak, Sulfonated poly (ether sulfone)/poly (vinyl alcohol) blend membranes customized with tungsten disulfide nanosheets for DMFC applications, Polymer. 155 (2018) 42–49. [28] S. Mohanapriya, S.D. Bhat, A.K. Sahu, A. Manokaran, R. Vijayakumar, S. Pitchumani, A.K. Shukla, Sodium-alginate based proton exchange membranes as electrolytes for DMFCs, Energy Environ. Sci. 3 (2010) 1746–1756. [29] M. Ranjani, D.J. Yoo, G.G kumara, Sulfonated Fe3O4@SiO2 nanorods incorporated sPVdF nanocomposite membranes for DMFC applications, J. Membr. Sci. 555 (2018) 497–506. [30] N.W. DeLuca, Y.A. Elabd, Direct methanol fuel cell performance of Nafion®/poly (vinyl alcohol) blend membranes, J. Power Sources 163 (2006) 386–391. [31] N.W. DeLuca, Y.A. Elabd, Nafion®/poly (vinyl alcohol) blends: effect of composition and annealing temperature on transport properties, J. Membr. Sci. 282 (2006) 217–224. [32] A. Anis, A.K. Banthia, S. Bandyopadhyay, Synthesis & characterization of PVA/STA composite polymer electrolyte membranes for fuel cell application, J. Mater. Eng. Perform. 17 (2008) 772–779. [33] D.K. Lee, J.T. Park, J.K. Choi, D.K. Roh, J.H. Lee, Y.G. Shul, J.H. Kim, Proton conducting crosslinked membranes by polymer blending of triblock copolymer and poly (vinyl alcohol), Macromol. Res. 16 (2008) 549–554. [34] P. Duangkaew, J. Wootthikanokkhan, Methanol permeability and proton conductivity of direct methanol fuel cell membranes based on sulfonated poly (vinyl alcohol) layered silicate nanocomposites, J. Appl. Polym. Sci. 109 (2008) 452–458. [35] K. Das, D. Ray, N.R. Bandyopadhyay, A. Gupta, S. Sengupta, S. Sahoo, A. Mohanty, M. Misra, Preparation and characterization of crosslinked starch/poly (vinyl alcohol) green films with low moisture absorption, Ind. Eng. Chem. Res. 49 (2010) 2176–2185. [36] C.C. Yang, Fabrication and characterization of poly (vinyl alcohol)/montmorillonite/poly (styrene sulfonic acid) proton-conducting composite membranes for direct methanol fuel cells, Int. J. Hydrog. Energy 36 (2011) 4419–4431.
[7] J.F. Blanco, Q.T. Nguyen, P. Schaetzel, Novel hydrophilic membrane materials: sulfonated polyethersulfone Cardo, J. Membr. Sci. 186 (2001) 267–279. [8] M.S. Boroglu, S. Cavus, I. Boz, A. Ata, Synthesis and characterization of poly (vinyl alcohol) proton exchange membranes modified with 4,4-diaminodiphenylether-2,2disulfonic acid, Express Polym Lett 5 (2011) 470–478. [9] J. Qiao, T. Hamaya, T. Okada, New highly proton conductive polymer membranes poly(vinyl alcohol)–2-acrylamido-2-methyl-1-propanesulfonic acid (PVA–PAMPS), Polymer. 46 (2005) 10809–10816. [10] D.S. Kim, H.B. Park, J.W. Rhim, Y.M. Lee, Proton conductivity and methanol transport behavior of cross-linked PVA/PAA/silica hybrid membranes, Solid State Ionics 176 (2005) 117–126. [11] A.M. Attaran, K. Hooshyari, M. Javanbakht, M. Enhessari, Fabrication and characterization of poly vinyl alcohol/poly vinyl pyrrolidone/MnTiO nanocomposite membranes for PEM fuel cells, Iran. J. Energy Environ. 4 (2013) 86–90. [12] C.W. Liew, S. Ramesh, A.K. Arof, A novel approach on ionic liquid-based poly (vinyl alcohol) proton conductive polymer electrolytes for fuel cell applications, Int. J. Hydrog. Energy 36 (2013) 2917–2928. [13] Y.W. Chang, E. Wang1y, G. Shin, J.E. Han, P.T. Mather, Poly (vinyl alcohol) (PVA)/ sulfonated polyhedral oligosilsesquioxane (sPOSS) hybrid membranes for direct methanol fuel cell applications, Polym. Adv. Technol. 18 (2007) 535–543. [14] F. Arranz, C.M. Sanchez, R. Martinez, Reaction of poly (viny1 alcohol) with n-butyl isocyanate, Angew. Makromol. Chem. 152 (1987) 79–91. [15] S.V. Caro Jr., C.S. Paik Sung, E.W. Merrill, Reaction of hexamethylene diisocyanate with poly (vinyl alcohol) films for biomedical applications, J. Appl. Polym. Sci. 20 (1976) 3241–3246. [16] K.J. Kim, S.B. Lee, N.W. Han, Effects of the degree of crosslinking on properties of poly (vinyl alcohol) membranes, Polym. J. 25 (1993) 1295–1302. [17] R. Rudra, V. Kumar, P.P. Kundu, Acid catalyzed cross-linking of poly vinyl alcohol (PVA) by glutaraldehyde: effect of crosslink density on the characteristics of PVA membranes used in single chambered microbial fuel cells, RSC Adv. 5 (2015) 83436–83447. [18] A.M. Abu-Saied, R. Wycisk, M.M. Abbassy, G.A. El-Naim, F. El-Demerdash, M.E. Youssef, H. Bassuony, P.N. Pintaro, Sulfated chitosan/PVA absorbent membrane for removal of copper and nickel ions from aqueous solutions fabrication and sorption studies, Carbohydr. Polym. 165 (2017) 149–158. [19] H. Awada, C. Daneault, Chemical modification of poly (vinyl alcohol) in water, Appl. Sci. 5 (2015) 840–850. [20] H. Beydaghi, M. Javanbakht, H.S. Amoli, A. Badiei, Y. Khaniani, M.R. Ganjali, M. Abdouss, Synthesis and characterization of new proton conducting hybrid membranes for PEM fuel cells based on poly (vinyl alcohol) and nanoporous silica containing phenyl sulfonic acid, Int. J. Hydrog. Energy 36 (2011) 13310–13316. [21] C.Y. Tseng, Y.S. Ye, K.Y. Kao, J. Joseph, W.C. Shen, J. Rick, B.J. Hwang, Interpenetrating network-forming sulfonated poly (vinyl alcohol) proton exchange membranes for direct methanol fuel cell applications, Int. J. Hydrog. Energy 36 (2011) 11936–11945.
