Graphene and Graphene Oxide for Fuel Cell Technology - American

Review pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2018, 57, 9333−9350

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

Ind. Eng. Chem. Res. 2018.57:9333-9350. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/08/18. For personal use only.

Structural Composite Fabrication Laboratory, Department of Metallurgical and Materials Engineering, Defence Institute of Advanced Technology (DU), Ministry of Defence, Girinagar, Pune-411025, India ‡ Department of Polymer Engineering, University College of Engineering, Thodupuzha, Idukki, Kerala-685587, India ABSTRACT: The proton exchange membrane fuel cell (PEMFC) converts chemical energy into electrical energy via electrochemical reaction between hydrogen and oxygen, with heat and water as byproducts. When a PEMFC is engineered with polymer electrolyte membrane, e.g., Nafion and polybenzimidazole (PBI), it helps to enhance the performance of the fuel cell under monitored environmental conditions, i.e., high proton conductivity, improved electrode kinetics, and tailoring of properties, along with low tolerance for carbon monoxide. Recently discovered “graphene” has enticed the scientific community, because of its exceptional properties. As per the literature, PEMFCs 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 polymer electrolyte membranes engineered using graphene and graphene oxide, as well as their synthesis routes and the influence on the performance of PEMFCs.

1. INTRODUCTION

At the anode: 2H 2 → 4H+ + 4e−

Current global energy needs are mainly sustained by fossil-fuelbased energy sources, and, because of unprecedented use, they are diminishing at higher rates; furthermore, they have raised environmental concerns, because of their poisonous emissions in Nature.1 Currently, hydroelectricity (∼7%), natural gas (∼22.5%), coal (∼23.3%), and oil (∼40%), are fulfilling the global energy needs.2 The limitation of fossil fuels and humankind’s need for alternative energy sources have led 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 (PEMFCs) (Figure 1) have emerged as the promising and environment friendly clean source of energy, and they have been categorically used for stationary and automobile applications, coupled with portable power energy devices. PEMFCs mainly utilize hydrogen (H2) and oxygen (O2) as the reactants and yield water as a byproduct.3,4 The primary components the PEMFC are anode, cathode, and proton (H+) conducting electrolyte membrane, collectively known as the membrane electrode assembly (MEA) (see Figure 1a).3−5 Generally, these proton-conducting polymer electrolyte membrane (PEMs) are coated with catalyst layers on the anode and cathode sides. These catalyst layers split the hydrogen (H+ and e−) and oxygen (H+ and OH−) molecules and facilitate the following chemical reactions through the PEM, as delineated in reactions 1−3:6−8 © 2018 American Chemical Society

(1)

At the cathode: O2 + 4e− + 4H+ → 2H 2O

(2)

Net reaction: 2H 2 + O2 → 2H 2O + heat

(3)

As compared to conventional sources of energy, the performance efficiency of the fuel cells is determined to be higher, with respect to power output, as shown in Figure 2.1,6−10 The electrolyte membrane, i.e., a thin permeable sheet, is the main component of the PEMFC, which facilitates proton conduction with the help of anode and cathode catalyst layers. Based on the operating temperature, they are categorized as a low-temperature membrane or a high-temperature membrane. The low-temperature membranes operate at a temperature range of 70−90 °C, and the high-temperature membranes operate at a temperature range of 150−200 °C,13,3 as elucidated in Table 1. Nafion (Figure 1b) is the most widely utilized electrolyte membrane in PEMFCs for low-temperature Received: Revised: Accepted: Published: 9333

May 24, 2018 June 22, 2018 June 26, 2018 June 26, 2018 DOI: 10.1021/acs.iecr.8b02326 Ind. Eng. Chem. Res. 2018, 57, 9333−9350

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Industrial & Engineering Chemistry Research

Figure 1. (a) Schematic of the polymer electrolyte membrane fuel cell (PEMFC) assembly. (b) Chemical structure of the Nafion membrane. (c) Chemical structure of PBI membrane.

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 using sulfonate groups (ionic nature).15,16 As per DuPont, only alkali metals (specifically sodium) are able to degrade Nafion under normal operating conditions. The conglomeration of the 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. Furthermore, Nafion PEM facilitates proton conduction up to 0.2 S cm−1, because of the operating temperature and humidification state. The proton conduction in Nafion membrane is possible through the Nafion membrane under humidified conditions, and, hence, Nafion-based PEMs can be used up to 100 °C for lowtemperature applications; beyond this temperature, water evaporates out.15−17 This drawback of Nafion PEMs was overcome by polybenzimidazole (PBI), which operates at 150−200 °C under acidic conditions (i.e., phosphoric acid). The PBI-based PEMs exhibit advantages such as high chemical kinetics at the electrode, easy thermal and water management, and further utilization of heat. Other advantages include decreased poisoning possibility of catalyst (due to impurities) and very low tolerance for CO and CO2.18−20 To enhance the performance efficiency of the PEMs, platinum (Pt)-based catalysts are considered to be the most efficient electrocatalysts for the oxidation of hydrogen and the oxygen reduction reactions (ORRs). However, despite being efficient in electrocatalysis, the Pt catalyst sometimes causes the emission of carbon monoxide (CO); furthermore, the fuel stream also sometimes contain traces of hazardous elements, such as CO, sulfur (S), and NH3.21 In one study, Gottesfeld et al. have reported that the low-temperature operation (below 150 °C) of PEMFCs can cause CO poisoning, because of the higher negative change in free energy (ΔG), which is favorable for CO adsorption on electrodes engineered from Pt (due to its affinity).22 In order to curtail the generation of CO, various

Figure 2. Efficiency comparison between power-generating systems.

