Functionalized fullerene embedded in Nafion matrix: A modified composite membrane electrolyte for direct methanol fuel cells

Accepted Manuscript Functionalized fullerene embedded in Nafion matrix: A modified composite membrane electrolyte for direct methanol fuel cells Gutru Rambabu, N. Nagaraju, Santoshkumar D. Bhat PII: DOI: Reference:

S1385-8947(16)30978-0 http://dx.doi.org/10.1016/j.cej.2016.07.032 CEJ 15481

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

29 April 2016 24 June 2016 8 July 2016

Please cite this article as: G. Rambabu, N. Nagaraju, S.D. Bhat, Functionalized fullerene embedded in Nafion matrix: A modified composite membrane electrolyte for direct methanol fuel cells, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.07.032

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Functionalized fullerene embedded in Nafion matrix: A modified composite membrane electrolyte for direct methanol fuel cells Gutru Rambabu, N. Nagaraju, Santoshkumar D. Bhat* CSIR-Central Electrochemical Research Institute-Madras Unit, CSIR Madras Complex, Chennai-600 113, India

ABSTRACT Methanol permeability is a major concern for Nafion® membranes used as polymer electrolyte in direct methanol fuel cells (DMFCs). In the present study, composite membranes are formed by incorporation of functionalized fullerene (FF) in Nafion® ionomer for its use as electrolytes in direct methanol fuel cells at various methanol concentrations. Fullerene was functionalized by 4-benzene diazonium sulfonic acid precursor formed via diazotization reaction of sulfanilic acid. Surface functionalization and sulfonation of fullerene is confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and elemental analysis. Structural changes in fullerene are determined by transmission electron microscopy (TEM) analysis. The composite membranes of Nafion®-FF are prepared by solvent casting technique and characterized for their physico-chemical properties in terms of water uptake, proton conductivity and methanol permeability. The morphological changes are observed by field emission scanning electron microscopy (FE-SEM) suggesting the FF distribution in Nafion® and atomic force microscopy (AFM) explaining the presence of FF in the ionic clusters of Nafion® thereby increasing the surface roughness. Nafion®-FF composite membranes show improved proton conductivity due to the presence of surface functional –SO3H groups. Composite membranes exhibit better electrochemical selectivity and as a result enhanced DMFC power output in comparison to recast Nafion®. In addition, methanol permeability and DMFC polarization for the optimized composite membrane are carried out at different methanol concentration. The peak power density of 146 mW cm-2 in DMFC is obtained for Nafion®-FF (1 wt. %) at 2 M methanol which is higher than the performance observed for Nafion-117. Open circuit voltage change is minimal with respect to time for Nafion®-FF (1 wt. %) confirming its better stability in comparison with recast Nafion® and on par with Nafion-117. Keywords: Fullerene, Ionomer, Membranes, Permeability, Fuel Cells * Corresponding author. Tel.: +91-44-22544564; fax: +91-44-22542456 E-mail address: [email protected] (S.D. Bhat)

1

1. Introduction In the present scenario, the rise of global energy demand and most of the energy being produced by burning fossil fuel has adverse environmental effects [1]. Hence need of clean energy is of primary importance. The research efforts are rigorous to develop the devices that convert energy, also being environmental benign and economically viable. Recent emerging trends on energy storage and conversion applications suggest that membrane electrolyte is one of the important component that can be fine-tuned to improve the performance of fuel cells, litihim and sodium-ion batteries, supercapacitors and solar cells in view of global energy scenario [2-5]. Presently, fuel cells are subjected as electrochemical devices that covert chemical energy of the fuel into electrical energy with high efficiency without any major pollutants [6]. Among various types of fuel cells, direct methanol fuel cells (DMFCs) are most promising energy conversion technology wherein methanol is used as fuel which is easy to handle compared to hydrogen [7]. In addition, DMFC does not require any combustion of fuel and hence there is no emission of greenhouse gases like methane, nitrous oxide and carbon monoxide which causes global warming and depletion of ozone layer. It is noteworthy that in methanol oxidation reaction (MOR), CO species are intermediate product which later oxidizes to CO2 through polishing Pt active site from CO poisoner species. However this will have a minimal impact on the environment during letout [6]. As far as applications are concerned, DMFC finds its real potential in powering portable and stationary sectors [7,8]. The power output in DMFC depends mainly on membrane electrode assembly (MEA) where proton exchange membrane (PEM) is sandwiched between two electrodes (anode and cathode). PEM acts as an electrolyte for ionic transport and also as a barrier to restrict methanol cross-over from anode to cathode [9]. Hence membranes used in DMFCs should

2

essentially have high proton conductivity, better methanol barrier characteristics and durability during long term operation [10]. Nafion® is widely used electrolyte for DMFCs due its high ionic conductivity and stability [11-14]. However, high methanol cross-over through the Nafion® is a major limitation in DMFCs, due to its larger ionic clusters through which methanol can permeate. This also originates the mixed potential at the cathode in turn reducing the overall cell performance [8]. Modified Nafion® composite membranes can significantly mitigate the methanol permeability through inorganic additives like oxides, zeolites and mesoporous materials [15-18]. Kingshuk Dutta et al prepared modified Nafion membranes using partially sulfonated poly aniline wherein improved IEC and reduced methanol permeability was observed in DMFCs [19]. Sudipta Mondal et al. reported another strategy to reduce methanol crossover of Nafion membrane wher in sulfonated PVdF and PBI were coated on the Nafion-117 surface and used as electrolyte in DMFCs [20]. These materials can have an impact on pore and channel size reduction in Nafion® by forming its hybrids [21]. In addition, these materials may also block the proton conducting paths in Nafion® where methanol permeability is reduced affecting ionic conductivity which is not desirable for DMFC operation [22]. Since methanol and proton transport in the membrane follow the same path, it is important to reduce the methanol permeability selectively without affecting the proton conductivity of the pristine Nafion® membrane in DMFCs [23]. Recently, functionalized carbon nanomaterials like carbon nanotubes (CNTs) and graphene oxide (GO) are used as additives in Nafion® to control the methanol permeability through Nafion® membrane in DMFCs. For instance, histidine functionalized CNTs were incorporated in Nafion® for its use as electrolyte in DMFCs and the resultant composite membrane exhibits better ionic transport properties due to the electrostatic interactions between the imidazole group of histidine and sulfonic acid group of Nafion® [24]. 3