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Influences of crosslink density on the performance of PVA-diphenylamine-4sulfonic acid sodium salt composite membranes Sajede Shabanpanah, Abdollah Omrani
T
⁎
Faculty of Chemistry, University of Mazandaran, P. O. BOX 453, Babolsar, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: PVA proton exchange membrane Diphenylamine-4-sulfonic acid sodium salt Crosslink density Mechanical properties Morphology
In the present paper, diphenylamine-4-sulfonic acid sodium salt and silica nanoparticles are introduced into poly (vinyl alcohol) (PVA) matrix to improve the thermal and mechanical properties through crosslinking with glutaraldehyde (GA) under acidic conditions. Proton conductivity, ion exchange capacity, and swelling ratio are examined as a function of the crosslinking degree. The membranes showed a degree of crosslinking between 6.1 and 33%. XRD experiments revealed that the amorphous character of the newly developed PVA membranes is improved by increasing the amount of GA. FE-SEM observations confirmed a major change of the surface morphology owing to the variation of GA content. It is also demonstrated that the proton conductivity decreased by increasing the crosslinker content and its maximum value, 6 × 10−2 (S/cm), is obtained for the membrane having 2 wt% of glutaraldehyde. Results showed that the methanol permeability decreased by increasing the concentration of GA. The prepared membrane consisting 5 wt% of silica nanoparticles showed the best thermal and mechanical properties.
1. Introduction Recently, interest in fuel cell technology as a replacement for fossil fuels is increased because of their limited reserves and eco-friendly aspects [1,2]. Among several types of fuel cells, polymer electrolyte membrane fuel cells (PEMFC) have been received as promising candidates for electrical vehicles. The vital component in PEMFC is proton conducting membrane. In this regard, the Nafion-based membranes are extensively used but, in spite of their high conductivity, they have weak resistance to methanol crossover [3,4]. This problem finally reduces the performance of commercially available fuel cells. Many polymer-based membranes have been suggested to overcome this problem and indeed to improve the fuel cell efficiency. Among polymers used in direct methanol fuel cells (DMFCs), PVA-based membranes are of great interest because of their suitable cost, safety, reliable performance in water/alcohol separations, and facile approach of film-forming. Compared with Nafion, PVA-based polymer electrolytes showed poor proton conductivity. Accordingly, attempts have been paid to improve proton conductivity of PVA membranes with simultaneous improvement of the other relevant properties like thermal and mechanical stabilities. As cited in the literatures, different types of possible candidates comprising hydroxyl, amine, carboxylate, sulfonate, and quaternary ammonium groups have been successfully incorporated into the
⁎
membranes to improve their hydrophilicity and charged properties [5–7]. Boroglu et al. developed a PVA-based membrane modified with 4,4-diaminodiphenylether-2,2-disulfonic acid salt [8]. It is demonstrated that the proton conductivity increased with the sulfonic acid group content. In other study conducted by Qiao et al., a novel poly (vinyl alcohol)–2-acrylamido-2-methyl-1-propanesulfonic acid (PVA–PAMPS) blend membranes proposed as suitable alternative materials to perfluorinated ionomers for application in polymer electrolyte fuel cells [9]. High proton conductivity of 0.09 to 0.12 S/cm is obtained for a blend composition of PVA–PAMPS with 1:2 ratio in mass. In 2005, Kim and co-workers prepared a series of new highly proton conductive poly (vinyl alcohol)/poly (acrylic acid)/sulfosuccinic acid/silica hybrid membranes according to blending and chemical crosslinking procedures [10]. Reports indicated that the proton conductivity is enhanced due to the presence of sulfonic acid groups. The hybrid membranes containing PVA/PVP (Poly vinyl pyrrolidone)/MnTiO3 nanocomposite are synthesized by Attaran et al. [11]. Owing to hydrophilic nature of MnTiO3 nanoparticles, the proton conductivity and water uptake of the membranes are increased. A novel approach presented by Liew and coworkers on ionic liquid-based poly (vinyl alcohol) proton conductive polymer electrolytes for fuel cell applications [12]. Detailed studies on proton conductivity of PVA/ammonium acetate/1-butyl-3-methylimidazolium chloride (BmImCl) system are reported. They observed that
Corresponding author. E-mail address: [email protected] (A. Omrani).
https://doi.org/10.1016/j.ssi.2019.05.003 Received 29 August 2018; Received in revised form 1 May 2019; Accepted 5 May 2019 0167-2738/ © 2019 Published by Elsevier B.V.
