Recent Progress Using Graphene Oxide and Its Composites for Supercapacitor Applications: A Review

Abstract

Supercapacitors are prospective energy storage devices for electronic devices due to their high power density, rapid charging and discharging, and extended cycle life. Materials with limited conductivity could have low charge-transfer ions, low rate capability, and low cycle stability, resulting in poor electrochemical performance. Enhancement of the device’s functionality can be achieved by controlling and designing the electrode materials. Graphene oxide (GO) has emerged as a promising material for the fabrication of supercapacitor devices on account of its remarkable physiochemical characteristics. The mechanical strength, surface area, and conductivity of GO are all quite excellent. These characteristics make it a promising material for use as electrodes, as they allow for the rapid storage and release of charges. To enhance the overall electrochemical performance, including conductivity, specific capacitance (C_s), cyclic stability, and capacitance retention, researchers concentrated their efforts on composite materials containing GO. Therefore, this review discusses the structural, morphological, and surface area characteristics of GO in composites with metal oxides, metal sulfides, metal chalcogenides, layered double hydroxides, metal–organic frameworks, and MXene for supercapacitor application. Furthermore, the organic and bacterial functionalization of GO is discussed. The electrochemical properties of GO and its composite structures are discussed according to the performance of three- and two-electrode systems. Finally, this review compares the performance of several composite types of GO to identify which is ideal. The development of these composite devices holds potential for use in energy storage applications. Because GO-modified materials embrace both electric double-layer capacitive and pseudocapacitive mechanisms, they often perform better than pristine by offering increased surface area, conductivity, and high rate capability. Additionally, the density functional theory (DFT) of GO-based electrode materials with geometrical structures and their characteristics for supercapacitors are addressed.

1. Introduction

High levels of energy consumption and the depletion of fossil fuel sources are all a consequence of fast global development and a significant increase in the rate of population growth around the globe. Consequently, there is a growing need for energy daily to power essential operations such as operating electronic gadgets and motors and supplying distant places with electricity. Currently, supercapacitors (SCs) are considered cutting-edge technology for storing energy because of their fast charging and discharging capabilities, high power density, safety, and extended cycle performances [1,2]. Nevertheless, the energy density (ED) of supercapacitors is comparatively lower than that of batteries, and a key challenge encountered by SCs is the sluggish ion transfer rate, which can be attributed to the comparatively low electrical conductivity of their electrode materials [3]. Batteries are believed to store energy by ion intercalation or deintercalation, resulting in slow ion migration into active electrode materials and having the capacity to keep high ED [4,5,6]. However, batteries also have some limitations, including diminished performance at elevated temperatures, lower cycle performance, poor power density, and safety issues. Owing to SCs’ safety, electrochemical researchers are consistently striving to advance the study to boost the ED in supercapacitors for commercial use [7], given that their ED is not competitive with batteries and fuel cells. Accordingly, Figure 1a shows the Ragone plot of fuel cells, which are observed as high-energy storage materials, while supercapacitors are regarded as high-power devices, and batteries exist between them. Recently, the application of high-charge storage in SCs has been the primary focus of electrochemistry-based scientists [4,8]. The reason SCs are being explored as battery substitutes is because they store energy electrostatically, which takes place promptly at the electrode/electrolyte interface or via redox processes at the electrode interface [9,10,11].

Figure 1b schematically shows that SCs consist of two electrodes separated by an ion-permeable membrane. Current collectors and an electrolyte are also used [12]. SCs can be divided into two types based on their configuration, i.e., symmetric and asymmetric. Symmetric SCs contain an identical electrode (both the positive and negative electrodes are made of the same materials), whereas asymmetric SCs contain dissimilar electrodes (positive and negative electrodes of different materials). The charge storage properties of SCs are significantly impacted by the type of electrolyte, which can be classified as either aqueous or non-aqueous. Charge storage is achieved through the active electrodes, which are facilitated by electrolytes. Aqueous electrolytes are capable of attaining high capacitance and conductivity [16]. Aqueous electrolytes, which are inexpensive and environmentally benign, are ideally suited for SCs on account of their high conductivity and capacitance. The electrode material is the most important component in supercapacitors and has received substantial attention. The design of electrode materials, which constitute the most crucial component of energy storage systems, plays a pivotal role in deciding electrochemical performance [17]. Hence, it is essential to fabricate electrode materials with the requisite composition to create high-performance devices for supercapacitor applications [18,19]. On the other hand, the separator is a component that is used in supercapacitors to separate electrode materials (cathode and anode) against a short circuit. It is present in the form of a porous structured membrane to facilitate the transfer of ions [20,21]. Based on the electrochemical reaction, SCs can be classified as an electrochemical double-layer capacitor (EDLC) or a pseudocapacitor (PC) [22,23,24]. An EDLC derives its capacitance from charge separation at the interface between the electrode and electrolyte. In contrast, a PC (redox-type capacitor) utilizes charge-transfer pseudocapacitance generated by reversible faradaic redox reactions that take place at the surface of the electrode material and electrolyte [4,25]. Materials for electrodes of the EDLC types include graphene, carbon aerosol, activated carbon, porous carbon, carbon fibers, carbon nanotubes (CNTs), etc. [26,27,28,29,30,31]. Electrode materials of the PC type are classified as conductive polymers, metal oxides, metal carbides, metal chalcogenides, metal sulfides, conducting MOFs, etc. [32,33,34,35,36,37,38].

Based on the differences in the energy storage process, SCs may be classified as EDLC, PC, and hybrid SCs [39,40,41,42]. To store charges, the EDLC mechanism depends on electrolyte ion adsorption–desorption at the interfaces or internal pores of materials, as shown in Figure 1c (i). The storing charges in the EDLC mechanism are produced by a physical adsorption process free of any redox reaction [13]. PC goes through a fast and reversible faradaic process on electrodes to store energy, and the charge storage mechanism is caused by a shift in the valence state of a component in the electrode caused by electron transport, as shown in Figure 1c (ii) [43]. On the other hand, hybrid SCs exhibit both charge storage mechanisms [44]. Hybrid supercapacitors are divided into asymmetric and symmetric SCs. To this degree, the asymmetric SCs’ two-electrode system has a distinct electrode material that may take advantage of multiple voltage windows to optimize the device’s operating voltage and attain a higher ED [38,45]. The two-electrode system of symmetrical SCs uses an identical electrode material, is simple to build, and is inexpensive [30,46]. Figure 1d depicts an asymmetric hybrid electrode system with battery-type cathode electrodes and double-layer anode electrode materials. The activated carbon (AC) electrode’s high cycle stability and the capacitance of LiNi_0.03Mo_0.01Mn_1.96O₄ (LNMMO) contribute to this [14]. However, the conductivity of the electrode may vary depending on whether the anode and cathode materials possess comparable characteristics.

Table 1. Comparison of EDLC, PC, and hybrid capacitors [24,44].

EDLC	PC	Hybrid
Carbon-based materials are utilized as electrode materials.	Metal oxides and conducting polymers are utilized as electrode materials	Electrode materials are used as both carbon and metal oxides/conducting polymers.
The charge storage mechanism is via the non-Faradaic process (electrochemical double-layer formation).	The charge storage mechanism is via the Faradaic process (redox reactions).	The charge storage mechanism is via both Faradaic and non-Faradaic processes.
It has low energy density, good rate capability, cyclic stability, and low specific capacitance (Cs).	High energy density, high power density, low rate capability, and high Cs.	With high energy and power density and good cyclability, the system could have a low or high cost, which depends on the design of electrode materials and modest stability.

shows a comparison of EDLC, PC, and hybrid SCs.

Electrodes for supercapacitors have not exhibited significant efficiency despite recent developments in the synthesis of metal oxide materials due to their exhibiting poor efficiency, low capacitance, and low cycle stability throughout the redox reaction as a result of their low electrical conductivity and surface area [47,48]. This may restrict their applicability to PCs in the industrial sector. Hence, the configuration of an innovative composite electrode material, characterized by exceptional electrochemical properties and a synergistic effect among various electrode materials, has garnered significant attention from an extensive array of scientific investigations. Consequently, it is pragmatic to accept that the combination of metal oxides and carbonaceous materials in SCs can produce favorable ED and PD. Due to its high surface area (2630 m² g⁻¹), mechanical stability, zero band gap, thermal conductivity, and electrical conductivity, graphene is an effective active material [49,50,51]. Due to their superior chemical stability and electrical conductivity in comparison to metal oxide electrodes, their popularity has been progressively increasing. It is a two-dimensional carbon allotrope family with a hexagonal lattice of atomic size. Chemical decomposition of graphite, epitaxial growth, chemical vapor deposition, and other processes have been utilized to produce graphene. Among these techniques, mechanical cleavage holds promise for the synthesis of high-quality graphene, while chemical methods have the potential to enhance graphene productivity. Graphene oxide (GO), which is a member of the graphene family, has a lower surface area than graphene. GO is a honeycomb-structured carbon material formed by the single-layered arrangement of carbon atoms with sp² bonds [52]. GO demonstrates a high specific capacitance (C_s) as a result of the abundance of oxygen-functional groups on its surface. This phenomenon results in an enhanced attraction of water towards GO, whereas graphene exhibits the opposite effect due to its oxygen functional group deficiency [53,54]. GO is more hydrophilic in water than graphene, making it easier to composite with other materials. This improves the electrochemical performance and makes it more useful in supercapacitor applications [55]. In SCs, GO is regarded as a more viable alternative to graphene due to its lower cost and superior performance. Graphene possesses remarkable conductivity and mechanical stability, rendering it a viable material for active materials. However, achieving high ED and power density (PD) with pure carbon materials for SCs is challenging. Nevertheless, their excellent surface area and superior conductivity continue to be crucial factors in developing high-performance SCs.

Recently, interest in graphene oxide-based electrode materials for supercapacitors has gradually increased [15,56,57]. GO was synthesized in the 19th century by the methods of Brodie, Staundenmaier, Offeman, and Hummers [15]. GO is a graphene derivative that can be directly synthesized from graphite material. Chemical modification is used to add oxygen-containing functionalities to GO, including hydroxyl, carboxyl, epoxy, and carbonyl groups. rGO is produced by reducing GO employing chemical, mechanical, thermal, and green procedures to eliminate the oxygen functional groups. A significant distinction between GO and rGO is brought about by the concentration of the oxygen functional groups [15]. GO’s and rGO’s chemical structure are illustrated in Figure 1e. GO is hydrophilic because oxygen functional groups are present on its surface. Due to the intolerable nature of complete reduction, rGO demonstrates a partially hydrophilic nature. GO is an outstanding material for energy storage due to its high conductivity, mechanical properties, specific surface area, and stability in basic and acidic environments [58]. By utilizing the structural and chemical variety of oxygenated functional groups, the physicochemical properties of GO can be effectively modified to achieve the desired properties for energy storage applications [59].

Recently, reviews on using graphene and GO-based composites in supercapacitor applications, including GO, AC/graphene, graphene/vanadium, graphene-based gels, graphene/nano cellulose, graphene/metal oxide, GO/MXenes, metal oxide/rGO, rGO/cellulose, iron oxide/graphene, rGO/polypyrrole, GO/MOFs, LDHs/rGO, GO-based aerogels, and poly(cyclotriphosphazene)/GO, have been published [15,49,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75]. In contrast, this review focuses on several types of materials, including metal oxides, metal sulfides, metal chalcogenides, MOFs, and MXene composites with GO, as well as fluorine, bacteria, and other organic functionalization with GO. It discusses their synthesis, as well as their surface, structural, and morphological properties for supercapacitor applications. GO-based composites’ synthesis conditions, time, and cost were thoroughly provided. In addition, their C_s, resistances, rate capability, cycle stability, energy, and power density for the application of supercapacitors were discussed, and the electrochemical results of the various GO composites utilizing three- and two-electrode setups were tabulated for the reader’s ease of access. To assist researchers in examining the properties of electrode materials for use in supercapacitors, earlier work using DFT calculations is also reviewed. Figure 2 shows the maximum C_s of GO and its composite made of various materials for application in supercapacitors, as measured by GCD in recent studies.

2. Synthesis

2.1. Fluorine-Functionalized GO

Fluorine’s (F⁻) strong electronegativity makes it possible to tweak and adapt graphene’s characteristics via fluorination. The fluorinated graphene-based materials exhibit increased electron transport kinetics to this extent, and since this functionalization creates defects on the graphitic surface, it may change the electrical properties of the graphene surface [76,77]. Thus, for the application of SCs, Sim et al. synthesized fluorine-functionalized graphene oxide (FGO) via plasma treatment [77]. In this procedure, GO was synthesized using chopped graphite fibers using Hummer’s method. The chemical reaction occurred in the presence of potassium permanganate (KMnO₄), sulfuric acid (H₂SO₄), and sodium nitrate (NaNO₃) in a cold bath. The reaction required five days to complete after the ice bath was removed and replaced with water. Hydrogen peroxide (H₂O₂) was employed to terminate the reaction, and GO was obtained through multiple washes with water. GO was subsequently functionalized with fluorine using a plasma reactor developed in-house. Plasma was generated in this procedure utilizing nitrogen (N₂), nitrogen trifluoride (NF₃), and hydrogen (H₂) as plasma gases via a dielectric barrier discharge reactor. NF₃ was utilized as the F-radical-generating gas in the reactor, and by acting as a catalyst, H₂ caused NF₃ to dissociate. When the reactor was consistently supplied with GO and plasma gases, FGO was generated. The morphology, surface area, and crystalline characteristics are significant factors that may greatly influence the electrochemical performances of electrode materials. Hence, it is important to thoroughly analyze and study the characteristics of electrode materials to determine their suitability in supercapacitor applications. Accordingly, the FGO morphology revealed very thin layers with interspaces, while GO seemed to be ruffled and flake layered structures. In addition, it could be either due to the morphology of GO or due to its having very few pores that it has a low surface area of 3.1 m² g⁻¹, and accordingly, no significant surface area was found on FGO (3.7 m² g⁻¹). The use of an in-house plasma reactor for the functionalization of fluorine on GO may require highly skilled workers and entail significant expenses due to using gases, electrodes, and Teflon coating in the reactor for corrosion prevention by NF₃, and FGO may be produced on large scales using this plasma reactor. Figure 3a (i and ii) illustrates the schematic design of the FGO production process and a photo image of the fluorine plasma treatment reactor.

2.2. GO-Metal Oxides

TiO₂ may create oxygen vacancies, which function as redox centers and improve the capacitive properties. Additionally, it is environmentally friendly, easily synthesized, electrically conductive, and has high pseudocapacitance. Further, the capacitance, high conductivity, and redox activity of TiO₂ can be enhanced by doping of other metal oxides. In light of this, El-Shafai et al. recently created GO@TiO₂ and doped it with several metal oxides (MgO, SiO₂, Co₃O₄, and V₄O₇) for use in supercapacitors [78]. Initially, TiO₂ was doped on GO using the reflux technique. After obtaining GO@TiO₂, it was combined with other nanoparticles (GO@TiO₂-NPs) to produce GO@TiO₂-MgO, GO@TiO₂-SiO₂, GO@TiO₂-Co₃O₄, and GO@TiO₂-V₄O₇ by grinding and a two-hour ultrasonication process. Figure 3b schematically depicts the reflux process used to produce the GO composite with TiO₂, followed by the composite of different nanoparticles to the GO@TiO₂. The produced composite particles have an aggregated form and a spherical shape on the GO sheet. All composite particles were highly crystalline except for GO@TiO₂, which showed low crystallinity. The introduction of additional nanoparticles with GO@TiO₂ may have been influenced by the high crystallinity. However, the composite’s preparation through physical grinding led to morphological damage and raised the possibility of bulk structure. This approach may seem straightforward. Still, the synthesis of different metal oxides and subsequent composites makes this kind of synthesis quite expensive.

The energy storage sector has focused much attention on activated carbon (AC) because of its unique surface area, porous structure, and surface functional groups. Also, supercapacitors have considered transition metal oxides (TMOs) as potential electrode materials; their charge storage mechanisms are predicated on pseudocapacitance. Therefore, Veeresh et al. designed two types of nanocomposites for the use of supercapacitor application: AC-GO-SnO₂ and AC-GO-TiO₂-Zn, which incorporate AC-GO with SnO₂ and TiO₂-Zn, respectively [79]. By combining carbon materials with conducting graphene and TMOs, the electrochemical properties of GO can be enhanced. Initially, AC was made in a muffle furnace using charcoal under 800 °C for 2 h. Then, GO was produced using a modified Hummer’s process using graphite powder. Later, nanocomposites AC-GO-SnO₂ and AC-GO-TiO₂-Zn were produced in water and thiourea reaction media using the hydrothermal technique at 180 °C. A schematic representation of the synthesis of AC from charcoal via chemical activation at 400 °C and the preparation of an AC-GO-SnO₂ composite via hydrothermal methods is depicted in Figure 3c. To produce AC-GO-TiO₂-Zn, the same hydrothermal method may be used. Typically, the morphologies of nanoparticles or composites produced via hydrothermal techniques are well-structured. However, in this procedure, the synthesized AC-GO-SnO₂ and AC-GO-TiO₂-Zn morphologies were found to be aggregated and irregular spherical morphologies solely due to AC showing an aggregated and planar structure. A hydrothermal process is used to prepare these composites’ highly hybrid structures. However, the purity of the composites was found to be quite poor, as shown by a large number of dust particles on the composite’s morphology. A poor crystalline structure was also seen in the composites that were formed, which may be attributed to the low crystallinity of AC. The composite may have a relatively low surface area as a result of the aggregated morphologies and poor crystallinity. This synthesized nanocomposite may be a simple and modest cost, but there are multiple processes, and a time-consuming technique may be seen.

Recently, Abdallah et al. created a ternary composite form of activated carbon-GO-TiO₂ (AC-GO-TiO₂) for supercapacitor use [80]. According to the synthesis, AC was originally produced by calcining H₂SO₄-treated corncob powder at 600 °C in a muffle furnace. Then, using corncob powder, GO was made via a modified Tour method. To make charcoal, corncob powder that had been diluted and treated with HCl, along with 0.1 N NaOH, was combined with H₂SO₄ and autoclaved for 8 h at 90 °C. The resulting charcoal was subsequently treated in an ice bath with H₂SO₄ and H₃PO₄, after which KMnO₄ was added gradually. After forming a reddish-brown hue, water was introduced into the solution, followed by the dropwise addition of 30% H₂O₂ to obtain a GO. Finally, the ternary composite was created by combining AC, GO, and TiO₂ with PVDF as an electrode for the supercapacitor. The morphologies of AC-TiO₂-GO were observed to be the GO sheet, and TiO₂ particles were deposited on the surfaces of AC, which blocked most of the pores. Following composites, the morphology of AC was found to be fluffy. During this technique, the level of crystallinity was reduced, but the surface area of the composites with AC increased at each stage. AC, AC-TiO₂, and AC-TiO₂-GO exhibited surface areas of 212.5, 344.5, and 439.2 m² g⁻¹, respectively. The decrease in crystallinity might be attributed to the amorphous nature of GO or perhaps its low quality. The AC-TiO₂-GO composites obtained were inexpensive. However, the synthesis of AC from corncob using chemical activation and the synthesis of GO from corncob using the modified Tour method can be costly due to the multi-step process, the use of multiple organic compounds, and the lack of an environmentally friendly approach.

Copper oxide (CuO) has shown that low conductivity and rapid capacitance drop cannot be used in a supercapacitor application as a consequence of the chemical ion insertion or extraction process causing CuO crystal structure degradation [81,82]. To address this concern, composites of CuO with other metal oxides and carbon materials may be able to solve the CuO issue in supercapacitor applications. Accordingly, El-Shafai et al. developed a composite of GO with CuO, CuO/ZnO, CuO/TiO_2, and CuO/ZnO/TiO₂ using a simple procedure [82]. To create GO-CuO, CuO nanorods were produced by reacting CuSO₄⋅5H₂O with 1.0 M KOH in ethanol. Hummer’s technique was used to synthesize GO, which was then composited with CuO (GO-CuO) by reacting it with CuSO₄⋅5H₂O and adding 1.0 M KOH at 90 °C for 3 h. GO-CuO/ZnO nanorods were produced by reacting Zn(CH₃COOH)₂⋅2H₂O with 1.0 M NaOH in ethanol. The resulting ZnO was then composited with GO-CuO under ultrasonic agitation for three hours to create GO-CuO/ZnO. For GO-CuO/TiO₂, TiO₂ was first produced using the reflux technique with titanium (IV) isobutoxide. The produced TiO₂ was then ultrasonically mixed with GO-CuO for 2 h to produce GO-CuO/TiO₂. Finally, a mixture of GO and all nanoparticles was subjected to a straightforward ultrasonication process to generate GO-CuO/ZnO/TiO₂. This approach included the observation of nanorod morphologies of CuO, ZnO, and aggregated TiO₂ irregular particles on a GO sheet. The structure of GO was found to be amorphous. However, the composite of CuO, ZnO, and TiO₂ resulted in an enhanced crystallinity of GO compared to CuO alone, as well as CuO/ZnO composites on GO. Therefore, the number of distinct nanoparticles has the potential to enhance crystallinity. Ternary compounds with structured morphologies and crystalline structures have the potential to be materials with a significant surface area. These synthesis processes seem relatively straightforward; nonetheless, synthesizing numerous metal oxides and composites with GO may be time-consuming.

Figure 3. (a) Synthesis procedure of (i) FGO and (ii) Photograph image of fluorine plasma treatment system for FGO synthesis (reproduced with permission from [77]). Copyright 2022, Elsevier. (b) GO@TiO₂-x composite (reproduced with permission from [78]). Copyright 2023, Elsevier. (c) AC/GO/SnO₂ composite (reproduced with permission from [79]). Copyright 2023, Elsevier. (d) NMGO composite (reproduced from [83]). Copyright 2023, Elsevier.

Figure 3. (a) Synthesis procedure of (i) FGO and (ii) Photograph image of fluorine plasma treatment system for FGO synthesis (reproduced with permission from [77]). Copyright 2022, Elsevier. (b) GO@TiO₂-x composite (reproduced with permission from [78]). Copyright 2023, Elsevier. (c) AC/GO/SnO₂ composite (reproduced with permission from [79]). Copyright 2023, Elsevier. (d) NMGO composite (reproduced from [83]). Copyright 2023, Elsevier.

