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Graphene and Graphene Oxide for Fuel Cell Technology Ramdayal Yadav, Akshay Subhash, Nikhil Chemmenchery, and Balasubramanian Kandasubramanian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02326 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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Industrial & Engineering Chemistry Research

Graphene and Graphene Oxide for Fuel Cell Technology Ramdayal Yadav a, Akshay Subhash b, Nikhil Chemmenchery b, Balasubramanian Kandasubramanian a,* a

Structural Composite Fabrication Laboratory, Department of Metallurgical and Materials

Engineering, Defence Institute of Advanced Technology (DU), Ministry of Defence, Girinagar, Pune-411025, India. b

Department of Polymer Engineering, University College of Engineering, Thodupuzha, Idukki, Kerala – 685587, India. * Corresponding Author E-Mail: [email protected]

Abstract The Proton Exchange Membrane Fuel Cell (PEMFC) converts chemical energy into electrical energy via electrochemical reaction between Hydrogen and Oxygen, with a byproduct as heat & water. When a PEMFC is engineered with polymer electrolyte membrane (PEM) e.g. Nafion & Polybenzimidazole (PBI), it helps in enhancing the performance of the fuel cell under monitored environmental conditions i.e. high proton conductivity, improved electrode kinetics, tailoring of properties, along with low tolerance for carbon monoxide. Recently, discovered ‘Graphene’ has enticed the scientific community, due to their exceptional properties. As per the literature, PEMFC’s engineered with Graphene can yield high power density, along with 38% enhanced current density, and 257% improved ionic conductivity. In this context, the present review gives the state-of-the-art and progress on the Graphene & Graphene oxide engineered Polymer Electrolyte Membranes, their synthesis routes, and the influence on the performance of PEMFC’s.

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

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Keywords: Polymer Electrolyte Membrane Fuel Cell; Graphene; Graphene Oxide; Power Density; Current Density. 1. Introduction The current global energy needs are mainly sustained by the fossil fuel based energy sources, and due to unprecedented use, they are diminishing at a higher rates, further they have raised environmental concerns due to their poisonous emissions in the nature1. Currently, hydroelectric~7%, natural gas~22.5%, coal~23.3%, and oil~40%, are catering the global energy needs2. The limitation of fossil fuels and the humankinds need for alternative energy sources, lead to the development of fuel cells, which converts chemical energy into electrical energy via electrochemical reactions using oxidants and reactants

1,2

. Recently, Polymer Electrolyte

Membrane Fuel Cells (PEMFC) (Figure 1) have emerged as the promising and environment friendly clean source of energy, and have been categorically used for stationary and automobile applications, coupled with portable power energy devices. PEMFC mainly utilizes Hydrogen (H2) and Oxygen (O2) as the reactants and yields water as a byproduct

3,4

. The primary

components the PEMFC are anode, cathode and proton (H+) conducting electrolyte membrane, collectively known as Membrane Electrode Assembly (MEA) [Figure 1 (a)] 3-5.


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Figure 1. (a) PEM Fuel Cell Assembly, (b) Chemical Structure of Nafion Membrane, (c) Chemical Structure of PBI Membrane. Generally, these proton conducting PEM’s are coated with catalyst layers on anode and cathodes sides. These catalyst layers split the hydrogen (H+ and e-) and oxygen (H+ and OH-) molecules and facilitate following chemical reactions through the PEM, as delineated in eq 1-3 6-8

.

At Anode: 2H 2 → 4H + + 4e-

(1)

At Cathode: O2 +4e- +4H + → 2H 2 O

(2)

Net Reaction: 2H 2 +O 2 → 2H 2 O+heat

(3)

As compared to conational, the performance efficiency of the Fuel Cells is found to be higher with respect to power output, as shown in Figure 2 1,6-10.



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Figure 2. Power Generating Systems Efficiency Comparison. Table 1. Different Categories of Fuel Cells. Type

Temperature °C

Electrolyte

Reference

Polymer Electrolyte

70–110, and 150-

Nafion, Polybenzimidazole (PBI)

11

Membrane Fuel Cell

200

Alkali Fuel Cell

100–250

Aqueous potassium hydroxide

11

Phosphoric Acid Fuel

150–250

Phosphoric acid

12

500–700

(Na,K)2CO3

12

700–1000

(Zr,Y)O2-δ

12

Cell Molten Carbonate Fuel Cell Solid Oxide fuel Cell


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The electrolyte membrane i.e. thin permeable sheet, is the main component of the PEMFC, which facilitates proton conduction with the help of anode & cathode catalyst layers. Based on the operating temperature, they are categorized as low temperature membrane and high temperature membrane. The low temperature membranes operate at a temperature range of 7090°C, and high temperature membranes operate at a temperature range of 150-200°C

13,3

as

elucidated in Table 1. Nafion [Figure 1 (b)] is the most widely utilized electrolyte membrane in PEMFC’s for low temperature applications, and Polybenzimidazole (PBI) [Figure 1 (c)] for high temperature operations . The chemical nature of Nafion facilitates hydration, which leads to enhanced proton transfer. The main advantages of the PEM’s are high proton conductivity, light weight, flexibility, enhanced thermal and mechanical strength compared to electrolytes present in ordinary fuel cells14. Further, the PEMFC’s operate at low pressure, which increases the safety and also provides quick start up3. The Nafion based PEM’s are widely used in PEMFC’s, which is attributed to their high conductivity to cations, making it suitable for many membrane applications. The Nafion PEM exhibits, polytetrafluoroethylene (PTFE) which is attached with sulfonate groups (ionic nature)

15,16

. As per the DuPont, only alkali metals (specifically sodium)

are able to degrade Nafion under normal operating conditions. The conglomeration of tetrafluoroethylene backbone, sulfonic acid groups, and the matrix make Nafion a robust PEM for low temperature operations.15,16 It exhibits great selectivity and permeability for water. Further, Nafion PEM

facilitates proton conduction up to 0.2 S/cm, owing to operating

temperature and humidification state. The proton conduction in Nafion membrane is possible through the Nafion membrane under humidified conditions, and hence Nafion based PEM’s can be used till 100°C for low temperature applications, beyond which water evaporates out 15-17.