Incorporation of carboxylic acid functionalized MWCNTs in Nafion® were also carried out wherein decrease in methanol permeability is observed. However proton conductivity decreased due to the absence of ion conducting functional groups in MWCNTs [25]. Graphene oxide due to its unique structural properties [26] improved the properties of Nafion® to form a composite membrane for its use as electrolyte in DMFCs [7, 27]. Nicotera et al. prepared nanocomposite membranes of Nafion using organo functionalized graphene oxide (GO) as additive and studied its effect as electrolyte in DMFCs [28]. Kai Feng et al prepared composite of Nafion using sulfonated graphene oxide-silica and studied its properties in terms of proton conductivity and methanol permeability [7]. Fullerene C60 subjected as additive in the present study has high surface functional group density due to its unique buckky ball structure, high electron affinity and radical scavenging property [29,30]. By considering these advantages, PEMs with fullerene as an additive was fabricated for fuel cell applications [31]. Ken Tasaki et al. prepared composite membranes of Nafion® using polyhydroxy fullerene as additive and studied its characteristics in terms of water uptake and proton conductivity [29]. Saga et al. reported polystyrenefullerene composite membranes for DMFCs wherein fullerene mitigated the methanol permeability and improved the oxidation stability. However mechanical stability was affected due to the poor compatibility between fullerene and host matrix [30]. Considering the above important literatures, it can be seen that fullerene modification and its compatibility with Nafion® is essential for improved stability and conductivity. However, functionalized fullerene as additive in Nafion® and its characteristics in DMFC in terms of power output and stability are not completely explored in the literature. Hence, in the present study we report simple and effective route for the sulfonation of fullerene which provides additional sulfonic acid groups and better compatibility with Nafion® matrix thereby improving the proton conductivity. Fullerene is functionalized with 4-benzene diazonium sulfonic acid and 4

embedded in Nafion® matrix to form Nafion-FF composite membranes for its use as electrolyte in DMFCs. DMFC performance in terms of power output is determined and the results are compared with recast Nafion®. 2. Experimental 2.1. Fullerene functionalization Functionalization of fullerene (Buckminster fullerene C60, Acros Organics) was carried out similar to our earlier reports [32]. Since direct functionalization of fullerene is difficult because of its hydrophobic nature and also the absence of reactive groups on the surface, fullerene was initially treated with the mixture of sulfuric acid and nitric acid (1:1 vol. ratio) to remove the impurities and to activate its surface. Further, the oxidized fullerene was treated with 4-benzene diazonium sulfonic acid which is formed by the diazotization reaction of sulfanilic acid (Acros Organics) [32]. In brief, required amount of sulfanilic acid is dispersed in 1 M HCl and 10% sodium nitrite (Acros Organics) solution is added drop-wise to the dispersion under constant stirring at 0-5 oC for 30 min and the white precipitate formed is filtered and dried. The obtained precursor 4-benzene diazonium sulfonic acid is dissolved in water-ethanol (2:3 vol. ratio) mixture and the above oxidized fullerene is added to the solution and the reaction was carried out in an ice bath with the temperature maintained at 0-5 °C and finally required amount of hypophosphorous acid (H3PO2) (Acros Organics) is added to the solution with continuous stirring for 1 h. After 1 h, similar amount of H3PO2 is added again and the reaction is further continued for another hour. The product was filtered and dried to obtain functionalized fullerene (FF). The reaction time and ratio of 4-benzene sulfonic acid to fullerene was optimized during the reaction to obtain the steady state for the sulfonation degree.

5

2.2. Membrane fabrication 5 wt. % 30 ml Nafion solution (Nafion dissolved in alcohol and water) (DuPont, USA) was heated at 60 oC to evaporate solvents and the obtained Nafion® resin was again dissolved in 30 ml of DMAc (Acros Organics). The required amount of FF (0.5-1.5 wt.%) is dispersed in DMAc through ultra-sonication and added to Nafion® solution and further sonicated for 1 h. The resultant brown dispersion (Nafion®-FF) was kept under stirring for 6 h and the mixture was cast on a flat Plexi-glas plate and dried in vacuum at 70 °C for 12 h. The membranes were peeled off from the glass plate and then treated with 0.5 M H2SO4 for the activation of proton conducting groups. Recast Nafion membrane is also prepared under similar condition for the comparison of results. Thickness of all the membranes was maintained at 170 µm. Fig. S1 (Supporting Information) shows the photographs of recast Nafion® and Nafion®-FF composite membranes. It is inferred that as the content of FF increases in the Nafion® matrix, colour change in the membranes are observed from transparent to brown. 2.3. Characterization of functionalized fullerene (FF) Elemental analysis (CHNS) for the samples were carried out on Elementarvario EL 111-Germany. X-ray photoelectron spectroscopy (XPS) analysis on the samples was performed using Thermo Scientific MULTILAB 2000 Base system with X-Ray, Auger and ISS attachments. The energy dispersive X-ray dpectroscopy (EDS) analysis on the samples was done using field rmission scanning electron microscopy (FE-SEM) instrument (Zeiss ultra FE-SEM instruments, Germany) with a separate EDS detector (INCA) connected to the instrument. Pristine fullerene and FF were analysed by Raman spectroscopy (RFS27, Bruker) employing a Nd:YAG laser wavelength of 1064 nm. TEM images of fullerene and

6

FF along with SAED pattern were recorded on a 200 kV Tecnai-20 G2 transmission electron microscope (TEM) at 50 nm scale to study the morphology of the samples. 2.4. Physico-chemical characterization for the membranes Morphologies for recast Nafion® and Nafion®-FF (1 wt. %) were analysed by FESEM instrument (Zeissultra FE-SEM instruments, Germany). The samples for FE-SEM analysis were prepared by dipping the membranes in liquid nitrogen and then precisely cut to the required size. Atomic force microscopy (AFM) analysis was done for recast Nafion® and Nafion®-FF (1 wt.%) membranes by tapping mode (AFM, PicoSPM-Picoscan 2100, Molecular Imaging, USA). Mechanical properties of the aforesaid membranes were measured on universal testing machine (UTM) (Model AGS-J, Shimadzu, Japan) with an operating head-load of 10 kN. Thermo-gravimetric analysis (TGA) of Nafion® and Nafion®-FF (1 wt.%) membranes was determined using NETZSCH STA 449F3 TGA-DSC instrument in the temperature range between 30 °C and 1000 °C at a heating rate of 5 °C min-1 with nitrogen flushed at 60 ml min-1. X-ray diffraction (XRD) analysis for recast Nafion® and Nafion®-FF (1 wt.%) composite membranes was carried out using Bruker D8 advanced X-ray diffractometer using Cu Kα radiation of wavelength 1.54 Å. Ion exchange capacity (IEC) for the membranes was measured by acid-base titration method [32]. Water and methanol-water mixture sorption for the membranes was determined by equilibrating the pre-weighed membrane samples for 24 h in deionised water/methanolwater in a sorption chamber at 30 oC [32]. Proton conductivity for the membranes was measured using four probe DC method under fully humidified condition (100%) as per our earlier literature [17]. The details of experimental conditions and equations for membrane characterization such as tensile strength, IEC, sorption and proton conductivity are given in the supporting information (Experimental Section).