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S. Shabanpanah and A. Omrani
the ionic conductivity could be increased with the ionic liquid content. The highest value of ionic conductivity (5.74 mS/cm) is obtained upon addition of 50 wt% of the ionic liquid. Moreover, the novel organic/ inorganic hybrid membranes based on PVA and sulfonated polyhedral oligosilsesquioxane (sPOSS) crosslinked by ethylenediaminetetraacetic dianhydride are fabricated and characterized by Chang et al. [13]. It is evidenced that the proton conductivity could be enhanced directly by the content of sPOSS in the hybrid membranes. On the other hand, stability (thermal and hydrolysis stability) is a critical parameter for the membranes having soluble components. Therefore, to enhance the stability of PVA-based membranes they should be crosslinked with an appropriate crosslinking agent. Pure PVA has high hydrophilicity and crosslinking reaction can affect the hydrophilicity and mechanical strength of the final products. Crosslinking of PVA occurs through its hydroxyl groups and sometimes resulting in the formation of water-insoluble hydrophilic networks which could be used for a wide range of applications [14–17]. The purpose of the present study is to correlate the conductivity, proton exchange capacity, morphology and structure, thermal stability, and mechanical properties with the crosslinking degree of PVA-based membranes composed of diphenylamine-4-sulfonic acid sodium salt and silica nanoparticles. SiO2 nanoparticles are selected for its better water retention properties. For that aim, PVA membranes synthesized with 2, 3, 4 and 5 wt% of glutaraldehyde and abbreviated as PVA1, PVA2, PVA3 and PVA4 samples, respectively. It is seen that high GA concentrations enhanced the overall stability of PVA membranes. Combining a potential strategy based on silica nanoparticles allowed further improvement of the membrane stability.
(flow rate of 50 ml/min). About 15 mg of the sample is used for the TGA tests. The surface morphology, elemental analysis, and X-ray mapping of the prepared membranes are studied through Field Emission Scanning Electron Microscopy (FE-SEM, Model Mira 3-XMU) equipped well with the required accessories. The samples are dried firstly and then coated with a thin layer of Au before imaging at an accelerating voltage of 15 kV. The X-ray diffraction patterns are obtained from XRD instrument (Model Philips PW1730) with Cu Kα radiation (λ = 1.5418 Å) at ambient temperature. X Ray Diffraction recorded in the 2θ range from 10 to 80° at scanning step of 0.05°. The mechanical strength of the composite membranes is evaluated by a Santam tensile tester machine (model STM-20, Iran) with a crosshead speed of 2 mm/ min. Atomic force microscopy (AFM) is used to examine the surface morphology and roughness of the prepared membranes. The morphological measurements are carried out on the atomic force microscope Ara (Ara. AFM, model N0.0101/A, Iran). Small squares of the prepared membranes (approximately 1 cm2) were cut and glued on glass substrate.
2. Experimental
The IEC values are determined by a titration approach. A small fraction of each sample is immersed in 1 M H2SO4 solution for 24 h. The samples are washed with distilled water to remove excess amount of H2SO4, then equilibrated with 50 mL of 1 M NaCl solution, and finally heated to 40 °C for 24 h allowing the exchange between protons and sodium ions to be completed. The resulting solution is titrated with 0.01 N NaOH solution in the presence of phenolphthalein. The IEC value is estimated using the following equation:
2.4. Water uptake and swelling ratios Water uptake of the membranes is monitored by the weight change of the samples due to hydration. In this way, a small piece of the membrane is immersed in deionized water for 24 h at 25 °C and then the wet sample weight obtained. Meanwhile, this test was performed three times for each sample. 2.5. Ion exchange capacity (IEC)
2.1. Materials Poly (vinyl alcohol) (PVA, Mw = 27,000 g/mol) and diphenylamine-4-sulfonic acid sodium salt are supplied from Fluka. Glutaraldehyde (25 wt% solution in water) and SiO2 nanoparticles, average particle size 10–20 nm, are purchased from Sigma-Aldrich. Sodium hydroxide, sodium chloride and sulfuric acid (H2SO4) are also purchased from Merck. De-ionized (DI) water is used for the all experiments.
IEC = (VNaOH × 0.01)/Wdry
(1)
where, VNaOH is the volume of NaOH used in the titration, and Wdry is the dry weight of the membrane in g. The ion exchange experiment was repeated three times for each sample.
2.2. Membrane preparation PVA aqueous solutions are prepared in double distilled water under stirring at 50 °C for 4 h by solution casting method. Then, 80 wt% of diphenylamine-4-sulfonic acid sodium salt is mixed with the solution under stirring for 4 h. The desired amount of GA (0.2, 0.3, 0.4 and 0.5 mL) is added to the PVA solutions under acidic conditions (pH = 2.5 to 3.1) and the resultant viscous solutions transferred to petri-glass dishes. The films are dried at ambient temperature and subsequently cured in a vacuum oven at 50 °C for 24 h. In order to highlight the role of silica nanoparticles on the membrane properties, 5 wt% of SiO2 nanoparticles is added to the PVA solution having the sulfonic acid salt and ultrasound samples taken for 2 h to obtain well homogeneous mixtures. Finally, the optimum value of the crosslinker (0.5 ml) is added to the solution and the membrane fabricated according to the same procedure as described earlier.
