Sulfonated Holey Graphene Oxide (SHGO) - ACS Publications

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Sulfonated holey graphene oxide (SHGO) filled Sulfonated Poly(Ether Ether Ketone) membrane: the role of holes in the SHGO in improving its performance as proton exchange membrane for direct methanol fuel cells Zhong-Jie Jiang, Zhongqing Jiang, Xiaoning NA Tian, Lijuan Luo, and Meilin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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ACS Applied Materials & Interfaces

Sulfonated Holey Graphene Oxide (SHGO) Filled Sulfonated Poly(Ether Ether Ketone) Membrane: The Role of Holes in the SHGO in Improving its Performance as Proton Exchange Membrane for Direct Methanol Fuel Cells Zhong-Jie Jiang*,†, Zhongqing Jiang*,‡, Xiaoning Tian‡, Lijuan Luo‡, and Meilin Liu†,§ † Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, College of Environment and Energy, South China University of Technology, Guangzhou 510006, China. ‡ Department of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, Zhejiang, China. § School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. ABSTRACT: Sulfonated holey graphene oxides (SHGOs) have synthesized by the etching of sulfonated graphene oxides with concentrated HNO3 under the assistance of ultrasonication. These SHGOs could be used as the fillers for the sulfonated aromatic poly(ether ether ketone) (SPEEK) membrane. The obtained SHGO incorporated SPEEK membrane has a uniform and dense structure, exhibiting the higher performance as proton exchange membranes (PEMs), for instance, higher proton conductivity, lower activation energy for proton conduction, and comparable methanol permeability, as compared to Nafion® 112. The sulfonated graphitic structure of the SHGOs are believed to be one of the crucial factors resulting in the higher performance of the SPEEK/SHGO membrane, since it could increase the local density of the –SO3H groups in the membrane and induce a strong interfacial interaction between SHGO and the SPEEK matrix, which improve the proton conductivity and lower the swelling ratio of the membrane, respectively. Additionally, the proton conductivity of the membrane could be further enhanced by the presence of the holes in the graphitic planes of the SHGOs, since it provides an additional channel for the transport of the protons. When used, direct methanol fuel cell with the SPEEK/SHGO membrane is found to exhibit much higher performance than that with Nafion® 112, suggesting the potential use of the SPEEK/SHGO membrane as the PEMs. KEYWORDS: Sulfonated holey graphene oxide, filler, blend membrane, activation energy, fuel cell

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INTRODUCTION As a clean source of energy, direct methanol fuel cells (DMFCs), which produce electricity directly via electrochemical oxidation reactions of methanol in the anode and oxygen reduction in the cathode, have attracted tremendous attention.1-5 The performance of a DMFC is largely determined by the proton exchange membrane (PEM), a key component of the DMFC playing the roles of the conduction of protons and the separation of the anode and the cathode.4,6-11 A typical PEM is made from ionomer polymers consisting of electrically neutral units and a fraction of ionized units covalently bonded to the hydrophobic polymer backbone as pendent moieties.12-15 The tetrafluorethylene based fluoropolymers, also known as Nafion, are currently the most widely used ionomers for the PEMs. These tetrafluorethylene based fluoropolymers are demonstrated to have high proton conductivity due to their high concentrations of sulfonic acid groups.16-20 Their widespread uses in the DMFCs have, however, been greatly hindered by the inherent disadvantages of high cost and methanol permeability.1,3,12 This has stimulated numerous studies to exploit alternative PEMs for the DMFCs, which could meet the requirement of low cost, high proton conductivity, low fuel permeability, and high mechanical and chemical stabilities.16,21-23 Recent work has demonstrate the potential use of sulfonated aromatic main-chain polymers as the PEMs for the DMFCs due to their low cost, high mechanical strength, and superior chemical and thermal stabilities.24-26 Among various sulfonated aromatic polymers reported, the sulfonated aromatic poly(ether ether ketone)s (SPEEKs) are the most studied due to their specific structure similar to Nafion, which comprises ionizable sulfonic acid groups connecting to a hydrophobic backbone.23,25,27-28 As reported previously,27-28 SPEEKs could be synthesized with a high concentrations of sulfonic acid groups, making them highly promising as PEMs with high proton conductivity. The high degree of sulfonation, however, makes the SPEEKs readily swollable and even soluble in the methanol/water solution, which has severely limited their direct use as the PEM in the DMFCs.29-30 The modifications used to increase the dimensional stability of the SPEEKs in the aqueous methanol solution are therefore required to improve their usability as the PEM for the DMFCs. Blending the SPEEKs with inorganic or organic fillers, such as TiO2, and SiO2, etc., is a widely used method to improve its performance.6-7,31 As demonstrated recently, ionomer membranes exhibit improved dimensional and chemical stability and mechanical strength when they are blended with inorganic or organic fillers.6-7,31 Graphene oxide (GO) is the oxidized derivative of graphene, possessing a layered structure with oxygenous groups, such as –OH, –COOH, and –O– groups, on its basal plane and edges.32-35 The specific structure which consists of graphitic plane and oxygenous groups gives GO with an amphiphilic property, making it highly soluble in various solvents to form homogenous solutions.32-33 Recent work has demonstrated that the GOs could be used as the fillers to the ionomer membranes since they have a large surface area and are electronically insulative.14,29-30,36-38 Owing to the flexibility and good compatibility of the GOs with the host membranes, the GO 2