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This drawback of Nafion PEM’s was overcome by Polybenzimidazole (PBI), which operate at 150-200°C under acidic conditions i.e. Phosphoric acid . The PBI based PEM’s exhibit advantages such as high chemical kinetics at the electrode, easy thermal & water management, and further utilization of heat. Another advantages include decreased poisoning possibility of catalyst (due to impurities), and very low tolerance for CO and CO2 18-20. For enhancing the performance efficiency of the PEM’s, the Platinum (Pt) based catalysts are considered to be the most efficient electrocatalysts for oxidation of Hydrogen and the Oxygen reduction reactions (ORR). Although, being efficient in electrocatalysis, the Pt catalyst sometimes causes emission of Carbon Monoxide (CO), further the fuel stream also sometimes contain traces of hazardous elements like CO, Sulphur (S), NH3

21

.

In one of the study, S.

Gottesfeld et al. have reported that the low temperature operation (below 150°C) of PEMFC’s can cause CO poisoning due to higher negative change in free energy (∆G), which is favorable for CO adsorption on Pt (due to its affinity) engineered electrodes

22

. In order to curtail the

generation of CO, various advanced methods have been proposed, such as oxidative surface environment

23,24

, and utilization of electrical pulse (these methods convert CO to CO2)

25

. In

addition to this, the high cost of the platinum based catalyst is another concern for commercial utilization of PEM fuel cell 26. In this context, materials like carbon based nano materials (nano tubes, nano fibres, bucky nano balls, graphite particles and graphite nano sheets) have been explored by researchers for minimizing the traces of CO, the material cost, and simultaneously improving the efficiency of the PEMFC’s 26-28. The discovery of two-dimensional single layered ‘Graphene’ brought a revolution in the materials science field

29,30

specific area (2630 m2/g)

. Graphene is considered as an important material due to its steep

31

, exceptionally high electrical conductivity (104.63 S/cm)


32

, high

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flexural strength (44.28 MPa) 33 and enhanced chemical stability 34. For the large-scale synthesis of graphene, techniques such as hydrogen exfoliation, 35 focused solar exfoliation 36,37 have been used. The rise of Graphene based nanomaterials opened a new window for the fabrication of low cost electro catalyst system

37,38

. In one of the study, Lee et al. have fabricated GO/Pt

nanoparticle system microwave method, along with Nafion/GO electrolyte membrane. The reported Nafion/GO membrane shows improvement in performance of the cell i.e. 0.802 A, 1.27A, and 0.827A at 0.6 voltage under 100% relative humidity (RH)for 0.5, 3.0 and 4.5wt% loadings of GO content in the Nafion/GO membrane, respectively, compared to pristine Nafion 115 membrane

39

. In another study, Chien et al. have fabricated Nafion membrane engineered

with sulphonated Graphene oxide (exfoliated) (SGO) via blending. Their XRD and rheological characterizations enhanced results for SGO/Nafion membrane in for 0.05 to 0.5 wt% loadings of GO. The nano/-micro sized SGO particles in the SGO/Nafion membrane showed higher selectivity via steric hindrance effect and reducing the ionic clusters. The enhanced proton conductivity (0.0367 S.cm-1) at low relative humidity (65 RH%) and a water retention capability are attributed to largely functionalized SGO 40. Xue et al. reported a fabrication of polybenzimidazole (PBI) membrane via synthesis of 3, 3′diaminobenzidine (GO/PBI), and 5-tert-butyl isophthalic acid (GO/BuIPBI), and isocyanate engineered graphite oxide/BuIPBI (iGO/BuIPBI) membranes as electrolytes, where proton conductivity was achieved using phosphoric acid. The SEM analysis revealed fine dispersion of GO, and strong connection with BuIPBI membrane resulting in enhanced stability and compatibility with larger acid content. The electrochemical analysis revealed a improved proton conductivities of 0.016 and 0.027 S/cm, for GO/BuIPBI and iGO/BuIPBI electrolyte membranes, respectively, at a temperature of 140°C,under no humidity 41. In yet another study, Üregen et al.



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have described the fabrication of phosphoric acid doped PBI/GO electrolyte membranes for high temperature PEMFC’s. Their characterizations revealed an presence of GO into PBI matrix, which helped in enhancing the doping of acid, and proton conductivity. The SEM characterization revealed uniform distribution of GO in PBI. They tested GO/PBI electrolyte membranes in a single cell type PEMFC system with a 5 cm2 active area at 165°C temperature, under no humidification. The electrochemical analysis of GO/PBI membrane revealed that wt.% loading of GO improved the performance of the membrane under no humidification. Further, the GO/PBI membranes showed a power density of 0.38 W/cm2, current density of 0.252 A/cm2 H2 & Air flow at 0.6 V, at 165°C temperature under ambient pressure 42. In this context, the present review article gives the state-of-the-art and progress on Graphene engineered electrolyte membranes for PEMFC’s. Further, the review article discusses in detail the properties, synthesis routes and the applications of Graphene and Graphene oxide (GO), and their influence on the functionalized polymer electrolyte membranes

38,43-45

. Finally, the review

concludes with the future scope, and challenges associated with the development of the high performance PEMFC’s.

2.

Graphene based Nanomaterials Graphene is an emerging material in the field of material science, nanotechnology and

condensed matter physics. The separation of Graphite to a single layer by Scotch tape exfoliation technique resulted in the invention of Graphene (Figure 3)

45-50

. The sp2 hybridized carbon

atoms present in the Graphene consist of tightly arranged hexagonal crystal lattice

46

. The

excellent properties of Graphene forced the science world to discover its derivative. The unique properties are observed for the derivatives e.g. Graphene Nanoplatelets (GNP), Few Layer Graphene (FLG), Graphene Nano-Onions (GNO), Graphene Nano Ribbons (GNR), Graphene


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Oxide, and Reduced Graphene Oxide (rGO), which varies upon different functions such as defect density, number of layers, surface chemistry, lateral dimension, composition, purity and quality

of

graphene

6,

sheets

45-58

.

Figure 3. Synthesis Routes of Graphene. The Graphene can be synthesized via various methods such as Chemical Vapour Deposition (CVD)

59

, Liquid Phase Exfoliation (LPE)

60,61

, Mechanical Exfoliation

62

,

Electrochemical Exfoliation 62, and Bottom Up Synthesis 59-68 (Figure 3). Graphene is considered as a good reinforcing agent in composites because of its excellent mechanical properties

69

. Recently, researchers have found using interferometric profilometry,

that the Graphene is greatly softened by out-of-plane buckling, whereas the in-plane firmness is found to vary between 20–100 N.m-1 at room temperature 70.The intrinsic strength of the single layered thin lamina is 42 N/m, which yields an intrinsic strength of 130 GPa 71. Elegantly, it has been discovered that the robustness and firmness of graphene are managed continuously at greater densities of sp3-type irregularities, although the fracturing strength is only 14%



underneath the pristine graphene, in the sp3- defect establishment. It has been observed that, when Graphene runs in the direction of through to the gap-irregularity area, its strength reduces remarkably. As a conclusion of the geometrical locking effect, wrinkled multilayer graphene is more boosted related to its plane analogy

71

. It is reported that wrinkled graphene shows the

shear modulus of 1100 MPa and strength of 610 MPa with a corresponding aspect ratio of 0.177. The reuniting of corrugated graphene can point to the build-up of a thin lamina with a tensile strength of over 12 GPa, as a consequence of its formidable interlayer shear modulus and strength 70-80.