7

2.6. Membrane electrode assembly (MEA) preparation Membrane electrode assemblies (MEAs) were prepared by sandwiching the membranes between anode and cathode followed by hot press (Flow Mech) at 130 °C with a compaction pressure of 20 kg cm-2. Electrodes with anode comprising Pt-Ru/C (60 wt.% in 1:1 atomic ratio, Alfa Aesar, Johnson Matthey) and cathode comprising Pt/C (40 wt.% Pt on carbon support, Alfa Aesar, Johnson Matthey) with 2 mg cm-2 loading on either side were prepared following the procedure similar to our earlier reports [33]. 2.5. Methanol permeability measurements Methanol permeability for the membranes was measured ex situ at 30 °C and in situ at 60 °C using two compartment diffusion cell and cell mode at open circuit voltage (OCV), respectively. In a diffusion cell, one compartment is filled with 2 M methanol and other with water separated by a membrane. Concentration gradient is developed with respect to time and methanol permeates through the membrane from one compartment to the other and the concentration of the methanol was measured by gas chromatography (GC) equipped with flame ionization detector (FID) and capillary column. Methanol permeability is measured using the following equation [32]. P=

k2 × V2 × L (C1 − C2 ) × A

(1)

where P is methanol permeability, k2 is the slope of concentration profile considered from water compartment, C1, C2 are the concentration of methanol in methanol and water compartments respectively. L, A are the thickness and area of the membrane, respectively. Further the optimized Nafion®-FF (1 wt.%) composite membrane along with recast Nafion® is tested for methanol permeability at different concentrations of methanol (3 and 5 M) using the same protocols as discussed above.

8

In cell mode operation at 60 oC, 2 M methanol is passed at the anode and cathode was fed with oxygen at the rate of 300 mL min-1. The difference in volume of inlet and outlet at the anode is noted and their respective concentration is measured by GC. Methanol permeability through the membrane electrode assembly (MEA) is measured using following equation [32]:

P=

Cout × T × Vout

(t − t0 ) A× Cin

(2)

where P is methanol permeability, t-t0 time duration of methanol supply, Cin ,Cout are the concentration of methanol at inlet and outlet respectively, Vout is the volume of outlet methanol, T, A are the thickness and area of the membrane, respectively. Single cell compromising MEAs of recast Nafion, Nafion-117 and Nafion-FF composite membranes are subjected to linear sweep voltammetry (LSV) studies [34, 35], wherein anode was fed with 2 M methanol and cathode was fed with nitrogen (200 mL min-1). In this mode, anode act as reference while cathode act as working electrode, wherein methanol crossover from anode to cathode and gets oxidised at the cathode. The methanol oxidation current is directly proportional to the quantity of methanol crossover from anode to cathode. The experiments were conducted from 0-0.9 V with a scan rate of 50 mV s-1 using potentiostat (Biologic Science Instruments model: VMP3B-20). Electromchemical impedance spectra (EIS) was recorded for the MEAs comprising recast Nafion, Nafion-117 and Nafion-FF composite membranes assembled in a single cell wherein anode was fed with 2 M methanol and cathode was fed with oxygen (300 mL min-1) . EIS was recorded in potentiostat mode at 0.3 V in the frequency range of 10 MHz to 10mHz using Biologic Science Instruments (model: VMP3B-20).

9

Electrochemical selectivity is the key factor to determine the performance of PEM in DMFC and is defined as the ratio between proton conductivity and methanol crossover of the membranes calculated using the following equation: Electrochemical selectivity (S s  ) =

  ! " (# $ %& ) '()*+ ,($(*- + " ($ . / %& )

(3)

2.7 DMFC performance evaluation and stability The prepared MEAs were evaluated in conventional fuel cell fixture with serpentine flow field machined on graphite plates (Fuel Cell Technology, US) with an active area of 4 cm2. The dimensions of the experimental setup are represented in Table S1( Supporting information). The fuel methanol was passed at the anode with a flow rate of 2 mL min-1 and oxygen was passed at the cathode with a flow rate of 300 mL min-1. All the MEAs were tested at 60 °C using electronic load Model-LCN4-25-24/LCN 50-24 from Bitrode Instruments (US). Cell polarization studies were perfomed in Galvanostatic mode wherein constant current was drawn from the cell and the corresponding voltage value is noted in relation to individual current in each step. Further power density values were calculated by considering the cell voltage, current and active area of the MEA. The stability test for the MEAs comprising recast Nafion® and Nafion®-FF (1 wt.%) is carried out in DMFC under similar flow rate (Anode: MeOH 2 mL min-1; Cathode: oxygen mL min-1) by measuring the OCV in relation to time for 50 h. The cell polarization for the Nafion®-FF (1 wt.%) was also carried out at 3 and 5 M methanol concentration and the performance was compared with recast Nafion® under similar conditions. The optimized Nafion®-FF (1 wt.%) membrane was also tested for its stability under constant voltage mode by keeping cell at 0.3 V and with current change as function of time.

10

3. Results and discussion 3.1. Characterization of FF Raman spectra for pristine fullerene and functionalized fullerene (FF) are presented in Fig. 1. The most significant peak of fullerene appears at 1464 cm-1 referred as pentagonal pinch mode of fullerene and this mode is active for intermolecular bonding [36,37]. Fullerene is highly symmetrical molecule and surface functionalization normally involves substitution across π-bonds introducing asymmetry in the molecule. In the present case, the pentagonal pinch mode appears at 1458 cm-1 for functionalized fullerene (FF), a downshift of 6 cm-1 as compared to pristine fullerene suggesting the interaction of fullerene with functional groups. Further the broadening of peaks in FF in comparison with fullerene is due to the asymmetry introduced by the surface functional groups. Fig. 2 shows the XPS analysis for FF. The binding energies in the range between 166 eV and 172 eV is considered for sulphur and the deconvolued spectra shows three distinct peaks at 168.4, 169.1 and 169.8 eV assigned to sulphur (S 2p) in S-C, S-O, S=O states. The deconvoluted carbon (C1s) spectra also shows 3 distinct peaks at 284.5, 285, 285.5 and 287 eV assigned to C-O, C-C, C=C and –COOH states, respectively. The deconvoluted spectra of oxygen (O 1s) show peaks at 530.8, 531.6, 532.3, 532.9 and 533.6, assigned to O=S, C=O, C-O, O-H, and C-OH, respectively [38]. For comparision, C1s spectra of pristine fullerene and oxidised fullerene and O1s spectra of oxidised fullerene are presented in supporting information (Fig. S2) wherein C1s spectra of pristine fullerene shows only C-C, C=C and some amount of oxygen species from impurities and oxidised fullerene shows C1s and O1s spectra due to the presence of hydroxyl and carboxylic acid groups. Surface functionalization of fullerene is evident from the above results. Table 1 shows CHNS analysis for pristine fullerene and FF. 2.79 % of sulphur is present in FF and also the increased hydrogen content confirms the presence of sulfonic acid