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filled membranes show the reinforced mechanical stability.36-38 This is different from inorganic fillers, whose incorporation makes the ionomer membranes highly brittle, especially when inorganic fillers are excessively incorporated. Because of the absence of –SO3H groups, the GO filled ionomer membranes, however, usually exhibit low proton conductivity, which lowers the performance of the DMFCs equipped with theses GO filled ionomer membranes.39-40 In the most cases, therefore, the sulfonated GOs (SGOs) are employed as the fillers to improve the performance of the ionomer membranes. As reported recently, the sulfonation could not only facilitate the homogenous distribution of the GOs in the membranes owing to the good compatibility between SGO and –SO3H in the membrane matrix and increase the proton conductivity of the membranes because of the improvement of the local density of the –SO3H groups in the membranes, but also enhance the mechanical stability of the membranes through a strong interfacial interaction between the graphitic plane of the SGO and the membrane matrix.22-23,36,41 GO, however, is a sheet-like material and has a large aspect ratio, which may block the transport of protons through the membranes, especially when the basal plane of the GO is coincidently oriented in the way that is parallel to the plane of the PEM. In this case, the transport of proton through the membranes needs to circumvent the basal plane of the GOs, making the proton conductivity of the PEMs not as high as expected. The introduction of holes into the basal plane of the GO might be a promising strategy to promote the transport of the protons through the membranes since the holes in the basal plane of the GO could construct an addition path for the transport of the proton.35,42-46 This well avoids the case that the transport of the protons through the membranes is blocked by the graphitic plane of the GO with the high aspect ratio, especially when the membranes are highly incorporated with the GO, in which a larger amount of the GO is oriented in the way that is parallel to the membrane plane. To demonstrate that the holes in the basal plane of the GO could improve the proton conductivity of the membranes, we fabricated sulfonated holey GOs (SHGOs) in this work, which were then used as the fillers for the SPEEK membranes. The results show that the SHGO incorporated SPEEK (SPEEK/SHGO) membrane could exhibit higher proton conductivity (σ), lower activation energy for proton conduction (Ea), and comparable methanol permeability (P), than Nafion® 112. The holes in the basal plane of the graphitic structure of the SHGOs were found to make big contributions in improving the proton conductivity of the SPEEK/SHGO membrane and lowering its Ea. The practical application showed that the DMFCs with the SPEEK/SHGO membrane could exhibit greatly improved performance than those with Nafion® 112, which strongly suggests that the SPEEK/SHGO membrane could be used as the PEM for the DMFCs. EXPERIMENTAL SECTION Materials synthesis. Preparation of SPEEK. SPEEK used in this work was sulfated from PEEK (Scheme 1a). Typically, ~ 5 g of the vacuum dried PEEK were mixed with 100 mL of 95 % 3



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concentrated H2SO4. After 7 h of the vigorous stirring at room temperature, the mixture was blended with ice-cold water under mechanical agitation and left overnight. The resulting SPEEK was collected by filtration, then thoroughly washed with water, and finally vacuum dried at 60 °C for ~24 h. Preparation of sulfonated holey graphene oxide (SHGO). The aryl diazonium salt was employed as the sulfonation agent, which was synthesized using the following procedure: in a 100 mL beaker with a warm water bath, 40 mL of a 2% NaOH solution and 1.5 g of sulfanilic acid (SA) were added. After the complete dissolution of SA, 0.6 g of NaNO2, 50 mL of water, and 10 mL of concentrated HCl were successively added under stirring. The obtained mixed solution was then cooled to 0 °C and reacted for 15 min, which led to the formation of a diazonium salt. The diazonium salt solution was then injected dropwise into 200 mL of the GO solution (1 mg mL-1, prepared by a Hummers method with a small modification as described elsewhere.29,47). After 4 h of vigorous stirring in an ice water bath, the resulting SGO was purified by centrifugation and washing with water several times and then redispersed in water for use. Scheme 1b briefly illustrates the procedure for the preparation of the SGO.

Scheme 1. (a) Sulfonation of the PEEK through its reaction with the concentrated sulfuric acid. (b) Synthetic procedure of the SGO and the SHGO. The fabrication of the SHGO was achieved through the etching of the SGO by the concentrated HNO3 under the assistance of ultrasonication (Scheme 1b). Specifically, 250 mL of 70% concentrated HNO3 and 50 mL of the SGO in water (2 4


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mg mL-1) was successively added into a 500 mL conical flask. A bath sonicator with a power of 100 W and a frequency of 50/60 Hz was used to ultrasonicate the mixture. Typically, the mixture was ultrasonically treated for 1 h after the sealing of the reaction flask. The resulting product was settled for another 1 h before it was poured into 200 mL of water. The SHGO was collected through centrifugation and washing of the product with water to well remove the residual acid. Membrane synthesis. The membrane synthesis was carried out using a procedure reported in our previous work.29-30 Typically, 5mL of homogenous solution prepared by dissolution of SHGO in DMF were blended with 5mL of a SPEEK/DMF solution. The contents of the SHGO added were controlled to give 5 wt.% of the SHGO relative to the SPEEK. After 24 h of stirring, the mixture was cast onto a glass plate. The blend membrane was formed after drying at 80 °C overnight and subsequently at 100 °C for 12 h. The SPEEK/SHGO membranes were then obtained through soaking the dried membranes in methanol for 24 h and thoroughly washing with DI water. Scheme 2 briefly illustrates the structure of the SPEEK/SHGO blend membrane and the operation principle of DMFCs. For comparison, the plain SPEEK and blend membranes consisting of 5 wt. % GO, 5 wt. % SGO, and 5 wt. % HGO (the preparation procedure of the HGO was described detailedly in the supporting information) were also synthesized using the same procedure described above. The thicknesses of all the membranes were well controlled and the membranes with the comparatively similar thicknesses were used for the DMFC evaluation.

Scheme 2. The structure of the SPEEK/SHGO blend membrane and the operation principle of DMFCs. Characterizations. Fourier transform infrared (FTIR) spectroscopy was used to characterize the structures of the materials synthesized in this work. The FTIR spectra 5



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were obtained by a Perkin Elmer BX FTIR spectrometer in the wavenumber ranging from 4000 to 400 cm−1. A SEM microscope (Hitachi S5500) with a 30.0 kV operating voltage was employed to investigate the sample morphologies. High-resolution TEM images of the samples were collected on a JEM-2100F microscope with a 200 kV accelerating voltage. 1H NMR spectra of the samples were collected on a Varian Unity spectrometer at 500 MHz at 30 °C using tetramethylsilane and DMSO-d6 as the internal standard and the solvent, respectively. The microstructure of the membranes was analyzed by the small angle X-ray scattering (SAXS). The chemical structure of the materials was analyzed using X-ray photoelectron spectroscopy (XPS) analyses. 29-30 Raman spectra of the samples were recorded by a Renishaw inVita Raman microscope using an excitation energy of 2.41 eV. The laser power was controlled to be below 1 mW with the exposure time of ~ 5 s to avoid the damage to the samples by the laser-induced heating. A laser beam size of ~ 1 µm with ×100 objective lens was used. The water uptake (WU%), swelling ratio (SR%), ion exchange capacities (IEC), proton conductivity (σ), and methanol permeability (P) of the blend membranes were determined as reported elsewhere.10-11,29-30 Fuel cell evaluation. The membrane electrode assemblies (MEAs) for the fuel cell evaluation were fabricated through hot-pressing the catalyst coated anode and cathode gas diffusion layers (25 BC, SGL) onto both sides of the membrane according to the previous reported methods.29-30 The performance of the MEAs was evaluated in the single DMFC. The flow rate of the methanol solution was controlled to 1 mL cm-1. The humidified oxygen was fed into the cathode without the back pressure. The flow rate of the humidified oxygen was 200 mL min-1. The temperature of the whole cell was controlled to 65 °C. In all the cases, the data of the performance of the DMFCs were collected after 12 hs of the activation. Additionally, all the single cells were performed three times. The results here were their average value.

