Membrane prepared by incorporation of crosslinked ...

Applied Energy 123 (2014) 66–74

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Membrane prepared by incorporation of crosslinked sulfonated polystyrene in the blend of PVdF-co-HFP/Nafion: A preliminary evaluation for application in DMFC Piyush Kumar, Kingshuk Dutta, Suparna Das, Patit Paban Kundu ⇑ Advanced Polymer Laboratory, Department of Polymer Science & Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Fabrication of low cost sulfonated

polystyrene/PVdF-co-HFP/Nafion semi-IPN PEM. Enhancement of water uptake value and ion exchange capacity compared to Nafion-117. A maximum current density of 120 mA cm2 at 0.2 V was obtained. 2 A cell efficiency of 24 mW cm at 60 °C was obtained while using air at cathode. Enhanced proton conductivity over Nafion-117 was recorded.

a r t i c l e

i n f o

Article history: Received 24 October 2013 Received in revised form 28 January 2014 Accepted 22 February 2014

Keywords: Direct methanol fuel cell Ion exchange capacity Polymer electrolyte membrane Cross-linked interpenetrating network Sulfonated polystyrene

a b s t r a c t Sodium salt of sulfonated styrene (SS) was polymerized in situ within the polymeric blend of PVdF-coHFP/Nafion. The electrical efficiency of this cross-linked semi interpenetrating network membranes were evaluated for its potential application as a polymer electrolyte membrane in direct methanol fuel cell (DMFC). The characteristic aromatic peaks obtained in the FT-IR spectra confirmed the successful incorporation of SS within the polymeric blend. X-ray diffraction analyses were conducted to determine the presence of crystalline and amorphous domains within the structure of the blend membrane. Water uptake measurements at room temperature indicate that above a threshold value of 20 wt% of incorporated SS (S-20), water uptake of the semi-IPN membranes increases up to 24%, with an IEC value equal to Nafion, i.e. 0.8 meq g1. The maximum current density was recorded to be 120 mA cm2 at 0.2 V, with a cell efficiency (power density) of 24 mW cm2 at 60 °C. In addition, proton conductivity and methanol permeability test results indicate that the prepared membrane S-20 is comparable to that of Nafion117 membrane. Ó 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +91 2350 1397; fax: +91 2352 5106. E-mail address: [email protected] (P.P. Kundu). http://dx.doi.org/10.1016/j.apenergy.2014.02.060 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

P. Kumar et al. / Applied Energy 123 (2014) 66–74

1. Introduction Direct methanol fuel cell (DMFC) is one of the most important members of the polymer electrolyte membrane fuel cell (PEMFC) family, and therefore, its development has attracted a considerable research over the years [1–3]. Considering the prevailing energy crisis in our planet, recent research works have focused on the urge of developing alternative power sources [4–6]. In this respect, DMFC has evolved as a potential candidate, and its importance lies in the fact that it is an electrochemical device that converts chemical energy into electrical energy directly by redox reactions between methanol and air/oxygen. It has also received attention as potential energy source for powering portable devices and vehicles [7–11]. However, irrespective of the huge amount of active research that has been done in this area, fabrication of a low cost and highly efficient DMFC still remains a challenge for researchers [12–14]. A complete DMFC system consists of a polymer electrolyte membrane (PEM), bipolar plates, current collector plates, gaskets, end plates and reactants (methanol and oxygen/air). Among these several constituents, PEM is often considered as the heart of a DMFC, since its function is critical to the overall performance of the cell. Until date, Nafion (a perfluorosulfonic acid polymer) has been the most preferable PEM material in view of its several unmatchable properties, including proton conductivity, thermal and chemical stability, and stability towards oxidation [15–17]. However, its high cost, performance failure at higher temperatures, and methanol crossover posed serious limitations over its extensive use and have left the scope for further developmental research [18–20]. In recent years, the focus of research concerning PEMs has been the modification and substitution of Nafion as the membrane material [21–30]. Poly(vinylidene fluoride) (PVdF), its copolymers and blends, as prospective PEM constituents, have exhibited promising results [23,27,31–35]. For example, PEMs fabricated from PVdF-co-hexafluoropropylene (HFP)/Nafion blends have shown excellent chemical resistance and reduced methanol crossover [34,36], along with improved mechanical strength and dimensional stability [37]. However, the PVdF-co-HFP copolymer is associated with low water uptake and proton conductivity values due to its extreme hydrophobicity [36,38]. An alternative, as shown by Kundu et al., can be provided by a Nafion/sulfonated polystyrene based PEM. It was shown that formation of a semi-interpenetrating network (semi-IPN), by cross-linking sulfonated polystyrene within the pores of Nafion membrane, led to enhanced water uptake by as much as 40%. This result was a direct consequence of the modified membrane’s lower interfacial resistance. It was also reported that the modified membrane’s high temperature (>100 °C) performance exhibited an improvement over that of the virgin Nafion membrane [39]. Based on the above observations, we intuited that a PEM prepared by in situ polymerization of sulfonated styrene (SS) within the blend of PVdF-co-HFP/Nafion can exhibit characteristic advantages of each constituent, and therefore, can potentially improve the properties in terms of ion exchange capacity (IEC), reduction of membrane cost, water uptake, proton conductivity, and DMFC performance. We have characterized the synthesized membranes with a number of tests, namely water uptake, FT-IR, X-ray diffraction (XRD), IEC, proton conductivity, oxidative stability, methanol crossover, and have finally analyzed their DMFC performances. The primary objective of this work was to reduce the overall cost involved inthe operation of a DMFC. We have tried to achieve this possibility by reducing the Nafion content of the membrane, coupled with utilization of air as a reactant at the cathode. It should be noted in this respect that the PVdF-co-HFP copolymer utilized

