Environmental and Energy Applications of Graphene-Based Nanocomposites: A Brief Review

Abstract

Chemically stable two-dimensional nanostructured graphene with huge surface area, high electrical conductivity and mechanical excellence has gained significant research attention in the past two decades. Its excellent characteristics make graphene one of the important materials in various applications such as environmental and energy storage devices. Graphene no doubt has been a top priority among the carbon nanomaterials owing to its structure and properties. However, the functionalization of graphene leads to various nanocomposites where its properties are tailored to be suited for various applications with more performance, environmental friendliness, efficiency, durability and cost effectiveness. Graphene nanocomposites are said to exhibit more surface area, conductivity, power conversion efficiency and other characteristics in energy devices like supercapacitors. This review was aimed to present some of the applications of graphene-based nanocomposites in energy conversion devices like supercapacitors and Li-ion batteries and some of the environmental applications. It was observed that the performance of supercapacitors was obstructed due to restacking and agglomeration of graphene layers. This was addressed by combining MO (metal oxide) or CP (conducting polymer) with graphene as material for electrodes. Electrodes with CP or MO/graphene composites are summarized. Heterogeneous catalysts were of environmental concern in recent years. In this context, graphene-based nanocomposites gained significance due to expansion in structural diversity. A minimum overview is presented in this paper in terms of structural aspects and properties of GO/rGO-based materials used in supercapacitors and environmental applications like dye removal. Continuous efforts towards synthesis of productive graphene-based nanocomposites might lead to significant output in applications related to environment and energy sectors.

1. Introduction

Graphene, discovered in 2004, is an allotrope of carbon with a honeycomb lattice structure. Innovative experiments on 2D graphene fetched a Nobel prize in 2010 [1]. As per the literature, it was observed that in 2004, a pair of scientists, Andre Geim and Konstantin Novoselov, produced single-layer graphene but the credit was given to Hanns-Peter Boehm and his team who experimentally discovered graphene in 1962 in continuation to its theoretical exploration by Wallace P.R. in 1947. Since 2004 thousands of papers have been published on graphene and its composites owing to their unique properties. Graphene exhibits very high electron mobility (250,000 cm²/Vs) at normal room temperature. It has an abnormal quantum hall effect, high thermal conductivity (5000 Wm⁻¹K⁻¹) and good mechanical strength. These characteristics make graphene significant in various applications in sensors, energy storage, electrodes, solar cells, nanocomposites, etc. It is reported that a polystyrene nanocomposite formed by the addition of a 1% volume of graphene and polymer obtained a conductivity of approximately 0.1 Sm⁻¹, which is enough for various electrical applications [2]. These nanocomposites could withstand high stress with good strength and toughness [3,4,5]. Hence, graphene–polymer nanocomposites are considered to be of great importance as novel functional materials. Large SSA (specific surface area) enhances mechanical properties in these nanocomposites and is capable of opposing cracks in a much better way as compared to zero-dimensional (nanoparticle) fillers and one-dimensional nanotubes [5]. It is reported that structural defects in graphene influence graphene–polymer interfacial behavior, which needs to be addressed [6,7].

1.1. History of Graphene

The history of graphene was reported to start way back in 1859 through studies on a highly lamellar structure of thermally reduced graphene oxide [8,9]. Later, in 1916, the graphite structure was identified through the powder diffraction method [10,11], whose properties were studied in 1918 [12] and structure with single crystal diffraction in 1924 [13]. Graphene and its theory were first explored to understand the electronic properties of 3D graphite in 1947 [14,15]. TEM images related to multi-layer graphite were published in 1948 [16] and through electron microscopy, uni-layer graphene was produced [17]. Single-layer flakes of rGO were produced in 1962 [18,19,20,21], followed by growth of graphene epitaxially on other materials starting in the 1970s [22]. Graphene (combination of graphite and the suffix -ene, which refers to polycyclic aromatic hydrocarbons) was first introduced in 1986 by Hanns-Peter Boehm, Ralph Setton and Eberhard Stumpp [23]. Even though graphite film of a thin size using mechanical exfoliation was first seen in 1990 [24], it was confined to more than 50 to 100 layers until the year 2004 [25]. The first patent on graphene production was filed in 2002 and was granted in 2006 [26]. The extraction of single-atom-thick crystallites from bulk graphite was reported in 2004 [27]. A Nobel prize was awarded in 2010 to Geim and Novoselov for the extraordinary work on graphene [28,29]. The commercialization of graphene began with the announcement of National Graphene Institute in order to support research in 2014 [30,31].

