Essay: Dielectric performance and response on electrical charge transport of nanocomposite SPEEK electrolyte membrane for Fuel Cell Applications

Abstract
Polymer nanocomposite proton exchange membranes (PEMs) based on Sulfonated Poly (ether ether ketone) and varied content of MMT nanoclay is developed by solution casting method. The degree of sulfonation of synthesized SPEEK polymer is 64% obtained from the 1H-NMR peak integration. The chemical and structural interactions of the membranes are explored by using FT-IR, XRD. The water uptake and methanol uptake of the films are studied and reported. The thermal stability of the membrane is evaluated by TGA. The composite membrane with 1 wt.% of MMT nanoclay exhibits the enhanced proton conductivity of 1.108610-1 S cm-1 at 70 °C under 100% relative humidity (RH) condition. The dielectric properties such as dielectric constant (ε’), dielectric loss (ε”), dissipation factor (tan δ), electrical modulus (M’ and M”) and Scaling behavior of the membranes is analyzed as a function of frequency with the support of conductivity data. The obtained results demonstrate that the developed composite membrane serving as a potential candidate for electrochemical applications.
Keywords: Organic polymer, composite materials, Proton exchange membrane, Energy conversion, Dielectric properties.
1. Introduction
Energy crisis due to rapid depletion of fossil fuel, global warming, and environmental pollution are the major critical problems to the modern society. Today the world is urging to discover a new clause of renewable energy with free of pollution to protect the environment, low cost and high efficiency in energy conversion. Fuel cell is the candidate in which no need to charge and discharge like batteries [1, 2]. Among the various fuel cells, Direct Methanol Fuel Cells (DMFC) possess advantageous features such as high energy density, sustained operation, low operating temperature, low in cost, volume, pollution and fast start up. Methanol is one of the most promising fuel in comparison with the other organic fuels due to its high energy density, low cost, high solubility in aqueous electrolyte and easier to store and transport. DMFC converts the chemical energy of methanol to electricity with reduced pollution [3]. Polymer electrolyte membrane(PEM) is a unique component which not only conducts protons between anodic and cathodic compartment but also acts as a barrier to avoid the crossover of fuel, separating the two catalytic electrodes where the fuel oxidation and comburent reduction takes place [1, 4]. It is recognized that Nafion is the most widely used electrolyte membrane due to their excellent proton conductivity, mechanical and chemical stability. However, it suffers from high methanol permeability, reduction in proton conductivity at low relative humidity and high cost. In such a way that Nafion makes the DMFC system too expensive for targeted application [5-7]. Researchers explored the feasibility of hydro-carbon based membranes as an alternative to eliminating the shortcomings of Nafion from the past recent days. Therefore, the development of a PEM material which exhibits higher selectivity, substantially cheaper compared to Nafion is the need of the hour [8, 9]. The number of sulfonated polymers have been developed for this purpose among them Poly Ether Sulfone (PES)[10], Poly Ether Ether Ketone (PEEK) [11, 12], Polyimide (PI) [13], Polysulfone (PS) [14], Poly (vinyl alcohol) (PVA) [15], Poly (vinylidene fluoride) (PVdF) [16] have been synthesized and substituted as potential replacement with Nafion.
The basic polymer is Poly Ether Ether Ketone (PEEK) have high thermo-oxidative stability and excellent chemical resistance [17]. In order to provide hydrophilicity to the hydrophobic PEEK polymer, it is sulfonated by the use of sulfuric acid. Sulfonated PEEK is obtained next to electrophilic substitution of the sulfonic group in the polymer backbone whose degree of sulfonation can be controlled by reaction kinetics [18]. The hydrophilic/hydrophobic phase separation in SPEEK is narrower and more branched, results in lower electro-osmatic drag and permeation coefficient in comparison to Nafion. SPEEK displays more appealing advantages such as superior film forming property, suitable thermal stability, high mechanical strength, lower fuel cross over and sufficient conductivity, all of which depend upon the degree of sulfonation [8, 19]. Incorporation of nano scale clay particles within the polymer matrix may enhance the water retention, mechanical stability, withstand relatively at high temperatures. The nanoscale dispersion of inorganic nano layers into polymer matrix strongly affects the permeation rate of fuel across the membrane [18]. Clays can be easily exfoliated to individual platelet inducing extremely large surface area, a fine interface between the filler and the polymer matrix. Different inorganic fillers such as Silica [20], Titania [21], Montmorillonite [22, 23], and Zirconia [24] are used to alter the properties of a pristine polymer. In the present work SPEEK based nanocomposite membranes are prepared with sulfonated Poly Ether Ether Ketone and MMT nanoclay has been analyzed by different characterization techniques such as XRD, TGA, AC impedance, etc., and their results are reported.