67

in this study has been widely reported as a low-cost membrane material compared to Nafion [23,28,37,40,41]. It is expected that a blend comprising of a lower content of costly Nafion, and at the same time exhibiting comparable performance to that of pristine Nafion, can serve better as a PEM material for DMFCs. 2. Materials and methods 2.1. Chemicals used PVdF-co-HFP (Mw: 455,000), sodium salt of styrene, and Nafion resin (5 wt% solution in a mixture of lower aliphatic alcohols and water, density: 0.924 Kg m3) were obtained from Sigma Aldrich. Dimethyl formamide (DMF) and divinyl benzene (DVB) were bought from Merck Millipore India. Nafion-117 membrane was purchased from M/S Anabond Synergy India Pvt. Ltd. All chemicals, except Nafion membranes, were used as received. The chemical structures of the polymers utilized in this work are depicted in Fig. 1. 2.2. Preparation and pre-treatment of PEM PVdF-co-HFP copolymer was fully dissolved in DMF. Nafion resin was then added drop wise with continuous stirring at 80 °C for 2 h. The mixture was stirred vigorously until the solution turned homogeneous, transparent and viscous. The resulting viscous solution of the polymeric blend was casted on a flat glass plate and kept in an oven overnight at 80 °C. The obtained blend film was then redissolved in DMF and the resulting blend solution was transferred into a three-necked round bottom flask containing sodium salt of styrene. This was followed by the addition of initiator AIBN and cross-linker DVB. The resultant mixture was left under stirring condition at 110 °C to allow the SS to polymerize in situ [42]. Finally, a viscous solution was obtained, which was again casted on the flat glass plate and kept in the oven at 80 °C for 24 h. The constituent details of the different samples prepared are represented in Table 1. In order to obtain a proper protonation, the prepared membranes were first treated with a mixture of water and H2SO4 (50:50) for a period of 3 h. Then, the membranes were transferred into beakers containing de-ionized (DI) water, and washed until a neutral pH was obtained. Next, they were vacuum-dried in an oven for 12 h at 80 °C. On the other hand, the Nafion-117 membrane

Fig. 1. Chemical structures of Nafion, PVdF-HFP and sodium salt of sulfonated styrene.

68


Table 1 Chemical composition of the prepared membranes. S. no.

Sample designation

PVdF-HFP/Nafion (4:1) (%)

SS (%)

AIBN (wt% of SS)

DVB (wt% of SS)

1 2 3 4 5

S-5 S-10 S-15 S-20 S-30

95 90 85 80 70

5 10 15 20 30

0.1 0.2 0.3 0.4 0.6

0.2 0.4 0.6 0.8 1.2

was treated by first immersing in a 5 M H2O2 solution, followed by treating with a mixture of water and H2SO4 (7:3) for 2 h with continuous stirring, and finally washed with DI water. Schematic illustration of the processes involved in membrane preparation is depicted in Fig. 2.

2.3. FT-IR and XRD Both Nafion and synthesized membranes were subjected to FTIR characterization using a Bruker Alpha ATR FT-IR spectrophotometer (Model: Alpha E), and employing a wave number range of 500–3000 cm1. The XRD spectra of the membranes were determined using an X-ray diffractometer, using an angle of 2h and a fixed scan rate of 1° min1.

2.4. Water uptake A cut piece from each membrane was dried at 80 °C under vacuum, weighed and immersed into beakers containing DI water. After leaving undisturbed for 24 h, respective cut pieces were taken out, wiped in order to remove the unabsorbed water, and weighed. Respective water uptakes of the prepared membranes were calculated from the following Eq. (1):

Water uptake ð%Þ ¼ ðW wet W dry Þ 100=W dry

ð1Þ

where Wwet represents the respective weights of wet membranes soaked in DI water for 24 h, and Wdry is the respective weights of dry membranes.

2.5. Swelling ratio A cut piece from each membrane, having dimensions of 2 2 cm2, was first dried at 80 °C under vacuum. Their thicknesses were then measured by a thickness gauge, and they were subsequently immersed into beakers containing DI water for proper dousing. After leaving undisturbed for 24 h, respective cut pieces of each sample were taken out, and their thicknesses were once again measured in order to determine their swelling extent. Respective swelling ratios of the prepared membranes were calculated from the following Eq. (2):

Swelling ratio ð%Þ ¼ ðT wet T dry Þ 100=T dry

ð2Þ

where Twet represents the respective thicknesses of wet membranes soaked in DI water for 24 h, and Tdry is the respective thicknesses of dry membranes. Since proton and methanol transport from anode to cathode takes place along the thickness of the membrane, therefore swelling ratios have been calculated in terms of changes of the membrane thicknesses upon water absorption. 2.6. Ion exchange capacity (IEC) Conventional titration technique was employed to determine the ion exchange capacities (IECs) of the membranes. Square pieces of each membrane were soaked for 24 h in a large volume of 1 M H2SO4 solution, followed by repetitive washing with distilled water in order to remove excess acid. For the purpose of replacing the protons with sodium ions, the samples were placed in 50 mL of 1 M NaCl solution, heated to 40 °C, and equilibrated for at least 24 h. The remaining solution was then titrated with 0.01 N NaOH solution, using phenolphthalein as the indicator. The IEC value (in meq g1) was calculated using the following equation:

IEC ¼ ðV NaOH ÞðSNaOH Þ=W dry

ð3Þ

where VNaOH is the volume of NaOH used in the titration, Wdry is the dry weight of the membrane in g, and SNaOH is the strength of NaOH used for the determination of IEC. 2.7. Oxidative stability The oxidative stabilities of the prepared membranes were determined by measuring the changes that occur in their respective weights upon exposure to H2O2 and temperature for a given time [43]. In brief, a cut piece of size 3 3 cm2 of each membrane was immersed in a 3 wt% aqueous solution of H2O2 at a temperature of 25 °C. After every 24 h, the samples were taken out, quickly wiped with a tissue paper to remove the excessive surface liquid, and were weighed immediately using an electronic balance. The reduction that occurred in the weight of each sample was used as a measure to evaluate its oxidative stability. 2.8. Proton conductivity

Fig. 2. A schematic illustration of the process involved in membrane preparation.

Proton conductivity was measured by ac impedance spectroscopic technique, employing a potentiostat (Gamry potentiostat600). Frequency range employed was 0.01–10,000 Hz. A schematic

69


illustration of the setup used for analyzing the proton conductivities of the membranes has been illustrated in Fig. 3. Before the test, the membranes were activated in 1 M H2SO4 solution for 24 h. The conductivities of all the prepared samples (r) were measured in the transverse direction at 20 °C and under 70% relative humidity, and were calculated from the following equation:

r ¼ T=R A

ð4Þ

where T is the thickness of the sample, A is the surface area of the sample, R is the resistance (derived from the low intersect of the high frequency semi-circle on a complex impedance plane with the Real (Z) axis). 2.9. Methanol crossover

Fig. 4. A schematic illustration of the diffusion cell utilized for analyzing the methanol crossover of the membranes.

Methanol crossover tests were conducted in a glass diffusion cell at 20 °C. The cell was divided in two equal sized glass cylinders. Part-A was filled with the fuel (2 M methanol), while Part-B was filled with DI water, both having equal volume. The membrane, having an area of 15.2 cm2, was placed vertically between the two cylinders. A schematic illustration of the employed setup has been illustrated in Fig. 4. After an equal time duration (i.e. every 30 min), the crossover methanol (from Part-A to Part-B) was collected from Part-B and mixed with an equal volume of a chromogenic reagent, known as ‘‘SNP’’ (prepared by mixing 5 g of sodium nitroprusside in 50 mL water, 5 g of potassium ferrocyanide in 50 mL water, and 2.5 g of NaOH in 50 mL water at a temperature of 4 °C), and was examined by UV–vis spectroscopic technique, employing an Optizen UV–vis spectrophotometer [24,44]. The methanol permeability was calculated by using the Eq. (5), as mentioned below:

jA ¼ V B dC B =dt ¼ A D K ðC A C B Þ=l

ð5Þ

where jA is the flux of methanol from Part-A to Part-B; VB is the volume of the reservoir B; CB and CA are the methanol concentration in the reservoirs B and A, respectively; dCB is the change in methanol concentration occurring at reservoir B; dt is the time interval between two successive readings; A is the membrane area; D is the methanol diffusivity (which was assumed to be constant inside the membrane); K is the partition coefficient (constant); D K is the membrane permeability; and l is the thickness of the membrane. 2.10. Fabrication of membrane electrode assembly (MEA)

ion exchange resin (5 wt% Nafion solutions), followed by sonication for a period of 30 min separately until a fine ink of catalyst mixture was obtained. The obtained ink was then carefully deposited onto the surface of carbon paper by employing a simple painting technique. A total of 4 mg cm2 of the respective metal catalysts were then loaded onto the respective electrodes and baked at 80 °C for 12 h. The entire set up was finally hot pressed for 6 min at 135 °C, applying a pressure of up to 1.5 ton, to get the final MEA. 2.11. DMFC cell performance For DMFC performance test, a single cell stack was loaded and well connected with the fully automated DMFC test station (K-PAS Electronic India Ltd). The cell was allowed to equilibrate for 12 h. Fuel was then pumped at the anode side at a flow rate of 2 mL min1; while at the cathode, air was supplied at a constant flow rate of 50–60 mL min1. A DC programmable load bank (KPAS Electronic India Ltd.), attached to the DMFC Test Station, was used to measure the performance of the cell. 3. Results and discussion 3.1. FT-IR analysis Upon subjecting the membranes to FT-IR analysis, we obtained the spectra depicted in Fig. 5, which confirms the presence of characteristic groups within the semi-IPN membranes. In case of Nafion-117 (used as a reference), two stronger and wider peaks

60 wt% of Platinum (Pt)/Ruthenium (Ru) and 40 wt% of Carbon (C) (30:30:40) mixture was used as the anode and 60 wt% of Pt and 40 wt% of C (60:40) mixture was used as the cathode. The catalyst mixtures were magnetically stirred with isopropanol and

Fig. 3. A schematic illustration of the setup utilized for analyzing the proton conductivities of the membranes.

Fig. 5. FT-IR spectra of the samples.

