A Review of Graphene Oxide and Reduced Graphene Oxide Applications: Multifunctional Nanomaterials for Sustainable Environmental and Energy Devices

Abstract

Graphene oxide (GO) and reduced graphene oxide (rGO) have solidified their role as cornerstone nanomaterials in the pursuit of sustainable technology. This review synthesizes recent advances in harnessing the unique properties of GO and rGO such as their tunable surface chemistry and exceptional electrical conductivity for applications spanning environmental remediation and energy storage. In the environmental domain, they function as superior adsorbents and catalysts for the removal of hazardous pollutants. Concurrently, in the energy sector, their integration into supercapacitors and battery electrodes significantly enhances energy and power density. The adaptability of these materials also facilitates the creation of highly sensitive sensors and biosensors. However, the transition from laboratory research to widespread industrial application is hindered by challenges in scalable production, environmental health and safety concerns, and long-term stability. This review enhances the understanding of GO and rGO’s diverse applications and paves the way for future sustainable technologies in energy and environmental sectors.

1. Introduction

Graphene oxide (GO) and reduced graphene oxide (rGO) are the two most important types of graphene, with chemical and physical properties that can be changed, which makes them useful for many scientific and industrial purposes [1]. In the last ten years, research on GO and rGO has grown quickly because there is a strong need for new materials that can help solve global problems with energy and the environment. GO and rGO are very useful for things like adsorption, catalysis, sensors, and energy storage because they have high surface area and modifiable oxygen functional group [2]. GO and rGO are being looked at as eco-friendly replacements for traditional materials in water purification, pollutant removal, and air filtration in the field of environmental technology because they have very high surface reactivity and sorption capacity. In the energy sector, their high electrical conductivity and stability have made them good materials for electrodes in batteries, supercapacitors, and fuel cells [3]. GO/rGO is versatile because it can form hybrids or composites with metals, metal oxides, polymers, and other carbon-based materials, this makes it more useful [4]. The chemical structure of GO/rGO and the reduction process of GO to rGO can be seen in Figure 1.

The growing environmental concerns associated with industrialization, such as heavy metal pollution, organic dye contamination, and the increasing global energy demand, have further fueled interest in GO and rGO [3]. Traditional remediation and energy storage techniques often rely on expensive, inefficient, or environmentally harmful materials, prompting researchers to explore sustainable nanomaterial-based solutions [6]. GO and rGO, with their tunable chemical composition and electronic structure, have emerged as leading candidates to fill this gap. Their synthesis is relatively low-cost, scalable, and adaptable, making them suitable for both laboratory and industrial use [7]. Moreover, their ability to act as multifunctional materials simultaneously serving as adsorbents, catalysts, or conductive agents positions them as pivotal components in green technology innovation [3]. This combination of versatility, environmental compatibility, and scalability makes GO and rGO highly valuable in the pursuit of sustainable development goals related to clean water, renewable energy, and pollution control [8].

Although they have great potential, various significant issues persist in understanding and improving the application of GO and rGO. A significant research challenge involves discovering how to balance the structural and chemical characteristics to achieve the desired performance across various application fields [9]. For instance, while the elevated oxygen content in GO aids in drawing in contaminants via hydrogen bonding and electrostatic interactions, it also diminishes electrical conductivity, an essential property for energy storage and electrochemical apparatus [10]. In contrast, the partial restoration of the sp² carbon framework during the reduction in rGO enhances its conductivity; nevertheless, this process simultaneously diminishes its capacity to draw in water and decreases the number of active functional sites that are accessible for adsorption [11]. Consequently, researchers face a trade-off between minimizing structure and enhancing surface reactivity, necessitating tailored strategies according to the intended application. Additionally, the persistent issues of durability, regenerative capacity, and recyclability of GO/rGO-based systems remain unresolved, limiting their broader use. To tackle these difficulties, many research efforts have focused on creating hybrid systems and functional composites that integrate the benefits of both GO and rGO. For instance, composites made from metal oxides and GO, like TiO₂/GO and ZnO/GO, have shown enhanced photocatalytic effectiveness in breaking down organic pollutants when exposed to visible light. Additionally, composites formed from polymers and GO exhibit enhanced mechanical flexibility and increased adsorption efficiency [12]. Likewise, in the area of energy storage, rGO combined with transition metal oxides or conductive polymers has demonstrated improved electrochemical efficiency owing to the cooperative effects between conductivity and active surface areas [13]. These developments demonstrate that combining different materials is a useful method to address the fundamental drawbacks of pure GO or rGO [14]. However, the design principles that control these cooperative behaviors are still being actively investigated.

In environmental uses, GO and rGO have shown remarkable effectiveness as adsorbents for eliminating heavy metals (such as Pb²⁺, Cd²⁺, Cr⁶⁺), dyes (including methylene blue and rhodamine B), and organic contaminants [15]. The significant presence of oxygen-rich functional groups, such as hydroxyl, epoxy, and carboxyl, allows GO to have strong interactions with both ionic and molecular pollutants [16]. The addition of magnetic nanoparticles or biopolymers has been demonstrated to enhance the selectivity, recovery, and reusability of GO-based adsorbents [17]. Conversely, rGO having increased water-repelling properties and a restored connected structure, demonstrates better ability to adsorb aromatic and nonpolar pollutants due to π-π interactions [18]. These results show that both GO and rGO can be precisely modified to focus on particular types of contaminants, which makes them very flexible materials for treating wastewater and cleaning up the environment.

Alongside these advancements, materials based on rGO have become increasingly important in energy applications. Their excellent ability renders them suitable for application as electrodes in lithium-ion batteries, supercapacitors, and fuel cells [19]. Specifically, rGO composites combined with metal oxides (like MnO₂, Fe₃O₄, and CO₃O₄) and conductive polymers (including polyaniline and polypyrrole) have demonstrated elevated energy and power densities along with enhanced cycling stability [20]. Additionally, the three-dimensional structure of GO/rGO aerogels and foams has been utilized to enhance ion movement and electrolyte availability, resulting in better electrochemical performance [21]. These findings highlight the capability of GO and rGO to serve as advanced materials for energy storage systems that are efficient, lightweight, and high-performing [22].

