Proton Conducting Nafion-Sulfonated Graphene ... - ACS Publications

Dec 17, 2015 - for Direct Methanol Fuel Cells with Reduced Methanol Crossover ... a hybrid membrane with Nafion for a direct methanol fuel cell (DMFC)...
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Proton Conducting Nafion-Sulfonated Graphene Hybrid Membranes for Direct Methanol Fuel Cells with Reduced Methanol Crossover V. Parthiban,† Srinu Akula,† S. Gouse Peera,† Nazrul Islam,‡ and A. K. Sahu*,† †

CSIR - Central Electrochemical Research Institute-Madras Unit, CSIR Madras Complex, Taramani, Chennai - 600113, India Department of Chemistry, Workers College, Jamshedpur - 831012, India

‡

S Supporting Information *

ABSTRACT: Sulfonic acid functionalized graphene (S-graphene) is explored as a potential inorganic filler as well as a solid acid proton conducting medium to realize a hybrid membrane with Nafion for a direct methanol fuel cell (DMFC). The simple, but effective, functionalization of graphene is performed by sulfonic acid containing aryl radicals to increase the number of sulfonate groups per unit volume of graphene domain. Nafion−S-graphene hybrid membranes increase compactness of ionic domains and enhanced proton conductivity while restricting the methanol crossover across the membrane. DMFCs with a Nafion−S-graphene (1 wt %) hybrid membrane deliver a peak power density of 118 mW cm−2 at a load current density of 450 mA cm−2 while operating at 70 °C under an ambient pressure. By contrast, operating under identical conditions, a peak power density of 54 mW cm−2 at a load current density of 241 mA cm−2 is obtained with the pristine recast Nafion membrane. The Nafion−S-graphene hybrid membranes are extremely beneficial and useful for DMFCs in addressing many critical problems associated with commercial Nafion membranes.

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INTRODUCTION Direct methanol fuel cells (DMFCs) are considered as alternative power sources for portable and stationary applications as they allow liquid fuel without the requirement of any fuel-processing units.1−5 This helps simple and compact designs of DMFC systems which can be operated at the temperature range between 25 and 120 °C without any membrane dehydration. Membrane electrode assemblies (MEAs), composed of a proton exchange membrane (PEM) and the electrodes (anode and cathode), are the key components that determine the electrochemical performances of DMFC.6−8 To date, perfluorinated sulfonic acid (PFSA) membranes (e.g., Nafion) have been widely used as PEM materials because of their excellent proton conductivity and durability.2 In a Nafion membrane, the hydrophobic polytetrafluoroethylene backbone offers mechanical strength and hydrophilic perflouorinated pendant side chains ending with sulfonic acid moieties provide ionic conductivity. While in contact with water, the sulfonic acid moieties swell and form hydrophilic domains with a size of ∼40 Å.9−13 However, Nafion exhibits high methanol permeability referred to as methanol crossover from anode to cathode through the polymer electrolyte membrane as a result of electro-osmotic drag and concentration gradients. The methanol crossover lowers the performance of the DMFC due to a mixed potential at the cathode. In addition, the unoxidized methanol that reaches the cathode also complicates further the activity of the cathode catalyst by decreasing oxygen reduction and anode catalyst poisoning due to carbon monoxide adsorption.14,15 To reduce methanol crossover, Nafion membranes with a higher thickness such as Nafion-115 (thickness of 125 μm) or Nafion-117 (thickness of 175 μm) are generally used. However, a thicker membrane increases the resistance for proton conduction and affects the overall DMFC performance. Various © 2015 American Chemical Society

