Graphitic Carbon Nitride Nanosheets - Nafion as Methanol Barrier

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C: Energy Conversion and Storage; Energy and Charge Transport

Graphitic Carbon Nitride Nanosheets - Nafion as Methanol Barrier Hybrid Membrane for Direct Methanol Fuel Cell Parthiban Velayutham, and Akhila Kumar Sahu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06042 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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The Journal of Physical Chemistry

Graphitic Carbon Nitride Nanosheets - Nafion as Methanol Barrier Hybrid Membrane for Direct Methanol Fuel Cell Parthiban Velayutham†,‡, A.K. Sahu†,‡ ,* †

CSIR - Central Electrochemical Research Institute - Madras Unit, CSIR Madras Complex, Taramani, Chennai 600113, India. ‡

Academy of Scientific and Innovative Research (AcSIR), CSIR - Central Electrochemical Research Institute, Karaikudi 630003, India. ABSTRACT: Sulfonated graphitic carbon nitride (s-GCN) is first time realized as effective

bi-functional filler in modifying the Nafion membrane for direct methanol fuel cell (DMFC) application. Carbon nitride with amino (-NH2) and imino (-NH) functional groups was successfully synthesized through one step calcination of melamine (C3H6N6) and thereafter surface functionalized to incorporate sulfonic acid (-SO3H) groups. The proton transfer in the Nafion-sGCN hybrid membrane is enhanced by the presence of sulfonic acid groups in the filler and also by acid-base interaction between -NH2 groups present on the GCN with -SO3H groups of hybrid membrane. Nafion-s-GCN hybrid membrane with optimized filler content exhibits proton conductivity of 183 mS cm-1 which is about 36 % higher in relation to the pristine Nafion membrane. In addition, porous nature of GCN nanosheets in the hybrid membrane enhances movement of hydronium ions while acting as a barrier for methanol molecule. As a result, the Nafion-s-GCN hybrid membranes displayed much lower methanol crossover current density than that of the pristine Nafion membrane. The DMFC comprising Nafion-sGCN (0.5 wt%) hybrid membrane delivers a peak power density of 125 mW cm-2 at 70 °C under ambient pressure while a peak power density of 65 mW cm-2 is realized for pristine Nafion membrane under similar operating conditions.

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INTRODUCTION

Direct methanol fuel cells (DMFCs) receiving pervasive attention as an alternative energy source for most of the portable and stationary electronics because of their system simplicity as it does not require humidifier and reformer.1,2 The DMFCs utilize methanol as fuel whose volumetric energy density nearly double of pure hydrogen leads to the high theoretical system efficiency compared to that of proton exchange membrane fuel cell fuelled with hydrogen.3-5 Besides the advantages, there are many key problem needs to be addressed to harvest useful energy from a DMFCs which includes electrode kinetics and methanol crossover. Methanol crossover from anode to cathode through polymer electrolyte membrane (PEM) results in undesirable mixed potential at cathode resulted in low fuel utilization and cathode catalyst poisoning affecting overall DMFCs performance.6-8 Though the extensive research have been focused to address methanol crossover issue by developing new PEM materials or modifying the existing PEM, the DMFCs performance is still inferior for commercial use. The PEM is an essential component of DMFC which needs to have high proton conductivity, mechanical and thermal stability, and essentially low methanol crossover.9,10 Among various types of PEM, perfluorinated Nafion membrane is widely employed PEM in fuel cell for commercial applications because of its high proton conductivity (~0.1 S cm-1) and exceptional stability.11 However, the application of Nafion membrane in DMFCs is limited because of its high methanol crossover as a consequence of electro-osmatic drag and concentration gradient between the anode and cathode compartment of the cell.12 One of the most effective way to mitigate methanol crossover through Nafion membrane is to employ inorganic materials such as graphene,13 carbon nanotubes,14 porous carbon,15 graphene oxide,16,17 and metal oxides18-20 as fillers to modify the Nafion membrane. This approach specifically blocks the methanol passage while increasing the thermal and mechanical 2 ACS Paragon Plus Environment

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stability. However, the incorporation of fillers without any surface functionalization leads to the poor dispersion in polymer matrix and greatly affects the proton conductivity of hybrid membranes. Therefore, the use of surface functionalized filler is an effective strategy to modify the Nafion membrane as it unravels the aforementioned downsides. Among the different fillers, the two dimensional (2D) nanosheets including graphene, graphene oxide, boron nitride and MoS2 is explored as potential fillers in Nafion membrane because of its excellent physical and chemical stability. Moreover, the layered structure of the 2D material could effectively reduce the methanol crossover in DMFC.21 Graphitic carbon nitride (GCN) is a most stable allotrope of carbon nitrides with a similar stacked 2D structure of Graphene. The GCN is attractive for various application, including catalysis, photocatalysis and CO2 capturing due to its unique properties such as high thermal and hydrothermal stability and presence of basic surface sites arises from surface functionalities such as –imino (-NH) and –amino (-NH2) groups.22-26 Mingyue G et al explored the potential application of GCN in modifying the Sulfonated poly (ether ether ketone) (SPEEK) membrane for fuel cell applications. The acid-base interaction between the amino groups of GCN and –SO3H groups of SPEEK resulted in improved proton conductivity while reducing the methanol crossover. However, the application of this membrane is not evaluated in DMFC.27 Fei W et al utilized the oxidized g-C3N4 (OCN) to modifiy the SPEEK membrane fabricated via a solution-casting method for vanadium redox flow battery (VRFB) applications. Niu R et al also explored the potential application of pristine g-C3N4/SPEEK hybrid membrane for VRFB applications. It was found that the resultant membranes showed a low vanadium ion permeability and high ion selectivity with improved VRFB performance.28, 29 However, the potential application of GCN in modifying the Nafion membrane and its use in DMFC is remained unexplored.



Herein, we report the application of surface functionalized GCN as effective filler to modify the Nafion membrane for DMFCs. The proton transfer in the Nafion-s-GCN hybrid membrane is enhanced by the presence of sulfonic acid groups in the filler as well as by acidbase interaction between -NH2 groups present on the GCN with -SO3H groups of hybrid membrane. The SAXS characterization also revealed that the addition of s-GCN filler to Nafion matrix significantly alter the microstructure by increasing the size of ionic clusters facilitating proton conduction in the hybrid. In addition, the periodic vacancies in the s-GCN with the geometric area lesser than that of methanol molecule selectively allow the proton with suppressed fuel crossover. This together, helped in achieving power density of 125 mW cm-2 for the DMFCs employed Nafion-s-GCN (0.5 wt %) hybrid membrane which is about 90% higher compared to power density of DMFC comprising pristine Nafion membrane under identical operating conditions.