Table 1. Different Categories of Fuel Cells type of fuel cell

temperature (°C)

polymer electrolyte membrane

70−110 and 150−200

alkali

100−250

phosphoric acid molten carbonate solid oxide

150−250 500−700 700−1000

electrolyte Nafion, polybenzimidazole (PBI) aqueous potassium hydroxide phosphoric acid (Na,K)2CO3 (Zr,Y)O2−δ

ref 11 11 12 12 12

applications, and polybenzimidazole (PBI) (Figure 1c) for high-temperature operations. The chemical nature of Nafion facilitates hydration, which leads to enhanced proton transfer. The main advantages of the PEMs are high proton conductivity, light weight, flexibility, and enhanced thermal and mechanical strength, compared to electrolytes present in ordinary fuel cells.14 Furthermore, the PEMFCs operate at low pressure, which increases the safety and also provides quick startup.3 The Nafion-based PEMs are widely used in PEMFCs, 9334

DOI: 10.1021/acs.iecr.8b02326 Ind. Eng. Chem. Res. 2018, 57, 9333−9350

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Figure 3. Schematic depictions of the synthesis routes of graphene.

(65 RH%) and water retention capability are attributed to largely functionalized SGO.40 Xue et al. reported a fabrication of polybenzimidazole (PBI) membrane via the 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. Scanning electron microscopy (SEM) analysis revealed a fine dispersion of GO, and strong connection with BuIPBI membrane, resulting in enhanced stability and compatibility with larger acid content. The electrochemical analysis revealed improved proton conductivities of 0.016 and 0.027 S cm−1, for GO/BuIPBI and iGO/ BuIPBI electrolyte membranes, respectively, at a temperature of 140 °C, under no humidity.41 In yet another study, Ü regen et al. described the fabrication of phosphoric-acid-doped PBI/ GO electrolyte membranes for high-temperature PEMFCs. Their characterizations revealed the presence of GO in the PBI matrix, which helped to enhance the doping of acid and the proton conductivity. SEM characterization revealed a 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 a temperature of 165 °C, under no humidification. The electrochemical analysis of GO/PBI membrane revealed that the wt % loading of GO improved the performance of the membrane under no humidification. Furthermore, the GO/PBI membranes showed a power density of 0.38 W cm−2 and a current density of 0.252 A cm−2 H2, in an air flow at 0.6 V, at a temperature of 165 °C under ambient pressure.42 In this context, the present review article gives the state-ofthe-art and progress on graphene-engineered electrolyte membranes for PEMFCs. Furthermore, the review article discusses, in detail, the properties, synthesis routes, and applications of graphene and GO, and their influence on the functionalized polymer electrolyte membranes.38,43−45 Finally, the review concludes with the future scope and challenges

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 Pt-based catalyst is another concern for commercial utilization of PEMFCs.26 In this context, materials such as carbon-based nanomaterials (nanotubes, nanofibers, bucky nano balls, graphite particles and graphite nano sheets) have been explored by researchers to minimize the traces of CO, reduce the material cost, and simultaneously improve the efficiency of the PEMFCs.26−28 The discovery of two-dimensional single layered “graphene” brought a revolution in the materials science field.29,30 Graphene is considered to be an important material, because of its steep specific area (2630 m2 g−1),31 exceptionally high electrical conductivity (104.63 S cm−1),32 high flexural strength (44.28 MPa),33 and enhanced chemical stability.34 For the large-scale synthesis of graphene, techniques such as hydrogen exfoliation and35 focused solar exfoliation36,37 have been used. The rise of graphene-based nanomaterials opened a new window for the fabrication of a low-cost electrocatalyst system.37,38 In one study, Lee et al. have fabricated a graphene oxide (GO)/Pt nanoparticle system microwave method, along with a Nafion/GO electrolyte membrane. The reported Nafion/GO membrane shows improvement in performance of the cell (i.e., 0.802, 1.27, and 0.827 A at a voltage of 0.6 V under 100% relative humidity (RH) for 0.5, 3.0, and 4.5 wt % loadings of GO content in the Nafion/GO membrane, respectively, compared to the pristine Nafion 115 membrane.39 In another study, Chien et al. have fabricated Nafion membrane engineered with sulfonated graphene oxide (exfoliated) (SGO) via blending. Their X-ray diffraction (XRD) and rheological characterizations enhanced results for the SGO/Nafion membrane for GO loadings of 0.05−0.5 wt %. The nanosized/microsized SGO particles in the SGO/ Nafion membrane showed higher selectivity via steric hindrance effect and reduced the ionic clusters. The enhanced proton conductivity (0.0367 S cm−1) at low relative humidity 9335