Graphene Oxide for Fuel Cell The Graphene oxide (GO) is a derivative of Graphene, which exhibits carbon to oxygen ratio of 2:1 81. The presence of various oxygen groups such as epoxide, hydroxide, carbonyls and carboxyls turns GO into insulating and hydrophilic material (Figure 4)82, with the retention of other properties like mechanical strength, gas impermeability and surface area electrical insulation, gas permeability, hydrophilicity, and proton conductivity make GO promising material for electrolyte membranes in PEMFC

81-83

. The disruption of the lattice in the GO is

emulated as an increase in the interlayer spacing from graphene (0.335nm) to graphene oxide (more than 0.625nm) 82-84.


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Figure 4. Generation of GO, and the resulting Proton conductivity.



Commercially, Hummers method has been widely used for the production of GO 85. The GO production is accomplished using series of treatments with H2SO4, NaNO2, KMnO4, and DI water, followed by vacuum drying as shown in figure 4.

Why Graphene and Graphene Oxide (GO) for Fuel Cell Applications? Recently, Graphene based materials have been widely used for fuel cell applications, due to their exceptionally high conductive properties and their stability during processing of electrolytes 30,40-45. In one of the study, Seger and Kamat have reported that Carbon based nanomaterials such as Graphene enhances the nano-scaled electrocatalyst surface area for improved transfer of electrons, and further help in accelerating the mass transport of reactants i.e. fuels, to the electrocatalytically active surface. Further, the enhanced conductivity helps in accumulation and transport of electrons to the electrocatalytically active surfaces (Figure 5). The Graphene can be surface modified with functional groups such as carboxyl and epoxies e.g. Graphene oxide (GO). Graphene sheets can also help in holding semiconductor particles like TiO2. The ability of these sheets to uphold nanoparticles has opened new window for development of electrocatalysts for fuel cell applications 86.


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Figure 5. Graphene oxide (GO) based Electrocatalyst/Electrode system. Graphene could be produced at low cost in bulk quantity if the source materials are Graphite, its oxide, and sub-derivatives. It is also presumed that the planar 2D planar carbonsheet permit the edge planes to connect with catalyst nanoparticles. Further, the planar structure of these materials could promote active and large surface area for expediting the catalyst particles. Considering these highly enhanced properties of Graphene based materials, they can also be utilized as effective catalysts for cathodes of PEM fuel cells, due to their greater catalytic affinity towards Oxygen reduction reactions (ORR) Therefore, considering these attributes of Graphene anchored nanomaterials, they can be propitious contenders for metal free electrocatalyst systems, due to their exceptional catalyst upholding capacity 87.



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Some properties typical properties of Graphene, and modified Graphene oxides have been represented in Table 2, and Table 3, respectively. Table 2. Properties of Graphene. Properties

Maximum Observed Values

References

Carrier transport velocity

40 GHz

53

Current carrying capacity

109 A/cm2

53

Specific area

2630 m2/g

53

Superior thermal conductivity

5300 W/Mk

54

Ultra-high electron mobility

250,000 cm2/Vs

54

Modulus of elasticity

1 TPa

54

Fracture strength

130 GPa

54

Elastic modulus

32 GPa

54

Table 3. Electrolyte based Properties of Graphene oxide.88 Graphene Oxide Synthesized

Chemically reduced GO

Specific Surface Specific Capacitance References Area

(in Electrolytes)

705 (m2.g-1)

135 (in KOH) 99 (in 89 Organic)

Chemically reduced GO

320 (m2.g-1)

205 (in KOH)

90

Microwave expansion of GO

463 (m2.g-1)

191 (in KOH)

91


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Thermal reduction of GO in -

122 (in Organic)

92

PC Thermal expansion of GO 925 (m2.g-1)

117 (in H2SO4) 75 93

(1050°C)

(Ionic Liquid)

Thermal expansion of GO 368 (m2.g-1)

264 (in KOH) 122 (in 94

(200°C, vacuum)

Organic)

rGO-SnO2 composite

-

43 (in H2SO4)

95

GO-MnO2 composite

-

216 (in Na2SO4)

96

1% GO doped Polyaniline

-

531 (in H2SO4)

97

Considering the zero band gap in Graphene, it exhibits semiconductor characteristics, along with a bipolar effect, where charge carriers can be adjusted continually within electrons and holes exhibiting concentrations up to 1013 cm− 2 , with movability at room temperature up to 15 000 cm2.V-1.s-1. The perceived movability in Graphene is less dependent at room temperature, which suggests that very high movability can be achieved at this temperature for Graphene. The movability being more than 200000 cm2.V-1.s-1 has been accomplished by reducing scattering impurities. The movability of Graphene is found to be higher for large carrier densities observed in electrically & chemically doped devices, showing ballistic transport capability at sub-micron range 88. In another study, Li et al. have reported a catalyst system based on reduced Graphene oxide (rGO) for Oxygen reduction reactions (ORR) for fuel cell applications. The catalyst system prepared by mixing Platinum (Pt) loaded rGO with carbon black, thus forming a hybrid composite system. They claimed that, the incorporation of Carbon black particles helps in