approaches have been proposed in the literature to prepare a PEM with low methanol permeability, which includes modification of the Nafion membrane, development of nonfluorinated membranes, etc.6 Among all, Nafion-hybrid membranes suitably modified with ceramic/inorganic fillers, such as SiO2,16,17 TiO2,18,19 ZrO2,20,21 zirconium phosphate,22 and zeolites,23,24 look attractive and are extensively studied in DMFC applications with their methanol crossover behavior. The addition of inorganic fillers into Nafion led to restructured hydrophilic channels, composed of the pendant sulfonic acid groups of Nafion and the imbedded hydrophilic inorganic fillers.25 However, incorporating these hygroscopic materials without any sulfonic acid groups often reduces the proton conductivity of the Nafion hybrid membranes owing to a decrease in the number of sulfonate groups per unit volume of each domain.26−28 Additives functionalized with sulfonic acid groups increase the net acid content and are beneficial in implementing a Nafion hybrid membrane.29−34 For instance, Kannan et al.32 introduced sulfonic acid functionalized multiwalled carbon nanotubes (s-MWCNTs) into a Nafion matrix to form a hybrid membrane. In this work, s-MWCNTs act as an anchoring backbone for −SO3H groups to enrich proton conductivity and a blending agent to improve mechanical properties of the hybrid membrane. Liu et al.34 functionalized MWCNTs with Nafion and later added them to the Nafion matrix to prepare a hybrid membrane. They reported that the outer surface of MWCNTs is covered with Nafion chains, which facilitates MWCNT dispersion in the Nafion matrix and improves their compatibility. Received: September 23, 2015 Revised: December 17, 2015 Published: December 17, 2015 725

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(1:1) mixture maintained at 3−5 °C with continuous stirring. Then, 150 mg of graphene was added to the above mixture, maintaining the same temperature. Subsequently, 100 mL of 50 wt % H3PO2 aqueous solution was added dropwise, and the mixture was stirred for another 3 h. The obtained sulfonated graphene (S-graphene) was washed with a copious amount of deionized water and dried under vacuum at 90 °C for 12 h. The required amount of S-graphene was added into 5 wt % Nafion ionomer with mass ratios of 0.5, 1, and 1.5%, to realize Nafion hybrid membranes of different filler contents. The resultant admixtures were ultrasonicated for 30 min, followed by mechanical stirring for 12 h. Hybrid membranes were prepared by a solution casting process on flat Plexiglas plates and allowed to dry overnight at 80 °C under vacuum. The hybrid membranes thus formed were peeled off and further dried at 110 °C for 5 h. For comparison, Nafion ionomer was cast in a similar manner without any filler material. The dry membrane thicknesses for all the hybrid membranes were measured at 5 random points over the surface using a digital micrometer and were found to be ∼160 μm. Before use, all the hybrid membranes were pretreated with H2SO4 (0.5 M, 80 °C) for 1 h, followed by washing with deionized water until the pH of the washing water reaches neutral. Physico-Chemical Characterizations. Sulfonic acid functionalized graphene was examined by Raman spectroscopy (RFS27, Bruker) employing a Nd:YAG laser of wavelength 1064 nm. Surface morphologies of graphene and S-graphene samples were characterized by scanning electron microscopy (SEM) (Hitachi S4800) and fieldemission transmission electron microscopy (FE-TEM) (Hitachi, HF3300). Corresponding quantitative elemental mappings for graphene and S-graphene samples were performed by using an energy-dispersive X-ray spectroscope (EDS) (Hitachi S4800) coupled to an SEM instrument. A morphological difference of Nafion−S-graphene hybrid membranes in comparison with pristine recast Nafion was observed by atomic force microscopy (AFM) (AFM, Pico SPM-Picoscan 2100, Molecular Imaging, USA). The imaging was performed in tapping mode with a diamond-like carbon-coated ultrasharp silicon tip. For quantitative and qualitative observations, topological and phase images were analyzed along with the line profile, which gives the height variation at a certain cross-section on the membrane surface. Water-uptake measurements for all the membranes were conducted by immersing the membrane samples into deionized water at room temperature for 24 h to attain equilibrium. The experiments were repeated for at least three different membrane samples, and the average value was considered for calculation. Subsequently, the membranes were surface blotted with a tissue paper and weighed immediately on a microbalance with an accuracy of ±0.01 mg. The samples were then dried in a hot air oven at 100 °C for 12 h, and their weights were measured. The percentage of water uptake was calculated from eq 1

