Sulfonated GrapheneâNafion Composite Membranes for Polymer

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Sulfonated Graphene-Nafion Composite Membranes for Polymer Electrolyte Fuel Cells Operating under Reduced Relative Humidity Akhila Kumar Sahu, Kriangsak Ketpang, Sangaraju Shanmugam, Osung Kwon, Sang Cheol Lee, and Hasuck Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11674 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on April 4, 2016

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The Journal of Physical Chemistry

Sulfonated Graphene-Nafion Composite Membranes for Polymer Electrolyte Fuel Cells Operating under Reduced Relative Humidity Akhila Kumar Sahua,b, , Kriangsak Ketpanga, Sangaraju Shanmugama,*, Osung Kwonc,d,, Sangcheol Leec, Hasuck Kima,* a

Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988 KOREA

b

CSIR-Central Electrochemical Research Institute-Madras Unit, CSIR Madras Complex, Taramani, Chennai 600113 INDIA

c

Robotics Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988 KOREA d

Colleage of Liberal Education, Keimyung University, 1095 Dalgubeol-daero, Daegu 42601 KOREA.

Abstract Sulfonic acid-functionalized graphene (S-graphene) is employed as a promising inorganic filler as well as a solid acid proton conducting medium to realize a composite membrane with Nafion for polymer electrolyte fuel cell (PEFC) applications under reduced relative humidity (RH). The functionalization of graphene is performed by sulfonic acid-containing aryl radicals to increase the number of sulfonate groups per unit volume of a domain. A Nafion-S-graphene composite membrane is obtained by embedding S-graphene in Nafion, which provides high absorption of water and fast proton-transport across the electrolyte membrane under low RH values. The proton conductivity of the Nafion-S-graphene (1%) composite membrane at 20% RH is 17 mS cm-1, which is five times higher than that of a pristine recast Nafion membrane. PEFCs incorporating the Nafion-S-graphene composite membrane deliver a peak power density of 300 mW cm-2 at a load current density of 760 mA cm-2 while operating at optimum temperature of 70 °C under 20% RH and ambient pressure. By contrast, operating under identical conditions, a peak power density of 220 mW cm-2 is achieved with the pristine recast Nafion membrane. The Nafion-S-graphene composite membrane could be used to address many critical problems associated with commercial Nafion membranes in fuel cell applications. ______________________________________________________________________________ * Corresponding authors. Tel.: +82-53-785-6410; fax: +82-53-785-6409 E-mail addresses:[email protected] (S.Shanmugam), [email protected] (Hasuck Kim).

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1. Introduction Polymer electrolyte fuel cells (PEFCs) are considered one of the most promising power sources for realizing a hydrogen economy and for alleviating issues such as environmental pollution. PEFCs are, however, not in widespread commercial use and researchers have accordingly attempted to improve their performance and minimize expensive auxiliary components such as humidifiers.1 A pivotal component that underpins the overall performance of fuel cells is the proton exchange membrane (PEM). Critical characteristics of a good PEM include fast proton conduction, good water transport, thermomechanical stability, and sustained durability under various operating conditions.2-4 The state-of-the-art PEM for fuel cells is Nafion due to its high proton conductivity and mechanical strength and excellent progress has been achieved in membrane development, focusing on both academic and automotive applications.5-9

A Nafion membrane is composed of a hydrophobic polytetrafluoroethylene backbone and hydrophilic perflouorinated pendant side chains ending with sulfonic acid moieties. When in contact with water, the sulfonic acid moieties swell and form hydrophilic domains with a size of ∼40 Å.10-13 The proton conductivity of Nafion membranes is very sensitive to water content and is maximal when fully saturated with water.14,15 Proton conductivity decreases dramatically by orders of magnitude at low relative humidity (RH) values due to membrane dehydration and operation of PEFCs at low RH is thus limited. Nafion membrane based fuel cells consequently rely on external humidifiers and associated water management units for humidification of reactant gases before entry to the cell.16-19 PEFCs consume about 20% of the generated power in supporting these auxiliary units.20 One method to reduce the auxiliary power demand for PEFCs is to employ a membrane that can operate under low RH conditions, thus making the system both 2


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simpler and cost-effective. To this end, Nafion-composite membranes suitably modified with ceramic/inorganic fillers, such as SiO2,21-26 TiO2,27-29 zirconium phosphate,30,31 and zeolites,32,33 are extensively used to facilitate proton conductivity at low RH values. The addition of inorganic fillers into Nafion led to restructured hydrophilic channels, composed of the pendant sulfonic acid groups and the imbedded hydrophilic inorganic fillers.34 The tiny hydrophilic fillers are critical to retaining moisture and facilitating proton transfer in the composite membranes. However, incorporating these hygroscopic materials without any sulfonic acid group in Nafion often reduces the proton conductivity owing to a decrease in the number of sulfonate groups per unit volume of each domain.35-37

Additives with sulfonic acid groups have attracted attention for preparing composites with Nafion as they can increase the net sulfonic acid content.38-43 Kannan et al.41 introduced multi-walled carbon nanotubes (s-MWCNTs) which were functionalized with sulfonic acid into a Nafion matrix for a composite membrane. S-MWCNTs act both as an anchoring backbone for -SO3H groups to enhance proton conductivity and as a blending agent to improve mechanical properties of the composite. Liu et al.43 used Nafion-functionalized MWCNTs to a Nafion matrix to prepare a composite membrane. They reported that Nafion chains covered the surface of MWCNTs to facilitate the dispersion in the Nafion matrix and to improve their compatibility.