Graphene and other carbon-based materials like nanotubes and quantum dots [8,32,33,34] gained huge significance in the past few decades in view of their properties and significant applications in sensors and electronics. Graphene is a single layer of sp²-hybridized carbon atoms arranged in a 2D honeycomb lattice. It has high thermal and electron conductivity and it is highly flexible and optically transparent with an open band gap, making it suitable for usage in opto-electronic devices [35,36,37,38]. It is highly compatible with silicon. It can be used for transparent electrodes, FETs, solar cells and energy storage applications [31,39,40,41,42]. Graphene can be easily obtained through the exfoliation of pyrolytic graphite [37]. This technique reduces the area [43] limiting its applications. Hence, the CVD technique (chemical vapor deposition) was employed in producing single-layer graphene of high quality [1,44]. This technique uses methane as precursor molecules [45] for graphene deposition on a metal-catalyst substrate (Cu) [46] on which the formation of active carbon species takes place. This leads to the nucleation of a honeycomb-like sp²-hybridized carbon bunch, then graphene domain growth followed by coalescence to form a deposit of single-layer graphene [47]. Another aspect of graphene is to act as a substrate for other compounds such as TMDC [48].

1.2. Characteristics of Graphene

Graphene is in the form of a 2D hexagonal lattice, and its composite is of much strength compared to composites of CNT. It can be treated as a semiconductor with a band gap of zero. It consists of unusual charge carriers, which act to be mass-less (Dirac fermions) on the application of a magnetic field with abnormal integer QHE (quantum hall effect) [49]. This was also seen at room temperature [50]. Graphene when applied with gate voltage can tune charge carriers between electrons and holes (effect of ambipolar electric field) [51]. It is reported that processing of graphene introduces impurities and defects strongly influencing thermal, mechanical and electrical properties [52,53,54,55,56]. It is also reported that imperfections in graphene may be utilized to alter its properties to obtain new functions [57,58]. Defects in graphene may be intrinsic (without impurities) or extrinsic (with impurities) and may be point defects or line defects [59]. This paper mainly concentrated on applications related to graphene-based composites, which have been published recently. We did not try to eloborate on the differences between graphene and graphite as we mainly focused on specific applications.

1.3. Graphene-Based Materials

In recent years, there has been a lot of emphasis towards the investigation of advances in graphene and its derivatives. Applications using graphene-based materials and their derivatives in various sectors make them significant [60,61,62]. In this context, polymer nanocomposites derived from graphene are of importance. The surface area of graphene is much larger, and hence capable of an enhanced reaction between polymer and graphene sheets, and hence can be applied in areas of biomedical and electronics [63,64]. Graphene composites gain importance due to their structural fabrication resulting in novel applications. Development of polymer composites with graphene has been in a good phase for the past twenty years. A graphene and polymer combination improves the properties and overall performance of these composites [65]. Excellent thermal, electrical and mechanical properties of graphene encourage researchers regarding its usage [66,67]. Figure 1 shows the basic structure of graphene in various dimensions. It is noteworthy that significant applications of graphene and its associates are mainly driven by various graphene-based materials such as GO, rGO, fGO, frGO and mG with specific attention on particular applications [68]. Various processes exist in order to produce graphene and its associates depending on particular applications. Figure 2 shows some of the existing processes below.

1.4. Functionalization of Graphene

Functionalization is a process of doping new functional groups by physical or chemical means onto the structure of graphene derivatives (Figure 3 and Figure 4). This can be obtained by surface modification. The mechanism of functionalization is shown in Figure 3.