Graphene, a two-dimensional sheet of sp2-hybridized carbon discovered by Geim et al.,35 in 2004, has been widely used in the fields of nanoelectronic devices,36 sensors,37 catalysis,38−40 adsorption,41 and energy storage42−44 due to its excellent thermomechanical stability, superior electrical conductivity, and high degree of exposed surface active sites. Graphene-based fillers have also been used in polymer nanocomposites and hold potential for a variety of possible applications. Various polymer base matrixes with graphene-based fillers have been proposed in the literature, which include polystyrene,45−47 poly(methyl methacrylate),48 polyvinyl alcohol (PVA),49−51 polypropylene,52 polyester,53 poly(vinyldiene fluoride),54 and poly(ethylene oxide),55 for various applications. It was reported that, at 0.7 wt % graphene loading, a PVA−graphene nanohybrid membrane exhibits a 76% increase in tensile strength and a 62% increase in Young’s modulus; these results were attributed to effective load transfer to the graphene filler via interfacial hydrogen bonding.49 Recently, Chien et al.7 prepared various compositions of sulfonated graphene oxide (GO)−Nafion hybrid membranes for DMFC application and obtained improved proton conductivity with reduced methanol crossover. This was attributed to chemical interactions between the different kinds of oxygen functionalities of GO and Nafion. In the present study, we explored graphene functionalized with sulfonic acid groups, as a potential filler material into the Nafion matrix to reduce the methanol crossover and to improve the DMFC performance. A chemical strategy for effective functionalization of graphene by using sulfonic acid containing aryl radicals is adopted, wherein the number of sulfonate groups per unit volume of each graphene domain increased, which facilitates proton conducting pathways between filler and polymer matrix. After incorporating in the Nafion matrix, the hydrated S-graphene can tune the hydrophilic domains and provides proton conducting pathways between the filler and the polymer matrix to increase proton conductivity. The presence of filler preferentially restricts the methanol molecules and reduces its crossover in the hybrid membranes. DMFC performance of these hybrid membranes is evaluated and found to be much superior in relation to the pristine Nafion membrane under identical operating conditions.

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EXPERIMENTAL SECTION

Materials. Sulfanilic acid was procured from Acros Organics, India. Sodium nitrite (NaNO2), hydrochloric acid (HCl), and ethanol were obtained from Merck, Germany. Graphene (N002-PDR) was obtained from Angstron Materials Inc., USA, and H3PO2 was procured from Sigma-Aldrich. Nafion ionomer (5 wt %) was procured from Du Pont, USA, and Pt/C (40 wt % Pt on Vulcan XC-72R carbon) was obtained from Alfa Aesar (Johnson Matthey) Ltd. All the chemicals were used without further purification. The deionized water (18 MΩ cm) was used throughout the experiments. Synthesis of 4-Benzenediazoniumsulfonate and Functionalization of Graphene. 4-Benzenediazoniumsulfonate was synthesized by diazotization of sulfanilic acid, similar to a process reported elsewhere.56 Briefly, 5.2 g of 0.03 M sulfanilic acid was ultrasonically dispersed in 300 mL of 1 M HCl aqueous solution in a round-bottom flask. The flask was then transferred to an ice water bath, and the temperature was controlled at 3−5 °C with continuous stirring. To the admixture was added 33 mL of 1 M NaNO2 aqueous solution dropwise. A clear solution was obtained after all the NaNO2 was added. After stirring for another 1 h at the same temperature, a white precipitate was formed. This was filtered and washed with a copious amount of deionized water. The obtained 4-benzenediazoniumsulfonate was transferred to a round-bottom flask and dissolved in 120 mL of an ethanol and water

⎛ W − Wo ⎞ Water uptake (%) = ⎜ ∞ ⎟ × 100 ⎝ Wo ⎠

(1)

where W∞ and Wo refer to the weights of sorbed and dried membranes, respectively. Ion-exchange capacity (IEC) indicates the number of milliequivalents of ions in 1 g of dry polymer. The IEC for membrane samples was obtained by a titration method for three samples in each case and taking the average value for calculation. The required amount of membrane samples was soaked in 50 mL of a 3 M NaCl solution for 24 h to exchange H+ ions with Na+ ions. To measure displaced H+ ions in the sample, 10 mL of the above solution was titrated against a 0.01 N aq. NaOH solution using phenolphthalein indicator. By measuring the amount of NaOH consumed in the titration, the molar quantity of the sulfonic acid groups (−SO3H) contained in the H+ sample was determined. By using this value, the IEC (mequiv·g−1) was calculated by eq 2. IEC =

Volume of NaOH consumed × Normality of NaOH Dry weight of membrane (2)