Graphene is a two-dimensional sheet of sp2-hybridized carbon discovered by Geim et al.,44 has been widely employed in many areas such as nanoelectronic devices,45 sensors,46 catalysis,47-49 adsorption,50 and energy storage51-53 because of its excellent thermo-mechanical stability, high electrical conductivity, and a large number of exposed active sites. Graphene based fillers have been used in polymer nanocomposites and showed potential for a variety of 3



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possible applications. Several polymers were explored with graphene based fillers.54-64 For example, it was reported that at 0.7 wt.% graphene loading, a PVA-graphene nanocomposite exhibits an increase in tensile strength by 76% and in Young’s modulus by 62%, and these results were due to effective load transfer to the graphene filler through interfacial hydrogen bonding.58 Recently, Kumar et al.65 prepared various compositions of graphite oxide (GO)Nafion composite membranes for PEFC application and obtained improved proton conductivity. This was attributed to chemical interactions between the different kinds of oxygen functionalities of GO and Nafion.

Herein, we present a chemical strategy to functionalize graphene directly by using sulfonic acid-containing aryl radicals. The number of sulfonate groups per unit volume of each graphene domain increases. The sulfonated graphene dispersed in the Nafion matrix increases the surface roughness and can tune the extent of hydrophilic domains, thereby increasing the proton conductivity of the composite membranes. The former helps to increase adhesion and compatibility with the electrodes during fabrication of membrane electrode assemblies, while the latter minimizes the ohmic drop under stringent operating conditions especially at low RH environments. Increased water retention properties and a better network for proton transport for Nafion-S-graphene composite membranes are interesting finding for application of composite membranes to PEFCs.

2. Experimental 2.1. Materials

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Sulfanilic acid, NaNO2, and HCl were procured from Daejung Chemicals, Korea. Graphene (N002-PDR) was obtained from Angstron Materials Inc, USA and H3PO2 was procured from Sigma-Aldrich. The Nafion ionomer (15%) was purchased from Ion Power Inc., USA. Pt/C (40 wt.% Pt on Vulcan XC-72 R carbon) was obtained from Alfa Aesar (Johnson Matthey Ltd.). All above chemicals were used as received.

2.2. Synthesis of 4-Benzenediazoniumsulfonate and functionalization of graphene 4-Benzenediazoniumsulfonate was synthesized by diazotization of sulfanilic acid, similar to a process reported elsewhere.66,67 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 oC with continuous stirring. To the admixture, 33 mL of 1 M NaNO2 aqueous solution was added drop wise. 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 1:1 mixture of ethanol and water at 3-5 oC with continuous stirring. Then, 150 mg of graphene was added to the solution at same temperature. Subsequently, 100 mL of aqueous 50 wt.% H3PO2 solution was added drop wise and 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 oC for 12 h. The required amount of S-graphene was then impregnated with Nafion ionomer with weight ratios of 0.5%, 1%, and 1.5% to realize Nafion composite

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membranes of different filler content. The resultant mixtures were ultra-sonicated for 30 min followed by mechanical stirring for 12 h. Composite membranes were prepared by casting these solutions on flat Plexiglass plates and leaving them to dry overnight at 80 oC under vacuum. The composite membranes thus formed were peeled off and further dried at 110 oC for 5 h. For comparison, Nafion ionomer was cast in a similar manner without any filler material. The dry membrane thicknesses of all the composite membranes were measured at 5 random points over the surface using a digital micrometer to find them to be 50 m. Finally, the membranes were pre-treated by boiling in 5% H2O2, H2O, 0.5 M H2SO4, and H2O in sequence for 1 h in each case.

2.3. Physico-chemical characterizations Routine procedures for physico-chemical characterization were followed.67 Raman spectroscopy (RFS27, Bruker) employing an Nd:YAG laser of wavelength 1064 nm was used to examine sulfonic acid functionalized graphene. Surface morphology of the graphene and Sgraphene was observed by scanning electron microscopy (SEM) (Hitachi S4800) and fieldemission transmission electron microscopy (FE-TEM) (Hitachi, HF-3300). Surface morphology and corresponding quantitative elemental mappings for graphene and S-graphene samples along with the membranes were performed by using an energy dispersive X-ray spectroscope (EDS) (Hitachi S4800) coupled to the SEM instrument.

