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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.32 No.6 pp.411-426
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2022.32.6.411

Role of Graphene Derivatives in Anion Exchange Membrane for Fuel Cell: Recent Trends

Manoj Karakoti*, Sang Yong Nam*,**†
*Research Institute for Green Energy Convergence Technology, Gyeongsang National University, Jinju 52828, Korea
**Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju 52828, Korea
†Corresponding author(e-mail: walden@gnu.ac.kr; http://orcid.org/0000-0002-6056-2318)
October 31, 2022 ; December 19, 2022 ; December 19, 2022

Abstract


Energy plays a significant role in modern lifestyle because of our extensive reliance over energy-operating devices. Therefore, there is a need for alternative and green energy resources that can fulfill the energy demand. For this, fuel cell (FCs) especially anion exchange membrane fuel cells (AEMFCs) have gained tremendous attention over the other (FCs) due to their fast reaction kinetics without using noble catalyst and allow to use of cheaper polymers with high performance. But lack of highly conductive, chemically, and mechanically stable anion exchange membrane (AEM) still main obstacle to the development of high performance AEMFCs. Therefore, graphene-based polymer composite membranes came into the existence as AEMs for the FCs. The exceptional properties of the graphene help to improve the performance of AEMs. Still, there are lot of challenges in the graphene derivatives based AEMs because of their high tendency of agglomeration in polymer matrix which reduced their potential. To overcome this issue surface modification of graphene derivatives is necessary to restrict their agglomeration and conserved their potential features that can help to improve the performance of AEM. Therefore, this review focus on the surface modification of graphene derivatives and their role in the fabrication of AEMs for the FCs.


graphene , graphene oxide , surface modification , anion exchange membrane , fuel cell

연료전지용 음이온교환막에서 그래핀 유도체의 역할: 최근 동향

마노즈 카라코티*, 남 상 용*,**†
*경상대학교 그린에너지융합연구소(RIGET)
**경상대학교 나노신소재융합공학과

초록


다양한 장치에 대한 광범위한 의존으로 에너지는 현대 생활에서 중요한 역할을 하고 있다. 전통적인 에너지원은 많은 환경 및 건강 문제를 안고 있어, 환경과 건강에 미치는 영향을 최소화하면서 에너지 수요를 충족할 수 있는 대체 가능 한 에너지원이 시급히 필요하다. 이러한 관점에서 연료전지, 특히 음이온 교환막 연료전지는 고가의 촉매를 사용하지 않는 빠 른 반응속도, 컴팩트한 디자인, 수소 이외의 연료 선택 가능성 및 보다 저렴한 연료의 사용이 가능한 특성으로 인하여 다른 연료전지에 비해 큰 주목을 받고 있다. 그럼에도 이온전도성이 높고 화학적, 기계적으로 안정적인 음이온 교환막의 개발이 부 진한 것이 주요 장애물이 되어 왔고, 그래핀 기반 고분자 복합막이 AEMFC용 전해질막으로 등장하게 되었다. 2D 구조, 높은 기계적 강도, 높은 내화학성 및 표면적과 같은 그래핀의 견고한 구조 및 물리적 특성은 음이온교환막의 성능 개선에 도움이 된다고 보고되고, 그래핀 및 그 유도체를 사용하는 전해질막의 연구가 중요하게 되었으나, 그래핀 재료의 높은 잠재력에도 지 나친 응집 경향으로 나타나는 문제점이 지적되고 있어 그래핀 유도체의 표면 개질은 응집을 완화하고 그래핀이 가지는 잠재 적 성능을 이끌어내는데 꼭 필요하다. 따라서 본 고에서는 그래핀과 유도체의 표면 개질과 연료 전지용 AEM 제조에서 그 역 할에 초점을 맞추어서 논의하고자 한다.


    1. Introduction

    Energy is a basic need of our life because nowadays we are heavily dependent on energy consumption appliances. In addition to this, continuous growth in the industrial revolution increases energy need. Fossil fuels currently supply most of the world's energy needs; however, they are not a green source of energy because their use produces air pollution and greenhouse gas emissions. Therefore, there is a requirement for green and alternative energy sources that can replace conventional ones. In this context, recently fuel cells have gained a lot of attention due to their high efficiency (as high as 60%) and almost zero carbon emission (>90% reduction in harmful pollutants) over the other green energy sources[1]. A fuel cell is an electrochemical device that converts chemical energy into electrical energy, and it is made of mainly three components, anode, cathode, and electrolyte. During this conversion, generally, two chemical reactions (one is oxidation and the other is reduction) are occurring at the interface of the above-mentioned components and generate electricity. Generally, a fuel cell can be divided into the five categories according to the choice of electrolyte used in these cells: alkaline fuel cells (AFCs), proton exchange membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs)[2].

    In above-mentioned fuel cells, AFC is the most popular and significantly researched fuel cell. This fuel cell is coming into the limelight after being used by NASA for the Gemini, Apollo, and Space Shuttle missions in the mid-1960s[3]. ACF, generally operates with an alkaline electrolyte such as potassium hydroxide (KOH). KOH is the best among the other alkaline hydroxide because it showed more ionic conductivity than others. In addition, ACF provides more pros over the other fuel cells in terms of easy handling due to lower operating temperature of around 23~70°C, greater extent of reaction kinetics at the electrode than acidic medium, use of other metal catalysts than the expensive platinum as well as low cost[4]. Besides these advantages, one major drawback of ACF is the use of liquid KOH electrolyte because it is very sensitive against the CO2 which forms HCO3-2 and CO3-2 which reduced the performance of the fuel cell[5,6]. Therefore, there is a need to replace this liquid aqueous KOH with more sophisticated and easier-to-handle anion exchange membranes (AEMs) which is analogous to proton exchange membranes (PEM) with the additional advantages of being all solid-state alkaline fuel cells.

    Liquid aqueous KOH replaced by polymer based AEM in AFC is called AEMFC. It has mainly an anode and cathode which are electronically separated by the polymer electrolyte membrane apart from the other necessary components of the fuel cell (Fig. 1)[7]. Also, it has advantages over the PEMFC in terms of fuel cross-over restriction. In PEMFC, fuel crossover is higher because the movement of ions and direction of fuel crossover is the same which promotes the higher fuel crossover. On the other hand, in AEMFC, the direction of fuel crossover and movement of hydroxide ion are opposite that inhibited the electro-osmotic drag which reduced the fuel crossover. The AEMFC is noticed for the first time when Cheng et al.[8] replace the conventional electrolyte with a polymer-based electrolyte. After that, lots of development have been made in AEMFCs research, especially in the fabrication of better AEMs.

    AEM is made of solid polymer electrolytes that consist of polymers matrix (polyvinyl alcohol (PVA), poly(arylene ether sulfone) (PAES), poly(arylene ether ether ketone) (PEEK), poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), etc.)[7], positive cationic groups (quaternary ammonium, imidazolium, guanidinium, pyridinium, spirocyclic quaternary ammonium, tertiary sulfonium, phosphonium, phosphazenium, phosphatranium, benzimidazolium, pyrrolidinium, and metal-cations, etc.)[9] and mobile negatively charged anions. Based on these, a lot of research has been done in recent years with different-different compositions, however, AEMs still have many problems such as low hydroxide ionic conductivity and low chemical and mechanical stabilities in high alkaline medium.