726

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Energy & Fuels Proton conductivity of the membranes was measured in the longitudinal direction with a four-probe method using a membrane conductivity cell (Bekktech) with gas flowing options.57 The membrane sample with an area of 1 cm × 3 cm was assembled in the cell in contact with two platinum electrodes placed at fixed positions. The potentiostat was set to apply specific voltages between two Pt electrodes, and the resulting currents were measured. The resistance (R) was derived from the slope of the line that connects the current−voltage data points. The membrane conductivity (σ) under fully humidified conditions (100% RH) and at different temperatures was calculated by eq 3. The above specified experiments were repeated for at least three different membrane samples, and the average resistance values are taken to calculate the proton conductivity

σ=

L R×W×T

the amount of methanol collected from the outlet. Methanol permeability was calculated using the following eq 5. MeOH permeability = MeOH in − MeOHout

Before starting the experiment, the anode inlet, outlet tubes and pumps were filled with fresh 2 M methanol. For each measurement, density meter tubes and sample vials were cleaned with water and acetone and completely dried.

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RESULTS AND DISCUSSION Figure 1 illustrates the functionalization process by direct anchoring of sulfonic acid containing aryl radicals to the

(3)

where L = 0.425 cm, the fixed distance between two Pt electrodes; R is the membrane resistance in Ω; W is the width of the sample in cm; and T is the thickness of the membrane in cm. Thermal behavior of the pristine Nafion membrane and Nafion−Sgraphene hybrid membranes was conducted by thermogravimetric analysis (TGA) using a NETZSCH STA 449F3 TGA-DSC instrument in the temperature range between 35 and 700 °C at a heating rate of 5 °C min−1 with nitrogen flushed at 60 mL min−1. Fabrication of Membrane Electrode Assemblies and DMFC Performance Evaluation. The membranes were used for performance evaluation in DMFCs by making membrane electrode assemblies (MEAs). Commercial SGL DC-35 gas-diffusion layers (GDLs) were used as the electrode on which the catalyst layer is applied. For the catalyst layers, 40 wt % Pt/C catalyst mixed with Nafion ionomer was dispersed in isopropyl alcohol and ultrasonicated for 30 min and coated onto one of the electrodes, which serves as the cathode. For the anode, 40 wt % Pt-Ru/C catalyst was used to form the slurry and coated in a similar manner. Catalyst loading on both the anode and the cathode was kept at 2 mg cm−2. MEAs were obtained by sandwiching the recast Nafion and Nafion−S-graphene hybrid membranes between the cathode and anode, followed by their hot-compaction under a pressure of 20 kg cm−2 at 130 °C for 3 min. The active area for the DMFCs was 4 cm2. Each MEA was coupled with Teflon gas sealing gaskets and placed in single-cell test fixtures separately with a parallel serpentine flow-field machined on graphite plates. Dry oxygen was passed on the cathode side at a flow rate of 250 mL min−1 and 2 M aqueous methanol fed to the anode side of DMFC at a flow rate of 2 mL min−1, respectively. After establishing and equilibrating the opencircuit voltage (OCV), measurements of cell voltage as a function of current density were conducted galvanostatically. The experiments were repeated several times in the given set conditions for equilibrium until the data are reproduced. The polarization data were collected point by point, and 1 min was provided for the system to reach a steady state. All the MEAs were evaluated in DMFCs at 70 °C under atmospheric pressure. Methanol permeability studies were carried at OCV conditions in cell mode as reported elsewhere.58 The amount of methanol crossover from anode to cathode can be calculated based on the mass balance between, amount of methanol supplied to the cell, amount of methanol consumed in the electrochemical reaction, and amount of methanol collected in the outlet. Methanol (2 M) was fed to the anode of the DMFC, maintaining the cell temperature at ∼70 °C, and the unreacted methanol was collected from the anode outlet for a time period of 5 h. Subsequently, 20 mL samples were collected from both anode inlet and outlet, and their density measurements were carried out using a density meter (Metler Toledo DE51). The concentration of methanol was calculated by the following eq 4

⎛ρ⎞ Molarity = 10 × wt % Ethanol⎜ ⎟ ⎝M⎠

(5)

Figure 1. Illustration for the preparation of sulfonated graphene.