Water-uptake measurements for all membranes were carried out by immersing the membrane samples into deionized water at room temperature for 24 h to attain equilibrium. Subsequently, the membranes were surface blotted with a tissue paper, and weighed immediately

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on a microbalance (± 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 uptake67 was calculated from Eq. (1).  W  W   x 100 Water uptake (%) =    W 

(1)

Where, W∞ and Wo refer to the weights of sorbed and dried membranes, respectively.

Ion-exchange capacity (IEC) indicates the number of milli-equivalents of ions in 1 g of dry polymer. The detailed procedures to get the ICE can be found in the reference.67 The IEC was calculated by Eq. (2).

IEC 

Volume of NaOH consumed  Normality of NaOH (meq. g-1) Dry weight of membrane

(2)

The proton conductivity of membranes was measured in the longitudinal direction with a four-probe method in a membrane conductivity cell (Bekktech) with gas flowing options.67 The membrane conductivity () as a function of relative humidity percentage (% RH) at 80 oC was calculated by Eq. (3).



L R W  t

(3)

Where, L is the fixed distance between two Pt electrodes, 0.425 cm; 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. The surface morphology of the pristine Nafion and Nafion-S-graphene composite membranes was also characterized by SEM with quantitative EDS elemental mappings. Morphological difference on pristine Nafion and Nafion-S-graphene composite membranes was 7



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observed by atomic force microscopy (AFM) (PSIA NX-10). The imaging was performed in contact mode and a diamond-like carbon coated ultra-sharp silicon tip was used throughout the study. The tip radius was around 20 nm and the spring constant of the tip was 40 N m-1. For quantitative and qualitative observations, line profile, histogram, and surface roughness parameters were used, where the line profile gives the height variation at a certain cross-section on membrane surface and the histogram shows the probability density function of the topography. Surface roughness is defined as the deviation of the actual membrane surface topography (Z(x, y)) from an ideal atomically smooth surface (µ).68 In case of membrane, the surface morphology varies significantly from area to area depending on the local surface roughness and the crumple surface of the membrane, the root-mean-square (RMS) roughness (Sq) may provide useful information to the membrane surface topography and is analysed in the study as shown in Eq. 4 Sq 

1 MN

2

M 1 N 1

  z x

k 0 l 0

k

 yl    

(4)

Where, M and N, are respectively, x and y points in the sample area; Z(x, y) is the surface; and



1 M1  MN k 0

N1

 Z( x

k

 yl ) is the mean height.

(5)

l0

2.4. Fabrication of membrane electrode assemblies and fuel cell performance evaluation The membranes were tested for a performance evaluation in PEFCs by making membrane electrode assemblies (MEAs). Generally the same procedures were followed as in the case of DMFC.67 Diffusion-layer coated carbon papers (SGL DC-35) were used as the electrodes. For the reaction layers, 40 wt.% Pt/C catalysts (Johnson Matthey) were dispersed in isopropyl alcohol and ultrasonicated for 30 min followed by the addition of a 30 wt.% Nafion solution. The resultant slurries were ultrasonicated for another 30 min and coated onto one of the electrodes, 8


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which serves as the cathode. For the anode, 40 wt.% Pt/C was dispersed in isopropyl alcohol with 7 wt.% of Nafion and coated in a similar manner. Catalyst loading on both the anode and cathode was kept at 0.5 mg cm-2. The active area for the PEFCs was 25 cm2. MEAs were obtained by sandwiching the membrane between the cathode and anode followed by hot-pressing under a pressure of 60 kg cm-2 at 130°C for 3 min. Each MEAs were coupled with Teflon gassealing gaskets and placed in single-cell test fixtures separately with a parallel serpentine flowfield machined on graphite plates. The PEFC was maintained at ca.100% RH by passing hot and wet hydrogen and oxygen gases to its anode and cathode sides, respectively, at a flow rate of 450 mL min-1 and 600 mL min-1 measured by mass flow controllers. The RH of the PEFC depends on the mass of water vapor (D), saturated vapor pressure of water (Psat), and moisture content (mH2O). In order to operate the PEFC at varying RH values between 20% and 100%, parameters such as dew-point temperature (DPT), gas temperature (GT)/gas supply temperature (GST), and dew-point-humidification temperature (DPHT) were adjusted using a Fuel Cell Test Station, model PEM-FCTS-158541 (Arbin Instruments, US), as described in the literature.24 After establishing and equilibrating the desired humidity level, 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 till 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 PEFCs under atmospheric pressure. The permeability of H2 gas measurements were also carried out for pristine Nafion and Nafion-S-graphene (1%) composite membrane in cell configuration with the active area of 25 cm2 by linear sweep voltammetry (LSV) technique similar to earlier works.16,69 The H2 crossover limiting current was measured using a potentiostat (Autolab, PGSTAT 30) by sweeping the cell