2.6. Solvent extraction The solvent extraction method is employed to determine the degree of crosslinking of the membranes. A small portion of the samples is covered in filter paper and kept in distilled water at room temperature. The solvent replaced in every 15 h until no further solubility is observed in the polymer. The remaining part of the samples is then dried and weighed. 2.7. Proton conductivity measurement Proton conductivity of the fabricated membranes is obtained using impedance spectroscopy. A frequency response detector (EG&G model 1025) with an electrochemical setup that works under M 398 software is used for the measurements over a frequency range of 0.005 Hz to 100 kHz and at a sinusoidal potential of 10 mV. Prior to the impedance tests, each membrane sample is cut into pieces with dimensions of 1 cm × 1 cm and then kept immersed in 1 M H2SO4 solution for 24 h. The membranes are sandwiched between two brass plates acting as anode and cathode electrodes, and the proton conductivity obtained using the following equation:
2.3. Membranes characterization The FT-IR spectra of the fabricated PVA membranes are recorded over the frequency range 500–4000 cm−1 by a spectrometer (Thermo Nicolet, model Avatar 370). Thermal stability of the fabricated composite membranes is evaluated by a thermogravimetry analyzer (BÄHR Thermoanalyse GmbH STA 450, Germany). The programmed heating rate was 10 °C/min from 25 to 600 °C under a dry nitrogen atmosphere
σ = L/RA 13
(2)
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S. Shabanpanah and A. Omrani
where, σ is the proton conductivity (S/cm), L is the membrane thickness (cm), R is the resistance from the impedance data (Ω), and A is the cross-sectional area of the membrane (cm2).
considerably with an increase in the content of glutaraldehyde confirming major part of the hydroxyl groups converted to the acetal rings and ether linkages.
2.8. Methanol permeability measurement
3.2. Morphology of the prepared membranes
Resistances to methanol crossover of the membranes are evaluated using gas chromatography equipped with a thermal conductivity detector (GC-7890A with a DB-5 column, Agilent, USA). For this purpose, a glass diffusion cell is employed to measure methanol permeability. The membranes are clamped between two compartments A and B (40 ml, respectively). Compartments of (A) and (B) are filled by 2 M methanol solution and double distilled water, respectively. Magnetic stirrers are used in each compartment to ensure uniform concentration in each compartment during the experiment. The methanol concentration in the receiving compartment as a function of time is given by the following equation:
SEM photographs of the crosslinked PVA membranes are presented in Fig. 2. The PVA membrane showed a flat and smooth surface morphology as evidenced in Fig. 2a. Crosslinking by GA results in a considerable change of the surface morphology (Fig. 2b and c). A distinct and uniform layered morphology is the characteristic of crosslinked PVA which could be seen in Fig. 2b. When the concentration of GA increased from 2 wt% (case of PVA1 membrane) to 5 wt% (case of PVA4/SiO2 membrane) the surface morphology changed significantly. The layered structure has been completely destroyed in the case of PVA1 membrane and a spongy structure with some hollow pores (Fig. 2c) was obtained for the PVA4/SiO2 membrane. The decrease in the available free volume of the membranes with addition of the crosslinker content has some contributions to the formation of acetal covalent linkages after crosslinking reaction. Energy-dispersive X-ray (EDXA) spectrum of the crosslinked PVA membrane consisting SiO2 nanoparticles is shown in Fig. 2d. The presence of Si, Na, and S elements in the spectrum is a directly evidence for the existence of the sulfonic acid salt and silica nanoparticles within the composite membrane.
P = CB (t) VB L/A CA (t − t 0)
(3)
where, CB and CA are the methanol concentrations in the two compartments. A, L, and VB are the membrane effective area, membrane thickness, and volume of permeated compartment, respectively. 3. Results and discussion 3.1. FT-IR ATR spectroscopy
3.3. XRD results Attenuated total reflection Fourier transform infrared spectroscopy (FT-IR ATR) is used to characterize the existence of specific chemical groups in the membranes. Fig. 1 represents the ATR FT-IR spectra of the pure PVA, PVA1, PVA2, PVA3 and PVA4 samples. The large bands are observed at around 3000 and 3400 cm−1 (region (I)) attributed to –OH group stretching of the PVA chain, which indicates the presence of hydroxyl groups [18]. The band occurring at around 2924 cm−1 (region (II)) is corresponding to the stretching of –CH groups [19]. Obviously, after cross-linking with GA, intensity of the -OH peak is abridged and shifted towards higher frequency. The formation of ether bonds (C-O-C) as the result of reaction between the hydroxyl groups of PVA and –CHO groups of GA is previously demonstrated in the literatures [20]. The characteristic bands at 1250–1270 cm−1 are accordingly attributed to the formation of C-O-C linkage. Moreover, the carbonyl group stretching at 1720 cm−1 (region (III)) is observed in the crosslinked membranes. Besides, the absorption peaks at 1030–1145 cm−1 and 830 cm−1 are assigned to S]O and SeO stretching vibrations of SO3− groups [21]. The observed bands at around 800 and 1050 cm−1 are characteristic of Si-O-Si symmetric and asymmetric vibrations, respectively, which confirms the presence of silica in PVA4/SiO2 membrane. However, the intensity of the OeH vibration peak decreased