graphene surface. SEM images on surface morphologies of graphene before and after sulfonation are shown in Figure 2. Obvious crumpling features with many layers are noted for the pristine graphene sample (Figure 2a,b); in the case of the Sgraphene sample, the layers looks more prominent with sharp and open active edges (Figure 2c,d). This provides a large surface area for S-graphene and offers more facile conducting pathways through sulfonic acid groups while its composite with the Nafion membrane. More SEM images of graphene and Sgraphene samples are shown in the Supporting Information (Figure S1a−d). The density and distribution of S and O from −SO3H groups are EDS mapped for graphene and S-graphene samples (Figure 2e−l). The selected area of the samples used for imaging is shown in the square box in Figure 2 (e and i). It should be noted that S is distributed homogeneously on the entire surface of S-graphene rather than only being located at the graphene edge sites. Further, the S and O content is denser in the case of S-graphene compared to the pristine graphene sample. Sulfur content is also obtained from CHNS analysis, and it is about 0.26% and 2.5% for the pristine graphene and S-graphene samples, respectively. This further confirms the presence of a greater number of sulfonate groups per unit volume in the case of the S-graphene domain. TEM images further corroborate the SEM observation, indicating that the graphene microstructures are not destroyed by the sulfonation reaction (Figure 3a,b); rather, the active sharp edges are exposed and look more prominent for the S-graphene sample. It is noteworthy that the high surface area of graphene with crumpling features and active edge sites is beneficial for the hybrid membrane in view of water uptake and retention. This feature also facilitates in anchoring −SO3H groups to enrich the proton conductivity and act as a blending agent to improve the surface roughness. The peak at around 2.3 keV (Figure 3d) from the EDAX analysis further confirms the presence of sulfur on the Sgraphene material.

(4)

where ρ is the density of the sample and M is the molecular weight of the methanol. The amount of permeated methanol is equal to the difference between the amount of methanol supplied to the cell and 727

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Figure 2. SEM images for (a, b) graphene and (c, d) S-graphene. Quantitative EDS elemental mapping performed on a particular area for graphene (e) and S-graphene (i). Corresponding quantitative EDS elemental mappings for (g) carbon, (h) oxygen, (i) sulfur for graphene and corresponding quantitative EDS elemental mappings for (j) carbon, (k) oxygen, (l) sulfur for S-graphene.

Figure 3. TEM images for (a) graphene, (b) S-graphene with EDAX elemental analysis for (c) graphene, (d) S-graphene.

mode for the sp2 carbon lattice. The D band at 1353 cm−1 corresponds to the vibrations of carbon atoms with associated defects and disorders.59 The intensity ratio, ID/IG, is generally used to evaluate the structural changes of S-graphene during functionalization. In our study, the ID/IG ratio for S-graphene (1.36) is slightly higher in relation to that for the pristine graphene (1.23), indicating successful functionalization of the latter by the −HSO3 group to the sp2 carbon network. Further,

Raman spectroscopy is an effective tool to study the properties of carbonaceous materials and can provide useful information about the degree of ordering of these materials and the state of carbon hybridization. Raman spectroscopy data for pristine graphene and S-graphene are shown in Figure 4. The Raman spectra display two characteristic bands, at around 1353 and 1600 cm−1, for the pristine graphene sample. The G band at 1600 cm−1 is attributed to the first-order scattering of the E2g 728

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Figure 4. Raman spectra of graphene and sulfonated graphene.

the G band for S-graphene shifted toward a lower frequency of about 7 cm−1 in relation to that for pristine graphene. This blue shift of the G band may be attributed to the introduction of abundant aryl radicals to graphene, which might change in the electronic structure/density in the graphene sheet after functionalization. The change of alternating single−double carbon bonds in sp2 carbon after sulfonation could also be a reason for the G band blue shift.58,60 After ascertaining the successful functionalization of graphene, the functionalized graphene is incorporated into the Nafion ionomer and cast hybrid membranes with varying filler loadings. Proton conductivity of the membrane depends to a large extent on the amount of water uptake and ionexchange capacity. It is observed that the Nafion−S-graphene hybrid membranes absorb more water in relation to the pristine recast Nafion membrane (Table 1). This is due to the high surface area of the graphene material and strong interactions between the absorbed water molecules with the −SO3H group of S-graphene. IEC values for S-graphene also increase in relation to those for the pristine Nafion membrane. This is again due to the extensive spreading of high surface area Sgraphene powders over the membrane surface and associated acidic groups within the membrane framework. However, an excessive amount of S-graphene slightly decreases the IEC, may be due to disruption of the ion mobility in the hybrid membrane. The proton conductivity data for the recast Nafion membrane, Nafion−pristine graphene and Nafion−S-graphene hybrid membranes as a function of temperature is shown in Figure 5a. The proton conductivity of the hybrid membranes increases with S-graphene content and reaches a maximum value at 1 wt % of the graphene loading. The conductivity values decrease for the Nafion−S-graphene (1.5 wt %) hybrid membrane, which may be due to the excess amount of graphene, which could hinder/disrupt the continuity of proton