70


appeared at 1202 cm1 and 1140 cm1 due to the Nafion backbone’s CAF stretching vibration; while a sharp peak at 1054 cm1 can be assigned to the SAO stretching vibration of ASO3H groups [45]. On the other hand, for the synthesized membranes, peaks appearing at 1068 cm1 and 1162 cm1aredueto the symmetric and asymmetric stretching vibrations of ASO3 groups. It can be clearly seen from the figure that the intensities of both these peaks increased continuously from S-5 to S-30, due to the increasing concentration of ASO3H group (SS contains ASO3H group in each unit, see Fig. 1). The peak obtained at 981 cm1for Nafion is due to the stretching vibration of CAOAC bonds. A characteristic peak, appearing at 659 cm1, can be assigned to the out-of-plane bending vibration of styrene rings. The presence of a sharp peak at 1681 cm1 is due to the C@C stretching vibration of the aromatic benzene ring of sulfonated polystyrene (SPS), and this peak became more prominent as the concentration of SS increased, reaching the highest value corresponding to S-30 [39]. This characteristic peak is absent in case of pristine Nafion, which confirms the successful incorporation of SPS within the blend. 3.2. XRD analysis X-ray diffraction was conducted to analyze the presence of crystalline and amorphous zones within the structure of semi-IPN membrane. XRD spectra of samples S-20 and S-30 are given in Fig. 6, along with PVdF-co-HFP/Nafion blend and pure PVdF-coHFP membrane as references. From the figure, it can be clearly observed that for the reference blend sample of PVdF-co-HFP/Nafion, the 2h peak has almost completely disappeared; while for sample S-20, a sharp 2h peak appeared in the region of 20–30°. This is due to the fact that the presence of Nafion (recast) causes the semi-crystalline nature of PVdF-co-HFP to disappear [46], while SS reforms the crystalline structure within the blend of PVdF-coHFP/Nafion. From the figure, it is also clear that for sample S-30 the intensity of the peak is relatively higher, compared to both S20 and S-15. This is due to the greater constraints imposed by DVB (Table 1), by the way of forming more linkages within the membrane structure. Hence, the XRD results confirm that the prepared semi-IPN membrane possesses a semi-crystalline structure. 3.3. Analysis of water uptake capacities and swelling ratios of the membranes The water uptake analysis of the different samples, including the reference (Nafion-117) was done by using Eq. (1), and the

Fig. 6. XRD spectra of the samples.

results are plotted in Fig. 7. From the figure, it can be seen that at room temperature the water uptake values for samples S-5, S-10, S-15 and S-20 are 8%, 15%, 18% and 24%, respectively. These results reveal an enhancement of water uptake capacity upon an increase in the SPS content within the membrane. This increased affinity for water is due to the greater polar nature of the SPS polymer. SPS absorbs water on its surface by virtue of strong interactions between its surface ASO3 groups and water molecules via formation of strong hydrogen bonds, which essentially promote the liquid retention within the PVdF-co-HFP/Nafion blend membrane. However, the sample S-30, containing the highest percentage of SS (30%), exhibited a slightly reduced water uptake value of 18% at room temperature, compared to that obtained for S-20. This is due to the greater constraints imposed by the cross-linker DVB by forming additional cross-links within the membrane structure, which essentially led to increased rigidness. Nevertheless, samples S-15 and S-20 showed an improvement in water uptake capacity compared to the values of 16% at room temperature obtained for Nafion-117 (Fig. 7). Similarly, the swelling ratio analyses revealed that the sample S-20 exhibited the maximum swelling, followed by the sample S-15 (Fig. 7). Both these samples showed swellings higher than Nafion-117. This result can be explained on a similar basis, and was expected from the results obtained for water uptakes. This enhancement, in turn, shall provide a favorable condition for proton conduction in fuel cells. 3.4. IEC analysis Fig. 8 represents the IEC values as a function of the percentage of SS incorporated within the membrane structure. The IECs obtained were about 0.9 meq g1 and 0.8 meq g1 for the samples S-30 (containing 30% SS) and S-20 (containing 20% SS), respectively. The value of 0.8 meq g1, corresponding to the sample S-20, is equal to that obtained for the reference membrane Nafion-117; while the value of 0.9 meq g1 obtained for S-30 is slightly higher. Fig. 8 further reveals that the IEC value of the membrane increases with an increase in SS content. This is due to a corresponding increase in the concentration of ASO3 groups (present within the structure of SS, see Fig. 1) with increasing amount of incorporated SS. 3.5. Analysis of oxidative stability The oxidative stabilities of the membranes were analyzed by immersing a cut piece of each sample into a 3 wt% aqueous solution of H2O2, and the result has been plotted in Fig. 9. From the figure it can be seen that Nafion-117 shows the highest stability and

Fig. 7. Water uptake and swelling ratio values (%) of the samples.