    Therefore, in recent years AEMs based research is orientated towards the incorporation of different types of nanofillers in the polymer matrix that are having the potential to increase the above properties and performance of AEMs. These kinds of combinations generate an organic-inorganic or hetero-phase polymers membrane that are showing performance enhancement in AEMs due to the synergic effect between the filler and polymer matrix that is evolved by physical crosslinking and partial crystallization of the polymer adjacent to the inorganic phase[10]. Till date, there are a number of fillers such as titanium dioxide nanotube[11], carbon nanotubes (CNTs) (single and multiwalled)[12-14], MXenes[15,16], graphene (graphene oxide (GO), reduced graphene oxide (rGO))[17-22], carbon dots[23,24], graphitic carbon nitride (g-C3N4) nanosheets[ 25,26], boron nitride (BN)[27], silica and silicates[ 28-31], metal oxides and sulphides[32-36], metal– organic frameworks[37,38], and heteropoly acid[39], etc. have been used in the fabrication of AEMs. Among these fillers, recently graphene and its derivatives such as GO and rGO have gained tremendous attention as a filler in the development AEMs due to their fascinating structural and inherent properties.

    Graphene is a two-dimensional (2D) thin single-atom sheet of sp2 hybridized carbon atoms that are set up in a hexagonal lattice[40]. It is also known as a fundamental component of graphite and other carbon materials such as CNTs and Fullerenes (Fig. 2)[41]. Graphene is an extraordinary material because of its enormous and astounding characteristics such as high flexibility, high transparency, high mechanical and thermal properties as well as high surface area and chemical inertness[ 42,43]. Due to these tremendous properties of graphene, it become an immensely popular and emerging material for the development of high-performance AEMs. Besides these, it has the nature of quick agglomeration in different solvents and polymer matrices due to its high surface energy and van der Waals force between the layers which reduced its potential for further applications. Different forms of graphene such as single-layer graphene, few layers graphene, multilayer graphene, rGO and GO have been used in various applications. Among these, GO having the oxidative groups such as hydroxide, epoxy, and carboxyl, is very popular for the fabrication of AEMs due to its high dispersibility in aqueous and organic solvents, better dispersion in polymer matrix because of strong interfacial interaction between GO and polymer, film-forming properties and ease of surface modification which enhance the overall performance of the composite.

    Also, 2D morphology of GO provides continuous and long-range transport channels in the composites for anions[44]. These properties of GO can be valuable to the development of highly stable and high-performing AEMs in near future. Graphene and its derivatives such as GO and rGO show a strong tendency for agglomeration which decrease the potential of final polymer nanocomposite[45]. Therefore, there is a need for surface modification of graphene/rGO/GO through physical or non-covalent and covalent functionalizations that helps to restrict their agglomeration tendency in the polymer matrix[46]. In recent years, these graphene derivatives either functionalized or non-functionalized are used in the polymer matrix as filler. Finally, fabrication of graphene/rGO/GO/functionalized GO (FGO) based AEMs is done in such a way where the different ratios of these derivatives are added in the polymer matrix to enhance the overall performance of the fuel cell. In addition, these components of AEM need to be quaternized either graphene derivatives or polymer or both through previously mentioned cationic species via in-situ or ex-situ. These cationic species improve the alkaline stability as well as increase the hydroxide ion conductivity[47,48]. Overall, the initial research and published reports on graphene-based derivatives for AEMs application creates hope that these materials have the potential to fabricate high-performance AEMs for fuel cells in near future.

    Therefore, this review is focused on the recent development of surface modification graphene/GO/rGO through its surface and the presence of oxidative groups as well as graphene derivatives-based composites with different anion exchange polymers for AEMFCs applications.

    2. Surface Modification of Graphene/GO/ rGO

    To access the potential features of graphene/GO/rGO in polymer composites there is a need for surface modification of these materials. Surface modification generally restricts the agglomeration of these graphene and its derivatives and enhances the multifold dispersion in the polymer matrix. Surface modification of these materials can be done through covalent and non-covalent functionalization according to the nature of graphene/ GO/r-GO[46]. Graphene has no functional group so it can only interact through its π network with the other functional moieties. rGO has a limited number of functional groups apart from the π network that is available for functionalization with the other functional moieties. At the last, GO has been extensively used in the fabrication of AEMs because it has large numbers of oxidative functional groups such as epoxides, hydroxyl, and carboxyl apart from the π network. These groups make GO is very susceptible to desire functionalization according to the applications. Details discussion related to the surface modification of graphene/ GO/r-GO is as follows.

    2.1. Covalent modification

    The carbon-carbon double bonds (C=C) or oxygen- containing groups on the surface of graphene/ GO/rGO are exploited as reaction sites in covalent surface modification. A covalent or permanent bond form between the modifier and one of these functional sites that are responsible for the surface modification of graphene-based materials[49-51]. On the surface and margins of GO several oxygen-containing groups, including hydroxyl, carboxyl, and epoxy groups, can be found. These oxidative functional groups have a high degree of reactivity[52], which increases the potential for covalent surface modification[ 53]. The covalent modification of these graphene and its derivatives can achieve either by creating a covalent bond between an amphiphilic or free radical molecule and π network of these materials[54], or by creating a covalent bond between an organic functional group and an oxygen-containing group on the surface of these materials. The following section describes the surface modification of graphene/GO/rGO for the development of graphene-based composites for AEMs application.

    2.1.1. Modification through epoxy group

    The epoxide ring opening of GO plays a significant role to modify its surface and makes it suitable for further applications. In this order, GO is modified with 4-(tri-fluoromethylthio)aniline (TFMA) through the epoxide ring-opening reaction in the presence of EtOH/H2O at 80°C[44]. This modified GO was further used for the fabrication of AEM with quaternized polybenzimidazolium (QPBI). Lin et al.[55] and Wang et al.[56] reported the surface modification of GO through epoxide ring opening by using 1-(3-aminopropyl)-3- methylimi-dazolium bromide in the basic medium at 80°C under refluxing condition. This FGO was used for the fabrication of AEM with PBI. FGO increases the interaction with the polymers as well as provide a higher surface area and long 2D channel for hydroxide ion conductance.

    2.1.2. Modification through carboxyl group

    The carboxylic group assisted with the modification of GO through its corners by making amide, ester, and Si-O linkages. The activation of -COOH groups of GO was done by the 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) or N,N'-dicyclohexylcarbodiimide in several earlier published reports[57,58]. It is an easy and common method which was used for the functionalization of GO. In this context, GO was modified by the (3-chloropropyl) trimethoxy silane (CTMS)[59]. In this process, GO dispersed into toluene then add a certain amount of CTMS in the dispersion and reflex at 90°C for 6 h. Further, this FGO was quaternized with multi- cationic oligomer brushes-decorated graphene oxide (QBGO) and used as filler for PAES-based membrane for AEM application.

