Expanding Horizons: Taking Advantage of Graphene’s Surface Area for Advanced Applications

Abstract

Graphene, being a two-dimensional monolayer of carbon, exhibits an exceptionally increased surface-to-volume ratio due to its atomic thinness and high aspect ratio, making it a material of considerable interest in advanced technology applications. Recent developments have leveraged their unique surface characteristics, such as nanoscale ripples and grooves, to enhance energy storage, sensing, catalysis, and environmental remediation performance. Its extensive surface area enables rapid ion adsorption and desorption, significantly improving energy and power densities in supercapacitors and lithium-ion batteries while enhancing stability over prolonged cycles. In sensing, the high surface-to-volume ratio supports the immobilization of biomolecules and nanoparticles, improving sensitivity in detecting gases, biomarkers, and pollutants, thereby advancing diagnostic and environmental monitoring applications. Its expansive surface area and unique electronic properties contribute to high catalytic efficiencies, enabling sustainable chemical processes, such as hydrogen production, water treatment, and pollutant degradation. Unlike many review articles that primarily explore the functionalization of graphene, this study mainly emphasizes the evaluation of methodologies aimed at augmenting graphene’s surface area. This review systematically evaluates recent advancements in the optimization of graphene surface characteristics, with a primary focus on their role in enhancing energy storage systems while also addressing emerging applications in healthcare and environmental sustainability.

1. Introduction

One of the most recognized, highly promising, and transformative materials in the 21st century is graphene. Its popularity is attributed to its exceptional properties, which include remarkable mechanical strength, outstanding electrical conductivity, and impressive thermal stability. These qualities make it extremely desirable for a wide range of applications in several areas, such as energy storage, materials research, and electronics. Its widespread applicability and significant contributions to technological advancements originated from its high surface-to-volume ratio combined with its unique two-dimensional structure. As a result, it continues to garner significant attention and research interest in the scientific community. While these attributes have attracted significant attention, the recent focus has shifted toward an equally remarkable feature: its surface area. This inherent characteristic makes it an excellent candidate for electrode materials in electrochemical devices, particularly in applications like batteries and supercapacitors [1]. It is important to note that two crucial qualities typically found in conventional electrode materials are not strictly inherent to pure, single-layer graphene. These qualities are affordability in production and the presence of a highly developed surface area with a well-defined pore structure. Theoretically, it can offer a surface area potential of 2640 m²/g; however, real-world samples, primarily in the form of nanoplatelets, typically have a significantly less developed surface area ranging from 10 to 750 m²/g [1,2,3]. Its expansive surface area renders it an ideal platform for the adsorption of ions and molecules, significantly boosting the capacity and efficiency of electrochemical energy storage systems. In the context of supercapacitors (SCs), its extensive surface area enables high charge storage capacity, rapid charge and discharge rates, and an extended cycle life. At the interface between the electrode and the electrolyte, its high surface-to-volume ratio facilitates effective ion adsorption and desorption. As a result, SCs incorporating graphene-based (G-b) materials demonstrate enhanced energy and power density compared to their traditional counterparts. G-b anodes and cathodes also enhance the capacity of batteries by accommodating more lithium ions during charge and discharge cycles. Its large surface area contributes to improved electrode stability, reducing the risk of electrode degradation over time. Its special qualities have shown their importance in the field of energy generation and storage devices, including fuel cells, battery cells, conventional dielectric capacitors, and SCs. Batteries are favored for their superior energy density, while traditional dielectric capacitors excel in high-power density. SCs serve as a bridge between batteries and conventional dielectric capacitors, as they offer a blend of both energy density and power density characteristics. Unlike batteries, which excel in energy storage but may lack rapid energy release, SCs prioritize fast energy discharge alongside adequate energy storage. This unique combination makes SCs a perfect device for applications requiring greater capacity in energy storage and quick energy delivery; for example, electric automobiles use them for reverse braking. This helps with peak power shaving in renewable energy systems. Consequently, SCs are receiving more and more attention because of their adaptability and efficiency in a variety of domains, including energy storage devices [4,5], electric vehicles [6], and semiconductor devices [7].

With the rapid growth of renewable energy, there is a strong need to improve the energy density of batteries and SCs. Because of its exceptional electrical conductivity, durability, and chemical resistance, graphene—a two-dimensional single layer of graphite—stands out. The key to improving its energy storage performance is increasing the area of its surface (per unit mass or volume). A larger surface area allows for more ion interaction and charge storage, boosting energy density and power output [8,9]. This makes graphene highly suitable for meeting the energy demands of renewable technologies while maintaining fast and efficient charging. Methods such as chemical exfoliation, 3D graphene foams, chemical activation for porosity, and mechanical buckling or wrinkling effectively increase its surface area [10,11]. These approaches improve the surface-to-volume ratio, enabling better performance in next-generation energy storage devices. It distinguishes itself through its exceptional attributes, such as high electrical conductivity, mechanical stability, thermal conductivity, etc. Its versatility makes it ideal for a multitude of applications spanning numerous industries. It has demonstrated remarkable performance in various applications, including catalysis, catalyst support, carbon dioxide capture, and energy conversion and storage devices. Its characteristics position it as a key contributor to advancing energy storage and conversion technologies [12,13,14,15].