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Figure 4. Schematic of the generation of GO, and the resulting proton conductivity.

layer graphene (FLG), graphene nano-onions (GNO), graphene nanoribbons (GNR), graphene oxide (GO), and reduced graphene oxide (rGO), which varies based on different functions, such as defect density, number of layers, surface chemistry, lateral dimension, composition, purity, and quality of graphene sheets.6,45−58 Graphene can be synthesized via various methods, such as chemical vapor deposition (CVD),59 liquid-phase exfoliation (LPE),60,61 mechanical exfoliation,62 electrochemical exfoliation,62 and bottom-up synthesis59−68 (see Figure 3). Graphene is considered to be a good reinforcing agent in composites, because of its excellent mechanical properties.69 Recently, researchers have determined, by using interfero-

associated with the development of high-performance PEMFCs.

2. GRAPHENE-BASED NANOMATERIALS Graphene is an emerging material in the field of materials science, nanotechnology, and condensed matter physics. The separation of graphite to a single layer via a cellophane tape exfoliation technique resulted in the invention of graphene (see 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), few9336


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Figure 5. Schematic of the graphene oxide (GO)-based electrocatalyst/electrode system.

using series of treatments with H2SO4, NaNO2, KMnO4, and deionized (DI) water, followed by vacuum drying, as shown in Figure 4. 2.2. Why Graphene and Graphene Oxide (GO) for Fuel Cell Applications? Recently, graphene-based materials have been widely used for fuel cell applications, because of their exceptionally high conductive properties and their stability during the processing of electrolytes.30,40−45 In one study, Seger and Kamat have reported that carbonbased nanomaterials such as graphene enhances the nanoscaled 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. Furthermore, the enhanced conductivity helps in the accumulation and transport of electrons to the electrocatalytically active surfaces (see 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 such as TiO2. The ability of these sheets to uphold nanoparticles has opened a new window for the development of electrocatalysts for fuel cell applications.86 Graphene could be produced at low cost in bulk quantity if the source materials are graphite, its oxide, and subderivatives. It is also presumed that the two-dimensional (2D) planar carbon sheet permits the edge planes to connect with catalyst nanoparticles. Furthermore, 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, because of their greater catalytic affinity toward oxygen reduction reactions (ORRs). Therefore, considering these attributes of gfaphene-anchored nanomaterials, they can be

metric profilometry, that graphene is greatly softened by outof-plane buckling, whereas the in-plane firmness is determined to vary in the range of 20−100 N m−1 at room temperature.70The intrinsic strength of the single-layered thin lamina is 42 N m−1, 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 is oriented in the direction through the gapirregularity 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 buildup 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 2.1. Graphene Oxide for Fuel Cell. Graphene oxide (GO) is a derivative of graphene, which exhibits a C:O ratio of 2:1.81 The presence of various oxygen groups such as epoxide, hydroxide, carbonyls, and carboxyls turns GO into a insulating and hydrophilic material (see Figure 4),82 with the retention of other properties such as mechanical strength and gas impermeability and surface area electrical insulation, gas permeability, hydrophilicity, and proton conductivity make GO a promising material for electrolyte membranes in PEMFCs.81−83 The disruption of the lattice in the GO is emulated as an increase in the interlayer spacing from graphene (0.335 nm) to GO (>0.625 nm).82−84 Commercially, Hummers’ method has been widely used for the production of GO.85 The GO production is accomplished 9337


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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 to elevate the diffusion of the oxygen (O2) molecules, 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 rGObased catalyst and still can retain an active surface area of Pt nanocrystals, i.e., >95%, until 20 000 ADT cycles, much larger than the commercial catalyst system. Furthermore, they reported that the uniquely ordered 2D structure of rGO acts as an obstacle to Pt nanocrystals, thereby impeding its 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 is dependent on the structural disorder. Generally, the GO sheets exhibit ian nsulating nature, because of the electronic density of states, which leads to an energy gap, thus leading to a sheet resistance (RS) of ≥1012 Ω □−1.99−101 The progressive removal of O2, i.e., the formation of reduced GO (via various treatments), promotes the transport of carriers,102,103 resulting in reduced RS by many fold, thus imparting semiconductor characteristics to rGO, i.e., semimetals similar to graphene.99−112 The electrical conductivity of rGO can attain values up to ∼1000 S m−1.109,112 Theoretical calculations demonstrate that the oxidation-level-dependent approximate local density band gap of GO can fluctuate over a 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 electrochemical capacitance capability with great cycle efficiency, thus demonstrating its applicability as an ultracapacitor. Shao et al. claimed that the rGO reveals exceptionally high electrochemical capacitance (ECC) and cycle durability much greater than carbon nanotubes (CNTs). They demonstrated the specific capacitance to be ∼165 F g−1 and ∼86 F g−1for rGO and CNT, respectively.114,115 Considering the enhanced properties demonstrated by graphene and 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

propitious contenders for metal-free electrocatalyst systems, because of their exceptional catalyst upholding capacity.87 Some properties typical properties of graphene, and modified GOs, have been represented in Tables 2 and 3, respectively. Table 2. Properties of Graphene property

maximum observed value

ref

carrier transport velocity current carrying capacity specific area superior thermal conductivity ultrahigh electron mobility modulus of elasticity fracture strength elastic modulus