    The -COOH group of GO is also activated by converting it into acyl chloride (-COCl) which was reported by Staser et al.[60] Further, GO-COCl was treated with the 1-(amino propyl)imidazole at 100 °C for 24 h, during this reaction amide bond is formed (O=C-NH). In the next step, quaternization of this product was done in the presence of 1-bromobutane at 90°C for 24h (Fig. 3). Finally, this functionalized and quaternized GO used for the development of AEM for fuel cell with Fumaperm® membrane. Liu et al.[61] reported the functionalization of GO by using ethylenediamine (EDA) in the presence of EDC and NHS. In this process, EDA connected with GO-COOH through the amide linkage (Fig. 4). Further, FGO was used for the synthesis of an ion exchange membrane with sulfonated SPEEK polymer.

    2.1.3. Modification through hydroxyl group

    GO and rGO contained the numbers hydroxyl (-OH) groups over their edges. However, the latter one has a limited extent of -OH groups. These -OH groups are very feasible for the reaction with other functional moieties and form a permanent covalent bond. As per our knowledge, selective -OH group functionalized GO was used limited as filler in polymer matrix for AEM. However, recently one research group reported, GO is functionalized by using -OH group with 3-(triethoxysilyl) propyl spirocyclic ammonium bromide. In this process, trimethoxy groups of trimethoxysilane were hydrolyzed into silanols, thereafter these reacted with the -OH group of GO to form 6-azonia-spiro[5.5]undecane-GO (ASU-GO)[62]. Further, this ASU-GO used for the fabrication of AEM with PPO for fuel cell membrane application which exhibited excellent hydroxide conductivity along with alkaline stability. In another report, Shukla et al.[63] chemically grafted dimethyloctadecyl [3-(trimethoxysilyl)propyl] ammonium chloride (DMOTACl) and 3-chloropropane (trimethoxysilane) (CTS) over the GO surface by using -OH groups in tetrahydrofuran (THF) under the refluxing condition for 15 h. Thereafter, this FGO was used as filler in QPES polymer for the development of AEM which showed better IEC and water uptake capacity than pure polymer.

    2.1.4. Modification through π network

    Graphene/GO/rGO has an extensive π network to its surface which is open for the reaction with other functional moieties. Graphene and rGO have a more conjugated π network as compared to the GO. The 2D surface structure allows for reactions can occur through both surfaces (above and below). For this, nitrene, diazonium coupling, and 1,3 dipolar cycloadditions reactions are very useful for the functionalization of graphene derivatives through their π network[ 64-66]. In this context, diazonium salt reactions have been adopted wieldy for the addition of aryl group by using diazonium salt over the π network of Graphene/GO/rGO (Fig. 5). Therefore, researchers have shown their interest in this diazonium chemistry for the functionalization of graphene derivatives[67-69]. However, this kind of approach is not very much explored in AEM research. But recently, GO was modified through its basal plane with TFMA via diazonium chemistry[44]. During this reaction, TFMA is reacted through the π network of the GO in the presence of NaNO2/HCl. This modified GO was further used for the development of AEM with QPBI polymer.

    2.2. Non-covalent modification

    Apart from the covalently functionalized graphene/ rGO/GO, the non-covalent method of functionalization is also very useful to modify the surface of graphene/ rGO/GO, then these can be potential fillers for the polymer matrix. This kind of approach enhances the properties and performance of the final composites. The functionalization can be done through the π-π interaction between the extensive π-network of graphene/ rGO/GO with π-network functional moieties as well as can be done through the hydrogen bonding between rGO/GO and functional moieties. Movil et al.[70] to develop the poly(diallyldimethylammonium chloride (PDDA) functionalized GO through a non-covalent approach. In this process, they mixed the 1.20 wt% of GO and PDDA under stirring conditions for 24 hr. During this, GO and PDDA attached to each other through the H-bonding. Further, this FGO was used for the fabrication of AEM with polyvinyl alcohol/PDDA semi-interpenetrating polymer networks (PVA/PDDA SIPNs). In addition, rGO also used directly as filler in the polymer matrix in which they are attached with each other through the π- π bonding. The final composite has a better performance than the pure one[71].

    3. Graphene/GO/ rGO Based Polymers Composites for AEM for Fuel Cell

    In recent years, to exploit the 2D surface properties and inherent physical properties of graphene and its derivatives in the fabrication of better and high-performing AEM. There are some reports available that showed the potential and utility of graphene and its derivatives in AEMFCs application. In this area, Chu et al. [72] prepared the quaternary ammonium functionalized GO (Q-GO) into quaternized poly(arylene ether) (QPAE) random copolymer composite for AEMs. The incorporation of QGO in the QPAE matrix improved the performance along with the dimensional stability and mechanical properties of the resultant membrane. Incorporating only 0.7% of QGO in QPAE matrix showed the hydroxide conductivity of 114.2 mS/cm at 90°C along with IEC of 1.45 mmol/g which was 2.07 times higher than that of the pure membrane. This synthesized membrane-based cell showed a maximum peak power density (MPPD) of 135.8 mW/cm2. Long et al.[62] reported the N-spirocyclic ammonium-functionalized GO-based composite with PPO. This functionalized GO/PPO composite exhibited higher hydroxide conductivity of 73.4 mS/cm at 80°C which was 3 times higher than that of unfunctionalized GO and PPO composite. The final membrane displayed the MPPD of 102 mWcm2 with high alkaline stability up to 700 h in 1M KOH at 80°C. The reduction in hydroxide conductivity was found only 10.8% after the alkaline stability test.

    Multi-cations oligomer modified GO was also reported as filler in the polymer matrix, later that was used for the development of AEM. This modified GO provides better crosslinking with the polymer matrix that directly contributes to the impressive performance of polymer membranes. In this context, Lu et al.[59] reported synthesized multi-cationic oligomer brushes- decorated GO (QBGO) by using 1,4-diazabicyclo [2,2,2]octane and 1,6-dibromohexane. Further, this QBGO used with quaternized poly (arylene ether sulfone) (QPAES) to develop well-defined phase separation microstructures along with ion-conducting channels that shown excellent ion transport and water uptake than the pure polymer. 2.0 wt% of QBGO increases the ionic conductivity is 1.90 times higher and as well as showed better chemical stability in alkaline medium than the QPAES. Also, this membrane based single H2/O2 fuel cell showed the MPPD of 78.7 mW/cm2 which was higher than that of QPAES membrane (MPPD of 53.1 mW/cm2).

    In this order, Mao et al.[73] reported the fabrication of ImGO based composite membrane with imidazolium- functionalized bisphenol A-type polysulfone (ImPSF). This developed AEM created an organic-inorganic interface in composite by manipulating the aggregation of conductive groups using the dense Im groups over GO. These composite membranes exhibited well-defined micro-phase structures and low-resistance channels for ionic transport. Due to this, the membrane showed better performance in terms of IEC, activation energy, and hydroxide conductivity of 3.05 mmol/g, 13.62 kJ/mol (28.63 kJ/mol for ImPSF), and 22.02 mS/cm at 30°C, respectively with 0.2% of ImGO. In addition, this composite membrane-based single fuel cell structure showed the MPPD of 78.7 mW/cm-2 at 60°C.