The CuFe₂O₄ nanoparticles (anode material) have been attracted to the design of supercapacitor devices due to their ease of availability and environmental friendliness [84]. However, CuFe₂O₄ particles are hardly used for supercapacitor applications due to their low conductivity, which quickly weakens capacitance. Furthermore, heteroatoms like nitrogen (N), sulfur (S), phosphorus (P), and boron (B) may significantly influence the electrical characteristics of GO, resulting in rapid ion diffusion at the electrolyte–electrode interface and a considerable amount of charge storage. Therefore, the best approach could be to use N-doped GO to enhance the capacitance characteristics of CuFe₂O₄. Accordingly, Lashkenari et al. studied NGO composited with CuFe₂O₄ nanocomposite by hydrothermal methods [85]. For this approach, they first created GO using Hummer’s method. Later, N was doped into GO (NGO) and employed by thermal breakdown procedure under the reaction of urea at 350 °C for 30 min in a tubular furnace. Furthermore, NGO/CuFe₂O₄ was created using the hydrothermal technique. As a result, the produced NGO was reacted with ferric chloride hexahydrate and copper (II) chloride dihydrate in ethanol while stirring. A total of 6.0 M of NaOH was used to adjust the pH of the solution to 12, and the solution was then autoclaved at 180 °C for 24 h to produce a composite NGO/CuFe₂O₄. Due to thermal expansion during the thermal decomposition process, the GO’s sheet shape became wrinkly after N doping, and rough surfaces were found. For structural stability, it could be preferable to dope N on GO surfaces using low-temperature processing. CuFe₂O₄ is produced using a hydrothermal method, which results in the formation of nanocubes on the NGO sheet. Upon a blend with CuFe₂O₄, it was found that the crystallinity of the NGO was improved. The surface area of this composite may be substantial owing to the nanocube structure of CoFe₂O₄ and the NGO sheet. This approach might be expensive because of the several steps involved, such as the synthesis of GO using different organic compounds, thermal breakdown to dope a heteroatom into GO, and, finally, the hydrothermal procedure for composite formation.

Fe₃O₄ is one of the metal oxides used for supercapacitors due to its low toxicity, simple preparation, cost-effectiveness, high capacitance, and wide range of operational windows [86]. However, its conductivity is low, which hinders the rapid electron transfer in the electrode materials. On the other hand, sodium tungstate (Na₂WO₄) has a high electrochemical redox property and a high conductivity, and it has the potential to be used in the enhancement of the characteristics of Fe₃O₄. In addition, ionic liquids (ILs) are used as an efficient electrode material for supercapacitors. These liquids possess a variety of distinctive characteristics, including high electrical conductivity and a broad range of electrochemical window functioning. Due to this, Dalvand et al. studied GO-modified Fe₃O₄ on IL@tungstate (GO/Fe₃O₄-IL@W) to develop a supercapacitor [58]. Consequently, IL was produced through a reaction involving 1,4-diazabicyclo [2.2.2] octane (DABCO), 1.0 mmol of 3-(chloropropyl) trimethoxy silane, and sodium iodide in methyl formamide at 120 °C under N₂ atmosphere for 48 h. Then, GO/Fe₃O₄@IL was produced by combining ammonia, FeCl₂∙4H₂O (0.75 g), and FeCl₃∙6H₂O in a toluene solution at 80 °C. GO/Fe₃O₄ was acquired via aqueous rinsing after the reaction. IL and GO@Fe₃O₄ were additionally combined with TEOS and DMF/H₂O at 80 °C while agitating to produce GO/Fe₃O₄@IL. A product of GO/Fe₃O₄-IL@W was subsequently produced by combining Na₂WO₄ with GO/Fe₃O₄@IL while agitating for 24 h. After functionalization of Fe₃O₄-IL-W, the GO sheet was folded, indicating that the sp³ defect was caused by the introduction of oxygenated groups into the GO surfaces, and the spherical shape of Fe₃O₄ and W particles may be adsorbed randomly on GO surfaces. However, the cloud of particles was found to be low on the GO surfaces, whereas the composite exhibits a highly crystalline structure owing to the crystalline properties of Fe₃O₄ and W. Based on the morphologies and crystalline properties, this composite may have a moderate surface area. This process incurs significant costs due to the observation of numerous processes and the use of numerous compounds in the synthesis of a composite.

Due to its strong electrochemical stability, pseudocapacitive behavior, and outstanding electrical conductivity, bismuth oxide (Bi₂O₃) has received a lot of attention as a material for supercapacitors [87]. It may also be used instead of traditional carbon materials to increase the C_s and high energy density of supercapacitors. The electrochemical reaction and electrochemical capacity are hampered, nonetheless, by the particle agglomeration. One potential solution to address the issue is to include highly electrically conductive elements in Bi₂O₃. Thus, Hounkanrin et al. recently created a Bi₂O₃-GO composite by using a microwave process [88]. This compound is very easy to synthesize from raw materials, such as sugar and pentahydrate bismuth nitrate. The mixture was thereafter transferred to a glass beaker and heated for 120 s at 600 W in a microwave oven. Following this procedure, the powder was crushed and heated to 450 °C to produce the yellow Bi₂O₃-GO composite product. In this study, sugar was used as a source for GO. This method yields high-purity composite particles. GO morphology revealed sheet and layered structures but collapsed following composite with Bi₂O₃ at 600 W for 120 s. Increasing Watts (800 W) and time (90 s and 120 s) strongly impacted morphologies. Thus, a microwave under 600 W with a short processing time is best. Elevated crystallinity was observed as a result of the composite’s high purity. As a result of the dominance of Bi₂O₃ on GO surfaces, no GO peaks were witnessed. Additionally, the highly crystalline and purity levels of this study might account for its high surface area. Using a microwave to create a composite or nanoparticle might be a relatively straightforward process.

A composite material consisting of NiO, MnO₂, and GO (NiO/MnO₂/GO) was recently created by ultrasonication for the supercapacitor application [83]. To this degree, Tour’s approach was first used to produce GO for the composite preparation. Secondly, NiO was subsequently synthesized via co-precipitation using nickel chloride hexahydrate at a pH of 10. Thirdly, MnCl₂∙4H₂O was employed in the co-precipitation process to produce MnO₂. The various ratios of prepared materials, including GO, NiO, and MnO₂, were mixed with water and ultrasonicated for two hours to make a NiO/MnO₂/GO (NMGO) hybrid. Figure 3d shows a schematic diagram that illustrates the NMGO composite formation using sonication. The shape of GO was found to be a thin nanosheet layer, and the very dense particles were completely adsorbed on the GO surfaces after they were composited with NiO and MnO₂. However, nanoparticles on the GO sheet did not exhibit any noticeable shape. Electrostatic interaction or other chemical bonds might be the mechanism by which the adsorbing nanoparticles bind to GO. When compared to other chemical techniques, this study might be advantageous to producing GO-based composites without the use of solvents or organic media. The shape of the particles showed that the study was unable to produce particles of high purity on GO surfaces. A low crystalline NiO/MnO₂ peak and a strong crystalline GO peak were noticed since GO was the dominant component [89]. It was determined that the produced GO and nanoparticle may have high and poor purity, respectively, by analyzing their crystalline structure. Following the composite with NiO/MnO₂, the surface area of GO increased due to the high surface-to-volume ratio of nanoparticles. Accordingly, the surface areas of GO and NMGO were observed to be 94 and 102 m² g⁻¹, respectively. This ultrasonication process for making composites is very easy, quick, and cheap.

Abdolahi et al. recently synthesized a hierarchical Ni_0.5Co_0.5WO₄ (NCWO₄)/f-MWCNTs/GO via a hydrothermal technique [90]. NCWO₄ was produced using a hydrothermal process at 220 °C for 12 h using trisodium citrate, water, cobalt chloride, and nickel chloride precursors. Following the synthesis, the procured GO and functionalized multi-walled carbon nanotubes (f-MWCNTs) were disseminated in water using sonication. The prepared NCWO₄ was added to the solution while continuously sonicating for 2 h to create the composite NCWO₄/f-MWCNTs/GO. Through the use of this procedure, NCWO₄ was shown to have an urchin-like morphology and to be heavily adsorbed randomly on a composite made of GO thin sheet and f-MWCNTs in the form of wires. The electrostatic interaction between NCWO₄ and f-MWCNTs/GO may be the cause of the adsorption. However, the clouds of urchin-like structure were low on the f-MWCNTs/GO. To enhance clouds, the NCWO₄ ratio must be optimized. Furthermore, it was noted that the composite NCWO₄/f-MWCNTs/GO had low crystallinity, perhaps as a result of low NCWO₄ purity. As a result, high-purity composite particles could not be produced via this synthesis method. The predominance of NCWO₄ resulted in the appearance of the majority of crystalline peaks, while f-MWCNT and GO exhibited only two peaks. To increase the surface area of NCWO4/f-MWCNTs, GO is introduced. As a result, NCWO₄/f-MWCNTs/GO had more surface area (172.2 m² g⁻¹) than NCWO₄@f-MWCNTs (146.4 m² g⁻¹). The surface area of the composite may be enhanced due to the high porosity of GO. Lastly, procuring GO and f-MWCNTs may be costly, and the many steps involved make the process time-consuming.

Similarly, a manganese ferrite (MnFe₂O₄)/GO composite was produced using a modified Hummer technique [91]. In this process, MnFe₂O₄ was initially produced via co-precipitation utilizing precursors Fe(NO₃)₃ and Mn(NO₃)₂ in the presence of KOH to modify the pH to 12 at 70 °C, after which the sample was calcinated at 600 °C. Furthermore, synthesized MnFe₂O₄ was mixed with GO, which was made using Hummer’s method to create a MnFe₂O₄/GO hybrid. The modified Hummer’s approach demonstrates heavily adsorbed fine spherical MnFe₂O₄ on GO’s stacked sheet. The stacked morphology may improve the electrochemical reaction rate by allowing ions access to high electroactive sites in the electrode materials. It is noteworthy that the co-precipitation-synthesized morphology of MnFe₂O₄ showed high stability since the synthesized nanoparticle composites with GO by Hummer’s technique did not significantly alter their morphology. However, MnFe₂O₄ and GO were found to have low and high purity, respectively. Therefore, the MnFe₂O₄ and GO were found to have a low and a highly crystalline structure, respectively. After compositing with GO, MnFe₂O₄ crystallinity enhanced significantly. The composite synthesis using a modified Hummer’s method may increase MnFe₂O₄ powder purity. The combination of dense MnFe₂O₄ nanoparticles and thin GO sheets may have a high surface area. Making this composite seems like a simple, low-cost way to make an electrode for a supercapacitor. In another investigation, GO-modified graphite felt (GF) was hydrothermally combined with MnO₂ to form GO/GF-MnO₂ [92]. In this procedure, GF that had been treated with sulfuric acid, water, sodium nitrate, and nitrate was functionalized with GO (GO/GF) using Hummer’s method at a reaction temperature of 180 °C for 12 h. The hydrothermal method was employed in the synthesis of GO-GF/MnO₂. In this step, KMnO₄, MnSO₄, and GO/GF were added to water and agitated for five minutes. They were subsequently transferred to an autoclave for a 12 h reaction at 140 °C to acquire the composite. The high-purity GF was found to have a cylindrical tube structure and hydrothermally produced a sheet-like structure with uneven development on its surface after MnO₂ modification was found. On the other hand, MnO₂ with GO composite on GF using the same technique exhibits rod forms that are almost covered on the GF surfaces. Consequently, the shape of MnO₂ changes when it is combined with other materials. As a result of GF’s predominance, the composites exhibit a single peak corresponding to GF/GO; no peaks of significance were detected for MnO₂. The broad appearance of the crystalline peaks suggests a low degree of crystallinity. The synthesized composites, MnO₂-GO/GF, may have a greater surface area than MnO₂/GF because of the high clouds of MnO₂/GO nanorods produced on GF when compared to the uneven sheet shape of MnO₂. Due to the procurement of GF and the multiple processes required to synthesize the composite and its application in batteries, the synthesis procedure may be moderately expensive. Nevertheless, the synthesized composite has the potential to be utilized in supercapacitors.

The heterojunction structure of transition metal oxides (TMOs) has the potential to enhance the material’s overall conductivity through favorable electronic effects [93]. As a result of its advantages, chemical stability, low cost, and simplicity of synthesis, Cu₂O has been the subject of extensive research as a supercapacitor material [94]. CoO has a favorable theoretical capacitance and is made from cost-effective materials. To further optimize the electrochemical characteristics, metal oxide composites may incorporate carbon-based materials recognized for their exceptional electrical conductivity. For this reason, Yu et al. utilized hydrothermal methods to produce Cu₂O-CoO composited with GO (Cu₂O-CoO/GO) to investigate supercapacitors [95]. Therefore, the precursors CoCl₂ and CuCl₂ were added to water together with the synthesized GO solution, and the mixture was agitated for 10 min before KOH was added. The combination was then placed in an autoclave to undergo a reaction for eight hours at 160 °C, producing black powder. Moreover, they were calcined for two hours at 350 °C in an N₂ environment to produce Cu₂O-CoO/GO. Formed like a folded sheet, the Cu₂O/GO shape was seen, but the Cu₂O on or with GO could not be recognized. In contrast, irregular CoO particles can be found on GO. Additionally, particles of spherical-like Cu₂O-CoO are visible on GO. Consequently, the hydrothermal method exhibits particle formation on GO exclusively under conditions involving CoO and Cu₂O-CoO. In comparison to CoO/GO, the samples of Cu₂O/GO and Cu₂O-CoO/GO may have a higher purity because CoO/GO lacks a smooth morphology. Therefore, Cu₂O/GO and Cu₂O-CoO/GO were found to have a high crystallinity. These materials may also have a high surface area. The process for producing this kind of composite via hydrothermal means is simple and inexpensive.

Tungsten oxide (WO₃) has drawn a lot of interest because of its benefits, which include cheap cost, many oxidation states, fast ion insertion characteristics that enable electrochemical activity, significant absorption in different electrolytes, and good ion transport properties. Therefore, using hydrothermal methods, Shembade et al. investigated the use of supercapacitors on the GO/WO₃ composite [96]. GO was first produced via this process using a modified version of Hummer’s technique from graphite flakes. The various percentages (5, 10, and 15%) of GO were then hydrothermally synthesized to form a composite with WO₃. Thus, 0.1 M sodium tungstate was dissolved in water. HCl and H₂O₂ were added to the aforementioned solution and served as pH adjusters and reducing agents, respectively. GO was subsequently added to the solution, which was sonicated for a few minutes before being transferred to an autoclave for a 12 h reaction at 180 °C; after drying and rinsing the solution several times with ethanol and water, a GO/WO₃ composite was obtained, and finally, it was annealed at 500 °C. GO/WO₃ nanorod hybrids were observed via this method; as the ratio of GO to WO₃ was increased, thus were the particle sizes and minor morphological changes. The high degree of hybrid formation hinders the identification of WO₃ particles and GO sheets based on their morphologies. We found that after increasing the GO ratio from 5% to 10%, the crystallinity of WO₃ significantly decreased due to the dominance of GO. As a result, optimization studies on crystalline and morphological properties are of critical importance. According to the morphology and crystalline characteristics of GO/WO₃, all composites have the potential to be very pure. Because 10%GO/WO₃ has a strong crystalline quality, the surface area was calculated and found to be low at 33.9 m² g⁻¹. The low surface area of the composite may be due to the lack of pores on its surfaces. This procedure appears to have a moderate cost and allows for the synthesis of fine-sized particles with high purity, as well as the synthesis of highly hybrid particles through systematic investigation.

Carbon-based materials (GO or CNT) typically have problems with self-aggregation, which results in poor electrochemical performances [97]. Consequently, using carbon-based materials that can be composited with metal oxides has improved supercapacitor performance. Consequently, because of its exceptional electrochemical reversibility, high conductivity, capacitance value, and cycle efficacy, cobalt ferrite (CoFe₂O₄) is primarily used in supercapacitors. To enhance the electrochemical performance in supercapacitor applications, Verma et al. investigated a composite CoFe₂O₄ with GO/CNT (GO/CNT/CoFe₂O₄) [97]. Initially, Hummer’s technique was adapted to synthesize GO for this procedure. Second, cobalt nitrate hexahydrate and ferric nitrate nonahydrate were combined with glycine to produce CoFe₂O₄ using the solution-combustion technique. A precipitate of black cobalt ferrite was produced by undergoing a reaction in water at 80 °C; this precipitate was subsequently calcined at 800 °C to yield CoFe₂O₄. Using this solution-combustion method to produce nanoparticles could be an inexpensive and straightforward process. Composites were synthesized utilizing a simple stirring method. Further pure composites were obtained by combining varying weight percent (wt.%) of GO, CNT, and CoFe₂O₄ with water while agitating for six hours. The mixture was then washed with 10% HCl and dried at 100 to 120 °C to obtain GO/CNT/CoFe₂O₄. The morphologies of GO and CNT were found to be tubes and sheets, respectively. In contrast, GO/CNT/CoFe₂O₄ shows the tubular shape of the CNT and the spherical shape of the CoFe₂O₄ on the GO sheets. On GO sheets, however, very few CoFe₂O₄ particulates were observed in comparison to CNTs. Further optimization of the composite’s ratio is required to achieve a more balanced formation of all materials. In addition to several crystalline peaks associated with CoFe₂O₄, the presence of GO and CNTs is distinguished by a single crystalline peak each. The crystalline quality of composites serves as an indicator of their purity. The presence of CNTs and CoFe₂O₄ nanoparticles on GO could potentially account for its considerable surface area. In this study, a low-cost method was used to prepare the composite. However, it might be more expensive to use commercial graphite and CNTs, as well as to synthesize GO using the modified Hummer’s method and CoFe₂O₄ by the solution-combustion method. Simply stirring different materials in water may not effectively produce hybrid particles.

2.3. GO-Metal Sulfide

Because of their high van der Waals forces and pi–pi interactions, GO sheets are bound and layered. By functionalizing nanoparticles with GO surfaces, the degree of GO layer restacking can be substantially reduced through a blend of GO and transition metal compounds. In consequence, transition metal sulfides (TMSs) are extensively employed as potential materials in the field of supercapacitor technology owing to their favorable electrochemical characteristics. Furthermore, because their electrochemical properties are superior to those of TMOs, TMSs are employed in pseudocapacitors. As a result, Chandraraj et al. have recently investigated the use of a composite of CoS/GO and NiS/GO in a supercapacitor application [98]. For CoS/GO, GO that had been synthesized hydrothermally was composited with CoS. To acquire a CoS/GO composite, they combined GO with CoCl₂ and Na₂SO₃ in a solution of water and ethanol while agitating. The resulting mixture was then autoclaved at 190 °C for one night. GO was likewise incorporated into NiS/GO via a hydrothermal process involving NiSO₄, NaOH, triacetonamine, and water, followed by its autoclaving at 175 °C for three days. Furthermore, this study did not describe the synthesis of CoS/NiS/GO. The NiS/GO and CoS/GO composites, made using hydrothermal methods, had a very hybrid structure. The particles formed a uniform thin coating on the GO sheets, thereby preventing the nanoparticles from agglomerating. GO sheets retain their exceptional stability even after composites. However, no discernible nanoparticle form was seen on GO. In addition, GO’s crystallinity increased after composites with CoS and NiS. As a result, the produced nanoparticles are very pure and crystalline. These composite particles may have a modest surface area because they appear as coatings on GO, which is related to their low porosity rather than their actual particle form. This procedure may be straightforward, but it may be moderately priced due to the need for multiple syntheses and the impossibility of mass production.

Similarly, lanthanum (La₂S₃) and manganese (MnS) sulfides make excellent pseudocapacitive materials because of their cheap cost, strong C_s, and simplicity of production [99]. Additionally, these materials possess high power density, a broad voltage window operation range, and high redox properties due to their diminished electrical conductivity, low energy density, and poor kinetics during cycle testing. For these reasons, it may be possible to effectively enhance the voltage window, capacitance, electrical conductivity, and cyclic stability by combining MnS and La₂S₃ with carbon-based conducting materials in the form of a composite. As a result, Mane et al. investigated the composite of MnS and La₂S₃ with GO for use in supercapacitors [100]. Consequently, films can be fabricated using GO dispersed in a water solution in an ultrasonic chamber. By performing 100 SILAR (Successive Ionic Layer Adsorption and Reaction) cycles on a stainless steel (SS) substrate, GO film was produced. A MnS film was subsequently deposited onto another SS substrate using the 90 SILAR cycles method and a solution of MnCl₂ and Na₂S. For the MnS-La₂S₃ film, MnCl₂ and LaCl₃ were dissolved in water following the addition of Na₂S. The film was subsequently coated by dipping the SS substrate in the solution for 90 SILAR cycles. The 90 SILAR cycle method was employed to fabricate a MnS-La₂S₃/GO film on an SS substrate using the identical procedure. Figure 4a illustrates the fabrication process of composite film using the SILAR method. The application of this film manufactured using the SILAR method may be straightforward and efficient. The candy-like morphology of MnS on SS was observed, in contrast to the cotton-like morphology of densely aggregated particles in MnS-La₂S₃ composites, which may have a high porosity. The composite of these particles with GO does not exhibit a discernible MnS-La₂S₃ morphology. When the porous cotton-shaped particles interact with GO, they transform into a film or sheet morphology. Despite this, the morphology of the layered structure of GO following composites appears unstable. This type of porous morphology may facilitate the adsorption and desorption of electrolyte ions on electrode materials by providing a large number of active sites, thereby enhancing electrochemical performance. The considerable enhancement of the surface area of MnS (47.0 m² g⁻¹) following composite with La₂S₃ (82.0 m² g⁻¹) and GO (156.0 m² g⁻¹) is intriguing. It may account for the increase in porosity between composite stages; in particular, GO, which is a highly porous material, may be the cause. Nevertheless, the synthesized composites exhibited a decreased degree of crystallinity, which may indicate a lower degree of purity. The process of anion and cation coating on an SS substrate may appear straightforward and inexpensive; however, 100 SILAR cycles are required to achieve a clean coating.

2.4. GO-Transition Metal Chalcogenide

Because of its excellent redox characteristics, large surface area, crystal structure, and electrical conductivity, transition metal chalcogenides, or TMDs, have drawn interest in the field of energy storage. The benefit of including sulfur and selenium is that they enhance the metal-to-carbon-based material ion exchange. Thus, carbon materials may be useful for supercapacitor applications because of the TMDs’ excellent ion exchange capability. As a result, Yasoda et al. used a straightforward mixing procedure to create a manganese sulfoselenide (MnSSe) composite with GO (GO-MnSSe) [104]. Consequently, manganese anodium sulfide was incorporated into GO (GO-SW) using graphite flakes in a modified Hummer’s process. By agitating a GO-SW in water for a whole night, repeatedly washing it with ethanol, and drying it at 60 °C, the GO-MnS was produced using GO-SW. To prepare GO-MnSSe, GO-SW and Na₂S were dissolved in water, and then a Se compound was added while continuously stirring. Following the reaction, it was repeatedly washed in ethanol and dried at 60 °C to obtain the composite. The wrinkled appearance of GO-MnS can be attributed to the potential film-like formation of MnS. On the other hand, finely spherical MnSSe nanoparticles were observed to form on GO surfaces. The interaction between Se and MnS to form particles may be the cause of the uneven particle formation on GO surfaces and the lack of particles on some GO surfaces. Therefore, the ratio of GO to MnSSe should be optimized to ensure the particle’s uniform formation. Enhancement of ion transport through energy storage in the structure’s attributes may be contingent upon the number of nanoparticles present on the GO surfaces, thereby leading to improved electrochemical performances. Furthermore, the nanoparticles that form between the GO layers potentially contribute to a favorable conductivity, resulting in enhanced electrochemical performance. Additionally, the low crystallinity observed in both composites may be attributed to the possibly low purity of the particles acquired in this investigation. Both composites may have poor surface area due to irregular GO particles and low crystallinity. The synthesis of this composite might follow a relatively straightforward and quick approach, but since it involves many steps and takes a lot of time, the process could be quite expensive.