Hybrid supercapacitors (HSCs) are intricate energy storage systems that typically consist of three fundamental components [16]. These are electrodes, electrolytes, and separators, as illustrated in Figure 1. The overall performance characteristics of HSCs are predominantly influenced by electrochemical activity, as well as the kinetic properties inherent to the electrodes utilized within the device. It is crucial to concentrate on improving the kinetics of ion and electron transport, which takes place inside the electrodes and at the connection among the electrolyte and electrode materials, to greatly increase the energy density and capacity of these HSCs [17]. Thus, the selection and optimization of electrode materials, which can be regarded as the fundamental essence or “soul” of these energy storage devices, play an undeniably crucial role in determining the overall operational performance and efficiency of HSCs. This system is characterized as a sophisticated energy storage apparatus that ingeniously integrates complementary types of electrode materials, thereby enabling it to attain high energy density alongside rapid charge–discharge capabilities, which are essential for various applications. Within this innovative configuration, transition metal oxides, sulfides, or hydroxides are typically employed as positive electrode materials, chosen specifically for their impressive specific capacitance and significant energy storage potential, which can be attributed to the reversible redox reactions they facilitate. On the other hand, materials based on carbon, like reduced graphene oxide (rGO), carbon nanotubes (CNTs), and activated carbon (AC), are strategically used for the negative electrode because of their exceptional electrical properties, vast surface area, and intrinsic balance under operating conditions. This carefully designed configuration not only allows the supercapacitor to effectively balance the dual demands of energy density and power output but also positions it as an optimal choice for applications that necessitate efficient, rapid energy delivery coupled with an extended cycle life, thereby underscoring the versatility and practicality of hybrid supercapacitor technology in contemporary energy storage solutions.

A biosensor is a sophisticated device designed to harness the capabilities of biological sensing components by linking them to a transducer. The principal function of the transducer is to convert discernible physical or chemical changes into quantifiable signals, typically in the form of electronic output. The precise desired concentration of chemicals or a preset set of chemical compounds determines how strong these signals are. Biosensors’ effectiveness is largely dependent on the inherent properties of the materials used to make them. These substances are essential for many reasons, including the design of the transducer, the creation of matrices for immobilizing enzymes, and the incorporation of stabilizing and mediating elements. Collectively, these components profoundly influence a biosensor’s overall performance [11,12,13]. They have gained significant prominence as a viable and versatile option for conducting various medical and biological analyses. Their applications span a diverse spectrum, including environmental monitoring, biomedical research, food security assurance, and agricultural enhancement. Conventional analytical techniques, such as high-performance liquid chromatography (HPLC), gas chromatography–mass spectrometry (GC-MS), and liquid chromatography–mass spectrometry (LC-MS), have faced significant obstacles in meeting stringent regulatory requirements for quantifying analyte levels in diverse sample matrices. Despite their widespread use and effectiveness in many applications, these methods often struggle to achieve the necessary precision, accuracy, and sensitivity that regulatory bodies demand. As a result, there is a growing need for more advanced analytical approaches to reliably measure analyte concentrations across various sample types while meeting regulatory standards. Limitations, such as low sample throughput, the necessity for solvents, the need for sample pretreatment, and large sample volumes, often constrain these methods. This has led to an increasing demand for analytical platforms that can efficiently and simultaneously detect one or more analytes. Recent advancements in wearable sensors, lab-on-a-chip technologies, and high levels of throughput assays have shown the ability to deliver precise and timely analyses in a short amount of time. These advancements signify a significant leap in analytical capabilities, enabling faster decision-making processes and more efficient workflows. Such technologies hold promises for various applications, from medical diagnostics to environmental monitoring. The development of compact and portable biosensor devices has expanded their applicability to various fields, including defense, biomedical research, agriculture, crime investigation, and drug screening at multiple entry points [14,15]. These innovations enhance analytical capabilities and make biosensors more accessible and versatile for addressing the diverse and evolving demands of modern science and industry [16]. Figure 2 outlines the main components of a biosensor device, including the biorecognition element, transducer, and signal processing unit. A biochemical reaction is triggered by the biorecognition element’s particular interaction with the intended substance [17]. The transducer subsequently transforms this response into an optical, electrical, or mechanical signal. The signal processing unit amplifies and interprets this signal, allowing for precise detection and quantification of the analyte. Each component works in sequence to enable accurate, real-time analysis in the biosensor [18].

The utilization of its substantial surface area extends its applications into the domains of catalysis and environmental remediation. G-b catalysts have demonstrated heightened catalytic activity due to their expansive surface area, facilitating efficacious interactions with reactant molecules. These catalysts have proved effective in a variety of applications, such as the generation of hydrogen, the purification of water supplies, and the breakdown of contaminants, providing long-term answers to pressing global issues. Its remarkable surface area has been strategically harnessed for environmental remediation objectives. Its ability to adsorb various organic and inorganic pollutants from atmospheric and aquatic sources holds considerable importance. This characteristic underscores the significant potential of G-b adsorbents as an efficacious solution for environmental remediation. Demonstrating efficient and cost-effective contaminant removal, they present a promising strategy for fostering a cleaner and safer environment.

This review article uniquely addresses gaps in the literature by focusing on surface area modifications of graphene, specifically, the impact of structural features like ripples, grooves, and other nanoscale surface-enhancing structures. Unlike previous reviews, which primarily examine chemical functionalization or bulk properties, this article explores how physical surface modifications enhance surface area and reactivity. Increasing the effective surface area through controlled modifications can significantly expand its potential for advanced applications, including high-performance energy storage, catalysis, and sensing. To the authors’ knowledge, no existing review has systematically examined its aspect, making this article the first to detail the critical role of surface structuring in optimizing graphene in cutting-edge technologies. This study seeks to contribute to the broader knowledge base, promote innovation, and provide insights into addressing pressing global challenges across scientific disciplines by conducting a comprehensive analysis of its surface area and its implications.