EXPERIMENTAL

Materials. Melamine was procured from Acros Organics, India. Chlorosulfonic acid was procured from Spectrochem, India and 5 wt% Nafion ionomer was purchased from Du Pont, USA. The Pt/C (40 wt% Pt on Vulcan XC-72R carbon) and Pt-Ru/C (40 wt% Pt and 20 wt% Ru on Vulcan XC-72R carbon) catalyst was purchased from Alfa Aesar (Johnson Matthey) Ltd. The Gas diffusion layers (Sigracet DC-35 GDL) were obtained from SGL group, Germany. All the chemicals were used without further purification and Milli-Q water (resistivity is 18.2 MΩ cm) is used throughout the experiments. Preparation of graphitic carbon nitride (GCN) and functionalization. Graphitic carbon nitride (GCN) was obtained by a simple one step calcination of melamine at 550 °C for 4 h at a heating ramp of 5 °C min-1 under air atmosphere. The resultant yellow powder was grinded and processed for surface functionalization for potential filler materials while making hybrid membrane. Briefly, 1 g of as-prepared GCN was added to 50 mL of dichloromethane under 4 ACS Paragon Plus Environment

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stirring. To this admixture, a 0.5 mL of chlorosulfonic acid was added drop wise for about 10 minutes and continued the stirring for another 3h. The yellow GCN powder changed into white colour which was filtered off washed with water followed by methanol and dried under vacuum at 50 °C overnight.30 The resultant white powder is labelled as sulfonated GCN (sGCN) and employed as bi-functional filler to modify the Nafion membrane. Fabrication of Nafion- s-GCN hybrid membrane. Solution casting method is adopted to fabricate Nafion hybrid membranes. Particular amounts of s-GCN were ultrasonically dispersed into a specific volume of Nafion ionomer which was mechanically stirred further to attain a homogeneous mixture. The hybrid membrane was obtained by transferring the above admixture to a glass petri-dish and subsequent vacuum drying at 80 °C for 8 h. After cooling to ambient temperature, the membrane was peeled off from the dish by adding water. Prior to any characterizations, the resultant membrane was pre-treated in 0.5 M sulphuric acid at 80 °C for 1 h and then washed with water until the pH reaches neutral. The content of s-GCN was varied to optimize the filler content in the Nafion matrix in order to achieve maximum performance. Pristine Nafion membrane also was fabricated by the similar procedure without addition of s-GCN filler. Thickness of all the membrane was maintained around 170 µm. Physico-chemical characterizations. The phase purity of graphitic carbon nitride (GCN) and sulfonated GCN (s-GCN) was characterized by powder X-ray diffraction (XRD), Philips Pan Analytical X-ray diffractometer using Cu-Kα (λ = 1.541 Å) radiation. The morphology and corresponding quantitative elemental composition of GCN and s-GCN was examined in a Field Emission Scanning Electron Microscope (FE-SEM, MIRA3, TESCAN) equipped with an energy dispersive X-ray (EDX) spectrometer. Nitrogen adsorption and desorption isotherms were evaluated by N2 physisorption at 77 K using Autosorb iQ-MP, Quantachrome. Total surface area and pore size distribution (PSD) were determined using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The presence 5 ACS Paragon Plus Environment


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of functional groups were characterized by Fourier transform infrared (FTIR) spectra recorded in the frequency range 400-4000 cm-1 by Bruker TENSOR 27 infrared spectrophotometer using KBr pellets sample matrix. Information on elemental composition and their electronic states was acquired from X-ray photoelectron spectroscopy (XPS) surface analysis using a MULTILAB 2000 XPS system. Small-angle X-ray scattering (SAXS) profiles were recorded to understand the microstructure of the membranes on a Rigaku smart lab guidance diffractometer with a Cu-Kα (λ = 1.541 Å) radiation operating at 45 kV, 200 mA. Thermal stability of pristine Nafion and Nafion-s-GCN membranes was evaluated using NETZSCH STA 449TG-DSC instrument in the temperature range between 30 and 900 °C at a heating rate of 5 °C min-1 under N2 atmosphere. Proton conductivity. Electrochemical impedance spectroscopy (EIS) method was employed to evaluate in-plane proton conductivity at different temperatures using four-probe conductivity cell (Bekktech, BT-112). The membrane samples with area of 4 cm2 placed in contact with four platinum electrodes. The temperature and humidity was maintained by maintaining the water vapor pressure using humidity chamber (Espec, SH-242). The EIS spectra were recorded in the frequency range between 1 MHz and 1 Hz with amplitude of 10 mV using potentiostat (Biologic, SP-150). The resistance value corresponding to the xintercept of the real axis at high frequency region (HFR) is used to calculate the proton conductivity according to the following equation;

σ=

L R ×W ×T

(1)

Where, σ is the proton conductivity of the membrane in S cm-1; L = 0.425 cm, the fixed distance between two Pt electrodes; R is the membrane resistance in Ω; W is the width of the sample in cm; and T is the thickness of the membrane in cm.


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Water uptake and ion-exchange capacity of the membranes. To obtain the dry weigh of membrane, a piece of membrane sample is dried at 80 °C for 6 h and weight. The dried membrane sample is transferred to a water filled glass container and kept for 24 h at room temperature for complete hydration. Then, the wet sample was removed from the container and weighed after removing of surface water to obtain wet weigh of the membrane. From the weight difference between wet and dry samples, the water uptake capacity (WUC) is calculated using the following equation;

Water uptake (%) =

× 100

(2)

Where, Wd and Ww are the weight of wet and dry membranes, respectively. Ion-exchange Capacity (IEC) was measured by the simple acid-base titration method. In brief, a 0.5 g of the sample was soaked in 30 mL of 3 N NaCl for 24 h to exchange H+ with Na+ ions. The displaced H+ ions in sample was measured by titrating the known volume of above solution against 0.01 N NaOH solution using phenolphthalein indicator. IEC of the membrane samples was calculated using the equation (3).

IEC =

× !" " #

mmol/g

(3)

Fabrication of the membrane electrode assemblies (MEAs) and fuel cell performance evaluation. The anode and cathode electrode were prepared using commercial gas diffusion layer (GDL, DC-35). For the anode electrode, certain amount of Pt-Ru/C (Pt-40 wt. % and Ru-20 wt. %) catalyst was ultrasonically dispersed in isopropyl alcohol with 10 wt% Nafion binder to obtain the homogeneous catalyst slurry. Subsequently, the resultant slurry was uniformly applied on the GDL to reach the catalyst loading of 2 mg cm-2. The cathode electrode was fabricated by same procedure with the 40 wt. % Pt / C catalyst and 30 wt % Nafion binder with the catalyst loading of 2 mg cm-2. The MEAs were fabricated by hot 7 ACS Paragon Plus Environment