71

value in spite of possessing the highest IEC value (Fig. 8). Since the proton conductivity is directly proportional to the water uptake capacity of the membrane, which in turn depends upon the available free volume, therefore the presence of the highest density of crosslinking agent (DVB) in S-30 led to a decrease in available free volume. This resulted in lowering the mobility of ions, and thus, the proton conductivity. 3.7. Analysis of methanol crossover

Fig. 8. Ion-exchange capacities of the samples.

flexibility, without undergoing any physical changes, among all the membranes studies, as expected and reported [43]. In case of the samples S-15, S-20 and S-30, the weight initially exhibited a rapid increase up to 72 h. In addition, a sharp reduction in weight was observed after 72 h, especially in case of the samples S-15 and S30, since membranes containing SS underwent continuous swelling and decomposition inside the solution [43]. The initial gain in weight can be attributed to the decomposition of crosslinked structure within the membrane [47]. However, after 72 h, the degradation rate of the crosslinked structure was higher than the rate of swelling; resulting in the observed sharp weight loss. From this analysis it appeared that, among all the samples studied, S-20 exhibited the highest swelling rate and the lowest degradation rate.

3.6. Proton conductivity analysis The proton conductivities of the prepared membranes, along with Nafion-117, were determined by employing the impedance spectroscopic technique, and the obtained results have been presented in Table 2. From the table it can be clearly observed that the Nafion-117 membrane exhibited a proton conductivity value of 3.16 102 S cm1, which is close to the literature values reported earlier [37,48–52] under the same operating conditions (i.e. 20 °C and 70% relative humidity). The sample S-20 exhibited the highest proton conductivity (i.e. 3.16 102 S cm1) among all the samples; while the sample S-30 exhibited a slightly lower

The methanol permeabilities of the samples were determined in a glass diffusion cell setup, by using UV–vis spectroscopic technique, and the obtained results have been presented in Table 2. From the table, it appears that the permeability of each sample is following its respective water uptake (Fig. 7) and IEC values (Fig. 8), except in case of S-30. Since, the sample S-30 contains a higher concentration of the cross-linker (DVB), therefore, this imparted rigidity and a packed structure to this membrane (as explained in Sections 3.3 and 3.6). In effect, a reduction in the free volume in the sample S-30 is responsible for causing a decrease in the mobility of liquid through it. However the maximum value of methanol permeability was recorded for the sample S-20 (1.76 106 cm2 s1), which is comparable to that obtained for Nafion-117 (1.22 106 cm2 s1) in our study and is also close to the reported values for Nafion-117 in the literature (Table 3) [37,46,48,49,53–55]. The fact that S-20 exhibited the highest methanol permeability among all the composite membranes is primarily due to it possessing the maximum free volume within the membrane structure, as indicated by its highest swelling ratio among all the samples, which is responsible for allowing high liquid mobility. It should be noted in this respect that with the increase in DVB (the crosslinker) content, the crosslink density increases while the free volume decreases. Along with the increase in DVB content in the compositions, the content of sodium salt of SS also increases. The increase in crosslink density (due to increase in DVB content) and increase in content of SS have opposing effects in terms of water penetration or swelling (the increase in crosslink density will reduce it, whereas the increase in SS content will increase it). These two opposing effects have been found to be optimum in terms of swelling for the S-20 composition (Fig. 7). Nevertheless, the substantially low methanol permeability of S30, by about one order of magnitude compared to pristine Nafion, suggests that judicious optimization of the weight ratio of the different constituents of the composite membrane can result in producing a balance between proton conductivity and methanol permeability in order to realize higher membrane selectivity. In fact, it has been reported that by increasing the content of PVdFco-HFP, as well as, the extent of DVB cross-links, the methanol diffusivity of PEMs can be substantial reduced [36,47]. 3.8. Determination of membrane selectivity Membrane selectivity, which is an important parameter to gauge the efficiency of a PEM, is defined as the ratio between the proton conductivity and the methanol permeability of the PEM. Therefore we have calculated the selectivity values of the synthesized membranes, and have presented them in Table 2. It can be realized from these tabulated values that the selectivities of the synthesized membranes S-20 and S-30 (i.e. 1.80 104 Ss cm3 for both the samples) are about 73% of that of pristine Nafion117 membrane (2.47 104 Ss cm3). 3.9. DMFC performance test

Fig. 9. Plots representing the oxidative stabilities of the samples.

The DMFC performance tests for all the membrane samples were conducted on a single cell stack, having an MEA area of

72


Table 2 Membrane thickness, proton conductivities, methanol permeabilities and membrane selectivities of the samples. [Proton conductivity and methanol crossover tests were conducted at 20 °C.] S. no. 1 2 3 4

Sample designation Nafion-117 S-15 S-20 S-30

Membrane thickness (lm)

Proton conductivity (Scm1) 2

190 201 204 202

Methanol crossover (cm2 s1)

3.02 10 1.32 102 3.16 102 1.82 103

Membrane selectivity (Ss cm3) 2.47 104 1.17 104 1.80 104 1.80 104

6

1.22 10 1.12 106 1.76 106 1.01 107

Table 3 A comparison of methanol crossover value obtained for Nafion-117 membrane in this study with other reported values. S. no.

Author name

Techniques used

Temp. (°C)

Methanol conc. (M)

Methanol crossover (cm2 s1)

Ref.

1 2 3 4 5 6 7 8 9

Cho et al. Ramya and Dhathathreyan Every et al. Dutta et al. Bello Bello Bello Tricoli et al. This study

Potentiometry at OCV Cyclic Voltammetry/chronoamperometry Modified NMR UV spectroscopic Cyclic Voltammetry Chronoamperometry Potentiometry at OCV Gas chromatography UV spectroscopic

30 30 30 20 22 22 22 22 20

2 3 1–5 2 2.5 2.5 2.5 2 2

2.32 106 3.52 106 1.30 106 1.22 106 1.27 106 1.11 106 1.13 106 1.15 106 1.22 106

[37,46] [48] [49] [53] [54] [54] [54] [55] –

Fig. 10. Plots representing the current density of the samples.