The structural changes created by the presence of GA are evaluated using X-ray diffraction. Fig. 3 shows the diffraction patterns of the PVA membranes crosslinked by 2 to 5 wt% of GA. According to Fig. 3, the peak occurring at 2θ = 20° is attributed to the pure PVA membrane. Clearly, this characteristic peak is somewhat broadened and its intensity reduced by an increase in the crosslink density. The decreased concentration of the hydroxyl groups of PVA owing to crosslinking reaction is responsible for the reduced crystallinity [22]. This indicates that the crosslinked PVA membrane becomes more amorphous and the amount of amorphous portion increases by the crosslinking reaction. Also, the amorphous character is developed in the presence of SiO2 nanoparticles. The decreased crystallinity could be described based on a reduction in the hydroxyl groups concentration of the PVA side-chain because of the reaction with hydroxyl groups of silica nanoparticles and GA. 3.4. Water uptake and swelling ratio analysis Fig. 4 shows the changes in water uptake and swelling ratio of the composite membranes in terms of GA content. What is clear is that water absorption capacity of the membranes is decreased with increasing the crosslinker concentration. Apparently, the crosslinking reaction decreased the PVA's hydrophilicity based on a reduction in the number of water molecules adsorbed on the membrane surface. In other words, the reduced free space in the membranes will make its structure harder, hence, the water absorption capacity decreased. The similar trend is observed for the changes in swelling ratio by GA content as can be seen in Fig. 4. Again, an increase in the crosslinker concentration results in a decrease of the swelling ratio. This can be described based on a reduction in the polymer chains mobility causing the membrane structure be more compact. Such compact structures have the limited available free space for swelling. 3.5. IEC evaluation Changes in the value of IEC with the crosslinker concentration in the composite membranes are shown in Fig. 5. The highest IEC value is obtained for the PVA1 sample where the content of GA was the
Fig. 1. FT-IR spectra of: (a) PVA, (b) PVA1, (c) PVA2, (d) PVA3, (e) PVA4 and (f) PVA4/SiO2 membranes. 14
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Fig. 2. FE-SEM images of the pure PVA (a), PVA1 (b) and PVA4/SiO2 (c) membranes, and EDXA profile of the PVA4/SiO2 membrane (d).
Fig. 3. X-ray diffraction patterns of: (a) pure PVA, (b) PVA1, (c) PVA2, (d) PVA3, (e) PVA4 and (f) PVA4/SiO2 membranes.
Fig. 4. Water uptake and swelling ratios of the PVA membranes.
3.6. Crosslink density assessment
minimum value of 2 wt%. This can be described according to the membrane structure. The membrane structure becomes more compact as the content of GA increased. Therefore, the available free sites in its structure, where the protons could be exchanged, are decreased.
The variation of crosslink density with GA concentration is shown in Fig. 6. Clearly, the crosslink density improved as the content of GA increased. The PVA4 sample showed the highest amount of crosslink density, i.e. 33%. 15
Solid State Ionics 338 (2019) 12–19
S. Shabanpanah and A. Omrani
Fig. 8. The Arrhenius plot of a) PVA1 b) PVA2, c) PVA3, d) PVA4.and e) PVA4/ SiO2 membranes.
Fig. 5. Ion exchange capacity of the PVA membranes.
3.7. Proton conductivity measurement Fig. 7 represents the effect of GA concentration on the membrane's AC impedance spectra and proton conductivity at 25 °C. When the crosslinker content in the membrane composition is increased from 2 to 5 wt% a significant decrease in the conductivity observed (about 463.6%). The observed decreasing trend of proton conductivity suggests that an increase in GA content makes compression of the membrane structure via filling its pores preventing the proton transfer process. This phenomenon ultimately reduces the proton conductivity of the membranes having higher amount of GA [17,24,25]. Temperature dependence of the conductivity for the fabricated membranes are determined. Generally, the change in the proton conductivity with temperature is complied with an Arrhenius type equation which can be used to obtain the activation energy (Ea) of proton transfer. The Ea values are obtained from the linear regression of lnσ vs 1000/T plots. The results are shown in Fig. 8 and Table 1. Obviously, the ionic conductivity is decreased by increasing the degree of crosslinking. As it is expected, the proton transfer facilitates at higher temperatures. Due to development of a compact structure, the activation energy is increased by increasing the content of GA. As evidenced earlier, the water uptake is diminished and hence, the Ea values increased.
Fig. 6. Crosslink density of the synthesized PVA membranes.
As it is demonstrated, the formation of acetal ring networks in the crosslinked membranes reduces their solubility considerably via forming a hydrophobic protective layer [23].
Fig. 7. The Nyquist plots of (1) PVA1, (2) PVA2, (3) PVA3, (4) PVA4 and (5) PVA4/SiO2 membranes (a) and the corresponding proton conductivities (b). 16
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Table 1 Apparent activation energy (Ea) of proton conduction for the different PVA membranes. Membrane
% additive
Ea/kJ mol−1
PVA/GLA/salt PVA/GLA/salt PVA/GLA/salt PVA/GLA/salt PVA/GLA/salt/SiO2
GA GA GA GA GA
9.8 10.3 10.6 10.9 11.7
2 3 4 5 5/SiO2 5
Fig. 10. Changes in the tensile strength, elongation, and modulus of the crosslink PVA membranes as a function of the sample composition.
obtained for the PVA1 membrane which is 95.6% of Nafion 117 conductivity. 3.9. Tensile test results The mechanical properties are one of the key parameters for polymeric membranes. Fig. 10 shows the effect of crosslink density on the membrane's tensile strength, elongation, and Young's modulus at failure point. With further increase in the crosslink density, the tensile strength of the membranes is enhanced with the maximum value of 48.39 MPa for the PVA4 sample. Also, the percent elongation at break is decreased by increasing the crosslink density. The enhancement of mechanical strength is due to the change of the membrane structure after making crosslinked networks which restricted the mobility of the PVA chains [32]. Similarly, the addition of silica nanoparticles to the membrane composition having 5 wt% of the crosslinker improved the mechanical stability as well.