1.5. Graphene–Polymer Nanocomposites

Polymers can achieve multi-functional properties by strengthening with graphene. This property in composites makes them significant in various sectors. A polymer nanocomposite is a combination of a polymer and nano-filler. They are new composites with inorganic nanoparticles dispersed in an organic polymer matrix improving their performance. Composites of polymer nanocomposites filled with graphene and its associate derivatives find significant applications in automotive, electronic, green energy and aerospace industries. Various methods of synthesizing these nanocomposites include solution blending [69,70,71,72], which utilizes some techniques like lyophilization [73], phase transfer [74], surfactants [75,76], melt mixing, in situ polymerization, etc. [77,78]. Mechanical properties of these nanocomposites depend on agglomeration, aspect ratio, distribution of nano-filler in the matrix and interface bonding. Enhanced fracture strength in a graphene–PS nanocomposite was reported with 0.9 wt% graphene [79]. Similarly, when graphene is used as filler in an insulating polymer matrix, electrical conductivity of the composite will be enhanced. Unusual thermal characteristics in graphene improve conductivity and thermal stability in nanocomposites. The two-dimensional structure of graphene offers low thermal resistance, high thermal conductivity and anisotropy in nanocomposites [80]. Enhanced thermal conductivity was reported in graphene–epoxy nanocomposites with high filler loading [81,82]. As per the above literature it is observed that graphene and graphene composites exhibit significant properties making them suitable for various applications in energy storage, environment etc. Hence, it is very appropriate to review some of the applications of graphene-based nanocomposites in a detailed manner.

1.6. Graphene/GO/rGO-Based Nanocomposites

Expanded structural diversity makes graphene-based nanocomposites significant for energy storage, environmental protection, etc. Graphene has revolutionized ultra-filtration with two-dimensional layers and huge surface area. Graphene has become a broad base for nanostructured materials used in various applications [83,84,85,86,87]. Different methods have been reported to create functionalized graphene nanocomposites [88,89]. Reduced graphene oxide (rGO) is one of the important materials for photocatalytic applications. Reports indicated low electrical conductivity in rGO compared to pure graphene [90]. Graphene/rGO nanocomposites can be used in solar applications, electrochemical energy systems, energy appliances and environmental applications such as the detection of heavy metal ions, bacteria, oxidation, radioactive waste, etc. [91]. Various advances related to rGO composites have been reported [92,93,94].

2. Applications of Graphene and Their Composites

Figure 5 displays the application areas of graphene-based composites. Applications like energy storage, automobile components, conductive inks, biomedicine, solar cells, sensors, etc., were clearly noted from the literature [92].

Environmental Applications

New techniques and refined processes enhanced the performance of nanomaterials. As per the existing literature it is very clear that nanomaterials play vital role in various applications. Challenges related to the environment could be addressed with nanomaterials. It is reported that environmental decontamination can be taken up with graphene and its base materials by using them as sorbent materials. Large values of electronic mobility make these materials more attractive in environmental applications. Graphene-based nanomaterials that use GO are of a low cost and can be used in removing metal ions, CO₂ and organic compounds from aquatic environments. They perform as photocatalytic materials in removing contaminants in water. Silver nitrate graphene nanocomposites can be used for photocatalysis. Graphene-based nanocomposites have impermeable nature and act as barrier between liquid and gas. Figure 6 shows ultrathin membranes (of nonporous graphene membrane and stacked graphene oxide membrane) used for the separation of water. Reports indicated antimicrobial activity by graphene-based nanomaterials through reactions with lipid membranes of microbes [95].

Membrane separation is one of the technologies that can solve environmental problems. The functionalization of the graphene membrane can be easily performed as it contains nanopores. By altering the shape and pore size, stability and selectivity may be improved. If the thickness of the membrane is reduced, permeability will be increased and achieve better performance [96]. This paper mainly concentrates on graphene-based membranes with multiple layers reported after 2015. Figure 7 shows graphene-based membranes.