Page 16 of 67

preventing the stacking of the rGO sheets, which exhibits the tendency to stack-together under the influence π−π interactions even when loaded with nanocrystal systems i.e. Pt. Further, they reported that the stacking of rGO sheets impedes significant quantity of catalytic locations on nanocrystals, thus ultimately imparts obstruction for the diffusion of the reacting molecules, thereby retarding the efficiency of the catalyst system. Thus, the rGO loaded with Pt catalyst and supported with Carbon black helped in elevating the diffusion of the oxygen (O2) molecules, and further improving the efficiency of the catalytic activity. Their accelerated durability test i.e. ADT, demonstrated that the hybrid supporting material can impressively increase the durability of the rGO based catalyst and still can retain active surface area of Pt nanocrystals i.e. >95%, till 20,000 ADT cycles, much larger than the commercial catalyst system. Further, they reported that the uniquely ordered two dimensional structure of rGO acts as an obstacle to Pt nanocrystals, thereby impeding it’s leaching in the electrolyte, and in addition the presence of Carbon black particles provide an active location for recapturing the disintegrated Pt nanocrystals 98. It is reported that, the electronic/electrocatalytic properties of GO depends on the structural dis-arrangement. Generally, the GO sheets exhibit insulating nature due to the electron density of state, which leads to energy gap, thus leading to a sheet resistance (RS) of ≥1012 Ω /sq 99-101.The progressive removal of O2 i.e. formation of reduced GO (via various treatments), promotes the transport of carriers,102,103 resulting in reduced RS by many folds , thus imparting semiconductor characteristics to rGO i.e. semimetal similar to Graphene

99-112

. The electrical

conductivity of rGO can attain up to ~1000 S/m,109,112. Theoretical calculations demonstrate that the oxidation level dependent approximate local density band gap of GO can fluctuate over few electron volts (eV). 91, thereby implying its capability to tune the energy gap via monitored reduction processes. The researchers have shown that the GO possesses outstanding


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electrochemical capacitance capability with great cycle efficiency, thus demonstrating its applicability as ultracapacitor. Shao et al. claimed that the rGO reveals exceptionally high electrochemical capacitance (ECC) and cycle durability much greater than carbon nanotubes (CNT’s). They demonstrated the specific capacitance to be around ~165 F/g, & ~86 F/g for rGO & CNT, respectively 114,115. Considering, the enhanced properties demonstrated by Graphene and Graphene oxide (GO), it can be clearly concluded that these Graphene based materials are efficient and effective catalyst systems (Figure 5) for the functionalization of electrolyte membranes for PEMFC applications 86-115.

3.

Graphene and Graphene Oxide in Fuel Cells 3.1 Direct Methanol Fuel Cell (DMFC)

Figure 6. Schematic of DMFC 116.



DMFC is a type of PEM Fuel Cell with simple operation, design and high efficiency as shown in Figure 6. Even though the development of proton conductive membrane in DMFC is difficult, Nafion membranes are widely used. Along with Nafion, there are other polymeric membranes like poly (ether ether ketone) (PEEK), polybenzimidazole (PBI), as well as polysulfone are also used as a membrane. But, Graphene based membranes received more attention in this field

116

. After PEMFCs, GO has proven its worth as a membrane material in

DMFC as well; it provides a unique two dimensional structure with flexible functionalization and admirable thermal stability as per criteria for PEM. The presence of various functional group makes it more flexible for functionalization compared to other organic material 117. By combining graphene oxide with the Nafion it is observed that a decrease in methanol permeability and ionic conductivity by 70% and 22% respectively

118

, also it has better selectivity than a recent Nafion

membrane. The graphene oxide engineered membranes preserves the ionic conductivity and its methanol crossover gets decreased. The incorporation of graphene monolayer with Nafion resulted in increased proton permeability and decreases methanol permeability by 68.6% in comparison to natural Nafion membrane 119. It is observed that Nafion/GO membrane has 2 times slower methanol permeability and 5 times higher selectivity than the recast Nafion 117 membrane in the same conditions. Nafion/GO composite membrane has also shown an exceptional open circuit voltage of 0.67 V with a maximal power density 64.38 mW/cm2 along with 200mA/cm2 current density at 600C

120

. Sulphonated poly (ether ether ketone) also has an excellent affinity

with GO. Recently GO was modified with sodium dodecylbenzene sulphonate (SDBS) and this modified filler was incorporated into SPEEK membrane for DMFC application 121. The basic principle to increase the property of electrolyte membrane is by combining basic imidazole group (accept proton) from histidine molecules and acidic –SO3H groups (donate


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proton) from SPEEK to form acid base pairs. Thus, it can be concluded that the proton conductivity of combined membrane was increased by 30.2% as compared with pure SPEEK membrane. Low methanol permeability of 1.32-3.91 x 10-7 cm2/s, an excellent selectivity of 5.14 x 10-5 S s/cm3 and 80.7% maximal power density is also acquired by the hybrid membrane than the pure SPEEK membrane

122

. By adding Zwitter coated GO (ZC-GO) to PBI for DMFCs it is

observed that the proton conductivity and methanol permeability got values 4.12 x 10-2 s/cm and 1.38 x 10-7 cm2/s respectively which is better than Nafion 117. The obtained PBI/ZC-GO composite membrane is an exceptional component for DMFC application 123. GO paper can also act as an electrolyte in fuel cell due to its higher methanol permeability, which is around 18.2 x 10-6 cm2/s higher than that of Nafion 115 (3.36 x 10-6 cm2/s) 124. Graphene itself can be used as a proton exchange membrane for DMFC application

125

. Synthesis of GO with 3-mercaptopropyl

trimethoxy silane will result in sulfonated organosilane functionalized GO (SSi-GO), which is added to SPEEK and used in DMFC application. Optimal and controlled SSi-GO content exhibited around 17% and 38% higher power densities as compared to Nafion 115 and Nafion 112 membranes respectively 125.

3.2 Microbial Fuel Cell (MFC)



Figure 7. Microbial fuel cell 126. MFCs generates electricity from biodegradable substrate that are present in wastewater (Figure 7). As a result of the reaction protons are generated, which must pass through the membrane 126-131. The utilization of graphene as microbial fuel cell has been extensively reviewed by Pant et al. and they have demonstrated that microbial fuel cells are sustainable green energy source that exploits available chemical energy in organic waste to produce electricity 132-134. The conversion of chemical energy of organic waste into electrical energy by the virtue of microbial metabolism which release electron and subsequently passed to solid electrode via direct or indirect extracellular electron transfer mechanism. It has been advocated that graphene possess the ability to promote microorganism to express signaling molecules and act further as a mediator to improve the electron transfer efficiency. Pant et al. have reported that the microbial fuel cell


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electrode fabricated by hybrid graphene (HG) exhibited improved power density for graphite paste- hybrid graphene (GP-HG) (220 mW/m2), compared to 30 mW/m2 for GP electrode and 80 mW/m2 for GP-TiO2

135

. In addition, the modified electrodes demonstrated lower charge transfer

resistance (CTR) compared to the bare electrode. To improve the membrane property GO is incorporated into polyvinyl alcohol silicotungstic (PVA-STA) solution to obtain a 100µm semi permeable membrane. This addition of graphene results in the improved conductivity of PVA-STA-GO membrane up to 0.065 s/cm, however, it was around 0.046 s/cm of pure PVA-STA membrane and 0.062 s/cm of Nafion 117 membrane. Also, this addition leads to increase the mechanical strength to a value of 39.1 MPa. Moreover, the power density of GO filled membrane was excellent with 1.19 W/m3, whereas 0.88 W/m3 is observed for pure Nafion 117 membrane 136. Graphene is also added with SPEEK to prepare more functionally advanced membrane for MFCs. The addition of GO with SPEEK will provide a proton conductivity of 1.48 x 10-3 S/cm and oxygen diffusion coefficient of 1.154 x 10-6 cm2/s. Highest columbic efficiency of 16.86% is also observed with GO/SPEEK composite membrane. The pure SPEEK membrane will only provide a power density of 812 mW/m2 whereas the SPEEK/GO composite membrane exhibit a power density of 902 mW/m2 137.



