Graphene oxide based nanohybrid proton exchange ...

Advances in Colloid and Interface Science 240 (2017) 15–30

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Graphene oxide based nanohybrid proton exchange membranes for fuel cell applications: An overview Ravi P. Pandey a,b, Geetanjali Shukla a,b, Murli Manohar a,b, Vinod K. Shahi a,b,⁎ a Electro-Membrane Processes Division, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Gijubhai Badheka Marg, Bhavnagar 364 002, Gujarat, India b Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Gijubhai Badheka Marg, Bhavnagar 364 002, Gujarat, India

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Available online 15 December 2016 Keywords: Graphene oxide Nanohybrid proton exchange membrane Fuel cell Ion-exchange membrane

a b s t r a c t In the context of many applications, such as polymer composites, energy-related materials, sensors, ‘paper’-like materials, field-effect transistors (FET), and biomedical applications, chemically modified graphene was broadly studied during the last decade, due to its excellent electrical, mechanical, and thermal properties. The presence of reactive oxygen functional groups in the grapheme oxide (GO) responsible for chemical functionalization makes it a good candidate for diversified applications. The main objectives for developing a GO based nanohybrid proton exchange membrane (PEM) include: improved self-humidification (water retention ability), reduced fuel crossover (electro-osmotic drag), improved stabilities (mechanical, thermal, and chemical), enhanced proton conductivity, and processability for the preparation of membrane-electrode assembly. Research carried on this topic may be divided into protocols for covalent grafting of functional groups on GO matrix, preparation of free-standing PEM or choice of suitable polymer matrix, covalent or hydrogen bonding between GO and polymer matrix etc. Herein, we present a brief literature survey on GO based nano-hybrid PEM for fuel cell applications. Different protocols were adopted to produce functionalized GO based materials and prepare their free-standing film or disperse these materials in various polymer matrices with suitable interactions. This review article critically discussed the suitability of these PEMs for fuel cell applications in terms of the dependency of the intrinsic properties of nanohybrid PEMs. Potential applications of these nanohybrid PEMs, and current challenges are also provided along with future guidelines for developing GO based nanohybrid PEMs as promising materials for fuel cell applications. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . Different types of graphene oxide based PEMs . . . . . . 2.1. Free-standing GO based PEMs . . . . . . . . . . 2.2. GO based nanohybrid PEMs . . . . . . . . . . . 2.2.1. Perfluorosulfonic acid-GO nanohybrid PEMs

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Abbreviations: APTES, 3-aminopropyl-triethoxysilane; CS, chitosan; DMF, dimethyl formamide; DMFC, direct methanol fuel cell; DGO, polydopamine modified graphene oxide; F-GO, functionalized graphene oxide; GO, graphene oxide; GtO, graphite oxide; iGtO, isocyanate modified GtO; IEMs, ion-exchange membranes; IEC, ion-exchange capacity; LbL, layer-by-layer; MEA, membrane electrode assembly; MGO, modified GO; MPS, 3-(methacryloxy) propyltrimethoxysilane; MPTMS, 3-mercaptopropyl trimethoxysilane; NMP, N-methyl-2-pyrrolidinone; OCV, open circuit voltages; OGO, ozonated GO; PEM, proton exchange membrane; PDDA, poly(diallyldimethylammonium chloride); PDHC, 1,4-phenyl diamine hydrochloride; PEMFC, proton exchange membrane fuel cell; PW-mGO, phosphotungstic acid coupled graphene oxide; PEEKs, poly-(ether ether ketone)s; PAESs, poly(arylene ether sulfone)s; PPA, polyphosphoric acid; PBI, polybenzimidazoles; PGO, phosphorylated GO; PVA, poly(vinyl alcohol); PVC, poly(vinyl chloride); RGO, reduced graphene oxide; SGO, sulfonated GO; SGON, sulfonated GO/Nafion nanohybrid; SPI, sulfonated polyimide; S-GO–SiO2, sulfonated graphene oxide–silica; SDBS, sodium dodecylbenzene sulfonate; SGtO, sulfonated GtO; SDBS, sodium dodecylbenzenesulfonate; SSi-GO, sulfonated organosilane functionalized graphene oxide; SCS, sulfonated chitosan; SDBS-HGO, sodium dodecylbenzenesulfonate adsorbed holey graphene oxide; SPB-FGO, SPES, sulfonated poly(ether sulfone) sulfonated polymer brush functionalized graphene oxide; THF, tetrahydrofuran; XRD, X-ray diffraction; ZC-GO, Zwitterion-coated GO. ⁎ Corresponding author at: Electro-Membrane Processes Division, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Gijubhai Badheka Marg, Bhavnagar 364 002, Gujarat, India. E-mail addresses: [email protected], [email protected] (V.K. Shahi).

http://dx.doi.org/10.1016/j.cis.2016.12.003 0001-8686/© 2016 Elsevier B.V. All rights reserved.

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R.P. Pandey et al. / Advances in Colloid and Interface Science 240 (2017) 15–30

2.2.2. Non-fluorinated nanohybrid PEMs . 3. Summary and future perspective . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Carbon allotropes have attracted significant attention for use in electrochemical applications, due to their enormous abundance, easy processability, excellent stability, and environmental adaptability [1]. Chemical (acid, base) and thermal stabilities of carbon allotropes are attractive features for their applications as electrode materials. Graphene is an exciting carbon allotrope with sp2-hybridized two-dimensional monolayer lattice [2]. A single-atom-thick carbon sheet with honeycomb arrangement showed improved stiffness and thermal and electrical conduction. Due to excellent thermal, mechanical, and electrical properties, graphene has been considered as a promising nanostructured carbon allotrope in the field of quantum mechanics and fundamental physics, materials science and condensed matter physics [3–7]. Further, the high mobility of charge carriers (~ 200,000 cm2 v−1 s−1) [8–10], surface area (~ 2600 m2 g− 1) [1], Young's modulus (~1100 GPa) [11], thermal conductivity (~5000 Wm−1 K−1) [12], optical transmittance (∼97.7%), and electrical conductivity make graphene an attractive material for electrochemical applications. In comparison with graphite and carbon nanotubes (1300 m2 g−1), graphene shows a high surface area (~10 m2 g−1) [13], and is considered as a basic building block for graphitic materials [2]. A wide range of nanostructured materials based on modified graphene [10,14,15]were developed for diversified applications such as: polymer composites [13,16], energy-related processes [17], sensors and probes [18,19], ‘paper’-like materials [14], field-effect transistors (FET) [20], photo-electronics [21,22], electro-mechanical systems [23],

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hydrogen storage [24], and biomedical applications [25]. Further, inexpensive manufacturing sources (graphite) and the excellent properties of graphene encouraged considerable interest in developing cost-effective, high-performance polymer composites materials [26]. Graphene oxide (GO) exfoliated from graphite oxide (GtO), was prepared by strong oxidation using a strong mineral acid (conc. H2SO4) in the presence of an oxidizing agent (KMnO4) (Hummers method), or by adding “potash of chlorate” (potassium chlorate; KClO3) to a slurry of graphite in fuming nitric acid (HNO3) as in the Staudenmaier or Brodie methods Fig. 1 [27–29]. GO contains closely located oxygenated functional groups (\\O\\, \\OH, and\\COOH) and a two-dimensional flat layer suitable for the formation of hydrogen-bonded proton conduction channels [30]. GO showed a large surface area and the presence of hydrophilic functional groups, providing a facile environment for proton conduction, by a “hopping” mechanism (improved water-retention, necessary for proton conduction in non-humid conditions). GO has been considered as an attractive organic filler for a polymer electrolyte membrane (PEM), and improved proton conductivity and water retention ability [31,32]. Further oxidation of GO ultimately disrupts the delocalized electronic structure of graphite and provides an electron-insulating environment [30]. The GO is well dispersed in several polar and nonpolar solvents, including water that make easy processing [33,34]. GO contains a high concentration of epoxide and hydroxyl functional groups on the basal plane, along with carboxylic acid groups around the periphery of the sheets (Fig. 2) [34,35]. GtO shows long-drawn-out interlayer distances (between the GO sheets) in comparison with graphite. Interlayer spacing depends on the humidity, and increases with hydrophilicity, due to the intercalation of water molecules between the sheets [14,36].

2. Different types of graphene oxide based PEMs

Fig. 1. Outlined oxidization/intercalation process for the preparation of GO.