pressing the pre-treated membranes sandwiched between the anode and cathode electrodes at 130°C under 10 kg cm-2 for 3 min. The DMFC performance of MEAs was evaluated in a single fuel cell fixture with the active area of 4 cm2 obtained from Fuel cell technologies Inc. USA. Oxygen with the flow rate of 250 mL min-1 and 2 M aqueous methanol solution with the flow rate is 2 mL min-1 were supplied as the oxidant and fuel on cathode and anode inlet of the DMFC, respectively. The DMFC performance of all the hybrid membrane was evaluated at 70°C under ambient pressure and compared with the performance of pristine Nafion membrane under similar operating conditions. Methanol permeability and Selectivity. Linear sweep voltammetry (LSV) technique is employed to measure the methanol crossover of pristine Nafion and Nafion-s-GCN membranes in cell mode using potentiostat/galvanostat (Biologic, VSP/VMP 3B-20) at room temperature. LSV was recorded from 0 to 1.0 V at the scan rate of 2 mV s-1. The MEAs fabricated with the pristine and hybrid membranes were assembled separately in a DMFC single cell (active area is 4 cm2) and conditioned for one hour at open circuit voltage (OCV) before the measurement. During the course of measurement, the oxygen on cathode is replaced by nitrogen at a flow rate of 100 mL min-1 and methanol was supplied at a flow rate of 2 mL min-1. Cathode of the cell was served as working electrode and anode of the cell was served as counter and reference electrodes of the potentiostat. The diffused methanol from the anode to cathode electrode through PEM will get oxidized at the cathode which produces the current. This current known as methanol crossover current which gradually increases as voltage sweep from 0 to 1 V and reaches the plateau after particular voltage when the diffused methanol get completely oxidized in the steady state condition. The maximum current corresponding to the plateau region is considered to quantify the methanol crossover of hybrid membrane. 8 ACS Paragon Plus Environment

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The selectivity of hybrid membranes and pristine Nafion membrane was calculated from the membrane resistance and the methanol crossover limiting current density according to the following equation;31

)=

* +, × -./0,,1023/

4 56*

(4)

Where, S is the electrochemical selectivity (in S mA-1); Icross-over is the Methanol crossover limiting current density (in mA cm-1); Rs area-specific resistance (in ohm cm2). The areaspecific resistance is calculated by the following equation; 9

78 = ;ℎ5 =5> :

(5)

Where, t is the membrane thickness (in cm); σ is the proton conductivity (in S cm-1).

RESULTS AND DISCUSSION

The phase purity of as-prepared GCN and s-GCN is characterized by powder XRD technique and resultant XRD pattern is shown in Figure 1a. The XRD pattern of GCN exhibited a strong diffraction peak at 2ϴ=27.4° corresponding to the (0 0 2) plane arises from the interplanar stacking of aromatic units.32,33 The interlayer spacing (d002) of 0.325 nm is close to that of graphite (0.33 nm). A weak diffraction peak at 2ϴ=13.2° is due to in-plane structural packing moiety of tri-s-triazine of CN unit with interplanar sapcing d100=0.669 nm which is smaller than the theoretically estimated tris-s-triazine unit (0.713 nm), which could be attributed to the presence of a tilt angularity in the structure.34 The above result matches the reports available with the literature and hence the formation of GCN with a tri-s-triazine based structure is confirmed. There is no obvious changes observed in the crystallinity of GCN after sulfonation as seen from the XRD analysis. The specific surface area and porous nature of GCN before and after sulfonation was investigated by nitrogen adsorptiondesorption analysis shown in Figure 1 (b & c). Both GCN and s-GCN sample exhibits the 9 ACS Paragon Plus Environment


type-II isotherms with H3-type hysteresis loop, suggesting the presence of micro and mesopores in the materials. As-synthesized GCN possess specific surface area of 40.97 m2 g-1 while the s-GCN sample possess a specific surface area of 19.64 m2 g-1. Decrease in surface area for sGCN might be due to immobilization of sulfonic acid groups into GCN during sulfonation process.30 Further the existence of pores primarily in the meso range is also observed for both GCN and s-GCN samples (inset to Figure 1b &c). The average pore diameter of GCN (1.88 nm) is increased to 3.32 nm after sulfonation. The cumulative effect of pores and presence of hydrophilic groups in s-GCN reflects absorbing more water molecule in the hybrid membrane. Figure 2a and b shows the FT-IR spectra of GCN and s-GCN, which implies the formation of graphitic carbon nitrite and its successful functionalization. FT-IR of GCN exhibited two band at 1640 and 1571cm-1 corresponding to the C=N stretching. Another three bands at 1416, 1313 and 1247 cm-1 are attributed to the aromatic C-N stretching vibrations. A sharp peak at 804 cm-1 is the characteristics peak of triazine ring system and it correspond to breathing mode of the triazine ring.35,36 The broad peak centred at 3171 cm-1 is associated to the N-H stretching vibration of uncondensed amino or imino groups.37 The above results confirm the formation of graphitic carbon nitride with uncondensed amino or imino groups from melamine during calcination process. In FT-IR spectrum of s-GCN, addition to the aforementioned vibration bands, three new bands at 3420, 1080 and 1195 cm-1 appeared as seen in Figure 2a and b (magnified view of Figure 2a). A band at 1195 cm-1 is characteristic peak for sulfonated graphitic carbon nitride30 and at 1080 cm-1 is assigned to the symmetric stretching of S=O of sulfonic acid groups. The peak corresponding to the –OH group is appeared at 3420 cm-1 and this particular region is much broader compared to the FT-IR spectra of pristine GCN which strongly evident the sulfonation of GCN.38


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As exemplified in Figure 2c, the XPS survey spectrum of GCN revealed that it contains only C, N and O while the survey spectrum of sulfonated GCN showed a new peak corresponding to the sulphur atom. A peak centred at 532.6 eV is corresponding to the O 1s which likely arises from C-O bond of the adsorbed CO2 and this peak become more intense in the survey spectrum of s-GCN as a result of the additional oxygen atoms comes from the – SO3H moieties.39 In the case of s-GCN, the S 2p peak cantered at 167.4 eV (Figure 2d) is corresponding to the -SO3H groups which evident the successful sulfonation of GCN.40,41 The C 1s spectra of sulfonated GCN can be deconvoluted into four peaks as shown in Figure 2e. The peak at 284.20 eV is arises from the sp2 hybridized C-C graphitic carbon commonly observed for carbon nitrides. Carbon from -C-NH2 bonding is observed at 286.30 eV and the peak at 288.10 eV is comes from the sp2 hybridized carbon in N-C=N centre. Sulfonated GCN also exhibited a peak at 285.08 eV as result of C-S bonding similar to the findings observed elsewhere.42 Figure 2f displays the deconvoluted N 1s spectra of s-GCN in which three peaks at 398.61, 399.50 and 401.29 eV are observed. These peaks are associated to the nitrogen atom from N-C=N coordination, N-(C)3 coordination and amino groups (C-N-H). The condensation of melamine into graphitic carbon nitride can be conformed from the existence of three coordinated nitrogen atom (N-(C)3).43,44 The surface morphology of GCN and s-GCN are examined by FESEM analysis as shown in Figure 3. Submicron sized flaky nano-sheets are densely packed in pristine GCN (Figure 3a) and the morphology is not much altered in the case of sulfonated GCN (Figure 3b) owing to the mild and effective surface functionalization process. The elemental composition of GCN and s-GCN is also examined by EDAX analysis and found that the GCN is mainly composed of carbon (42.15 at. %) and nitrogen (57.85 at. %) with the C/N ratio of 0.73 which is close to the theoretical value of g-C3N4 (0.75).45 EDAX analysis of sGCN sample shows the existence of sulfur atom (0.2 at. %) further confirm the successful