2.5. GO-Layered Double Hydroxide

Layered double hydroxides (LDHs) are synthetic two-dimensional (2D) nanostructured anionic clays [105,106]. It is an ionic, positively charged brucite structure with interlayer portions that contain charge compensation anions. It features a multilayer structure with a large surface area, many active sites, and rich redox characteristics [107]. The spacing between layers is more favorable to ion deintercalation, and insertion contributes to illuminating its C_s. Thus, the C_s of GO may be increased by combining it with Co₂-Ni₁ LDH (GO/Co₂-Ni₁ LDH) for supercapacitor applications. Thus, Zhao et al. produced GO/Co₂-Ni₁ LDH using the reflux technique at 90 °C [106]. The mixture of water, hexamethylenetetramine, GO, 2.0 M NiCl₂·6H₂O, and 1.0 M CoCl₂·6H₂O was refluxed at 90 °C. After a 3 h reaction, the mixture was washed with water and ethanol to yield a GO/Co₂-Ni₁ LDH. The sample exhibited a sheet morphology with a wrinkly structure, which may account for the solid sheet structure of LDHs; it may have exerted a contraction force on the GO surfaces. On the other hand, the type of composite particles may possess high conductivity and active sites, allowing for rapid ion movement and efficient adsorption of electrolyte ions, which is advantageous for high-energy storage. This composite showed low crystallinity, and the GO/Co₂-Ni₁ LDH composite showed no GO crystalline peak. It can be a highly thin sheet of LDH and govern in terms of quantity as compared to GO. In general, the high-porosity nature of LDHs results in high surface area [108,109,110]. Accordingly, GO/Co₂-Ni₁ LDH was attributed to a high surface area (84.6 m² g⁻¹). The approach for this composite seems to be low-cost and simple.

2.6. GO-Organic Material

Graphene sheets are experiencing aggregation via the process of stacking two graphene sheets on top of one other, which results in a large surface area that is not achievable for high electrical conductivity [111]. On the other hand, GO that has been generated by the sp³ hybridization of carbon atoms is not conductive and exhibits poor charge transfer, which contributes to its unstable cycling [112]. Therefore, to solve this problem, one active method for supercapacitor applications is to increase the performance of GO materials by including organic molecules as an interlayer in graphene sheets. In light of this, Biradar et al. produced a composite material called 2,5-bis((3-(6-amino-9H-purin-9-yl)propyl)amino)cyclohexa-2,5-diene-1,4-dione (ABQA)-GO by using Hummer’s technique, which was then followed by a chemical approach [101]. The reaction of 1,4-benzoquinone, 3-bromopropylamine hydrobromide, and adenine in dimethyl sulfoxide (DMF) under the N₂ environment was the process that was followed to make ABQA. They were able to obtain the ABQA using the process of low-pressure distillation. To generate acyl chlorides of GO surface (G-COCl), the synthesized GO was then combined with SOCl₂ and DMF in a reflux procedure while the environment was controlled to be N₂. To produce an ABQA-GO, G-COCl and ABQA were first reacted into DMF under reflux conditions. This was then followed by washing in methanol and freeze-drying. Figure 4b depicts the schematic synthesis of ABQA-GO, which began with graphite and subsequently reacted with SoCl₂ to form GO-Cl under reflux. GO-Cl was then organically reacted with ABQA under reflux to obtain the final product. The synthesized GO was sheet-structured and unsmooth. However, GO became wrinkled and weakened after ABQA functionalization. It may be because organic solvents damage GO surfaces during ABQA functionalization. Thus, this method affects the GO structure when modified or functionalized. Functionalized GO has poor crystallinity due to bio-inspired organic molecules’ low crystalline characteristics. But, it slightly enhances the GO’s crystallinity. The strong redox property, quick ion transfer, and superior conductivity of ABQA allow the GO to store more charges. In this study, the use of organic molecules to composite GO in this technique may be both expensive and not environmentally friendly.

MOFs perform poorly in supercapacitors due to their weak conductivity and electrolyte instability. Trimesic acid, which is readily accessible, stable, and inexpensive, has been used recently to synthesize neodymium (Nd)-MOF. This material was then combined with GO for use in supercapacitors [113]. In the media of water and ethanol mixtures, a GO was composited with Nd-MOF using the hydrothermal technique. Consequently, trimesic acid and neodymium nitrate hexahydrate were added to the GO-containing mixture above, and the mixture was sonicated to disperse the particles further. The solution was placed in an autoclave set at 200 °C for 12 h. Following the reaction, the precipitate was repeatedly cleaned with ethanol and water to create composite GO/Nd-MOF. For Nd-MOF, smooth hexagonal-rod-shaped crystals were seen in this process; these crystals adhere to GO after compositing with it. However, the MOF crystal is slightly damaged and unsmooth after the composite. Furthermore, the GO reveals wrinkled and broken sheet morphology. The composite Nd-MOF/GO demonstrates a strong crystalline property because of the crystallinity of Nd-MOFs. Fast ion transport to the pores of MOFs/GO and highly active sites for the adsorption of high electrolyte ions may be made possible by the combination of the sheet structure of GO and rod-shaped MOFs to allow high energy storage applications. The high purity of the materials synthesized via this method for MOFs and their composites may facilitate superior electrochemical performance. Additionally, MOFs containing GO composites may possess a substantial surface area. The procedure of synthesizing GO and its composites seems to be low-cost, yields a well-structured MOF morphology, and produces particles with highly active sites and purity, and it could be a time-consuming process.

An easy chemical procedure was employed by Chen et al. to fabricate a Ni-BTC MOF/GO composite [102]. Firstly, GO was synthesized via a modified Hummer’s method. A subsequent solvothermal reaction was used to produce Ni-BTC MOFs. This was accomplished by sonicating a solution of Ni(NO₃)₂∙6H₂O, 1,3,5-trimesic acid (BTC), and 2-methylimidazole in DMF. The resulting solution was then transferred to an autoclave at 170 °C for a duration of 48 h. For subsequent use, the obtained particulates were rinsed with DMF and ethanol. Subsequently, composite GO/Ni-BTC MOFs were produced by sonicating the reaction of GO in DMF. Following that, 2-methylimidazole, BTC, and Ni(NO₃)₂·6H₂O were introduced into the GO solution. Additionally, the solution was autoclaved at 170 °C for 48 h to obtain composite. Furthermore, the quantity of GO varied from 0 to 40 mg when producing a composite for comparison reasons. The schematic representation of the Ni-BTC@GO process, which involves the hydrothermal reaction of GO, BTC, and nickel nitrate precursor, is illustrated in Figure 4c. The morphology of this Ni-BTC MOF is octahedral, with smooth surfaces. As the amount of GO on Ni-BTC MOF increased from 10 to 40 mg, the morphology was observed to be unevenly adherent GO on a smooth MOF surface for 10 mg GO. Furthermore, rough surfaces, GO aggregation, and unevenness on MOF surfaces were found for the 20 and 40 mg doped GO. As a result, increasing the GO caused the MOF surfaces to become rougher. However, partly covered GO on the MOF surface will enable Ni²⁺ to gain certain advantages, such as interaction with electrolyte ions, shortening the ion diffusion route, and protecting the active site of Ni²⁺ in the electrochemical process. Interestingly, the crystalline characteristics of Ni-BTC MOF grew when the GO dosage was raised from 10 to 20 mg, and after 40 mg of GO, the crystalline peaks of MOF were severely reduced owing to excess GO, which terminated the crystallinity of MOF. As a result, combining higher-dosage GO with Ni-BTC MOF may be ineffective for supercapacitor applications. Based on the crystallinity, the resulting particles may be of high purity in the investigations. Inevitably, the prepared composite may possess a higher surface area as a result of its elevated crystallinity, high purity, and potentially porous characteristics. The synthesis process appears to be straightforward and economical. However, it is not an environmentally friendly procedure because of the use of organic solvents in the hydrothermal reaction, and it is a time-consuming process due to the 48 h reaction and drying particles maintained for 24 h. Similarly, Khakpour et al. produced sulfur-doped GO/ZIF-8 MOFs (SGZ) through a straightforward chemical process [114]. As a result, a modified version of Hummer’s method was employed to prepare the GO. Sulfur (S) was introduced into GO (S-GO) via the hydrothermal process. This included sonicating Na₂S and GO in water and stirring for an hour. After that, the mixture was put in an autoclave to undergo a hydrothermal reaction for 10 h at 200 °C, yielding S-GO. For the SCZ composite, an hour was spent sonicating a mixture of S-doped-GO and 2-methylimidazole in methanol. After that, Zn salt and methanol were combined separately and left for 10 min under stirring to create another solution. To create the SCZ composite, the two solutions mentioned above were combined, agitated for two hours, and then centrifuged. In this work, no significant morphology or irregular shape was found for the ZIF-8 MOF, and the purity of the MOF seems poor. Probably, the MOF synthesis carried out in an aqueous solvent with mechanical stirring for 1 h in an open environment affected the morphology and maybe contaminated dust particles from the air. For GO, the morphology was severely wrecked after S doping. The broken structure might be because the GO was sonicated and mechanically stirred with Na₂S before the hydrothermal reaction, which could affect the shape of the GO. Similarly, no notable morphology or irregular shape was seen in composite SCZ. As a result, this approach may not be successful in producing morphology for MOFs and/or composite particles. The crystallinity of the SGO composite with ZIF-8 MOF decreased dramatically as a result of the presence of GO, which reduced the crystalline quality of the MOF. The process for SCZ composite might be inexpensive, straightforward, and time-saving. However, in the process of preparing composites, the hydrothermal method should produce GO and sulfur-doped GO (SGO). This method may have a modest overall cost, but it results in particles with low purity and no distinct shape.

2.7. GO-MXene

MXene is a two-dimensional material extensively used in several applications, including energy storage, water treatment, and sensors. MXene has strong electrical conductivity, high hydrophilicity, and a multilayer structure, enabling it to function as a current collector [115,116]. Compared to stacked layers of GO or MXene materials, the conductivity of MXene-GO composites may significantly enhance ion transit and charge storage for supercapacitor applications. The MXene-GO composite has considerable promise for development in energy storage applications. Fu et al. created an MXene-GO film using drop casting [103]. GO and MXene were combined in a specific ratio while being continuously stirred. The solution was applied onto a glass plate using the drop-casting method and then dried in a vacuum. Subsequently, the undesired section of the film was eliminated using a CO₂ laser to serve as an electrode for supercapacitor use. Figure 4d shows a schematic diagram of the process of making the MXene-GO composite film using drop casting and further electrode assembly for electrochemical testing. The drop-coated morphology of the MXene-GO reveals rough surfaces, and MXene particles were discovered to form islands on the GO. Because of the electrostatic interaction, GO and MXene may bond strongly together. The flat composite thin film surfaces may provide high interaction with electrolyte ions and quicker ion motions into the MXene-GO electrode, allowing for efficient energy storage. Though the synthesis method appears straightforward, developing the electrode design using a CO₂ laser could be an expensive process.

Conducting polymers were investigated for their potential application as energy storage materials due to their high C_s, rapid redox protonation/deprotonation processes, and cheap cost of operation. Following this, poly (1,5-diaminoanthraquinone) (PDAAQ) is classified as a conducting polymer. It has favorable structural properties, including a π-conjugated structure, making it an ideal candidate for use as novel electrode material in supercapacitors. As a result of the exfoliation of MXene (Ti₃C₂T_X), nanosheets tend to self-restack. This is because of the intermolecular force, which impedes the movement of electrons and reduces the surface area. An et al. designed cetyltrimethylammonium bromide (CTAB)-modified graphene oxide (GO)-coated PDAAQ nanotubes@MXene for use in supercapacitors to circumvent this issue [117]. CTAB was initially incorporated into the GO (CGO) surface via ultrasonication in water for 15 min. Then, using an initiator, 1,5-diaminoanthraquinone monomer (DAAQ) was dissolved in a mixture of dichloromethane and acetonitrile. Following the solution’s dissolution of ammonium persulfate and sodium dodecyl benzene sulfonate, CGO was added to the mixture. CGO/PDDAQ was produced by combining the DAAQ solution with the CGO solution and observing the reaction for 48 h. Before undergoing modification with CGO/PDDAQ, an organic process was employed to etch MXene utilizing lithium fluoride and hydrochloric acid. Following the etching process, CGO/PDDAQ and MXene sheets were combined using ultrasonication in water to produce CGO/PDAAQ/MXene by the force of electrostatic interaction. The synthesis of CGO/PDAAQ@MXene film-type electrode is illustrated schematically in Figure 5a. This is achieved by coating CTAB-functionalized graphene oxide with poly(1, 5-diaminoanthraquinone) nanotubes via interfacial polymerization and then connecting the GO and MXene through vacuum-assisted filtration and electrostatic self-assembly. The produced CGO/PDDA shows the morphology of CGO adsorbed on PDDA nanotubes in the form of a sheet. After modification with an MXene solution, it exhibits an uneven morphology, thick, wrinkled textures, and a rough morphology. However, this morphology may have a highly active site, high purity, strong structural stability, offer high electrons to attract electrolyte ions, and allow for much quicker ion transportation to effectively store charges. The crystallinity of the CGO/PDAAQ/MXene was discovered to have a weak crystalline structure owing to the polymer nature of PDAAQ, which may be an amorphous structure. According to the synthesis, it may be time-consuming, expensive, and environmentally unfriendly due to the use of organic compounds and the involvement of multiple processes.

Recently, dip coating was utilized to implement an MXene-GO composite film onto polyamide material, followed by laser-induced graphene (LIG) to etch the film [118]. GO and MXene, both of which were obtained in solution form, were initially combined while agitating. The composite material was subsequently coated onto a polyamide substrate via dip coating. The interdigital design or pattern was subsequently etched into the film via a single laser-induced graphene process. The resulting film was subsequently utilized for studies of supercapacitors, and the pattern generated through this method appeared to be effective. For comparison, they also developed LIG on bare polyamide surfaces for electrochemical studies. The goal of this study was to demonstrate how to combine GO and MXene to create films with improved microstructure modification and ordered MXene particle distribution. They also wanted to demonstrate how to use laser-induced graphene to create patterned porous graphene structures on MXene/GO-polyamide surface electrodes that can make good contact with electrolyte ions for high electrochemical performance. The schematic representation of Figure 5b shows the film fabrication process using a single laser-induced graphene structure on the MXene-GO-polyamide film composite for the supercapacitor application. According to the morphology of the composite film, the polyamide film coated with a highly porous MXene/GO composite had a uniform distribution of MXene fine particles with twinkled GO. On the other hand, the absence of GO results in the observation of MXene particles aggregating on the polyamide film. Following the laser-induced graphene process, the composite film acquires a fibrous structure that is densely layered. The porous structure of this structured electrode film may facilitate contact with electrolyte ions, while the presence of graphene structure (after the LIG process) in the composite film contributes to its high conductivity. As a result, this electrode film has the potential to be significantly effective in supercapacitors. Because the polyamide material contains MXene and GO, it may have a low-crystalline property. This procedure for making films and fabricating electrodes with laser-induced graphene could provide highly pure film electrodes. Nevertheless, the procedure of procuring MXene and GO compounds and employing laser techniques might be quite costly.

Because of its vast surface area, high crystallinity, and quick ion diffusion, tungsten sulfide (WS₂) has high electrochemical activity. On the other hand, WS₂ exhibited inadequate electrical conductivity, specific capacitance, and cycling stability. To enhance the electrochemical characteristics of WS₂, Hussain et al. fabricated a composite material consisting of MXene-GO and WS₂ (WS₂@MXene-GO) intended for use in supercapacitors [119]. Initially, MXene was produced for this composite by reacting commercially available Ti₂AlC₂ powder with hydrofluoric acid to remove Al content. To produce WS₂/MXene-GO, commercially available GO and the as-synthesized MXene were combined with water, sonicated, and stirred at 60 °C for 1 h. The mixture was then supplemented with ammonium tungsten oxide hydrate and CH₄N₂S for two hours. They were then transferred to an autoclave set to 200 °C overnight to obtain WS₂@MXene-GO. The acquired particles were subsequently annealed at 400 °C in an H₂/Ar atmosphere to eliminate any remaining impurities. Figure 5c depicts a schematic layout of the process of converting Ti₃AlC₂ MAX to Ti₃C₂TX MXene, followed by a hydrothermal reaction of MXene, GO, and ammonium tungsten oxide hydrate with thiourea to produce WS₂@MXene-GO, which is subsequently annealed at 400 °C. The treated MXene was found to have stacked sheet structures, while WS₂ showed layered sheet structures. After compositing MXene/WS₂, the morphology reveals agglomerated sheet-shaped grains. The GO composite with WS₂@MXene morphology showed that the grains were distributed throughout the GO surfaces. The morphology of the composite potentially offers an abundance of active sites for electrolyte ion interaction, thereby improving electrochemical performance. Furthermore, this composite exhibits a high degree of crystallinity due to the crystalline properties of the individual particles, such as MXene, WS₂, and GO. The prepared composite (WS₂@MXene/GO (12.9 m² g⁻¹) reveals a very low surface area due to the low surface area of the individual particles (MXene (2.7 m² g⁻¹), WS₂ (5.2 m² g⁻¹) and GO (2.9 m² g⁻¹)). The acquisition of MXene precursors, GO, and other chemicals may contribute to the high cost of the synthesis process. Additionally, the involvement of multiple processes can be time-consuming.

2.8. GO-Bacteria

To save energy and safeguard the environment, most synthesis processes for electrode materials use poisonous substances or unfavorable reaction conditions, making them expensive and toxic. Finding a low-cost, environmentally friendly method that uses precursors that might be found in nature would be beneficial. As a result of their robust cell walls, bacteria belong to the class of prokaryotic organisms that can endure a variety of harsh environments. Bacteria are inexpensive, widely distributed, and safe for the environment. By converting the material in the electrolyte, bacteria can raise C_s and electrical conductivity. Thus, to develop supercapacitors, Verma et al. investigated the interaction of bacteria and graphene oxide (B@GO). They, therefore, utilized Bacillus subtilis, a Gram-positive bacterium [120]. For four hours, the produced Bacillus subtilis solution and the synthesized GO solution were combined by stirring. After that, the solutions were carbonized at 600 °C in an N₂ environment and cleaned with water and ethanol to produce B@GO. Figure 5d depicts a schematic representation of bacteria functionalization on GO utilizing GO produced from graphite powder and stirring with bacterial solution for 4 h, followed by calcination at 600 °C under N₂ gas for heteroatom doping. The resulting GO has an unsmooth and sheet-like morphology, while B@GO has an irregular and wrecked sheet shape with a larger size. This might be the reason why bacterial solution produces highly wettable GO, which is attributed to agglomeration. Furthermore, it was mechanically stirred for 4 h; thus, the morphology was affected. This B@GO composite was found to have semi-crystalline properties owing to structural deformation and interlayer gap caused by the bacterial solution connection with a heteroatom (N)-doped GO. The bacterial component with the GO materials may have good electrochemical performance because of their abundant porosity and plenty of foreign atoms. The composite method may be easy to use; however, further optimization is needed when utilizing bacterial cell culture in conjunction with carbon materials.

2.9. Discussion

Supercapacitor electrode materials must be low-cost, be easy to synthesize, porous, have highly active sites, be conducive, pure, have a well-structured particle shape, chemical stability, structural stability, and reproducibility. Based on the reported studies,

Table 2. Advantages and disadvantages of synthesis technique for various GO-based composite electrode materials, including their morphology, surface area, and structural properties.