2. Increasing the Surface Area of Graphene

The phrase “increasing the surface of graphene” describes a variety of changes and treatments made to the material to increase its usable surface area, which makes it ideal for use in sensors, energy storage, and catalysts. Recent advancements have introduced methods such as wrinkling, folding, and buckling its sheets to achieve this [11,19,20]. Its surface area is mostly determined by its physical size and dimensions and is composed of a single layer of carbon atoms organized in a two-dimensional hexagonal lattice. This distinctive honeycomb structure is fundamental to its properties, with its surface area primarily determined by its geometric characteristics [21,22,23,24]. Chae et al. [25] reported three ways of enlarging its surface area. For practical applications, large-area sample preparation is crucial, and three methods have been explored to achieve this. The first method is micromechanical cleavage, which involves exfoliating its layers from graphite [26]. The second method is the thermal decomposition of a silicon carbide (SiC) substrate, which is produced by heating SiC to high temperatures, allowing the silicon to sublimate and leaving a graphene layer behind [27]. Finally, the carburization and annealing of a metal substrate method involves introducing carbon into a metal surface and using heat treatment to form them [28]. Each approach has distinct advantages and challenges in producing large, continuous graphene sheets. Its surface area is of paramount importance in a myriad of applications due to its ability to provide a vast number of active sites for catalysis, enhance sensitivity and detection limits in sensors, increase capacity and performance in energy storage devices, facilitate efficient filtration and adsorption processes, offer versatile drug delivery platforms, optimize supercapacitor performance, aid in efficient gas storage and separation, enable sensitive biosensing, and promote various chemical modifications and functionalizations, ultimately making it a versatile and indispensable material in a wide range of cutting-edge technologies and scientific advancements. Increasing the size of the surface further can be viewed as a novel challenge. However, the surface area can be increased in a number of ways.

2.1. Functionalization

A significant increase in the effective surface area of graphene is achieved through the process of functionalization, which involves the covalent or non-covalent bonding of different chemical moieties, molecules, or nanoparticles to its surface [29]. This augmentation of the surface area is of paramount significance in diverse applications for several fundamental reasons. The introduction of these functional entities results in a higher degree of surface coverage, manifesting as an expanded lattice structure replete with additional binding sites. These augmentations of its surface possess the potential to facilitate a plethora of interactions and reactions, consequently amplifying the available surface area. The functional groups or molecules grafted onto its surface invariably exhibit unique chemical reactivity or binding affinities. These entities serve as loci for subsequent chemical reactions, molecular adsorption, or targeted molecule binding. This enhanced chemical reactivity imparts a significant increase in the effective surface area with specific propensities for particular interactions. The chemical diversity brought about by functionalization endows graphene with custom-tailored surface properties. Such tailoring enables the modulation of wettability, hydrophilicity, hydrophobicity, or selective reactivity toward distinct molecular species. Thus, the diversity in surface chemistry extends the range of applications and renders it an adaptable platform for multifarious requirements [30]. Its functionalization can bestow it with multifaceted functionalities, including catalysis, sensing, and controlled drug delivery. This further augments the realm of applications and broadens the potential utility of functionalized graphene. In addition to these benefits, functionalization also affords improvements in dispersion and its stability within solvents or matrices. Such enhancements are crucial for incorporation into composite materials and coatings, thereby ensuring an extended effective surface area for interactions with other materials. The realm of biological and biomedical applications exploits the functionalized ability to interface with biomolecules. This leads to the augmentation of the effective surface area for biological interactions and is instrumental in applications such as biosensors, targeted drug delivery, and tissue engineering. Functionalized graphene finds application in chemical and gas sensing due to the selectively reactive nature of the introduced chemical functional groups. These functional groups instigate chemical interactions with target analytes, thus affording heightened sensitivity and specificity of the sensor, correspondingly expanding the effective surface area dedicated to molecular recognition events. Its functionalization, by covalent or non-covalent means, results in a profound increase in the effective surface area, concomitant with diverse surface properties and functionalities [31,32]. This augmented surface area is important in rendering it as a versatile and indispensable material in a wide array of scientific, industrial, and technological domains.

2.2. Surface Enhancement by Creating Ripples

Two-dimensional (2D) crystals are expected to be unstable because of thermodynamic requirements that require the formation of out-of-plane bending, which creates a theoretical conundrum when combined with interatomic interactions. The substantially isolated stretching and bending processes result in the production of ripples, which provide durability intrinsic to fictitious 2D materials [33,34]. Transmission electron microscopy (TEM) investigations revealed that free-suspended graphite fails to adhere to the two-dimension paradigm. These tests showed that suspended graphene membranes exhibited notable out-of-plane distortions (ripples) with altitudes as high as 1 nm [35]. Scanning probe microscopy (SPM) studies of graphene in SiO₂ surfaces also revealed nanometer-scale fluctuations that were analogous to those found in independent graphene [36]. The AFM images in Figure 3a,b illustrate the buckling-induced formation of ripples in 500 nm wide monolayer graphene ribbon arrays, achieved by releasing a 30% pre-strain. The buckled ribbons consistently exhibited a regular surface morphology characterized by periodic ripple structures with an approximate amplitude of 80 nm and a periodicity of 340 nm. Rippling was consistently observed on the graphene-covered areas of the ribbons, which typically had a length of 10 μm and a width of up to 1.2 μm, following the strain release. Additionally, the study explored how varying the pre-strain influenced the resulting surface morphologies of these monolayer graphene ripples [37].