2. Experimental
2.1. Materials and Methods
Poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4phenylene) (PEEK) was obtained from Sigma-Aldrich. N,N-Dimethyl sulfoxide (DMSO), Sulfuric acid (H2SO4) (98%) was purchased from Merck, Montmorillonite (MMT) from Himedia. PEEK pellets were ground to reduce the dissolution time of polymer and dried in vacuum oven for 24 hours at 100 °C to remove the moisture content. 100 ml of (98%) sulfuric acid was firstly transferred into a conical flask at room temperature, and then 5 gm of PEEK was slowly added (≈1hr) under continuous vigorous stirring was maintained during the course of reaction. The mass remained heterogeneous initially and later it is completely dissolved. The degree of sulfonation can be varied by changing the residence time and temperature in the reaction mixture then it was precipitated by dropwise addition of SPEEK polymer solution into excess of de-ionized ice water under continuous agitation to terminate the sulfonation reaction and it was left overnight. The sulfonated PEEK in the form of numerous fibers were gathered from filter paper and washed repeatedly with de-ionized water until reaching neutral pH. As a result, the obtained sulfonated PEEK was dried at room temperature overnight and was further dried in vacuum oven at 80 °C for 24 hours. Now for the PEM preparation, the SPEEK polymer was dissolved in DMSO to form 10 wt.% solution and appropriate weight percentages of MMT were then added to the dissolved polymer solution. The resulting mixture was stirred at 50 °C for 6 hours. It is then inferred that slow and gradual drying procedure resulted in the free-standing and flexible membrane. The thickness of the membranes is estimated around 6-8 mm by the micrometer screw gauge.
The degree of sulfonation (DS) was determined by using 1H-NMR Spectroscopy. The spectra were recorded using a Brucker 300 MHz spectrometer. The DS values of the sulfonated polymers were determined by comparing the intensities of specific signals. To identify the functional groups present and their interaction in the composites, the membranes are subjected to FTIR analysis. FTIR spectra were recorded with Perkin Elmer Pyris FTIR Spectrophotometer at a resolution of 4 cm-1 over a frequency range of 4000 cm-1 to 400 cm-1. The structural stability and molecular level miscibility of the membrane were revealed by Powder X-ray diffraction (XRD) technique (X’pert PAN analytical diffractometer). Thermo Gravimetric analysis (TGA) of the membranes was performed with NETZSCH STA 449F3 under nitrogen atmosphere with scanning rate at 10 °C min-1. Liquid uptake of the membrane was intended by measuring the weight difference under completely dry and wet condition. Transverse direction proton conductivity and dielectric response of the membrane samples were investigated over the frequency range of 10 Hz to 10 MHz with 5 mV oscillating voltage using frequency response analyzer (FRA) (HI604D).
3. Results and Discussion
3.1. Degree of Sulfonation
Sulfonation modifies the chemical character of PEEK, reduces the crystallinity and stability of the polymer. The Degree of Sulfonation (DS) has been determined quantitatively by 1H-NMR spectra. For analysis, 3 wt.% polymer solution is prepared and the test is conducted at room temperature with Tetra Methylsilane (TMS) as internal standard reference. The 1H-NMR spectrum of SPEEK polymer is shown in Fig. 1. The presence of sulfonic acid group causes a characteristic singlet (H13) at 7.5 ppm. The intensity at H13 position is equivalent to the content of SO3H group. The DS is determined by comparative integration of distinct aromatic signals [3, 11]. The ratio between peak area of distinct signal (AH13), and the integrated peak area (∑AH1,2,3,4,5,6,7,8,9,10,12,14,15,16 ) of the signals corresponding to all other aromatic hydrogens can be expressed by the following equation
n⁄((12-2n))=〖AH〗_13⁄(∑AH_(integrated signal) ) (0 ≤ n ≤ 1) … (1)
where n is the number of H13 per repeat unit.
The degree of sulfonation is obtained from the equation of
DS%=n×100
It is estimated that the degree of sulfonation for the prepared SPEEK is 64%. The DS determines the solubility of SPEEK among the solvents. The SPEEK produced for polymer electrolyte membrane have high mechanical and thermal stability only if the DS is maintained between 40-70% [28]. Ion Exchange capacity (IEC) of SPEEK has been calculated theoretically from the DS of SPEEK and its value is 1.886 meq/g.