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voltage ranging between 0 and 0.45 V vs. SHE at a scan rate of 10 mV s-1. During the experiment, the cell was conditioned at 60 ᵒC and 100% RH with hydrogen (200 mL min-1) and nitrogen (25 mL min-1) feeds on the anode and cathode side, respectively. The hydrogen diffuses from the anode electrode through the polymer electrolyte membrane and reaches the cathode electrode, where it is oxidized electrochemically. The current that measured at cathode electrode is referred as hydrogen crossover limiting current. The crossover current was determined at the steady state voltage range between 0.15 V and 0.35 V. The mechanical stability of pristine Nafion and Nafion-S-graphene (1%) composite membrane was evaluated from the stress–strain curve obtained with a Shimadzu universal testing instrument Autograph (AGS-J10kN) at 30 °C. The membranes were placed in a sample holder and pulled at a cross head speed of 1 mm min-1 similar to earlier test procedures.69

3. Results and discussion Figure 1 depicts the functionalization process by directly attaching sulfonic acidcontaining aryl radicals to the graphene surface. The XRD patterns for graphene and S-graphene are shown in Fig. 2a. The characteristic peaks corresponding to the diffractions from (002) basal plane and (100) plane, respectively are found at 2θ=25.7° and 43.1° for both graphene and Sgraphene. The corresponding d-spacing is calculated to be 3.462 Å for (002) basal plane and 2.096 Å for (100) plane for both samples. Figure 2b shows the Raman spectra of pristine graphene and S-graphene which provides information on defects, degree of ordering of graphene materials as well as the state of carbon hybridization. Two characteristic bands, at around 1353 cm-1 and 1600 cm-1 are observed for pristine graphene sample. The G band at 1600 cm-1 attributes to first-order scattering of E2g mode of sp2 carbon lattice while the D band at 1353 cm-1

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is the vibrations of carbon atoms with associated defects involved in it.67,70 The intensity ratio, I(D)/I(G) for pristine graphene and S-graphene is 1.23 and 1.36, respectively, which indicats surface functionalization of the former by -HSO3 group and more defects in S-graphene. Furthermore, the G band for S-graphene shifts towards a lower frequency by ~7 cm-1 with respect to pristine graphene which indicates the presence of abundant aryl radicals in the Sgraphene sample. The change of alternating single - double carbon bonds in sp2 carbon lattice of S-graphene could also be a reason of G band blue shift.66,67,71 The XPS spectra of graphene and S-graphene are shown in Figure 2c. The peak at 168 eV corresponds to S 2p of S-graphene which is otherwise not noticed in the pristine graphene sample. This further confirms the presence of sulphur which arises upon sulfonation of graphene. The de-convoluted S 2p spectra of S-graphene sample are shown in Figure 2d. It is clearly seen that there are multiple peaks located at the binding energies between 166 and 169 eV which are characteristic peaks of -SO3H groups. This observation further confirms the presence of -SO3H groups from the sulfonation of graphene. In addition, the second S 2p peaks are found at 164.28 eV and 163.32 eV which are attributing to R–SH/R–S–R bonds at the surface of S-graphene.72,73

Scanning electron microscopy (SEM) images for graphene before and after sulfonation are shown in Figs. 3a and 3b, respectively. The surface morphologies of S-graphene show crumpling features similar to the pristine graphene, indicating that there is no obvious structural deformation took place after sulfonation. Transmission electron microscopy (TEM) images further show clear multilayer graphene sheets for both samples without much morphological variations (Figs. 3c & 3d). Above observations confirmed that the graphene microstructures were not destroyed after the sulfonation reaction. The crumpling features of graphene are beneficial in 11



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anchoring -SO3H groups through their active edge sites which help in enhancing proton conductivity and mechanical characteristics of the composite membranes. The peak at about 2.3 keV in the EDS data (Figure 3f) further confirms the presence of sulphur atom in S-graphene material. The density and distribution of -SO3H groups on the S-graphene are also evaluated by quantitative energy dispersive elemental mapping. It is seen that the residual sulphur is uniformly distributed on entire surface of pristine graphene rather than only being located at the edge sites (Figure 4d). Further, the sulphur content is denser in case of S-graphene which further confirms anchoring of -SO3H moieties during surface functionalization of graphene (Figure 5d). For more quantitative information on sulphur, CHNS analysis is performed and the sulphur content in pristine graphene and S-graphene is found to be 0.26% and 2.5%, respectively. This further confirms the presence of more sulfonate groups per unit volume in case of S-graphene material. After ascertaining successful functionalization of graphene, the functionalized graphene is incorporated in the Nafion ionomer and cast composite membranes with varying filler loadings. Proton conductivity of the membrane to a large extent depends on the amount of water uptake and ion exchange capacity. It is observed that the Nafion-S-graphene composite membranes absorb more water with respect to the pristine recast-Nafion membrane (Table 1). In addition to the high surface area of the filler material, strong interactions between absorbed water and -SO3H group of S-graphene lead to increased water content of the composite membranes. IEC values for S-graphene also increase compared to those of the pristine Nafion membrane. This is again due to the extended spreading of high surface area S-graphene powders over the membrane surface and associated acidic groups within the membrane framework. A large amount of Sgraphene, however, decreases the IEC slightly, probably due to disruption of ion mobility in the composite membrane.