During recent years, nanostructured PEMs received great attention due to their unique structural features, improved stabilities, and versatile processing techniques. For developing nanostructured materials, generally elastomeric and flexible polymer matrices were considered, for achieving good elasticity, strength, flexibility, controlled surface and bulk properties. Among the structural polymeric matrices, the advanced nanohybrids, elastomers, thermoplastics, epoxy, block copolymers, and hydro/aerogels are widely used due to their unique physical and chemical properties. For significant improvement in the performance of PEMs (water retention capacity, ionic conductivity, fuel cross-over and stability) at high operating temperature, various approaches were endeavoured. Extensive efforts were rendered to develop the GO based nanohybrid PEMs with a better understanding of molecular-level chemistry, morphology, transport behaviour, and polymer degradation. Incorporation of GO in the PEM matrix provides a more facile environment for proton conduction, by a “hopping” mechanism, and improved water-retention properties (necessary for proton conduction in non-humid conditions) due to the large surface area and the presence of hydrophilic functional groups [31,32]. These properties of GO could be further improved by chemical grafting of charged functional groups. Improved mechanical stability of modified GO has also been achieved by developing nanohybrid membranes. Herein, we provide a broad literature survey on recent developments of modified GO based nanohybrid PEMs for fuel cell applications. Novel synthetic routes for nanohybrid membrane forming materials


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Fig. 2. Schematic chemical structure of graphite oxide with the structural difference between layered and exfoliated graphene oxide (GO) platelets [35]. Copyright © 2010, Elsevier Ltd.

with ion conducting functional groups were discussed in light of recent innovations and breakthroughs in the field of membrane science and technologies along with a need for frontier research to achieve the important objective of developing high performance PEMs for fuel cells and other diversified electro-membrane applications. 2.1. Free-standing GO based PEMs Scott and co-workers prepared a GtO paper electrolyte from natural flake graphite by a modified Hummers method for direct methanol fuel cells [37]. A free-standing GtO paper electrolyte was fabricated by filtration of a colloidal solution using a cellulose acetate membrane filter (47 mm in diameter and 0.2 μm pore size). The XRD spectra of the GtO paper electrolyte showed a peak at 26.45° corresponding to the

interplanar distance between the different graphene layers, whereas chemically oxidized graphite (GtO) showed a peak at 11.26°, due to the grafting of different functional groups (epoxide, hydroxyl, carboxyl, etc.) (Fig.3) [37]. The presence of oxygen containing moieties increased the interplanar distance between the sheets. A cross-sectional scanning electron microscopy (SEM) image of the GtO paper electrolyte showed a dense stacking of GtO with 100–200 nm thick layers (Fig. 4). The reported GtO paper electrolyte exhibited 4.1–8.2 × 10−2 S cm−1 proton conductivity within 25–90 °C, and membrane electrode assembly (MEA) showed 8 mW cm− 2 peak power density against 35 mA cm− 2 at 60 °C, corresponding to 18.2 × 10−6 cm2 s−1 fuel crossover [37]. To circumvent the high fuel crossover and mechanical stability, a free standing sulfonated GtO (SGtO) PEM was prepared via surface modification with aryl diazonium salt. The reported PEM exhibited 0.04 S cm−1

Fig. 3. XRD spectra of graphite, and graphite oxide [37].

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Fig. 4. SEM cross-section image of GtO paper electrolyte [37].

(in-plane) and 0.012 S cm−1 (through-plane) proton conductivity at 303 °K, and 1.1 meq g−1 ion-exchange capacity (IEC). Further, about 7.2 kJ mol−1 (in-plane) and 16.6 kJ mol−1 (through plane) activation energy suggested a suitable proton conducting membrane in plane rather than through plane. The PEMFC performance revealed a 113 mW cm−2 maximum power density of 0.39 V. But high fuel crossover across the PEM was the serious disadvantage of the SGtO based PEM and affected the long-term durability for practical applications [38]. In-plane ionic conductivity of pristine GO nanosheets was compared with bulk GtO and graphene oxide/proton based nanohybrid (GO-H) PEMs. The proton conductivity (κm) of a GO nanosheet (κm = ~ 10− 2 S cm− 1) was amazingly high in comparison with GtO (κm ∼ 10−4 S cm−1) and GO-H (κm = ∼10−5 S cm−1) at 100% humidity. The high conductivity of GO reveals a diversified possibility for perfect

two-dimensional electrochemical devices from GO-based proton-conductive materials such as fuel cells and sensor applications [30]. A freestanding ozonated GO (OGO) film was synthesized via chemical modification of GO. A prepared membrane showed improved proton conductivity at 100% relative humidity (RH). This was attributed to the improved number of oxygen containing functional groups responsible for the formation of a facile pathway for proton transport. The long term durability test (100 h at 35 °C) also revealed superior performance of the OGO membrane in comparison with a pristine GO membrane [39]. Chen and co-workers developed the sodium dodecylbenzenesulfonate adsorbed holey graphene oxide (SDBS-HGO) based PEMs for air-breathing direct methanol fuel cell applications [40]. The SDBS-HGO was prepared via a two-step process: fabrication of holey GO (HGO) through the ultra-sonication of GO in the presence of conc. HNO3; and adsorption of sodium dodecylbenzenesulfonate (SDBS) onto the HGO surface. The SDBS-HGO based PEM was fabricated via the filtration of the SDBS-HGO colloidal solution through a membrane filter (Fig. 5). The HGO facilitates easy transport of protons across the GO nanosheet. The SDBS-HGO paper electrolyte exhibited 1.84 meq g−1 IEC and 9.18 S cm−1 proton conductivity [40]. 2.2. GO based nanohybrid PEMs 2.2.1. Perfluorosulfonic acid-GO nanohybrid PEMs The state-of-the-art hydrated perfluorosulfonic acid membrane (Nafion; registered trademark of DuPont Co.) was widely used as PEM materials and served as benchmarks because of excellent mechanical, thermal and chemical stabilities, as well as its relatively high proton conductivity [41–43]. The structural features of the Nafion membrane consist of a hydrophobic poly(tetra-fluoroethylene) main backbone which is responsible for mechanical strength and perflourinated pendant side chains with hydrophilic terminated sulfonic acid groups responsible for superior conductivity [44]. Sulfonic acid groups formed an intermediate region of inverted hydrophilic domains with a size of ∼ 40 Å [45,46]. The proton conductivity of Nafion is very sensitive to the water retention capacity, as it requires severe humidification above 80 °C due to water evaporation from the membrane structure.

Fig. 5. Schematic illustration of the synthetic procedure of the SDBS-HGO paper [40]. Copyright © 2014, Royal Society of Chemistry.


Due to inherent water properties (freezing or boiling), the performance of the Nafion membrane is not satisfactory below 0 °C and above 100 °C [47,48]. Further, Nafion also suffers from other drawbacks, such as high methanol crossover, deterioration in stability (mechanical, chemical and thermal) at elevated temperatures, and a complicated expensive manufacturing process [44]. To overcome these problems, considerable research efforts were devoted to alter and improve the physical and chemical properties of the Nafion membrane. Modification of Nafion membranes were also explored to improve the IEC, and thus proton conductivity, by chemical grafting of ionic groups. Nevertheless, Nafion with high IEC exhibited poor mechanical properties because of swelling. Incorporation of graphene oxide/inorganic materials, such as silica [49– 52], zirconia [53,54], titania [55,56], zeolite [57], clay [58], and montmorillonite [59], and various polymers, such as chitosan [60], poly(vinylidene fluoride) [61], and polybenzimidazole [62], was also attempted to improve the humidity of Nafion membranes. Hygroscopic inorganic fillers reduced methanol permeation and improved water retention at high temperature and low humid conditions, which facilitate the proton conduction process across the PEM [49,50,53]. Kumar et al. reported GtO/Nafion nanohybrid PEMs with varied GtO contents and studied the effect of GtO on the fundamental properties of a Nafion membrane as well as the overall PEMFC performance [63]. These PEMs showed improved proton conductivity and mechanical stability. The proton conductivity of the GtO (4 wt%)/Nafion nanohybrid membrane (κm = 0.078 S cm−1) was comparatively higher than Nafion 212 (κm = 0.068 S cm−1) and Nafion recast (κm = 0.043 S cm−1) membranes at 30 °C and 100% humidity. Polarization and power density

Fig. 6. PEMFC performance curves for a H2 (humidified)/O2 (dry) fuel cell: (a) at 60 °C under 100% RH and (b) at 100 °C and 25% RH [63]. Copyright © 2012, Royal Society of Chemistry.