Fig. 9. Methanol permeability of the PVA membranes.
3.8. Evaluation of methanol permeability Resistance to methanol crossover of the membranes are calculated using Eq. (3), and the results shown as a function of the crosslinker concentration in Fig. 9. Methanol permeability of Nafion 117 was measured under the same experimental conditions yielding a value of 1.83 × 10−6 cm2/s which is in good agreement with the value reported in the literature [25]. Interestingly, the methanol permeability of all the crosslinked PVA membranes is lower than that of Nafion 117. The methanol permeability is found to be 4.38 × 10−7 cm2/s for the crosslinked membrane containing 2 wt% of GA (PVA1 membrane) and decreased continuously until reaching a value of 1.37 × 10−7 cm2/s for the crosslinked membrane having 5 wt% of GA (PVA4 sample). Generally, the change in the methanol permeability of the membranes might be attributed to the crosslinking effect of the PVA matrix by glutaraldehyde. Further decrease of the methanol permeability is achieved by incorporation of SiO2 nanoparticles (5 wt%) within the membrane composition. The lowest amount of 1.20 × 10−7 cm2/s is obtained for the PVA4/SiO2 hybrid membrane. However, it may be implied that the simultaneous actions of GA and SiO2 nanoparticles in the PVA membranes result in a blocking effect for methanol transport. Data in Table 2 highlights that the highest proton conductivity is
3.10. Evaluation of the membranes thermal stability The thermal stability of the membranes is evaluated with TGA and the results are shown in Fig. 11. According to the previously reports, thermal degradation of the PVA membranes occur in three main steps: loss of water or moisture absorbed from the membrane, degradation of inter-component bonds by breaking of the side chains, and breakdown of the polymer backbone [33–36]. To compare thermal stability of the synthesized membranes, the TGA curves are analyzed further by defining two parameters of Tonset and Wresidual. Tonset is the temperature at which the weight loss of the membrane sample is 5% while Wresidual shows the value of char yield at 600 °C. The values of Tonset are 136.9, 143.6, 157.5, 160.2, and 170.3 °C for the PVA1, PVA2, PVA3, PVA4, and
Table 2 Properties of Nafion 117 and various PVA membranes reported in references at ambient temperature. Membrane
Conductivity (S/cm)
IEC (m equiv g−1)
Water uptake (%)
Methanol permeability (cm2/s × 10−6)
Ref
PVA/SSA PVA/SSA/GA PVA/SiO2/SSA PVA/PAMPS PVA/SPES PVA/SA/GA PVA/CS/GA Nafion 117 PVA/Nafion PVA membrane
0.001–0.01 0.01–0.0526 0.001–0.01 0.099–0.11 0.0021–0.0051 0.01 0.0099 0.064 0.0009–0.02 0.0129–0.0612
0.5–2.24 0.576–2.676 0.2–1.1 1.71–1.91 0.62–1.24 0.79 0.62 0.91 0.09–0.91 0.12–0.52
10–80 – 45–60 136–163 29.3–35 – – 33.6 13–43 90–250
0.1–1 0.14–2.9` 0.01–0.1 – 0.0468–0.0614 0.069 0.094 2 0.31–0.65 0.12–0.438
[25] [23] [26] [9] [27] [28] [28] [29] [30,31] This work
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Solid State Ionics 338 (2019) 12–19
S. Shabanpanah and A. Omrani
Fig. 11. TGA profiles of (a) PVA1, (b) PVA2, (c) PVA3, (d) PVA4, and (e) PVA4/SiO2 composite membranes.
the membranes. A possible interpretation is that the reduce of hydrophilicity can lead to lower water content and ion exchange rate, and decrease the proton conductivity.
Table 3 Results of the PVA membranes from AFM experiment. Membranes
Ra (nm)
Rq (nm)
PVA PVA1 PVA2 PVA3 PVA4 PVA4/SiO2
0.9875 1.5885 1.8947 3.9222 4.0171 7.0363
1.423 2.0936 2.6408 4.8107 4.9843 9.1658
4. Conclusions The novel crosslinked PVA/diphenylamine-4-sulfonic acid sodium salt membranes with varied glutaraldehyde concentration are synthesized via solution casting. Increased crosslink density resulted in reductions of water uptake, swelling, IEC, and proton conductivity while thermal and mechanical properties of the membranes are improved. Also, methanol permeability of the PVA membranes is decreased by increasing the crosslinker concentration and in the presence of silica nanoparticles. It is demonstrated that by increasing the crosslink density the membrane sites are filled and hence, its mobility rate decreased. Observations also revealed that the presence of 5 wt% silica nanoparticles increases the thermal and mechanical stability of the membranes, therefore, the membrane is stable. It is also noticed that adding silica nanoparticles did not change the amount of ion exchange, water uptake, and proton conductivity of PVA4 membrane. In this case, the presence of crosslinker seems to have a greater impact on the properties of the system.
PVA4/SiO2 membranes, respectively. These results showed that both the crosslinking reaction with GA and reinforcement by SiO2 nanoparticles enhanced the thermal stability of PVA-based membranes. Correspondingly, the values of Wresidual at 600 °C are 12.1%, 14.7%, 17.2%, 20.7%, and 25.3% for the PVA1, PVA2, PVA3, PVA4, and PVA4/ SiO2 membranes, respectively. The highest value of char yield for the PVA4/SiO2 membrane confirmed that the flame retardancy property of the membrane is improved owing to crosslinking and reinforcement. The strong interaction between PVA and GA reduces the mobility of polymer segments resulting in an improvement of the thermal stability. Moreover, results from this part highlights that the incorporation of SiO2 nanoparticles in the membrane composition leads to an enhancement of the decomposition temperature.