    N, N-dimethyl-p-phenylenediamine (DMPD) modified GO (DMPD-GO) was also used for the development of AEM with QPPO by Zhang et al.[74]. This DMPD-GO and QPPO-based membrane showed less water uptake and swelling ratio along with higher ionic conductivity as well as better alkaline stability. The membrane showed a higher ionic conductivity of 123 mS/cm at 80°C and which was reduced at 83 mS/cm after being immersed in 2M KOH solution at 60°C for 384h. On the other hand, pure QPPO membrane showed fragility after 144h. Also, this membrane demonstrated the highest MPPD of 397.8 mW/cm2 at 0.61 V which was higher than pure QPPO membrane (289.3 mW/cm2 at 0.53V).

    rGO-based composite membranes were also used as AEM for fuel cell. The π-π interaction between the π framework of rGO and polymer backbone provides better mechanical stability, avoids fuel crossover and restricts membrane swelling, and improves ionic conductivity along with excellent alkaline stability[75]. Therefore, many reports are available for the rGO-based composites for AEMs application. Chu et al.[71] used rGO as a filler for PAEK polymer to prepare AEM which showed outstanding alkaline stability. The different ratio of rGO (1.0 wt%, 3.0 wt%, and 5.0 wt%) was used in the PEAK matrix. After the synthesis of composites membrane quaternization and anionization were done. Among these different percentages of rGO, 1.0 wt% based membrane showed better performance than others. The membrane showed highest hydroxide conductivity of 115 mS/cm which was higher than that of other percentages of rGO. This is because rGO has a smaller number of oxidative groups, therefore, it has the tendency to agglomerate in the polymer matrix in higher percentages, as a result, reduced the ionic conductivity. In addition to this, the highest IEC showed by 1.0 wt% of rGO of 1.57 mmol/g with which was higher than the quaternized PEAK of 1.28 mmol/g.

    Apart from these, graphene quantum dots were also reported as filler for polymers matrix to increase their performance. In this order, Dong et al.[76] reported the synthesis of nitrogen-doped graphene quantum dots (N-GQDs) which were further modified with quaternized branched polyethyleneimine (Fig. 6 (a)). These quaternized N-GQDs further used for the fabrication of AEM with quaternized polysulfone (QPSU) polymer (Fig. 6 (b)). The fabricated membrane demonstrated ionic conductivity of 76.18 mS/cm with 0.7% of quaternized N-GQDs which was higher than that of pure QPSU of 46.14 mS/cm at 80°C. This combination- based membrane also showed an IEC of 2.56 mmol/g along with 70% of alkaline stability after 700h in Fenton's reagent. Furthermore, the single fuel cell configuration of this AEM showed a higher MPPD of 85.2 mW/cm2 with an open circuit voltage of (OCV) of 1.02 V (Fig. 6 (c)).

    Nanoribbons shaped graphene that are usually strips of graphene which also have greater mechanical and physical properties along with high surface area[77]. Therefore, graphene nanoribbons have a wide range of applications in composites, fuel cells and solar cell, etc.[79-81]. Therefore, graphene nanoribbons were used as filler in polymer composite for AEM application. In this context, Liu et al.[82] reported the use of graphene nanoribbons (GNRs) as a filler for polymer based AEM. In this work, they have prepared the GNRs by oxidative cutting of MWCNTs using KMnO4 and H2SO4, and further GNRs sample was functionalized with 3-aminopropyltrimethoxysilane (APTMS) to make NH2-GNRs. After that, this functionalized GNRs treated with ionic liquid, 1-methyl-3-(oxiran-2-ylmethyl)- 1H-imidazol-3-iumchloride (MIMC) to prepare the cationic GNRs (IGNRs). Further, prepared cationic IGNRs were mixed in ionic-liquid-functionalized perfluorinated anion polymer (I-PFSO2NH2-Cl). The composite membrane exhibited the highest ionic conductivity of 120.5 mS/cm at 80°C which was 1.7 folds higher than that of the pure membrane. This composite membranebased single fuel cell showed the highest MPPD of 197.2 mW/cm2 which was 1.9 times greater than pure membrane.

    Organic dye such as methyl orange (MO) functionalized GO was also used as a filler for the matrix of anion exchange polymers which help increase the ionic conductivity and alkaline stability along with the performance of the AEMFC. MO functionalized GO and QPAEK composite AEM used in alkaline fuel cells[83]. In this work, initially GO is modified with 3-trimethoxysilylpropyl chloride (CPTMS) through silanization and further reacted with the MO (Fig. 7(a)). In this process, the disubstituted nitrogen atom of MO directly attacks the halide group attached carbon through the Menshutkin reaction. Further, this GO-MO used in a different ratio in the QPAEK polymer matrix for AEM application (Fig. 7(b)). The developed composite membrane displayed good ion conductive channels (Fig. 7(c)). Due to this only 0.7% of GO-MO incorporated in QPAEK showed a higher ionic conductivity of 118.5 mS/cm. Also, this membrane exhibited a 1.3 times higher alkaline stability than the pure membrane in alkaline KOH for 500 h (Fig. 7 (d)). Further, this membrane based H2/O2 single fuel cell demonstrated the MPPD of 303.2 mW/cm2 at 60°C (Fig. 7(e)).

    4. Direct Coating of Graphene/GO/ rGO over Polymer and Commercially Available AEM film

    The efficiency of graphene and its derivatives were also tested by their direct coating over the polymer and commercially existing membranes to improve their performance. In this order, Chen et al.[84] reported the GO sandwich for the covalent organic framework (COF) based composite with QPPO for AEMFC application. Initially, they prepared the quaternary ammonium- modified COF-LZU1 (QAmCOF-LZU1) which was incorporated into QAPPO to prepare the QAmCOF-LZU1/QPPO composite membrane. In addition, sulfonated graphene oxide (SGO) was coated on both sides of the QAmCOF-LZU1/QPPO membrane via ionic interactions between sulfonic acid and quaternary ammonium groups and π-π interactions between benzene rings, which can effectively prevent the leakage of ion-conducting groups, further improved the stability of the composite membrane. GO sandwich composite membrane achieved desirable properties such as high hydroxide conductivity, alkali stability, and dimensional stability for AEMFC applications. SGO-coated membrane of QAmCOF-LZU1/QPPO high tensile strength of 25.3 MPa which was higher than the pure QPPO of 18.3 MPa. Also, SGO coating decreases the swelling ratio from 36.7% for pure QPPO to 16.4% for SGO-QAmCOF-LZU1/QPPO membrane. This membrane showed excellent alkaline stability and only a 16.9% reduction was observed in conductivity in NaOH solution for 840 h which was better than a 28.7% reduction for pure QPPO membrane. Further, SGO-modified membrane was tested for a single H2/O2 fuel cell which exhibited MPPD of 242 mW/cm2 at 60°C. Calderon et al.[85] also used the partially reduced GO coating over Sustainion® X37-50 AEM for the enhancement of its performance for fuel cells. Initially, they partially reduced the GO into rGO (prGO) by using 6, 12, and 24 mmol sodium borohydride (NaBH4) solutions. Further, this prGO coated over the Sustainion® X37-50 AEM by using spray coating technique. The 12 mmol solutions reduced prGO coated membrane showed better performance and increased approximately 37.6% MPPD (0.589 W/cm2) than the pure membrane. In addition to this, 6 mmol, 24 mmol solution reduced prGO and GO-based coated membrane showed a MPPD of 0.415, 0.443, and 0.467 W/cm2, respectively. This study indicated that the degree of reduction and presence of oxidation group at the surface of GO also play a significant role in the performance of AEM fuel cells.