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curves for the GtO (4%)/Nafion nanohybrid, recast Nafion and Nafion 212 membranes have been depicted in Fig. 6a,b [63]. The maximum power density for a GtO (4 wt%)/Nafion nanohybrid PEM (415 mW cm−2) at 60 °C improved to 212 mW cm−2 at 100 °C. These values are better than those for Nafion recast and Nafion 212 under 100 and 25% relative humidity [63]. In a new strategy for the preparation of GO/Nafion nanohybrid PEMs, GO sheets were rolled up by evaporation in the mixed solvent (H2O/THF, or H2O/NMP, or H2O/DMF) to a remote aluminum foil surface. GO/Nafion nanohybrid membranes were successfully prepared by casting the Nafion solution on the surface of the hole-like self-assembled rolled up GO layer [64]. These nanohybrid PEMs exhibited excellent proton conductivity in comparison with the recast Nafion membrane, especially under low-humidity conditions. Reduced graphene oxide (RGO)-Nafion nanohybrid PEMs were also prepared via a solution casting method followed by a hot press thermal reduction process. These PEMs showed 30 times higher proton conductivity (2.20 × 10−1 S cm−1) in comparison with the recast Nafion membrane (7.30 × 10−3) [65]. The GO-laminated Nafion 115 nanohybrid PEM was prepared for DMFC application. The laminated membrane was fabricated via transfer printing followed by hot-pressing on ordered GO paper with a Nafion 115 membrane. A GO-laminated Nafion 115 membrane exhibited about 70% lower methanol permeability, and 40% higher selectivity (i.e. ratio of proton conductivity to methanol permeability) in comparison with pristine Nafion 115 [66]. The DMFC results for the GO-laminated Nafion 115 membrane revealed 100% higher peak power density in comparison with the pristine Nafion 115 membrane under 8.0 M methanol feed. Lee et al. prepared Nafion/GO nanohybrid PEMs with varied GO contents (0.5–4.5 wt%) for a low humidified fuel cell. These PEMs showed significant improvement in single cell PEMFC performance (H2/O2 fuel cell) under various RHs such as 100%, 60% and 40%. At 0.6 V applied voltage, Nafion/GO-0.5, Nafion/GO-3, and Nafion/GO-4.5 membranes showed 0.802, 1.27, and 0.827 A cm−2 current densities under RH 100% respectively, in comparison with 0.435 A cm−2 for pristine Nafion membrane [67]. Lai and co-workers developed well-aligned bilayer Nafion/GO nanohybrid membranes for direct liquid fuel cells using a spin coating method. The spin-coated Nafion/GO nanohybrid with 0.067% GO loading exhibited low fuel permeability in comparison with pristine Nafion. Further, reported PEM showed double peak power densities (113, 35, and 163 mW cm−2) in direct methanol, ethanol, and formic acid fuel cells respectively, compared with pristine Nafion at 80 °C. However, negative effects were observed on the water uptake, IEC and proton conductivity of spin-coated Nafion/GO nanohybrids [68]. Sulfonated GO (SGO)/Nafion nanohybrid (SGON) PEMs were prepared by innovative chemical strategy (Fig. 7). Sulfonation of GO was carried out by microwave-assistance using conc. sulfuric acid. The SGON membranes were prepared by solution casting and exhibited physically and chemically modulated ion-conducting channels. SGOs provide nanoscale manipulation of ion-conducting channels and controlled the state of water in the PEM matrix [69]. The SGON nanohybrid PEM exhibited about two-fold high proton conductivity, with reduced methanol permeability (about 35%) in comparison with a pristine Nafion membrane. Further, enhanced proton conductivity and low methanol permeability of SGON resulted in high selectivity and good fuel cell performance. Single cell DMFC performance revealed 132 mW cm−2 maximum power density for the SGON membrane in comparison with Nafion 112 (101 mW cm−2) and GO/Nafion (GON) membranes (120 mW cm−2) at 60 °C [69]. An organo-modified GO layered nanostructured material with numerous hydrophilic functional groups (\\NH2, \\OH, \\SO3H) was reported for the fabrication of graphene-based Nafion nanohybrid membranes [70]. The reported PEMs exhibited an exceptional water retention property compared to a pristine Nafion membrane. Further, these membranes are much stiffer and can withstand high temperature in comparison with pristine Nafion.

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Fig. 7. Schematic physicochemical strategy for nanoscales manipulation of ion conducting channels in Nafion membrane using functionalized graphenes [69]. Copyright © 2011, American Chemical Society.

Chang and co-workers carried out sulfonation of GO in sulfanilic acid and prepared sulfonated GO (SGO)/Nafion nanohybrid membranes. These membranes show reduced methanol-permeability, marginal swelling ratio, and high proton conductivity under low relative humidity compare to pristine Nafion. The single cell DMFC tests of a 0.05 wt% SGO/Nafion nanohybrid membrane revealed superior performance than that of the commercial Nafion 115 in 1 M and 5 M methanol solutions [71]. Methanol impervious composite membranes were also prepared by layer-by-layer assembly of poly(diallyldimethylammonium chloride) (PDDA) and GO nanosheets onto the surface of a Nafion membrane. The bilayers with a methanol-blocking behaviour were formed on the surface of Nafion membranes. The reported membrane exhibited 63%

reduction in methanol permeability and 60.6% improvement in maximum power density compared to the pristine Nafion membrane [72]. Nicotera et al. modified the surface of GO by intercalation chemistry to organo-modified GO containing sulfonilic terminal groups (GOSULF) and incorporated in Nafion by solution intercalation to prepare Nafion/GOSULF nanohybrid membranes. An amine derivative containing a sulfonic acid functional end group was covalently bonded via the amide functionality on the GO surface (Fig. 8). GOSULF showed a methanol impervious behaviour along with improved proton mobility and water retention capacity at high temperatures. Nafion/GOSULF nanohybrid membranes showed better DMFC single cell performance in comparison with pristine Nafion [73].

Fig. 8. Schematic reaction procedure for the synthesis of GOSULF nanofiller [73]. Copyright © 2014, American Chemical Society.


Fig. 9. Proton conductivity (σ) comparison between GO and F-GO powders [74]. Copyright © 2011, American Chemical Society.

Sulfonic acid functionalized GO (F-GO) was synthesized by grafting 3-mercaptopropyl trimethoxysilane (MPTMS) onto the GO surface and followed by mercapto groups which were oxidized using H2O2. Proton conductivity of GO increased by 1 to 3 orders after grafting of sulfonic acid groups (F-GO) at 20 and 80 °C, respectively (Fig. 9). A high concentration of sulfonic acid on the GO surface improved the water retention capacity. Incorporation of F-GO into the Nafion matrix significantly improved the proton conductivity and single cell fuel cell performance (4 times) over pristine Nafion at 120 °C with 25% humidity [74]. The F-GO/Nafion nanohybrids offer a promising strategy for the fabrication of PEMs. Wu and co-workers also developed sulfonated graphene oxide–silica (S-GO–SiO2) nanohybrid particles and prepared S-GO-SiO2/Nafion nanohybrid PEMs via solution casting. The reported nanohybrid membranes showed improved water uptake and proton conductivity along with a methanol impervious nature due to the two-dimensional structure. The improved selectivity parameters of these membranes in comparison with pristine Nafion under high methanol concentration and/or temperature, make them a potential candidate for DMFC applications [75]. Sulfonic acid grafted graphene (S-

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graphene) was also synthesized via diazotization and a Nafion-Sgraphene nanohybrid membrane was fabricated for DMFC application Fig. 10. A Nafion-S-graphene (1 wt%) nanohybrid membrane with improved proton conductivity and reduced methanol permeability, was assessed as a promising candidate for DMFC application with 118 mW cm−2 peak power density in comparison with 54 mW cm−2 for a pristine Nafion membrane at 70 °C [76]. A phosphotungstic acid (H3PW12O40; PW) coupled graphene oxide (PW-mGO) based nano-hybrid material was synthesized by reducing GO using hydrazine hydrate followed by grafting the 3-aminopropyltriethoxysilane (APTES) and a Nafion/PW-mGO based nano-hybrid membrane and was reported for fuel cell applications at low relative humidity operation (Fig. 11). The reported PEM showed improved proton conductivity (10.4 mS cm−1) in comparison with a pristine Nafion membrane (6.5 mS cm− 1) at 25% RH. Further, the maximum power density for Nafion/PW-mGO PEM (841 mW cm− 2) was about 4-fold higher in comparison with a pristine Nafion membrane (210 mW cm−2) at 80 °C under 20% RH [77]. Layer-by-layer (LbL) fabrication strategy was also adopted for developing GO/Nafion nanohybrid PEMs, in which GO nanosheets were cross-linked with a Nafion 117 surface using 1,4-phenyl diamine hydrochloride (PDHC) as the cross-linking agent (Fig. 12). The reported GO/Nafion nanohybrid membrane showed good stability, and low methanol permeability (6.7 × 10−8 cm2 s−1), in comparison to the Nafion 117 under a similar environment. The reported PEM with improved selectivity showed 64.38 mW cm−2 maximum power density in comparison to pristine Nafion 117 (41.60 mW cm−2) [78]. 2.2.2. Non-fluorinated nanohybrid PEMs Sulfonated aromatic and aliphatic hydrocarbon polymers were widely used as alternative PEMs due to their excellent stability and easy functionalization [79]. For developing PEMs, aromatic polymers, such as poly-(ether ether ketone)s (PEEKs) [80], poly(arylene ether sulfone)s (PAESs) [81,82], polyimides [83,84], polyphosphazenes [85], polybenzimidazoles (PBIs) [86,87], and polyphenylenes [88] were either sulfonated or blended with mineral acids/inorganic precursors. Further, aliphatic polymers such as chitosan [89–91], poly(vinyl alcohol) (PVA) [92,93] and poly(vinyl chloride) (PVC) [94,95] etc. were also used as an alternative polymer matrix for the preparation of PEMs. 2.2.2.1. SPI based nanohybrid PEMs. SPIs are thermally stable polymers with stiff aromatic backbones and considered as promising membrane forming materials, due to their high thermal stability, mechanical and chemical properties, good film-forming ability and reasonable cost

Fig. 10. Schematic reaction route for the preparation of S-graphene [76]. Copyright © 2016, American Chemical Society.

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Fig. 11. Schematic reaction route for the preparation of PW-mGO and composite membrane (Nafion/PW-mGO) [77].