Ions such as Cl, Li⁺, K⁺, Na⁺ and Br can be diffused by monolayer graphene with functionalized nanopores of diameter 5 Å [97]. Simulation studies indicated that nanoporous graphene monolayers are viable as ion separation membranes for desalination. In another simulation study, water desalination was investigated via single-layer graphene nanopores [98]. It was reported that water selectivity was better in the case of nanopores without hydrogen. Permeability of water through nanoporous graphene was estimated as 66 Lcm², which is two to three times more than regular reverse osmosis membranes. This clearly demonstrates the strength of functionalized nanoporous graphene sheets in desalination membranes of high permeability [99]. Functionalized graphene nanocomposites find various environmental applications that include sensing, monitoring and treatment. Graphene and its nanocomposites can detect biomolecules, creatures, in organic ions and also remove dangerous compounds from the environment. They can be used for removal of heavy metal ions, removal of CO₂, degradation of organic species as well as for bacterial studies. Removal of dye (organic pollutant) is one of the challenges in environmental application. Removal of dye using GO was investigated. Dye photodegradation can be performed with TiO₂–graphene composites [100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123]. Methylene Blue (MB) is a cationic dye, which is used for coloring cotton, wool, etc. It is harmful for human health, which needs to be eliminated [124]. Various techniques have been employed in removal of MB from water. It was reported that zero-loaded GO, activated carbon, CNT and rGO exhibit good degradation efficiency [125,126]. The efficiency of elimination for cationic dyes was 50%, 99% and 100% in the case of rGO, GO nanostructures and 3D rGO-based hydrogels, respectively [127,128,129].

Table 1. Degradation of MB, Copyright [93].

S. No.	Nanocomposite	% Degradation
1	GO-NiFe2O4	96.2
2	Mn2O3-G	84
3	Bi2O3-rGO	96
4	G-SnO2	100
5	rGO-ZnO nanorod	99
6	ZnO-NPs/rGO	99.5
7	ZnFe2O4/G	100

displays some of the graphene/GO/rGO composites that are tested for MB degradation in an effective manner. Another toxic dye, which is harmful to both fish and humans, is Rhodamine Blue (RhB). It can be degraded by using rGO, GO and thermal rGO [130,131,132,133,134,135,136,137,138,139,140,141,142].

Table 2. Degradation of Rhodamine Blue, Copyright [93].

S. No.	Nanocomposite	% Removal
	MRGO	91
1	ZnFe2O4/G	100
2	CdSe-Graphene	98
3	CdSe-Graphene-TiO2	85
4	Ag3PO4/GO	100
6	GO-BiOBr	95

shows some of the reported graphene composites that can remove RhB.

In continuation, various other cationic dyes [89,90,91,92,93,94,95] are being removed with the help of graphene-based nanocomposites. Methyl Orange (MO) is an anionic dye which is harmful. This can be eliminated from contaminated water through adsorption by using rGO and GO.

Table 3. Degradation of MO [Methyl Orange], Copyright [93].

S. No.	Nanocomposite	% Removal
1	bismuth oxide/rGO	93
2	ZnO nanorod/rGO	78
3	ZnFe2O4/G	78
4	CdSe-G-TiO2	71
5	β-SnWO4-rGO	90
6	Cu2O/GO/RC	92

displays some composites used in the removal of Methyl Orange (MO). In continuation, various other ionic dyes are removed by using graphene composites [143,144].

3. Energy Applications

Functionalized GO/rGO nanocomposites in the conversion of solar energy and energy devices gained significant interest. Supercapacitors, batteries with lithium and lithium ions, photovoltaic cells etc., use these composites.