40 GHz 109 A cm−2 2630 m2 g−1 5300 W M−1 K−1 250 000 cm2 V−1 s−1 1 TPa 130 GPa 32 GPa

53 53 53 54 54 54 54 54

Table 3. Electrolyte-Based Properties of Graphene Oxidea synthesized graphene oxide

specific surface area

specific capacitance (in electrolytes)

chemically reduced GO

705 m2 g−1

chemically reduced GO microwave expansion of GO thermal reduction of GO in PC thermal expansion of GO (1050 °C)

320 m2 g−1 463 m2 g−1

135 (in KOH), 99 (in organics) 205 (in KOH) 191 (in KOH)

90 91

122 (in organics)

92

93

(200 °C, vacuum)

368 m2 g−1

117 (in H2SO4), 75 (ionic liquid) 264 (in KOH), 122 (in organics) 43 (in H2SO4) 216 (in Na2SO4) 531 (in H2SO4)

rGO-SnO2 composite GO-MnO2 composite 1%-GO-doped polyaniline

925 m2 g−1

ref 89

94 95 96 97

a

Data taken from ref 88.

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 mobility at room temperature up to 15 000 cm2 V−1 s−1. The perceived mobility in graphene is less dependent at room temperature, which suggests that very high mobility can be achieved at this temperature for graphene. The mobility, being >200 000 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 and chemically doped devices, showing ballistic transport capability at the submicrometer range.88 In another study, Li et al. have reported a catalyst system based on reduced graphene oxide (rGO) for ORRs for fuel cell applications. The catalyst system prepared by mixing Pt-loaded rGO with carbon black, thus forming a hybrid composite system. They claimed that the incorporation of carbon black particles helps to prevent the stacking of the rGO sheets, which exhibits the tendency to stack together under the influence of π−π interactions, even when loaded with nanocrystal systems (i.e., Pt). Furthermore, they reported that the stacking of rGO sheets impedes a significant quantity of catalytic locations on nanocrystals, thus ultimately imparting obstruction for the

3. GRAPHENE AND GRAPHENE OXIDE IN FUEL CELLS 3.1. Direct Methanol Fuel Cell (DMFC). The direct methanol fuel cell (DMFC) is a type of PEMFC with simple operation, simple design, and high efficiency, as shown in Figure 6. Even though the development of a proton-conductive membrane in DMFCs is difficult, Nafion membranes are widely used. Along with Nafion, there are other polymeric membranes, such as poly(ether ether ketone) (PEEK) and polybenzimidazole (PBI), as well as polysulfone, that are also used as a membrane. However, 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 2D structure with flexible functionalization and admirable thermal stability, as per criteria for PEM. The presence of various functional groups makes it more flexible for functionalization, compared to other organic material.117 By combining GO with the Nafion, decreases in methanol permeability and ionic conductivity (by 70% and 22%, respectively) are observed;118 also; it has better selectivity 9338


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Figure 6. Schematic of the direct methanol fuel cell (DMFC).116

than a recent Nafion membrane. The GO-engineered membranes preserve the ionic conductivity and its methanol crossover gets decreased. The incorporation of a graphene monolayer with Nafion resulted in increased proton permeability and decreased methanol permeability by 68.6%, in comparison to the natural Nafion membrane.119 It is observed that the Nafion/GO membrane has 2 times slower methanol permeability and 5 times higher selectivity than the recast Nafion 117 membrane under the same conditions. The Nafion/GO composite membrane has also shown an exceptional open circuit voltage of 0.67 V with a maximal power density of 64.38 mW cm−2, along with a 200 mA cm−2 current density at 60 °C.120 Sulfonated poly(ether ether ketone) also has an excellent affinity with GO. Recently, GO was modified with sodium dodecylbenzene sulfonate (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 the basic imidazole group (accept proton) from histidine molecules and acidic −SO3H groups (donate 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%, compared with pure SPEEK membrane. Low methanol permeability of (1.32−3.91) × 10−7 cm2 s−1, an excellent selectivity of 5.14 × 10−5 S s cm−3, and 80.7% maximal power density is also acquired by the hybrid membrane, relative to that of 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 presented values of 4.12 × 10−2 s cm−1 and 1.38 × 10−7 cm2 s−1, respectively, which are better values than those observed for 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 cells, because of its higher methanol permeability, which is ∼18.2 × 10−6 cm2 s−1 higher than that of Nafion 115 (3.36 × 10−6 cm2 s−1).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 ∼17% and 38% higher power densities, compared to Nafion 115 and Nafion 112 membranes, respectively.125 3.2. Microbial Fuel Cell (MFC). MFCs generates electricity from biodegradable substrate that are present in wastewater (see Figure 7). As a result of the reaction, protons are generated, which must pass through the membrane.126−131