Figure 8. Schematic diagram showing incorporation of MWCNT into graphene 137. To fabricate electrode in MFC, Carbon nano tube(CNT) and graphene can be used, among these two graphene is preferred because for CNT it is difficult to ensure enough space for bacterial growth due to the lack in its 3D structure. Polytetrafluoroethylene solution is mixed with a 1% weight solution to obtain a paste form, which is applied on both sides carbon cloth, used as the anode. To improve the performance of anode, multi-walled carbon nanotube (MWCNT) is inserted into a bundle of graphene (Figure 8). This insertion inhibits the accumulation of generating electron as well as maintain enough surface area needed for bacterial production and the electrochemical reaction. Also, this incorporation of MWCNT into graphene helps to enhance the microbial growth on anode surface leads to increase in the cell performance 80

. The combined effect of graphene derivative and carbon nanotube results in an increase of

higher capacitive current as well as redox peak current 86-115.

3.3 Alkaline Fuel Cell (AFC)


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Figure 9. Diagram of an Alkaline fuel cell 138. AFCs as described in figure 9 were mainly used in space shuttle programs and the Apollo space program due to its high operational temperature (100 & 250°C) and higher efficiency (around 60%). The alkaline condition of this cell will enhance the electrode reaction kinetics

138

. For improving the overall performance of this cell graphene modified polyvinyl

alcohol (PVA/GO) composite membranes are used. The existence of graphene in membrane increases ionic transport by providing a well-connected and a continuous ionic channel in the membrane morphology. The ionic conductivity of GO engineered membrane improved by 126% and point out a 55% decrease in methanol permeability with 0.7wt% graphene loading. This



composite membrane also shows an incredible increase in power density (148%) and tensile strength (73%) 139,140. For the preparation of alkaline exchange membrane, a multilayer GO was modified with KOH, which will be effective in the AFC 139,140. As a result of the incorporation of this membrane into the cell, maximum ionic conductivity of 6.1 mS/cm at 700C and an ion exchange capability of 6.1mmol/g is obtained 139,140. The vacancies present on graphene surface help the metal ions to correlate with it, which is the difference of graphene from another carbon based materials. By taking this advantage of graphene, Nitrogen is added at the time of preparation leads to the formation of a catalyst with nitrogen doped reduced graphene (rGO(N)) which possess high oxygen reduction reaction property 122. Usually in alkaline fuel cell Pt/carbon composite material is used as the cathode, to enhance the performance of the cell rGO(N) is added along with cobalt oxide results in the formation of newly efficient cathode. This hybrid cathode will upgrade the oxygen reduction reaction and electron selectivity of the cell. The mingled effect of Nitrogen doping and the presence of a defect will help the graphene to hold more Co which results in the formation of a narrow cathode layer with improved mass transfer 141.

3.4 Direct Glucose Fuel Cell (DGFC)


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Figure 10. Direct Glucose Fuel Cell. The main advantage of this fuel cell is its non-toxic nature as well as simple operation, because the main reaction takes place in this cell is the oxidation of glucose to carbon dioxide which will result in an open circuit potential of 1.24V (Figure 10). The cathode of DGFC is fabricated using graphene, to enhance the cell performance by the addition of graphene oxide with 5% polytetrafluoroethylene. Activated carbon based electrode can also use as an electrode instead of graphene based materials

142

. The DGFC consisting of graphene oxide with

microporous silica (MPS) membrane have a peak power density of 5.3µW/cm2. Also, this GO/MPS hybrid fuel cell exhibit large open circuit potential of 314.6mV, whereas for with activated carbon electrode cell has only a potential of 160mV, which indicates the reduced passage of oxygen between anode and cathode 143.

3.5 Direct Ethanol Fuel Cell (DEFC)



Figure 11. Direct Ethanol Fuel Cell. In this type of cell, ethanol is mainly used as a fuel (Figure 11). In DEFC, electrode fabrication is based on Pt catalyst, but its toxic nature and high cost are a question mark in its use. High surface area and robust electro conductivity of graphene makes it as a good candidate for electro catalyst 144. Strong activeness towards electro oxidation of alcohols as well as glucose make metal loaded graphene as a better potential catalyst. In this cell Pd-Pt is added to graphene aerogel along with Nafion for the construction of the electrode. It is observed that the maximum output power achieved is 3.6mW/cm2,which is higher than the individual values of Pd-Pt electrodes 145.

3.6 Bipolar Plates


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Figure 12. Fuel Cell with Bipolar Plates 146. For the uniform distribution of fuel gas, air, remove heat from the active area, conduct electrical current from one cell to another and arrest the omission of coolant, there is a component in PEM known as Bipolar plates which accounts about 80% of total weight (Figure 12). These plates must be of light weight material, inexpensive and easy to manufacture. The materials used in its fabrication are electro graphite, sheet metal and graphite

146

. Polymer

electrolyte membranes provide an optional track to bipolar plate materials, because of its lightweight and easiness in manufacturing. Carbon composite bipolar plates have been manufactured using thermoplastic or thermosetting resins along with additives, and further can be reinforced with fiber. It has been observed that 1% graphene heightened the in-plane and through-plane electrical conductivities of the composite from 415.05 to 435.31 S/cm and 99.70 to 130.17 S/cm, correspondingly 146. The reinforcement of 1% graphene to these plates increases the peak power output from 397 to 437 mW/cm2 at current densities 752 and 827 mA/cm2



correspondingly. The convincing hike in in-plane along with through-plane electrical conductivities around 4% and 23% of the composite bipolar plate, the performance of the fuel cell gets improved 147.