3.2. Structural Analysis
The comparative FTIR spectra with PEEK, SPEEK, MMT and SPEEK/MMT composite membranes, which are shown in Fig. 2(a). The appearance of broadband at 3435cm-1 in the sulfonated samples is assigned to O-H vibration from sulfonic acid groups. The peak, which is observed at 1484 cm-1 is due to C-C aromatic ring in the PEEK. The sulfonation process causes the intensity of aromatic C-C band to be decreased and the peak is seen to split to absorption band at 1479 cm-1 and 1413 cm-1. The bands appeared at 1224 cm-1 and 1082 cm-1 are assigned to the asymmetric O=S=O and symmetric O=S=O vibrations of sulfonic acid groups confirming the successful sulfonation of PEEK. The other bands related to sulfonic acid groups in SPEEK appears at 1024 cm-1 and 710 cm-1 for S=O stretching and S-O stretching vibrations respectively [3,8]. The broad bands at 3625 cm-1, 3434 cm-1, and 1636 cm-1 are the stretching and bending vibrations for the hydroxyl groups of water molecules present in the MMT particles. The presence of bands at 3518 cm-1 (H-O-H stretching) and 1640 cm-1 (H-O-H bending) indicate the formation of composite membranes. The anti-symmetric Si-O stretching is observed at 1053 cm-1, 466 cm-1 and Si-O-Al vibrations at 797 cm-1 and 529 cm-1 [29,22].
XRD analysis is made to provide information about the structural changes due to the polymer sulfonation and addition of MMT nanoclay particles. Fig. 2(b) shows the XRD pattern of PEEK, which illustrates that the polymer chain of PEEK might have enough degree of freedom to fold and crystallize. The four main peaks of pure PEEK are found at 2θ equal to 18.87°, 20.7°, 23°, 28.9° and 33° these correspond to the diffractions of (110), (111), (200) and (211) crystalline planes respectively. Sulfonation of PEEK alters the chain conformation and packing thus bringing about a loss of crystallinity it is reflected in the XRD pattern of SPEEK, which explicit a mixture of both amorphous and crystalline nature with a broad peak around 20° [3, 30]. Zhang et al. reported that the crystalline peak of MMT is appeared at 2θ = 5.82° with silicate layer spacing of 1.51 nm in the crystalline plane d001. By observing the intensity, position and shape of the basal reflections from the silicate layers the nanocomposite structure (intercalated or exfoliated) may be identified [25]. From the XRD pattern of SPEEK/MMT, The reduction in the crystallinity of the composite matrix while the MMT is dispersed throughout the surface.
3.3. Thermal stability
Thermal stability of the membrane is an essential property to be studied to know the lifetime of proton exchange membrane in fuel cell. Fig. 3(a) shows the TGA profile of PEEK, SPEEK, and Fig. 3(b) shows TGA profile of SPEEK/MMT composite membranes. PEEK is a highly thermostable polymer with a decomposition temperature of 550 °C under moisture-free condition. A three step degradation profile is evident for SPEEK and SPEEK/MMT composite membranes. The first degradation below 220 °C corresponds to the loss of physically absorbed moisture and solvent within the membrane during sulfonation process. The second weight loss from 220-450 °C is due to decomposition of – SO3H groups of SPEEK. The third step thermal degradation started at about 450 °C is ascribed to the main chain degradation of host polymer matrix [32]. The range of thermal degradation start’s earlier in SPEEK than that of PEEK because the catalytic degradation of the polymer chain and the enhanced asymmetry in the SPEEK structure caused by SO3H group that makes it less regular, and therefore, less stable. All the other composite membranes were shown the improved thermal stability by means of adding nanoclay particles may be slowed down the out-diffusion of water from the SPEEK matrix which acts as barrier [33]. A decrease in weight loss is observed in the composite membrane with MMT filler, showing that not only the amount of clay was responsible but also the nanocomposite morphology, as these membranes presented an exfoliated structure which hindered the mass transfer efficiently [29]. Hence it is inferred that the nanoclay addition can improve thermal stability of composite matrix.