11

(-SO3H) groups. Degree of sulfonation calculated from CHNS analysis by considering the weight of the sample and percentage of sulphur content is found to be 14 %. Fig. S3 (supporting information) shows the EDS analysis for pristine fullerene and FF and the corresponding data is presented in Table 1. From the EDS analysis, it is evident that 0.7 at.% sulphur is present in FF and the oxygen content is increased in comparison with pristine fullerene confirming the presence of –SO3H group in fullerene. Fig. 3 shows the TEM analysis for pristine fullerene and FF. Crystalline structure (SAED pattern) of fullerene is not disturbed by functionalization. In addition, FF moiety expansion is seen in Fig. 3b, due to the electrostatic interaction between the functional groups present in FF [39]. This may also provide a difficult path for methanol to transport when FF is used as additive in Nafion® matrix for DMFCs. 3.2. Physico-chemical properties of Nafion®-FF composite membranes Tensile strength and elongation for Nafion®-FF composite membranes are measured and compared with recast Nafion® and Nafion-117 to understand the stability of the composite membranes. As shown in Table 2, tensile strength of Nafion®-FF increases with the increased content of FF due to the interfacial interaction between Nafion® and FF and the compatibility between additive and host matrix [22]. In contrast, percentage elongation for the composite membranes is reduced in comparison with recast Nafion® due to the modification of microstructure of Nafion® by FF inducing the rigidity. Fig. S4 (supporting information) shows the thermogravimetric analysis (TGA) for Nafion®-FF (1 wt. %) and recast Nafion®. Incorporation of FF does not affect the thermal stability of the membrane confirming the compatibility between the additive and Nafion® matrix. Further the desulfonation temperature (250-400 °C) [40] and backbone degradation temperature (500-600 °C) are slightly lowered in composite membranes (Nafion®-FF) in comparison to Nafion® due to the reduction in ionic cluster size of Nafion® [7].

12

Fig. 4 shows the surface morphologies for recast Nafion® and Nafion®-FF (1 wt.%) composite membranes and cross-sectional morphology for Nafion®-FF (1 wt.%). FF dispersion in Nafion® matrix results in surface roughness in comparison with the smooth morphology observed for Nafion® (Fig. 4a). The uniform distribution of FF throughout the Nafion® matrix is seen with minimal agglomeration (Fig. 4b). This also suggests the proper dispersion of the additive in DMAc solvent used for fabricating the membranes. In addition, cross-sectional SEM images reveal that FF is homogeneously embedded in the Nafion® matrix in addition to its presence on the surface (Fig.4c) and zoom in portion of the FF distribution is seen in Fig. 4d. Fig. 5 shows the AFM analysis for recast Nafion® (a and b) and Nafion®-FF (1 wt.%) (c and d) composite membranes. Distribution of FF is clearly seen through the Nafion® matrix which leads to the surface roughness. Brighter regions are assigned to hydrophilic domain and darker regions are assigned to hydrophobic domain, respectively. Increased brighter regions are seen in the composite membrane compared to pristine Nafion® due to the addition of fullerene which provides better connectivity of the hydrophilic domains thereby improving the conductivity due to the interaction between hydrophilic groups of Nafion® and FF [7]. Also, FF presence in hydrophobic domain of the composite may be beneficial in controlling the methanol transport in DMFCs because methanol follows the torturous path through these sites before it diffuses through the Nafion® membrane [7]. XRD pattern for FF, recast Nafion® and Nafion®-FF (1 wt.%) composite membranes are shown in Fig. S5 (supporting information). For FF, the diffraction peaks are observed at 2θ values of 10.7 ,17.9, 20.6, 21.6, 27.3 and 28.1 degree assingned to (002), (100), (200), (004), (114) and (201) planes respectively [41]. The average particle size of FF calculated from the Scherrer equation is 50 nm. The crystalline reflections of Nafion® are observed in both the membranes and peak intensity is directly proportional to the electron density

13

difference between the ionic cluster and the backbone [27]. The peak intensity is lowered for composite membrane of Nafion®-FF than Nafion®, wherein FF may act as a shield between backbone and ionic cluster providing better interaction between FF and ionic cluster for improved proton transport [27]. The new peak at 2θ=11 o for the composite membranes is due to the graphitic nature of fullerene [37]. Ion exchange capacity for Nafion®-FF, recast Nafion® and Nafon-117 is calculated and represented in Table 2. It is inferred that Nafion®-FF shows higher IEC than recast Nafion® suggesting the participation of ion-exchangeable groups from FF in addition to the sulfonic acid groups of Nafion® matrix in the ion exchange process. The IEC values obtained in the present study are comparable with theoretical IEC values reported in the literature [42]. Water sorption for Nafion®-FF recast Nafion® and Nafion-117 is measured at 30 °C and 60 °C. The composite membranes exhibit less water sorption than recast Nafion® membrane due to the reduction of free volume for water molecules in the membrane [27]. In addition, methanol sorption for the membranes at different methanol concentration is also measured. It is to be noted that composite membranes of Nafion®-FF show less affinity towards methanol due to the absence of free volume for methanol molecules in the membrane similar to the results observed by H. C.Chien et al. for Nafion®-GO composite membranes [38]. It is noteworthy that water sorption for the membranes at 60 oC is higher due to the better chain mobility and void formation in the membranes, wherein water transport is facile [32]. 3.3. Transport properties of Nafion ®-FF composite membranes Interconnected ionic clusters are responsible for the methanol and proton transport in the polymer matrix. Since methanol and protons transport through the Nafion® via same path, it is important to design a composite that allows only protons and restricts methanol and this

14

is possible by modification of microstructure of Nafion® with nanomaterials that selectively allows protons [22]. To understand the transport properties, proton conductivity and methanol permeability of Nafion®-FF are measured and compared with recast Nafion® and Nafion-117. Proton conductivity values for the membranes at 30 and 60 °C are shown in Table 3. Proton conductivity of the membranes is enhanced by the incorporation of FF wherein sulfonic acid groups from FF act as proton hoping site in addition to the proton conducting channels available with Nafion®. It is interesting to note that the proton conductivity is 32 % higher than recast Nafion® at 30 oC, while at 60 °C it is higher by 24 %. However beyond an optimum value of FF loading, i.e. at 1.5. wt.%, decrease in proton conductivity is observed which may be due to the aggregation of FF blocking the proton conducting channels in Nafion® [36]. Conductivity measured beyond optimum level (1.5 wt.% loading) of FF in Nafion®, shows 9 % decrement at 30 oC and 60 oC compared with 1 wt.% of FF. Methanol permeability of Nafion®-FF composites is evaluated and compared with recast Nafion® and Nafion-117 at 30o and 60 °C as shown in Table 3 and Fig. 6 (a). Embedded FF in Nafion® matrix may lead to ionic cluster shrinkage and barrier effect may be more prominent with significant restriction in permeability [27,37]. Incorporation of FF in Nafion® may also provide the tortuous path for methanol to permeate through the membrane. This effect is seen at both 30 and 60 °C. It is known that methanol permeability is more at elevated temperature and concentration due to the high energy and concentration gradient, respectively [43]. Hence in the present study, higher methanol permeability is observed at 60 °C than 30 °C for all the membranes. Further to understand the methanol barrier properties of the composite membrane, methanol permeability at different concentrations are studied with Nafion®-FF (1 wt.%) and the results are compared with recast Nafion® as shown in Table 3, while the corresponding concentration profile is shown in Fig. S6 (a, b) (supporting 15