RESULTS AND DISCUSSION SPEEK. PEEK is an organic polymer consisting of a repeating unit as shown in Scheme 1a. The dissolution of the PEEK in concentrated sulfuric acid would lead to the formation of the SPEEK through the grafting of aromatic ring with sulfonic acid group between two ether linkages (Scheme 1a). The FTIR spectrum of the SPEEK in Figure 1a shows the peaks at the wavenumbers of 1024 and 1080 cm-1, corresponding to the symmetrical and asymmetric stretching vibrations of S–O and O=S=O in the sulfonic acid groups, respectively. This clearly demonstrates the sulfonation of the PEEK during its reaction with concentrated sulfuric acid. Indeed, the sulfonation of the PEEK could further be demonstrated by the splitting of aromatic C–C band in the PEEK. As displayed in Figure 1a, the SPEEK exhibits two aromatic C–C bands at 1494 and 1474 cm-1 in its the FTIR spectrum, which is different from that of the PEEK, where only one peak at 1489 cm-1 corresponding to the aromatic C–C band could be observed. We used the 1H NMR spectroscopic analysis to determine the sulfonation degree of SPEEK. As shown in Figure 1b, the 1H NMR spectrum of 6


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SPEEK exhibits a significant peak (HE) corresponding to protons in the sulfonic acid group. Since the intensity of this peak is closely correlated with the content of the sulfonic acid group in the SPEEK, the sulfonation degree of the SPEEK could be calculated using the following equation:

A E n = H (0 ≤ n ≤ 1) (12 − 2n) ∑ AH

(1)

where n and AH E represent the number of HE in each repeat unit and its peak area, respectively, and

∑A

H

corresponds to the peak area sum of all the other aromatic

protons. Based on the result shown in Figure 1b, the sulfonation degree of the SPEEK in this work is estimated to be 89%. 3200

(b)

(a)

2800

SPEEK

Transmittance / %

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


2400 -1

1494 cm 1474 cm -1

1024 cm

-1

-1

A'

A

H H

H

E

H

B'

B

H D

H

PEEK C

C'

H 1489 cm

1800

1080 cm

1600

H

1600 1200 800 400

-1

1400

2000

1200

1000

Wavenumber / cm

0

800

7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0

-1

ppm

Figure 1. (a) FTIR spectra of PEEK and SPEEK; (b) 1H NMR spectrum of SPEEK using DMSO-d6 as the solvent.

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Figure 2. TEM images of (a) GO, (b) SGO and (c) SHGO. Some in-plane holes in the SHGO are marked red. (d) SEM image of SHGO. Top view SEM images of (e) SPEEK/SHGO and (f) plain SPEEK membranes. The insets in e and f are their corresponding cross-section SEM images.

GO

O1s

SGO

1720 1050 1420 1620 1220

C1s

SHGO

Intensity / a.u.

1244 1065

Intensity / a.u.

SGO

(d)

(c)

(b)

S2s S2p

SHGO

Intensity / a.u.

(a) Transmittance / %

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S2p

GO 3300

1000 1500 2000 2500 3000 3500 -1

Wavelength / cm

800 1000 1200 1400 1600 1800 2000 -1

1000 800 600 400 200

Binding Energy (eV)

Raman Shift / cm

0

176 174 172 170 168 166 164 162

Binding Energy (eV)

Figure 3. (a) FTIR and (b) Raman spectra of GO, SGO, and SHGO. (c) XPS survey spectrum of the SHGOs, and (d) High resolution XPS spectrum of S 2p in the SHGOs. SHGO. SHGOs used in this work were prepared through a procedure involving the synthesis of the GOs, the sulfonation of the GOs through their reaction with a sulfonated diazonium salt, and the etching of the sulfonated GOs with concentrated HNO3 under the assistance of ultrasonication. The TEM image in Figure 2a shows 8


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that the GOs synthesized possesses a sheet-like structure with wrinkles and crumples observable on their sheet plates. The FTIR spectrum in Figure 3a indicates the presence of -OH, =C-O, -COOH, and phenol groups in the GOs, as demonstrated by the absorbance at 3310, 1725, 1622, 1420, 1220 and 1049 cm-1, assignable to the stretching vibration of O-H, the stretching vibration of C=O, the stretching vibration of sp2 hybridized C=C bond, the bending vibration of OH, the stretching vibration of C-O-C, and the stretching vibration of C-O, respectively.48 The existence of these oxygen containing groups renders the GOs highly reactive and capable of being sulfonated through their reaction with the sulfonated diazonium salt. The sulfonation would not change the morphological feature of the GOs. As shown in Figure 2b, the SGOs exhibit as crumpled papers, similar to the morphology of the pristine GOs shown in Figure 2a. The FTIR spectra in Figure 3a shows that the intensity of the peak corresponding to the stretching vibration of -OH is greatly reduced after the sulfonation. The appearance of new peaks at 1065 and 1244 cm-1, assignable to the stretching bands of the sulfonic acid groups, clearly demonstrates the sulfonation of the GOs. Figure 3b shows an increase in the ratio of ID/IG after the sulfonation (ID is the Raman intensity of the D band corresponding to the vibrations of disordered carbon at the edge and defect sites of the graphitic structure and IG is the Raman intensity of the G band corresponding to the in-plane bond stretching of sp2 carbon atoms), suggesting that the sulfonation produces more disordered carbon in the graphitic structure of the SGOs. To make them holey, the SGOs obtained were etched by the concentrated HNO3 under the assistance of ultrasonication. It has been reported that the cavitation bubbles formed during the ultrasonication would produce high strain rate upon implosion. This would lead to the generation of strong frictional forces between the moving liquid, resulting in the stretching of the sheet plates of the SGOs and the damage of the framework and the fracture of the bonds. The formation of the in-plane holes is therefore achieved by partially detaching and removing carbon atoms in the nanosheet of the SGOs due to the attack of the coordinatively unsaturated carbon atoms at the damaged and edge sites by the nitric acid molecules. Since the coordinatively unsaturated carbon atoms are distributed throughout the sheet plate of the SGO, the etching process occurs across the entire SGO nanosheets to produce abundant in-plane holes in the SHGOs (Figure 2c). The TEM image in Figure 2c shows that the etching with assistance of ultrasonication does not change the sheet-like morphology of the SGOs, but produces the in-plane holes in its sheet plates. Due to their holey structure, the SHGOs are highly pliable. The SEM image in Figure 2d reveals that the SHGO nanosheets are prone to stick together, forming a solid with a cotton-like appearance. This is different from the SGOs, whose aggregation leads to the formation of a solid with a paper-like structure, due to the rigidity of the relatively intact graphitic plane in the SGOs, as shown in Figure S1. Due to the formation of the holes, the SHGOs exhibit an increase in the ratio of ID/IG in comparison to the SGOs (Figure 3b), due to the production of more edge atoms, which increases the disorder of the graphitic structure. 9