Fig. 11. Maximum power density of the samples obtained at +0.2 V.

5 5 cm2, and at a temperature of 60 °C. The obtained results are represented in Fig. 10, as a characteristic plot of potential vs. current density. The maximum current density achieved for the samples S-10, S-15, S-20 and S-30 were 55 mA cm2, 70 mA cm2, 120 mA cm2 and 90 mA cm2 respectively, when recorded at a potential of +0.2 V. These values, when compared with the value of 105 mA cm2 obtained for Nafion-117 at the same potential, reveals that the samples which exhibited higher water uptakes and IECs also showed better cell performances. It should be noted in this respect that the current density obtained using Nafion-117 membrane in this study is somewhat lower compared to that

reported in some other works due to the following reasons (Table 4) [56–58]. (a) no additional treatment was conducted on graphite bipolar plates before DMFC testing; (b) atmospheric air was used, without preheating or humidification, as a reactant at the cathode; and (c) locally obtained catalysts, which are of much lower cost compared to commercial E-TEK catalysts, were used. Power density values, plotted in Fig. 11, quantify the cell efficiency. From the figure, it is observed that the maximum power density was realized for the sample S-20. This further confirms that this particular sample composition have the potential to be further

Table 4 A comparison of the DMFC performance obtained in this study using Nafion-117 membrane with other reported values. S. no.

Author

Catalyst loading at anode (mg/cm2)

Methanol conc. (M)

Oxidant (O2/air)

Temp. (°C)

Power density (mW/cm2)

Ref.

1 2 3 4 5 6 7 8

Cho et al. Cho et al. Dutta et al. Dutta et al. Liu et al. Achmad et al. Oliveira et al. This study

3 5 4 4 4 8 4 4

2 2 2 2 4 5 0.75 and 2 2

O2 O2 Air Air O2 Air Air Air

30 30 20 60 40 27 20 and 60 60

42 50 14 22 29.5 25 14 and 35 22

[37] [46] [53] [53] [56] [57] [58] –


exploited as a partial/complete replacement of traditional Nafion for fabrication of PEMs. 4. Conclusions In summary, we have prepared a semi-IPN membrane and evaluated its potential to be applied as a PEM for application in DMFC. For this purpose, sodium salt of SS along with DVB and initiator were impregnated into the polymeric blend of PVdF-co-HFP/Nafion and subsequently polymerized in situ. We determined the electrical efficiency of this semi-IPN membrane, and characterized it with FT-IR and XRD. The obtained results confirmed the successful incorporation of SS within the polymeric blend of PVdF-co-HFP/ Nafion. From the water uptake analysis, we realized that above a threshold value of 20 wt% of incorporated SS, water uptake of the semi-IPN membrane increases up to 24%, with an IEC value equal to Nafion, i.e. 0.8 meq g1. The sample S-20 exhibited the highest swelling rate and the lowest degradation rate amongst all membranes. This sample also exhibited a slightly higher proton conductivity and similar membrane selectivity compared to that of pristine Nafion-117. Moreover, for this membrane, the maximum current density was observed to be 120 mA cm2 at +0.2 V, with a corresponding power density of 24 mW cm2 at 60 °C. In effect, the synthesized membrane (S-20) exhibited higher water uptake, swelling ratio, proton conductivity, current density and maximum power density, as well as, comparable IEC, methanol permeability and membrane selectivity to that of Nafion-117 membrane. This, coupled with the use of air in place of oxygen as a cathode reactant, resulted in realizing a cheaper overall DMFC operation. We believe that this novel semi-IPN membrane will set the trend for more such compounds and formulations, and consequently the spectrum of materials used for fabrication of PEM for DMFC will increase. We are presently working on (a) varying the ratio of different constituents of this semi-IPN membrane in order to realize a lower methanol permeability and a higher membrane selectivity, (b) the high temperature operation aspects of this semi-IPN membrane, and (c) the structural characterizations of the different synthesized membranes, so that, a clear understanding of the structure–property relationships can be realized. Acknowledgments PPK is thankful to the Ministry of New Renewable Energy (MNRE, GOI) for a grant-in-aid. PK would like to thank the Department of Science and Technology (DST, GOI) for providing an INSPIRE fellowship. KD thanks the Council of Scientific and Industrial Research (CSIR, India) for a Senior Research Fellowship. References [1] Kumar P, Dutta K, Kundu PP. Enhanced performance of direct methanol fuel cells: a study on the combined effect of various supporting electrolytes, flow channel designs and operating temperatures. Int J Energy Res 2014;38:41–50. [2] Chen CY, Liu DH, Huang CL, Chang CL. Portable DMFC system with methanol sensor-less control. J Power Sources 2007;167:442–9. [3] Qian W, Wilkinson DP, Shen J, Wang H, Zhang J. Architecture for portable direct liquid fuel cells. J Power Sources 2006;154:202–13. [4] Armand M, Tarascon J-M. Building better batteries. Nature 2008;451:652–7. [5] Grätzel M. Dye-sensitized solar cells. J Photochem Photobiol C 2003;4:145–53. [6] Dutta K, Kundu PP. A review on aromatic conducting polymers-based catalyst supporting matrices for application in microbial fuel cells. Polym Rev 2014. http://dx.doi.org/10.1080/15583724.2014.88137. [7] Shimizu T, Momma T, Mohamedi M, Osaka T, Sarangapani S. Design and fabrication of pumpless small direct methanol fuel cells for portable applications. J Power Sources 2004;137:277–83. [8] Patil AS, Dubois TG, Sifer N, Bostic E, Gardner K, Quah M, et al. Portable fuel cell systems for America’s army: technology transition to the field. J Power Sources 2004;136:220–5.