information). It is to be noted that as the concentration of methanol increases, permeability gets increased drastically for Nafion® membrane. In contrast, for Nafion®-FF (1 wt.%) composite membrane, increase in methanol permeability is less pronounced even at the higher concentration suggesting that Nafion®-FF is an effective barrier for methanol and these results can also be correlated with water-methanol mixture sorption. In addition, methanol crossover is also validated by LSV as shown in Fig. 6 (b) wherein methanol crossover current density is measured by scanning the cathode potential from 0 to 1 V under nitrogen atmosphere . Since cathode was fed with nitrogen (inert gas), the current generated at the cathode is due to the electro-oxidation of methanol and is directly proportional to amount of methanol crossover from anode to cathode. From Fig. 6 (b), both recast Nafion and Nafion-117 shows high methanol crossover current density. On the other hand, crossover current density for Nafion-FF composites decrease with increased content of FF and Nafion-FF (1.5 wt%) shows the lowest crossover current density suggesting the methanol barrier properties of FF [34,35]. It is noteworthy that the obtained results are in good agreement with methanol permeability values represented in Table 3. EIS measurents for the MEA containing aforesaid membranes is shown in Fig. 6 (c) wherein high frequency intercept represents series resistance (Rs) in terms of membrane properties and the charge transfer resistance (Rct) at low frequency intercept is related to cathode activity [44]. It is infered that high frequency intercept values are high for recast Nafion and Nafion-117, whereas for composite membranes, the values are reduced [45]. Since all the other components are same, except membrane, change in series resistance may be due to the difference in proton conductivity of the membranes and these results are in good agreement with proton conductivity values reported in Table 3. On the other hand, low frequency intercept values are in the same order for the aforesaid membranes which relates to methanol crossover from anode to cathode. When methanol cross-over from anode to cathode

16

through the membrane, it gets oxidized at cathode (Pt/C), produces oxidation current thus reducing the cathode potential and poisioning Pt surface which evatually leads to poor ORR kinetics [44]. Hence for the membranes with reduced methanol crossover, charge transfer resistance values are lower and these results are in agreement with methanol permeability values reported in Table 3. 3.4. DMFC performance All the composite membranes along with recast Nafion® and Nafion-117 are subjected to DMFC polarization studies at 60 °C. Nafion®-FF shows higher OCV in comparison with recast Nafion® due to reduced methanol crossover. As the FF content increases in the Nafion® matrix, improved DMFC power output is observed attributing to interaction between the sulfonic acid groups of FF and Nafion® increasing the proton conductivity wherein the hydrogen bonding networks between FF and Nafion® forms facile path for proton conduction thereby minimizing the Ohmic resistance of the composite membranes. Further due to the high electronegativity of fullerene, electron delocalizes through pi system of fullerene. In fact, fullerene derivatives are superacids due to charge stabilization through electron delocalization, as a result, the sulfonic acid attached to the fullerene becomes more acidic and act as solid acid proton conductor improving the transport of protons. Further the barrier effect of embedded FF increases the tortuosity for methanol transport through the composite membranes leading to better electrochemical selectivity as shown in Fig.7 (a) . As seen in Fig. 7 (b), the optimized Nafion®-FF (1 wt.%) membrane show higher peak power density of 146 mW cm-2 at a current density of 550 mA cm-2 and is almost twice the value than that of recast Nafion. Lower power output for Nafion®-FF (1.5 wt.%) composite is due to the aggregation of FF in the Nafion® matrix as discussed in the above section. This data is comparable to other important Nafion®-carbon based composite membranes, wherein functionalized GO and CNTs are used as additives [24, 25, 27, 38]. It is

17

to be noted that Nafion-117 has shown higher DMFC performance than recast Nafion due to the higher proton conductivity and stability than the later. However, Nafion-FF composites have shown onpar performance with Nafion-117 due to the reduced methanol permeability and better electrochemical selectivity. Further the Nafion®-FF (1 wt.%) composite membrane and recast Nafion® is subjected to DMFC polarization at 3 and 5 M methanol concentration as shown in Fig. 8. It is to be noted that at 0.3 V recast Nafion shows current density of 243 mA cm-2 with 2 M methanol, for 3 M and 5 M methanol, it is 189 mA cm-2 and 131 mA cm-2 . Whereas Nafion-FF (1 wt.%) at 0.3 V shows current density of 475 mA cm-2 at 2 M methanol, for 3 M and 5 M it is 375 mA cm-2 and 312 mA cm-2 . The peak power density for Nafion®-FF is much higher than the pristine Nafion® membrane at 5 M methanol where methanol permeability is also higher compared to 3 M as shown in Table 3. The better DMFC performance at high methanol concentration confirms the barrier effect of FF. 3.5. Stability of the membranes Recast Nafion and Nafion-FF (1 wt.%) are subjected to stability under OCV condition for 50 h and compared with Nafion-117 as shown in Fig. 9 (a). Recast Nafion ® shows lower OCV due to the high methanol permeability through its ionic clusters eventually creating the mixed potential at cathode thereby reducing the cell efficiency, hence at the end of 50 h recast Nafion® shows much higher loss in OCV. On the other hand, Nafion®-FF shows higher OCV due to the microstructural modification of Nafion® with FF wherein FF offers tortures path to methanol thereby by reducing the permeability and hence even after 50 h of stability test, Nafion®-FF shows only marginal loss in OCV proving its long-term stability on par with Nafion-117. In addition to OCV stability, potenstiostatic stability for Nafion®-FF (1 wt. %) at 0.3 V is also performed for 50 h to estimate its stability under dynamic operation as shown in Fig.9 (b). At 0.3 V, Nafion®-FF delivers 500 mA cm-2 current 18

density and even at end of 50 h, only 75 mA cm-2 drop is observed suggesting the dynamic stability of the composite. 4. Conclusions Microstructural modification of Nafion® is successfully achieved by the incorporation of functionalized fullerene. Functionalization of fullerene improves the compatibility with Nafion® forming uniformly distributed Nafion®-FF composite membranes. Tuning of ionic domains with FF greatly improved the properties of Nafion® in terms of sorption, proton conductivity and methanol permeability. The composite membranes show better electrochemical selectivity due to selective transport of protons rather than methanol. DMFC power output for Nafion®-FF composite membranes is demonstrated using 2, 3 and 5 M methanol. In all the cases, the composite membranes show better power output than the pristine Nafion® attributing to its better methanol impermeable characteristics. The composite membranes also show better stability both under OCV and potentiostatic conditions. Finally, this study provides insights to design advanced polymer electrolytes for electrochemical devices that would deliver improved performance. Future scope of the present study will be in air/methanol condition to understand the behaviour of composite membranes in passive DMFCs where the methanol permeability is a critical issue. Acknowledgements Authors thank CSIR for the financial support under HYDEN programme (CSC-0122) in 12th Five Year Plan for the funding. Authors thank Scientist-In-Charge, CECRI Madras Unit and Director, CSIR-CECRI, Karaikudi for the support.