While considerable advancements have been made, a thorough review of the current literature reveals that numerous studies continue to focus on synthesis techniques rather than optimizing applications. The present research field shows a disjointed comprehension of the basic mechanisms that control the performance of GO/rGO in certain environmental or energy situations [23]. For example, the relationship between changes in surface chemistry and the preference for adsorbing pollutants has not been sufficiently studied. In a similar manner, the long-term stability and mechanisms of degradation of GO/rGO-based electrodes in energy devices under actual operating conditions need additional examination [3]. Furthermore, although many studies have emphasized that GO/rGO is outstanding as materials for laboratory use, their adaptation to industrial-scale applications is still limited due to production expenses, challenges in replicating results, and possible environmental effects [24]. These research gaps demonstrate the necessity for organized, practical studies that connect laboratory advancements with practical use in the real world.

Given these gaps, this review seeks to gather and thoroughly evaluate the recent progress made in the use of GO and rGO in environmental cleanup and energy technologies from 2015 to 2025. A key novelty of this review lies in its shift in focus from the synthesis techniques to the recent application of GO/rGO in these fields, instead it focuses on practical performance, relative efficiency, and new developments in adsorption, catalysis, energy storage, and sensing. The uniqueness of this research is found in its comprehensive viewpoint, linking various application areas through a cohesive framework that emphasizes the relationships among structure, properties, and performance. Additionally, it recognizes the current obstacles to widespread use, such as issues related to environmental safety, cost efficiency, and sustainability challenges. This review aims to present a clear summary of recent discoveries, highlight future research goals, and emphasize the essential role of GO and rGO in creating sustainable technologies for environmental and energy applications.

2. Overview of Research Trends

In the last ten years, the study of graphene oxide (GO) and reduced graphene oxide (rGO) has shifted from focusing primarily on improving their production to investigating their practical uses [25]. This change highlights the increasing awareness of these materials as flexible solutions for tackling worldwide environmental and energy issues [26]. At first, scientific research focused on comprehending the distinct structure and characteristics of graphene, which include its exceptional electrical conductivity, mechanical strength, and thermal stability [27]. Nevertheless, the practical difficulties associated with the large-scale production of flawless graphene have redirected focus to its derivatives, namely GO and rGO [28]. These derivatives provide comparable benefits while allowing for adjustable surface properties and variations in chemical reactivity. From 2015 to 2025, research on GO and rGO has risen dramatically, with bibliometric studies indicating that the yearly publication rates in this area have more than tripled [29]. This swift development highlights a change in focus from creating materials in a lab setting to practical research aimed at developing sustainable technologies.

The global growth in GO and rGO research is driven by increasing environmental concerns and the urgent need for sustainable materials that can help manage pollution and renewable energy needs [30]. Their carbon-based hybrid structures which include both sp2 and sp3 bonds allow for the merging of electrical conductivity with plentiful oxygen-containing functional groups, thus rendering them suitable for various uses [31]. Researchers have utilized these characteristics to create materials that function as adsorbents, catalysts, electrodes for energy storage, and platforms for sensing [32]. The natural adaptability of GO and rGO, combined with the relative simplicity of their production and alteration, has positioned them as excellent choices for large-scale environmentally friendly technologies [33]. Consequently, the emphasis of research in both academic and industrial settings has gradually moved from investigating methods of synthesis to enhancing performance in real-world applications, including wastewater treatment, pollutant adsorption, photocatalysis, supercapacitors, and battery technologies [3].

A bibliometric analysis of works published between 2015 and 2025 (Figure 2) shows a distinct development in subject matter. Earlier publications, especially those published before 2018, focused on the structural analysis and chemical reduction methods [29]. These efforts were mainly aimed at understanding how oxidation and reduction influence the morphology, defect density, and surface chemistry of GO. Following 2019, application-focused studies started to lead the field, indicating a development in the research of GO/rGO. The move towards applied science shows a wider understanding that a material’s performance relies not just on how it is made but also on how its physical and chemical properties work with the environments it is intended for. For example, the amount of oxygen in GO is important for adsorption and catalysis, as it creates active sites for attracting pollutants [34]. Meanwhile, the conductivity of rGO makes it useful for storing electrochemical energy [35]. This change from research focused on synthesis to research aimed at practical applications emphasizes the growing complexity of the field and how it corresponds with actual technological needs.

Another notable feature of contemporary research on GO/rGO is the increase in collaboration across different fields. Electrochemists, environmental engineers, and materials scientists are currently working together to develop these nanomaterials for specific applications [36]. Because of worldwide funding initiatives that encourage sustainable material innovation, there has been a rise in international collaborations and partnerships across various sectors, especially between universities in Asia and Europe [37]. By combining computational modeling, machine learning (ML), and on-site characterization techniques, this collaboration has enhanced research capabilities and broadened methodological strategies. Predictive algorithms are increasingly utilized to model electrochemical behavior and adsorption effectiveness, while machine learning-supported synthesis design enables the optimization of reduction conditions and the prediction of performance, all while reducing experimental costs [38]. These improvements indicate a shift in the industry from hands-on testing to materials design based on data, illustrating an increasing digital transformation.

Further analyses illustrate that research on GO and rGO encompasses several thematic categories, particularly focusing on adsorption and pollutant elimination, photocatalysis, and energy storage, with sensing emerging as the most favored area of study. About 70% of the published works concentrate on energy storage and the cleanup of environmental issues [39]. Because there is a high amount of oxygen-containing groups, most studies on adsorption have focused on employing GO and its derivatives to remove heavy metals and dyes [15]. achieve substantial interactions through electrostatic and hydrogen bonding. In the meantime, most studies on rGO have concentrated on its applications in energy storage, particularly in lithium-ion batteries and supercapacitors, where its electrical conductivity and mechanical stability enhance performance metrics [33]. such as charge–discharge stability and specific capacitance. These two areas environmental and energy applications are distinct, yet they have common foundational links between structure and properties, particularly regarding the adjustment of electronic conductivity and surface chemistry.