3.4. Liquid uptake and Proton Conductivity of the composite membrane
The transportation of proton from anode to cathode is supported by the water molecule is a desirable parameter in the fuel cell. Moreover, too little water absorption reduces proton transport while too much water absorption results in mechanical instability of the membrane [34]. The main purpose of sulfonating aromatic PEEK is to increase its acidity and hydrophilicity. The hydrophilic/hydrophobic phase separation of SPEEK is narrower and more branched with less hydrophilic interconnection channels passing through the hydrophobic domains. This minimizes the water uptake as well as swelling in SPEEK membrane [35]. The uptake responses were listed in Table 1. The membrane with low level of MMT showed higher water uptake than the others. The higher water uptake values might be due to the hydrophilic groups in the MMT. Agglomeration of the nano-clay particles may suppress the water sorption in the other membranes. The water content plays an important role in methanol permeability across the membranes because water absorption will enhance the flexibility of polymer chains and enlarge the size of water channels inside the membrane. Additionally more water content will cause more diffusion of methanol molecules, leading to an unfavorable effect on the methanol rejection ability of the membrane [36]. MMT is composed of silica tetrahedral and alumina octahedral sheets which help in preventing the transport of fuel such as methanol through the membrane [25]. The clay embedded in the polymer matrix prohibits the extreme swelling of the composite membrane.
Proton conductivity is the essential requirement to enable membranes to be applicable in DMFC. During measurement, the measuring cell with the membrane was immersed into water to assure the condition of 100% relative humidity at desired temperature. The conductivity (σ) of the samples are calculated from the impedance data by the following equation
σ=d/RS .…(2)
Where d, S, R are the thickness, Surface area, bulk resistance of the membrane sample.
The proton conductivity of membranes might depend on the level of DS, pretreatment of membrane, hydration state, ambient relative humidity, and temperature. The mechanism of proton transport is based on the surface and chemical properties of the interface between the inorganic and organic phases in the hybrid system. The proton conductivity results of different SPEEK/MMT composite membranes were listed in Table 1 and put on view in Fig. 4. Thus, the membranes charge transport responses were noted as the function of the varied concentration of MMT and temperature. Herein that, the Sulfonation hoists the conducting process of PEEK, not only by increasing the number of sulfonic acid (-SO3H) groups but also by forming water-mediated pathways for the proton transport. It can be observed that the pristine SPEEK membrane has lower proton conductivity as compared to other membranes with MMT. The formation of aggregates in the membranes of higher additive concentration retards ionic transportation through the membrane. In addition, the incorporation of silicate particles into the polymer may restrict the accessible nanometric channel for migration of polar molecules such as water, hydrogen, and methanol [37]. In contrast, the proton conductivity of all the membranes increases as the temperature increases and the membrane SPM1 approaches the highest value of 1.1086 × 10-1 S cm-1 at 70 °C.
3.5. Dielectric Studies
The dielectric polarization of the polymer electrolyte membrane can be desired by the following facts, the dipole alignment in the polymer, migration of ions within the materials and injection from electrodes [39]. In the PEEK, the main polarization mechanism is contributed by electronic and ionic polarization. Furthermore, the ionic polarization is dominated for sulfonated PEEK due to the addition of sulfonic acid groups in the polymer network [40]. The dielectric parameters are strongly influenced by the nature of additives and ambient conditions. Fig. 5 a&b shows the dielectric response of the electrolyte membranes over the frequency range of 10 Hz to 10 MHz. It is well known that the dielectric permittivity is strongly temperature dependent at low frequencies but not at high frequencies. Now the both, real (ε’) and imaginary (ε”) part of dielectric permittivity rises sharply at low frequencies indicating the space charge polarization effects at the electrode-electrolyte interface and confirms the non-Debye type of behavior. A rapid decrease in dielectric permittivity at high frequencies is due to the inability of dipoles to rotate themselves leading to a lag between oscillating frequency of the dipoles and that of applied field [42]. The membrane (SPM1) having high conductivity has higher value of dielectric constant and also it increases with increase in temperature due to migration polarization of mobile ions. The frequency dependence of dielectric loss can provide both the relaxation and polarization properties of proton exchange membrane. Fig. 6 reveals the dielectric loss(ε”) or conduction loss for the membrane of SPEEK/MMT shows higher value at lower frequencies may due to free charge motion in the electrolyte material but decreases with increasing frequency [43,44] besides the dielectric loss increases with increase of temperature. The perturbation of phonon by the applied electric field leads to dielectric loss, addition of nanocomposite particles modifies the perturbation of phonon during the application of electric field.