[96–99]. For medium/high temperature applications, SPIs were utilized as the matrix polymer for holding functionalized GO/inorganic materials for the preparation of nano-hybrid PEMs [100–102]. This type of membrane was generally prepared by solution casting with varied GO contents and exhibited improved proton conductivity, mechanical properties and reduced methanol permeability. For example, the SPI/ GO (0.5 wt%) nanohybrid membrane exhibited higher proton conductivity (6.67 × 10− 1 S cm−1) in comparison with Nafion 117 (2.31 × 10−1 S cm−1), at 90 °C. In addition, methanol permeability

was significantly reduced by 4.8-fold in comparison to a pristine SPI membrane at 30 °C. These observations were attributed to the excellent methanol blocking behaviour and proton conducting nature of GO [31]. Further, SPI/GO nanohybrid PEMs with different particle sizes of GO were also prepared by the modified Hummers method and their DMFC performance was assessed. It was observed that under the same reaction conditions, the smallest particle size of GO showed a high degree of oxidation. The SPI–0.5%-GO1 nanohybrid membrane (with the smallest size of GO) exhibited 1.2 S cm−1 proton conductivity (at

Fig. 12. Schematic illustration of (a) LbL deposition procedure to synthesize the GO/Nafion composite membranes, (b) the mechanism of reactions between Nafion and PDHC, and (c) the mechanism of reactions between GO and PDHC [78]. Copyright © 2015, Elsevier.


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Fig. 13. Preparation of sulfonated propylsilane graphene oxide [105]. Copyright © 2014, American Chemical Society.

80 °C and RH 100%), and 1.07 × 10−7 cm2 S−1 methanol permeability (at 25 °C) along with excellent fuel cell performance in comparison to pristine SPI [103].

Phosphoric acid-doped ionic liquid-functionalized graphene oxide (FGO) was prepared by functionalization of GO with 1-methylimidazole followed by doping of phosphoric acid. FGO/SPI based nano-hybrid

Fig. 14. Preparation scheme for sulfonated imidized graphene oxide (SIGO) [106]. Copyright © 2015, Elsevier.

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PEMs were prepared by the incorporation of the desired FGO content into the SPI matrix. The SPI/FGO-5% exhibited comparatively high proton conductivity in comparison with pristine SPI membrane [104]. Pandey et al. synthesized sulfonated propylsilane graphene oxide (SPSGO) by chemical grafting of (3-mercaptopropyl)trimethoxysilane (MPTMS) on GO and subsequently oxidation of mercapto groups with hydrogen peroxide (Fig. 13). SPI/SPSGO based nanohybrid PEMs were prepared via incorporation of various contents of SPSGO into SPI. The SPI/SPSGO-8 membrane (with 8 wt% of SPSGO) showed improved proton conductivity κm = 9.62 × 10−2 S cm−1, and thermal, mechanical, and chemical stabilities along with water retention capacity responsible for slow dehydration of the membrane matrix. Further, the single-cell direct methanol fuel cell test performance of the SPI/SPSGO-8 nanohybrid membrane showed 75.06 mW cm−2 maximum power density, which is comparable to the Nafion 117 membrane (62.40 mW cm−2 maximum power density) under 2 M methanol fuel at 70 °C [105]. A new strategy has been developed by Pandey et al. for the preparation of sulfonated imidized graphene oxide (SIGO) (GO tethered SPI) based nanohybrid membranes to avoid the leaching out of nanofillers. They chemically grafted GO into the SPI polymer chains via a polycondensation reaction (Fig. 14). These types of PEMs showed excellent stability and conductivity. This strategy is also helpful for the incorporation of a high content of nanofillers into the membrane matrix [106]. 2.2.2.2. PEEK based nanohybrid PEMs. Poly (ether ether ketone) (PEEK) belongs to the family of poly (aryl ether ketone)s and is a linear aromatic semi-crystalline polymer of an oxy-1,4-phenylene-oxy-1,4phenylene-carbonyl-1,4-phenylene repeating unit. It can be easily sulfonated via an electrophilic substitution reaction using concentrated sulfuric acid. The degree of sulfonation (DS) can also be manipulated by varying the reaction time and temperature [107,108]. An electrophilic substitution reaction depends on the groups present in the aromatic chains. In the case of PEEK sulfonation occurs only on the phenyl ring located between two ether groups because the other two phenyl rings are deactivated by the electronegative effect of the adjacent carbonyl group. Sulfonated PEEK (SPEEK) is a promising polymeric material for the fabrication of nano-hybrid membranes due to good thermal, chemical and mechanical stabilities [109–111]. SPEEK membranes showed an excellent methanol blocking property but lower conductivity in comparison with Nafion. All desired properties of the SPEEK membrane (IEC, ionic conductivity, and stability) can be also improved by the incorporation of various inorganic fillers, such as heteropolyacids, nanoclays, inorganic functionalized materials, etc. [112–116].

Manthiram et al. reported that sodium dodecylbenzene sulfonate (SDBS) adsorbed with GO based nanohybrid PEMs (SDBS-GO/SPEEK) with improved IEC, water uptake, and proton conductivity for DMFC application. Incorporation of SDBS-GO into the SPEEK matrix significantly reduced the methanol permeability. The single cell DMFC performance of a well optimized SPEEK/SDBS-GO (5 wt%) membrane exhibits about 2 times higher power density in comparison with a pristine SPEEK membrane, while the power density of the previous one was about 20% high in comparison with a Nafion 112 membrane [117]. Further, sulfonated GO (s-GO) was prepared by grafting propane sultone onto the GO surface (Fig. 15) and s-GO/SPEEK based nanohybrid PEMs were fabricated by a solution blending method. Proton conductivity and mechanical properties of these PEMs were significantly improved, while mass transport (methanol/water) across the membranes was highly reduced with incorporation of s-GO into the SPEEK matrix. Relatively speaking, a high selectivity parameter makes the reported nanohybrid membrane a suitable candidate for DMFC application [118]. Sulfonic acid functionalized GO (FGO) was synthesized and FGO/SPEEK nanohybrid PEMs were prepared for PEMFC applications. They functionalized the surface of GO by sulfanilic acid to FGO and incorporated this into the SPEEK matrix for the preparation of nanohybrid membranes. Incorporation of FGO greatly influenced the membrane properties such as proton conductivity, IEC, and fuel cell performance. The SGO-SPEEK nanohybrid membrane showed 0.055 S cm− 1 proton conductivity, 2.3 meq g − 1 IEC, and 378 mW cm − 2 maximum power density, which were higher than that of pristine SPEEK [119]. Polydopamine modified graphene oxide (DGO)/SPEEK based nanohybrid anhydrous PEMs were also reported with unique nanophase-separation morphology with chain packing. Interfacial electrostatic attraction between free amino groups (DGO) and sulfonic groups (SPEEK) provide a facile pathway for proton hopping, imparting an enhanced proton transfer via the Grotthuss mechanism (Fig. 16). Proton conductivity and fuel cell performance at an elevated temperature of the SPEEK/DGO nanohybrid membrane were comparatively higher than the pristine SPEEK membrane under hydrated and anhydrous conditions [120]. Iyengar and co-workers prepared a SPEEK membrane by the incorporation of graphene and phosphotungstic acid into the SPEEK matrix, a new self-humidifying nanohybrid membrane with improved water retention and proton conductivity which was reported for PEMFC applications [121]. Sulfonated organosilane functionalized graphene oxide (SSi-GO) was also synthesized via the grafting of MPTMS with GO followed by oxidation of mercapto groups, and used as a filler in SPEEK membranes.

Fig. 15. Synthetic procedure for sulfonated graphene oxide (s-GO) [118]. Copyright © 2012 Elsevier.


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Fig. 16. Synthesis and structure of DGO and SPEEK/DGO membrane and proton-hopping behaviour between acid–base pairs at the SPEEK–DGO interface [120]. Copyright © 2014, Royal Society of Chemistry.

Incorporation of SSi-GO in the SPEEK matrix greatly improved the proton conductivity, methanol blocking capacity, and fuel cell performance of nanohybrid membranes [122]. In another report, FGO was prepared by grafting p-aminobenzene sulfonic acid with GO and incorporated in a phosphoric acid doped quaternized PEEK (QPEEK) membrane matrix for high temperature PEM applications [123]. Incorporation of FGOs in a SPEEK membrane matrix significantly improved the membrane properties. Further, a SPEEK/FGO nanohybrid membrane impregnated with imidazole-type ionic liquid (IL) was prepared and a SPEEK/FGO (7.5%) nanohybrid membrane exhibited 21.9 mS cm−1 anhydrous conductivity at 150 °C, which was about 30 times higher in comparison with a pristine SPEEK membrane (0.69 mS cm−1) [124]. Sulfonated polymer brush modified GO (SP-GO) was also prepared via facile distillation precipitation polymerization, and incorporated into a SPEEK matrix to fabricate SP-GO/SPEEK nanohybrid membranes, in which SP-GO formed proton-conductive pathways within polymeric membranes via uniform dispersion thus tend to lie perpendicularly to the cross-section surface of the whole membrane [125]. Sulfonated GO (SGO)/Fe3O4 nanosheets were incorporated to a SPEEK/ poly(vinylalcohol) blend and the reported nanohybrid membranes exhibited enhanced mechanical stability, proton conductivity and methanol blocking properties. As a reference, an optimized SPEEK/PVA/SGO/ Fe3O4 (5%) nanohybrid membrane exhibited 0.084 S cm−1 proton conductivity at 25 °C, 8.83 × 10− 7 cm2 s−1 methanol permeability, 51.2 MPa tensile strength, and 122.7 mW cm−2 power density at 80 °C [126]. 2.2.2.3. Poly ether sulfone based nanohybrid PEMs. The surface morphology and property of sulfonated polyarylene ether sulfone (SPAES) and adequately sulfonated GO (SGO) based nanohybrid membranes were critically assessed. SPAES/SGO nanohybrid membranes showed a methanol impervious nature with improved mechanical properties. Nevertheless, the high content of SGO (N 5 wt%) caused mechanical instability to nanohybrid membranes. Further, it was found that the properties of the nanohybrid membrane with a 5% blended SGO is