Figure 7. Schematic of the microbial fuel cell.126

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 sources that exploit the available chemical energy in organic waste to produce electricity.132−134 The conversion of chemical energy of organic waste to electrical energy by virtue of microbial metabolism, which releases electrons that are subsequently passed through the solid electrode via direct or indirect extracellular electron transfer mechanisms. It has been advocated that graphene possess the ability to promote microorganism to express signaling molecules and further act as a mediator to improve the electron transfer efficiency. Pant et al. have reported that the microbial fuel cell electrode fabricated by hybrid graphene (HG) exhibited improved power density for graphite paste−hybrid graphene (GP-HG) (220 mW m−2), compared to 30 mW m−2 for the GP electrode and 80 mW m−2 for GP-TiO2.135 In addition, the modified electrodes demonstrated lower charge transfer resistance (CTR), compared to the bare electrode. To improve the membrane properties, GO is incorporated into poly(vinyl alcohol) silicotungstic (PVA-STA) solution to obtain a 100 μm semipermeable membrane. This addition of graphene results in the improved conductivity of PVA-STAGO membrane up to 0.065 s cm−1; however, it was ∼0.046 s cm−1 for the pure PVA-STA membrane and 0.062 s cm−1 for the Nafion 117 membrane. Also, this addition leads to an increase the mechanical strength to a value of 39.1 MPa. Moreover, the power density of GO filled membrane was excellent (1.19 W m−3), whereas a value of 0.88 W m−3 is observed for pure Nafion 117 membrane.136 Graphene is also added with SPEEK to prepare a more functionally advanced membrane for MFCs. The addition of GO with SPEEK will provide a proton conductivity of 1.48 × 10−3 S cm−1 and an oxygen diffusion coefficient of 1.154 × 10−6 cm2 s−1. The highest columbic efficiency of 16.86% is also observed with the GO/SPEEK composite membrane. The pure SPEEK membrane will only provide a power density of 812 mW m−2, whereas the SPEEK/GO composite membrane exhibits a power density of 902 mW m−2.137 To fabricate electrodes in MFC, carbon nanotubes (CNTs) and graphene can be used; among these two, graphene is preferred, because, for CNT, it is difficult to ensure enough space for bacterial growth, because of the lack of a threedimensional (3D) structure. A polytetrafluoroethylene (PTFE) 9339


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shows an incredible increase in power density (148%) and tensile strength (73%).139,140 For the preparation of an 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, a maximum ionic conductivity of 6.1 mS cm−1 at 70 °C and an ion exchange capability of 6.1 mmol g−1 is obtained.139,140 The vacancies present on the graphene surface help the metal ions to correlate with it, which is the difference between graphene and other carbon-based materials. By taking advantage of this property of graphene, nitrogen is added at the time of preparation, leading to the formation of a catalyst with nitrogen-doped reduced graphene (rGO(N)), which possess high oxygen reduction reaction property.122 Usually, in an AFC, Pt/carbon composite material is used as the cathode, and to enhance the performance of the cell, rGO(N) is added, along with cobalt oxide, resulting in the formation of a 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). The main advantage of this fuel cell is its nontoxic nature, as well as simple operation, because the main reaction that occurs in this cell is the oxidation of glucose to carbon dioxide, which will result in an open circuit potential of 1.24 V (see Figure 10). The cathode of direct glucose fuel cell (DGFC) is fabricated using graphene, to enhance the cell performance by the addition of graphene oxide with 5% PTFE. An activatedcarbon-based electrode, instead of graphene-based materials, can also be used as an electrode.142 The DGFC consisting of graphene oxide with microporous silica (MPS) membrane have a peak power density of 5.3 μW cm−2. Also, this GO/ MPS hybrid fuel cell exhibit large open circuit potential of 314.6 mV, whereas for with activated carbon electrode cell has only a potential of 160 mV, which indicates the reduced passage of oxygen between anode and cathode.143 3.5. Direct Ethanol Fuel Cell (DEFC). In this type of cell, ethanol is mainly used as a fuel (Figure 11). In a direct ethanol fuel cell (DGFC), electrode fabrication is based on a Pt catalyst, but its toxic nature and high cost discourage its use. The high surface area and robust electroconductivity of graphene makes it as a good candidate for electrocatalyst.144 Strong activeness toward the electro-oxidation of alcohols, as well as glucose, make metal-loaded graphene 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 3.6 mW cm−2, which is greater than the individual values of Pd−Pt electrodes.145 3.6. Bipolar Plates. For the uniform distribution of fuel gas, air, removal of heat from the active area, conduction of electrical current from one cell to another, and arresting of the omission of coolant, there is a component in PEM known as bipolar plates, which accounts for ∼80% of the total weight (see Figure 12). These plates must be of lightweight material, inexpensive, and easy to manufacture. The materials used in its fabrication are electrographite, sheet metal, and graphite.146 Polymer electrolyte membranes provide an optional track to bipolar plate materials, because of its lightweight nature and ease of manufacture. Carbon composite bipolar plates have

solution is mixed with a 1 wt % solution to obtain a paste form, which is applied on both sides of the carbon cloth and used as the anode. To improve the performance of the anode, a multiwalled carbon nanotube (MWCNT) is inserted into a bundle of graphene (Figure 8). This insertion inhibits the