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Figure 6a shows the proton conductivity of pristine recast Nafion membrane, Nafionpristine graphene and Nafion-S-graphene composite membranes as a function of RH at 80 oC. Proton conductivity of the pristine recast Nafion membrane under a fully humidified condition (ca. 100% RH) is 60 mS cm-1. Proton conductivity of the Nafion membrane decreases as the RH value decreases; at 20% RH, the conductivity of the Nafion membrane is only 3.5 mS cm-1. The proton conductivity of the Nafion-S-graphene composite membranes is determined to be higher in relation to the pristine Nafion membrane at all RH values. The proton conductivity of the composite membranes increase with S-graphene content and reaches a maximum value at 1% of the graphene loading. The conductivity value is higher in relation to the composite membrane with presence of unmodified graphene which proves that introduction of sulfonic groups in the graphene helps rise in proton conductivity. The decrease in conductivity values for Nafion-Sgraphene (1.5%) composite membrane may be due to the excess amount of graphene which could hinder/disrupt the continuity of proton conduction paths in the Nafion membrane.7 Under a fully humidified condition, a maximum proton conductivity of 105 mS cm-1 is exhibited by the Nafion-S-graphene (1%) composite membrane. Similar to the pristine Nafion membrane, proton conductivity of Nafion-S-graphene composite membranes decreases with RH. However, the conductivity of composite membranes is higher with respect to the pristine Nafion membrane at all RH values. It is noteworthy that at 20% RH, the conductivity of Nafion-S-graphene (1%) composite membrane is about 5 times higher than the corresponding conductivity value of pristine Nafion membrane. At lower RH values, desorption of water molecules on sulfonic acid sites in pristine Nafion leads to reduced proton mobility as well as channel pinch-off; a properly ordered microphase separation is consequently not achieved, thereby lowering the overall proton

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conductivity. By contrast, Nafion-S-graphene composite membranes have enhanced waterholding capacity due to stronger absorption of water on the high surface area S-graphene and their edge sites, which serves to retain proton conductivity. The proton conductivity data for recast Nafion membrane, Nafion-pristine graphene and Nafion-S-graphene composite membranes as a function of temperature are shown in Figure 6b. Proton conductivity of the pristine Nafion membrane increases gradually with temperature and is maximum at 80 oC beyond which the conductivity values start to decrease. Thus water retention capacity of the membrane at higher temperatures is crucial for proton conductivity. In the case of Nafion-Sgraphene composite membranes, the proton conductivity values keeps increasing even above 80 oC which indicates that the presence of graphene as inorganic fillers provides additional sites to maintain adequate proton conductivity. From the temperature dependent Arrhenius-type proton conductivity behaviour, the activation energy (Ea), which refers to the minimum energy required for proton transfer from one free-site to another, is calculated for each membrane from slope of the Arrhenius plot using the equation:

σ = σ0 exp (- Ea/ RT)

(6)

Where, σ is the proton conductivity (in S cm-1), σ0 is the pre-exponential 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).

Recast Nafion membrane exhibits Ea value of 14.50 kJ mol-1 which is higher in relation to Nafion-S-graphene composite membranes (10 - 13 kJ mol-1).65,67 Accordingly, it can be concluded that the composite membrane with S-graphene filler material provides more acidic

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sites which lowers the energy barrier for proton transfer from one free-site to another. Further, due to large surface area and high aspect ratio of S-graphene, small amount of filler spread across the composite membranes are sufficient to achieve percolation and improve proton transfer along with water molecules.57 Increased proton conductivity of the composite membranes is also attributed to Grotthus type mechanism, in which reorganization of hydrogen bonds is vital in hydrated graphene.65 From the foregoing, the optimum filler content of Sgraphene is optimized to 1% in this study to realize the best composite membrane and accordingly this composite membrane is considered for further study and finally evaluating its performance in PEFC at different humidity conditions. The surface morphologies of recast Nafion and Nafion-S-graphene composite membranes are shown in Figures 7a and 7b1, 7b2, respectively. S-graphene is dispersed over the Nafion ionomer and tightly held in the polymer matrix due to strong interfacial interactions. Wrinkled graphene features in some regions are also observed in the polymer matrix due to the presence of S-graphene (Figures 7b1 and 7b2). The topography of the pristine Nafion and Nafion-S-graphene composite membranes is observed by AFM and results are shown in Figures 7c and 7d, respectively. The membranes were scanned under ambient conditions: room temperature (25 oC) and 30% RH. The image size is 10 μm × 10 μm. Colors in the topography represent height variation on the membrane. The topography shows significant morphological difference on the membrane before and after S-graphene is mixed with Nafion. Pristine Nafion shows a detailed structure with many irregular and random spikes of approximately 1 nm to 4 nm height and 100 nm to 200 nm width distributed over the membrane surface. The Nafion-S-graphene composite membrane, however, shows entirely different surface behaviour with respect to pristine Nafion. It displays huge protrusions of 80 nm height at the left and right sides of the membrane. Also, it