Electrode Materials	Methods	Advantages	Disadvantages	Morphology	Surface Area (m2 g−1)	Structure	Ref.
GO-Fluorine
FGO	Direct plasma reactive treatment	Achieving uniform coating of fluorine on GO and rich active sites, large-scale preparation, and operation under atmospheric pressure could be the best surface coating method for powdery materials.	High-cost process and methods may not be environmentally friendly due to the use of NF3 gas, which is toxic. Skilled labor is required to handle the reactor, possible corrosion in the reactor through NF3 gas, the plasma generates high electron temperature affects the sensitive material, maintenance of the reactor, low surface area of material, and high energy consumption.	Thin layers with large interspaces	3.7	-	[77]
GO-Metal oxides
GO@TiO2-NPs	Grinding	Low-cost, simple, easy to composite, rich active sites, no energy consumption, large-scale production, and environmentally friendly procedure.	High aggregation, particle sizes could be very high, poor quality, the interaction between nanoparticles and GO may be weak, morphological shape damages during grinding, possibly low surface area, and different techniques used to synthesize different nanoparticles consume the cost of the process and time.	Aggregation and spherical shape	-	High crystalline	[78]
AC-GO-SnO2	Hydrothermal	Low-cost, simple, safe, environmentally friendly, doping, high-purity particles, highly active sites, easy to composite, low-temperature process, and well-structured morphologies.	Small-scale production, time-consuming process, moderate surface area, moderate energy consumption, and this method requires a vacuum or unvacuumed oven to synthesize particles.	Aggregated and spherical shape	-	Low crystalline	[79]
AC-TiO2-GO	Simple reaction	Cheap, easy-to-obtain composite using a binder, large-scale production, rich active sites, high hybrid structure, and no energy consumption.	Synthesizing AC, TiO2, and GO separately and then combining them may be a costly, time-consuming process; there is poor composite quality owing to dust particles during outdoor mixing; and it is not environmentally friendly since organic chemicals are added during particle mixing.	Micro-TiO2 and GO sheet hybrid AC (Fluffy)	439.2	High crystalline	[80]
GO-CuO/ZnO/TiO2	Ultrasonication	Cheap, very simple process, easy to operate instrument, has no energy consumption, has highly active sites, time-saving and room-temperature process, applies to all common materials for composites, the composite surfaces may exhibit high surface functional groups, low particle aggregation, and homogeneity of particles.	Mechanical forces cause morphological damage to particles and produce heat while performing small-scale production, and long-term operation may affect the instrument; high-time consuming procedures; this technique may produce low bonding between particles, and synthesizing many nanoparticles using chemical procedures might be expensive and expensive.	Nanorods of CuO and ZnO and irregular shape TiO2 on GO sheet	-	High crystalline	[82]
NGO-CuFe2O4	Hydrothermal	Cheap, simple, environmentally friendly, high-purity particles, easy to hybrid particles, rich active sites, doping, particles could be of high surface area, low-temperature process, well-structured morphologies, and particles produced by this method are nanometer in size.	Commonly used to prepare metal oxides, temperatures below 220 °C could be used for particle synthesis, particles cannot be produced if the autoclave is not tightly closed, small-scale production, time-consuming process, doping N on GO by thermal decomposition in the tubular furnace could be high energy consumption and expensive, and high-temperature thermal decomposition can affect the stability of the material.	CuFe2O4 nanocubes on GO sheet	-	High crystalline	[85]
GO/Fe3O4-IL@W	Simple reaction	Low-cost procedure, highly hybrid particles, high-purity particles, high structural stability, highly active sites, large-scale production, the composite surfaces may exhibit high surface functional groups, low-temperature process, and no energy consumption.	Synthesis of GO and its functionalization with Fe3O4-IL may be time-consuming and energy consumption due to the large number of organic compounds and processes involved, and GO-Fe3O4-IL reaction under inert conditions.	Fe3O4/IL/W presents an irregular spherical shape on the GO sheet	-	High crystalline	[58]
Bi2O3-GO	Microwave	Dust-free synthesis, nanoparticles can be synthesized, high-purity particles, a time-saving process, high yields, no chemicals or water were used, no solvents or water are needed to purify the product after synthesis, sugar used as a source for GO, and chemical-free GO is synthesis.	Energy consumption, expensive technique, maintenance cost, morphology can be damaged under high watts operation, limited penetration depth of the heat radiation into the reaction precursors/medium, and heat-sensitive material cannot be used for surface modification or functionalization.	Sheet morphology	-	High crystalline	[88]
NMGO	Ultrasonication	The process is inexpensive, produces highly hybrid particles, high-surface-area particle formation, uses only water as a medium for composite formation, consumes no energy, operates at room temperature, and saves time. It also facilitates the interaction between GO and nanoparticles, enhances composite structural stability, allows for the adsorption of high-density particles on GO surfaces, and does not require the use of solvents or organic media.	Specific morphology cannot be obtained; low-purity particles form on GO due to atmospheric dust contamination; sonication process under long-term operation and high amplitude may develop defects on the particle surface and break GO layers; instrument maintenance; and synthesis of GO, NiO, and MnO2 to prepare composite structure are expensive and time-consuming.	Nanoparticles on the GO sheet have no specific shape	102.0	Moderate crystalline	[83]
NCWO4/f-MWCNTs/GO	Ultrasonication	Only uses water to produce composites. For ideal composite preparation, first ultrasonicate f-MWCNTs and GO in water. After that, NCWO4 was added to the solution, and the particles had a substantial surface area.	Low-purity particles were observed. It could be an expensive process due to the synthesis of NCWO4 by hydrothermal and then ultrasonication used for composite f-MWCNTs and GO. Additionally, using commercial MWCNTs and their functionalization and commercial GO in this procedure seems to be expensive, and a 2 h continuous ultrasonication process may affect the instrument’s functionality.	Urchin-like structure of NCWO4 on the wire-like shape of f-MWCNTs and GO sheet composite	172.2	Low crystalline	[90]
MnFe2O4/GO	Modified Hummer’s method	Carbon-based materials can be synthesized by this technique, a low-temperature process, a time-saving procedure, highly layered GO sheets, high-purity GO sheets, highly dense composite particles, high-surface-area composites, high crystallinity composites, and high structural and morphological stability	The use of the coprecipitation technique to synthesize MnFe2O4 revealed low purity, moderately expensive procedures, and the need for an ice bath for GO/composite preparation; the precipitation process uses a high molarity ratio for the precursors, commercial graphite is used for the preparation of GO and its composites, vacuum drying is used for the composites, and a high amount of KMnO4 is used for the reaction of GO and its composites.	The MnFe2O4 has a fine spherical shape on the GO sheet	-	High crystalline	[91]
MnO2-GO/GF	Hydrothermal	The synthesis process is simple, involving only water as the medium for MnO2-GO formation on GF, an environmentally friendly procedure, a well-structured morphology, the possibility of high-surface-area composites, the use of major precursors for composite preparation, and the absence of any toxic solvents or liquids in the procedure.	The process is costly due to the utilization of commercial GF, a 12 h process, the lack of a detailed explanation for the synthesis procedure, the use of toxic chemicals for pretreating the GF, the production of low-purity particles or composites, the requirement for pretreated GF to dry under vacuum conditions, the absence of a purification procedure after synthesis, the potential expense of using a high-pressure reactor for the synthesis of GO, and potential maintenance issues.	Nanorod shape of MnO2/GO on GF	-	Low crystalline	[92]
Cu2O-CoO/GO	Hydrothermal	This process used only an 8 h reaction and may have high-surface-area composites, moderate energy consumption, high-purity composites, highly active sites, and synthesized GO that can directly be added to the precursor for composite preparation using hydrothermal.	The process was moderately expensive, revealed no significant morphology, and revealed low-purity particles. The addition of KOH to the precursor solution for alkaline titration may not be environmentally friendly, the interaction between GO and nanoparticles could be low, the resulting black gel product requires soaking in water for 24 h to purify it, this process is time-consuming, and the final composite particles require calcination in a tubular furnace under an N2 atmosphere.	No significant morphology; however, GO shows the sheet-layered structure	-	High crystalline	[95]
10%GO/WO3	Hydrothermal	Using H2O2 as a reducing agent in the synthesis could make fine particles, the reaction could directly add GO powder to make the composite, and a methodical process could find the best ratio of GO to WO3 for the composite formation. Fine-size particles were synthesized, along with high-purity particles, and it has a moderate energy consumption and a high hybrid particle formation.	It is a 12 h process, low-surface-area particles, annealing at 500 °C may damage the morphology, low-activity sites, a high GO ratio may affect the crystallinity of WO3, using commercial graphite flake for GO synthesis may be costly, and using HCl and H2O2 in the synthesis may be an environmentally unfriendly and time-consuming procedure.	Nanorods	33.9	High crystalline	[96]
GO/CNTs/CoFe2O4	Simple reaction	The process is low-cost, simple, energy-free, and time-saving. It uses only water as a medium for composite formation under stirring, has rich active sites, is environmentally friendly, and does not use any solvents or organic chemicals.	The process may be costly because it involves synthesizing GO using a modified Hummer’s method, which necessitates the use of numerous chemicals, acids, solvents, and an ice bath setup. The solution-combustion method synthesizes CoFe2O4 but requires calcination at 800 °C. Further, commercial CNTs and graphite flakes are expensive. The preparation of composites in water with simple mixing may not be effective, the particles may not hybridize strongly, and the use of 10% HCl to wash the composites may damage their structure and morphology.	The spherical shape of CoFe2O4 and the tube shape of CNTs are present on GO sheets	-	High crystalline	[97]
GO-Metal sulfide
NiS/GO and CoS/GO	Hydrothermal	The process is environmentally friendly and time-saving, as it directly adds GO powder to the precursor solution for composite synthesis, eliminating the need for any additional pH adjustments or reducing agents. This technique can achieve a uniform particle size distribution and deposition. The particle interaction with the GO sheet could be strong, ensuring high stability and purity.	The particles on the GO sheets did not exhibit any notable morphology. They might have a low surface area, undergo a 12 h process, and have few active sites. The process involves the consumption of chemicals for GO, NiS, and CoS synthesis, as well as hydrothermal treatment at high temperatures, which can be somewhat expensive.	No significant shape of nanoparticles was observed on the GO sheet	-	High crystalline	[98]
MnS-La2S3/GO	SILAR	Thin film dip coating can be performed at low cost, simple, room temperature, time-saving, large-scale production, no energy consumption, highly active sites, high composite structures, only water used to remove the unbound anion/cations on the film surfaces, an environmentally friendly process, multiple dip coating can be performed, no drying process is required, and no chemical reaction is required to coat the film on the SS substrate.	It takes at least 100 SILAR cycles to obtain a perfect coating. There are concerns about the coating’s stability, difficulties regulating the film’s thickness, and the possibility of airborne dust particles contaminating thin film surfaces. Handling thin film devices by hand or in a closed pack is challenging due to their susceptibility to scratches or cracks. And dip-coating film on one side of the substrate is challenging.	Thin film/sheet morphology	-	Low crystalline	[100]
GO-Transition metal chalcogenide
GO-MnSSe	Simple reaction	Low-cost, simple, environmentally friendly process, no complicated technique or method used, room-temperature process, no energy consumption, mass production, no solvents or reducing agents used for the composite preparation, and low-temperature drying process.	The overnight process used 1 g of GO to prepare Go-MnSSe, low-activity sites, low purity, and potentially low-surface-area particles. The reaction under only water may not form particles on GO, leading to uneven particle formation on GO and no particle formation on GO for the composite GO-MnS. The composite preparation did not calculate a ratio, limiting the possibility of incorporating Mn into GO synthesis using a modified Hummer’s method.	The GO sheet contains fine spherical MnSSe nanoparticles	-	Low crystalline	[104]
GO-Layered double hydroxide
GO/Co2-Ni1 LDH	Reflux	The procedure is simple and involves high-surface-area particles, rich active sites, a low-temperature process, high hybrid formation, a sheet type of morphology, highly porous composites, and no energy consumption.	The process is not environmentally friendly, it takes a long time, the particles are not very pure, it uses high molarity ratios, it is moderately expensive because it synthesizes them through reflux in an oil bath, and the clay-like nature of the LDHs may damage the shape of the GO.	Sheets with a wrinkled structure	84.6	Low crystalline	[106]
GO-organic material
ABQA-GO	Organic reaction	It is simple to synthesize, ABQA and GO are strongly covalently bonded, it has inert atmosphere processes, the process is carried out at low temperatures, there are a lot of active sites and functional groups on the surface of the functionalized GO, and ABQA has a high redox property.	The process is time-consuming, not environmentally friendly, and costly due to the use of expensive organic chemicals and commercial graphite, as well as the use of toxic chemicals and the freeze-drying process. The product’s purity may be low, necessitating self-safety precautions before synthesis. The distillation process can be energy-consuming, and only synthesis can take place in fume exhaust hoods. And there are morphology stability issues.	Broken and wrinkled sheet structure	-	Low crystalline	[101]
Nd-MOF/GO	Hydrothermal	Easy to synthesize, environmentally friendly, low cost, high-purity particles, well-structured morphology, water and ethanol utilized for reaction, drying particle at less than 90 °C, low-energy consumption, and highly active sites.	The process involves the use of an ice bath before the hydrothermal reaction, treatment at elevated temperatures, and a prolonged duration. High temperatures could not be used to dry the particles; hybrid particles could not form properly, and the morphology was slightly damaged after the composite was mixed with GO.	Hexagonal-rod-shaped Nd-MOFs on the GO sheet	-	High crystalline	[113]
Ni-BTC@GO	Hydrothermal	Cheap, simple to synthesize, drying particles at less than 90 °C, rich active sites, low energy consumption, high-purity particles, high surface area and porous surfaces, and well-structured morphology particles.	It is a time-consuming technique owing to the 48 h hydrothermal process and the 24 h particle drying time. The organic solvents employed for washing the particles may affect their surfaces. Adjusting the dosage of GO in the composite preparation for comparison makes it a bit expensive. Higher doses of GO could affect the crystallinity of MOF, and the procedure is not environmentally friendly because of the usage of organic solvents for the reaction.	Sheet-structured GO on octahedral-shaped Ni-BTC MOF.	-	High crystalline	[102]
SGZ	Simple reaction	The process is cheap, simple, easy to synthesize, conducted at 40 °C, consumes no energy, has highly active sites, may have a high surface area, and uses only methanol for reaction and particle purification.	It is not an environmentally friendly procedure and also not an effective procedure for making composites due to the possibility of contamination, which leads to low purity. Sonication and mechanical stretching destroy the particle shape. Overall, processes such as GO fabrication, S doping on GO, and ZIF-8 MOF to make composite particles are quite expensive. GO affects the crystallinity of MOF.	Irregular shape	-	Low crystalline	[114]
GO-MXene
CGO/PDAAQ/MXene film electrode	Vacuum-assisted filtration	The composite structure exhibits high flexibility and high stability due to the electrostatic interaction between CGO/PDAAQ and MXene. The preparation of the composite film at room temperature and the conducting composite at water are both easy and simple processes.	The procedure is expensive due to the numerous processing stages involved, and the final product requires the use of a freeze-drier. It is not an environmentally friendly procedure for CGO/PDDAQ, and the whole synthesis study seems time-consuming. The pH of the solution should be 6–7 for MXene preparation, ice bath ultrasonication, and vacuum-assisted filtration to obtain the final composite may damage the morphology by the force of the vacuum.	Unsmooth nanotube-blended sheet morphology	-	Low crystalline	[117]
MXene/GO film electrode	Drop-coating and laser-induced graphene technology	Drop-coating is a simple method for achieving film uniformity. GO is used to prevent MXene aggregation. The polyamide substrate provides excellent film deformation-resisting properties, along with good stability and flexibility. The laser-induced graphene (LIG) process was able to demonstrate perfect patterning to obtain a graphene-structured film, a programmable fabrication process, a highly porous structure, and a fibrous structure.	A time-consuming procedure. Laser-induced technology may make it costly. The film drying process should be carried out in inert gas. High-power-watt lasers can damage morphology and contaminate film during drop coating. Self-precaution may be needed to process the patterning electrode. Electrode operation and patterning demand skilled researchers.	Fibrous with stacked structure film	-	-	[118]
WS2@MXene/GO	Hydrothermal	The procedure is environmentally friendly and easy to synthesize, the composite surface is rich in active sites, and it uses only water for the hydrothermal reaction, resulting in high purity and particle purification through a green process.	The impregnation process for GO with MXene (1 h) and the subsequent addition of sodium tungsten hydrate (2 h) before the reaction result in a moderately time-consuming procedure. The reaction process operates at a high temperature of 200 °C, has a small surface area, produces laboratory-scale samples, and lacks a well-organized morphological structure. This procedure could be moderately expensive due to the use of commercial GO and Ti2AlC2 powder.	GO sheets contain grain-shaped WS2@MXene particles	12.9	High crystalline	[119]
GO-Bacteria
B@GO	Simple reaction	The procedure is cheap, simple, environmentally friendly, and time-saving.	Using N2 gas under calcination may not be effective in dope N on GO. The stirring process for 4 h changes the shape and structure of the GO, resulting in low-quality particles and active sites. Cell culture and incubation of Gram-positive bacteria may be costly and have low-purity particles and active sites. The bacterial solution and the GO were not optimized.	Wrecked sheets	-	Semi-crystalline	[120]

summarizes the advantages and disadvantages of synthesis methods for GO-based composite electrode materials and their characteristics. This review shows that the hydrothermal technique was mostly used for producing GO-based composites. Because of its low cost, ease of processing, ease of the composite, environmentally friendly process, autoclave reaction at 120–220 °C, high purity and crystalline particles, structural stability, packed hybrid particles, well-structured shape particles, that water is mostly used for the reaction media, low energy consumption, and rich active sites’ nanostructured materials, it enables high electrochemical performance. However, the hydrothermal reaction could happen for at least 12 h and up to 48 h for particles to form, and the produced particles may have a low surface area. In this technique, the particles might not form if the autoclave is not tightly locked. Using a mortar and pestle to grind two particles together to make a composite could also be very cheap and easy with highly crystalline particles and a large-scale production method that does not use any energy. However, there is a chance of serious structural and morphological damage, high particle contamination, particle agglomeration, and blockage of the pores on the particle surface, which could affect the electrochemical performance and particle contact with the eyes and nose during grinding, which can cause health problems. Utilizing direct plasma reactive treatment to produce doping materials enables large-scale production and uniform distribution and is ideally suited for doping powdery substances due to its high stability and the ability of the numerous functional groups on the doped GO materials to facilitate a rapid electrochemical reaction and improve performance. However, the global applicability of this technique may be hindered by its proficient complexity, costly equipment, demand for highly experienced personnel, high maintenance expenses, a doping process that could affect the surface area of the composites, and substantial energy consumption. The simple reaction employed for the synthesis of composite particles may be inexpensive, straightforward, energy-free, large-scale, operate at room temperature, and produce a high number of active sites with substantial surface area particles and high or semi-crystalline particles. However, this approach to composite preparation proves to be ineffective due to its time-consuming procedures, inability to produce well-structured morphology as a result of reaction under a mechanical stirrer, and there is a possibility for low interaction between materials, which leads to poorly hybridized particles, and a high risk of particle contamination that blocks surface pores, thereby resulting in low electrochemical performance. Ultrasonication may not be the optimal method for preparing composite particles for supercapacitors due to the possibility for high-frequency ultrasound to break the interfacial bond, introduce surface defects and contamination, damage morphology and low crystalline particles, and particles with a moderate surface area, all of which could contribute to suboptimal electrochemical performance.

The microwave method is a rapid and efficient method for synthesizing particles with well-structured morphologies, high-purity and crystalline particles, and no reaction medium requirement. The synthesis of composites using this method may result in particles that are well hybridized, and the abundance of active sites may facilitate rapid electrochemical reactions and high performance. Nonetheless, this is a costly method, and prolonged operation at high wattages can damage particles. Many processes for combining GO with other materials include first preparing GO using a modified Hummer’s method and then using another approach for combining it. This method may be costly, time-consuming, and energy-intensive. To address these difficulties, GO and its composite with MnFe₂O₄ were synthesized utilizing a single procedure, a modified Hummer’s approach. This technique revealed good structural stability and highly crystalline particles with no morphological issues. Thus, this type of process may be utilized to construct a GO composite with other materials using a low-cost, easy, and energy-efficient technique and a superior electrode material for electrochemical performance. SILAR technique may be a relatively simple, room temperature process that produces homogeneous thin film electrodes, a variety of coatings on the substrate, and a highly active site and conductivity electrode film. This process is much less expensive than other coating techniques, such as spray coating, spin coating, sputtering, thermal evaporation, pulsed laser deposition, atomic layer deposition, and so on. However, this SILAR may have issues with contamination on the film surfaces, structural stability, the film’s low crystallinity and thickness, and the coating process occurring on both sides of the substrate, which is one of its disadvantages. The preparation of thin film electrodes using vacuum-assisted filtration techniques can produce low-cost, easily obtained composites with high particle interaction due to the vacuum force, high flexibility, a highly active site film, and highly porous properties, all of which contribute to high electrochemical performance. However, using a strong vacuum force to filter the particles could damage their morphology and collapse their pores, leading to a poor electrochemical reaction, low ion transportation, and low electrochemical performance. On the other hand, drop-coating is a simple, low-cost method that produces a high-conductivity electrode layer. However, it is not possible to adjust the thickness of the film. The laser-induced graphene technique, which patterns the composite electrode film after drop-coating, is highly effective, fabricating a high fibrous structure and porous morphology that allow contact electrolyte ions to produce quick electrochemical reactions for high electrochemical performance. The reflux/organic chemical approach is a simple, low-temperature way to make particles that have a lot of active sites, form well-hybrid particles, make high-porosity composites, and do not use any energy. However, this approach lacks environmental friendliness; hinders the development of high-purity, crystalline, and surface area particles; and risks damaging particle morphology due to its extended processing time. Therefore, the electrochemical performance of the chemical or organic technique that generates composites may be modest. Therefore, further investigation into the development of technologically advanced composite electrodes may yield cost-effective solutions for implementation in supercapacitors. Such electrodes would possess the following characteristics: a simplified one-step composite preparation process, environmental friendliness, time efficiency, large-scale synthesizing capability, well-structured morphology, substantial surface area, highly hybrid particles, higher crystalline properties, abundant active sites, high structural stability, and high conductivity.

3. GO and Its Composites in Supercapacitor Application

3.1. Fluorine-Functionalized GO

Surface-modified GO might enhance the wettability of the electrodes, so leveraging the pseudocapacitance effect enhances the capacitance of the device. Fluorination is an excellent method to alter the characteristics of graphene since fluorine (F⁻) has the greatest electronegativity. Carbon–fluorine bonding in the highest polar states is advantageous for electrochemical reactions because of the interaction between ionic and covalent bonds. Thus, Sim et al. investigated fluorine-functionalized GO (FGO) using a three-electrode configuration in a 1.0 M potassium hydroxide (KOH) solution [77]. Figure 6a shows the CV curves of the FGO and GO electrodes analyzed in the negative potential region at a scan rate of 10 mV s⁻¹. This test shows that FGO had both EDLC and redox activity because F⁻ functional groups react with OH⁻ groups. On the other hand, GO only showed EDLC action. The FGO exhibited significantly higher C_s (514 F g⁻¹) in comparison to the GO (231 F g⁻¹). The galvanostatic charge–discharge (GCD) curves of GO and FGO for potential (V) and time (s) are illustrated in Figure 6b. The charge–discharge duration of FGO was longer than that of GO, indicating that FGO has a high charge-storing capacity, which might be attributable to its high C_s. At a current density (CD) of 6 A g⁻¹, the charging and discharging activities of the FGO and GO were 276 and 90 F g⁻¹, respectively. Using electrochemical impedance spectroscopy (EIS), the solution resistance (R_s) and charge-transfer resistance (R_ct) of the FGO and GO electrodes were subsequently determined. As shown in Figure 6c, the R_s and R_ct values of FGO (0.58 Ω and 0.71 Ω) were observed to be less than GO (1.69 Ω and 2.61 Ω). The diameter of the semi-circular from the EIS plot may be used to compute the R_ct values. For ion-diffusion resistance, both electrodes exhibit a distinct vertical line, thereby confirming the better kinetics of the electrodes. Therefore, the FGO electrode stimulates the movement of ions between the electrode and electrolyte, enhancing fast charge-transfer processes. Therefore, FGO exhibits better electron transfer properties than GO. According to the FGO’s morphology, the thinner fluorine layer on GO sheets has highly porous (fine pores) structures that facilitate charge storage via EDLC production, while the interconnecting porous channels enhance redox reactions. Furthermore, FGO surfaces may include a large number of fluorine ions, which are responsible for significant redox reactions with OH⁻ ions, resulting in good electrochemical performance. To facilitate practical implementation, a symmetric hybrid FGO (FGO//FGO) was designed and tested over a potential range of 1.0 to 1.6 V. A sheet of polypropylene serves as the anode in this device. The device also observed EDLC and PC operations. The designed device achieves an energy density (ED) of 25.8 Wh kg⁻¹ and power density (PD) of 1280.0 W kg⁻¹ at a CD of 1 A g⁻¹. This device demonstrated excellent cycle stability, with no loss of specific capacity after 20,000 cycles, as shown in Figure 6d. The increased stability of the FGO may be due to the improved surface activation resulting from higher levels of F⁻ sites during electrochemical action, which could be very stable electrochemically. The inset of Figure 6d reveals that the pictures of the hybrid device were programmed to use yellow, blue, green, and white light-emitting diodes (LEDs) at 2, 3, and 4 V, respectively.

3.2. GO-Metal Oxides

Recently, the Ni₀.₅Co₀.₅WO₄/f-MWCNT/GO (NCWO₄/f-MWCNT/GO) composite was used for the three- and two-electrode systems in 6.0 M KOH for supercapacitor application [90]. The enlarged CV curves of NCWO₄/CNT/GO, which resulted from the bimetallic synergy, provide insight into the PC mechanism and indicate that the electrode material can operate at high scan rates (100 mV s⁻¹) within the potential ranges of 0.0 to 0.6 V. Figure 6e shows the calculated C_s using the GCD curves under various current densities (CDs). Accordingly, the CD increases from 1 to 30 A g⁻¹, and the C_s decreases slightly from 1166.6 to 950.0 F g⁻¹. As a consequence of this outcome, the composite electrode demonstrated a notable rate capability of 90.3%. It was determined that the electrode exhibited cycle stability with a capacity retention of 95.6% over 5000 cycles. By employing EIS, it was determined that the NCWO₄/f-MWCNT/GO electrode possessed excellent conductivity with values of 0.67 Ω and 1.2 Ω, respectively. Accordingly, these values are lower in comparison to NCWO₄@f-MWCNTs (0.69 Ω and 1.53 Ω). As illustrated in Figure 6f, a semicircle at high-frequency areas discloses the charge-transfer resistance (R_ct) of the electrodes associated with the faradaic processes, while a linear line at lower-frequency regions suggests the EDLC behavior of materials. According to Figure 6f, the NCWO4@f-MWCNTs/GO electrode exhibits a sharper angle than other electrodes, suggesting that the composite is capable of fast ion diffusion. This composite’s higher electrochemical properties could be due to the crystalline structure of the electrodes, highly active sites, and the synergistic morphology effect of urchin (NCWO₄) and nanotubes (f-MWCNT) on sheets (GO). Further development of an asymmetric device (NCWO₄@f-MWCNTs/GO//AC) was implemented within the voltage range of 0.0–1.5 V. Figure 6g depicts a schematic representation of an asymmetric device made from nickel foam (current collector) modified with NCWO₄@f-MWCNTs/GO (positive electrode) and AC (negative electrode) and a separator was located between the electrodes to transport ions from the negative electrode to the positive electrode and prevent a short circuit. The device had a sandwich-like circuit design. Additionally, PC behavior was evident due to the presence of redox peaks in CV curves at various scan rates. The device exhibited a high C_s value (266.2 F g⁻¹) at a CD of 1 A g⁻¹, as determined by GCD. The shallow values of R_s (0.22 Ω) and R_ct (0.9 Ω) for the device suggest high conductivity. The number of cycles increases to reduce the capacitance retention (%) of the electrode material, as shown in Figure 6h. It was determined that the developed device maintained 87% of its capacity for 5000 cycles. The asymmetric device’s NCWO₄@f-MWCNTs/GO electrode may be electrochemically stable and have a solid bond to the current collector, resulting in good capacitance retention during the cycle test. The device’s maximum PD and ED values were 703.1 W kg⁻¹ and 83.3 Wh kg⁻¹, respectively.