sulfonation of GCN. The incorporation of sulfonated GCN in Nafion polymer matrix with – SO3H and amino/imino groups is expected to show bifunctional activity in improving the proton conductivity of resultant hybrid membrane. After detailed characterizations, required amount of s-GCN was incorporated in Nafion matrix to realize the Nafion-s-GCN hybrid membranes with 0.25, 0.5 and 0.75 wt% of filler with respect to dry Nafion membrane weight. Surface morphology of pristine Nafion shows the plain and neat surfaces (Figure 3c), while in case of hybrid membrane, uniform dispersion of s-GCN fillers over Nafion matrix are seen and they are tightly held in the polymer matrix (Fig. 3d). This uniform dispersion of s-GCN contributes improved proton conductivity with reduced methanol permeability. The effect of s-GCN on thermal stability of hybrid membrane are analysed and the characteristic thermo gravimetric analysis (TGA) and derivative thermo gravimetric analysis (DTG) of Nafion-s-GCN hybrid membrane with reference to pristine Nafion membrane is shown in Figure 4a and b. All the membranes exhibit a typical three stage weight loss process.46,47 The initial weight loss up to 150 °C is arises from the loss of surface and bound water present in membrane and thereafter membrane looks stable with no obvious weight loss till 280 °C. First weight loss between 280-380 °C is observed due to the loss of sulfonic acid groups and the second weight loss (380-450 °C) is attributed to ether side-chain degradation. Thermal degradation of PTFE backbone can be seen from third weight loss (about 70 %) after 450 °C in Figure 4a. The strong hydrogen bonding interaction between the surface functionalized GCN filler and Nafion membrane upturns the thermal stability of Nafion-s-GCN hybrid membranes compared to pristine Nafion membrane. The improved thermal stability of hybrid membrane compared to pristine Nafion membrane can also be observed from the DTG curves (Figure 4b). Good compatibility of s-GCN filler with Nafion matrix positively alters the degradation


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temperature by about 2 % at all three stage of weight loss in the case of Nafion-s-GCN hybrid membrane with 0.75 wt% filler content. The mechanical stability of membranes also evaluated and noticed that the incorporation of s-GCN filler improves the mechanical stability as displayed in Figure-4c. The hybrid membrane with 0.5 wt% filler content exhibited a tensile strength of 7.0 MPa with about 20.2% elongation while the pristine Nafion possess only tensile strength of 5.0 MPa with 8.5% elongation. About 40 % increase in tensile strength for Nafion-s-GCN hybrid membrane is mainly due to the reduced phase separation as a result of uniform dispersion and good compatibility of s-GCN filler to the Nafion polymer matrix. Small-angle X-ray scattering (SAXS) is used to investigate the structural changes in Nafion matrix on incorporation of the s-GCN filler (Figure 4d). The ionic cluster of Nafion membrane is usually seen in the range between q of 0.11 and 0.13 Ǻ-1 and is repsented as “ionomer peak”. The peak at about 0.05 Ǻ-1 is arises from the fluorocarbon crystallites which are recognised as “matrix knee”.48 In the present study, the ionomer peak and matrix knee is observed at ~0.13 Ǻ-1 and ~0.04 Ǻ-1 respectively, for pristine Nafion membrane. For the Nafion-s-GCN (0.5 wt%) hybrid, the ionomer peak shifted to lower q value of ~0.11 Ǻ-1 with no change in matrix knee peak position. The Bragg spacing (d) tells the inter-cluster distance or the size/density of ionic clusters which can be calculated form the q value using the equation d=2Π/q. The d spacing is increased from 4.8 nm for pristine Nafion to 5.7 nm for Nafion-s-GCN (0.5 wt%) hybrid membrane. This implies that the addition of s-GCN filler with hydrophilic groups increases the size of ionic clusters. This alteration in the microstructure of the hybrid membrane is attributed to the absorbtion of more water molecule in the hybrid membrane, thus facilitate facile proton transport.49,50 In addition, the observed higher intensity of SAXS pattern of Nafion-s-GCN hybrid membrane expresses the higher



degree of crystallinity of hybrid membrane which is beneficial in long-term fuel cell operation. Water uptake capacity (WUC) of a PEM is crucial as it directly influence the proton transfer by both the GrӦtthuss and vehicle type mechanisms. All the Nafion-s-GCN hybrid membrane showed higher WUC compared to the pristine Nafion membrane as shown in Table 1. Noticeably, the Nafion-s-GCN hybrid membrane with 0.5 wt% filler content exhibits a WUC of 25.7 % which is about 40 % higher than that of pristine Nafion membrane. The higher WUC of the hybrid membrane is owed to high affinity of water molecules to the hydrophilic groups (i.e. sulfonic acid moieties) present in sulfonated GCN filler. The increased WUC is expected to form larger ionic clusters which can enhance the proton transport as a result of more number of hydrogen bonding. Nafion-s-GCN (0.5 wt%) hybrid membrane displayed ~19% higher IEC than that of pristine Nafion membrane (see Table 1). Higher IEC is mainly arises from two factor; (i) facile proton transfer caused by acid-base interaction between amino groups of s-GCN and sulfonic groups of Nafion membrane; (ii) the additional sulfonic groups present in the s-GCN filler. The reason for low WUC and IEC of Nafion-s-GCN (0.75 wt.%) hybrid membrane may be due to trivial aggregation of filler beyond the threshold limit in the polymer matrix. Proton conductivity of a PEM is an important parameter in determining the fuel cell performance and is measured by ac impedance analysis. Figure 5a exemplify the recorded ac impedance spectra of pristine Nafion membranes and Nafion-s-GCN hybrid membranes at 70 °C under fully hydrated condition. The resistance value associated to at high frequency region (HFR) was considered to calculate the proton conductivity. All the hybrid membrane exhibited lower resistance value compared to pristine Nafion membrane and is solely credited to the s-GCN which act as an effective filler in modifying the property of Nafion membrane. Proton conductivity of all the membranes is increases as temperature increased from 30-90