superior to that of Nafion 117 [127]. Sulfonated polymer brush functionalized graphene oxide (SPB-FGO) was synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization, and incorporated in sulfonated poly (ether sulfone) (SPES) matrix for fabrication of nanohybrid PEMs. Incorporation of SPB-FGO in the SPES matrix significantly improved the membrane properties such as proton conductivity, methanol blocking, thermal stability, mechanical property and oxidative stability as compared with that of a pristine SPES membrane [128]. Membrane properties of the SPAES matrix was also improved by the incorporation of thermally treated GO and poly (2,5benzimidazole)-grafted GO (ABPBI-GO). ABPBI-GO was prepared by grafting 3,4-diaminobenzoic acid with GO via an imidization reaction at 160 °C (Fig. 17). Dimensional stability and mechanical strength were significantly improved with the incorporation of ABPBI-GO. The SPAES/ABPBI-GO 1.0 nanohybrid membrane exhibited the highest proton conductivity of 152.5 mS cm−1 at 80 °C and 90% RH condition compare to pristine SPAES and SPAES/GO nanohybrid membranes [129]. SGO/SPES nanohybrid PEM with improved IEC, water retention, proton conductivity and methanol resistance was also reported for DMFC application [130]. 2.2.2.4. Chitosan based nanohybrid PEMs. Chitosan (CS, a deacetylated form of chitin), is an abundant, low cost, and non-toxic biodegradable polymer, and contains amino/hydroxyl groups [131,132]. Recently, CS was explored as a promising membrane due to its excellent film forming, high mechanical and inherent low fuel permeability properties [133,134]. To prepare CS/SGO based nanohybrid membranes, the SGO was synthesized via the grafting of 3-(methacryloxy) propyltrimethoxysilane (MPS) onto the surface of GO. A polymeric layer (poly(Divinylbenzene-co-Styrene)) (poly(DVB-co-St)) was formed through distillation-precipitation polymerization, and sulfonic acid groups were grafted to the phenyl groups within the polymeric layer. The CS/SGO based nanohybrid membranes showed enhanced proton conductivity, mechanical, and thermal stabilities. As a representative, the CS/SGO-2 (2 wt% SGO) nanohybrid membrane exhibited a

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Fig. 17. Schematic reaction route for the synthesis and modification of GO [129]. Copyright © 2015, Royal Society of Chemistry.

122.5% increase in hydrated conductivity and a 90.7% increase in anhydrous conductivity when compared with a pristine CS membrane [135]. SGO was also synthesized via an epoxide ring opening reaction of GO with taurine and studies the effect of the incorporation of sulfonated chitosan (SCS) and SGO on the electrochemical properties of the CS membrane. The CS/SCS/SGO nanohybrid membranes showed a 454% increase in proton conductivity and 23% reduction in methanol permeability, therefore, the selectivity is increased by 650%, relative to the pristine CS membrane, at an SGO content of 5 wt% [136]. Phosphorylated GO (PGO)/CS based nanohybrid anhydrous PEM was also reported for elevated temperature fuel cells. Thermal and mechanical stabilities of a pristine CS membrane were significantly enhanced for a PGO doped nano-hybrid PEM. The CS/PGO-2.5 (2.5% PGO) nanohybrid PEM exhibited 5.79 mS cm−1 proton conductivity at 160 °C and 0% RH [137]. Sharma et al. synthesized the highly stable and water retentive sulfonated CS (SCS) modified with a GO nanohybrid membrane (MGO-SCS-5; 5 wt% of MGO), which exhibited 11.2 × 10−2 S cm−1 proton conductivity at 90 °C along with improved thermal and mechanical stabilities [138]. Pandey et al. synthesized N,N-dimethylene phosphonic acid propylsilane GO (NMPSGO) and incorporated it into a functionalized biopolymer (N-o-sulfonic acid benzyl chitosan (NSBC)) for the preparation of multifunctionalised nanohybrid PEMs. NMPSGO was prepared by chemical grafting of 3aminopropyltrimethoxysilane (APTMOS) followed by phosphorylation of amino groups (Fig. 18). Incorporation of NMPSGO into NSBC significantly enhanced the proton conductivity, water retention properties and stability [139].

3. Summary and future perspective GO based nanohybrids are a promising candidate for developing high performance materials of technological relevance. However, many technological challenges must still be addressed to achieve the full potential of nanohybrids. Different types of free-standing GO based PEMs were reported using modified or natural flake graphite sheets, but these PEMs suffer due to mechanical instability. To avoid the problem, different protocols were adopted to produce functionalized GO nanohybrid PEMs by dispersing modified GO in to various polymer matrices with suitable chemical interactions. Serious research was carried out for a deeper understanding of the surface chemistry in reinforcement direction. To avoid these problems, surface imidization was considered as a novel approach for the preparation of functionalized GO tethered sulfonated polyimide. Polyimide-tethered silica and graphene have been reported in the literature. Moreover, the defects introduced into GtO platelets by either the oxidation to convert graphite to GO or the processing to generate GtO platelets, might ultimately limit the electrical conductivity and mechanical properties relative to pristine and defect-free graphene platelets. Thus, methods of graphene platelet production which preserve its extended, conjugated structure may find favour for certain demanding applications of graphene-based nanohybrids. Thus, properties of GO-based composites may be influenced by improved morphological control. The hydrolytic stability of GO is worthy for consideration as a GOpolymer based nanohybrid for PEMs in fuel cell applications. Several properties of these PEMs are widely reported in the literature, which


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Fig. 18. Schematic route for the preparation of N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO) [139]. Copyright © 2014, Royal Society of Chemistry.

supports the versatile future of the GO-polymer based nanohybrid. Indeed, there may be some agreements and disagreements on the method of preparation of the PEMs, observed results and proton transport mechanism across the PEM. But, in the near future with continuous efforts these points may be reasonably solved. In comparison with other membrane forming materials, GO based PEMs have the leading edge because of their easy preparation and processing, with uniform surface morphology and controllable extent of functionalization. Further, the structure of the GO based PEMs can be modified to tailor the morphological structure and interlayer spacing between the GO layers. Layered GO based PEMs exhibit good conductivity, water retention ability, chemical, mechanical and thermal stabilities accompanied with problems related to durability, scalability and reproducibility. The important parameters such as durability and stability of membranes need improvement for practical applicability. Despite of many these challenges, graphene polymer nanocomposites have already found diversified industrial applications and their commercial impact is expected to significantly rise in the future. In the absence of chemical interactions between two components (covalent bonding, non-covalent bonding, or hydrogen bonding), poor interfacial adhesion between graphene and the polymer matrix is the serious problem to utilize the advantages of nanohybrid PEMs. Thus care should be rendered to modify GO by covalent bonding to avoid deterioration in stability/durability and membrane surface morphology. For further improvements, GO based materials are classical fillers in polymer nanohybrids and can be exploited with advantage by covalent

functionalization of GO and the choice of a suitable polymer matrix for developing PEMs. The feasibility and processability of these PEMs as a membrane electrode assembly could improve the life-time and efficiency of fuel cells. Notably, no significant effort was rendered for the scaleup of GO-polymer nanohybrid PEMs to alleviate these problems. Acknowledgements Registration number: CSIR-CSMCRI-182/2016. Authors are thankful to the Department of Science and Technology, New Delhi, Govt. of India, for providing financial assistance by sanctioning project no. SR/S1/PC62/2012. Instrumental support received from the Analytical Science Division, CSIR-CSMCRI, is also gratefully acknowledged. References [1] Hou J, Shao Y, Ellis MW, Moore RB, Yi B. Graphene-based electrochemical energy conversion and storage: fuel cells, supercapacitors and lithium ion batteries. Phys Chem Chem Phys 2011;13:15384–402. [2] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183–91. [3] Zhu BY, Murali S, Cai W, Li X, Suk JW, Potts JR. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22:3906–24. [4] Compton OC, Nguyen SBT. Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials. Small 2010;6:711–23. [5] Nair RR, Blake P, Grigorenko AN, Novoselov KS, Booth TJ, Stauber T, et al. Fine structure constant defines visual transparency of graphene. Science 2008;320:1308. [6] Neto AHC, Guinea F, Peres NMR, Novoselov KS, Geim AK. The electronic properties of graphene. Rev Mod Phys 2009;81:109. [7] Geim AK. Graphene: status and prospects. Science 2009;324:1530.