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The FTIR spectra in Figure 3a show that the SHGOs exhibit the absorption peaks with the number and positions similar to those of the SGOs, indicating that the SHGOs possess the IR sensitive functionalities similar to those of the SGOs. Their sulfonated feature could be corroborated by the absorption peaks at 1065 and 1244 cm-1, corresponding to the stretching bands of the sulfonic acid groups. To further demonstrate the sulfonated structure of the SHGOs, their XPS spectra were recorded. As shown Figure 3c, the XPS survey spectrum of the SHGOs shows the peaks corresponding to S, C, and O. The presence of -SO3H groups could be clearly demonstrated by the high-resolution S2p peak at ~168.3 eV (Figure 3d). Based on the XPS survey spectrum in Figure 3c and Table S1, the estimated atomic percentage of S in the SHGOs is 2.53%, corresponding to 15.1 wt.% of -SO3H groups in the SHGOs. SPEEK/SHGO membranes. Due to their specific pliable structure consisting of the graphitic planes with oxygeneous and sulfonic groups functionalized, the SHGOs possess an amphiphilic nature, which makes them readily dispersible in the DMF solution of the SPEEK and compatible with the SPEEK matrix. This enables the fabrication of the SPEEK/SHGO membranes with high quality through the solution casting of the SPEEK and the SHGOs in DMF. The SEM image in Figure 2e shows that the SPEEK/SHGO membrane has a uniform and dense structure without cracks and pinholes observed on their surfaces, although its overall surface is slightly rougher compared to that of the plain SPEEK membrane (Figure 2f) due to the embedment of the SHGOs. The cross-sectional SEM image in the inset of Figure 2e indicates the homogeneous distribution of the SHGOs in the SPEEK matrix without significant aggregation. The formation of the uniform SPEEK/SHGO membranes mainly benefit from the good dispersibility of the SHGOs in the DMF solution and their high compatibility with the SPEEK matrix. The good dispersibility makes the SHGOs well exfoliated in the DMF solution to form a homogeneous solution, even during the solvent evaporation. This would facilitate the formation of the uniform SPEEK/SHGO membranes with the SHGOs distributed homogeneously due to the high compatibility and the strong interfacial interaction between SHGO and SPEEK. Worthnoting is that although the introduction of the GO and the SGO in the matrix of the SPEEK could also result in the formation of the membranes with relatively uniform structures (the SEM images of the SPEEK/GO and SPEEK/SGO membranes are given in Figure S2), their overall surfaces are much rougher compared to those of the SPEEK/SHGO membranes. This might be due to the reason that the increased pliability due to the in-plane holes makes the SHGOs more compatible with the SPEEK matrix. It can be further confirmed by the relatively uniform structure of SPEEK/HGO membranes as shown in Figure S2.

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ln σ

5.0 4.5 4.0 3.5

1

2

3

-1

Scattering vector / nm

4

3.0 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 1000/T / K-1

(c)

0.06 0.05 0.04

/ cm2

5.5

0.07

Nafion SPEEK/GO SPEEK/HGO SPEEK/SGO SPEEK/SHGO

(Cr,tVrL )/(Cd,0S)

SPEEK SPEEK/GO SPEEK/HGO SPEEK/SGO SPEEK/SHGO Nafion 112

(b)

2

SPEEK SPEEK/GO SPEEK/SGO SPEEK/HGO SPEEK/SHGO

/ cm

6.0

(a)

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


(Cr,tVrL)/(Cd,0S)

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0.20

SPEEK

0.15 0.10 0.05 3000

4000 5000 t/s

6000

0.03 0.02 0.01 0.00 2800

3500

4200

4900

5600

6300

t/s

Figure 4. (a) SAXS patterns of the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO, and SPEEK/SHGO membranes. (b) Arrhenius plot showing the temperature dependent-proton conductivity. (c) Plots of Cr,tVrL/(Cd,0S) vs. the diffusion time for the membranes. In both b and c, the dots are the results obtained experimentally. The straight lines are their fitting. To unravel the influence of the SHGOs on the microstructure of the SPEEK/SHGO membranes, the SAXS patterns of the plain SPEEK and SPEEK/SHGO membranes were recorded. Figure 4a shows that both the plain SPEEK and SPEEK/SHGO membranes exhibit a distinct ionomer peak in their corresponding SAXS pattern, suggesting the presence of the self-organized networks of ionic clusters in the membranes. The average ionic cluster dimension (daverage) in the membranes could be estimated based upon the following equation:

daverage =2π / q

(2)

where q is the scattering vector, equal to 4π/λsinθ (2θ is the scattering angle) and λ the X-ray wavelength. Based on the results in Figure 4a, the daverage estimated is 1.97 nm for the SPEEK membrane, which is decreased to 1.80 nm, when the SHGOs are incorporated. This indicates that the introduction of the SHGOs would decrease the daverage in the SPEEK membrane. In PEMs, the generation of ionic clusters could be attributed to the counterbalance between the electrostatic and the elastic free energies arising from the release of ion-dipole interactions and the backbone chain deformation, respectively.29-30 In this sense, the reduction in the daverage in the SPEEK membrane with the introduction of the SHGOs could be a result of the compatibilizing effect of the amphiphilic SHGOs on the hydrophilic and hydrophobic domains of the SPEEK membranes through the strong interactions between the conjugation of the SHGO graphitic plane and the hydrophobic backbone of the SPEEK and between the sulfonic acid groups of the SHGOs and the hydrophilic clusters of the SPEEK matrix because of the high surface area of the SHGO nanosheets and the good compatibility of the SHGOs with the SPEEK matrix. This is well consistent with the observation that the incorporation of the GO, the HGO, and the SGO also decreases the daverage in the membrane (the daverage in the SPEEK/GO, SPEEK/SGO, and SPEEK/HGO membranes are 1.90, 1.88, and 1.85, respectively). Since the GO, the HGO, and the SGO are all amphiphilic and have a graphitic plane, the strong interactions between the conjugation of the graphitic plane and the hydrophobic backbone of the SPEEK 11