Significant optical and electrical characteristics of graphene and GO nanocomposites (Figure 8) make them promise in PV and PEC systems. Initially, solar cells used transparent electrodes made with pure graphene, which was dominated by graphene nanocomposites [145,146,147,148,149,150,151,152,153,154,155,156,157,158]. It is observed that electrodes made with graphene nanocomposites in photovoltaic devices make these devices exhibit a high short circuit current, power conversion efficiency and fill factor as compared to pure graphene electrodes. Dye-sensitive solar cells are reported to use poly doped with a graphene/polystyrene sulfonate composite instead of a Pt electrode [159]. This could give a wavelength transparency of more than 80% with much higher conversion efficiency as compared to Pt electrodes [160]. The 2.7% efficiency was obtained in organic solar cells with an upper electrode made from composites of graphene/gold nanoparticles/PEDOT as compared to graphene electrodes with greater transmittance [161]. Graphene nanocomposites play useful role in PEC (Photoelectrochemical) photovoltaic systems. It is reported that photocurrent increased in PEC solar cells when photoelectrodes made of CdS QD-sensitized graphene nanocomposites were used instead of pure quantum dot PEC cells [162]. In continuation, many reports indicated the differences in using graphene nanocomposites over pure graphene [163,164,165]. Graphene/TiO₂ nanocomposite photoelectrodes were found to be better than pure TiO₂ nanocrystal photoelectrodes [166,167]. Lithium-ion batteries (LIBs) use graphitic carbon as anode material. The energy capacity of a LiC₆ battery is reported to be 372.0 mA h g ⁻¹ [168], in which the electrons are stored between Li and carbon. More ions are retained if Li₂ covalent sites are added to carbon, improving the energy capacity [169]. A low conductivity and lifetime limit usage of graphite anodes [170,171]. Hence, rGO may be used to overcome this limit. Still, graphene’s low efficiency is a major drawback [54,172,173,174,175,176,177,178,179,180]. Graphene’s stability, surface area, low weight and conductivity make it a good base for an LIB electrode. Graphene nanocomposites are reported to increase stability and performance of LIB [181,182,183,184,185,186,187,188,189,190,191,192,193]. Supercapacitors are more beneficial over lithium batteries in terms of environmental friendliness, high-power density, fast charging and discharging, durability, etc. [194]. Graphene is observed to have capacitance between 100 and 200 F/g, which is much higher than in other activated carbons of high surface area [195,196,197,198]. Since graphene can be restacked for usage of the entire surface area, the addition of graphene to supercapacitor electrodes enhances their conductivity and hence is widely used [199,200,201,202,203,204,205,206,207]. Supercapacitors designed with mesoporous graphene nanoballs, nanosheets and yarn [208,209,210] show good stability and electrochemical performance. It is reported that a yarn-type supercapacitor exhibits low energy density [211] as compared to a supercapacitor with graphene yarn [212].

3.1. Supercapacitors

A supercapacitor is one of the important energy storage devices in today’s world [213,214,215,216,217]. This is of a light weight, and can operate between a wide temperature range with long durability. They can be used in aircrafts, electronics and vehicles [216,217]. Large specific area mandates supercapacitors to have electrodes of activated carbon. In the process of searching for new electrode materials for efficient supercapacitors graphene-based nanocomposites play a vital role. Energy may be stored in supercapacitors through electrochemical double layer capacitance (EDLC) or pseudocapacitance. The efficiency may be improved by incorporating both the mechanisms in a single electrode material. Graphene-based nanocomposites will be one of the best options for energy storage because of graphene’s properties mentioned earlier [218,219]. Metal oxide supercapacitors are of a low cost and environmentally friendly, exhibiting high theoretical basic capacitance [8,220,221,222,223,224,225,226]. Fast reactions between electrode–electrolyte interfaces in metal oxides give rise to large specific capacitances [227]. Low electronic and ionic conductivities limit cycling stability and power density limiting their applications. Hence, graphene and metal oxide combinations address these drawbacks. In this context, graphene combined with MnO₂, Fe₃O₄, NiO₄ and Co₃O₄ is of importance. The MnO₂–graphene composite is widely used as material for an electrode in supercapacitors [228,229,230]. The main mechanism involved in charge storage of MnO₂ is the oxidation of manganese from state III to IV [231,232]. It is reported that 1.42 F/cm² capacitance was exhibited by a 3D graphene network filled with MnO₂ electrode material [233]. Similarly, graphene/MnO₂ composites obtained by the chemical reduction of GO/MnO₂ show 327.5 F/g and 278.6 F/g specific capacitances, respectively [234]. In continuation, various combinations of graphene and MnO₂ could produce electrodes for supercapacitors of large efficiency [235]. Low toxicity and high thermal stability make Fe₃O₄ a potential electrode material in supercapacitors [182,183]. In order to enhance the power density and cyclic stability, Fe₃O₄ was combined with graphene. The rGO-Fe₃O₄ nanocomposite was reported to have almost twice the specific capacitance compared to rGO [236]. Similarly, Fe₃O₄–graphene nanocomposites with high power density were reported [237]. Electrochemical performance of graphene and iron oxide nanocomposites makes them highly suitable for applications in energy. NiO is another important material for electrodes in supercapacitors [238,239]. It is said to have poor performance due to low electrical conductivity. Hence, by adding it to graphene its efficiency may be enhanced. In this direction, a three-dimensional NiO/graphene aerogel nanocomposite was proposed [240], which attained 587.3 F/g specific capacitance. A similar combination attained a big specific capacitance of 950 F/g and significant stability. It is due to interaction between the rGO network and NiO nanoparticles, as well as a highly porous structure of 3D-RGNi. Hence, graphene–NiO nanocomposites are assumed to be favorable for energy storage. CO₃O₄ exhibits a high value of 3560 F/g theoretical capacitance and is environmentally friendly [241,242]. Co₃O₄/rGO composites are prepared for supercapacitor applications using hydrothermal techniques [243]. Electrodes made from Co₃O₄/rGO nanocomposites attained 754 F/g specific capacitance with only 4% reduction in initial capability after 1000 continuous cycles. Similarly, CO₃O₄ nanowires on a 3D graphene foam electrode exhibited a high specific capacitance of 1100 F/g with good cycling stability [244,245,246]. The GO-based nanocomposites and carbon-based nanocomposites showed applications in energy storage and other advantages like efficient removal of heavy metal ions and visible photocatalytic activity [247,248].