Figure 5. Proton conductivity of (a) recast Nafion, Nafion−pristine graphene, and Nafion−S-graphene hybrid membranes as a function of temperature. (b) Thermogravimetric analysis of pristine recast Nafion and Nafion−S-graphene hybrid membranes.

conduction paths in the Nafion membrane.61 In the case of the pristine Nafion membrane, proton conductivity increases gradually with temperature and attains a maximum value at 80 °C and seems to be a saturation level of proton conductivity, beyond which the decay in the conductivity values is observed. Thus, not only the capacity of water uptake but also the capacity of the membrane to retain water at higher temperatures is seminal for proton conductivity. In the case of hybrid membranes, the continuous rise in the proton conductivity beyond 80 °C indicates that the presence of S-graphene helps the adequate hydration level and provides additional sites in the hybrid to maintain the proton conductivity. To evaluate the significance of sulfonation of graphene material, proton conductivity data for the Nafion−pristine graphene (1 wt %) hybrid membrane are also analyzed. It is seen that the conductivity of the Nafion−pristine graphene (1 wt %) hybrid membrane is lower in relation to that of the Nafion−S-

Table 1. Water Uptake, Ion-Exchange Capacity, Proton Conductivity, Methanol Permeability, and Peak Power Density Values for Different Membranes membrane types recast Nafion Nafion−S-graphene (0.5 wt %) Nafion−S-graphene (1 wt %) Nafion−S-graphene (1.5 wt %)

water uptake (%) at 25 °C

ion-exchange capacity (mequiv g−1) at 25 °C

proton conductivity at 80 °C and 100% RH (mS cm−1)

methanol crossover at OCV (×10−7 mol s−1 cm−2)

DMFC peak power density (mW cm−2)

20.12 ± 0.5 24.56 ± 0.3

0.88 ± 0.01 0.92 ± 0.01

65.3 ± 0.5 82.0 ± 0.7

5.53 ± 0.1 4.94 ± 0.2

54 80

27.32 ± 0.7

0.96 ± 0.01

104.0 ± 1.0

3.72 ± 0.1

118

29.17 ± 0.4

0.95 ± 0.01

94.0 ± 0.8

2.88 ± 0.3

95

729

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Figure 6. SEM images for (a) pristine recast Nafion membrane and (b) Nafion−S-graphene hybrid membrane. (c) Topography of pristine recast Nafion membrane. (d) Topography of Nafion−S-graphene hybrid membrane. (e) Phase image of pristine Nafion membrane. (f) Phase image of Nafion−S-graphene hybrid membrane. (g) Line profile of pristine Nafion membrane. (h) Line profile of Nafion−S-graphene.

graphene (1 wt %) hybrid membrane. This is due to the fact that the pristine graphene has no sulfonated functional groups to impart any additional conduction sites to the base Nafion matrix. However, a larger amount of sulfonic acid groups present in the S-graphene causes increased proton conductivity in the case of Nafion−S-graphene composite membranes. All the membrane samples exhibit an Arrhenius-type temperature dependence of proton conductivity, suggesting thermally activated proton conduction. The activation energy, which is the minimum energy required for proton transfer from one free site to another, can be obtained for each membrane

from the slope of the Arrhenius plot, obeying the relationship as shown in eq 6 σ = σ0 exp( −Ea /RT )