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and between the oxygenous groups in the GO, the HGO, and the SGO and the hydrophilic clusters of the SPEEK matrix could also result in the reduction of the daverage in the membranes. Due to the presence of the holes in the graphitic plane which increases the pliability of the SHGOs and the HGO, the SPEEK/SHGO and SPEEK/HGO membrane exhibits the smaller daverage than the SPEEK/SGO and SPEEK/GO membranes, respectively. This suggests that the increased pliability could make embedment of the SHGOs and HGO in the membrane in a more compacted way. Table 1. Properties of the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO,

SPEEK/SHGO, and Nafion® 112 membranes Samples

a

Thickness (µm)

WU%

IEC

(%)

(mequiv -1

q )

σ

SR%

-1

(%)

o

o

25 C

65 C

o

(mS cm ) o

25 C

65 C

o

25 C

o

65 C

Ea (kJ mol-1)

Pa (×10-6 cm2 s-1）

SPEEK

71.7±3.6

1.805

39.1

120.4

8.95

61.97

28.4

56.9

19.83

52.7

SPEEK/GO

71.8±3.6

1.383

20.7

23.4

2.73

3.95

27.1

55.6

21.35

0.435

SPEEK/HGO

71.6±3.6

1.398

19.5

22.9

2.49

3.89

40.2

71.2

15.89

0.803

SPEEK/SGO

71.2±3.6

1.935

45.3

50.6

4.65

5.92

61.2

91.6

10.48

3.31

SPEEK/SHGO

72.3±3.6

1.941

46.7

54.8

4.55

5.98

90.5

135.9

6.34

3.83

Nafion® 112

53.2±0.5

0.94

33.2

38.7

6.92

15.7

88.1

126.3

7.36

4.75

P is the methanol permeability of the membranes at 25 oC.

Properties of the SPEEK/SHGO membranes. To demonstrate the usability of the SPEEK/SHGO membrane as the PEMs, its IEC, WU%, SR%, and proton conductivity were measured and the results are summarized in Table 1. For comparison, the IECs, WU%, SR%, and proton conductivity of the plain SPEEK, SPEEK/GO, SPEEK/HGO and SPEEK/SGO membranes and Nafion® 112 were also measured. Table 1 shows that although the plain SPEEK membrane has higher IEC and WU% relative to Nafion® 112, its proton conductivity is much lower than Nafion® 112. The highly sulfonated structure makes the plain SPEEK membrane readily swellable in the aqueous solution (as demonstrated by its higher SR% shown in Table 1), lowering its transport of proton. This has been considered as the main failure for the potential application of the plain SPEEK membrane as the PEM. The embedment of the SHGOs greatly increases the usability of the SPEEK membrane as the PEM. As shown in Table 1, the SPEEK/SHGO membrane exhibits higher IEC, WU%, and proton conductivity, but lower SR% than the plain SPEEK membrane. Indeed, Table 1 shows that the SPEEK/SHGO membranes could exhibit even higher IECs and proton conductivity, but lower SR% than Nafion® 112. This clearly suggests that the SPEEK/SHGO membrane could be used as the PEM for the DMFCs. We would attribute the presence of the SHGOs in the SPEEK matrix to the main reason leading to the lower SR%, but the higher IEC, WU% and proton conductivity of the SPEEK/SHGO membrane. Since the SHGOs consist of a graphitic structure, their embedment could therefore facilitate a strong interfacial interaction between SHGO and SPEEK, reducing the solubility of the SPEEK in the aqueous solution and lowering the SR% of the membrane. Additionally, the highly sulfonated structure of the SHGOs could also 12


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enhance the local density of the sulfonic acid groups in the membranes, improving the IEC, WU% and proton conductivity of the SPEEK/SHGO membrane. This is unlike the GO and the HGO, whose embedment decreases the local densities of the sulfonic acid groups in the membranes due to the lack of the –SO3H groups in the GO and HGO. Accordingly, the SPEEK/GO and SPEEK/HGO membranes exhibit relatively lower IECs and WU% than the plain SPEEK membranes (Table 1), although the proton conductivity of these membranes is slightly higher than that of the plain SPEEK membrane due to their reduced SR% resulting from the strong interfacial interactions between GO/HGO and SPEEK. Wothnoting is that the proton conductivity of the SPEEK/SHGO membrane is much higher than that of the SPEEK/SGO membrane, although both of them exhibit comparable IECs, WU%, and SR% since they are fabricated by the SPEEK membranes embedded with the same amount of the fillers consisting of the sulfonated graphitic planes. This observation makes us believe that the holes in the graphitic planes of the SHGOs play an important role in the higher proton conductivity of the SPEEK/SHGO membrane. These holes might be able to construct an addition path for the transport of protons through the graphitic planes of the SHGOs, enabling the ready transport of protons through the membrane. This could effectively avoid the case that the proton conductivity of the membranes is lowered by the blockage of the proton transport by the unholey graphitic planes with high aspect-ratios, especially that the unholey graphitic planes embedded are coincidently oriented in the way that is parallel to the plane of the membrane. Indeed, the improvement of the proton conductivity of the SPEEK membrane by the embedment of the holey graphitic materials could further be demonstrated by the higher proton conductivity of the SPEEK/HGO membrane than that of the SPEEK/GO membrane although both of them have comparable IECs, WU%, and SR%, as shown in Table 1. Activation energy for proton conduction (Ea) of the SPEEK/SHGO membrane. Ea is an important parameter of an ionomer membrane, which reflects the minimum energy required for proton transport. Since the proton transport through the ionomer membrane is a thermally activated process, its proton conductivity is a function of temperature following a simple Arrhenius-type law. Ea (kJ mol-1) of the membrane can then be extracted by linearly fitting the temperature-dependent proton conductivity using Eq. 3:

ln σ = ln σ 0 −

Ea RT

(3)