Figure 8. Schematic diagram showing incorporation of MWCNT into graphene.137

accumulation of generated electrons, 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 the anode surface, which leads to an 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). Alkaline fuel cells (AFCs), as described in Figure 9, were mainly used in the space shuttle

Figure 9. Diagram of an alkaline fuel cell (AFC).138

program and the Apollo space program, because of its high operational temperature (100 and 250 °C) and higher efficiency (∼60%). The alkaline condition of this cell will enhance the electrode reaction kinetics.138 To improve the overall performance of this cell, graphene-modified poly(vinyl alcohol) (PVA/GO) composite membranes are used. The existence of graphene in the 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 is improved by 126% and a 55% decrease in methanol permeability with 0.7 wt % graphene loading is noted. This composite membrane also 9340


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Figure 10. Schematic of the direct glucose fuel cell (DGFC).

through-plane electrical conductivity (∼4% and 23%, respectively) 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 components for gas diffusion electrode. Primarily, three components are present in an electrode: 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 substrates. Above all, carbon-based materials such as graphene, carbon fiber paper, and crisscross carbon are used as a base for the electrode laminas. Carbon fiber paper is a powdered material at elevated temperature, has an almost rigid structure, and is an outstanding conductor of electricity. Although crisscross carbon is extra extensible, it is an excellent conductor and immense power execution can be attained through more-desirable water management. For the construction of the electrode in microbial fuel cells, two different morphologies of graphene are applied: flat lamina of paper and rumpled paper balls.80 The bulky surface width, along with an 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 (ORR) Catalyst. In PEMFCs, Pt and its alloys are contemplated to be the most efficient oxygen reduction reaction (ORR) catalysts, because of their low overpotential and high current density; however, serious intermediate tolerance, time-dependent drift, anode crossover, sluggish kinetic, poor stability in an electrochemical environment, and high cost circumvent their effective commercial utilization.148,149 Wang et al. elucidated that alternative catalysts such as 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, a nanostructured catalyst support such as carbon (active carbon, porous carbon, carbon nanotube, graphene), metal carbide, mesoporous silica, and conducting polymers has been extensively exploited to augment the catalyst performance and durability during cell operation.151 Jafri et al. have exemplified that the carbon support leads to the improved

Figure 11. Schematic of the direct ethanol fuel cell (DEFC).

Figure 12. Schematic depiction of the fuel cell with bipolar plates.146

been manufactured using thermoplastic or thermosetting resins along with additives, and can be further 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 S cm−1 to 435.31 S cm−1 and 99.70 S cm−1 to 130.17 S cm−1, correspondingly.146 The reinforcement of 1% graphene to these plates increases the peak power output from 397 mW cm−2 to 437 mW cm−2 at current densities of 752 mA cm−2 and 827 mA cm−2, correspondingly. Based on the convincing hike in in-plane electrical conductivity, along with 9341


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Industrial & Engineering Chemistry Research interaction with Pt, which further stabilizes the Pt nanoparticle and subsequently augments durability and catalyst performance.152 Gong et al. reported vertically aligned nitrogen-doped CNTs in ORRs and demonstrated that ORRs were facilitated by virtue of the electron-accepting capability of nitrogen, which generates a net positive charge on the CNT surface to augment oxygen adsorption.153 In contrast to Gong et al., Li et al. have elucidated that CNT-based electrocatalysts possess lower ORR activity in acidic environments while nitrogen-doped multiwalled CNTs or aligned CNT arrays have exhibited improved ORR activity in alkaline media but reduced efficiency under acidic conditions.154 Therefore, to enhance the overall performance of the fuel cell, the anode and cathode must be modified, to improve the ORR. This improvement in ORR can be achieved via 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 a 3D Ndoped graphene aerogel supported with Fe3O4 nanoparticle and exhibited a positive onset potential, higher cathodic density, and higher electron transfer number in alkaline media, compared to Fe3O4-supported nitrogen-doped carbon black or nitrogen-doped graphene sheets.156 In another study, Yang et al. tailored the electronic arrangement of graphene by doping similar electronegative elements, such as sulfur and selenium, and subsequently demonstrated excellent catalyst activity, longterm 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 sulfur-doped graphene (Figure 13a) and further confirmed the presence of sulfur by virtue of the EDX pattern illustrated in Figure 13b.

Figure 14. (a) Cyclic voltammograms 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. (Reproduced with permission from ref 157. Copyright 2012, American Chemical Society, Washington, DC.)