Acknowledgement The financial support from University of Mazandaran, Iran, (grant ID: 93172322102) is gratefully acknowledged.
3.11. Atomic force microscopy (AFM) observations In order to have deep insight on the surface morphology of the synthesized membranes, the atomic force microscopy (AFM) test is carried out. Table 3 shows the results of average roughness (Ra) and root mean square (RMS or Rq) roughness obtained from the AFM images. By comparing the results shown in Table 3, it can be noticed that the surface of pure PVA membrane is quite smooth. However, by increasing the crosslinker concentration the surface of membranes become further rough. Images recorded for PVA4/SiO2 membrane confirm that SiO2 nanoparticles were successfully coated onto the membrane and produced rougher surfaces. The average surface roughness for this membrane is 7.0363 nm which is considerably high when compared to the rest of the membranes demonstrating that incorporation of SiO2 nanoparticles can significantly improve surface roughness. AFM analysis reveals a possible correlation between the average roughness and hydrophilicity of
References [1] J. Pandey, A. Shukla, PVDF supported silica immobilized phosphotungstic acid membrane for DMFC application, Solid State Ionics 262 (2014) 811–814. [2] S.L. Chavan, D.B. Talange, Modeling and performance evaluation of PEM fuel cell by controlling its input parameters, Energy. 138 (2017) 437–445. [3] Y. Devrim, S. Erkan, N. Baç, I. Eroglu, Nafion/titanium silicon oxide nanocomposite membranes for PEM fuel cells, Int. J. Energy Res. 37 (2012) 435–442. [4] F.A. Zakil, S.K. Kamarudin, S. Basri, Modified Nafion membranes for direct alcohol fuel cells: an overview, Renew. Sust. Energ. Rev. 65 (2016) 841–852. [5] D.S. Kim, M.D. Guiver, M.Y. Seo, H.I. Cho, D.H. Kim, J.W. Rhim, G.Y. Moon, S.Y. Nam, Influence of silica content in crosslinked PVA/PSSA-MA/silica hybrid membrane for direct methanol fuel cell (DMFC), Macromol. Res. 15 (2007) 412–417. [6] H.B. Park, S.Y. Nam, J.W. Rhim, J.M. Lee, S.E. Kim, J.R. Kim, Y.M. Lee, Gas transport properties through cation-exchanged sulfonated polysulfone membranes, J. Appl. Polym. Sci. 86 (2002) 2611–2617.
18
Solid State Ionics 338 (2019) 12–19
S. Shabanpanah and A. Omrani
[22] C. Shao, H.Y. Kim, J. Gong, B. Ding, D.R. Lee, S.J. Park, Fiber mats of poly (vinyl alcohol)/silica composite via electrospinning, Mater. Lett. 57 (2003) 1579–1584. [23] C.E. Tsai, C.W. Lin, B.J. Hwang, A novel crosslinking strategy for preparing poly (vinyl alcohol)-based proton-conducting membranes with high sulfonation, J. Power Sources 195 (2010) 2166–2173. [24] V. Kumar, P. Kumar, A. Nandy, P.P. Kundu, Crosslinked inter penetrating network of sulfonated styrene and sulfonated PVdF-co-HFP as electrolytic membrane in a single chamber microbial fuel cell, RSC Adv. 5 (2015) 30758–30767. [25] J.W. Rhim, H.B. Park, C.S. Lee, J.H. Jun, D.S. Kim, Y.M. Lee, Crosslinked poly (vinyl alcohol) membranes containing sulfonic acid group: proton and methanol transport through membranes, J. Membr. Sci. 238 (2004) 143–151. [26] D.S. Kim, H.B. Park, J.W. Rhim, Y.M. Lee, Preparation and characterization of crosslinked PVA/SiO2 hybrid membranes containing sulfonic acid groups for direct methanol fuel cell applications, J. Membr. Sci. 240 (2004) 37–48. [27] D. Kumar, S.S.A.S. Meenakshi, A. Subbiah, N. Alagumalai, R. Dipak, Sulfonated poly (ether sulfone)/poly (vinyl alcohol) blend membranes customized with tungsten disulfide nanosheets for DMFC applications, Polymer. 155 (2018) 42–49. [28] S. Mohanapriya, S.D. Bhat, A.K. Sahu, A. Manokaran, R. Vijayakumar, S. Pitchumani, A.K. Shukla, Sodium-alginate based proton exchange membranes as electrolytes for DMFCs, Energy Environ. Sci. 3 (2010) 1746–1756. [29] M. Ranjani, D.J. Yoo, G.G kumara, Sulfonated Fe3O4@SiO2 nanorods incorporated sPVdF nanocomposite membranes for DMFC applications, J. Membr. Sci. 555 (2018) 497–506. [30] N.W. DeLuca, Y.A. Elabd, Direct methanol fuel cell performance of Nafion®/poly (vinyl alcohol) blend membranes, J. Power Sources 163 (2006) 386–391. [31] N.W. DeLuca, Y.A. Elabd, Nafion®/poly (vinyl alcohol) blends: effect of composition and annealing temperature on transport properties, J. Membr. Sci. 282 (2006) 217–224. [32] A. Anis, A.K. Banthia, S. Bandyopadhyay, Synthesis & characterization of PVA/STA composite polymer electrolyte membranes for fuel cell application, J. Mater. Eng. Perform. 17 (2008) 772–779. [33] D.K. Lee, J.T. Park, J.K. Choi, D.K. Roh, J.H. Lee, Y.G. Shul, J.H. Kim, Proton conducting crosslinked membranes by polymer blending of triblock copolymer and poly (vinyl alcohol), Macromol. Res. 16 (2008) 549–554. [34] P. Duangkaew, J. Wootthikanokkhan, Methanol permeability and proton conductivity of direct methanol fuel cell membranes based on sulfonated poly (vinyl alcohol) layered silicate nanocomposites, J. Appl. Polym. Sci. 109 (2008) 452–458. [35] K. Das, D. Ray, N.R. Bandyopadhyay, A. Gupta, S. Sengupta, S. Sahoo, A. Mohanty, M. Misra, Preparation and characterization of crosslinked starch/poly (vinyl alcohol) green films with low moisture absorption, Ind. Eng. Chem. Res. 49 (2010) 2176–2185. [36] C.C. Yang, Fabrication and characterization of poly (vinyl alcohol)/montmorillonite/poly (styrene sulfonic acid) proton-conducting composite membranes for direct methanol fuel cells, Int. J. Hydrog. Energy 36 (2011) 4419–4431.