Even with major progress, many lasting problems still affect the direction of present research. A key issue that needs attention is reproducibility. Important alterations in the structure and function of GO/rGO may occur due to differences in manufacturing conditions, including oxidation duration, temperature, or the quality of the precursor used [40]. This lack of consistency complicates the comparison of research findings from different studies and impedes the standardization process within the industry. Furthermore, the long-term environmental and toxic effects of GO and rGO are still not well understood, even though they are often advertised as eco-friendly materials [41]. Concerns have been expressed about the safety of large-scale implementation, as various studies have suggested potential issues related to bioaccumulation and cytotoxicity in aquatic environments [42]. These uncertainties highlight the importance of conducting lifecycle assessments and environmental risk assessments alongside material development.

Another major challenge is scalability. Although numerous laboratory-scale demonstrations have proven GO/rGO’s functional potential, translating these findings into cost-effective industrial processes remains difficult [30]. Issues such as batch-to-batch variability, high production costs, and the environmental burden of chemical reduction processes hinder commercial adoption [43]. Researchers are increasingly investigating alternative synthesis routes based on bio-derived or waste carbon sources and green reduction methods utilizing plant extracts or benign chemical agents. These approaches not only reduce environmental impact but also align with circular economy principles, promoting sustainable material production.

At the same time, new research directions have begun to emerge that extend GO/rGO’s role beyond traditional applications. Hybrid composites, incorporating metal oxides, polymers, or other carbon nanostructures, represent one of the most promising strategies for enhancing multifunctionality [44]. Such hybrids often exhibit synergistic effects combining the high surface area and adsorption capacity of GO with the conductivity and durability of rGO [45]. Additionally, the integration of GO/rGO materials with renewable energy technologies, such as photocatalytic water splitting or CO₂ reduction, has opened new possibilities for their utilization in energy conversion systems [46]. These advancements reflect a growing emphasis on multifunctional systems capable of performing simultaneous roles, such as pollutant degradation coupled with energy generation [3].

Overall, the trajectory of GO and rGO research from 2015 to 2025 illustrates a maturing scientific field characterized by increasing sophistication, interdisciplinarity, and application relevance. The initial phase of synthesis optimization has evolved into a complex research ecosystem that combines theoretical modeling, experimental validation, and technological integration. As the focus shifts toward sustainability and scalability, future research will likely prioritize eco-friendly synthesis routes, standardized characterization protocols, and comprehensive environmental assessments [37]. The convergence of computational methods, green chemistry, and hybrid material design marks a significant turning point for the field. GO and rGO are no longer viewed merely as laboratory materials but as pivotal components of global strategies aimed at clean energy production, environmental remediation, and sustainable technological advancement [29]. The continued evolution of this research area will depend on collaborative innovation that balances material performance with ecological responsibility and economic feasibility.

3. Applications of GO and rGO

3.1. Adsorption-Based Pollutant Removal

Graphene oxide (GO) and reduced graphene oxide (rGO) have been thoroughly studied as highly efficient materials for environmental remediation [47]. Due to the significant presence of oxygen-containing functional groups on GO, such as hydroxyl, epoxy, and carboxyl groups, there are active areas available for electrostatic interactions with various pollutants [16]. The properties of GO enable it to eliminate a wide range of pollutants, such as heavy metals, dyes, organic compounds, and even new contaminants like pharmaceuticals and tiny plastic particles [48]. In comparison, rGO has a decreased amount of oxygen and a partially regenerated sp² carbon structure, which enhances π–π and hydrophobic interactions [49]. Consequently, both GO and rGO exhibit distinct yet complementary mechanisms for pollutant adsorption, making them versatile for diverse environmental remediation applications [50].

Over the last decade, many experimental studies have shown that GO and rGO have remarkable abilities to adsorb various pollutants [18]. For instance, rGO composites have shown greater effectiveness in adsorbing lead and cadmium ions, while GO-based adsorbents have achieved adsorption capacities exceeding 400 mg/g. the uptake of dyes such as methylene blue and rhodamine B [51,52]. The basic adsorption processes are significantly affected by the conditions of the solution, including pH, ionic strength, and temperature, along with surface modifications [53]. At lower pH levels, the attraction between opposite charges mainly governs the adsorption of positively charged species [54]. In contrast, at higher pH levels, the bonding of metal ions with carboxyl and hydroxyl groups becomes more significant. The effectiveness of adsorbing organic dyes and aromatic pollutants is significantly improved by the π-π interactions occurring between the graphene base plane and the attached dye molecules [55]. This adaptability emphasizes the effectiveness of GO and rGO in different environmental conditions.

The effectiveness of GO and rGO adsorbents has been improved further through functionalization and the creation of composites. The reusability of the materials has been enhanced through the use of magnetic nanoparticles (such as Fe₃O₄), allowing for their easy retrieval and regeneration with the help of external magnetic fields [56]. Polymer modification, for instance with chitosan, polyaniline, or polyethyleneimine, has enhanced selectivity, improved mechanical stability, and increased adsorption kinetics [57]. These changes enhance performance and address a significant challenge associated with nanocarbon adsorbents, namely the difficulties in recycling and separating them. Combinations of GO and rGO with TiO₂ and ZnO are two instances of metal oxides that have shown dual capabilities, functioning as both photocatalysts for the decomposition of pollutants and as adsorbents [58]. By merging photocatalytic oxidation with adsorption-based concentration, this combined method enhances the overall efficiency of the purification process [59]. The application of GO/rGO and its composites as adsorbents, along with their adsorption capacities, can be seen in

Table 1. Comparison of maximum adsorption capacities of different GO/rGO-based adsorbents for MB or RhB adsorption.