It has been observed that several graphene-supported materials have been used as improved ORR catalysts, as tabulated in Table 4. Table 4. Graphene-Based Materials for Oxygen Reduction Reaction (ORR) Activity material PtCo/GNs PtNi/GNs Pd/GNs Pd3Y/GNs Au/GNs Au/GNs

properties and performance power density ≈ 1378 mW m−2, efficiency ≈ 71.4% current density ≈ −1.63 mA cm−2 current density ≈ 17.5 mA g m2 current density ≈ 2 mA cm−2 maximum current ≈ 17 μA maximum current ≈ 10 μA, maximum current density ≈ 15 μA cm−2)

fuel cell type

ref

microbial fuel cell (singlecell type)

158

alkaline fuel cell (single-cell type) direct formic acid fuel cell (half-cell type) acid-based PEMFC (halfcell type) three-electrode-based cell

159 160, 161 162

three-electrode-based cell

164

163

To fabricate a graphene-engineered cathode, the graphene isopropyl alcohol mixture is sprayed over the cathode surface. This process leads to an increase in the electrochemical surface area value up to 38.2 m2 g−1, and it shows a higher power density (303 mW cm−2). 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 cm−2) of the Pt/ carbon electrode (376 mW cm−2).165 For the ORR, some metal oxides, such as manganese oxide (MnOx), are used. The insufficient electrical conductivity and low dissipation power make MnO2 as a bad candidate, with regard to being a 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 that, among other composite material with MnO2, a graphene-engineered MnO2 electrode shows the least resistance (12.4 Ω), 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 the Nafion/GO blended membrane has been determined to be slightly higher, compared to Nafion/Pt-G blended membrane,

Figure 13. (a) TEM image of S-graphene-1050 and (b) EDX of Sgraphene. (Reproduced with permission from ref 156. Copyright 2012, American Chemical Society, Washington, DC.)

Guo et al. have reported graphene-based FePt nanoparticles for active and durable catalyst for the ORR and compared the cyclic voltammograms with C/FePt and commercially available C/Pt catalyst as shown in Figure 14a.157 The obtained results have demonstrated that double-layer capacitance of G/FePt nanoparticles are much larger than those of 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 (0.557 V) is higher than C/FePt (0.532 V) and commercially available C/ Pt catalyst (0.512 V), which indicated improved ORR activity for G/FePt nanoparticles, in contrast to the C/FePt and commercially available C/Pt catalyst (Figure 14b). 9342


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membrane also reaches values up to 21.37 MPa, with a 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 a solution mixing method.176 Sufficient protons are provided for the membrane by the partially ionized −COO− and H+ ions present in the −COOH group of GO sheets. At 60 °C, the GO-added PEO composite membrane exhibits a proton conductivity of 0.0951 cm and a power density of 53 MW cm−2. 3.10. Effect of Graphene and Graphene Oxide on Polymer Electrolyte Membranes. It is the processing condition that imparts imperfection to the crystalline system, in addition to the second law of thermodynamics.177−181 Since graphene can be manufactured via many methods, including thermal decomposition, chemical vapor deposition (CVD) growth, mechanical cleavage of the graphite, unzipping of CNTs, and electrochemical exfoliation. It has been observed that, based on the processing conditions and methods, graphene structure may contain defects such as the Stone− Wales defect (heptagon/pentagon bonded carbon atom network), multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities (such as boron or nitrogen), and dislocation-type defects.177−181 It has been extensively reported that the structure can be altered by introducing defects such as the Stone−Wales defect. Zhang et al. have examined the catalytic effect of graphene structure with a cluster of dopants and defects, as elucidated in Figure 15.182

because of the hydrophilic features of GO. With different weight percentages 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 100 °C), it avoids the excessive use of Pt catalyst which will help to improve the cell performance.168 Graphene-modified ionicliquid polymer electrolyte membranes and proton ionic liquid composite membranes show an excellent ionic conductivity of 7.5 × 10−3s cm−1, with a graphene loading of 0.5 wt % at 160 °C,168 which is very high, compared with sulfonated membranes. These membranes have also shown an increase of ∼127% in the Young’s modulus, along with a 345% increase in tensile strength, at a graphene loading of 0.9 wt %.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 a low loading of phosphoric acid. The ILGO/PBI membrane shows a maximum ionic conductivity of 0.035 S cm−1 at 175 °C.169 Nafion, which is a frequently used PEM, exhibits excellent chemical stability, great mechanical strength, and good ionic conductivity. GO exhibits excellent compatibility with Nafion polymer, because of their strong interfacial attraction and the enhanced thermal and mechanical properties of Nafion when GO is added, which is due to the fact that GO has the ability to improve side chains and both backbones of Nafion. In 2012, the first composite membrane with GO/Nafion was 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-fold, compared to that of recast Nafion membrane.171 SPEEK is one of the most commonly used polymers for PEMFC. The addition of GO into a Nafion/SPEEK matrix has demonstrated excellent proton conductivity of 322.2 mS cm−1 and a power density of 621.2 mW cm−2 at 90 °C.172 The incorporation of polydopamine-modified GO (DGO) sheets into SPEEK polymer matrix will determine the development of an anhydrous proton exchange membrane.173 This GOincorporated membrane fuel cell shows an increase of 47% in maximum power density and 38% in maximum current density. Thus, we can conclude that the GO-SPEEK membrane achieves better performance in fuel cells, even at elevated temperature. In addition, PBI is a polymeric membrane that is used in fuel cells that can perhaps be handled over a wide temperature range. The incorporation of GO into PBI results in the formation of a new membrane with better tensile strength and proton conductivity, 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 0.016 S cm−1 at 140 °C for the iGO/BUIPBI and GO/BUIPBI membranes, respectively.41 GO at different concentrations were added into a PVA/ Chitosan matrix, followed by the characterization of prepared polymer electrolyte membranes. The proton conductivity of GO-filled membrane was 11.2 × 10−2 S cm−1, whereas that of pure membrane exhibits only a conductivity of 6.77 × 10−2 S cm−1 at 90 °C. The elastic modulus of the composite

Figure 15. Graphene structures with several dopants and defects. N is 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. (Reproduced with permission from ref 182. Copyright 2012, American Chemical Society, Washington, DC.)