The loss is induced by the friction during rotation or movement of atoms or molecules in an alternating electric field. The frequency dependence of loss tangent at various temperatures of SPM1 composite membrane is shown in Fig. 7. The presence of peak indicates the relaxing dipole is present and is slightly shifted towards higher frequency with rising in temperature which indicates the reduction in relaxation time i.e., fast segmental motion and hence higher ionic conductivity [45]. The higher relaxation peak indicates increase in amorphous phase and lowering of glass transition temperature [46]. Although, these composites PEMs exhibit almost zero dielectric loss or heat loss which suggests that composites PEMs are loss-less materials at low frequencies. The Arrhenius behavior, the frequency of maximum loss tangent is plotted as a function of temperature for the SPM1 electrolyte membrane is displayed in Fig. 7 (inset). The tan δ spectra have the peak value corresponding to electrode polarization (EP) at relaxation frequency which is used to evaluate the EP relaxation time τ = 1/ω where ω is the angular frequency of the applied signal [42]. The data is fitted to a straight line which indicates that the dielectric relaxation is thermally activated process and obeys the Arrhenius activities.
The Electrical modulus is used to investigate conductivity and its associated relaxation in ionic conductors and polymers[47]. The dielectric modulus can be calculated by the following equation
M\’= ε\’/(ε^\’2+ ε^\’\’2) ….(3)
M\” = ε\’\’/(ε^\’2+ ε^\’\’2) ….(4)
The frequency dependence of real (M’) and imaginary (M”) part of modulus formalism of SPEEK/MMT composite membranes at different concentration are shown in Fig. 8(a)&(b). The imaginary part of electrical modulus is an indicative of energy loss in the sample under electric field. The modulus values (M’ and M”) are very small indicating that the electrode polarization at the interface is negligible at low frequencies and confirms the large equivalent capacitance [49, 50]. Nevertheless, this is rising at high frequencies and frequency dispersion is observed, which may be due to distribution of relaxation process. The peak shifted towards high frequencies due to increase in charge carrier concentration hence reduces the relaxation time. Relaxation peak at high frequencies is not visible in the plot due to experimental frequency limitation [51]. Fig. 8(c)&(d) depicts real (M’) and imaginary (M”) part of Modulus formalism of SPM1 membrane at different temperatures. A shrinkage in intensity is observed with rising in temperature. As the temperature increases, the possible peak maxima shifted to higher frequencies indicates that the conductivity of charge carrier has been thermally activated.
The scaling of modulus data (M”) provides an information about relaxation dynamics with charge carrier concentration and temperature. In the present study of scaling-modulus, the frequency is normalized by fmax and the modulus by M”max and their results are displayed in Fig. 9. The occurrence of perfect spectral overlapping may suggest, a common relaxation mechanism in all the electrolytes. The temperature dependence of scaling behavior of modulus spectra is also recorded. All the spectra were merged on single master curve is indicating the dynamic relaxation process occurring at different temperatures analogous to the thermal activation energy for a particular composition [52]. The above consequences conveys the perfect merging on modulus spectra is the another beneficial effect to polymer composite system.
4. Conclusion
A series of composite membranes has been prepared by adding MMT into sulfonated PEEK polymer and characterized. The 64% of level sulfonation is obtained with 1H-NMR spectral analysis for the synthesized SPEEK polymer. The structural functionalities and the polymer chain intercalation into the silicate layers can be discovered through FTIR, XRD studies. The results of TGA analysis reveal that thermal stabilities of SPEEK based composite membranes are high enough to meet the requirements of PEM applications. The composite membrane SPM1 shows the maximum proton conductivity of 1.1086 ×10-1 S cm-1 under the conditions of 70°C and 100% RH. Further, the ability to retain water is very much important to PEM operation at an elevated temperature for ionic transportation and also electrochemical processing. The electrode polarization influences the membrane dielectric constant and dielectric loss. The dielectric permittivity of the membrane decreases with increase in frequency due to high polarity of sulfonic acid groups. The Modulus spectra of the composite membrane exhibit scaling behavior and M” values successfully collapse into a single master curve indicating the temperature and composition independent relaxation behavior. The temperature dependent loss spectra specify the charge carrier is thermally activated. Due to low cost, easy fabrication, desired conductivity, significant water uptake, methanol uptake of the SPEEK/MMT composite membranes offer great promise for DMFC applications.
Acknowledgement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for- profit sectors.
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