    5. Direct use of Graphene/GO/rGO film for AEM

    Graphene and its derivatives are showing exceptional structural and physical properties. Therefore, these are the ideal candidate for direct use in the fabrication of AEMs for fuel cells. Although, very limited work has been done in this area because these graphene derivatives are tending agglomeration due to their high surface energy and strong van der Waals force between the layers. In that case, among these GO is the ideal candidate for making an AEM film without using any polymer matrix. Thanks to its oxidative functional groups which are makes it is dispersible in various solvents. Also, these groups help to improve the water uptake capacity and hydroxide ion conduction. Due to this, Bayer et al.[86] synthesized the KOH-modified multilayer GO (KOH-GO). Further, the KOH-GO membrane was fabricated with the help of the traditional filtration process. This KOH-GO membrane showed a large extent of swelling. However, this technique is very convenient and useful for the fabrication of varying thicknesses of membranes. Further, KOH-GO membrane was used as an AEM. This KOH-GO membrane demonstrated the ICE of 6.1mmol/g, maximum anion conductivity of 6.1 mS/cm at 70°C with a high tensile strength of 24.5 MPa. This membrane also exhibited the MPPD of ~1 mW/cm2 with OCV of 0.94 V.

    6. Conclusion and Future Prospectus of Graphene Derivatives for AEMFCs

    Presently, several types of polymers and polymers- based composites with filler have been used as AEMs for fuel cells. However, these materials are unable to perform well in terms of hydroxide ion conductivity, alkaline stability, mechanical stability, water uptake capacity as well as the performance of fuel cells. Therefore, graphene-based derivatives gained lot of attention in recent years as a filler in the polymer matrix and increase their properties and performance. This happens due to the extraordinary structural and physical properties of graphene-based materials which makes them alluring and potential candidates as fillers for the polymer matrix, this also confirmed the above results of developed graphene-based AEMFCs. Apart from the exceptional properties of these materials, they are also having the tendency to quick agglomeration which reduced the performance of the final composites. Therefore, there is a need for surface functionalization of these derivatives which enhances their dispersibility in the solvents and increases the homogeneity in the polymer’s matrix. As far as the surface functionalization of these materials, GO is the best candidate because it has a variety of oxygen-containing groups that are present in abundance on the surface giving GO excellent modifiability and multifunctionality, which improve the overall performance of the composites. The composites are produced with continuous and longrange transport channels for ion conduction thanks to the 2D shape of GO. Due to the robust interfacial contacts between the polymer matrix and GO, GO nanosheets are simple to disperse uniformly in a range of polymer matrices. Therefore, the performance of composites made of polymer and GO significantly enhance by such homogenous dispersion.

    For the future prospectus, there is a need for extensive research in this area because a lot of irregularities were found in the previous research. Consequently, an appropriate formulation and what kind of graphene derivatives could be used in the polymer matrix that will significantly increase the properties of composites as well as the performance of the fuel cell, need to be find. Many reports showed the higher amount of graphene derivatives reduced the performance of composites due to the agglomeration of these materials. Thus, research in finding appropriate amounts and suitable surface modifying agents for graphene derivatives is also necessary which increases the dispersion in solvent as well as in the polymer matrix to increase the properties and performance of the final composite membrane for fuel cells.

    Acknowledgments

    This research is endorsed by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R1A6A03038697) and the Korean government (MSIP) (No. NRF-2021M1A2A2038115).

    Figures

    MEMBRANE_JOURNAL-32-6-411_F1.gif

    Schematic representation of H2/O2 AEMFC (Reprint with the permission of Das et al. 2022, MDPI[7]).

    MEMBRANE_JOURNAL-32-6-411_F2.gif

    Graphene and graphene-based 0D, 1D, and 3D structured carbons.

    MEMBRANE_JOURNAL-32-6-411_F3.gif

    Surface modification of GO via carboxylic group activation.

    MEMBRANE_JOURNAL-32-6-411_F4.gif

    Functionalization of GO via activation of carboxylic group.

    MEMBRANE_JOURNAL-32-6-411_F5.gif

    Functionalization of graphene through diazonium chemistry by using diazonium salt.

    MEMBRANE_JOURNAL-32-6-411_F6.gif

    (a) Synthesis of QBPEI@N-GQDs (b) QPSU/QBPEI@N-GQDs (c) the single fuel cell performance of pure QPSU and QQ-0.7%-N-GQDs AEMs at 80°C. (Reprint with the permission of Dong et al. 2022, Wiley & Sons[76]).

    MEMBRANE_JOURNAL-32-6-411_F7.gif

    (a) Schematic illustration for the synthesis process of GO derivative, (b) composite membrane, (c) cross section FESEM image representing ionic channels, (d) alkaline stability, (e) single Cell performance (Reprint with the permission of Lee et al. 2021, American Chemical Society[83]).