3.7 Graphene as Electrocatalyst In a fuel cell, the electrode is one of the key component for gas diffusion electrode. Mainly 3 components are present in an electrode, they are; a porous layer of gas, a gas diffusion layer capable of conducting electricity and an electrode supporting material. For the construction of fuel cell electrodes, various materials are used as substrate. Above all, carbon based material such as graphene, carbon fiber paper along with crisscross carbon are used as a base for the electrode laminas. Carbon fiber paper is a powdered material at elevated temperature, has almost a rigid structure and outstanding channel of electricity. Although crisscross carbon is extra extensible, it is an excellent conductor, in addition to immense power execution attained by more desirable water management. For the construction of the electrode in microbial fuel cell two different morphologies of Graphene are invented, they are of flat lamina of paper as well as rumpled paper balls

80

. The bulky surface width along with open structure for easy transport of

fuels and ions makes crumpled graphene more useful than flat sheet paper, which will result in steep electrical conductivity and catalyst activity of oxygen reduction 86-115.

3.8 Graphene as An Oxygen Reduction Reaction Catalyst (ORR) In PEM fuel cell, Pt and its alloys are contemplated to be the most efficient oxygen reduction reaction (ORR) catalyst due to their low over potential and high current density but serious intermediate tolerance, time dependent drift, anode crossover, sluggish kinetic, poor stability in an electrochemical environment and high cost circumvent their effective commercial


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utilization

148,149

. Wang et al. elucidated that alternative catalyst like metal or metal oxides

endure from dissolution, sintering, and agglomeration which results in degraded catalyst performance during operation of the fuel cell 150. In this context, nanostructured catalyst support like carbon (active carbon, porous carbon, carbon nanotube, graphene), metal carbide, mesoporous silica and conducting polymers have been extensively exploited to augment the catalyst performance and durability during cell operation 151. Jafri et al. have exemplified that the carbon support lead to the improved interaction with Pt which further stabilize Pt nanoparticle and subsequently augment durability and catalyst performance 152. Gong et al. reported vertically aligned nitrogen doped carbon nanotube as oxygen reduction reaction and demonstrated that ORR was facilitated by the virtue of electron accepting capability of nitrogen which generates net positive charge on CNT surface to augment oxygen adsorption 153. In contrast to Gong et al., Li et al. have elucidated that carbon nanotube based electrocatalyst possess lower ORR activity in acidic environment while nitrogen doped multiwalled carbon nano tube or aligned carbon nanotube arrays have exhibited improved ORR activity in alkaline medium but reduced efficiency in acidic condition

154

. Therefore, for enhancing the overall performance of fuel cell

anode and cathode need to be modified, to improve the oxygen reduction reaction (ORR). This improvement in ORR can be achieved by the incorporation of graphene into the electrodes of cells which allows the utilization of its rich macroporosity and multidimensional electron transport pathways

155

. Wu et al have reported the fabrication of three-dimensional N-doped

graphene aerogel supported with Fe3O4 nanoparticle and exhibited positive onset potential, higher cathodic density and higher electron transfer number in alkaline medium compared to Fe3O4 supported N-doped carbon black or N-doped graphene sheets

156

. In another study, Yang

et al. have tailored the electronic arrangement of Graphene by doping similar electronegative



Page 30 of 67

elements like sulfur and selenium and subsequently demonstrated excellent catalyst activity, long term stability and high methanol tolerance in alkaline media compared to the conventionally doped graphene system

156

. The authors have elucidated the transparent sheet and folded feature

of S doped Graphene Figure 13 (a) and further confirmed the presence of S by the virtue of EDX patter as illustrated in Figure 13 (b).

Figure 13. (a) TEM Image of S-graphene-1050 (b) EDX of S-Graphene

156

. (Copyright

permission has been provided as per the guidelines from American Chemical Society. Image taken from ACS Nano, 2012, 6, 205-211). Guo et al. have reported graphene based FePt nanoparticles for active and durable catalyst for oxygen reduction reaction and compared the cyclic voltammograms with C/FePt and commercially available C/Pt catalyst as shown in Figure 14 (a)

157

. The obtained results have

demonstrated that double-layer capacitance of G/FePt nanoparticles are much larger than C/FePt and commercially available C/Pt catalyst. In addition to this, the ORR measurement of the various sample exhibited that the half wave potential of G/FePt nanoparticles (0.557V) is higher


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than C/FePt (0.532V) and commercially available C/Pt catalyst (0.512V), which indicated improved ORR activity for G/FePt nanoparticles in contrast to the C/FePt and commercially available C/Pt catalyst [Figure 14 (b)].

Figure 14. (a) CVs in N2-saturated 0.1 M HClO4 solution at a scan rate of 50 mV/s. (b) Polarization curves for ORR in O2-saturated 0.1 M HClO4 solution at 295 K. The potential scan rate was 10 mV/s and the electrode rotation speed was 1600 rpm

157

. (Copyright permission has

been provided as per the guidelines from American Chemical Society. Image taken from J. Am. Chem. Soc. 2012, 134, 2492-2495). It has been observed that number graphene supported materials have been exploited for the improved oxygen reduction reaction catalyst as tabulated in Table 4. Table 4. Graphene Based Materials for Oxygen Reduction Reaction (ORR) activity 158-164. Materials

Properties & Performance

Fuel Cell Type

Power Density~(1378 mW/m2),

Microbial fuel cell

References

158

PtCo/GNs Efficiency~(71.4%),

(Single cell type)



Page 32 of 67

Alkaline fuel cell PtNi/GNs

2

159

Current density~(-1.63 mA/cm ) (single cell type) Direct formic acid

Pd/GNs

Current density~(17.5 mA.g/m2)

fuel cell (Half-cell

160,161

type) Acid based PEMFC Pd3Y/GNs

2

162

Current density~(2 mA/cm ) (Half-cell type) Three Electrode

Au/GNs

163

Maximum current~(17 µA) based Cell Maximum current~(10 µA)

Three Electrode 164

Au/GNs Maximum current density~(15 µA/cm2)

based Cell

For the fabrication of graphene engineered cathode, the graphene isopropyl alcohol mixture is sprayed over the cathode surface. This process lead to increase the electrochemical surface area value up to 38.2 m2/g, as well as shows higher power density of 303 mW/cm2. Pt is one of the main electrodes used in fuel cell, thus the graphene modification of Pt electrode possesses twice the power density (661 mW/cm2) of Pt/carbon electrode (376 mW/cm2) 165. For ORR some metal oxides like Manganese oxide (MnOx) ae used. The insufficient electrical conductivity and low dissipation power make MnO2 as a bad candidate being catalyst. To make it more conducting an excellent conducting material such as graphene can be engineered into it. This overall impact of graphene on metal oxide increases the catalytic activity of the fuel cell. Graphene is used as a predecessor in the synthesis of metal oxide-graphene and it is observed


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that among other composite material with MnO2, graphene engineered MnO2 electrode shows least resistance of 12.4Ω, which makes it as a good ORR catalyst 166.