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shows a deep subsidence at the top center of the membrane. From the larger scale topography (15 μm × 15 μm), the protrusions and subsidence are randomly distributed over the membrane. Detailed structure with spikes on the membrane might be hidden because the protrusions are relatively higher than spikes in case of the composite membrane. Figures 7e and 7f present the line profiles for the Nafion and Nafion-S-graphene composite membranes. Figures 7g and 7h show the histograms for the pristine Nafion and Nafion-S-graphene composite membranes, respectively. The histogram represents the probability density function of each pixel. It is directly related to the surface roughness and filler distribution. Figure 7g shows a clear Gaussianlike distribution. This illustrates that the pristine Nafion membrane surface is smooth. Figure 7h shows two sharp peaks at around -10 nm and 10 nm. They correspond to protrusions and subsidence on the composite membrane. The roughness analysis for the pristine Nafion and Nafion-S-graphene composite membranes were measured from Eq. 4. The RMS roughness for pristine Nafion is approximately 2 nm, similar to uniform polymer materials. However, the RMS roughness for the Nafion-S-graphene composite membrane is approximately 28 nm. This result shows that RMS roughness increases significantly when Nafion incorporates S-graphene. It is noteworthy that high surface area of graphene with crumpling features is responsible for higher surface roughness of the composite membrane which helps increase in water uptake capacity and brings compatibility with the electrodes while fabrication of membrane electrode assemblies. The hydrogen crossover data for pristine Nafion and Nafion-S-graphene (1%) composite membranes are shown in Figure 8a. The higher limiting current density indicates the higher fuel crossover through membrane. It was observed that the hydrogen crossover current density for the Nafion-S-graphene is 0.75 mA cm-2 which is only 20% of that for pristine Nafion membrane (3.94 mA cm-2). The reduction of hydrogen crossover in case of composite membrane is

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attributed to the presence of high surface area S-graphene fillers over the polymer matrix wherein molecular hydrogen is made to follow a tortuous path across the composite membranes. Mechanical properties of the pristine Nafion and optimized Nafion-S-graphene composite membrane are shown in Figure 8b. The tensile strength of 9.375 and 12.78 MPa is obtained for pristine Nafion and composite membrane, respectively which indicates the higher mechanical stability of the later. Accordingly, Nafion-S-graphene hybrid membranes are extremely beneficial for low RH and elevated temperature PEFC operation. The Nafion-S-graphene composite membrane with optimized S-graphene content was further studied through a fuel cell performance evaluation together with a recast Nafion membrane and Nafion-pristine graphene composite membrane for comparison. Figure 9a shows the polarization and performance curves for the PEFCs comprising recast Nafion membrane, Nafion-pristine graphene, and Nafion-S-graphene composite membrane under ∼100% RH at 70 °C and ambient pressure. The open circuit voltage (OCV) of PEFCs comprising composite membranes is about 0.98 V indicating that there is no significant electronic conductivity effect due to the presence of small amount of graphene in Nafion matrix. Peak power densities of 667 mW cm-2 and 680 mW cm-2 are achieved for the PEFCs with the recast Nafion membrane and Nafion-pristine graphene composite membrane, respectively. By contrast, the Nafion-S-graphene composite membrane delivers a peak power density of 720 mW cm-2 at relatively higher current with identical operating conditions. Due to the high density of -SO3H groups on S-graphene, the ion exchange capacity of Nafion-S-graphene is significantly higher with respect to the pristine Nafion and Nafion-pristine graphene membrane (Table 1). The high density of -SO3H groups on S-graphene also causes S-graphene to act as a solid acid proton conducting medium.