Adsorbent	Type of Dye	Adsorption Capacity (mg/g)	Reference
Graphene oxide (GO)	Methylene blue (MB)	1635.5	[60]
GO–chitosan composite granules	Methylene blue (MB)	951.35	[61]
Reduced graphene oxide (rGO)	Methylene blue (MB)	276.06	[62]
Pristine reduced graphene oxide (rGO)	Methylene blue (MB)	121.95	[63]
Ni/Al layered double hydroxide–graphene oxide composite (Ni/Al–GO)	Methylene blue (MB)	61.35	[64]
Graphite oxide/graphene oxide (GO)	Rhodamine B (RhB)	1655	[65]
EDTA/chitosan/magnetic graphene oxide nano-sheets	Rhodamine B (RhB)	1085.3	[66]
reduced graphene oxide (rGO) (fixed-bed column)	Rhodamine B (RhB)	195.24	[67]
nZVI/rGO composite	Rhodamine B (RhB)	87.72	[68]

.

Although GO and rGO possess excellent adsorption abilities, several challenges remain. The stability of these materials in water is an important factor, as too much dispersion of GO can hinder their recovery and reuse [59]. Furthermore, the carbon structure might be partially damaged by the chemical or thermal techniques employed for regenerating the adsorbent, which would reduce its effectiveness over time [69]. Recent research has explored the attachment of GO/rGO onto solid materials such as membranes, foams, and aerogels to overcome these limitations. These established systems ensure a high ability to adsorb while also enhancing mechanical strength and user-friendliness [70]. Another important consideration is the potential that GO particles could pose a risk when released into the environment. Concerns have been raised regarding the safe disposal and reuse of GO, as studies show it may interact with aquatic organisms and microbial membranes [41]. Future research should consequently integrate the assessment of environmental risks, the sustainability throughout the lifecycle, and the enhancement of performance.

Despite these limitations, the advantages of GO and rGO as next-generation adsorbents are undeniable. Their superior adsorption capacities, chemical versatility, and structural tunability make them strong candidates for addressing complex environmental challenges [17]. The combination of experimental insights, computational modeling, and hybrid composite design continues to push the boundaries of their application in wastewater treatment and pollution control [3]. As research advances, integrating GO/rGO adsorbents into scalable, modular treatment systems remains a key direction for achieving sustainable water purification technologies [71].

3.2. Catalytic and Photocatalytic Applications

Beyond adsorption, GO and rGO have demonstrated significant potential as catalytic and photocatalytic materials in environmental remediation [72]. Their structural and electronic characteristics, including large surface area, high electron mobility, and abundant defect sites, make them ideal candidates for supporting or enhancing catalytic reactions [73]. GO’s oxygenated groups facilitate electron transfer and provide anchoring sites for metal and metal oxide nanoparticles, while rGO’s restored conjugated network improves charge transport, minimizing electron-hole recombination [74]. These synergistic features are particularly valuable in photocatalytic degradation and advanced oxidation processes (AOPs) targeting persistent organic pollutants [75].

In photocatalytic systems, GO and rGO are frequently integrated with semiconductor photocatalysts such as TiO₂, ZnO, and g-C₃N₄ to improve visible-light response and overall degradation efficiency [76]. When coupled with these materials, GO/rGO acts as an electron mediator, promoting charge separation by capturing photogenerated electrons and reducing recombination losses [76,77]. This enhances the generation of reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and superoxide anions (O2·−), which are responsible for the oxidative breakdown of pollutants [76,77]. Photocatalytic efficiencies exceeding 90% have been reported for dye and phenol degradation under visible-light irradiation, with rGO-TiO₂ composites exhibiting improved activity compared to pure TiO₂ due to enhanced electron transfer and extended light absorption range [78].

Furthermore, GO and rGO play crucial roles in heterogeneous catalytic systems for AOPs. Their surfaces can host transition metal nanoparticles (e.g., Fe, Cu, Co) that catalyze Fenton-like or peroxymonosulfate activation reactions, generating powerful oxidizing radicals capable of degrading refractory organic compounds [79]. rGO-based catalysts, in particular, demonstrate superior stability and electron conduction, maintaining activity over multiple cycles. The strong interactions between metal nanoparticles and rGO prevent aggregation, ensuring uniform dispersion and prolonged catalytic performance [80]. These systems have proven effective in treating industrial wastewater and reducing the concentration of toxic organics such as chlorophenols, pharmaceuticals, and pesticides.

Another emerging application involves metal-free catalysis using GO and rGO as active components themselves [81]. The intrinsic redox activity of oxygen-containing groups in GO enables it to activate oxidants like hydrogen peroxide or persulfates without requiring metal catalysts [82]. This metal-free approach eliminates secondary pollution associated with metal leaching and offers a more environmentally benign alternative. However, the catalytic efficiency of such systems depends heavily on the oxidation degree and defect density of GO, both of which govern its electron transfer capability [83]. Therefore, fine-tuning synthesis parameters and post-treatment processes remains critical for optimizing catalytic performance.

While catalytic and photocatalytic applications of GO and rGO have achieved substantial success in laboratory settings, several challenges hinder their industrial translation. Catalyst deactivation, poor recyclability, and scalability issues persist as major obstacles [84]. Metal nanoparticle leaching can contaminate treated water, while photocatalytic degradation under real environmental conditions may be limited by turbidity, natural organic matter, and light penetration [85]. Additionally, the high cost of producing high-quality GO and rGO composites restricts their widespread deployment. To address these challenges, researchers are developing immobilized catalyst systems, such as GO-coated membranes or rGO-supported aerogels, which combine high reactivity with mechanical durability and reusability [24,86].

Recent studies have also emphasized the integration of GO/rGO-based photocatalysts into hybrid technologies, such as photoelectrocatalytic and sonophotocatalytic systems [87,88]. These advanced configurations exploit external stimulielectric fields or ultrasonic vibrations to further enhance charge transfer and reactive species generation [89]. For example, rGO-g-C₃N₄ photoelectrocatalysts have achieved superior degradation rates for complex organic mixtures by simultaneously improving photon utilization and electron mobility [90]. Such hybrid systems exemplify the direction of contemporary research, which seeks to couple GO/rGO’s structural and electronic features with external energy inputs for maximized environmental performance [91].