3.9 Graphene Oxide As A Membrane In Fuel Cell Pt nanoparticles are modified with GO and incorporated into fuel cells for improved performance. The water uptake of Nafion/GO blended membrane have been found to be a bit higher as compared to Nafion/Pt-G blended membrane by cause of hydrophilic feature of GO. With different weight percentage of graphene several SiO2/Nafion/G composite membranes were prepared and their proton conductivities are tested. The results show that cell performance increases only at below 1.5 weight percentage of Pt-G content and above 1.5 weight percentage the cell performance decreases

167

. Nowadays, GO based ionic liquid modified composite

membranes are effectively used due to the fact that at high temperature (greater than 100°C) it avoid the excessive use of Pt catalyst which will help to improve the cell performance

168

.

Graphene modified ionic liquid polymer electrolyte membranes and proton ionic liquid composite membranes show an excellent ionic conductivity of 7.5x10-3s/cm with 0.5 weight percentage

168

of graphene loading at 160°C, which is very high compared with sulphonated

membranes. These membranes have also shown a rise about 127% young’s modulus along with 345% tensile strength at 0.9 weight percentage of graphene 169. On the other hand it is noted that the incorporation of ionic liquid GO (ILGO) into PBI membrane resulted in better proton conductivity with low loading of phosphoric acid. ILGO/PBI membrane shows a maximum ionic conductivity of 0.035S/cm at 175°C 169. Nafion, the frequently used PEM exhibits excellent chemical stability, great mechanical strength and good ionic conductivity. GO exhibit excellent compatibility with Nafion polymer due to their strong interfacial attraction the enhanced thermal and mechanical properties of



Page 34 of 67

Nafion when GO is added, due to the fact that GO has an ability to improve side chains and both backbones of Nafion. In 2012 the first composite membrane with GO/Nafion is successfully applied in fuel cells 170. The retention of water through hydrogen bonding facilitate the transport of proton through the membrane. As a result the composite membrane had improved proton conductivity by 1.6 folds compared to recast Nafion membrane 171. SPEEK is one of the most commonly used polymer for PEMFC. The addition of GO into Nafion/SPEEK matrix has demonstrated excellent proton conductivity of 322.2mS/cm and power density of 621.2mW/cm2 at 90°C

172

. The incorporation of polydopamine modified GO

(DGO) sheets into SPEEK polymer matrix will decide on the development of an anhydrous proton exchange membrane

173

. This GO incorporated membrane fuel cell shows an increase in

47% maximum power density and 38% maximum current density. Thus, we can conclude that GO-SPEEK membrane achieve better performance in fuel cell even at elevated temperature. On the top of that, PBI is a polymeric membrane used in fuel cells, which perhaps handled at wide temperature range. The incorporation of GO into PBI result in the formation of a new membrane with better tensile strength and proton conductivity as compared with pure PBI

174

. For the

preparation of Isocynate modified GO (iGO) /polybenzimidazole (BUIPBI), electrolyte membrane 3,5-diaminobenzidine is added to 5-tert-butyl isophthalic acid

41

. Finally the

composite membrane exhibit proton conductivities of 0.027 and 0.016S/cm at 140°C for iGO/BUIPBI and GO/BUIPBI membrane respectively 41. GO at different concentration were added into PVA/Chitosan matrix followed by the characterization of prepared polymer electrolyte membrane. The proton conductivity of GO filled membrane was 11.2x10-2S/cm, whereas that of pure membrane exhibits only a conductivity of

6.77x10-2S/cm at 900C. The elastic modulus of the composite membrane is also reached up


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to a value of 21.37MPa with maximum stress of 54 MPa 175. For the low temperature application of PEMFCs, a composite membrane is produced by the addition of GO into poly (ethylene oxide) (PEO/GO) using solution mixing method

176

. Sufficient protons are provided for

membrane by the partially ionized –COO- and H+ ions present in the –COOH group of GO sheets. At 600C GO added PEO composite membrane shows a proton conductivity of 0.0951cm and a power density of 53Mw/cm2.

3.10

Effect of Graphene and Graphene Oxide on Polymer Electrolyte

Membranes It is the processing condition which impart imperfection to the crystalline system in addition to the second law of thermodynamics

177-181

. Since graphene can be manufactured via

number of methods including thermal decomposition, CVD growth, mechanical cleavage of the graphite, unzipping of carbon nanotube, and electrochemical exfoliation. It has been observed that based on the processing condition and methods, graphene structure may contain defects like Stone-Wales defect (heptagon/pentagon bonded carbon atom network), multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities like boron or nitrogen and dislocation type defects 177-181. It has been extensively reported that can be altered by introducing defects like Stone-Wales defect. Zhang et al. have examined the catalytic effect of graphene structure with cluster of dopants and defect as elucidated in Figure 15 182.



Figure 15. Graphene structures with a number of dopants and defects. N = the number of nitrogen dopants. Gray, blue, and small white balls represent carbon, nitrogen, and hydrogen atoms, respectively. The structures in the first row contain no defects, while the structures in the second row do 182. (Copyright permission has been provided as per the guidelines from American Chemical Society. Image taken from Langmuir 2012, 28, 7542−7550). They have demonstrated that nitrogen clustering renders reversible potential of each reaction step closer to the ideal reversible potential and hence possess the ability of increasing the reaction rate in ORR. In order to further examine the effect of doping and defects on catalytic attributes of graphene, authors have calculated energy separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as exemplified in Figure 16.


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Figure 16. HOMO−LUMO energy gap as a function of the number of nitrogen-doped atoms in cluster with and without Stone-Wales defects

182

. (Copyright permission has been provided as

per the guidelines from American Chemical Society. Image taken from Langmuir 2012, 28, 7542−7550). It has been observed that the presence of Stone-Wales defect rendered reduced energy gap between HOMO and LUMO compared to the defect free graphene system. In another abstraction, Okamoto et al. have concluded that the interaction between graphene and Pt13 cluster is enhanced in the presence of carbon vacancies compared to the defect free system 183. Graphene oxide (GO) has been used with various polymeric membranes for enhancing the performance of the fuel cell. Table 5 represents various properties of Fuel cells engineered with Graphene and polymeric membranes 184-195. Table 5. Effect of Graphene oxide (GO) on PEMFC performance.