19

References [1] G.W. Crabtree, M.S. Dresselhaus, The hydrogen fuel alternative, MRS Bull. 33 (2008) 421-428. [2] F. Bella, F. Colò, J.R. Nair, C. Gerbaldi, Photopolymer electrolytes for sustainable, upscalable,

safe,

and

ambient-temperature

sodium-ion

secondary batteries,

ChemSusChem 8 (2015) 3668-3676. [3] N. Hui, F. Chai, P.Lin, Z. Song, X. Sun, Y. Li, S. Niu, X. Luo, Electrodeposited Conducting Polyaniline Nanowire Arrays Aligned on Carbon Nanotubes Network for High Performance Supercapacitors and Sensors, Electrochimi. Acta 199 (2016) 234241. [4] J.R. Nair, F. Bella, N. Angulakshmi, A. M. Stephan, C. Gerbaldi, Nanocellulose-laden composite polymer electrolytes for high performing lithium–sulphur batteries, Energy Storage Materials 3 (2016) 69-76. [5] F. Bella,

N.Vlachopoulos,

K. Nonomura, S.Zakeeruddin, M. Grätzel, Claudio

Gerbaldi, A.Hagfeldt, Direct light-induced polymerization of cobalt-based redox shuttles: an ultrafast way towards stable dye-sensitized solar cells, Chemical Communications 51 (2015) 16308-16311. [6] R. Devanathan, Recent developments in proton exchange membranes for fuel cells, Energy Environ. Sci. 1 (2008) 101-119. [7] K. Feng, B. Tang, P. Wu, Sulfonated graphene oxide-silica for highly selective Nafion based proton exchange membranes, J. Mater. Chem. A 2 (2014) 16083-16092. [8] C.W. Lin, Y.S. Lu, Highly ordered graphene oxide paper laminated with a Nafion membrane for direct methanol fuel cells, J. Power Sources 237 (2013) 187-194.

20

[9] S.M. Priya, S.D. Bhat, A.K. Sahu, S. Pitchumani, P. Sridhar, A.K. Shukla, new mixed-matrix membrane for DMFCs, Energy Environ. Sci. 2 (2009) 1210-1216. [10] L. Zhang, S.R. Chae, Z. Hendren, J.S. Park, M.R. Wiesner, Recent advances in proton exchange membranes for fuel cell applications, Chem. Eng. Journal 204-206 (2012) 87-97. [11] Y. He, C. Tong , L. Geng, L. Liu, C. Lü, Enhanced performance of the polyimide proton exchange membranes by graphene oxide: Size effect of graphene oxide, J. Membr. Sci. 458 (2014) 36-46. [12] D.K. Paul, K. Karan, A. Docoslis, J. B. Giorgi, J. Pearce, Characteristics of SelfAssembled Ultrathin Nafion Films, Macromolecules 46 (2013) 3461-3475. [13] Y.L. Liu, Y.H. Su, C.M. Chang, Suryani, D.M. Wang, J.Y. Lai, Preparation and applications of Nafion-functionalized multi walled carbon nanotubes for proton exchange membrane fuel cells, J. Mater. Chem. 20 (2010) 4409-4416. [14] D. B. Spry, A. Goun, K. Glusac, D.E. Moilanen, M. D. Fayer, Proton transport and the water environment in Nafion fuel cell membranes and AOT reverse micelle, J. Am. Chem. Soc. 129 (2007) 8122-8130. [15] R. Jiang, H. R. Kunz, J.M. Fenton, Composite silica/Nafion® membranes prepared by tetraethylorthosilicate sol-gel reaction and solution casting for direct methanol fuel cells, J. Membr. Sci. 272 (2006) 116-124. [16] V. Baglio, A.S. Aricò, Di Blasi, V. Antonucci, P.L. Antonucci S. Licoccia, E. Traversa, F. Serraino

Fiory, Nafion-TiO2 composite DMFC membranes: physico-

chemical properties of the filler versus electrochemical performance, Electrochim. Acta 50 (2005) 1241-1246.

21

[17] S. Meenakshi, A.K. Sahu, S.D. Bhat, P. Sridhar, S. Pitchumani, A.K. Shukla, Mesostructured-aluminosilicate-Nafion hybrid membranes for direct methanol fuel cells, Electrochim. Acta 89 (2013) 35-44. [18] J.Zheng, Q.He, C. Liu, T. Yuan, S. Zhang, h. yang, Nafion-microporous organic polymer networks composite membranes, J. Membr. Sci. 476 (2015) 571-579. [19] K. Dutta, S. Das, P.P. Kundu, Partially sulfonated polyaniline induced high ionexchange capacity and selectivity of Nafion membrane for application in direct methanol fuel cells, J. Membr. Sci. 473 (2015) 94-101. [20] S. Mondal, S. Soam, P. P. Kundu, Reduction of methanol crossover and improved electrical efficiency in direct methanol fuel cell by the formation of a thin layer on Nafion 117 membrane: Effect of dip-coating of a blend of sulphonated PVdF-co-HFP and PBI, J. Membr. Sci. 474 (2015) 140-147. [21] E. Gerasimova, E. Safronova, A. Ukshe, Y. Dobrovolsky, A. Yaroslavtsev, Electrocatalytic and transport properties of hybrid Nafion membranes doped with silica and cesium acid salt of phosphotungstic acid in hydrogen fuel cells, Chem. Eng. Journal (2015) in press. [22] B.G. Choi, Y.S. Huh, Y. C. Park, D.H. Jung, W. H. Hong, H. Park, Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets, Carbon 50 (2012) 5395-5402. [23] N. Miyake, J. S. Wainright, R. F. Savinell, Evaluation of a Sol-Gel derived Nafion/Silica bybrid membrane for polymer electrolyte membrane fuel cell applications: II. methanol uptake and methanol permeability, J. Electrochem. Soc. 148 (2001) A905-A909.

22

[24] M.S.

Asgari, M.

Nikazar, P.

Molla-abbasi,

M.M.