In summary, GO and rGO serve as multifunctional platforms for catalytic and photocatalytic environmental applications, offering efficient pollutant degradation and energy-assisted remediation pathways [6]. Their high reactivity, electronic conductivity, and structural flexibility enable their integration into diverse catalytic systems [3]. However, realizing their full potential requires addressing challenges related to stability, scalability, and environmental compatibility. Future work must focus on developing durable, recyclable, and low-cost GO/rGO-based catalysts, guided by mechanistic understanding and life-cycle considerations. These efforts will be essential to transition from laboratory demonstrations to practical, sustainable environmental technologies.

3.3. Supercapacitors

Due to the unique structural, electrical, and mechanical characteristics of GO and rGO, they have garnered considerable attention as active components for supercapacitors [22]. Supercapacitors, referred to as electrochemical capacitors, are devices that store energi [92]. Supercapasitor serve as a bridge between batteries and standard capacitors by offering a high power density, an extended operational lifespan, and a broad range of working temperatures. rapid charging and discharging speeds, in addition to lifespan of cycles [93]. The excellent electrochemical properties of rGO and GO arise from their extensive surface area, adjustable functional groups, and varying electrical conductivity, which support both electrical techniques involving pseudocapacitance and electric double-layer capacitance (EDLC) [31]. The quest for energy storage systems that are lightweight, adaptable, and eco-friendly, suitable for the upcoming generation of wearable and portable devices, has resulted in the development of supercapacitors utilizing GO/rGO [94].

GO sheets in their unmodified form contain numerous oxygenated functional groups that enhance their affinity for water and assist in the distribution of aqueous electrolytes [95]. However, an excessive amount of oxygen disrupts the sp² carbon structure, resulting in decreased electrical conductivity [95]. The incomplete transformation of GO into rGO reinstates electrical conductivity while maintaining an optimal level of oxygen functionalities that enhance interaction with electrolytes and facilitate ion transport [96]. It is crucial to keep a balance between surface chemistry and conductivity in order to achieve the highest possible capacitance [97]. For example, chemically reduced GO has demonstrated specific capacitances of about 150–250 F/g, whereas GO that has been reduced through thermal or hydrothermal methods may exhibit higher capacitance values, contingent on the reduction technique employed. the level of reduction and pore configuration [97]. The structured porosity of rGO, consisting of micropores for storing charge and mesopores for facilitating ion movement, enhances both electrochemical access and the efficiency of charge transport [98].

Recent studies have focused on improving the electrochemical efficiency of rGO by developing composites with metal oxides and conductive polymers [99,100]. Metal oxides such as MnO₂, along with polymers like polyaniline (PANI) and polypyrrole (PPy), enhance the efficiency of charge transfer and offer greater mechanical flexibility [101]. Additionally, Fe₂O₃ and CO₃O₄ contribute additional pseudocapacitance through reversible redox reactions [102]. For instance, specific rGO-MnO₂ and rGO-PANI composites have achieved capacitances reaching up to 185 F/g at 1 A/g and excellent capacitance retention of approximately 88.54% over 4000 charge–discharge cycles [103]. The cooperative interaction between the redox-active elements and the conductive structure of rGO enables greater energy and power densities, while also maintaining stability over a long period [103,104]. The capacitance of several supercapacitors based on GO/rGO are shown in

Table 2. Comparison of Capacitance for Supercapacitors Based on GO and rGO.

Material	Model Supercapacitor	Specific Capacitance (F/g)	Reference
Graphene Oxide (GO)	EDLC electrode	26.6 at 1 A/g	[105]
Reduced Graphene Oxide (rGO)	EDLC electrode	301.7 at 1 A/g	[105]
RGO/CCF15A composite	Supercapacitor electrode	74.1 at 0.2 A/g	[106]
rGO paper nanocomposite	Flexible supercapacitor electrode	255 (aqueous, KOH)	[107]
NiCo-MOF/rGO hybrid	Supercapacitor electrode	1553 at 1 A/g	[108]
rGO composite (pseudocapacitive)	Supercapacitor electrode	176.83 F/g at 0.5 A/g	[109]

.

Three-dimensional (3D) structures such as hydrogels, foams, and aerogels have been developed to solve the issue of sheet restacking, a common limitation that reduces the surface area of graphene-based materials [110]. These structures create linked networks that encourage electrolyte movement and efficient ion diffusion, leading to improved rate capability and cycle efficiency [110]. For example, aerogels composed of rGO with complex pore structures have shown a capacitance retention of more than 96% after 5000 cycles, indicating that they are stable and reversible [111]. Furthermore, the application of rGO films or fibers in the development of wearable and flexible supercapacitors has enabled the incorporation of energy storage into fabrics and flexible electronic devices [112]. The potential of the material for future use in flexible energy systems is highlighted by its ability to maintain high energy densities [112].

Despite these developments, the large-scale production and uniformity of rGO-based supercapacitors still present difficulties. The electrochemical performance can differ significantly based on the reduction methods employed, the quality of the precursor, and the conditions under which synthesis occurs [113]. Furthermore, the application of hazardous chemical reducing agents and the high cost of graphene derivatives present obstacles to sustainable production [114]. Recent studies have suggested eco-friendly synthesis methods that make use of plant extracts or ascorbic acid as reducing agents, alongside carbon sources obtained from biomass [115]. These approaches reduce the negative effects of creating electrodes on the environment and align with the goals of sustainable development. Future studies will probably focus on integrating these environmentally friendly production methods with scalable designs for electrodes to guarantee both efficiency and commercial viability.

3.4. Batteries

GO and rGO have been extensively explored as electrode materials in battery systems due to their excellent electrical conductivity, high mechanical stability, and tunable surface chemistry [116]. GO/rGO two-dimensional lamellar structure provides pathways for efficient electron and ion transport while accommodating volume changes during charge–discharge cycles [117]. The applications of GO and rGO in batteries span multiple systems, including lithium-ion (Li-ion), sodium-ion (Na-ion), lithium-sulfur (Li-S), and zinc-ion (Zn-ion) batteries, each leveraging distinct advantages of the materials [117,118,119]. Schematic illustration of the synthesis rGO based material for lithium-ion batteries illustrated in Figure 3.