Figure 4b shows the temperature-dependent proton conductivity for the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO, SPEEK/SHGO, and Nafion® 112 membranes. Their corresponding Ea obtained using Eq. 3 are summarized in Table 1. Table 1 shows that Ea of the SPEEK/SHGO membrane is 6.34 kJ mol-1, which is much lower than that of the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO membranes. This indicates that the embedment of the SHGOs would greatly promote the transport of the protons through the membrane. The –SO3H groups and in-plane 13



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holes in the graphitic planes of the SHGOs could be attributed to the main reasons leading to the lower Ea of the SPEEK/SHGO membrane. This could be easily understood by the fact that the presence of the –SO3H groups in the graphitic planes of the SHGOs could increase the number of the proton exchange groups participating in the proton transport, while the in-plane holes could generate the additional path for the proton transport, both of which would lead to the lower Ea of the membrane. In addition, Table 1 also shows that Ea of the SPEEK/SHGO membrane is lower than that of Nafion®112 membrane. This observation further indicates that the SPEEK/SHGO membrane could be used as the PEM for DMFCs, since the low Ea would be beneficial to improve the energy utilization of DMFCs due to the loss of less energy by the ionic resistance of the membranes. Methanol permeability of the SPEEK/SHGO membrane. To further demonstrate that the SPEEK/SHGO membrane is usable as the PEM for the DMFCs, its methanol permeability was measured. It is well known that the methanol crossover through a PEM is a major issue causing the performance degradation of DMFCs, because the permeated methanol can deteriorate the activity of the cathodic catalyst towards oxygen reduction reaction,10-11,29-30 which would inevitably result in a performance reduction in DMFCs. Low methanol permeability is therefore required for the practical use of the PEM in DMFCs. In the case of methanol diffusion through a membrane with a thickness of L, the methanol concentration (Cr, t) in the receptor compartment is closely related with the diffusion time (t), as described in Eq. 4.

Vr L L2 Cr ,t = P(t − ) Cd ,0 S 6D

(4)

Where S is the effective area of the membrane (m2), Vr the volume of the receptor compartment (m3), Cd,0 the initial methanol concentration in the donor compartment, P the methanol permeability, and D the diffusion coefficient of methanol, respectively. This indicates that plotting Cr,tVrL/(Cd,0S) vs. t will yield a straight line with the slope corresponding to the methanol permeability. Figure 4c clearly shows that Cr,tVrL/(Cd,0S) is linearly dependent on the diffusion time. The methanol permeability of the membranes in Table 1 shows that the methanol permeability of the SPEEK/SHGO membranes is much lower than that of the plain SPEEK membrane, suggesting that the embedment of the SHGOs would greatly reduce the methanol crossover through the membranes. This is well consistent with the lower SR% of the SPEEK/SHGO membrane resulting from the strong interfacial interaction between SPEEK and SHGO (Table 1). Additionally, Table 1 also shows that the methanol permeability of the SPEEK/SHGO membrane is higher than that of the SPEEK/GO, SPEEK/HGO, and SPEEK/SGO membranes. This is reasonable, since the embedment of the SHGOs increases the local density of the –SO3H groups in the membrane and enhances its SR%, while the presence of the holes into the graphitic planes of the SHGOs provides an additional channel for the diffusion of methanol, both of which could improve the methanol permeability of the membrane. The most important is 14


Page 15 of 24

that the methanol permeability of the SPEEK/SHGO membrane is lower than that of Nafion®112 membrane, which further demonstrates the great potential of using it as the PEM for the DMFCs. Fuel cell performance evaluation. The potential use of the SPEEK/SHGO membrane as the PEM were evaluated by the polarization curves of a single DMFC equipped with a MEA comprising the SPEEK/SHGO membrane in 1 M methanol at 65 °C. Figure 5a shows that the cell with the SPEEK/SHGO membrane has an open circuit voltage comparable to that with Nafion® 112 due to the close proximity of the methanol permeability between the SPEEK/SHGO membrane and Nafion® 112. The higher power output of the cell with the SPEEK/SHGO membrane than that of the cell with Nafion® 112, as shown in Figure 5a, clearly corroborates the outperformance of the SPEEK/SHGO membrane over Nafion® 112. This is consistent with the higher proton conductivity, lower Ea, and comparable methanol permeability of the SPEEK/SHGO membrane in comparison to those of Nafion® 112. That is because the higher proton conductivity minimizes the ohmic resistance of the membrane, while the lower Ea decreases the energy loss by the ionic resistance of the membrane. Indeed, as shown in Figure 5a, the cell with the SPEEK/SHGO membrane can deliver an maximum power density of 80.3 mW cm-2 at the current density of 285 mA cm-2, which is ~25% higher than that of the cell with Nafion® 112 (the maximum power density of the cell with Nafion® 112 is 60.8 mW cm-2, which appears at the current density of ~205 mA cm-2). Additionally, Figure 5a also shows the outperformance of the cell with the SPEEK/SHGO membrane over those with the plain SPEEK, SPEEK/GO, SPEEK/HGO, and SPEEK/SGO membranes. This is in good agreement with the results in Table 1, which indicates that the SPEEK/SHGO membrane has higher proton conductivity and lower Ea.

60

0.4

40

0.2

20

0.0 0

100

200

300

150

(b) Temperature:

0.8

0 400

120

o

blue: 80 C o red: 65 C o black: 25 C

0.6

90

0.4

60

0.2

30

0.0

0 0

-2

100

200

300

400 -2

Current density / mA cm

Current density/ mA cm

15


500

-2

SPEEK/SHGO Nafion Power density / mW cm

0.6

80

Voltage / V

0.8

1.0

100

SPEEK/SHGO SPEEK/SGO SPEEK/HGO SPEEK/GO SPEEK Nafion 112

-2

(a)

Power density / mW cm

1.0

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



0.6

150

1.0

(c)

0.6

90

0.4

60

0.2

30

0

100

200

300

400

88.6% 82.2% 88.8%

0.4 Nafion SPEEK SPEEK/GO SPEEK/HGO SPEEK/SGO SPEEK/SHGO

0.3

0.2

0

0.0

(d)

0.5 Voltage / V

black: 1 M red: 2 M blue:5 M

Power density / mW cm

120

-2

SPEEK/SHGO Nafion

Methanol concentration:

0.8

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

Page 16 of 24

500

0

10

20

30

80.9%

76.5%

61.7%

40

50

60

Time / h

-2

Current density / mA cm

Figure 5. (a) Polarization curves of the DMFCs equipped with the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO, SPEEK/SHGO, and Nafion® 112, as PEMs. The methanol concentration and cell temperature are 1 M and 65 °C, respectively. (b) Polarization curves of the DMFCs equipped with the SPEEK/SHGO (circle) and Nafion® 112 (solid triangle) at 25oC (black), 65oC (red) and 80oC (blue). (c) Methanol concentration dependent polarization curves of the DMFCs equipped with the SPEEK/SHGO membrane (circle) and Nafion® 112 (solid triangle) at 65 °C. (d) Stability of the DMFCs with the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO, SPEEK/SHGO, and Nafion® 112 under the current density 50 mA cm−2. The methanol concentration and cell temperature are 1 M and 65 °C, respectively. To further demonstrate the outperformance of the SPEEK/SHGO membrane, the influences of the temperature and the concentration of methanol on the performance of the cells with the SPEEK/SHGO were investigated. Previous work has suggested that the improvement of the cell operation temperature would increase the proton conductivity of the membrane and enhance the catalytic activity and durability of the electrode catalyst toward the methanol poisoning.29-30 Figure 5b shows an improvement of the power output from the cell with the SPEEK/SHGO membrane with increase of the operation temperature. The most interesting is the cell with the SPEEK/SHGO membrane always exhibits higher power output than that with Nafion® 112 at the temperature range present in this work. The increase of methanol concentration increases the number of methanol molecules available for the electrochemical reactions at the low methanol concentration, which could improve the power output of the fuel cells. At the high methanol concentration, however, the enhanced methanol crossover would decrease the catalytic activity of the cathodic catalyst, degrading the performance of the fuel cells. Figure 5c shows an initial increase and a subsequent decrease in the power output of the cell with the SPEEK/SHGO membrane with increase of the methanol concentration. The similar result could also be observed in the cell with Nafion® 112, as shown in Figure 5c. Additionally, Figure 5c shows that the performance of the cell with the SPEEK/SHGO membrane is always higher than that of the cell with Nafion® 112. This, along with the higher performance of the cell with the SPEEK/SHGO membrane at the various temperature (shown in Figure 5b), further suggests that the SPEEK/SHGO membrane could be used as the PEM for the DMFCs. 16


Page 17 of 24

The impedance spectra were used to better understand the electrochemical behavior of the cells with the SPEEK/SHGO membrane and Nafion® 112. The Nyquist plots in Figure 6 show that the cell with the SPEEK/SHGO membrane exhibits a resistance (~7.6 mΩ) lower than that of the cell with Nafion® 112 (~13.6 mΩ). This suggests that the SPEEK/SHGO membrane offers less electrolyte resistance than Nafion® 112. In addition, Figure 6 also shows that the diameter of the semicircle in its Nyquist plot of the cell with the SPEEK/SHGO is smaller than that of the cell with Nafion® 112, indicating that the SPEEK/SHGO membrane possesses a lower charge transfer resistance than Nafion® 112. These results are in good accordance with the observations in Table 1, which has demonstrated that the SPEEK/SHGO membrane exhibits higher proton conductivity, lower Ea, and comparable methanol permeability than Nafion® 112. 0.05

Nafion SPEEK/SHGO

0.04

-Z'' / Ω

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


0.03

0.02

0.01

0.00 0.00

0.02

0.04

0.06

0.08

0.10

0.12

Z' / Ω

Figure 6. Nyquist impedance (Z) plots of the DMFCs with the SPEEK/SHGO membrane and Nafion® 112. The cell with the SPEEK/SHGO membrane was then subjected to the durability test to examine its long-term stability. Figure 5d shows that the cell with the SPEEK/SHGO membrane could output a stable voltage with ~88.6% of the retention after 60 h of operation at a constant current density. The voltage retention of the cell with the SPEEK/SHGO membrane is comparable to that of the cell with Nafion ® 112, but is much higher than those of the cells with the plain SPEEK, SPEEK/GO, SPEEK/HGO, and SPEEK/SGO membranes. This observation suggests that the SPEEK/SHGO membrane has a stability comparable to Nafion® 112, but higher than the plain SPEEK, SPEEK/GO, SPEEK/HGO, and SPEEK/SGO membranes. The higher stability of the SPEEK/SHGO membrane could be ascribed to the compatibilizing effect of the amphiphilic SHGOs on the hydrophilic and hydrophobic domains of the SPEEK membranes through the strong interactions between the conjugation of the SHGO graphitic plane and the hydrophobic backbone of the SPEEK and between the sulfonic acid groups of the SHGOs and the hydrophilic clusters of the SPEEK matrix. The holes in the graphitic plane which facilitates the embedment of the SHGOs in the SPEEK membranes in a more compacted way also make a big contribution on the higher stability of the SPEEK/SHGO membrane. As demonstrated in Figure 5d, the stability of the SPEEK/SHGO and SPEEK/HGO 17



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

membranes is higher than that of the SPEEK/SGO and SPEEK/GO membranes, respectively. Most interestingly, the output voltage of the cell with the SPEEK/SHGO membrane is always higher than those of the cell with the other membranes as shown in Figure 5d. This, in combination with its higher stability, further demonstrates the outperformance of the SPEEK/SHGO membrane over the plain SPEEK, SPEEK/GO, SPEEK/HGO, SPEEK/SGO, and Nafion® 112. The higher output voltage of the cell with the SPEEK/SHGO membrane is also ascribable to the higher proton conductivity, lower Ea, and lower permeability of the SPEEK/SHGO membrane, since they reduce the ohmic resistance of the membrane, the energy loss induced by the ionic resistance, and the methanol crossover, respectively. These results therefore further demonstrate the potential use of the SPEEK/SHGO membrane as the PEM for the DMFCs. CONCLUSIONS In summary, the SHGOs synthesized by the etching of sulfonated GOs with concentrated HNO3 under the assistance of ultrasonication have been used as the filler for the SPEEK membrane. The obtained SPEEK/SHGO membrane is found to exhibit higher IEC and proton conductivity, lower SR% and Ea, and comparable methanol permeability, when compared Nafion® 112, which make it highly promising as the PEM for the DMFCs. Their specific pliable structure which consists of the graphitic planes with oxygeneous and sulfonic groups functionalized could be attributed to the main reason leading to the high performance of the SPEEK/SHGO membranes. That is because the sulfonated structure could increase the local density of the –SO3H groups in the membrane, which increases the IEC of the membrane and its proton conductivity as well. The proton conductivity of the membrane could be further enhanced by the presence of the holes in the graphitic planes of the SHGOs, since it provides an additional channel for the transport of the protons. Additionally, the strong interfacial interaction between the graphitic plane of the SHGOs and the SPEEK matrix could decrease the SR% and methanol permeability of the membrane and increase its stability. When used as the PEM in the DMFCs, the cell with the SPEEK/SHGO membrane could exhibit greatly improved performance than that with Nafion® 112. This, along with its comparable stability to that with Nafion® 112, strongly suggests that the SPEEK/SHGO membrane could be used as the PEM for the DMFCs. The results shown here is of great interest since it provides a new and simple method to obtain the DMFCs with high performance. ASSOCIATED CONTENT Supporting Information This file provides more detailed information regarding certain Chemicals and reagents; the preparation procedure of the HGO; SEM image of SGO; Top view SEM images and the corresponding cross-section SEM images of SPEEK/GO, SPEEK/SGO, and SPEEK/HGO membranes; relative atomic percentages of C, O and S in GO and SHGO analyzed based on the XPS spectra. These materials are available free of 18