73

[9] Wee J-H. Which type of fuel cell is more competitive for portable application: direct methanol fuel cells or direct borohydride fuel cells? J Power Sources 2006;161:1–10. [10] Höhlein B, Biedermann P, Grube T, Menzer R. Fuel cell power trains for road traffic. J Power Sources 1999;84:203–13. [11] Kamarudin SK, Achmad F, Daud WRW. Overview on the application of direct methanol fuel cell (DMFC) for portable electronic devices. Int J Hydrogen Energy 2009;34:6902–16. [12] Kamarudin SK, Daud WRW, Ho SL, Hasran UA. Overview on the challenges and developments of micro-direct methanol fuel cells (DMFC). J Power Sources 2007;163:743–54. [13] Ahmed M, Dincer I. A review on methanol crossover in direct methanol fuel cells: challenges and achievements. Int J Energy Res 2011;35:1213–28. [14] Kumar P, Dutta K, Das S, Kundu PP. An overview of unsolved deficiencies of direct methanol fuel cell technology: factors and parameters affecting its widespread use. Int J Energy Res 2014. http://dx.doi.org/10.1002/er.3163. [15] Haubold H-G, Vad Th, Jungbluth H, Hiller P. Nano structure of NAFION: a SAXS study. Electrochim Acta 2001;46:1559–63. [16] Cappadonia M, Erning JW, Stimming U. Proton conduction of NafionÒ 117 membrane between 140 K and room temperature. J Electroanal Chem 1994;376:189–93. [17] Mauritz KA, Moore RB. State of understanding of Nafion. Chem Rev 2004;104:4535–85. [18] Shin S-J, Balabanovich AI, Kim H, Jeong J, Song J, Kim H-T. Deterioration of Nafion 115 membrane in direct methanol fuel cells. J Power Sources 2009;191:312–9. [19] Wang Y, Chen KS, Mishler J, Cho SC, Adroher XC. A review of polymer electrolyte membrane fuel cells: technology, applications, and needs on fundamental research. Appl Energy 2011;88:981–1007. [20] Seo SH, Lee CS. A study on the overall efficiency of direct methanol fuel cell by methanol crossover current. Appl Energy 2010;87:2597–604. [21] Zhang H, Huang H, Shen PK. Methanol-blocking Nafion composite membranes fabricated by layer-by-layer self-assembly for direct methanol fuel cells. Int J Hydrogen Energy 2012;37:6875–9. [22] Wu QX, Zhao TS, Chen R, An L. A sandwich structured membrane for direct methanol fuel cells operating with neat methanol. Appl Energy 2013;106:301–6. [23] Navarra MA, Panero S, Scrosati B. A composite proton-conducting membrane based on a poly(vinylidene)fluoride-poly(acrylonitrile), PVdF-PAN blend. J Solid State Electrochem 2004;8:804–8. [24] Das S, Kumar P, Dutta K, Kundu PP. Partial sulfonation of PVdF-co-HFP: a preliminary study and characterization for application in direct methanol fuel cell. Appl Energy 2014;113:169–77. [25] Dutta K, Kumar P, Das S, Kundu PP. Utilization of conducting polymers in fabricating polymer electrolyte membranes for application in direct methanol fuel cells. Polym Rev 2014;54:1–32. [26] Jaafar J, Ismail AF, Matsuura T. Effect of dispersion state of Cloisite15AÒ on the performance of SPEEK/Cloisite15A nanocomposite membrane for DMFC application. J Appl Polym Sci 2012;124:969–77. [27] Wootthikanokkhan J, Seeponkai N. Methanol permeability and properties of DMFC membranes based on sulfonated PEEK/PVDF blends. J Appl Polym Sci 2006;102:5941–7. [28] Zhang H, Shen PK. Recent development of polymer electrolyte membranes for fuel cells. Chem Rev 2012;112:2780–832. [29] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349–84. [30] Li N, Zhang S, Liu J, Zhang F. Synthesis and properties of sulfonated poly[bis(benzimidazobenzisoquinolinones)] as hydrolytically and thermooxidatively stable proton conducting ionomers. Macromolecules 2008;41:4165–72. [31] Quartarone E, Mustarelli P, Magistris A. Transport properties of porous PVDF membranes. J Phys Chem B 2002;106:10828–33. [32] Inan TY, Dog˘an H, Unveren EE, Eker E. Sulfonated PEEK and fluorinated polymer based blends for fuel cell applications: investigation of the effect of type and molecular weight of the fluorinated polymers on the membrane’s properties. Int J Hydrogen Energy 2010;35:12038–53. [33] Mokrini A, Huneault MA. Proton exchange membranes based on PVDF/SEBS blends. J Power Sources 2006;154:51–8. [34] Kumar GG, Shin J, Nho Y-C, Hwang Is, Fei G, Kim AR, et al. Irradiated PVdF-HFPtin oxide composite membranes for the applications of direct methanol fuel cells. J Membr Sci 2010;350:92–100. [35] Jung H-Y, Park J-K. Long-term performance of DMFC based on the blend membrane of sulfonated poly(ether ether ketone) and poly(vinylidene fluoride). Int J Hydrogen Energy 2009;34:3915–21. [36] Kumar GG, Lee DN, Kim P, Nahm KS, Elizabeth RN. Characterization of PVdFHFP/Nafion/AlO[OH]n composite membranes for direct methanol fuel cell (DMFC). Eur Polym J 2008;44:2225–30. [37] Cho K-Y, Eom J-Y, Jung H-Y, Choi N-S, Lee YM, Park J-K, et al. Characteristics of PVdF copolymer/Nafion blend membrane for direct methanol fuel cell (DMFC). Electrochim Acta 2004;50:583–8. [38] Zhou H, Wang H, Niu H, Gestos A, Lin T. Robust, self-healing superamphiphobic fabrics prepared by two-step coating of fluoro-contaning polymer, fluoroalkylsilane, and modified silica nanoparticles. Adv Funct Mater 2013;23:1664–70.