Hasani-Sadrabadi,

Nafion®/histidine functionalized carbon nanotube: High-performance fuel cell membranes, Int. J. Hydrogen Energy 38 (2013) 5894-5902. [25] J.M. Thomassin, J. Kollar, G. Caldarella, A. Germain, R. Jérôme, C. Detrembleur, Beneficial effect of carbon nanotubes on the performances of Nafion membranes in fuel cell applications, J. Membr. Sci. 303 (2007) 252-257. [26] A. K. Geim, K. S. Novoselov, The rise of graphene, Nature Materials 6 (2007) 183191. [27] B.G. Choi, J. Hong, Y.C. Park, D. H. Jung, W. H. Hong, P.T. Hammond, H. Park, Innovative polymer nanocomposite electrolytes: Nanoscale manipulation of ion channels by functionalized graphene, ACS Nano 5 (2011) 5167-5174. [28] I. Nicotera, C. Simari, L. Coppola, P. Zygouri, D. Gournis, S. Brutti, F. D. Minuto, A. S. Aricò, D. Sebastian, V. Baglio, Sulfonated Graphene Oxide Platelets in Nafion Nanocomposite Membrane: Advantages for Application in Direct Methanol Fuel Cells, J. Phys. Chem. C, 118 (2014) 24357-24368. [29] K. Tasaki , J. Gasa, H. Wang, R. DeSousa, Fabrication and characterization of fullerene-Nafion composite membranes, Polymer 48 (2007) 4438-4448. [30] S. Saga, H. Matsumoto, K. Saito, M. Minagawa, A. Tanioka, Polyelectrolyte membranes based on hydrocarbon polymer containing fullerene, J. Power Sources 176 (2008) 16-22. [31] H. Wang, R. DeSousa, J. Gasa, K. Tasaki, G. Stucky, B. Jousselme, F. Wudl, Fabrication of new fullerene composite membranes and their application in proton exchange membrane fuel cells J. Membr. Sci. 289 (2007) 277-283. [32] G. Rambabu, S.D. Bhat, Sulfonated fullerene in SPEEK matrix and its impact on the membrane electrolyte properties in direct methanol fuel cells, Electrochim. Acta 176 (2015) 657-669. 23

[33] G. Rambabu, S.D. Bhat, Simultaneous tuning of methanol crossover and ionic conductivity of sPEEK membrane electrolyte by incorporation of PSSA functionalized MWCNTs: A comparative study in DMFCs, Chem. Eng. Journal 243 (2014) 517-525. [34] W. Li, A. Manthiram, M. D. Guiver, B. Liu, High performance direct methanol fuel cells

based

on

acid-base

blend

membranes

containing

benzotriazole,

Electrochem.Comm. 12 (2010) 602-610. [35] Y. Zhu,

S. Zieren, A. Manthiram, Novel crosslinked membranes based on

sulfonated poly(ether ether ketone) for direct methanol fuel cells, Chem. Commun. 47 (2011) 7410-7412. [36] M. Sathish, K. Miyazawa, Synthesis and characterization of fullerene nanowhiskers by liquid-liquid interfacial precipitation: Influence of C60 solubility, Molecules 17 (2012) 3858-3865. [37] B.Y.G. Guo, C.J. Li, L.j. Wan, D.M. Chen, C.R. Wang, C.L. Bai, Y.G. Wang, Well defined fullerene nanowire arrays, Adv. Funct.Mater. 13 (2003) 626-630. [38] H.C. Chien, L.D. Tsai,C. P. Huang, C.Y. Kang, J. N. Lin, F.C. Chang, Sulfonated graphene oxide/Nafion composite membranes for high-performance direct methanol fuel cells, Int. J. Hydrogen Energy 38 (2013) 13792-13781. [39] Y. Kanbur, Z. Küçükyavuz, Synthesis and characterization of surface modified fullerene, Nanotubes and Carbon Nanostructures 20 (2012) 119-126. [40] L. Li, Y. Zhang, J.F. Drillet, R. Dittmeyer, K.M. Juttner, Preparation and characterization of Pt direct deposition on polypyrrole modified Nafion composite membranes for direct methanol fuel cell applications, Chem. Eng. Journal 133 (2007) 113-119.

24

[41] B.M. Ginzburg, S. Tuœchiev, S.K. Tabarov, A.A. Shepelevskiœ, L.A. Shibaev, X-ray Diffraction Analysis of C60 fullerene powder and fullerene soot, Technical Physics 50 (2005) 1458-1461. [42] C. H. Park, H.K. Kim, C.H. Lee, H.B. Park, Y.M. Lee, Nafion® nanocomposite membranes: Effect of fluorosurfactants on hydrophobic silica nanoparticle dispersion and direct methanol fuel cell performance, J. Power Sources 194 (2009) 646-654. [43] J. Auimviriyavat, S. Changkhamchom, A. Sirivat, Development of poly(ether ether ketone) (PEEK) with inorganic filler for direct methanol fuel cells (DMFCc), Ind. Eng. Chem. Res. 50 (2011) 12527-12533. [44] S. Siracusano, A. Stassi, V. Baglio, A.S. Aricò, F. Capitanio, A.C. Tavares, Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells, Electrochim. Acta 54 (2009) 4844-4850. [45] A.S. Aricò, D. Sebastian, M. Schuster, B. Bauer, C. D’Urso, F. Lufrano, V. Baglio, Selectivity of Direct Methanol Fuel Cell Membranes, Membranes 5 (2015) 793-809.

25

Figure captions Fig. 1. Raman spectra for fullerene before and after functionalization. Fig. 2. XPS analysis of FF. Deconvoluted spectra for (a) C 1s, (b) O 1s and (c) S 2p states. Fig. 3. TEM morphology of (a) pristine fullerene and (b) FF. Fig. 4. FE-SEM images representing surface morphology for (a) recast Nafion® (b) Nafion®FF, (c and d) cross-sectional morphology of Nafion®-FF (1wt.%). Fig. 5. AFM images for (a and b) recast Nafion® and (c and d) Nafion®-FF (1wt.%). Fig. 6. (a) Concentration profile representing methanol concentration as function of time in water compartment (B) of the diffusion cell, (b) Linear sweep voltammetry studies for the MEAs containing recast Nafion, Nafion-117 and Nafion-FF composite membranes, (c) EIS for DMFC containing recast Nafion Nafion-117 and Nafion-FF composite membranes Fig. 7. (a) Electrochemical selectivity measured from proton conductivity and methanol permeability of corrosponding membranes, (b) DMFC polarization plots for recast Nafion®, Nafion®-FF composite and Nafion-117 membranes at 60 oC (Anode: Pt-Ru/C, 2 mg cm-2 and Cathode: Pt/C, 2 mg cm-2. Anode fuel: 2 M Methanol 2 mL min-1, Cathode : Oxygen 300 mL min-1). Fig. 8. DMFC polarization for (a) recast Nafion® and (b) Nafion®-FF (1 wt.%) at different methanol concentration at 60 oC (Anode: Pt-Ru/C, 2 mg cm-2 and Cathode: Pt/C, 2 mg cm-2. Anode fuel: 2 M Methanol 2 mL min-1, Cathode : Oxygen 300 mL min-1). Fig.9. (a) Stability data representing open circuit voltage (OCV) as function of time at 60 oC, (b) Potentiostatic stability for Nafion®-FF (1 wt.%) composite at 0.3 V for 50 h at 26

60 oC (Anode Pt-Ru/C, 2 mg cm-2 and Cathode: Pt/C, 2 mg cm-2. Anode fuel: 2M Methanol 2 mL min-1, Cathode : Oxygen 300 mL min-1).