    Tables

    References

    1. T. Wilberforce, Z. El-Hassan, F. N. Khatib, A. Makky, A. Baroutaji, J. G. Carton, and A. G. Olabi, “Developments of electric cars and fuel cell hydrogen electric cars”, Int. J. Hydrog. Energy, 42, 25695 (2017).
    2. M. Géraldine, W. Matthias, and N. Kitty, “Anion exchange membranes for alkaline fuel cells: A review”, J. Membr. Sci., 377, 1 (2011).
    3. M. Warshay and P. R. Prokopius, “The fuel cell in space: yesterday, today and tomorrow”, In: Grove Anniversary (1839-1989) Fuel Cell Symposium (No. E-5084), (1989).
    4. T. B. Ferriday and P. H. Middleton, “Alkaline fuel cell technology-A review”, Int. J. Hydrog. Energy, 46, 18489 (2021).
    5. E. Gülzow and M. Schulze, “Long-term operation of AFC electrodes with CO2 containing gases”, J. Power Sources, 127, 243 (2004).
    6. M. Schulze and E. Gülzow, “Degradation of nickel anodes in alkaline fuel cells”, J. Power Sources, 127, 252 (2004).
    7. G. Das, J. H. Choi, P. K. T. Nguyen, D. J. Kim, and Y. S. Yoon, “Anion exchange membranes for fuel cell application: A Review”, Polymers, 14, 1197 (2022).
    8. X. Cheng, Z, Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z. S. Liu, H. Wang, and J. Shen, “A review of PEM hydrogen fuel cell contamination: Impacts, mechanisms, and mitigation”, J. Power Sources, 165, 739-756 (2007).
    9. M. G. Marino and K. D. Kreuer, “Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids”, ChemSusChem., 8, 513-523 (2015).
    10. R. Narducci, E. Sgreccia, P. Knauth, and M. L. Di Vona, “Anion exchange membranes with 1D, 2D and 3D fillers: A review”, Polymers, 13, 3887 (2021).
    11. V. Elumalai and D. Sangeetha, “Preparation of anion exchangeable titanate nanotubes and their effect on anion exchange membrane fuel cell”, Mater. Des., 154, 63 (2018).
    12. W. H. Pan, S. J. Lue, C. M. Chang, and Y. L Liu, “Alkali doped polyvinyl alcohol/multi-walled carbon nano-tube electrolyte for direct methanol alkaline fuel cell”, J. Membr. Sci., 376, 225 (2011).
    13. S. J. Lue, W. H. Pan, C. M. Chang, and Y. L Liu, “High-performance direct methanol alkaline fuel cells using potassium hydroxideimpregnated polyvinyl alcohol/carbon nano-tube electrolytes”, J. Power Sources, 202, 1 (2012).
    14. T. Zhou, M. Wang, X. He, and J. Qiao, “Poly(vinyl alcohol)/Poly(diallyldimethylammonium chloride) anion-exchange membrane modified with multiwalled carbon nanotubes for alkaline fuel cells”, J. Mater., 5, 286 (2019).
    15. X. Zhang, C. Fan, N. Yao, P. Zhang, T. Hong, C. Xu, and J. Cheng, “Quaternary Ti3C2Tx enhanced ionic conduction in quaternized polysulfone membrane for alkaline anion exchange membrane fuel cells”, J. Membr. Sci., 563, 882 (2018).
    16. L. Wang and B. Shi, “Hydroxide conduction enhancement of chitosan membranes by functionalized MXene”, Materials, 11, 2335 (2018).
    17. Y. S. Ye, M. Y. Cheng, X. L. Xie, J. Rick, Y. J. Huang, F. C. Chang, and B. J. Hwang, “Alkali doped polyvinyl alcohol/graphene electrolyte for direct methanol alkaline fuel cells”, J. Power Sources, 239, 424 (2013).
    18. J. M. Yang and S. A Wang, “Preparation of graphene- based poly(vinyl alcohol)/chitosan nanocomposites membrane for alkaline solid electrolytes membrane”, J. Membr. Sci., 477, 49 (2015).
    19. P. P. Sharma, S. Gahlot, B. M. Bhil, H. Gupta, and V. Kulshrestha, “An environmentally friendly process for the synthesis of an fGO modified anion exchange membrane for electro-membrane applications”, RSC Adv., 5, 38712 (2015).
    20. Z. Zakaria, S. K. Kamarudin, and S. N. Timmiati, “Influence of graphene oxide on the ethanol permeability and ionic conductivity of QPVA- based membrane in passive alkaline direct ethanol fuel cells”, Nanoscale Res. Lett., 14, 1 (2019).
    21. W. T. Ma, S. R. Kumar, C. T. Hsu, C. M. Shih, S. W. Tsai, C. C. Yang, Y. L. Liu, and S. J. Lue, “Magnetic field-assisted alignment of graphene oxide nanosheets in a polymer matrix to enhance ionic conduction”, J. Membr. Sci., 563, 259 (2018).
    22. Y. Bai, Y. Yuan, L. Miao, and C. Lü, “Functionalized rGO as covalent crosslinkers for constructing chemically stable polysulfone-based anion exchange membranes with enhanced ion conductivity”, J. Membr. Sci., 570, 481 (2018).
    23. Y. Yuan, B. Hu, C. Tong, Y. Bai, and C. Lü, “Novel quaternized carbon dots modified polysulfone- based anion exchange membranes with improved performance”, Int. J. Hydrog. Energy, 44, 22181 (2019).
    24. V. Yadav, S. K. Raj, N. H. Rathod, and V. Kulshrestha, “Polysulfone/graphene quantum dots composite anion exchange membrane for acid recovery by diffusion dialysis”, J. Membr. Sci., 611, 118331 (2020).
    25. P. B. Ingabire, X. Pan, A. Haragirimana, N. Li, Z. Hu, and S. Chen, “Enhanced conduction capability of nanocomposite membrane of quaternized poly (arylene ether sulfone)s covalently bonded with graphitic carbon nitride nanosheets for fuel cells”, React. Funct. Polym., 144, 104260 (2019).
    26. Y. Lu, X. Pan, N. Li, Z. Hu, and S. Chen, “Improved performance of quaternized poly(arylene ether ketone)s/graphitic carbon nitride nanosheets composite anion exchange membrane for fuel cell applications”, Appl. Surf. Sci., 503, 144071 (2019).
    27. N. H. Rathod, V. Yadav, A. Rajput, J. Sharma, D. Shukla, and V. Kulshrestha, “New class of composite anion exchange membranes based on Quaternized poly (phenylene oxide) and functionalized boron nitride”, Colloid Interface Sci. Commun., 36, 100265 (2020).
    28. E. Sgreccia, R. Narducci, P. Knauth, and M. Di Vona, “Silica containing composite anion exchange membranes by Sol–gel synthesis: A short review”, Polymers, 13, 1874 (2021).
    29. X. Zuo, S. Yu, X. Xu, J. Xu, R. Bao, and X. Yan, “New PVDF organic–inorganic membranes: The effect of SiO2 nanoparticles content on the transport performance of anion-exchange membranes”, J. Membr. Sci., 340, 206-213 (2009).
    30. E. D. Wang, T. S. Zhao, and W. W. Yang, “Poly (vinyl alcohol)/3-(trimethylammonium) propyl-functionalized silica hybrid membranes for alkaline direct ethanol fuel cells”, Int. J. Hydrog. Energy, 35, 2183 (2010).