28


[8] Bolotin KI, Sikes KJ, Jiang Z, Klima M, Fudenberg G, Hone J, et al. Ultrahigh electron mobility in suspended graphene. Solid State Commun 2008;146:351. [9] Morozov SV, Novoselov KS, Katsnelson MI, Schedin F, Elias DC, Jaszczak JA, et al. Giant intrinsic carrier mobilities in graphene and its bilayer. Phys Rev Lett 2008; 100:16602. [10] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nat Nanotechnol 2009;4:217–24. [11] Zhao G, Wen T, Chen C, Wang X. Synthesis of graphene-based nanomaterials and their application in energy related and environmental-related areas. RSC Adv 2012;2:9286–303. [12] Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [13] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442(7100):282–6. [14] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature 2007;448:457. [15] Watcharotone S, Dikin DA, Stankovich S, Piner R, Jung I, Dommett GHB, et al. Graphene–silica composite thin films as transparent conductors. Nano Lett 2007; 7:1888–92. [16] Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M, Piner RD, et al. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol 2008;3(6):327–31. [17] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8(10):3498–502. [18] Elias DC, Nair RR, Mohiuddin TMG, Morozov SV, Blake P, Halsall MP, et al. Control of graphene's properties by reversible hydrogenation: evidence for graphane. Science 2009;323(5914):610–3. [19] Shan C, Yang H, Song J, Han D, Ivaska A, Niu L. Direct electrochemistry of glucose oxidase and biosensing for glucose based on graphene. Anal Chem 2009;81(6): 2378–82. [20] Li X, Zhu Y, Cai W, Borysiak M, Han B, Chen D, et al. Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Lett 2009; 9(12):4359–63. [21] Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V, et al. Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Lett 2009;9(1): 30–5. [22] Shi Y, Kim KK, Reina A, Hofmann M, Li L-J, Kong J. Work function engineering of graphene electrode via chemical doping. ACS Nano 2010;4(5):2689–94. [23] Chen C, Rosenblatt S, Bolotin KI, Kalb W, Kim P, Kymissis I, et al. Performance of monolayer graphene nanomechanical resonators with electrical readout. Nat Nanotechnol 2009;4(12):861–7. [24] Balog R, Jørgensen B, Nilsson L, Andersen M, Rienks E, Bianchi M, et al. Bandgap opening in graphene induced by patterned hydrogen adsorption. Nat Mater 2010;9(4):315–9. [25] Nanda SS, Papaefthymiou GC, Yi DK. Functionalization of graphene oxide and its biomedical applications. Crit Rev Solid State Mater Sci 2015;40(5):291–315. [26] Fang M, Wang K, Lu H, Yang Y, Nutt S. Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites. J Mater Chem 2009;19:7098–105. [27] Brodie BC. Ann Chim Phys 1860;59:466–72. [28] Staudenmaier L. Dtsch Ber Chem Ges 1898;31:1481–99. [29] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80: 1339. [30] Karim MR, Hatakeyama K, Matsui T, Takehira H, Taniguchi T, Koinuma M, et al. Graphene oxide nanosheet with high proton conductivity. J Am Chem Soc 2013; 135(22):8097–100. [31] Tseng CY, Ye YS, Cheng MY, Kao KY, Shen WC, Rick J, et al. Sulfonated polyimide proton exchange membranes with graphene oxide show improved proton conductivity, methanol crossover impedance, and mechanical properties. Adv Energy Mater 2011;1:1220–4. [32] Shukla G, Pandey RP, Shahi VK. Temperature resistant phosphorylated graphene oxide–sulphonated polyimide composite cation exchange membrane for water desalination with improved performance. J Membr Sci 2016;520:972–82. [33] Bourlinos AB, Gournis D, Petridis D, Szabo T, Szeri A, Dékány I. Graphite oxide: chemical reduction to graphite and surface modification with primary aliphatic amines and amino acids. Langmuir 2003;19:6050–5. [34] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39:228–40. [35] Potts JR, Dreyer DR, Bielawski CW, Ruoff RS. Graphene-based polymer nanocomposites. Polymer 2011;52:5–25. [36] Buchsteiner A, Lerf A, Pieper JA. Water dynamics in graphite oxide investigated with neutron scattering. J Phys Chem B 2006;110:22328–38. [37] Kumar R, Mamlouk M, Scott KA. Graphite oxide paper polymer electrolyte for direct methanol fuel cells. Int J Electrochem 2011:34186 [2011]. [38] Kumar R, Scott K. Freestanding sulfonated graphene oxide paper: a new polymer electrolyte for polymer electrolyte fuel cells. Chem Commun 2012;48:5584–6. [39] Gao W, Wu G, Janicke MT, Cullen DA, Mukundan R, Baldwin JK, et al. Ozonated graphene oxide film as a proton-exchange membrane. Angew Chem Int Ed 2014; 53:3588–93. [40] Jiang Z, Shi Y, Jiang ZJ, Tian X, Luo L, Chen W. High performance of a free-standing sulfonic acid functionalized holey graphene oxide paper as a proton conducting polymer electrolyte for air breathing direct methanol fuel cells. J Mater Chem A 2014;2:6494–503. [41] Yu S, Liu Q, Yang W, Han K, Wang Z, Zhu H. Graphene–CeO2 hybrid support for Pt nanoparticles as potential electrocatalyst for direct methanol fuel cells. Electrochim Acta 2013;94:245–51.

[42] Saito M, Arimura N, Hayamizu K, Okada T. Mechanisms of ion and water transport in perfluorosulfonated ionomer membranes for fuel cells. J Phys Chem B 2004;108: 16064–70. [43] Miyake N, Wainright JS, Savinell RF. Evaluation of a sol-gel derived Nafion/silica hybrid membrane for polymer electrolyte membrane fuel cell applications. J Electrochem Soc 2001;148:A905–9. [44] Zhang H, Shen PK. Recent development of polymer electrolyte membranes for fuel cells. Chem Rev 2012;112:2780–832. [45] Hickner MA, Ghassemi H, Kim YS, Einsla BR, McGrath JE. Alternative polymer systems for proton exchange membranes (PEMs). Chem Rev 2004;104:4587–611. [46] Jang SS, Molinero V, Cagin T, Grapheneddard WA. Nanophase-segregation and transport in Nafion 117 from molecular dynamics simulations: effect of monomeric sequence. J Phys Chem B 2004;108:3149–57. [47] Pineri M, Gebel G, Davies RJ, Diat O. Water sorption–desorption in Nafion® membranes at low temperature, probed by micro X-ray diffraction. J Power Sources 2007;172:587–96. [48] Hou JB, HM Y, Wang LA, Xing DM, Hou ZJ, Ming PW, et al. Conductivity of aromaticbased proton exchange membranes at subzero temperatures. J Power Sources 2008;180:232–7. [49] Mistry MK, Choudhury NR, Dutta NK, Knott R, Shi Z, Holdcroft S. Novel organic-inorganic hybrids with increased water retention for elevated temperature proton exchange membrane application. Chem Mater 2008;20:6857–70. [50] Tang HL, Pan M. Synthesis and characterization of a self-assembled Nafion/silica nanocomposite membrane for polymer electrolyte membrane fuel cells. J Phys Chem C 2008;112:11556–68. [51] Sahu AK, Bhat SD, Pitchumani S, Sridhar P, Vimalan V, George C, et al. Novel organic–inorganic composite polymer-electrolyte membranes for DMFCs. J Membr Sci 2009;345:305–14. [52] Zhang YW, Cai WW, Si FZ, Ge JJ, Liang L, Liu CP, et al. A modified Nafion membrane with extremely low methanol permeability via surface coating of sulfonated organic silica. Chem Commun 2012;48:2870–2. [53] Kim T, Choi YW, Kim CS, Yang TH, Kim MN. Sulfonated poly(arylene ether sulfone) membrane containing sulfated zirconia for high-temperature operation of PEMFCs. J Mater Chem 2011;21:7612–21. [54] Rodgers MP, Shi Z, Holdcroft S. Ex situ characterisation of composite Nafion membranes containing zirconium hydrogen phosphate. Fuel Cell 2009;9:534. [55] Baglio V, Arico AS, Blasi AD, Antonucci V, Antonucci PL, Licoccia S, et al. Nafion–TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance. Electrochim Acta 2005;50:1241–6. [56] Ketpang K, Lee K, Shanmugam S. Facile synthesis of porous metal oxide nanotubes and modified Nafion composite membranes for polymer electrolyte fuel cells operated under low relative humidity. ACS Appl Mater Interfaces 2014;6:16734–44. [57] Li X, Roberts EPL, Holmes SM, Zholobenk V. Functionalized zeolite A–Nafion composite membranes for direct methanol fuel cells. Solid State Ion 2007;178:1248–55. [58] Zhang W, Li MKS, Yue PL, Gao P. Exfoliated Pt-clay/Nafion nanocomposite membrane for self-humidifying polymer electrolyte fuel cells. Langmuir 2008;24: 2663–70. [59] Silva RF, Passerini S, Pozio A. Solution-cast Nafion®/montmorillonite composite membrane with low methanol permeability. Electrochim Acta 2005;50:2639–45. [60] Xiang Y, Yang M, Guo ZB, Cui Z. Alternatively chitosan sulfate blending membrane as methanol-blocking polymer electrolyte membrane for direct methanol fuel cell. J Membr Sci 2009;337:318–23. [61] Choi SW, YZ F, Ahn YR, Jo SM, Manthiram A. Nafion-impregnated electrospun polyvinylidene fluoride composite membranes for direct methanol fuel cells. J Power Sources 2008;180:167–71. [62] Wycisk R, Chisholm J, Lee J, Lin J, Pintauro PN. Direct methanol fuel cell membranes from Nafion–polybenzimidazole blends. J Power Sources 2006;163:9–17. [63] Kumar R, Xu C, Scott K. Graphite oxide/Nafion composite membranes for polymer electrolyte fuel cells. RSC Adv 2012;2:8777–82. [64] Feng K, Tang B, Wu P. “Evaporating” graphene oxide sheets (GOSs) for rolled up GOSs and its applications in proton exchange membrane fuel cell. ACS Appl Mater Interfaces 2013;5:1481–8. [65] Aragaw BA, WN S, Rick J, Hwang BJ. Highly efficient synthesis of reduced graphene oxide–Nafion nanocomposites with strong coupling for enhanced proton and electron conduction. RSC Adv 2013;3:23212–21. [66] Lin CW, Highly LYS. Ordered graphene oxide paper laminated with a Nafion membrane for direct methanol fuel cells. J Power Sources 2013;237:187–94. [67] Lee DC, Yang HN, Park SH, Kim WJ. Nafion/graphene oxide composite membranes for low humidifying polymer electrolyte membrane fuel cell. J Membr Sci 2014; 452:20–8. [68] Lue SJ, Pai YL, Shih CM, Wu MC, Lai SM. Novel bilayer well-aligned Nafion/graphene oxide composite membranes prepared using spin coating method for direct liquid fuel cells. J Membr Sci 2015;493:212–23. [69] Choi BG, Hong J, Park YC, Jung DH, Hong WH, Hammond PT, et al. Innovative polymer nanocomposite electrolytes: nanoscale manipulation of ion channels by functionalized graphenes. ACS Nano 2011;5:5167–74. [70] Enotiadis A, Angjeli K, Baldino N, Nicotera I, Gournis D. Graphene-based Nafion nanocomposite membranes: enhanced proton transport and water retention by novel organo-functionalized graphene oxide nanosheets. Small 2012;8:3338–49. [71] Chien HC, Tsai LD, Huang CP, Kang CY, Lin JN, Chang FC. Sulfonated graphene oxide/ Nafion composite membranes for high-performance direct methanol fuel cells. Int J Hydrogen Energy 2013;38:13792–801. [72] Yuan T, Pu L, Huang Q, Zhang H, Li X, Yang H. An effective methanol-blocking membrane modified with graphene oxide nanosheets for passive direct methanol fuel cells. Electrochim Acta 2014;117:393–7.