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°C and decreases thereafter (Figure 5b). All the hybrid membrane displayed higher proton conductivity compared to that of pristine Nafion at all temperature and the observed trend in proton conductivity corroborates the IEC and WUC results. Among all, the Nafion-s-GCN (0.5 wt %) hybrid membrane showed nearly 36% higher proton conductivity that pristine Nafion membrane at 70 °C. The higher proton conductivity of Nafion-s-GCN hybrid membrane expresses the potential application of s-GCN in Nafion matrix. As shown in scheme 1, the higher water uptake capacity and additional sulfonic acid groups can elevate the proton transfer by both vehicle and hopping mechanism, respectively. Furthermore, the facile proton transport results from acid-base interaction as explained in IEC section responsible for higher proton conductivity. However, the Nafion-s-GCN hybrid membrane with filler content beyond 0.5 wt% showed a decrease in proton conductivity as it disrupts the ionic path of the proton movement. The methanol permeability of Nafion-s-GCN hybrid and pristine Nafion membranes is quantified in terms of methanol cross-over current density as shown in Figure 6a. Methanol cross-over current density is linearly decreased as s-GCN content increases. Particularly, the Nafion-s-GCN (0.75 wt%) demonstrated much lower cross-over current density of 94 mA cm-2 which is about 44% lower than that of pristine Nafion (168 mA cm-2). The suppressed methanol crossover in case of Nafion-s-GCN membrane is attributed to layered structure of GCN with the triangular pores with lower geometric diameter of 0.311 nm compared to kinetic diameter of methanol molecule (0.385 nm) which particularly restricts the methanol transport.27 Furthermore, the achieved uniform dispersion of hydrophilic sulfonated GCN fillers over the Nafion matrix can provide the longer tortuous path for methanol diffusion. The membrane selectivity is a ratio of proton conductivity and methanol permeability which can tell us the comprehensive performance of a membrane. As seen in Figure-6b, the Nafion-s-GCN (0.5 wt. %) hybrid membrane displayed about 162 % higher electrochemical



selectivity compared to pristine Nafion membrane. Though, the Nafion-s-GCN (0.75 wt%) presented lower methanol crossover than Nafion-s-GCN (0.5 wt %) membrane, the inferior selectivity of that membrane can be understood from its lower proton conductivity value. The Nafion-s-GCN (0.5 wt. %) with high proton conductivity and low methanol crossover can be considered as the optimized hybrid membrane in this study. The application of Nafion-s-GCN hybrid membrane in comparison with pristine Nafion membrane is studied in single cell DMFC at 70 °C under ambient pressure. The corresponding polarization and power density curves are displayed in Figure 7a. The Nafions-GCN (0.5 wt%) hybrid membrane delivered a peak power density of 125 mW cm-2 at a load current density of 503 mA cm-2 while DMFC assembled with pristine Nafion membrane delivered a peak power density of only 65 mW cm-2 under identical operating conditions. The increase in DMFC performance by about 90 % with Nafion-s-GCN (0.5 wt%) hybrid membrane resulted from its greater electrochemical selectivity as discussed. The stability of the Nafion-s-GCN (0.5 wt%) hybrid membrane also evaluated in cell mode at 70 °C and compared with pristine Nafion membrane in Figure 7b. After 100 h run, the loss in open circuit voltage (OCV) values of pristine Nafion and Nafion-sGCN hybrid membrane is about 20 % and 15 % respectively. The superior performance of Nafion-s-GCN hybrid membrane compared to pristine Nafion membrane is likely arises from the good interfacial interaction of s-GCN filler with Nafion matrix and the potential application of s-GCN filler in preparing the Nafion hybrid membrane can be realized from the higher DMFC performance of Nafion-sGCN hybrid membranes with improved stability.


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CONCLUSIONS

The Nafion hybrid membrane with sGCN as potential filler is successfully explored as a alternate methanol barrier membrane useful for DMFC. The SAXS characterization revealed that the addition of s-GCN filler with –NH2 / -NH and –SO3H functional groups to Nafion matrix significantly alter the microstructure by increasing the size of ionic clusters facilitating proton conduction in the hybrid. The high proton conductivity and low methanol crossover of Nafion-s-GCN hybrid membrane together resulted in higher electrochemical selectivity. Consequently, the DMFC employed with optimized Nafion-s-GCN hybrid membrane delivers about 90 % higher peak power density than that of pristine Nafion membrane. The Nafion-sGCN hybrid membrane reported in this study may bestow a competitive methanol barrier alternative polymer electrolyte membrane useful for DMFC technology.

AUTHOR INFORMATION

Corresponding author *E-mail: [email protected], [email protected] Phone: +91-44-22544554; fax: +91-44-22542456 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

Financial support by CSIR, New Delhi, is gratefully acknowledged. We thank Dr.Vijayamohanan K. Pillai, Director, CECRI, Karaikudi, for his constant encouragement and support. V P thanks CSIR for awarding Senior Research Fellowship (31/20 (158)/2017EMR-I) to pursue research at CSIR-CECRI.



REFERENCES

(1) Chen, M.; Wang, M.; Yang, Z.; Ding, X.; Li, Q.; Wang, X. A Novel Catalyst Layer Structure Based Surface-Patterned Nafion® Membrane for High-Performance Direct Methanol Fuel Cell. Electrochim. Acta 2018, 263, 201–208. (2) Xia, Z.; Sun, R.; Jing, F.; Wang, S.; Sun, H.; Sun, G. Modeling and Optimization of Scaffold-like Macroporous Electrodes for Highly Efficient Direct Methanol Fuel Cells. Appl. Energy 2018, 221, 239–248. (3) Devi, A. U.; Divya, K.; Kaleekkal, N. J.; Rana, D.; Nagendran, A. Tailored SPVdFCo-HFP/SGO Nanocomposite Proton Exchange Membranes for Direct Methanol Fuel Cells. Polymer (Guildf). 2018, 140, 22–32. (4) Li, J.; Xu, G.; Luo, X.; Xiong, J.; Liu, Z.; Cai, W. Effect of Nano-Size of Functionalized Silica on Overall Performance of Swelling-Filling Modified Nafion Membrane for Direct Methanol Fuel Cell Application. Appl. Energy 2018, 213, 408–414. (5) Sahu, A. K.; Bhat, S. D.; Pitchumani, S.; Sridhar, P.; Vimalan, V.; George, C.; Chandrakumar, N.; Shukla, A. K. Novel Organic–inorganic Composite PolymerElectrolyte Membranes for DMFCs. J. Memb. Sci. 2009, 345 (1–2), 305–314. (6) Jang, S.; Kim, S.; Kim, S. M.; Choi, J.; Yeon, J.; Bang, K.; Ahn, C.-Y.; Hwang, W.; Her, M.; Cho, Y.-H et al. Interface Engineering for High-Performance Direct Methanol Fuel Cells Using Multiscale Patterned Membranes and Guided Metal Cracked Layers. Nano Energy 2018, 43, 149–158. (7) Velayutham, P.; Sahu, A. K.; Parthasarathy, S. A Nafion-Ceria Composite Membrane Electrolyte for Reduced Methanol Crossover in Direct Methanol Fuel Cells. Energies 2017, 10 (2), 259. (8) Meenakshi, S.; Sahu, A. K.; Bhat, S. D.; Sridhar, P.; Pitchumani, S.; Shukla, A. K. Mesostructured-Aluminosilicate-Nafion Hybrid Membranes for Direct Methanol Fuel Cells. Electrochim. Acta 2013, 89, 35–44. (9) V., V.; Khastgir, D. Fabrication and Comprehensive Investigation of Physicochemical and Electrochemical Properties of Chitosan-Silica Supported Silicotungstic Acid Nanocomposite Membranes for Fuel Cell Applications. Energy 2018, 142, 313–330. (10) Guo, Y.; Jiang, Z.; Ying, W.; Chen, L.; Liu, Y.; Wang, X.; Jiang, Z.-J.; Chen, B.; Peng, X. A DNA-Threaded ZIF-8 Membrane with High Proton Conductivity and Low Methanol Permeability. Adv. Mater. 2018, 30 (2). (11) Nguyen, H.-D.; Jestin, J.; Porcar, L.; Iojoiu, C.; Lyonnard, S. Aromatic Copolymer/Nafion Blends Outperforming the Corresponding Pristine Ionomers. ACS Appl. Energy Mater. 2018, 1 (2), 355–367. (12) Ranjani, M.; Yoo, D. J.; Gnana kumar, G. Sulfonated Fe3O4@SiO2 Nanorods Incorporated sPVdF Nanocomposite Membranes for DMFC Applications. J. Memb. Sci. 2018, 555, 497–506. (13) Parthiban, V.; Akula, S.; Peera, S. G.; Islam, N.; Sahu, A. K. Proton Conducting Nafion-Sulfonated Graphene Hybrid Membranes for Direct Methanol Fuel Cells with Reduced Methanol Crossover. Energy & Fuels 2016, 30 (1), 725–734.