They have demonstrated that nitrogen clustering renders reversible potential of each reaction step closer to the ideal reversible potential and, hence, possess the ability to increase the reaction rate in ORR. In order to further examine the effect of doping and defects on the 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. It has been observed that the presence of Stone−Wales defects rendered a reduced energy gap between HOMO and 9343


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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 GO has been used with various polymeric membranes to enhance the performance of the fuel cell. Table 5 represents various properties of fuel cells engineered with graphene and polymeric membranes.184−195

4. CHALLENGES AND OPPORTUNITIES For the fabrication of graphene in enormous volume, the currently available methods are less efficient, which leads to a delay in the bulk supply of graphene.196 The graphene fabrication procedure sounds ambitious because graphene is available as a monolayer, bilayer, or multiple layers, but the issue is that it will be in a flatlike graphite structure, although it approaches 10 laminas. As yet, for the fabrication of graphene, the frequently used procedure has been a vigorous oxidative

Figure 16. HOMO−LUMO energy gap, as a function of the number of nitrogen-doped atoms in cluster with and without Stone−Wales defects. (Reproduced with permission from ref 182. Copyright 2012, American Chemical Society, Washington, DC.)

Table 5. Effect of Graphene Oxide (GO) on PEMFC Performance No.

electrolyte

graphene oxide

1

polybenzimidazole (PBI)/SGO electrolyte membrane

graphite oxide and sulfonated graphene oxide (SGO)

(1) it can reach a power density of 0.38 W cm−2

results

184

ref

2

Nafion/GO electrolyte membrane

rolled-up graphene oxide sheets (GOs)

offers new degrees of freedom

185

3

Chitosan (CS)/SGO electrolyte membrane

sulfonated graphene oxide (SGO)

(1) conductivity increased by 454%

186

(2) permeability decreased by 23% (3) selectivity increased by 650% 5

polyaniline/GO electrolyte membrane

graphene oxide

maximal power density of 0.756 mW cm−2 at 0.42 V

187

6

Chitosan/GO electrolyte membrane

graphene oxide functionalized with polymer brushes containing quaternary phosphonium

maximum power density reached at ∼110 mW cm−2

188

7

IL/GNR electrolyte membrane

ionic liquid functionalized graphene nanoribbons

(1) peak power density of 197.2 mW cm−2

189

(2) highest conductivity of 120.5 mS cm−1 8

Aquivion ionomer

graphene oxide

high power density of 1.6 W cm−2

190

9

polypropylene/GO membrane

graphene

(1) electrical conductivity of 104.63 S cm−1 (2) flexural strength of 44.28 MPa

191

10

carboxylated poly(styrene-bisoprene-b-styrene)/graphene membrane

graphene

power density of 20 μW cm−2

192

11

Nafion/S-graphene electrolyte membrane

sulfonic acid-functionalized graphene

crown power density of 300 mW cm−2 at a load current density of 760 mA cm−2 at an optimal temperature of 70 °C, under 20% RH and medium pressure

193

12

polyethylene glycol−borate ester/GO electrolyte membrane

graphene oxide

(1) ionic conductivity of 10−3 S cm−1;

194

(2) thermal stability is 273 °C; (3) electrochemical stability at 4 V 13

palladium-polypyrene/GO electrolyte membrane

graphene oxide

performance of the electrocatalytic activity was substantially improved

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automobile, and portable power devices, because of their high performance and light weight. The performance efficiency of the PEMFCs is mainly dependent on their polymer electrolyte membranes, which are mostly based on Nafion and PBI polymers, because of their improved conduction properties and high durability. The recently invented graphene materials have attracted researchers because of their exceptional properties, which improves the durability and performance of PEMFCs 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. This Review has discussed the properties, synthesis routes, and applications of graphene and graphene oxide (GO), and their influence on the performance of PEMFCs, along with their future prospects for real-world applications.

exfoliation of graphite in a solution medium, which comes 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 that 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 because of mechanical instability. To avoid the problem, different protocols were adopted to produce functionalized GO nanohybrid PEMs by dispersing modified GO into various polymers with 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. Therefore, care should be rendered to modify GO by covalent bonding to avoid deterioration in stability/durability and membrane surface morphology.199

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Balasubramanian Kandasubramanian: 0000-0003-4257-8807 Notes

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 enhances the performance of the fuel cell and is widely used in several energy applications, such as use as supercapacitors and in 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 GO sheets in the lateral area, controlled size and high quality, thereby decreasing the performance of the graphene-based composites. The limited recovery of sp2 conjugated graphene network can be resolved by using a 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 concerns. By exploring the potential of graphene, it can be used in the conversion of solar energy to chemical energy in photocatalysts.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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 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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