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The polarization and power density curves for PEFCs with recast Nafion membrane, Nafion-pristine graphene, and Nafion-S-graphene composite membrane under a near-dry condition (∼20% RH) are shown in Figure 9b. At this RH value, the PEFCs with the recast Nafion membrane and the Nafion-pristine graphene membrane yield a peak power density of 220 mW cm-2 and 246 mW cm-2 at load current densities of 515 mA cm-2 and 575 mA cm-2, respectively. The low performance of pristine Nafion membrane is primarily due to the poor conductivity at this RH value. In contrast, under identical conditions, the PEFC employing the Nafion-S-graphene composite membrane delivers a peak power density of 300 mW cm-2 at a load current density value of 760 mA cm-2. A large improvement in fuel cell performance at high current density region is clearly noticed for the Nafion-S-graphene composite membrane at this RH value. From the foregoing, it is obvious that the PEFC performance is closely related to the issue of water management. In unmodified Nafion membranes, due to the limited availability of water at the anode, electro-osmotic drag of water from the anode to the cathode and insufficient water back-diffusion from the cathode to the anode cause the MEA to dehydrate. Membrane dehydration results in an increase in the ohmic resistance of the cell, leading to decreased cell performance. By contrast, Nafion-S-graphene composite membrane is responsible to hold more generated water and helps the composite membrane sufficiently wet which maintains the proton conductivity at such a low RH value. At high load current densities, due to more product water, cathode flooding occurs in case of a PEFC with pristine Nafion membrane thereby the effective area of gas-diffusion layers inside the electrode may decrease. However, due to the high water uptake nature of the Nafion-S-graphene composite membrane, the water flooding is mitigated. Moreover, impregnation of S-graphene into Nafion reduces the susceptibility of the composite membranes to high-temperature damage, mitigating their shrinkage at low RH values. The

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presence of S-graphene within the membrane apparently obviates structural changes, and the proton conductivity is retained even at low RH values. Accordingly, the present study provides a novel Nafion-S-graphene composite membrane with filler particles that absorb water and keep the membrane hydrated even at low RH values. This helps the PEFC to sustain periods of inletstream draught without excessive loss of membrane conductivity. Consequently, auxiliaries conventionally used to humidify PEFCs could be avoided, thereby making the system simpler and more cost effective.

4. Conclusions This study describes sulfonation of graphene by anchoring sulfonic acid-containing aryl radicals and their subsequent impregnation to a perfluorosulfonic acid ionomer to form a composite membrane. Due to the high density of -SO3H groups on S-graphene, the ion exchange capacity and proton conductivity of the composite membrane significantly increase. Combination of high surface area and strong-acid functionality of S-graphene, the composite membrane ameliorates the PEFC performance at low RH values in relation to PEFCs employing a pristine Nafion membrane.

Acknowledgments: This work was supported by the DGIST R & D program of the Ministry of Science, ICT and Future Planning of Korea (15-BD-01 and 16-RS-04). A.K.S. thanks to the Director of CSIR-CECRI, Karaikudi for constant encouragement and support.

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Electrolyte Fuel Cells under Reduced Relative Humidity. J. Membr. Sci. 2016, 499, 503514 (70) Liu, F.; Sun, J.; Zhu, L.; Meng, X.; Qi, C.; Xiao, F.S. Sulfated Graphene as an Efficient Solid Catalyst for Acid-Catalyzed Liquid Reactions. J. Mater. Chem. 2012, 22, 5495-5502. (71) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prud’homme, R. K.; Aksay I. A.; Car, R. Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets. Nano Lett., 2008, 8, 36-41. (72) Duesberg, G. S.; Graupner, R.; Downes, P.; Minett, A.; Ley, L.; Roth, S. Hydrothermal Functionalization of Single-Walled Carbon Nanotubes. Synth. Met. 2004,142, 263-266. (73) Barroso-Bujans, F.; Fierro, J. L. G.; Rojas, S.; Sanchez-Cortes, S.; Arroyo, M.; LopezManchado, M. A. Degree of Functionalization of Carbon Nanofibers with Benzenesulfonic Groups in an Acid Medium. Carbon 2007, 45, 1669-1678.

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Table 1. Water uptake, ion exchange capacity, and proton conductivity and area specific resistance values for different membranes

Membrane types

Water uptake* (%) at 25 oC

Ion-exchange capacity* (meq. g-1) at 25 oC

Proton conductivity* at 80 ◦C and 100% RH (mS cm-1)

Area specific resistance (Ω cm2) at 100% RH

Area specific resistance (Ω cm2) at 20% RH

Recast Nafion

20.1

0.88

65.3

0.26

0.78

24.5

0.92

82.0

---

---

27.3

0.96

104.0

0.24

0.53

29.2

0.95

94.0

---

---

21.4

0.89

67.2

0.25

0.73

Nafion-S-graphene (0.5%) Nafion-S-graphene (1%) Nafion-S-graphene (1.5%) Nafion-pristine graphene (1%)

* Values in part were taken from reference 67 for easy comparison.