In Li-ion batteries, rGO has primarily been utilized as a conductive additive or active anode material to enhance charge transfer and stabilize electrode structures [19]. rGO high electrical conductivity ensures rapid electron mobility, while the residual oxygen functionalities facilitate electrolyte interaction and ion diffusion [96]. Composite electrodes combining rGO with transition metal oxides (e.g., Fe₃O₄, CO₃O₄, SnO₂) showcasing higher reversible capacities of 1021 and 773 mAhg⁻¹ after 100 cycles at 100 mAg⁻¹, along with excellent rate capability [116]. Similarly, rGO–Si anode composites mitigate the volume expansion issue associated with silicon electrodes, thereby improving cycle life and mechanical integrity [121]. The incorporation of GO as a binder or coating material on cathodes also enhances interfacial adhesion and structural cohesion, contributing to improved capacity retention over extended cycling [121].

In the case of sodium-ion and zinc-ion batteries, GO and rGO have shown promise as conductive frameworks that stabilize active materials and suppress dendrite formation [122,123]. rGO’s high electrical conductivity and structural flexibility make it particularly suitable for flexible and aqueous-based energy storage systems [119]. For example, Zn-ion batteries employing rGO-modified cathodes have achieved high energy densities and long-term stability due to the uniform ion distribution and efficient charge transfer facilitated by rGO layers [119]. Additionally, the hydrophilic nature of GO can enhance electrolyte wettability and interfacial contact, thereby reducing internal resistance and improving overall efficiency [124].

Lithium-sulfur (Li–S) batteries represent another domain where GO and rGO play crucial roles. These materials effectively trap polysulfide intermediates, mitigating the notorious “shuttle effect” that leads to capacity fading [125]. rGO’s conductive framework not only immobilizes sulfur species through physical adsorption and chemical bonding but also provides efficient electron pathways for redox reactions [126]. rGO–sulfur composites at 500 mA g⁻¹ delivered an initial discharge capacity of 552 mA h g⁻¹, and there was no capacity loss in its initial five cycles, maintaining a stable capacity of 390 mA h g⁻¹ till 300 cycles with 73% capacity retention [126]. Moreover, the porous structure of rGO helps accommodate volume expansion during cycling, maintaining electrode integrity [127].

While the electrochemical performance of GO/rGO-based batteries is impressive, several challenges must be addressed before large-scale application becomes feasible. The synthesis of uniform, defect-controlled graphene derivatives remains complex, and the scalability of high-quality rGO production is limited by cost and environmental concerns [113]. Furthermore, optimizing the electrode–electrolyte interface and minimizing unwanted side reactions are ongoing areas of research. To overcome these limitations, recent studies have focused on hybridization strategies that combine GO/rGO with other carbon materials, such as carbon nanotubes (CNTs) and activated carbon, to create hierarchical architectures that enhance conductivity and mechanical stability [128].

Looking forward, the integration of GO and rGO into next-generation batteries aligns with global efforts to achieve high-performance, sustainable, and flexible energy storage technologies [129]. As the demand for renewable energy systems continues to grow, the versatility of GO and rGO offers a platform for tailoring electrode materials across diverse chemistries [117]. Through these approaches, GO and rGO will continue to play a pivotal role in advancing battery technology toward higher efficiency, durability, and environmental sustainability.

3.5. Sensing Applications

GO/rGO have emerged as highly promising materials for use in sensing applications because their exceptional electrical, mechanical, and chemical characteristics [130,131]. GO/rGO are ideal for identifying many different substances, including gases, because of their large surface area relative to their volume, customizable surface properties, and excellent ability to conduct charge, as well as metallic elements to biological molecules and carbon-based substances [131]. The partially restored sp² network in rGO enhances the attraction of specific molecules and enables chemical modifications, as GO contains oxygen-rich functional groups. The movement of charge carriers and the ability to conduct electricity [11]. Because of the interaction between their electronic and structural features, both rGO and GO can serve as very sensitive transduction platforms for different types of sensing methods, including electrochemical sensing. Field-effect transistor (FET)-based sensors, optical sensors, and field-effect transistor (FET)-based sensors [132].

One of the most thoroughly studied electrochemical sensors is those that utilize rGO and GO due to their ability to transform molecular interactions into quantifiable electrical signals [133]. In these devices, GO and rGO typically act as modifications for electrodes, improving the adsorption of analytes, accelerating electron transfer rates, and enlarging the effective surface area [133]. The oxygen functional groups present in GO enable significant interactions with polar organic molecules and metal ions [134]. In contrast, the conductive structure of rGO supports efficient charge transfer between the analyte and the surface of the electrode [135]. For instance, electrodes modified with rGO have demonstrated outstanding sensitivity in detecting heavy metals such as lead, mercury, and cadmium, achieving detection limits as low as the nanomolar range [136]. The primary functioning method for these sensors is stripping voltammetry, where metal ions are initially concentrated on the surface of rGO and subsequently anodically dissolved, resulting in high signal-to-noise ratios [136]. In the same way, composites made with functionalized rGO and GO, using polymers or nanoparticles like gold, silver, and platinum, have been developed to detect pharmaceutical residues, pesticides, and organic pollutants. integrating selectivity with minimal detection limits in water environments [137].

The application of GO and rGO in biosensing is another area that is rapidly expanding. Their compatibility with living tissues, ability to adapt functions, and capability to hold biomolecules make them particularly appropriate for creating enzyme-based, DNA-based, and immunosensors [138]. The ability of GO to quench fluorescence is used in optical biosensing systems, and its high oxygen content aids in the physical absorption or covalent attachment of biomolecules [138]. Conversely, electrochemical transduction advantages arise from the increased electrical conductivity of rGO. For instance, glucose biosensors that utilize rGO and glucose oxidase was tested for glucose detection, the sensitivity of glucose detection was shown to be 57.3 µA/(mM·cm²) with a detection limit of 86.8 µM [139]. Furthermore, immunosensors that are functionalized with GO have been employed to selectively detect disease biomarkers, including cardiac proteins and cancer antigens; this high binding specificity is achieved due to the antibody being attached to the surface of the GO [140]. Besides that, the performance of the biosensor has been enhanced by incorporating GO and rGO with nanostructured materials such as quantum dots and metal nanoparticles, resulting in improved signal transduction efficiency and stability [141].