Carbon atoms occupy the third spatial dimension because of the spatial and temporal variability in carbon–carbon bond dimensions, which are influenced by vibrations caused by heat and interatomic connections [34]. This facilitates the formation of active ripples as well as a reduction in the total independent energy, as demonstrated in autonomous graphene. Additionally, the unequal distribution of bond lengths is a result of electron delocalization inside the π-cloud along with the resulting e-hole puddle. To reduce free energy, this asymmetry forces the lattice to take on a non-planar form. Notably, scanning tunneling microscopy (STM) shows that heat fluctuation-induced ripples on free-standing graphene display temporal dynamics. Graphene’s bond length asymmetry is most noticeable at the edges and near defects, which increases the density of ripples in these areas [34]. In a thorough study of edge-stress-induced ripples using a deformation warping mode, Shenoy et al. [38] used graphene-simulated edges as elastic strings attached to the material. The stretching of atomic connections caused by the out-of-plane movement of carbon reduced compressible edge stresses.

Polycrystallinity and defect development are common outcomes of large-scale graphene sheet synthesis using chemical vapor deposition (CVD) techniques. Wrinkles and ripples are examples of out-of-plane deformations that have the potential to greatly reduce the number of in-plane stresses caused by stacking faults [39]. Thus, wrinkles and ripples on graphene can be caused by atomic line defects, such as grain boundaries (or dislocations). To characterize ripples that are adjacent to defects, such as disclinations (heptagons or pentagons) and dislocations (heptagon–pentagon di-poles), in graphene, and to predict large-scale configurations of graphene under specific defect distributions, a theoretical model was generalized by the von Karman equation for a flexible solid membrane. In the study by Wang et al. [40], these defect-guided ripples in graphene were further simulated and examined. Using a line defect, which was defined as a long one-dimensional periodic Stone–Wales defect, the researchers found that the amplitude (the height difference between successive peaks) and the wavelength (the distance between periodic ripple peaks) decreased with an increasing angle between the line defect and the direction of strain [41,42]. When graphene is characterized by flaws or exposed to thermal fluctuations, which change the C-C bonds inside the lattice both temporally and spatially, these ripples may arise. Scholars at Radboud University conducted a simulation aimed at elucidating the characteristics of the ripples by examining graphene’s behavior at ambient (300 K) and elevated (1000 K, 2000 K, and 3500 K) temperatures, ultimately concluding that intrinsic ripples could emerge as a result of thermal fluctuations. This discovery is exceptionally promising as it suggests that the ripples can be manipulated artificially through the modulation of temperature [34]. These modulations, such as variations in temperature or the presence of defects, result in the carbon atoms occupying additional spatial dimensions, thereby facilitating the formation of ripples. This alteration in the carbon configuration prompts the electrons within the p-cloud to reallocate, leading to variations in the symmetries of the bond lengths within the structure [43]. The disruption of symmetry engenders a non-planar configuration, which subsequently reduces the free energy associated with the graphene. Consequently, ripples serve to preserve the structural stability of graphene in the face of architectural changes [44]. Figure 4 shows a schematic that depicts the formation of ripples on a graphene surface to achieve an enhanced surface area. Starting with a smooth graphene sheet, a stimulus, such as ion beam irradiation, is applied. This stimulus induces surface instability, leading to the self-organization of the graphene into a periodic ripple pattern. The resulting structure exhibits a significantly increased surface area compared to the initial smooth graphene.

2.3. Layering

The increase in surface area achieved through the layering, or stacking, of graphene involves the superposition of multiple sheets or layers atop one another in a manner that generates a three-dimensional structure [33]. This stacking leads to significant augmentation in the effective surface area for several fundamental reasons. Primarily, the superimposed graphene layers generate a stratified, interlocking structure with the potential for creating additional interlayer spacing and, therefore, additional surface area between the layers [34]. This interlayer spacing can be manipulated to accommodate guest molecules, solvents, or ions, further enhancing the overall surface area accessible for interactions. Secondly, the layering of sheets in three dimensions can produce unique geometries, such as mesoporous structures, within the interlayer spaces. These mesopores create an extended surface topology, featuring numerous pores and channels that amplify the surface area available for adsorption, catalysis, and molecular interactions. The layering of sheets can result in the creation of edges and corners where the individual layers meet. These edges and corners provide high-energy sites that are more chemically reactive, thus offering increased opportunities for chemical reactions, binding, and surface-related processes. Consequently, the increase in surface area due to layering is not solely a consequence of the interlayer spacing but also the emergence of these high-energy sites at layer interfaces. The substantial augmentation in surface area by layering makes it particularly advantageous in applications such as gas storage, adsorption, and separation. The three-dimensional structure enhances the capacity for gas adsorption within the interlayer spaces and mesoporous regions, amplifying the storage capabilities for gases, including hydrogen and methane. In applications like filtration and environmental remediation, the layering method provides a larger area of contact for the adsorption of different compounds and ions. This increased surface area is key in enhancing the performance of G-b materials and devices in applications that require gas storage, adsorption, catalysis, and molecular interactions, further solidifying the significance of graphene in multidisciplinary fields of science and technology.