(6)

where σ is the proton conductivity (in S cm−1), σ0 is the preexponential factor, Ea is the activation energy (in kJ mol−1), R is the universal gas constant (8.314 J mol−1 K), and T is the absolute temperature (K). The Ea value of 14.50 kJ mol−1 is observed for the pristine Nafion membrane, which is higher compared to the Ea values for Nafion−S-graphene hybrid membranes (10−13 kJ mol−1). 730

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membrane. The methanol permeability of the Nafion hybrid membrane with 1.5 wt % S-graphene is 2.88 × 10−7 mol s−1 cm−2, which is 50% lower than that of the pristine recast Nafion membrane (5.53 × 10−7 mol s−1 cm−2). The interaction of Sgraphene in the polymer matrix increases the compactness of the membrane and blocks the pathway for methanol passage, which attributed to the lower methanol permeability. From the data, it is observed that a compromise needs to be made between proton conductivity and methanol crossover in any hybrid membrane. In the present study, the S-graphene content is optimized to 1 wt % in the hybrid membrane to realize a good membrane for the DMFC applications. Figure 7a shows the polarization and performance curves for the DMFCs comprising recast the Nafion membrane and

From this observation, it can be concluded that S-graphene provides more acidic sites in the hybrid membranes and exhibits a lower energy barrier for ion transfer from one free site to another. Proton conductivity in the hybrid membranes is attributed to protons transferred through hydrogen bonding with water-filled ion pores. Because of the large interfacial area and the high aspect ratio of S-graphene material, low loadings of filler in the hybrid are sufficient to achieve percolation and improve the proton conductivity.62 The high proton conductivity of the hybrid membrane is attributed to a Grötthustype mechanism, wherein reorganization of hydrogen bonds plays a vital role in hydrated graphene.63 The presence of acidic groups provides more facile hopping of protons, thereby increasing proton transport. Thermal stability of pristine recast Nafion and Nafion−Sgraphene hybrid membranes is studied by TGA shown in Figure 5b. A weight loss of 5% for pristine recast Nafion and Nafion−S-graphene hybrid membranes in the temperature range between 25 and 80 °C is due to loss of water molecules from the membranes. It is observed that the weight loss for all the membranes is less than 10% up to a temperature of about 280 °C. After 280 °C, all the membranes start to decompose and lose weight quite rapidly due to the degradation of sulfonic groups in the side chain of the membranes. It is noteworthy that the thermal behavior of Nafion−S-graphene hybrid membranes shifts toward higher temperature throughout the study and is more pronounced at the temperature range in between 280 and 400 °C. The lower weight loss in the hybrid membrane is a clear manifestation of higher stability of the hybrid membranes comprising S-graphene filler materials. The surface morphologies of recast Nafion and Nafion−Sgraphene hybrid membranes are shown in Figure 6a,b, respectively. S-graphene is dispersed over the Nafion ionomer and tightly held in the polymer matrix due to strong interfacial interactions. Wrinkled features with agglomeration in some regions are also observed due to the presence of S-graphene in the hybrid matrix. Topography, phase image, and line profile of the pristine Nafion and Nafion−S-graphene hybrid membrane are studied by AFM, as shown in Figure 6c−h. The image size of each membrane is 10 μm × 10 μm. The microstructure can have significant impact on the properties of the membranes, particularly in the spatial distribution of the ionic sites. The dark regions correspond to hydrophilic domains of membrane grafted with −SO3H groups, and the bright regions are accounted for by the polymeric chain. It is noteworthy that the surface of the pristine Nafion membrane is relatively smooth and the formation of bicontinuous ionic hydrophilic channels embedded in a hydrophobic matrix is seen. In the case of the Nafion−S-graphene hybrid membrane, the hydrophilic channels are widened randomly with the increase in the surface roughness. This appears to be advantageous toward more water uptake properties and high proton conductivity in the case of hybrid membranes. Moreover, the increased surface roughness of the hybrid membrane is highly beneficial for more adhesion and compatibility with the electrodes during fabrication of membrane electrode assemblies. The methanol crossover is an important factor in DMFCs, which severely affects the overall performance of fuel cell. The methanol permeability values for recast Nafion and hybrid membranes with various weight percentages of S-graphene are shown in Table 1. Hybrid membranes with S-graphene incorporated in the Nafion polymer matrix show much lower methanol permeability than that of the recast pristine Nafion

Figure 7. (a) DMFC performance of pristine Nafion and Nafion−Sgraphene hybrid membrane at 70 °C under ambient pressure. (b) Stability test of the membranes for 50 h under fuel cell configuration at OCV.