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Figure captions

Fig. 1. Illustration for the preparation of sulfonated graphene. Fig. 2. XRD patterns for graphene and S-graphene (a); Raman spectra for graphene and Sgraphene (b); XPS spectra for graphene and S-graphene (survey spectra) (c); Deconvoluted XPS S 2p spectra for S-graphene (d). Fig. 3. SEM images for graphene (a), and S-graphene (b); TEM images for graphene (c) and Sgraphene (d); EDS spectrum for graphene (e) and S-graphene (f). Fig. 4. SEM image for graphene of selected area (a), and corresponding quantitative EDS elemental mappings for carbon (b), oxygen (c), sulphur (d) and overlapping of all the elements (e). Fig. 5. SEM image for S-graphene of selected area (a), and corresponding quantitative EDS elemental mappings for carbon (b), oxygen (c), sulphur (d) and overlapping of all elements (e). Fig. 6. Proton conductivity of recast Nafion, Nafion-graphene and Nafion-S-graphene composite membranes as a function of relative humidity (a); Proton conductivity of recast Nafion, Nafiongraphene and Nafion-S-graphene composite membranes as a function of temperature (b).

Fig. 7. SEM images for pristine recast Nafion membrane (a), Nafion-S-graphene composite membrane (b1 & b2); topography of pristine recast Nafion membrane (c), topography of NafionS-graphene composite membrane (d); line profile of pristine Nafion membrane (e), line profile of Nafion-S-graphene composite membrane (f), histogram of pristine Nafion membrane (g), black line is histogram based on the topography of pristine Nafion membrane and red line is Gaussian31



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fit of histogram; histogram of Nafion-S-graphene composite membrane (h), black line is histogram based on the topography of Nafion-S-graphene composite membrane, green line is individual peaks and red line is sum of individual peaks. Fig. 8. H2 cross-over current for the PEFCs employing pristine Nafion and Nafion-S-graphene composite membrane (a); Stress-strain curves for pristine Nafion and Nafion-S-graphene composite membrane at ambient condition (b). Fig. 9. Performance of H2/O2 PEFC with recast Nafion, Nafion-graphene and Nafion-S-graphene composite membranes at 100% RH at 70 oC under atmospheric pressure (a); Performance of H2/O2 PEFC with recast Nafion, Nafion-graphene and Nafion-S-graphene composite membranes at 20% RH at 70 oC under atmospheric pressure.

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Fig. 1.

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(002)

(a)

(b)

Page 34 of 42

D - Band

Intensity (a. u.)

Intensity (a. u.)

(100)

G - Band

S-Graphene

(100)

(002)

-1

1593 cm

S-Graphene

ID/IG = 1.36

-1

1600 cm Graphene

ID/IG = 1.23

Graphene

10

20

30

40

50

60

70

80

1000

1200

2(Deg.) C 1s

1600

S 2p

1800

Counts

S-Graphene 162

165

168

171

174

Binding energy (eV)

O 1s

SO3H

(d) Intensity (a. u.)

Intensity (a. u.)

(c)

1400

Raman shift (cm-1)

S 2p

C 1s

R-SH

Graphene O 1s

700

600

500

400

300

200

100

0

172

Binding energy (eV)

170

168

166

Binding energy (eV)

Fig. 2.

34


164

162

Page 35 of 42

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Fig. 3.

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(a)

(b)

(c)

(d)

(e)

Fig. 4.

36


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(a)

(b)

(c)

(d)

(e)

Fig.5

37



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Proton conductivity (mS cm-1)

100

Page 38 of 42

(a)

10 Recast Nafion Nafion-S-graphene (0.5%) Nafion-S-graphene (1%) Nafion-S-graphene (1.5%) Nafion-pristine graphene (1%)

1 20

40

60

80

100

Relative humidity (%)

-1550 -1

Log conductivity (mS cm )

-1600

Recast Nafion Nafion-S-graphene (0.5%) Nafion-S-graphene (1%) Nafion-S-graphene (1.5%) Nafion-pristine graphene (1%)

(b)

-1650 -1700 -1750 -1800 -1850 -1900 -1950 -2000 2.6

2.8

3.0

3.2 -1

1000/T (K ) Fig. 6.

38


3.4

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(a)

(b2)

(b1)

(a)

Fig. 7.

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Fig. 8.

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800

(a)

100% RH 600

Cell voltage (V)

0.8 0.6

400

0.4 200 Recast Nafion

0.2

Nafion-pristine graphene composite (1%) Nafion-S-graphene composite (1%)

0.0 0

500

1000

1500

2000

Power density (mW cm-2)

1.0

0 2500

Current density (mA cm-2) 350

Cell voltage (V)

20% RH

300


(b)

1.0 0.8

250

0.6

200 150

0.4

100 Recast Nafion

0.2

50


0.0

0 0

200

400

600

800

Current density (mA cm-2)

Fig. 9.

41


1000

1200


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Page 42 of 42

Table of Contents (TOC) Graphic

350

Cell voltage (V)

20% RH

300


(b)

1.0 0.8

250

0.6

200 150

0.4

100 Recast Nafion

0.2

50


0.0

0 0

200

400

600

800

Current density (mA cm-2)

42


1000

1200

Sulfonated GrapheneâNafion Composite Membranes for Polymer

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