Gas sensing represents another field where both rGO and GO show significant potential. Because of their large surface area and customizable electronic features, they are highly selective and responsive to different gas molecules, such as nitrogen and ammonia (NH₃), dioxide (NO₂) and volatile organic compounds (VOCs) [142]. The primary function of the sensor relies on changes in electrical resistance caused by gas absorption, which subsequently influences the density of charge carriers within the material [143]. Due to its surface being rich in oxygen, GO interacts effectively with polar gas molecules; however, its low conductivity might restrict its sensitivity [144]. Conversely, the elevated conductivity and presence of defect sites in rGO facilitate the movement of electrons, resulting in faster response and recovery times [144]. Research indicates that rGO-based NO₂ sensors have high sensitivity, rapid response, and a recovery time of 31 s and 81 s, respectively, when exposed to 250 ppm NO₂ at room temperature (RT). The sensor demonstrated good repeatability, excellent 60-day stability, and linearity from 50 to 500 ppm of NO₂ concentration [145]. Metallic nanoparticles such as Pd, Pt, or Cu can enhance functionalization and, in turn, increase selectivity by serving as catalysts. Furthermore, heterostructure designs that integrate rGO along with cooperative electronic interactions involving metal oxides such as ZnO and SnO₂ enhance the sensitivity for specific gas adsorption reactions [142,146].

Optical sensing platforms utilizing GO and rGO benefit from their distinctive optical and photoluminescent properties [147]. GO demonstrates remarkable fluorescence quenching capability, which can be applied in fluorescence resonance energy transfer (FRET)-based biosensors for the detection of DNA, proteins, and small molecules [148]. The fluorescence of biomolecules that are labeled with fluorescent tags decreases when they come into contact with GO because energy moves from the fluorescent tag to GO’s conjugated π system [149]. However, the fluorescence returns to normal when the target is attached. This system enables detection that is highly sensitive and does not require labels. In a similar manner, surface plasmon resonance (SPR) sensors utilizing rGO as an active layer demonstrate enhanced refractive index and signal strength [150]. Improved surface plasmon coupling leads to increased sensitivity to the index. These optical sensing techniques have been employed in monitoring the environment and diagnosing medical conditions, offering fast, non-damaging, and efficient detection abilities [150]. The advantages of GO/rGO as a sensor are shown in the Figure 4.

Overall, rGO and GO have proven to be highly versatile materials for use in sensor applications, including optical, electrochemical, and gas detection systems. Their unique combination of adjustable properties, excellent conductivity, and strong mechanical durability positions them as future materials for advanced sensors. Future enhancements in the management of hybrid synthesis are expected. It is anticipated that their sensitivity, selectivity, and stability will improve through advancements in material design and the integration of systems. The upcoming wave of advanced sensing technologies will be characterized by the integration of data analysis, eco-friendly manufacturing, adaptable electronics, and the material science of GO/rGO materials [151]. This combination will enable applications in smart infrastructure, healthcare diagnostics, and environmental monitoring.

4. Comparative Analysis of GO and rGO Performance

To understand the respective benefits, drawbacks, and potential combinations of GO and rGO in various environmental and energy-related fields, a comparison between the two is required. Although both materials originate from the same carbon precursor, variations in oxygen levels, defect density, and electronic structure result in notable differences in their physical and chemical characteristics, these differences in structure affect their efficiency in adsorption, catalysis, sensing, and energy storage [2]. A methodical examination reveals that rGO outperforms GO in terms of electrical conductivity, stability, and mechanical strength, while GO excels in surface functionalization and chemical reactivity. The distinct characteristics of these materials underscore the importance of selecting them thoughtfully or combining them according to the specific needs of the application [152].

From a chemical and structural viewpoint GO is characterized by its significant presence of oxygen-containing groups such as hydroxyl, carboxyl, and epoxy which disrupt the extensive sp² network of graphene. The addition of oxygen makes the substance more water-attracting, which enhances its ability to mix well in polar solvents and facilitates chemical modifications [153]. These surface groups serve as active sites for the adsorption and covalent bonding processes, which enhance the remarkable capability of GO to immobilize biomolecules, facilitate reactions, and eliminate pollutants [9]. However, the specific qualities that increase the reactivity of GO also reduce its conductivity. Electrochemical localization occurs as sp² carbon changes to sp3 hybridization, resulting in reduced mobility of charge carriers. Consequently, the low conductivity of GO restricts its application in electrochemical fields such as supercapacitors and batteries, where efficient electron transfer is essential [154].

In contrast, rGO is made by chemically, thermally, or electrochemically reducing GO, which partially returns the sp² conjugated structure and enhances its electrical conductivity [155]. The oxygen level in rGO is reduced; however, it still has imperfections that provide it with some degree of chemical reactivity. Due to its blend of conductivity and a morphology rich in defects, rGO is ideally suited for applications in energy storage and sensing [11]. rGO water-repellent properties and the prolonged π-conjugation enhance π–π interactions with aromatic compounds, making it more effective than GO in capturing nonpolar organic pollutants and acting as a sensing interface for gaseous and biochemical substances [144,156]. Nevertheless, the limited oxygen functionality of rGO hinders its ability to absorb water and interact with substances in water, thus complicating its dispersion and the formation of composites without additional modifications to its surface [157].