3.2. Role of Microstructure in Enery Storage

It is well known that microstructural species like kinds of shape, size, etc., can play a vital role in energy storage applications. Several researchers focused on this kind of application. Field emission scanning and transmission electron microscopic images were reported for the morphological study of CNTs grown using various nickel films of different thicknesses (20 nm to 60 nm) with methane as a carbon source [249]. On nickel catalyst film with the methane presence, the CNTs have formed at 900 °C but weak coverage due to scant nickel ice lands formed on the Cu surface and catalyzed the CNT growth. In the presence of methane at 650 °C, the CNTs possess a larger diameter than that of CNTs in the presence of an acetylene source. The CNTs developed in the acetylene at 650 °C contain fine coverage and a small diameter of nanorods. The CNTs formed possess good conductivity. Similarly, nanospheres and nanocubes were formed, which are used in energy storage devices. In a similar way, SEM and TEM images related to growth of nanotubes and carbon nano-onions synthesized with a Ni/Al catalyst at 450–600 °C in the presence of methane were reported [249]. These pictures confirmed the presence of nanotubes and nano-onions. The morphology depends on the Ni/Al catalyst and maintenance of temperature. Pure nano-onions were obtained at a higher temperature of 550–600 °C and lower composition of Ni/Al with 60% of weight. The FESEM and TEM analysis quantifies the dependence of CNT growth on the rate of ethylene flow and CVD process rate. The CNT growth depends on the flow rate of the precursor’s gas. Multiwalled carbon nanotubes (MWCNTs) are grown on Fe₂O₃ and Al₂O₃ catalysts at constant temperature, 800 °C. At a flow rate of 100 sccm and CVD rate of 60 min., they obtain dense, uniform growth and relatively pure CNTs of a long length and lower diameter. Under the optimal conditions, the diameter is found as a minimum of the 20–25 nm range. Furthermore, the lowest surface defects and impurities and a high yield percentage have been noted [249].