Sr.

Page 38 of 67

Electrolyte

Graphene Oxide

Results

Reference

Polybenzimidazole

Graphite oxide &

1). It can reach power density of

184

(PBI)/SGO

Sulfonated graphene

0.38W/cm2

Electrolyte

oxide (SGO)

No.

1

Membrane

2

Nafion/GO

Rolled up graphene

Electrolyte

oxide sheets (GOs)

Offers new degree of freedom

185

186

Membrane 3

Chitosan

Sulfonated graphene

1). Conductivity increased by 454%

(CS)/SGO

oxide (SGO)

2). Permeability decreased by 23%

Electrolyte

3). Selectivity increased by 650%

Membrane

5

Polyaniline/GO

Graphene Oxide

Maximal power density of

0.756

187

mW·cm -2 at 0.42 V.

Electrolyte Membrane

6

Chitosan/GO

Graphene oxide

Maximum power density reached

Electrolyte

functionalized

approximately 110 mW/ cm2

Membrane

with polymer brushes containing quaternary phosphonium


188

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7


IL/GNR

Ionic Liquid

1). Peak power density of

Electrolyte

functionalised

197.2mWcm-2

Membrane

graphene nanoribbons

2). Highest conductivity of 120.5

189

mScm-1 8

Aquivion ionomer

Graphene oxide

High power density of 1.6W/cm2

190

9

Polypropylene/GO

Graphene

1). Electrical

191

Membrane

conductivity of 104.63 S/cm 2). Flexural strength of 44.28 MPa

10

Graphene

Power density of 20 µW cm−2

192

Nafion/s-Graphene

Sulfonic acid-

A crown power density of 300 mW

193

Electrolyte

functionalized graphe

cm-2 at load current density of 760 mA

Membrane

ne

cm-2 at optimal temp of 70 °C under

Carboxylated Poly(styrene-bisoprene-bstyrene)/Graphene Membrane

11

20% RH and medium pressure 12

Polyethylene

Graphene oxide

1). Ionic conductivity 10-3S/cm 2).

glycol-Borate

Thermal stability is 2730C

ester/GO

3). Electrochemical stability at 4V



194


13

Palladium-

Graphene oxide

Polypyrene/GO

Page 40 of 67

Performance of the electro catalytic

195

activity was substantially improved


4. Challenges and Opportunities For the fabrication of graphene in enormous volume the currently available methods are less efficient, which leads to delay in bulk supply of graphene

196

. The graphene fabrication

procedure sounds ambitious because graphene is available as a mono, di or multiple layers, but the issue is that it will be in flat like graphite structure although it approaches 10 laminas. As yet, for the fabrication of graphene the frequently used procedure has been vigorous oxidative exfoliation of graphite in a solution medium come after chemical reduction of GO. While this path is sufficient for bulk manufacturing of graphene, it surely generates an excessive number of irregularities in the graphene layers by virtue of transformation in its aromatic structure along with the formation of oxygen-containing groups. Cost is another important factor which affects the graphene production

197

.

Extreme conductivity of graphene is an advantage in many

occasions, but vacancies of band gap are a drawback 198. It is challenging to prevent the electric current discharge of graphene when it is charged. GO based nanohybrids are a promising candidate for developing high performance materials of technological relevance. 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 polymers with


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suitable chemical interactions. Properties of GO-based composites may be affected by enhanced morphological control. Good conductivity, thermal and mechanical stability, and water retention ability is exhibited by layered GO based membranes. The poor interfacial bond 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 199.

5. Future and Application The main applications of graphene engineered materials consists of transparent flexible electrodes, graphene/polymer electrolyte composites for energy storage, organic electronics and sensors

200

. The research on graphene in a polymer membrane, enhance the performance of the

fuel cell and are widely used in several energy applications such as super capacitors, automobiles, sensors, satellites, submarines, etc. The lowest leaking threshold for electrical conductivity and upgraded thermal, mechanical and gas barrier properties can be obtained by graphene/polymer electrolyte membranes. Improved techniques are used for the preparation of graphene or graphene oxide sheets in lateral area, controlled size and high quality, thereby declining the performance of the graphene based composites. The limited recovery of sp2 conjugated graphene network can be resolved by cutback procedure. From the recent studies, the production of graphene has reported a comparatively minute rise in the generation of graphene with lateral area, tailored thickness, shape and size 201. Therefore, in order to advance and attain the capabilities of graphene and graphene based composite, the actual description and synthesis of free standing graphene must be taken into account and paid sufficient attention. Graphene based composites are potent material that are capable of addressing various energy related and



environmental concern. By exploring the Graphene’s potential, it can be used in the conversion of solar to chemical energy in photocatalysts.

Conclusion Polymer Electrolyte Membrane Fuel Cells (PEMFC) have emerged as the promising and environment friendly clean source of energy, and are categorically used in stationary, automobile, and portable power devices due to their high performance and light weight. The performance efficiency of the PEMFC’s is mainly dependent on their polymer electrolyte membranes, which are mostly based on Nafion and PBI polymers, due to their improved conduction properties, and high durability. The recently invented Graphene materials have attracted researchers due to their exceptional properties which improves the durability and performance of PEMFC’s up to ~3.7 times, greater than the catalyst. When Graphene is functionalized with polymer electrolyte membranes such as Nafion, PBI, SPEEK etc., it is found that the ionic conductivity can be greatly improved. Future research focus will rely on the effective use of Graphene based nanomaterials in polymer electrolyte membrane modification for enhancing the operation efficiency of PEMFC’s. The present review article has discussed the technological developments and progress of Graphene engineered electrolyte membranes for PEMFC’s. The review has discussed the properties, synthesis routes and the applications of Graphene and GO, and their influence on the performance of PEMFC’s, along with their future prospects for real world applications.

Acknowledgements The authors acknowledge Dr. Hina Gokhale, Vice Chancellor of DIAT (DU), Pune, and Dr. Geethamma VG, Principal of University College of Engineering, Thodupuzha, Idukki, Kerala for


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their continuous encouragement and support. The authors are also thankful to Mr. Prakash Gore, and Mr. Swaroop Gharde for providing their continuous technical support. The authors are also thankful to anonymous reviewers for improving the quality of the manuscript by their valuable suggestions and comments.

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GRAPHICAL ABSTRACT


Graphene and Graphene Oxide for Fuel Cell Technology - American

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