Another study examined the three-electrode systems in a 3.0 M KOH electrolyte using AC, AC/GO/SnO₂, and AC/GO/TiO₂-Zn electrodes [79]. The results revealed quasi-triangular CV curves, indicating that all electrodes operated as EDLC-type supercapacitors. Figure 6i plots the C_s of the electrodes using CV curves at various scan rates. Compared to AC (265.3 F g⁻¹) and AC/GO/SnO₂ (295 F g⁻¹), the C_s of the AC/GO/TiO₂-Zn (329.9 F g⁻¹) was higher at 10 mV s⁻¹. The synergistic effect of the electrode materials, along with their highly porous structure and flower-like morphology, could be attributed to the high C_s of the AC/GO/TiO₂-Zn composites, each of which has good crystalline properties. Triangle curves are found by GCD analyses of the electrodes AC and AC/GO/SnO₂, which point to high EDLC performance. On the other hand, because of the faradic charge-transfer reaction, PC behavior was seen in the AC/GO/TiO₂-Zn electrode. The faradaic reaction of the AC/GO/TiO₂-Zn electrode resulted in a longer discharge time than other electrodes, indicating higher C_s. Due to storage processes via the electrochemical diffusion process, the C_s of the electrode materials generally decrease as CD rises. Accordingly, plotting the C_s of the electrodes using GCD curves at different CDs in Figure 6j helps to understand it. The AC, AC/GO/SnO₂, and AC/GO/TiO₂-Zn electrodes had maximum C_s of 467.8 F g⁻¹, 595.2 F g⁻¹, and 1491.6 F g⁻¹ at 2.5 A g⁻¹, respectively, at a CD of 2.5 A g⁻¹. Furthermore, compared to AC (11.5 Ω) and AC/GO/SnO₂ (4.4 Ω), the R_s of the AC/GO/TiO₂-Zn (192.2 Ω) electrode was higher. The symmetrical device (AC/GO/TiO₂-Zn//AC/GO/TiO₂-Zn) was built based on the best result obtained from the AC/GO/TiO₂-Zn. A symmetrical device of cycle stability profile is shown in Figure 6k, which demonstrates how capacitance retention steadily declines as the number of cycles increases. It demonstrated low capacitance retention of 61.8% over 10,000 cycles. The device’s low capacitance retention rate could be caused by the electrode structure collapsing due to the volume expansion and contraction of the electrode material during the charging and discharging cycles. As a result, while synthesizing ternary compounds, careful consideration should be given to the significant effects on structural suitability and chemical stability during electrochemical reactions. Furthermore, as depicted in Figure 6l, the Ragone plots of the AC/GO/TiO₂-Zn//AC/GO/TiO₂-Zn device’s PD vs. ED profile indicate that the device generated a substantial PD of 3572.0 W kg⁻¹ despite having an ED of 99.0 Wh kg⁻¹ when compared to other reported devices. Due to the Faradaic reaction in 6.0 M KOH, pseudocapacitance (PC) behavior was seen in the Bi₂O₃-GO electrode from the CV analysis in the different scan (5–100 mV s⁻¹) rate [88]. At a CD of 1 A g⁻¹, the electrode’s C_s were measured to be 1029.0 F g⁻¹. However, the C_s slightly decreased from 1029 to 808 F g⁻¹, while the CD increased from 1 to 10 A g⁻¹, indicating that as the CD increased, the electrode’s C_s remained highly sustained. Though the Bi₂O₃-GO composite did not show significant morphology, they did, however, obtain high C_s due to the highly active sites, high purity, and highly crystalline structure. The R_s values for various wattages, 600 W 120 s, 800 W 90 s, and 800 W 120 s, were 0.91, 0.83, and 1.37 Ω, respectively. The resistance of the electrode exhibited an increase as the wattage was raised. GCD analysis also revealed PC behavior, with the C_s of Bi₂O₃-GO (1029 F g⁻¹) being greater than that of bare Bi₂O₃ (314 F g⁻¹) at a CD of 1 A g⁻¹. Therefore, the electrochemical performance is enhanced by the combination of GO and Bi₂O₃. The electrodes’ structural stability contributes to their cycling performance. In this work, the decrease in capacitance retention observed as the number of cycles increases may be caused by an irreversible reaction between Bi₂O₃-GO and KOH electrolyte, which causes structural degradation over a long cycle life. It was found that the electrode maintained approximately 80% of its initial C_s over 3000 cycles. It would be preferable if the cyclic study for the supercapacitor application included at least 10,000 cycles. Consequently, the cycle stability of this particular GO-composite might be compromised, necessitating the optimization of the ratio or functionalization to enhance its structure.

Foroutan et al. investigated the electrochemical performance of MnFe₂O₄/GO in 6.0 M KOH and demonstrated the possibility of a redox reaction by obtaining PC behavior as indicated by rectangular CV curves [91]. Semi-rectangular shapes were maintained in the CV curves despite the increase in scan rates (10–200 mV s⁻¹). As determined by GCD analysis, the quasi-rectangular and triangular structures of GO and MnFe₂O₄ correspond to EDLC and PC behavior, respectively. In contrast, MnFe₂O₄/GO exhibits semi-triangular shapes and demonstrates characteristics of both EDLC and PC. At a CD of 1 A g⁻¹, the C_s of MnFe₂O₄/GO (298 F g⁻¹) was found to be greater than that of MnFe₂O₄ (285 F g⁻¹) and GO (188 F g⁻¹). Hence, MnFe₂O₄ exhibits promise as a material for enhancing the electrochemical characteristics of GO. The high C_s could be the reason for the large number of MnFe₂O₄ spherical nanoparticles on GO sheets; their strong crystalline property and highly porous surface may make it easy to connect with the electrolyte ion and promote quicker ion movement to produce high electrochemical performance. Conversely, increasing the CD (0.5–8.0 A g⁻¹) results in a significant reduction in the C_s of the MnFe₂O₄/GO (405.0–57.0 F g⁻¹). This could be due to the limitation of ion-diffusion electrolytes to electrode surfaces at higher CDs. Moreover, according to the EIS results, MnFe₂O₄/GO (2.16 and 2.45 Ω) may have a higher conductivity than GO (4.7 and 13.8 Ω) and MnFe₂O₄ (3.4 and 5.7 Ω), as their R_s and R_ct resistance values were lower. In only the 500 cycles, the capacitance retention was found to be 92%. It is possible that strong cycle stability was responsible for the chemical stability of MnFe₂O₄/GO in the electrolyte. However, a supercapacitor application that would have performed 10,000 cycles might not find the 500-cycle research relevant or predictable. Regarding the practical applicability, no device preparation or electrochemical investigations were conducted in this study. Similarly, CV analysis showed that the ternary hybrid of AC/TiO₂/GO behaved in both EDLC and PC. In this study, 2.0 M H₂SO₄ was used as the electrolyte solution [80]. However, the authors said that using H₂SO₄ as an electrolyte greatly affects the capacitance rate. The electrochemical studies showed that AC and AC-TiO₂ behaved like EDLC and PC, respectively. At a scan rate of 10 mV s⁻¹, the electrodes’ C_s show an AC of 69.7 F g⁻¹, AC-TiO₂ of 159.2 F g⁻¹, and AC-TiO₂-GO of 219.3 F g⁻¹. Additionally, GCD was utilized to examine the C_s of the AC, AC/TiO₂, and AC/TiO₂/GO, which were 130.3, 195.4, and 617.0 F g⁻¹, respectively, at 1 A g⁻¹. Therefore, adding GO to the AC/TiO₂ combination improves its electrochemical properties. The good C_s of the AC/TiO₂/GO was found to possess many nano-scale pores, a large surface area (439.2 m² g⁻¹), and good crystallinity, enabling the surface to attract higher amounts of electrolyte ions and achieving high charge storage. This ternary composite’s porous structure is highly beneficial for electrical conductivity because it allows for high-ion transport. Figure 7a illustrates the Nyquist plots of the electrode’s resistance recorded from 0.01 Hz to 100 kHz at a potential voltage of 10 mV. At the high- and medium-frequency ranges, the plots show semicircles for the different electrodes caused by R_ct at the electrode and electrolyte interface. On the other hand, the plots reveal aberrations in the straight-line slope in the low-frequency zone. This demonstrates a decreased diffusion barrier for electrolyte ions. According to Figure 7a, AC-TiO₂-GO has a lower internal resistance than other electrodes. This might be attributed to the faradaic reactions and high conductivity of TiO₂, while the good conductivity of GO contributes to the low charge-transfer resistance. Accordingly, AC/TiO₂/GO showed lower R_s and R_ct (1.87 and 2.68 Ω) than AC (5.34 and 7.76 Ω) and AC/TiO₂ (2.67 and 3.40 Ω). Therefore, the addition of GO to AC/TiO₂ improves electrical conductivity. Figure 7b shows the cycle stability profile of the various electrodes, revealing that the electrode’s capacitance retention decreased gradually as the number of cycles increased. The ternary composite (AC/TiO₂/GO) structured electrode demonstrated remarkable stability over 1000 cycles of testing. As a result, AC, AC/TiO₂, and AC/TiO₂/GO retained 57.2%, 72.6%, and 80.1% of their capacitance, respectively. A synergistic impact of GO and TiO₂, high conductivity, structural stability, high chemical stability, and extremely low volume expansion and contraction may be responsible for the high cycle stability of ternary composite electrodes. The influence of the structural degradation and non-conductivity of AC causes no synergistic effect on the binary composite electrode; therefore, the moderate cycle stability is achieved exclusively by the effort of TiO₂ owing to structural and chemical stability. The non-conductive sites and the electrochemical processes may be the cause of the poor cycle stability of AC and the collapse of its pore structure. Therefore, only the use of GO improves the cycle stability in this study, which was observed through the cycle profile. This investigation also did not find any devices in preparation for practical applications.

Recently, Lashkenari et al. conducted an electrochemical investigation on GO, CuFe₂O₄, N-doped GO (NGO), and N-doped GO/CuFe₂O₄ (NGO/CuFe₂O₄) using a various electrolyte, such as 1.0 M Na₂SO₄, 1.0 M H₂SO₄, and 1.0 M KOH [85]. NGO/CuFe₂O₄ electrode CV profile at 100 mV s⁻¹ scan rate in various electrolyte solutions is shown in Figure 7c. Based on the large CV curve and wide working voltage potential, 1.0 M H₂SO₄ outperformed others, and this electrolyte was optimized for the investigations. This study exhibited both EDLC and PC behavior on NGO/CuFe₂O₄ due to faradaic reaction, whereas GO, NGO, and CuFe₂O₄ demonstrated EDLC, EDLC, and PC behaviors, respectively. Both behaviors on NGO/CuFe₂O₄ were observed as a result of the Faradaic process of CuFe₂O₄ in the electrolyte (redox reaction) and electrostatic charges on the GO or NGO surface. Because of the synergic impact, NGO/CuFe₂O₄’s CV curves were greater than others, suggesting that C_s may be high. Accordingly, NGO/CuFe₂O₄ had higher C_s (374.2 F g⁻¹) than GO (138.0 F g⁻¹), NGO (181.2 F g⁻¹), and CuFe₂O₄ (214.0 F g⁻¹) at a CD of 1 A g⁻¹. Electrode materials that are a combination of NGO and CuFe₂O₄ may exhibit enhanced redox properties, structural stability, conductivity, surface area, active sites, and porousness due to the nanocube shape of CuFe₂O₄ on GO sheets. Adding NGO to CuFe₂O₄ may also greatly improve the C_s because NGO makes the electrode more electrically conductive and may also increase the specific surface area and porosity. It has a great potential to sustain the C_s from 436 to 201.5 F g⁻¹ while increasing the CD from 0.5 to 10 A g⁻¹. The electrodes’ Cs decrease with increasing CDs, which could be due to not having enough active material for the redox reaction. The conductivity test results indicated that the R_s of NGO/CuFe₂O₄ (6.85 Ω) was marginally lower than that of NGO (7.25 Ω) and CuFe₂O₄ (7.1 Ω). Since NGO/CuFe₂O₄ has low resistance, it must have a high conductivity. This is because the NGO framework is bonded together, which increases the electrode material’s overall conductivity. However, NGO/CuFe₂O_4’s R_s value was higher than that of other reported electrode materials [80,95]. In the same way, the R_ct values of the electrodes indicate NGO/CuFe₂O₄ (1.1 Ω), NGO (2.8 Ω), and CuFe₂O₄ (1.3 Ω). Therefore, NGOC may exhibit more capacity for charge transfer than other reactions owing to its low resistance. Figure 7d shows the capacitance retention (%) vs. cycle number profile of CuFe₂O₄ and NGO/CuFe₂O₄. This study found that increasing the cycle numbers led to a decrease in the capacitance retention of both electrode materials. However, NGO/CuFe₂O₄ shows high cycle stability when compared to CuFe₂O₄. Interlayer ferrite particles may behave in an unstable form during the charge and discharge processes, which causes CuFe₂O_4’s poor cycle stability and high capacitance loss. The NGO-modified CuFe₂O₄ particles may help to preserve their rigidity, and CuFe₂O₄ can inhibit the restacking of NGO sheets and produce a high-porosity medium channel for the interaction of electrolyte ions. Accordingly, over 2000 cycles, the cycle stability of NGO/CuFe₂O₄ (82%) was greater than that of CuFe₂O₄ (58.4%). Furthermore, an asymmetric device (NGO/CuFe₂O₄//AC) was developed that operated in the voltage range of 0.0–1.7 V. The appearance of quasi-rectangular CV curves caused NGO/CuFe₂O₄ and AC to exhibit PC and EDLC characteristics, respectively. The device showed a C_s of 89.1 F g⁻¹ at a CD of 1 A g⁻¹. The device achieved a capacitance retention of 78.6% after 500 cycles. Furthermore, this device demonstrated its great potential in high-performance energy storage applications, as it was able to produce ED and PD of 35.7 Wh kg⁻¹ and 883.0 W kg⁻¹, respectively, at a CD of 1 A g⁻¹. As seen in Figure 7e, the asymmetric device (NGO/CuFe₂O₄//AC) was able to light up the LED, confirming its possible use in practical applications.

Similarly, the hydrothermal synthesis of the hierarchical Cu₂O-CoO-modified GO electrode showed PC behavior at 6.0 M KOH due to the formation of redox peaks [95]. Figure 7f shows the electrochemical CV analysis of Cu₂O-CoO/GO with others in the voltage ranges between 0.0 V and 0.5 V. According to the CV curves, Cu₂O/GO and CoO/GO both exhibit a couple of redox peaks, while Cu₂O-CoO/GO has two pairs of redox peaks, indicating that both single-type electrode materials are participating in their pseudocapacitance processes and therefore they could pseudocapacitive materials. The GCD confirms that it exhibits the greatest C_s and excellent reversibility. Therefore, Figure 7g demonstrates the calculation of C_s at various CD plots using GCD curves. The inadequate response is caused by the redox reaction, which only happens on the electrode surface during the charging and discharging process. As the CD rises, the C_s of the electrode materials gradually decrease. However, Cu₂O-CoO/GO still exhibits high C_s when compared to others, which could be the reason for the fast response to the redox reaction. As a result, at a CD of 1 A g⁻¹, Cu₂O-CoO/GO (723 F g⁻¹) was achieved with higher C_s than Cu₂O/GO (576 F g⁻¹) and CoO/GO (533 F g⁻¹). Cu₂O-CoO/GO demonstrated good retention of capacitance, increasing the CD from 1 to 10 A g⁻¹ while decreasing the CD gradually from 723 to 484 F g⁻¹. The high C_s of Cu₂O-CoO/GO could be due to its highly active sites, high conductivity, and high specific surface area, which could be attributed to the spherical kind of Cu₂O-CoO nanoparticles on GO sheets. Cu₂O-CoO/GO (0.1 Ω) has a lower resistance (R_s) than Cu₂O/GO (0.5 Ω) and CoO/GO (0.31 Ω), potentially making it a high-conductivity electrode. These resistance values have shown that Cu₂O-CoO/GO had lower resistance than other electrode materials, such as Cu₂O and CoO. This suggests that the TMO composite on GO can greatly improve the material’s ability to conduct electricity while also increasing the pseudocapacitive response, which may show high electrochemical performance. Furthermore, an asymmetric device (Cu₂O-CoO/GO//GO) was developed and tested in the voltage range of 0.0–1.6 V. It showed a C_s of 125.0 F g⁻¹ at a CD of 1 A g⁻¹. This was able to make a middling ED of 44.1 Wh kg⁻¹ and a low PD of 794.0 W kg⁻¹. Figure 7h illustrates the cycle stability profile of an asymmetric device, demonstrating low capacitance reduction and high coulombic efficiency over 10,000 cycles, suggesting high structural stability in the electrochemical reaction. Accordingly, it kept its capacitance well (89.3% after 10,000 cycles) at a CD of 10 A g⁻¹. Another study found that the best GO/CNT/COF weight ratio (2.4: 2.1: 1.0) had a higher C_s of 544.91 F g⁻¹ at 1 mV s⁻¹ and 175.09 F g⁻¹ at 1 A g⁻¹ [97]. Due to their porous structure, high electrolyte ion adsorption could be attributed to the high C_s achievement, which may account for the CNT and GO materials’ high specific surface area, while the COF contributes to their high redox activity. In general, GO and CNTs are high-conductivity materials. However, COF in the composite structure could be the reason that affects the overall conductivity of GO/CNT/COF, which is attributable to R_ct (11.34 Ω) and R_s (2.25 Ω), which were found to be somewhat greater than the reported literature, suggesting moderate conductivity. Because of its high structural stability in the electrolyte solution, the electrode demonstrated high cycle stability (84.76% over 8000 cycles), and there may be no volume expansion or contraction of the GO, CNT, and COF combinations. Furthermore, the developed symmetric supercapacitor (GO/CNT/COF//GO/CNT/COF) produced 24.3 Wh kg⁻¹ of mild ED and 248.95 W kg⁻¹ of low PD; therefore, this means that this kind of device could be used to store low amounts of energy.

Another ternary composite was investigated in 1.0 M H₂SO₄ and designated GO@Fe₃O₄-ionic liquid@Na₂WO₄ (GO@Fe₃O₄-IL@W) [58]. Figure 7i,j show the CV curves for GO and GO@Fe₃O₄-IL-W at different scan rates. According to the CV curves, when the scan rate rises, the current steadily increases, confirming the electrode materials’ good capacitive performance and both electrodes’ pseudocapacitive behaviors. GO@Fe₃O₄-IL@W showed higher stability and reversibility in comparison to GO, and larger CV curves were caused by the faradaic reaction. At a CD of 0.2 A g⁻¹, the high C_s were identified as GO@Fe₃O₄-IL@W (332.4 F g⁻¹). In contrast, GO showed only 122 F g⁻¹. The composite has high C_s when compared to GO because IL and tungstate (W) have qualities such as high electrical conductivity and high redox properties, whereas Fe₃O₄ nanoparticles are structurally stable but have low conductivity. Therefore, the combination of Fe₃O₄, IL, and W could enhance the electrochemical performance of GO. In addition, high Fe₃O₄-IL and W nanoparticles on the GO sheet could allow highly active sites, a high synergistic effect, and a high surface area for a high electrochemical reaction. Even with a significant fall in the electors’ C_s due to increased CD, the ternary composite retained a higher retention rate than GO. It was found that the ternary composite’s resistance (R_s and R_ct) was less than GO’s due to the high conductivity of IL and W. Thus, GO@ Fe₃O₄-IL@W (5.47 and 9.35 × 10⁻⁵ Ω) and GO (7.60 and 0.002 Ω) will also be. In Figure 7k, the cycle stability profile of GO and its composite shows that the capacitance retention (%) of both electrode materials decreased with increasing cycles. However, the GO@Fe₃O₄-IL@W exhibits higher cycle performance than the GO. Accordingly, it was shown that GO@Fe₃O₄-IL@W exhibited notable cycle stability (91.3% capacitance retention over 10,000 cycles). Moreover, the capacitance retention of GO is only 80%. The high number of nanoparticles on the GO sheet, which are associated with its high chemical stability and high reversibility, may be the cause of the high cycle stability of GO@Fe₃O₄-IL@W. The GO also did well in cycles but not as well as the composite because it has less conductivity and only has a sheet-like structure, which means it may undergo some moderate structural deformation due to volume expansion and contraction. Figure 7l shows the Ragone plot, which measured the ED and PD of the GO@Fe₃O₄-IL@W. It shows that as PD increased, the electrode materials’ ED decreased. The inset picture reveals that the composite electrode can light up a red LED using the cell setup. The electrode’s ED and PD were determined to be very low using the three-electrode method, measuring 7.38 Wh kg⁻¹ and 40.0 W kg⁻¹, respectively. These low values indicate that the electrode material may have low energy storage, but it may provide high values when it is in device form with a negative electrode (symmetric or asymmetric configuration). Similarly, the ternary alloy NiO/MnO₂/GO (NMGO) was tested in 1.0 M KOH and demonstrated PC behavior when compared to GO and MnO₂ (EDLC behavior) [83]. This means that NiO, not GO and MnO₂, can start the faradaic reaction in the electrolytic process. NMGO had a high C_s (402 F g⁻¹) compared to MGO (297.0 F g⁻¹), NiO/GO (283.0 F g⁻¹), and GO (82.0 F g⁻¹) at a CD of 1 A g⁻¹. Thus, MnO₂ and NiO have the potential to improve the electrochemical efficacy of GO. The high C_s of NMGO might also be a synergistic impact of (NiO and MnO₂) nanoparticles densely accomplished on GO surfaces covered evenly, which may increase conductivity, highly active sites, and high surface area (102 m² g⁻¹) when compared to merely MnO₂ and NiO₂ nanoparticles on GO. The low C_s of GO may be related to its sheet-structured shape, which is characterized by weak conductivity, low activity sites, and low surface area (94 m² g⁻¹). Additionally, it was observed that NMGO (34 Ω) had a lower R_ct value than MGO (68 Ω), NGO (87 Ω), and GO (138 Ω). It was determined that the low conductivity of GO may have contributed to the reaction’s restricted charge transfer and, consequently, low electrochemical permeances. Moreover, it was observed that the ternary composite (NMGO) exhibited superior capacitance retention (93%) and cycle stability (over 14,000 cycles). The cycle stability of NMGO was good because it had dense nanoparticles on GO sheet shapes that made it more conductive, had a lot of surface area, was chemically and structurally stable, and did not expand or contract structurally during the electrochemical reaction. Additionally, in 1.0 M Na₂SO₄, an asymmetric device (NMGO//MWCNT) was fabricated and investigated. At a CD of 1 A g⁻¹, it exhibits C_s of 90.0 F g⁻¹. On the other hand, the capacitance retention of the device is low as a result of the increase in CD from 1 to 4 A g⁻¹, which gradually reduces C_s from 90.0 to 42.0 F g⁻¹. Because of its good cycle stability (retaining 88% of capacitance after 6000 cycles), the developed asymmetric device could have high chemical and structural stability in the electrolyte solution. Therefore, the high structural combination of NMGO and MWCNT devices could be very effective in electrochemical performance. The asymmetric device may have moderate energy storage because it yielded values of ED and PD, measuring 28.0 Wh kg⁻¹ and 750 W kg⁻¹, respectively. Consequently, the proposed ternary composite may be utilized as a material for supercapacitors instead of the raw materials.