2.4. Buckling Behavior of Graphene

The relationship between graphene’s surface area and its buckling behavior can be connected through the adhesive interactions with the substrate, which play a crucial role in determining the material’s structural stability. The study by Yang et al. [11] showed that for graphene sheets supported on a substrate, the critical buckling stress could reach 4.39 N/m, and the critical buckling strain could be as high as 1.58%, which was about ten times greater than those for free-standing sheets [11]. This enhancement was primarily due to increased adhesion rather than the size of the graphene itself, as the critical buckling strain remained largely independent of its size for sheets longer than 20 nm. Additionally, the energy analysis indicated that the competition between adhesion energy and strain energy was crucial, suggesting that a larger surface area could improve adhesion and thus reduce the likelihood of buckling under compression. The buckling behavior was more directly correlated with the sheet’s size, even though a larger surface area may improve adhesive interactions. It was subjected to compressive loads that tend to wrinkle, with the wavelength, amplitude, and direction of the wrinkles being adjustable by modifying the boundary conditions at the edges of its sheets [20]. Its modification to increase its surface area not only improves its potential for applications like energy storage [35] and catalysis [36] but also strengthens its mechanical resilience against buckling due to enhanced adhesion with the underlying substrate [38]. The schematic shown in Figure 5 illustrates the formation of graphene with an increased surface-to-volume ratio through buckling under compressive load. A compressive force is applied uniaxially on the graphene sheet. As the tensile load increases, the graphene sheet becomes unstable and buckles, forming a periodic, wrinkled structure. This buckling deformation significantly increases the surface area of the graphene compared to its initial flat state. The resulting morphology, including the amplitude and wavelength of the buckles, is influenced by factors such as the magnitude and direction of the tensile load, the number of graphene layers, the presence of defects, and the interaction with the substrate.

2.5. Nanostructures on Graphene Surfaces

The introduction of nanostructures on graphene surfaces entails the deposition, growth, or attachment of nanoscale materials, such as nanowires, nanotubes, or nanoparticles, onto the graphene substrate. This process leads to a substantial increase in its effective surface area, with profound implications for various applications. First and foremost, the deposition of nanoscale materials onto its surfaces introduces a three-dimensional aspect to the otherwise two-dimensional lattice [45]. The nanostructures create an intricate topography, producing a wealth of additional surface features, including protrusions, pits, and facets. This increased surface complexity enhances the surface area available for interactions, making it particularly well-suited for adsorption, catalysis, and sensing applications. The presence of nanostructures on the graphene surface augments the density of active sites and edge sites. Nanostructures often have high surface-to-volume ratios, which means that the active surface area per unit volume is significantly increased [46]. As a result, there are significantly more active sites available for chemical reactions or adsorption processes. The intricate spatial arrangement of nanostructures also introduces a level of roughness and porosity to the surface [47]. These surface irregularities offer additional opportunities for molecular adsorption, making them well-suited for applications such as gas storage, sensing, and catalysis. In addition to providing extra surface area for molecular interactions, nanostructures can offer specific functionalities due to the unique properties of the nanostructures themselves [48]. For example, CNTs can be highly conductive, while metal or metal oxide nanoparticles can catalyze reactions. These functional nanostructures not only increase the surface area but also contribute to the overall performance of the G-b materials in applications like electrocatalysis or electrochemical sensing [49]. Nanostructures on graphene are particularly advantageous in fields like energy storage and conversion. The integration of metal nanoparticles into it can improve the electrocatalytic activity of fuel cells, while the attachment of semiconductor nanostructures can enhance the performance of photovoltaic devices [46]. The incorporation of nanostructures onto its surfaces results in a substantial increase in the effective surface area due to the added topographical complexity, higher mass in a unit volume of active locations, and the introduction of surface irregularities.

3. Properties of Graphene with Enhanced Surface Area

Karaphun et al. [50] conducted a study with samples of graphene, rGO, and enhanced-surface-area graphene, named rGO_An, created by an annealing method. They compared the Brunauer–Emmett–Teller (BET) properties of the normal rGO with rGO_An. Using nitrogen adsorption–desorption isotherms, the particular surface area of the rGO and rGO_A materials was methodically investigated. The rGO and rGO_An samples’ isotherm curves were classified as type IV isotherms because they displayed a hysteresis loop, indicating the existence of significant mesopores and macropores [51,52]. Furthermore, the loops of hysteresis linked to the rGO_A sample were noticeably wider than those of the rGO sample, suggesting that the severely crumpled rGO sheets contained a substantial number of pores. The rGO and rGO_An samples were found to have specific BET surface areas of 130.2 and 672.1 m²/g, respectively. Additionally, the BJG method revealed that the rGO and rGO_An samples had mean pore size variations of 19.5 nm and 49.2 nm, respectively. Notably, the rGO_An sample’s overall pore size ranges were higher than the rGO samples. The pore volumes of the rGO sample (0.635 cm³/g) and the rGO_A sample (0.826 cm³/g) were almost equal, according to another finding. Larger BET-specific surface areas, smaller pore diameters, and advantageous scattering states are known to make it easier for electrolyte ions to enter the electrodes’ pore surfaces, which raises the specific capacitance [52]. Therefore, the rGO_An sample activated a greater number of electrodes than the rGO sample in supercapacitor applications. Graphene derivatives, such as GO and rGO, have undergone significant advancements, improving their surface area in the field of electronic structure and properties for diverse applications. The reduction of GO enhances electrical conductivity by restoring its graphene-like structure and reducing oxygen functional groups, resulting in smoother, thinner films with superior performance [53]. Additionally, their unique quantum properties, including Dirac electron interactions and the anomalous Hall effect, can be tuned to create advanced nanomaterials and metamaterials for electronic applications [54]. These substances are perfect for batteries, supercapacitors, and sensors because they also have outstanding electrochemical qualities, such as an increased surface area and improved electron mobility [55].