74


[39] Kundu PP, Kim BT, Ahn JE, Han HS, Shul YG. Formation and evaluation of semi-IPN of nafion 117 membrane for direct methanol fuel cell 1.Crosslinkedsulfonated polystyrene in the pores of nafion 117. J Power Sources 2007;171:86–91. [40] Neburchilov V, Martin J, Wang H, Zhang J. A review of polymer electrolyte membranes for direct methanol fuel cells. J Power Sources 2007;169:221–38. [41] Song M-K, Kim Y-T, Fenton JM, Kunz HR, Rhee H-W. Chemically-modified NafionÒ/poly(vinylidene fluoride) blend ionomers for proton exchange membrane fuel cells. J Power Sources 2003;117:14–21. [42] Brijmohan SB, Swier S, Weiss RA, Shaw MT. Synthesis and characterization of cross-linked sulfonated polystyrene nanoparticles. Ind Eng Chem Res 2005;44:8039–45. [43] Liu Q, Song L, Zhang Z, Liu X. Preparation and characterization of the PVDFbased composite membrane for direct methanol fuel cells. Int J Energy Environ 2010;1:643–56. [44] Zhana Y-Y, Zhang Y, Lia Q-M, Duc X-Z. Novel visible spectrophotometric method for the determination of methanol using sodium nitroprusside as spectroscopic probe. J Chinese Chem Soc 2010;57:230–5. [45] Gruger A, Regis A, Schmatko T, Colomban P. Nanostructure of NafionÒ membranes at different states of hydration an IR and Raman study. Vib Spectrosc 2001;26:215–25. [46] Cho K-Y, Jung H-Y, Sung KA, Kima W-K, Sung S-J, Park J-K, et al. Preparation and characteristics of Nafion membrane coated with a PVdF copolymer/recast Nafion blend for direct methanol fuel cell. J Power Sources 2006;159:524–8. [47] Lehtinen T, Sundholm G, Sundholm F. Effect of crosslinking on the physicochemical properties of proton conducting PVDF-g-PSSA membranes. J Appl Electrochem 1999;29:677–83. [48] Ramya K, Dhathathreyan KS. Methanol crossover studies on heat-treated NafionÒ membranes. J Membr Sci 2008;311:121–7.

[49] Every HA, Hickner MA, McGrath JE, Zawodzinski Jr TA. An NMR study of methanol diffusion in polymer electrolyte fuel cell membranes. J Membr Sci 2005;250:183–8. [50] Wang H, Holmberg BA, Huang L, Wang Z, Mitra A, Norbeck JM, et al. Nafionbifunctional silica composite proton conductive membranes. J Mater Chem 2002;12:834–7. [51] Elabd YA, Walker CW, Beyer FL. Triblock copolymer ionomer membranes: Part II. Structure characterization and its effects on transport properties and direct methanol fuel cell performance. J Membr Sci 2004;231:181–8. [52] Deluca NW, Elabd YA. Polymer electrolyte membranes for the direct methanol fuel cell: a review. J Polym Sci: Part B: Polym Phys 2006;44:2201–25. [53] Dutta K, Das S, Kumar P, Kundu PP. Polymer electrolyte membrane with high selectivity ratio for direct methanol fuel cells: a preliminary study based on blends of partially sulfonated polymers polyaniline and PVdF-co-HFP. Appl Energy 2014;118:183–91. [54] Bello M. Assessment of electrochemical methods for methanol crossover measurement through PEM of direct methanol fuel cell. Int J Eng Tech 2011;11:92–111. [55] Tricoli V, Carretta N, Bartolozzi M. A comparative investigation of proton and methanol transport in fluorinated ionomeric membranes. J Electrochem Soc 2000;147:1286–90. [56] Liu JG, Zhao TS, Liang ZX, Chen R. Effect of membrane thickness on the performance and efficiency of passive direct methanol fuel cells. J Power Sources 2006;153:61–7. [57] Achmad F, Kamarudin SK, Daud WRW, Majlan EH. Passive direct methanol fuel cells for portable electronic devices. Appl Energy 2011;88:1681–9. [58] Oliveira VB, Rangel CM, Pinto AMFR. Effect of anode and cathode flow field design on the performance of a direct methanol fuel cell. Chem Eng J 2010;157:174–80.