Redox activity (pseudocapacitive behavior) owing to Faradaic reactions occurring at the electrode/electrolyte interface was detected on the electrodes of 5%GO/WO₃, 10%GO/WO₃, and 15%GO/WO₃ composite in an electrolyte of 2.0 M KOH [96]. CV study revealed that the C_s values for 5%, 10%, and 15% GO/WO₃ were 508, 668, and 475 F g⁻¹ at 5 mV s⁻¹, respectively. Increasing the Wt.% of GO increases the C_s; however, after 10wt.%, the C_s drops, which might be due to a limiting of charge transport from an electrolyte to the electrode surface. As the wt.% of GO on WO₃ nanorods increases from 5 to 10, C_s may rise. This might be because the surfaces have a large number of active sites and pores, shorten the distance for electrolyte ions diffusion, and increase the electrolyte ion transportation, resulting in rapid redox reactions. The decreasing C_s for the composites with 15% GO/WO₃ was due to a high wt.% of GO sheets restacking with WO₃, which limited the ion diffusion and conductivity, resulting in low performance. The GCD analysis confirmed that 10%GO/WO₃ (738.0 F g⁻¹) had a higher C_s than 5%GO/WO₃ (590 F g⁻¹) and 15%GO/WO₃ (341 F g⁻¹). All prepared electrodes exhibited solution resistance (R_s) values ranging from 1.05 to 2.35 Ω; consequently, the 10% GO/WO₃ electrode showed a low R_s value of 1.05 Ω, suggesting that its high electrical conductivity may contribute to its high electrochemical performances. Moreover, over 7000 cycles, the 10%GO-WO₃ maintained 88% of its charge storage, demonstrating its exceptional cycle stability. The electrode’s high cycle stability may be due to the decorated WO₃ nanorod morphology with GO, which may have active sites for high interfacial connections for the electrolyte ions and chemically strong structures. Moreover, the asymmetrical device (GO/WO₃//GO-NF) was developed, exhibiting a C_s of 213 F g⁻¹ at a rate of 5 mV s⁻¹. It was found that 25 Wh kg⁻¹ of mild ED and 1000 W kg⁻¹ of high PD. Over 3000 cycles, the device’s cycle stability showed that 87% of its capacitance was still in it. Due to their structural and chemical stability, GO/WO₃ electrode materials are also capable of producing high cycle stability in device applications.

3.3. GO-Metal Sulfide

For supercapacitor applications, metal sulfides like MnS and La₂S₃ perform poorly because of their sluggish kinetics, low ED, low conductivity, and poor cyclic performance. Metal sulfides could be composited with GO to enhance electrochemical performance. Thus, Mane et al. investigated the electrochemical performance of MnS, MnS-La₂S₃, and MnS-La₂S₃/GO in a 1.0 M Na₂SO₄ [100]. In Figure 8a, the CV curve for MnS-La₂S₃/GO was wider than the others, and it looked like a quasi-rectangular shape. At 100 mV s⁻¹, the PC behavior was found for all electrodes. Through GCD analysis, it was discovered that, at a CD of 2 A g⁻¹, the C_s for MnS-La₂S₃/GO (890 F g⁻¹) was greater than that of MnS-La₂S₃ (694 F g⁻¹) and MnS (666 F g⁻¹). The sheet-like MnS-La₂S₃ particles on GO morphology could have a high porosity, high surface area, highly active sites, high conductivity, and high synergistic effect, which may be ascribed to shorter ion path and ease of electrolyte ion adsorption and make high quicker to boost the electrochemical performance and thus they obtained high C_s than other electrode materials. Regarding MnS-La₂S₃, their morphology resembles cotton candy, and they may have a good surface area, porosity, active sites, conductivity, synergistic effect, and moderate ion route that contributes to moderate electrochemical performance and C_s. Further, no significant morphology was observed on MnS. Its low surface area, low porosity, low-activity sites, low conductivity, and lack of a synergistic effect may be attributed to its higher ion path, which results in slower ion transportation. This, in turn, leads to a low electrochemical performance and low C_s. Moreover, as the CD was increased from 2 to 5 A g⁻¹, the C_s of the electrodes MnS-La₂S₃/GO (890 to 545 F g⁻¹), MnS-La₂S₃ (694 to 415 F g⁻¹), and MnS (666 to 320 F g⁻¹) exhibited a gradual decrease. This can be attributed to the fact that at low CD, ions can penetrate extensively into the electrode surface, given sufficient time to do so. In contrast, migration of ions into the electrode surface is restricted at higher CD values due to time constraints. Figure 8b shows Nyquist plots of various electrode materials to study their resistances, and the plots show electrodes with semi-circular curves in the higher frequency region. The straight line in a low-frequency zone is caused by diffusive resistance, which acknowledges short ionic routes. This is advantageous for rapid ion transportation and great electrochemical performance. The electrodes’ diameter of the semi-circle equals the actual axis of the Nyquist plot and the R_ct of the electrodes caused by the reversible redox reaction on their surfaces. Subsequently, the resistance values (R_s and R_ct) of MnS-La₂S₃/GO (0.6 and 18.2 Ω cm⁻²) were found to be poorer than those of MnS-La₂S₃ (0.9 and 39.7 Ω cm⁻²) and MnS (1.1 and 80.4 Ω cm⁻²). As a result, a charge-transfer mechanism that is more rapid than others may be feasible on the MnS-La₂S₃/GO. The MnS-La₂S₃/GO material exhibited remarkable cycle stability, retaining 89% of its capacitance over 4000 cycles. The high cycle stability could be due to the electrodes’ composite nanosheet structure, which works well together and maintains shape during electrochemical reactions and cycles. It is also possible that the electrodes had good conductivity, had no binder on their surface, had a shorter ion path, and were highly porous with active sites. Moreover, an investigation was conducted on the symmetrical device (MnS-La₂S₃/GO//MnS-La₂S₃/GO) using a PVA-Na₂SO₄ electrolyte with an operational voltage range of 0.0–1.8 V. A high C_s of 151 F g⁻¹ was produced by the device at a CD of 7 A g⁻¹. A device was discovered to exhibit high cycle stability, with capacitance retention of 92.5% over 10,000 cycles. The developed electrode materials exhibit high cycle stability in three and two electrode setups, potentially due to their high chemical stability and lack of structural degradation during the cycle test. It also has been determined that the improved interface between the MnS-La₂S₃/GO and PVA-Na₂SO₄ gel electrolyte contributes to excellent cyclic stability. Figure 8c depicts the Ragone plot of the developed symmetric device, which represents that the device’s ED decreased gradually with an increase in PD. This device achieved moderate ED and PD values of 54.2 Wh kg⁻¹ and 1300 W kg⁻¹, respectively. To implement a real-time application, two symmetric devices were linked in a series of LEDs. Figure 8d shows a picture of the LED setup panel at two different discharge times (30 s and 150 s), along with two symmetric devices (MnS-La₂S₃/GO//MnS-La₂S₃/GO) that were connected in series. Each device was charged at 3.2 V for 30 s and discharged for 150 s via the LEDs’ illumination. As a result, the fabricated symmetrical device can be utilized in industrial and real-time settings.

In another study, NiS/CoS/GO was investigated for supercapacitors in 3.0 M KOH and demonstrated PC behavior [98]. At a CD of 5 A g⁻¹, high C_s (1114 F g⁻¹) were observed. This high accomplishment by NiS/CoS-modified GO might be highly pours morphology, homogeneous particle distribution on GO sheets, structurally stable, highly active sites, high conductivity, and a very shorter ion route that may enable very faster ion transmission for high C_s. Resistance (R_ct) was found to be only 1.99 Ω cm², and therefore, it may have had a high charge-transfer process in an electrolyte. High cycle stability also revealed that 80% of capacitance was retained after 8000 cycles. Decorating CoS and NiS particles around GO might make the structure more stable, increase the surface area, and create more active sites. This could make it easier for high-ion movement during the cycle test’s electrochemical reaction. Therefore, they could maintain stability for a long time. However, single compositions such as CoS/GO, NiS/GO, and GO show lower cycle stability when compared to CoS/NiS/GO. There may be structural volume changes during the charge–discharge test that contribute to low chemical stability. When all materials combine to enhance both structural and chemical stability, this could potentially lead to high cycle stability. The higher the ED, the higher the device’s PD, which may be a high-energy storage material. Therefore, this CoS/NiS/GO electrode demonstrated high ED and PD values of 674.12 Wh kg⁻¹ and 8089.44 W kg⁻¹, respectively. However, the device preparation for practical application was not found in this study, and the actual ED and PD can be measured from either symmetrical or asymmetrical devices.

3.4. GO-Transition Metal Chalcogenides

Transition metal chalcogenides (TMDs) have a 2-D sheet-like layered structure, a large surface area, good redox properties, and low electronegativity, among other characteristics. However, the poor electrical conductivity and high-volume variations make it unsuitable for commercial uses. Thus, it might be used with high electrical conductivity-based materials to boost electrochemical performance. In addition, adding sulfur (S) and selenide (Se) compounds to the metals improved charge transfer between the metals and carbon-based materials. Yasoda et al. have investigated a GO composite with MnSSe (GO-MnSSe) in a 1.0 M KCl solution for supercapacitor applications [104]. As a result, EDLC and PC behavior were identified for the GO-MnSSe, while GO only demonstrated EDLC behavior. As a result of MnSSe’s redox properties, a faradaic reaction occurred when GO-MnSSe was used. The C_s of the GO-MnSSe (300 F g⁻¹) was found to be high at 10 mV s⁻¹, and it showed excellent reversibility since the C_s were highly sustained even at a high scan rate of 100 mV s⁻¹. The GCD study revealed EDLC and PC in GO-MnSSe, with a C_s of 603 F g⁻¹ at 0.1 A g⁻¹. The cycle stability was modest, with 67% capacitance retention after 9000 cycles. The insufficient cycle stability of GO may account for its poor chemical stability and structural deterioration, as the decorated MnSSe nanoparticles did not succeed in improving the functionality of GO on account of their uniform and limited layering on GO sheets. Consequently, the potential benefits of adjusting the ratio of MnSSe to GO include better electrochemical performance and cycle stability. This research did not include an investigation of the two-electrode setup. The research found that using a three-electrode setup for supercapacitor applications yielded superior results due to high C_s and cycle stability.

3.5. GO-Layered Double Hydroxides

LDH has a layered structure with a large surface area, allowing for numerous active sites and strong redox characteristics. LDHs’ layered structure with large layer spacing promotes ion insertion and deintercalation, which aids in C_s improvement. However, LDHs have poor electrical conductivity and sluggish ion diffusion, which may compromise cycle stability [121]. It may be feasible to improve the conductivity and other characteristics by combining carbon-based materials. Therefore, Zhao et al. developed an optimal ratio of Co₂Ni₁ LDH composite with GO (Co₂Ni₁-GO) for the electrochemical study in 3.0 M KOH [106]. The electrode exhibited PC behavior due to the redox reaction caused by LDH in the electrolytes. The GCD curves of Co₂Ni1 LDH-GO’s charging and discharging concerning potential vs. time are shown in Figure 8e. These curves show that the electrode’s discharging time drastically decreased as the CD increased. However, it retains an acceptable symmetry curve, strong charge–discharge properties, and reversibility while increasing the CD. At 0.1 A g⁻¹, the electrode Co₂Ni₁-GO had a very high C_s (3317.5 F g⁻¹), whereas bare Co₂Ni₁ exhibited only 793 F g⁻¹. LDH nanosheets on GO layers might improve the shorter ion route, resulting in much quicker ion transport and the ability to store more charges. The significant storage charges may be due to the LDHs-GO combination having a high surface area, porosity, active sites, conductivity, structural stability, and chemical stability when compared to bare LDHs. GO might be a useful adjunct electrode material for LDH to enhance its C_s for supercapacitor applications. Furthermore, the produced composite electrode may have great conductivity owing to its very low resistance. The Co₂Ni₁-GO electrode has R_s and R_ct values of 0.88 and 0.029 Ω, respectively. In contrast, bare Co₂Ni₁ LDHs exhibited a higher resistance than Co₂Ni₁-GO, which may imply poor conductivity. Figure 8f depicts Co₂Ni₁-GO’s cycle stability profile, which plots capacitance retention (%) against cycle number at a CD of 5 A g⁻¹. It shows that the capacitance retention (%) decreases with increasing cycle number. However, Co₂Ni₁-GO has very good cycle stability, with 91.8% capacitance retention after 10,000 cycles. Both sheets (Co₂Ni₁-GO LDH and GO) in composite form may have excellent structural stability, highly active sites, mechanical strength, and chemical stability, and, therefore, an excellent response to the charging–discharging cycle test. The asymmetric device (Co₂Ni₁-GO//AC) was developed for practical use and functioned throughout a voltage range of 0–1.6 V. When the scan rate (5–50 mV s⁻¹) is increased, the CV curves exhibit high symmetry, indicating that the device can transport electrons effectively as shown in Figure 8g. The device achieved a C_s of 328.7 F g⁻¹ at a CD of 1 A⁻¹ and maintained a high C_s (114.2 F g⁻¹) at a CD of 50 A g⁻¹. Figure 8h shows the cycle stability profile of an asymmetric device by plotting capacitance (%) against cycle numbers. As the cycle numbers increase from 0 to 10,000, the capacitance (%) slowly decreases. The device demonstrated 91.3% cycle stability over 10,000 cycles at a CD of 5 A g⁻¹. Furthermore, the device’s ED and PD were found to be high, at 94.7 W h kg⁻¹ and 750.0 W kg⁻¹_, respectively. According to its high C_s, high cycle stability, and high ED, Co₂Ni₁-GO proved to be a potential candidate for energy storage.

3.6. GO-Organic Materials

MOFs have a large surface area and active surface; however, their poor conductivity limits their electrochemical effectiveness [122]. Additionally, sulfur doping into GO (SGO) impacts graphene’s electroneutrality and develops charged ion sites, resulting in increased electrical conductivity. Recently, ZIF-8 MOFs were composited with SGZ (ZIF-8/SGO) for supercapacitor applications in 6.0 M KOH [114]. Figure 8i represents the CV analysis of ZIF-8, SGO, and ZIF-8/SGO (SGZ) electrodes at a scan rate of 100 mV s⁻¹. According to the CV plots, SGZ showed PC behavior due to sulfur-doped GO contributing to the redox reaction, and its CV rectangular curve was steep. ZIF-8 and SGO, on the other hand, demonstrated EDLC and PC tendencies, respectively, as shown by small CV rectangular and quasi-rectangular curves. However, the synergistic effect of SGZ showed a high area CV curve when compared to ZIF-8 and SGO, indicating that it may have high electrochemical properties that enhance the electrochemical reaction for high energy storage. At the CD of 1 A g⁻¹, the SGZ (261 F g⁻¹) had greater C_s than ZIF-8 (150 F g⁻¹) and SGO (246 F g⁻¹). However, the C_s of the SGZ (298–80 A g⁻¹) rapidly reduced when the CD increased from 0.5–8.0 A g⁻¹. This electrode material of moderate C_s and high reduction of C_s at elevated CD could be due to morphological and purity issues with SGZ. Because no significant morphological structure was found, it may have low porosity, low-activity sites, and moderate conductivity, which could contribute to moderate electrochemical performance. Furthermore, the R_ct-related semicircle in the high-frequency area assigned to the redox reactions and the straight line in the low-frequency region represented by Warburg impedance are attributed to the electrolyte diffusion within the SGZ electrode, as shown in Figure 8j. SGZ exhibited lower resistance (R_ct) (14.7 Ω) than SGO (29.6 Ω) and ZIF8 (35.4 Ω). Thus, SGZ may have higher conductivity than SGO and ZIF-8. However, these resistance values are relatively high compared to the other known electrode materials [96,102,106,113]. The poor conductivity of the ZIF-8 MOF could reduce the total composite electrode material’s electrical conductivity, affecting electrochemical performance. Figure 8k depicts Ragone plots of SGZ vs. other reported electrode materials, demonstrating that SGZ has more ED than the others. However, the electrode may be a low-energy-storage electrode material since its high resistance caused it to exhibit poor PD (231 W kg⁻¹) and ED (13.4 Wh kg⁻¹) values. As shown in Figure 8l, there was no loss in capacitance retention (%) up to 700 cycles; However, after 1000 cycles, the capacitance (%) rose to 102%, owing to the SGZ sites’ self-activation over time. The good cycle stability of SGZ was shown, which might be attributed to chemical stability. Due to its high resistance, moderate C_s, and cycle stability, SGZ may not be ideal for high-energy-storage applications, and it may be necessary to optimize the ratio of ZIF-8 and SGO for the efficient composite to enhance the electrochemical characteristics. The two-electrode setup has not been found in this investigation.

Similarly, neodymium-MOF-modified GO (Nd-MOFs/GO) was investigated for supercapacitor applications in 3.0 M KOH [113]. In this study, Nd-MOFs and Nd-MOFs/GO exhibited EDLC behavior; however, the rectangular CV curve was larger for Nd-MOFs/GO; therefore, the composite may have high conductivity and electrochemical performance. Accordingly, Nd-MOFs/GO may have greater C_s than Nd-MOFs themselves. Modified MOFs (677.6 F g⁻¹) demonstrated greater C_s than Nd-MOFs (11.1 F g⁻¹) at a scan rate of 10 mV s⁻¹. The GCD also revealed that modified MOFs had larger C_s (633.5 F g⁻¹) than Nd-MOFs (11.3 F g⁻¹). Despite having highly active sites, the nanorod morphology of Nd-MOFs may contribute to their low conductivity, resulting in a very low C_s value. Conversely, composites containing GO can achieve high C_s because of their highly active sites, high porosity, high conductivity, and synergistic effect, which accelerates the electrochemical reaction through faster ion transportation. However, when exploring the morphology of Nd-MOFs/GO, the nanorod structure of MOFs was found to be loosely associated with GO, and the composition of GO was found to be higher than that of Nd-MOFs. As a result, GO is responsible for the majority of the outstanding electrochemical performance. As a result, GO might be used in tandem with Nd-MOFs to store large amounts of electrochemical energy. The modified MOFs demonstrated high cycle stability (88.7% capacitance retention over 4000 cycles), and the resistance (R_s) of modified MOFs (0.8 Ω) was lower than Nd-MOFs (0.92 Ω). As a result, modified MOFs may have very high conductivity, allowing for improved electrochemical performance. This finding indicates that Nd-MOFs/GO is a long-lasting electrode material in the electrolyte solution that can successfully serve as a super-capacitive electrode that has high cycle stability. The electrode setup resulted in high ED (31.4 Wh kg⁻¹) and PD (1500 W kg⁻¹), and these values indicate that this composite electrode material may be useful in energy storage.

Another study examined the optimized Ni-BTC MOFs/GO 2 composite in 3.0 M KOH [102]. In this case, the Faraday reaction of the nickel (Ni²⁺) ion in the electrolyte caused the GO composite with MOFs to exhibit PC behavior. Using GCD, Ni-BTC@GO 2 demonstrated a high C_s of 1199.0 F g⁻¹, whereas Ni-BTC only showed 625.6 F g⁻¹ at a CD of 1 A g⁻¹. Therefore, incorporating GO into MOFs may be a viable way to improve the electrochemical reactivity of MOFs. It was found that the electrochemical performance of MOFs was not dependent on morphology but rather on particle size, conductivity, and crystallinity. As a result, MOFs composited with GO had high C_s, but bare MOFs exhibited poor C_s despite having an octahedral shape and micron-sized particles. It is also possible that the MOF’s low performance was due to their bulk structure, which contributed to their poor electrochemical performance. As a result of their high conductivity and crystalline structure, the thin layer of GO sheets over micro-sized octahedral MOFs had the greatest contribution to electrochemical performance. However, when a large number of GO aggregates on MOF surfaces, the conductivity may be reduced. The Ni-BTC MOFs/GO 2 were able to retain high capacitance (1199 to 676 F g⁻¹) when the CD was increased from 1 to 20 A g⁻¹. Because the resistance of R_s and R_ct was discovered to be 0.79 and 7.0 Ω, the modified MOFs may have a moderate conductivity, whereas the naked Ni-BTC MOFs may have a poor conductivity material because their resistance was 0.66 and 7.64 Ω. As a result, the higher conductivity of Ni-BTC/GO may serve as a better electrode material for energy storage than bare MOFs. The Ni-BTC MOFs/GO 2 showed high cycle stability, with 84.47% capacitance retention even after 5000 cycles. The high cycle stability was due to the GO sheets improving the structural properties of Ni-BTC, which increased the contact region between the electrode and the electrolyte, reducing the ion diffusion path and retaining the activated sites of Ni-BTC MOFs for energy storage during the electrochemical reaction. With the assistance of GO functionalization on MOFs, the electrochemical reaction between electrolyte ions reduces the dimension change of Ni-BTC MOFs, which improves the cycle stability of composite electrode materials. Additionally, during operation in the 0–1.6 V range, the asymmetric device (Ni-BTC MOFs/GO 2//AC) demonstrated a moderate ED of 42.89 Wh kg⁻¹ at a PD of 800 W kg⁻¹. Over 10,000 cycles, this device was able to achieve 70% cycle stability in capacitance retention. The Ni-BTC MOFs/GO 2 have shown promising properties as an electrode material for a device application, including good conductivity, high-electrolyte contact surfaces, and high stability. When compared to other MOF/GO composites, the findings suggest that Ni-BTC MOFs/GO 2 might be an exceptional electrode.