4. Surface Area in Supercapacitors and Lithium-Ion Batteries

Within the ever-evolving realm of energy storage technologies, graphene, a two-dimensional carbon allotrope, has emerged as a transformative material, harnessing its exceptional properties to revolutionize the performance of SCs and lithium-ion batteries. Because of its extraordinary surface area, graphene is becoming a major focus of research and development efforts as the need for more efficient and sustainable solutions for storing energy increases globally. This extraordinary material holds the promise of enabling energy storage systems with augmented capacities, swifter charge and discharge rates, extended service life, and superior overall efficiency. This scientific investigation embarks on a comprehensive exploration of graphene’s surface area, delving into the fundamental principles, experimental findings, and practical applications that underpin its pivotal role in the relentless pursuit of enhanced energy storage capabilities within the framework of scientific inquiry and innovation. Some of the applications of its surface area in the field of supercapacitors and lithium-ion batteries are discussed here.

4.1. Surface Area and Electrode Capacitance

Laxman et al. [39] conducted a study on capacitive deionization with asymmetric electrodes, where they compared the electrode capacitance with the electrode surface area. They used a 10 mM solution of zinc acetate dihydrate to prepare a solution in deionized water. After being cleaned and immersed in the zinc acetate solution for 20 min, an activated carbon cloth (ACC) was dried for 30 min at 100 °C in an oven. To confirm that a thick layer of zinc ions was formed on the ACC surface, this procedure was repeated three times. Figure 6A illustrates the electrochemical impedance spectroscopy for AA and ZnO-ACC, and Figure 6B illustrates the desalination performance of the symmetric and asymmetric arrangements of the ZnO-ACC and ACC electrodes. The zinc-coated ACC was then subjected to calcination at 350 °C for 5 h to produce ACC surfaces with seeded ZnO nanoparticles. By analyzing Nyquist plots, the electrode-specific capacitance values were confirmed. It was clear that in contrast to the uncoated ACC electrodes, the electrodes coated with ZnO nanorods (ZnO NR) on an ACC displayed unique electrochemical properties. Notably, the ACC sensors coated with ZnO NR showed reduced series electrode resistance and increased charge transfer resistance. The Nyquist plots were used to extract important electrical properties, such as coating capacitance, double-layer capacitance, and pore resistance, using a modified Randles equivalent circuit model. After coating the ACC with ZnO NRs, the results showed a significant increase in coating capacitance, from 12.36 mF to 600 mF, for a 1 cm² electrode [40], along with a slight rise in the combined layer capacitance, from 4 F to 4.6 F. The transfer resistance of the ZnO NR-ACC electrodes exhibited a significant elevation when compared to the uncoated ACC electrodes. This heightened resistance posed an additional impediment to undesirable Faradaic currents and simultaneously amplified the electrode pore resistance [39,41].

The Warburg impedance curve showed a delayed response from the ZnO NR-ACC electrode surface, suggesting that, even at lower frequencies, ions could readily reach the electrode surface. This was explained by the uniformly dispersed electric field and the ZnO NRs’ adsorption-accessible surface. When the ZnO NR-coated ACC electrodes were paired with the uncoated ACC electrodes as counter electrodes, they exhibited distinct physical, chemical, and electrical properties. The electrodes were set up in three distinct configurations to examine the main factor affecting salt adsorption in an asymmetric capacitive deionization (CDI) cell [56,57,58]. The results revealed that capacitance, rather than the surface area, played the dominant role. When compared to the symmetric ACC electrode devices, the ZnO NR-coated symmetric ACC electrode devices continuously showed a greater specific salt removal capability. However, when the symmetry of the electrodes was disrupted, the salt adsorption capacity decreased significantly [59,60,61]. A comparable investigation into large-scale pattern-generation graphene sheets for transparent electrodes was conducted by Kim et al. [42]. They created a simple technique that uses CVD on nickel layers to produce and transfer stretchable, high-quality graphene films on a large scale. By varying the thickness of the catalytic metals, growth time, and UV treatment duration, the number of layers in these patterned films can be easily transferred to stretchable substrates using straightforward contact procedures. Scaling up production is possible because the only restriction on the film’s dimensions is the size of the CVD growing chamber. These graphene films’ remarkable optical, electrical, and mechanical qualities create opportunities for use in flexible, stretchable, and foldable transparent electronics on a wide scale [43,44].