    31. S. R. Kumar, C. H. Juan, G. M. Liao, J. S. Lin, C. C. Yang, W. T. Ma, J. H. You, and S. J. Lue, “Fumed silica nanoparticles incorporated in quaternized poly(vinyl alcohol) nanocomposite membrane for enhanced power densities in direct alcohol alkaline fuel cells”, Energies, 9, 15 (2015).
    32. C. C. Yang, S. J. Chiu, W. C. Chien, and S. S. Chiu, “Quaternized poly(vinyl alcohol)/alumina composite polymer membranes for alkaline direct methanol fuel cells”, J. Power Sources, 195, 2212 (2010).
    33. M. T. Pérez-Prior, T. García-García, A. Varez, and B. Levenfeld, “Preparation and characterization of ammonium-functionalized polysulfone/Al2O3 composite membranes”, J. Mater. Sci., 50, 5893 (2015).
    34. R. Vinodh, M. Purushothaman, and D. Sangeetha, “Novel quaternized polysulfone/ZrO2 composite membranes for solid alkaline fuel cell applications”, Int. J. Hydrog. Energy, 36, 7291 (2011).
    35. K. Rambabu, G. Bharath, A. F. Arangadi, S. Velu, B. Fawzi, and P. L. Show, “ZrO2 incorporated polysulfone anion exchange membranes for fuel cell applications”, Int. J. Hydrog. Energy, 45, 29668 (2020).
    36. X. C. Jiang; Y. J. Sun, H. X. Zhang, and L. X. Hou, “Preparation and characterization of quaternized poly(vinyl alcohol)/chitosan/MoS2 composite anion exchange membranes with high selectivity”, Carbohydr. Polym., 180, 96 (2018).
    37. P. Y. Hsu, T. Y. Hu, S. R. Kumar, C. H. Chang, K. C. W. Wu, K. L. Tung, and S. J. Lue, “Highly zeolite-loaded polyvinyl alcohol composite membranes for alkaline fuel-cell electrolytes”, Polymers, 10, 102 (2018).
    38. B. Wu, L. Ge, D. Yu, L. Hou, Q. Li, Z. Yang, and T. Xu, “Cationic metal–organic framework porous membranes with high hydroxide conductivity and alkaline resistance for fuel cells”, J. Mater. Chem. A, 4, 14545 (2016).
    39. K. H. Lee and D. J. Yoo, “Effect of silicotungstic acid as inorganic filler on the properties of anion exchange composite membranes”, KHNES, 33, 28 (2022).
    40. M. Karakoti, S. Pandey, R. Jangra, P. S. Dhapola, P. K. Singh, S. Mahendia, A. Abbas, and N. G. Sahoo, “Waste plastics derived graphene nanosheets for supercapacitor application”, Mater. Manuf. Process., 36, 171-177 (2021).
    41. Y. Hu and X. Sun, “Chemically functionalized grapheme and their applications in electrochemical energy conversion and storage”, In: Advances in Graphene Science, pp. 161, Intechopen, Rijeka, Croatia (2013).
    42. M. Karakoti, R. Jangra, S. Pandey, P. S. Dhapola, S. Dhali, S. Mahendia, P. K. Singh, and N. G. Sahoo, “Spray dryer processed graphene oxide/reduced graphene oxide for high‐performance supercapacitor”, Int. J. Appl. Ceram. Technol., 17, 1899 (2020).
    43. A. K. Geim, “Graphene: status and prospects”, Science, 324, 1530 (2009).
    44. L. Cseri, J. Baugh, A. Alabi, A. AlHajaj, L. Zou, R. A. Dryfe, P. M. Budd, and G. Szekely, “Graphene oxide – polybenzimidazolium nanocomposite anion exchange membranes for electrodialysis”, J. Mater. Chem. A, 6, 24728 (2018).
    45. Z. Baig, O. Mamat, and M. Mustapha, “Recent progress on the dispersion and the strengthening effect of carbon nanotubes and graphene-reinforced metal nanocomposites: a review”, Crit. Rev. Solid State Mater. Sci., 43, 1 (2018).
    46. M. Karakoti, S. Dhali, S. Rana, S. R. V. S. Prasanna, S. P. S. Mehta, and N. G. Sahoo, “Surface modification of carbon-based nanomaterials for polymer nanocomposites”, In: Carbon-based polymer nanocomposites for environmental and energy applications, pp. 27-56, Elsevier, Hampshire, Cambridge, USA (2018).
    47. P. Goel, E. Bhuvanesh, P. Mandal, V. K. Shahi, A. Bandyopadhyay, and S. Chattopadhyay, “Diquaternized graphene oxide based multi-cationic cross-linked monovalent selective anion exchange membrane for electrodialysis”, Sep. Purif. Technol., 276, 119361 (2021).
    48. S. Changkhamchom, P. Kunanupatham, K. Phasuksom, and A. Sirivat, “Anion exchange membranes composed of quaternized polybenzimidazole and quaternized graphene oxide for glucose fuel cell”, Int. J. Hydrog. Energy, 46, 5642 (2021).
    49. F. Karlický, K. R. K. Datta, M. Otyepka, and R. Zbořil, “Halogenated graphenes: rapidly growing family of graphene derivatives”, ACS Nano, 7, 6434 (2013).
    50. M. Quintana, E. Vazquez, and M. Prato, “Organic functionalization of graphene in dispersions”, Acc. Chem. Res., 46, 138 (2013).
    51. P. Di Pietro, G. Forte, R. Snyders, C. Satriano, C. Bittencourt, and D. Thiry, “Sulphur functionalizion of graphene oxide by radiofrequency plasma”, Plasma Processes Polym., 17, 2000039 (2020).
    52. S. Stankovich, R. D. Piner, S. T. Nguyen, and R. S. Ruoff, “Synthesis and exfoliation of isocyanatetreated graphene oxide nanoplatelets”, Carbon, 44, 3342 (2006).
    53. I. A. Kinloch, J. Suhr, J. Lou, R. J. Young, and P. M. Ajayan, “Composites with carbon nanotubes and graphene: an outlook”, Science, 362, 547 (2018).
    54. Y. Feng, H. Liu, W. Luo, E. Liu, N. Zhao, K. Yoshino, and W. Feng, “Covalent functionalization of graphene by azobenzene with molecular hydrogen bonds for long-term solar thermal storage”, Sci. Rep., 3, 3260 (2013).
    55. B. Lin, H. Shang, F. Chu, Y. Ren, N. Yuan, B. Jia, S. Zhang, X. Yu, Y. Wei, and J. Ding, “Ionic liquid-tethered graphene oxide/ionic liquid electrolytes for highly efficient dye sensitized solar cells”, Electrochim. Acta, 134, 209 (2014).
    56. C. Wang, B. Lin, G. Qiao, L. Wang, L. Zhu, F. Chu, T. Feng, N. Yuan, and J. Ding, “Polybenzimidazole/ ionic liquid functionalized graphene oxide nanocomposite membrane for alkaline anion exchange membrane fuel cells”, Mater. Lett., 173, 219 (2016).
    57. S. Guo, J. Raya, D. Ji, Y. Nishina, C. Ménard- Moyon, and A. Bianco, “Is carboxylation an efficient method for graphene oxide functionalization?”, Nanoscale Advances, 2, 4085 (2020).
    58. C. Wang, Q. Yan, H. B. Liu, X. H. Zhou, and S. J. Xiao, “Different EDC/NHS activation mechanisms between PAA and PMAA brushes and the following amidation reactions”, Langmuir, 27, 12058 (2011).