R.P. Pandey et al. / Advances in Colloid and Interface Science 240 (2017) 15–30 [73] Nicotera I, Simari C, Coppola L, Zygouri P, Gournis D, Brutti S, et al. Sulfonated graphene oxide platelets in Nafion nanocomposite membrane: advantages for application in direct methanol fuel cells. J Phys Chem C 2014;118:24357–68. [74] Zarrin H, Higgins D, Jun Y, Chen Z, Fowler M. Functionalized graphene oxide nanocomposite membrane for low humidity and high temperature proton exchange membrane fuel cells. J Phys Chem C 2011;115:20774–81. [75] Feng K, Tang B, Sulfonated WP. Graphene oxide–silica for highly selective Nafionbased proton exchange membranes. J Mater Chem A 2014;2:16083–92. [76] Parthiban V, Akula S, Peera SG, Islam N, Sahu A. Proton conducting Nafion-sulfonated graphene hybrid membranes for direct methanol fuel cells with reduced methanol crossover. Energy Fuel 2016;30(1):725–34. [77] Kim Y, Ketpang K, Jaritphun S, Park JS, Shanmugam S. A polyoxometalate coupled graphene oxide–Nafion composite membrane for fuel cells operating at low relative humidity. J Mater Chem A 2015;3:8148–55. [78] Wang LS, Lai AN, Lin CX, Zhang QG, Zhu AM, Liu QL. Orderly sandwich-shaped graphene oxide/Nafion composite membranes for direct methanol fuel cells. J Membr Sci 2015;492:58–66. [79] Pandey RP, Shahi VK. Aliphatic-aromatic sulfonated polyimide and acid functionalized polysilsesquioxane composite membranes for fuel cell applications. J Mater Chem A 2013;1:14375–83. [80] Tripathi BP, Kumar M, Shahi VK. Highly stable proton conducting nanocomposite polymer electrolyte membrane (PEM) prepared by pore modifications: an extremely low methanol permeable PEM. J Membr Sci 2009;327:145–54. [81] Kazeroonian FK, Iranagh SA, Modarress H. Molecular dynamics simulation study of carboxylated and sulfonated poly(arylene ether sulfone) membranes for fuel cell applications. Int J Hydrogen Energy 2015;40:15690–703. [82] Lee SY, Kwon Y, Kim BH, Chae J, Jang JH, Henkensmeier D, et al. Synthesis of high molecular weight sulfonated poly(arylene ether sulfone) copolymer without azeotropic reaction. Solid State Ion 2015;275:92–6. [83] Chen JC, Wu JA, Lee CY, Tsai MC, Chen KH. Novel polyimides containing benzimidazole for temperature proton exchange membrane fuel. J Membr Sci 2015;483: 144–54. [84] Pan H, Chen S, Zhang Y, Jin M, Chang Z, Pu H. Preparation and properties of the cross-linked sulfonated polyimide containing benzimidazole as electrolyte membranes in fuel cells. J Membr Sci 2015;476:87–94. [85] Guoa Q, Pintaurob PN, Tangb H, O'Connor S. Sulfonated and crosslinked polyphosphazene-based proton-exchange membranes. J Membr Sci 1999;154: 175–81. [86] Liua C, Lib X, Zhang S, Li Z, Cao Y, Jian X. Synthesis and characterization of sulfonated polybenzimidazoles containing 4-phenyl phthalazinone groups for proton exchange membrane. Solid State Ion 2014;261:67–73. [87] Jheng LC, Hsu SLC, Lin BY, Hsu YL. Quaternized polybenzimidazoles with imidazolium cation moieties for anion exchange membrane fuel cells. J Membr Sci 2014;460:160–70. [88] Yang S, Gong C, Guan R, Zou H, Dai H. Sulfonated poly(phenylene oxide) membranes as promising materials for new proton exchange membranes. 2006;17: 360–5. [89] Li PC, Liao GM, Kumar SR, Shih CM, Yang CC, Wang DM, et al. Fabrication and characterization of chitosan nanoparticle-incorporated quaternized poly(vinyl alcohol) composite membranes as solid electrolytes for direct methanol alkaline fuel cells. Electrochim Acta 2016;187:616–28. [90] Shaari N, Kamarudin SK. Chitosan and alginate types of bio-membrane in fuel cell application: an overview. J Power Sources 2015;289:71–80. [91] Wu Q, Wang H, Lu S, Xu X, Liang D, Xiang Y. Novel methanol-blocking proton exchange membrane achieved via self-anchoring phosphotungstic acid into chitosan membrane with submicro-pores. J Membr Sci 2016;500:203–10. [92] Merle G, Hosseiny SS, Wessling M, Nijmeijer K. New cross-linked PVA based polymer electrolyte membranes for alkaline fuel cells. J Membr Sci 2012;409-410:191–9. [93] Erkartal M, Usta H, Citir M, Sen U. Proton conducting poly(vinyl alcohol) (PVA)/ poly (2-acrylamido-2-methylpropane sulfonic acid) (PAMPS)/zeolitic imidazolate framework (ZIF) ternary composite membrane. J Membr Sci 2016;499:156–63. [94] Allan JTS, Prest LE, Easton EB. The sulfonation of polyvinyl chloride: synthesis and characterization for proton conducting membrane applications. J Membr Sci 2015;489:175–82. [95] Woo JJ, Seo SJ, Yun SH, RQ F, Yang TH, Moon SH. Enhanced stability and proton conductivity of sulfonated polystyrene/PVC composite membranes through proper copolymerization of styrene with -methylstyrene and acrylonitrile. J Membr Sci 2010;363:80–6. [96] Chen BK, TY W, Kuo CW, Peng YC, Shih IC, Hao L. 4.4′-Oxydianiline (ODA) containing sulfonated polyimide/protic ionic liquid composite membranes for anhydrous proton conduction. Int J Hydrogen Energy 2013;38:11321–30. [97] Akbarian-Feizi L, Mehdipour-Ataei S, Yeganeh H. Survey of sulfonated polyimide membrane as a good candidate for Nafion substitution in fuel cell. Int J Hydrogen Energy 2010;35:9385–97. [98] Chen BK, Wong JM, TY W, Chen LC, Shih I. Improving the conductivity of sulfonated polyimides as proton exchange membranes by doping of a protic ionic liquid. Polymer 2014;6:2720–36. [99] Lee SY, Yasuda T, Watanabe M. Fabrication of protic ionic liquid/sulfonated polyimide composite membranes for non-humidified fuel cells. J Power Sources 2010; 195:5909–14. [100] Miyatake K, Tombe T, Chikashige Y, Uchida H, Watanabe M. Enhanced proton conduction in polymer electrolyte membranes with acid-functionalized polysilsesquioxane. Angew Chem Int Ed 2007;46:6646–9. [101] Liu D, Geng L, Fu Y, Dai X, Lu C. Novel nanocomposite membranes based on sulfonated mesoporous silica nanoparticles modified sulfonated polyimides for direct methanol fuel cells. J Membr Sci 2011;366:251–7.