4.2. Reduced Diffusion Path Length

Graphene’s larger surface area brings the active material closer to the electrode–electrolyte interface. This reduces the diffusion path length for ions and electrons, which is critical for achieving fast charge–discharge rates. In a study by Uthaisar and Barone [62], they initially investigated the adsorption energies of lithium (Li) along different pathways within two-dimensional graphene. Given the honeycomb lattice’s high symmetry, they identified two distinct diffusion paths [62,63]. The result depended on whether the lithium adatom moved from the hexagon’s center (H) by moving across the middle of the bonding (M) or if it landed on the highest point of a molecule of carbon (T). However, upon reducing its dimensionality to create an armchair nanoribbon, the lattice’s symmetry was disrupted, causing hexagons near the ribbon’s edge to differ from those at the center. As a result, the primary diffusion pathways (A and B) seen in graphene branched into multiple paths due to this loss of symmetry. Figure 7 illustrates a computational study of directional movement and associated energy barriers within graphene and crisscross graphene nanoribbons (GNRs). It shows how atoms or particles move along specific paths (labeled as A, A₁–A₄, and A′) in both graphene and GNR structures. By examining these paths, the diagrams reveal the energy barriers encountered during movement along different directions. As shown in Figure 7, this created variations in barrier profiles, especially near the edges. For instance, the authors observed two distinct paths (A₁ and A₂) of type A, which exhibited different energy barriers, particularly near the edges. They found that the activation energy (Ea) was slightly lower in the center of the ribbon compared to graphene, but it increased significantly near the edges for A₁. Conversely, the same energies for A₂ were higher, with no apparent edge effects. Paths A₁ and A₂ produced comparable results at the margins at temperatures up to 3000 K. The measured length of the ribbon determined the range of distinct types of B pathways. Seven different paths of this sort were pinpointed in the research were depicted because they all showed similar barrier properties. For diffusion toward the axis of the ribbon, they identified distinct diffusion pathways with barrier profiles resembling those observed in graphene.

The rate at which lithium (Li) ions moved within the pathways at ambient conditions was only approximately six times faster compared to that of graphene. However, in the case of the perpendicular direction, there was a consistent reduction in the obstacles toward the edges. Specifically, for pathways A′, as shown in Figure 7e, and B, as shown in Figure 8c, the obstacles in energy toward the side corners were notably less compared to those for pathways along the axis parallel to the ribbon, resulting in a coefficient of diffusion toward the pathways that can be as high as two orders of magnitude compared to graphene. Li atoms capable of overcoming the initial obstacles can diffuse easily toward the side corners [64]. Its atoms may efficiently egress from carbon atoms, suggesting potential for the Li anodes with greater power made from these elements compared to traditional graphitic materials [65]. It is worth mentioning that this trend of edge effects contrasts with the recent findings of one study on silicon nanowires, where the adsorption energies were greater, which could be observed in the middle of the structures, and there were greater barriers to migration along the surface [66]. The study discovered that in graphene nanoribbons with edges in the shape of armchairs, diffusion probably takes place toward the side corners rather than toward the nanoribbon axis. In zigzag nanoribbons, edge effects were even more pronounced. The authors observed different diffusion channels depending on the ribbon’s width. The binding energies of lithium atoms atop hexagons were sensitive to their proximity to the side corners, but their activation energies remained fairly consistent. This resulted in the formation of a direction conducive to lithium conduction along the length of the ribbon [67]. The rate of lithium movement along these paths was only about six times higher than in graphene at room temperature. Moreover, the energy obstacles toward the boundary regions were significantly lesser than those for pathways along the ribbon’s length, indicating the possibility of creating high-power lithium anodes from these materials. Nevertheless, carbon electrodes tend to develop a solid electrolyte interface, potentially impacting the battery’s capacity. If this interface forms predominantly at the edges, it may have a more substantial impact on the battery’s energy and power density compared to traditional graphite electrodes. Nevertheless, the possible enhancement in diffusion rates along particular pathways and the increased reactivity of edges for lithium attachment in comparison to graphite necessitate experimental exploration. Conducting experiments is crucial to investigate potential enhancements in the diffusion coefficient toward specific pathways and the increased reactivity of edges for lithium adsorption, as opposed to graphite. Figure 8 presents a computational analysis of lithium atom movement and adsorption sites within graphene and GNRs. In graphene, movement direction B is analyzed with specific adsorption positions H, M, and T, where lithium can attach above a hexagon, between two carbon atoms, or directly on top of a carbon atom, respectively. The crisscross GNR illustration in Figure 7b demonstrates the B movement toward the side corners and the corresponding barriers to energy for this movement in the nanoribbon and graphene, highlighting the influence of atomic positioning and structural features on lithium mobility and binding energy in these materials.

5. Environmental Concerns in Utilizing the Surface Area of Graphene

The endeavor to functionalize graphene while retaining its fundamental structural and electrical properties poses significant scientific challenges [68]. These challenges encompass not only the optimization of the functionalization process but also the paramount requirement to maintain the long-term stability of functionalized graphene materials. Several studies [69,70,71] have been conducted to overcome these issues during the last few decades. Considerable effort has been dedicated to the creation of high-quality graphene electrodes, reflecting the widespread interest in harnessing their remarkable properties for various applications. Ke et al. [69] effectively analyzed the diverse approaches and made advancements in this field. These materials are excellent resources for researchers and professionals who want to learn about the cutting-edge methods and processes used to create graphene electrodes with exceptional quality and functionality. More details on the difficulties with scalability and the manufacturing techniques are provided below. The current methods for producing high-quality graphene sheets, such as CVD [72,73,74] and mechanical exfoliation [75,76,77,78], are often suitable for research but face challenges in scaling up for industrial applications. Achieving uniformity, high yield, and cost-effectiveness in large-scale production remains a significant scientific challenge. Furthermore, adding functional groups to graphene can change its stability even though it can customize its characteristics for particular uses [47,79,80]. The scientific challenge lies in optimizing the functionalization process to maintain the desirable properties of graphene while ensuring long-term stability. The potential toxicity of graphene and its derivatives, particularly GO, is a subject of ongoing investigation. Understanding the mechanisms of interaction between graphene and biological systems and elucidating toxicity thresholds and biocompatibility in a precise and quantifiable manner are scientific challenges [81,82,83].