Nafion−S-graphene hybrid membranes at 70 °C and ambient pressure. Hybrid membranes show higher open-circuit voltage (OCV) compared to recast pristine Nafion, indicating that the methanol crossover is less in the former. Higher OCV for the hybrid membranes also indicates no significant electronic conductivity effect on the presence of a small amount of graphene in the hybrid membrane, which would otherwise adversely affect the OCV. The hybrid membrane with optimized S-graphene content (1 wt %) delivers a peak power density of 118 mW cm−2, which is much higher than that of the recast Nafion membrane, which delivers only a peak power density of only 54 mW cm−2. The high density of −SO3H groups on S-graphene acts as a solid acid proton 731

DOI: 10.1021/acs.energyfuels.5b02194 Energy Fuels 2016, 30, 725−734

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Energy & Fuels conducting medium and helps the hybrid membrane to achieve higher proton conductivity. The presence of S-graphene in the hybrid membranes blocks the pathway for methanol passage and reduces methanol crossover. The combination of both of these factors is responsible for this membrane in achieving high power density in the DMFC. Further increase in the filler content beyond 1 wt % did not help in achieving higher performance, as Nafion−S-graphene (1.5 wt %) and Nafion−Sgraphene (2 wt %) composite membranes deliver the peak power densities of 95 and 85 mW cm−2, respectively, which are lower in relation to that of the Nafion−S-graphene (1.0 wt %) composite membrane. This is likely due to the fact that higher filler content hinders the ionic conducting pathways and reduces the conductivity. Hence, it can be concluded that 1 wt % of S-graphene is the optimum level of filler content in the Nafion polymer matrix to achieve maximum peak power density. DMFC performance of the Nafion−S-graphene (1 wt %) composite membrane developed in this study is also compared with the performance of various membranes reported in the literature (see the Supporting Information, Table S1). Among them, the Nafion−S graphene composite membrane is a potential candidate in achieving high DMFC performance to that of state-of-the-art methanol fuel cell technology. Accordingly, the present study provides a novel Nafion hybrid membrane with S-graphene filler particles that helps to reduce methanol permeability and improve ionic conductivity. The stability test for both Nafion−S-graphene (1 wt %) and pristine recast Nafion membranes is shown in Figure 7b under OCV conditions at 70 °C for 50 h using 2 M methanol and oxygen on the anode and cathode of the DMFC, respectively. After 50 h of operation, the DMFC comprising the Nafion−Sgraphene hybrid membrane shows only a 5% drop on the OCV value. However, under similar operating conditions, the drop in OCV is 10% in the case of the DMFC comprising the pristine Nafion membrane. In light of the foregoing, it can be surmised that functionalized graphene is a potential inorganic filler to realize a Nafion hybrid membrane which helps in reducing the methanol crossover issue and improves the overall performance in a DMFC.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-44-22544554. Fax: +91-44-22544556. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge CSIR, New Delhi, for financial support through HYDEN (CSC-0122). We thank Dr. V. V. Giridhar, Scientist-in-Charge, CECRI Madras Unit, and Dr. Vijayamohanan K. Pillai, Director, CECRI, Karaikudi, for their constant encouragement and support.

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REFERENCES

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CONCLUSIONS This study describes sulfonation of graphene by anchoring sulfonic acid containing aryl radicals and its subsequent impregnation to a perfluorosulfonic acid ionomer to form a hybrid membrane. The presence of S-graphene blocks the pathway for methanol, which helps in reduction of methanol permeability. The simple and effective functionalization process of graphene significantly increases the ion-exchange capacity and proton conductivity of the hybrid membranes. The combination of high surface area and strong-acid functionality of S-graphene in the hybrid membrane ameliorates the DMFC performance in relation to DMFCs employing a pristine Nafion membrane.

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SEM images for graphene and S-graphene samples and table of DMFC performances of various Nafion composite membranes (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02194. 732

DOI: 10.1021/acs.energyfuels.5b02194 Energy Fuels 2016, 30, 725−734

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