In the electrolyte of 1.0 M H₂SO₄, the bioinspired organic molecule ABQA-modified GO/carbon paper (ABQA-GO/CP) showed both PC and EDLC behavior [101]. On the other hand, ABQA and GO/carbon paper, respectively, were the cause of the PC and EDLC behaviors. The ABQA-GO/CP electrode performs well because of the synergistically high charge transfer that results from the covalent binding of GO and ABQA. Accordingly, by using CV measurement, the maximum C_s of the ABQA-GO/CP was identified to be 203.2 F g⁻¹ at 5 mV s⁻¹, while GCD revealed 235.5 F g⁻¹ at 0.5 A g⁻¹. The rising scan rates and CDs were 203.2 to 7.8 F g⁻¹ and 235.5 to 205.5 F g⁻¹, respectively, from 5 to 500 mV s⁻¹ and 0.5 to 10 A g⁻¹. The findings reveal that raising the scan rate did not sustain the electrode’s C_s, while increasing the CD accomplished it. The cause might be insufficient time for ion diffusion from the electrolyte to the electrode surface at a higher scan rate or CD. In contrast to other composites consisting of GO-organic materials, the electrochemical performance of this organic molecule functionalized GO/CP was notably poor, as evidenced by its low capacitance (C_s), increased scan rate, and CDs that were incapable of sustaining its initial capacitance. The potential cause for the collapsed or deformed morphology of GO following ABQA functionalization is the presence of low-activity sites, fewer pores, reduced ion diffusion, and the potential for delayed ion transportation, all of which contribute to diminished electrochemical performance. The designed symmetric device (ABQA-GO/CP//ABQA-GO/CP) was evaluated in the range of −0.2 to 1.2 V and indicated lower C_s than the three-electrode system. Accordingly, the GCD investigation revealed 134.1 F g⁻¹ at 0.5 A g⁻¹ but with good capacitance retention (113.4 F g⁻¹) even at 10 A g⁻¹. On the other hand, the device’s organic molecule-functionalized GO/CP exhibits a lower resistance, potentially enhancing its electrochemical performance. Accordingly, the device’s low resistances (R_s and R_ct) of 0.63 and 0.79 Ω cm² suggest the potential for excellent conductivity. The device achieved high ED and PD of 32.8 Wh kg⁻¹ and 1256 W kg⁻¹, respectively, at 0.5 A g⁻¹. This device demonstrated good cycle stability, retaining 106% of capacitance over 5000 at 5 A g⁻¹. The ABQA-GO/CP electrodes may have good chemical stability, in addition to the combined contribution of the faradaic redox reaction by ABQA and EDLC by GO, which may account for the overall symmetric device performance. As a result, the organic material composite containing GO demonstrated excellent electrochemical performance, making it potentially valuable in industrial applications. Overall, the MOF performance.

3.7. GO-MXene

Laser-induced graphene (LIG) has been consistently investigated recently to analyze graphene’s scalability in energy storage applications. LIG has found applications in various domains, including energy storage devices and wearable sensors, owing to its elementary fabrication process. Electrodes based on LIG have a more porous structure and superior conductivity. Despite this, supercapacitors continue to struggle with high capacitance issues as a result of the porous structure of laser-induced graphene electrode materials, which restricts conductivity and is inadequate for ion transport. Therefore, to improve electrochemical performance, high-conductivity composites should be used to increase the conductivity of laser-induced graphene. As a result, Fu et al. designed the self-assembled MXene-GO composite on polyamide film-enhanced LIG (LIG-C) to investigate electrochemical energy storage in the H₃PO₄-PVA electrolyte [118]. They also studied LIG on bare polyamide (LIG-PI) surfaces for comparison. Accordingly, the electrodes based on LIG-PI and LIG-C exhibited high capacitance retention of 98.75% and 102.4%, respectively, over 1000 cycles, indicating excellent stability. They also tested that the LIG-C film electrode was capable of showing a high capacitance retention of 98.9% even after 6 months. The MXene-GO uniformly drop-coated polyamide film could be the reason for its chemical stability due to its high resistance to chemicals and high structural stability. They also use LIG technology to shape the film electrode, which has a graphene structure that is made up of very tightly stacked layers with porous structures that can effectively interact with many electrolyte ions. During the cycle test, the film structure may not be involved in volume expansion and contraction, thus achieving its high cycle stability. In contrast, the resistance values of these electrode materials are significantly higher than those of other GO composites. As a result, LIG and LIG-C have respective R_s values of 242 and 182 Ω. These high resistivity values may be due to the polyamide film’s insulating properties, which could make MXene and GO less conductive. Consequently, the overall conductivity of the film is very low. However, LIG-C film has a lower resistivity than LIG-PI film. This might be because laser-induced graphene technology creates unique atomic structures of graphene on MXene-GO-polyamide film, which makes it possible for it to have low resistivity. Despite the use of LIG, there are still challenges to obtaining a low resistivity for the films, which may affect their electrochemical performance. Additionally, the LED lights were illuminated by the series-connected optimized, integrated device operating within the voltage range of 0.8–2.4 V. Similarly, utilizing CO₂ laser-induced patterning, a self-assembled MXene-GO electrode was fabricated and investigated in H₃PO₄-PVA electrolyte [103]. In contrast to MXene, the MXene-GO electrode exhibited elevated integral CV curves, which resulted in correspondingly lower C_s for MXene. The capacitance retention of this composite electrode remains constant at 92.6% for 1000 cycles. The elevated capacitance retention observed in the MXene-GO can be attributed to the efficient ion transport facilitated by the GO composite. In addition, it may be because the MXene-GO electrode has a highly uniform film coating, is structurally and chemically stable, has conductivity, has many active sites, and has channels for moving high ions. Moreover, an MXene-GO (planar 3) supercapacitor device was devised, demonstrating exceptional capacitance retention (94.8%) throughout 1000 cycles. The prepared MXene-GO film proved capable of maintaining high cycle stability in device applications, which could be the reason for uniform film morphology, structural stability, and chemical stability. The results of this investigation demonstrated that an MXene-GO composite featuring optimized mass ratios exhibited superior ion transportation and higher C_s compared to bare MXene. The LIG or other laser-induced method for etching the film electrodes for supercapacitors may be used in flexible or other electronic applications. Better electrochemical performances, however, need to be optimized in terms of the material ratio, film morphology, and film structure to increase porosity and conductivity.

In 1.0 M H₂SO₄, PC behavior was observed on CTAB-functionalized GO@polyaminoanthraquinone-MXene (CGO/PDAAQ-MXene) film [117]. According to Figure 9a, the CV curves of MXene and CGO/PDAAQ films were substantially identical, indicating that they exhibited excellent capacitance properties. However, CGO/PDAAQ produced a slightly larger curve, indicating that it may function better than MXene. On the other hand, the strong synergistic impact of CGO/PDAAQ-MXene might be a greater electrochemical performance with quite a larger CV integral area than MXene and CGO/PDAAQ-MXene. Consequently, the composite electrode’s C_s could be higher than that of MXene and CGO/PDAAQ. The authors claimed that the elevated capacitance was a result of the substantial electron transfer facilitated by benzoquinone in PDAAQ, followed by the MXene contribution. At 1 A g⁻¹, the C_s of CGO/PDAAQ-MXene was determined to be 346 F g⁻¹, while CGO/PDAAQ exhibited 183 F g⁻¹. The high C_s are also due to the morphology of the composite film, which shows a high number of nanotubes (PDAAQ) with sheets (CGO) on MXene film. Rough surfaces may have highly active sites and high porosity, allowing for high electrolyte ion contact and shorter ion paths for faster ion transportation for high energy storage. The C_s may be contingent on the electrodes’ conductivity. Nevertheless, the resistance (R_ct) of the CGO/PDAAQ-MXene (1.15 Ω) was higher than that of MXene (0.89 Ω) in this study. The poor conductivity of the composite film might be due to a difference in the electronic structure of the materials that were mixed to produce a composite form, resulting in conductivity variations. Figure 9b depicts the cycle stability profile of a composite film electrode, illustrating capacitance retention (%) and coulombic efficiency vs. cycle numbers. It shows that increasing the cycle numbers from 0 to 5000 steadily decreases capacitance retention while maintaining coulombic efficiency. Accordingly, CGO/PDAAQ-MXene demonstrated exceptional cycle stability, retaining 83% of its capacitance and 100% of its coulombic efficiency after completing 5000 cycles. The chemical and structural stability of CTAB-modified GO functionalized with conducting polymer (PDAAQ) decorated on MXene structures may be attributed to its high cycle stability. An additional asymmetric device (CGO/PDAAQ-MXene//rGO) was utilized for electrochemical performance at 1.8 V. At 0.5 A g⁻¹, the device’s C_s was measured at 114 F g⁻¹, and it maintained capacitance retention of 70.3% over 10,000 cycles. The Ragone plots of ED and PD values were calculated from the GCD curves for this asymmetric device and are shown in Figure 9c. The CGO/PDAAQ-MXene/rGO comparison with reported devices shows these plots. As the device’s ED decreased, there was an increase in the PD. Generally, supercapacitors have such high PD that they can release energy rapidly, but due to their low ED, they store only low energy. Therefore, the research could focus on improving the ED of the supercapacitor by making an effective electrode design and structure for more energy storage to use fast charging and discharging. The device observed a low PD of 404 W kg⁻¹ and a high ED of 41 Wh kg⁻¹, and this device shows higher values compared to other reported devices. Furthermore, Figure 9d depicts the LED function achieved by connecting two devices in series, which may light up a series of LED bulbs. Therefore, based on the C_s, cycle stability, and device application, this CGO/PDAAQ-MXene film may be a suitable candidate for high-energy-storage supercapacitor applications.

The electrochemical use of MXene has been severely hampered by several issues, including restacking due to weak hydrogen bonding and van der Waals forces, as well as oxidation weaknesses in colloidal solutions [119,123,124]. Furthermore, tungsten sulfide (WS₂) exhibits inadequate ion transport, low electrical conductivity, and deficient C_s. Carbon-based materials, characterized by high electrical conductivity, excellent active sites, and high structural stability, ought to be incorporated into MXene and WS₂ composites to mitigate these concerns and attain desirable electrochemical properties. Therefore, Hussain et al. synthesized a WS₂-MXene hybrid composite (WS₂@MXene/GO) containing GO for an electrochemical investigation in 1.0 M KOH [119]. This composite material exhibited battery-like activity, as determined by CV analysis. The C_s of the WS₂@MXene/GO electrode (1111 F g⁻¹) was found to be higher than that of MXene (270 F g⁻¹), GO (138 F g⁻¹), WS₂ (229 F g⁻¹), and WS₂@MXene (852 F g⁻¹) at a CD of 2 A g⁻¹. Comparatively, WS₂@MXene/GO exhibited the highest electrochemical performance because GO functioned exceptionally well with the WS₂@MXene composite. Furthermore, the composite material exhibits a high conductivity due to sufficient exposure to electroactive sites, elevated ion diffusion, and high porosity. Although the composite material composition (WS₂@MXene/GO) lacked distinct morphology, exhibited low crystallinity, and had a small surface area (12.9 m² g⁻¹), it achieved a high C_s, which could potentially account for its low resistance, highly active sites, and structural stability. Even though the CD increased from 2 to 20 A g⁻¹, the C_s remained high at 601 F g⁻¹. Additionally, the electrode’s conductivity may be quite high as a result of the comparatively low resistance values (R_s and R_ct, 2.3 and 0.41 Ω, respectively). The cycle stability profile of WS₂@MXene/GO is illustrated in Figure 9e. From 0 to 5000 cycles, the capacitance retention (%) decreased very slightly. Accordingly, the WS₂@MXene/GO composite maintained 97.15% of its capacitance over 5000 cycles. The material composition of the composite contributed to its long-cycling stability and facilitated rapid ion transport in the electrolyte. Figure 9f depicts a schematic representation of the structural arrangement of an asymmetric device with anode and cathode electrodes of WS₂@MXene/GO and activated carbon (AC), respectively, sandwiched by a KOH-dipped Whatman filter paper as a separator. An asymmetric device (WS₂@MXene/GO//AC) was built and examined in 1.0 M KOH, showing a C_s of 320 F g⁻¹ at a CD of 2 A g⁻¹. Furthermore, this asymmetric device achieved high ED and PD values of 95 Wh kg⁻¹ and 1000.4 W kg⁻¹, respectively. The device also demonstrated high cycle stability, with a capacitance retention of 93.1% after 15,000 cycles. For practical application, Figure 9g depicts the initial and after 20 min of red and green LED illumination by five serially connected WS₂@MXene/GO//AC devices. This serially connected asymmetric device was tested to light up the red and green 4 V LEDs for 20 min after charging for 40 s. Finally, the GO composite with MXene-based electrode films or materials demonstrated very high cycle stability owing to their composition and structural stability during charging–discharging cycles, as well as high ED. Because the film electrode is coated over insulated materials like glass, polyamide, and other low-conductivity polymer films, its conductivity is lower than that of powder-formed electrodes. As a result, these substrates may influence the conductivity of active materials on them. Thus, the powder form of electrode materials exhibits strong conductivity without the assistance of base substrates. Thus, the powder electrode material form of GO-MXene may provide higher performance than film-type GO-MXene-based electrodes.

3.8. GO-Bacteria

The robust cell walls of bacteria enable them to endure a wide variety of hostile environments. They are renewable resources that are inexpensive, abundantly available, and environmentally friendly. Consequently, microorganism-based materials have gained prominence as appealing biological substances for supercapacitor applications in recent times. Also, a bacterial cell culture derived from Bacillus subtilis was employed as the N₂ source for the electrode material [125]. In light of the merits associated with this biological material, Verma et al. investigated the electrochemical properties of a Bacillus subtilis@GO composite (BGO) [120]. Figure 9h depicts the CV curves of GO and BGO, indicating that the rectangular form area of B@GO was greater than GO. As a result, BGO might have a stronger electrochemical performance than GO. Thus, the C_s of GO were observed to be improved after its composite with Bacillus subtilis. As a result, B@GO (111 F g⁻¹) exhibited higher C_s than GO (38.1 F g⁻¹). High capacitance values of bacteria composites with GO may be attributable to the presence of bacterial heteroatoms (N), which contribute to surface area and sufficient active sites. However, the weakened sheet shape of BGO continues to offer a highly active site for electrochemical reactions. Furthermore, Figure 9i depicts Nyquist plots of GO and B@GO, with straight and sloping lines in the high- and low-frequency ranges. With a small slope and vertical line, the BGO electrode may have capacitive performance and low charge-transfer resistance. With a steeper slope and a less vertical line, GO electrodes demonstrate poor capacitive performance and high charge-transfer resistance. Accordingly, it was observed that the charge-transfer resistance of B@GO (29.2 Ω) was comparatively lower than that of GO (47.6 Ω), indicating that B@GO potentially possesses a good conductivity. As a result, the bacterial solution may have strong conductivity properties that may boost the conductivity of GO. Figure 9j displays the cycle stability profile of B@GO, showing that capacitance retention (%) steadily declines as the number of cycles increases from 0 to 5000. According to the cycle stability profile, B@GO demonstrated significant capacitance retention (86.9%) over 5000 cycles at a CD of 1 A g⁻¹. ED and PD were found to be lower than expected, at 15.4 Wh kg⁻¹ and 249 W kg⁻¹, respectively, at a CD of 1 A g⁻¹, through this electrode. Figure 9k shows the Ragone plots of the B@GO electrode at different CDs for plotting the ED against PD. The ED of B@GO gradually decreased with increasing CDs (1 to 20 A g⁻¹) at the corresponding PD, whereas GO shows very low ED compared to B@GO. Conversely, GO only showed an ED of 5.2 Wh kg⁻¹ at a PD of 248 W kg⁻¹. On the other hand, B@GO revealed 15.42 Wh kg⁻¹ at a PD and CD of 249 W kg⁻¹ and 1 A g⁻¹, respectively. For practical applications, the B@GO has not been utilized to construct any devices for electrochemical investigations. As a result, the B@GO electrodes performed better than the GO electrodes due to their typical compositional properties, surface affinity by bacteria functionalization, improved conductivity, structural stability, and highly active sites. Also, the heteroatom doping by bacteria on GO surfaces helps with electrochemical kinetics.

3.9. Discussion

The electrochemical results of GO and its different composites were studied using three- and two-electrode systems, and the results are shown in

Table 3. Results of GO and its composites in three and two-electrode systems.

	Three-Electrode System				Two-Electrode System
	CV	GCD	EIS	Cycles
Electrodes	Cs		Rs and Rct	CS and CR	Device	Cs	ED	PD	CS and CR	EL	Ref.
GO-Fluorine
FGO	514.0	276.0	0.58; 0.71	-	FGO//FGO (Symmetry)	-	25.8	1280.0	20,000; 99.0%	1.0 M KOH	[77]
GO-metal oxides
NCWO4/f-MWCNT/GO	-	1166.6	0.67; 1.2	5000; 95.6%	NCWO4/f-MWCNT/GO//AC (Asymmetry)	266.2	83.3	703.1	5000; 87.0%	6.0 M KOH	[90]
AC/GO/TiO2-Zn	329.9	1491.6	192.2; -	-	AC/GO/TiO2-Zn//AC/GO/TiO2-Zn (Symmetry)	-	99.0	3572.0	10,000; 61.8%	3.0 M KOH	[79]
Bi2O3/GO	-	1029.0	0.91; -	3000; 80%	-	-	-	-	-	6.0 M KOH	[88]
MnFe2O4/GO	-	298.0	2.16; 2.45	500; 92.0%	-	-	-	-	-	6.0 M KOH	[91]
AC/TiO2/GO	219.3	617.0	1.87; 2.68	1000; 80.1%	-	-	-	-	-	2.0 M H2SO4	[80]
NGO/CuFe2O4	-	374.2	6.85; 1.1	2000; 82%	NGO/CuFe2O4//AC (Asymmetry)	89.1	35.7	883.0	500; 78.6%	1.0 M H2SO4	[85]
Cu2O-CoO/GO	-	723.0	0.1; -	-	Cu2O-CoO/GO//GO (Asymmetry)	125.0	44.1	794.0	10,000; 89.3%	6.0 M KOH	[95]
GO/CNT/COF	544.9	175.0	2.25; 11.34	8000; 84.7%	GO/CNT/COF//GO/CNT/COF (Symmetry)	-	24.3	248.9	-	1.0 M H2SO4	[97]
GO@ Fe3O4-IL@W	-	332.4	5.47; 9.35 × 10−5	10,000;91.3%	-	-	7.3	40.0	10,000; 91.3%	1.0 M H2SO4	[58]
NMGO	-	402.0	-; 34.0	14,000; 93%	NMGO//MWCNT (Asymmetry)	90.0	28.0	750.0	6000; 88%	1.0 M KOH or Na2SO4	[83]
10%GO/WO3	668.0	738.0	1.05; -	7000; 88.0%	GO/WO3//GO (Asymmetry)	213.0	25.0	1000.0	3000; 87%	2.0 M KOH	[96]
GO-metal sulfides
MnS-La2S3/GO	-	890.0	-	4000; 89.0%	MnS-La2S3/GO//MnS-La2S3/GO (Symmetry)	151.0	54.2	1300.0	10,000; 92.5%	1.0 M Na2SO4 or PVA-Na2SO4	[100]
NiS/CoS/GO	-	1114.0	-	8000; 80.0%	-	-	674.1	8089.4	-	3.0 M KOH	[98]
GO-Transition metal chalcogenides
GO-MnSSe	300.0	603.0	-	9000; 67.0%	-	-	-	-	-	1.0 M KOH	[104]
GO-Layered double hydroxides
Co2Ni1-GO	-	3317.5	0.88; 0.029	10,000; 91.8%	Co2Ni1-GO//AC (Asymmetry)	328.7	94.7	750.0	10,000; 91.3%	3.0 M KOH	[106]
GO-organic materials
SGZ	-	261.0	-; 14.7	1000; 102%	-	-	-	-	-	6.0 M KOH	[114]
Nd-MOFs/GO	677.6	633.5	0.8; -	4000; 88.7%	-	-	-	-	-	3.0 M KOH	[113]
Ni-BTC MOF/GO 2	-	1199.0	0.79; 7.0	5000; 84.4%	Ni-BTC MOF/GO 2//AC (Asymmetry)	-	42.8	800.0	10,000; 70%	3.0 M KOH	[102]
ABQA-GO/CP	203.2	235.5	-	-	ABQA-GO/CP//ABQA-GO/CP (Symmetry)	134.1	32.8	1256.0	5000; 106%	1.0 M H2SO4	[101]
GO-MXene
LIG-C film	-	-	182; -	1000; 102.4%	LIG-C-based supercapacitor	-	-	-	1000; 103%	H3PO4-PVA	[118]
MXene-GO film	-	-	8.5; -	1000; 92.6%	Planar supercapacitor	-	-	-	1000; 94.8%	H3PO4-PVA	[103]
CGO/PDAAQ-MXene film	-	346.0	-; 1.15	5000; 83%	CGO/PDAAQ-MXene//rGO (Asymmetry)	114.0	41.0	404.0	10,000; 70.3%	1.0 M H2SO4	[117]
WS2@MXene/GO	-	1111.0	2.3; 0.41	5000; 97.1%	WS2@MXene/GO// AC (Asymmetry)	320.0	95.0	1000.4	15,000; 93.1%	1.0 M KOH	[119]
GO-Bacteria
BGO	-	111.0	-; 29.2	5000; 86.9%	-	-	-	-	-	0.5 M Na2SO4	[120]

C_s = specific capacitance (F g⁻¹); CV = cyclic voltammetry; GCD =galvanostatic charge–discharge; R_s = solution resistance (Ω); R_ct = charge-transfer resistance (Ω); ED = energy density (Wh kg⁻¹); PD = power density (W kg⁻¹); CS&CR = cycle stability (cycles) and capacitance retention (%); EL = electrolyte.

. Using three electrodes, it is evident that GO/CNT/COF, 10%GO/WO₃, and Nd-MOFs/GO provide better CV results when compared to other composites, with high C_s of 544.9, 688.0, and 677.0 F g⁻¹, respectively. The reason for this might be that certain electrode material compositions have a highly active site, porosity, capacitive and redox activity, and electrolyte ion permeability. The AC/TiO₂/GO and ABQA-GO/CP had lower C_s than the others because they may have slow-moving charge-transfer ions, fewer electrochemical sites, a lower redox property, etc. In addition, GCD analysis showed that some other electrodes worked better on high C_s. These included NCWO₄/f-MWCNT/GO (1166.6 F g⁻¹), AC/GO/TiO₂-Zn (1491.0 F g⁻¹), Bi₂O₃/GO (1029.0 F g⁻¹), Co₂Ni₁-GO (3317.5 F g⁻¹), Ni-BTC MOF/GO 2 (1199.0 F g⁻¹), and WS₂@MXene/GO (1111.0 F g⁻¹). High-conductivity electrodes, high charge-transfer ions, high electrochemical properties, and highly active sites that lead to high conductivity and allow the high electrochemical performances and quick ion transport from electrolyte to electrode surfaces might be the cause of the high C_s. Generally, C_s values of electrode materials vary with scan rate during CV. As a result, at high scan rates, the sluggish migration of electrolyte ions toward the electrode’s active sites induces a drop in C_s. Low scan rates, on the other hand, provide for easy passage through electro-active regions as well as rapid diffusion of electrolyte ions, resulting in high C_s. The same may be observed for C_s at high and low current densities during GCD. Low C_s at higher current density might be due to inadequate time for electrolyte ions to reach electro-active sites, resulting in low C_s, while high C_s at low-current density indicate enough time for electrolyte ions to acquire electro-active sites and high C_s. Because studies use different scan rates and current densities during CV and GCD analysis, respectively, the C_s values compared to electrode materials seem quite strange. However, the electrode materials that may exhibit high C_s at fast scan rates and high-current density may have significant potential for energy storage.