The interaction of graphene with environmental components is a scientific concern. Its increased surface-to-volume ratio can lead to the effective adsorption of environmental pollutants, and the subsequent release of these adsorbed species under varying conditions necessitates in-depth study. Investigating these interactions and their ecological implications is crucial [84,85,86,87].

Table 1. Summary of the different methods used to increase the surface area of graphene.

References	Modification Method	Increased Surface Area	Key Findings
[2]	Exfoliation method	1100 m2/g	-
[68]	Hydrothermal method	165.95 m2/g	Higher specific capacitance with CoFe2O4 addition to rGO
[95]	Hydrothermal carbonization	2106 m2/g	High packing density and capacitance
[96]	Carbon nanoparticle functionalization	1256 m2/g	High specific capacitance of 324.6 F/g
[97]	Tape casting	>400 m2/g	High electrical conductivity and tensile strength
[98]	Hydrazine reduction	745 m2/g	High electrical conductivity and specific capacitance
[99]	Chemical reduction	195.97 m2/g	Low-cost production method
[100]	Hydrazine reduction with rGO	891 m2/g	Useful for supercapacitors
[101]	Active rGO method	1000 to 3000 m2/g	Increased surface-to-volume ratio
[102]	GO reduction	670.98 m2/g	Increased surface-to-volume ratio provides high electric conductivity

summarizes the increased surface-to-volume ratio of graphene sheets. This benefit can be used to ensure less toxicity. Scientifically ensuring the purity, uniformity, and quality of G-b materials is a challenge in graphene production. The methods for characterizing and controlling material defects, grain boundaries, and other imperfections require continued scientific investigation [88,89,90,91]. Graphene’s exceptional mechanical properties are direction-dependent, and maintaining mechanical strength and durability in real-world applications is a scientific challenge [92,93,94]. Understanding the influence of structural defects, environmental conditions, and strain on its mechanical behavior is essential.

6. Toxicity Related to Graphene Derivatives

Most graphene research highlights its positive physical and biological traits. Studies often emphasize biocompatibility and cell growth. However, the negative impacts of graphene appear less investigated and should be considered as well. The primary research highlights the good biocompatibility of various graphene derivatives (GDs) across different cell lines, including fibroblasts, osteoblasts, cervical carcinoma cells, vascular smooth muscle cells, and mesenchymal stem cells. Multiple studies [103,104] demonstrated that GD-like graphene paper, graphene oxide–hydroxyapatite nanocomposites, chitosan-based graphene oxide scaffolds, and poly-sulfone/GO membranes generally support cell attachment and proliferation and even enhance cell viability without inducing significant cytotoxicity. While the focus is largely on positive outcomes, literature note that biocompatibility can vary depending on the specific properties of GD, such as surface chemistry, morphology, and synthesis route, emphasizing the need for comprehensive material characterization and standardized production methods. Despite the overall positive trend, this section implicitly acknowledges the importance of further investigation into potential negative outcomes, as most research has historically emphasized the beneficial aspects of graphene [105,106].

Controlling the potential toxicity of GD involves a multifaceted approach focusing on material design, processing, and application. Careful control over the synthesis process to minimize impurities and tailor the size, shape, and surface functionalization of graphene materials is crucial, as these properties significantly influence their biological interactions. Surface modification with biocompatible coatings or functional groups can reduce direct cellular interactions and enhance dispersibility, preventing agglomeration, which can lead to localized toxicity [107]. Rigorous in vitro and in vivo testing using standardized protocols is essential to thoroughly evaluate the toxicological profile of specific graphene derivatives under relevant exposure conditions, ultimately guiding the safe development and application of these promising materials [108].

7. Conclusions

This review outlines recent advancements in the development of graphene-based materials with enhanced surface area, emphasizing their potential in energy storage and other emerging applications. The main insights from the literature are summarized below:

• Surface area and capacitance: Extensive research demonstrates that enhancing the surface area of carbon-based materials decreases resistance in supercapacitors by offering a greater number of electrochemically active sites, which, in turn, leads to improved capacitance. Hybrid nanomaterials have significantly enhanced the performance of electrochemical materials in energy storage and conversion. However, more research is needed to understand the synthesis, structure, and properties of these materials.
• Synergistic interactions: Potential synergistic effects may result from these interactions, which alter the structure and geometry of materials produced atop graphene and carbon nanotubes. To comprehend charge transfer mechanisms, it is also essential to investigate the electrical and chemical structures at the interface of graphene, carbon nanotubes, and supporting materials.
• Physical property modulation: An increased surface-to-volume ratio in nanomaterials significantly enhances their optical, electrical, thermal, and mechanical properties. It improves light absorption (including surface plasmon resonance), boosts conductivity, and increases active sites for reactions and adsorption. It also affects surface energy and wettability, influencing behaviors like adhesion and liquid spreading. These effects are highly dependent on the material’s composition, structure, and environment.

In conclusion, this review highlights the pivotal influence of surface area on optimizing the properties and functionality of graphene-based materials, particularly for energy storage and related technologies. It advocates for future studies to delve deeper into the relationships between surface architecture, material interfaces, and device performance, paving the way for the strategic development of advanced energy systems.