Similar to the above study, the hydrothermal method was used to grow MoS₂ (Molybdenum Disulphide) nanoflakes on CNT [250]. This study analyzed the electrochemical properties by bringing a change in heterostructure by varying CNT and MoS₂. Electrochemical performance was enhanced with a high specific capacitance of ∼436 F/g at 1 A/g. Using this MoS₂/CNT heterostructure, a supercapacitor was fabricated, which exhibited a specific capacitance of ∼164 F/g at 1 A/g, losing only 4% of its capacitance after 1000 cycles, similar to the above case. Figure 9 shows the morphological analysis of MoS₂. Figure 9a shows micro-flower-type morphology. Figure 9b discloses a CNT diameter ranging between 70 nm and 150 nm with a length of many microns. Figure 9c displays the image of prepared sample MoS₂/CNT40 where the MoS₂ nanoflake was grown over CNT. Here, MoS2 does not surround the nanotubes due to poor binding between CNT and MoS₂. Figure 9d represents the morphological structure of d-MoS₂/CNT40. Here, the MoS₂ nanoflakes are found to have uniformly grown over the nanotubes along the horizontal axis without any aggregation. Electrochemical measurements indicated capacitive and diffusive characteristics by the electrode. They also observed EDLC behavior at higher scan rates, > 85% with ∼436 F/g specific capacitance at 1 A/g. These reports confirmed d-MoS₂/CNT40 to be a promising candidate as a solid-state supercapacitor for efficient energy storage applications.

Another study synthesized Cu-MOF/CNT nanomaterial using the hydrothermal method for usage as a battery-grade electrode in supercapacitor devices [251]. The authors achieved an excellent specific capacity of 1875 C/g with excellent cyclic stability and 98.3 % retention after 15,000 consecutive charge–discharge cycles. Figure 10a displays the structural image of a Cu-MOF/CNT composite having a Cu-MOF framework with a network of connected nodes and struts. Incorporated CNT was spread over the MOF, completely highlighting interaction between two components, indicating the possibility of enhanced performance. Figure 10b shows the XRD analysis of the prepared sample with the integration of Cu-MOF and CNT inside the composite. Figure 10c represents the BET curve, which indicates the presence of mesopores within the material's pores, which can analyze adsorption capabilities. Figure 10d shows the Raman spectra analysis of the composite with integrated Cu-MOF inside.

4. Conclusions

This review summarized some of the reported graphene derivatives in the past few years. It is mainly observed that the chemical functionalization of a graphene surface will lead to good interfacial interaction, enhancing its applicability in energy and environmental sectors. It was observed that graphene derivative and polymer interaction led to graphene-based polymer nanocomposites with certain limitations. Even though graphene has excellent mechanical, thermal and electrical properties suitable for sensors, the usage of electronic circuits is still enhanced through the functionalization of graphene. This leads to graphene-based nanocomposites with good durability, energy density and power density. In the above sections, we discussed some of the applications related to graphene nanocomposites. Mainly, we concentrated on materials used in energy storage applications like lithium-ion batteries and supercapacitors, and environmental applications like dye removal using graphene composites. From the reported literature, it is very clear that GO/rGO sheets alone cannot be used as electrodes for devices in energy storage. Instead of combining active materials such as metals, polymers, etc., improve the performance significantly. Also, the role of graphene composites in environmental applications was reviewed. The role of graphene composites in the degradation or removal of cationic and anionic dyes was presented. This review might help researchers in creating high-performance graphene-based nanocomposites for various applications. Even though various researchers produced high-quality graphene derivatives using various techniques, still, various limits exist in using graphene for practical applications. Various studies indicated that chemically functionalizing the surface of graphene achieves good interfacial interaction, enhancing its role in energy and environmental sectors. It is also observed that interaction between graphene derivatives and polymers plays a significant role in the development of technology. However, graphene-based polymer nanocomposites need to overcome various challenges and are yet to attain a full shape. Extraordinary properties of graphene can replace traditional nano-fillers in polymer matrices. This makes graphene significant in sensors, solar cells, electronic circuits and much more. Even though this review represented minimum applications of graphene-filled polymer nanocomposites, it can be inferred that more research towards graphene-based nanocomposites is essential. These need further improvement in structure by achieving elongated morphology, influencing their mechanical properties. A large scope for usage of graphene and its derivatives in all sectors cannot be ruled out. As already discussed in previous sections, the role of carbon-based nanomaterials in anti-bacterial activity with rGO was seen. Hence, further research needs to be focused on biocompatibility of nanocarriers, and enhanced stability, size and toxicity reduction. It is inferred that graphene can be used for environmental applications with low-cost production techniques. This might be achieved by reducing GO to graphene, which is challenging, in the near future. It can be concluded that, in any application, graphene and its composites cannot be replaced in the near future in view of their unique properties.