Page 18 of 31

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Chang, C.-M.; Li, H.-Y.; Lai, J.-Y.; Liu, Y.-L. Nanocomposite Membranes of Nafion and Fe3O4-Anchored and Nafion-Functionalized Multiwalled Carbon Nanotubes Exhibiting High Proton Conductivity and Low Methanol Permeability for Direct Methanol Fuel Cells. RSC Adv. 2013, 3 (31), 12895–12904. (15) Parthiban, V.; Akula, S.; Sahu, A. K. Surfactant Templated Nanoporous CarbonNafion Hybrid Membranes for Direct Methanol Fuel Cells with Reduced Methanol Crossover. J. Memb. Sci. 2017, 541, 127–136. (16) Lin, C. W.; Lu, Y. S. Highly Ordered Graphene Oxide Paper Laminated with a Nafion Membrane for Direct Methanol Fuel Cells. J. Power Sources 2013, 237, 187–194. (17) Mohamad, Y. Y. C. and K. S. L. and W. R. W. D. and A. B. Synthesis and Characterization of Sulfonated Graphene Oxide Nanofiller for Polymer Electrolyte Membrane. IOP Conf. Ser. Mater. Sci. Eng. 2016, 160 (1), 12035. (18) Zhang, Y.; Cai, W.; Si, F.; Ge, J.; Liang, L.; Liu, C.; Xing, W. A Modified Nafion Membrane with Extremely Low Methanol Permeability via Surface Coating of Sulfonated Organic Silica. Chem. Commun. 2012, 48 (23), 2870–2872. (19) Sahu, A. K.; Meenakshi, S.; Bhat, S. D.; Shahid, A.; Sridhar, P.; Pitchumani, S.; Shukla, A. K. Meso-Structured Silica-Nafion Hybrid Membranes for Direct Methanol Fuel Cells. J. Electrochem. Soc. 2012, 159 (11), F702–F710. (20) Zhengbang, W.; Tang, H.; Mu, P. Self-Assembly of Durable Nafion/TiO2 Nanowire Electrolyte Membranes for Elevated-Temperature PEM Fuel Cells. J. Memb. Sci. 2011, 369 (1), 250–257. (21) Feng, K.; Tang, B.; Wu, P. Selective Growth of MoS2 for Proton Exchange Membranes with Extremely High Selectivity. ACS Appl. Mater. Interfaces 2013, 5 (24), 13042–13049. (22) Zhu, J.; Xiao, P.; Li, H.; Carabineiro, S. A. C. Graphitic Carbon Nitride: Synthesis, Properties, and Applications in Catalysis. ACS Appl. Mater. Interfaces 2014, 6 (19), 16449–16465. (23) Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Z. Graphitic Carbon Nitride Materials: Controllable Synthesis and Applications in Fuel Cells and Photocatalysis. Energy Environ. Sci. 2012, 5 (5), 6717–6731. (24) Zhang, Y.; Mori, T.; Niu, L.; Ye, J. Non-Covalent Doping of Graphitic Carbon Nitride Polymer with Graphene: Controlled Electronic Structure and Enhanced Optoelectronic Conversion. Energy Environ. Sci. 2011, 4 (11), 4517–4521. (25) Miller, T. S.; Jorge, A. B.; Suter, T. M.; Sella, A.; Corà, F.; McMillan, P. F. Carbon Nitrides: Synthesis and Characterization of a New Class of Functional Materials. Phys. Chem. Chem. Phys. 2017, 19 (24), 15613–15638. (26) Mansor, N.; Miller, T. S.; Dedigama, I.; Jorge, A. B.; Jia, J.; Brázdová, V.; Mattevi, C.; Gibbs, C.; Hodgson, D.; Shearing, P. R et al. Graphitic Carbon Nitride as a Catalyst Support in Fuel Cells and Electrolyzers. Electrochim. Acta 2016, 222, 44–57. (27) Gang, M.; He, G.; Li, Z.; Cao, K.; Li, Z.; Yin, Y.; Wu, H.; Jiang, Z. Graphitic Carbon Nitride Nanosheets/sulfonated Poly(ether Ether Ketone) Nanocomposite Membrane for Direct Methanol Fuel Cell Application. J. Memb. Sci. 2016, 507, 1–11. 19 ACS Paragon Plus Environment