[7] J.F. Blanco, Q.T. Nguyen, P. Schaetzel, Novel hydrophilic membrane materials: sulfonated polyethersulfone Cardo, J. Membr. Sci. 186 (2001) 267–279. [8] M.S. Boroglu, S. Cavus, I. Boz, A. Ata, Synthesis and characterization of poly (vinyl alcohol) proton exchange membranes modified with 4,4-diaminodiphenylether-2,2disulfonic acid, Express Polym Lett 5 (2011) 470–478. [9] J. Qiao, T. Hamaya, T. Okada, New highly proton conductive polymer membranes poly(vinyl alcohol)–2-acrylamido-2-methyl-1-propanesulfonic acid (PVA–PAMPS), Polymer. 46 (2005) 10809–10816. [10] D.S. Kim, H.B. Park, J.W. Rhim, Y.M. Lee, Proton conductivity and methanol transport behavior of cross-linked PVA/PAA/silica hybrid membranes, Solid State Ionics 176 (2005) 117–126. [11] A.M. Attaran, K. Hooshyari, M. Javanbakht, M. Enhessari, Fabrication and characterization of poly vinyl alcohol/poly vinyl pyrrolidone/MnTiO nanocomposite membranes for PEM fuel cells, Iran. J. Energy Environ. 4 (2013) 86–90. [12] C.W. Liew, S. Ramesh, A.K. Arof, A novel approach on ionic liquid-based poly (vinyl alcohol) proton conductive polymer electrolytes for fuel cell applications, Int. J. Hydrog. Energy 36 (2013) 2917–2928. [13] Y.W. Chang, E. Wang1y, G. Shin, J.E. Han, P.T. Mather, Poly (vinyl alcohol) (PVA)/ sulfonated polyhedral oligosilsesquioxane (sPOSS) hybrid membranes for direct methanol fuel cell applications, Polym. Adv. Technol. 18 (2007) 535–543. [14] F. Arranz, C.M. Sanchez, R. Martinez, Reaction of poly (viny1 alcohol) with n-butyl isocyanate, Angew. Makromol. Chem. 152 (1987) 79–91. [15] S.V. Caro Jr., C.S. Paik Sung, E.W. Merrill, Reaction of hexamethylene diisocyanate with poly (vinyl alcohol) films for biomedical applications, J. Appl. Polym. Sci. 20 (1976) 3241–3246. [16] K.J. Kim, S.B. Lee, N.W. Han, Effects of the degree of crosslinking on properties of poly (vinyl alcohol) membranes, Polym. J. 25 (1993) 1295–1302. [17] R. Rudra, V. Kumar, P.P. Kundu, Acid catalyzed cross-linking of poly vinyl alcohol (PVA) by glutaraldehyde: effect of crosslink density on the characteristics of PVA membranes used in single chambered microbial fuel cells, RSC Adv. 5 (2015) 83436–83447. [18] A.M. Abu-Saied, R. Wycisk, M.M. Abbassy, G.A. El-Naim, F. El-Demerdash, M.E. Youssef, H. Bassuony, P.N. Pintaro, Sulfated chitosan/PVA absorbent membrane for removal of copper and nickel ions from aqueous solutions fabrication and sorption studies, Carbohydr. Polym. 165 (2017) 149–158. [19] H. Awada, C. Daneault, Chemical modification of poly (vinyl alcohol) in water, Appl. Sci. 5 (2015) 840–850. [20] H. Beydaghi, M. Javanbakht, H.S. Amoli, A. Badiei, Y. Khaniani, M.R. Ganjali, M. Abdouss, Synthesis and characterization of new proton conducting hybrid membranes for PEM fuel cells based on poly (vinyl alcohol) and nanoporous silica containing phenyl sulfonic acid, Int. J. Hydrog. Energy 36 (2011) 13310–13316. [21] C.Y. Tseng, Y.S. Ye, K.Y. Kao, J. Joseph, W.C. Shen, J. Rick, B.J. Hwang, Interpenetrating network-forming sulfonated poly (vinyl alcohol) proton exchange membranes for direct methanol fuel cell applications, Int. J. Hydrog. Energy 36 (2011) 11936–11945.
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