27

Intensity (a.u)

Pristine Fullerene

FF

1000

1200

1400

1600

Wavenumber (cm-1) Fig. 1

29

1800

2000

O1s

(a)

C1s

294

292

Intensity (CPS)

OH C=O C-OH C-O S=O

290

288 286 284 Binding Energy (eV)

S2p

282

280

540

538

536

534

532

530

528

Binding Energy (eV)

(c)

(a)

(b)

S-C S-OH S=O

Intensity (CPS)

Intensity (CPS)

C-O C-C C=O C=C

(b)

176 175 174 173 172 171 170 169 168 167 166 165 164 Binding Energy (eV)

Fig. 2

(b) 30

526

(a)

(b) (b)

(a)

Fig. 3

31

(a)

(b)

(c)

(d)

Fig. 4

32

(a)

(b)

(c)

(d)

Fig. 5

33

-1

Methanol concentration (mol L )

0.7

(a)

Recast Nafion Nafion-117 Nafion-FF (0.5 wt.%) Nafion-FF (1 wt.%) Nafion-FF (1.5 wt.%)

0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1

-2 Methanol crossover current density (mA cm )

0

10000 15000 Time (s)

20000

25000

180

140 120 100

30000

(b)

Recast Nafion Nafion-117 Nafion-FF (0.5 wt.%) Nafion-FF (1 wt.%) Nafion-FF (1.5 wt.%)

160

80 60 40 20 0 -20 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Applied potential (V)

0.7



Nafion -117  Recast Nafion  Nafion -FF (0.5 wt.%)  Nafion -FF (1 wt.%)  Nafion -FF (1.5 wt.%)

0.6 0.5 2 Z'' (ohm cm )

5000

(c)

0.4 0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2 2 Z' (ohm cm )

Fig. 6 34

1.4

1.6

1.8

2.0

Elec12 troc hem 10 ical selec tivit 8 y (10- 6 4Ss cm- 4 3 )

(a)

2 0 Recast Nafion0.5 wt.% FF 1 wt.% FF 1.5 wt.% FF Nafion-117 ®

FF (0.5 wt.%) Nafion -117 ® (0.5 wt.%)FF (0.5 wt.%) Recast NafionFF

Recast Nafion Nafion-117 Nafion-FF 0.5 wt.% Nafion-FF 1.5 wt.% Nafion-FF 1 wt.%

Cell voltage (V)

0.7

160 140 120

0.6

100

0.5

80

0.4

60

0.3

40

0.2 20 0.1 0

100

200

300

400

500

-2 Current density (mA cm ) Fig. 7 35

600

700

0 800

-2

0.8

(b)

Power density (mW cm )

0.9

2M Methanol 3M Methanol 5M Methanol

Cell voltage (V)

(a)

70

0.6

60

0.5

50

0.4

40

0.3

30

0.2

20

0.1

10

0.0

0 0

50

100

150

200

250

300

-2

0.7

80

Power density (mW cm )

0.8

350

-2

Current density (mW cm )

160

0.8 0.7

(b)

120

0.6 Cell voltage (V)

140

100

0.5 80 0.4 60

0.3

40

0.2

20

0.1 0.0

0 0

100

200

300

400

500

600 Current density (mA cm-2)

Fig. 8

36

700

800

-2

5 M Methanol 3 M Methanol 2 M Methanol

Power density (mW cm )

0.9

0.85

(a)

Recast Nafion Nafion-117 Nafion-FF (1 wt.%)

0.80

Cell voltage (V)

0.75 0.70 0.65 0.60 0.55 0.50 0

10

20

30

40

50

Time (h) 600

(b)



Nafion -FF (1 wt. %) -2

Current density (mA cm )

500 400 300 200 100 0 0

10

20

30

40

Time (h)

Fig. 9

37

50

60

Table 1 Elemental analysis of fullerene and functionalized fullerene. Sample Name

EDS analysis (At. %)

CHNS analysis (%) C

H

N

S

C

S

O

Pristine Fullerene

96.9

0.096

0.035

0.059

97.98

-

1.96

Functionalized Fullerene

92.49

2.650

0.042

2.791

95.98

0.73

3.08

Table 2 Properties of the membranes.

Tensile strength (MPa)*

Elongation (%)

Sorption (%)

Membrane type

IEC (meq g-1)

Recast Nafion®

0.86± 0.03

18.1±0.1

32.0±0.5

0.98± 0.02

19.3±0.2

30.8±0.2

17.1±0.2

Nafion -FF (1 wt. %)

1.09± 0.03

20.6±0.1

29.6±0.3

15.4±0.1

Nafion®-FF (1.5 wt. %)

1.15± 0.04

22.3±0.3

26.8±0.2

13.9±0.1

Nafion®-117

0.96± 0.02

25±0.1

43±0.4

19.4±0.1

Water At 30 °C At 60 °C 25.1±0.2 22.2±0.1

®

Nafion -FF (0.5 wt.%) ®

* Measured under sorbed condition.

38

19.30.1

17.6±0.1

16.3±0.2

22.±0.1

2M MeOH

3M MeOH

5M MeOH

39.0± 0.3

43.0± 0.2

53.4±0.3

33.6± 0.2

36.4± 0.3

45.0±0.2

29.2± 0.2

32.9± 0.2

38.3±0.2

23.5± 0.3

27.05± 0.3

30.5±0.2

35± 0.2

39± 0.3

48± 0.3

Table 3 Proton conductivity and methanol permeability for the membranes. Membrane

Methanol permeability

Proton conductivity (mS cm-1)

(10-7 cm2 s-1) 2M MeOH

30 °C

60 °C

30 °C

60 °C

Recast Nafion®

54.6±1

78.3±1

10.9

14.3

Nafion®-FF (0.5 wt.%)

61.2±2

87.1±1

8.7

11.2

Nafion®-FF (1 wt.%)

72.4±2

97.4±1

7.6

8.5

Nafion®-FF (1.5 wt.%)

64.8±1

89.2±2

5.1

7.4

Nafion-117

65.3±1

94.5±2

9.74

11.56

39

3M MeOH 5M MeOH 30 °C

30 °C

13.0

19.9

-

-

10.1

12.3

-

-

-

-

Graphical abstract

Nafion backbone

Nafion backbone

Schematic representation of Nafion-FF composite membrane, wherein FF acts as methanol barrier and helps in transport of protons through the sulfonic acid groups attached to the surface of FF.

40

Research highlights

•

Nafion-Functionalized fullerene (FF) composite membrane is prepared.

•

FF improves proton conductivity of composite membrane.

•

FF mitigates methanol permeability of composite membrane.

•

Nafion-FF show stable DMFC power output at different methanol concentration.

•

Nafion-FF show better stability than recast Nafion membrane.

41