    59. Y. Lu, L. Liu, Y. Pu, Y. Liu, N. Li, Z. Hu, and S. Chen, “Towards performance improved anion exchange membrane: Cross-linking with multi-cations oligomer modified graphene oxide”, Int. J. Hydrog. Energy, 46, 23855 (2021).
    60. J. A, Staser and C. Movil, “Ionic liquid-functionalized graphene oxide-based nanocomposite anion exchange membrane”, US Patent: US 20190044 169A1, February 7 (2019).
    61. S. Liu, D. Li, L. Wang, H. Yang, X. Han, and B. Liu, “Ethylenediamine-functionalized graphene oxide incorporated acid-base ion exchange membranes for vanadium redox flow battery”, Electrochim. Acta, 230, 204 (2017).
    62. C. Long, C. Lu, Y. Li, Z. Wang, and H. Zhu, “N-spirocyclic ammonium-functionalized graphene oxide-based anion exchange membrane for fuel cells”, Int. J. Hydrog. Energy, 45, 19778 (2020).
    63. G. Shukla and V. K. Shahi, “Amine functionalized graphene oxide containing C16 chain grafted with poly (ether sulfone) by DABCO coupling: Anion exchange membrane for vanadium redox flow battery”, J. Membr. Sci., 575, 109 (2019).
    64. T. A. Strom, E. P. Dillon, C. E. Hamilton, and A. R. Barron, “Nitrene addition to exfoliated graphene: a one-step route to highly functionalized graphene”, Chem. Commun., 46, 4097 (2010).
    65. J. R. Lomeda, C. D. Doyle, D. V. Kosynkin, W. F. Hwang, and J. M. Tour, “Diazonium functionalization of surfactant-wrapped chemically converted graphene sheets”, J. Am. Chem. Soc., 130, 16201 (2008).
    66. V. Georgakilas, A. B. Bourlinos, R. Zboril, T. A. Steriotis, P. Dallas, A. K. Stubos, and C. Trapalis, “Organic functionalisation of graphenes”, Chem. Commun., 46, 1766 (2010).
    67. E. Bekyarova, M. Itkis, and P. Ramesh, “Chemical modification of expitaxial graphene: spontaneous grafting of aryl groups”, J. Am. Chem. Soc., 131, 1336 (2008).
    68. A. Sinitskii, A. Dimiev, and D. Corley, “Kinetics of diazonium functionalization of chemically converted graphene nanoribbons”, ACS Nano, 4, 1949 (2010).
    69. H. He and C. Gao, “General approach to individually dispersed, highly soluble, and conductive graphene nanosheets functionalized by nitrene chemistry”, Chem. Mater., 22, 5054, (2010).
    70. O. Movil, L. Frank, and J. A. Staser, “Graphene oxide–polymer Nanocomposite anion-exchange membranes”, J. Electrochem. Soc., 162, F419 (2015).
    71. J. Y. Chu, K. H. Lee, A. R. Kim, and D. J. Yoo, “Graphene-mediated organic-inorganic composites with improved hydroxide conductivity and outstanding alkaline stability for anion exchange membranes”, Compos. B. Eng., 164, 324 (2019).
    72. J. Y. Chu, K. H. Lee, A. R. Kim, and D. J. Yoo, “Improved electrochemical performance of composite anion exchange membranes for fuel cells through cross linking of the polymer chain with functionalized graphene oxide”, J. Membr. Sci., 611, 118385 (2020).
    73. X. Mao, Z. Li, G. He, Z. Li, J. Zhao, Y. Zhang, and Z. Jiang, “Enhancing hydroxide conductivity of anion exchange membrane via incorporating densely imidazolium functionalized graphene oxide”, Solid State Ion., 333, 83 (2019).
    74. D. Zhang, S. Xu, R. Wan, Y. Yang, and R. He, “Functionalized graphene oxide cross-linked poly (2, 6-dimethyl-1, 4-phenylene oxide)-based anion exchange membranes with superior ionic conductivity”, J. Power Sources, 517, 230720 (2022).
    75. B. Hu, L. Miao, Y. Zhao, and C. Lü, “Azide-assisted crosslinked quaternized polysulfone with reduced graphene oxide for highly stable anion exchange membranes”, J. Membr. Sci., 530, 84 (2017).
    76. D. Dong, M. Zhang, Y. Xiao, Z. Yang, K. Wang, and M. Fan, “Quaternized branched polyethyleneimine modified nitrogen‐doped graphene quantum dots/ quaternized polysulfone composite anion exchange membranes with improved performance”, Macromol. Mater. Eng., 307, 2100787 (2022).
    77. A. Sinitskii, A. Dimiev, D. A. Corley, A. A. Fursina, D. V. Kosynkin, and J. M. Tour, “Kinetics of diazonium functionalization of chemically converted graphene nanoribbons”, ACS Nano, 4, 1949 (2010).
    78. W. Lu, G. Ruan, B. Genorio, Y. Zhu, B. Novosel, Z. Peng, and J. M. Tour, “Functionalized graphene nanoribbons via anionic polymerization initiated by alkali metal-intercalated carbon nanotubes”, ACS Nano, 7, 2669 (2013).
    79. D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii, J. R. Lomeda, A. Dimiev, B. K. Price, and J. M. Tour, “Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons”, Nature, 458, 872 (2009).
    80. C. Xiang, N. Behabtu, Y. Liu, H. G. Chae, C. C. Young, B. Genorio, D. E. Tsentalovich, C. Zhang, D. V. Kosynkin, and J. R. Lomeda, “Graphene nanoribbons as an advanced precursor for making carbon fiber”, ACS Nano, 7, 1628 (2013).
    81. C. Wang, H. Gao, H. Li, Y. Zhang, B. Huang, J. Zhao, Y. Zhu, W. Z. Yuan, and Y. Zhang, “Graphene nanoribbons hybridized carbon nanofibers: remarkably enhanced graphitization and conductivity, and excellent performance as support material for fuel cell catalysts”, Nanoscale, 6, 1377 (2014).
    82. X. Liu, X. Chen, Y. Hu, T. Gong, H. Li, and Y. Zhang, “Ionic-liquid-functionalized graphene nanoribbons for anion exchange membrane fuel cells”, J. Electrochem. Soc., 164, F433 (2017).
    83. K. H. Lee, J. Y. Chu, A. R. Kim, and D. J. Yoo, “Fabrication of high-alkaline stable quaternized poly (arylene ether ketone)/graphene oxide derivative including zwitterion for alkaline fuel cells”, ACS Sustain. Chem. Eng., 9, 8824 (2021).
    84. J. Chen, M. Guan, K. Li, and S. Tang, “High-performance COF-based composite anion exchange membrane sandwiched by GO layers for alkaline H2/O2 fuel cell application”, J. Ind. Eng. Chem., 104, 136 (2021).
    85. J. Calderon, N. Huq, Y. Zhou, M. Zheng, A. Raut, H. Fang, M. Rafailovich, and M. R. Isseroff, “Power enhancement of anion exchange membrane fuel cells (AEMFCs) through spray-coating of membrane with partially reduced graphene oxide”, J. Undergrad. Chem. Eng. Res., 75 (2022).
    86. T. Bayer, B. V. Cunning, R. Selyanchyn, T. Daio, M. Nishihara, S. Fujikawa, K. Sasaki, and S. M. Lyth, “Alkaline anion exchange membranes based on KOH-treated multilayer graphene oxide”, J. Membr. Sci., 508, 51 (2016).
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