Fast charge/electron transfer made possible by the conductivity of the electrode materials is a key characteristic linked to high energy storage. The electrical structure of the electrode materials affects the high and low conductivities. In general, the zero-bandgap electronic structure of carbon-based materials results in excellent conductivity. The conductivity is also affected by the electro materials’ impurities. Thus, the material used to synthesize or fabricate an electrode should be of high purity. To obtain better electrochemical performance, it is preferable to develop electrode material composites with high conductivity that include carbon-based materials. Furthermore, film-type electrodes (CGO/PDAAQ-MXene film) exhibit higher resistance in comparison to powder-based electrode materials (FGO, NCWO₄/f-MWCNT/GO, 10%GO/WO₃, Bi₂O₃/GO, Cu₂O-CoO/GO, Co₂Ni1-GO, Nd-MOFs/GO and WS₂@MXene/GO). This is attributed to the formation of coatings on insulating substrate materials, including glass, polyamide, and other low-conductivity polymer materials. Consequently, it may be advantageous to enhance the total conductivity through the use of the coated substrate’s conductivity characteristic. On the other hand, film-type electrode materials are more expensive to fabricate than powder-based electrode materials. However, the film-type electrode might be practical for flexible electronic applications. So, films with high conductivity and low cost should be the core of research.

The cycle test is useful for studying the electrode materials to determine how much initial capacitance they maintain with increasing cycle numbers at a particular current density. The capacitance (%) of the electrode materials is almost equal to 100%, while increasing the cycle numbers might mean those electrode materials are highly structurally stable during charging–discharging, and the energy storage is accomplished over the physical adsorption and desorption of ions on the electrode and electrolyte interface, which can be observed in electrode materials such as NCWO₄/f-MWCNT/GO, MnFe₂O₄/GO, GO@ Fe₃O₄-IL@W, NMGO, Co₂Ni₁-GO, WS₂@MXene/GO, and MXene-GO film. In another case, the capacitance retention (%) increases by more than 100% as the cycle numbers increase due to self-activation; however, after the self-activation step, the capacitance retention rate gets to the initial capacitance value or is slightly less than 100%, as seen in SGZ, ABQA-GO/CP, and LIG-C films. On the other hand, the capacitance (%) of electrode materials (Bi₂O₃/GO, NGO/CuFe₂O₄, NiS/CoS/GO, and GO-MnSSe) progressively decreases as the cycle number increases owing to modest structural change but no deterioration; these materials are relatively stable when charging and discharging. It might also be the cause of charge transfer during chemical reactions and phase transformations, as well as volume expansion and contraction of electrode materials throughout the cycling process. Therefore, the electrode materials synthesized should have high structural stability and chemical resistance. Although the structure and physio-chemical properties of electrodes have an impact on cyclic stability, EDLC-based electrodes exhibit greater cyclic stability than PC or battery behavior-type electrodes, and test conditions also play a role [126]. In this review, the performance of GO-metal oxide, GO-MXene, and GO-Layered double hydroxides electrode materials in the three-electrode system was found effective.

The PD and ED results of the device might be determined by its overall electrochemical performance. However, in general, the PD of the supercapacitor is high; consequently, it may be more prudent to concentrate on improving the device’s ED, given that its energy storage is determined by its ED values. Higher and lower energy storage on the device corresponds to higher and lower ED, respectively. Consequently, devices that have higher ED can be found, including AC/GO/TiO₂-Zn//AC/GO/TiO₂-Zn, Co₂Ni₁-GO//AC, WS₂@MXene/GO//AC, NCWO₄/f-MWCNT/GO//AC, and MnS-La₂S₃/GO//MnS-La₂S₃/GO. The electrolyte is an essential part of supercapacitors that transfers and balances energies between the two electrodes. The choice of electrolyte is critical to the safety and performance of supercapacitor devices since most electrode materials are stable in base electrolytes rather than acids. Aqueous electrolytes have higher conductivity than acids and ionic electrolytes, which helps to reduce resistance and improve device performance. One potential explanation for the solution’s elevated conductivity, leading to a greater abundance of electrolyte ions, is the potential for increased energy and power density. Because of this, good conductivity and a lot of electrolyte solutions could make the devices work better overall. The ternary composite structure-based devices were also found to be more stable over a wide range of cycles than other structured composites. The additive effect of the ternary-type compound may be what makes the electrolyte solution so good at conducting electricity and moving charges around. The overall performance of the three-electrode system was better than the two-electrode system. This is because the three-electrode system uses a single working electrode, which allows for high conductivity and fast charge transfer. However, the precise measurement of energy storage may be considered for the two-electrode system’s use. High cycle stability of the devices for the composition of GO with fluorine, metal sulfides, layered double hydroxides, and MXene was shown to be good due to high structural stability and faster ion transportation. According to device ED findings, GO-metal oxides, GO-layered double hydroxides, and GO-MXene performed better than other types of devices in the two-electrode setup due to their good C_s and high cycle performance.

4. Simulation Studies

The electrode materials’ electrical structure, which can influence the energy storage performance, can be obtained by simulation study. The design of materials for supercapacitors that have higher ED and PD may now be assisted by density functional theory (DFT) simulation studies. DFT simulations, often used to study the physics and chemistry of solid states, provide a computationally efficient method for calculating structural, electrical, and thermodynamic aspects from quantum mechanics. The basis of this simulation is the solution of the system’s body’s Schrodinger equation using different wave functions. As can be shown in the previous reports, there are two DFT theorems for studying the system’s energy states and the electron density using the Kohn–Sham equation [127,128]. Additionally, the widely used and accessible density functional computation to examine the electron density location is the local density approximation (LDA) [129]. Later, the accuracy of LDA was enhanced by the use of the generalized gradient approximation (GGA) functional theory. In solids computing, the most often used GGA functional is the Perdew–Burke–Ernzerhof (PBE) functional [130]. The self-interaction error that occurs when electrons are randomly transferred between many positions is not eliminated by GGA or LDA, which may lead to multiple calculation mistakes in the redox energy. The Hubbard-U correction approach was used to resolve this problem. The exchange (J) and Coulomb energies (U) may be used to describe the on-site Coulomb interactions. Furthermore, in comparison to the Hubbard-U correction technique, the hybrid DFT methodology is employed to explore the independence of structure and more precise predictions for electrode attributes. The Hartree–Fock (HF) exchange technique was introduced to the hybrid DFT to correct the electron self-interaction mistakes and avoid the problem of excessive delocalized states in the local approximation. The Heyd–Scuseria–Ernzerhof (HSE06) functional is the only one that combines fractional PBE–GGA exchange and exact partial exchange to complete PBE correlation. It is also used to forecast precise poor gap values. As a result, DFT simulations provide fundamental properties of materials, including charge transport, lattice stability, and electronic structure.

To predict the physical properties of the electrode material, such as crystal structure, electronic interaction, ion diffusivity, and phase transition, as well as the atomic-level structures of the electrode material, DFT calculations are used to study the interactions between nuclei atoms and electrons and the bonding between atoms. Software like Quantum Espresso (version 6.3), VASP (version 6.4.1), DMol³ (version 7.0), Gaussian 16 (Revision C.01), CASTEP (version 23.1.1), WIEN2K (version 23.2), Turbomole (version 7.7), etc., are used for DFT calculations. An essential factor in controlling an electrode material’s electrical characteristics is its electronic density of states (DOS). Accurate band structure and electronic states were studied using quantum mechanical techniques, and the DOS investigations immediately provide insight into a material’s conductivity, magnetic properties, bonding mechanism, and doping impact [127]. Partial density of states, or PDOS, is a study of orbital hybridization and bonding process. It describes the contribution of a particular atom’s orbital to the overall DOS of the corresponding material. Moreover, the increase or decrease in the number of states at the Fermi level of a particular orbital determines the transfer of charge.

A thorough understanding of the electrochemical performance of materials may be gained by theoretical research on the electrode–electrolyte kinetic behavior by estimating various parameters using DFT. Consequently, the ions’ mobility, which contributes to improving the charge transfer in the supercapacitor, may be developed by a diffusion energy barrier of ions that has little impact on the electrode surface. Therefore, a better way to understand the mobility of electrolytic ions and maybe value their charge storage ability is to forecast their diffusion energy using simulation tools. Additionally, Luryi introduced the concept of quantum capacitance (QC) for two-dimensional electron gas in 1988 [131]. The differential in electric charge that relates to the electrochemical potential is also known as chemical capacitance. QC predominates in 2D systems-based materials (carbon) used as supercapacitor electrode materials [132]. Measuring the energy storage capability of materials requires an understanding of the relationship between electrode voltage and quantum capacitance. It includes a kinetic constant by the DOS as well as an electrostatic component that analyses the interactions between electrons and their connections. While the molarity of the solution affects the electrochemical capacitance, the inner quantum structure of the electrode determines the QC [119]. The equation for the quantum capacitance of electrode materials may be found in the previous study [133,134]. Furthermore, the charge storage capability of a material may be theoretically assessed by determining the voltage induced by ions [135].

Karmakar et al. recently conducted a study on the structure and electrical characteristics of NiGa₂O₄/rGO using VASP combined with a PAW-GGA exchange-correlation functional code [132]. The study revealed that the weak van der Waals forces were responsible for the interactions between the (311) surface of NiGa₂O₄ and rGO. Figure 10a depicts the DFT-optimized structure of (311) surface rGO-supported NiGa₂O₄. According to this figure, the distance between NiGa₂O₄ and rGO was measured to be 0.33 nm. The electronic state of NiGa₂O₄/rGO was determined to be greater than that of NiGa₂O₄ around the Fermi level in the DOS analysis. Composite with rGO may enhance the conductivity of NiGa₂O₄ by improving the electron states around the Fermi level. Consequently, increased conductivity results in larger C_s of the material. In the partial density of states (PDOS), the electronic states associated with the carbon 2p orbital are found to be more pronounced around the Fermi level in the hybrid structure compared to the unmodified rGO. This enhancement is attributed to the substantial transfer of electrons from NiGa₂O₄ to rGO. The charge transfer occurred due to the orbital interactions involving the 3d orbital of Ni, as well as the 4s and 4p orbitals of Ga, with the 2p orbital of C. In addition, Bader charge analysis revealed that a charge of 0.072e was transferred to rGO. The quantum capacitance (QC) is higher for the low-dimensional system with a hybrid structure compared to the bare electrode materials when considering a broad range of potential, as shown in Figure 10b. Hence, the orbital interactions and charge transfer from NiGa₂O₄ to rGO significantly contribute to enhancing the electrochemical performances of this hybrid material.

Another work utilized SIESTA version 4 to analyze the DOS and QC of graphene treated with epoxide–hydroxyl and N-doped graphene [139]. The electrical conductivity of both functionalized graphene may be higher than that of bare graphene because electronic states are very near to the Fermi level, which intensely modifies its DOS owing to the presence of functional groups on the graphene’s surface. Furthermore, QC was computed from the DOS, and greater QC values were seen for both functionalized graphene and naked graphene. The simulation studies determined that N-doped graphene (CQ = 0) and epoxide–hydroxyl-treated graphene (CQ = 34.6 μF cm⁻²) could potentially serve as positive and negative electrodes, respectively, in an asymmetric supercapacitor, based on their respective CQ values. Islam et al. recently employed DFT simulation to examine the electronic structure of single-layer (GO/rGO) and double-layer (rGO-GO, rGO-rGO, GO-GO, and GO-rGO) screen-printed electrodes by analyzing their highest occupied molecular orbital (HOMO) and highest occupied molecular orbital (LUMO) energy gaps [136]. The energy gap of HOMO and LUMO was discovered to be greater in a double-layer screen-primed electrode than in a single layer, as shown in Figure 10c. Additionally, the energy gap of HOMO and LUMO was observed to be greater in hydrated electrodes (containing K⁺ and Cl⁻ ions) compared to non-hydrated electrodes, as shown in Figure 10d. This suggests that hydrated electrodes may possess a more stable electronic structure.

DFT simulations were recently used to investigate the performance and calculation of OH ios adsorption energy and its adsorption on CoS₂@C sites for the CoS₂@gC/rGO electrode material [137]. Before and after OH⁻ ion adsorption on the surfaces of CoS₂(200) and CoS₂(200)@C are depicted in Figure 10e. The adsorption energy of -OH ions on the Co site of the CoS₂(200) surface was observed to be higher in comparison to the CoS₂(200)@C surface. Moreover, according to the Bader effective charge values, CoS₂(200) (−0.079 eV) acquired negative charges as a result of a greater number of electrons being adsorbed, whereas CoS₂(200)@C exhibited positive Bader effective charges (0.029 eV) as a consequence of electrons being neglected on Co atoms. Thus, OH⁻ adsorption towards CoS₂(200)@C may be higher than that of the CoS₂(200) slab surface. Additionally, the material’s total density of states (TDOS) was investigated using PBE + U_eff. Accordingly, Figure 10f depicts the total TDOS of each unit cell and Co 3d contributions in CoS₂ and CoS by PBE + U_eff functional. The TDOS of CoS₂ was observed to be higher than that of CoS, possibly because the energy peaks near the Fermi level are higher in CoS₂. Consequently, the electronic conductivity of CoS₂ may be higher. High electronic conductivity and high electron transfer in the electrode material will, therefore, permit high electrochemical performance in supercapacitor applications.

On the plane of (001), a DFT first-principles calculation was performed on the electrode materials WS₂-embedded MXene/GO [119]. The bond lengths of GO (C–C (1.42 Å) and C–O (1.46 Å)) were observed to be comparatively shorter than MXene ((Ti–C (2.03 Å)) and WS₂ (W–S (2.42 Å)). This discrepancy in bond length was attributed to the covalent bond between WS₂ and MXene. Additionally, Figure 10g represents the PDOS results, which demonstrate that the high conductivity of WS₂@MXene/GO is due to the high overlap between the valence and conduction bands near the Fermi level, which confirms the metallic character of the electrodes. Furthermore, Figure 10h shows quantum capacitance (QC) values for the WS₂@MXene/GO structure (1024.3 μF cm⁻²) were significantly higher than those for WS₂@MXene (931.6 μF cm⁻²), owing to the dominance of Ti d-states at the Fermi level. However, it produces an exceptionally high QC value of 1404.4 μF cm⁻² while operating at a negative potential voltage of −0.16 V. WS₂@MXene/GO may be a promising electrode material to boost the electrochemical performance in supercapacitors because of its high energy levels and good QC. Similarly, the van der Waals interaction between CoSe₂ and rGO resulted in zero band gap values for CoSe₂ and CoSe₂/rGO was observed by DOS calculation, and the DOS peak intensity was higher for CoSe₂-rGO near the Fermi level than for CoSe₂, suggesting that both are metallic and may have high conductivity [140]. The difference in charge density indicates that both positive and negative charges are collected near the CoSe₂ and rGO surfaces. Consequently, as compared to rGO, a higher amount of OH⁻ ion could be adsorbed in the electrolyte using CoSe₂ surfaces. Therefore, compared to CoSe₂-rGO (−3.0 eV), the adsorption energy of the OH⁻ ion on CoSe₂ (−2.6 eV) was lower. Moreover, for CoSe₂-rGO and CoSe₂, the distance between the OH-ion and the Co site was 1.999 Å and 2.035 Å, respectively. After wrapping CoSe₂ with rGO, these DFT analyses demonstrate that OH⁻is strongly adsorbed on the CoSe₂ surface. Because of this beneficial effect, CoSe₂-rGO may be a suitable electrode material to improve electrochemical performances. This is because of its metallic nature, higher adsorption energy, and short distances to adsorb OH⁻ ions.

Yang et al. investigated chrysin-modified rGO (CHY-rGO) for use in supercapacitors [141]. To do this, they used DFT modeling to study the adsorption of chrysin on GO. They found that the π–π parallel approach is very stable when chrysin is combined with rGO and that 1.18 eV is the maximum adsorption energy. It was discovered that there was a significant amount of charge (electrons) accumulating on the CHY-rGO surfaces as a result of the oxygen atom on the chrysin surfaces, which readily causes electrostatic interaction between rGO and chrysin. The CHY-rGO electrode’s strong charge buildup and high adsorption energy may thus contribute to its high C_s, ED, PD, and cycle stability. Additionally, CHY-rGO’s DOS peak intensity was greater than that of bare rGO, suggesting that CHY-rGO may be a high capacitance electrode. Furthermore, the DOS and PDOS of carbon (C) on CHY-rGO overlap (π-π stacking interactions) close to the Fermi level, suggesting that there exist forces between the CHY and rGO’s C-2p orbitals in the CHY-rGO. It may be an electrode material with high conductivity due to the overlap around the Fermi level. DFT was used to examine the combinations of GO-MnO₂-CoNi for the supercapacitor application [138]. As a result, the positive charges of CoNi and the negative charges of GO and MnO₂ electrostatically attracted to one another. Figure 10i illustrates the study of the surface model of electrodes using electrostatic potential mapping. Accordingly, compared to GO-MnO₂ (0.02693 a.u.) and MnO₂-CoNi-LDH (0.01842 a.u.), the energy level difference between HOMO and LUMO of GO-MnO₂-CoNi (0.01837 a.u.) was less, indicating that the combined three electrodes had high conductivity. According to DFT research, GO, MnO₂, and CoNi-LDH are linked by electrostatic interactions, proving that this heterostructure may be constructed and used in supercapacitors.

A supercapacitor was recently fabricated using highly oxidized GO, and the DFT analysis was conducted to determine how this GO was formed from a graphite sheet [142]. Accordingly, a bilayer graphite sheet revealed that the surface of the material had negative charges and that the C-C, C-H, and interlayer bond lengths were, respectively, 1.588, 1.105, and 1.60 Å. Furthermore, there was a repulsion between negatively charged graphite and negatively charged acids as well as KMnO₄ due to the high electronegativities of H₂SO₄, H₃PO₄, and KMnO₄. On the other hand, graphite molecules could adsorb via an O (H₂SO₄), OH (H₃PO₄), or O&K (KMnO₄). The electronic energy of bilayer graphite is around 7708.251 Hartree, but bilayer graphite takes on an additional −72.061 Hartree upon adsorption with H₂SO₄, H₃PO₄, and KMnO₄. Upon adsorption of H₂SO₄ with graphite, the S–O bond lengths may increase from 1.567 to 1.628 Å, as confirmed by the HOMO pattern. The energy required for the formation of GO with a single O atom is −7830.079 Hartree. The presence of O and OH on the graphite leads to the formation of GO through the disruption of interlayer bonds.

Similarly, for supercapacitor use, guanidine-functionalized GO (G-GO) was studied through an amide process [143]. Also, the ease of designing electrodes (functionalization), their electronic states, the distance between layers, and the quality were all confirmed using the Quantum-ESPRESSO by DFT. At first, the carbon atoms of GO and dicyclohexylcarbodiimide react with each other, creating a double bond. Then, 4-dimethylaminopyridine was used to get rid of the dicyclohexylurea molecule by attacking the carbonyl group. Finally, it was predicted that guanidine would functionalize GO by attacking the carbonyl and substituting 4-dimethylaminopyridine in the structure. Because functionalization increases interlayer distance and prevents GO from restacking, it may be a useful strategy for GO synthesis. It was found that the interlayer gap between guanidine and GO was 2.17 Å. It was found that the QC value rose along with the DOS value increasing at the Fermi level and becoming positively charged electrodes as a result of electron loss after guanidine functionalization. Consequently, G-GO’s QC value (3572 F g⁻¹) was higher than G‘s (2290 F g⁻¹). Owing to the superior QC of G-GO electrode material, supercapacitor applications may be possible.

Supercapacitor electrode design is still beset with difficulties, such as finding stable, suitable architectures and persistent features. Therefore, efforts may be made to develop better designs for high-performance electrodes with unique features with the use of DFT modeling. In light of this, simulated studies based on DFT predictions were examined alongside recent reports on GO-based electrode materials for supercapacitors. To comprehend electrode design and electronic structure, electrode interaction, adsorption energies, charge density difference, interlayer distance, and quantum capacitance, the DFT simulation is very helpful. To identify innovative materials, DFT-assisted simulation may be used. Finding viable candidates with the capacity to have high energy storage, high electrical conductivity, and high stability is made easier with the use of a simulation-based computational analysis before the synthesis of electrode materials.

5. Conclusions and Future Perspectives

This review represents a recent study on synthesizing graphene oxide (GO) and its composites with diverse materials to develop electrode materials for supercapacitor applications. As a sign of recent progress in the field of GO material-based electrode material for supercapacitor uses, the need for energy storage devices has been brought up for discussion. The experimental conditions, processing time, and cost associated with the synthesis of a variety of materials, including fluorine, metal oxides, metal sulfide, metal chalcogenides, LDHs, organic material, MXene, biological material, and its composite with GO, were evaluated. It was found that metal oxides, metal sulfides, metal chalcogenides, and LDHs with GO composites are inexpensive and straightforward to produce. The present review outlines the diverse composites of GO electrode materials and electrolytes employed in electrochemical investigations for supercapacitors. To this degree, the scan rates, charging–discharging rates, resistances, and cyclic stability of a three-electrode setup are provided. To achieve Faradaic processes, GO composites containing additional 2D materials exhibit either pseudocapacitive or battery-like characteristics. The primary determinant of electrochemical performance is the choice of electrode materials featuring nanostructured surfaces; this should result in a reduction of the supercapacitor’s capital expenditure. Based on the current analysis, it is evident that GO incorporating electrode materials, including metal oxides, LDHs, MOFs, and MXene, results in exceptionally high supercapacitor performance. Compared to undoped GO materials, GO functioning with other substances exhibit high C_s. Thus, GO could be a viable material for improving the conductivity, C_s, energy and power density, cycle performances, and conductivity of other low-cost materials to enhance their electrochemical performance. Furthermore, it could serve as a strategy for fabricating inexpensive and high-energy storage devices.

Supercapacitor devices based on GO composites with metal oxides, sulfides, carbides, polymers, MOFs, MXene, biological materials, etc., should prioritize easy and cost-effective preparation procedures in light of prospects. Furthermore, GO nanosheets encounter extra difficulties, such as agglomerations and self-restacking, which hinder performance by decreasing surface area, redox sites, and ion transport. Composites of GO and other nanomaterials may improve the materials’ surface area and function as separators between the GO sheets, mitigating these problems and making them ideal for use as supercapacitors. Establishing pores through the introduction of organic or heteroatom dopants can serve as a viable strategy to promote accelerated ion diffusion in GO and other composite materials, thereby augmenting the overall activity. Also, more research should be done on the design features, and new ways of making GO and its hybrid materials work more effectively combined to improve total electrochemical performance. Concerning the electrochemical performance of a variety of GO-based composite electrode materials, effective electrodes for energy storage devices of the next generation should be developed.

To optimize the design of electrodes with particular capacitance, ion diffusion, and stability, DFT might be utilized to precisely tailor the electrode materials at the nanoscale level. It may be accomplished by investigating how supercapacitor performances are impacted by size, structure, shape, and functionalization. DFT may assist researchers studying energy storage in identifying effective electrode materials with high energy storage capacities. With DFT modeling, novel electrolytes, surface functionalization, or electrode hybrids may be investigated to enhance the electrochemical performance of electrode materials.