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charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author ∗ E-mail: [email protected] or [email protected]; [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors acknowledge the financial supports from the Chinese National Natural Science Foundation (No. U1532139, and 11474101), the Guangdong Innovative and Entepreneurial Research Team Program (No. 2014ZT05N200), the Zhejiang Provincial Public Welfare Technology Application Research Project (2015C31151), the Zhejiang Provincial Natural Science Foundation (No. LY14B030001), and the “Outstanding Talent and Team Plans Program” of South China University of Technology. REFERENCES 1. Rhee, C. H.; Kim, H. K.; Chang, H.; Lee, J. S. Nafion/Sulfonated Montmorillonite Composite: A New Concept Electrolyte Membrane for Direct Methanol Fuel Cells. Chem. Mater. 2005, 17 (7), 1691-1697. 2. Bauer, F.; Willert-Porada, M. Characterisation of Zirconium and Titanium Phosphates and Direct Methanol Fuel Cell (DMFC) Performance of Functionally Graded Nafion(R) Composite Membranes Prepared out of Them. J. Power Sources 2005, 145 (2), 101-107. 3. 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. 4. Jiang, Z.; Jiang, Z.-J. Plasma Techniques for the Fabrication of Polymer Electrolyte Membranes for Fuel Cells. J. Membrane Sci. 2014, 456, 85-106. 5. Jiang, Z.; Jiang, Z.-J. Plasma-Polymerized Membranes with High Proton 19



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Conductivity for a Micro Semi-Passive Direct Methanol Fuel Cell. Plasma Process. Polym. 2016, 13 (1), 105-115. 6. Mohtar, S. S.; Ismail, A. F.; Matsuura, T. Preparation and Characterization of SPEEK/MMT-STA Composite Membrane for DMFC Application. J. Membrane Sci. 2011, 371 (1-2), 10-19. 7. Nunes, S. P.; Ruffmann, B.; Rikowski, E.; Vetter, S.; Richau, K. Inorganic Modification of Proton Conductive Polymer Membranes for Direct Methanol Fuel Cells. J. Membrane Sci. 2002, 203 (1–2), 215-225. 8. Lee, K. H.; Cho, D. H.; Kim, Y. M.; Moon, S. J.; Seong, J. G.; Shin, D. W.; Sohn, J.-Y.; Kim, J. F.; Lee, Y. M. Highly Conductive and Durable Poly(Arylene Ether Sulfone) Anion Exchange Membrane with End-Group Cross-Linking. Energy Environ. Sci. 2017, 10, 275-285. 9. Jiang, Z.; Jiang, Z.-J. Synthesis and Optimization of Proton Exchange Membranes by A Pulsed Plasma Enhanced Chemical Vapor Deposition Technique. Int. J. Hydrogen Energy 2012, 37 (15), 11276-11289. 10. Jiang, Z.; Jiang, Z.-j. Preparation of Proton Exchange Membranes with High Performance by A Pulsed Plasma Enhanced Chemical Vapor Deposition Technique (PPECVD). RSC Adv. 2012, 2 (7), 2743-2747. 11. Jiang, Z.; Jiang, Z.-j.; Meng, Y. Optimization and Synthesis of Plasma Polymerized Proton Exchange Membranes for Direct Methanol Fuel Cells. J. Membrane Sci. 2011, 372 (1-2), 303-313. 12. Bébin, P.; Caravanier, M.; Galiano, H. Nafion®/clay-SO3H Membrane for Proton Exchange Membrane Fuel Cell Application. J. Membrane Sci. 2006, 278 (1-2), 35-42. 13. Hsu, W. Y.; Gierke, T. D. Ion Transport and Clustering in Nafion Perfluorinated Membranes. J.Membrane Sci. 1983, 13 (3), 307-326. 14. 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. Energ. Fuel. 2016, 30 (1), 725-734. 15. Tsai, J.-C.; Cheng, H.-P.; Kuo, J.-F.; Huang, Y.-H.; Chen, C.-Y. Blended Nafion®/SPEEK Direct Methanol Fuel Cell Membranes for Reduced Methanol Permeability. J. Power Sources 2009, 189 (2), 958-965. 16. Choi, B. G.; Hong, J.; Park, Y. C.; Jung, D. H.; Hong, W. H.; Hammond, P. T.; Park, H. Innovative Polymer Nanocomposite Electrolytes: Nanoscale Manipulation of Ion Channels by Functionalized Graphenes. ACS Nano 2011, 5 (6), 5167-5174. 17. Jang, S. S.; Molinero, V.; Çagin, T.; Goddard Iii, W. A. Effect of Monomeric Sequence on Nanostructure and Water Dynamics In Nafion 117. Solid State Ionics 2004, 175 (1–4), 805-808. 18. Escudero-Cid, R.; Montiel, M.; Sotomayor, L.; Loureiro, B.; Fatás, E.; Ocón, P. Evaluation of Polyaniline-Nafion® Composite Membranes for Direct Methanol Fuel Cells Durability Tests. Int. J. Hydrogen Energy 2015, 40 (25), 8182-8192. 19. Lee, D. C.; Yang, H. N.; Park, S. H.; Kim, W. J. Nafion/Graphene Oxide Composite Membranes for Low Humidifying Polymer Electrolyte Membrane Fuel Cell. J. Membrane Sci. 2014, 452, 20-28. 20. Yan, X. H.; Wu, R.; Xu, J. B.; Luo, Z.; Zhao, T. S. A Monolayer Graphene – 20


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Sulfonated Holey Graphene Oxide (SHGO) - ACS Publications

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