(28) Wang, F.; Wang, G.; Zhang, J.; Li, B.; Zhang, J.; Deng, J.; Chen, J.; Wang, R. Novel Sulfonated Poly(ether Ether Ketone)/oxidized G-C3N4 Composite Membrane for Vanadium Redox Flow Battery Applications. J. Electroanal. Chem. 2017, 797, 107–112. (29) Niu, R.; Kong, L.; Zheng, L.; Wang, H.; Shi, H. Novel Graphitic Carbon Nitride Nanosheets/sulfonated Poly(ether Ether Ketone) Acid-Base Hybrid Membrane for Vanadium Redox Flow Battery. J. Memb. Sci. 2017, 525, 220–228. (30) Baig, R. B. N.; Verma, S.; Nadagouda, M. N.; Varma, R. S. Room Temperature Synthesis of Biodiesel Using Sulfonated Graphitic Carbon Nitride. Sci. Rep. 2016, 6, 39387. (31) Arico, A. S.; Sebastian, D.; Schuster, M.; Bauer, B.; D’Urso, C.; Lufrano, F.; Baglio, V. Selectivity of Direct Methanol Fuel Cell Membranes. Membranes (Basel). 2015, 5 (4), 793–809. (32) Yuan, B.; Chu, Z.; Li, G.; Jiang, Z.; Hu, T.; Wang, Q.; Wang, C. Water-Soluble Ribbon-like Graphitic Carbon Nitride (G-C3N4): Green Synthesis, Self-Assembly and Unique Optical Properties. J. Mater. Chem. C 2014, 2 (39), 8212–8215. (33) Guo, Q.; Xie, Y.; Wang, X.; Lv, S.; Hou, T.; Liu, X. Characterization of WellCrystallized Graphitic Carbon Nitride Nanocrystallites via a Benzene-Thermal Route at Low Temperatures. Chem. Phys. Lett. 2003, 380 (1), 84–87. (34) Lei, B.; Zhang, Y.; He, Y.; Xie, Y.; Xu, B.; Lin, Z.; Huang, L.; Tan, S.; Wang, M.; Cai, X. Preparation and Characterization of Wood–plastic Composite Reinforced by Graphitic Carbon Nitride. Mater. Des. 2015, 66, 103–109. (35) Kim, M.; Hwang, S.; Yu, J.-S. Novel Ordered Nanoporous Graphitic C3N4 as a Support for Pt-Ru Anode Catalyst in Direct Methanol Fuel Cell. J. Mater. Chem. 2007, 17 (17), 1656–1659. (36) Guo, Y.; Chen, J. Photo-Induced Reduction of Biomass-Derived 5Hydroxymethylfurfural Using Graphitic Carbon Nitride Supported Metal Catalysts. RSC Adv. 2016, 6 (104), 101968–101973. (37) Tonda, S.; Kumar, S.; Kandula, S.; Shanker, V. Fe-Doped and -Mediated Graphitic Carbon Nitride Nanosheets for Enhanced Photocatalytic Performance under Natural Sunlight. J. Mater. Chem. A 2014, 2 (19), 6772–6780. (38) Zheng, F.-C.; Chen, Q.-W.; Hu, L.; Yan, N.; Kong, X.-K. Synthesis of Sulfonic Acid-Functionalized Fe3O4@C Nanoparticles as Magnetically Recyclable Solid Acid Catalysts for Acetalization Reaction. Dalt. Trans. 2014, 43 (3), 1220–1227. (39) Liu, J.; Zhang, T.; Wang, Z.; Dawson, G.; Chen, W. Simple Pyrolysis of Urea into Graphitic Carbon Nitride with Recyclable Adsorption and Photocatalytic Activity. J. Mater. Chem. 2011, 21 (38), 14398–14401. (40) Kang, S.; Zhang, G.; Yang, X.; Yin, H.; Fu, X.; Liao, J.; Tu, J.; Huang, X.; Qin, F. G. F.; Xu, Y. Effects of P-Toluenesulfonic Acid in the Conversion of Glucose for Levulinic Acid and Sulfonated Carbon Production. Energy & Fuels 2017, 31 (3), 2847– 2854. (41) Anderson, J. M.; Johnson, R. L.; Schmidt-Rohr, K.; Shanks, B. H. Hydrothermal Degradation of Model Sulfonic Acid Compounds: Probing the Relative Sulfur–carbon Bond Strength in Water. Catal. Commun. 2014, 51, 33–36. 20 ACS Paragon Plus Environment

Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Yi, M. T.; Youhong, T.; Sheng, D.; Zhang, Q. S. Proton‐Functionalized Two‐ Dimensional Graphitic Carbon Nitride Nanosheet: An Excellent Metal‐/Label‐Free Biosensing Platform. Small 2014, 10 (12), 2382–2389. (43) Fang, J.; Fan, H.; Li, M.; Long, C. Nitrogen Self-Doped Graphitic Carbon Nitride as Efficient Visible Light Photocatalyst for Hydrogen Evolution. J. Mater. Chem. A 2015, 3 (26), 13819–13826. (44) Gu, S.; Xie, J.; Li, C. M. Hierarchically Porous Graphitic Carbon Nitride: LargeScale Facile Synthesis and Its Application toward Photocatalytic Dye Degradation. RSC Adv. 2014, 4 (103), 59436–59439. (45) Li, X.; Zhang, J.; Shen, L.; Ma, Y.; Lei, W.; Cui, Q.; Zou, G. Preparation and Characterization of Graphitic Carbon Nitride through Pyrolysis of Melamine. Appl. Phys. A 2009, 94 (2), 387–392. (46) Zhang, Z.; Désilets, F.; Felice, V.; Mecheri, B.; Licoccia, S.; Tavares, A. C. On the Proton Conductivity of Nafion–Faujasite Composite Membranes for Low Temperature Direct Methanol Fuel Cells. J. Power Sources 2011, 196 (22), 9176–9187. (47) Di Palma, L.; Bavasso, I.; Sarasini, F.; Tirillò, J.; Puglia, D.; Dominici, F.; Torre, L. Synthesis, Characterization and Performance Evaluation of Fe3O4/PES Nano Composite Membranes for Microbial Fuel Cell. Eur. Polym. J. 2018, 99, 222–229. (48) Joseph, J.; Tseng, C.-Y.; Hwang, B.-J. Phosphonic Acid-Grafted Mesostructured silica/Nafion Hybrid Membranes for Fuel Cell Applications. J. Power Sources 2011, 196 (18), 7363–7371. (49) Li, H.-Y.; Liu, Y.-L. Nafion-Functionalized Electrospun Poly(vinylidene Fluoride) (PVDF) Nanofibers for High Performance Proton Exchange Membranes in Fuel Cells. J. Mater. Chem. A 2014, 2 (11), 3783–3793. (50) Li, G.; Zhao, C.; Li, X.; Qi, D.; Liu, C.; Bu, F.; Na, H. Novel Side-Chain-Type Sulfonated Diphenyl-Based Poly(arylene Ether Sulfone)s with a Hydrogen-Bonded Network as Proton Exchange Membranes. Polym. Chem. 2015, 6 (32), 5911–5920.



Scheme 1. Schematic representation of proton transport via vehicular and hopping mechanisms in Nafion-s-GCN hybrid membrane. In vehicular mechanism, the proton combining with water molecule and transferred as H3O+; in hopping mechanism, proton is transferred through hydrogen bonding interaction and hope along the side chains of –SO3H groups.


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(a)

(1 0 0)

Intensity (a.u.)

(0 0 2)

GCN

s-GCN 0

10

20

30

40

50

60

70

80

90

)

2θ (deg.)