29

[102] Geng L, He Y, Liu D, Dai X, Lu C. Facile in situ template synthesis of sulfonated polyimide/mesoporous silica hybrid proton exchange membrane for direct methanol fuel cells. Microporous Mesoporous Mater 2012;148:8–14. [103] He Y, Tong C, Geng L, Liu L, Lü C. Enhanced performance of the sulfonated polyimide proton exchange membranes by graphene oxide: size effect of graphene oxide. J Membr Sci 2014;458:36–46. [104] Kowsari E, Zare A, Ansari V. Phosphoric acid-doped ionic liquid-functionalized graphene oxide/sulfonated polyimide composites as proton exchange membrane. Int J Hydrogen Energy 2015;40:13964–78. [105] Pandey RP, Thakur AK, Shahi VK. Sulfonated polyimide/acid-functionalized graphene oxide composite polymer electrolyte membranes with improved proton conductivity and water-retention properties. ACS Appl Mater Interfaces 2014;6: 16993. [106] Pandey RP, Shahi VK. Sulfonated imidized graphene oxide (SIGO) based polymer electrolyte membrane for improved water retention, stability and proton conductivity. J Power Sources 2015;299:104–13. [107] Mikhailenko SD, Robertson GP, Guiver MD, Kaliaguine S. Properties of PEMs based on cross-linked sulfonated poly(ether ether ketone). J Membr Sci 2006;285: 306–16. [108] Muthu Lakshmi RTS, Choudhary V, Varma IK. Sulfonated poly(ether ether ketone): synthesis and characterization. J Mater Sci 2005;40:629–36. [109] Xing P, Robertson GP, Guiver MD, Mikhailenko SD, Wang K, Kaliaguine S. Synthesis and characterization of sulfonated poly(ether ether ketone) for proton exchange membranes. J Membr Sci 2004;229:95–106. [110] Mikhailenko SD, Wang K, Kaliaguine S, Xing P, Robertson GP, Guiver MD. Proton conducting membranes based on cross-linked sulfonated poly(ether ether ketone) (SPEEK). J Membr Sci 2004;233:93–9. [111] Li L, Zhang J, Wang Y. Sulfonated poly(ether ether ketone) membranes for direct methanol fuel cell. J Membr Sci 2003;226:159–67. [112] Irfan MA, Zaidi SMJ, Shakeel A. Proton conducting composites of heteropolyacids loaded onto MCM-41. J Power Sources 2006;157:35–44. [113] Zaidi SMJ, Ahmed MI. Novel SPEEK/heteropolyacids loaded MCM-41 composite membranes for fuel cell applications. J Membr Sci 2006;279:548–57. [114] Zhang H, Fan X, Zhang J, Zhou Z. Modification research of sulfonated PEEK membranes used in DMFC. Solid State Ion 2008;179:1409–12. [115] Bello M, Zaidi SMJ, Rahman SU. Proton and methanol transport behavior of SPEEK/ TPA/MCM-41 composite membranes for fuel cell application. J Membr Sci 2008; 322:218–24. [116] Prehn K, Adelung R, Henien M, Nunes SP, Schulte K. Catalytically active CNT–polymer-membrane assemblies: from synthesis to application. J Membr Sci 2008;321: 123–30. [117] Jiang Z, Zhao X, Fu Y, Manthiram A. Composite membranes based on sulfonated poly(ether ether ketone) and SDBS-adsorbed graphene oxide for direct methanol fuel cells. J Mater Chem 2012;22:24862–9. [118] Heo Y, Im H, Kim J. The effect of sulfonated graphene oxide on sulfonated poly (ether ether ketone) membrane for direct methanol fuel cells. J Membr Sci 2013; 425-426:11–22. [119] Kumar R, Mamlouk M, Scott K. Sulfonated polyether ether ketone–sulfonated graphene oxide composite membranes for polymer electrolyte fuel cells. RSC Adv 2014;4:617–23. [120] He Y, Wang J, Zhang H, Zhang T, Zhang B, Cao S, et al. Polydopamine-modified graphene oxide nanocomposite membrane for proton exchange membrane fuel cell under anhydrous conditions. J Mater Chem A 2014;2:9548–58. [121] Lee SH, Choi SH, Gopalan SA, Lee KP, Anantha-Iyengar G. Preparation of new selfhumidifying composite membrane by incorporating graphene and phosphotungstic acid into sulfonated poly(ether ether ketone) film. Int J Hydrogen Energy 2014;39:17162–77. [122] Jiang Z, Zhao X, Manthiram A. Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells. Int J Hydrogen Energy 2013;38:5875–84. [123] Zhang N, Wang B, Zhang Y, Bu F, Cui Y, Li X, et al. Mechanically reinforced phosphoric acid doped quaternized poly(ether ether ketone) membranes via cross-linking with functionalized graphene oxide. Chem Commun 2014;50:15381–4. [124] Wu W, Li Y, Chen P, Liu J, Wang J, Zhang H. Constructing ionic liquid-filled proton transfer channels within nanocomposite membrane by using functionalized graphene oxide. ACS Appl Mater Interfaces 2016;8(1):588–99. [125] Zhao L, Li Y, Zhang H, Wu W, Liu J, Wang J. Constructing proton-conductive highways within an ionomer membrane by embedding sulfonated polymer brush modified graphene oxide. J Power Sources 2015;286:445–57. [126] Beydaghi H, Javanbakht M, Bagheri A, Salarizadeh P, Ghafarian Zahmatkesh H, Kashefi S, et al. Novel nanocomposite membranes based on blended sulfonated poly(ether ether ketone)/poly(vinyl alcohol) containing sulfonated graphene oxide/Fe 3 O 4 nanosheets for DMFC applications. RSC Adv 2015;5:74054–64. [127] Miao S, Zhang H, Li X, Wu Y. A morphology and property study of composite membranes based on sulfonated polyarylene ether sulfone and adequately sulfonated graphene oxide. Int J Hydrogen Energy 2016;41(1):331–41. [128] Zhao Y, Fu Y, He Y, Hu B, Liu L, Lü J, et al. Enhanced performance of poly(ether sulfone) based composite proton exchange membranes with sulfonated polymer brush functionalized graphene oxide. RSC Adv 2015;5:93480–90. [129] Ko T, Kim K, Lim MY, Nam SY, Kim TH, Kim SK, et al. Sulfonated poly(arylene ether sulfone) composite membranes having poly(2.5-benzimidazole)- grafted graphene oxide for fuel cell applications. J Mater Chem A 2015;3:20595–606. [130] Gahlot S, Sharma PP, Kulshrestha V, Jha PK. Dramatic improvement in water retention and proton conductivity in electrically aligned functionalized CNT/SPEEK nanohybrid PEM. ACS Appl Mater Interfaces 2014;6:5595–601.

30


[131] Muzzarelli RAA. Potential of chitin/chitosan-bearing materials for uranium recovery: an interdisciplinary review. Carbohydr Polym 2011;84:54–63. [132] Yang JM, Chiu HC. Preparation and characterization of polyvinyl alcohol/chitosan blended membrane for alkaline direct methanol fuel cells. J Membr Sci 2012;419420:65–71. [133] Srinophakun T, Martkumchan S. Ionic conductivity in a chitosan membrane for a PEM fuel cell using molecular dynamics simulation. Carbohydr Polym 2012;88: 194–200. [134] Smitha B, Sridhar S, Khan AA. Chitosan–sodium alginate polyion complexes as fuel cell membranes chitosan–sodium alginate polyion complexes as fuel cell membranes. Eur Polym J 2005;41:1859–66. [135] Liu Y, Wang J, Zhang H, Ma C, Liu J, Cao S, et al. Enhancement of proton conductivity of chitosan membrane enabled by sulfonated graphene oxide under both hydrated and anhydrous conditions. J Power Sources 2014;269:898–911.

[136] Shirdast A, Sharif A, Abdollahi M. Effect of the incorporation of sulfonated chitosan/ sulfonated graphene oxide on the proton conductivity of chitosan membranes. J Power Sources 2016;306:541–51. [137] Bai H, Li Y, Zhang H, Chen H, Wu W, Wang J, et al. Anhydrous proton exchange membranes comprising of chitosan and phosphorylated graphene oxide for elevated temperature fuel cells. J Membr Sci 2015;495:48–60. [138] Sharma PP, Kulshrestha V. Synthesis of highly stable and high water retentive functionalized biopolymer-graphene oxide modified cation exchange membranes. RSC Adv 2015;5:56498–506. [139] Pandey RP, Shahi VK. A N-o-sulfonic acid benzyl chitosan (NSBC) and N,Ndimethylene phosphonic acid propylsilane graphene oxide (NMPSGO) based multi-functional polymer electrolyte membrane with enhanced water retention and conductivity. RSC Adv 2014;4:57200–9.