In environmental remediation, GO and rGO exhibit distinct yet complementary adsorption mechanisms. GO’s oxygenated functional groups favor electrostatic attraction, hydrogen bonding, and surface complexation, leading to high adsorption capacities for heavy metals and ionic contaminants [158]. Conversely, rGO demonstrates superior performance in removing organic and aromatic pollutants through hydrophobic interactions and van der Waals forces [159]. Hybrid materials combining GO and rGO have been found to exploit the strengths of both: GO contributes surface polarity and functional reactivity, while rGO enhances adsorption kinetics and structural stability. This dual-functionality approach represents a promising route for developing multifunctional adsorbents capable of treating complex pollutant mixtures [160].

The unique electronic properties of GO and rGO lead to different roles in photocatalytic and catalytic systems. Due to its abundant surface functional groups that facilitate the attachment of metal nanoparticles and enhance charge transfer activities, GO often functions as a catalyst support or co-catalyst [161]. When combined with semiconductors such as TiO₂, ZnO, or g-C₃N₄, its semiconducting properties and oxygen-rich surface render it highly efficient in photocatalysis under visible light [162]. rGO primarily serves as a conductive platform and electron mediator, enhancing charge transport and reducing the recombination of electrons and holes [163]. For example, rGO–TiO₂ composites enhance photocatalytic activity by allowing efficient electron movement, whereas GO–TiO₂ composites boost the surface adsorption of reactants [164]. Consequently, when compared to each component alone, the combined use of GO and rGO can achieve greater degradation rates and enhanced catalytic stability.

Because of its greater electrical conductivity and strength, the advantages of rGO are more apparent in energy storage applications [6]. Due to rGO excellent specific capacitance, outstanding rate performance, and long cycling life, electrodes made from rGO are ideal for use in batteries and supercapacitors [107]. In comparison, GO is often used as a starting material, binder, or structural component to improve ion movement, while MnO₂ or conductive polymers are utilized and the ability to bend easily [165]. The functional groups present in GO enhance the ability of electrolytes to spread and strengthen the bonding between interfaces in lithium-ion and sodium-ion batteries, whereas rGO acts as the conductive framework essential for swift electron transfer [166]. These additional functions emphasize how GO and rGO can be collaboratively engineered to achieve a balance between ionic and electronic conductivity in composite electrodes.

In sensing applications, GO and rGO demonstrate distinct yet interconnected roles. GO is ideal for chemical and biosensing applications due to its significant ability to undergo functionalization, enabling it to selectively engage with specific target analytes [49]. Due to GO ability to bind biomolecules through both covalent and non-covalent interactions, it finds applications in DNA detection systems and immunosensors [167]. Conversely, rGO possesses the efficiency in signal transduction and the electrical conductivity essential for immediate detection. When combined, GO and rGO may enhance sensing abilities, as GO facilitates the binding of analytes and rGO enables signal amplification through efficient electron transport [168]. In hybrid systems that combine electrochemical and optical sensing, where both selectivity and sensitivity are required, this additional method is particularly advantageous.

Despite these performance advantages, there remain several important trade-offs between rGO and GO. Because of the uneven removal of oxygen and remaining functional groups that happen during the reduction process for creating rGO, there is variation in the electronic and structural properties from one batch to another [169]. Conversely, excessive oxidation during the production of GO can lead to a breakdown of its structure, diminishing its mechanical strength and limiting its ability to be reused [170]. These factors complicate the process of standardization and hinder the ability to reproduce published performance metrics. Moreover, both substances present concerns regarding environmental safety and durability over time. Additional research on the life-cycle effects of GO and rGO is essential due to the potential toxicity associated with the oxidative surface of GO and the uncertain biodegradability of rGO [171]. For GO and rGO technologies to transition from small laboratory demonstrations to widespread industrial application, it is essential to tackle these challenges.

The growing application of GO/rGO in energy and environmental technologies should be supported by Life Cycle Assessment (LCA). An LCA framework can be introduced by defining a functional unit relevant to the application (e.g., per kWh delivered over battery lifetime), selecting system boundaries (cradle-to-gate for material synthesis or cradle-to-grave including use and end-of-life), and compiling a life cycle inventory covering oxidants/reductants, washing and purification steps, energy-intensive drying/thermal treatments, and wastewater management. Hotspot and sensitivity analyses are particularly important because the overall impacts are strongly influenced by synthesis yield, electricity mix, and the number of purification steps. Importantly, potential environmental burdens in material production may be offset by use-phase benefits such as extended cycle life, improved reusability, or reduced operational energy, which should be quantified whenever possible [172].

In summary, the comparison shows that there is no clear champion between GO and rGO, and the most appropriate choice relies on the specific application and context of use. rGO is ideal for use in electrochemical and electronic devices due to its excellent conductivity and stability [33]. In contrast, GO plays a vital role in adsorption and catalytic systems because of its ability to attract water, its chemical reactivity, and its functional surface properties [173]. The most effective method for future technological advancement is to integrate these various features through the design of hybrid materials [45]. Additional research should focus on enhancing synthesis control, improving reproducibility, and broadening mechanistic insights to fully leverage the complementary capabilities of GO and rGO in sustainable energy technologies.

5. Conclusions and Future Directions

This review has examined the recent advancements and emerging applications of GO and rGO, highlighting their unique physicochemical characteristics and the growing range of their environmental and energy-related applications. The comparative analysis demonstrates that GO, with its high degree of functionalization and hydrophilicity, excels in adsorption and catalytic systems, while rGO, characterized by superior electrical conductivity and structural stability, is better suited for electrochemical and sensing applications. The integration of both materials in hybrid systems provides synergistic improvements in performance, establishing them as pivotal components in next-generation sustainable technologies. Despite substantial progress, challenges such as synthesis reproducibility, environmental safety, and large-scale manufacturing remain critical barriers to practical implementation. The study underscores the importance of interdisciplinary innovation combining green chemistry, digital materials design, and advanced manufacturing to address these issues and to enable the development of scalable, eco-friendly GO/rGO-based systems. By consolidating the current state of knowledge and identifying key research gaps, this review contributes to a deeper understanding of GO and rGO’s multifunctional roles and sets a foundation for future research toward sustainable material technologies that support global energy and environmental goals.