)

-1

-1

dV(d) (cm 3 nm g )

Volume adsorbed @ STP (cm3 g

-1

(b)

0.012

60

40

Pore diameter=1.88 nm

0.010 0.008 0.006 0.004 0.002 0.000

0

5

10

15

20

25

Pore diameter (nm)

20

-1 BET surface area=40.97 m2 g

adsorption desorption

0 0.0

0.2

0.4 0.6 0.8 1.0 Relative pressure (P/Po)

80

-1

Pore diameter=3.32 nm -1 -1

60

40

1.2

(c)

0.012

dV(d) (cm 3 nm g )

Volume adsorbed @ STP (cm3 g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.010 0.008 0.006 0.004 0.002 0.000

0

5

10

15

20

25

Pore diameter (nm)

20

-1 BET surface area=19.64 m2 g

0 0.0

adsorption desorption 0.2 0.4 0.6 0.8 Relative pressure (P/Po)

1.0

1.2

Figure 1. (a) X-ray diffraction pattern of GCN and s-GCN; (b & c) are the nitrogen adsorption-desorption isotherms of GCN and s-GCN respectively (pore size distribution analysis are shown in the insets). 23 ACS Paragon Plus Environment


(b) GCN 804 cm-1

3171 (-NH2/-NH)

1640 1571 1416 1313 1247

(CN heterocycle)

s-GCN

Transmittance (%)

Transmittance (%)

(a)

GCN

s-GCN 1080 cm

3420 (-OH)

1195 cm

4000 3500 3000 2500 2000 1500 1000

Wave number (cm-1)

500 1500

(c)

N 1s

s-GCN

S 2p

0

200

400

800 165

600

C-C N-C=N

286

288

750

500

S 2p

167.4 eV

166

167

168

169

170

171

290

N 1s Raw Intensity N-C=N N-(C)3

Intensity (a.u.)

Raw Intensity C-S C-NH2

284

1000

Wave number (cm -1)

(f)

C 1s

282

-1

Binding energy (eV)

Binding energy (eV)

(e)

-1

Intensity (a.u.)

O 1s

C 1s

Intensity (a.u.)

GCN

280

1250

(d)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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292 394

NH

396

398

400

402

404

Binding energy (eV)

Binding energy (eV)

Figure 2. (a) FT-IR spectrum in full view and (b) in magnified view of GCN and sulfonated GCN XPS; (c) XPS survey spectrum of GCN and sulfonated GCN, (d) S 2p, (e) C 1s, (f) N 1s spectrum of sulfonated GCN.


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Figure 3. FE-SEM images of (a) GCN; (b) s-GCN; (c) Pristine Nafion membrane and (d) Nafion-s-GCN (0.5 wt%).



(a)

(I)

(b) Pristine Nafion

80 60 40 Pristine nafion Nafion-s-GCN (0.25 wt%) Nafion-s-GCN (0.5 wt%) Nafion-s-GCN (0.75 wt%)

20

0

200

400

600

0

800

Temperature (°C) 0.012

Nafion-s-GCN (0.5 wt%)

200

400 600 Temperature (°C)

Intensity (a.u.)

0.004

800

(d)

(c)

0.008

0.000 0.00



0

Load (kN)

(II) (III)

DTG (% min-1)

Weight loss (%)

100

Pristine Nafion Nafion-s-GCN (0.5 wt%)

Pristine Nafion Nafion-s-GCN (0.5 wt%)

0.05

0.10

0.15 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

ÅÅÅÅ q

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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( -1)

Tensile Strain (mm/mm)

Figure 4. (a) TGA and (b) DTG curves of pristine Nafion and Nafion-s-GCN hybrid membranes; (c) stress-strain curves and (d) SAXS analysis of pristine Nafion and Nafion-sGCN (0.5 wt%) hybrid membranes.


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100

(a)

-Im (Z) (ohm)

80

Pristine Nafion Nafion-sGCN (0.25 wt%) Nafion-sGCN (0.50 wt%) Nafion-sGCN (0.75 wt%)

60 40 20 0 80

90

100

110

120

130

140

150

Re (Z) (ohm) Proton conductivity (S cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25

(b) 0.20

0.15

0.10

Pristine Nafion Nafion-sGCN (0.25 wt%) Nafion-sGCN (0.50 wt%) Nafion-sGCN (0.75 wt%)

0.05 30

40

50

60

70

80

90

100

Temperature (°C) Figure 5. (a) EIS of Pristine Nafion and Nafion-s-GCN hybrid membranes at 70 °C under 100% relative humidity; (b) Proton conductivity of pristine Nafion and Nafion-s-GCN hybrid membranes at different temperatures under 100% relative humidity.


200

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(a)

160 Pristine Nafion Nafion-s-GCN (0.25 wt%) Nafion-s-GCN (0.50 wt%) Nafion-s-GCN (0.75 wt%)

120 80 40 0 0.0

0.2

0.4

0.6

0.8

1.0

-1

)

Applied voltage (V)

Electrochemical selectivity (S mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crossover current density (mA cm-2)


0.05

(b)

0.04

0.03

0.02 0.00

0.25

0.50

0.75

1.00

s-GCN content (%) Figure 6. (a) Methanol crossover current density; (b) Electrochemical selectivity of Pristine Nafion and Nafion-s-GCN hybrid membranes.


Cell voltage (V)

1.0

140

Pristine Recast Nafion Nafion-sGCN (0.25 wt%) Nafion-sGCN (0.5 wt%) Nafion-sGCN (0.75 wt%) Nafion-117

0.8

(a)

120 100

0.6

80

0.4

60 40

0.2

20

0.0 0

0 100 200 300 400 500 600 700 800

Current density (mA cm-2)

1.0

(b) Cell voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Power density (mW cm-2)

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0.8 0.6 0.4 0.2 Nafion-s-GCN (0.5 wt%) Pristine Recast Nafion

0.0 0

10 20 30 40 50 60 70 80 90 100

Time (h) Figure 7. DMFCs performance of Pristine recast Nafion and Nafion-s-GCN hybrid membranes at 70 °C under ambient pressure; (b) Stability test for pristine recast Nafion and Nafion-s-GCN (0.5 wt%) hybrid membrane for 100 hrs at 70˚C and ambient pressure under OCV condition.



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Table 1. Water uptake, Ion-exchange capacity, Peak power density, Methanol crossover current density and Electro-chemical selectivity of Pristine Nafion and Nafion-s-GCN hybrid membranes. Membrane Type

Water Uptake (%) (±0.5)

Ionexchange capacity (mmol g-1) (±0.01)

Proton conductivity (mS cm-1) @ 70°C

Methanol crossover current density (mA cm-2)

Electrochemical selectivity (× 10-2 S mA-1) (±0.1)

Peak power density (mW cm-2)

Pristine Nafion

18.4

0.90

135

168

1.81

65


23.2

0.95

155

128

3.29

111


25.7

1.07

183

110

4.75

125


24.1

1.00

164

94

4.55

115


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Graphitic Carbon Nitride Nanosheets - Nafion as Methanol Barrier

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